Design optimization of printed spiral coils and impedance matching networks for load-variable wireless power transfer systems

The inductively coupled coil in wireless power transfer (WPT) system is the key to optimizing the power transfer efficiency. Based on an iterative printed spiral coil (PSC) design procedure, this paper proposes a load-based design method to improve the efficiency stability for a load-variable system. The efficiency stability coefficient is defined to choose the optimal PSCs. Several pairs of PSCs are designed and fabricated. Theoretical calculation, simulation and measurement are in good agreement. With the load ranging from 50Ω to 500Ω at 500kHz (transfer distance D=1cm), maximal and minimal measured efficiency of the pair of PSCs derived by the proposed method is 60.1% and 42.5% respectively. Besides, an adaptive impedance matching network is presented, which realizes more stable efficiency.