Comparison of Single-Layer and Multi-Layer Windings with Physical Constraints or Strong Harmonics

Abstract

Single-layer and multi-layer transformer and inductor windings are compared. Multi-layer windings (including litz wire) can typically achieve lower loss, but several situations that may favor single-layer windings are discussed. As shown in previous work, for sinusoidal waveforms, if the minimum practical layer thickness is 1.5 skin depths or greater, single-layer windings are preferred. If the maximum number of practical layers is limited, increasing the number of layers decreases the loss approximately inversely proportional to the square root of the number of layers. For waveforms with strong harmonic content, it becomes more difficult to reduce losses with multi-layer windings. We study typical waveforms for power converter circuits and conclude that, for waveforms used in power electronics, multi-layer windings have significant advantages even with only a few layers. The number of layers needed can be as few as two layers for many waveforms; other waveforms, such as square waveforms with fast rise times, typically require four to six layers to effect a significant improvement over a single-layer design.