Feedback circuit with tl431 and optocoupler

Question

I was examining the feedback circuit on a power board belonging to a machine and I roughly drew the circuit on the feedback side as follows.

This circuit board belongs to a very old device and therefore there is no datasheet.

I checked the circuit on the board many times and such a diagram emerged.

I have 2 questions according to the diagram below:

• You can see the compensation elements between the feedbcak and the cathode end of the tl431. An output was taken from where the capacitor, which is the compensation element, meets the cathode of the tl431 and is transmitted to a transitor and this npn transistor is controlled by the DSP (Digital signal processor). Does anyone know why this transistor has such a connection here or what it does?
• A feed was taken from the transformer to the input side (anode) of the optocoupler and the LM323 (voltage regulator) was directed to the cathode with a diode. I don't fully understand why the anode of the optocoupler is given 2 different inputs, which one will it work with? If there is such an application, can you explain how it works?

Answer 1

The intention of the circuit is to disable the SMPS with the DSP OUTPUT signal.

If the transistor is conducting, the LED in the coupler is bright on, which signals the SMPS regulator a "sufficiently high output voltage / no more energy needed".

But if the SMPS then stops, there will no longer be a secondary side voltage and therefore the stop signal can not stay active. This would produce on/off oscillations.

To feed the coupler LED while the SMPS is not switching, a second supply path for the LED via LM323 was added. The diode in this path avoids reverse current flow to the LM323 while the SMPS is operating.