New Quasi-Resonant Inverter with Short-circuit Switches across the Resonant Capacitor and its Operating Characteristics

Abstract

Two frequencies are often used in induction heating, that is, a lower frequency f_L for preheating and a higher frequency f_H for main heating in order to decrease thermal stress. In this case, each of f_L and f_H is generated by the different inverter, and f_H approximately equals several times to ten times of f_L. Then we propose a new quasi-resonant inverter that two-way short-circuit switches across the resonant capacitor. The proposed inverter has features that its operating frequency is adjustable and the frequency range is so wide that the higher frequency can be ten times as much as the lower frequency. It is considered that the output current has only a fundamental wave by properly manipulating the shorting time of the resonant capacitor even though the output current is not a sinusoidal wave. More over the output power of the inverter can be controlled, which is obtained by subtracting the loss power from the input DC power.