Fairchild 变压器设计

ApplicationNoteAN4140TransformerDesignConsiderationforoff-lineFlybackConvertersusingFairchildPowerSwitch(FPSTM)©2003FairchildSemiconductorCorporationRev.1.0.01.IntroductionForflybackcoverters,thetransformeristhemostimportantfactorthatdeterminestheperformancesuchastheefficiency,outputregulationandEMI.Contrarytothenormaltransformer,theflybacktransformerisinherentlyaninductorthatprovidesenergystorage,couplingandisolationfortheflybackconverter.Inthegeneraltransformer,thecurrentflowsinboththeprimaryandsecondarywindingatthesametime.However,intheflybacktransformer,thecurrentflowsonlyintheprimarywindingwhiletheenergyinthecoreischargedandinthesecondarywindingwhiletheenergyinthecoreisdischarged.Usuallygapisintroducedbetweenthecoretoincreasetheenergystoragecapacity.Thispaperpresentspracticaldesignconsiderationsoftransformersforoff-lineflybackconvertersemployingFairchildPowerSwitch(FPS).Inordertogiveinsighttothereader,practicaldesignexamplesarealsoprovided.2.GeneralTransformerdesignprocedure(1)ChoosethepropercoreCoretype:FerriteisthemostwidelyusedcorematerialforcommercialSMPS(Switchiedmodepowersupply)applications.VariousferritecoresandbobbinsareshowninFigure1.Thetypeofthecoreshouldbechosenwithregardtosystemrequirementsincludingnumberofoutputs,physicalheight,costandsoon.Table1showsfeaturesandtypicalapplicationofvariouscores.Figure1.Ferritecore(TDK)Table1.FeaturesandtypicalapplicationsofvariouscoresCoresize:Actually,theinitialselectionofthecoreisboundtobecrudesincetherearetoomanyvariables.Onewaytoselectthepropercoreistorefertothemanufacture'scoreselectionguide.Ifthereisnoproperreference,usethetable2asastartingpoint.Thecorerecommendedintable1istypicalfortheuniversalinputrange,67kHzswitchingfrequencyand12Vsingleoutputapplication.Whentheinputvoltagerangeis195-265Vac(Europeaninputrange)ortheswitchingfrequencyishigherthan67kHz,asmallercorecanbeused.Foranapplicationwithlowvoltageand/ormultipleoutputs,usuallyalargercoreshouldbeusedthanrecommendedinthetable.Table2.Corequickselectiontable(Foruniversalinputrange,fs=67kHzand12Vsingleoutput)CoreFeaturesTypicalApplicationsEEEI-LowcostAux.powerBatterychargerEFDEPC-LowprofileLCDMonitorEER-Largewindingwindowarea-VariousbobbinsformultipleoutputCRTmonitor,C-TVDVDP,STBPQ-Largecrosssectionalarea-RelativelyexpensiveOutputPowerEIcoreEEcoreEPCcoreEERcore0-10WEI12.5EI16EI19EE8EE10EE13EE16EPC10EPC13EPC1710-20WEI22EE19EPC1920-30WEI25EE22EPC25EER25.530-50WEI28EI30EE25EPC30EER2850-70WEI35EE30EER28L70-100WEI40EE35EER35100-150WEI50EE40EER40EER42150-200WEI60EE50EE60EER49AN4140APPLICATIONNOTE2©2002FairchildSemiconductorCorporationOncethecoretypeandsizearedetermined,thefollowingvariablesareobtainedfromthecoredatasheet.-Ae:Thecross-sectionalareaofthecore(mm2)-Aw:Windingwindowarea(mm2)-Bsat:Coresaturationfluxdensity(tesla)Figure2showstheAeandAwofacore.ThetypicalB-HcharacteristicsofferritecorefromTDK(PC40)areshowninFigure3.Sincethesaturationfluxdensity(Bsat)decreasesasthetemperatureincreases,thehightemperaturecharacter-isticsshouldbeconsidered.Ifthereisnoreferencedata,useBsat=0.3~0.35T.Figure2.WindowAreaandCrossSectionalAreaFigure3.TypicalB-Hcharacteristicsofferritecore(TDK/PC40)(2)Determinetheprimarysideinductance(Lm)ofthetransformerInordertodeterminetheprimarysideinductance,thefollowingvariablesshouldbedeterminedfirst.(Foradetaileddesignprocedure,pleaserefertotheapplicationnoteAN4137.)-Pin:Maximuminputpower-fs:SwitchingfrequencyofFPSdevice-VDCmin:MinimumDClinkvoltage-Dmax:Maximumdutycycle-KRF:Ripplefactor,whichisdefinedattheminimuminputvoltageandfullloadcondition,asshowninFigure4.ForDCMoperation,KRF=1andforCCMoperationKRF1.TheripplefactoriscloselyrelatedwiththetransformersizeandtheRMSvalueoftheMOSFETcurrent.EventhoughtheconductionlossintheMOSFETcanbereducedthroughreducingtheripplefactor,toosmallaripplefactorforcesanincreaseintransformersize.Consideringbothefficiencyandcoresize,itisreasonabletosetKRF=0.3-0.5fortheuniversalinputrangeandKRF=0.4-0.8fortheEuropeaninputrange.Meanwhile,inthecaseoflowpowerapplicationsbelow5Wwheresizeismostcritical,arelativelylargeripplefactorisusedinordertominimizethetransformersize.Inthatcase,itistypicaltosetKRF=0.5-0.7fortheuniversalinputrangeandKRF=1.0fortheEuropeaninputrange.Figure4.MOSFETDrainCurrentandRippleFactor(KRF)Withthegivenvariables,theprimarysideinductance,LmisobtainedasAwAwAwAwAeAeAeAe100500400300200800160000MagneticfieldH(A/m)FluxdensityB(mT)MagnetizationCurves(typical)Material:PC40100℃℃℃℃120℃℃℃℃60℃℃℃℃25℃℃℃℃I∆EDCIEDCRFIIK2∆=CCMoperation:KRF1I∆EDCIEDCRFIIK2∆=DCMoperation:KRF=1peakdsIpeakdsIDmaxDmaxLmVDCminDmax⋅()22PinfsKRF----------------------------------------------=(1)APPLICATIONNOTEAN41403©2002FairchildSemiconductorCorporationwhereVDCministheminimumDCinputvoltage,Dmaxisthemaximumdutycycle,Pinisthemaximuminputpower,fsistheswitchingfrequencyoftheFPSdeviceandKRFistheripplefactor.OnceLmisdetermined,themaximumpeakcurrentandRMScurrentoftheMOSFETinnormaloperationareobtainedasWiththechosencore,theminimumnumberofturnsforthetransformerprimarysidet