Switching losses in a GTO inverter for induction heating

The application of gate-turn-off thyristors (GTOs) to parallel resonant inverters for induction heating is investigated. Three different modes of operation of the GTOs are explained and tested in a test circuit. The optimum mode is selected by switching loss measurement. It is shown that due to the small inductance in the commutation circuit, the gate-assisted turn-off technique leads to very high turn-on losses and is not the optimum way to drive the GTO in this application. However, it can be used under different conditions in order to reduce turn-off losses. It is suggested that a mixed turn-off of the devices by both gate action and natural commutation is the optimum mode of operation. Then, the load is driven at unity power factor. In this mode of operation, no snubbers are necessary in principle. A small capacitor in parallel to the GTO is recommended for only the staring procedure. An estimation of the total power loss in the GTOs leads to the result that the GTO tested can be operated at frequencies between 10 kHz and 20 kHz with a reasonable inverter efficiency