An integrated circuit for a power supply circuit that includes a transformer and a transistor. The integrated circuit includes a first terminal receiving a voltage corresponding to a coil voltage across an auxiliary coil of the transformer when the transistor is off, a second terminal receiving a feedback voltage corresponding to an output voltage of the power supply circuit, a third terminal receiving a voltage that corresponds to a current flowing through the transistor and the coil voltage respectively when the transistor is on and off, a detection circuit configured to detect whether the voltage at the third terminal when the transistor is off is lower than a reference voltage, and a control circuit configured to control switching of the transistor based on the feedback voltage, the voltage at the third terminal when the transistor is on, and a detection result of the detection circuit.