Quasi Variable Capacitor for Efficient Heating by Induction Heating

This paper presents some quasi variable capacitors applied for resonance tracking control. Generally, ferromagnetic metals are easily heated in the case of using Induction Heating (IH) and resonant inverters are applied to power supply for IH. However, equivalent circuit parameters of IH, such as inductance and resistance, are changed at Curie temperature. For this reason, each switch is changed to hard switching from soft switching since resonant frequency is changed in accordance with equivalent inductance. Simultaneously, each switching stress is increased and there is a risk of causing a power supply failure. To prevent a power supply failure, each switch should always switch with a soft switching, even if parameters are changed. In order to maintain a soft switching, it is necessary to track a resonance. Although PFM (Pulse Frequency Modulation) is usually utilized as a resonance tracking control, there is a problem in the case of IH. It is that the heating part is changed in accordance with frequency control. To solve this problem, we have proposed quasi variable capacitor. The heating part is not changed since a resonance is tracked at constant frequency by quasi variable capacitor. Consequently, it is useful for IH appliances. Moreover, quasi variable capacitor can be widely used for various conventional inverters such as SEPP (Single Ended Push Pull) inverter, Half bridge inverter and Full bridge inverter. In this paper, validity of the proposed quasi variable capacitor is verified from the simulated results and experimental results.