High Frequency PCB Winding Transformer Design for On Board Battery Charger

摘要

Traditionally, copper sheet or litz-wire transformers are used in on board battery charger systems to reduce the winding loss. However, the complexity of manufacturing the transformer often involves extensive labor work. With advanced wide band gap devices, the switching frequency can be greatly increased, providing the opportunity to use PCB as transformer windings. Also, the use of a novel two stage structure and control can greatly reduce the required resonant inductance, providing the opportunity to integrate leakage into the transformer serving as resonant inductor. In this paper, the impact of magnetizing inductance and fringing effect on PCB winding loss is analyzed and a novel transformer structure is proposed for the second stage of the on board battery charger so that leakage inductance can be controlled. By adopting center post as leakage path, leakage flux is well confined within the center post, avoiding additional eddy current loss. A transformer loss model is built and a design procedure is provided to get the lowest loss within the target footprint. A 6.6kW 500kHz CLLC resonant converter prototype with 97.7% efficiency is built to verify the feasibility of the proposed transformer.