An Analysis of a Double-layer Electromagnetic Shield for a Universal Contactless Battery Charging Platform

A patented double-layer planar structure is employed to shield the electromagnetic (EM) field at the bottom of a universal charging platform. The double-layer consists of a layer of soft ferromagnetic material and a layer of conductive material. Thin 4F1 ferrite plate and the copper sheet are used in this example. This shielding technique is analyzed with the extended transmission line theory. With the formulas presented, the shielding effectiveness (SE) of both single-layer shield and double-layer shield could be calculated out directly, without using finite-element software. A classical transmitting-receiving loop measurement has been carried out to evaluate the SE of single ferrite shield, single copper shield and double-layer shield respectively. Calculated and measured results in the working frequency range of the charging platform from 100 kHz to 500 kHz confirm that the double-layer shield has obviously better SE than single-layer shield and can achieve a SE above 40 dB, which is commonly accepted by the industry. Finally, the potential of this double-layer shielding technique for a wide range of power electronic applications is discussed