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UCC28950: TL431 BASED ISOLATED FEEDBACK

Part Number: UCC28950

HELLO

Vout feedback is taken through TL431 as like for PMP8606 design. Now i am getting some waveforms that i am attaching.

My Vout is getting settled at required 24V. But transient behavior i am not understanding. especially behavior of tl431 output. I have configured the TL431 ref to be 2.5V when output is 24V.

what we see in transient is that LED CURRENT(all primary side ICE)  IS NOT AT ALL FLOWING till the output reaches 24V. so there is no control available till then? is it not digital isolation behavior? What exactly happens to the output of TL431 when REF pin of it reaches 2.5V. I have tried doing something on similar lines to PMP8606.tidrbj0.pdf

CTR is 1.

Is there any biasing problem.? what is the significance of R31?

please someone make me understand how the feedback is travelling along and then the compensation ?

thanks

samrat