Abstract: A power supply device includes: a first transistor that switches a first current flowing between a first terminal and a second terminal of the first transistor; a second transistor that has a first terminal connected to the first terminal of the first transistor and a control terminal connected to a control terminal of the first transistor and is formed in a chip in which the first transistor is formed; a current source that supplies a second current between the first and second terminals of the second transistor; and a control part that performs a turn-on and turn-off control of the first transistor on the basis of voltages of the second terminals of the first and second transistors.

Abstract: A control device includes: a control circuit that controls a power supply circuit by a feedback of an output from an output terminal, the power supply circuit having a first transistor, the first transistor including gallium nitride material and having a source and a drain connected so that one of the source and the drain is connected to a first power supply and an other of the source and the drain is connected to the output terminal; and a first terminal via which a first control signal output by the control circuit is output to a gate of the first transistor.