Double-Sided Conduction: A Loss-Reduction Technique for High Frequency Transformers

Power converters are increasingly being operated in the high-frequency (HF) regime (3–30 MHz), where proximity-effect losses in transformers are difficult to contain. Litz wire does not scale well to multi-MHz operation where sub-skin-depth strands are prohibitive to manufacture. Interleaving is more scalable to HF and, in its best implementations, causes a current equal to the net current to flow on a single surface of a conductor or layer of conductors (e.g., a copper turn in a planar transformer), with no “negative” current or eddy currents. In this paper, we propose a field-shaping technique that results in the even distribution of current on two sides of each conductor layer, yielding two skin-depths worth of conduction. We present one method to achieve this by paralleling the first and last layer of a foil-wound transformer. We demonstrate this technique theoretically, through FEA simulation, and through an experimental prototype.