a power supply device is disclosed that is able to satisfy the power requirements of a device in service and has high efficiency . the power supply device includes a first power supply ; a voltage step - up unit that steps up an output voltage of the first power supply ; a voltage step - down unit that steps down an output voltage of the voltage step - up unit ; and a load that is driven to operate by an output voltage of the voltage step - down unit . the voltage step - up unit steps up the output voltage of the first power supply to a lower limit of an operating voltage of the voltage step - down unit .