Short-circuit critical frequency for induction heating parallel resonant inverters

In induction heating applications, the component being heated is very close to the heating coil. Due to the high temperatures reached during the process, isolation is not always possible and short-circuits may happen. Two main types of short-circuit should be considered: complete inductor´s short-circuit and short-circuit between windings. Parallel resonant oscillators are widely used in induction heating applications together with a current fed inverter to generate the alternating current. Parasitic inductance in the connection between the inverter and the oscillator tank may generate high over-voltages if the switching is uncontrolled. The aim of the paper is to describe the protections needed to avoid the failure of the transistors due to over-voltages which may occur during the short-circuit process. Short-circuit critical frequency is calculated and determines whether over-voltages will appear after short-circuit occurs.