Reliability

The operation of a thyristor is greatly affected by temperature. Its reliability is ensured only when it is subjected to voltages and currents within specified limits based on permissible junction temperatures. These limits are called the thermal ratings of the device. The different types of thyristor ratings, such as the continuous RMS or averge rating, the intermittent or recurrent peak rating, and the surge or nonrepetitive rating, have already been discussed in Chapter 2. These ratings relate to the anode-to-cathode main power circuit and the gate-to-cathode control circuit. If, for any reason, the device carries voltage and current greater than its ratings, the junction temperature may rise beyond the safety limit and permanently damage the device. Therefore, some form of protection against such hazards is essential.