We've all been taught that power factor (PF) is the ratio of real power to apparent power, or the cosine of the phase angle between sinusoidal current and voltage waveforms. When the current isn't sinusoidal (and the voltage is), PF includes a distortion factor related to total harmonic distortion (THD). Specifically, PF relates to THD by:

where THD is expressed as a decimal.

When the switch closes in a non-PFC boost converter like the one in Figure 1 in the main article, the current through the inductor increases linearly and the voltage across it grows exponentially until it equals the input voltage. Then the switch opens, causing the inductor current to fall, while the voltage across it changes sign. Together with the voltage from the rectifier bridge, it charges the output capacitor through the diode. In the steady state, the capacitor is charged to a higher voltage than the voltage from the bridge.

Without PFC, the input current waveform in a boost converter draws pulses of current only at the peaks of the voltage waveform. Therefore, it generates many rather powerful harmonics. In some cases, the third harmonic can be nearly as large as the fundamental.