Auxiliary power supply systems in traction vehicles are like air brakes, cooling system, air pressures, fans, etc., supplied from a standard 3×400 VAC on-board power grid. The grid is generated by a structure of dedicated converters which usually also provides battery charging functionality. The design of such auxiliary power supply has to respect the fact that the catenary voltage of nominal value 600 VDC or 750 VDC may vary in the range of 400 VDC – ۹۵۰ VDC. Also the outputs, either the power grid or battery charger, must be isolated in order to maintain high level safety of the vehicle. The soft switching DC-DC converters in these systems with improved efficiency are reasonable solution rather than using hard switching topologies. The Power electronics transformer (PET) converter is presented in [1, 2], based on half bridge topology. The PET converter has improved power density and better efficiency over conventional converter [3]. Then researchers concentrated on efficient usage of fuel cell and solar cells based full bridge and half-bridge resonant converters [4]. Consequently, in [5], the peak gain approximation made on full bridge topology using LLC, which is used for low power applications with higher resonant frequency. A hybrid series resonant, full bridge converter [6] achieved the zero voltage switching (ZVS) turn-on and turn-off operations by means of simple series resonant elements such as a series inductor and additional snubber capacitors. However, the converter has poor efficiency, due to its conduction losses. A high power density, compact LLC resonant converter [7] with buck converter principle, operated at higher switching frequency, doubled resonant frequency and incorporated with a passive integration was used to integrate L-L-C-T components. The discontinuous conduction mode (DCM) based phase modulated series resonant full bridge converter [8] with an analytical approach effectively derived the critical load resistance by defining the continuous conduction mode (CCM) and DCM boundary conditions. To simplify the transformer, a self-sustained oscillator controller (SSOC) is used and it leads to increase the value of mutual inductance in a resonant full bridge converter [9]. An isolated resonant boost converter with synchronous rectifier [10] and achieved 93.3% efficiency at 1.5 kW. Nevertheless, it has a poor efficiency and expensive. An LLC FB step-down converter [11] achieved 94% efficiency at 67 kHz switching frequency under 500 W output power. The present paper deals with a high efficiency resonant DCDC converter and it is compared with the conventional hard switched full bridge topology. In this paper, we obtained a ZVZCS turn on and ZCS turn off of all the power switches, and also it improved the overall system efficiency.