Answer 5—A power factor controller is basically a boost-mode switching regulator that converts the raw full-wave rectified AC power line voltage into a relatively constant high DC voltage. A special control circuit operates the regulator so that the load on the AC line appears to be purely resistive, with a power factor within a few percent of 1.00.

The control circuit contains two separate feedback loops. A high-speed loop makes sure that the input current waveform is proportional to the input voltage waveform. A low-speed loop varies the proportionality constant of the first loop to maintain the average output voltage at a constant level (ignoring the ripple at two times the line frequency).

A simple commercial example of this type of power factor controller is the Siemens/Infineon TDA4862. Search the ’Net for its datasheet to see a complete circuit that covers some of the other subtleties of this application, including start-up issues. The TDA4862’s application note is also useful (W. Frank and M. Herfurth, “TDA4862: Technical Description,” AN-PFC-TDA4862-1, Infineon Technologies, April 2001).

Contributor: David Tweed

Published December 2003