Articles: Cases/PSU
 

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That’s why Power Factor Correction devices are becoming ever more popular. The simplest and most widespread device like that performs passive power factor correction. This device is an ordinary choke of a rather high inductance, attached to the circuit in series with the power supply.


Power Supply with passive Power Factor Correction

This oscillogram shows that a passive PFC device smoothes out the pulsation of the electric current somewhat, stretching it out in time, but the inductance of a choke which can be packed into a PSU cannot seriously affect the power factor, so the power factor of PSUs with passive PFC is about 0.75.

Not only the dimensions, but also the influence of the choke on the operation of the PSU does not permit to use a choke of a greater inductance. A high inductance attached in series with the power supply worsens the dynamic characteristics of the latter, i.e. its reaction to quick changes of the load as well as to sudden surges in the power grid.

The PFC choke can also suppress interference, but only low-frequency one. Due to its high inductance, it lets high-frequency noise pass through.

Thus, the role of passive PFC is ambiguous. It does very little to improve the power factor, but worsens the dynamic characteristics of the PSU. So, when choosing between two PSUs – with and without passive PFC, you should base your choice on other factors, rather than on the presence/absence of passive PFC.

Unlike a passive PFC, an active PFC device is yet another switching power source, which increases the voltage. An active PFC is attached between the power grid and the main regulator, providing a constant voltage of about 380-400v on the input of the latter. Unlike the main switching regulator, an active PFC device is designed in such a way that it doesn’t require a smoothed-out voltage on its input, so it doesn’t require capacitors. Thus, the active PFC switching power source doesn’t put a capacitive load on the circuit and, accordingly, has a power factor close to 1.


Power Supply with active Power Factor Correction

As you see, the shape of the current consumed by a PSU with active PFC differs but little from the consumption of an ordinary resistive load – the resulting power factor of such a PSU may be 0.95-0.98 at full load. The power factor diminishes at smaller loads, to 0.7-0.75 at the minimum, i.e. to the level of units with passive PFC. Still, peak values of the consumption current are much smaller with active-PFC PSUs than with any other type of power supply, even under small loads.

 
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