A power processing unit suitable for Hall electric propulsion systems is designed to meet the high input voltage requirements of space missions such as deep space exploration. The LLC resonant converter is studied for half bridge full bridge dynamic topology switching and soft start, achieving high efficiency and wide range output. By establishing a fundamental model of LLC resonant converter, analyzing the characteristic curves of frequency and gain, selecting the values of K and Q, variable frequency control output voltage is achieved, and a low peak full bridge half bridge switching mode based on simultaneous frequency conversion and variable duty cycle is designed. In order to verify the design performance, a 5 KW prototype was designed, with an input of 400 V and an output of 200 V to 1000 V. PSIM simulation was conducted on the LLC resonant converter built, and the results showed that it can achieve a wide range and high efficiency output of the LLC resonant converter.