Let’s first go over some common PFC terminology and some basics about power factor correction. Power Factor (PF) is defined as the phase relationship between the AC line voltage vs. AC line current. A PF of 1 or unity, means there is no phase shift difference between the voltage and current. In many switch mode power supplies, the current is drawn from the line only when the AC line voltage swings above the voltage of the bulk storage capacitors. A bulk storage capacitor is a very common element in the front end of a power supply. This asymmetrical current draw introduces harmonics into the line which a PFC circuit aims to eliminate. The basic PFC diagram in the illustration is fairly simple where the PFC will try to pull current from the line in a sinusoidal fashion in phase with the voltage. The PFC controller will turn the MOSFET switch on and off pulling current through the inductor in a way that simulates what the natural in phase sinusoidal current draw would be if the load was purely resistive with no reactive components. This produces a near unity power factor for the power supply. Total Harmonic Distortion (THD) is measured as a percentage of a pure sinusoidal wave where the lower the percentage of distortion, the better the power factor. In many instances, excessive distortion can very readily be seen on an oscilloscope as the waveform will be distorted. The two most popular modes of operation for power factor correction controllers are Transition Mode (TM) which is sometimes referred to as Critical Conduction Mode, and Continuous Conduction Mode (CCM). The differences between them will be discussed in detail on the following slides.