Behavior of thyristors under transient conditions

The capability of gate-triggered thyristors to withstand steep wavefront, high-current pulses (i.e., di/dt capability) is a function of both junction temperature and frequency of operation. Localized internal heating occurs during turn-on and may lead to thermal runaway. The conditions required for this to occur have been determined by destructively testing many devices. The initial conducting area of a thyristor largely determines di/dt capability, which is not necessarily related to the size of the device but is a function of the design of the gate region. Gate drive is very important for determining the di/dt capability of a thyristor having a conventional gate design. Two devices which have been designed to increase the initial conducting area are discussed. One of these devices, if improperly designed, can lose its effectiveness with high gate drive. This characteristic can be studied by observing the reverse recovery current immediately following short forward current pulses.