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1、HowtoReducetheSpikeVoltageforSynchronousRectifierBuckConverterBYJamesYehandHunterHoAbstractSynchronousrectifierbuckoperationinhighswitchfrequencywillgeneratethespikevoltageandringingduetothestrayinductanceandcapacitanceexistinpracticalPCB.Inthispaperwilldis
2、cussthesourceofthespikevoltageindetailandhowtoselectanddesignsuitablesnubbercircuittodecreaseit.Theoreticalanalysis,simulationandexperimentalresultcouldprovidesomeinformation,forrelatedengineer.1.IntroductionSynchronousrectifierbuckiswidelyusedinthepowersup
3、plyofmanyelectronicequipment,duetoithasfastresponse,simpletopology,higherefficiencyforalowvoltageandhighcurrentneed.Butitstillhasaseriousproblem“EMI”.Whiletheswitchturnonandturnoffinahighfrequency,thestrayinductanceandcapacitanceexistinpracticalPCB,itwillge
4、neratehighfrequencyringingandspikevoltage.Thehighfrequencyringingandspikevoltagewillinterferencetheequipmentordamagetheswitch.Inordertosolvethisproblem,therearesomemethodssuchas“softswitching”,“snubbercircuit”,and”addEMIfilter”.Inthispaper,wewilldiscusswhat
5、causespikeandringingindetail,andhowtoplaceasimplepassivecomponent(snubbercircuit)tosolvethisdrawback.2.SourseanalysisofspikevoltageandhowtosuppressFigure1showsatypicalsynchronousrectifierBuckconverter,whereLs1andLs2arethestrayinductanceduetothefinitesizeoft
6、hecircuitlayout.Inpracticalcircuitsoperation,whiletheVg2signalhasbeenlowandVg1hasnotreachhighyet,itiscalledthedeadtime.TheloadcurrentflowsthroughthebodydiodeD2andstrayinductanceLs2.Duringthedeadtime,itemtheVg1hasbeenalreadyhigh,thentheM1hasbeenturnedonandD2
7、starttoaddreversevoltage.Butthebodydiodewithreversevoltageexistareverserecoverycurrent(Irr),itpracticalturnoffhastoacrossareverserecoverytime(trr)andhasthesnap-offcurrentduringtheextremelyshorttimets.Thesnap-offbodydiodecurrentasshowinFigure2canproducethetr
8、ansienthighpeakvoltage,andringing.ItmaybegenerateEMItoinferencethecontrollerorothersequipmentandthehighpeakvoltagemaybetodamagetheMOSFET(M2).Fordifferentpositionofstrayinductance,ahighfrequencycapacito
