This is Lecture-7 on power electronics. In this lecture we discuss power BJTs

1. Semiconductor Power Switching Devices-4
(Lecture-7)
R S Ananda Murthy
Associate Professor and Head
Department of Electrical & Electronics Engineering,
Sri Jayachamarajendra College of Engineering,
Mysore 570 006
R S Ananda Murthy Semiconductor Power Switching Devices-4

2. Structure of Power BJT
IB
IC
vBE
vCE
(c)(a)
C
B E
−
+
−
+
C
B
(b)
C
EB
E
n
p
n n+
n−
p
n+
Structure shown in Figure (a) is preferred for low voltage
ratings.
Structure shown in Figure (b) is preferred for high voltage
ratings. But this tends to increase the on-state voltage
drop.
R S Ananda Murthy Semiconductor Power Switching Devices-4

3. BJT as a Power Switching Device
CE
V
VBE
RB
VCB
IE
VCE 1 VCE 2
VBE
+
− VBB
IB
IC
IB
RC
VCC
+
−
+
−
(b)(a)
0
B
E
+
−
+
−
>
C
CE configuration is always used as it offers high input
impedance and higher current gain.
When transistor is on IC = ICS = [VCC −VCE(sat)]/RC.
IBS = ICS/β is the base current that just turns on the BJT.
For faster turn on usually we make IB > IBS.
R S Ananda Murthy Semiconductor Power Switching Devices-4

4. CE Output Characteristics of Power BJT
VCCVCE(Sat)
VCE
Quasi−
saturation
zone
IB
IC
RC
VCC
IB4
IB3
IB2
IB1
IB
increasing
Linear zone
Hard saturation line
Cutoff zone
Load Line
= 0
b
a
d
e
c
Off
On
Since hard saturation increases the turn-off time, power
BJTs are always operated in cut-off and quasi-saturation
regions.
R S Ananda Murthy Semiconductor Power Switching Devices-4

5. Darlington Connection for Higher Current Gain
So overall current gain is
and voltage drop is
T1
T2
Darlington connection gives higher β but it increases
leakage current, on-state voltage drop, and reduces the
switching frequency.
R S Ananda Murthy Semiconductor Power Switching Devices-4

6. Switching Times of Power BJT
iB
ICS
ICS
ICS
td
tr
tn
ts
tf
ton
toff
iC
IB1
vCE
B2−I
to
VCC
VCE(Sat)
0.9
0.1
t
t
t
(c)
(b)
(a)
R S Ananda Murthy Semiconductor Power Switching Devices-4

7. Preferred Base Current Waveform for BJT
By reducing
like this, BJT is prevented
from entering into
hard saturation
0
By making IB1 > IBS for a brief duration during turn on both
td and tr and hence ton is reduced.
To turn off BJT gradually apply a negative base current
with a peak of −IB2 which falls to zero after some time.
This reduces ts and tf and hence toff .
R S Ananda Murthy Semiconductor Power Switching Devices-4

8. Baker’s Clamping Circuit to Reduce Storage Time
Q
D1
D2 D3
D4
C
B
E
0
X
D2, D3, D4 are low voltage diodes.
D1 should be a high voltage diode.
D4 is needed to provide path for negative
base current while turning off the BJT.
When BJT turns on, VCE tends to drop causing D1 to turn
on. This decreases IB preventing BJT going into hard
saturation.
When D1 is on, as all diode voltages are equal, by KVL we
get VCE = VX −VD1 = VBE +VD2 +VD3 −VD1 = VBE +VD2 .
R S Ananda Murthy Semiconductor Power Switching Devices-4

9. FSOA and RSOA of Power BJT
FSOA shown in Fig (a) is applicable when the BJT is
turned on by applying a positive base current.
RSOA shown in Fig (b) is applicable when the BJT is
turned off by applying a negative base current.
R S Ananda Murthy Semiconductor Power Switching Devices-4

10. Parallel Operation of BJT
T1 T2
Let permissible
Let permissible
By KVL
Let T1 and T2 be identical transistors. When they are on
Then
Suppose V
and A
then,
If IC1 increases, the voltage across emitter resistor gives a
negative feedback and reduces VBE1. Then, IB1 decreases
causing a reduction in IC1. But emitter resistors cause power
loss which reduces the efficiency of the circuit.
R S Ananda Murthy Semiconductor Power Switching Devices-4

11. Base Drive Isolation using Opto-coupler
Logic
drive
circuit
Amplifier
VCC
B
Q
E
COpto−coupler
Since BJT needs a continuous base current, isolation of
base drive is possible only by using an opto-coupler.
But this needs additional VCC and amplifier which makes
the base drive circuit bulkier if more power BJTs are to be
used.
R S Ananda Murthy Semiconductor Power Switching Devices-4

12. Specifications of Power BJT BU208D
R S Ananda Murthy Semiconductor Power Switching Devices-4

13. Specifications of Power BJT BU208D
R S Ananda Murthy Semiconductor Power Switching Devices-4

14. Specifications of Power BJT BU208D
R S Ananda Murthy Semiconductor Power Switching Devices-4

15. Specifications of Power BJT BU208D
Safe Operating Area increases for pulsed operation.
R S Ananda Murthy Semiconductor Power Switching Devices-4

16. Specifications of Power BJT BU208D
R S Ananda Murthy Semiconductor Power Switching Devices-4

17. Demerits of Power BJT
Though BJT is fully-controlled, it has the following demerits —
Since it is continuously triggered, it needs opto-coupler
isolated bulky base drive circuit.
It cannot block high reverse voltage.
It has low current gain (β) which varies with collector
current and temperature. Due to this, typically base current
of the order of few amperes is needed for controlling the
BJT.
β can be increased using Darlington connection. But this
increases leakage current, on-state voltage drop, and
reduces switching frequency.
R S Ananda Murthy Semiconductor Power Switching Devices-4

18. Demerits of Power BJT
It is prone to fail due to ‘Second Breakdown’ — due to hot
spots caused by concentration of current in a narrow area
in the emitter junction when the device is turned ON and in
the collector junction when it is turned OFF.
Its switching frequency is typically within 20 kHz.
Parallel operation of BJTs is difficult due to their negative
temperature coefficient of resistance.
BJT cannot be protected against overload using a fuse.
It cannot withstand overload. So it should be turned off
immediately when there is overload. This requires sensing
of overload by some method.
Due to the demerits mentioned above BJTs are not preferred in
modern power converters. Instead MOSFETs and IGBTs are
used.
R S Ananda Murthy Semiconductor Power Switching Devices-4

19. Next Lecture...
In the next lecture we will discuss some more power
semiconductor switching devices used in power electronics.
Thank You.
R S Ananda Murthy Semiconductor Power Switching Devices-4