Conversion apparatus with three-level switching circuit and method of operating the same

A conversion apparatus with a three-level switching circuit, comprising:

• a DC conversion module having a DC input end and an output end, the DC input end configured to receive an input voltage, and the output end having a positive output end and a negative output end,
• a three-level circuit, comprising:
• a bridge arm assembly having a first switch unit, a second switch unit, a third switch unit, and a fourth switch unit connected in series;
• a first node between the first switch unit and the second switch unit coupled to the positive output end, and a second node between the third switch unit and the fourth switch unit coupled to the negative output end, and
• a capacitor assembly having a first capacitor and a second capacitor connected in series;
• a positive end of the first capacitor coupled to the first switch unit and a negative end of the second capacitor coupled to the fourth switch unit, and a middle end between the first capacitor and the second capacitor coupled to a third node between the second switch unit and the third switch unit, and
• a control unit configured to control the bridge arm assembly to operate in a DC mode,
• wherein in the DC mode, the control unit is configured to switch the second switch unit and the third switch unit so that the three-level circuit operates in a first state of connecting the first capacitor to the positive output end and the negative output end, and operates in a second state of connecting the second capacitor to the positive output end and the negative output end,
• wherein in the first state, the control unit turns on the third switch unit so that the DC conversion module charges the first capacitor to maintain a first capacitor voltage across the first capacitor to a bus voltage;
• in the second state, the control unit turns on the second switch unit so that the DC conversion module charges the second capacitor to maintain a second capacitor voltage across the second capacitor to the bus voltage, and
• wherein when the bridge arm assembly operates in the DC mode, the control unit switches a switching frequency of the second switch unit and the third switch unit to be a first frequency, and the first frequency is less than or equal to 1 kHz, and
• wherein when the bridge arm assembly operates in the DC mode, the control unit switches a switching frequency of the DC conversion module to be a second frequency which is higher than the first frequency.