A Novel Low-Winding-Loss Circulated Interleaving Structure for Planar Transformers Based on Current Invariance Principle

With the increase in frequency, the winding loss of planar transformers (PTs) rises significantly due to the skin and proximity effects, making it one of the major contributors to converter loss. Paralleling layers is commonly adopted in high-current applications to reduce winding loss. However, the most widely used structure, symmetric interleaving, is not the minimum-loss structure according to Ferreira's model, even though it can achieve current sharing. In this article, a new interleaving structure named circulated interleaving is proposed. This structure is invented based on the current invariance principle proposed in this article and has only a 50% loss of symmetric interleaving. The current sharing condition and the minimum winding loss of the two structures are analyzed and compared systematically by mathematical derivation. The 2-D simulations validate the consistency with theory under ideal conditions and explore the variations in current distribution and losses under nonideal conditions, demonstrating that the structure exhibits the same level of advancement under nonideal conditions. Vias effect is also evaluated under 3-D simulations, indicating only 50% inner-core winding loss compared to the symmetric interleaving. Six transformers with different interleaving structures and different copper thicknesses are built and tested and the advantages of the proposed interleaving structure are validated. Practical considerations such as capacitance, leakage inductance, core shape, and heat are also discussed.