Soft switching inverter for induction heating with power factor control function using quasi variable capacitor

Recently, reduction of toxic substances included in exhaust gas, and conversion to a low carbon society are demanded for the purpose of environment protection. To prevent global warming, it is important to progress energy-saving technologies and environment protection technologies. IH (Induction heating) technology is useful for energy-saving because it is an efficient heat source. In this study, induction heating is utilized for purifying the toxic substances included in exhaust gas from marine diesel engine. Ferromagnetic metals are suitable for metallic filter within this system. However, because ferromagnetic metals have Curie temperature, equivalent circuit parameters of induction heating are rapidly changed at Curie temperature. For this reason, automatic resonance tracking control is applied to prevent breakdown of power supply. In general, although resonant frequency is tracked by PFM (Pulse Frequency Modulation), the heating piece is changed because a penetration depth is changed in accordance with controlling frequency. Consequently, it is desirable that the power supply for IH is operated at constant frequency. In this paper, to heat the metals at constant frequency, quasi variable capacitor is proposed and is applied to power supply for IH as resonance tracking control. The proposed quasi variable capacitor performs the same function as a variable capacitor. As a result, the proposed inverter can control a power factor. This is an advantage that resonant frequency is tracked and that metallic filter is also heated efficiently. Moreover an output power is controlled, without changing frequency.