A 1kW 450kHz induction heating system consisting of a series resonant circuit fed by a voltage-source inverter using power MOSFET's.

Abstract

This paper describes an induction heating system of 1kW 450kHz which is developed for dental casting machines. This system consists of a single-phase diode rectifier, a voltage-source inverter using four power MOSFET's, and a series resonant circuit with a matching transformer of turn ratio 8:1. No capacitor is connected on the DC side of the rectifier except for a high frequency capacitor of 0.47 μ F. Thus, the DC voltage across the high frequency capacitor is fluctuating at 100Hz, that is, twice as high as the line frequency of 50Hz as if no capacitor were connected. Not only does the fluctuation have no effect on heating or melting but also the fluctuation plays an essential role in shaping the line current of the rectifier into sinusoid and unity power factor, because it is seen from the DC side of the rectifier that the inverter plus the resonant circuit acts as a pure resistance.The power control covering a wide range from 50W to 1kW is achieved by power factor control of the inverter which takes the place of DC voltage control of the rectifier.The system efficiency is estimated to be 76% by sophisticated measurement of the actual heating power, which is based on Lissajous's figures for voltage and charge.