Field-Balanced Windings for High-Frequency Transformers

Abstract

High-frequency operation is a well-established strategy for miniaturizing power converters, but eddy-current losses in magnetics thermally limit its effectiveness in high-power converters. Interleaved windings, which reduce the magnetic fields around the windings, have therefore become standard practice. However, at high frequency, the current crowds to a single conductor surface, limiting the optimal copper thickness and causing excessive winding losses. This paper introduces field-balanced windings (FBW), a family of windings that establish balanced magnetic fields that are both reduced and symmetric about the center of the conductors, leading to current flow on both surfaces of the conductor (“double-sided conduction”). This yields a series of benefits for the transformer design, reducing its footprint, core loss, winding loss, parasitic capacitance, and leakage inductance. A Pareto-optimization case study shows that the FBW transformer design is up to 39% more compact than its interleaved counterpart. Three novel, scalable methods for realizing field-balanced windings are introduced and validated, supported by design guidelines for high-frequency transformers. The concept is demonstrated in two 400/800 V LLC-resonant onboard-charger DC-DC converters: a fixed-frequency 800 kHz DCX converter that delivers 7.4 kW at 97.7 % peak efficiency and 13.5 kW/L, and a frequency-regulated converter that covers the full 720–920 V battery range over 380–800 kHz at 98.4 % peak efficiency and 11.2 kW/L. Among reported transformers, these prototypes achieve the highest operating frequency at the presented power level and realize state-of-the-art surface power density.