Please tell about this circut

[vicky.pace]

Dear All,

Please see attached file and tell me what is the work of Q1 and Q2 in this circuit.
Many times I made smps designs but never seen this cofiguration in opto bias.
Please tell me what is need of this circuit in SMPS.

Thanks

 

[Eugen_E]

Q1, Q2 form a constant current source that gives VBE/R2≈10mA current to the opto LED.
Depending on the output voltage:
- when it is too low, the LED in opto will be off;
- when too high, the LED is biased at 10mA.
I think instead of using PWM to control voltage, here the circuit is activated/de-activated, depending if the output voltage is too low or too high.

The TL431 programable zener is used as reference at 2.5V(1+R4/R5)=13.86V.