Planar transformers are often used in LLC resonant converters due to their unique advantages. Modern switching power supplies typically require compact and lightweight power conversion systems, and research on planar transformers provides an ideal solution for these applications, improving efficiency and reducing costs. At high frequencies, the impact of losses and parasitic parameters in planar transformers on performance becomes significantly more pronounced, making the optimization of winding structures particularly critical. Fractional-turn transformers, as an emerging winding configuration, have primarily been studied in the context of step-down planar transformers. This paper addresses the gap by proposing a “pseudo-fractional-turn” structure for the primary winding of step-up planar transformers. Based on the working principles of fractional-turn transformers, the differences between applying fractional-turn techniques to the primary and secondary sides are highlighted. A simulation model was developed, and the losses and parasitic parameters were measured. The results demonstrate that the proposed fractional-turn transformer reduces winding losses by 28% and total losses by 7.3% compared to traditional integer-turn transformers. Additionally, while maintaining minimal changes in leakage inductance, the parasitic capacitance of the transformer is effectively reduced.