IGBT Tutorial 2.docx
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IGBT Tutorial 2.docx
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IGBTTutorial2
IGBTTutorialPart2-Static,dynamiccharacteristics
onathanDodge,P.E.,SeniorApplicationsEngineer;JohnHess,VicePresident,Marketing,Microsemi'sAdvancedPowerTechnology
3/13/20073:
05PMEDT
InPart1ofthistwo-parttutorial,IGBTswerereviewedandtradeoffsconsidered.WenowtakealookatanIGBTdatasheettogiveyouanin-depthunderstandingofitscharacteristics.
StaticElectricalCharacteristics
BVCES&mdashCollector-EmitterBreakdownVoltage
Measuringtheactualcollector-emitterbreakdownvoltageispracticallyimpossiblewithoutdestroyingthedevice.Therefore,BVCESisthecollector-emittervoltageatwhichnomorethanthespecifiedcollectorcurrentwillflowatthespecifiedtemperature.Thistrackstheactualbreakdownvoltage.
AsshowninFigure8,BVCEShasapositivetemperaturecoefficient.Atafixedleakagecurrent,anIGBTcanblockmorevoltagewhenhotthanwhencold.Infact,whencold,theBVCESspecificationislessthantheVCESrating.FortheexampleshowninFigure8,at-50°C,BVCESisabout93%ofthenominal25°Cspecification.
Figure8NormalizedBreakdownVoltagevs.JunctionTemperature
RBVCES—ReverseCollector-EmitterBreakdownVoltage
Thisisthereversecollector-emitterbreakdownvoltagespecification,i.e.,whentheemittervoltageispositivewithrespecttothecollector.AswithBVCES,RBVCESistheemitter-collectorvoltageatwhichnomorethanthespecifiedemittercurrentwillflowatthespecifiedtemperature.Atypicalvalueisabout15Volts,howeverRBVCESisoftennotspecifiedsinceanIGBTisnotdesignedforreversevoltageblocking.EventhoughintheoryanNPTIGBTcanblockasmuchreversevoltageasforwardvoltage,ingeneralitcannotduetothemanufacturingprocess.APTIGBTcannotblockverymuchreversevoltageduetothen+bufferlayer.
VGE(th)—GateThresholdVoltage
Thisisthegate-sourcevoltageatwhichcollectorcurrentbeginstoflow.Testconditions(collectorcurrent,collector-emittervoltage,junctiontemperature)arealsospecified.AllMOSgateddevicesexhibitvariationinVGE(th)betweendevices,whichisnormal.Therefore,arangeofVGE(th)isspecified,withtheminimumandmaximumrepresentingtheedgesoftheVGE(th)distribution.VGE(th)hasanegativetemperaturecoefficient,meaningthatasthedieheatsup,theIGBTwillturnonatalowergate-emittervoltage.Thistemperaturecoefficientistypicallyaboutminus12mV/C,thesameasforapowerMOSFET.
VCE(on)—Collector-EmitterOnVoltage
Thisisthecollector-emittervoltageacrosstheIGBTataspecifiedcollectorcurrent,gate-emittervoltage,andjunctiontemperature.SinceVCE(on)istemperaturedependent,itisspecifiedbothatroomtemperatureandhot.
Graphsareprovidedthatshowtherelationshipsbetweentypical(notmaximum)collector-emittervoltageandcollectorcurrent,temperature,andgateemittervoltage.Fromthesegraphs,acircuitdesignercanestimateconductionlossandthetemperaturecoefficientofVCE(on).ConductionpowerlossisVCE(on)timescollectorcurrent.ThetemperaturecoefficientistheslopeofVCE(on)versustemperature.NPTIGBTshaveapositivetemperaturecoefficient,meaningthatasthejunctiontemperatureincreases,VCE(on)increases.PTIGBTsontheotherhandtendtohaveaslightlynegativetemperaturecoefficient.Forbothtypes,thetemperaturecoefficienttendstoincreasewithincreasingcollectorcurrent.Ascurrentincreases,thetemperaturecoefficientofaPTIGBTcanactuallytransitionfromnegativetopositive.
ICES—CollectorCutoffCurrent
Thisistheleakagecurrentthatflowsfromcollectortoemitterwhenthedeviceisoff,ataspecifiedcollectoremitterandgate-emittervoltage.Sinceleakagecurrentincreaseswithtemperature,ICESisspecifiedbothatroomtemperatureandhot.LeakagepowerlossisICEStimescollector-emittervoltage.
IGES—gate-emitterleakagecurrent
Thisistheleakagecurrentthatflowsthroughthegateterminalataspecifiedgate-emittervoltage.
Dynamiccharacteristics
Figure9showsanequivalentIBGTmodelthatincludesthecapacitancesbetweentheterminals.Input,output,andreversetransfercapacitancesarecombinationsofthesecapacitances.SeeapplicationnoteAPT0103formoredetails.Testconditionstomeasurecapacitancesarespecifiedinthedatasheet.
Figure9IGBTCapacitancesCies—InputCapacitance
ThisistheinputcapacitancemeasuredbetweenthegateandemitterterminalswiththecollectorshortedtotheemitterforACsignals.Ciesismadeupofthegatetocollectorcapacitance(CGC)inparallelwiththegatetoemittercapacitance(CGE),or
Cies=CGE+CGC
Theinputcapacitancemustbechargedtothethresholdvoltagebeforethedevicebeginstoturnon,anddischargedtotheplateauvoltagebeforethedevicebeginstoturnoff.Therefore,theimpedanceofthedrivecircuitryandCieshaveadirectrelationshiptotheturnonandturnoffdelays.
Coes—outputcapacitance
ThisistheoutputcapacitancemeasuredbetweenthecollectorandemitterterminalswiththegateshortedtotheemitterforACvoltages.Coesismadeupofthecollectortoemittercapacitance(CCE)inparallelwiththegatetocollectorcapacitance(CGC),or
Coes=CCE+CGC
Forsoftswitchingapplications,Coesisimportantbecauseitcanaffecttheresonanceofthecircuit.
Cres—reversetransfercapacitance
Thisisthereversetransfercapacitancemeasuredbetweenthecollectorandgateterminalswiththeemitterconnectedtoground.Thereversetransfercapacitanceisequaltothegatetocollectorcapacitance.
Cres=CGC
Thereversetransfercapacitance,oftenreferredtoastheMillercapacitance,isoneofthemajorparametersaffectingvoltageriseandfalltimesduringswitching.Figure10showsanexamplegraphoftypicalcapacitancevaluesversuscollector-emittervoltage.Thecapacitancesdecreaseoverarangeofincreasingcollector-emittervoltage,especiallytheoutputandreversetransfercapacitances.Thisvariationisthe'raisond'tre'forgatechargedata,aswillbeexplained.
Figure10Capacitancevs.Collector-EmitterVoltage
VGEP—Plateauvoltage
Figure11showsthegate-emittervoltageasafunctionofgatecharge.Theturn-onsequencetraversesthiscurvefromlefttoright,turn-offtraversesfromrighttoleft.ThemethodformeasuringgatechargeisdescribedinJEDECstandard24-2.ThegateplateauvoltageVGEPisdefinedasthegate-emittervoltagewhentheslopeofthegate-emittervoltagefirstreachesaminimumduringtheturn-onswitchingtransitionforaconstantgatecurrentdrivecondition.Inotherwords,itisthegate-emittervoltagewherethegatechargecurvefirststraightensoutafterthefirstinflectioninthecurve,asshowninFigure11.Alternatively,VGEPisthegate-emittervoltageatthelastminimumslopeduringturn-off.
Theplateauvoltageincreaseswithcurrentbutnotwithtemperature.BewarewhenreplacingpowerMOSFETswithIGBTs.A10or12VoltgatedrivemightworkfineforahighvoltagepowerMOSFET,butdependinguponitsplateauvoltage,anIGBTathighcurrentmightswitchsurprisinglyslowlyornotevencompletelyturnonunlessthegatedrivevoltageisincreased.
Figure11vGEasaFunctionofGateChargeQge,Qgc,andQg—GateCharge
ReferringtoFigure11,Qgeisthechargefromtheorigintothefirstinflectioninthecurve,Qgcisthechargefromthefirsttosecondinflectioninthecurve(alsoknownasthe"Miller"charge),andQgGEequalsthepeakdrivevoltage.Gatechargevaluesvarywithcollectorcurrentandcollector-emittervoltagebutnotwithtemperature.Testconditionsarespecified.Inaddition,agraphofgatechargeistypicallyincludedinthedatasheetshowinggatechargecurvesforafixedcollectorcurrentanddifferentcollector-emittervoltages.Thegatechargevaluesreflectchargestoredontheinterterminalcapacitancesdescribedearlier.Gatechargeisoftenusedfordesigninggatedrivecircuitrysinceittakesintoaccountthechangesincapacitancewithchangesinvoltageduringaswitchingtransient.
Switchingtimes
SwitchingTimesandEnergies
SwitchingtimesandenergiesarenotalwayseasytopredictforIGBTs,soMicrosemiprovidesswitchingtimesandenergiesinthedatasheetforhard-switchedclampedinductiveswitching.Testcircuitsanddefinitionsareincludedineachdatasheet.Figure12showsatestcircuitusedtomeasureswitchingtimesandenergiesandFigure13showstheassociatedwaveformsanddefinitions.ThefollowingtestconditionsarespecifiedintheDynamicCharacteristicstable:
VCCinFigure12,inductorcurrent,gatedrivevoltage,gateresistance,andjunctiontemperature.NotethatgateresistanceincludestheresistanceofthegatedriverIC.Sinceswitchingtimesandenergiesvarywithtemperature(exceptEon1),dataisprovidedbothatroomtemperatureandhot.Graphsarealsooftenprovidedshowingtherelationshipsbetweenswitchingtimesandenergiestocollectorcurrent,junctiontemperature,andgateresistance.
Figure12InductiveSwitchingLossTestCircuit
Figure13Turn-onWaveformsandDefinitions
Ingeneral,turn-onspeedandenergyarerelativelyindependentoftemperature,oractuallyincreasinginspeed(decreasinginenergy)veryslightlywithincreasingtemperature.Diodereverserecoverycurrentincreaseswithtemperature,resultingintheincreaseinEon2withtemperature.Eon1andEon2aredefinedbelow.Turn-offspeeddecreaseswithincreasingtemperature,correspondingtoanincreaseinturn-offenergy.Switchingspeed,bothturn-onandturn-off,decreaseswithincreasinggateresistance,correspondingtoanincreaseinswitchingenergies.Switchingenergycanbescaleddirectly
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