Power Factor

Power Factor is one of the most misunderstood concepts in PSUs. For a more in-depth, technical article read here.

To understand Power Factor, some basic terms have to be defined.

• Working Power: This is the power that does some real work, ie. work of heating, lighting and motion.
• Reactive Power: In order to get a motor moving, the motor must build (and sustain) a magnetic field in the motor coils, which requires power. This power does not "do" anything, but it is required for operation.
• Apparent Power is the sum of Working Power and Reactive Power.

The formula for Power Factor is Power Factor = Working Power [W] / Apparent Power [VA]. For example if a circuit has a Working Power of 275W and an Apparent Power of 300VA, its Power Factor is 275W / 300VA = 0.92

The problem with Reactive Power is, that it does not do anything, and is thus not billed by your power company (unless you are an industrial customer, who has to pay additional charges if his power factor is low). The power company DOES have to provide your household with the Reactive Power, so their generator output must be higher and the grid must be able to take the increased load, overall this costs the power company money. It is also possible that low Power Factor, high-load devices have an effect on the power quality in your house.

In order to increase Power Factor, passive PFC uses a capacitor in parallel with the AC mains, because the effects of capacitance are exactly opposite those of inductance.
Active PFC uses a special circuit which acts like a resistive load (which has no Reactive Power) and feeds this power to the PSU circuit, isolating it from the mains. One effect this has, is that the overall conversion efficiency is a bit lower, since this circuit inside the PSU consumes power, which is not converted into DC voltage (this is in the range of 3%-8%).

Overall, you, as end-customer will not directly benefit from PFC. In the long-term it will save you money and problems. Less load on the grid means less likeliness of black-outs. Less load on the power company's generators means that their prices will not go up as fast as they would when they needed to buy bigger generators.

In the European Union all power supplies >70W must have either Passive or Active PFC.

Value and Conclusion