Frequency control of a current-fed inverter for induction heating

A miniature high frequency induction furnace has been developed for the melting and alloying of small quantities (50g) of metal for laboratory and jewellery applications. The induction heating load forms part of a resonant circuit whose frequency is determined by the electrical characteristics of the work-piece placed inside the heating coil. The dynamic nature of the induction-heating load is such that it causes a shift in the natural resonant frequency of the load circuit. Automatic frequency control of the power source enables continuous operation at the resonant frequency of the tank circuit, resulting in a high conversion efficiency due to zero-voltage switching (ZVS) and maximum power transfer to the load at all times. This paper describes a PLL frequency control system for the prototype miniature induction furnace. Some basic design criteria as well as the actual implementation of the system are presented. Experimental results are presented to illustrate the effectiveness of the automatic frequency control system.