Power Supply FAQs
  • Questions about active power factor correction (PFC):

    • What is power factor correction?

    • Poor power factor can be corrected by adding some form of power factor correction to the AC input of the power supply. Power Factor Correction comes in two forms: Active Power Factor Correction, or APFC, and Passive Power Factor Correction.

      Computer power supplies can create harmonics of the same frequency as the input current, due to the non-linear load caused by the bridge-rectifier doing the AC to DC conversion, and typically have poor power factor (typically 0.55 to 0.65).

      Passive Power Factor Correction uses a filter that kills any harmonic current and passes current only at line frequency (typically 60Hz in the U.S.) The filters typically come in the form of large, high-value inductors.

      Active Power Factor Correction is done by using a boost converter in between the bridge-rectifier and main input capacitors. The boost converter attempts to maintain a constant output voltage while drawing a current that is always in phase and at the same frequency as the line voltage.

      Power factor correction won’t make your power supply more efficient (convert more DC output power with less AC input power), but can allow for more devices to be plugged into the same circuit.  If you have a number of PCs on the same circuit, say in the event of a LAN party where a number of computers are plugged into a single power strip, it is easier to overload that circuit if a number of the PCs have poor power factor.  Say for example you have a 20A breaker and there are five PCs plugged into the outlets on this breaker.  Let’s assume the PCs are each drawing 115V at 3A from the wall, or 345W each, for a total of 1725W.  This isn’t a lot of power and something the breaker should be able to handle without problem, but if the computers in question lack power factor correction, the “apparent” current draw could be as high as 27A (assuming a power factor of .55)!  This will easily trip the breaker.

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