Articles :: Power Supplies 101: A comprehensive guide ::

Jon Gerow · 06-26-2006 · Category: Guides

Power Factor Correction

Something 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 "non-linear 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 non-PFC. 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.

Current and voltage in a linear load have both identical sinusoidal waveforms.

Some electronics, like motors, have a current waveform that lags behind the waveform for the voltage. The waveforms are still sinusoidal, but they are out of phase. This affects their power factor. These loads are called "inductive."

Current and voltage waveforms in inductive loads are sinusoidal, but out of phase.

Because a switch-mode power supply, like a computer's power supply, conducts current in small pulses, it has waveforms that are in phase, but the current waveform is not sinusoidal. So naturally, this isn't good for power factor either and this is why the load is called "non-linear."

A non-linear load has a non-sinusoidal current waveform.

Non-linear loads are probably the loads your utility company hates the most because a non-linear load draws harmonic currents from the AC mains. Luckily, the power factor correction used in power supplies reduces these harmonics. This reduction in harmonics can in turn clean up the power for everything else plugged into this circuit.

The Active PFC circuit board out of an Ultra X-Finity 500W.

Active PFC uses a switching regulator with a good deal of complex circuitry to reduce harmonics. Components such as FET's, diodes and even IC's are used to improve power factor to 95% or better! Active PFC can also automatically adjust the input voltage of a power supply.

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%.

Note the large coil in this Thermaltake TR2 power supply at the bottom of the photo. This is passive PFC.

Unfortunately, any means of power factor correction is going to use some of your effective power. So although your computer may pull less VA with power factor correction, it's going to pull more Watts. So I suppose in that sense your PFC power supply is actually less efficient than a non-PFC model, but this penalty tends to only be around 5%. And this use of power becomes less with the higher your input voltage is (because you draw less amperage at higher voltages and less amperage means less resistance.) This is why some power supplies in the EU, where 230V is used, they will sell 230V only APFC power supplies. To make a full range APFC power supply, considerations need to be taken to cool the PFC circuitry.


  1. Introduction
  2. The PC power supply:
  3. The PC power supply label:
  4. Defining the connectors of an ATX power supply
  5. ATX power supplies DO NOT turn on at the flip of a switch
  6. Testing your power supply's voltage: Software vs. Multimeter
  7. Power Supply Efficiency
  8. The Derating Curve
  9. Power Factor Correction
  10. The resistance of modular connectors, adapters and splitters
  11. Conclusion

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