an-990

  ApplicationNoteAN-990IGBTCharacteristicsTableofContentsPage1.GateDriveRequirements..................................................................................22.SwitchingTrajectoriesandSafeOperatingArea..............................................73.ConductionLosses............................................................................................94.LossesinHardSwitching..................................................................................105.Trade-offbetweenConductionandSwitchingLosses:DeviceOptimization.....126.ThermalDesign.................................................................................................147.ReplacingMOSFETswithIGBTs......................................................................158.GuidelinesonParalleling..................................................................................15ThisapplicationnotecoverssomeofthemajorissuesnormallyencounteredinthedesignofanIGBTpowercircuits.ItisthecompaniontoAN-983,IGBTCharacteristics,whichcoversthedetailsofthede-vice,ratherthanitsapplication.  1.GatedriveRequirementsA.ImpactoftheimpedanceofthegatedrivecircuitonswitchinglossesAsshownintheequivalentcircuitofFigure1,theIGBTconsistsofaPNPdrivenbyanN-ChannelMOSFETinapseudo-Darlingtonconfiguration.ThegatedrivecircuitcontrolsdirectlytheMOSFETchanneloftheIGBTand,throughthedraincurrentoftheMOSFET,thebasecurrentofitsbipolarportion.Sincetheturn-onchar-acteristicsofanIGBTaredetermined,toalargeextent,byitsMOSFETportion,theturn-onlosseswillbesig-nificantlyaffectedbythegatedriveimpedance.Turn-offcharacteristics,ontheotherhand,arechieflydeter-minedbytheminoritycarrierrecombinationmechanism,whichisonlyindirectlyaffectedbytheMOSFETturn-off.Asaresult,gatechargeparametersofanIGBTdonotprovidemeaningfulindicationofswitchingperfor-mance,astheydoinpowerMOSFETs.Gatechargeremains,however,ausefulparametertodesignthegatedrivecircuit.Figure1.IGBTequivalentcircuit.TheterminalcalledcollectorisactuallytheemitterofaPNPtransistor.TheMOSFETdrivesthebaseofthePNPanddeterminestheturn-onspeedoftheIGBTanditsvoltagedrop.AnincreaseingatedriveimpedanceprolongstheMillerplateauanddelaysthecurrentfall.TheimpactofthegatedriveimpedanceontotalswitchinglossesdependsonthedesignoftheIGBTanditsspeed.Theimpactonturn-onlossesisappreciableforallIGBTsfromInternationalRectifier,regardlessofspeed.Theimpacton  turn-offlossesdependsonthespeedofthedeviceanditstechnology:trenchIGBTandhighspeedIGBTsaremoresensitivetogatedriveimpedance.Inanyevent,additionalgatedriveimpedancehasalowermar-ginalimpact,i.e.thesameamountofadditionaldriveimpedancewillhavealowereffectifthegatedriveim-pedanceisalreadyhigh.Gatedriveimpedanceisfrequentlyincreasedforapracticalreason:toreducethecurrentspikeandringingatturn-oncausedbythereverserecoveryofthediode.Thisresistancecanbesafelybypassedwithananti-paralleldiodetoreducetheturn-offlosses,asshowninFigure28ofAN-978.Inseveralinstancesajudicioususeofthisnetworkmayactuallyreducetheturn-onlosses.Thespecificdependenceoftheswitchingenergyongatedriveresistanceisshowninmostdatasheets.B.ImpactofthegatedriveimpedanceonnoisesensitivityInMOS-gateddevices(IGBTs,FETs,etc.)anydv/dtthatappearsonthecollectoriscoupledtothegatethroughacapacitivedividerconsistingoftheMillercapacitanceandthegate-to-emittercapacitance(Figure2a).Figure2a.Adv/dtonthecollectorofthelowerIGBTisreflectedtothegatethroughthecapacitivedividerCrs-s/Cgss.Thespikemaybehighenoughtoturn-ontheIGBT,dependingontheratioofthesetwocapacitancesandtheinternalimpedanceofthegatedrive(ZG).  Ifthegateisnotsolidlyclampedtotheemitter,alargeenoughdv/dttakesthegatevoltagebeyonditsthresh-oldandtheIGBTconducts.AstheIGBTgoesintoconductionitclampsthedv/dtthatiscausingittoconductsothatthegatevoltagenevergoesmuchbeyondthethresholdvoltage,asitisevidentfromFigure2b.Theendresultisalimitedamountofshoot-throughcurrent,withanincreaseinpowerdissipation.Noticethatthe“shoot-throughcurrent”inFigure2bincludesacapacitivechargingcurrentthatcannotbecompletelyseparatedfromtheshoot-throughcurrent.Theshoot-throughcurrentonlyflowsafterthegateex-ceedsthethresholdvoltage(approximately3to5V),whilethecapacitivecurrentflowsassoonasthedv/dtappearsonthecollector.Figure2b.Thedv/dtonthecollectorofthelow-sideIGBTiscoupledtothegateandcausessomeshoot-throughcurrenttoflow.Partoftheshoot-throughcurrentchargesthedevicecapacitances.Toreducenoisesensitivityandtheriskofthisdv/dt-inducedturn-on,thegatedriveimpedancemustbeverylowwhenthedeviceisintheoff-stateandthegatevoltageisclosetozero.ThesmallPNPtransistorshowninFigure2aisfrequentlyusedtoclampapositive-goingdv/dtthatappearsonthegate.Gatedrivecircuitsinhighpowerapplicationswingpositive(+15)tonegative(-5or-15V).Thisprovidesanaddedmeasureofnoiseimmunityandimprovesswitchingperformance.However,thisrequiresadditionaliso-latedsuppliesforthehigh-sideIGBTsandincreasesthecostofthegatedrive.Iftheonlyconcernisimmunity