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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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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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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