功率半导体器件基本原理第10章精.docx

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）

1030FUNDAMENTALSOFPOWERSEMICONDUCTORDEVICES

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

1032FUNDAMENTALSOFPOWERSEMICONDUCTORDEVICES

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