A voltage supply unit includes a control circuit, a capacitor circuit and a unidirectionally-conductive circuit. The control circuit controls an output end of the control circuit to be electrically connected to a first input end or a second input end of the control circuit under the control of a voltage signal from a first voltage output end. The capacitor circuit controls a potential at a first input end. When a difference between a first level and a potential at the first input end is greater than or equal to a predetermined on-state voltage, the unidirectionally-conductive circuit allows a unidirectional current flowing from a first level end to a first end thereof, and when the difference between the first level and the potential at the first input end is smaller than the predetermined on-state voltage, controls the first level end to be electrically disconnected from the first input end.