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

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 d Phi/dt-induced voltage cancellation in planar-Litz winding can minimize circulating current. A 3.0-kW and a 3.2-kW 500-kHz LLC 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.

Automated summary

This article introduces a new 3-D parallel winding method with low resistance and low capacitance. Through experimental comparisons and finite element analysis, it demonstrates the effectiveness of this method in improving transformer efficiency and reducing winding loss.