Based on a half-bridge and full-bridge hybrid modular multilevel converter（MMC）

the pre-charging start-stop process of the PV DC boost converging system is analyzed. During the start process

the voltage of the half-bridge and the full-bridge sub-module in the uncontrolled charging stage can not rise synchronously

and the DC bus voltage is excessively low. This reason is analyzed and the soft start scheme is given. During the stop process with the packet discharge strategy used in the traditional MMC system

the larger impulse current will be generated at the beginning of the discharge and the switching time of the discharge group. Moreover

an energy allocation stage is proposed to reduce the DC bus voltage value during the stop process and reduce the discharge resistance and withstand voltage level. In the discharge stage

the balanced discharge strategy and the energy allocation stage are adopted to effectively suppress the above impulse current. Finally

a photovoltaic high DC collection system optimized by hybrid MMC is built in MATLAB/Simulink

which verifies the effectiveness of the proposed control strategy.