Adjustable Frequency Quasi-Resonant InverterCircuits Having Short-Circuit SwitchAcross Resonant Capacitor

Recently dual frequency induction heating is often performed for hardening an uneven work-piece such as a gear which has tips and bottoms, and two resonant inverters are necessary for such heating. In this paper, new quasi-resonant inverter circuits including a resonant capacitor with a short-circuit switch, which can operate at various frequencies are discussed. First, an equivalently variable resonant capacitor having a short-circuit switch in parallel is described for varying output frequency of the quasi-resonant inverter. Then, two types of adjustable frequency quasi-resonant inverter circuits including the variable resonant capacitor are reported in every desired range of the adjustable output frequency. Additionally a new indirect power control method is proposed since the power to the work-piece cannot be directly detected in operation and the equivalent output resistance of the heating circuit is unknown.