Maximum Output Power Improvement Using Negative Coil in Over-Coupled WPT System

Conventional two-coil wireless power transfer (WPT) systems achieve the maximum output power at the optimal distance. In the over-coupled state, in which the distance between coils is shorter than the optimal distance, the input impedance is significantly higher than that with the optimal distance. As a result, the output power at the load is dramatically decreased. In this letter, we propose a method to improve the maximum output power by applying a negative-impedance converter (NIC) to an additional coil. The proposed system can drastically improve the output power at a load even in the over-coupled state because the negative coil eliminates the impedance reflected by mutual coupling between coils. To validate the proposed method, the NIC was implemented with an operational amplifier and four identical coils. In the experiment, the proposed system transferred enhanced power, which was three times higher than the maximum output power of the conventional system.