A power supply device includes a power supply module, a voltage regulator module and an over-voltage protection (OVP) module. The power supply module is utilized to provide a standby voltage signal. The voltage regulator module includes a phase modulator circuit. The phase modulator circuit includes a high-side power transistor, a low-side power transistor and a phase inductor. The OVP module is utilized to transmit the standby voltage signal to the phase modulator circuit and detect a detection potential on the phase modulator circuit. The OVP module selectively conducts the low-side power transistor of the phase modulator circuit according to the detection potential.