Articles: Cases/PSU

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Power Factor

The AC electric mains can be considered as having two types of power: active and reactive. Reactive power is generated in two cases: when the load current and the mains voltage are out of phase (that is, the load is inductive or capacitive) or when the load is non-linear. The PC power supply is a pronounced example of the second case. It will normally consume the mains current in short high impulses that coincide with the maximums of the mains voltage.

The problem is that while active power is fully transformed into useful work in the PSU (meaning both the power the PSU yields into the load and its own heating up), reactive power is not consumed at all. It is driven back into the mains. It is kind of wandering to and fro between the generator and the load, but it heats up the connecting wires as well as active power does. That’s why reactive power must be got rid of.

The circuit called active PFC is the most efficient way to suppress reactive power. It is in fact an impulse transformer that is designed in such a way that its instantaneous consumed power is directly proportional to the instantaneous voltage in the mains. In other words, it is linear and consumes active power only. The voltage from the output of the active PFC device goes right to the switching transformer of the power supply which used to be a reactive load due to its non-linearity. But now that it receives direct voltage, the non-linearity of the second transformer doesn’t matter anymore because it is detached from the electric mains and cannot affect it.

The power factor is the measure of reactive power. It is the ratio of reactive power to the total of active and reactive power. It is about 0.65 for an ordinary PSU, but PSUs with active PFC have a power factor of 0.97-0.99. So, the active PFC device reduces reactive power almost to zero.

Users and even hardware reviewers sometimes make no difference between the power factor and the efficiency factor. Although both these terms describe the effectiveness of a power supply, it is a gross mistake to confuse them. The power factor describes how efficiently the PSU uses the AC electric mains, i.e. what percent of power the PSU consumes from it is actually put to use. The efficiency factor describes how efficiently this consumed power is transformed into useful work. There is no connection between these two things because, as I said above, reactive power, which determines the value of the power factor, is not transformed in the PSU into anything. You cannot apply the term “conversion efficiency” to it, so it has no effect on the efficiency factor.

Generally speaking, it is the power supply companies rather than the users that profit from active PFC because it reduces the computer’s load on the electric mains by a third or more. This amounts to big numbers today when there is a PC standing on every office desk. From an ordinary user’s point of view, active PFC makes no difference even when it comes to electricity bills. Home electricity supply meters measure only active power as yet. The manufacturers’ claims that active PFC can in any way help your computer are nothing but marketing noise.

A side effect of active PFC is that it can be easily designed to support a full range of input voltages, from 90 to 260V, thus making it a universal PSU that can work in any power grid without a manual selection of the input voltage. Moreover, PSUs with manual switches can only work in two input voltage ranges, 90-130V and 180-260V, and you cannot start them up at an input voltage of 130-180V. A PSU with active PFC covers all those input voltage ranges without any gaps. So, if you have to work in an environment with unstable energy supply, when the AC voltage may often bottom out to below 180V, a PSU with active PFC will allow you to do without a UPS or will make the UPS’ battery life much longer.

Well, the availability of active PFC does not guarantee that the PSU supports the whole range of input voltages. It can be designed to support a range of 180-260V only. This is sometimes implemented in PSUs to be sold in Europe because the use of such narrow-range active PFC helps reduce the manufacturing cost of the PSU somewhat.

Besides active, there are passive PFC devices. Passive PFC is the simplest way to correct the power factor. It is just a large choke connected in series with the power supply. Its inductance is smoothing out the pulsation of the current consumed by the PSU and is thus reducing the level of non-linearity. There is a very small effect from passive PFC – the power factor grows only from 0.65 to 0.7-0.75. But while implementing active PFC requires a deep redesign of the PSU’s high-voltage circuitry, passive PFC can be easily added into any existing power supply.

We measure the power factor of a PSU by the same method as the efficiency – increasing the load from 50W to maximum in steps. The results are presented together with the efficiency in a single diagram.

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