PFC decoded - #31

Is active better than passive? Not if you’re a submarine, and Dan Rutter believes it’s the same with PSUs.

So you're shopping for a new PC power supply unit (PSU), and along with outrageous wattage ratings, multiple fans, funky cables, gold-plated connectors and little lights, some of them have 'Power Factor Correction' (PFC).

What the heck, I hear you ask, is that?

A PC PSU doesn't have to have PFC, but practically all of them do these days. Most PSUs have passive PFC; fancier models have active PFC.

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

PFC is, essentially, what you do to complex alternating current (AC) loads (like switch-mode PC power supplies) to make them act more like simple loads (like toasters).

Simple 'resistive' loads draw their maximum current at the points in the AC cycle when there's the maximum voltage across them. When the voltage reverses during each oscillation, so does the current. If you multiply the root-mean-square (RMS) voltage and the RMS current of a simple AC circuit like this, you get the power being drawn, in watts. Simple.

Complex AC loads are not nearly 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.

The more complex a load is, the more out of sync the current will be with the voltage, and the worse the device's 'power factor' will be. The worse the power factor, the more apparent AC power you'll need to run it.

Multiplying a reactive load's RMS voltage and RMS current will give you the apparent power in volt-amps (VA, which you may remember seeing on the spec sheets for uninterruptible power supplies), not the real power. Power equals VA times power factor.

Proper power meters, like the induction wheel meters that the electricity company uses to figure out how much you owe them, are meant to measure true power, not apparent power. The meters 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, particularly seeing as mere 'phase' isn't actually an adequate description for the differences between the non-sinusoidal waveforms that lousy power factors create. But the meters do more or less get it right.

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

The apparent power involves real current flow, though. A 1,000VA device with a power factor of 0.5 may only be drawing 500 watts, but it'll be causing as much current to flow through the wires as a real 1,000W device.

Electricity companies don't like this. The more 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.

Industrial power customers are, for this reason, 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. It can iron out lousy power factor better, but it's [i]less[/i] efficient, not more. An active PFC circuit will waste some power 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, and also reduces their power-factor-linked electricity bill.

But if you're not being billed by power factor -- if you're a home or small business, you're probably not -- then an Active PFC PSU isn't going to consume any less real power than any other kind. It'll actually probably consume a little bit more, and that little bit more will be noticed by your electricity meter.

So by all means, buy an Active-PFC-equipped PSU if you like. They do no harm, and they're generally likely to be high quality units, less likely to do 'Something Horrible' to your PC. 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.