PROBLEM TO BE SOLVED: To provide a power supply device that can control an output voltage value and an available output current capacity (MAX value), an AC adapter, an electronic apparatus and a power supply system.SOLUTION: A power supply device 4A includes: a DC/DC converter 13 arranged between an input and an output; a primary side controller 30 for controlling an input current to the DC/DC converter 13; and a secondary side controller 16 AC-coupled to the output to feed power information about the output back to the primary side controller 30. The primary side controller 30 controls the input current on the basis of the power information fed back from the secondary side controller 16, so that an output voltage value and an available output current capacity of the DC/DC converter 13 are variable. An AC adapter, an electronic apparatus and a power supply system are mounted with the power supply device 4A.
