Rebuilding The Elektor 400W Power Supply: Pre-Regulator

As described in the introduction to this series, the pre-regulator used in the Elektor 400W power supply project is very similar to a light dimmer, but designed to handle the inductive load of the transformer.  I will attempt to describe the theory of operation below, based on my understanding of the circuit and the description provided in the Elektor articles.  Additionally, this application note from SGS-Thomson describes a similar topology. Continue reading