High-Efficiency Self-Resonant Three-Coil Wireless Power Transfer System for Biomedical Implants

Self-resonant wireless power transfer (SWPT) systems eliminate external compensation components by exploiting the intrinsic parasitic properties of planar coils, making them attractive for compact receiver designs. This article proposes a self-resonant three-coil WPT architecture that enables high inductance generation and efficient power transfer within a limited receiver footprint. The receiver is implemented using two interconnected double-layer printed circuit board (PCB) coils, enabling a substantial increase in effective inductance without enlarging the coil area. An auxiliary resonant path is formed using the inherent inductive (L_m) and capacitive (C_m) properties of the coil structure, resulting in enhanced impedance characteristics and improved power transfer robustness. On the transmitter side, an air-gap configuration is employed to allow flexible resonance tuning through coil spacing while reducing dielectric losses associated with conventional substrates. The proposed system is systematically evaluated and compared with conventional self-resonant three-coil and self-resonant two-coil configurations under identical operating conditions. Comprehensive simulation and experimental results show that the proposed architecture achieves a peak efficiency of 92%, compared to 73% for a conventional self-resonant three-coil system, along with a voltage gain of 25.1 and an output power of 360 mW, while maintaining a compact, capacitor-free receiver structure.