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I'm looking at getting a UPS for my home computer. So far the APC BR700G looks very promising, except for one thing: one of the reviews on Newegg says that this UPS does not work with a power supply with Active PFC.

Pros: Unit looks great, built well, very heavy, was excited to use it.

Cons: Didn't research enough - many newer power supplies like my corsair 750w (and yes dells and other mainstreamers sell them too) that I bought last year have a feature called active pfc (power factor corrected). The signal for this backup battery doesn't fully support that feature and can cause issues.

You can find an article on APCs site if you search their user forums for PFC.

And the power supply in my computer is, in fact, an Active PFC PSU.

I've already found one answer on this site claiming that it's not an issue, that "most quality supplies these days have PFC and work just fine with a UPS." That disagrees with the review on Newegg. Can someone explain this discrepancy? Also, what is it exactly about a UPS that makes it incompatible with an Active PFC PSU? (if anything) Is there some way to tell based on the technical specifications, or do I just have to hunt for reviews online to avoid wasting my money?

While any input would be appreciated, I would prefer to get an answer from someone with actual experience with similar UPS's and Active PFC power supplies, who can tell me whether it works or not.

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2 Answers 2

I know this is an old thread, but if there are still people struggling with this subject, you can find more information in this Application Note by APC. (You can also do a Google search for "APC active PFC white paper" should the link expire.)

Here is a summary:

Because of the way active PFC’s operate, they can sometimes overload the UPS with momentary high inrush current. This can occur when the UPS transfers from online to on-battery operation, creating a momentary loss of power (<8ms). The PFC supply may respond by temporarily drawing an excessive amount of current. Also some PC’s, when awoken from standby (or ‘sleep’) mode, will draw a momentary high inrush current, potentially overloading the UPS if it happens to be running on battery. All APC UPS’s are designed to protect themselves when there is a severe overload while on battery. Some general serverclass UPS’s such as APC’s core Smart-UPS® models will protect itself by actively limiting the overload to a level that it can manage. Other, more economical UPS designs such as Back-UPS® or the Smart-UPS® SC will protect itself by shutting down quickly when it detects a severe overload. This potential for incompatibility should be considered when selecting a UPS – sometimes the most economical choice is not the best one. It is important to note that not all PFC power supplies will cause the UPS overload. However, the incompatibility is most acute in the one of the following situations: • A large server class PFC supply (e.g. rated 500W or more) is used with a Back-UPS or Smart-UPS SC. • The server is equipped with redundant PFC supplies (has two line cords) that are powered by the same UPS. • More than one PFC supply is plugged into the same UPS, bringing the total power rating (nominal) of the power supplies to 500W or more. • A workstation class PC (or high-end gaming PC) is equipped with a PFC power supply rated 500W or more. In any of these situations, APC recommends that a true, pure sine wave, server class UPS be used. Acceptable models include APC’s Smart-UPS®, Smart-UPS® XL and Smart-UPS® RT family of UPSs. However if, a Smart-UPS SC or Back-UPS RS is to be used, the UPS should be sized accordingly.

A critical factor to consider in avoiding an overload trip fault is the ‘nominal’ power rating of the power supplies, not the actual steady state power consumption. For example, a server may have two 600W power supplies in parallel-redundant mode, for a total power rating of 1200W. But the steady state power consumption in this case will be less than 600W. In another example, a high-end workstation with an 850W PFC power supply may only consume 350W under normal operation. So proper sizing of a UPS with active PFC power supplies, to better handle momentary overloads, must take into consideration the maximum power rating of the power supply, not just the actual power consumption of the load. Also keep in mind that if a power supply is rated for 600W output, it’s maximum ‘input’ power will be higher depending on its efficiency. For example, an Energy Star 4.0 compliant power supply has to be more than 80% efficient. That means when it is delivering 600W output power, its input power can be as high as 750W. This ‘input’ power should be the basis for sizing the UPS. Currently not all UPS selectors take these factors into consideration when recommending a proper UPS for servers with active PFC power supplies. Therefore the following guidelines should be followed when recommending a UPS for a PFC load.

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I can say with certainty that my PSU is compatible with my UPS. I have done some research before buying my UPS and upon finding the mentioned information, I bought a UPS that can supply the amount of power my PC would need when switching to battery. This is my Power Supply: seasonic s12 II 620W (With Active PFC) This is my UPS: APC Back-UPS 1100VA (BX1100CI) (With stepped approximation to sinewave) –  WJK Nov 8 at 19:56

There relly shouldn't be any problem with active PFC, because if it is done in a really good way they UPS shouldn't see the PFC, it'll just see the voltage and the current almost in phase with each other.

The problem with that UPS would rather be the low power rating it has, it will not be able to power a high powered computer.

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