Editing Power factor (section)

==== Active PFC ====
[[File:Active pfc PSU packaging.svg|right|thumb|Specifications taken from the packaging of a 610 W [[Power supply unit (computer)|PC power supply]] showing active PFC rating]]

Active PFC is the use of [[power electronics]] to change the waveform of current drawn by a load to improve the power factor.<ref>{{Citation | publisher = Fairchild Semiconductor | year = 2004 | type = application note | number = 42047 | title = Power Factor Correction (PFC) Basics | url = http://www.fairchildsemi.com/an/AN/AN-42047.pdf | access-date = 2009-11-29 | archive-url = https://web.archive.org/web/20140611063712/http://www.fairchildsemi.com/an/AN/AN-42047.pdf | archive-date = 2014-06-11 | url-status = dead }}</ref>  Some types of the active PFC are [[Buck converter|buck]], [[Boost converter|boost]], [[Buck-boost converter|buck-boost]] and [[synchronous condenser]].  Active power factor correction can be single-stage or multi-stage.

In the case of a switched-mode power supply, a [[boost converter]] is inserted between the bridge rectifier and the main input capacitors. The boost converter attempts to maintain a constant voltage at its output while drawing a current that is always in phase with and at the same frequency as the line voltage. Another switched-mode converter inside the power supply produces the desired output voltage from the DC bus. This approach requires additional semiconductor switches and control electronics but permits cheaper and smaller passive components. It is frequently used in practice.

For a three-phase SMPS, the [[Vienna rectifier]] configuration may be used to substantially improve the power factor.

[[Switched-mode power supply|SMPSs]] with passive PFC can achieve power factor of about 0.7–0.75, SMPSs with active PFC, up to 0.99 power factor, while a SMPS without any power factor correction have a power factor of only about 0.55–0.65.<ref>{{Citation |last1=Sugawara |first1=I. |last2=Suzuki |first2=Y. |last3=Takeuchi |first3=A. |last4=Teshima |first4=T. |contribution=Experimental studies on active and passive PFC circuits |title=INTELEC 97, 19th International Telecommunications Energy Conference |date=19–23 Oct 1997 |pages=571–78 |doi=10.1109/INTLEC.1997.646051|isbn=978-0-7803-3996-5 |s2cid=109885369 }}</ref>

Due to their very wide input voltage range, many power supplies with active PFC can automatically adjust to operate on AC power from about 100&nbsp;V (Japan) to 240&nbsp;V (Europe). That feature is particularly welcome in power supplies for laptops.