This paper presents a comparison of four different isolated DC-DC converters: the Dual active bridge, the LLC resonant, the series LC resonant and the Phase-shift converter. The comparison evaluates three characteristics for a 3kW high/low voltage application: efficiency, power density and reliability. Time domain simulations are carried on each topology with Simetrix and Octave environmental to integrate the reliability analysis based on an output power profile yielding to select a converter topology as a function of these parameters. The difficult in to determine an accurate model to estimate the reliability of GaN power transistors is also discussed and by using the FIDES model, three different transistors are compared, two GaN power transistors and one Si mosfet. The analysis and simulation results identify, when designed with one of the GaN, the LC resonant and the Dual Active Bridge as the most efficient converters, and the LC resonant converter as the more reliable.