A Matter Of Light Part2

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1、AMatterofLightPart2BuckWheneverPossibleSamehSarhanChrisRichardsonApplicationEngineerApplicationsEngineerNationalSemiconductorCorporationNationalSemiconductorCorporationEmail:Sameh.Sarhan@nsc.comEmail:Chris.Richardson@nsc.comPhone:+1408-7215641Phone:+1408-7214322Partoneofthisseriesthrashed

2、outthebasicsofLEDlightingsourcesandtheirdrivingrequirements.Itwasconcludedthattheperformanceofsimpledrivingtechniques,suchasvoltagesources/ballastresistorsorlinearregulatorswouldfallshortasthecomplexityandinputpowerrequirementsofLEDbasedlightingsourcesincrease.Consequently,thiscreatesanee

3、dformoresophisticatedswitch-modeLEDdrivers.Sowhatwouldbethetopologyofchoice?PartTwoofthisserieswilldiscusswhyaconstantcurrentbuckconvertershouldbethefirstpreferencewhenitcomestoswitch-modeLEDdriversor,inotherwords,whythebuckshouldbeusedwheneverpossible.IntroductionTherapidadoptionofLEDsby

4、variouscomplexapplicationsmakessimpledrivesolutionssuchaslinearregulatorsimpracticalinmanycases.Ingeneral,simpledriveschemescontinuouslydeliverpowerfromtheinputsourcetothedriver'soutputwhileusingresistiveelementstoprogramthedesiredLEDforwardcurrent.ForthesameLEDcurrent,thelossesintheseres

5、istiveelementsincreaseconsiderablyasthelinevoltagesincrease.Forexample,alinearregulatorbasedLEDdriveryields70%efficiencywhensupplying1Afroma5VinputsourcetoatypicalwhiteInGaNLED(VF=3.5V).Underthesameoperatingconditions,thedriver’sefficiencywilldroptoapproximately30%whentheinputvoltageincre

6、asesto12V.InadditiontodegradingtheoverallperformanceofanLEDbasedapplication,suchpoorefficiencieswouldrequireimpracticalthermalmanagementschemes.SwitchingRegulatorsToimprovetheconversionefficiency,switchingregulatorsinterruptthepowerflowwhilecontrollingtheconversiondutycycletoprogramthedes

7、iredoutputvoltageoroutputcurrent.Interruptingthepowerflowresultsinpulsatingcurrentandvoltageandthereforeitnecessitatestheuseofenergystorageelements(inductorsand/orcapacitors)tofilterthesepulsatingwaveforms.Contrarytolinearregulators,switchingregulatorscanbeconfigure