A power supply system includes a power converter, a switch, and a controller. The power converter is operable for providing power to a load. The switch is coupled to the power converter and is operable for adjusting the power to the load. The switch has a gate, a source and a drain. The controller is coupled to the switch and is operable for controlling a state of the switch. The controller has a first control terminal for providing a gate voltage to the gate of the switch, a second control terminal for providing a source voltage to the source of the switch, and an input terminal for receiving a supply voltage via the source of the switch. The controller is also operable for limiting the supply voltage within a predetermined maximum level by regulating the gate voltage to a preset level.