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MitigationMethodsforParasiticTurn-oneffectduetoMillerCapacitorWhitePaperIntroductionOneofthecommonproblemsfacedwhenswitchinganIGBTisparasiticturn-onduetoMillercapacitor.Thiseffectisnoticeableinsinglesupplygatedriver(0to+15V).Duetothisgate-collectorcoupling,ahighdV/dttransientcreatedduringIGBTturn-offcaninduceparasiticturn-on(Gatevoltage,VGE)whichispotentiallydangerous(Figure1).RDRIVERRGMillerCapacitorCCGVGE=(RDRIVER+RG)*ICGICG=CCG*dVCE/dt+HVDCDRIVERS1S2GateVoltageSpikeHighdV/dtFigure1.BottomIGBTParasiticTurn-OnduetoMillerCapacitorParasiticTurn-onduetoMillerCapacitorWhenturningontheupperIGBT,S1inahalf-bridge,avoltagechangedVCE/dtoccursacrossthelowerIGBT,S2.AcurrentflowsthroughtheparasiticMillercapacitorCCGofS2,thegateresistorRGandinternaldrivergateresistor,RDRIVER.Figure1showsthecurrentflowthroughthecapacitor.Thiscurrentvaluecanbeapproximatedbythefollowingformula:dtdVCICECGCG(1)Thiscurrentcreatesavoltagedropacrossthegateresistor.IfthisvoltageexceedstheIGBTgatethresholdvoltage,aparasiticturn-onoccurs.DesignersshouldbeawarethatrisingIGBTchiptemperaturewouldleadtoaslightreductionofgatethresholdvoltage,usuallyintherangeofmV/°C.Thisparasiticturn-oncanalsobeseenonS1whenS2isturnedon.ParasiticTurn-onMitigationSolutionsTherearethreeclassicalsolutionstotheaboveproblem;thefirstbeingtovarythegateresistor(Figure2),secondtoaddacapacitorbetweengateandemitter(Figure3)andthirdtousenegativegatedrive(Figure4).AfourthsimpleandeffectivesolutionistheActiveClamptechnique(Figure5).Separategateresistorforturn-onandturn-offTheon-gateresistor,RGONinfluencesthevoltageandcurrentchangeduringIGBTturn-on.Increasingthisresistorreducesthevoltageandcurrentchangesbutincreasesswitchinglosses.Parasiticturn-oncanbepreventedbyreducingtheoff-gateresistor,RGOFF.ThesmallerRGOFFwillalsoreduceswitchinglossduringIGBTturn-off.However,thetradeoffishigherover-shootVceandoscillationduringturn-offduetostrayinductances.Duetotheabove,somedesignoptimizationofbothgateresistorswouldberequired(Figure2).RGON+HVDCCCGRDRIVERDRIVERRGOFFFigure2.SeparateOnandOffGateresistorAdditionalgateemittercapacitortoshunttheMillercurrentTheadditionalcapacitorCGbetweengateandemitterwillinfluencetheswitchingbehavioroftheIGBT.CGistotakeupadditionalchargeoriginatingfromtheMillerca-pacitance.DuetothefactthatthetotalinputcapacitanceoftheIGBTisCG||CCG,thegatechargenecessarytoreachthethresholdvoltageisincreased(Figure3).RG+HVDCGateCapacitorCGECCGRDRIVERDRIVERFigure3.AdditionalcapacitorbetweengateandemitterDuetothisadditionalcapacitor,therequireddriverpowerisincreasedandtheIGBTshowshigherswitchinglossesforthesameRG.NegativePowerSupplytoIncreaseThresholdVoltageTheusageofnegativegatevoltagetosafelyturn-offandblockIGBTistypicallyusedinapplicationwithnominalcurrentabove100A.Duetocost,negativegatevoltageisoftennotutilizedinIGBTapplicationbelow100A.Figure3showsatypicalcircuitusingnegativesupplyvoltage.RG+HVDC+_RDRIVERDRIVERVGE=(RDRIVER+RG)*ICG+VNEG-SUPPLYCCGFigure4.NegativeSupplyVoltageActiveMillerClampSolutionInordertoavoidRGoptimizationproblem,efficiencylossduetoCGandadditionalcostfornegativesupplyvoltage,anothermeasuretopreventtheunwantedIGBTturn-onisproposedbyshortingthegatetoemitterpath.Thiscanbeachievedbyanadditionaltransistorbetweengateandemitter.This‘switch’shortsthegate-emitterregionafteracertainVGEisreached.TheoccurringcurrentsacrosstheMillercapacitanceareshuntedbythetransis-torinsteadofflowingthroughtheoutputdriverpin,Vout.ThistechniqueiscalledActiveMillerClamp(Figure5).RG+HVDCCCGRDRIVERDRIVERAdditionalTransistorTheadditionofnegativesupplyvoltageincreasesdesigncomplexityandsizeofthedesign.Figure5.ActiveMillerClampingusingadditionaltransistorTheadditionofthetransistorwouldincreasedesigncom-plexityofthedrivercircuit.CONCLUSIONTable1showsthecomparisonbetweenthefourtechniquespresentedearlier.Table1.ComparisonofParasiticTurn-onDuetoMillerCapacitorSolutionsEffectonreducingMillercapacitorparasiticturn-onCostofSolutionSwitchingLossesOtherConsiderationReducingRGOFF+Low▼Highover-voltage,OptimizationrequiredAdditionalCGE+Low▲Efficiencyreduce,OptimizationrequiredNegativeSupplyVoltage++High▼IncreasedesigncomplexityActiveMillerClamp++Medium▼IncreasedesigncomplexityInrecentyears,integratedIGBTgatedrivershaveincludedtheactivemillerclampsolutionalongwithdesaturationprotectionandunder-voltagelock-out,namelyAvagoTechnologiesACPL-331JandACPL-332J.Thishashelpedtoreducedesigncomplexityandproductsizeformanypowerdesignersandindustrial/consumermanufacturers.References1.AvagoGateOptocouplerDatasheetACPL-332J2.AvagoApplicationNote,ActiveMillerClamp3.SemikronApplicationManual,Chapter3.5Driver4.InternationalRectifierApplicationNoteAN983.IGBTCharacteristicForproductinformationandacompletelistofdistributors,pleasegotoourwebsite:©007AvagoTechnologiesLimited.Allrightsreserved.AV0-0599EN-August8,007