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The LLC resonant converter topology is widely used in dc-dc converter applications due to its advantages in achieving high efficiency and high power density. However, due to the complexity in the analysis, the converter lacks a clear design guideline on the selection of resonant tank parameters. In this paper, an optimization method is developed based on the operation mode analysis and peak gain placement. Following this approach, the converter can minimize the conduction loss while maintaining the required gain range. A 400-W, 400-V output and 25-38 V input LLC converter is built using the proposed method, which achieves above 98% peak efficiency. As a comparison, the conventional searching method is used to reselect the LLC parameters for the same specifications, and the experimental results show that the optimal design has better performance.