JP-A-2009-142146 discloses a power supply device that calculates a square current value of a detection value proportional to a current value flowing to a semiconductor switch coupled with an electric wire, compares a temperature rise corresponding value of the electric wire obtained from the square current value with an abnormality determination value, and turns off the semiconductor switch when the temperature rise corresponding value exceeds the abnormality determination value.
In the power supply device, a transitional thermal change of a heat flow rate corresponding to a thermal change of the semiconductor switch is expressed by a predetermined logical expression, and the logical expression is calculated by a digital operation.