Powering Your PC: Truths and Misconceptions
A Guide To Power Supplies

Power Factor Correction:

Poor power factor can be corrected both by active and passive means. Remember a few pages ago the discussion on how power travels through a power supply? One of the early stages mentioned power is held in a pair of large capacitors before it is fed to the primary field transistor. These capacitors make a PSU naturally lean more to the capacitive side, so you would need an equal amount of inductance to counter-act the effects. A simple & effective inductor (as mentioned before) is a choke. Conversely, if the load was more inductive, then the reactance could be cancelled by adding a capacitor. In theory it may sound simple, but depending on the complexity of the device a passive solution may not work very efficiently or as desired.

On the other hand, Active PFC is a secondary switching circuit placed before the primary switching circuit in a power supply. It switches the power without using capacitors and feeds a more constant supply to the primary switching circuit than would normally be without any PFC. One little bonus with active PFC is the ability to remove the 115/230 switch on that back of a power supply (not really a big deal, but worth mentioning).

A PC power supply with no PFC generally has a PF around .6 - .7 (give or take) under regular loading conditions. Passive PFC usually bumps this up to something in the neighborhood of .75 - .85 (again, give or take depending on the power supply & implementation). Finally, active PFC can easily bump up the power factor to .98 or greater.

Is a power supply with active PFC more efficient? No!!! Technically it is another circuit, which requires power itself to operate, thus the power supply could be considered slightly less efficient. However the amount is relatively so minute that the benefits of active PFC greatly outweigh any downside. Don't confuse "Power Factor" with "Efficiency", they are two totally different things.

Uninterruptible Power Supplies:

Now is a good time to mention power ratings for Uninterruptible Power Supplies (UPS), which are typically rated in Volt-Amps (ex: 750VA), but also have maximum Watt ratings (which most people ignore or are unaware of). For example, I own an APC SU1000XL, it has a maximum rating of 1000VA & 670W, neither of these ratings should be exceeded.

It is a de-facto standard in the industry that the Watt rating is approximately 60% of the VA rating for small UPS systems, this being the typical power factor of common personal computer loads. In some cases, UPS manufacturers only publish the VA rating of the UPS. For small UPS designed for computer loads, which have only a VA rating, it is appropriate to assume that the Watt rating of the UPS is 60% of the published VA rating.⁴

In its most simplest terms, a UPS is really nothing more than a DC-AC Inverter that uses large rechargeable batteries as a power source when the line power goes out. Battery runtime varies based on the capacity of the UPS and the load (most UPS manufacturers have charts on their website with this information). The worse the PF, the less devices you will be able to plug into the UPS. Having a PC power supply with Active-PFC makes good sense to help maximize the use of your UPS as it is typically one of the bigger power hogs.