The TL431 in Switch-Mode Power Supplies loops is a classic TL431 paper by Christophe Basso and Petr Kadanka. This explains the TL431 from the internals through loop compensation to an actual power supply. It includes a detailed schematic and description of how the TL431 operates internally. ...
Output regulation and isolation is achieved by the use of the TL431 as an output reference and comparator, and a 4N35 optocoupler. As the 5.0 V output reaches its nominal level, the TL431 will start to conduct current through the LED in the 4N35. This in turn will cause the optoreceiver ...