psu features

[ervinshiznit]

how important r dual +12v rails and pfc's in psus, esp. when it comes 2 oc'ing?

 

[doolittle]

the dual rails promote stability between the gpu and other periphs like cpu and hdd, if a sudden rush of current on your gpu occurs it won't starve your cpu & hdd since they are isolated between the rails. Of course it is better to have enough +12v power regardless of the number of rails so if you have a high single +12v that is probably more important.

+12v combined wattage > dual rail config

PFC or power factor correction can be completely ignored for the typical consumer in the USA (in many countries is mandated so you have no choice). In USA if you are running a data center that has hundreds of PCs then I would worry about PFC since the cumulative effect can be dramatic to the AC power as far as noise and harmonics are concerned. Efficiency is more important so that is a better rank. Not to mention the utility company may bill your business / industry on "apparent power" which factors in PFC vs the typical consumer power meter which is "real power" only and not PFC related.

efficency > power factor correction

For a PF primer check out Lessons In Electric Circuits -- Volume II, Chapter 11 @ ibiblio.org it does a great job at explaning the current / voltage relationship in complex AC circuits. It's difficult to visualize the phase angle difference and how it relates to the "power triangle" without seeing it in graphs.

 

[phreejak]

Power Factor Correction - protects against things like voltage fluctuations and electrical irregularities that can affect the efficiency of your power supply. There are three types of PFC: active, passive, and non-PFC. Active PFC provides the best protection, while non-PFC provides the worst. With a more efficient PSU there is less waste and, therefore, less heat produced by the PSU. This, in turn, means a reduction in it's contribution to overall system heat.

 

[ervinshiznit]

so when i look for a psu, i should look for dual +12v rails but not really pfc?

 

[phreejak]

Well, all things considered...

If you find two PSU with near identical specs, with the exception of one having PFC and the other not, You'd be better off getting the one with PFC (active PFC is the best). It may not be a critical aspect as to powering your rig but, it's always better to have it than to not have it. Besides, when comparing like PSUs, the ones with PFC produce less heat and, therefore, contribute less heat to the overall system heat.

As I said, though, not everyone really places alot of importance on PFC so it's personal preferance.

As for 12v rails...

With the exception of PC P&C and Athena, most reputable brands tend to have more rails the higher in wattage you go. However, don't just pay attention to watts or rails or PFC. Amps are every bit as important as any other spec.

Take my Silverstone 750 that is powering the rig I am on now - Active PFC, quad 12v rails, 18amps per rail.

 

[doolittle]

Power Factor Correction - protects against things like voltage fluctuations and electrical irregularities that can affect the efficiency of your power supply.

This implies voltage regulation & correction as part of the PFC circuitry, and I don't think that is the case. Active PFC is an additional circuit in the psu to bring the apparent power as close to the real power as much as possible (bring the PF as close to 1 as possible) by opposing the reactive AC load with an equivalent reactive load in the opposite phase-shifting direction. Hence, if your non-pfc load is capacitive it will apply an inductive reactance as an active circuit to "correct" or reduce the phase angle.

A more correct way to put it would be:

Power Factor Correction - reduces the effects of voltage fluctuations and electrical irregularities that can be caused by a low power factor in an AC network.

edit: for example, if your environment is already noisy, it will not help much to have PFC if the harmonics are so bad and your waveform is so distorted it barely resembles a sine wave. The only savior in that case is a power conditioner that performs automatic voltage regulation. Very common in industrial environments.

 

[phreejak]

You still need to mention "efficiency" in that definition because that is what PFC specifically deals with.

 

[merc14]

The Dual rail spec isn't really there to increase stability and efficiency, it is a mandated safety requirement. The spec was developed to keep the VA (or watts DC) below 240 on the 12v side in order to reduce the danger of the shock hazard. Of course how many folks break open their PSU and tinker with it. Not many.

Yes, by splitting off part of the 12v and using to deliver power only to the more sensitive components such as the CPU and RAM, you do theoretically isolate these components from noisy things like fan and drive motors. Of course, somewhere up inside the PSU they all get connected back together anyways.

The problem is that limiting a 12v rail to 240 watts means you can only put 20 amps on that rail. Do you see the problem here. How many vid cards recommend a minimum of 21 amps on the 12v rail for stability.

So most PSU makers "cheat" and violate the standard. The more sophisticated jut put in a circuit that puts more amps on whatever rail requires it at the time. Others just hook the rails together. Some just say screw the standard and offer one 12v rail with all the amps on it (see PC Power and Cooling)

See this artcle for problems encountered with dual 12v rails:
http://www.extremetech.com/article2/0,1558,1932947,00.asp

 

[phreejak]

I think you misread what I said. I was referring to the PFC aspect and it's relation to efficiency. I didn't say anything about 12v rails having to do with efficiency.