A converter circuit is provided. The converter circuit includes a capacitor module, bridge arms, a voltage measuring module and an open-circuit detection module. Each bridge arm is connected to the capacitor module in parallel and includes an upper arm and a lower bridge arm connected at a middle point. The voltage measuring module measures voltage differences between each two bridge arms. The open-circuit detection module transmits test impulse signals to the bridge arms to activate at least one upper-conducted bridge arm and at least one lower-conducted bridge arm. The open-circuit detection module retrieves the voltage differences of each pair of the upper-conducted and lower-conducted bridge arms to make comparison with a reference value to determine an operation state thereof, and makes comparison of the operation state determined according to the different groups of test impulse signals to determine whether the upper and the lower bridge arms are actually open circuit.