Analysis and Design of Novel High-Frequency Integrated Transformer with Adjusting Leakage Inductance for LLC Resonant Converter

The LLC resonant converter, widely used in DC-DC converter topologies for EV charging systems, is an isolated converter with separated primary and secondary sides, making effective transformer design essential. When applying an integrated transformer for high-efficiency and high-density LLC resonant converters, it is crucial to adjust the leakage inductance to approximate the design value in order to achieve the desired output voltage range of the converter. Although various integrated transformer structures and design techniques have been researched to adjust leakage inductance, they often require additional devices or complex winding structures, making them difficult to apply to wound transformers. This paper proposes a new wound-type integrated transformer structure that enables adjustment of leakage inductance without additional devices through sections where primary and secondary windings are completely separated and sections where they partially overlap. The proposed integrated transformer structure is elaborated based on design techniques that consider not only efficiency and power density but also reliability factors such as flux and temperature saturation. The proposed structure and design techniques were validated through Finite Element Analysis (FEA) simulations and experiments with a 10kW LLC resonant converter.