Feasible development of soft-switched SIT inverter with load-adaptive frequency-tracking control scheme for induction heating

This paper describes the latest feasible developments of a voltage-source-type series-capacitor-commutated load-resonant high-frequency bridge inverter using static induction transistors (SITs) for industrial induction-heating and melting applications. A load-adaptive variable-frequency tuned controller, composed of a phase-locked loop integrated circuit (PLLIC) device, a pulse amplitude modulation (PAM) power controller employing a phase-controlled AC/DC converter, and protective control schemes for overvoltage and/or overcurrent generation, is also demonstrated for this series resonant inverter. In addition to these, a short-circuit test of a high-power SIT incorporated into the bridge leg high-frequency series resonant inverter is carried out from a practical point of view and evaluated, in order to obtain important design data considering reliable switching characteristics of large-capacity SITs and to realize cost-effective protective circuits for SITs