Optimal Design Methodology for LLC Resonant Converter in Battery Charging Applications Based on Time-Weighted Average Efficiency

In this paper, an LLC resonant converter design methodology for battery charging applications is proposed aiming at achieving high efficiency. Compared with traditional resistive or constant power load applications, the battery voltage and current are nonlinear and vary with the charging profiles, making the optimal design of battery charger more difficult and complicated. Based on the characteristics of the battery charging profiles, a new time-weighted average efficiency (TWAE) index is proposed, which represents the average weight of conversion efficiency during battery charging period. Converter's losses are calculated based on the standard loss models using the current and voltage information derived from the LLC steady-state model, which is solved utilizing the numerical non-linear programming techniques. In addition, the TWAE is achieved serving as the objective function to optimize the converter parameters. To reduce the search space and speed up the search algorithm, a variable-step exhaustive search algorithm is applied considering the constraints of operation and variable range. Finally, a 3 kW prototype LLC converter, which converts 220 VAC from the grid to a wide output voltage range from 60 to 100 V is built and a TWAE of 94.74% and a peak efficiency of 95.19% are achieved, validating the effectiveness of the presented method.