TL431 is usually used in the feedback section of the switching power supply
What is the difference between the two?
if TL431 is in follower mode and compensation is done on the primary side
Or the internal OPAMP of the IC switching is in the follower mode and the secondary compensation is done by TL431 ( Usually this is seen )
Yes i am also interested in what you mean by "TL431 in follower mode".
But i will stick my neck out and say that the TL431 must act as error amplifier, (and the primary side IC as "follower"), since only the TL431 can "see" the output voltage. The controller on the primary side, has no way of knowing what is the output voltage.
Unless you are speaking of non isolated SMPS?...but then you probably wouldnt be using an opto