Increasing frequency using GTO in gate-assisted-turn-off mode

The main advantage of GTO over thyristors is to reduce tq (turn-off time) to zero but at the cost of increasing Woff losses due to the tail current that sweeps residual stored charge off the thick N base while the voltage is restored across the device. Nevertheless, no significant improvement is obtainable if tq is much shorter than the carriers' life time in the N base, and auxiliary circuits' disposals, being considered equivalent for thyristors, are not equivalent in the case of GTO, some of them being dangerous for the device. To demonstrate this, a practical problem has been experienced with the aim of 25 kHz, 500 V, 230 A application. Four different GAT mode circuits (A, B, D, E), using the same 2 mu F capacitor as that used for the GTO mode, allowing approximately 4 mu s tq, have been tested (one shot Woff measurements at Tj=110 degrees C) and results have been compared to the losses of the GTO mode and of a thyristor mode (tq=8 mu s). Nine GTO and two thyristors have been cross tested that way.<
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