PROBLEM TO BE SOLVED: To provide a power supply device, AC adapter, electronic apparatus, and power supply system which are capable of controlling an output voltage value and outputtable current capacity (MAX value).SOLUTION: There are provided a power supply device 4A, and AC adapter, electronic apparatus, and power supply system which mount the power supply device 4A. The power supply device 4A comprises: a DC/DC converter 13 arranged between first and second terminals; a primary side controller 30 for controlling input current of the DC/DC converter; an insulation two-way circuit 34A which is DC-coupled with the second terminal and feeds back power information on the second terminal to the primary side controller; a capacitor CA for AC coupling connected to the insulation two-way circuit; and a low-pass filter 36A connected to the insulation two-way circuit. The primary side controller controls input current on the basis of the power information fed back so that an output voltage value and outputtable curr
