Active PFC-Enabled PSUs Are Not Compatable 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. If you look at it on an oscilloscope, it would look something like this (this is a pure sine wave, 5V @ 10kHtz):
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:
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.
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Did you discover this during a power outage, or is there some method of testing that simulates an outage?
Curious…
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.
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.