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

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
Zn
_0.85–x
Cu
_0.15
Fe
₂
O
₄
(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 Ni
_0.38
Zn
_0.47
Cu
_0.15
Fe
₂
O
₄
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.