Protecting device for a gate turn-off thyristor

摘要

A protecting device includes a discriminating circuit and a protecting circuit. The GTO thyristor is operated in such a manner that a carrier storing is completed from a first time point at which the supply of a negative gate current is started to a second time point, an anode-cathode voltage increases from the second time point to a third time point and decreases from the third time point to a fourth time point, and increases again from the fourth time point. The discriminating circuit includes a circuit for obtaining an amount of change between the anode-cathode voltages at the third and fourth time points, a circuit for obtaining a ratio of the amount of change to the anode-cathode voltage at the third time point, and a comparing circuit for producing a control signal when the ratio is smaller than a given value. The protecting circuit, when receiving the control signal, stops the conduction of the GTO thyristor. It is judged whether or not the GTO thyristor is operated at a critical point to its break-down.