Relation of gap length and resonant frequency about a double-coil drive type IH cooker

This paper studies possibility of soft switching operation through the calculation of the circuit constants in the double-coil drive type induction heating (IH) cooker, which had been proposed by us. Therefore, we experimentally clarified the effect of (a) gap length between metal load and heating coil and (b) kinds of the metal loads, on the characteristic of the drive circuit in the double-coil drive system. Double-coil drive system is an excellent new technique that can control the increase of switching loss and heating coil loss. Power device used in the IH cooker needs soft switching operation with a view to reduce the switching loss. However, soft switching operation will become impossible under the condition of (a) change of the gap length between the metal load and heating coil; and (b) change of the kinds of metal loads, because these condition brings the change of the inductance-value. So, it becomes very important to evaluate the heating coil inductance in securing the stable soft switching operation. We have focused our attention on the change of inductance-value of the heating coil caused by (a) change of the gap length between the metal load and heating coil; and (b) change of the kinds of metal loads. As the first step of deriving inductance-value, we measured both (a) resonance frequency and (b) equivalent impedance under the resonance. Then, we compared these characteristics with data on a single-coil system, and investigated the possibility of applying skills used in the single-coil system. As the result, it becomes clear that the equivalent impedance on the single-coil system and on the double-coil system shows the strong resemblance. On the other side, the equivalent impedance of the single-coil is lower than the equivalent impedance of the double-coil, and their difference becomes larger when the gap length becomes large. Resonant frequencies of the double-coil system for each gap length are about 0.7 times that of the single-coil system. From theses results, we arrived at a conclusion that the stable soft switching can be realized on the double-coil system, for the practical cooking application.