Dual high frequency quasi-resonant inverter circuit by using power MOSFET for induction heating

This paper presents the dual high frequency quasi-resonant single inverter circuit that can output the frequency of 100kHz or more by Power Metal Oxide Semiconductor Field Effect Transistor (MOSFET) for induction heating. The induction heating is often used for the heat-treatment of a metal work-piece. In hardening of a gear, since the bottom or tips are heated at lower frequency fL or higher frequency fH respectively, the heating at two different frequencies for uniform heat-treatment is required. Therefore, two inverters are necessary to output different resonant frequency each other. To solve this problem, a single adjustable frequency quasi-resonant inverter circuits which have the short-circuit switch across a capacitor have been reported [1]. This inverter circuit includes the first resonant capacitor and that of the second with a one-way short-circuit switch. Synthetic series capacitance is varied by manipulating the on-time of the switch and then the resonant frequency can be adjusted. Insulated Gate Bipolar Transistor (IGBT) is used as the switching device of the circuit in [1]. The proposed circuit cannot realize with the IGBT to output the high frequency of 100kHz or more. The circuit uses the Power MOSFET instead of the IGBT. Otherwise, the device has the disadvantages of withstand voltage and current. However, this problem can be solved by the methods of connecting switching devices to series and using step down transformer. As an example, we could obtain the dual frequencies of 400kHz and 160kHz with the proposed single inverter composed of the Power MOSFETs.