Transformer Design Consideration for off-line Flyback Conver.pdf

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1、www.fairchildsemi.comApplicationNoteAN4140TransformerDesignConsiderationforoff-lineFlybackConvertersusingFairchildPowerSwitch(FPSTM)1.IntroductionForflybackcoverters,thetransformeristhemostimportantCoreFeaturesTypicalApplicationsfactorthatdeterminestheperformancesuchastheEE-LowcostA

2、ux.powerefficiency,outputregulationandEMI.ContrarytotheEIBatterychargernormaltransformer,theflybacktransformerisinherentlyanEFD-LowprofileLCDMonitorinductorthatprovidesenergystorage,couplingandisolationEPCfortheflybackconverter.Inthegeneraltransformer,theEER-LargewindingwindowareaCR

3、Tmonitor,C-TVcurrentflowsinboththeprimaryandsecondarywindingat-VariousbobbinsformultipleDVDP,STBthesametime.However,intheflybacktransformer,theoutputcurrentflowsonlyintheprimarywindingwhiletheenergyPQ-Largecrosssectionalareainthecoreischargedandinthesecondarywindingwhilethe-Relative

4、lyexpensiveenergyinthecoreisdischarged.UsuallygapisintroducedTable1.Featuresandtypicalapplicationsofvariousbetweenthecoretoincreasetheenergystoragecapacity.coresThispaperpresentspracticaldesignconsiderationsoftransformersforoff-lineflybackconvertersemployingCoresize:Actually,theinit

5、ialselectionofthecoreisFairchildPowerSwitch(FPS).Inordertogiveinsighttotheboundtobecrudesincetherearetoomanyvariables.Onereader,practicaldesignexamplesarealsoprovided.waytoselectthepropercoreistorefertothemanufacture'scoreselectionguide.Ifthereisnoproperreference,usethetable2asastar

6、tingpoint.Thecorerecommendedintable1istypicalfortheuniversalinputrange,67kHzswitching2.GeneralTransformerdesignprocedurefrequencyand12Vsingleoutputapplication.Whentheinputvoltagerangeis195-265Vac(Europeaninputrange)orthe(1)Choosethepropercoreswitchingfrequencyishigherthan67kHz,asmal

7、lercorecanCoretype:Ferriteisthemostwidelyusedcorematerialbeused.Foranapplicationwithlowvoltageand/ormultipleforcommercialSMPS(Switchiedmodepowersupply)outputs,usuallyalargercoreshouldbeusedthanapplications.Variousferritecoresandbobbinsareshowninrecommendedinthetable.Figure1.Thetypeo

8、fthecoreshouldbecho