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ECE 442 Power Electronics 1
Bipolar Junction Transistors (BJT)
NPN PNP
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ECE 442 Power Electronics 2
BJT Cross-Sections
NPN PNP
Emitter
Collector
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ECE 442 Power Electronics 3
Common-Emitter NPN Transistor
Forward bias the BEJ
Reverse bias the CBJ
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ECE 442 Power Electronics 4
Input Characteristics
Plot IB as f(VBE, VCE)
As VCE increases,
more VBE required to
turn the BE on so thatIB>0.
Looks like a pn
junction volt-ampere
characteristic.
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ECE 442 Power Electronics 5
Output Characteristics
Plot IC as f(VCE, IB)
Cutoff region (off) both BE and BC
reverse biased Active region
BE Forward biased
BC Reverse biased
Saturation region (on) both BE and BC
forward biased
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ECE 442 Power Electronics 6
Transfer Characteristics
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ECE 442 Power Electronics 7
Large-Signal Model of a BJT
KCL >> IE = IC + IB
F = hFE = IC/IB
IC = FIB + ICEO
IE = IB(1 + F) + ICEO
IE = IB(1 + F)
IE = IC(1 + 1/F)
IE = IC(F + 1)/F
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ECE 442 Power Electronics 8
(1 ) ( 1)
111
1 1
E B C
CF FE
B
C F B CEO
E B F CEO B F
FE C C
F F
C F E
F FF F
F F
I I I
Ih
II I I
I I I I
I I I
I I
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ECE 442 Power Electronics 9
Transistor Operating Point
B BEB
B
CE CC C
C C
CE CC C C
V VI
RV V
I
R RV V I R
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ECE 442 Power Electronics 10
DC Load Line
VCC
VCC/RC
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ECE 442 Power Electronics 11
BJT Transistor Switch
B BEB
B
CE CC C C
CE CB BE
CB CE BE
V VI
R
V V I R
V V VV V V
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ECE 442 Power Electronics 12
BJT Transistor Switch (continued)
CC CE CC BE CM
C C
CMBM
F
V V V V I
R R
II
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ECE 442 Power Electronics 13
BJT in Saturation
( )CC CE sat
CS
C
CSBS
F
B
BS
CSforced
B
V VI
R
I
I
IODF
II
I
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ECE 442 Power Electronics 14
Model with Current Gain
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ECE 442 Power Electronics 15
Miller Effect
vbe vceiout
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ECE 442 Power Electronics 16
Miller Effect (continued)
( ) ( )
[1 ] [1 ]
[1 ]
out cb be ce cb be be
out cb be cb be
cb cb
d di C v v C v Av
dt dt d d
i C A v C A vdt dt
C C A
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ECE 442 Power Electronics 17
Miller Effect (continued)
Miller Capacitance, CMiller= Ccb(1 A)
since A is usually negative (phase inversion),
the Miller capacitance can be much greater
than the capacitance Ccb
This capacitance must charge up to the
base-emitter forward bias voltage, causing
a delay time before any collector currentflows.
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ECE 442 Power Electronics 18
Saturating a BJT
Normally apply more base current than
needed to saturate the transistor
This results in charges being stored in the
base region
To calculate the extra charge (saturating
charge), determine the emitter current
1cse B BS BS BS I
I I ODF I I I ODF
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ECE 442 Power Electronics 19
The Saturating Charge
The saturating charge, Qs
( 1)s s e s BSQ I I ODF storage time constantof the
transistor
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ECE 442 Power Electronics 20
Transistor Switching Times
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ECE 442 Power Electronics 21
Switching Times turn on
Input voltage rises from 0 to V1
Base current rises to IB1
Collector current begins to rise after the
delay time, td
Collector current rises to steady-statevalue ICS
This rise time, trallows the Millercapacitance to charge to V1
turn on time, ton = td + tr
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ECE 442 Power Electronics 22
Switching Times turn off
Input voltage changes from V1 toV2
Base current changes toIB2
Base current remains atIB2 until theMiller capacitance discharges to zero,
storage time, ts
Base current falls to zero as Millercapacitance charges toV2, fall time, tf
turn off time, toff= ts + tf
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ECE 442 Power Electronics 23
Charge Storage in Saturated BJTs
Charge storage in the Base Charge Profile during turn-off
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ECE 442 Power Electronics 24
Example 4.2
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ECE 442 Power Electronics 25
Waveforms for the Transistor Switch
VCC = 250 VVBE(sat) = 3 V
IB = 8 A
VCS(sat) = 2 V
ICS = 100 A
td = 0.5 s
tr= 1 s
ts = 5 stf= 3 s
fs = 10 kHz
duty cycle k = 50 %
ICEO = 3 mA
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ECE 442 Power Electronics 26
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ECE 442 Power Electronics 27
Power Loss due to IC for ton = td + tr
During the delay time, 0 t td
Instantaneous Power Loss
Average Power Loss
0 0
1( )
(250 )(3 )(10 )(0.5 ) 3.75
d dt tCC CEO
d c CC CEO s d
d
V IP P t dt dt V I f t
T T
P V mA kHz s mW
( )
( ) (250 )(3 ) 0.75c CE C CC CEO
c
P t v i V I
P t V mA W
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ECE 442 Power Electronics 28
During the rise time, 0 t tr
( )
( )
( )
max
( )
( )
( ) ( )
( )( )
( ) @
2[ ]
c CE c
CSc CC ce sat CC
r r
ce sat CC c CS CSCC ce sat CC
r r r r
c m
r CCm
CC ce sat
P t v i
ItP t V V V t
t t
V VdP t I Itt V V V
dt t t t t
P t P t tt V
tV V
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ECE 442 Power Electronics 29
2
max
( )
2
max
(1 )(250 ) 0.5042[250 2 ]
4[ ]
(250 ) (100 ) 63004[250 2 ]
m
CC CS
CC CE sat
s Vt sV V
V IPV V
V AP WV V
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ECE 442 Power Electronics 30
Average Power during rise time
( )
0
1( )
2 3
(250 ) (2 250 )(10 )(100 )(1 )
2 3
42.33
rt
CE sat CC CCr c s CS r
r
r
V VVP P t dt f I t
T
V V VP kHz A s
P W
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ECE 442 Power Electronics 31
Total Power Loss during turn-on
0.00375 42.33 42.33375
42.33
on d r
on
on
P P P
P W
P W
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ECE 442 Power Electronics 32
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ECE 442 Power Electronics 33
Power Loss during
the Conduction Period
( )
( ) ( )
0 0
0
( ) 100
( ) 2( ) (100 )(2 ) 200
1
( )
(2 )(100 )(10 )(48.5 ) 97
n n
n
c CS
CE CE sat
c c CE
t t
n c CE sat CS s CE sat CS s n
n
t t
i t I A
v t V V P t i v A V W
P P t dt V I f dt V I f tT
P V A kHz s W
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ECE 442 Power Electronics 34
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ECE 442 Power Electronics 35
Power Loss during turn off
Storage time
( )
( )
( ) ( )
0 0
0
( ) 100
( ) 2
( ) (2 )(100 )
( ) 200
1 ( )
(2 )(100 )(10 )(5 ) 10
s s
s
c CS
CE CE sat
c CE c CE sat CS
c
t t
s c CE sat CS s CE sat CS s s
s
t t
i t I A
v t V V
P t v i V I V A
P t W
P P t dt V I f dt V I f tT
P V A kHz s W
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ECE 442 Power Electronics 36
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ECE 442 Power Electronics 37
Power Loss during Fall time0
( ) 1 , 0
( ) , 0
( ) 1
( ) 11 0
3( ) @ 1.5
2 2
(250 )(100 )
4 4
f
c CS CEO
f
CCCE CEO
f
c CE c CC CS
f f
c CC CS
f f f
f
c m
CC CS m
t t
ti t I I
t
Vv t t I
t
t tP t v i V I
t t
dP t V I tt
dt t t t
t sP t P t s
V I V AP
6250W
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ECE 442 Power Electronics 38
Power Loss during Fall time (continued)
0
( )
1( )
6
(250 )(100 )(3 )(10 ) 1256
6
10 125 135
ftCC CS f s
f c
f
CC f
off s f CS s s CE sat
off
V I t f P P t dt
T
V A s kHz P W
V t
P P P I f t V
P W
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ECE 442 Power Electronics 39
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ECE 442 Power Electronics 40
Power Loss during the off time
0
0( )
( )
( ) (250 )(3 ) 0.75
1
(250 )(3 )(10 )((50 5 3) )
0.315
o
o
CE CC
c CEO
c CE C CC CEO
t
o CC CEO CC CEO s o
o
o
t tv t V
i t I
P t v i V I V mA W
P V I dt V I f tT
P V mA kHz s
P W
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ECE 442 Power Electronics 41
The total average power losses
42.33 97 135 0.315
274.65
T on n off o
T
T
P P P P P
P
P W
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ECE 442 Power Electronics 42
Instantaneous Power for Example 4.2
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ECE 442 Power Electronics 43
BJT Switch with an Inductive Load
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ECE 442 Power Electronics 44
Load Lines