Improved high frequency quasi-resonant inverter for induction heating using power MOSFET

This paper describes the work heating power efficiency characteristics of a high frequency quasi-resonant inverter (QR-INV) and a normal resonant inverter (NR-INV) using power Metal Oxide Semiconductor Field Effect Transistor (MOSFETs), and also introduces a full-bridge type of QR-INV. The output frequency of NR-INV depends on resonant frequency in inverter load and cannot operate with variable frequency. Otherwise, the QR-INV 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 switch and then the resonant frequency can be adjusted. For the sake of the heating in metal stick that the inside diameter is about 2 mm, we are now developing a QR-INV that can vary the output frequency from 160 kHz to 400 kHz [1-5]. Therefore, the power MOSFETs are used as switching devices. 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 f_L or higher frequency f_H respectively, the heating at two different frequencies for uniform heat-treatment is required. Therefore, two NR-INVs are necessary to output different resonant frequency each other. To achieve saving space and the simplicity in the control circuit, a single adjustable frequency half-bridge QR-INV (HB-QR-INV) circuit has been proposed [6-7]. In this paper, power conversion efficiency from inverter input power to work heating power in HB-QR-INV and half-bridge type NR-INV (HB-NR-INV) are investigated in the experimental results. Moreover, we will propose the full-bridge type QR-INV (FB-QR-INV). This type will be able to expect the improvement of the increase of output power compared with HB-QR-INV of the same capacity.