A power supply apparatus mainly includes a secondary side controller, a digital isolator, a plurality of isolation sensing units, and a driving unit. The secondary side controller is configured to isolatedly measure voltages and currents, and is configured to isolatedly control the driving unit. The power supply apparatus includes a controller only, i.e. the secondary side controller 114. Therefore, the power consumption is lower, and the cost is lower. Only one controller is required to update software. There is no communication delay problem between two controllers.