PFC

PFC stands for Power Factor Controller.  It is a required feature for all power supplies shipped to Europe since 1/2002.  Basically it allows power to be distributed more efficiently from the electrical company to your PC system.  The following article is a detail technical review on the subject of PFC.

One of the leading topics in line-operated power-converter design is to take power from the grid but not give any back. In other words, wall plugs should present to the ac source a resistive (and not a reactive) load. A power-factor corrector is the interface between the ac line and power converter that achieves this. So what is power-factor correction and why does it matter?

Why PFC?

A power-factor controller (PFC) is a resistive load to the ac source. It provides a regulated dc output as input to an ordinary converter. Typical power supplies have as input a full-wave bridge rectifier followed by a storage capacitor. While the bridge diodes conduct, the line is driving an electrolytic capacitor - a nearly reactive load for the line. A reactive load causes line voltage and current to be out of phase, which is suboptimal for power distribution. Maximum power is delivered when they are in phase. The power factor is the cosine of the phase angle. A resistive load has a phase angle of zero and a PF of one.

Furthermore, instead of dissipating (that is, keeping) power, reactive loads store it and give some back later. This causes waveform distortion and harmonics on the ac line, defiling its purity. Line noise, surges, and dips reduce power quality. The developed world is now at the stage where "electrical environmentalism" is requiring clean treatment of the line from its users.

What is a PFC?

A PFC appears resistive to its source. This implies that the input current must differ from the sinusoidal source voltage by only a scaling factor. Their waveforms must be identical, though scaled by the effective input resistance of the PFC, by Ohm's Law.