Although pleasantly simple and robust, a passive PFC rarely achieves

low Total Harmonic Distortion (THD). Also, because the circuit operates

at the low line power frequency of 50Hz or 60Hz, the passive

elements are normally bulky and heavy.

Active PFC

Active PFC offers better THD and is significantly smaller and lighter

than a passive PFC circuit. To reduce the size and cost of passive

filter elements, an active PFC operates at a higher switching

frequency than the 50Hz/60Hz line frequency.

Background

More than 90 million notebook computers, 150 million LCD monitors

and 50 million LCD televisions are expected to ship in 2008. With

such a fast growing number of power electronics, actions must to be

taken to ensure the functionality of the nationwide power grid.

In 2001, the European Union put EN61000-3-2 into effect to set the

harmonic regulation standard on any power grid supplied application

with power consumption over 75 watts. This essentially requires

power factor correction (PFC). Additionally, a standby power

dissipation limit is set to conserve power when a load is OFF.

“80 PLUS” is an initiative funded by electric utilities to integrate more

energy efficient Power Supply Units (PSUs) - especially for desktop

computers and servers. 80 PLUS certifies to more than 80% energy

efficiency at 20%, 50% and 100% of rated load. To meet the 80

PLUS certification, PSUs require a PFC of 0.9 or greater at 100%

load. This means PSUs that waste 20% or less electric energy (as

heat at the specified load levels) will lead to reduced electricity

consumption and lower bills. Rebates are sometimes given to

manufacturers who use 80 PLUS certified PSUs.

Implementing power factor correction (PFC) into switch mode power

supplies will maximize:

• The power handling capability of the power supply

• Current handling capacities of power distribution networks

Input power factor (PF) is defined as:

PF =

Real Power (watts)

Apparent Power (VA)

PF is expressed as decimal number between zero and one (0 and 1).

A non-corrected power supply with a typical PF equal to 0.65 will

draw approximately 1.5 times greater input current than a PFC

supply (PF = 0.99) for the same output loading. The non-corrected

supply requires additional AC current to be generated which is not

network.

There are two types of PFCs:

1. Active

2. Passive

Passive PFC

The simplest form of PFC is passive (Passive PFC). A passive PFC

uses a filter at the AC input to correct poor power factor. The passive

PFC circuitry uses only passive components — an inductor and

some capacitors (Figure. 1).

Power Factor Correction (PFC) Application Notes

AC

+

-

PF C Inductor

DC Bus

FFiigguurree 1

1: A passive PFC circuit requires only a few components to increase efficiency,

but they are large due to operating at the line power frequency.

PFC

Co ntrol

A C

PFC

Inductor

+

DC

Bu s

-

FFiigguurree 2

2:: An active PFC circuit produces low THD and uses relatively small passive

components.