Proposal of a Figure of Merit for the Characterization of Soft-Magnetic Shielding Material Used in Inductive Wireless Power Transmission Systems

In inductive wireless power transmission systems, often soft-magnetic shielding is used to avoid lossy eddy currents being induced in electrically conducting components like batteries or ground layers of electronic circuits. Datasheet information on such shielding materials are often limited to magnetic permeability and sometimes exemplary loss information. For designing inductive wireless power systems, e.g., at variable frequency, detailed loss information are of interest. Therefore, it is proposed to measure the impact of these materials on the power transmission in a standardized setup, which is closely related to the real application. This consists of two coils, a transmitting and a receiving coil. Here, a configuration as described in the Qi standard for wireless charging of mobile devices published by the wireless power consortium is used as reference. A figure of merit, i.e., the product of the coupling factor k and the geometric average of the coil´s quality factors Q, is proposed to qualify materials concerning both: shielding against conducting components on the backside of the receiving coil; and establishing a high mutual inductance of the transformer coils, resulting in higher system efficiencies. Furthermore, considerations on the measurement setup as well as qualifications of shielding materials are presented.