EE-321 N

Lecture-5

Thyristor Protection Through Snubber Circuits

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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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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di/dt snubber

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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

sss

L

V

dt

di

dt

diLV

dv/dt Snubber

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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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Polarized Snubber

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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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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

ss

I

VR

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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Solution on Board

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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