How this regulator works?

[Shahriar]

Hi
Look at the this OLD ATX power supply.
Can somebody explain how the 3.3V regulatosr built around TL431 IC works. It looks like a
Shunt regulator but very strange. I know How TL431 works but can't understand this circuit. i.e. if Q13 Current changes, how does this affect 3.3V rail. (look at R71)

http://www.4shared.com/file/4564161/afd71557/atxps.html

ThanX
Shahriar

 

[indulis]

As the 431 starts to draw current through R72 it will start to turn on Q13 which will load the 3.3V output in the event "it" has a light load and hold the output within load regulation limits.

 

[aartak]

Q13 Paase negative charge to filter capacitor at every pulse of supply
so regulation of charge regulates output voltage

 

[Shahriar]

Hi, ThanX both of you, I Understood it now.  ;D

But what is the roll of L6 there?

Shahriar