A Simple Wireless Power Charging Antenna System: Evaluation of Ferrite Sheet

Abstract

We report a simple wireless power charging (WPC) antenna system (50 mm $\times40$ mm $\times0.1$ mm) for use in simulating power transfer efficiency ( $\eta $ ) with and without ferrite and a metallic object (battery case), thereby identifying suitable magnetic parameters and ferrite sheet thickness. The simulation results suggest that magnetic loss tangent (tan $\delta _{\mu }$ ) and permeability ( $\mu '$ ) need to be less than 0.05 and higher than 125 at 13.56 MHz, respectively, to achieve at least 75% of the transfer efficiency ( $\eta _{\mathrm {max}}$ ) of the WPC antenna without ferrite and the metallic object. Accordingly, we have fabricated Ni x Zn0.85– x Cu0.15Fe2O4 (x = 0.32 – 0.38) spinel ferrites using a one-step sintering process and obtained relatively high $\mu '$ of 169 and low tan $\delta _{\mu }$ of 0.1 with a nickel (Ni) concentration of 0.38. This magnetic loss is still too high to achieve 75% of the $\eta _{\mathrm {max}}$ value. In order to further reduce the magnetic loss, a two-step sintering process was used. The two-step sintered Ni0.38Zn0.47Cu0.15Fe2O4 shows $\mu '$ of 132 and a tan $\delta _{\mu }$ of 0.03 at 13.56 MHz, respectively. Therefore, this ferrite meets criteria identified by the transfer efficiency simulation and is a good candidate for 13.56-MHz wireless power transfer charging antenna system. In addition, our designed WPC system is capable of evaluating other magnetic materials that are suitable for WPC antenna applications.