A power supply apparatus includes a trans circuit configured to convert an input voltage into an output voltage by adjusting switching of the input voltage; an output voltage detector configured to detect the output voltage; a compensator configured to generate a control value controlling a duty ratio of the switching such that a value of the detected output voltage coincides with a target value for the output voltage; an input voltage detector configured to detect the input voltage; an estimator configured to estimate an allowed range of the control value in accordance with the target value and a value of the detected input voltage; and an adjuster configured to adjust the control value so as to stop the switching when the control value falls outside the allowed range.