Optimization for High-Frequency Power Transformers with Multi-Winding Current Ballasting

This work explores optimization of multi-winding transformers which may be used for high-frequency isolated power conversion and wireless power transfer. We extend the concept of current ballasting previously used to reduce AC losses in multi-winding inductors, allowing efficient operation at higher frequencies. 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. Design insights for planar spiral transformers include that the optimal current distribution depends on both loss and coupling of each winding but interestingly, the relative loss fraction of each winding should be the same. The analytical model has been verified with numerical optimization and experimental measurements using wire-wound multi-winding transformer.