Reduction of AC Winding Losses Due to Fringing-Field Effects in High-Frequency Inductors With Orthogonal Air Gaps

In high-frequency inductors, ac winding losses are affected by skin and proximity effects, including uneven current distribution due to fringing magnetic fields around air gaps. It is well known that fringing effects can be mitigated using distributed air gaps. This article is focused on an orthogonal air-gap approach, which is a distributed air-gap technique where gaps are placed in core segments parallel with the windings and in core segments perpendicular to the windings. The orthogonal air-gap approach is developed using a one-dimensional analytical framework and validated by two-dimensional and three-dimensional finite-element simulations. Analytical guidelines are presented to optimize the air-gap distribution to achieve minimum ac resistance. As a case study, a planar inductor is designed for an 8-kW SiC-based buck converter operating at 250 kHz. It is shown how the orthogonal air gaps result in approximately 50% reduction in ac resistance and substantially reduced inductor losses compared to the design using standard air gaps. Experimental validation includes measurements of losses on the converter prototype as well as quality factor measurements in a resonant-circuit test setup.