FlyBack Fundamental & Design Example外文

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1、FlybackversusForward•Flyback–Lowestcostsolutionforoutputcurrents<~6Amps–Advantages:1.Simplecircuit-nooutputenergystorageinductorrequired2.Simpletoimplementmultipleoutputdesigns–Disadvantages:higheroutputripplecurrent-higheroutputcapacitorcosts•Forward–Lowestcostsolutionforoutpu

2、tcurrents>~6Amps–Advantages:lowoutputripple-lowercostoutputcapacitors–Disadvantages:1.Morecomplexcircuit-outputenergystorageinductorrequired2.Coupledoutputinductorrequiredformultipleoutput75Filename:GXFlyback03242004.ppt•Flybackistheidealsolutionforapplicationswith0Ato6Aoutputc

3、urrent•Mostoftheseapplicationsareinthepowerrangeof0Wto100W.However,applicationsbeyond100Wcouldstillbesuitableforflybackifoutputvoltageishighenoughtokeeptheoutputcurrentlow.(Foragivenoutputpower,higheroutputvoltagemeansloweroutputcurrent).FlybackFundamentals•LinkSwitchisdesigned

4、fordiscontinuousmodeFlybackoperation–Allenergyintransformertransferredtosecondaryduringswitchoff-time•DuringdiodeconductionVistransformedtoprimaryasVOOR–V≈V×N/NOROPS20Lowpwr022404•VORontheprimarysideisacloserepresentationoftheoutputvoltageforflybackconverters•Unstableoperationm

5、ayresultifaLinkSwitchdeviceisusedinthecontinuousconductionmode(CCM).Therefore,CCMoperationisnotrecommended.High-sideMOSFETallowsdirectVsensingORHigh-sideSwitchReferenceLow-sideSwitchReference•SensingVisdifficultwithlow•SensingViseasywithhighORORsideMOSFETsideMOSFET–canonlysense

6、VOR+VINwith–cansenseVORdirectlywithrespectrespecttosourcetosource21Lowpwr022404•(≈VOasoutputdiodedropneglected)High-sideMOSFETWaveformsSOURCEtoRTNvoltageLeakageinductancespikeVFB≈VORRTNtoSOURCEvoltageDRAINtoSOURCEvoltagePI-3299-09100222Lowpwr022404•ReferencedtoSourceVFBcanbesen

7、seddirectly•(LeakageinductancespikecausesanerrorinVFB(aboveVOR))•(Forillustration,rippleonVFBexaggerated)LinkSwitchIndirectlySensesVfromVOORNDV≈V=V×PCLAMPFBORONSCCLAMPDiodedropneglected•CsamplesandholdsV≈VCLAMPFBOR•RconvertsVintofeedbackcontrolcurrentIFBFBC•Clampcircuit(D,C,R)a

8、lso:CLAMPCLAMPFB–LimitsvoltageacrossMOSFETduetoleakage