TL431 Problem in SMPS circuit

[godfreyl]

Yes you're right, there is a mistake somewhere. Maybe the +5V is supposed to be +3V, or maybe the bottom of the TL431 is not connected to ground. I can't see where that wire goes.

 

[FvM]

and another question is when i put 2 100Kohm resistor in series the output voltage of Tl431 follow the increasing voltage until it wants to be more than Vref*2 and after that it decrease.why it follow the input voltage? because as i know in the comparator mode of op-amp if the voltage at V(+) is infinitesimal bigger than the V(-) it goes to Max saturation and doesn't transfer the input voltage to output

If I understand right, you are referring to the circuit in post #18.

It's correct to analyze it as a comparator (with 2.5V threshold). But it's output is open collector and unlike a regular comparator it misses an independent supply voltage. So positive saturation implies "open" output and negative saturation 1.9 V.