The rapid evolution of electric vehicles (EVs) has prompted extensive exploration into higher voltage regimes to enhance their overall performance. Within an 800-volt architecture, the 48-volt level has emerged as the next frontier for auxiliary power supply enhancement. This paper introduces an 800V to 48V Auxiliary Power Unit (APU). To efficiently manage the elevated input voltage while alleviating primary-side device voltage stress by 50%, a stacked half-bridge (SHB) LLC converter is designed, capitalizing on the favorable attributes of 650V Gallium Nitride (GaN) wide-bandgap semiconductor devices. Furthermore, the study conducts a comparative analysis between matrix transformers and fractional-turn transformers, seeking to determine the optimal number of transformer cells and the requisite primary and secondary turns. Subsequently, in pursuit of heightened power density, the conventional circular central post within the planar transformer (PT) is substituted with a square configuration, accompanied by a reduction in the PT's height. The viability of the proposed transformer design is substantiated through validation on a prototype.