An output stage circuit of a power conversion circuit includes a first power switch, a driving circuit, a first current source, a second current source and a combining circuit. The first power switch is coupled to a second terminal of a bootstrap capacitor. The driving circuit is coupled to the first terminal of the bootstrap capacitor and the first power switch and provides a control signal to the first power switch. The first current source generates a first current according to the control signal. The second current source generates a second current according to a reference voltage which is a first voltage at the first terminal or a second voltage at the second terminal. The combining circuit, coupled to the driving circuit, the first current source and the second current source, generates a switch operation indicating signal to the driving circuit according to the first current and second current.