A Methodology for Magnetic Structure Design Based on Hierarchical Parallel-Coil in Wireless Power Transfer Systems

While parallel multilayer coils enhance current-carrying capacity, transmission performance, and power density, they inherently suffer from current imbalance due to impedance disparities among parallel branches. This imbalance can induce localized coil overheating and additional power losses. The previous solutions either had high-frequency circulating currents (HFCs) or required the addition of extra inductive components resulting in excessive system volume. To address this limitation, this article proposes a novel structural design methodology for multilayer parallel coils that resolves current imbalance. First, circuit modeling and theoretical derivation establish precise current-balancing conditions. Second, a detailed coil design process determines both integer and fractional turns per layer to satisfy these conditions. Finally, a three-layer parallel coil structure was designed for both primary and secondary sides. A 2.1 kW experimental platform was built based on this design, demonstrating balanced current distribution and a peak efficiency of 89.6%. The proposed coil structure achieves a 1.1% efficiency improvement over conventional designs under heavy-load conditions.