IGBT tutorial 1.docx

IGBT tutorial 1.docx

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IGBTtutorial1

IGBTtutorial:

Part1-Selection

JonathanDodge,P.E.,SeniorApplicationsEngineer;JohnHess,VicePresident,Marketing,Microsemi'sAdvancedPowerTechnology

3/8/20074:

07PMEST

Theinsulatedgatebipolartransistors（IGBTs）combinesaneasilydrivenMOSgateandlowconductionloss,andisquicklydisplacingpowerbipolartransistorsasthedeviceofchoiceforhighcurrentandhighvoltageapplications.Thebalanceintradeoffsbetweenswitchingspeed,conductionloss,andruggednessisnowbeingfinelytunedsothatIGBTsareencroachinguponthehighfrequency,highefficiencydomainofpowerMOSFETs.Infact,theindustrytrendisforIGBTstoreplacepowerMOSFETsexceptinverylowcurrentapplications.Part1helpsyouunderstandthetradeoffsandhelpswithIGBTdeviceselection,applicationandarelativelypainlessoverviewofIGBTtechnology.Part2providesanexamplewalkthroughofIGBTdatasheetinformation.

HowtoselectanIGBT

Thissectionisintentionallyplacedbeforethetechnicaldiscourse.AnswerstothefollowingsetofburningquestionswillhelpdeterminewhichIGBTisappropriateforaparticularapplication.ThedifferencesbetweenNonPunch-Through（NPT）andPunch-Through（PT）devicesaswellastermsandgraphswillbeexplainedlater.

1.Whatistheoperatingvoltage?

ThehighestvoltagetheIGBThastoblockshouldbenomorethan80%oftheVCESrating.

2.Isithardorsoftswitched?

APTdeviceisbettersuitedforsoftswitchingduetoreducedtailcurrent,howeveraNPTdevicewillalsowork.

3.Whatisthecurrentthatwillflowthroughthedevice?

Thefirsttwonumbersinthepartnumbergivearoughindicationoftheusablecurrent.Forhardswitchingapplications,theusablefrequencyversuscurrentgraphishelpfulindeterminingwhetheradevicewillfittheapplication.Differencesbetweendatasheettestconditionsandtheapplicationshouldbetakenintoaccount,andanexampleofhowtodothiswillbegivenlater.Forsoftswitchingapplications,theIC2ratingcouldbeusedasastartingpoint.

4.Whatisthedesiredswitchingspeed?

Iftheansweris"thehigher,thebetter",thenaPTdeviceisthebestchoice.Again,theusablefrequencyversuscurrentgraphcanhelpanswerthisquestionforhardswitchingapplications.

5.Isshortcircuitwithstandcapabilityrequired?

Forapplicationssuchasmotordrives,theanswerisyes,andtheswitchingfrequencyalsotendstoberelativelylow.AnNPTdevicewouldberequired.Switchmodepowersuppliesoftendon'trequireshortcircuitcapability.

IGBToverview

AnN-channelIGBTisbasicallyanN-channelpowerMOSFETconstructedonap-typesubstrate,asillustratedbythegenericIGBTcrosssectioninFigure1.（PTIGBTshaveanadditionaln+layeraswellaswillbeexplained.）Consequently,operationofanIGBTisverysimilartoapowerMOSFET.Apositivevoltageappliedfromtheemittertogateterminalscauseselectronstobedrawntowardthegateterminalinthebodyregion.Ifthegate-emittervoltageisatorabovewhatiscalledthethresholdvoltage,enoughelectronsaredrawntowardthegatetoformaconductivechannelacrossthebodyregion,allowingcurrenttoflowfromthecollectortotheemitter.（Tobeprecise,itallowselectronstoflowfromtheemittertothecollector.）Thisflowofelectronsdrawspositiveions,orholes,fromthep-typesubstrateintothedriftregiontowardtheemitter.ThisleadstoacoupleofsimplifiedequivalentcircuitsforanIGBTasshowninFigure2.

Figure1N-ChannelIGBTCrossSection

Figure2IGBTSimplifiedEquivalentCircuits

ThefirstcircuitshowsanN-channelpowerMOSFETdrivingawidebasePNPbipolartransistorinaDarlingtonconfiguration.ThesecondcircuitsimplyshowsadiodeinserieswiththedrainofanN-channelpowerMOSFET.Atfirstglance,itwouldseemthattheonstatevoltageacrosstheIGBTwouldbeonediodedrophigherthanfortheN-channelpowerMOSFETbyitself.ItistrueinfactthattheonstatevoltageacrossanIGBTisalwaysatleastonediodedrop.However,comparedtoapowerMOSFETofthesamediesizeandoperatingatthesametemperatureandcurrent,anIGBTcanhavesignificantlyloweronstatevoltage.ThereasonforthisisthataMOSFETisamajoritycarrierdeviceonly.Inotherwords,inanNchannelMOSFETonlyelectronsflow.Asmentionedbefore,thep-typesubstrateinanN-channelIGBTinjectsholesintothedriftregion.Therefore,currentflowinanIGBTiscomposedofbothelectronsandholes.Thisinjectionofholes（minoritycarriers）significantlyreducestheeffectiveresistancetocurrentflowinthedriftregion.Statedotherwise,holeinjectionsignificantlyincreasestheconductivity,ortheconductivityismodulated.TheresultingreductioninonstatevoltageisthemainadvantageofIGBTsoverpowerMOSFETs.

Nothingcomesforfreeofcourse,andthepriceforloweronstatevoltageisslowerswitchingspeed,especiallyatturn-off.Thereasonforthisisthatduringturn-offtheelectronflowcanbestoppedratherabruptly,justasinapowerMOSFET,byreducingthegate-emittervoltagebelowthethresholdvoltage.However,holesareleftinthedriftregion,andthereisnowaytoremovethemexceptbyvoltagegradientandrecombination.TheIGBTexhibitsatailcurrentduringturn-offuntilalltheholesaresweptoutorrecombined.Therateofrecombinationcanbecontrolled,whichisthepurposeofthen+bufferlayershowninFigure1.Thisbufferlayerquicklyabsorbstrappedholesduringturn-off.NotallIGBTsincorporateann+bufferlayer;thosethatdoarecalledpunch-through（PT）,thosethatdonotarecallednonpunch-through（NPT）.PTIGBTsaresometimesreferredtoasasymmetrical,andNPTassymmetrical.

TheotherpriceforloweronstatevoltageisthepossibilityoflatchupiftheIGBTisoperatedwelloutsidethedatasheetratings.LatchupisafailuremodewheretheIGBTcannolongerbeturnedoffbythegate.LatchupcanbeinducedinanyIGBTthroughmisuse.ThusthelatchupfailuremechanisminIGBTswarrantssomeexplanation.

Basicstructure

ThebasicstructureofanIGBTresemblesathyristor,namelyaseriesofPNPNjunctions.ThiscanbeexplainedbyanalyzingamoredetailedequivalentcircuitmodelforanIGBTshowninFigure3.

Figure3IGBTModelShowingParasiticThyristor

AparasiticNPNbipolartransistorexistswithinallNchannelpowerMOSFETSandconsequentlyallN3channelIGBTs.Thebaseofthistransistoristhebodyregion,whichisshortedtotheemittertopreventitfromturningon.Notehoweverthatthebodyregionhassomeresistance,calledbodyregionspreadingresistance,asshowninFigure3.TheP-typesubstrateanddriftandbodyregionsformthePNPportionoftheIGBT.ThePNPNstructureformsaparasiticthyristor.IftheparasiticNPNtransistoreverturnsonandthesumofthegainsoftheNPNandPNPtransistorsaregreaterthanone,latchupoccurs.LatchupisavoidedthroughdesignoftheIGBTbyoptimizingthedopinglevelsandgeometriesofthevariousregionsshowninFigure1.

ThegainsofthePNPandNPNtransistorsaresetsothattheirsumislessthanone.Astemperatureincreases,thePNPandNPNgainsincrease,aswellasthebodyregionspreadingresistance.VeryhighcollectorcurrentcancausesufficientvoltagedropacrossthebodyregiontoturnontheparasiticNPNtransistor,andexcessivelocalizedheatingofthedieincreasestheparasitictransistorgainssotheirsumexceedsone.Ifthishappens,theparasiticthyristorlatcheson,andtheIGBTcannotbeturnedoffbythegateandmaybedestroyedduetoover-currentheating.Thisisstaticlatchup.Highdv/dtduringturn-offcombinedwithexcessivecollectorcurrentcanalsoeffectivelyincreasegainsandturnontheparasiticNPNtransistor.Thisisdynamiclatchup,whichisactuallywhatlimitsthesafeoperatingareasinceitcanhappenatamuchlowercollectorcurrentthanstaticlatchup,anditdependsontheturn-offdv/dt.Bystayingwithinthemaximumcurrentandsafeoperatingarearatings,staticanddynamiclatchupareavoidedregardlessofturn-offdv/dt.Notethatturn-onandturn-offdv/dt,overshoot,andringingcanbesetbyanexternalgateresistor（aswellasbystrayinductanceinthecircuitlayout）.

PunchthroughvsNPT

PTversusNPTtechnology

Conductionloss

Foragivenswitchingspeed,NPTtechnologygenerallyhasahigherVCE（on）thanPTtechnology.ThisdifferenceismagnifiedfurtherbyfactthatVCE（on）increaseswithtemperatureforNPT（positivetemperaturecoefficient）,whereasVCE（on）decreaseswithtemperatureforPT（negativetemperaturecoefficient）.However,foranyIGBT,whetherPTorNPT,switchinglossistradedoffagainstVCE（on）.HigherspeedIGBTshaveahigherVCE（on）;lowerspeedIGBTshavealowerVCE（on）.Infact,itispossiblethataveryfastPTdevicecanhaveahigherVCE（on）thanaNPTdeviceofslowerswitchingspeed.

Switchingloss

ForagivenVCE（on）,PTIGBTshaveahigherspeedswitchingcapabilitywithlowertotalswitchingenergy.Thisisduetohighergainandminoritycarrierlifetimereduction,whichquenchesthetailcurrent.

Ruggedness

NPTIGBTsaretypicallyshortcircuitratedwhilePTdevicesoftenarenot,andNPTIGBTscanabsorbmoreavalancheenergythanPTIGBTs.NPTtechnologyismoreruggedduetothewiderbaseandlowergainofthePNPbipolartransistor.ThisisthemainadvantagegainedbytradingoffswitchingspeedwithNPTtechnology.ItisdifficulttomakeaPTIGBTwithgreaterthan600VoltVCESwhereasitiseasilydonewithNPTtechnology.AdvancedPowerTechnologydoesofferaseriesofveryfast1200VoltPTIGBTs,thePowerMOS7��'IGBTseries.

Temperatureeffects

ForbothPTandNPTIGBTs,turn-on