Articles: Cases/PSU

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Operation with UPS’s

The above-described active PFC has one drawback. Some of its implementations cannot work normally with uninterruptible power supplies. When the UPS switches to its batteries, such an A-PFC device increases its consumption greatly, making the UPS shut down to prevent overload.

To check out the implementation of active PFC in each particular PSU we connect it to an APC SmartUPS SC 620VA and test them both in two modes: powered from the mains and powered from the batteries. In both cases the load on the PSU is being steadily increased until the UPS reports overload.

If the PSU is compatible with the UPS, the max load on the PSU is typically 340-380W and 320-340W when powered from the mains and batteries, respectively. When the load is higher at the moment of switching to the batteries, the UPS reports overload but doesn’t shut down.

If the PSU has the mentioned problem, the maximum load the UPS can work from the batteries at is far below 300W, and if it is exceeded, the UPS shuts down right after switching to the batteries or five to ten seconds afterwards. You should avoid such PSUs if you are planning to use a UPS.

Fortunately, there are fewer power supplies incompatible with UPS’s these days. For example, this incompatibility had been a problem with FSP Group’s PLN/PFN series but was eliminated in the subsequent GLN/HLN series.

If you have a PSU that is unable to work with UPS’s normally, you have two solutions (besides improving the PSU itself which would call for a good knowledge of electronics): change the PSU or change the UPS. The former solution is usually cheaper since the UPS would have to have a large reserve of wattage or even be the online variety, which is expensive and not really appropriate for home applications.

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