Active Clamp Forward Converter and TI Solutionsppt课件

ActiveClampForwardConverterandTISolutions April2009 Outline ForwardconverterandtransformerresettechniquesActiveClampForwardConverterOperationanalysisDesignguidelineTIsolutionsforActiveClampForwardConverter ForwardConverter ForwardconverterisanIsolatedBuckconverter MagnetizingCurrent VIN tONisconstantinregulationduetoVolt SecondbalancingThepeakmagnetizingcurrentisconstantoverlinevoltagechangeswhenthesupplyisinregulationThemagnetizingenergymustberemovedeachcyclefromthetransformeroritwillsaturate TransformerReset TransformerresetmustbecompletedeveryswitchingcycleThemagnetizingcurrentmustbereturnedtozeroeverycycleorthetransformerwillsaturateResettechniquesare RCD ClampResetwindingusinginputvoltageResonantresetActiveResettechnique ThemagnetizingenergyislostasheatintheRCDThemagnetizingenergyisreturnedtothesystemintheResonantResetandActiveClampapproaches TraditionalTransformerResetTechniques AdvantagesLowComplexityLowVoltageStressRecycledInductiveEnergyDisadvantagesHard switching50 DutyCycleLimitLowerTransformerTurnsRatioIncreasedTransformerLeakage Advantages 50 DutyCycleSimpleTransformerDesignDisadvantagesHard switchingHigherVoltageStressDissipativeInductiveEnergyClampRLosses A ThirdWinding B RCDClamp C ResonantReset AdvantagesFewestComponentsSimpleTransformerDesignRecycledInductiveEnergyReducedEMI 50 DutyCycleDisadvantagesHard switchingParasiticLandCVariationHigherVoltageStressSelf DrivenSynchronousRectificationnotIntuitive ActiveClampandResetTechnique AdvantagesNaturalZVSforHighestEfficiencyFixedFrequencyOperationHigherFrequencyOperationPossible 50 DutyCycleRecyclesInductiveEnergyReducedEMISelf DrivenSynchronousRectificationDisadvantagesAdditionalMOSFETSwitchRequiredforActiveResetGateDriveCircuitryComplexPrecisionMaxDutyCycleClampisCriticalAdvancedPWMControlTechniqueRequired IRESET IRESET OR High Side Flyback ActiveClamp AdvantagesVclLowerthanLow SideClampQ2isN Channel MoreSelectionDisadvantagesGateDriveTransformerforQ2 Low Side Boost ActiveClamp AdvantagesDirectGatedrivetoQ2PreciseDelayTimingforZVSSameVdsasHigh SideDisadvantagesHigherClampCapacitorVoltagethanHigh SideQ2isP Channel LessSelection OperationModeAnalysis S1ON D1ONPrimarysidetransfersenergytotheload OperationModeAnalysis S1OFF D1ONPrimarysidecurrentchargesupswitchnodevoltageContinuestransferringenergytotheload OperationModeAnalysis SwitchingnodebecomeshigherthaninputvoltageSecondaryfreewheelingLeakageinductorresonateswithjunctioncapacitors OperationModeAnalysis SwitchingnodeclampedbyclampcapacitorvoltageS2bodydiodeconductsLeakageinductordischargedbyclampcapacitor OperationModeAnalysis Leakageinductorcurrentbecomesthesameasmagnetizinginductorcurrent TransformerstopstransferringcurrentSCturnsONwithZVS OperationModeAnalysis SCturnsOFFLeakageandmagnetizinginductorsresonantwithjunctioncapacitorsSwitchnodevoltagedecreases OperationModeAnalysis SwitchnodevoltagedropslowerthaninputvoltageSecondarysidefreewheelingDifferencebetweenmagnetizingcurrentandleakageinductorcurrentistransferredtosecondaryside OperationModeAnalysis SwitchnodedropsbelowgroundS1bodydiodeconductingS1canbeturnedonwithZVS WhatisZeroVoltageSwitching ZVS ZeroVoltageSwitchingisatechniquethatpositionsthemainswitchtohavezerovoltageacrossitsdrain to sourcebeforeturn onZVSminimizesturn onswitchinglossofthemainMOSFETpowerswitchZVSlosslesslyrecyclesthetransformer smagnetizingcurrent andvoltageontheparasiticcapacitanceZVSenableshigherswitchingfrequencyZVSreducesEMI RFIZVSreducesgatedrivechargeneededforMOSFETswitching ZeroVoltageSwitchingusesparasiticelementsaspartoftheresonanttankcircuit Idealvs RealTransformer EL EC Thestoredmagnetizingenergy EL MUSTbeGREATERthantheopposingcapacitiveenergy EC orZVSdoesNOToccur NOZVS LimitationforZVS ZeroVoltageSwitchingDesignConsideration ClampswitchcanalwaysrealizeZVSbecausethetransformerneedstoreset onlyneedtosetupappropriatedelaytimeMainswitchZVScanbeachievedbasedonMagnetizinginductanceMagnetizingcurrentneedstobelargerthanthereflectedloadcurrentDifficultforheavyloadconditionEasytoachieveZVSatlightloadconditionLeakageinductanceNeedstodischargethejunctioncapacitorbecausecurrentchangedirectionExtrainductanceisneededforsoftswitching ZVSDesignConsideration DesignthetransformerbasedonitsmaximumefficiencyDoaniterationprocesstofindthesuitablemagnetizinginductanceandleakageinductanceDecreasemagnetizinginductancecouldincreasethemagnetizingcurrent andhelprealizeZVS whileitincreasesconductionlossIncreaseleakageinductorcanhelponZVS whileitequivalentlyintroducedutycycleloss whichrequiresincreaseturnsratioanddiodevoltageratingItisatradeoffbetweentheswitchinglossesandconductionlossesActiveclamprecycleslargeportionofthestoredenergyandimprovessystemefficiencyevenwhenZVSisnotachievedDon tforceZVS itmayhurtyourefficiency ActiveResetPWMControllersUCC2891 2 3 4 7ACurrentModeUCC3580 1 2 3 4VoltageMode ActiveReset ClampBenefits HigherefficiencybyreducingswitchinglossesRecyclesmagnetizingenergyBHcurvecanextendintothirdquadrantNodissipativeprimary sidesnubbersLossless zerovoltageturn onoftheswitchesSquarewaveanddeadtimeallowsself drivensynchronousrectificationLowerEMIwithcontrolleddV dttransitionsTheclamp resetswitchonlyhandlesmagnetizingcurrent notloadcurrentDedicatedPWMcontrollersareavailable Features CurrentModeControlCyclebyCycleCurrentLimitDedicatedAUXOutputProgrammableTimingDelayPreciseMaxDutyCycleClampProgrammableInternalSlopeCompensationSynchronizableOscillatorHVInternalStart UpDeviceProgrammableSoft StartProgrammableLineMonitorTrueDrive2ASink SourceOutput UCC2891 2 3 4 7ACurrentModeActiveClampandResetPWMController UniqueProtectionFunctions AccuratemaximumdutyratioevenwhensynchronizedInputvoltagemonitoringwithuserprogrammablehysteresisAdvancedclampvoltagemanagementtopreventtransformersaturationOver voltage maintainclampvoltagebydisablingOUTandAUXoutputsatthesametimeUnder voltage UVLO dischargingclampcapacitorbeforere startClampdischargeisPWMmodulated seetimingdiagramFullcyclesoft start CSSmustbedischargedbelow0 5VHiccupmodeoperationthroughbias JFETcontrolCurrentmodecontrol limitonprimarycurrent UCC2891 2 3 4 7PWMControllersforActiveClampandResetinForwardConverters UCC2897A GateDriverswithProgrammableDelay MainMOSFETDrive Pin13 OUTClampMOSFETDrive Pin12 AUX TrueDriveSource Sink 2A 2A ThedelaybetweenmainswitchingandclampswitchissetupbysingleresistortogroundonRDELpin PWMconfigurationpins MaximumOn time Pin2 RONMinimumOff time Pin3 ROFF SwitchingFrequencyandMaximumDutyCycle Byusingthisconfiguration theerrorcausedbythethresholds voltagereferencesandtimingcapacitorsarecancelledoutThemaximumdutycycletoleranceisdeterminedbyexternalresistortolerance Synchronization Frequencysynchronization SYNCUCC2891 2 3 4 Pin5 unidirectionalUCC2897 Pin7 bidirectional Synctohigherfrequency Syncpulsewidthisbetween50nsand 1 Dmax xTsync Dmaxischangedtohigher ifpulsewidth 1 Dmax xTsync GroundthePinifnotused Evenwithsynchronization themaximumdutycycleisstillclampedbyitsownRONandROFFresistorvalues CurrentSensingandSlopeCompensation Currentsensingandslopepins Currentsensing Pin7 CSUCC2891 3 0 75V cycle by cyclecurrentlimit UCC2892 4 1 27V cycle by cyclecurrentlimit UCC2897A 0 5V HiccupOCP 0 75V UseexternalRCtofilteroutthenoiseCurrentslopecompensation Pin8 RSLOPEMinimizeexternalparasiticcapacitanceandinductance forcurrentmodecontrol becomingvoltagemodecontrol LineOVandUVProtection LineUV OVsensingpins LineUV Pin15 LINEUVUCC2897 Pin18 LINEOV Pin16 LINEOVUCC2897 Pin19 where VDDandBias ICbiaspins HVstart Pin16 VIN AvailableinUCC2891 3 7 HVstartvoltagebetween18Vand110V Provides15mAstartcurrentICbias Pin14 VDD UVLOON13 5V UVLOOFF8 5V Operation8 5Vto14 5V Biasingcurrentneeded3mA drivingcurrent OtherFunctions Soft start Shut down Pin10 SS SD FeedbackLoopControl Pin9 FB Controlvoltagerangebetween1 25Vand4 5V FB4 5V maximumdutycycleThelowerofSSandFBdeterminesthecontrol Referencevoltage Pin4 VREF Loadcapabilitymaximum5mA Atypical0 1to0 33uFbypassingcapacitorneeded UCC2891 2 3 4 7APWMControllersforActiveClampandResetinForwardConverters ActiveClampsingle endedforwardconverterintelecomapplications UCC2897AEVM UCC2891EVM EVMSpecs Vin 36Vto75V Vo 3 3V Io 30A Po 100W EVMLineUVoffVoltageRingingacrossQ1 ObservationVoltageringingacrossQ1whenlineUVhappensandtheringingmaydamageQ1 Causeoftheringing Un controlledsecondaryself drivensynchronousrectifierdrivesQ1andQ2fromcyclingtheenergystoredintheprimaryinputandclampcapacitorsandthesecondaryoutputfiltercapacitors Solutions Controlledsecondarysynchronousrectifier Softstop RatingQ1withhigheravalancheenergy Q1withhighervoltagerating Oscillationfromsecondaryenergyfeedbacktotheprimaryfromself drivenSRduringpowershutdownLineUV powershutdown T1a andc Q4onandreverseLocurrent T1b andd Q3on energytransfertoprimary Magnetizingcurrentreduction T1a andc Loop MechanismoftheOscillationduringPowerShutdown T1 PowerShutdown LineUV Oscillation Oscillationappearsduringpowershutdownwithoutsoftstop Oscillationdoesnotappearduringpowershutdownwithsoftstop Withsoftstop bothQ1andQ2arecontrolledduringpowerdown Thesecondarystoredenergywillbedischargedincontrolledmanner Theoscillationiseliminated LoadTransientRingingacrossQ1withSecondarySelfDrivenSynchronousRectifier Causesfastloopcompensationhighmaximumdutycycle SolutionsslowerloopcompensationlowermaximumdutycycleHighervoltagerating Vds gates Vo UCC2897APersistencetime 19 6ns LM5025 6Persistencetime 1