Thermal Design, Modeling and Evaluation of Liquid-cooling High-power Planar Transformer

This paper addresses the thermal management challenges of planar transformers. A novel liquid-cooling structure is proposed, featuring direct cooling of the PCB windings and indirect cooling of the magnetic core for improved thermal management. Special attention is given to the core limb—the common thermal weakness of planar transformers—during the design process. The proposed structure achieves outstanding cooling performance with minimal volume increase and no additional loss caused by the cooling structure. To further optimize the parameters of the structure, a three-dimensional (3D) thermal resistance network model of the liquid-cooled planar transformer is developed. Key heat transfer coefficients in the model are calibrated using data from multiple physical test units, which emulate winding and core losses as well as the resulting temperature rise under various operating conditions. After calibration the model achieves improved accuracy and better reflects real-world thermal behavior. The calibrated model is then employed to evaluate and optimize the transformer's thermal performance under varying power losses and coolant flow rates. The detailed heat flow paths and temperature distribution within the transformer are predicted. And a thermal vertical optimization is conducted focusing on the thermal weakness of the transformer. And FEA simulation is further used to optimize the detailed structure of the transformer. With the help of the cooling structure, a 25 kW, 200 kHz planar transformer prototype is built with a basic structure and an enhanced structure. Good agreement is observed between the thermal model and experimental measurements, validating the effectiveness of the proposed approach.