In this paper, a DC-DC resonant converter with high voltage gain and small size is studied. To reduce the size and weight of this converter, the operating frequency should be as high as possible. In this research, the frequency changes from 120 to 200 kHz. Due to the high operating frequency, it is necessary to reduce the switching losses using soft switching methods. Resonant converters are suitable choice for this purpose. Since the output voltage is high, the series structure can be a good choice.