Thyristor protection is needed to insure that its specified ratings are not exceeded during its operation.
We apply following measures in order to protection of thyristor.
When VAK> 0 and Ig> 0, conduction of anode current begins near gate cathode junction. After that anode current spreads in the whole area of gate cathode junction (as explained earlier). If the rate of rise of anode current (di/dt) is greater than spread velocity of carriers, local hot spots will be formed near the gate due to high current density which may destroy the thyristor.
The di/dt is maintained within limit by using inductor in series with anode circuit. An inductor opposes the change in current.
Current in SCR to turn on transient can be reduced by connecting a small inductor in series with the anode.
If the rate of rise of VAK is high then thyristor may get turn ON (explained earlier in dv/dt triggering). If we don’t want to turn ON the thyristor then we have to reduce dv/dt. So, accidental turn ON due to dv/dt can be check by using snubber circuit shown in figure.
When switch S is closed, sudden voltage is applied to the circuit. Capacitor CS acts as short circuit so voltage across SCR is zero. As time passes voltage across CS builds up at a slow rate. Before SCR is fired by gate pulse, CS charges to full voltage Vs. When SCR is on then the capacitor discharges through SCR. This discharging current is very high which may destroy SCR as resistance of local path formed by CS and SCR is low. A current limiting resistor RS is introduced in the circuit which helps to limit this discharging current and protect SCR.
Normally RS, CS and load circuit form an under damped circuit to limit dv/dt.
A fuse is attached in series with SCR so that when current go above rated current, the fuse get blow off.
For over voltage protection we attach a varistor in parallel to SCR. Varistor is metal oxide non-linear resistor whose v-i characteristics is nonlinear. So as voltage increases, the slope of v-I characteristics of varistor decreases or resistance decreases or it offers less resistance. So it offers high resistance at low voltage and low resistance at high voltage.
Suppose voltage across thyristor is high, then varistor offer low resistance at this voltage. It means voltage drop across varistor is low as it is in parallel with SCR so voltage across SCR will also be low.
To keep temperature within limit, a suitable heat sink is provided in SCR.
We place a Zener diode parallel to SCR which maintain constant voltage across SCR and protect it from overvoltage. A resistance is also placed which limits gate current and protect SCR from overcurrent. Capacitor and a resistor combination is also placed to increase noise immunity of SCR.
Heat Sink for Thyristor Protection
We mount the heat sink on the thyristor and in order to get best results per unit cost, we use aluminum heat sinks.
Notes on Thyristor Protection
In thyristor, high rate of rise of voltage does not cause damage to the SCR, it will make the SCR turn ON (without gate signal).