Question on TL431 Opto Feedback, how does it work?

https://i53.tinypic.com/300qt8l.jpg

I have searched for this a lot (on this forum and google). Can someone verify/explain how this circuit works?

Here is my understanding: TL431 has a ~2.5V reference. The resistor divider of R1/Rlower creates a divided down output voltage. The difference between the output and 2.5V reference is the "error signal". C1 acts as an integrator to TL431, so any error gets bigger quick (as C1 charges) and regulates the TL431 output. RLed limits the current through the opto, and the CTR ratio of the opt provides the current on the collector of Opto. The drop across Rbias provides a bias current to the opto, not sure why? When the opto turns on (with enough current flow in the diode), the output of the opto is: Vfb = (Vd0 - V_Rpullup). C2 is for high frequency switching noise.

Is this how this circuit works? In general, whats the purpose of this circuit? When the opto turns on, does it mean the "smps controller" stops switching till the opto-turns off?

it seems only two people in the world write good articles on this (as well as edaboard correspondents)

The two are Christophe Basso and Ray Ridley

happy googling and you will find out your answers.

C2 is a pole...its for stability as well as noise
I think C1 gives a zero
The TL431 is a pain as its cathode and supply node are the SAME pin.

...so you have to put its operating current into the cathode.

The TL431/opto combo gives an absolutely awful transient response......probably about 2KHz max. if youre lucky.

Is that OK for a Fender guitar amp power supply?...i dont know.

In some cases like tny264(PI) yes. It will on/off the switching pulses.
But in some cases(high power application) the control loop is stabilized and the Vfb will change until proper duty cycle is generated.