In this paper, quarter-turn planar transformer structure is proposed and implemented in an LLC resonant converter for server power supply applications. Main power losses of high-frequency resonant converters are secondary-side winding loss and transformer core loss. Conventional transformer winding structures limit the minimum number of turns on the primary and secondary sides, such that the core size and secondary-side conduction loss cannot be reduced by increasing the frequency. To achieve high efficiency and high power density, a fractional-turn transformer structure can reduce core volume and conduction loss in the windings. The optimized transformer design also enables the turn ratio of the fractional-turn to achieve normal coupling of primary and secondary sides without generating flux imbalance. The effects of different fractional-turn windings on efficiency are examined, and Maxwell simulation results are used to verify the structure and operation of the planar transformer. The printed circuit board layout is analyzed using Q3D Extractor to reduce the uneven current generated by parasitic components on the secondary-side windings. Wide-bandgap switches are utilized to implement a resonant converter with 1-MHz operating frequency, 380-V input voltage, 12-V output voltage, 1-kW output power, 37.6-W/cm³ power density, and 97% maximum efficiency.