Planar Transformer With Integrated Direct Liquid Cooling

Planar transformers (PTs) are becoming increasingly popular due to the growing demand for compact medium frequency transformers in electric vehicles (EVs) and data centres. Reducing the size of such converters by increasing the power density of the PT remains the most significant challenge. In particular, thermal management of the internal printed circuit board (PCB) layers, which are difficult to access for cooling, poses the primary limitation. Therefore, this paper proposes three integrated direct liquid cooling methods for the PT layer stack. The first uses an additional hollow copper conductor as a cooling channel and the second uses a plastic spacer with a channel for coolant guidance. In these designs, the liquid flows inside the hollow copper conductor and, for the plastic spacer, comes into direct contact with the winding layers. Due to the nature of the copper conductor, additional losses will be induced. As current models cannot correctly predict losses in rectangular hollow conductors, a novel closed form solution has been developed to evaluate additional losses for efficiency and optimal placement. Additionally, the conductor thickness and channel height are optimized to achieve the best thermal performance. Then, a 1D thermal network is developed to estimate the possible temperature reduction of the proposed solutions. The proposed solutions are then compared, and the use of a hollow conductor as an active turn with cooling for high current capability is introduced as a third solution to reduce losses and the problem of current sharing in high current systems with many parallel layers. An experimental setup was built to validate the proposed cooling solutions and a temperature reduction of 54 % was achieved.