adlsongForwardConverterForwardConverter1開關(guān)管導(dǎo)通

LdiL/dt=Vin–Vo

I=(Vin–Vo)ton/L2開關(guān)管關(guān)斷

LdiL/dt=–Vo

I=–Votoff/LI:輸出電感伏秒平衡.輸出電壓:Vo=Vin1Conventionalclamp&resettechnical–MagnetizingcurrentresetbyanextrawindingthatinparallelwithPri-winding2RCDtypeclamp&resettechnicalLosslesssnubber-LCDSnubberCircuitSelfresonantresetSoftSwitching–ActiveClamp/ResetTheforwardconvertertransfersdirectlytheenergyfromtheinputsourcetotheloadduringtheon-timeofthepowerswitch.Duringoff-timeofthepowerswitch,theenergyisfreewheelingthroughtheoutputinductorandtherectifierD2,likeinachopperAforwardregulatorcanberealizedwithasingleswitchstructureorwithadoubleswitchstructure,accordingtothewaytheenergystoredinthetransformerprimaryinductanceisdemagnetized.Forward

convertersarecommonlyusedforoutputpowerupto250WNVinVout=dVinsingleswitches,andupto1kWindoubleswitchstructures.Singleswitchvs.doubleswitchforwardForwardConverterInthesingleswitchforward,themagnetizingenergystoredintheprimaryinductanceisrestoredtotheinputsourcebyademagnetizationwindingNd.Mostcommonly,theprimaryandthedemagnetizationwindingshavethesamenumberofturns.So,atturn-off,thepowerswitchhastowithstandtwicetheinputvoltageduringthedemagnetizationtime,andthen,oncetheinputvoltage.Thedemagnetizationandprimarywindingshavetobetightlycoupledtoreducethevoltagespike-morethanthetheoretical2Vin-occuringatturn-offacrossthepowerswitch.ForwardConverterForwardConverterPowerSwitch:VDDS>Vinmax(1+n)+leakageinductancespikeIDrms>1.2Pin/VinminDmaxIDrms>1.2Pin/VinminDmax1/2Rectifiers:ForwardD1:VRRM>Vinmax/n+leakageinductancespikeIF(AV)>IoDmaxFreewheelingD2:VRRM>Vinmax(Vo+VFD)/VinminDmaxIF(AV)>IoDemagnetizationD3:VRRM>(1+N3/Np)Vinmax(Vo+VFD)/VinminDmaxIF(AV)>ImagpeakDmax/2ForwardConverterDoubleswitchforward,alsocalledasymmetricalhalfbridgeforward,themagnetizingenergystoredintheprimaryinductanceisautomaticallyreturnedtothebulkcapacitorbythetwodemagnetizationdiodesD1andD2.ThetwopowerswitchesanddemagnetizationdiodeshavetowithstandonlyoncetheinputvoltageVin.Asforthedoubleswitchflyback,theasymmetricalhalfbridgeneedsafloatinggatedriveforthehighsideswitch.PowerSwitch:VDDS>VinmaxIDrms>1.2Pin/VinminD1/2Rectifiers:ForwardD1:VRRM>Vinmax(Vo+VFD)/VinminDmaxIF(AV)>IoDmaxFreewheelingD2:VRRM>Vinmax(Vo+VFD)/VinminDmaxIF(AV)>IoForwardConverterDualSwitchesConverters

SelfReverseVoltageClamp&

MagnetismCurrentResetMaxDutybeenlimitsandnotexceeds50%becauseofthemagnetizingcurrentresetLm=LPondualswitchesconvertersForwardConverterTheForwardConverterAReviewofTransformers:Voltageappliedacrossprimaryistransformedintoavoltageacrossthesecondary,withpolarityfollowingthedottedterminals,inaccordancewiththerelation:Vp/Vs=Np/NsCurrentgoingintothedottedprimaryterminalistransformedandgoesoutofthesecondaryterminal,followingtherelation:Ip/Is=Ns/NpAnidealtransformerdoesnotstoreenergy,hence:Pin=PoutorPp=Ps.ForwardConverterTheforwardtopologyisoneofthemostcommonlyused,andhasseveralvariations,themostbasicofwhichisshowninFigure1.9.Theforwardconverterisessentiallyanisolatedversionofthebuckconverteroperatinginthedirectmodeandthebasicsingleswitchversionshowncanbesuccessfullyoperatedoverawidepowerrange.Duetothetransformer,theforwardtopologycanbeusedaseitheranuporadownconverter,althoughthemostcommonapplicationisdownconversion.Thepowertransferredtothesecondaryduringtheon-timeisconductedthroughdiodeD1totheoutputLCfilter.Duringtheoff-time,thesecondarycurrentcirculatesthroughdiodeD2.Thetransformerisresetduringtheofftimebymeansoftheauxiliarywinding,Naux,anddiodeD3.Themainadvantagesoftheforwardtopologyareitssimplicityandflexibility.OutputRippleFrequencyFRelativeCostLowOnecommonvariationontheforwardconverteristhetwotransistorforward.Thisconfigurationaddsanotherswitchelementontheothersideofthetransformerprimaryandtwoclampingdiodes,onefromeachsideoftheprimarytotheoppositeinputvoltageterminal.Inexchangeforthisadditionalcomplexitythetwotransistorforwardreducesthevoltagestressontheswitchelements.ForwardConverter輔助繞組復(fù)位減小Dmax可以開關(guān)管的最大電壓,但會導(dǎo)致次級二極管的電壓應(yīng)力大.寬電壓輸入時:

Dmax=0.45andNp=Nr.開關(guān)管的最大電壓最大占空比RCD復(fù)位DissipativetypeClamp/ResettechnicalVerylowcostdesignofresetcircuitwithdiscretecomponentsR,CandDMax.operatingdutycouldbeslightlyhigherthan50%(upto55~65%),dependingthemagnetizingcurrentresetbyRCconstantCriticalpoint:Higherconsumptionbecausetheenergystorageon“C”shoulddissipatesbypassivecomponent“R”Thetransformervolt/secondbalanceshouldtradeoffbyadjustthevalueof“C”and“R”forClamp/ResetIncaseVDS≤2Vinbutverypossiblehigherthan2VINinworstcaseatopenloopoperatingandmaxdutyexceed50%ToavoidtransformersaturationneedtoevaluatetheDsetmaxofPWMincaseofshortcircuit

RCDresetThesnubbercapacitorvoltageisfixedandalmostindependentoftheinputvoltage,theMOSFETvoltagestresscanbereducedcomparedtotheresetwindingapproachwhentheconverterisoperatedwithawideinputvoltagerange.AnotheradvantageofRCDresetmethodisthatitispossibletosetthemaximumdutyratiolargerthan50%withrelativelylowvoltagestressonMOSFETcomparedtoauxiliarywindingresetmethod,whichresultsinreducedvoltagestressonthesecondaryside.ThemaximumvoltagestressthenominalsnubbercapacitorvoltageElementsofprimaryzenerclampcircuitD1currentPrimarywindingleakagecurrentwithreversecurrentpulloutduetoslowdiodeD1/C2LeakagespikeNormalizedandinvertedsecondarywindingvoltageMagnetizingvoltage

ringatdrainMagnetizingcurrentLCDResetSelfResonantReset要高壓的MOSFETAddAuxiliarySwitchSaBasicRCDClampRemoveResistorComparedwithRCD,CccandischargetoinverselydeeplymagnetizethemagnetizationinductanceviaSc.ActiveClampResetForwardConverter

Sa:

auxiliaryswitch

S1:

mainswitch

Da:

intrinsicparasiticdiodeinsideSa

D1:intrinsicparasiticdiodeinsideS1

C1:intrinsicparasiticcapacitorinsideSa

andexternalcapacitorActiveClampResetForwardConverter

1.Usuallyneedhighsidedriveforhighsideactiveswitch.NeedPchannelMOSFETforLowsidedrive.2.Deadtimeadjustmentinbetweentwoswitches3.ResonantTransitionmodeneedcontroltheinductanceofthemaintransformer.4.ActiveswitchneedselecthighvoltageMOSFETbecausetheenergystoreinCc5.AllowmuchhigherefficiencyoperationbecauseenergyoftransformermagnetizingrecyclesandZVTofthemainswitch.6.Lowervoltageclampwouldreducedthevoltagestressonmainswitch,similarpowertransfertoconventionalsquarewaveswitching7.ReducedEMI/RFIviasoftswitching8.Operateatfixedswitchingfrequency9.Dutycyclesbeyond50%maxareobtainable10.ActivelyresetsmaintransformertothirdquadrantofBHcurveActiveClampResetForwardConverter

S1SavCsipimiCcVinVCcActiveClampResetForwardConverter

M1:t0~t1

S1/DrisonandSa/DcisoffActiveClampResetForwardConverter

LfissohighthatitcanbeactedasconstantcurrentsourceIoM2:t1~t2

S1/Sa/DcisoffandDrisonNormalPWM

Im:Risefrom–to0to+Resonant

Im:RiseM3:t2~t3

S1/Sa/DrisoffandDcison

VNpisnegativesoDrisoffandIocannotreflectedtoprimaryside.

C1andLmresonateandVc1resonateduptoVin+VCcatt3andDaison.ActiveClampResetForwardConverter

Resonant

Im:DownResonant

Im:DownM4:t3~t4

S1/DrisoffandDa/Dcison

VNpis–VCcsoImisdemagnetizedto0andDaisturnedoffnaturallyatt4.SaisZVSduringt3~t4.M5:t4~t5

Sa/DcisonandSa/Drisoff

VNpis-VCcandImincreasesinverselyfrom0.Saisturnedoffatt5.ActiveClampResetForwardConverter

Resonant

Im:RiseinverselyM6:t5~t6

S1/Sa/DrisoffandDcison

LmandC1resonateandC1dischargeandVC1declinedtoVinatt6.Resonant

Im:RiseinverselyM7(veryshort):t6~t7

Sa/S1isoffandDr/Drison

VNpispositivesoDrison.PrimarycurrentisnotenoughtoprovideloadcurrentsoDcisstillonandsecondarysideisshortcircuit.PrimaryinductanceismagnetizedbyVin-VC1andiLmincreasesfromnegative.LeakageinductanceLkismagnetizedbyVin-VC1anditscurrentrisessharplyandisreflectedtosecondaryside.SoiDrincreasesgraduall