Free-Positioning Wireless Charging System for Small Electronic Devices Using a Bowl-Shaped Transmitting Coil

This paper presents a wireless power transfer (WPT) system using a bowl-shaped transmitting (Tx) coil and its application for charging small electronic devices such as hearing aids and wearable electronic devices. The receiving (Rx) electronic devices are placed in the Tx coil and charged effectively with spatial freedom. The WPT system includes three coils for power transfer: a source coil, a Tx resonant coil, and a parallel-resonance Rx coil. The proposed Tx resonant coil is bowl shaped and made of spiral and conical-shaped helical coils in series. A compact box-like Rx coil with ferrite sheets in the box is designed to improve magnetic coupling. To analyze the proposed system, an equivalent circuit of the system is presented. Magnetic fields of the bowl-shaped Tx coil, as well as the mutual inductance between the Tx and Rx coils are derived. For the verification of theoretical analyses and calculations, a wireless charging system for hearing aids is fabricated. It is shown that the proposed system can uniformly charge a Li-ion battery with 30% overall efficiency in the parallel arrangement. Furthermore, even in the perpendicular arrangement, the battery can be charged, except for a narrow dead zone near the center of the Tx coil.