Coil design for 100 KHz and 6.78 MHz WPT system :Litz and solid wires and winding methods

Coil design is one of the critical part to get high power transfer efficiency for wireless power transfer (WPT) system. There exists coil figure of merit (FOM), which is used to evaluate coil design in the aspect of WPT system efficiency. WPT coil FOM is function of coil quality factor (Q-factor) and coupling coefficient, k and higher Q and k guarantees higher coil-to-coil efficiency. Coil DC loss, skin effect, proximity effect, and displacement current between turns are key factors to determine Q-factor and its effects changes depending on wire-type, winding method, and target frequency. In this paper, we focus on transmitter coil design to maximize Q-factor in both 100 MHz and 6.78 MHz. Coil Q-factor equations are provided to explain and analyze the physics and measurement was done for the validation of the equation. Finally optimal coil winding methods are proposed to maximize coil Q-factor