Optimal inner diameter ratio of spiral coils for wireless charging

Abstract

Many electronic devices such as smartphones can now utilize wireless charging. Most of the power transmitting and receiving coils built into these devices are single-layer planar spiral coils to make them lighter and thinner. However, manufacturers have difficulty determining whether or not to wind the coil all the way to the center and what the inner diameter of the coil should be. In this paper, we clarify the various electrical and physical properties given by different inner diameters and present the optimal inner diameter ratio for power transfer efficiency. The analysis results show that the optimal inner diameter ratio for obtaining the maximum power transfer efficiency in the resonant frequency range of 100 to 200 kHz is 0.288 to 0.391 when the outer diameter is 43 mm.