A novel high step-down DC/DC converter is presented, and its operating characteristics and design principles of significant component parameters are described to meet the design requirements of the low-voltage and high-current power supply module in data centers. The converter is a quasi-parallel Sigma structure made up of the LLC converter and PCL-Buck. Every switch in the topology operates in a soft switching state. The accuracy of the system's steady-state measurements and capacity for dynamic response is significantly increased by the closed-loop control system's optimization for the stability of PCL-Buck voltage regulation. In PSIM, a simulation model with a 100 W, 360 V input and 5 V output was built. The RT-LAB test bench is used to conduct semi-physical experiments. The experimental platform's parameters and simulation all compare favorably. The practicality of the converter is confirmed, and the simulation results mostly match the theoretical study.