Abstract: At present, high-reliability DC/DC power supply modules with medium and low voltage input generally use Si-based power switches, with a typical input voltage of 28V and a switching frequency of 500kHz. With the development of miniaturization of module power supply and the continuous increase of switching frequency, the switching loss increases greatly, which seriously affects the power conversion. efficient. The application of soft-switching technology can greatly reduce the switching loss caused by high frequency. However, the structure of the soft-switching control circuit is complex, and there is currently no high-end integrated controller available in the military high-reliability field. GaN devices have extremely low gate charge, output capacitance and zero reverse recovery charge characteristics, which can effectively reduce switching losses caused by high-frequency applications without increasing circuit complexity. successful application in the device. However, in the medium and low voltage input military high-reliability power modules, with the substantial reduction of bus voltage and the increase of switching frequency, whether the high-speed characteristics of GaN devices can effectively reduce switching losses and improve conversion efficiency remains to be verified. In this paper, a single-ended flyback power topology and synchronous rectification technology are used to design a principle prototype with a typical input of 28V, output of 5V/30W, and a switching frequency of 1MHz. The loss and efficiency curve of the base device at a switching frequency of 1MHz shows that under the condition of medium and low voltage and high frequency, the conversion efficiency of the GaN device is improved by 4% compared with the Si-based device, and the voltage stress of the power switch is controlled within a reasonable range. It is of great significance for the miniaturization and high frequency development of medium and low voltage input DC/DC converters in the field of military high reliability.