AC Resistance Reduction Using Orthogonal Air Gaps in High Frequency Inductors

This paper presents a relatively simple technique to reduce winding losses due to fringing fields in high-frequency inductors. In high-frequency power electronics, ac inductor winding losses are affected by skin and proximity effects, including uneven current distribution due to fringing magnetic fields around airgaps. It is well known how fringing effects can be mitigated using distributed airgaps, at the expense of non-standard core or winding geometry. The orthogonal-airgap approach proposed in this paper combines airgaps in core segments parallel with the windings with airgaps in segments perpendicular to the windings. The approach is developed using a 1D analytical framework and validated by 2D finite-element simulations. Analytical guidelines are presented to optimize the airgaps 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 airgaps result in more than 45% reduction in ac resistance and substantially reduced inductor losses compared to the design using standard airgaps. The results are verified by loss measurements on an experimental converter prototype.