AC-DC power supplies with outputs of more than 75 W may require power factor correction (PFC) to comply with standards such as IEC61000-3-2 and ENERGY STAR. This application note describes techniques for measuring the effectiveness of a power factor correction circuit, both in terms of power factor improvement and reduction of current harmonics. Measurements are made using an oscilloscope equipped with voltage and current probes. It describes manual techniques for making power factor and harmonics measurements, as well as techniques using automatic power analysis tools on the oscilloscope. It also covers measurements of gate drive signals (modulation), switching loss, and magnetic loss in the PFC circuit.