Active PFC-Enabled PSUs Are Not Compatible With Most Low-End UPS

It turns out that most PC power supply units with active power factor correction (PFC) do not work well with lower-cost uninterrupted power supplies (UPS), e.g. battery back-ups. This I learned the hard way, as my Antec EarthWatts EA 500 PSU, which I adore, will go dead the second the battery on my APC Back-UPS BX1300LCD kicks in.

That’s because APC’s Back-UPS units output power, when on battery, as a modified sine wave, rather than a true sine wave.

In short, if you send a high-end PSU the current created by a lower-end UPS, the PSU’s built-in power factor correction hates it, and cuts power to the PC immediately. As in, the exact thing you bought the UPS to prevent in the first place is exactly what happens.

While I understand the value in having a high-performing PSU — less heat, more power, protection for internal components — a straight-up power off of the PC is never a good thing. So your choices are either to forgo a UPS, purchase a really expensive UPS, or purchase a low-cost PSU.

U.S. household electricity generally comes out of the wall  at 60 hertz; that is, 60 “cycles,” or “waves,” of current per second, alternating between 120 volts of positive current, and 120 volts of negative current. This is what a sine wave looks like, an elegant arc of current alternating over time:

Sine and Cosine

For various technical reasons, rather than making a pure sine wave, it’s easiest, when creating a power inverter, to use what is called a “modified sine wave.” Rather than producing elegant parabolic waves, what is produced more closely resembles stair steps, like this:

Modified sine wave vs pure sine wave
A modified sine wave (the red square) vs. a pure sine wave (the red line). Courtesy BD Batteries.

When a power supply unit that has active power factor correction sees this “square wave,” it doesn’t pass the internal tests the power supply uses to maintain high efficiency and protect your system against power fluctuations. So, in most cases, such as mine, it simply turns off.

Probably the best thing to do is go all in, and spend about $500-$600 for a true sine wave UPS, such as APC’s Smart-UPS products. Or, you could do what I did: purchase a different PSU, one that is generally reliable but doesn’t have PFC built into it. I’m going with a Cooler Master eXtreme Power Plus 500 Watt PSU.

I don’t usually blog on help desk / techie stuff, but this was annoying enough to warrant a post.


  1. Did you discover this during a power outage, or is there some method of testing that simulates an outage?

    1. Thorn: APC has software called “Power Chute” that will let you run a test on the unit, which includes switching over to the battery. And you can press and hold down the power button on an APC unit for 5 seconds to switch it over to battery.

      The problem with that is, if your PC is plugged in to that unit, and your PSU isn’t compatible, then your PC will shut off immediately. So it makes the most sense to assume that if your PSU has PFC, it needs a pure sine wave UPS. Check your PSU’s documentation.

  2. Some method of testing? lol, just pull the power cord out of the wall socket or flip the breaker to the circuit and see if the PC stays running.

    I could not say it makes sense to assume you need a pure sine wave UPS with an APFC (merely calling it PFC is incorrect as many passive PFC PSU are labeled as “PFC” by their manufacturers) UPS, do check the documentation or ask the manufacturer if you don’t already own it so you can test that yourself.

    However, this writeup is lacking one crucial detail in that it is not always the PSU that shuts down, instead with some pairs of PSU and UPS the problem is the modified sine wave causes too great an inrush current and the UPS shuts off its output, which of course shuts down the PSU too but it was the UPS that shut down first.

    Lastly, I wouldn’t go with most of the Cooler Master PSUs. Low quality fan and capacitors give them shorter lifespans, but I suppose if you feel like relubing the fan every 18 months or so you might lengthen lifespan a bit.

  3. BTW, because of the problem CyberPower now offers a new lower cost product line to consumers Active PFC compatible battery backed up UPS products, such as the CyberPower Intelligent PFC LCD CP850PFCLCD uninterruptible power supply (UPS) which is around $150 bucks instead of $400+

  4. My power supply did the exact same thing in my $7000.00 Desktop. Turns out the power supply was damaged by the Battery Back Up. Fortunately the power supply has a three year warranty and the power supply guys honored the warranty, had to pay $30.00 for shipping though. Went from 1320 watt BBU to a 900 Watt Cyberpower pure sine wave back up. Been two months and still working like a champ.

  5. Dear friend,
    I also thinking to buy new power supply, It is that no PFC is require at all or Passive PFC can also work,

    But for sure no to active PFC.

  6. Thank you so much for this post. I’ve had an issue with Dell XPS 8500 randomly turning off out of the blue since I bought it in 2012. I always thought it was a PSU or mobo issue, but after reading this I realize it’s probably the UPS.

    1. @iralki: The short answer is no, you generally cannot disable active PFC.

      Power factor correction is hard-wired within the power supply unit itself; it’s not something you can control via a setting in the computer. The computer itself does not communicate with a PSU and is completely unaware whether a PSU uses active PFC.

      In theory, you could rebuild the power supply to remove the circuitry that enables PFC.

  7. Just wanted to thank you for this post. I just avoided buying the wrong UPS (as my PC has a power supply with active PFC), which would defeat the very purpose of buying one in the first place. More manufacturers should point that out, as more and more PC enthusiasts will have rigs with active PFC power supplies.

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