The topology of the thyristor semi-controlled rectifier is shown in figure 2-1. The first bridge arm is composed of two thyristors T1 and T2, and the second bridge arm is composed of two diodes D1 and D2. The auxiliary converter directly draws power from the secondary windings of the transformer and outputs a fluctuating dc power after semi-controlled rectification by the thyristor. After LDCCDC filtering on the dc bus, it becomes a relatively stable dc voltage on the capacitor CDC and outputs a stable dc power supply in the subsequent stages. By controlling the trigger Angle of the thyristor, the intention of steady output voltage can be achieved. In this project, the thyristor semi-controlled rectifier is required to convert 1000V single-phase ac to 750V dc.