Part of Lecture series on EE321N, Power Electronics-I delivered by me during Fifth Semester of B.Tech. Electrical Engg., 2012 Z H College of Engg. & Technology, Aligarh Muslim University, Aligarh Please comment and feel free to ask anything related. Thanks!

1. EE-321 N
Lecture-5
Thyristor Protection Through
Snubber Circuits

2. Introduction
• A thyristor needs protection against
overvoltages, overcurrents, high dv/dt and
high di/dt
• O/V and O/C protection can generally be
achieved by zener diodes and fuses etc.
• However, device is protected against high
dv/dt and high di/dt using snubber circuits
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3. Types of Snubbers
1. Turn-ON snubbers to reduce sudden rise of
current (di/dt)
2. Turn-OFF snubbers to reduce sudden rise of
voltage (dv/dt)
3. Snubbers for reducing v & i stresses on the
device during switching (for power
transistors)
4. Energy recovery snubbers
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4. di/dt snubber
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5. di/dt snubber
• Under steady state operation, Dm will conduct
when thyristor T1 is OFF
• If T1 is triggered when Dm is still conducting,
di/dt can be very high
• In practice, the di/dt is limited by adding a
series inductor Ls (also includes stray
inductance)
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s
s
ss
L
V
dt
di
dt
di
LV 

6. dv/dt Snubber
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7. dv/dt Snubber
• Series combination of R & C shunted across the
device
• C takes care of dv/dt while R limits the discharge
current when thyristor is switched ON
• A more effective variation is the polarized
snubber in which there is additional diode in
parallel with R
• The diode bypasses the resistor to reduce the
response time of RC for +ve spikes (which may
turn ON the device)
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8. Polarized Snubber
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9. Analysis
• If switch S1 is closed at t = 0, a step voltage will
be applied across the thyristor T1
• The dv/dt may be high enough to turn on the
device
• The dv/dt can be limited by connecting a
capacitor Cs across T1
• When the thyristor T1 is turned on, the
discharge current of capacitor is limited by
resistor Rs
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10. Analysis
• The circuit dv/dt can be found approximately
from:
• The snubber circuit can be designed based on
the known value of the dv/dt for a device
• The value of Rs is found from the discharge
current ITD
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ss
ss
CR
VV
dt
dv 632.0632.0


TD
s
s
I
V
R 

11. Design Example (Ex. 7.4 MHR)
For the circuit configuration shown, input voltage is Vs =
200 V with load resistance R = 5 Ω. The load and stray
inductances are negligible and the thyristor is operated
at a frequency of fs = 2 kHz. If the required dv/dt is 100
V/µs and the discharge current is to be limited to 100 A.
Determine: (a) the values of Rs, Cs,
(b) snubber loss, and
(c) the power rating of snubber resistor.
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12. Solution on Board
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13. Try Yourself
For an RL load with R = 2.5 Ω, find the min.
value of L so that the thyristor switch is not
damaged due to high di/dt. The di/dt limit for
the thyristor is 50 A/µs.
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+
−
i
500 V