A 3-D Winding Structure for Planar Transformers and Its Applications to LLC Resonant Converters

Abstract

This article proposed a new 3-D parallel winding method (also named planar-Litz), which just requires minimal interleaving between primary and secondary windings. Low ac resistance and low parasitic capacitance can both be achieved. A general 1-D analytical model is developed to predict parallel winding circulating/eddy current in planar transformers. The analytical model shows that there is high circulating current in direct-parallel windings in high-frequency planar transformers. Direct-parallel windings and planar-Litz windings have been compared both in finite-element analysis (FEA) and experimental prototypes, which proves that the <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$d\Phi / dt$ </tex-math></inline-formula> -induced voltage cancellation in planar-Litz winding can minimize circulating current. A 3.0-kW and a 3.2-kW 500-kHz <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$LLC$ </tex-math></inline-formula> resonant converter are built to compare the direct-parallel winding and planar-Litz winding structures. More than 98.5% peak efficiencies are achieved in both converters. The comparisons of thermal images and efficiencies further verify the effectiveness of planar-Litz winding. The 98.6% peak efficiency and the 97.8% full-load efficiency are measured. The planar-Litz windings planar transformer demonstrated a 26% winding loss reduction and 0.25% converter efficiency improvement over the direct-parallel winding planar transformer on 3 kW. The hot spot inside the transformer can be relieved and hence allow more power delivery.