This paper presents practical design aspects of a high-power DC-DC converter which is applicable to renewable sources for dc grid. The output voltage of a dc-dc resonant converter can be varied by adjusting the switching frequency. In order to maintain output voltage constant from full load to no load using conventional frequency control, the range of switching frequency variation is very high. This consequently increases switching losses, electromagnetic interference which deteriorates the system performance especially for light loads. This paper proposes an algorithm to maintain output voltage constant by variable switching frequency and duty cycle with combined effect. The response using proposed algorithm is tested with 250 W, 110/480V dc-dc resonant converter and performance is analyzed using PSIM simulation.