Integrated Fractional-Turn Planar Transformer for MHz and High-Current Applications

In MHz and high current applications, planar transformers face problems such as significant winding loss and limited winding current-carrying capacity, which seriously restrict converter's efficiency and power density improvement. This paper proposes a fractional-turn planar transformer structure and embeds key power devices into the transformer. The proposed method can significantly shorten the winding length, thus reducing winding losses. At the same time, the high integration of the transformer and key power devices can effectively reduce the overall volume of the converter and eliminate terminal losses. The analysis and optimization of the proposed integrated fractional-turn transformer are presented in detail. Finally, a 380V-12V 1kW unregulated LLC is built with the proposed integrated transformer. The full-load efficiency is 97.7%, and the power density is 99W/cm ³ , both of which are the highest among the state-of-the-art prototypes with similar specifications.