dr a k kapoor

8/14/2019 Dr a K Kapoor

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Power BJT

Dr A K Kapoor


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Power BJT1. Have controlled turn-on & turn-off

characteristics.

2. Have to withstand large blocking voltage in offstate.

3. Have high current carrying capacity in the on-state.

These requirements leads to modified structurethan its logic/signal level counterpart.

There are significant differences in the i-vcharacteristics and switching behavior of the twotypes, that lead to different drive circuits.

Structure:

A power BJT has a vertical three layer structure. This structuremaximizes the cross sectional area through which thecurrent in the device flows.

The large cross-sectional area minimizes the on-state


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Basic Geometry of Power BJTs

Features to Note

Multiple narrow emitters - minimize emitter current crowding.

Multiple parallel base conductors - minimize parasitic resistance in

series with the base.


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BJT Construction Parameters

Features to Note

Wide base width - low (hfe< 10) beta.

Lightly doped collector drift region - large breakdown voltage.


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Darlington-connected BJTs

IC

IB

B

C

D1

D

M

= =C

B

+ +D M D M

Composite device has respectable or hfe


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The doping levels in each of the layers have a significant

effect on the characteristics of the device.

The thickness of the drift region determines thebreakdown voltage of the Power BJT (ranges from tens to

hundreds of microns)

Small base thickness reduces the breakdown voltage

capability of the BJT

BVSUS, BVCEO, BVCBO

Primary breakdown voltage is due to conventional

avalanche

breakdown of the collector base junction. The region is

avoided. The region labeled 2nd breakdown is avoided. The major observable difference is the quasi saturation

region.


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Power BJT I-V Characteristic

Features to Note

2nd breakdown

must be

avoided.

Quasi-

saturation unique to

powerBJTs

BVCBO > BVCEO

extendedblocking


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BJT Safe Operating Areas

Forward bias safe operating area Reverse bias safe operating area

Voltage break downlimit

Thermal

powerdissipationlimit

Maxmpermissibl

ecombination ofvCE.ic


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Generic BJT Application - Clamped Inductive Load

RB

DF I o

Q

V dc

vi

+

-

Model of an

inductively-loaded

switching circuit

Current source Io models an inductive load with an L/Rtime constant >> than switching period.

Positive base current turns BJT on (hard saturation). So-called forward bias operation.

Negative base current/base-emitter voltage turns BJToff. So-called reverse bias operation.

Free wheeling diode DF prevents large inductiveovervoltage from developing across BJT collector-emitter terminals.


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Power BJT Turn-on Waveforms

t

t

t d,on

Io

VBE,on

tri

Vdc

tfv1

tfv2

v (t)CE

i (t)C

v (t)BE

i (t)

B

IB,on

VBE,off

VCE,sat


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Turn-off Waveforms with Controlled Base Current

t

ts

Io

VBE,on

tfi

trv

1

trv

2

v (t)

CE

i (t)

C

i (t)B

IB,on

Vdc

VBE,off

I B,offdi /dtB

VCE,sat

Base currentmust make acontrolledtransition(controlled

value of-diB/dt) frompositive tonegativevalues in order

to minimizeturn-off timesand switchinglosses.


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Power BJT Breakdown Voltage

Blocking voltage capability of BJT limited by breakdown ofCB junction.

BVCBO = CB junction breakdown with emitter open.

BVCEO = CB junction breakdown with base open.

BVCEO = BVCBO/()1/n ; n = 4 for npn BJTs and n = 6for PNP BJTs

BE junction forward biased even when base current = 0

by reverse current from CB junction.

Excess carriers injected into base from emitter and

increase saturation current of CB junction.

Extra carriers at CB junction increase likelihood of impact

ionization at lower voltages , thus decreasing breakdown

voltage.

Wide base width to lower beta and increase BVCEO.


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In BJT maximum collector current in

the active region is obtained for

VCB = 0 and VBE = VCE

Icmax = (Vcc-VCE)/RC = (Vcc

VBE)/Rc

Where, Vcc, VCE, VBE, Icmax and

Rc have the standard meaning.

Therefore, IBmax = Icmax/

For base current > Ibmax, VBE and


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Base Drive Control of BJT

Commonly used techniques for optimizing the base drive of a power transistor are:

1. Turn on control

2. Turn-off control3. Proportional Base control

4. Anti-saturation control

Turn-on control:Current peaking at turn on to reduce turn on

time.

Fig.1 Base drive currentwaveform


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Click to edit Master text stylesSecond level Third level Fourth level Fifth level

Turn-oncontrol

Initial value of current, IB0 = (V1 -VBE)/R1

Final value of base current, IB1 = (V1 -VBE)/(R1+R2) andVC V1.{R2/(R1+R2)}

Charging time constant of C1, 1 = (R1.R2.C1)/(R1+R2)

Discharging time constant of C1, 2 = R2.C1

Maxm switching freq., fs =1/T = 1/(t1+t2) = 1/5(1+ 2) =

0.2/(1+ 2)

t1 51, t2 52

Turn-off control:

During turn-off, the applied base voltage is V2, the capacitor

voltage Vc is added to V2 as a reverse voltage and results in

turn-off current peaking.

Fig.2 - Base current peaking duringturn-on


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Click to edit Master text stylesSecond level Third level

Fourth level Fifth level

Fig. 3 - Base current peaking during turn-on andturn-off

If different turn-on and turn-off characteristics are

required, a turn-off ckt. (C2,R3,R4) as shown in Fig.3may be added. The diode D1 isolates the turn-on basedrive circuit from turn off base drive circuit.


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Proportional Base drive Circuit

This type of base drive has the advantage that if

the collector current changes due to load current

the base current also changes in proportion. To

initiate the circuit operation Switch S1 is closed a

pulse current of short duration flows through Q1;

and Q1 turns on into saturation. When thecollector current begins a corresponding base

current is induced due to transformer action. The

Q1 would latch-on itself. The switch S1 has no

further role and can be turned off.Base Drive Current changes in proportion to


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To maintain the power BJT in quasi saturation, byclamping the collector emitter voltage to a pre-determined level. The collector current is given by:

IC = (VCE-VCM)/RC, where VCM is the clampingvoltage & VCM>VCEsat

Base drive current: IB = I1= (VB -Vd1-VBE)/RB andIC = IB = IL

Antisaturationcontrol(Bakers clamp)


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When clamping takes place due to conduction of D2

VCE = VBE + Vd1-Vd2 and IL= (VCC-VCE)/RC = (VCC-

VBE -Vd1+Vd2)/RC

and collector current with clamping is

IC = IB = (I1-IC+IL) = (I1+IL)./(1+)

For clamping Vd1>Vd2; This can be achieved by

connecting two or more diodes in place of d1.

The load resistance should satisfy the condition

IB > IL Also,

IB RC > (VCC - VBE Vd1 + Vd2)

The clamping action results in reduced collector currentand almost negligible storage time, resulting in fast turn-

on. However, due to increased VCE, the on-state power in

the BJT increases, whereas the switching losses decrease.


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Series and Parallel Operation ofpower BJTPower BJTs may be operatedin series and parallel similar

to Power diodes andThyristors. BJTs have vethermal coefficient. The seriesand parallel connections maybe made similar to SCRs.

For series connection thedevices should be closelymatched for gain,transconductance,threshold voltage, onstate voltage, turn-ontime and turn-off time.

For parallel connectionreasonable current sharingcan be obtained byconnecting series resistance

and coupled inductors Current sharing of paralleled BJT duringturn-off

Dynamic current sharing inparalleled BJT


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Base Drive Isolation

There are two ways to achieve the base drive isolation:

1. Pulse transformer isolation

2. Optocoupler/optoisolator based


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Current Crowding Enhancement of 2nd Breakdown Susceptibility

Emitter current crowding

during either turn-on or

turn-off accentuates

propensity of BJTs to 2nd

breakdown.

Minimize by dividingemitter into many narrow

areas connected

electrically in parallel.


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Second Breakdown avoidance:

To avoid second Breakdown (SB) total power

dissipation should be under control during turn-onand turn-off, when the instantaneous powerdissipation is largest, current density nonuniformities shall be avoided.

Emitter current crowding above specific currentlevels leads to localized thermal runaway.

During turn-off the flow of ve base current causescrowding of the emitter current towards the centre

of the emitter. The severity of the current crowdingis reduced with multiple narrow emitter fingers.

Controlled rate of change of base current use ofsnubbers, freewheeling diodes and maintaining the

switching trajectory under SOA limits are the key

dr a k kapoor

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