A 6.78-MHz Free-Positioning Wireless Charging Bowl with Optimized Passive Electromagnetic Shield for Wearable Devices

This paper presents a wireless charging bowl with a double-layer electromagnetic shield comprising a magnetic layer and a conductive layer to reduce the leakage magnetic field. A complete parametric analysis of the wireless power transfer (WPT) system with the double-layer shield structure is performed using Ansys High Frequency Structure Simulator (HFSS) software. The simulation results show that the quality factor of the transmitter (Tx) is effectively improved by increasing the gap between the shield and the Tx, while the shielding characteristic is preserved. An optimal gap of 5 mm is selected to achieve a balance between the system performance and the Tx size. The measured maximum power transfer efficiency (PTE) of the WPT system with the proposed Tx decreases only 3.5%, from 95 % to 91.5%, when applying the double-layer shield with an optimal gap of 5 mm, while the leakage magnetic field is reduced by 98%.