Turn on switching losses analysis for Si and SiC diodes in induction heating inverters

In high frequency induction heating applications, the inverters used to generate the high frequency alternating current are implemented with MOSFET. In order to avoid the failure of the MOSFET intrinsic diode, each switch is formed by two extern diodes which are connected to the MOSFET, one in series, and the other one in anti-parallel with both devices. If the power regulation is achieved by using the frequency variation control strategy, the MOSFET turn-off causes the turn-on of the anti-parallel diode in the same arm of the inverter. In given conditions of load (null load), the MOSFET turns-off the maximum current. Hence the diode turns-on with maximum current. On switching losses in fast Silicon diodes cannot be neglected for switching frequencies around 400kHz. In this paper, a solution to reduce the on switching losses in the diodes is proposed by replacing Si diodes by SiC diodes with similar characteristics. For this purpose, the on switching losses have been characterised for several Silicon Carbide diodes. Finally, the results were verified in a 25kW inverter working at 400kHz.