Characteristics of a high frequency quasi-resonant inverter for induction heating using power MOSFET

In this paper, the frequency characteristics of a high frequency quasi-resonant inverter using power Metal Oxide Semiconductor Field Effect Transistors (MOSFETs) as switching device is described. This inverter circuit includes the first resonant capacitor and that of the second resonant capacitor with a one-way short-circuit switch and can output the variable frequency from 160kHz to 400kHz by using power MOSFETs. The induction heating is often used for the heat-treatment of a metal workpiece. 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 inverters are necessary to output different resonant frequency each other. To solve this problem a single adjustable frequency quasi-resonant inverter circuits. 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 switch and then the resonant frequency can be adjusted. The power MOSFET has the disadvantages of withstand voltage and current. To solve this problem by the methods of connecting switching devices to series [1]. Voltage across the series connected power MOSFETs are balanced by magnetically coupling all gate lines using a simple two-winding transformer. This inverter circuit has the frequency characteristics that the initial current time T₀ changed, the size of the current can be changed and the dead time T_d changed, the output frequency can be changed.