Articles: Cases/PSU
 

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Power Factor Correction

The following kinds of power are generally recognized in alternating current circuits: 1) instantaneous power is the product of the currant by the voltage in the particular moment of time; 2) active power is the power generated on a purely resistive load and is measured watts. Active power fully goes to useful work (heating, mechanical motion) and is usually referred to as consumed power; 3) since a real load usually has inductive and capacitive constituents, reactive power accompanies active power. Reactive power is measured in reactive volt-amperes and is not consumed by the load. Received during one half-cycle of the line voltage, it is fully returned back into the power grid during the next half-cycle, just uselessly loading the power wires. Thus, reactive power is absolutely useless and various corrective devices are employed to oppose it.

Power factor is the ratio of active power to full power, i.e. to the vector sum of the active and reactive power.

A switch-mode power supply without any additional correction circuitry is a high capacitive load – as the schematic I showed you earlier shows, there are two capacitors (of a rather high capacitance) right after the diode bridge D1. On the PSU’s connection to the power grid, the first quarter-wave of the voltage loads the capacitors to 300 with something volts, then the voltage goes down quickly (the second quarter-wave), while the capacitors are more slowly discharging into the load (i.e. into the switching regulator). As the result, when the voltage starts to grow up again (the third quarter-wave), the voltage on the not-fully-discharged capacitors is about 250 volts, and the charge current will be zero while the voltage in the power grid is smaller than that (the rectifier’s diodes are locked by the applied reverse voltage which equals the voltage difference between the capacitors and the power grid). During the last third of the quarter-wave (of course, I give just approximate numbers since in reality they depend on the load and the capacitance of the capacitors) the voltage in the grid is again higher than the voltage in the capacitors, and the charge current starts to flow. The charge stops as soon as the voltage in the grid is again smaller than on the capacitors – in the first half of the fourth quarter-wave. As the result, the PSU is consuming power from the power grid in short pulses, approximately coinciding with the peaks of the sinusoid of the voltage in the power grid:


Power Supply without Power Factor Correction

The green line in the oscillogram above denotes the line voltage, the yellow line marks the current consumed by the PSU from the line. The power factor equals roughly 0.7 here, i.e. about one third of the power goes to heat up the cables, without doing any useful work. Home users shouldn’t bother much about this number, as home electric meters only measure the active power, but a low power factor may become a problem for large offices and rooms where there are many computers running at the same time, because the electric wiring and the accompanying equipment should be made according to the full power. In other words, it should be one third higher (at power factor = 0.7) than it would be if the PSU didn’t consume reactive power. A low power factor may also affect the choice of an Uninterruptible Power Supply (UPS) since they are limited by the full rather than active power, too.

 
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