AbstractOptimum conditions for the on‐resistance and output capacitance of a power MOSFET used as the main switching element of a single‐ended forward converter are derived by circuit simulation. In the simulation, a device model of the power MOSFET suitable for analysis of a switching power supply is introduced, enabling analysis of the factors contributing to generation of main switch loss. First, the simulation clarifies that the power considered to become a loss in the conventional concept is partly recovered. A new evaluation method is proposed for the main switching loss, taking this phenomenon into account. Next, the main switch loss is analyzed by using as an independent variable the on‐resistance or the output capacitance, which have high device sensitivity to the main switch loss. The results are shown by an equi‐loss diagram. Finally, this diagram is used to derive the optimum on‐resistance and output capacitance with the largest CR product to make fabrication easiest within the allowable main switch loss.