Design and modeling of a PCB coil with reduced copper trace for self-resonant WPT systems

Inductive wireless power transfer (WPT) systems require compensating capacitors to operate efficiently. However, these capacitors can introduce additional costs, weight, and losses, particularly at MHz-range operating frequencies. To mitigate these issues, self-resonant coils can be used by exploiting the parasitic capacitances inherent in double-layer printed circuit boards (PCBs). In this work, a double-layer coil for a self-resonant series WPT system is developed for operation at the AirFuel standard frequency of 6.78 MHz. Unlike conventional design methods adopted for self-resonant coils, mainly based on analytical formulations, the proposed approach employs Finite Element Method simulations that couple electro-quasistatic and magneto-quasistatic formulations. This approach enables accurate evaluation of the self-resonant frequency (SRF) as well as the current distribution along the PCB layers, revealing significant non-uniformity. Building on this insight, copper trace length is reduced to save material while maintaining performance in terms of operating frequency. Both the full-trace and reduced-trace coils are fabricated and tested, showing good agreement with simulations. Notably, the proposed reduction in the copper trace affects the SRF by only 3% compared with the full-trace design.