Protection of thyristors against overvoltage with breakover diodes

Overvoltage protection is achieved by connecting the breakover diode (BOD) between anode and gate of a thyristor in such a way that it is turned on when excessive voltage occurs. The turn-on delay time of the thyristor is dependent on the magnitude of the supplied BOD current. The delay time can be less than 0.3 μs, so that protection against steep transient is possible. The BOD is, in many cases, indispensable for thyristor strings when the load conditions are unpredictable