Design Principles and Optimization Considerations of a High Frequency Transformer in GaN Based 1 MHz 2.8 kW LLC Resonant Converter with over 99% Efficiency

LLC resonant converter is widely used for isolated dc-dc power conversion applications due to its soft switching characteristics and simple structure. With the help of wide bandgap (WBG) devices, the switching frequency can be pushed to MHz range and the transformer loss becomes a more critical factor for the efficiency of the whole LLC converter. The high frequency effects, such as skin effect, proximity effect and fringing effect, make the transformer design even more challenging. Many research have been performed to evaluate core loss and copper loss from calculation, simulation or direct measurement in specific tester circuits. However, they are different from real circuit conditions. In this paper, the design principles and optimization considerations for the transformer in MHz LLC converter are presented. An accurate test-based method for core loss and copper loss estimation is proposed. Based on the proposed method, several optimization considerations are provided. To verify the optimized transformer design and its loss prediction method, a GaN based 1 MHz/2.8 kW LLC converter prototype is built, which can achieve 99.04% peak efficiency.