A Square-Shaped Omnidirectional Wireless Charing Bowl with a Double Layer Electromagnetic Shield for Portable Device Applications
Wireless power transfer (WPT) is a promising solution for delivering power to a battery in a variety of applications, and near-field inductive power transfer technology has been widely adopted in consumer electronics applications. A three-dimensional WPT system with omnidirectional power transfer capability is preferred due to the spatial charging freedom. In this paper, a square-shaped wireless charging bowl is proposed; the transmitter coil structure is shown in Fig. 1.
An important design issue of an inductive power transfer system is minimizing human exposure to electromagnetic fields (EMFs). Several regulation documents offer guidance for preventing adverse health effects due to EMFs. To comply with these standards, a ferrite shield is added to reduce the stray field level; in the proposed setup, the ferrite shield layer is wrapped around the charging bowl to confine the magnetic field. Cases for the field distribution on the yz plane for a system with a ferrite shield and without a ferrite shield are shown in Fig. 2(a) and (b) respectively. Herein, the stray field outside the charging bowl decreases, but the attenuation is not enough due to the low permeability of the available high frequency (6.78 MHz) ferrite material.
To further reduce the stray field level, a conductive layer is added to the ferrite layer. The resulting yz plane field distribution is shown in Fig. 3. The stray field outside the bowl is attenuated effectively through the induction of an eddy-current within the conductive layer. Thanks to the ferrite layer, the magnetic field inside the bowl doesnt change much; therefore, the power transfer capability wouldnt suffer even with the conductive layer. And, according to Finite Element Analysis simulation, the eddy current loss in the conductive layer is around 1 mW. However, the ferrite loss is around 0.3 W at 5 W output, and the system efficiency drops from 82% to 78% after adding the double-layer shield.