PFC decoded

Originally published 2003 in Atomic: Maximum Power Computing
Last modified 03-Dec-2011.

 

Companies that make PC Power Supply Units (PSUs) find it difficult to make their products stand out from the crowd. On top of the increasingly outrageous wattage ratings (a quality 300 watt PSU is enough to run practically every PC out there, but lots of people buy something with a much higher rating just for the heck of it*), there are multiple fans, funky cables, gold-plated connectors, little lights...

...and Power Factor Correction (PFC).

A PC PSU doesn't have to have PFC, but practically all of them do these days, because many countries have regulations that require some kind of PFC. Most PSUs have passive PFC; fancier models have active PFC.

Active PFC, your friendly shiny-suited PC salesman will explain, is more efficient. He may or may not also say that it'll save you money on your electricity bill.

Either way, he's full of crap.

Power factor correction (PFC) is, essentially, what you do to complex AC loads (such as PC switchmode power supplies) to make them act more like simple loads (such as toasters).

Alternating current oscillates continuously. 50 times a second, here in Australia and in most other 220/240 volt countries; 60 times a second, everywhere else (people living in East Elbonia and using 153-volt DC mains and four-pin plugs need not e-mail me to complain about this generalisation).

If you plot voltage versus current drawn for a simple ("resistive") load in an AC circuit, the current will neatly follow the voltage, perfectly in sync. At the points in the AC cycle when there's the maximum voltage across the load, the maximum current will flow. And when the voltage reverses during each AC oscillation, so does the current. This is all very simple and sensible; it's what you'd expect AC to do, by applying the volts equals amps times ohms rule you learned for direct current circuits in high-school physics. Or should have learned, anyway.

If you multiply the root-mean-square (RMS) voltage by the RMS current of a simple AC circuit like this, you get its power in watts. Again, this all works like DC electricity.

Complex AC loads are not this simple. Their current draw doesn't follow the voltage; it's out of sync. This is because the load is capacitive or inductive - "reactive". Reactive loads can even be both capacitive and inductive, in different mixtures over time.

If you think of a resistive load as just a length of hose, that needs a certain constant pressure to get a certain constant amount of water flow, then a reactive load is a contraption involving buckets and water-balloons. Things are filling up and emptying at their own rates, in response to the water that's being pumped in.

The more complex a load is, the more out of sync the current can be with the voltage, and the worse the device's "power factor" will be. The current waveform doesn't even have to look like the voltage waveform; it can be all sorts of funny shapes. The worse the power factor, the more apparent AC power you'll need to run the device.

Multiplying a reactive load's RMS voltage and RMS current will give you the circuit's "volt-amps" (VA, which you may remember seeing on the spec sheets for uninterruptible power supplies) rating. This is its apparent power, but not its real power. Power equals VA times power factor, and power factor is the cosine of the phase angle between voltage and current.

(You're allowed to not spend time thinking about this bit. I won't be asking questions later. The phase angle thing can end up very difficult to calculate, anyway, when a load's current waveform is a funny shape, and not just a nice constant sine wave like the voltage waveform.)

A device that draws an apparent power of 1000VA and has a 0.5 power factor is consuming 500 watts of power. Not 1000. Put that device in a thermally insulated room and measure the temperature rise and it'll be the same as if you put a nice resistive 500-watt heater in there.

The VA rating is how much power the device seems to be consuming, if you don't look at the volts-versus-amps graph. But it's actually storing some power in its reactance during one portion of each AC oscillation, and returning it in the next. All of this current flow looks like real power consumption to someone who's just hooked up a couple of multimeters.

Actually, it's even more confusing than this, because the current waveform is unlikely to be sinusoidal, so it can't really be said to have a phase relationship with the voltage waveform, and frequency analysis and other evils are called for. Handwave, handwave; this stuff needn't detain us here.

Proper power meters, like the induction-wheel meters that the electricity company uses to figure out how much money household power customers owe them, are meant to measure true power, not apparent power. They're supposed to compensate for differences in phase between voltage and current. How well they do that is a topic for animated discussion among people who seldom have anything better to do on a Saturday night, but the meters do more or less get it right.

So it doesn't matter much how bad the aggregate power factor of your various appliances is, at least as far as a domestic electricity bill goes.

(This doesn't stop rip-off artists from selling various magic boxes that're supposed to, A, correct power factor, and thus, B, save you money. They probably don't do A, and even if they do, B does not follow unless you're actually billed by power factor, which you almost certainly aren't.)

Devices with low power factors, however, pollute the mains. Their odd current draw shifts the mains voltage around in similarly odd ways.

Consider a device with a power factor of zero. That'd be a perfect inductor - a thing that can only exist in physics-experiment-land - but it serves to illustrate what's going on here.

Feed AC through a perfect inductor and you'll be able to measure a current flow, perfectly out of phase with the mains supply (a "90 degree phase angle"). This means no actual power will be consumed; the inductor will draw current on one quarter of the cycle and deliver it back again on the next.

The AC supplier won't be happy with this, though, because it'll have to deliver current one half of the time, and handle that same current coming back again the other half of the time. This current doesn't indicate real power consumption, but it is real current. And the more of this real current the mains grid has to handle, the thicker the wires have to be, the bigger the distribution transformers have to be, the more power will be wasted thanks to cable resistance, and so on.

Lots and lots of reactive loads - an office full of PCs, say, or various other gear - can leave the mains waveform looking very weird indeed. This may be bad for other devices trying to run from mains power, and is annoying to the electricity company, for the abovementioned reasons.

Industrial power customers are, for these reasons, commonly billed according to their equipment's power factor, as well as its power consumption. The more of a mess they make of the mains, the more they pay.

Well, that's the theory, anyway; the formulae used to figure this stuff out can be baroquely complex. If you want mystifying equations, always look to accountancy before physics.

And so, Power Factor Correction (PFC) is used. PFC makes reactive loads look more like resistive ones, from the outside.

Passive PFC is just compensatory capacitance or inductance across inductive or capacitive loads; it tries to iron out the oddities with passive components.

Active PFC is an actual second circuit. It sucks power from the mains in a resistive way, and feeds it to the low power factor circuit on the other side, isolating the mains from whatever that circuit is doing. Active PFC can iron out lousy power factor better, but it's less efficient, not more; an active PFC circuit will waste some power (at least 10%, in this case) as heat, just like every other circuit in the world.

This can still work out as a good deal for industrial customers, because improving the power factor of componentry reduces the amount of power generation and distribution infrastructure needed to support it. Thinner wires, smaller transformers, smaller generators, et cetera.

But if you're not being billed by power factor - and if you're a home or small business, you're probably not - then an Active PFC PSU, or any other kind of power factor corrected hardware, isn't going to consume any less real power than cheaper gear without PFC. It'll actually probably consume a little bit more, and that little bit more will be noticed by your electricity meter, though the cost per year of this extra is unlikely to be more than you can find down the side of the couch.

If you want to do your part to clean up the mains, then PFC PSUs are a good idea. This might also qualify as enlightened self-interest, because the rate that domestic power consumers are charged per kilowatt-hour is no doubt influenced by their overall power factor. Everyone who swaps out low-power-factor gear for PFC gear lightens the load on the grid, and this may delay power price rises (or, if you live in Happy Land, actually cause the price per kilowatt-hour to drop).

Active PFC PSUs may also deal with lousy mains power better than passive PFC units, but PC PSUs generally handle spikes and surges and dropouts pretty well already, and a proper outboard power conditioner (as shown in this piece) is a better solution to that problem, anyway.

So by all means, buy an Active-PFC-equipped PSU if you like; they do no harm, and they're generally high quality in other respects as well. But don't think that PFC of any kind is going to save you any money, if you're using ordinary domestic power.

The difference between car salesmen and computer salesman is that car salesmen know when they're lying, so someone who tells you Active PFC makes a PSU more efficient is not necessarily trying to pull the wool over your eyes. You'd still probably do well to buy from someone else, though.


* I wrote this article in 2003, when a good 300-watt PSU really was enough to run most PCs. As time's gone by, the power draw of some computer components - video cards, mainly - has increased enough that many current PCs really do need more than 500 watts, for peak-load times like particularly busy moments in 3D games.

This isn't an environmental disaster, though, because the average power draw of a computer that's doing ordinary boring desktop stuff hasn't increased much, if at all.

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