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Chapter27：SnubberCircuits27-1FunctionandTypesofsnubberCircuits27-2DiodesSnubber27-3SnubberCircuitsforThyristors27-4NeedforSnubberswithTransistors27-5Turn-offSnubber27-6Overvoltagesnubber27-7Turn-onSnubber27-8SnubbersforBridgeCircuitConfigurations27-9GTOSnubberConsiderations27-1OverviewofSnubberCircuitsforHard-SwitchedConvertersFunction:Protectsemiconductordevicesby:Limitingdevicevoltagesduringturn-offtransientsLimitingdevicecurrentsduringturn-ontransientsLimitingtherate-of-rise(di/dt)ofcurrentsthrough thesemiconductordeviceatdeviceturn-onLimitingtherate-of-rise(dv/dt)ofvoltagesacross thesemiconductordeviceatdeviceturn-offShapingtheswitchingtrajectoryofthedeviceasit turnson/offTypesofSnubberCircuits1. UnpolarizedseriesR-Csnubbers ? Usedtoprotectdiodesandthyristors2. PolarizedR-Csnubbers ? Usedasturn-offsnubberstoshapetheturn-off switchingtrajectoryofcontrolledswitches. ? Usedasovervoltagesnubberstoclampvoltages appliedtocontrolledswitchestosafevalues. ? Limitdv/dtduringdeviceturn-off3. PolarizedL-Rsnubbers ? Usedasturn-onsnubberstoshapetheturn-on switchingtrajectoryofcontrolledswitches. ? Limitdi/dtduringdeviceturn-on27-2NeedforDiodeSnubberCircuit1Thecauseofovervoltages

TheovervoltagesoccurduetothestrayorleakageinductanceLσinserieswiththediodeandthesnap-offofthediodereverserecoverycurrent.-+DfRsCsLsVdIoSwIrrdtVdLs=IottiDf(t)DiodevoltagewithoutsnubberdtLsdiLsvDf(t)diDfVd27-2NeedforDiodeSnubberCircuit2overvoltageprotectionIntroducingaRs-CssnubbercircuitIntroducingabaselinecapacitanceCbaseandbaselineresistanceRbase,-+DfRsCsLsVdIoSw3PerformanceofR-CSnubberVmax=themaximumreversevoltageacrossthediode.ThereisanoptimumvalueofRs=Ropt=1.3RbasethatminimizesVmax.0123012VdVmaxRsRbaseRsIrrVdCs=CbaseRs,optRbase=1.34Implementation(2)Step-down,half-bridgeandfull-bridgeconvertersmaynotrequireadiodesnubberifLσisminimizedbypropercircuitlayout.(1)Theconvertersthoseconsistofanisolationtransformerthatintroducedasubstantialleakageinductanceneeddiodesnubbercircuits.(3)Sinle-phaseline-frequencydiodeconverters.+Vs-+Vd-isD1D4D3D2Ls+Vs-+Vd-isD1D4D3D2LsLfLfNoneedfordiodesnubbersIfthereverse-recoverycurrentofD1snapsoff,thenD4providesapathfortheinductivecurrent,andreverse-voltageacrossD1isclampedtotheVd.Needfordiodesnubbers+Vs-+Vd-isD1D4D3D2LsLsmoothsnubber27-3SnubberCircuitforThyristors3-phasethyristorcircuitwithsnubbers1Thecauseofovervoltages

TheovervoltagesoccurduetotheseriesinductanceLc(linereactances+transformerleakageinductance)andthesnap-offofthethyristorreverserecoverycurrent.id135246+++---RsABCPLsvcnvanvbnLsLsCsPhase-to-neutralwaveformsavanwt1vbnvLL=vbn-van=vbaEquivalentCircuitforSCRSnubberCalculationsiLc+Vba(ωt1)=1.44VLL-CsRsiT1T1afterRecoveryT3on2LcPAEquivalentcircuitafterT1reverserecoveryAssumptionsComponentValuesforThyristorSnubber27-4NeedforSnubberswithControlledSwitchesSnubbercircuitsareusedtoprotectthetransistorsbyimprovingtheirswitchingtrajectory.Threebasictypesofsnubbers:1Turn-offsnubbers2Turn-onsnubbers3OvervoltagesnubbersStep-downconverterSwitchcurrentandvoltagewaveformsL1,L2,L3=strayinductanceLσ=L1+L2+L3t0t1t3t4t5t6VdI0iSWI0vSWAtt=t0,theswitchvoltagebeginstorise,Att=t1,thediodebeginstoconduct,Att=t3,thevoltagecomesdowntoVd,Att=t4,theswitchcurrentbeginstorise,Att=t5,thefreewheelingdioderecovers,Att=t6,theswitchvoltagedecreasestozero.SwitchingtrajectoryofswitchBothvoltageandcurrentoftheswitcharehighsimultaneouslyatturn-onandturn-off,thuscausingahighinstantaneouspowerdissipation.ThestrayinductancesresultinovervoltagebeyondVd.Thediodereverse-recoverycurrentcausesovercurrentbeyondIo.turn-onturn-offidealizedswitchinglocit0VdiswvswOvervoltageatturn-offduetostrayinductanceOvercurrentatturn-onduetodiodereverserecoveryt1t3t4t5t6Fig.27-11(b)27-5Turn-offSnubberforControlledSwitchesStep-downconverterwithturn-offsnubberThegoalofaturn-offsnubberistoprovideazerovoltageacrosstheswitchwhilethecurrentturnsoff.DsshortsoutRs

duringSwturn-off.DuringSwturn-on,Dsreverse-biasedandCsdischargesthroughRs.DfDsCsRsVdIo+-iDfiCsTurn-offsnubberSwFig.27-12(a)Equivalentcircuitduringswitchturn-off. Simplifyingassumptions(1)Nostrayinductance(2)iSW(t)=Io(1-t/tfi)(3)iSW(t)uneffectedbysnubbercircuitCsVdiswDfIoIo-iswFig.27-12(b)CircuitwaveformsforvariousvaluesofCsiCsVdvCsiswIoCs<Cs1iswiswiDfiDfiDftfitfitfiVdVdCs=Cs1Cs>Cs1Fig.27-12(c)Theturn-offSwitchingtrajectoryCs<Cs1Cs=Cs1Cs>Cs1IoVdvswiswRBSOAFig.27-13Switchingtrajectoryduringturn-offwithvariousvaluesofsnubbercapacitanceCs.BenefitsofSnubberResistanceatSwitchTurn-oniswtrr0DischargeofCs

VdIovswtrit2tri+trrFig.27-14(a)Turn-onwithRs=0EnergystoredonCsdissipatedinSw.ExtraenergydissipationinSwbecauseoflengthenedvoltagefalltime.vswIrrIoIrriDftrrVdRsiswFig.27-14(b)Turn-onwithRs>0EnergystoredonCsdissipatedinRsratherthaninSw.Voltagefalltimekeptquite short.BenefitsofSnubberResistanceatSwitchTurn-onAllthecapacitorenergyisdissipatedintheresistor,whichiseasiertocoolthantheswitch.Noadditionalenergydissipationoccursintheswitchduetotheturn-offsnubber.Thepeakcurrenttheswitchmustconductisnotincreasedduetotheturn-offsnubber.EffectofTurn-offSnubberCapacitanceEnergydissipationWR=dissipationinresistorWT=dissipationinswitchWtotal=WR+WTWbase=0.5VdIotfi00.20.40.60.8100.20.40.60.811.21.4Cs/Cs1WR/WbaseWtotal/WbaseWT/WbaseTurn-offSnubberDesignProcedureSelectionofCsMinimizeenergydissipation(WT)inSwitchatturn-onMinimizeWR+WTKeepswitchinglocuswithinRBSOAReasonablevalueisCs=Cs1SelectionofRsLimiticap(0+)=Vd/Rs<Irr(27-31)UsuallydesignerspecifiesIrr<0.2Io,soVd/Rs=0.2Io(27-32)Snubberrecoverytime(switchinon-state)CapacitorvoltagevCs=Vd·exp(-t/RsCs)(27-33)TimeforvCstodropto0.1Vdis2.3RsCsSwitchmustremainonforatimeof2.3RsCs,thus,tonstate>2.3RsCs(27-34)27-6OvervoltageSnubberStep-downconverterwithovervoltage

snubbercomprisedofDov,Cov,andRov.Overvoltage

snubberlimitsovervoltage(duetostrayInductance)acrossSwasitturnsoff.+-VdLsDfIoRovCovDovSwFig.27-16(a)Overvoltagesnubber

tfi=switchcurrentfalltimekVd=overvoltageonSwwithoutCov.kVdvswiLσtfi0VdIoSwitchwaveformswithoutCovEquivalentcircuitwhileswitchcurrentdecaystozerovCov(0+)=VdiLs(0+)=IoiLs(t)=Icos[]

Ls

CovtoOperationofOvervoltageSnubberDov,CovprovidealternatepathforinductorcurrentasSwturnsoff.SwitchcurrentcanfalltozeromuchfasterthanLσcurrent.DfforcedtobeonbyIoafterSwisoff.Equivalentcircuitafterturn-offofSw.+-VdLsRovCovDoviLsvCov+-Fig.27-16(b)+-VdLsCoviLs+-vCovFig.27-16(c)withCov.vswtfi0iLσLσCovchargingRovCovdischargeVdOperationofOvervoltageSnubberEnergytransferfromLσtoCovSwitchwaveformswithCovOperationofOvervoltageSnubber27-7Turn-onSnubberStep-downconverterwithturn-onsnubberSnubberreducesVswatswitchturn-onduedropacrossinductorLs.Willlimitrate-of-riseofswitchcurrentifLsissufficientlylarge.Turn-onSnubberVd+-LsDLsDfRLsIoSwVd-LsDLsRLsIoSwDf+SnubbercircuitFig.27-17(a)and(b)Switchingtrajectorywithandwithoutturn-onsnubber.Turn-onSnubberiswvswVdIoLsdiswdtWithout