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PC Power Supply Discussion Troubleshooting and discussion of computer power supplies

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Old 03-28-2008
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Default Power Factor Correction FAQ

What is "power factor"?

Power factor, or “PF” for short, is the ratio of the real power to the apparent power.

Real power is the capacity of the circuit for performing work in a particular time and is measured in Watts.

Apparent power is the product of the voltage and current (V x A) of the circuit and is measured in volt-amperage (or “VA”.)

I know that it almost sounds as if Watts and VA are the same thing, and in DC they are (240W DC is equal to 240VA DC, for example) but because energy stored in the load of a device using alternating current (AC) is returned to the source, or due to a non-linear load that distorts the wave shape of the current drawn from the source, the apparent power can actually be greater than the real power. This would give you a power factor of less than 1. Power factor below .70 is generally considered poor power factor.

So what if I have a poor power factor?

Most consumers are charged per kWh (kilowatt hour) by their utility company. Fortunately, for the customer, poor power factor does not typically affect how much wattage your computer uses. But poor power factor does has an affect on how much power the utility companies can deliver. This means that the utility companies either have to increase their grid's capacity to compensate for the increased power load, charge per kVA instead of per kWh (some commercial/industrial accounts are charged per kVA while residential customers are still charged per kWh), charge a "power factor penalty charge" (which can be applied to customers with power factors even as high as .95!) or impose a martial law of sorts requiring all appliances sold in the country to have a power factor of .96 or better.

The European Union believed the latter of these to be the best solution, so as of January 1st of 2001 the EN61000-3-2 was put into place imposing limits on the harmonic currents drawn from the mains. In other words, if you're in the EU, you are REQUIRED to have a power supply with power factor correction. Power factor correction is not (yet) a requirement in the U.S.

Poor power factor can also limit how much current you can draw from a circuit. If you’re using a 20A breaker and are drawing a total of 15A in “real power” and the power factor is only .70, then you are drawing an apparent 21.4A, thus overloading the breaker.

Also, one of the requirements for a computer power supply to be considered "Energy Star" compliant is that it has a power factor of at least .90.

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.

So I'm in the U.S. PFC is a non-issue, right?

Well... yes and no.

You may not NEED power factor correction, but "green" is more marketable and it costs a PSU factory less to make a bunch of the same platform, even if it includes PFC, then split production up between non-PFC and PFC designs.

These days, PFC is typically integrated into the design of the platform. So much so that the PSU manufacturers couldn't even really REMOVE the PFC in an effort to cut costs. That's ok because they're making up for it in larger quantities being able to sell their product around the globe. It also reduces returns because with active PFC it's impossible to plug the PSU into 230V while the switch is set to 115V (BOOM!)

If you're looking at a PSU over 550W continuous and it DOES NOT have PFC, be suspicious of it. It's likely an old, recycled platform that's inefficient and probably not suitable for the high +12V loads of today's computers. Or worse: It's really a 500W PSU labeled as a 700W!
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Last edited by jonnyGURU; 05-12-2008 at 03:06 PM.
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Old 03-29-2008
g1raffe g1raffe is offline
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Nice read, power factor is always a tricky one to explain.
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Old 03-29-2008
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Stefan555 Stefan555 is offline
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Suggested addition:
A passive PFC increase the Power Factor to 0.70-0.75

An active PFC increase the Power Factor to 0.95-0.99
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Old 04-04-2008
ianm2 ianm2 is offline
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Sticky's please Jon!!!!! all the useful info.
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