Recent advances in GaN switching devices pave the way for realizing miniaturized high efficiency DC-DC converters. This device technology combines with LLC conversion topology, one of the most prominent topology in industrial power electronics, is evaluated in this paper for its suitability and high efficiency performance for satellite applications. A 50W 400 - 500kHz LLC DC-DC converter is designed for spacecraft bus voltage of 70/+ - 2V and with output of 7V/6A; to cater typical SSPA applications. Initially the design was carried out using conventional silicon devices and subsequently replaced with GaN devices and verified the efficiency improvements. Extensive simulation and analysis carried out using modern software tools to optimize it for high efficiency performance.