Power BJT.docx

Power BJT.docx

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PowerBJT

PowerBJT-BiploarJunctionTransistor

BJThasthreeterminals:

Powertransistorofnpntypesareeasytomanufactureandcheaper.

Usedinhigh-voltageandhighcurrentapplication.

Working

Thebase-emitterdiode（forward）actsasaswitch.

Whenv1>0.7itletstheelectronsflowtowardcollector,sowecancontrolouroutputcurrent（Ic）withtheinputcurrent（Ib）byusingtransistors.

PowerTransistorStructure

VerticalCrossSection

STEADYSTATECHARACTERISTICSOFSIGNALLEVELBJT

IBversusVBEinputcharacteristics

ICversusVCEoutputcharacteristics

SteadyStateCharacteristicsofPowerTransistor

Intransistor,basecurrentiseffectivelytheinputcurrentandcollectorcurrentisoutputcurrent.

OUTPUTCHARACTERISTCS

CurveI?

IB=0

CurveII?

IB?

0

InitialpartofthecurveII,characterizedbyVCE?

calledsaturationregion.Inthisregiontransistoractslikeswitch.

Flatpartofthecurve?

withincreasingVCE,almostICisconstant?

calledactiveregion.Inthisregiontransistoractsasamplifier.

Almostverticallyrisingcurveisthebreakdownregion,whichmustbeavoidedat

allcost.

TheloadlineIC=（VCC-VCE）/RC.ThelinejoiningAandB.

WhentransistorisON,VCE=0,theIC=VCC/RC.Thiscollectorcurrentisshownby

pointA

WhentransistorisOFF,orincut-off,Vccappearsacrosscollector-emitterand

thereisnocollectorcurrent.ThisvalueisindicatedbypointB.

TRANSFERCHARACTERISTICS

Forwardcurrentgaina=Ic/IE

Theratioofcollectorcurrent（O/P）IcandbasecurrentIB（I/P）calledcurrentgain.

Formulas

Working

Transistorsworkin3regions

Active:

Alwayson--Ic=BIb

Saturation:

Ic=IsaturationOnasaswitch

Off:

Ic=0Offasaswitch

Transistorshavethreeterminals:

TransistorasaSwitch

Transistorscanbeusedasswitches.

Transistorscaneitherconductornotconductcurrent.

TransistorSwitchingExample

WhenVBEislessthan0.7Vthetransistorisoffandthelampdoesnotlight.

WhenVBEisgreaterthan0.7Vthetransistorison

andthelamplights.

Transistoroperationasswitchmeansthattransistoroperateseitherinsaturationregionorincut-offregionandnowhereelseontheloadline.

AsanidealswitchoperateatA.AtpointBincut-offstateasanopenswitch.

Largebasecurrentwillcausethetransistorworkinsaturationregionat

pointA’withsmallsaturationvoltageVCES.

Whenthecontrolorbaseisreduceto0,thetransistoristurn-offanditsoperationisshifttoB’inthecut-offregion.AsmallleakagecurrentICEOflowinthecollectorcircuitwhenthetransistorisoff.

IfVCE（S）isthecollector–emittersaturationvoltage,thenthecollectorcurrent

ICSis:

TheratioofICStoIBiscalledforcedcurrentgainandlessthanß.

andthecorrespondingvalueofIBS:

IfthebasecurrentislessthanIBS,thetransistoroperateinactiveregioni.e.somewherebetweenthesaturationandcut-offpoint.

IfthebasecurrentismorethanIBS,VCESisalmostzero.Thisshowsthat

collectorcurrentatsaturationremainsubstantiallyconstantevenifbasecurrentisincreased.

Totalpowerlossinthetwojunctionofthe

transistoris:

WithbasecurrentmorethanIBS,harddriveoftransistorisobtained.

TheratioofIBtoIBSisoverdrivefactor:

UndersaturationconditionVBES>VCES,meansbaseemitterjunctionisforwardbias.IeVCBis–ve.UndersaturationconditionCJBisalsoFB.SoatSC,BEJ&CBJareFB.

SWITCHINGCHARACTERISTICSOFBJT

Whenthebasecurrentisapplied,atransistordoesnotturnoninstantlybecauseofthepresenceofinternalcapacitance.

WheninputvoltageVBtobasecircuitismade–v2att0,junctionEBisreverse

biasedVBE=-V2,atransistorisOFFi.e.IB=IC=0andVCE=VCC.

Attimet1,inputvoltageVBismade+V1andIBrisestoIB1.

Aftertimet1,baseemittervoltageVBEbeginstorisegraduallyfrom-V2and

collectorcurrentIcbeginstorisefromzeroandcollector-emittervoltagestartfallingfrominitialvalueVCC.

Aftersometimetdcalleddelaytime,thecollectorcurrentrisesto0.1ICSthis

delaytimeisrequiredtochangethebaseemittercapacitancetoVBES=0.7V.This

delaytimetdisdefinedastimeduringwhichthecollectorcurrentrisesfrom0too.1Icsandcollector–emittervoltagefallVccto0.9Vcc.

Afterdelaytimetd,collectorcurrentrisesfrom0.1Icsto0.9IcsandVCEfallfrom0.9VCCto0.1VCCintimetr.Thistimetrisknownasrisetimewhichdependsupontransistorjunctioncapacitance.Therisetimetrisdefinedasthetimeduringwhichcollectorcurrentrisesfrom0.1Icsto0.9Ics.Thisshowsthetotalturnontimeton=td+tr.

SAFE-OPERATINGAREA

Thesafeoperatingareaofapowertransistorspecifiesthesafe-operatinglimitsofcollectorcurrentversuscollectoremittervoltage.

Forreliableoperationofthepowertransistor,thecollectorcurrentandvoltagemustalwaysliewithinthisarea.

Twotypesofsafe-operatingareasarespecifiedbymanufacturer:

FBSOA（Forward-biasedsafe-operatingarea）

RBSOA（Reverse-biasedsafe-operatingarea）

FBSOA?

belongstothetransistoroperationwhenbase-emitterjunctionis

forwardbiasedtoturn-onthetransistor.

FORWARDBIASEDSAFEOPERATINGAREA

（FBSOA）DCASWELLASSINGLEPULSEOPERATION

FBSOAINCREASES?

PULSE-WIDTHDECRESASES

Reversebiasedsafeoperatingarea（RBSOA）

Reversebiasedsafeoperatingarea（RBSOA）

Duringturn-off,atransistorissubjectedtohigh-currentandhighvoltagewithBEJreversebiased.

Safe-operatingareaoftransistorduringturn-offisspecifiedas

RBSOA.

RBSOAspecifiesthelimitoftransistoroperationatturn-offwhenthebasecurrentiszeroorwhenthebaseemitterjunctionisreversedbiased（with–vebasecurrent）.Withincreasingreversebias,areaRBSOAdecreasesinsize.

fontsize=6color=black>ADVANTAGESOFBJTs

•Havehighswitchingfrequencies.

•Turn-onlossesaresmall.

•Controlledturn-on&turn-offcharacteristics.

•Nocommutationcircuitrequired.

fontsize=6color=black>DEMERITSOFBJT

•Drivecircuitiscomplex.

•Hastheproblemofchargestorage.

•Hastheproblemofsecondbreakdown.

•Cannotbeusedinparalleldueto

•Problemsofnegativetemperature

coefficients.

Review

Introduction

Working

PowerTransistorStructure

SteadyStateCharacteristics

OutputCharacteristics

TransferCharacteristics

TransistorasaSwitch

SwitchingCharacteristics

SafeOperatingAreas

AdvantagesandDemeritsofPowerBJTS