Optimal Current Distribution in Multi-Winding Transformers for Isolated and Wireless Power Transfer

This work explores the modeling and optimization of multi-winding transformers which may be used for high-frequency isolated power conversion and wireless power transfer. An analytical framework, based on a multi-port impedance matrix, is used to determine the optimal current for each winding that maximizes overall power transfer efficiency. The resulting expressions are used to provide insight into the combined effects of magnetic coupling and power loss among the windings, and is especially relevant for planar-spiral transformers where different windings have different loss and power transfer properties. We apply the model to representative high-frequency planar air-core transformers and show how the optimal current distribution depends on coil geometry and spacing. This is used to show the benefits of the multi-winding approach as compared to conventional single-winding multi-turn spiral transformers. The model is verified with experimental measurements on coreless and magnetic-core multi-winding transformers.