PSU Question: Active PFC vs Passive PFC

[Wedge1]

I need some input from you guys who know what this is:
Active PFC (Power Factor Correction) vs. Passive PFC.

I already understand that Active PFC is the better of the two methods, but I need to know more about it as I am still shopping for a new Power Supply.

What I'm really asking is if somebody could speak in layman's terms of the advantages of having a PSU that uses Active PFC.

Would there be any perceptible differences between PSUs that use each method? Or, is the Active PFC kind of one of those hidden benefits, where the better technology is being used but you're never really aware of it in practice, i.e., you wouldn't be able to tell if a given PSU uses active or passive without reading the tech specs.

Truth is, I have recently purchased a passive PFC power supply that I am thinking of exchanging for one with active PFC.

 

[Mark R]

PFC means power factor correction. Power factor is a strange property of AC circuits. It is possible to draw current in such a way that not all of the current does useful work. As such, it is possible for some equipment to take a lot of current from the mains, but only a small amount of power. Power factor is the ratio of 'actual power' to 'apparent power'. PFC is used to adjust current flow, so that it is able to do more work.

The difference between active and passive PFC is really only apparent to the electricity companies. The active PFC does a better job of correcting high frequency 'harmonic' currents. Excessive harmonic currents are a major problem to the suppliers as they cause distortion of the voltage waveform.

With the increasing number of devices that use switched-mode PSUs (TVs, PCs, VCRs, etc.) power quality is becoming a problem and new legislation is being considered that would make active PFC mandatory for new products in the future. In Europe Passive PFC has been mandatory for PC PSUs for at least 3 years.

 

[CTho9305]

As long as you aren't a large industrial facility that has special pricing based on power factor of the load, you really don't care (they care because they get lower rates for PFs closer to 1, since that means less energy is wasted in the power company's lines)

 

[Wedge1]

So, there is no reason to buy a PSU because it would be quieter with active PFC?

Nor would I save on my monthly electric bill with a PSU using active PFC?

Nor is there any perceptible performance gain, say, in Windows or the games I play using active PFC?

 

[CTho9305]

Power supply won't affect performance. Worst case it will affect stability, but only if it is much too small or very low quality.

 

[Wedge1]

Sounds like I might be wasting time and money to exchange my 350w passive PFC for a 400w active PFC, other quality issues notwithstanding.

 

[RadioactiveMagpies]

I was under the impression active pfc reduces buzz and coil whine noise in the psu, is that true?

 

[RaynorWolfcastle]

Since this is the Highly Technical forum, I'll elaborate a bit on what the PF actually is, I won't go in the details of compex power but here's the idea.

If you're supplying AC power to a load whose equivalent output Thevenin impedance is properly complex there will be some inefficiencies. That is, if your load is either capacitative or inductive, your circuit will be storing some energy and returning it to the source on some portions of the cycle.

In a perfect world with superconducting wire, that wouldn't be a problem because the actual power used in your system wouldn't increase. However, in real life the transmission lines have a resistance so you get losses when your system is too capacitative or too inductive.

Because of their nature, the impedence caused by inductors and capacitors is dependant on the frequency at which you are supplying them the voltage. Also, adding and removing components changes the topology of your system and thus changes the impedence.

This is where PFC comes in:

- In the passive case, you add reactive elements in such a way that you correct for a given output impedence at a given frequency. You sorta pick average expected parameters and say that's what you'll be correcting for. By definition, if the parameters are off from what you expected, your correction will not be 100% correct and you will still have inefficiencies

- In the active case, you add a circuit that detects the actual output impedence of the circuit and dynamically readjusts your correction factor. As a result, you are always correcting as best you can.

So the question is, how does this affect your computer? Well to be perfectly honest, I'm not sure how PC power supplies actually go about transforming your 60 Hz AC voltage to the DC voltage that your electronic components need. Depending on how they do it, I guess there might be some benefit, more likely however it's just a gimmick.

As was stated earlier, usually PFC is an issue for large industrial electricity consumers than it is for individual users.

Hope that helps

 

[Wedge1]

Interesting and informative stuff, RaynorWolfcastle.

Thank you for your input, and yes it does help. I'm inclined to read more about it now, regardless of its bearing on whether I buy and active PFC or passive PFC for my power supply unit.

 

[blahblah99]

Jeez, 350W and you're concerned with PFC? We use PFC on our 9000W audio amplifier.

 

[Mday]

basically for something as small as 350W, PFC considerations can be ignored.

 

[Wedge1]

Thanks guys.

I didn't realize this was more of an issue with industrial strength machines, rather than desktop PCs.