Design of Low-Loss Magnetic-Core Toroidal Inductor for Multi-MHz Wireless Power Transfer Systems

Matching network inductors are the most lossy components in multi-MHz capacitive wireless power transfer (WPT) systems and are traditionally implemented using air-core inductors to avoid high-frequency core losses. Although air-core inductors can achieve high quality factors, they are bulky and could pose electromagnetic interference (EMI) concerns. To address these issues, this paper investigates a magnetic-core-based toroidal inductor structure to achieve high-power, high-frequency, high-Q cored inductors. The proposed inductor structure leverages NiZn ferrites with better core loss characteristics at MHz frequencies, quasi distributed air-gaps and interleaved multi-layer foil windings to enhance performance. Quasi-distributed air-gaps help minimize fringing losses, while interleaved foil windings reduce winding losses by paralleling two or more windings. Furthermore, design guidelines for such inductors are discussed with appropriate examples. To validate the proposed inductor structure, two 5 μH identically shaped magnetic core toroidal inductors, one with single-layer foil winding and the other with two-layer interleaved foil winding are built and tested at 13.56 MHz up to 20 A peak current. The inductor with single-layer winding achieves a quality factor of ∼725 experimentally at 20 A peak current whereas two-layer interleaved foil winding inductor achieves a quality factor of ∼1190, resulting in 65% improvement.