This paper presents an analytical bus-voltage optimization method for two-stage Vienna–LLC AC–DC converters. The goal is to improve efficiency across a wide operating range. A simple analytical loss model reveals that the total loss and bus voltage exhibit a non-monotonic relationship. This leads to a closed-form solution for the optimal point. The proposed control adjusts the bus voltage in real-time, eliminating the need for curve fitting or extensive measurements. It can also guide converter design. Tests on a 60 kW prototype demonstrate that the method increases peak efficiency by 1.31% with a fixed bus voltage and by 0.91% with the resonant LLC method. The results approach those of a full grid search with minimal computation. This method offers a practical solution for high-efficiency EV fast chargers and industrial power supplies.

DC-link voltage,Energy Loss Analysis,Optimization Design,Vienna rectifier,LLC resonant converter