lecture-7 : semiconductor power switching devices-4
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Semiconductor Power Switching Devices-4(Lecture-7)
R S Ananda Murthy
Associate Professor and HeadDepartment of Electrical & Electronics Engineering,
Sri Jayachamarajendra College of Engineering,Mysore 570 006
R S Ananda Murthy Semiconductor Power Switching Devices-4
Structure of Power BJT
IBIC
vBE
vCE
(c)(a)
C
B E−
+−
+
C
B
(b)
C
EBE
np
nn+n−
pn+
Structure shown in Figure (a) is preferred for low voltageratings.Structure shown in Figure (b) is preferred for high voltageratings. But this tends to increase the on-state voltagedrop.
R S Ananda Murthy Semiconductor Power Switching Devices-4
BJT as a Power Switching Device
CEV
VBE
RB
VCB
IE
VCE 1 V
CE 2
VBE
+
−V
BB
IB
IC
IB
RC
VCC
+
−
+
−
(b)(a)
0
B
E
+
−
+
−
>
C
CE configuration is always used as it offers high inputimpedance 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
CE Output Characteristics of Power BJT
VCCVCE(Sat)
VCE
Quasi−
saturation
zone
IB
IC
RC
VCC
IB 4
IB 3
IB 2
IB 1
IB
increasing
Linear zone
Hard saturation line
Cutoff zone
Load Line
= 0
b
a
de
c
Off
On
Since hard saturation increases the turn-off time, powerBJTs are always operated in cut-off and quasi-saturationregions.
R S Ananda Murthy Semiconductor Power Switching Devices-4
Darlington Connection for Higher Current Gain
So overall current gain is
and voltage drop is
T1
T2
Darlington connection gives higher β but it increasesleakage current, on-state voltage drop, and reduces theswitching frequency.
R S Ananda Murthy Semiconductor Power Switching Devices-4
Switching Times of Power BJT
iB
ICS
ICS
ICS
tdtr
tn
tstf
tontoff
iC
IB 1
vCE
B 2−I
to
VCC
VCE(Sat)
0.9
0.1
t
t
t
(c)
(b)
(a)
R S Ananda Murthy Semiconductor Power Switching Devices-4
Preferred Base Current Waveform for BJT
By reducinglike this, BJT is preventedfrom entering intohard saturation
0
By making IB1 > IBS for a brief duration during turn on bothtd and tr and hence ton is reduced.To turn off BJT gradually apply a negative base currentwith 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
Baker’s Clamping Circuit to Reduce Storage Time
QD1
D2 D3
D4
C
B
E0
X
D2, D3, D4 are low voltage diodes.D1 should be a high voltage diode.D4 is needed to provide path for negativebase current while turning off the BJT.
When BJT turns on, VCE tends to drop causing D1 to turnon. This decreases IB preventing BJT going into hardsaturation.When D1 is on, as all diode voltages are equal, by KVL weget VCE = VX −VD1 = VBE +VD2 +VD3 −VD1 = VBE +VD2 .
R S Ananda Murthy Semiconductor Power Switching Devices-4
FSOA and RSOA of Power BJT
FSOA shown in Fig (a) is applicable when the BJT isturned on by applying a positive base current.RSOA shown in Fig (b) is applicable when the BJT isturned off by applying a negative base current.
R S Ananda Murthy Semiconductor Power Switching Devices-4
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 anegative feedback and reduces VBE1. Then, IB1 decreasescausing a reduction in IC1. But emitter resistors cause powerloss which reduces the efficiency of the circuit.
R S Ananda Murthy Semiconductor Power Switching Devices-4
Base Drive Isolation using Opto-coupler
Logic
drive
circuit
Amplifier
VCC
BQ
E
COpto−coupler
Since BJT needs a continuous base current, isolation ofbase drive is possible only by using an opto-coupler.But this needs additional VCC and amplifier which makesthe base drive circuit bulkier if more power BJTs are to beused.
R S Ananda Murthy Semiconductor Power Switching Devices-4
Specifications of Power BJT BU208D
R S Ananda Murthy Semiconductor Power Switching Devices-4
Specifications of Power BJT BU208D
R S Ananda Murthy Semiconductor Power Switching Devices-4
Specifications of Power BJT BU208D
R S Ananda Murthy Semiconductor Power Switching Devices-4
Specifications of Power BJT BU208D
Safe Operating Area increases for pulsed operation.
R S Ananda Murthy Semiconductor Power Switching Devices-4
Specifications of Power BJT BU208D
R S Ananda Murthy Semiconductor Power Switching Devices-4
Demerits of Power BJT
Though BJT is fully-controlled, it has the following demerits —Since it is continuously triggered, it needs opto-couplerisolated bulky base drive circuit.It cannot block high reverse voltage.It has low current gain (β ) which varies with collectorcurrent and temperature. Due to this, typically base currentof the order of few amperes is needed for controlling theBJT.β can be increased using Darlington connection. But thisincreases leakage current, on-state voltage drop, andreduces switching frequency.
R S Ananda Murthy Semiconductor Power Switching Devices-4
Demerits of Power BJT
It is prone to fail due to ‘Second Breakdown’ — due to hotspots caused by concentration of current in a narrow areain the emitter junction when the device is turned ON and inthe collector junction when it is turned OFF.Its switching frequency is typically within 20 kHz.Parallel operation of BJTs is difficult due to their negativetemperature coefficient of resistance.BJT cannot be protected against overload using a fuse.It cannot withstand overload. So it should be turned offimmediately when there is overload. This requires sensingof overload by some method.
Due to the demerits mentioned above BJTs are not preferred inmodern power converters. Instead MOSFETs and IGBTs areused.
R S Ananda Murthy Semiconductor Power Switching Devices-4
Next Lecture...
In the next lecture we will discuss some more powersemiconductor switching devices used in power electronics.
Thank You.
R S Ananda Murthy Semiconductor Power Switching Devices-4
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