Power conversion device comprising three-level inverter and balance circuit and control method thereof

A control method of a power conversion device, comprising steps of:

• (a) providing a power conversion device, wherein the power conversion device comprises a three-level inverter, a first positive DC terminal, a first negative DC terminal, a first capacitor, a second capacitor, a balance circuit and a bidirectional DC-DC converter, the three-level inverter has a first AC terminal, a second AC terminal and two DC terminals, the three-level inverter is configured for AC-DC power conversion, the first positive DC terminal and the first negative DC terminal are coupled to the two DC terminals respectively, a capacitor midpoint is between the first capacitor and the second capacitor, two terminals of the first capacitor are coupled to the first positive DC terminal and the capacitor midpoint respectively, two terminals of the second capacitor are coupled to the capacitor midpoint and the first negative DC terminal respectively, the balance circuit is electrically connected between the first positive DC terminal and the first negative DC terminal, the balance circuit has a neutral terminal electrically connected to the capacitor midpoint, the bidirectional DC-DC converter is configured for DC-DC voltage conversion, a first side of the bidirectional DC-DC converter is electrically connected to the first positive DC terminal and the first negative DC terminal, when the three-level inverter receives an AC power, a second side of the bidirectional DC-DC converter provides a DC power correspondingly, and when the second side of the bidirectional DC-DC converter receives a DC power, the three-level inverter provides an AC power correspondingly;
• and
• (b) controlling switches of the balance circuit according to a first capacitor voltage across the first capacitor and a second capacitor voltage across the second capacitor to keep the first capacitor voltage and the second capacitor voltage equal,
• wherein the balance circuit comprises a first balance switch, a second balance switch and an inductor, a first terminal of the first balance switch is electrically connected to the first positive DC terminal, a second terminal of the first balance switch is electrically connected to a first terminal of the second balance switch and a first terminal of the inductor, a second terminal of the second balance switch is electrically connected to the first negative DC terminal, a second terminal of the inductor is electrically connected to the neutral terminal, a control signal of the first balance switch is complementary to a control signal of the second balance switch, and the step (b) of the control method comprises a sub-step of:
• (b1) comparing the first capacitor voltage with the second capacitor voltage, and regulating a duty ratio of the control signal of the first balance switch according to a comparison result, wherein the duty ratio is less than 0.5 when the first capacitor voltage is greater than the second capacitor voltage, and the duty ratio is greater than 0.5 when the first capacitor voltage is less than the second capacitor voltage, wherein a magnitude of the duty ratio of the control signal of the first balance switch is determined using a proportional-integral controller receiving an average current value of the inductor and an intermediate result based on a difference value between the first capacitor voltage and the second capacitor voltage.