A control circuit for a main switch is provided. The control circuit includes an output voltage tracker, a main switch bias generator, and a reference current device. The output voltage tracker is coupled to the main output end and generates a first tracking voltage positively correlated to an output voltage. The main switch bias generator, in response to the first tracking voltage, generates a second tracking voltage substantially equal to the output voltage. The reference current device is coupled to the main switch bias generator and is used to generate a control voltage on a main control end. The reference current device is used to limit the maximum value of the output current. The main switch and a duplicating switching element of the main switch bias generator form a current mirror configuration circuit. The consuming current of the output voltage tracker is positively correlated to the output current.