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Thread: Active PFC Power Supply + UPS Compatibility

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    WareTernal is offline

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    Default Active PFC Power Supply + UPS Compatibility

    I recently upgraded my PSU to a Corsair TX650. After the upgrade I started to notice some issues with my APC Back-UPS RS 900. The APC software reports a 17 minute runtime on battery, but power interuptions of less then 1 second are causing the PC to hibernate, which shouldn't happen until the battery has only 5 minutes left.

    Searching has turned up some information about Active PFC and "square" sine waves coming from UPS inverters. Most of the information seems vague, probably due to me not paying attention in electronics. From what I understand there is an issue with high "inrush" to recharge the caps in the PSU at the point when the powers switches from the mains to the battery, and this may be ralated to the "square" wave, with it's long and flat peak, and the way the caps charge.

    The PureOC article about the OCZ Pro-Source 1500 touches on the issue, mentioning "A pure sine wave is essential to a professional-grade back-up power source, while an inferior "step" sine wave plays havoc with today's sophisticated and sensitive power factor correction (PFC) circuitry, making it possible for your PSU to reject the back-up power source and shut down."

    To make things worse, I've read in other forums that "square" waves can actually damage high efficiency capacitors. Some people are reporting whining or buzzing from PSU's when running on battery. I don't really want to test my battery now, because I'm afraid it might be hurting my PSU.

    So how true is all this?
    Why don't the PSU manufacturers say that we need certain type of UPS? or the other way around?
    Will the new Energy Star requirements end up making all our (not so) old UPS's obsolete?
    Has anyone fried a PSU from this?

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    Yblad is offline

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    Capacitors behave very different when a square wave is applied compared to a sinusoidal waveform. Square waves mess up all the reactance formulas, meaning that break frequencies change in filters and there are different reactances than expected if the product has been designed for sinusoidal waveforms. Reactance, by the way, is essentially resistance in layman's terms. But we call it reactance because it is different depending on the frequency of the AC waveform, hence it "reacts" rather than just resisting.

    This is linked with affect on capacitor charge and discharge, I'll avoid using the actual formulas for this as it will be easier for most to understand. Discharge and charge depends on both the capacitance and the voltage being applied across the capacitor at any given time. Now, im going to focus on discharge as a discharging capacitor will almost certainly govern the psu/battery switching. With a sinusoidal wave, the discharge rate is much lower than with a square wave. With a sinusoidal wave, the voltage across the capacitor gradually decreases, meaning that it discharges slowly at first, and increases in its rate of discharge as the voltage reduces. With a square wave the voltage reduction is near instantaneous, so the capacitor discharges at its maximum rate right from the start, so it discharges much more quickly. This is probably why your battery is kicking in way before it should.

    So, in short, if your backup hardware is set up to work with a sinusodial waveform then it wont work as intended if you apply a square wave to it.
    Last edited by Yblad; 04-04-2010 at 19:49.
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    WareTernal (04-04-2010)

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