Improved Thin Heating Coil Structure of Copper Foil Feasible for Induction Cookers

Abstract

Litz wire is widely utilized for heating coils of the induction cookers to suppress the proximity effect. However, low space factor of the Litz wire often significantly increases the heating coil height, which may be an obstacle of wide-spread of the induction cookers. In order to overcome this problem, a preceding study has proposed a heating coil structure of the copper foil, which suppresses the proximity effect without using the Litz wire. The preceding study has proven effective suppression of the proximity effect, when the material to be heated is placed to cover the heating coil. However, this heating coil structure still suffers from large AC resistance, when the material to be heated does not exist or partially covers the heating coil. Furthermore, the ferrite walls, installed to suppress the proximity effect, need larger height than the coil, hindering effective reduction of the total heating coil height. The purpose of this paper is to propose an improved heating coil structure. The proposed heating coil structure further incorporates a thin layer of low-permeability soft-magnetic material covering the top of the coil. This layer enables elimination of the proximity effect regardless of the existence or the disposition of the material to be heated. Furthermore, this layer enables the ferrite walls to have the same height as the coil, thus further reducing the total heating coil height. The FEM based simulation was carried out to verify the operating principle of this structure. The result supported reduction of the heating coil height and effective suppression of the AC resistance regardless of the existence and the disposition of the material to be heated.