8.1 Power Factor Correction

Power factor correction, or PFC, is a term used with AC mains powered circuits. A good power factor is when the AC current is in phase with the AC voltage. A pure resistive load has a power factor of 1, but active loads tend to have power factors closer to 0.5, unless special measures are taken to correct this.

The most common power factor correction circuit is a boost converter. The AC line voltage is boosted to about 400V and the amplitude of the current pulses into a storage capacitor is arranged to be sinusoidal. This is achieved by switching the current on for short but constant periods: as the supply voltage rises and falls, so does the amplitude of the current. A typical PFC circuit is shown in Figure 8.1.

Figure 8.1: PFC Circuit

A simple alternative is to use a fly-back supply. It is common to switch the primary current off when a certain current level is reached, but this leads to constant average current. To give a good power factor, the primary current should be switched with a constant on-time , so that the current amplitude rises and falls in phase with the supply voltage. The secondary current will rise and fall at double the AC line frequency and so a large secondary capacitor is required to absorb this ripple, to prevent significant ripple in the output voltage.

Driving an LED from a power factor corrected supply usually requires a simple buck converter, since...