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  • TI Thinks Resolved

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

  • Hi Samrat,

    The TL431 is a shunt regulator, when the voltage on its ref pin is below 2.5V the TL431 is essentailly open circuit i.e. it draws no current. Once the voltage on the ref pin exceeds 2.5V the TL431 device sinks current and will pull the current through the optocoupler diode.

    So if the output voltage is below 24V, the TL431 is not active, once the output voltage hits 24V the TL431 will start to operate and sink current.

    Regards

    Peter

  • In reply to Peter Meaney:

    Hello Peter

    If we use TL431 and Optocoupler as compensating or error amplifier, the control is not available till the output reaches the desired 24V as feedback itself is not available (TL431 is not active) to the internal error amplifier of ucc28950?

    That means we cannot shape the transient till the output reaches the desired value of 24V?

    Can you please elaborate how to use this isolated feedback in conjuction with the internal error amplifier of ucc28950?

    thanks
    samrat
  • In reply to SAMRAT PUJARI:

    Hi Samrat,

    The output voltage in the simulation waveform is montonic and is well controlled so I do not understand what transient you are trying to control before the output comes into regulation at 24V?

    The UCC28950 is intended to be used on the secondary side of the but configuration you have appears to be working well in simulation. It should be possible to put all the control loop compensation around the TL431 and make the EA on the UCC28950 a simple voltage follower with unity gain.

    Regards

    Peter

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