PFC in Power Supplies?

[opiuman]

What exactly is the benefit of having active PFC in a PSU compared to none/passive PFC in PSU? It is worth the added cost and what does it buy you?

 

[Mark R]

Originally posted by: opiuman
What exactly is the benefit of having active PFC in a PSU compared to none/passive PFC in PSU? It is worth the added cost and what does it buy you?

PFC reduces allows the PSU to take fewer Amps from the wall for the same number of Watts.

If we assume 120V mains supply, a PSU using 200 W of power would need about 2.6 A of current if non-PFC, or 1.7 A if PFC.

For home users, you only pay for the Watts used - so it doesn't get you much.

For business users, PFC is beneficial because you can put more PCs on one circuit (because they use less amps), and because business users are billed for Amps used as well as Watts.

PFC is required in some countries (e.g. European countries) because non-PFC PC PSUs cause severe pollution of the mains supply due to their 'harmonic waveform'. These polluted grids cause reduced performance of equipment connected to that grid - especially transformers and motors which run hotter, noisier and less efficiently - as well as strain the grid reducing its capacity to supply power. In the US, there are no such requirements and so power companies simply have to eat the cost of grid inefficiency and cleaning up the 'harmonic' pollution.

The traditional method of PFC in PSUs has been 'passive PFC' - this consists of a series of inductors and capacitors. The problem is these inductors and capacitors are heavy and bulky. As PSUs get more powerful, it becomes impractical to use this method - the necessary components become too big, heavy and expensive.

The modern approach is 'active PFC'. Active PFC uses an electronic circuit to supply power to the main PSU - although more expensive, active PFC is more effective, more energy efficient, smaller and lighter than passive. It also has a couple of side benefits - the active PFC circuit is a voltage regulator, this means that the main PSU always gets reasonably clean power, even if the mains voltage is fluctuating, it also means that you don't need a voltage selector switch on the PSU, the active PFC regulator can automatically adjust over a wide voltage range (typically 90 - 270 V).

There is another benefit to PFC, and that is if you are using a UPS. UPS systems have to work extra-hard to deal with the awkward demands of non-PFC PSUs, this means they run hotter and battery run-time is reduced. The use of PFC can optimise UPS performance.

 

[Howard]

You don't really need it unless you're running it from a UPS, but good luck finding a quality PSU that doesn't have it.

 

[Zepper]

Basically your PSU will be a friendlier load to the power grid and you get the autoranging AC sensing/compensation for free so it can handle minor line side disturbances on its own - a fancy, line-interactive UPS isn't necessary - one of the cheap battery back-up/surge suppressor units is good enough. You usually get it along with aPFC as a cost-saving measure. Either alone would require an AC side PCB (circuit board) so they kill both birds with one PCB... You will find aPFC/AC compensation only in a PSU mfr's top line products (they still offer passive PFC models in U-Rope so they can have a lower cost product to sell while staying within the law), so if top line is what you want - there you are.

As Howard suggested, if you can find a truly top-line PSU without it, let me know. You don't have to pay over $100. to get it any more. iStar markets a 350W unit that can be had for under $40. on sale. 500W can be had for under $70. and 600W for around $100. + shipping. To get it all with the 80 Plus cert as well will cost a bit more.

.bh.

 

[GPett]

Here is a good article that explains what PFC actually is:Dans Data PFC Decoded.

So basically active PFC stresses your power company less but costs you slightly more money and more heat.

BUT, the heat part is debatable since only quality psus have active pfc. The quality psus are more efficient by better design and more circuitry.

I would be interested in a highly efficient non pfc psu if such a beast existed. But they do not.

 

[dantonic]

I am also interested in this topic.

So What PSU would you say is more efficient a NON PFC 70% efficiency PSU
OR
an Active PFC 80% efficiency PSU?
How much energy does the active PFC actually waste?

 

[Trippytiger]

One thing to consider that didn't seem to be addressed in that article is that the closer a power factor is to unity, the less current is being drawn from the wall. This means that less power is lost to resistance as the electricity travels through the wiring in your house. While such losses are probably not very significant, I'd bet that the advantages of the slightly lower current draw at least compensates for the extra power consumed by the active PFC circuitry.

And I'm completely sure that an 80% efficiency PSU with active PFC will use less energy than a 70% efficient PSU without active PFC. There's no way the circuitry can use that much power.

 

[Howard]

Originally posted by: dantonic
I am also interested in this topic.

So What PSU would you say is more efficient a NON PFC 70% efficiency PSU
OR
an Active PFC 80% efficiency PSU?
How much energy does the active PFC actually waste?

Efficiency is efficiency. The number is all that matters.

 

[Mark R]

Originally posted by: dantonic
I am also interested in this topic.

So What PSU would you say is more efficient a NON PFC 70% efficiency PSU
OR
an Active PFC 80% efficiency PSU?
How much energy does the active PFC actually waste?

The 80% efficiency is more efficient. The efficiency figure takes into account the energy wasted in the active PFC.

Active PFC circuits are typically between 92 and 97% efficient - so they waste between 3 and 8% of the power they convert. Just how efficient depends on the design. I'd expect PSUs for PCs to use the lowest grade of active PFC they can get away with. However, efficiency of aPFC depends on mains supply voltage - at 110V efficiency is much lower than at 230 V. e.g. 92% at 110, 96% at 230. (A side effect of Power = I^2.R).

This inefficiency is partly compensated for because active PFC has a side effect of enhancing the efficiency of the rest of the PSU, although with low-grade aPFC, its likely losses in the aPFC outweigh the benefits in the rest of the PSU.

The big difference comes from improved design of the main power converter. Manfuacturers have made a lot of effort to improve the designs of their PSUs to meet modern energy efficiency standards, and there have been impressive gains. Average efficiency was less than 70% 6 years ago, but some designs now get mid 80s, even when active PFC is added as well.

There probably is still a fair bit of room for improvement - but the issue is cost. Most PC PSUs use a fairly traditional design, albeit using with modern components; specifically, most use a 'forward converter' to produce 12V and 5V, and a 'magnetic amplifier' regulator to convert 5V into 3.3V

Forward converters and magamp regulators are cheap, simple, old, well-understood circuits. The problem is that they both have poor efficiency when compared to more modern designs. The redesign of PSUs to use 'active clamp' converters and 'synchronous buck' regulators should be able to push efficiencies (even with active PFC) to the high 80s, maybe even over 90%. The catch is that these modern designs are more complex and replace cheap parts (e.g. diodes) with expensive MOSFETs.

 

[13Gigatons]

PFC has nothing to do with efficency in fact active PFC actually will use a little bit of power itself. The point of PFC is to turn a complex load into a simple one like a toaster. It's a law in Europe in the USA were the voltage is lower and the wires are thicker it's not as big of a problem but it is better to have if your computer is plugged into a UPS.

http://www.dansdata.com/gz028.htm