A Phase-Shift-Controlled Direct AC-to-AC Converter for Induction Heaters

This paper proposes a new control method of a direct ac-to-ac converter consisting of two half-bridge converters and a series-resonant circuit. This circuit converts the ac input power from the utility grid to a high-frequency ac output without any dc stage. This circuit does not require a diode bridge rectifier and, thus, makes it possible to reduce forward voltage drops and power losses in the diodes. A new switching sequence for phase-shift control is proposed to regulate the capacitor voltage in each half-bridge converter and to achieve both zero-voltage switching and synchronous rectification. The proposed phase-shift control is theoretically discussed and is also verified by experimental setup using superjunction power MOSFETs. As a result, the proposed direct ac-to-ac converter exhibits a good power conversion efficiency as high as 97.7% at the rated power of 1.3 kW.

I. Introduction

High inverters are widely applied to many applications such as induction heating systems  [1]–[8] , wireless power transfer systems [9], and so on. Various soft switching techniques have been proposed to reduce the switching power losses in the high-frequency inverters because they perform a high-frequency switching operation. Especially, inverters with a series- or parallel-resonant load have widely been used to achieve zero-voltage and/or zero-current switching operations by tuning of the switching and/or resonant frequencies for reduction of the switching power losses, surge voltages, and spike currents  [10].