


Articles :: Power Supplies 101: A comprehensive guide :: Motherboards.org
Jon Gerow · 06262006 · Category: Guides Power Factor CorrectionSomething that seems to need a lot of clearing up when it comes to power supplies is "power factor correction," or "PFC." Some people say that power factor correction makes a power supply more efficient, but as you should now know, for a power supply to be more efficient it needs to pull less wattage from the wall. Power factor does not suddenly make a power supply pull less wattage from the wall. It does allow a power supply to pull less VA. And in some parts of the world that might save you on your electric bill. When talking about DC, VA is the same as Wattage: V * A = VA. But when talking about AC, VA takes power factor in consideration: (V * A) / PF = VA. So VA can only equal Watts if power factor is 1. Some electronics, like heaters, toasters, coffee makers, ranges, are what is called a "linear load" (or "simple load" or even "resistive load") and have a power factor of 1. While those with a "nonlinear load", like a computer's power supply, have a power factor as low as .60. Let's say you have a power supply that's nonPFC. We're going to assume that your PC typically puts a 250W load on this power supply. Let's also say that the efficiency of the power supply at 250W is 83%. That would mean it's pulling 300W from the wall. If you're billed by the KWh, like so many of us residential customer in the U.S. are, then that's how many Watts it's pulling from the wall is all you need to worry about. But if you're in the EU, or even a commercial customer in some parts of the world, and are charged per KVA, then you'll be concerned with the fact that your power supply is drawing 500VA from the wall. If we were to switch this power supply out for one with active PFC and a power factor of .99, assuming it's the same efficiency at your typical load, you're only pulling 303VA from the wall. So now that you understand what power factor correction is for, now I'm going to try to explain to you what power factor is... Power factor is the ratio of real power consumed to apparent power. Or W over VA, which is another way to look at the equation I use above to figure out VA based on Watts and PF (simple algebra here, people.) Traditionally, PF is known as the phase difference between the sinusoidal voltage and current (amperage) waveforms. Something that has a "linear load," like a simple resistor, has identical voltage and current waveforms, are sinusoidal and are in phase with each other.
Passive PFC uses a passive element, such as a large ferrite core, to dampen the harmonics. Unfortunately, power factor using such means can only be corrected to about 80%.
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