Answer 8
The key concept is to create a switched-mode current regulator on the primary side of the power supply such that the short-term average current (over one switching cycle, a tiny fraction of the AC power cycle) is proportional to the line voltage over that same period of time.

For the digital or analog domain, you need a multiplier circuit to accomplish this. One input to the multiplier is the line-voltage measurement, and the output sets the peak current for the current regulator. The other input to the multiplier is a proportionality factor that is derived from the rest of the power supply, indicating the amount of power currently required by the load, averaged over several AC cycles. Usually, this is just voltage feedback from the output of the main rectifier/filter, reducing the primary current when the voltage rises too high and increasing the current when it falls. In fact, this provides a certain amount of preregulation ahead of the main DC regulator(s).

Generally, the switched-mode current regulator is used in conjunction with a full-wave rectifier, so that it only needs to work with one polarity of current. This works out well with most switched-mode power supplies, because they normally rectify the line voltage to 350 VDC anyway before feeding it to a high-frequency transformer.

Contributor: David Tweed

Published July 2003