Design of an Integrated Transformer with Parallel Windings for a 30-kW LLC Resonant Converter

Transformer design is critical to high-power high-frequency dc/dc resonant converters. For the design of a high-current transformer with parallel windings, current sharing, resonant inductance integration as well as performance optimization are still quite challenging. Therefore, a comprehensive analytical model of the current distribution between the parallel litz wire windings is developed in this paper. Based on this model, a novel winding structure is proposed to eliminate the current discrepancy. Furthermore, the performances of two typical resonant inductance integration methods are compared when applied to parallel windings. With the selected transformer structure, loss and volume optimization is also conducted. It is worth mentioning that the proposed transformer structure and design principles have good scalability and can be extended to higher power ratings easily. Finally, the analysis and design are verified in two 100-kHz transformers for a 30-kW LLC resonant converter. The current difference between parallel windings does not exceed 3.6%. The efficiency and power density of the transformer at 30 kW is up to 99.6% and 22.4 kW/L, respectively.