PURPOSE:To easily ensure a DC breakdown voltage of a GTO even when a gate driving power supply is lost by short-circuiting a gate and a cathode of the GTO when a means detecting the voltage drop of the gate driving power supply is activated. CONSTITUTION:When interruption of a gate driving power supply 51a of a GTO gate driving circuit 10 takes place, an auxiliary relay 100 is deenergized at the time point when the voltage is less than a prescribed threshold voltage, the contact B of the auxiliary relay 100 short-circuits the gate and the cathode of a GTO 1, resulting that the resistance between the gate and the cathode of the GTO 1 goes to zero. Thus, the DC breakdown strength is improved and the DC breakdown voltage of the GTO 1 is ensured similarly to the case when a negative bias voltage is normal. Thus, even when the loss of the gate driving power supply takes place, the DC breakdown strength of the GTO is easily ensured.
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