功率半导体器件基本原理第10章精.docx
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功率半导体器件基本原理第10章精
Chapter10
Synopsis
PowerdevicesarerequiredforsystemsthatoperateoverabroadspectrumofpowerlevelsandfrequenciesasdiscussedinChap.1.Avarietyofpowerrectifierandtransistorstructureswerediscussedinpreviouschaptersforservingtheseapplications.Althoughthebipolarpowertransistorandthethyristorwerethefirsttechnologiesdevelopedforthispurpose,theyhavebeenreplacedbypowerMOSFETandIGBTstructuresinmodernapplicationsduetotheresultingsimplificationofthecontrolcircuitandeliminationofsnubbers.Thechoiceoftheoptimumdevicesuitableforanapplicationdependsuponthedevicevoltageratingandthecircuitswitchingfrequency.Fromthepointofviewofpresentingaunifiedtreatment,itisconvenienttoanalyzeatypicalpulse-width-modulated(PWM)motorcontrolcircuitasanexamplebecauseitisutilizedforbothlow-voltageapplications,suchasdiskdrivesincomputers,andhigh-voltageapplications,suchasthedrivetraininhybridelectricvehiclesandelectriclocomotives.
10.1TypicalH-BridgeTopology
ThecontrolofmotorsusingPWMcircuitsistypicallyperformedbyusingtheH-bridgeconfigurationshowninFig.10.1.Inthisfigure,thecircuithasbeenimplementedusingfourIGBTdevicesastheswitchesandfourP–i–Nrectifiersasthefly-backdiodes.ThisisthecommonlyusedtopologyformediumandhighpowermotordriveswheretheDCbusvoltageexceeds200V.WhentheH-bridgetopologyisusedforapplicationsthatoperatefromalowDCbusvoltage,itistypicallyimplementedusingfourpowerMOSFETdevicesastheswitchesandfourSchottkyrectifiersasthefly-backdiodes.
ThedirectionofthecurrentflowinthemotorwindingcanbecontrolledwiththeH-bridgeconfiguration.IfIGBT-1andIGBT-4areturnedonwhile
B.J.Baliga,FundamentalsofPowerSemiconductorDevices,doi:
10.1007/978-0-387-47314-7_10,©SpringerScience+BusinessMedia,LLC2008
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maintainingIGBT-2andIGBT-3intheirblockingmode,thecurrentinthemotorwillflowfromtheleftsidetotherightsideinthefigure.ThedirectionofthecurrentflowcanbereversedifIGBT-3andIGBT-2areturnedonwhilemaintainingIGBT-1andIGBT-4intheirblockingmode.Alternately,themagnitudeofthecurrentflowcanbeincreasedordecreasedbyturningontheIGBTdevicesinpairs.Thismethodallowssynthesisofasinusoidalwaveformacrossthemotor
1
Synopsis1029
Thetypicalwaveformsforthecurrentandthevoltageacrossthepowertransistorandthefly-backdiodeareillustratedinFig.10.2duringjustonecycleofthePWMoperation.Thesewaveformshavebeenlinearizedforsimplificationoftheanalysis.2Thecyclebeginsattimet1whenthetransistoristurnedonbyitsgatedrivevoltage.Priortothistime,thetransistorissupportingtheDCsupplyvoltageandthefly-backdiodeisassumedtobecarryingthemotorcurrent.Asthetransistorturnson,themotorcurrentistransferredfromthediodetothetransistorduringthetimeintervalfromt1tot2.InthecaseofhighDCbusvoltages,whereP–i–Nrectifiersareutilized,thefly-backdiodewillnotbeabletosupportvoltageuntilthestoredchargeinitsdriftregionisremovedasdiscussedinChap.5.Toachievethis,theP–i–Nrectifiermustundergoitsreverserecoveryprocess.Duringreverserecovery,substantialreversecurrentflowsthroughtherectifierwithapeakvalueIPRreachedattimet2.Thelargereverserecoverycurrentproducessignificantpowerdissipationinthediode.Moreover,thecurrentintheIGBTattimet2isthesumofthemotorwindingcurrentIMandthepeakreverserecoverycurrentIPR.Thisproducessubstantialpowerdissipationinthetransistorduringtheturn-ontransient.Thepowerdissipationinboththetransistorandthediodeisthereforegovernedbythereverserecoverycharacteristicsofthepowerrectifier.
Thepowertransistoristurnedoffattimet4allowingthemotorcurrenttotransferfromthetransistortothediode.Inthecaseofaninductiveload,suchasmotorwindings,thevoltageacrossthetransistorincreasesbeforethecurrentisreducedasillustratedinFig.10.2duringthetimeintervalfromt4tot5.Subsequently,thecurrentinthetransistorreducestozeroduringthetimeintervalfromt5tot6.Theturn-offdurationsaregovernedbythephysicsofthetransistorstructureasdiscussedinpreviouschapters.Consequently,thepowerdissipationinboththetransistorandthediodeduringtheturn-offeventisdeterminedbythetransistorswitchingcharacteristics.
Inadditiontothepowerlossesassociatedwiththetwobasicswitchingeventswithineachcycle,powerlossisincurredwithinthediodeandthetransistorduringtheirrespectiveon-stateoperationduetoafiniteon-statevoltagedrop.Itiscommonpracticetotradeoffalargeron-statevoltagedroptoobtainasmallerswitchinglossinthebipolarpowerdevices.Consequently,theon-statepowerlosscannotbeneglectedespeciallyiftheoperatingfrequencyislow.Theleakagecurrentforthedevicesisusuallysufficientlysmall,sothatthepowerlossintheblockingmodecanbeneglected.
10.2PowerLossAnalysis
Thetotalpowerlossincurredinthepowertransistorcanbeobtainedbysummingfourcomponents:
PL,T(total)=PL,T(on)+PL,T(off)+PL,T(turn-on)+PL,T(turn-off).(10.1)
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Thepowerlossincurredinthetransistorduringtheon-statedurationfromtimet3tot4isgivenby
PL,T(on)=t4−t3IMVON,T.T(10.2)
Thepowerlossincurredinthetransistorduringtheoff-statedurationbeyondtimet6untilthenextturn-oneventisgivenby
PL,T(off)=T−t6IL,TVDC.T(10.3)
Theleakagecurrent(IL,T)forthetransistorsisusuallyverysmallallowingthistermtobeneglectedduringthepowerdissipationanalysis.
Thepowerlossincurredinthetransistorduringtheturn-oneventfromtimet1tot3canbeobtainedbyanalysisofthesegmentsbetweenthetimeintervalst1tot2andt2tot3.Thepowerlossincurredduringthefirstsegmentisgivenby
PL,T−1(turn-on)=1t2−t1IPTVDC,2T(10.4)
wherethepeaktransistorcurrentisdependentonthepeakreverserecoverycurrentoftheP–i–Nrectifier:
IPT=IM+IPR.(10.5)
Inthepowerlossanalysis,itwillbeassumedthatthetimeduration(t2−t1)isdeterminedbythereverserecoverybehavioroftheP–i–Nrectifierandisindependentoftheoperatingfrequency.Thepowerlossincurredduringthesecondsegmentisgivenby
PL,T−2(turn-on)=1t3−t2⎛IPT+IM⎜2T⎝2⎞⎟VDC.⎠(10.6)
Inthepowerlossanalysis,itwillbeassumedthatthetimeduration(t3−t2)isalsodeterminedbythereverserecoverybehavioroftheP–i–Nrectifierandisindependentoftheoperatingfrequency.
Thepowerlossincurredinthetransistorduringtheturn-offeventfromtimet4tot6canbeobtainedbyanalysisofthesegmentsbetweenthetimeintervalst4tot5andt5tot6.Thepowerlossincurredduringthefirstsegmentisgivenby
PL,T−1(turn-off)=1t5−t4IMVDC.2T(10.7)
Thetimeinterval(t5−t4)isdeterminedbythetimetakenforthetransistorvoltagetorisetotheDCpowersupplyvoltage.Thistimedurationwasanalyzedforeach
Synopsis1031
transistorinthepreviouschapters.Thepowerlossincurredduringthesecondsegmentisgivenby
PL,T−2(turn-off)=1t6−t5IMVDC.2T(10.8)
Thetimeinterval(t6−t5)isdeterminedbythetimetakenforthetransistorcurrenttodecaytozero.Thistimedurationwasanalyzedforeachtransistorinthepreviouschapters.
Inasimilarmanner,thetotalpowerlossincurredinthepowerrectifiercanbeobtainedbysummingfourcomponents:
PL,R(total)=PL,R(on)+PL,R(off)+PL,R(turn-on)+PL,R(turn-off).(10.9)Thepowerlossincurredinthepowerrectifierduringtheon-statedurationfromtimet6totheendoftheperiodisgivenby
PL,R(on)=T−t6IMVON,R.T(10.10)
Inwritingthisexpression,itisassumedthatthecyclebeginsattimet1.Thepowerlossincurredinthepowerrectifierduringtheoff-statetimeduration(t4−t3)isgivenby
PL,R(off)=t4−t3IL,RVDC.T(10.11)
Theleakagecurrent(IL,R)forthepowerrectifierwillbeassumedtobeverysmall(evenforthesiliconSchottkyrectifier)allowingthistermtobeneglectedduringthepowerdissipationanalysis.
Thepowerlossincurredinthepowerrectifierduringtheturn-oneventfromtimet1tot3canbeobtainedbyanalysisofthesegmentsbetweenthetimeintervalst1tot2andt2tot3.Thepowerlossincurredduringthefirstsegmentismuchsmallerthanduringthesecondsegmentduetothesmallon-statevoltagedropforthepowerrectifiers.Thepowerlossincurredduringthesecondsegmentisgivenby
PL,R−2(turn-on)=1t3−t2IPRVDC.2T(10.12)
Thepowerlossincurredinthepowerrectifierduringtheturn-offeventfromtimet4tot6canbeobtainedbyanalysisofthesegmentsbetweenthetimeintervalst4tot5andt5tot6.Thepowerlossincurredduringthefirstsegmentisnegligibleduetothelowleakagecurrentforthepowerrectifier.Thepowerlossincurredduringthesecondsegmentisgivenby
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PL,R−2(turn-off)=1t6−t5IMVON,D.(10.13)2T
Thispowerlossisalsosmallduetothelowon-statevoltagedropofpowerrectifiers.
10.3LowDCBusVoltageApplications
Inthissection,theabovepowerlossanalysisisappliedtoamotorcontrolapplicationusingalowDCbusvoltagewithadutycycleof50%.TheDCbusvoltage(VDC)willbeassumedtobe20Vaspertainstothebackplanepowersourceindesktopcomputers.Inthiscase,thedeviceblockingvoltageratingistypically30V.Thecurrentbeingdeliveredtothemotorwinding(IM)willbeassumedtobe10A.Duetothelowblockingvoltagerequiredforthisapplication,itis