The dilemma between wide input range and narrow frequency band is a classic problem for the frequency modulated LLC converters. Using reconfiguring bridge can mitigate this issue effectively. However, there is a lack of systematic consideration in converter design and optimization. To address this challenge, this work introduces a six-mode LLC converter based on a reconfigurable H5 bridge, and systematically presents its optimal design methodology in ultra-wide input range applications. The dual LLC resonant tanks are driven by identical switching frequency, and provide a normalized gain range from 1 to 5. Synchronized rectification is employed on the secondary side to improve the gain and efficiency. The load distributions between dual tanks are analyzed in detail. It indicates that the equivalent output capacitance of the primary-side MOSFETs is reduced. This enhances the ZVS performance and reduces the circulating loss. The optimal design of transformer ensures a continuous voltage gain over adjacent modes. A 48V, 500W laboratory prototype is designed to validate the concept. The designed prototype is adapted to an 80V∼400V input range. The maximum efficiency is 96.95%.