Power Supplies for LED Drivers - 2008.pdf

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ElsevierUSPLDPrelimsH83411422008Page:1Trim:7.59.25inPowerSuppliesforLEDDriving ElsevierUSPLDPrelimsH83411422008Page:2Trim:7.59.25in ElsevierUSPLDPrelimsH83411422008Page:3Trim:7.59.25inPowerSuppliesforLEDDrivingSteveWinderAMSTERDAM•BOSTON•HEIDELBERG•LONDONNEWYORK•OXFORD•PARIS•SANDIEGOSANFRANCISCO•SINGAPORE•SYDNEY•TOKYONewnesisanimprintofElsevier ElsevierUSPLDPrelimsH83411422008Page:4Trim:7.59.25inNewnesisanimprintofElsevier30CorporateDrive,Suite400,Burlington,MA01803,USALinacreHouse,JordanHill,OxfordOX28DP,UKCopyrightÓ2008byElsevierInc.Allrightsreserved.Nopartofthispublicationmaybereproduced,storedinaretrievalsystem,ortransmittedinanyformorbyanymeans,electronic,mechanical,photocopying,recording,orotherwise,withoutthepriorwrittenpermissionofthepublisher.PermissionsmaybesoughtdirectlyfromElsevier’sScience&TechnologyRightsDepartmentinOxford,UK:phone:(þ44)1865843830,fax:(þ44)1865853333,E-mail:permissions@elsevier.com.YoumayalsocompleteyourrequestonlineviatheElsevierhomepage(http://elsevier.com),byselecting‘‘Support&Contact’’then‘‘CopyrightandPermission’’andthen‘‘ObtainingPermissions.’’Recognizingtheimportanceofpreservingwhathasbeenwritten,Elsevierprintsitsbooksonacid-freepaperwheneverpossible.LibraryofCongressCataloging-in-PublicationDataApplicationsubmittedBritishLibraryCataloguing-in-PublicationDataAcataloguerecordforthisbookisavailablefromtheBritishLibrary.ISBN:978-0-7506-8341-8ForinformationonallNewnespublicationsvisitourWebsiteatwww.books.elsevier.comPrintedintheUnitedStatesofAmerica08091011121310987654321Workingtogethertogrowlibrariesindevelopingcountrieswww.elsevier.com|www.bookaid.org|www.sabre.org ElsevierUSPLDPrelimsH83411422008Page:5Trim:7.59.25inTableofContentsPreface.....................................................................................................ixChapter1:Introduction................................................................................11.1ObjectivesandGeneralApproach.............................................................11.2DescriptionofContents.............................................................................2Chapter2:CharacteristicsofLEDs................................................................72.1ApplicationsforLEDs...............................................................................82.2LightMeasure..........................................................................................122.3EquivalentCircuittoanLED..................................................................132.4VoltageDropVersusColorandCurrent.................................................142.5CommonMistakes...................................................................................15Chapter3:DrivingLEDs............................................................................173.1VoltageSource.........................................................................................173.2CurrentSource.........................................................................................243.3TestingLEDDrivers................................................................................293.4CommonMistakes...................................................................................303.5Conclusions..............................................................................................31Chapter4:LinearPowerSupplies................................................................334.1Introduction.............................................................................................334.2AdvantagesandDisadvantages................................................................374.3Limitations...............................................................................................374.4CommonErrorsinDesigningLinearLEDDrivers.................................37Chapter5:Buck-BasedLEDDrivers............................................................395.1AnExampleBuckConverterControlIC.................................................405.2BuckCircuitsforDCApplications..........................................................415.3BuckCircuitsforACInput......................................................................46www.newnespress.com ElsevierUSPLDPrelimsH83411422008Page:6Trim:7.59.25inviTableofContents5.4BuckCircuitsPoweredbyanACPhaseDimmer....................................525.5CommonErrorsinACInputBuckCircuits............................................545.6DoubleBuck............................................................................................555.7HystereticBuck........................................................................................59Chapter6:BoostConverters.......................................................................616.1BoostConverterOperatingModes..........................................................626.2HV9912BoostController........................................................................636.3DesignofaContinuousConductionModeBoostLEDDriver..............676.4DesignofaDiscontinuousConductionModeBoostLEDDriver..........836.5CommonMistakes...................................................................................986.6Conclusions..............................................................................................98Chapter7:Boost-BuckConverter................................................................997.1TheCukConverter................................................................................1007.2SEPICBuck-BoostConverters..............................................................1317.3Buck-BoostTopology............................................................................1397.4CommonMistakesinBoost-BuckCircuits............................................1397.5Conclusions............................................................................................140Chapter8:LEDDriverswithPowerFactorCorrection................................1418.1PowerFactorCorrection.......................................................................1418.2Bi-Bred...................................................................................................1428.3Buck-Boost-Buck(BBB)........................................................................1448.4CommonMistakeswithPFCCircuits...................................................1478.5Conclusions............................................................................................147Chapter9:Fly-BackConverters.................................................................1499.1TwoWindingFly-Back..........................................................................1509.2ThreeWindingFly-Back........................................................................1539.3SingleWindingFly-Back(Buck-Boost).................................................158Chapter10:EssentialsofSwitchingPowerSupplies.....................................16110.1LinearRegulators..................................................................................16110.2SwitchingRegulators.............................................................................162Chapter11:SelectingComponentsforLEDDrivers....................................17511.1DiscreteSemiconductors........................................................................17511.2PassiveComponents..............................................................................18211.3ThePrintedCircuitBoard(PCB)..........................................................19111.4OperationalAmplifiersandComparators.............................................193www.newnespress.com ElsevierUSPLDPrelimsH83411422008Page:7Trim:7.59.25inTableofContentsviiChapter12:MagneticMaterialsforInductorsandTransformers..................19512.1FerriteCores..........................................................................................19712.2IronDustCores.....................................................................................19712.3SpecialCores..........................................................................................19812.4CoreShapesandSizes...........................................................................19812.5MagneticSaturation..............................................................................19912.6CopperLosses........................................................................................200Chapter13:EMIandEMCIssues.............................................................20313.1EMIStandards......................................................................................20413.2GoodEMIDesignTechniques..............................................................20513.3EMCStandards.....................................................................................21413.4EMCPractices.......................................................................................215Chapter14:ThermalConsiderations..........................................................21714.1EfficiencyandPowerLoss.....................................................................21714.2CalculatingTemperature.......................................................................21814.3HandlingHeat–CoolingTechniques....................................................220Chapter15:SafetyIssues.........................................................................22515.1ACMainsIsolation...............................................................................22515.2CircuitBreakers.....................................................................................22615.3CreepageDistance.................................................................................22615.4CapacitorRatings..................................................................................22615.5LowVoltageOperation.........................................................................227Bibliography...........................................................................................229Index.....................................................................................................231AuthorBiography.....................................................................................233www.newnespress.com 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ElsevierUSPLDPrelimsH83411422008Page:9Trim:7.59.25inPrefaceTheLEDhasbeenavailableformanyyearsnow,initiallyasaredcoloredindicator.Later,yellow/amber,greenandfinallybluecoloredLEDsbecameavailable,whichtriggeredanexplosioninapplications.Applicationsincludedtrafficlights,vehiclelightsandwall-washes(moodlighting).RecentlybluecoloredLEDshavebeencombinedwithyellowphosphortocreatewhitelight.TheamountoflightavailablefromLEDshasalsoincreasedsteadily,andnowpowerlevelsof1W,3Wand5Warefairlycommon.DrivingasingleLED,oralongstringofLEDsconnectedinseries,hasrelativelyfewproblemswhenthecurrentislow(maybe20mA).HighcurrentLEDsaretoughertodrive,requiring350mA,700mA,1Aorhigher.Ofcourse,asimplelinearregulatorcouldbeusedifpowerdissipationwasnotanissue,orasimpleresistorifcurrentregulationisnotcritical.However,inmostapplications,anefficientswitchingregulatorisused.AswitchingregulatorisessentialiftheLEDstringvoltageishigherthanthesupplyvoltage,orifthesupplyvoltagehaswidevariation.Butswitchingmeansthatelectro-magneticinterference(EMI)hastobeconsideredtoo.ThisbookdescribesanumberofLEDdrivingmethods.Themainaimsofthisbookare:(1)toshowsuitabletypesofLEDdrivertopologiesforgivenapplications;(2)toworkthroughsomeexamples;and(3)toavoidsomeofthemistakesthatsomeengineersmakewhencreatingtheirowndesigns.However,thecontentisnotexhaustiveandfurtherreadinginsomeperipheraltopicswillbenecessary.Significantdatatocreatethisbookhavebeendrawnfromthedatasheets,applicationnotes,trainingmaterialanddiscussionsprovidedbymycolleaguesinSupertex,particularlyRohitTirumalaandAlexMednik.SteveWinder,2007.www.newnespress.com CHAPTER1IntroductionAsafieldapplicationsengineerforoneofthepioneeringdevelopersofintegratedcircuitsfordrivingpowerLEDs,ImeetmanypotentialcustomerswhohavelittleornoideaofhowtodriveanLEDproperly.TheoldertypeofLEDrequiringa20mAsupplycanbeabusedtosomeextent.However,powerrequirementshavebeenincreasing;currentratingsof30mA,50mA,100mA,350mAandhigherarebecomingcommon.Thereareseveralmanufacturersthatproducepowerlevelsupto20W,andmore;thesehigherpowersuseLEDchiparrays.IfapowerLEDisabused,ittendstodieveryquickly.PowerLEDsarebeingusedinincreasingnumbers;inchannellighting(signage),trafficlights,streetlights,automotive,moodlighting(colourchanging‘wallwash’),theatrelightingforstepsandemergencyexits.NamessuchasHB-LEDs(highbright)andUB-LEDs(ultra-bright)arebecomingmeaninglessasthepowerlevelscontinuetorise.ThisbookwillcoveralltypesofLEDdrivers,fromlowpowertoUB-LEDsandbeyond.IspowerLEDdrivingsimple?No,notusually.Inafewcasesalinearregulatorcanbeused,whichissimple,butmostcasesrequireaswitchingpowersupplywithaconstantcurrentoutput.Lineardrivingisinefficientandgeneratesfartoomuchheat.Withaswitchingsupply,themainissuesareEMIandefficiency,andofcoursecost.Theproblemistoproduceadesignthatmeetslegalrequirementsandisefficient,withminimalcost.1.1ObjectivesandGeneralApproachTheapproachofthisbookwillbeverypractical,althoughsometheoryisintroducedwhennecessaryforunderstandingoflaterchapters.Itisimportanttounderstandthecharacteristicsofcomponentsbeforetheycanbeusedeffectively.www.newnespress.com 2Chapter1Inmostchapters,Iwillincludeasectioncalled‘CommonErrors’.Thissectionwillhighlighterrorsthatengineershavemade,andhowthesecanbeavoided,withthehopethatreaderswillnotmakethesamemistakes.Itissaidthatpeoplelearnfromtheirmistakes,butitisalsotruethatwecanlearnfromthemistakesofothers.Ourownmistakesaremorememorable,butalsomorecostly!Usuallythefirstproblemforadesigneristochoosebetweendifferenttopologies.Whenisabuckpreferredtoabuck-boostoraboost?WhyisaCukboost-buckbetterthanafly-backtype?Thisbookwillcoverthesetopicsatthebeginningoftheswitchingsuppliessection.Powersupplydesignequationswillbegivenandexampledesignsofpracticalsupplieswillbeworkedthrough.Withswitchingpowersupplies,equationsareneededtomakethecorrectcomponentchoice;awrongcomponentcanmakeapoorpowersupplyandrequirealotofcorrectiveaction.PowerLEDsgeneratealotofheatinasmallarea,whichmakesthermalmanagementdifficult,soanadjacentpowersupplyshouldbeefficientandnotaddtoomuchheatingeffect.Theimplicationsofchangingthecalculatedcomponentvaluesintostandardvalues,whichismorepractical,willbediscussed.Inmanycases,customerswanttousestandardoff-the-shelfparts,becauseofeaseofpurchaseandcost.Calculationsrarelyproduceastandardvalue,soacompromisehastobemade.Insomecasesthedifferenceisnegligible.Inothersitmaybebettertochooseahigher(orlower)value.Allcomponentvaluechangeswillintroducesome‘error’inthefinalresult.Havingprovenworkedexamplesinthebookwillhelpthereadertounderstandthedesignprocess:theorderinwhichthedesignprogresses.Itwillalsoshowhowthecalculatedcomponentvaluecompareswiththeactualvalueused,andwillincludeadescriptionofwhythechoicewasmade.1.2DescriptionofContentsInChapter2,thedescriptionofsomeLEDapplicationswillshowthebreadthoftheLEDdrivingsubjectandhowLEDs’physicalcharacteristicscanbeusedtoanadvantage.ItisalsoimportanttounderstandthecharacteristicsofLEDsinordertounderstandhowtodrivethemproperly.Oneofthecharacteristicsiscolour;anLEDemitsaverynarrowbandofwavelengthssothecolourisfairlypure.TheLEDcolordeterminesthedifferentvoltagedropacrosstheLEDwhileitisconducting,andIwillwww.newnespress.com Introduction3showhowthatvarieswiththecurrentlevel.Butthecurrentleveldeterminesthelightoutputlevel:highercurrentsgivehigherluminosityfromagivenLED.Thelightoutputhasthecharacteristicofintensityandtheamountofbeamspreading.Chapter3willshowthatthereareseveralwaystodriveLEDs.Becausemostelectroniccircuitshavetraditionallybeendrivenbyavoltagesource,itisnaturalfordesignerstocontinuethiscustomwhendrivinganLED.ThetroubleisthatthisisnotagoodmatchfortheLEDpowerrequirement.Aconstantcurrentloadneedsaconstantvoltagesource,butaconstantvoltageload(whichiswhatanLEDis)needsaconstantcurrentsupply.So,ifwehaveaconstantvoltagesupply,weneedtohavesomeformofcurrentcontrolinserieswiththeLED.Withaseriesresistororactiveregulatorcircuitwearetryingtocreateaconstantcurrentsupply.Infact,ashortcircuitinanypartofthecircuitcouldleadtoacatastrophicfailuresowemayhavetoprovidesomeprotection.DetectinganLEDfailureispossibleusingacurrentmonitoringcircuit.Thiscouldalsobeusedtodetectanopencircuit.Insteadofhavingaconstantvoltagesupply,followedbyacurrentlimiter,itseemssensibletojustuseaconstantcurrentsupply!Therearesomemeritsofusingbothconstantvoltagesupplyandacurrentregulator,whichwillbedescribedinChapter4.Chapter3continuesdescribingfeaturesofconstantcurrentcircuit.Ifwehaveaconstantcurrentsource,wemayhavetoprovidesomevoltagelimitingarrangement,justincasetheloadisdisconnected.Opencircuitprotectioncantakemanyforms.Afailure(short)wouldmakenodifferencetothecurrentlevel,sovoltagemonitoringwouldbeapreferredfailuredetectionmechanism.Ifthecircuitfailedopenthevoltagewouldriseuptotheleveloftheopencircuitprotectionlimit,whichcouldalsobedetected.Chapter4describeslinearpowersupplies,whichcanbeassimpleasavoltageregulatorconfiguredforconstantcurrent.AdvantagesincludenoEMIgeneration,sonofilteringisrequired.Themaindisadvantageisheatdissipationandthelimitationofhavingtoensurethattheloadvoltageislowerthanthesupplyvoltage;thisleadstoafurtherdisadvantageofonlyallowingalimitedsupplyvoltagerange.Chapter5describesthemostbasicofswitchingLEDdrivers:thebuckconverter.Thebuckconverterdrivesanoutputthathasalowervoltagethantheinput;itisastep-downtopology.Thereaderwillbetakenthroughthedesignprocess,followedbyanexampledesign.Chapter6describesboostconverters.TheseareusedinmanyapplicationsincludingLCDbacklightsfortelevision,andcomputerandsatellitenavigationdisplayscreens.www.newnespress.com 4Chapter1Theboostconverterdrivesanoutputthathasahighervoltagethantheinput;itisastep-uptopology.Thereaderwillbetakenthroughthedesignprocess,followedbyanexampledesign,forbothcontinuousmodeanddiscontinuousmodedrivers.Chapter7describesboost-buckconverters.Thesehavetheabilitytodrivealoadthatiseitherhigherorlowervoltagecomparedtotheinput.However,thistypeofconverterislessefficientthanasimplebuckorboostconverter.Chapter8describesspecialistconverters:buck-boostandbuck(BBB),andBi-Bred.TheseconvertersareintendedforACinputapplications,suchastrafficlights,streetlightsandgenerallighting.Theycombinepowerfactorcorrectionwithconstantcurrentoutput,butinmanycasescanbedesignedwithoutelectrolyticcapacitorsandsoareusefulforhighreliabilityapplications.Thisextrafunctionalitydoescomeatacost–theefficiencyismuchlowerthanastandardoff-linebuckconverter.Chapter9describesfly-backconverters.Thischapterdescribessimpleswitchingcircuitsthatcanbeusedforconstantvoltageorconstantcurrentoutput.Theuseoftwowindingsormoreinaninductorpermitsisolationoftheoutput.Asinglewindinginductorisanon-isolatedbuck-boostcircuitthatissometimesusedfordrivingLEDs,althoughtheCukandSEPICgenerallyproducelessEMI(atthecostofanadditionalinductor).Chapter10coverstopicsthatareessentialwhenconsideringaswitchmodepowersupply.Themostsuitabletopologyforanapplicationwillbediscussed.Theadvantages,disadvantagesandlimitationsofeachtypewillbeanalyzedintermsofsupplyvoltagerangeandtheabilitytoperformPFC(powerfactorcorrection).DiscussionwillincludesnubbertechniquesforreducingEMIandimprovingefficiency,limitingswitch-onsurgesusingeitherin-rushcurrentlimitersorsoft-starttechniques.Chapter11describeselectroniccomponentsforpowersupplies.Thebestcomponentisnotalwaysanobviouschoice.Therearesomanydifferenttypesofswitchingelements:MOSFETs,powerbipolartransistorsanddiodes,eachwithcharacteristicsthataffectoverallpowersupplyperformance.Currentsensingcanbeachievedusingresistorsortransformers,butthetypeofresistorortransformerisimportant;similarlywiththechoiceofcapacitorsandfiltercomponents.Magneticcomponentsareoftenamysteryformanyelectronicengineers,andthesewillbebrieflydescribedinChapter12.First,therearedifferentmaterials:ferritecores,irondustcoresandspecialmaterialcores.Thentherearedifferentcoreshapeswww.newnespress.com Introduction5andsizes.Oneofthemostimportantphysicalcharacteristicsfromapowersupplydesignpointofviewismagnetisationandavoidingmagneticsaturation.EMIandEMCissuesarethesubjectsofChapter13.ItisalegallybindingrequirementthatequipmentshouldmeetEMIstandards.GoodEMIdesigntechniquescanreducetheneedforfilteringandshielding,soitmakessensetocarefullyconsiderthisinordertoreducethecostandsizeofthepowersupply.MeetingEMCstandardsisalsoalegalrequirementinmanycases.Itisnousehavinganotherwiseexcellentcircuitthatisdestroyedbyexternallyproducedinterference.Inmanyareas,EMCpracticesarecompatiblewithEMIpractices.Chapter14discussesthermalissuesforboththeLEDsandtheLEDdriver.TheLEDdriverhasissuesofefficiencyandpowerloss.TheLEDitselfdissipatesmostoftheenergyitreceives(voltstimesamps)asheat:verylittleenergyisradiatedaslight,althoughmanufacturersareimprovingproductsallthetime.Handlingtheheatbyusingcoolingtechniquesisalargelymechanicalprocess,usingametalheatsinkandsometimesairflowtoremovetheheatenergy.Calculatingthetemperatureisimportantbecausethereareoperatingtemperaturelimitsforallsemiconductors.Anotherlegalrequirementissafety,whichiscoveredinChapter15.Theproductmustnotinjurepeoplewhenitisoperating.ThisisrelatedtotheoperatingvoltageandsomedesignerstrytokeepbelowSELV(safetyextralowvoltage)limitsforthisreason.WhentheequipmentispoweredfromtheACmainssupply,theissuesofisolation,circuitbreakersandcreepagedistancesmustbeconsidered.www.newnespress.com CHAPTER2CharacteristicsofLEDsMostsemiconductorsaremadebydopingsiliconwithamaterialthatcreatesfreenegativecharge(N-type),orfreepositivecharge(P-type).Thefixedatomshavepositiveandnegativecharge,respectively.Atthejunctionofthesetwomaterials,thefreechargescombineandthiscreatesanarrowregiondevoidoffreecharge.This‘intrinsicregion’nowhasthepositiveandnegativechargeofthefixedatoms,whichopposesanyfurtherfreechargecombination.Ineffect,thereisanenergybarriercreated;wehaveadiodejunction.InorderforaP-Njunctiontoconduct,wemustmaketheP-typematerialmorepositivethantheN-type.ThisforcesmorepositivechargeintotheP-typematerialandmorenegativechargeintotheN-typematerial.Conductiontakesplacewhen(insilicon)thereisabout0.7VpotentialdifferenceacrosstheP-Njunction.Thispotentialdifferencegiveselectronsenoughenergytoconduct.AnLEDisalsomadefromaP-Njunction,butsiliconisunsuitablebecausetheenergybarrieristoolow.ThefirstLEDsweremadefromgalliumarsenide(GaAs)andproducedinfraredlightatabout905nm.Thereasonforproducingthiscoloristheenergydifferencebetweentheconductionbandandthelowestenergylevel(valenceband)inGaAs.WhenavoltageisappliedacrosstheLED,electronsaregivenenoughenergytojumpintotheconductionbandandcurrentflows.Whenanelectronlosesenergyandfallsbackintothelowenergystate(thevalenceband),aphoton(light)isoftenemitted.SeeFigure2.1.www.newnespress.com 8Chapter2ElectronmigrationPNHoleMigrationRadiativetransitionsNon-radiativeTransitionsFigure2.1:BandDiagramofP-NJunctionSemiconductors.2.1ApplicationsforLEDsSoonnewsemiconductormaterialsweredevelopedandgalliumarsenidephosphide(GaAsP)wasusedtomakeLEDs.TheenergygapinGaAsPmaterialishigherthanGaAs,sothelightwavelengthisshorter.TheseLEDsproducedaredcolorlightandwerefirstjustusedasindicators.Themosttypicalapplicationwastoshowthatequipmentwaspowered,orthatsomefeaturesuchas‘stereo’wasactiveinaradio.Infactitwasmainlyconsumerproductslikeradios,taperecordersandmusicsystemsthatusedredLEDsinlargenumbers.WhenyellowandgreenLEDsbecameavailable,thenumberofapplicationsincreased.Nowthecolorcouldchange,togiveadditionalinformation,orcouldindicatemoreurgentalarms.Forexample,green=OK,yellow=requiresattention,red=faulty.MostimportantwastheabilitytohaveLEDlampsintrafficlights.OnecharacteristicofthelightfromanLEDisthatitoccupiesanarrowspectrumabout20nmwide;thecolorisfairlypure.Bycontrast,asemiconductorlaserusedfortelecommunicationsoccupiesaspectrumabout2nmwide.Theverynarrowspectrumofalaserisimportantbecause,whenusedwithopticalfibersystems,thenarrowspectralwidthallowsawidesystembandwidth.Ingeneral-purposeLEDapplications,thespectralwidthhasverylittleeffect.AnotherimportantcharacteristicofLEDlightisthatcurrentisconvertedintolight(photons).Thismeansthatdoublingthecurrentdoublesthelightamplitude.Sowww.newnespress.com CharacteristicsofLEDs9dimminglightsbyloweringthecurrentispossible.ItshouldbenotedthatthespecifiedwavelengthemittedbyanLEDisatacertaincurrent;thewavelengthwillchangealittleifthecurrentishigherorlowerthanthespecifiedcurrent.Dimmingbypulsewidthmodulation(PWM)isaviablealternativeusedbymanypeople.PWMdimmingusesasignal,typicalfrequency100Hz–1000Hz,toturntheLEDonandoff.Thepulsewidthisreducedtodimthelight,orincreasedtobrightenthelight.The‘holygrail’wasblueLEDs,whicharemadefromindiumgalliumnitride(InGaN).Whenaddingcoloredlight,red,greenandbluemakelightthatappearswhitetothehumaneye.Thereasonforonly‘appearing’whiteisthattheeyehasreceptors(cones)thatdetectred,greenandblue.Therearebiggapsinthecolorspectrum,buttheeyedoesnotnotice.WhiteLEDsaresometimesmadeusingblueLEDswithayellowphosphordotovertheemittingsurface.Theyellowphosphorcreatesawidespectrumand,whencombinedwiththeblue,appearswhite.AninterestingapplicationforblueLEDsisindentistry.Illuminatingmodernresinsusedintoothfillingmaterialswithbluelightwillhardentheresin.The465nmwavelengthhasbeenfoundtobeclosetooptimumforthisapplication,althoughtheintensityofthelightmustbehighenoughtopenetratethroughtheresin.SomeinterestingapplicationsrelyonthepurityoftheLEDcolor.TheilluminationoffreshfoodisbetterwithLEDs,becausetheyemitnoultravioletlight.Photographicdarkroomscanusecolorswherefilmisinsensitive–traditionallydarkroomshavebeenilluminatedbyredcoloredincandescentlamps.Eventrafficlightsmustemitalimitedrangeofcolors,whicharespecifiedinnationalstandards.ItshouldbenotedthatthecolorofanLEDwouldchangeastheLED’stemperaturechanges.Thetemperaturecanchangeduetoambientconditions,suchasbeinghousedadjacenttohotmachinery,orduetointernalheatingoftheLEDduetotheamountofcurrentflowingthroughit.Theonlywaytocontrolambienttemperatureistoaddacoolingfan,orbyplacingtheLEDawayfromthesourceofheat.MountingtheLEDonagoodheatsinkcancontrolinternalheating.TheearlyLEDswereallratedat20mAandtheforwardvoltagedropwasabout2Vforred,higherforothercolors;laterlowcurrentLEDswerecreatedthatoperatedfroma2mAsource.OvertimethecurrentratingofLEDshasincreased,sothat30mA,50mAandeven100mAarefairlycommon.LumiledswascreatedbyHPandPhilipsin1999andproducedthefirst350mALED.NowthereareanumberofpowerLEDmanufacturers,ratedat350mA,700mA,1Aandhigher.PowerLEDswww.newnespress.com 10Chapter2arebeingusedinincreasingnumbers;inchannellighting(signage),trafficlights,streetlights,automotive,moodlighting(colorchanging‘wallwash’),andalsointheatersforlightingstepsandemergencyexits.ChannellightingissocalledbecausetheLEDsaremountedinachannel;seeFigure2.2.Typicallythischannelisusedtoformletters,forilluminatedcompanynamesigns.Inthepast,channellightingusedcold-cathodeorfluorescenttubes,butthesehadreliabilityproblems.Healthandsafetylegislation,liketheRoHSDirective,bannedsomematerialslikemercurythatisusedintheconstructionofcold-cathodetubes.So,tocopewiththeshapesandenvironmentalconditions,themostviabletechnologyisLEDlighting.ChannelledSignageLEDLightingmodulesinsidechannelPowerSupplyFigure2.2:ChannelLighting.www.newnespress.com CharacteristicsofLEDs11TrafficlightshaveusedlowpowerLEDsforsomeyears,butnowsomemanufacturersareusingafewhighpowerLEDsinstead.Oneproblemwithtrafficlightsiscontrollingthewavelengthoftheyellow(amber)light.YellowLEDssufferfromagreaterwavelengthshiftthanothercolors,andthiscancausethemtooperateoutsidetheirpermittedspectralrange.Anotherproblemismakingthemfail-safe–authoritiespermitsomedegreeoffailure,butifmorethan20%oftheLEDsfail,theentirelampmustbeshutdownandafaultreportedtomaintenanceteams.HighambienttemperaturesinsidethelamphousingcanleadtoLEDdriverfailures.ThisisparticularlytrueiftheLEDdrivercircuitcontainselectrolyticcapacitors,whichventwhenhotandeventuallylosetheircapacitance.SomenovelLEDdrivershavebeendevelopedthatdonotneedelectrolyticcapacitorsandcanoperateforseveralyearsathightemperatures.FailingLEDdriverscangiveLEDlightsabadname–whyhaveLEDsthatcanworkforover100,000hoursiftheLEDdriverfailsafter10,000hours’operation?StreetlightshavebeenbuiltusingmediumandhighpowerLEDs.Althoughthiswouldseemtobeasimpleapplication,highambienttemperaturesandrelativelyhighpowerLEDscangiverisetodriverproblems.Insomecases,whiteandyellowLEDsareusedtogethertocreatea‘warm-white’light.TheproblemwithwhiteLEDs,madeusingablueLEDandayellowphosphor,isthatthehighbluecontentproducesa‘cold-white’light.Automotivelightinghasmanyapplications;internallights,headlights,stoplights,daylightrunninglights(DRL),rearfoglights,reversinglights,etc.ThegreatestproblemwithautomotiveapplicationsisthattheEMIspecificationsdemandextremelylowlevelsofemissions,whicharedifficulttomeetwithaswitchingcircuit.Lineardriversaresometimesusediftheefficiencyisnotacriticalrequirement.Connectingalineardrivertothemetalbodyofthevehiclecanbeusedtodissipatetheheatgenerated.AutomotivestoplightsusingLEDshaveasignificantsafetyadvantageoverthoseusingfilamentlamps.ThetimefromcurrentflowtolightoutputinanLEDismeasuredinnanoseconds.Inafilamentlamptheresponsetimeisabout300ms.At60mph(100km/h),avehicletravels1mile(1.6km)perminute,or88feetpersecond.In300ms,acarwilltravelover26feet(8meters).Stopping300mssooner,havingseenthepreviouscar’sbrakelightsearlier,couldavoiddeathorinjury.Also,LEDbrakelightsarelesslikelytofail.www.newnespress.com 12Chapter2Moodlightingisaneffectcausedbychangingthecolorofasurfaceanduseshumanpsychologytocontrolpeople’sfeelings.Itisusedinmedicalfacilitiestocalmpatients,andonaircrafttorelax(orwakeup!)passengers.Generallymoodlightingsystemsusered,greenandblue(RGB)LEDsina‘wallwash’projectortocreateanycolorinthespectrum.OtherapplicationsfortheseRGBsystemsincludediscolights!Backlightingdisplays,suchasflatscreentelevisions,alsouseRGBLEDarraystocreatea‘white’light.Inthiscasethecolorchangeslittle–ideallynotatall.However,acontrolsystemisrequiredtocarefullycontroltheamountofred,greenandblue,tocreatetheexactmixforaccuratetelevisionreproduction.Coldcathodetubesaresometimesusedtobacklightcomputerscreens,butheretheexactcolorisnotimportant.2.2LightMeasureThetotallightfluxismeasuredinunitsoflumens.Thelumenisthephotometricequivalentof1watt,weightedtomatchthenormalhumaneyeresponse.At555nm,inthegreen-yellowpartofthespectrumwheretheeyeismostresponsive,1W=683lumen.Thetermcandelaisalsoused.Thisisthelightproducedbyalamp,radiatinginalldirectionsequally,toproduce1lumenpersteradian.Asanequation,1cd=1lm/sr.1meterradius1cd1luxor12lm/m12mFigure2.3:LightMeasurement.www.newnespress.com CharacteristicsofLEDs13Asteradianhasaprojectedareaof1squaremeter,atadistanceof1meterfromthelight2source.Thelightfroma1cdsource,atmeterdistance,is1lux,or1lm/m,seeFigure2.3.Lightemissionefficiency(luminousefficacy)fromLEDsisdescribedintermsoflumensperwatt.ThereissomecompetitionbetweenLEDmanufacturerstogetthehighestluminousefficacy,butwhencomparingresultsitisimportanttomakeanoteoftheelectricalpowerlevelsused.Itiseasiertomakeanefficient20mALED,thananefficient700mALED.2.3EquivalentCircuittoanLEDAnLEDcanbedescribedasaconstantvoltageload.Thevoltagedropdependsontheinternalenergybarrierrequiredforthephotonsoflighttobeemitted,asdescribedearlier.Thisenergybarrierdependsonthecolor;thusthevoltagedropdependsonthecolor.WilleveryredLEDhavethesamevoltagedrop?No,becauseproductionvariationswillmeanthatthewavelength(color)willnotbethesame,andthusthevoltagedropwillhavedifferences.Thepeakwavelengthhastypicallya10%variation.IftherearetemperaturedifferencesbetweentwoLEDs,thiswillgiveacolorchangeandhencedifferencesinvoltagedrop.Asthetemperaturerises,itiseasierforelectronstocrosstheenergybarrier.Thusthevoltagedropreducesbyapproximately2mVperdegreeasthetemperaturerises.Sincethesemiconductormaterialisnotaperfectconductor,someresistanceisinserieswiththisconstantvoltageload,seeFigure2.4.Thismeansthatthevoltagedropwillincreasewithcurrent.TheESR(equivalentseriesresistance)ofalowpower20mALEDisabout20ohms,buta1W350mALEDhasanESRofabout1–2ohm(dependingonthesemiconductormaterialused).TheESRisroughlyinverselyproportionaltothecurrentratingoftheLED.TheESRwillhaveproductionvariationstoo.TheESRcanbecalculatedbymeasuringtheincreaseinforwardvoltagedropdividedbytheincreaseincurrent.Forexample,iftheforwardvoltagedropincreasesbyfrom3.5Vto3.55V(a50mVincrease)whentheforwardcurrentgoesfrom10mAto20mA(a10mAincrease),theESRwillbe50mV/10mA=5ohms.InFigure2.4,theZenerdiodeisshownasaperfectdevice.Inreality,ZenerdiodesalsohaveESR,whichcanbehigherthantheESRofanLED.ForinitialtestingofanLEDdriver,a5W,3.9VZenerdiodecanbeusedtoreplacethe(white)LED.Ifthewww.newnespress.com 14Chapter2LEDEquivalentCircuit(PerfectZenerdiode)Figure2.4:EquivalentCircuitforanLED.driverisnotworkingasplannedtheZenerdiodemaybedestroyed,butthisisfarlesscostlythandestroyingapowerLED.SincetheZenerdiodedoesnotemitlight,thetestengineerwillnotbedazzled.2.4VoltageDropVersusColorandCurrentThegraphinFigure2.5showshowtheforwardvoltagedropdependsonthelightcolorandontheLEDcurrent.Atthepointwhereconductionbegins,theforwardCurrent,IfSloperedbluegreendV/di=RyellowVf1Voltage,VfTypicalForwardVoltage,VfRed=2VBlue=3.5VFigure2.5:ForwardVoltageDropVersusColorandCurrent.www.newnespress.com CharacteristicsofLEDs15voltagedrop,Vf,isabout2VforaredLEDandabout3.5VforablueLED.Theexactvoltagedropdependsonthemanufacturer,becauseofdifferentdopantmaterialsandwavelengths.ThevoltagedropataparticularcurrentwillalsodependoninitialVf,butalsoontheESR.2.5CommonMistakesThemostcommonmistakeistobaseadesignonthetypicalforwardvoltagedropoftheLED,Vftyp.ThisincludesconnectingstringsofLEDsinparallel,withtheassumptionthattheforwardvoltagedropsareequalandthecurrentwillshareequallybetweenthetwoormorestrings.Infact,thetoleranceontheforwardvoltagedropisveryhigh.Forexample,a1WwhiteLuxeonStarhasatypicalVf=3.42V,buttheminimumvoltageis2.79Vandthemaximumis3.99V.Thisisover15%toleranceontheforwardvoltagedrop!www.newnespress.com CHAPTER3DrivingLEDs3.1VoltageSourceWehaveseeninChapter2thatanLEDbehaveslikeaconstantvoltageloadwithlowequivalentseriesresistance(ESR).ThisbehaviorislikeaZenerdiode–infactZenerdiodesmakeagoodtestload,ratherthanusingexpensivehighpowerLEDs!Drivingaconstantvoltageloadfromaconstantvoltagesupplyisverydifficult,becauseitisonlythedifferencebetweenthesupplyvoltageandtheloadvoltagethatisdroppedacrosstheESR.ButtheESRisverylowvalue,sothevoltagedropwillalsobelow.Aslightvariationinthesupplyvoltage,ortheloadvoltage,willcauseaverylargechangeincurrent;seecurveAinFigure3.1.Ifthevariationinsupplyvoltageandforwardkneevoltage(Vf)isknown,thevariationiscurrentcanbecalculated.RememberthattherearevariationsinLEDCurrent,IfCurveACurveBSlopeSlopedV/di=ESRdV/di=ESR+REXTVoltage,VSUPPLYFigure3.1:LEDCurrentVersusSupplyVoltage.www.newnespress.com 18Chapter3voltagedropduetomanufacturingtolerancesandoperatingtemperature.Mostsupplyvoltagesfromaregulatedsupplyhavea5%tolerance,butfromunregulatedsupplieslikeautomotivepower,thetoleranceisfargreater.VSOURCEMINVFMAXIMIN¼ESRVSOURCEMAXVFMINIMAX¼ESRTheseequationsassumethatESRisconstant.Inpractice,theVfandvoltagedropacrossESRarecombined,sincemanufacturersquotethevoltagedropatacertainforwardcurrent.TheactualVfcanbedeterminedfromgraphs,ormeasured.Ifthereisalargedifferencebetweenthesourceandloadvoltage,andahighESR,thereisverylittledifferencebetweenthemaximumandminimumLEDcurrent.ThismaybeperfectlyadequateforlowcurrentLEDs,upto50mA.However,inhighpowerLEDcircuits,alargevoltagedropacrossaseriesresistorwillbeinefficientandmaycauseheatdissipationproblems.Also,theESRofLEDsislowerasthepowerratingincreases.Astandard20mALEDmayhaveanESRof20ohms,buta350mALEDwillhaveanESRof1–2ohmstypically.Thusa1VdifferenceinsupplyvoltagecouldincreasetheLEDcurrentby1AinapowerLED.EveninlowcurrentLEDs,theproportionalchangeincurrentcanbehigh.3.1.1PassiveCurrentControlAlthoughtheLEDvoltagedropshiftsthecurveofthegraphtotheright,theslopeofthegraphisjustduetotheESR.Lowcurrentloadscanhavearelativelyhighvalueresistanceaddedinseries,inordertoreducetheslopeofthecurrentversusvoltagegraph;seecurveBinFigure3.1.Withaseriesresistoraddedweareabletocalculatethevariationincurrent,providedthatthevariationinsupplyvoltageandloadvoltageisknown.Intheequationsbelow,theloadvoltageincludesthevoltagedropacrossESR,attheratedcurrent,soonlytheexternalresistorvalueisneeded.VSOURCEMINVLOADMAXIMIN¼REXTVSOURCEMAXVLOADMINIMAX¼REXTwww.newnespress.com DrivingLEDs19Asanexample,letusdrivefromanautomotivesupply;thisisanominal13.5V,butforthisexercisewecansetthelimitsat12Vto16V.LetusselectaredLEDfortail-lights(LumiledsSuperfluxHPWA-DDOO),withaforwardvoltagedropof2.19Vto3.03Vat70mAforwardcurrent.ChoosingtoconnecttwoLEDsinseries,withaseriesresistor,wehaveatypicalvoltagedropof5V.Sothetypicalvoltagedropat70mAneedstobe8.5V;thismeansthattheseriesresistorshouldbe121.43ohms.Theneareststandardvalueresistoris120ohms,ratedat1Wsincewewillhaveatypicalpowerdissipationof588mW.VSOURCEMINVLOADMAX126:06IMIN¼¼¼49:5mAREXT120VSOURCEMAXVLOADMIN164:38IMAX¼¼¼96:83mAREXT120Atthehighlimitofsourcevoltage,theLEDisoverdrivenby38%.Butthereisalmosta2:1ratiobetweenIMAXandIMIN,soifweincreaseRby38%theworstcasecurrentlevelsare70mAmaximum,butonly35.78mAminimum.Inthepreviouscalculations,thevoltagedropacrossESR(0.672V)wasincludedintheminimumandmaximumloadvoltagevalues,soweignoredESR.Fromthemanufacturer’sdatasheetoftheLumiledsHPWA-DDOOLED,graphsshowthattheESRisabout9.6ohms.Supposewenowwanttooperateatalowercurrent.Usingthesameexample,butoperatingwithatypicalLEDcurrentof50mA,wemustmodifytheresults.NowthevoltageatthecurrentkneeisVf=1.518Vto2.358V.Withatypical13.5Vsupplyand50mA,thevalueforVfis1.828V.Thetotalresistanceneededis196.88ohms,butwealreadyhave9.6ohmsESR.Anexternalresistorvalueof180ohmsisthenearestpreferredvalueforacurrentof50mA.VSOURCEMINVLOADMAX124:716IMIN¼¼¼38:42mAESRþREXT189:6VSOURCEMAXVLOADMIN163:036IMAX¼¼¼61:85mAESRþREXT29:6Theseriesresistorhasahighervalue,sothevariationincurrentisreducedto1.6:1ratio.ThemaximumcurrentisnowbelowtheLEDcurrentratingof70mA.UnlesstheLEDsarematched(or‘binned’)toensurethesameforwardvoltagedrop,thecurrentthroughonestringcouldbeconsiderablydifferentfromthecurrentthroughanother.www.newnespress.com 20Chapter3WhenmultipleLEDsareusedtoprovidelightingforanapplication,theyarefrequentlyconnectedinanarray,consistingofparallelstringsofseriesconnectedLEDs.SincetheLEDstringsareinparallel,thevoltagesourceforallstringsisthesame.However,duetovariationsinforwardvoltageforeachLED,thetotalvoltagedropofeachstringdiffersfromtheotherstringsinthearray.Theforwardvoltagealsodependsontheambienttemperature.ToensureuniformlightoutputforallLEDs,equalcurrentshouldbedesignedtoflowthrougheachstringofLEDs.Thetraditionalwayistoconnectacurrentlimitingresistorinserieswitheachstringandpowerallthestringsusingasinglevoltagesource.Asubstantialvoltageneedstobedroppedacrosstheresistortoensurethatthecurrentwillstayinthedesiredrangeinthepresenceoftemperatureanddevice-to-devicevoltagevariations.Thismethodisinexpensive,butsuffersfrompowerinefficiencyandheatdissipation.Italsorequiresastablevoltagesource.AbetterwayofpoweringtheLEDarrayistoregulatethetotalcurrentthroughallthestringsanddeviseameanstodividethattotalcurrentequallyamongtheLEDstrings.Thisisactivecurrentcontrolandisthesubjectofthenextsubsection.3.1.2ActiveCurrentControlSinceaseriesresistorisnotagoodcurrentcontrolmethod,especiallywhenthesupplyvoltagehasawidetolerance,wewillnowlookatactivecurrentcontrol.Activecurrentcontrolusestransistorsandfeedbacktoregulatethecurrent.HerewewillonlyconsiderlimitingLEDcurrentwhentheenergyissuppliedfromavoltagesource;drivingLEDsusingenergyfromcurrentsourceswillbediscussedinSection3.2.Acurrentlimiterhascertainfunctionalelements:aregulatingdevicesuchasaMOSFETorbipolartransistor;acurrentsensorsuchasalowvalueresistor;andsomefeedback(withorwithoutgain)fromthecurrentsensortotheregulatingdevice.Figure3.2showsthesefunctions.ThesimplestcurrentlimiterisadepletionmodeMOSFET;ithasthreeterminalscalledgate,drainandsource.Conductionofthedrain-sourcechanneliscontrolledfromthegate-sourcevoltage,likeanyotherMOSFET.However,unlikeanenhancementMOSFET,adepletionmodeMOSFETis‘normally-on’socurrentflowswhenthegate-sourcevoltageiszero.Asthegatevoltagebecomesnegativewithrespecttothesource,thedeviceturnsoff,seeFigure3.3.Atypicalpinch-offvoltageis–2.5V.www.newnespress.com DrivingLEDs21V+LoadLimiterCurrentFeedbackCurrentSenseV–Figure3.2:CurrentLimiterFunctions.+VIdIdgsV0VVgsVTHFigure3.3:DepletionMOSFETCharacteristics.AcurrentlimitingcircuitwithadepletionmodeMOSFETusesaresistorinserieswiththesourcetosensethecurrent(seeFigure3.4).Thegateisconnectedtothenegativesupply(0V).Ascurrentflowsthroughtheresistor,thevoltagedropacrossitwww.newnespress.com 22Chapter3+VIdgs0VFigure3.4:DepletionMOSFETCurrentLimiter.increases.ThevoltageattheMOSFETsourceincreasesinpotentialcomparedtothe0VrailandtheMOSFETgate.Inotherwords,comparedtotheMOSFETsource,thegatebecomesmorenegative.Atacertainpoint,whenthevoltagedropapproachestheMOSFETpinch-offvoltage,theMOSFETwilltendtoturnoffandthusregulatethecurrent.Themaindrawbackofusingdepletion-modeMOSFETsisthatthegatethresholdvoltage(Vth)hasawidetolerance.AdevicewithatypicalVthof–2.5Vwillhavethresholdrangeof–1.5Vto–3.5V.However,theadvantageisthathighdrain-sourcebreakdownvoltagesarepossibleandsoalimiterdesignedusingadepletion-modeMOSFETcanprotectagainstshorttransients(longerperiodsofhighvoltagewouldtendtooverheattheMOSFET).Asimpleintegratedcurrentlimiterisavoltageregulatorintheplaceofthedepletion-modeMOSFET,asshowninFigure3.5.Thisusesaninternalvoltagereferenceandsotendstobequiteaccurate.Thedisadvantageisthatthereisaminimumdropoutvoltageofabout3V.Thiscircuitcanbeusedforcurrentsinkorcurrentsourceregulation,dependingonwhethertheloadisconnectedtothepositiveornegativesupplyrail.TheLM317hasafeedbackpincalled‘REF’,andthiscontrolstheregulationofthecurrent.Whenthevoltagedropacrosstheresistortriestoexceed1.25V,thecurrentthroughtheLM317isreduceduntiltheoutputterminal(OUT)isreducedbelow1.25V.Ifaccuratecurrentlimitersareused,parallelstringsofLEDscanbeconnectedtothesamevoltagesourceandtheneachstringwillhaveapproximatelythesamecurrent.www.newnespress.com DrivingLEDs23V+INLoadADJLM317OUTINADJLM317CurrentOUTLoadV–Figure3.5:LinearRegulatorasCurrentLimiter.WiththesamecurrentflowingthrougheachLED,thelightproducedwillbealmostthesameforeachLEDandthusno‘brightspots’willbeseenintheLEDarray.ThecurrentlimitersdescribedherearepurelytoshowhowLEDscanbedrivenfromaconstantvoltagesupply.FurtherlinearregulatorsaredescribedinChapter4.SwitchingregulatorsaredescribedinChapters5–10.3.1.3ShortCircuitProtectionThecurrentlimitingcircuitsdescribedintheprevioussubsectionwillprovideautomaticshortcircuitprotection.IftheLEDgoesshortcircuit,ahighervoltagewillbeplacedacrossthecurrentlimiter.Powerdissipationisthemainissuethatneedstobeaddressed.Ifthepowerdissipationcannotbetoleratedwhentheloadgoesshortcircuit,avoltagemonitoringcircuitwillbeneeded.Whenahigherthanexpectedvoltageisplacedacrossthecurrentlimiter,thecurrentmustbereducedtoprotectthecircuit.IntheLM317circuitpreviouslydescribed,theregulatoritselfhasthermalshutdown.3.1.4DetectingFailuresIfwehaveashortcircuitconditionintheLEDs,thevoltageacrossthecurrentlimiterwillincrease.Wecanusethischangetodetectafailure.Inthecircuitshowninwww.newnespress.com 24Chapter3V++5VLoad10VINADJLM317OUTFAILURE0VV–Figure3.6:ShortedLoadIndication.Figure3.6,a10VZenerdiodeisusedinserieswiththebaseofanNPNtransistor.Whenthevoltageatthe‘IN’terminaloftheLM317reachesabout11V,theZenerdiodeconductsandturnsonthetransistor.Thispullsthe‘FAILURE’lineto0VandindicatesashortcircuitacrosstheLEDs.3.2CurrentSourceSinceanLEDbehaveslikeaconstantvoltageload,itcanbedirectlyconnectedtoacurrentsource.ThevoltageacrosstheLED,orstringofLEDs,willbedeterminedbythecharacteristicsoftheLEDsused.Apurecurrentsourcewillnotlimitthevoltage,socaremustbetakentoprovidesomelimit;thiswillbecoveredinmoredetailinthenextsubsection.IfthecurrentsourceproducesmuchmorecurrentthantheLEDrequires,current-sharingcircuitswillberequired.Thesimplestoftheseisacurrentmirror,whichsharesthecurrentequallybetweenstringsbasedonthecurrentflowingthroughtheprimarystring.Figure3.7showsasimplecurrentmirror.Thebasicprinciplereliesonthefactthatmatchedtransistorswillhavethesamecollectorcurrentiftheirbase-emitterjunctionshavethesamevoltageacrossthem.Byconnectingallthebasesandalltheemitterswww.newnespress.com DrivingLEDs25V+PrimaryString–MaximumVoltageDropQ1Q2Q3QnV–MATCHEDNPNFigure3.7:CurrentMirror.together,everybase-emitterjunctionvoltagemustbeequalandthereforeeverycollectorcurrentmustbeequal.TheprimaryLEDstringistheonethatcontrolsthecurrentthroughtheotherstrings.SincethecollectorandbaseoftransistorQ1areconnected,thetransistorwillbefullyconductinguntilthecollectorvoltagefallslowenoughforthebase-emittercurrenttolimit.Othertransistors(Q2toQn)havetheirbaseconnectionsjoinedtoQ1,andwillconductexactlythesamecollectorcurrentasQ1sincethetransistorsarematched.ThetotalcurrentthroughQ1toQnwillequalthecurrentsourcelimit.ThevoltagedropacrosstheLEDsintheprimarystringmustbehigherthananyotherstringinorderforthecurrentmirrortoworkcorrectly.Intheslavestrings,somevoltagewillbedroppedacrossthecollector-emitterjunctionofthetransistorsQ2–Qn.Theslavecircuitsadjustthecurrentbyraisingorloweringthissurplusvoltagedropacrossthetransistor.3.2.1Self-AdjustingCurrentSharingCircuitAsanalternative,thecurrentsharingcircuitshowninFigure3.8automaticallyadjustsforstringvoltage.www.newnespress.com 26Chapter3BIASBUSD1TRANSISTORBASEBUS1K1K1K1K51R51R51R51RD1HEADROOMADJUSToneormorediodesinseriesFigure3.8:Self-AdjustingCurrentSharingCircuit.AssumingthattheLEDarrayisdrivenfromacurrentsource,therewillbeequalcurrentdivisionamongallconnectedbranches.Ifanybranchisopenduetoeitherafailureornoconnectionbydesign,thetotalcurrentwilldivideevenlyamongtheconnectedbranches.Unlikethesimplecurrentmirror,thisoneautomaticallyadjustsforthemaximumexpectedvoltagedifferencebetweenstringsofLEDs,whichisafunctionofthenumberofLEDsinthestringandthetypeofLEDused.Thecomponentsmustbeabletodissipatetheheatgeneratedbythesumofeachstringcurrentandtheheadroomvoltagedropacrosstheregulatorforthatstring.Inhighreliabilityapplications,thefailureofasingleLEDshouldnotmateriallyaffectthetotallightoutput.Theuseofthecurrentdividerwillhelpthesituation.WhenanLEDfailsshort,thevoltageofthestringcontainingtheshortedLEDwillhavelessvoltage.Thecurrentdividerwillaccommodatethechangeinvoltageandstilldistributethecurrentequally.WhenthefailedLEDstringopens,thecurrentdividerwillautomaticallyredistributethetotalcurrentamongtheremainingstrings,thusmaintainingthelightoutput.Inthisapplication,anextradiodestringcanbeaddedforredundancy,sothatanysinglefailurewillnotcausetheremainingLEDstooperateinanover-currentcondition.www.newnespress.com DrivingLEDs27Equalityofcurrentdivisionamongthebranchesisdependentontheclosematchingofthetransistors,whichareinclosevicinity(ideallyasinglepackagewithseveralmatchedtransistors).Whenanyofthetransistorssaturateduetolargevariationofthestringvoltages,equalcurrentdivisionwillbelost.Diodesconnectedtoeachcollectordetectthevoltageofeachbranch.Thehighestbranchvoltage(correspondingtotheLEDstringwiththelowestforwardvoltage)isusedtobiasthetransistorsinthelinearoperatingregion.Thecathodeofeachdiodeisconnectedtoacommon‘biasbus’.Toaccommodatevariationsinstringvoltagesandkeepthecurrentdividertransistorsfromsaturation,diodesareconnectedbetweenthe‘biasbus’andthe‘transistorbasebus’.Morethanoneexternaldiodecanbeusedtoaccommodatelargevoltagevariations.Ifthestringvoltagevariationislessthanonediodedrop,thetwobusescanbejoined.Whenabranchisnotconnected,therewillbehigherbasecurrentflowingintheassociatedregulatingtransistor.Thiscouldinterferewiththecurrentdivisionintheconnectedbranches,soaresistor(about1kohm)isconnectedfromthe‘transistorbasebus’toeachtransistorbasetoensurecorrectoperationoftheoverallcircuit.3.2.2VoltageLimitingIntheory,theoutputvoltageofaconstantcurrentdriverisnotlimited.Thevoltagewillbetheproductofthecurrentandloadresistanceinthecaseofalinearload.InthecaseofanLEDload,thevoltagelimitwilldependonthenumberofLEDsinastring.Inpractice,therewillbeamaximumoutputvoltage,becausecomponentsinthecurrentsourcewillbreakdowneventually.LimitingtheLEDstringvoltageisnecessarytopreventcircuitdamageandthevoltagelevelwilldependontheparticularcircuit.SafetyregulationswillbecoveredinChapter10,butUnderwritersLaboratories(UL)Class2andSafetyElectricalLowVoltage(SELV)requirementslimitanypotentialto60VDC,or42.4VAC,soequipmentdesignedtomeettheserequirementsshouldconsiderbothmainssupplyisolation(ifapplicable)andoutputvoltagelimiting.ThenumberofLEDsinastringwillberestrictedinthiscase,sothatthetotalstringvoltageremainsbelow60V.www.newnespress.com 28Chapter33.2.3OpenCircuitProtectionSomeconstantcurrentdrivers,especiallyswitchingboostconverters,willproduceasufficientlyhighvoltagetodestroythedrivercircuit.Forthesetypesofdriverashutdownmechanismisrequired.UsingaZenerdiodetogivefeedbackwhentheoutputvoltageexceedsacertainlimitisthestandardmethod.Someover-voltagedetectorswithinintegratedcircuits(ICs)havealatchedoutput,requiringthepowersupplytobeturnedoffandthenonagainbeforeLEDdriverfunctionsareenabled.Othercircuitswillauto-restartwhentheopencircuitconditionisremoved(i.e.whentheLEDsarereconnected).SomeICshaveanover-voltagedetector(internalcomparator)thatdisablestheLEDdrivercircuitwhenthevoltageattheinputexceedsthereferencevoltage.Apotentialdividercomprisingtworesistorsisusuallyusedtoscaledowntheoutputvoltagetothereferencevoltagelevel.3.2.4DetectingLEDFailuresInaconstantcurrentcircuit,afailureofanLEDcanmeanthateitherawholestringisoff(opencircuitLED)orasingleLEDisoff(shortcircuitLED).InthecaseofanopencircuitLED,theloadisremovedandsotheoutputvoltagefromthecurrentsourcerises.Thisriseinvoltagecanbedetectedandusedtosignalafailure.Incircuitswhereover-voltageprotectionisfitted,thiscanbeusedtoindicateafailure.IfacurrentmirrorisusedtodriveanarrayofLEDswithanumberofstrings,theresultofanopencircuitLEDwilldependonwhichstringtheLEDislocated.Inabasiccurrentmirror,asshowninFigure3.7,afailureintheprimarystringwillcausealltheLEDstohavenocurrentflowandnotbelit.Detectionoftheriseinoutputvoltagewouldbeasolution.However,ifthefailurewereinasecondarystring,therewouldbehighercurrentflowingintheotherstringsandtheoutputvoltagewouldnotriseverymuch(onlyduetotheextracurrentflowingthroughtheESR).Thevoltageatthetransistorcollectorofthebrokenstringwouldfalltozerosincethereisnoconnectiontothepositivesupply,andthiscouldbedetected.Anothertechnique,forlowcurrentLEDs,istoconnecttheLEDofanopto-couplerinserieswiththeLEDstring.Abasicopto-couplerhasanLEDandaphoto-transistorinthesamepackage.Currentthroughtheopto-couplerLEDcausesthephoto-transistortoconduct.ThuswhencurrentisflowingthroughtheLEDstringwww.newnespress.com DrivingLEDs29andtheopto-coupler’sinternalLED,thephoto-transistorisconducting.Ifthestringgoesopen-circuit,thereisnocurrentthroughtheopto-coupler’sLEDandthephoto-transistordoesnotconduct.3.3TestingLEDDriversAlthoughtestinganLEDdriverwiththeactualLEDloadisnecessary,itiswisetouseadummyloadfirst.Therearetwomainreasonsforthis:(1)costofanLED,especiallyhighpowerdevices,canbegreaterthanthedrivercircuit;and(2)operatinghighbrightnessLEDsforalongtimeundertestconditionscancauseeyestrainandtemporarysightimpairment(ifLEDsviewedatcloserange).Afurtherreasonisthatsomedummyloadscanbesettolimitthecurrentandsoenablefault-findingtobemadeeasier.3.3.1ZenerDiodesasaDummyLoadFigure3.9showshowZenerdiodescanbeusedasadummyload.Thisisthesimplestandcheapestload.The1N5334Bisa3.6V,5WZenerdiode(3.6Vtypicalat350mA).Thisisnottheperfectdummyload.Thisreversevoltageisslightlyhigherthanthetypicalforwardvoltageof3.42VofaLumileds‘LuxeonStar’1WLED.The1N5334Bhasadynamicimpedanceof2.5ohms,whichishigherthantheLuxeonStar’s1ohmimpedance.TheimpedancewillhaveaneffectonsomeswitchingLEDdriversthathaveafeedbackloop.Forsimplebuckcircuits,theimpedanceonlyhasasmalleffect.4×1N5334B=4×1WLED=Figure3.9:ZenerDiodeDummyLoad.Anactiveloadismoreprecise.Aconstantvoltageloadwillhave(intheoryatleast)zeroimpedance,sosimplyaddingasmallvalueseriesresistorwillgivethecorrectwww.newnespress.com 30Chapter3impedance.Commercialactiveloadscanbesettohaveconstantcurrentorconstantvoltage–aconstantvoltagesettingisrequiredtosimulateanLEDload.AconstantvoltageloadbuiltusingalowcostdiscretesolutionisshowninFigure3.10.Thisisaself-poweredloadandsocanbeisolatedfromground.TheZenerdiodecanbeselectedtogivethedesiredvoltage(add0.7Vfortheemitter-basejunctionofthetransistor).Thetransistorshouldbeapowerdevice,mountedonaheatsink.RVz2N3055Figure3.10:ActiveDummyLoad.ThecircuitisFigure3.10haslowimpedance.AlthoughtheZenerdiodedoeshaveafewohmsimpedance,thecurrentthroughitisverysmallandtheeffectofthetransistoristoreducetheimpedancebyafactorequaltothegainHFE.SupposethetransistorHFE=50at1AandtheZenerdiodeimpedanceZd=3ohms.Changingthecollectorcurrentfrom500mAto1Awillcausethebasecurrenttorisefrom10mAto20mA.A10mAchangeincurrentthroughtheZenerdiodewillcause30mVvoltagerise.Thischangeatthetransistorcollectorisequivalenttoanimpedanceof30mV/0.5A=0.06ohm.Inotherwords,thecircuitimpedanceisequaltotheZenerdiodeimpedancedividedbythetransistorgain.Animpedanceof0.06ohmisunrealisticallylow,butapowerresistorcanbeaddedinseriestogivethedesiredloadimpedance.Becauseofthepotentiallyhighloadcurrent,boththetransistorandseriesresistorshouldberatedforhighpower.Thetransistorshouldbemountedonalargeheatsink.3.4CommonMistakesThemostcommonmistakeistouseexpensivehighpowerLEDswhentestingaprototypecircuit.Instead,3.6V,5WZenerdiodesshouldbeusedinplaceofeachLED.OnlyoncethecircuithasbeentestedunderallconditionsshouldLEDsbeused.www.newnespress.com DrivingLEDs313.5ConclusionsAvoltageregulatedLEDdriverispreferredwhenthereareanumberofLEDmodulesthatcanbeconnectedinparallel.Eachmodulewillhaveitsownlinearcurrentregulator.Anexamplewouldbechannellighting,asusedinshopnameboards.AcurrentregulatedLEDdriverispreferredwhenitisdesirabletohaveanumberofLEDsconnectedinseries.AseriesconnectionensuresthatalltheLEDshavethesamecurrentflowingthroughthemandthelightoutputwillbeapproximatelyequal.AswitchingdriverwithconstantcurrentoutputisthefavoredoptionwhendrivinghighpowerLEDs,forreasonsofefficiency.Anefficiencyof75–90%canbeachieved.Ifaconstantvoltagesourcewereused,theLEDswouldalsoneedahighcurrentlinearregulatorinseries,whichisveryinefficientandwouldincreaseheatdissipationproblems.www.newnespress.com CHAPTER4LinearPowerSupplies4.1IntroductionLinearpowersuppliesfordrivingLEDsarepreferredforanumberofreasons.ThecompleteabsenceofanyEMIradiationisoneimportanttechnicalreason.Lowestcostisanimportantcommercialreason.However,theyalsohavedisadvantages:insomeapplicationstheyhavelowefficiencyandhencetheintroductionofthermalproblems;inotherapplications,suchaswhenpoweredfromtheACmainssupply,theyhavethedisadvantageoflargesize.4.1.1VoltageRegulatorsManyvoltageregulatorsarebasedontheLM317originallyfromNationalSemiconductor,butwhichisnowmadebyanumberofmanufacturers.InsidetheLM317are:(1)apowerswitch,whichisanNPNtransistor;(2)avoltagereferencesettoproduce1.25Vand(3)anoperationalamplifier(op-amp)tocontrolthepowerswitch,asshowninFigure4.1.Theop-amptriestokeepthevoltageattheoutputequaltothevoltageattheadjust(ADJ)pinminusthereferencevoltage.Toproduceacertainoutputvoltage,afeedbackresistorisconnectedfromtheoutput(OUT)totheADJpinandasinkresistorisconnectedfromtheADJpintoground,thuscreatingapotentialdivider.Usuallythefeedbackresistorissetto240ohms,inordertodrawaminimumof5mAfromtheregulatorandhelptomaintainstability.www.newnespress.com 34Chapter4LM317INOUT+–ADJR1R2Figure4.1:LM317Regulator.Acapacitorontheoutputterminalalsohelpswithstability.Theoutputvoltageisgivenbytheequation:1þR2VOUT¼1:25þIADJR2R1Note,IADJ=100mA,worstcase.VariationsoftheLM317regulatorincludefixedpositivevoltageversions(LM78xx)andnegativevoltageversions(LM79xx),where‘xx’indicatesthevoltage;i.e.LM7805isa+5V1Aregulator.TheLM317anditsvariantsneedaminimuminputtooutputvoltagedifferencetooperatecorrectly.Thisistypicallyintherange1Vto3V,dependingonthecurrentthroughtheregulator(highercurrentrequiresahighervoltagedifferential).Thisinputtooutputvoltagedifferenceisequaltothevoltageacrosstheinternalconstantcurrentgenerator,sincetheOUTpinisatthesamepotentialasthevoltagereference.LowdropoutvoltageregulatorsuseaPNPtransistorasthepowerswitch,withtheemitterconnectedtotheINterminalandthecollectorconnectedtotheOUTterminal,seeFigure4.2.Theyalsohaveagroundpinthatenablesaninternalreferencevoltagetobegeneratedindependentoftheinputtooutputvoltagedifferential.Adropoutvoltageoflessthan1Vispossible.www.newnespress.com LinearPowerSupplies35LP2950INPUTOUTPUT+–GROUNDFigure4.2:LowDropoutVoltageRegulator.4.1.2VoltageRegulatorsasCurrentSourceorSinkInFigure4.3areshowntwocircuitsusingavoltageregulatorasacurrentlimiter,oneisconfiguredasacurrentsourceandtheotherasacurrentsink.V+INLoadADJLM317INOUTADJLM317R1OUTR1LoadV–CURRENTSINKCURRENTSOURCEFigure4.3:ConstantCurrentCircuitsUsingtheLM317.Aspreviouslydescribed,theLM317regulateswhenthereis+1.25VbetweentheOUTandADJpins.InFigure4.3,acurrentsenseresistor(R1)isconnectedbetweentheOUTandADJpins.CurrentflowingthroughR1willproduceavoltagedrop,withthewww.newnespress.com 36Chapter4OUTpinbecomingmorepositivethantheADJpin.WhenthevoltagedropacrossR1reaches1.25V,theLM317willregulatethecurrent.Thusthecurrentlimitis1:25I¼.R14.1.3ConstantCurrentCircuitsTherearemanyconstantcurrentcircuits;someusingintegratedcircuits,someusingdiscretecomponents,andothersusingacombinationofbothICsanddiscretedevices.Inthissubsection,wewillexaminesomeexamplesofeachtype.Asimpleconstantcurrentsinkusesanop-ampwithaninputvoltagerangethatextendstothenegativerail,asshowninFigure4.4.Inordertosetthecurrentlevel,avoltagereferenceisrequired.Thevoltagedropacrossacurrentsensingresistoriscomparedtothereferencevoltageandtheop-ampoutputvoltagerisesorfallstocontrolthecurrent.Thevoltagereferencecanbeatemperaturecompensatedprecisionreference,oraZenerdiode.AZenerdiodegenerallyhasasmallesttemperaturecoefficientandlowestdynamicimpedanceatabreakdownvoltageof6.2V.+5V1K233K350mALMV321350mV+VN3205–3V93K31ohm0VFigure4.4:ConstantCurrentSinkUsingOp-amp.www.newnespress.com LinearPowerSupplies374.2AdvantagesandDisadvantagesTheadvantageoflinearpowersuppliesisthattheyproducenoEMIradiation.Thisadvantagecannotbeoverstated.Aswitchingpowersupplymayappeartohavefewcomponents,butthisdoesnottakeintoaccounttheEMIfilteringandscreening.TheseadditionalcircuitscandoubletheoverallcostoftheLEDdriver.IftheLEDsaredistributed,suchasinchannellightingwherethereisnoopportunitytoshieldanyEMI,bothcommonmodeanddifferentialfilteringarerequired.Andcommonmodechokesareexpensive!OnedisadvantageofalinearLEDdrivercanbelowefficiency,whichistheratiooftheLEDvoltagetothesupplyvoltage.TheefficiencyislowonlyifthesupplyvoltageissomewhathigherthantheLEDvoltage.Inthesecases,poorinefficiencycausestheintroductionofthermalproblems.Aheatsinkmayberequired,whichisbulkyandmoderatelyexpensive.ItshouldbenotedthatwherethesupplyvoltageisonlyalittlehigherthantheLEDvoltage,theefficiencyofacircuitusinglinearregulatorcouldbehigherthanoneusingaswitchingregulator.LinearmainspoweredLEDdrivershavethedisadvantageoflargesize,becauseastep-downtransformerisalmostalwaysrequired(unlesstheLEDstringvoltageisveryneartothepeakACsupplyvoltage).A50Hzor60Hzmainstransformerisbulkyandheavy.Smoothingcapacitorsafterthebridgerectifierarealsoverybulky.TheefficiencywillvaryastheACsupplyvoltagerisesandfallsoveralongperiod,becausethedifferencebetweentherectifiedvoltageandtheLEDstringvoltagewillchange.4.3LimitationsThemainlimitationofalinearsupplyisthattheLEDvoltagewillalwaysbelowerthanthesupplyvoltage.Linearvoltageandcurrentsourcescannotboosttheoutputvoltagesothattheoutputishigherthantheinput.Wheretheoutputvoltagecouldbehigherthantheinputvoltageaswitchingregulatorisnecessary.Thesewillbediscussedinthenextfewchapters.4.4CommonErrorsinDesigningLinearLEDDriversThemostcommonerroristoignorethepowerdissipation.Powerdissipationissimplythevoltagedropacrosstheregulatormultipliedbythecurrentthroughit.www.newnespress.com 38Chapter4Ifthevoltagedropishigh,thecurrentmustbelimitedtostaywithinthedevicepackagepowerdissipationlimits.AsurfacemountD-PAKpackagemaybelimitedtoabout1W,evenwhenthereissomecopperareasolderedtothetabterminal.Heatsinksarenowavailableforsurfacemountpackages,whicheasestheproblem.Anothererroristoignorethestart-upconditions.Thevoltageratingoftheregulatormustbehighenoughtoallowfortheoutputbeingconnectedto0V(ground).Thisisbecauseatstart-up,theoutputcapacitorwillbeunchargedandthusat0V.Onlyafteroperatingforashortperioddoestheoutputcapacitorcharge,whichreducesthevoltagedropacrosstheregulator.Thevoltageratingoftheregulatorshouldalwaysbegreaterthanthemaximuminputvoltageexpected.www.newnespress.com CHAPTER5Buck-BasedLEDDriversThefirstswitchingLEDdriverthatwewillstudyisthebuckconverter.Thebuckconverteristhesimplestoftheswitchingdrivers,andisastep-downconverterforapplicationswheretheloadvoltageisnevermorethanabout85%ofthesupplyvoltage.Thelimitofabout85%isduetoswitchingdelaysinthecontrolsystem.Inabuckconvertercircuit,apowerMOSFETisusuallyusedtoswitchthesupplyvoltageacrossaninductorandLEDloadconnectedinseries.TheinductorisusedtostoreenergywhentheMOSFETisturnedon;thisenergyisthenusedtoprovidecurrentfortheLEDwhentheMOSFETisturnedoff.AdiodeacrosstheLEDandinductorcircuitprovidesareturnpathforthecurrentduringtheMOSFETofftime.AsimpleschematicisshowninFigure5.1.CIRCULATINGCURRENT(MOSFETOFF)V+SUPPLY350mALEDD1L1CURRENT(MOSFETON)CONTROLLERQ1Figure5.1:BuckLEDDriver.www.newnespress.com 40Chapter5BuckconvertersareanattractivechoiceforLEDdriversinofflineandinlowvoltageapplicationsastheycanproduceaconstantLEDcurrentatveryhighefficiencies.Apeak-current-controlledbuckconvertercangivereasonableLEDcurrentvariationoverawiderangeofinputandLEDvoltagesandneedsnodesigneffortinfeedbackcontroldesign.Coupledwiththefactthattheseconverterscanbedesignedtooperateatabove90%efficiencies,thebuck-baseddriverbecomesanattractivesolutiontodrivehighbrightnessLEDs.5.1AnExampleBuckConverterControlICTheSupertexHV9910BintegratedcircuitwasdesignedespeciallyforLEDdriving.Itisagoodexampleofalowcost,lowcomponentcountsolutiontoimplementthecontinuousmodebuckconverter(theICitselfneedsjustthreeadditionalcomponentstooperate).LinearorPWMdimmingcanalsobeeasilyimplementedusingtheIC.AdiagramoftheHV9910BisshowninFigure5.2.VINReg7.5VOSCRoscVDD250mV–CMS+QRGATELD–CM+CSPWM_D100kHV9910GNDFigure5.2:SupertexHV9910B.www.newnespress.com Buck-BasedLEDDrivers41TheHV9910Bhastwocurrentsensethresholdvoltages–aninternallyset250mVandanexternalvoltageattheLDpin.Theactualthresholdvoltageusedduringswitchingwillbethelowerofthetwo.Thelowvalueofsensevoltageallowstheuseoflowresistorvaluesforthecurrentsense,whichmeanshighefficiency.TheHV9910BICoperatesdownto8Vinput,whichisrequiredforsomeautomobileapplications,andcanacceptamaximumof450Vinput,whichmakesitidealforofflineapplications.TheIChasaninternalregulatorthatsupplies7.5VtopowertotheIC’sinternalcircuitsfromtheinputvoltage,eliminatingtheneedforanexternallowvoltagepowersupply.TheICiscapableofdrivingtheexternalMOSFETdirectly,withouttheneedforadditionaldrivercircuitry.5.2BuckCircuitsforDCApplicationsForDCapplications,theschematicshowninFigure5.3canbeused.10–30VDCC14.7µF350mALEDD110BQ0601L1470µH6VINVDDC32,2µF10VHV99108RTR15PWM_D100K74Q1LDGATEVN3205N82CS3GNDR20,62RFigure5.3:BuckConverterforDCApplications.www.newnespress.com 42Chapter55.2.1TargetSpecificationInputvoltage=10V–30VLEDstringvoltage=4–8VLEDcurrent=350mAExpectedefficiency=90%5.2.2ChoosingtheSwitchingFrequencyandResistor(R1)TheswitchingfrequencydeterminesthesizeoftheinductorL1.Alargerswitchingfrequencywillresultinasmallerinductor,butwillincreasetheswitchinglossesinthecircuit.Atypicalswitchingfrequencyforlowinputvoltageapplicationsisfs=150kHz,whichisagoodcompromise.FromtheHV9910Bdatasheet,thetimingresistorbetweentheRTpinandgroundthatisneededtoachievethisfrequencyis150k.However,inthiscase,theminimuminputvoltageisonly80%ofthemaximumoutputvoltage.Inabuckconverter,thedutycycleoftheMOSFETswitch(proportionofthetimethattheswitchisturnedon),isgivenbyD=VOUTandwillalsobe80%.However,inVINcontinuousconductionmode,instabilitywillresultwhenthedutycyclegoesover50%.Topreventinstability,itisnecessarytooperateinconstantoff-timemode.ThisisachievedwiththeHV9910BcircuitbyconnectingthetimingresistorbetweentheRTpinandthegatepin.Thetimingcircuitonlychargesaninternalcapacitorwhenthetimingresistorisconnectedto0V;thegatepinisat0VwhentheMOSFETisturnedoff.Thustheoff-timeisconstant,sotheswitchingfrequencyvariesastheloadvoltagechanges.Ifwechooseatimingresistorthatgivesaconstantoff-timeofsay5ms,withan80%dutycycletheon-timewillbe20ms.Theswitchingfrequencywillbe40kHz.Attheotherextreme,witha30Vsupplyanda4Vload,thedutycyclewillbejust13.33%,sotheon-timewillbe767ns.Nowtheswitchingfrequencyis173.4kHz.Theaverageswitchingfrequencywillbeabout100kHz,sowecanbasetheselectionofothercomponentsonthis.Thetimingresistortogive5msoff-timewillbe100k.5.2.3ChoosingtheInputCapacitor(C1)Anelectrolyticcapacitorisgoodtoholdthevoltage,butthelargeESRofthesecapacitorsmakesitunsuitabletoabsorbthehighfrequencyripplecurrentgeneratedbythebuckconverter.Thus,metallizedpolypropylenecapacitorsorceramicwww.newnespress.com Buck-BasedLEDDrivers43capacitorsinparallelareneededtoabsorbthehighfrequencyripplecurrent.TherequiredhighfrequencycapacitancecanbecomputedasIoTOFFC1¼ðÞ0:05VminInthisdesignexample,thehighfrequencycapacitancerequiredisabout4.7mF50V.ThiscapacitorshouldbelocatedclosetotheinductorL1andMOSFETswitchQ1,tokeepthehighfrequencyloopcurrentwithinasmallareaonthePCB.Inpractice,twosuchcapacitorswithasmallinductorbetweenthem(tomakeaPIfilter)areneededtolimitEMIemissions.5.2.4ChoosingtheInductor(L1)TheinductorvalueweusedependsontheallowedlevelofripplecurrentintheLEDs.Assumethat–15%ripple(atotalof30%)isacceptableintheLEDcurrent.ThefamiliarequationforaninductorisE¼Ldi.ConsideringthetimewhendttheMOSFETswitchisoff,sothattheinductorissupplyingenergytotheLEDs,E¼V¼V¼Ldi.AnotherwayofwritingthisisL¼Vdt.Here,LEDo,maxdto,maxdidiistheripplecurrent=0.3Io,maxanddtistheoff-time.Then,theinductorL1canbecomputedattherectifiedvalueofthenominalinputvoltageasVo,maxTOFFL1¼0:3Io,maxInthisexample,L1=380mHandtheneareststandardvalueis470mH.Sincethisvalueisalittlehigherthanthecalculatedvalue,theripplecurrentwillbelessthan30%.Thepeakcurrentratingoftheinductorwillbe350mAplus15%ripple:ip¼0:351:15¼0:4A:TheRMScurrentthroughtheinductorwillbethesameastheaveragecurrent(i.e.350mA).www.newnespress.com 44Chapter55.2.5ChoosingtheMOSFET(Q1)andDiode(D2)ThepeakvoltageseenbytheMOSFETisequaltothemaximuminputvoltage.Usinga50%safetyrating,VFET¼1:530V¼45VThemaximumRMScurrentthroughtheMOSFETdependsonthemaximumdutycycle,whichis80%inourexample.Hence,thecurrentratingoftheMOSFETisIFETIo,max0:8¼0:28A:TypicallyaMOSFETwithaboutthreetimesthecurrentischosentominimizetheresistivelossesintheswitch.Forthisapplication,choosea50V,>1AMOSFET;asuitabledeviceisaSupertexpart,VN3205N8,ratedat50V1.5A.ThepeakvoltageratingofthediodeisthesameastheMOSFET.Hence,Vdiode¼VFET¼45VTheaveragecurrentthroughthediodeunderworstcaseconditions(minimumdutycycle)isIdiode¼0:87Io,max¼0:305AChoosea60V,1ASchottkydiode.TheInternationalRectifier10BQ060isasuitabletype.5.2.6ChoosingtheSenseResistor(R2)Thesenseresistorvalueisgivenby0:25R2¼1:15Io,maxThisistrueiftheinternalvoltagethresholdof0.25Visbeingused.Otherwise,substitutethevoltageattheLDpininsteadofthe0.25Vintotheequation.Notethatthecurrentlimitissetto15%abovethemaximumrequiredcurrent,duetothetotal30%ripplespecified.www.newnespress.com Buck-BasedLEDDrivers45Forthisdesign,R2=0.625.TheneareststandardvalueisR2=0.62.Ifastandardvalueisnotclosetothevaluecalculated,orifalowerpowerdissipationinthesenseresistorisrequired(perhapstoincreaseefficiency),apotentialdividercanbeconnectedtotheLDpintosetitatalowervoltage.Saywewanttousea0.47resistor;thenwewouldscalethe0.25VattheLDpinby0.47/0.625=0.752,sothatitbecomes188mV.NotethatcapacitorC3isabypasscapacitorforholdinguptheHV9910BinternalsupplyVDDduringMOSFETswitching,whenhighfrequencycurrentpulsesarerequiredforchargingthegate.AtypicalvalueforC3of2.2mF,16Visrecommended,althoughinthisdesigntheMOSFETgatechargeisverylow,soa1mF,16Vcanbeusedinstead.5.2.7CommonErrorsinLowVoltageBuckDesign1.Usinganinductorthathastoohighinductance.Althoughincreasingtheinductorvaluemayseemtobetheanswertoreducecurrentripple,itactuallycausesproblemsbecausethecurrentdoesnotfallenoughbetweenswitchingcyclesforpropercontrolbythecontrollerIC.Thevoltageseenacrossthecurrentsenseresistoratswitch-onwillbealmostatthecurrentsensecomparatorreferencevoltage.Atswitch-ontherewillbeacurrentsurge,causedbytheflywheeldiodereversecurrentandthecurrentthroughtheinductor’sparasiticcapacitance.Thesmallestcurrentsurgewillcreateavoltagespikeacrossthecurrentsenseresistorandhencethecurrentsensecomparatorwilltrip.ThismeansthattheMOSFETwillswitchoffalmostimmediatelyafterswitch-on.Atypicalswitchingpatternisoneproperswitchingcycle,whereenergyisstoredintheinductor,followedbyoneshortswitchingpulse.Thisswitchingpulseprovidesverylittleenergytotheinductor,butgenerateshighswitchinglosses.TheresultisalessefficientcircuitthatcouldsufferfromoverheatingandEMIproblems.2.Usingthewrongtypeofflywheeldiode.ASchottkydiodehasalowforwardvoltagedrop,whichwillgivelowpowerdissipation.However,inlowdutycycleapplicationstheLEDcurrentisflowingintheflywheeldiodemostofthetime.Aforwardvoltageofsay0.45Vwww.newnespress.com 46Chapter5at350mAresultsin157.5mWconductionlosses,soanSMAsizepackageworkswell,butforhighercurrentapplicationsalargeSMBorSMCpackageshouldbeconsidered.NotethattheforwardvoltagedropofSchottkydiodesincreaseswiththeircurrentrating,soa30VSchottkyhasmuchlowerVfthana100VSchottky.5.3BuckCircuitsforACInputIwillnowdiscussthedesignofabuck-basedLEDdriverusingtheHV9910BwiththehelpofanACmainsinputapplicationexample.ThesameprocedurecanbeusedtodesignLEDdriverswithotherinputvoltageranges.TheschematicisshowninFigure5.4.LiveV+230VAC+C1C233uF330nF350mALEDNTCD11UF4005L16VIN4,7mHVDDC32,2uF10VHV99104Q17GATESTD2NM60T4LD2CS5PWM_DR20,62RR183RoscGND470KFigure5.4:UniversalMainsInputBuckCircuit.DesignsforanACinputhavetwoproblemareastoaddress.InadditiontoconsideringtheLEDdrivingaspects,wemustalsoconsiderthelowfrequencyand,usually,highvoltagesupply.Becauseweareapplyingalowfrequencysinusoidalhighvoltagesupply,highvalueinputcapacitorsareneededtoholdupthesupplywww.newnespress.com Buck-BasedLEDDrivers47voltageduringthecuspsbetweeneachhalf-cycleoftheinput.Applyinghighvoltageacrosshighvaluecapacitorscreatesalargeinrushcurrentthatcancausedamage,soaninrushlimiter(negativetemperaturecoefficientthermistor)isrequired.5.3.1TargetSpecificationInputvoltage=90Vto265VAC(nominal230VAC)LEDstringvoltage=20–40VLEDcurrent=350mAExpectedefficiency=90%5.3.2ChoosingtheSwitchingFrequencyandResistor(R1)TheswitchingfrequencydeterminesthesizeoftheinductorL1.Alargerswitchingfrequencywillresultinasmallerinductor,butwillincreasetheswitchinglossesinthecircuit.Atypicalswitchingfrequencyforhighinputvoltageapplicationsisfs=80kHz,whichisagoodcompromise.FromtheHV9910Bdatasheet,thetimingresistorneededtoachievethisis470k.5.3.3ChoosingtheInputDiodeBridge(D1)andtheThermistor(NTC)Thevoltageratingofthediodebridgewilldependonthemaximumvalueoftheinputvoltage.A1.5multiplicationfactorgivesa50%safetymargin.pffiffiffiVbridge¼1:52Vmax,ac¼562VThecurrentratingwilldependonthehighestaveragecurrentdrawnbytheconverter,whichisatminimuminputvoltage(DClevel,allowingfora‘droop’acrosstheinputcapacitorbetweentheAClinevoltagepeaks)andatmaximumoutputpower.TheminimuminputvoltagemustbemorethanhalfthemaximumLEDstringvoltage,tomakesurethatthedutycyclestaysbelow50%andthusremainsstable.Forthisexample,theminimumrectifiedvoltageshouldbewww.newnespress.com 48Chapter5Vmin,dc¼2Vo,max¼80V:Vo,maxIo,max14Ibridge¼¼¼0:194AVmin,dc72Forthisdesign,usinga230VACsupply,choosea600V1Adiodebridge.Thethermistorshouldlimittheinrushcurrenttonotmorethanfivetimesthesteadystatecurrent,assumingmaximumvoltageisapplied.Therequiredcoldresistanceis:pffiffiffi2Vmax,acRcold¼5IbridgeThisgivesusa380resistanceat25°C.Thecalculationssuggestthatwechooseathermistorwhoseresistanceisaround380andRMScurrentgreaterthan0.2A,butinpracticea120thermistorratedat1Awouldsuffice.5.3.4ChoosingtheInputCapacitors(C1andC2)ThefirstdesigncriteriontomeetisthatthemaximumLEDstringvoltagemustbelessthanhalftheminimuminputvoltage.Thisistosatisfythestabilityrequirementswhenoperatingataconstantswitchingfrequency.Aswehavealreadyseen,theminimumrectifiedvoltageshouldbeVmin,dc¼2Vo,max¼80VThehold-upcapacitorrequiredattheoutputofthediodebridgewillhavetobecalculatedattheminimumACinputvoltage.ThecapacitorcanbecalculatedasVo,maxIo,maxC12V2V2freqmin,acmin,dcInthisexample,C126:45mFwww.newnespress.com Buck-BasedLEDDrivers49Thevoltageratingofthecapacitorshouldbemorethanthepeakinputvoltage.pffiffiffiVmax,cap2Vmax,ac)Vmax,cap375VChoosea450V,33mFelectrolyticcapacitor.Theelectrolyticcapacitorisgoodtoholdthevoltage,butthelargeESRofthesecapacitorsmakesitunsuitabletoabsorbthehighfrequencyripplecurrentgeneratedbythebuckconverter.Thus,ametallizedpolypropylenecapacitorisneededinparallelwiththeelectrolyticcapacitortoabsorbthehighfrequencyripplecurrent.TherequiredhighfrequencycapacitancecanbecomputedasIo,max0:25C2¼fsðÞ0:05Vmin,dcInthisdesignexample,thehighfrequencycapacitancerequiredisabout0.33mF,400V.ThiscapacitorshouldbelocatedclosetotheinductorL1andMOSFETswitchQ1,tokeepthehighfrequencyloopcurrentwithinasmallareaonthePCB.5.3.5ChoosingtheInductor(L1)TheinductorvalueweusedependsontheallowedlevelofripplecurrentintheLEDs.Assumethat–15%ripple(atotalof30%)isacceptableintheLEDcurrent.ThefamiliarequationforaninductorisE¼Ldi:ConsideringthetimewhendttheMOSFETswitchisoff,sothattheinductorissupplyingenergytotheLEDs,E¼V¼V¼Ldi:AnotherwayofwritingthisisL¼Vdt.LEDo,maxdto,maxdiVo,max1pffiffi2Vac,nomHere,diistheripplecurrent=0.3Io,maxanddtistheoff-timedt¼f.sNote,abuckcircuitdutycycleisgivenbyD¼Vout,sotheoff-timeisdt¼ðÞ1D.VinfsThen,theinductorL1canbecomputedattherectifiedvalueofthenominalinputvoltageasVo,maxVo,max1pffiffi2Vac,nomL1¼0:3Io,maxfswww.newnespress.com 50Chapter5Inthisexample,L1=4.2mH.Theneareststandardvalueis4.7mH.Sincethisvalueisalittlehigherthanthecalculatedvalue,theripplecurrentwillbelessthan30%.Thepeakcurrentratingoftheinductorwillbe350mAplus15%ripple:Ip¼0:351:15¼0:4ATheRMScurrentthroughtheinductorwillbethesameastheaveragecurrent(i.e.350mA).Notethatwithalargeinductancevalue,theparasiticcapacitanceacrossthecoilcouldbesignificantandwillaffectswitchinglosses.5.3.6ChoosingtheMOSFET(Q1)andDiode(D2)ThepeakvoltageseenbytheMOSFETisequaltothemaximuminputvoltage.Usinga50%safetyrating,pffiffiffiVFET¼1:52265¼562VThemaximumRMScurrentthroughtheMOSFETdependsonthemaximumdutycycle,whichis50%bydesign.Hence,thecurrentratingoftheMOSFETispffiffiffiffiffiffiffiIFETIo,max0:5¼0:247ATypicallyaMOSFETwithaboutthreetimesthecurrentischosentominimizetheresistivelossesintheswitch.Forthisapplication,choosea600V,>1AMOSFET;asuitabledeviceisanSTpart,STD2NM60,ratedat600V2A.ThisMOSFEThas2.8on-resistance.With350mAbeingpassedupto50%ofthetime,theconductionlosseswillbe171mW.AlthoughaMOSFETwithloweron-resistancecouldbeusedtoreducetheconductionlosses,theswitchinglosses,whicharecausedbyparasiticcapacitanceanddiodereverserecoverycurrent,willthenbehigher.ThediodeD2passescurrentinthereversedirectionforashortperiod:imagineamechanicalvaluethatispassingafluid–whenthepressurereversesittakesashorttimeforthevalvetocloseandshutoffthereverseflow.Theanalogycanbeappliedtodiodes,becausetheyhavefreeelectronsintheirconductionbandthathavetobesweptoutbythereversepotentialbeforecurrentflowstops.EachtimetheMOSFETturnson,acurrentspikepasseswww.newnespress.com Buck-BasedLEDDrivers51throughtheMOSFET,butthecurrentislimitedbytheMOSFETcurrentrating,soalowercurrentratingcanreducetheswitchinglosses.ThepeakvoltageratingofthediodeisthesameastheMOSFET.Hence,Vdiode¼VFET¼562VTheaveragecurrentthroughthediodeisIdiode¼0:5Io,max¼0:175AChoosea600V,1Aultra-fastdiode.TheUF4005isalowcostultra-fasttype,butforgreatestefficiencyafasterdiodelikeSTTH1R06shouldbeused.Ifweassumeaforwardvoltagedropof1Vat350mA,theconductionlosswillbelessthan350mWatlowdutycycles.Theswitchinglosscouldbehigherthatthisvalue,butislessofaprobleminfasterdiodesbecausethereverseconductionisforashortertimeperiod.5.3.7ChoosingtheSenseResistor(R2)Thesenseresistorvalueisgivenby0:25R2¼1:15Io,maxThisistrueiftheinternalvoltagethresholdof0.25Visbeingused.Otherwise,substitutethevoltageattheLDpininsteadofthe0.25Vintotheequation.AlowervoltagecouldbeappliedtotheLDpintoenableaconvenientvalueofR2tobeused,asdescribedearlier.Forthisdesign,R2=0.625.TheneareststandardvalueisR2=0.62.NotethatcapacitorC3isabypasscapacitorforholdinguptheHV9910BinternalsupplyVDDduringMOSFETswitching,whenhighfrequencycurrentpulsesarerequiredforchargingthegate.AtypicalvalueforC3of2.2mF,16Visrecommended,althoughforACapplicationssmallercapacitorsaslowas0.1mFhavebeenusedsuccessfully.TheswitchingfrequencytendstobelowerandsotheMOSFETgatecurrentrequirementsarelow.Alsowithahighervoltageontheinputsupplypin,thevoltagedropacrosstheinternalregulatorduringMOSFETswitchingisunlikelytocauseunder-voltagedropout.www.newnespress.com 52Chapter55.4BuckCircuitsPoweredbyanACPhaseDimmerAnLEDdriverpoweredbyanACphasedimmerneedsspecialadditionalcircuits.Theseadditionalcircuitsarerequiredbecauseofthephasedimmercircuit.Phasedimmersusuallyuseatriacactivatedbyapassivephaseshiftcircuit.Becauseofswitchingtransients,whichwouldotherwisecauseseriousEMIproblems,thetriacisbypassedbyacapacitor(typically10nF)andhasaninductorinserieswithitsoutput.ThephasedimmercircuitisshowninFigure5.5.62nF47K220KBR1000.1mHTRIAC47nFFigure5.5:PhaseDimmerCircuit.TheinputofaninactiveLEDdriverishighimpedance,withalargecapacitorontheDCsideofthebridgerectifier.Thecapacitoracrossthetriacallowsasmallcurrenttoflowthroughthebridgerectifierandthesmoothingcapacitorstartstocharge.Whenthevoltagebuildsup,theLEDdriverwilltrytooperate.TheresultisanoccasionalflickeroftheLED.Whatisrequiredisadischargecircuit,tokeepthesmoothingcapacitorvoltagebelowthatrequiredtostarttheLEDdriver.A390resistorwasfoundtokeepthesmoothingcapacitorvoltagebelow5V.Topreventhighpowerlosswhenthecircuitisactive,asimplevoltagedetectorcanbeusedtodisconnectthe390resistorwhenavoltageaboveabout8Visdetected.ThiscircuitisshowninFigure5.6.www.newnespress.com Buck-BasedLEDDrivers53DCRAILACINPUT+1N40061N4006+9V390R220K47KVN2460N3VN0106N37V57V5100KFigure5.6:SmoothingCapacitorDischargeCircuit.Thetriacneedstoseeaload.Onceatriacistriggered,itistheloadcurrentthatkeepsitswitchedon;thetriacisaself-sustainingswitch.However,anLEDdriverprovidesnoloaduntiltheinputvoltagehasrisenabovetheLEDvoltage,andittakesalittletimeforthiscurrenttobestableatasufficientlyhighleveltokeepthetriacturnedon.Forthisreason,anadditionalloadmustbeswitchedacrosstheLEDdriverinputatlowvoltages.Testshaveshownthata2K2resistorworksasatriacloadandthatitshouldremainincircuituntilthesupplyvoltagehasrisentoabout100V,butshouldthenbeswitchedoffuntiltherisingedgeofthenexthalf-wave.AlatchingcircuittoprovidethisfunctionisshowninFigure5.7.Thesecircuitscanbecombined.ThevoltagedetectorforthesmoothingcapacitordischargecircuitcanalsobeusedtoprovideanenablesignalfortheLEDdriverwww.newnespress.com 54Chapter5DCRAILACINPUT+1N40061N40062K21W1N4148330K220K47K+9VVN2460N3100VVN0106N37V57V5100KFigure5.7:AdditionalLoadSwitch.(PWMinput).Thuswhenthetriacisoff,theLEDdriverisalsooff.ThecombinedcircuitisshowninFigure5.8.5.5CommonErrorsinACInputBuckCircuitsThemostcommonerroristryingtodriveasingleLEDfromtheACmainssupply.ThedutycycleisVout/Vin,soforuniversalACinput90Vto265VAC,therectifiedvoltageisabout100Vto375V.Theworstcaseisthehighervoltage;considerdrivingawhiteLEDwith3.5Vforwardvoltage.Thedutycyclewillbe3.5/375=0.9333%dutycycle.Iftheswitchingfrequencyis50kHz,with0.02mssecondperiod,theMOSFETon-timewillbejust186ns.Thistimeistooshortforthecurrentsensewww.newnespress.com Buck-BasedLEDDrivers55NTC100nF100nFV+X2X2+C1C2220K33uF330nF350mALED+9V9V1D11UF4005L14,7mH6VINC3VDD1N40061N4006HV99104Q17GATESTD2NM60T42K2LD2CS1W1N4148330K220K+9V5PWM_DR20,62R47K390R220K8R1+9V47K3GNDRoscVN2460N3470KVN2460N3VN0106N3100VVN0106N37V57V57V5100K7V5Figure5.8:CompletePhaseDimmableLEDDriver.circuittoreact;itneedstobeatleast300ns.Operatingat20kHzwillgiveanon-timeof466ns,whichisclosetothelimitforaccuratecontrol.Adoublebuckmaybeneeded(seenextsection).Anothererrorisnottakingintoaccounttheparasiticcapacitanceoftheinductorwindingsandthereversecurrentintheflywheeldiode.ThesefactorscanbeignoredinlowvoltageDCapplications,butnotinACapplicationswheretherectifiedsupplyishighvoltage.ThecurrentpeakthroughtheMOSFETcanbehighenoughtotripthecurrentsensecircuit,resultinginerraticswitching.AnRCfilterbetweenthecurrentsenseresistorandthecurrentsenseinputoftheintegratedcircuitmaybenecessary.A2.2kseriesresistorfollowedbya100pFshuntcapacitortogroundshouldbesufficient.5.6DoubleBuckThedoublebuckisanunusualdesign,asshowninFigure5.9.ItusesoneMOSFETswitch,buttwoinductors(L2andL3)inseries.DiodessteerthecurrentinL2,whichmustoperateindiscontinuousconductionmode(DCM)forcorrectoperation.www.newnespress.com 56Chapter5LiveL1L20.1mH2mHV+230VACC1C247nF47nFD2C3D1BYD57J470nFL336V0.33mHD3D4BYD57JBYD57J1350mALED6VIN+VDDC32,2uF10VHV99104Q17GATESTD2NM60T4LD2CS5PWM_DR1R2470K0,56R83RoscGNDFigure5.9:DoubleBuck.Thedoublebuckisusedwhentheoutputvoltageisverylowandtheinputvoltageishigh.AnexampleisdrivingasinglepowerLEDfromanACsupplyline.Asinglebuckstagecannotworkeasilybecausetheon-timeofthebuckconverteristoosmall,unlessaverylowswitchingfrequencyisused.Assumethemaximumdutycycle,Dmax,islessthan0.5;alsoassumethatthefirststage(L2)isinboundaryconductionmode(BCM)atDmax.Boundaryconductionmodemeansthatthecurrentthroughtheinductoronlyjustfallstozeroandthenextswitchingcyclebegins.VoVinmin¼D2maxOrtransposed,thisbecomes:rffiffiffiffiffiffiffiffiffiffiffiffiffiVoDmax¼VinminThisassumesthatL2isinBCMandL3isincontinuousconductionmode(CCM);attheminimumoperatinginputvoltage(Vinmin).www.newnespress.com Buck-BasedLEDDrivers57ThestoragecapacitorvoltageatVinminandDmaxisgivenbytheequation:Vcmin¼VinminDminThepeakcurrentthroughtheinputstageinductor,atVinminequals:IL2pk¼2IL2avgVoIo¼2VcminThustheprimarystageinductorL2hasavaluegivenby:ðÞVinminVcminDmaxTsL2¼IL2pkThetransferratioforaDCMbuckconverter(whereRisloadresistorseenbytheconverter)isgivenby:Vc2¼qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiVin1þ1þ8L22RTsDTheresistorRseenbythefirststage(andassumingsecondstageisinCCM)isgivenby:V2cR¼PoðÞVD2V22co)RD¼¼PoPoCombiningtheprevioustwoequations(whichturnouttobeaconstant):Vc2¼K¼qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiVin1þ1þ8L2PoTsVo2WefindthatDisinverselyproportionaltoVin:VoVoD¼¼VcKVinwww.newnespress.com 58Chapter5AndwecannowshowthatthepeakinductorcurrentthroughL2isaconstantovertheoperatinginputvoltage:SettingD=K0/V,K0=V/KinoK0isaconstant,sinceVisconstant.oðÞVinVcDTsiL2,pk¼L2K0VinðÞ1KVTs¼inL1ðÞ1KK0Ts¼L2Wecannowdefinetheaverageinputvoltageasthemaximuminputvoltagepffiffiffið2VacmaxÞandtheminimumoperatinginputvoltage:ðVinmaxþVinminÞVinavg¼2Thestoragecapacitorvalueiscomputedbasedon10%voltagerippleonthecapacitoratVinminandDmax:0:5IL2pkð1DmaxÞTsC¼0:1VcminThevoltageacrossthestoragecapacitor,withaveragevoltageinput,isgivenby:Ccavg¼KVinavgWecannowcomputetheaveragedutycycle(ataverageinputvoltage):VoDavg¼VcavgComputingthevalueofL3:ðVcavgVoÞDavgTsL3¼DIL3www.newnespress.com Buck-BasedLEDDrivers595.7HystereticBuckAsanalternativetothepeakcurrentcontrolbuck,hystereticcontrolcanbeused.ThisusesafastcomparatortodrivetheMOSFETswitch.Theinputtothecomparatorisahighsidecurrentsensecircuit,wherethevoltageacrossaresistorinthepositivepowerfeedtotheLEDloadismonitored.ThisisshowninFigure5.10.FlywheelDiodeVINRcsLEDLComparator–MOSFET+SwitchFigure5.10:HystereticCurrentControlCircuit.TheMOSFETisturnedonwhenthecurrentlevelisatorbelowaminimumreferencevoltage.TheMOSFETisturnedoffwhenthecurrentisatoraboveamaximumreferencevoltage.ThisisshowninFigure5.11.Bythismethod,theaverageLEDcurrentremainsconstant,regardlessofchangesinthesupplyvoltageorLEDforwardvoltage.Vcs(high)Vcs(Average)VcsVcs(Low)Figure5.11:CurrentSenseVoltage(CurrentinLEDLoad).www.newnespress.com 60Chapter5Thecurrentlevelissetbyasuitableresistorvalue,givenby:
1VCSðhighÞþVCSðlowÞRSENSE¼
2ILEDInwords,theaveragecurrentsensevoltage(midwaybetweenthehighandlowlevels)dividedbytheaverageLEDcurrentrequired.Thedatasheetofthehystereticcontrollerbeingusedwillgivetheupperandlowercurrentsensevoltagelevelsthatthecomparatoruses.www.newnespress.com CHAPTER6BoostConvertersBoostconverters(seeFigure6.1)areidealforLEDdriverapplicationswheretheLEDstringvoltageisgreaterthantheinputvoltage.Normally,aboostconverterwouldonlybeusedwhentheoutputvoltageminimumisabout1.5timestheinputvoltage.•Theconvertercaneasilybedesignedtooperateatefficienciesgreaterthan90%.•BoththeMOSFETandLEDstringareconnectedtoacommonground.ThissimplifiessensingoftheLEDcurrent,unlikethebuckconverterwherewehavetochooseeitherahighsideMOSFETdriverorahighsidecurrentsensor.•Theinputcurrentcanbecontinuous,whichmakesiteasytofiltertheinputripplecurrentandthuseasiertomeetanyrequiredconductedEMIstandards.L1D1V+CinCoutCONTROLLERQ1Figure6.1:SimplifiedBoostConverterCircuit.www.newnespress.com 62Chapter6Boostconvertershavesomedisadvantages,especiallywhenusedasLEDdrivers,duetothelowdynamicimpedanceoftheLEDstring.•Theoutputcurrentoftheboostconverterisapulsedwaveform.Thus,alargeoutputcapacitorisrequiredtoreducetherippleintheLEDcurrent.•ThelargeoutputcapacitormakesPWMdimmingmorechallenging.TurningtheboostconverteronandofftoachievePWMdimmingmeansthecapacitorwillhavetobechargedanddischargedeveryPWMdimmingcycle.ThisincreasestheriseandfalltimesoftheLEDcurrent.•OpenloopcontroloftheboostconvertertocontroltheLEDcurrent(asinthecaseofanHV9910-basedbuckcontrol)isnotpossible.Closedloopisrequiredtostabilizetheconverter.ThisalsocomplicatesPWMdimming,sincethecontrollerwillhavetohavealargebandwidthtoachievetherequiredresponsetimes.•Thereisnocontrolovertheoutputcurrentduringtheoutputshortcircuitconditions.Thereisapathfromtheinputtotheoutputviaadiodeandinductor,soturningofftheswitchingMOSFETwillhavenoeffectontheshortcircuitcurrent.•TherewillbeasurgeofcurrentintotheLEDsifaninputvoltagetransientraisestheinputvoltageabovetheLEDstringvoltage.Ifthesurgecurrentishighenough,theLEDswillbedamaged.6.1BoostConverterOperatingModesAboostconvertercanbeoperatedintwomodes–eithercontinuousconductionmode(CCM)ordiscontinuousconductionmode(DCM).Themodeofoperationoftheboostconverterisdeterminedbythewaveformoftheinductorcurrent.Figure6.2(a)istheinductorcurrentwaveformforaCCMboostconverterwhereasFigure6.2(b)istheinductorcurrentwaveformforaDCMboostconverter.TheCCMboostconverterisusedwhenthemaximumstep-upratio(ratioofoutputvoltagetoinputvoltage)islessthanorequaltosix.Iflargerboostratiosarerequired,theDCMboostconverterisused.However,indiscontinuousconductionmode,theinductorcurrenthaslargepeakvalues,whichincreasesthecorelossesintheinductor.ThusDCMboostconvertersaretypicallylessefficientthanCCMboostconverters,cancreatemoreEMIproblemsandareusuallylimitedtolowerpowerlevels.www.newnespress.com BoostConverters63(a)ContinuousConductionModeILt(b)Dis-continuousConductionModeILtFigure6.2:InductorCurrentCCMandDCM.6.2HV9912BoostControllerSupertex’sHV9912integratedcircuitisaclosed-loop,peakcurrentcontrolled,switch-modeconverterLEDdriver.TheHV9912hasbuilt-infeaturestoovercomethedisadvantagesoftheboostconverter.Inparticular,itfeaturesadisconnectMOSFETdriveroutput.TheexternalMOSFETdrivenfromthisoutputcanbeusedtodisconnecttheLEDstringsduringshortcircuit,orinputover-voltage,conditions.ThisdisconnectMOSFETisalsousedbytheHV9912todramaticallyimprovethePWMdimmingresponseoftheconverter(seePWMDimmingsection).TheLinearTechnologyLTC3783hassimilarfunctionality,althoughthispartoperatesfromalowvoltagesupply(6–16Vinput).ThemostsignificantfunctionswithintheHV9912areshowninFigure6.3.TheinternalhighvoltageregulatorintheHV9912providesaregulated7.75VVDDfroma9–90Vinput,whichisusedtopowertheIC.Thisvoltagerangeisgoodformostboostapplications,buttheICcanalsobeusedinbuckandSEPICcircuitswhenaccuratecurrentcontrolisrequired.Inahighvoltagebuckapplication,aZenerdiodecouldbeaddedinserieswiththeinputtoallowanevenhigheroperatingvoltage,ortoreducethepowerdissipatedbytheIC.TheVDDpinoftheICcanbeoverdriven(ifnecessary)withanexternalvoltagesourcefedthroughalowvoltage(>10V),lowcurrentdiode.Thediodewillhelptowww.newnespress.com 64Chapter6HV99121.25VVINReg7.75VRegREFVDDPORSYNCOSCRTCLIM–CMSCS+QRGATE–CM+PWM_DCOMPPORHICCUP/13RFAULTDIMMINGSSVref–RCM+OVPGMGNDFDBK–IREF+2–CM+GNDFigure6.3:HV9912InternalStructure(Simplified).preventdamagetotheHV9912iftheexternalvoltagebecomeslessthantheinternallyregulatedvoltage.ThemaximumsteadystatevoltagethatcanbeappliedtotheHV9912VDDpinis12V(withatransientvoltageratingof13.5V).Allowingforthediodeforwardvoltagedropa12V±5%powersupplywouldbeideal.TheHV9912includesabuffered1.25V,2%accuratereferencevoltage.Thisreferencevoltagecanbeusedtosetthecurrentreferencelevelaswellastheinputcurrentlimitlevel,byconnectingpotentialdividernetworksbetweentheREFpinandtheIREFandCLIMpins.Thisreferenceisalsousedinternallytosettheover-voltagesetpoint.Usinganexternalresistor,wecansettheoscillatortimingoftheHV9912.IftheresistorisconnectedbetweentheRTandGNDpins,theconverteroperatesinaconstantfrequencymode,whereasifitisconnectedbetweentheRTandGATEpins,theconverteroperatesinaconstantoff-timemode(slopecompensationisnotnecessarytostabilizethewww.newnespress.com BoostConverters65converteroperatinginaconstantoff-time).Inbothcases,theclockperiodoroff-timecanbesettoanyvaluebetween2.8msto40msusingtheequationgiveninSection6.3.12.MultipleHV9912ICscanbesynchronizedtoasingleswitchingfrequencybyconnectingtheSYNCpinsofalltheICstogether.ThisissometimesnecessaryinRGBlightingsystems,orwhenEMIfiltersaredesignedtoremoveacertainfrequency.ClosedloopcontrolisachievedbyconnectingtheoutputcurrentsensesignaltotheFDBKpinandthecurrentreferencesignaltotheIREFpin.TheHV9912triestokeepthefeedbacksignalequaltothevoltageontheIREFpin.Ifthefeedbackistoohigh,indicatingthatthecurrentisabovetherequiredlevel,theMOSFETswitchingisstopped.WhenthefeedbackfallsbelowthevoltageattheIREFpin,switchingisstartedagain.ThecompensationnetworkisconnectedtotheCOMPpin(outputofthetransconductanceop-amp).WhatisnotshowninFigure6.3isthattheoutputoftheamplifierhasaswitchcontrolledbythePWMdimmingsignal.WhenthePWMdimmingsignalislow,thisswitchdisconnectstheoutputoftheamplifier.Thus,thecapacitor(s)inthecompensationnetworkholdthevoltagewhilethePWMsignalislow.WhenthePWMdimmingsignalgoeshighagain,thecompensationnetworkisreconnectedtotheamplifier.ThisensuresthattheconverterstartsatthecorrectoperatingpointandaverygoodPWMdimmingresponseisobtainedwithouthavingtodesignafastcontroller.TheFAULTpinisusedtodriveanexternaldisconnectMOSFET(seeFigure6.4).Duringthestart-upoftheHV9912,theFAULTpinisheldlowandoncetheICstartsupthepinispulledhigh.ThisconnectstheLEDsinthecircuitandtheboostL1D1V+CinCoutQ1CONTROLLERFAULTDisconnectMOSFETFigure6.4:DisconnectMOSFET.www.newnespress.com 66Chapter6converterpowersuptheLEDs.Incaseofanoutputover-voltageconditionoranoutputshortcircuitcondition,theFAULTpinispulledlowandanexternalMOSFETswitchedofftodisconnecttheLEDs.TheFAULTpinisalsocontrolledbythePWMdimmingsignal,sothatthepinishighwhenthePWMdimmingsignalishighandviceversa.ThisdisconnectstheLEDsandmakessurethattheoutputcapacitordoesnothavetobecharged/dischargedeveryPWMdimmingcycle.ThePWMdimminginputtotheFAULTpinandtheoutputoftheprotectioncircuitryarelogicallyAND’edtomakesurethattheprotectioncircuitoverridesthePWMinputtotheFAULTpin.Outputshortcircuitprotectionisprovidedbyacomparatorthattriggerswhentheoutputcurrentsensevoltage(attheFDBKpin)istwicethatofthereferencevoltage(attheIREFpin).TheoutputovervoltageprotectionisactivatedwhenthevoltageattheOVPpinexceeds5V.Boththesefaultsignalsarefedintothehiccupcontrol.TheoutputofthishiccupcontrolturnsoffboththeGATEpinandtheFAULTpinwhenafaultconditionoccurs.OncetheICgoesintothefaultmode,eitherbyanoutputover-voltageconditionorashortcircuit,thehiccupcontrolisactivated.ThehiccupcontrolturnsoffthegatedrivestobothMOSFETs.Atthesametime,atimerisstartedtokeeptheoutputturnedoffforashortperiod(determinedbythecapacitanceontheCOMPpin).Oncethistimeperiodelapses,theHV9912attemptstorestart.Ifthefaultconditionpersists,theoutputisturnedoffagainandthetimerisreset.ThisrepeatsuntilthefaultconditionhasbeenremovedandtheHV99112returnstonormaloperation.LineardimmingisachievedbyvaryingthevoltagelevelattheIREFpin.ThiscanbedoneeitherwithapotentiometerfromtheREFpinorfromanexternalvoltagesourceandaresistordivider.Thisallowsthecurrenttobelinearlydimmed.However,aminimumoutputvoltagelimithasbeendeliberatelyaddedtotheoutputoftheGMamplifier,topreventfalsetriggeringofthefaultconditionthatcouldotherwiseplaceifverylowvoltagesareappliedtotheIREFpin.Thisoutputvoltagelimitrestrictsthelineardimmingrangetoabout10:1.ThefeaturesincludedintheHV9912helpachieveaveryfastPWMdimmingresponseinspiteoftheshortcomingsoftheboostconverter.ThePWMdimmingsignalcontrolsthreenodesintheIC.•GatesignaltotheswitchingMOSFET•GatesignaltothedisconnectMOSFET•Outputconnectionofthetransconductanceop-ampwww.newnespress.com BoostConverters67WhenPWMDishigh,thegatesofboththeswitchingMOSFETandthedisconnectMOSFETareenabled.Atthesametime,theoutputofthetransconductanceop-ampisconnectedtothecompensationnetwork.Thisallowstheboostconvertertooperatenormally.WhenPWMDgoeslow,theGATEoftheswitchingMOSFETisdisabledtostopenergytransferfromtheinputtotheoutput.However,thisdoesnotpreventtheoutputcapacitorfromdischargingintotheLEDscausingalargedecaytimefortheLEDcurrent.Thisdischargeofthecapacitoralsomeansthatwhenthecircuitrestarts,theoutputcapacitorhastochargeagain,causinganincreaseintherisetimeoftheLEDcurrent.Thisproblembecomesmoreprominentwithlargeroutputcapacitors.Thus,itisimportanttopreventthedischargeoftheoutputcapacitor.ThisisdonebyturningoffthedisconnectMOSFET.ThiscausestheLEDcurrenttofalltozeroalmostinstantaneously.Sincetheoutputcapacitordoesnotdischarge,thereisnonecessitytochargethecapacitorwhenPWMDgoeshigh.Thisenablesaveryfastrisetimeaswell.Sowhathappensifourcontrollerdoesnothaveaswitchontheoutputofthefeedbackamplifier?WhenPWMDgoeslow,theoutputcurrentgoestozero.Thismeansthatthefeedbackamplifierseesaverylargeerrorsignalacrossitsinputterminals,whichwouldcausethevoltageacrossthecompensationcapacitortoincreasetothepositiverail.Thus,whenthePWMDsignalgoeshighagain,thelargevoltageacrossthecompensationnetwork,whichdictatesthepeakinductorcurrentvalue,willcausealargespikeintheLEDcurrent.Thecurrentwillcomebackintoregulationdependingonthespeedofthecontroller.TheHV9912disconnectstheoutputoftheamplifierfromthecompensationnetworkwhenPWMDgoeslow,whichhelpstokeepthevoltageatthecompensationunchanged.Thus,whenPWMDgoeshighagain,thecircuitwillalreadybeatthesteadystatecondition,eliminatingthelargeturn-onspikeintheLEDcurrent.6.3DesignofaContinuousConductionModeBoostLEDDriverAsareminder,continuousconductionmodeisvalidwhentheoutputvoltageisbetween1.5and6timestheinputvoltage.www.newnespress.com 68Chapter66.3.1DesignSpecificationInputvoltagerange=22–26VLEDstringvoltagerange=40–70VLEDcurrent=350mALEDcurrentripple=10%(35mA)LEDstringdynamicimpedance=18ohmsDesiredefficiency>90%6.3.2TypicalCircuitAtypicalboostconvertercircuitisshowninFigure6.5.L1D1Vin+Vout+C1C2C3LEDR8RTVINOVPRTR-slopeHV9912R9SCREFQ1GATEPWM_DR7R3R5SYNCCSIREFR1LEDCLIMCOMPQ2VDDFAULTR4R6FDBKGNDR2Figure6.5:ContinuousModeBoostConverter.www.newnespress.com BoostConverters696.3.3SelectingtheSwitchingFrequency(fs)Forlowvoltageapplications(outputvoltage<100V),andmoderatepowerlevels(<30W),aswitchingfrequencyoffs=200kHzisagoodcompromisebetweenswitchingpowerlossandsizeofthecomponents.Athighervoltageorpowerlevels,theswitchingfrequencymighthavetobereducedtolowertheswitchinglossesintheexternalMOSFET.6.3.4ComputingtheMaximumDutyCycle(Dmax)Themaximumdutycycleofoperationcanbecomputedasmin
VinminDmax¼1Vomax¼0:717Note:IfDmax=0.85,thestep-upratioistoolarge.Theconvertercannotoperateincontinuousconductionmodeandhastobeoperatedindiscontinuousconductionmodetoachievetherequiredstep-upratio.6.3.5ComputingtheMaximumInductorCurrent(Iinmax)ThemaximuminputcurrentisVomax
IomaxIinmax¼min
Vinmin¼1:24A6.3.6ComputingtheInputInductorValue(L1)Theinputinductorcanbecomputedbyassuminga25%peak-to-peakrippleintheinductorcurrentatminimuminputvoltage.Vinmin
DmaxL1¼0:25
Iinmax
fs¼254mHwww.newnespress.com 70Chapter6Chooseastandard330mHinductor.Toachieve90%efficiencyattheminimuminputvoltage,thepowerlossintheinductorhastobelimitedtoaround2–3%ofthetotaloutputpower.Usinga3%lossintheinductorPind¼0:03
Vomax
Iomax¼0:735WAssumingan80–20%splitintheinductorlossesbetweenresistiveandcorelosses,theDCresistanceofthechoseninductorhastobelessthan0:8
PindDCR25k,andcanbecalculatedas:R5¼20k,1=8W,1%R6¼8:06k,1=8W,1%Note:ItisrecommendedthatnocapacitorbeconnectedattheCLIMpin.6.3.17CapacitorsatVDDandREFPinsItisrecommendedthatbypasscapacitorsbeconnectedtobothVDDandREFpins.FortheVDDpin,thecapacitorusedisa1mFceramicchipcapacitor.IfthedesignusesswitchingMOSFETsthathaveahighgatecharge(Qg>15nC),thecapacitorattheVDDpinshouldbeincreasedto2.2mF.FortheREFpin,thecapacitorusedisa0.1mFceramicchipcapacitor.6.3.18SettingtheOver-VoltageTripPoint(R8andR9)Theover-voltagetrippointcanbesetatavoltage15%higherthanthemaximumsteadystatevoltage.Usinga20%margin,themaximumoutputvoltageduringopenLEDconditionwillbeVopen¼1:2
Vomax¼84Vwww.newnespress.com BoostConverters79Then,theresistorsthatsettheover-voltagesetpointcanbecomputedas
2Vopen5R8¼0:1¼64kTheaboveequationwillallowustoselecta1/8Wresistorbylimitingthepowerdissipationintheresistor.R8R9¼

5VVopen5¼3:95kTheclosest1%resistorvaluesareR8¼68k,1=8W,1%R9¼3:9k,1=8W,1%Note:Theactualover-voltagepointwillvaryfromthedesiredpointby±5%duetothevariationinthereference(seedatasheet).Forthisdesign,itvariesfrom80Vto88.2V.6.3.19DesigningtheCompensationNetworkThecompensationneededtostabilizetheconvertercouldbeeitheraType-Icircuit(asimpleintegrator)oraType-IIcircuit(anintegratorwithanadditionalpole-zeropair).Thetypeofthecompensationcircuitrequiredwillbedependentonthephaseofthepowerstageatthecrossoverfrequency.Theloopgainoftheclosedloopsystemisgivenby11LoopGain¼Rs
Gm
ZcðsÞ


GpsðsÞ15RcsWhereGmisthetransconductanceoftheop-amp(435mA/V),Zc(s)istheimpedanceofthecompensationnetwork,andGps(s)isthetransferfunctionofthepowerstage.Pleasenotethatalthoughtheresistorsgivea1:14ratio,theoveralleffectwhenincludingthediodedropiseffectively1:15.www.newnespress.com 80Chapter6Forthecontinuousconductionmodeboostconverterinpeakcurrentcontrolmodeandforfrequencieslessthanonetenthoftheswitchingfrequency,thepowerstagetransferfunctionisgivenbyL11s
2ðÞ1DmaxðÞ1Dmax
RLEDGpsðsÞ¼
2RLED
Co1þs
2Forthepresentdesign,chooseacrossoverfrequency0.01*fs,fc=2kHz.ThelowcrossoverfrequencywillresultinlargevaluesforCcandCz,whichwillindirectlyprovideasoft-startforthecircuit.SincetheHV9912doesnotdependonthespeedofthecontrollercircuitforthePWMdimmingresponse,thelowcrossoverfrequencywillnothaveanadverseeffectonthePWMdimmingriseandfalltimes.330
1061s
20:283ðÞ0:283
18GpsðsÞ¼
218
2
1061þs
21s
2:28912
104GpsðsÞ¼0:1415
1þs
1:8
105Substitutings=i
(2p
fc),wherefc=2kHz,s=i
12566.1i
2:8766GpsðsÞ¼0:1415
1þi
0:226188Atthisfrequency,themagnitudeandfrequencyofthepowerstagetransferfunction(obtainedbysubstitutings=i
(2p
fc)inthepreviousequation)areGpsðsÞfc¼2kHz¼Aps¼0:40996ffGðsÞ¼¼83:57psfc¼2kHzpswww.newnespress.com BoostConverters81Togetaphasemarginofabout=45°(therecommendedphasemarginrangeis45°–60°),thephaseboostrequiredwillbe¼90boostmps¼38:57Basedonthevalueofthephaseboostrequired,thetypeofcompensationcanbedetermined.0)TypeIcontrollerboost090)TypeIIcontrollerboost90180)TypeIIIcontrollerboostType-IIIcontrollersareusuallynotrequiredtocompensateanHV9912-basedboostLEDdriverandthuswillnotbediscussedhere.TheimplementationsfortheType-IandType-IIsystemsforusewiththeHV9912aregiveninTable6.1.Table6.1:CompensationNetworks.TypeCircuitdiagramTransferfunctionCOMPI1CcZcðsÞ¼sCcCOMPCz11þs
Rz
CzIIZcðsÞ¼
C
CCsðCcþCzÞ1þs
zc
RczRzCzþCcDesigningwithType-Icontrollersissimple–adjustCcsothatthemagnitudeoftheloopgainequals1atthecrossoverfrequency.Forthepresentdesign,however,wewww.newnespress.com 82Chapter6needtouseaType-IIcontroller.TheequationsneededtodesigntheType-IIcontrolleraregivenbelow:boostK¼tan45þ2¼2:07712
p
fc!z¼¼Rz
CzK¼6050rad=secCzþCp!p¼¼ðÞ2
p
fc
KCz
Cp
Rz¼26100rad=secOnemoreequationcanbeobtainedbyequatingthemagnitudeoftheloopgainto1atthecrossoverfrequency.01pffiffiffiffiffiffiffiffiffiffiffiffiffiffiB11þK2C11Rs
Gm
@
qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiA


Aps¼12
p
fc
ðÞCzþCc215Rcs1þðÞ1=KCzþCc¼10nF!zCc¼ðCzþCcÞ
!p¼2:32nFCz¼7:68nF1Rz¼!z
Cz¼21:522kChooseCc¼2:2nF,50V,C0GcapacitorCz=6.8nF,50V,C0GcapacitorRz=22.0k,1/8V,1%resistorwww.newnespress.com BoostConverters836.3.20OutputClampingCircuitOneproblemencounteredwithacontinuousmodeboostconverter,whenoperatingwithVout<2Vin,isL-CresonancebetweentheinductorandCout.ClampingtheoutputtotheinputbyadiodefromVintoVoutcanpreventthisresonance.ThisdiodeisshownasD2inFigure6.8DiodeD2canbeastandardrecoverytimediodelike1N4002;thistypeofdiodeisbetterathandlingsurgecurrentsthatcouldbepresentatswitch-on.D2L1D1Vin+Vout+C1C2C4LED1R1VIN12R2OVP7RTR116HV9912C11R3SC103Q2REFGATEC613PWM_DR585R12R8SYNCCS15IREFR6LED9CLIM14COMPC5211Q1C7VDDFAULTR13R9416R14FDBKGNDC10R10Figure6.8:BoostConverterwithClampingDiode.ThiscompletesthedesignoftheHV9912-basedboostconverteroperatingincontinuousconductionmode.6.4DesignofaDiscontinuousConductionModeBoostLEDDriverAsareminder,discontinuousmodeisusedwhentheoutputvoltageismorethansixtimestheinputvoltage.www.newnespress.com 84Chapter66.4.1DesignSpecificationInputvoltagerange=9–16VLEDstringvoltagerange=30–70V(Note,with9Vinputand70Voutput,theVo/Vinratioisapproximately7.8)LEDcurrent=100mALEDcurrentripple=10%(10mA)LEDdynamicimpedance=55ohmsEfficiency>85%6.4.2TypicalCircuitAtypicalcircuitforadiscontinuousmodeboostconverter,usingtheHV9912ICidenticaltothecontinuousmodecircuitshowninFigure6.5,butrepeatedhereforconvenience,inFigure6.9.6.4.3SelectingtheSwitchingFrequency(fs)Forlowvoltageapplications(outputvoltage<100V),andmoderatepowerlevels(<30W),aswitchingfrequencyoffs=200kHzisagoodcompromisebetweenswitchingpowerlossandsizeofthecomponents.Athighervoltageorpowerlevels,theswitchingfrequencymighthavetobereducedtolowertheswitchinglossesintheexternalMOSFET.6.4.4ComputingtheMaximumInductorCurrent(Iinmax)ThemaximuminputcurrentisVomax
IomaxIinmax¼min
Vinmin¼0:915Awww.newnespress.com BoostConverters85L1D1Vin+Vout+C1C2C3LEDR8RTVINOVPRTR-slopeHV9912R9SCREFQ1GATEPWM_DR7R3R5SYNCCSIREFR1LEDCLIMCOMPQ2VDDFAULTR4R6FDBKGNDR2Figure6.9:DiscontinuousModeBoostConverter.6.4.5ComputingtheInputInductorValue(L1)Assumingthatthesumoftheon-timeoftheswitchandtheon-timeofthediodeis95%ofthetotalswitchingtimeperiodatVinmin,110:95L1
iLpk
þ¼VinminVomaxVinminfs¼4:75mswhereiLPkisthepeakinputcurrent(seeFigure6.10).Vin/L1controlstherateatwhichcurrentincreasesandtherisingperiodisdeterminedbytheon-timeoftheMOSFET,whichisthedutycyclemultipliedbytheswitchingperiod.Therateoffalliscontrolledby(VoVin)/L1andthefallingperiodisthetimethatthediodeisconducting.www.newnespress.com 86Chapter6ILILpkVo-VinVinL1L1tTon_swTon_diode~D.TsTsFigure6.10:InductorCurrentWaveforminDCM.Theaverageinputcurrentattheminimuminputvoltageisequaltotheaverageinductorcurrentandcanbecomputedfrom14:75msIinmax¼
iLpk
25ms¼0:475
iLpkTransposingtheequation,thepeakinputcurrentisIinmaxiLpk¼0:4751:93ASubstitutingforiLpkintheequationforL10:959V
ðÞ70V9VL1¼
200k70V
1:93A¼19:3mHNotethatthevalueofL1computedistheabsolutemaximumvaluefortheinductor.Assuminga±20%variationintheinductance,thenominalinductorvaluehastobeL1L1nom¼1:2¼16:08mHThecloseststandardvalueisa15mHinductor.www.newnespress.com BoostConverters87TheRMScurrentthroughtheinductorisrffiffiffiffiffiffiffi0:9ILrms¼iLpk
3¼1:057AChoosea15mHinductor(±20%tolerance).Acustominductorwouldworkbestforthisapplicationgiventhelargeswingsintheinductorflux.However,ifastandardvalueinductorispreferred,thesaturationcurrentratingoftheinductorshouldbeatleast1.5timesthepeakcurrentcomputed,tokeepthecorelossestoanacceptablevalue.Theinductorchoseninthiscaseisa15mHinductorwithanRMScurrentratingof1.4Aandasaturationcurrentratingof3A.6.4.6ComputingtheOnandOffTimesoftheConverterTheon-timeoftheswitchcanbecomputedasL1nom
iLpktonsw¼Vinmin¼3:22msTheon-timeofthediodeisL1nom
iLpktondiode¼VomaxVinmin¼467nsThemaximumdutycyclecanthenbecomputedasDmax¼tonsw
fs¼0:644ThediodeconductiontimeratiocanbeexpressedasD1¼tondiode
fs¼0:0934www.newnespress.com 88Chapter66.4.7ChoosingtheSwitchingMOSFET(Q1)ThemaximumvoltageacrosstheMOSFETinaboostconverterisequaltotheoutputvoltage.Usinga20%overheadtoaccountforswitchingspikes,theminimumvoltageratingoftheMOSFEThastobeVFET¼1:2
Vomax¼84VTheRMScurrentthroughtheMOSFETisrffiffiffiffiffiffiffiffiffiffiffiDmaxIFETiLpk
3¼0:895ATogetthebestperformancefromtheconverter,theMOSFETchosenhastohaveacurrentratingaboutthreetimestheMOSFETRMScurrentwithminimumgatechargeQg.ItisrecommendedthatfordesignswiththeHV9912,thegatechargeofthechosenMOSFETbelessthan25nC.TheMOSFETchosenforthisapplicationisa100V,4.5AMOSFETwithaQgof11nC.6.4.8ChoosingtheSwitchingDiode(D1)ThevoltageratingofthediodeisthesameasthevoltageratingoftheMOSFET(100V).Theaveragecurrentthroughthediodeisequaltothemaximumoutputcurrent(350mA).Althoughtheaveragecurrentthroughthediodeisonly350mA,thepeakcurrentthroughthediodeisequaltoiLpk.Thus,itisabetterdesignapproachtochoosethecurrentratingofthediodesomewhereinbetweenthepeakinputcurrentandtheaverageoutputcurrent(preferablyclosertothepeakinputcurrent).Thus,forthisdesign,thediodechosenisa100V,2ASchottkydiode.6.4.9ChoosingtheOutputCapacitor(Co)ThevalueoftheoutputcapacitordependsonthedynamicresistanceoftheLEDstringaswellastheripplecurrentdesiredintheLEDstring.IndesignsusingtheHV9912,alargeroutputcapacitor(loweroutputcurrentripple)willyieldwww.newnespress.com BoostConverters89betterPWMdimmingresults.Thecapacitorrequiredtofilterthecurrentappropriatelywillbedesignedbyconsideringthefundamentalcomponentofthediodecurrentonly.TheoutputstageoftheboostconverterismodeledinFigure6.11,wheretheLEDsaremodeledasaconstantvoltageloadwithseriesdynamicimpedance.Icap+VJCoIIdiodeRLEDFigure6.11:ModelofBoostConverterOutput.ThewaveformofthecapacitorcurrentinsteadystateisshowninFigure6.12.IcapTimeFigure6.12:OutputCapacitorCurrent.Usingthe10%peak-to-peakcurrentripplegiveninthedesignparameterstable,themaximumvoltagerippleacrosstheLEDstringhastobeDvpp¼DIo
RLED¼0:55Vwww.newnespress.com 90Chapter6Assumingaconstantdischargingcurrentof350mAwhenthediodecurrentiszero,theequationforthevoltageacrossthecapacitorcanbewrittenasDvppIomax¼Co
Dmax
TsSubstitutingvaluesintotheaboveequation,Iomax
DmaxCo¼Dvpp
fs¼0:585mFTheRMScurrentthroughthecapacitorcanbegivenbyrffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiD12Irms¼ðÞ1D1
Iomax2þ
ðiLpkIomaxÞ3¼0:34AInthiscase,aparallelcombinationoftwo1mF,100Vmetalpolypropylenecapacitorsischosen.Note:Thepropertypeofcapacitortouseiseithermetalfilmcapacitorsorceramiccapacitors,sincetheyarecapableofcarryingthishighripplecurrent.Althoughceramiccapacitorsaresmallerinsizeandcapableofcarryingtheripplecurrent,theycausealotofaudiblenoiseduringPWMdimming.Highvalueceramiccapacitorsareusuallylimitedto50Vrating.Thusmetalpolypropylene(oranyothermetalfilm)capacitorsaretheidealchoiceforLEDdriversifPWMdimmingisrequired.6.4.10ChoosetheDisconnectMOSFET(Q2)ThedisconnectMOSFETshouldhavethesamevoltageratingastheswitchingMOSFETQ1.Theon-stateresistanceoftheMOSFETatroomtemperature(Ron,25C)hastobecalculatedbasedona1%powerlossinQ2atfullloadcurrent.Thus,0:01
VomaxRon,25C¼Iomax
1:4¼5The1.4multiplicationfactorisincludedtoaccountfortheincreaseintheon-resistanceduetoriseinjunctiontemperature.Inthiscase,ahighQgMOSFETwww.newnespress.com BoostConverters91canbechosenifdesired(asitisnotswitchingregularly),butahighQgMOSFETwillslowdowntheturn-onandturn-offtimes(whichmightbeallowablebasedonPWMdimmingfrequency).Inthiscase,theMOSFETchosenisa100V,0.7,SOT-23MOSFETwithaQgof2.9nC.6.4.11ChoosingtheInputCapacitors(C1andC2)ThevaluesofinputcapacitorsC1andC2havetobecalculatedtomeetclosedloopstabilityrequirements.Theconnectionfromthepowersourcetotheboostconvertercircuitwillhavesomeresistance,Rsource,andsomeinductance,Lsource.Thesefeedacrosstheinputcapacitors(C1andC2)andsoformanLCresonantcircuit.Topreventinterferencewiththecontrolloop,theresonantfrequencyshouldbearrangedtobe40%orlessoftheswitchingfrequency.Apairof22AWGconnectingwires1foot(30cm)longwillhaveaninductanceofabout1mH.Thisisagoodstartingpoint.Ifnecessary,thewirescanbetwistedtogethertoreducetheinductance.Witha200kHzswitchingfrequency,theresonantfrequencyshouldbelessthan80kHz.1CIN¼3:95mF2ðÞ2
p
fLC
LSOURCEC1¼C2¼2:2mF,50Vceramic:Themaximumsourceimpedanceisfoundusing:VO,MAX70M¼¼¼7:778VIN,MIN9M1RSOURCE,MAX¼
RLED¼1:404M2
ðM2Þ6.4.12ChoosingtheTimingResistor(RT)TheHV9912oscillatorhasan18pFcapacitorchargedbyacurrentmirrorcircuit.AnexternaltimingresistorRTprovidesareferencecurrentforthecurrentmirror.www.newnespress.com 92Chapter6WhenRTisconnectedto0V,currentflowsandthetimingprocessbegins.Whenchargedtoacertainvoltage,theRSflip-flopisset,thecapacitorisdischarged,andthetimingprocessstartsagain.Thetimingresistorcanbecalculatedbyusingthefollowingequation:1RT
18pFfsInthiscase,foraconstant200kHzswitchingfrequency,thetimingresistorvalueworksoutto274k.ThisresistorneedstobeconnectedbetweentheRTpinandGNDasshowninthetypicalcircuit.6.4.13ChoosingtheTwoCurrentSenseResistors(R1andR2)ThevalueoftheoutputcurrentsenseresistorR2canbecalculatedbylimitingitsvoltagedroptobelow0.4V.Usingthiscriterion,0:4VR2¼Iomax¼4Thepowerdissipationwillbe0.4V*Iomax=0.04W.Inthiscase,theresistorchosenisa3.9,1/8W,1%resistor.TheMOSFETcurrentsenseresistorR1iscalculatedbylimitingthevoltageacrosstheresistortoabout250mVatmaximuminputcurrent.0:25R1¼iLpk¼0:12ThepowerdissipatedinthisresistorisP¼I2
R1R1FET¼0:096WThus,thechosencurrentsenseresistorisa0.12,1/4W,1%resistor.www.newnespress.com BoostConverters936.4.14SelectingtheCurrentReferenceResistors(R3andR4)ThevoltageatthecurrentreferencepinIREFcanbeseteitherbyusingthereferencevoltageprovidedattheREFpin(throughavoltagedivider)orwithanexternalvoltagesource.Inthepresentdesign,itisassumedthatthevoltageattheIREFpinissetusingavoltagedividerfromtheREFpin.ThecurrentreferenceresistorsR3andR4canbecomputedusingthefollowingtwoequations:1:25VR3þR4¼25k50mA1:25V
R4¼Iomax
R2¼0:1
3:9¼0:39VR3þR4Forthisdesign,thevaluesofthetworesistorscanbecomputedtobeR3¼19:1k,1=8W,1%R4¼8:66k,1=8W,1%6.4.15SettingtheInductorCurrentLimit(R5andR6)Theinductorcurrentlimitvaluedependsontwofactors–themaximuminductorcurrentandtheslopecompensationsignaladdedtothesensedcurrent.AnotherresistordividerfromtheREFpin(R5andR6)isconnectedtotheCLIMpinandsetsthemaximuminductorcurrent.ThevoltageattheCLIMpincanbecomputedasVCLIM1:2
iIpk
R1Thisequationassumesthatthecurrentlimitlevelissetatabout120%ofthemaximuminductorcurrentIinmax.Forthisdesign,VCLIM¼1:2
1:93
0:120:278Vwww.newnespress.com 94Chapter6UsingamaximumcurrentsourcedoutofREFpinof50mA,thetworesistorscanbecalculatedasR5¼20k,1=8W,1%R6¼6:04k,1=8W,1%NocapacitorshouldbeconnectedattheCLIMpin,becausethiswillaffectthecircuitatstart-up.6.4.16CapacitorsatVDDandREFPinsItisrecommendedthatbypasscapacitorsbeconnectedtobothVDDandREFpins.FortheVDDpin,thecapacitorusedshouldbea10Vceramicchipcapacitor.Forlowpowerdesigns,a1mFisadequate.IfthedesignuseshighgatechargeswitchingMOSFETs(Qg>15nC),thecapacitorattheVDDpinshouldbeincreasedto2.2mF.FortheREFpin,thecapacitorusedisa0.1mFceramicchipcapacitor.6.4.17SettingtheOver-VoltageTripPoint(R8andR9)Theover-voltagetrippointcanbesetatavoltage15%higherthanthemaximumsteadystatevoltage.Usinga15%margin,themaximumoutputvoltageduringopenLEDconditionwillbeVopen¼1:15
Vomax¼80:5VThen,theresistorsthatsettheover-voltagesetpointcanbecomputedas
2Vopen5R8¼0:1¼57kTheaboveequationwillallowustoselecta1/8Wresistorbylimitingthepowerdissipationintheresistor.R8R9¼

1:25VVopen5¼3:77kwww.newnespress.com BoostConverters95Theclosest1%resistorvaluesareR8¼56:2k,1=8W,1%R9¼3:74k,1=8W,1%Note:Theactualover-voltagepointwillvaryfromthedesiredpointby±5%duetothevariationinthereference(seedatasheet).Forthisdesign,itvariesfrom76.67Vto84.52V.6.4.18DesigningtheCompensationNetworkThecompensationneededtostabilizetheconvertercouldbeeitheraType-Icircuit(asimpleintegrator)oraType-IIcircuit(anintegratorwithanadditionalpole-zeropair).Thetypeofthecompensationcircuitrequiredwillbedependentonthephaseofthepowerstageatthecrossoverfrequency.Theloopgainoftheclosedloopsystemisgivenby11LoopGain¼Rs
Gm
ZcðsÞ


GpsðsÞ15RcsWhereGmisthetransconductanceoftheop-amp(435mA/V),Zc(s)istheimpedanceofthecompensationnetwork,andGps(s)isthetransferfunctionofthepowerstage.Pleasenotethatalthoughtheresistorsgivea1:14ratio,theoveralleffectwhenincludingthediodedropiseffectively1:15.Tocomputethetransferfunctionforthediscontinuousconductionmodeboostconverterinpeakcurrentcontrolmode,weneedtodefineacoupleoffactors.Vomax
IomaxM¼Vomax
Iomax0:5
L1nom
iLpk2
fs70
0:1M¼70
0:10:5
15
106
1:932
200
1037M¼¼4:95521:41265M13:95522GR¼¼¼0:44392
M18:9104www.newnespress.com 96Chapter6Forfrequencieslessthanonetenthoftheswitchingfrequency,thepowerstagetransferfunctionisgivenbyIomaxGRGpsðsÞ¼2

iLpk1þs
RLED
Co
GR0:10:44390:4439GpsðsÞ¼2

¼1:931þs
55
2
106
0:44391þs
48:829
106Forthepresentdesign,chooseacrossoverfrequency0.01fs,orfc=2kHz.ThelowcrossoverfrequencywillresultinlargevaluesforCCandCZ,whichwillindirectlyprovideasoftstartforthecircuit.SincetheHV9912doesnotdependonthespeedofthecontrollercircuitforthePWMdimmingresponse,thelowcrossoverfrequencywillnothaveanadverseeffectonthePWMdimmingriseandfalltimes.Bysubstitutings=i
(2p
fc)=i
12566intothetransferfunction,weget:0:046GpsðsÞ¼1þs
0:6136Themagnitudeandfrequencyofthepowerstagetransferfunctionare:GpsðsÞfc¼2kHz¼Aps¼0:039ffGðsÞ¼¼31:5psfc¼2kHzpsTogetaphasemarginofaboutm=45°(therecommendedphasemarginrangeis45°–60°),thephaseboostrequiredwillbe¼90boostmps¼45þ31:590¼13:5Basedonthevalueofthephaseboostrequired,thetypeofcompensationcanbedetermined.0)TypeIcontrollerboost090)TypeIIcontrollerboost90180)TypeIIIcontrollerboostwww.newnespress.com BoostConverters97Type-IIIcontrollersareusuallynotrequiredtocompensateaHV9912-basedboostLEDdriverandthuswillnotbediscussedhere.TheimplementationsfortheType-IandType-IIsystemsforusewiththeHV9912aregiveninTable6.2.Table6.2:CompensationNetworks.TypeCircuitdiagramTransferfunctionCOMPI1CZcðsÞ¼csCcCOMP11þs
Rz
CzCzZcðsÞ¼
IIsðCcþCzÞCz
CcC1þs

RzcCzþCcRzForthepresentdesign,asimpleType-Icontrollerwillsuffice.Allthatisneededistoadjustthegainoftheloopgaintobe1atthecrossoverfrequency.Onemoreequationcanbeobtainedbyequatingthemagnitudeoftheloopgainto1atthecrossoverfrequency.111R2
Gm



Aps¼12
p
fc
Cc15R1Transposing,weget:111CC¼R2
Gm



Aps2
p
fc15R16111CC¼3:9
435
10



0:039¼2:92nF12566150:12ChooseaCC=3.3nF,50V,C0GcapacitorThiscompletesourDCMboostconverterdesign.www.newnespress.com 98Chapter66.5CommonMistakes1.Themostcommonmistakeisnothavingadequateover-voltageprotectionattheoutput.IftheLEDsaredisconnectedwhilethecircuitisoperating,theoutputvoltagewillriseuntilcomponentsstarttobreakdown.Theover-voltagelimitsetattheoutputoftheboostconvertershouldbelowerthanthebreakdownvoltageofanycomponentconnectedacrossit.2.TestingthecircuitwithashortstringofLEDs.Theforwardvoltagedropmaybelowerthanthesupplyvoltage,andinthiscasethereislittletopreventtheLEDsbeingdestroyedbythehighcurrentthatwillflow.6.6ConclusionsBoostconvertersareusedwhentheminimumoutputvoltageisatleast1.5timestheinputvoltage.Continuousconductionmodeshouldbeusedwhentheoutputvoltageisamaximumofsixtimestheinputvoltage.Discontinuousconductionmodeisnecessaryiftheoutputvoltageismorethansixtimestheinputvoltage.TheEMIproducedbyadiscontinuousmodeboostconverterishigherthanforacontinuousconductionmodeboostconverterofsimilarpoweroutput.www.newnespress.com CHAPTER7Boost-BuckConverterAboost-buckconverterisasingle-switchconverter,whichconsistsofacascadeofaboostconverterfollowedbyabuckconverter.Thepowertrainoftypicalboost-buckcircuittopology(usedasanLEDdriver)isshowninFigure7.1.L1C1L2VO–IL1+Vc–IL2+VIN–D1Q1VO+Figure7.1:Boost-Buck(Cuk)PowerTrain.Theconverterhasmanyadvantages:•Theconvertercanbothboostandbucktheinputvoltage.Thus,itisidealforcaseswheretheoutputLEDstringvoltagecanbeeitheraboveorbelowtheinputvoltageduringoperation.Thisconditionismostcommoninautomotiveapplications,orwhenacustomerwantsasingledriverdesigntocoverawiderangeofvoltagesupplyandloadconditions.www.newnespress.com 100Chapter7•Theconverterhasinductorsonboththeinputandoutputsides.Operatingbothstagesincontinuousconductionmode(CCM)willenablecontinuouscurrentsinbothinductorswithlowcurrentripple,whichwouldgreatlyreducethefiltercapacitorrequirementsatbothinputandoutput.ContinuousinputcurrentwouldalsohelpgreatlyinmeetingconductedEMIstandardsattheinput.•Alltheswitchingnodesinthecircuitareisolatedbetweenthetwoinductors.Theinputandoutputnodesarerelativelyquiet.ThiswillminimizetheradiatedEMIfromtheconverter.Withproperlayoutanddesign,theconvertercaneasilymeetradiatedEMIstandards.•Oneoftheadvantagesoftheboost-buckconverteristhecapacitiveisolation.Thefailureoftheswitchingtransistorwillshorttheinputandnotaffecttheoutput.Thus,theLEDsareprotectedfromfailureoftheMOSFET.•ThetwoinductorsL1andL2canbecoupledtogetherononecore.Whencoupledonasinglecore,therippleintheinductorcurrentfromonesidecanbetransferredcompletelytoanotherside(ripplecancellationtechnique).Thiswouldallow,forexample,theinputrippletobetransferredcompletelytotheoutputsidemakingitveryeasyfortheconvertertomeetconductedEMIstandards.7.1TheCukConverterInspiteofthemanyadvantagesoftheCukconverter,acoupleofsignificantdisadvantagesexistwhichpreventitswidespreaduse.•Theconverterisdifficulttostabilize.Complexcompensationcircuitryisoftenneededtomaketheconverteroperateproperly.Thiscompensationalsotendstoslowdowntheresponseoftheconverter,whichinhibitsthePWMdimmingcapabilityoftheconverter(essentialforLEDs).•Anoutputcurrentcontrolledboost-buckconvertertendstohaveanuncontrolledandundampedresonanceduetoanL-Cpair(L1andC1).TheresonanceofL1andC1leadstoexcessivevoltagesacrossthecapacitor,whichcandamagethecircuit.ThedampingofL1andC1caneasilybeachievedbyaddingadampingR-CcircuitacrossC1.However,theproblemofcompensatingthecircuitsothatitisstableismorecomplex.www.newnespress.com Boost-BuckConverter101TheSupertexHV9930solvestheproblemofcompensationandachievingafastPWMdimmingresponsebyusinghystereticcurrentmodecontrol.ThisusesfastcomparatorstocontrolaMOSFETgatebysettingupperandlowerlimits,whichensuresfastresponseandaccuratecurrentlevels.However,asimplehystereticcurrentmodecontrolwouldnotwork,astheconverterwouldnotbeabletostartup.Toovercomethisproblem,theHV9930hastwohystereticcurrentmodecontrollers–onefortheinputcurrentandanotherfortheoutputcurrent.Duringstart-up,theinputhystereticcontrollerdominatesandtheconverterisininputcurrentlimitmode.TheMOSFETturnsonandtheinputcurrentrisesuntiltheinputcurrentlimitisreached,itthenturnsoffsothattheinputcurrentdropsuntilalowercurrentlimitisreached.Thiscyclecontinuesuntiltheoutputcurrenthasbuiltuptotherequiredvalueandtheoutputhystereticcontrollercantakeover.Theoutputcurrentisthenmaintainedbetweenthesetupperandlowercurrentlimits.Unlikepeakcurrentmodecontroller,hystereticcontrolensuresthattheaverageoutputcurrentremainsconstantunderawiderangeofinputandoutputvoltageconditions.Thehystereticapproachwillalsohelpinlimitingtheinputcurrentduringstart-up(thusprovidingsoft-start);alsocurrentislimitedinthecaseofanoutputoverloadorinputunder-voltagecondition.Threeresistors(foreachofthetwohystereticcontrollers)arerequiredtosetboththecurrentrippleandtheaveragecurrent,whichenablesasimplecontrollerdesign.Thussixresistorsdeterminetheinputandoutputperformance.Thissectionwilldetailtheoperationoftheboost-buckconverterandthedesignofanHV9930-basedconverter.Thedesignexampleisspecificallydesignedforautomotiveapplications,butitcanalsobeappliedforanyDC/DCapplications.Atthetimeofwriting,thereisonlyoneotherdevicewiththesamefunctionalityastheHV9930,whichistheAT9933.TheAT9933hasanautomotivetemperaturespecification(upto125°Coperation),whereastheHV9930hasanindustrialtemperaturerange.7.1.1OperationofaCukBoost-BuckConverterThediagramofthepowertrainforaCukboost-buckconverterwasshownpreviously,inFigure7.1.www.newnespress.com 102Chapter7Insteadystate,theaveragevoltagesacrossbothL1andL2arezero.Thus,thevoltage,Vc,acrossthemiddlecapacitorC1isequaltothesumoftheinputandoutputvoltages.Vc¼VinþVoWhenswitchQ1isturnedon,thecurrentsinbothinductorsstartrampingup(seeFigure7.2).L1C1L2VO–+Vc–+VIN–IL1IL2VO+Figure7.2:CukCircuit,MOSFETOn.diL1L1¼VindtdiL2L2¼VcVo¼VindtWhenswitchQ1isturnedoff,thecurrentsinbothinductorsstartrampingdown(seeFigure7.3).L1C1L2VO–+Vc–+VIN–IL1IL2VO+Figure7.3:CukCircuit,MOSFETOff.www.newnespress.com Boost-BuckConverter103diL1L1¼VinVc¼VodtdiL2L2¼VodtAssumingthattheswitchisONforadutycycleDandusingthefactthat,insteadystate,thetotalvolt-secondsappliedacrossanyinductoriszero,wegetVin
ðÞ¼DVo
ðÞ1DVoD)¼Vin1DThus,thevoltagetransferfunctionobtainedfortheboost-buckconverterwillgivebuckoperationforD<0.5andboostoperationforD>0.5.ThesteadystatewaveformsfortheconverterareshowninFigure7.4.ThemaximumvoltageseenbyQ1andD1isequaltothevoltageacrossthecapacitorC1.VQ1¼VD1¼VcThestandardboost-buckconverterismodified,byaddingthreeadditionalcomponents,forproperoperationoftheHV9930(seeFigure7.5).AdampingcircuitRd-CdhasbeenaddedtodamptheL1-C1pair.Theseadditionalcomponentsstabilizethecircuit.Aninputdiode(D2)hasbeenadded.ThisdiodeisnecessaryforPWMdimmingoperation(incaseofautomobileapplications,thiscouldbethereversepolarityprotectiondiode).ThisdiodehelpstopreventcapacitorsC1andCdfromdischargingwhenthegatesignalsforQ1areturnedoff.Thus,whentheHV9930isenabled,thesteadystateoutputcurrentlevelwillbereachedquickly.7.1.2HystereticControloftheBoost-BuckConverterHystereticcontrolreferstothecontrolschemewherethecontrolledvariable(inthiscase,theinductorcurrentiL2)ismaintainedbetweenpre-setupperandlowerboundaries.AspreviouslyshowninFigure7.4,theinductorcurrentrampsupatawww.newnespress.com 104Chapter7GATEHI0tVL1,VINVL20t–VoIL1,IL20tIinIc0t–IoVc0tFigure7.4:CukConverterSteadyStateWaveforms.rateofVin/L2whentheswitchisONandrampsdownatarateofVo/L2whentheswitchisOFF.Thus,thehystereticcontrolschemeturnstheswitchOFFwhentheinductorcurrentreachestheupperlimitandturnstheswitchONwhenitreachesthelowerlimit.TheaveragecurrentininductorL2isthensetattheaverageoftheupperandlowerthresholds.TheONandOFFtimes(andthustheswitchingfrequency)varyastheinputandoutputvoltageschangetomaintaintheinductorcurrentlevels.However,www.newnespress.com Boost-BuckConverter105D2L1C1L2VO–IL1IL2CdRd++VIN–D1Q1VO+Figure7.5:ModifiedBoost-BuckCircuit.inanypracticalimplementationofhystereticcontrol,therewillbecomparatordelaysinvolved.TheswitchwillnotturnONandOFFattheinstanttheinductorcurrenthitsthelimits,butafterasmalldelaytime,asillustratedinFigure7.6.Turn-onDelayDesiredIoActualIoTurn-offDelayFigure7.6:CurrentintheOutputInductorL2.7.1.3TheEffectsofDelayinHystereticControlThisdelaytimeintroducestwounwantedeffects:•Italterstheaverageoutputcurrentvalue.Forexample,ifthedelayonthedownslopeoftheinductorcurrentismorethanthedelayontheupslope,thentheaveragecurrentvaluedecreases.•Itdecreasestheswitchingfrequency,whichmaymakeitmoredifficultforthecircuittomeetEMIregulations.www.newnespress.com 106Chapter7Theseeffectswillhavetobetakenintoconsiderationwhenchoosingtheoutputinductorvalueandthesettingthecurrentlimits.Assumeapeak-to-peakcurrentripplesettingofDio(usingtheprogrammingresistors)andadesiredaveragecurrentlo.Ahystereticcurrentcontrolledboost-buckconverteractsasaconstant-off-timeconverteraslongastheoutputvoltageisfixed,andtheoff-timeistheoreticallyindependentoftheinputvoltage.Thus,theconverterisdesignedassumingaconstantoff-timeToff(themethodtodeterminetheoff-timewillbediscussedlater).FortheHV9930,aslongastheswitchingfrequenciesarelessthan150kHz,thesedelaytimeshaveanegligibleeffectandcanbeignored.Inthesecases,theoutputinductorcanbedeterminedbyVo
ToffL2¼DioIftheinductorchosenissignificantlydifferentfromthecomputedvalue,theactualoff-timeToff,accanberecomputedusingthesameequation.However,inautomotiveapplications,itisadvantageoustosettheswitchingfrequencyoftheconverterbelow150kHzorintherangebetween300kHzand530kHz.ThiswillplacethefundamentalfrequencyoftheconductedandradiatedEMIoutsideoftherestrictedbandsmakingiteasierfortheconvertertopassautomotiveEMIregulations.Incaseswheretheswitchingfrequencyismorethan300kHz,thedelaytimescannotbeneglectedandhavetobeaccountedforinthecalculations.Figure7.7illustratestheoutputinductorcurrentwaveformandthevariousriseandfalltimes.Fromthisfigure,Toff¼Tf1þTf2þTfVin¼
TrþTf2þTfVoThedesiredoutputcurrentrippleDioandthedown-slopeoftheinductorcurrentm2determineTf2.ThedelaytimesoftheHV9930determineTrandTf.FortheHV9930,thedelaytimeofthecomparatorsisrelatedtotheoverdrivevoltagewww.newnespress.com Boost-BuckConverter107TrTf1ΔIo,acΔIo–m2=–VoL2m1=VinL2TfTf2ToffFigure7.7:HystereticControlwithComparatorDelays.(voltagedifferencebetweenthetwoinputterminalsofthecurrentsensecomparator)appliedasKTdelayqffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi3m
0:1=DioWhere‘m’istherisingorfallingslopeoftheinductorcurrent.6mpffiffiffiffiffiffipffiffiffiffiffiffisffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
33Tr¼L2¼K1
L23Vin
0:1DioDio
L2Tf2¼¼K2
L2Vo6mpffiffiffiffiffiffipffiffiffiffiffiffisffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
33Tf¼L2¼K3
L23Vo
0:1DioTofindthevalueofL2usingthetimedelayequationsaboveresultsinacubicequation.Thiscubichasonerealrootandtwocomplexroots.Theinductorvalueistherealrootofthecubicraisedtothethirdpower.www.newnespress.com 108Chapter7a¼K2Vinb¼
K1þK3Voc¼Toffrffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffipffiffiffi4
b3þ27
a
c2D¼12
3
a893>>"#1>>><=3>=122
bL2¼ðÞ108
cþD
a>>6
ahi1=>>>:23>;ðÞ108
cþD
aTheactualoff-timeToff,accanbecomputedbysubstitutingthechoseninductorvaluebackintotheequationsforTr,TfandTf2,togetTr,ac,Tf,acandTf2,ac.Toff;ac¼Tf1,acþTf2,acþTf;acVin¼
Tr,acþTf2,acþTf,acVoTheactualrippleintheinductorcurrentDio,acisVo
Toff,acDio,ac¼L27.1.4StabilityoftheBoost-BuckConverterThesingle-switchboost-buckconvertercanbeconsideredasseparateboostandbuckconverters(inthatorder),whicharecascaded,andbothswitchesbeingdrivenwiththesamesignal(seeFigure7.8).www.newnespress.com Boost-BuckConverter109L1D1AL2Q1BVO+IL1IL2++VIN–C1D1BQ1AVO–Figure7.8:Boost-BuckConverter.TherelationshipsbetweenthevoltagesinthesystemareVc1¼ðboostconverterÞVin1DVo¼DðbuckconverterÞVcThecapacitorvoltageVcandtheinput/outputrelationshipcanbothbederivedusingtheaboveequationsVoVoVcD¼
¼VinVcVin1DVinVinVc¼¼1D1Vo=Vc)Vc¼VoþVinForthepurposesofdesigningthedampingnetwork,itiseasiertovisualizetheconverterinitstwo-switchformatofFigure7.7ratherthanasthesingle-switchCukconverter.Hence,fortheremainderofthissection,thecascadedconverterwillbeusedtoderivetheequations.www.newnespress.com 110Chapter7Inhystereticcontroloftheboost-buckconverterusingtheHV9930,theoutputbuckstageiscontrolledandtheinputbooststageisuncontrolled.AnequivalentschematicoftheHV9930controlledboost-buckconverterisshowninFigure7.9.L1D1AL2Q1BVO+IL1IL2++VIN–C1D1BQ1AVO–+–REFFigure7.9:Boost-BuckController.ThehystereticcontrolofthebuckstageensuresthattheoutputcurrentiL2isconstantunderallinputtransientconditions.So,forthepurposesofaveragemodeling,theloadseenbythecapacitorC1canbemodeledasacurrentsourceequaltod
lo,wheredistheinstantaneousdutycycleandloistheconstantoutputcurrent.Thecontinuousconductionmodebuckstagealsoimposesonemoreconstraint:Vo¼d
vcwheredandvcarethetimedependentdutycycleandcapacitorvoltageandVoistheconstantoutputvoltage.Forthesystemtobestable,itisnecessarythatthecontrolsystemwillacttoreduceanydisturbanceincapacitorvoltage.Theloopgainofthesystemforaboost-buckconverterwithoutdampinghasanegativephasemargin(i.e.thephaseislessthan180°whenthemagnitudecrosses0dB).ThisisduetotheundampedLCpole-pairandcausesthesystemtobeunstable.Thus,anydisturbancetothecapacitorvoltagewillgetamplifiedandkeepincreasingtillwww.newnespress.com Boost-BuckConverter111thecomponentsbreakdown.Whentestingthecircuit,ifitisclosetobecomingunstable,theswitchingfrequencyrisesandfallswithalowfrequencybeatandalowfrequencyrippleintheaverageoutputcurrentcanbeseen.TheadditionofR-Cdampingofthisundampedpolepaircanstabilizethesystemandmakesurethatthedisturbanceinputisproperlydamped.Also,thepresenceofCdensuresthatRdwillnotseetheDCcomponentofthevoltageVcacrossit,reducingthepowerdissipatedinthedampingresistor(CdblockstheDCcomponentofthevoltage).AssumingCd>>C1,theloopgaintransferfunctionoftheR-Cdampedboost-buckconvertercanbederivedas!DL1
IoðÞ1þs
Rd
Cd
1s

2VDðÞ1DoGðsÞHðsÞ¼
!1DL1
CdðÞ1þs
Rd
C1
1þs
Rd
Cdþs2
2ð1DÞThus,theloophasaDCgainofD/(1D)andincludes:1.Damping(andESR)zeroat!¼1:ZRd
Cd22.RHPzeroat!¼ðÞ1D
Vo.RHPDL1
Io3.Complexdoublepolewithnaturalresonantfrequency!¼pffiffiffiffiffiffiffiffiffiffiffi1DandoqffiffiffiffiffiL1
Cddampingfactor¼ðÞ1D
Rd
Cd.L14.Highfrequencypoleat!¼1:PRd
C1Inordertoachievestableloop,the0dBcrossing(!c)mustbeplacedsuchthat!c<>C1.WecaneasilyobtainapproximatevaluesofCdandRdforthecaseof!c>>!o.Thisconditionisusuallymetfortheworst-casecalculationsatminimuminputvoltage,sincetheDCgainisthehighestatthiscondition.Set!c=!RHP/N,whereN>>1.Then!ocanbeapproximatelycalculatedfromrffiffiffiffiffiffiffiffiffiffiffiffirffiffiffiffiffiffiffiffiffiffiffiffi1D!RHP1D!¼!
¼
OCDNDwww.newnespress.com 112Chapter7Substitutingfor!oand!RHPin(21)givestheequationforcomputingCd:N2
D3L1
I2OCd¼ðÞ1D3V2OSelectingRdsuchthat!z=!cresultsinagoodphasemarginwithminimumpowerdissipation.Then,usingequationsfor!zand!RHPgivesasolutionforRd.N
DL1
IORd¼ðÞ1D2Cd
VOUsingtheequationsabove,theapproximatevaluesforthedampingnetworkcanbecomputedusingthefollowingequations:32DIoCd¼9

L1
1DVo3
DL1
IoRd¼
ðÞ1D2Cd
VoNotethatthedampingresistorvalueincludestheESRofthedampingcapacitor.Inmanycases,thedampingcapacitorischosentobeanelectrolyticcapacitor,whichwillhaveasignificantESR(sometimesafewohms).Insuchcases,thedampingresistorcanbereducedaccordingly.7.1.5DimmingRatioUsingPWMDimmingThelinearityinthedimmingratioachievablewiththeboost-buckdependsonboththeswitchingfrequencyandthePWMdimmingfrequency.Foraconverterdesignedtooperateataminimumswitchingfrequencyof300kHz,oneswitchingtimeperiodequals3.33ms.Thisistheminimumon-timeofthePWMdimmingcycle.AtaPWMdimmingfrequencyof200Hz(5msperiod),3.33msequalsaminimumdutycycleof0.067%.Thiscorrespondstoa1:1500dimmingrange.However,thesameconverterbeingPWMdimmedat1kHz(1mstimeperiod)willhaveaminimumdutyratioof0.33%oraPWMdimmingrangeof1:300.www.newnespress.com Boost-BuckConverter113Iftheminimumon-timeofthePWMdimmingcycleislessthantheswitchingtimeperiod,theLEDcurrentwillnotreachitsfinalvalue.Hencetheaveragecurrentwillbeless.Thus,theLEDswilldim,buttherewillbealossoflinearitybetweentheaverageLEDcurrentandthedutycycleofthePWMinput.7.1.6DesignoftheBoost-BuckConverterwithHV9930SpecificationInputvoltage:9–16V(13.5Vtypical)Transientvoltage:42V(clampedloaddumprating)Reversepolarityprotection:14VOutputvoltage:28VmaximumOutputcurrent:350mALEDresistance:5.6ohmsEstimatedefficiencies:72%minimum,82%maximum(80%typical)Theseefficiencyvaluesdonottakeintoaccountthepowerlossinthereverseblockingdiode.ASchottkydiodewilldropaboutVd=0.5Vacrossitandthuswilldissipatepowerintherange0.4–0.6W.Thisdiodevoltagedropwillbetakenintoaccountwhiledesigningtheconverter.Theefficiencyvaluesusedinthisdesignaretypicalvaluesforthegiveninputvoltagesandoutputpowerlevel.Higherefficienciescanbeobtainedatlowerinputcurrentlevels(i.e.higherinputvoltages):theefficiencydropatlowerinputvoltagesisduetoconductionlossescausedbythecorrespondinglylargerinputcurrents.Theefficiencyvalueswilldependontheoperatingconditionsand,exceptinveryhighpowerdesigns,thesevaluescanbeusedasagoodapproximation.Efficiencieshigherthan85%caneasilybeachievedwiththeHV9930controlledCukconverteriftheoperatingfrequencyiskeptbelow150kHz.However,becauseofautomotiveEMIrequirements,thehigherefficienciesaretradedoffforhigherswitchingfrequencies(whichincreaseswitchinglossesinthesystem).www.newnespress.com 114Chapter7Consideraboost-buckconvertercircuitasshowninFigure7.10.D2L1C1L2VIN+VO–CdRd+D3CinD1CoQ1Rcs1Rcs2VIN–VO+C2C3Rs1143Rs2a2VinGATEGND6CS1VDDPWM5HV99307Rs2bPWMCS2REF8Rref1Rref2Figure7.10:Boost-BuckConverterUsingHV9930.SwitchingFrequencyatMinimumInputVoltageAlthoughtheHV9930isavariablefrequencyIC,theselectionoftheminimumswitchingfrequencyisimportant.Inthecaseofautomotiveconverters,designingwithaswitchingfrequencyintherangebetween300kHzand530kHzwouldavoidtherestrictedradiobroadcastbandsandmakeiteasiertomeettheconductedandradiatedEMIspecifications.So,forthisapplicationwechooseaminimumswitchingfrequencyof300kHz(whichoccursatminimuminputvoltage).CalculatingtheDutyCycleTheswitchdutycyclewillhavetobecomputedattheminimuminputvoltage.1Dmax¼min
ðÞVin,minVd1þVo¼0:821www.newnespress.com Boost-BuckConverter115CalculatingtheInputCurrentTheinputcurrentlevelattheminimuminputvoltageshouldbecalculatedfirst,becausethisgivesthehighestcurrentlevel.Thevalueobtainedwillbeusedtoworkoutthecurrentratingsofthevariouscomponents.Vo
IoIin,max¼min
ðÞVin,minVd¼1:601ACalculatingtheOutputInductorThefirststepistocomputetheoff-time.Theoff-timeoftheconvertercanbecalculatedas1DmaxToff¼fs,min¼598nsAssuminga25%peak-to-peakrippleintheoutputcurrentðDio¼87:5mAÞ,andaccountingforthediodedropintheinputvoltagebysubstitutingVin,minVdinplaceofVin,yieldspffiffiffiffiffiffipffiffiffiffiffiffi33598ns¼0:887m
L2þ3:125m
L2þ1:89m
L2SolvingforL2gives3L2¼ðÞ0:052¼145mHThecloseststandardvalueisa150mH,0.35ARMS,and0.4Asaturationinductor.Sincetheinductancevalueisdifferentfromthecomputedvalue,theactualoff-timewillalsochangeaspffiffiffiffiffiffiffiffiffiffiT¼2:777m
3Lþ3:125m
Loff,ac2,ac2,ac¼616nswww.newnespress.com 116Chapter7TheactualrippleintheoutputcurrentisgivenbyVo
Toff,acDio,ac¼L2,ac¼0:115ANotethatalthoughtherippleintheoutputcurrentwasassumedtobeabout25%(or87.5mA),theactualrippleisalmostdoublethatvalue.Thisincreaseintherippleisduetothedelaysofthecomparators.Acapacitorwillberequiredattheoutputoftheconverter(acrosstheLEDs)toreducetherippletothedesiredlevel.Thiscapacitorwillbeverysmall,astheswitchingfrequenciesarelarge,butthecapacitorwillalsohelptoreduceoutputEMI.LargeoutputcapacitorsaretobeavoidedinapplicationsthatusePWMdimming,becausethestoredchargewillreducethedimmingratiothatcanbeobtained.Itisalsousefultocalculatetherippleovershootandundershootbeyondtheprogrammedlimits.Thiswillhelpdeterminehowtheaveragecurrentchangesduetothedelays.VoVin,minVdp3ffiffiffiffiffiffiffiffiffiffiDiover¼

K1
L2,acL2,acVo¼8:3mAVopffiffiffiffiffiffiffiffiffiffiDi¼
K
3Lunder32,acL2,ac¼19mAThus,theaverageoutputcurrentwillbereducedfromthesetvaluebyabout10.7mA.Inmostcases,duetotheinductorvaluesavailable,theactualoff-timewilldifferfromthecomputedvaluesignificantly.Thus,itisbettertousetheactualvalueoftheoff-timecalculatedinordertoworkouttherestofthecomponentvalues.Iftheswitchingfrequencyislessthan150kHz,theequationL¼Vo
Toffcanbeused2Diotocalculatetheoutputinductance(L2)value,simplifyingtheproceduregreatly.www.newnespress.com Boost-BuckConverter117CalculatingtheInputInductorWecanassumea15%peak-to-peakrippleintheinputcurrentatminimuminputvoltage(thislowinputripplewillminimizetheinputfilteringcapacitanceneeded).Theoff-timepreviouslycalculatedcanbeusedtofindthevalueoftheinputinductor.Vo
Toff,acL1¼0:15
Iin,max¼72mHThecloseststandardvalueinductorisan82mHinductor.Thecurrentratingofthisinductorwillbedecidedinthefinalstagesaftertheinputcurrentlimithasbeenset.Thepeak-to-peakrippleintheinputcurrentisVo
Toff,acDIin¼L1,ac¼0:21ACalculatingtheValueoftheMiddleCapacitor(C1)Assuminga10%rippleacrossthecapacitoratminimuminputvoltage(Dvc¼0:1
ðÞVin,minVdþVo¼3:65V),capacitorC1canbecalculatedasIin,max
Toff,acC1¼Dvc¼0:257mFqffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiIrms,C1¼I2
ðÞ1DmaxþI2o
Dmaxin,max¼0:72AThevoltageratingandtypeofthiscapacitorhavetobechosencarefully.Thiscapacitorcarriesboththeinputcurrentandtheoutputcurrent.Thus,topreventexcessivelossesandoverheatingofthecapacitor,itmusthaveaverylowESR.CeramiccapacitorsareanidealchoiceforthisapplicationduetotheirlowESRandhightransientvoltagelimit.Ifaceramiccapacitorcannotbeusedforreasonsofwww.newnespress.com 118Chapter7costoravailability,aplasticfilmcapacitorsuchasPETcanbeusedinstead,althoughtheseareconsiderablybulkier.Themaximumsteadystatevoltageacrossthecapacitoris44V(=28Vþ16V),andthemaximumtransientvoltageacrossthecapacitorVc,maxis70V(=28Vþ42V).Ceramiccapacitorscaneasilywithstandupto2.5timestheirvoltageratingforthedurationoftheloaddumpvoltage.Also,theactualcapacitancevalueofthesecapacitorsreducesbasedonthebiasvoltageapplied.CeramiccapacitortypesX7RandX5Raremorestableandthecapacitancedropisnotmorethan20%atfullratedvoltage.Thus,a0.22mF,50VX7Rceramicchipcapacitorcanbeselected.ChoosingtheSwitchingTransistor(Q1)ThepeakvoltageacrosstheMOSFETQ1is70V.Assuminga30%overheadonthevoltageratingtoaccountforleakageinductancespikes,theMOSFETvoltageneedstobeatleastVFET¼1:3
Vc,max¼91VTheRMScurrentthroughtheMOSFETwillbeatmaximumlevelatlowinputvoltage(highercurrentlevelsandmaximumdutycycle).ThemaximumRMScurrentthroughtheMOSFETispffiffiffiffiffiffiffiffiffiffiffiIFET,max¼ðÞIin,maxþIo
Dmax¼1:77AAtypicalchoicefortheMOSFETistopickonewhosecurrentratingisaboutthreetimesthemaximumRMScurrent.ChooseFDS3692fromFairchildSemiconductors(100V,4.5A,50mN-channelMOSFET).Notethatthecurrentratingisnormallyquotedat25°C;thecurrentratingreducesasthetemperaturerises.ThetotalgatechargeQgofthechosenMOSFETisamaximumof15nC.ItisrecommendedthattheMOSFETtotalgatechargeshouldnotexceed20nC,asthelargeswitchingtimeswillcauseincreasedswitchinglosses.Ahighergatechargewouldbeallowableiftheswitchingfrequencycanbereducedappropriately.www.newnespress.com Boost-BuckConverter119AresistorinserieswiththeMOSFETgatereducesEMIbyslowingdowntheturn-ontime.CurrenttransientsarelimitedwhentheMOSFETturnsonslowly,butthisreducesefficiency.APNPtransistortodischargetheMOSFETgatehelpstominimizethereductioninefficiencywithoutsignificantlyincreasingEMI.ChoosingtheSwitchingDiodeThemaximumvoltageratingofthediodeD2isthesameastheMOSFETvoltagerating.Theaveragecurrentthroughthediodeisequaltotheoutputcurrent.Idiode¼Io¼350mAAlthoughtheaveragecurrentofthediodeisonly350mA,theactualswitchingcurrentthroughthediodegoesashighas1.95A(Iin,maxþIo).(Note:thecalculationswerefor360mA,toallowfor10mAdropbecauseofdelays,buttheactualaveragecurrentis350mA.)A500mAdiodewillbeabletocarrythe1.79Acurrentsafely,butthevoltagedropatsuchhighcurrentlevelswouldbeextremelylarge,increasingthepowerdissipation.Thus,weneedtochooseadiodewhosecurrentratingisatleast1A.A100V,2ASchottkydiodewouldbeagoodchoice.Choosingavoltageratingsignificantlyhigherthanrequiredisnotagoodidea,sincegenerallytheforwardvoltagedropincreasesasthereversevoltageratingincreasesandthiscauseshigherconductionlosses.ChoosingtheInputDiodeTheinputdiodeservestwopurposes:1.Itprotectsthecircuitfromareversepolarityconnectionattheinput.2.IthelpsinPWMdimmingofthecircuitbypreventingC1fromdischargingwhentheHV9930isturnedoff.ThecurrentratingofthedeviceshouldbeatleastequaltoIin,max.Thevoltageratingofthedeviceshouldbemorethanthereverseinputvoltagerating.Ahighercurrentratingoftengivesalowerforwardvoltagedrop.Inthiscase,a30BQ015(15V,3ASchottkydiode)wouldbeagoodchoice.www.newnespress.com 120Chapter7IfneitherreverseprotectionorPWMdimmingisrequired,removingtheinputdiodefromtheLEDdrivercircuitwillincreasetheinputsupplyvoltageattheconverter,whichwillslightlyincreasetheefficiencyandslightlyreducethemaximuminputcurrent.CalculatingtheInputCapacitanceSomecapacitanceisrequiredontheinputsidetofiltertheinputcurrent.Thiscapacitanceismainlyresponsibleforreducingthe2ndharmonicoftheinputcurrentripple(whichinthiscasefallsintheAMradioband).AccordingtotheSAEJ1113specifications,thepeaklimitfornarrowbandemissionsinthisrangeis50dBmVtomeetClass3ataninputvoltageof13+0.5V.Assumingasawtoothwaveformfortheinputcurrentasaconservativeapproximation,theRMSvalueofthe2ndharmoniccomponentoftheinputcurrent(Iin,2)canbecomputedasDIinIin,2¼pffiffiffi¼0:024A2
2
pTheswitchingfrequencyoftheconverterat13Vinputcanbecomputedas1Dnom¼nom
ðÞVin,nomVd1þVo1¼0:8
ðÞ13:50:51þ28¼0:731Dnomfs,nom¼Toff,ac¼414kHzIin,2Cin¼4
p
f
106
1050=20s,nom¼14:6mFChooseaparallelcombinationofthree4.7mF,25V,X7Rceramiccapacitors.www.newnespress.com Boost-BuckConverter121CalculatingtheOutputCapacitanceTheoutputcapacitanceisrequiredtoreducetheLEDcurrentripplefrom115mAtoDILED=70mA(20%peaktopeakripple)canbeapproximatelycalculatedbyusingonlythefirstharmonicintheinductorcurrent.A70mApeak-to-peakrippleintheLEDresultsina392mV(Dvo=DlLED
RLED)peak-to-peakripplevoltage.ThenDvo8DiL2RLED¼

qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi2p2221þðÞ2
p
fs,min
RLED
CoTheoutputcapacitancerequiredcanthenbecalculatedfromthissffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi216
RLEDDiL2
1p2DvoCo¼2
p
fs,min
RLED¼0:178mFUsea0.22mF,35Vceramiccapacitor.CalculatingtheTheoreticalSwitchingFrequencyVariationThemaximumandminimumfrequencies(usingsteadystatevoltageconditions)canbenowbeworkedout:111þmin
ðÞVin,minVd=Vf¼os,minToff,ac¼291kHz111þmax
ðÞVin,maxVd=Vf¼os,maxToff,ac¼506kHzThetheoreticalfrequencyvariationforthisdesignis398kHz+27%.www.newnespress.com 122Chapter7DesignoftheDampingCircuitThevaluesforthedampingnetworkcanbecalculatedasfollows32DmaxIoCd¼9

L1,ac
1DmaxVo¼11mF3
DmaxL1,ac
IoRd¼
ðÞ1D2Cd
Vomax¼7:16ThepowerdissipatedinRdcanbecomputedasDv2cPRd¼12
Rd3:652¼¼0:155W12
7:16TheRMScurrentthroughthedampingcapacitorwillbeDvciCd¼pffiffiffi¼0:147A2
3
RdChoosea10mF,50Velectrolyticcapacitorthatcanallowatleast150mARMScurrent.AnexamplewouldbeEEVFK1H100PfromPanasonic(10mF,50V,SizeD).Thiscapacitorhasabouta1ESR,soRdcanbereducedtoabout6.2.InternalVoltageRegulatoroftheHV9930TheHV9930includesabuilt-in8–200Vlinearregulator.ThisregulatorsuppliesthepowertotheIC.ThisregulatorcanbeconnectedateitheroneoftwonodesonthecircuitasshowninFigure7.11.Inthenormalcase,whentheinputvoltageisalwaysgreaterthan8V,theVINpinoftheICcanbeconnectedtothecathodeoftheinputprotectiondiode(asshowninwww.newnespress.com Boost-BuckConverter123(A)(B)D2L1D2L1VIN+VIN+Q1Q1C2C21313VinGATEGND6VinGATEGND6CS1VDDCS1VDDHV9930HV9930PWMCS2PWMCS2REFREFFigure7.11:ConnectionPointsforVIN.Figure7.11A).Ifreverseprotectionisnotprovided,theVINpincanbeconnecteddirectlytothepositivesupply.Inconditionswheretheconverterneedstooperateatvoltageslowerthan8V,oncetheconverterisrunning(asinthecaseofcold-crankoperation),theVINpinoftheHV9930canbeconnectedasshowninFigure7.11B.Inthiscase,thedrainoftheMOSFETisatVinþVo,andhenceeveniftheinputvoltagedropsbelow8V,theICwillstillbefunctioning.However,inthiscase,morehold-upcapacitancewillberequiredattheVDDpintosupplythepowertotheICwhentheMOSFETisON.Inbothcases,a2.2mForgreatervalueceramiccapacitorisrecommendedattheVDDpin.InternalVoltageReferenceTheHV9930includesaninternal1.25V(+3%)reference.Thisreferencecanbeusedtosetthecurrentthresholdsfortheinputandoutputhystereticcomparators.Itisrecommendedthatthispinbebypassedwithatleasta0.1mFceramiccapacitor.www.newnespress.com 124Chapter7ProgrammingtheHystereticControllersandOverVoltageProtectionTheinputandoutputcurrentlevelsforthehystereticcontrollersaresetbymeansofthreeresistorsforeachcurrent–onecurrentsenseresistorandtwodividerresistors.TheequationsgoverningtheresistorsarethesameforboththeinputandoutputsidesandaregivenasDi0:05
þ0:1RsI¼RrefDi1:2
0:1IRs1:2
0:05RrefRcs¼ITheseequationsassumethatthe1.25VreferenceprovidedbytheHV9930isusedtosetthecurrent.IncaseswherelineardimmingoftheLEDsisrequired,itisrecommendedthattheinputcurrentthresholdsbebasedonthe1.25Vreferenceandtheoutputcurrentthresholdsaremodifiedusingthevariableinputvoltageavailable.Insuchacase,assumingthemaximumexternalvoltageVLDasthereference,theabovetwoequationscanbemodifiedasDi0:05
þ0:1RsI¼RrefDiðÞVLD0:05
0:1IRsðÞVLD0:05
0:05RrefRcs¼IInthisdesignexample,itisassumedthatlineardimmingisnotrequiredandthe1.25Vreferenceisusedforboththeinputandoutputprogramming.Note:TheHV9930cannotoperatetheboost-buckconverterinthediscontinuousconductionmode.TheminimumexternalvoltageisgivenbyRref2þRs2VLD¼0:1
:Rs2www.newnespress.com Boost-BuckConverter125Theprogrammingoftheoutputsideisalsolinkedtotheover-voltageprotection.Theboost-buckconverterisnotinherentlyprogrammedagainstopenLEDconditions,soexternalprotectionisrequired.ThisisachievedbyaddingZenerdiodeD3,andbysplittingtheresistorRs2intotwoparts–Rs2aandRs2b.Innormaloperation,theinductorcurrentwillflowonlythroughRcs2andthevoltagedropacrossRcs2issensedthroughRs2aandRs2binseries.WhenthereisanopenLEDcondition,theinductorcurrentwillflowthroughdiodeD3.ThiswillthenclamptheoutputtotheZenerbreakdownvoltage.However,sincethediodecannottakethefulldesigncurrent,thecurrentlevelhastobereducedtomoremanageablelevels.DuringopenLEDconditions,thecurrentwillflowthoughbothRcs2andRs2a.Thus,theeffectivecurrentsenseresistorseenbytheICisRcs2þRs2aandthevoltagedropacrossbothofthesewillbesensedthroughRs2b.This,ineffect,willreducetheprogrammedcurrentlevelandthuspreventthehighLEDcurrentsfromflowingintotheZenerdiode.ChoosingtheOutputSideResistorsFortheoutputcurrent,Io=0.36A(tocompensateforthe10mAdropduetothedelaytimes)andDlo=87.5mA.Notethatweareusingthevaluesassumedandnottheactualvaluescomputedfortheripplecurrent.Usingthesevaluesintheaboveequations,Rs2aþRs2b¼0:534Rref2Rcs2¼1:64P¼0:352
1:64¼0:2WRcs2Beforewecompletethedesignoftheoutputside,wealsohavetodesigntheover-voltageprotection.Forthisapplication,choosea33VZenerdiode.ThisisthevoltageatwhichtheoutputwillclampincaseofanopenLEDcondition.Fora350mWdiode,themaximumcurrentratingat33Vworksouttoabout10mA.Usinga5mAcurrentlevelduringopenLEDconditions,andassumingthesameRs/Rrefratio,Rs2aþRcs2¼120www.newnespress.com 126Chapter7Choosethefollowingvaluesfortheresistors:Rcs2¼1:65,1=4W,1%Rref2¼10k,1=8W,1%Rs2a¼100,1=8W,1%Rs2b¼5:23k,1=8W,1%DesignoftheInputSideResistorsFortheinputside,wefirsthavetodeterminetheinputcurrentlevelforlimiting.Thiscurrentlevelisdictatedbythefacttheinputcomparatormustnotinterferewiththeoperationofthecircuit,evenatminimuminputvoltage.ThepeakoftheinputcurrentatminimuminputvoltagewillbeDIinIin,pk¼Iin,maxþ2¼1:706AAssuminga30%peak-to-peakripplewhentheconverterisininputcurrentlimitmode,theminimumvalueoftheinputcurrentwillbeIlim,min¼0:85
Iin,limWeneedtoensurethatIlim,min>Iin,pkforproperoperationofthecircuit.Assuminga5%safelyfactor,i.e.,Ilim,min¼1:05
Iin,pkWecancomputetheinputcurrentlimittobeIin,lim=2.1A.Allowingfora30%peak-to-peakripple,wecancalculateRs1¼0:442Rref1Rcs1¼0:228P¼I2
RRcs1in,limcs1¼1Wwww.newnespress.com Boost-BuckConverter127Thispowerdissipationisamaximumvalue,whichoccursonlyatminimuminputvoltage.Atanominalinputvoltageof13.5V,wecancomputetheinputcurrentusingthenominalvaluesfortheefficiencyandtheinputvoltage.28
0:35Iin,nom¼0:8
ðÞ13:50:5¼0:942AP¼0:9422
0:228¼0:2WRcs1Thus,atnominalinputvoltage,thepowerdissipationreducesbyaboutfivetimestoareasonable0.2W.Choosethefollowingvaluesfortheresistors:Rcs1¼parallelcombinationofthree0:68,1=2W,5%resistorsRref1¼10k,1=8W,1%Rs1¼4:42k,1=8W,1%InputInductorCurrentRatingThemaximumcurrentthroughtheinputinductorisIlim,max¼1:15
Iin,lim¼2:4A:Thus,thesaturationcurrentratingoftheinductorhastobeatleast2.5A.Iftheconverterisgoingtobeininputcurrentlimitforextendedperiodsoftime,theRMScurrentratingneedstobe2A,elsea1.5ARMScurrentratingwillbeadequate.ImprovingEfficiencyTheinputcurrentsenseresistorcanbereducedinvalue,whichgivesreducedpowerdissipation(loss).Toallowthis,itisnecessarytoaddanextraresistor(RA)betweentheanodeoftheflywheeldiodeandthecurrentsenseinputoftheHV9930(AT9933).Thisresistorallowsareductioninthehysteresisrequiredbytheinputcomparator.TheadditionalresistorisshowninFigure7.12.InFigure7.12,RS1=R4,RREF1=R7,andRCS1=theparallelcombinationR1//R3.www.newnespress.com 128Chapter7RAD1L2C1L3VIN+VO–C5+R2D5R5D3C10LoadC2–C4Q3Q2VIN–R1//R3R8//R12VO+C9C8R4143R92VinGATEGND6CS1VDDPWM5HV99307R10PWMCS2REF8R7R11Figure7.12:ModificationoftheCukCircuit.ConsiderthecircuitduringtheperiodwhentheMOSFETisON,sothattheinputcurrentthroughL1isincreasingbyDIIN/2,untilitreachesIIN,LIMþDIIN/2.WiththeMOSFETturnedON,thepositivesideofthecapacitorC1isgroundedandtheothersideofC1,whichisconnectedtoresistorRA,isatpotentialVC1.NotethatthepotentialVC1,NOM=VIN,NOMþVO.ThevoltagereferenceforthecomparatorinputatCS1is0V.NowconsiderthenodeatCS1intermsofcurrentflow;CS1ishighimpedanceinput,sothesumofcurrentsequalzero:DIINIIN,LIMþ
R1==R3VREFVC1,NOM2¼þR7RAR4NowconsiderthecircuitwhentheMOSFETisOFF.NowtheflywheeldiodeD3isconducting,sothenegativesideofcapacitorC1isgrounded(thesmallforwardvoltageofthediodecanbeignored).WiththeMOSFETturnedOFF,thevoltagereferenceforthecomparatorinputatCS1is100mV.www.newnespress.com Boost-BuckConverter129DIIN0:1VþIIN,LIM
R1==R3VREF0:1V0:1V2¼þR7RAR4SinceR4isaverylargevalueandthevoltageacrossitissmall,wecanignoreitseffecttosimplifythecalculations:DIIN0:1VþIIN,LIM
R1==R3VREF0:1V2¼R7R4WecanthusignoretheadditionofRAduringtheperiodthattheMOSFETisturned
OFF.Clearly,thevalueofR1//R3canbereducedifthecurrentIDIINcanIN,LIM2beincreased,orifR4canbedecreased,orboth.ThemaximumcurrentsensevoltageoccurswhentheMOSFETisfirstturnedON.DIINVSENSE,MAX¼IIN,LIMþ
R1==R32ThisisafunctionofthevoltageacrossthecapacitorC1.IfwetakeanotherlookattheequationforcurrentflowwhentheMOSFETisturnedON:DIINIIN,LIMþ
R1==R3VREFVC1,NOM2¼þR7RAR4InaCuktopology,VC1=VINþVOUT.Atstart-up,VOUT=0V,soVC1,MIN=VIN,MIN.ThehighestinputcurrentoccursatVIN,MIN.DIINIIN,LIMþ
R1==R3VREFVIN,MIN2¼þR7RAR4
IfwesetthemaximumcurrentIþDIINinthemodifiedcircuittobeequaltoIN,LIM2theinductorL1saturationcurrent,ISAT,wegetVREFVIN,MINISAT
R1==R3¼þR7RAR4www.newnespress.com 130Chapter7
Inpracticewestartwiththedesignofanunmodifiedcircuit,soIþDIINareIN,LIM2thevaluescalculatedbeforetheadditionofRAisconsidered.Inthemodifiedcircuit,ISAT(ofL1)mustbemuchhigherthanthesevaluesinordertogainthelossreductionbenefit,whichgivesahigherinputrippleatstart-up.
VIN,MIN
ðÞIIN,LIMþDIINVIN;NOMþVOUTISATRA¼VREF1ðÞIIN,LIMþDIIN
1R7ISAT0:1VR4ðmodÞ¼VREF10:1VðÞIIN,LIMDIINVREF1VIN,MIN
R7ISATR7RAR4ðmodÞVREF1VIN,MINR1==R3ðmodÞ¼
ISATR7RAVREF1=1.25Vinthestandardconfiguration.MeetingConductedandRadiatedEMIDuetothenatureoftheboost-buckconverter,itiseasytomeetconductedandradiatedEMIspecifications.AfewprecautionsneedtobetakenduringdesignandPCBlayouttobeabletomeettheEMIstandards.1.Insomecases,whentheinputcurrentrippleistoolargeortheswitchingfrequencyoftheconverterisabove150kHz,itmightnotbepossibletomeettheconductedEMIstandardsusingonlycapacitorsattheinput.Insuchcases,aninputPIfiltermightberequiredtofilterthelowfrequencyharmonics.2.Shieldedinductorsortoroidalinductorsshouldalwaysbepreferredoverunshieldedinductors.Theseinductorswillminimizeradiatedmagneticfields.3.Duringlayout,theICandMOSFETgroundconnectionshouldbeconnectedtoacopperplaneononeofthePCBlayerswiththecopperplaneextendingundertheinductors.4.TheloopconsistingofQ1,C1andD1shouldbeassmallaspossible.ThiswouldhelpgreatlyinthemeetingthehighfrequencyEMIspecifications.www.newnespress.com Boost-BuckConverter1315.ThelengthofthetracefromGATEoutputoftheHV9930totheGATEoftheMOSFETshouldbeassmallaspossible,withthesourceoftheMOSFETandtheGNDoftheHV9930beingconnectedtotheGNDplane.Alowvalueresistor(10–47ohms)inserieswithGATEconnectionwillslowdowntheswitchingedgesandgreatlyreduceEMI,althoughthiswillcauseefficiencytodecreaseslightly.APNPtransistortodischargethegatequicklyhelpstolimitthedecreaseinefficiency,withoutaddinganysignificantEMI.6.AnR-CdampingnetworkmightbenecessaryacrossdiodeD1toreduceringingduetotheundampedjunctioncapacitanceofthediode.ThisconcludestheCukconverterdesign.Wecannowconsideracloselyrelatedcircuit;theSEPIC.7.2SEPICBuck-BoostConvertersTheabbreviationSEPICcomesfromthedescriptionSingleEndedPrimaryInductanceConverter.ASEPICisaboost-buckconverter,likeaCuk,soitsinputvoltagerangecanoverlaptheoutputvoltage.SEPICcircuitscanbedesignedforconstantvoltageorconstantcurrentoutput.TheSEPICtopologyhasbeenknownforsometime,butonlyrecentlyhastherebeenarevivalinitsapplicationbecause:(a)itneedslowESRcapacitorsandthesearenowwidelyavailableand(b)itcanbeusedtocreateACinputpowersupplieswithpower-factorcorrectionthatareusedtomeetworldwideEMIstandards.Inautomotiveandportableapplications,batteriesareusedasapowersourceforDC/DCconverters.A12Vsupplyusedinautomotiveapplicationscanhaveawiderangeofterminalvoltage,typically9Vto16Vduringnormaloperationusingalead-acidbattery,butcangoaslowas6.5Vduringcold-crankandashighas90Vduringload-dump(whenthebatteryisdisconnected).Thepeakvoltageisusuallyclampedtoabout40V,usingavoltagedependentresistortoabsorbtheenergy.Lithiumbatterieshavebeenverysuccessfulinportableapplications,thanksmostlytotheirimpressiveenergydensity.Asinglelithiumcellprovidesanopenvoltageof4.2Vwhenfullycharged,andreplacesuptothreeofthealternativeNiCdorNiMHcells.Duringdischargethecellstillretainssomeenergydownto2.7V.ThisinputvoltagerangecanbebothaboveandbelowtheoutputofmanyDC/DCconvertersandsodiscountsthepossibilityofusingboostorbuckconverters.www.newnespress.com 132Chapter7Internationalstandardsforpowersuppliesratedabove75Wrequirepower-factorcorrection(PFC).HavingagoodpowerfactormeansthatthecurrentwaveformfromtheAClineissinusoidalandinphasewiththevoltage.MostPFCcircuitsuseasimplestep-upconverterastheinputstage,implyingthattheinputstageoutputmustexceedthepeakvalueoftheinputwaveform.InEuropeACinputsof190–265VRMSarefound,whichimposeanoutputofatleast375V,forcingthefollowingconverterstoworkwithelevatedinputvoltages.TypicallyaPFCinputstagehasa400Voutput.ByusingaSEPICtopology,whichhasaboost-bucktopology,theboostsectionprovidesPFCandthebucksectionproducesaloweroutputvoltage.Thisprovidesacompactandefficientdesign.Itprovidestherequiredoutputlevelevenifthepeakinputvoltageishigher.7.2.1BasicSEPICEquationsTheboostorstep-uptopology,asshowninFigure7.13,isthebasisfortheSEPICconverter.Theboost-converterprincipleiswellunderstood:first,switchQ1conductsduringtheon-period,TON,whichincreasesthecurrentinL1andthusincreasesthemagneticenergystoredthere.Second,theswitchstopsconductingduringtheoff-period,TOFF,butthecurrentthroughL1cannotchangeabruptly–itcontinuestoflow,butnowthroughdiodeD1andintoCout.ThecurrentthroughL1decreasesslowlyasthestoredmagneticenergydecreases.CapacitorCoutfiltersthecurrentpulsethatwasgeneratedbyL1whenQ1turnedoff.L1D1VinVoutCinCoutQ1CONTROLLERFigure7.13:ThisBoost-ConverterTopologyistheBasisforSEPICPower-SupplyCircuits.ThediodeD1hastoswitchveryquickly,soadiodewithashortreverserecoverytime(Trrlessthan75ns)isneeded.IncaseswhereVoutisrelativelylow,thewww.newnespress.com Boost-BuckConverter133efficiencycanbeimprovedbyusingaSchottkydiodewithlowforwardvoltage(about400mV)forD1.Notethataboostconverterhasonemajorlimitation:VoutmustalwaysbehigherthanVin.IfViniseverallowedtobecomegreaterthanVout,D1willbeforwardbiasedandnothingcanpreventcurrentflowfromVintoVout.TheSEPICschemeofFigure7.14removesthislimitationbyinsertingacapacitor(Cp)betweenL1andD1.ThiscapacitorblocksanyDCcomponentbetweentheinputandoutput.TheanodeofD1,however,mustconnecttoaknownpotential.ThisisaccomplishedbyconnectingD1togroundthroughasecondinductor(L2).L2canbeseparatefromL1orwoundonthesamecore,dependingontheneedsoftheapplication.L1RL1CpRcpD1VinVoutCinL2CoutQ1RL2CONTROLLERRswFigure7.14:SEPICTopology.IfL1andL2arewoundonthesamecore,whichissimplyatransformer,onemightarguethataclassicalfly-backtopologyismoreappropriate.However,thetransformerleakageinductance,whichisnoprobleminSEPICschemes,oftenrequiresasnubbernetworkinfly-backschemes.Snubbernetworksaredescribedlaterinthischapter;putsimplytheyrequireadditionalcomponentsthatmustbecarefullyselectedtominimizelosses.ParasiticresistancesthatcausemostoftheconductionlossesinaSEPICareRL1,RL2,RSWandRCP,andareassociatedwithL1,L2,SW,andCPrespectively.TheseparasiticcomponentsarealsoshowninFigure5.27.AnadvantageoftheSEPICcircuit,besidesbuckandboostcapability,isacapacitor(Cp)thatpreventsunwantedcurrentflowfromVINtoVOUT.Thusthelimitationofthesimpleboostconverter,thatVINhadtoalwaysbelessthanVOUT,hasbeenovercome.www.newnespress.com 134Chapter7Thoughithasveryfewelements,theoperationofaSEPICconverterisnotsosimpletodescribebyequations;someassumptionshavetobemade.First,assumethatthevaluesofcurrentandvoltageripplearesmallwithrespecttotheDCcomponents.Second,assumethatatequilibriumthereisnoDCvoltageacrossthetwoinductancesL1andL2(neglectingthevoltagedropacrosstheirparasiticresistances).Byusingtheseassumptions,CpseesaDCpotentialofVinatoneside(throughL1)andgroundontheotherside(throughL2).TheDCvoltageacrossCpis:VCPðmeanÞ¼VINTheperiodofoneswitchingcycleisT=1/frequency.TheportionofTforwhichswitchQ1isclosedisthedutycycle,D,andtheremainingpartoftheperiodisthus1D.BecausethemeanvoltageacrossL1equalszeroduringsteadystateconditions,thevoltageseenbyL1duringD*T(i.e.theMOSFET‘ON’period)isexactlycompensatedbythevoltageseenbyL1during(1D)*T(i.e.theMOSFET‘OFF’period):D
T
VIN¼ðÞ1D
T
ðVOUTþVDþVCPVINÞWhereVDistheforwardvoltagedropofD1foradirectcurrentof(IL1þIL2),andVCPisequaltoVIN.Simplifyingthisweget:D
T
VIN¼ðÞ1D
T
ðVOUTþVDÞTransposingthis,weget:ðVOUTþVDÞD¼¼AiVIN1DAiiscalledtheamplificationfactor,where‘i’representstheidealcaseforwhichparasiticresistancesarenull.NeglectingVDwithrespecttoVOUT(asafirstapproximation),weseethattheratioofVOUTtoVINcanbegreaterthanorlessthan1,dependingonthevalueofD(withequalityobtainedforD=0.5).www.newnespress.com Boost-BuckConverter135ThemoreaccurateexpressionAa(amplification,actual)accountsforparasiticresistancesinthecircuit:VOUTþVDþIOUT
ðAi
RcpþRL2ÞAa¼VINAi
IOUT
ðRL1þRswÞRsw
IOUTThisformulaallowscomputationoftheminimum,typicalandmaximumamplificationfactorsforVin(Aamin,Aatyp,andAamax).Theformulaisrecursive(‘Aaxxx’appearsinboththeresultandtheexpression),butafewiterativecalculationsleadtothesolution.TheexpressionneglectsswitchinglossesduetotheswitchQ1andreverserecoverycurrentinD1.Thoselossesareusuallynegligible,especiallyifQ1isafastMOSFETanditsdrain-voltageswing(VinþVoutþVd)remainsunder30V.Insomecases,youshouldalsoaccountforlossesduetothereverserecoverycurrentofD1,andforcorelossesduetohigh-levelswingsinstoredmagneticenergy.YoucanextrapolatethecorrespondingvaluesofD:D¼Aa=ð1þAaÞOrmoregenerally:Dxxx¼Aaxxx=ð1þAaxxxÞ,wherexxxismin,typormax:TheDCcurrentthroughCpiszero,sothemeanoutputcurrentcanonlybesuppliedbyL2:IOUT¼IL2Thepower-dissipationrequirementforL2iseased,becausethemeancurrentintoL2alwaysequalsIOUTanddoesnotdependonvariationsofVIN.TocalculatethecurrentintoL1(IL1),wecanusethefactthatnoDCcurrentcanflowthroughCp.Thus,thecoulombchargeflowingduringD*Tisperfectlybalancedbyanoppositecoulombchargeduring(1D)*T.Whentheswitchisclosed(foranintervalD
T)thepotentialattheswitchnodeisfixedat0V.SincethecapacitorCpwaspreviouslychargedtovoltageVin,theanodeofD1willnowwww.newnespress.com 136Chapter7haveapotentialofVIN,whichreverse-biasesD1.CurrentthroughCpisthenIL2.Whentheswitchisopenduring(1D)*T,currentIL2flowsthroughD1whileIL1flowsthroughCp:D
T
IL2¼ðÞ1D
T
IL1KnowingthatIL2=IOUT,IL1¼Aaxxx
IOUTInputpowerequalsoutputpowerdividedbyefficiency,soIL1dependsstronglyonVIN.Foragivenoutputpower,IL1increasesifVINdecreases.KnowingthatIL2(andhenceIOUT)flowsintoCpduringD*T,wechooseCpsothatitsrippledeltaVcpisaverysmallfractionofVcp(gamma=1%to5%).TheworstcaseoccurswhenVinisminimal.IOUT
Dmin
TCpigamma
VINMINByusingahighswitchingfrequency,smallmulti-layerceramiccapacitorscanbeusedforCp.However,ensurethatCpisabletosustainthepowerdissipation(Pcp)duetoitsowninternalequivalentseriesresistance(Rcp):Pcp¼AaminRcpI2OUTTheMOSFETswitchdrain-to-sourceresistance,inserieswithacurrentsenseresistorforlimitingthemaximumcurrent,isgivenbythetermRsw.Thisincursthefollowingloss:Psw¼Aaminð1þAaminÞRswI2OUTLossesPRL1andPRL2,duetotheinternalresistancesofL1andL2,areeasilycalculated:P¼Aamin2RI2RL1L1OUTP¼RI2RL2L2OUTWhencalculatingthelossduetoD1,theaveragepowerlossisduetotheoutputcurrentandtheforwardvoltagedropofD1:PD1¼VDIOUTwww.newnespress.com Boost-BuckConverter137L1ischosensoitstotalcurrentripple(DIL1)isafraction(b=20%to50%)ofIL1.TheworstcaseforboccurswhenVINisatmaximum,becauseDIL1isatmaximumwhenIL1isatminimum.Assumingb=0.5:2
T
ðÞ1Dmax
VINMAXL1min¼IOUTChooseastandardvaluenearesttothatcalculatedforL1,andmakesureitssaturationcurrentmeetsthefollowingcondition:Aamin
IOUTþ0:5
T
Dmin
VINMINIL1SATiiIL1þ0:5
DIL1¼L1ThecalculationforL2issimilartothatforL1:2
T
Dmax
VINMAXL2min¼IOUTIOUTþ0:5
T
Dmax
VINMAXIL2SATiiIL2þ0:5
DI2¼L2IfL1andL2arewoundonthesamecore,youmustchoosethelargerofthetwocalculatedinductorvalues.Usingasinglecore,thetwowindingsshouldbebifilar(twistedaroundeachotherbeforebeingwoundonthecore)andthuswillhavethesamenumberofturnsandthesameinductancevalues.Otherwise,voltagesacrossthetwowindingswilldifferandCpwillactasashortcircuittothedifference.Ifthewindingvoltagesareidentical,theygenerateequalandadditivecurrentgradients.Inotherwords,therewillbemutualinductanceofequalvalueinbothwindings.Thus,theinductancemeasuredacrosseachisolatedwinding(whenthereisnothingconnectedtotheotherwinding)shouldequalonlyhalfofthevaluecalculatedforL1andL2.Becausenogreatpotentialdifferenceexistsbetweenthetwowindings,youcansavecostsbywindingthemtogetherinthesameoperation.Ifthewindings’cross-sectionsareequivalent,theresistivelosseswilldifferbecausetheircurrents(IL1andIL2)differ.Totalloss,however,islowestwhenlossesaredistributedequallybetweenthetwowindings,soitisusefultoseteachwinding’scross-sectionaccordingtothecurrentitcarries.Thisisparticularlyeasytodowhenthewindingsconsistofinsulatedstrandswww.newnespress.com 138Chapter7ofwire(Litz)forcounteractingtheskineffect.Finally,thecoresizeischosentoaccommodateasaturationcurrentmuchgreaterthan(IL1þIL2þDIL1)atthehighestcoretemperatureanticipated.Thepurposeoftheoutputcapacitor(COUT)istoaveragethecurrentpulsessuppliedbyD1duringTOFF.Thecapacitormustbeabletohandlehigh-levelrepetitivesurgecurrentswithlowESRandlowself-inductance.Fortunately,ceramicandplasticfilmcapacitorsmeettheserequirements.TheminimumvalueforCOUTisdeterminedbytheamountofripple(DVOUT)thatcanbetolerated:Aamin
IOUT
Dmin
TCOUTDVOUTTheactualvalueoftheoutputcapacitormayneedtobemuchlargerthanthatcalculatedusingtheaboveequation,especiallyiftheloadcurrentiscomposedofhighenergypulses.Theinputcapacitorcanbeverysmall,thankstothefilteringpropertiesoftheSEPICtopology.Usually,CINcanbeonetenththevalueofCOUT:CIN¼COUT=10OverallefficiencycanbepredictedfromVINandAa.Theresultcanbemisleading,becauseitdoesn’taccountfortheswitch-transitionlossesorcorelossesandtherealefficiencycouldbemuchlower:¼VOUT=AaVINFinally,theswitchSWanddiodeD1shouldberatedforbreakdownvoltageswitha15%margin:VDSðswitchÞ>1:15ðVOUTþVDþVINÞVRðdiodeÞ>1:15ðVOUTþVINÞExampleLetVIN=50150VandVOUT=15Vat1Amaximum.Letusoperateat200kHzswitchingfrequency,sothatT=5ms.NowVOUT¼D,soD=0.231andVIN1DmaxDmin=0.091.L1min¼2Tð1DmaxÞVINMAX=IOUTwww.newnespress.com Boost-BuckConverter139L1¼1050:769150=1¼1:15mH,letL1¼1:5mHminL2min¼2TDmaxVINMAX=IOUTL2¼1050:231150=1¼0:347mH,letL2¼0:47mHminCp>IOUT
DminT=ðgamma
VINMINÞCp>10:0912105=ð0:0550Þ¼728nF,letCp¼1mF:NowDxxx¼Aaxxx=ð1þAaxxxÞ,wherexxxismin,typormax.SoAaminoccursatDmin¼0:091andAamin¼0:1:COUTAamin
IOUT
Dmin
T=DVOUTC>0:110:0912105=0:1:OUTCOUT>>1:82mF:LetCOUT¼100mF:CIN>COUT=10:LetCIN¼10mF:So,thefundamentalcomponentvalueshavebeencalculated.Nowwhatremainsforthedesigneristhechoiceofsuitable(andavailable)parts.7.3Buck-BoostTopologyUnliketheboost-buckcircuitsusedbytheCukandSEPICtopologies,thebuck-boostusesasingleinductor.Itisafly-backcircuitandhencewillbecoveredinChapter9.7.4CommonMistakesinBoost-BuckCircuitsBoost-buckcircuitsoperatewithbothinductorsincontinuousconductionmode.Hencetheinductorshouldbechosenwithavaluehigherthanthatcalculated,toallowfortolerancesandforsaturationeffects(theinductancefallswithincreasingcurrent).Calculatethevalue,add20%,andthenpickthenexthigheststandardvalue.Currentratingsofinductorsaregivenforacertaintemperatureriseinthecore,typically40°C.Soiftemperatureriseisanissue,pickacomponentwithahighercurrentrating.www.newnespress.com 140Chapter77.5ConclusionsTheboost-buckisanidealtopologywheretheLEDloadvoltagecanbehigherorlowerthanthesupplyvoltage.Itshouldalsobeusedwhenthesupplyvoltageisnomorethan20%difference(worstcase)fromtheLEDloadvoltage.SoiftheLEDvoltage(maximum)is20Vandthesupplyvoltage(minimum)is23V,thedifferenceis3V,and3/20=0.15or15%,soaCukorSEPICshouldbeused.Ifthesupplyvoltageismorethan20%higher,useabucktopology.Ifthesupplyvoltageismorethan20%lower,useaboosttopology.Theboost-buckislessefficientcomparedtobuckorboosttopologies.www.newnespress.com CHAPTER8LEDDriverswithPowerFactorCorrection8.1PowerFactorCorrectionPowerfactorcorrection,orPFC,isatermusedwithACmainspoweredcircuits.AgoodpowerfactoriswhentheACcurrentisinphasewiththeACvoltage.Apureresistiveloadhasapowerfactorof1,butactiveloadstendtohavepowerfactorscloserto0.5,unlessspecialmeasuresaretakento‘correct’this.Themostcommonpowerfactorcorrectioncircuitisaboostconverter.TheAClinevoltageisboostedtoabout400Vandtheamplitudeofthecurrentpulsesintoastoragecapacitorisarrangedtobesinusoidal.Thisisachievedbyswitchingthecurrentonforshortbutconstantperiods:asthesupplyvoltagerisesandfalls,sodoestheamplitudeofthecurrent.AtypicalPFCcircuitisshowninFigure8.1.Asimplealternativeistouseafly-backsupply.Itiscommontoswitchtheprimarycurrentoffwhenacertaincurrentlevelisreached,butthisleadstoconstantaveragecurrent.Togiveagoodpowerfactor,theprimarycurrentshouldbeswitchedwithaconstant‘on-time’,sothatthecurrentamplituderisesandfallsinphasewiththesupplyvoltage.ThesecondarycurrentwillriseandfallatdoubletheAClinefrequencyandsoalargesecondarycapacitorisrequiredtoabsorbthisripple,topreventsignificantrippleintheoutputvoltage.DrivinganLEDfromapowerfactorcorrectedsupplyusuallyrequiresasimplebuckconverter,sincethevoltagesourcetendstobeveryhigh(about400V).However,alternativesolutionsexist;thesearetheBi-BredandtheBuck-Boost-Buck(BBB).www.newnespress.com 142Chapter8D2800uHSTTH2L06UC10R14R3R6D510nF100RR191M470K510KR3R7D4C11R10R201M470K18V470nF22K470KC5R91uF68KC12470nFC1347uF1450V8R14IC17M13L656264R5C9R11R21C6C713K100R476K210uF10uFnF0VFigure8.1:PFCCircuit.8.2Bi-BredTheBi-BredisverysimilartotheCukboost-buckthatwedescribedinthepreviouschapter,seeFigure8.2.ThemaindifferencebetweentheCukandtheBi-Bredisthat,inaBi-Bred,theinputinductorisindiscontinuousconductionmode(DCM)andoperationoftheoutputstageisincontinuousconductionmode(CCM).Theenergystoredineachinductorisproportionaltotheinductancevalue.Thismeansthatinthedesign,theinputinductorL1musthaveasmallenoughenergystoredtoensurethatconductionstopsbeforetheendofeachcycle.Thismeansthattheinputinductorvaluemustberelativelysmall.TheoutputinductorL2musthavelargeenoughenergystored(largeinductancevalue)sothatthecurrentonlyfallstoabout85%ofitsnominalvalueattheendofeachswitchingcycle.Whenpowerisfirstapplied,MOSFETM1isoffandwaitingforthefirstclocksignaltotriggerthegatedrivepulse.AtthistimethestoragecapacitorC3immediatelybeginstochargefromthesupplyvoltagethroughD1andL1,althoughthevoltagewillnotriseveryhighbecause,whentheMOSFETM1switcheson,thechargingcurrentisredirectedtothe0Vrail.WithM1conducting,currentcontinuestoriseinwww.newnespress.com LEDDriverswithPowerFactorCorrection143D1C3C2L1+L2OUTR9LoadC1M1D2C12(optional)Q1R2R7R1R3RTGATEGNDR4VINHV9931CS2CS1VDDPWMC6PWMR6R5Figure8.2:Bi-BredCircuit.amplitudethroughtheinductorL1untilthevoltagedropacrossR2issufficientfortheinternalcomparatorinsidetheHV9931totrigger,whichturnsM1off.NowtheinputcircuitactslikeaboostconverterbecausethecurrentthroughL1cannotchangeimmediatelyanditchargesC3toahighvoltage.TheenergyinC3isusedtodrivecurrentthroughtheLEDloadthenexttimethatM1switcheson.ThecurrentrisesininductorL2andtheloaduntilthevoltagedropacrossresistorR7issufficienttotripasecondinternalcomparatorandturnM1offagain.ThecurrentflowthroughL2passesthroughD2tokeepcurrentflowingintheLEDload.Noticethatthecurrentsenseresistorisnotinthispath,becausethecurrentlevelmeasurementisnotrequireduntiltheMOSFETturnsonagain;thisminimizespowerloss.TheoutputoftheBi-Bredisconfiguredasabuckstage.Energyissuppliedfromabulkstoragecapacitor,C3,withsufficientlylargecapacitancetoprovideamoreorlessconstantsupplyvoltageoveranAClinecycleperiod.AconstantcapacitorvoltagesupplyingpowertothebuckstagemeansaconstantswitchdutycyclewhenitisdrivingtheLEDload.TheBi-BreddrawsamoreorlesssinusoidalAClineinputcurrentwhendrivenfromaswitchoperatingatconstantdutycycle,hencealargecapacitancevalueforC3helpstoproduceagoodpowerfactor.www.newnespress.com 144Chapter8ThedutycycleoftheswitchingisgivenbyVO¼D.VI1DOrputanotherway,D¼VO.SoifV=350VandV=3.5V(atypicalwhiteVIþVOinoLED),D¼3:5¼3:5¼0:99%.Thisisclosetothe1%expecteddutycyclefora350þ3:5353:5simplebuckconverterandcanbedifficulttoswitchproperly.ThismeansthataBi-BredisnotreallysuitablefordrivingshortLEDstrings.8.3Buck-Boost-Buck(BBB)TheBuck-Boost-Buck(BBB)isaproprietarycircuit,patentedbySupertex,andisillustratedinFigure8.3.ItresemblestheBi-Bredinsomerespects,exceptfortwocurrentsteeringdiodesD1andD2.D1D2L2OUTC2D4L1+C3LoadD3C12R9C1M1Q1R2R8R10R3R4RTGATEGNDVINHV9931CS2CS1VDDPWMR11R5PWMC6Figure8.3:Buck-Boost-BuckCircuit.LiketheBi-Bred,theinputinductorisindiscontinuousconductionmode(DCM)andoperationoftheoutputstageisincontinuousconductionmode(CCM).Theenergystoredineachinductorisproportionaltotheinductancevalue.Thismeanswww.newnespress.com LEDDriverswithPowerFactorCorrection145thatinthedesign,theinputinductorL1musthaveasmallenoughenergystoredtoensurethatconductionstopsbeforetheendofeachcycle.Thismeansthattheinputinductorvaluemustberelativelysmall.TheoutputinductorL2musthavelargeenoughenergystored(largeinductancevalue)sothatthecurrentonlyfallstoabout85%ofitsnominalvalueattheendofeachswitchingcycle.Whenpowerisfirstapplied,MOSFETM1isoffandwaitingforthefirstclocksignaltotriggerthegatedrivepulse.Atthistime,thestoragecapacitorC3isnotcharged.WithM1conducting,currentbeginstoriseinamplitudethroughtheinductorL1untilthevoltagedropacrossR2issufficientfortheinternalcomparatorinsidetheHV9931totrigger,whichturnsM1off.Nowtheinputcircuitisinflywheelmode,becausethecurrentthroughL1cannotchangeimmediatelyanditchargesC3toamoderatelyhighvoltage.Thevoltageistypicallymidwaybetweentheinputandoutputvoltagelevels.TheenergyinC3isusedtodrivecurrentthroughD2,L2andtheLEDloadthenexttimethatM1switcheson.ThecurrentrisesininductorL2andtheloaduntilthevoltagedropacrossresistorR8issufficienttotripasecondinternalcomparatorandturnM1offagain.ThecurrentflowthroughL2passesthroughD2tokeepcurrentflowingintheLEDload.Noticethat,likeintheBi-Bred,thecurrentsenseresistorisnotinthispath,becausethecurrentlevelmeasurementisnotrequireduntiltheMOSFETturnsonagain;thisminimizespowerloss.TheoutputoftheBuck-Boost-Buck(BBB)isthebuckstage.Energyissuppliedfromabulkstoragecapacitor,C3,withsufficientlylargecapacitancetoprovideamoreorlessconstantsupplyvoltageoveranAClinecycleperiod.AconstantcapacitorvoltagesupplyingpowertothebuckstagemeansaconstantswitchdutycyclewhenitisdrivingtheLEDload.TheBBBdrawsamoreorlesssinusoidalAClineinputcurrentwhendrivenfromaswitchoperatingatconstantdutycycle,hencealargecapacitancevalueforC3helpstoproduceagoodpowerfactor.InpracticethereisalimittothevalueofC3,particularlyifplasticfilmcapacitorsareused.ThismeansthattherewillbesomevoltagerippleacrossC3,atafrequencydoublethatoftheACline(i.e.120Hzwhendrivenfroma60Hzline).Theeffectofthisripplevoltageistogeneratesecondharmonicsignalsintheinputcurrent,whichreducesthepowerfactor.Byaddingasimplecircuit,thesecondharmoniccanbereduced;theMOSFEToff-timeismodulatedbytheripplevoltageandthisactslikenegativefeedbacktoreducethesecondharmonic.TheadditionalcircuitsaregiveninFigure8.4.www.newnespress.com 146Chapter8D1D2L2OUT(negative)C2D4L1+C3C12D5C11–R9C1M1D3Load+Q1R8D6C5D10R1R7D7R10R6C7D9D8R3R4RTGATEGNDVINHV9931CS2CS1VDDPWMR11R5PWMC6Figure8.4:Buck-Boost-BuckwithHarmonicReduction.WhenMOSFETM1isconducting,thevoltageacrossC3isappliedtoC11,whichchargesC5.Betweeneachswitchingcycle,resistorR7dischargescapacitorC5.TheripplevoltageacrossC3willmodulatetheaveragevoltageacrossC5.CapacitorC7actsasaDCblock,toallowjustthemodulationacrossC5,ratherthananyDClevel,tovarytheMOSFEToff-time.AsthevoltageacrossC5risesandfalls,currentthroughR6risesandfalls,thusshorteningorlengtheningtheoff-time.VD2ThedutycycleoftheswitchinginaBBBconverterisgivenbyO¼.VI1DpffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiVOV2þ4
VI
VOOrputanotherway,D¼O.SoifV=350VandV=3.5V2
VIino(atypicalwhiteLED),D¼3:570¼66:5¼9:5%.Thisisaconsiderablygreaterduty700700cyclethantheBi-Bredorthebuckconverterwithasimilarlowvoltageload.ThismeansthattheBBBconverterismostsuitablefordrivingshortLEDstrings.www.newnespress.com LEDDriverswithPowerFactorCorrection1478.4CommonMistakeswithPFCCircuitsThemostcommonmistakeistouseastandardinductorforL1.Inductorsaresizedfortheirmagneticsaturationlevelandforresistiveheating.ThusaninductormaybespecifiedasI(av)=500mA,I(sat)=400mA.Thisinductorcanpass500mAwithatemperatureriseof,say,40°C.Itcanpass400mAbeforetheinductanceisreducedby,say,10%.IfthisinductorwereusedinaPFCstagewithapeakcurrentof400mAitwouldoverheat.Usinganinductorwithamuchhighersaturationcurrentratingwillbenecessary,togiveareasonablylowtemperatureriseduringoperation.Inductormanufacturersdonotnormallyspecifymagnetizinglosses.Themagneticsaturationlevelsarematerialdependent;themaximumfluxdensityofferriteisusually200mT,othermaterialscanbehigher.Sowhendesigningaferrite-basedinductor,amanufacturerwillmakehisdesignbasedonthislevel.Whenconsideringmagnetizinglosses,afluxdensityofabout50mTwouldbeabetterchoiceforaferrite-basedinductor.8.5ConclusionsDetaileddesignanalysishasnotbeengivenforthePFCcircuits.Thischapterhasbeenintendedtoshowreaderssomeoptionsandpointoutlimitations.Forexample,drivingasingleLEDwouldrequireaBuck-Boost-Buckcircuit,butlongerstringscanbedrivenfromaBi-BredoraPFCstagefollowedbyabuckconverter.ApplicationnotesfromSTMicroelectronicsandSupertexcoverthePFC,Bi-BredandBuck-Boost-Buckcircuitsindetail.Theseareproprietaryandspecializedsolutionsthatarestillevolving;interestedreadersshouldconsulttheseapplicationnotesforthelatestdesigns.www.newnespress.com CHAPTER9Fly-BackConvertersAtraditionalfly-backconverterusesaninductorwithatleasttwowindings(really,thisisatransformer).Considertwowindings;oneistheprimary,whichisconnectedtotheinputpowersupplyandaswitchtoground;theotheristhesecondary,whichisconnectedtotheload.Thecircuitisarrangedsothatmagneticenergyisstoredintheinductorduringthetimethattheswitchison,whencurrentincreasesintheprimarywinding.Whentheswitchisoff,themagneticenergyisreleasedbycurrentflowingoutofthesecondarywinding.ThisisshowninFigure9.1.Theenergyreleaseisthe‘fly-back’,socalledbecauseinearlytelevisionsetswithacathoderaytube,atransformerwindingwasusedtodeflecttheelectronbeambacktothestartingpointonthescreen.Theelectronbeamhadto‘flyback’quicklyaftercompletingascanacrossthescreen,toavoidmissingthenextlineofdatatobedisplayed.Fly-backpowersuppliesarerelativelyeasytodesign,butaremoresuitedtoconstantvoltageoutputs.Thisisbecausetheenergyisstoredinbursts,inalargereservoircapacitor,andcontrollingtheaveragevoltageacrossthecapacitorcanbeachievedwithsimplefeedback.DrivinganisolatedLEDloadisthenpossibleifthesecondarywindingisisolatedfromtheprimarywinding.Somegeneral-purposeapplicationscanusesimplecurrentlimittechniquesintheprimarywindingtocontroltheoutputcurrentfromthesecondarywinding.Anopto-couplerwillberequired,tomaintainisolationbetweenprimaryandsecondary,ifaccuratecontroloftheoutputcurrentisrequired.www.newnespress.com 150Chapter9V+LOADIPRIPrimarycurrenttISECSecondarycurrenttEStoredmagneticenergytFigure9.1:Fly-BackPrinciple.Somefly-backconvertersuseaninductorwithasinglewinding.Thesearebuck-boostcontrollersandareanalternativetotheboost-buckconverterslikeCukandSEPICtypesthatwerediscussedinChapter7.Clearly,isolationisnotpossiblewiththistypeofconverter.9.1TwoWindingFly-BackAschematicofatypicalfly-backcircuitfordrivingLEDsisshowninFigure9.2.Thedotalongsidethetransformerwindingindicatesthestartofthewinding.InthiscasethestartisconnectedtotheMOSFETdrain,whichalternatesbetweenagroundconnectionandopencircuit.Thevoltageatthedrain,andhencethewindingstartpoint,variesconsiderablyduringswitching.Conversely,theouterlayer(endofthewinding)isatafixedhighvoltageandtendstoscreentheinnerlayers,whichreducesradiatedEMI.www.newnespress.com Fly-BackConverters1511A800VLuxeonBYV26CStarX322uF100nFUF40061200V220uF100nF1W12VINVDD10M2,2uF8GTIRFBG204CS2K7100pF1HV9910NG13LD330K14RT9PWMGND5Figure9.2:Fly-BackCircuitforLEDs.Thesecondarywindingstartpointisconnectedtotheoutputdiode,whichpreventsconductionwhentheMOSFETison.Thestartpointofthesecondaryisconnectedtotheoutputdiode,buttheendpointisconnectedtogroundandthistendstoscreenthesecondarywindingforminimalEMIradiation.EnergythatisstoredduringtheMOSFETon-timeisreleasedduringtheoff-time,bycurrentflowingthroughtheoutputdiodeandintotheload.Calculationofthetransformercharacteristics,likeinductancevalueandprimarytosecondaryturnsratio,areveryimportantinthedesign.Inorderforcompletepowertransferfromtheprimarytosecondary,thevolt-secondsmustbeequal.Theequationis:VPRI
TONVSEC
TOFF¼NPRINSEC9.1.1Fly-BackExampleLetusmakeanisolated3WlampbyconnectingthreewhitepowerLEDsinseries.Supposewehaveaprimaryvoltageof48V,anon-timeof5microseconds,andtheprimarytosecondaryturnsratiois1:0.1.Ifwearedrivinga10VLEDload,thewww.newnespress.com 152Chapter9off-timewillbe240/100microseconds(2.4ms).Thustheswitchingperiodmustbegreaterthan12.4msinordertoallowcompleteremovalofthemagneticenergyinthetransformercore.Aswitchingfrequencyofbelow65kHzwillbesatisfactory,say60kHztogivesomemargin.With60kHzswitching,theperiodwillbe16.667ms.Iftheaverageoutputcurrentis350mA,theaveragein2.4mswillbe2.43A.Sincethiscurrentdecayslinearlyfromthetransformerwinding,thepeaksecondarycurrentwillbedoublethis:4.86A.ThesecondaryinductancewillbeE¼L
di.dtdt2:4
106L¼E
¼10
¼4:94mHdi4:86Sincetheprimaryhastentimestheturnsofthesecondary,theprimaryinductancewillbe100timesthatofthesecondary(theturnsratio,N,issquared).Inotherwords,theprimaryinductancewillbe494mH.Mostcurrent-modepowersuppliescontroltheswitchingsothattheMOSFETturnsoffwhenacertainpeakcurrentisreachedintheprimarywinding.Sincethepeakcurrentinthesecondaryis4.86Aandtheturnsratiois10:1,weneedapeakprimarycurrentof486mA.[Check:E¼L
di,soE=494*106*0.486/(5*106)=48V].dtTheproblemwiththedesignthatwehaveisthattheLEDcurrentwillchangeiftheLEDvoltagechanges,becausewehavebasedourdesignonacertainoutputvoltage.Actuallythisgivesaconstantpoweroutput,assumingaconstantvoltageinput,whichisfinefornon-criticaldesigns.Butwhatiftheinputvoltagechanges?Ahigherinputvoltagewillmeanthatthecurrentlimitwillbereachedinashortertime.Thismeansthatthedutycyclewillbereducedandhencethenumberofvolt-secondsontheprimarywillbeunchanged.Inpractice,inherentdelaysinthecurrentsensecomparatorwillcausetheinputcurrentto‘overshoot’thereferencelevel.Thisovershootincreaseswithincreasinginputvoltage,thisisbecausethedelayisconstantbuttherateofcurrentriseisincreasingwithinputvoltage.Compensationofthisovershootcanbeachievedbyconnectingaresistorbetweenthesupplyvoltagerailandthecurrentsensepin.ThisresistorinjectsasmallDCbiasthatincreaseswithincreasingsupplyvoltageandthustriggersthecomparatorearlierasthesupplyvoltagerises.www.newnespress.com Fly-BackConverters153The1:0.1turnsratioand10Voutputusedintheaboveexamplecauseareflectedvoltageof100Vintheprimarywindingwhenthesecondaryconductiontakesplace.Thisreflectedvoltageaddstothesupplyvoltage,soaMOSFETwitha200Vorhighervoltageratingisrequiredwhenpoweringthiscircuitfroma48Vsupply.Thedesignexampledoesnotallowforefficiency.Inpracticeafly-backconverterhasabout90%efficiency,sotheinputcurrentmustbeincreasedbyabout10%toallowforthis.Ifweweredesigningaconstantvoltagecircuit,wewouldallowthepeakprimarycurrenttobehigherthanthatgivenintheexample.Thismarginallowsfortheinputvoltagevariations.Wewouldthenusefeedbackfromtheoutputtocontroltheswitching,toreducethepowerintheprimary,asnecessary.9.2ThreeWindingFly-BackSomefly-backpowersuppliesuseathirdwinding,calledabootstraporauxiliarywinding,asshownifFigure9.3.ThisisusedtopowerthecontrolIC,oncethecircuitisoperating.Thebootstrapwindinghasthesameorientationasthesecondarywindingandthevoltageissimplydeterminedbytheturnsratioofthebootstrapcomparedtothesecondary.Inourexampleofa10Voutputfromthesecondary,thebootstrapcouldhavethesamenumberofturnsandthusgive(approximately)10VforthepoweringthecontrolIC.Atstart-up,thereisnopoweravailablefromthebootstrapwinding,soastart-upregulatorisrequired.Examplestart-upregulatorsaretheLR645andtheLR8fromSupertex;thesegivealowvoltage,lowcurrentoutputfromaninputwithavoltageashighas450V.Oncethebootstrapproducespower,thestart-upregulatorturnsoff.TheHV9120showninFigure9.3hasastart-upregulatorbuilt-in.9.2.1DesignRulesforaFly-BackConverterThissectiongivesdesignrulesforafly-backconverterbasedoneitherturnsratioselectiondeterminedbythemaximumdutycycleallowed(case1),orbytheoptimumturnsratiobasedonthemaximumworkingvoltageoftheMOSFETswitch(case2).Incase1,adesignbasedonthemaximumdutycycle(atthelowestinputvoltage)www.newnespress.com 154Chapter9D2C20VD1+20to300V+5VC4COMC3R7R9R11714156N135C18R12IC2R2IC1R65M1C7R10HV9120D39R8C8610161116R4R13R3C5C6R5R11ZD10VTL431Figure9.3:Fly-BackUsingaThree-WindingTransformer.allowsthewidestinputvoltagerange.Incase2,adesignbasedonthemaximumvoltageacrosstheMOSFETallowsapotentiallylowercostsolution.Alternatively,afly-backdesignbasedonanalreadyavailabletransformerwithaknown(andfixed)turnsratiomaybeconsidered.Thetransferfunctionofafly-backconverteris:VOD¼
NVIð1DÞSothedutycyclecanbefoundbytransposingtheequation:VO
ðÞ1D¼VI
D
NVO¼VI
D
NþVO
D¼D
ðVI
NþVOÞVOD¼ðVI
NÞþVOwww.newnespress.com Fly-BackConverters155Case1:TurnsRatioBasedonMaximumDutyCycleGiventheminimuminputvoltageVI_MIN,outputvoltageVOandmaximumdutycycleDMAX,theturnsratioNcanbecalculated:VO
ðÞ1DMAXN¼VIMIN
DMAXDMAXistypicallychosenas45%(0.45)foraPWMcontrollerwithamaximum49%dutycycle.WithDMAX<50%,thesystemisinherentlystableandthereisnocomplexcompensationrequired.Ifwetaketheearlierexampleof48Vinput(say,46Vminimum),10Voutput(add0.6Vfortheoutputdiode)andallow45%dutycycle,weget:10:6
ðÞ0:55N¼¼0:28246
0:45Thisistheminimumvalue.Atransformerwithaconvenientturnsratioof1:0.33(3:1)couldbeused.Themaximumdutycyclewouldthenbe:VO10:6D¼¼0:41ð41%ÞðVI
NÞþVOð15:33þ10:6ÞCase2:TurnsRatioBasedonMaximumSwitchVoltageTheoutputvoltageacrossthesecondarywindingisinducedintotheprimaryandmagnifiedbytheturnsratioN.Thiswasillustratedatthebeginningofthischapter,whena10Voutputcaused100Vtobeinducedintotheprimarywindingofa1:0.1turnsratiotransformer.Consideringthatthesupplyvoltagewasonly48V,thisforcedustousea200VMOSFETastheprimaryswitch.TheaimhereistominimisetheMOSFETswitchoperatingvoltagerequirement.Becausethevoltagereflectedintotheprimaryoftenhassomeringing,asnubbercircuitisusedtolimitthevoltageacrosstheprimarywinding.RingingisduetoresonancebetweentheMOSFETdraincapacitance,parasiticcapacitanceinthecircuitandparasiticinductanceofthetransformerprimary.Parasiticinductanceinthetransformerisoftenreferredtoas‘leakageinductance’becauseitistheproportionoftheprimaryinductancethatisnotcoupledintothesecondary,sothemagneticfield‘leaksout’.www.newnespress.com 156Chapter9AZenerdiodeissometimesusedasasnubber.ThevoltageacrosstheZenerdiodewillbegreaterthanthevoltageinducedintotheprimaryfromthesecondary(output)voltage,otherwisepowerdissipationandlosseswillbothbeveryhigh.VO¼N
ðVSWVZVINMAXÞInordertofindthesecondarywindingvoltage,theforwardvoltagedropoftheoutputdiode,VF,mustbeaddedtotheoutputvoltage.VOþVFN¼ðVSWVZVINMAXÞAsasafetymargin,ðVSWVZVINMAXÞ10V.Intheexampleweusedearlier,with48Vinput,wecouldhaveuseda100Vswitchanda33VZenerdiode.Theoutputis10V,soallowingforVFthisbecomes10.6Vacrossthesecondarywinding:10þ0:610:6N¼¼¼0:558ð1003348Þ19Wecoulduseatransformerwith1:0.5turnsratio(N=0.5).Theprimaryvoltageinducedfromthesecondarywindingwillbe21.2V,whichisbelowtheZenerdiodevoltageby11.8V,whichisareasonablemargintominimisepowerdissipation.ThepeakvoltageacrosstheMOSFETdrainwillbelimitedto48Vþ33V=81V.Withaturnsratioof1:0.5,themaximumdutycyclewitha46Vminimuminputvoltagewillbe:VO10:6D¼¼¼0:315ð31:5%ÞðVI
NÞþVO23þ10:6InductanceCalculationsNowwehavetheturnsratio(byeithermeansdescribedabove)andthemaximumdutycycle,wecannowdeterminetheinductanceandswitchingcurrent.Letususecase1,with41%asthemaximumdutycycle.POUTPIN¼www.newnespress.com Fly-BackConverters157Theoutputpoweris10V0.35A=3.5Wandtheefficiencycanbeguessedatasbeing85%.Theinputpoweristhen4.12W.Theinputcurrentatminimuminputvoltageisthen:PIN4:12IAV¼¼¼0:09AVIN462
IAVIPK¼DMAXAt46Vinand41%dutycycle:2
0:09IPK¼¼0:439A0:41With60kHzswitching,theperiodwillbe16.667ms.Witha41%dutycycle,theswitchon-timewillbe6.835ms.Soweneedtheprimarycurrenttoriseby439mAin6.835ms.V
dt46
6:835
106INLPRI¼¼¼716mHdI0:439Thesecondaryhas1/3thenumberofturnscomparedtotheprimary,sotheinductanceofthesecondarywillbe1/9,or79.55mH.TheotherdesignparameterforthetransformeristhesizeandALfactoroftheferritecore.Inafly-backtransformeranairgapbetweenthetwohalvesoftheferritecorearenecessarytopreventmagneticsaturation,astheairgapincreases,theALfactorreduces.Thefluxdensity(B)willdependonthecross-sectionalareaofthecore(Ac),giveninsquaremeters.SupposeinthiscasewehavesomeE20coresavailablefrom2Ferroxcube.ForE20/10/6cores,thecorecross-sectionalareais32mm.So62Ac=32*10m.Thenumberofturnscanbecalculated,basedonthedesignparametersaboveandusingB=200mTasthemaximumfluxdensity:LPRI
IPKN¼ðturnsÞAC
BMAX716
106
0:439N1¼¼4932
106
0:2L716
106PRIAL¼¼¼298nHN122401www.newnespress.com 158Chapter9Refertocoremanufacturer’sspecificationsandchooseacorewithalowerALvalue(largergap)thancalculatedusingtheaboveequation.Asuitablecore(3C90material,160mmairgap)hasanALvalueof250nH.Thenumberofturnscanthenbecalculatedas:rffiffiffiffiffiffiLN¼,whenLisexpressedinnH:Thus716mH¼716,000nH:ALNPRI=54(roundinguptothenexthighestvalue).ThisquiteconvenientlygivesthesecondaryturnsasNSEC=18,sinceitis1/3.9.3SingleWindingFly-Back(Buck-Boost)Inthebuck-boostconverter,asingleinductorwindingisusedfortheprimaryandsecondary.ThisisshowninFigure9.4.V+C1L1C4LED16VIND1VDDC2LED5PWM_D150K4Q1GATE2CS75KLDR2C3HV9910R183RoscGNDFigure9.4:Buck-BoostConverter.CurrentisforcedthroughtheinductorbyaMOSFETconnectingtheinductoracrossthepowersupplyrail.Thecurrentlevelrisesalmostlinearlywithtime.Atapredeterminedcurrentlevel,theMOSFETisturnedoffandthecurrentisforcedtoflowthroughadiodetochargetheoutputcapacitoranddrivetheload.Thecurrentintheinductorfallsbacktozeroandsodischargestheenergystoredinthewww.newnespress.com Fly-BackConverters159magneticcore.Likethetwo-windingfly-back,thesinglewindingfly-backcanbecalculatedfromthenumberofvolt-secondsonthechargecycleequallingthenumberofvolt-secondsonthedischargecycle.Thedutycycleofabuck-boostconverter(continuousconductionmode)isgivenbytheequation:VOD¼VI1DVO
ðÞ1D¼VI
DVO¼VI
DþVO
D¼D
ðVIþVOÞVOD¼VIþVOSo,ifwehaveVin=24VandVout=30V,D=30/54=0.555.Inpracticewewantdiscontinuousconductionmode,becausecontinuousconductionmodeisdifficulttostabilise.Thismeansthattheinductorcurrentfallstozeroattheendofeachcycle.So,assumewewant350mAoutputand100kHzswitchingfrequency.Theperiodis10ms,sotheon-timeis5.55msandtheoff-timeis4.45ms.Duringtheoff-time,thecurrentintheinductorfallslinearlyfromapeakleveltozero.Toaverage350mAoutput,theaveragecurrentduringtheoff-timemustbe350=0:445mA¼786:5mA,sothepeakcurrentmustbedoublethis,or1.573A.Thismeansthatduringtheon-time,thecurrentmustrisefromzeroto1.573A.Thevoltagefromthepowersupplyis24V,sousingthefamiliarequation:diE¼L
dtdt5:55
106L¼E
¼24
¼84:67mHdi1:573Inpracticethereshouldbesomedeadtimeallowed,whentheinductorcarriesnocurrent,toensurediscontinuousconductionmode.Thisdeadtimeistoallowforpowersupplytolerances,inductortolerances,etc.Toomuchdeadtimemeansthatthepeakcurrentishigherandthisreducestheefficiencyofthepowersupply.www.newnespress.com 160Chapter9Supposeweallow25%tolerance,sothattheon-timeis4.44ms;thiswillreducetheinductanceby25%.dt4:44
106L¼E
¼24
¼68mHdi1:573Theoff-timewillbereducedunlessthepeakcurrentisincreasedinproportion.diE¼L
dt6di30¼68
10
4:45
10630
4:45
106di¼¼1:963A68
106Increasingthepeakcurrentby25%givesthedesiredresult.ThepeakcurrentissetbythevalueofcurrentsenseresistorbetweentheMOSFETsourceandground.www.newnespress.com CHAPTER10EssentialsofSwitchingPowerSuppliesThischapterwillexaminetheadvantagesanddisadvantagesofthevariousdrivertechniques,whichhavealreadybeendescribed.Theissuesofefficiency,EMI,costandotherrequirementsthatareadditionaltothebasicfunctionoftheLEDdriver.10.1LinearRegulatorsInChapter4wesawhowtheuseoflinearregulatorscausedaheatdissipationproblembecauseoflowefficiency.AlinearLEDdriverisgenerallylessefficientthanaswitchingdriver.Sometimesalineardrivercanbemoreefficient.Forexample,ifyouhavea12VpowersourceandthreeLEDseachhavinga3.5Vforwarddrop,byconnectingtheminseriesthetotaldropis10.5V.Theefficiencyofalineardriver,droppingonly1.5Vwillbe87.5%.ItwouldbedifficultforaswitchingLEDdrivertoachievethislevelofefficiency.AndthereisnoEMItobefiltered.Ontheotherhand,drivingoneLEDfroma12Vsupplywouldgiveanefficiencyof3.5/12=29%withalinearLEDdriver.Hereabuckswitcherwouldgivecloserto90%efficiency.SeeFigure10.1.Efficiencyisimportantwhereheatdissipationmustbeminimized.OtherwisecostusuallytakesprecedenceandthecostofaswitchingregulatorwithEMIfilterswouldbesomewhathigher.www.newnespress.com 162Chapter1012V12VCL28.5V16VINVDDHV99108RT5PWM_D74LDGATE3.5VCS23GND0V(A)<30%Efficient(B)>90%EfficientFigure10.1:LinearvsSwitchingSolutions.10.2SwitchingRegulatorsInChapters5to9welookedatswitchingregulators,whichhavemuchhigherefficiency,butcangenerateelectro-magneticinterference(EMI)whichhastobesuppressedbycarefulcircuitboarddesign,screeningandfiltering.TheEMIreducingtechniquesaredescribedinChapter13.AlthoughSupertex’sLEDdriverintegratedcircuitsareusedinexamples,similardriversfromothermanufacturerscanalsobeused.Forexample,theLinearTechnologyLTC3783hassimilarfunctionstotheSupertexHV9912.TheNationalSemiconductorLM5020isabuckcontroller,liketheHV9910B.However,Supertexdeviceshaveaninternalhighvoltageregulator,whichmakesthemmoreversatile.Switchingpowersupplieshavethedisadvantageofproducingelectromagneticinterference(EMI).EMImustbelimited,topreventinterferencewithothersystems.Thisisalegalrequirementandproductcannotbesoldunlesstheequipmentmeetsthestandardslaiddowninlaw.DetailsofEMItechniquesaregiveninChapter13.Conversely,whereEMIrequirementsareverydemanding,suchasmedicalandautomotiveapplications,linearLEDdrivertechniquescanbeusedinstead.www.newnespress.com EssentialsofSwitchingPowerSupplies163Ofcoursetheefficiencymaysuffer,andsoaheatsinkwillbeneeded,butthisissometimesverymuchbetterthantryingtomakeaswitchingcircuitintermsofcostandphysicalsize.10.2.1BuckRegulatorConsiderationsInChapter5wefirstlookedatthesimplestswitchingregulator,thebuckconverter.Inabuckcircuittheloadvoltagemustbelessthan85%ofthesupplyvoltage,otherwisetheoutputbecomesdifficulttocontrol.BuckcircuitsareusedformainspoweredLEDdrivers,whendrivingalongstringofLEDs.Buckcircuitsarealsousedwheretheinputsupplyvoltageisrelativelylow,sayina12VDCautomotiveapplication,butwherejustoneLEDisbeingdriven.Buckregulatorscanbeveryefficient,maybe90–95%,especiallyiftheloadisalongstringofLEDswithamoderatelyhighforwardvoltage(i.e.highdutycycle).ThisisbecausethepowerdissipationintheflywheeldiodeisasmallerproportionofthetotalpowerbecausetheflywheeldiodeonlyconductsduringtheMOSFEToff-time,whichisasmallerproportionofthetotalswitchingcycle.TheMOSFETdissipatespowerduringtheon-time,whenitisconducting,butthevoltagedropacrosstheMOSFETswitchisusuallymuchlowerthantheforwarddropofafastrectifier.Inordertooperatecorrectlytheremustbesomerippleintheoutputcurrent.Theoutputcurrentneedstoreduceenoughtoallowthecurrentsensecomparatorstobereset.TheoutputripplecurrentDIOisnormallydesignedtobe20–30%ofIO;theoutputcurrentfallsfarenoughineachcyclesothatnoiseinthecurrentsensecomparatorhaslittleeffect.Iftheripplecurrentisbelow10%ofIO,theswitchingoftheMOSFETcanbeerratic.TheoutputcurrentintheLEDstring(IO)isgivenbytheequation:VTH1IO¼
DIORSENSE2HereVTHisthecurrentsensecomparatorthreshold,andRSENSEisthecurrentsenseresistor.Theripplecurrentcanintroduceapeak-to-averageerrorintheoutputcurrentsettingthatneedstobeaccountedfor.Whentheconstantoff-timecontroltechniqueisused,theripplecurrentisnearlyindependentoftheinputsupplyvoltagevariation.Therefore,theoutputcurrentwillremainunaffectedbythevaryinginputvoltage.www.newnespress.com 164Chapter10AddingafiltercapacitoracrosstheLEDstringcanreducetheoutputcurrentripple,thusallowingalowerinductorvalueoranapparentlymore‘constant’current.ThiscapacitorreducesEMIattheoutputbyprovidingabypasspathforanyswitchingcurrentspikes,whichmayalsoimprovetheLEDlifetime.However,keepinmindthatthepeak-to-averagecurrenterrorisaffectedbythevariationoftheMOSFEToff-time,TOFF.Therefore,theinitialoutputcurrentaccuracymightbesacrificedwithlargeripplecurrentlevelsintheinductor.AnotherimportantaspectofdesigninganLEDdriverisrelatedtocertainparasiticelementsofthecircuit,includingdistributedcoilcapacitanceoftheinductorCL,junctioncapacitanceCJ,andreverserecoveryoftheflywheeldiode,capacitanceoftheprintedcircuitboardtracesCPCBandoutputcapacitanceCDRAINoftheMOSFET.Theseparasiticelementsaffecttheefficiencyoftheswitchingconverterbecausetheycauseswitchinglosses.TheseparasiticelementsareshowninFigure10.2.Cj,junctioncapacitanceIrrReverseRecoveryCurrentDGHV9910NGParasiticPCBRGwinding8tracecapacitancecapacitance4MOSFETdrain-sourcecapacitanceRCFilter(limitsturn-oncurrentspike)Figure10.2:ParasiticElements.www.newnespress.com EssentialsofSwitchingPowerSupplies165ParasiticelementscouldpotentiallycausefalsetriggeringoftheLEDdriverIC’scurrentsensecomparator,especiallyifanRCfilterisnotfittedbetweentheMOSFETsourceandthecurrentsense(CS)pin.Minimizingparasiticelementsisessentialforefficientandreliableoperationofthebuckconverter.Coilcapacitanceofinductorsistypicallyprovidedinthemanufacturer’sdatabookseitherdirectlyorintermsoftheself-resonantfrequency(SRF).pffiffiffiffiffiffiffiffiffiffiffiffiffiSRF¼1=ð2L
CLÞ;HereListheinductancevalue,andCListhecoilcapacitance.CharginganddischargingthiscapacitanceeveryswitchingcyclecauseshighcurrentspikesintheLEDstring.Therefore,connectingasmallcapacitorCO(10nF)acrosstheLEDstringisrecommendedtobypassthesespikes,asmentionedearlier.Usinganultra-fastrectifierflywheeldiodeisrecommendedtoachievehighefficiencyandreducetheriskoffalsetriggeringofthecurrentsensecomparator.WhentheMOSFETturnsonthediodechangesfromforwardconductiontooff(reversebias),butthiscannothappenimmediatelybecausechargeshavetomoveinsidethesemiconductormaterial,whichtakestime.Thereisalwaysareverserecoverycurrentflowingintheoppositedirectionforashortperiod,TRR.Usingdiodeswithshorterreverserecoverytime,TRR,andlowerjunctioncapacitanceCJimprovesperformance.ThereversevoltageratingVRofthediodemustbegreaterthanthemaximuminputvoltageoftheLEDlamp.Theforwardvoltage-dropofdiodeswithveryfastrecoverytimesissometimesrelativelyhighandcanleadtohighconductionlosses,soalsoconsiderthiswhenmakingadiodeselection.ThetotalparasiticcapacitancepresentattheDRAINoutputoftheMOSFETcanbecalculatedas:CP¼CDRAINþCPCBþCLþCJWhentheswitchturnson,thetotalparasiticcapacitanceCPisdischargedintotheDRAINoutputoftheMOSFET.ThedischargecurrentislimitedtotheMOSFETsaturationcurrent,soMOSFETswithahighon-resistanceandalowersaturationcurrentcansometimesproduceloweroveralllosses.Thisisespeciallytrueifthedutycycleissmall,becausetheswitchisconductingforasmallproportionofthetimeandwww.newnespress.com 166Chapter10hencetheconductionlosseswillnotbesignificant.NotethatthesaturationcurrentinaMOSFETbecomesloweratincreasedjunctiontemperature.Thedurationoftheleadingedgecurrentspikecanbeestimatedas:VIN
CPTSPIKE¼þtrrISATInordertoavoidfalsetriggeringofthecurrentsensecomparator,CPmustbeminimizedinaccordancewiththefollowingexpression:
ISAT
TBLANKðMINÞtrrCP