A particularly important class of non-linear loads is the millions of personal computers that typically incorporate switched-mode power supplies
(SMPS) with rated output power ranging from a few watt to more than 1 kW. Historically, these very-low-cost power supplies incorporated a simple full-wave rectifier that conducted only when the mains instantaneous voltage exceeded the voltage on the input capacitors. This leads to very high ratios of peak-to-average
input current, which also lead to a low distortion power factor
and potentially serious phase and neutral loading concerns.
A typical switched-mode power supply
first makes a DC bus, using a bridge rectifier
or similar circuit. The output voltage is then derived from this DC bus. The problem with this is that the rectifier
is a non-linear device, so the input current is highly non-linear. That means that the input current has energy at harmonics
of the frequency of the voltage.
This presents a particular problem for the power companies, because they cannot compensate for the harmonic current by adding simple capacitors or inductors, as they could for the reactive power drawn by a linear load. Many jurisdictions are beginning to legally require power factor correction for all power supplies above a certain power level.
Regulatory agencies such as the EU
have set harmonic limits as a method of improving power factor. Declining component cost has hastened implementation of two different methods. To comply with current EU standard EN61000-3-2, all switched-mode power supplies
with output power more than 75 W must include passive PFC, at least. 80 PLUS
power supply certification requires a power factor of 0.9 or more.