Free-positioning wireless charging system for hearing aids using a bowl-shaped transmitting coil

This paper proposes a wireless power transfer system for charging hearing aids. A bowl-shaped transmitting (Tx) coil is to achieve free-positioning of wireless charging. The proposed Tx coil consists of both spiral and conical-shaped helical loops in series. A receiving coil is wound on a rectangular box made of ferrite sheets to improve coupling coefficients. It is shown that the proposed system can uniformly recharge a Li-Ion battery of the hearing aid with 30% overall efficiency in the parallel arrangement. Furthermore, even in the perpendicular arrangement, the battery can be recharged except a narrow dead zone near the center of the Tx coil.