A power supply configured to include a switch which performs a switching operation in accordance with a preset period in a standby mode, a transformer which supplies voltage from a primary side to a secondary side in response to the switching operation of the switch, and a constant-voltage controller which controls voltage supplied to the system by regulating the voltage at the secondary side of the transformer, the constant-voltage controller including a blocking diode which shuts off current flowing from an output of the power supply to the constant-voltage controller in an off-section of the switch. Thus, the power consumption is decreased in the off-section, thereby having an effect on reducing the standby power.