Articles: Cases/PSU

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The graph below shows you the experimentally measured dependence of the power factor on the load for three PSUs: without PFC, with passive PFC and with active PFC.

Besides providing a power factor closest to the ideal, active PFC improves the operation of the PSU, unlike passive PFC. Firstly, it additionally regulates the input voltage of the PSU’s main regulator. The unit becomes less sensitive to voltage sags in the power grid, and it is easy to design PSUs with a universal 110-230v power input that doesn’t require manual switching. Secondly, active PFC improves the reaction of the PSU to short-term (fractions of a second) sags in the ac voltage. In such moments the PSU works thanks to the power of the capacitors C1 and C2 of the high-voltage rectifier, and this power is proportional to the squared voltage in them. As I mentioned above, this voltage is 400 volts with active PFC instead of the ordinary 310 volts, so the efficiency of the capacitors improves by more than a half.

In fact, active PFC has two drawbacks only. First, like any other complication of the design, it reduces the reliability of the power supply unit. Second, the efficiency of the PFC device is not 100 percent, so it contributes to the heating-up of the PSU. Still, the advantages of active PFC usually counterweigh these drawbacks.

So, if you need a PSU with power factor correction, you should first consider models with active PFC. They alone provide a really good power factor, also improving other characteristics of the power supply. From the home user’s standpoint, PSUs with active PFC may come in handy for owners of low-power UPSes. Suppose you have a 500VA UPS – 50VA consumed by your LCD monitor and 450VA are left to your system case – and you want to upgrade the latter to the up-to-date level, knowing that a serious configuration may consume up to 300 watts at maximum from the PSU. In this case a PSU with a power factor of 0.7 and an efficiency factor of 80 percent (these are typical numbers for a good PSU) would give us a full power consumption of 300/(0.75*0.8) = 500VA, while the same PSU with a power factor of 0.95 would consume 300/(0.95*0.8) = 395VA. As you see, you have to upgrade your UPS for a power supply without PFC, since the current UPS won’t handle the load, while an active-PFC power supply will even give you a small reserve of 55VA. Well, we should also take into account the fact that the voltage on the output of cheap UPSes has a trapezoidal rather than sinusoidal shape – but the PSU with active PFC would still have an advantage, only the absolute numbers would be different.

Finishing this section of the article, I want to warn you against confusing two terms: power factor and efficiency factor. These denote two quite different things. Efficiency factor equals the ratio of the output power of the PSU to the active power it consumes from the power grid. Power factor is the ratio of the active power consumed from the power grid to the full power consumed from the same grid. The PFC circuitry in the PSU affects the amount of the consumed active power only indirectly – because the PSU itself consumes some power, plus the input voltage of the main regulator changes. The main purpose of PFC is a reduction of the consumption of reactive power by the PSU, but reactive power is not accounted for when calculating the efficiency factor. So, there’s no direct relation between power factor and efficiency factor.

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