Power supply device, power supply system and method for controlling the same

A power supply device to be connected to a power receiving device, the power supply device comprising:

• a power supply means for changing an input power into an output voltage;
• a voltage port for supplying the output voltage to the power receiving device;
• a ground port for matching a ground potential of the power receiving device with the power supply device;
• a control port for receiving a request signal, from the power receiving device, for determining an output voltage specification and sending a device information signal to the power receiving device, the control port being provided separately from the voltage port;
• a control means for controlling the power supply means based on the request signal so as to determine the output voltage specification and provide the output voltage to the voltage port;
• and
• device information output means for providing the device information signal indicating a type of the power supply device to the power receiving device through the control port,
• wherein the type of the power supply device indicates that the power supply device is an output-voltage variable device, and
• wherein, the voltage port, the ground port, and the control port are universal serial bus (USB) ports configured to be connected to a USB connector.