In MHz-frequency capacitive wireless power transfer (CWPT) systems, air-core magnetics is the preferred choice for realizing the compensation inductors due to absence of frequency-dependent core loss. Such inductors are typically realized using rectangular foil on air-core solenoids, and single-layer or interleaved multi-layer foils on toroids. However, higher power CWPT systems demand larger foil width, adversely affecting inductor size in solenoids, while increasing parasitic capacitance and proximity losses in interleaved structures. This digest examines different wire geometries for high-power air-core solenoidal inductors, demonstrating that the use of hollow cylindrical conductors results in significantly reduced AC resistance. It is further shown using FEM-based simulations that for a specified target inductance, there exists an optimal inter-turn spacing that minimizes the overall AC resistance. Two 1.1-kW, 6.78-MHz, 15-cm air-gap CWPT prototypes, one utilizing conventional foil-wire inductors and another utilizing the proposed inductors are developed and tested, with the proposed inductor resulting in 35% loss reduction.
