A Three-Coil Self-Resonant WPT System for Biomedical Applications

Wireless power transfer (WPT) is a reliable method for powering implantable medical devices. However, achieving high efficiency with a compact receiver (RX) remains a major challenge. Self-resonant wireless power transfer (SRWPT) systems enhance reliability by using parasitic capacitance within planar coils, eliminating external compensation components. This paper presents a fully self-resonant three-coil PCB system that eliminates external capacitors while improving power transfer efficiency for compact RX coils. The system utilizes three specially designed double-layer coil structures: two dedicated to generating high inductance values (L and L), and one configured as an auxiliary coil to further enhance impedance. This arrangement achieves inductances up to four times greater than those of a traditional coil with the same footprint. Through a series combination of mutual inductance (L_m) and mutual capacitance (C_m) within the auxiliary coil network, the resonant frequency and system impedance are significantly increased, leading to improved overall efficiency and system robustness. Moreover, the size mismatch between the transmitter (TX) and receiver (RX) coils necessitates precise synchronization of their resonant frequencies. To achieve this, an air-core structure is employed instead of FR4, allowing flexible frequency tuning through coil spacing adjustments while simultaneously reducing power losses. Simulation results demonstrate that the proposed system achieves a power transfer efficiency of 94.6% at a transmission distance of 10 mm.