反激式转换器13 flyback isolatednotes 3非隔离.pptx

Flybackconverter(1)FlybackconverterisextensivelyusedinpowersuppliesintherangeuptoafewhundredWatts–allTVs,somePCsetcLookatnon-isolatedversionfirst–easiertounderstandNon-isolatedFlybackConverterCircuitLOADQDLCOVSCSVOVLiOiQiDiLNote:QisshownasaBJTforeaseofdrawing–muchmorelikelytobeaMOSFETinpracticeapracticalimplementationhasacontrolcircuit(notshown)thatprovidestheON/OFFsignalforQviaitsbase(orgate)

Flybackconverter(2)AssumptionsforanalysisQandDideal–switchinzerotime–negligiblevoltagedropwhenconductingIdealL–noresistivepowerlossinwindings,nopowerlossduetohysteresisoreddycurrentsIdealCOSupplytoconvertermodelledasanidealvoltagesourceForthepurposesofcalculatingiL(andiD,iQ),assumeVOissmooth(constant)Thisisvalidsincemostpowersupplieswithhaveanoutputvoltageripplespecificationofafew%orlessripple=?VO/mean(VO)CSisneededinanypracticalconverter(seelaterforwhy)–butdoesnotaffecttheanalysissinceVSassumedideal–ignorefornow

Flybackconverter(3)QisoperatedwithFixedSwitchingFrequency(fS=1/T)Dutycycle(d=TON/T)isvariedtocontroltocircuitandachieveregulationWewillanalysethecircuitINSTEADYSTATEassumingdisconstant–andthenlookatwhathappensfordifferentvaluesofdAnalysisisbasedonthe“FourRules”–seeearliernotesCircuitoperation:WhenQisON,DisreversebiassedandisOFFEquivalentcircuitis:

Flybackconverter(4)Circuitoperation(cont–QON,DOFF):VL=VS?iLincreasesataCONSTANTratediL/dt=VS/LEnergyisstoredinL[W=(1/2)LiL2]COhastosupplyalloftheloadcurrentiOEnergyissuppliedfromCOtotheloadCOmustbebigenoughsothatthereductioninVOisverysmallNowturnQOFF,iLCOMMUTATES(seeearlierhandout)toDandDturnsON?equivalentcircuites:

Flybackconverter(5)Circuitoperation(cont–QOFF,DON):iLcontinuestoflow(Dremains