Feedback signal voltages in SMPS?

[eem2am1]

Hello

In the following TL431 SMPS regulation circuit.........

TL431 Feedback circuit:
https://i51.tinypic.com/30skly9.jpg

-supposing that the SMPS is on constant, maximum load
-also, that it is on steady state.

Is the voltage at the cathode of TL431 supposed to be "flat" DC with no AC component...?....or is it OK if its going up and down?

....likewise, i ask the same question for pin 2 voltage of the PWM controller?

 

[mtwieg]

The capacitor from the cathode to reference pin means that it is pretty much a short at high frequencies, and that the cathode will be a low impedance. Generally this is used as the high frequency feedback paths, while the resistor dividers from output to reference pin are the low frequency (DC) paths.

As for the control pin of the controller, that depends a lot on the actual structure of the control input. Could be either.