A main advantage of the series resonant inverter is its simple structure. It needs only a non-controlled rectifier as power source, because its output power can easily be changed through adjusting frequency or shifting phase of the inverter. Through calculation and experiment it is shown that by these methods, the when output power of the inverter in some range decreases, the turn-off loss increases because its turn-off current increases. This character on the one hand restricts the output capacity and frequency of the inverter, on the other hand reduces the efficiency of the equipment. In order to solve this problem, a novel control method (cycle control method), by which the turn-off current of the switch is always limited to a fairly small value, is presented. The results of simulation with PSpice and a 60 kHz 6 kW IGBT series inverter induction heating equipment demonstrate that the switches of the series resonant inverter by this conbtrol method have so small switching losses that they can work in high frequency and large capacity range.
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