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UCC28950: UCC28950 With Optocoupler Feedback

Ruben Iraheta
Prodigy 115 points
Part Number: UCC28950
Other Parts Discussed in Thread: TL431

Hello everyone,


I am currently trying to use the UCC28950 to regulate 360V output from 380V input and without synchronous rectification (just using diodes).  I am following along with the excel spreadsheet BUT the excel spreadsheet uses a simple voltage divider network to sample the output voltage.  I am going to use the IL211AT optocoupler in the standard feedback configuration and am having difficulty with the compensation network.  I am currently reading through the article provided by Mike O' (when someone asked a similar question (which I will attach)) but I don't understand how one comes about finding A, Wcz1, or Wcp1.  It just says that these values are known.  Any help would be much appreciated! 

Thank you very much in advance.

-Ruben

Optofeedback.pdf

  • 0
    TI__Mastermind 40030 points
    Hello Ruben,

    I have notified the appropriate team to look at your question. As it is currently quite late in the week, it may be a couple of days before you get a response on this. Thanks for your patience.

    Regards,
    Ulrich
  • 0 in reply to Ulrich Goerke
    Prodigy 115 points
    Thank you. I am understanding more and more as I read and re-read. If I come to some conclusion, would I be able to answer my own question (of course to be verified by a TI employee)? Perhaps it may help someone else in the future to hear the explanation from someone who was struggling with the subject matter.
  • 0
    TI__Expert 3200 points
    Hi Ruben,

    The pdf link is for the feedback design of an off-line flyback controller and I assume your "finding A, Wcz1, or Wcp1" question is related to equation #6 in the pdf. I am not verifying the pdf but it looks like they have a step by step procedure to follow.

    I think the article from Mike O'Loughlin you are referring too is www.ti.com/.../slua560c.pdf for the UCC28950. However, you want to use the UCC28950 with an opto isolator (and I do not have a readily available design for this configuration nor step by step instructions) I recommend you take the following steps although there are a couple of different ways to do this but one suggestion is:
    1) Follow the compensation design as outlined in the slua560 document (or in the UCC28950 datasheet section 8.2.2.10.1)
    2) The step 1 design assumes the error amp is contained in the UCC28950 controller, since you want to use an opto isolator, then use any method to design this same compensation frequency response using a TL431 on the secondary side.
    3) Design the opto-isolator and the error amp in the UCC28950 to have the gain characteristics you need but set their frequency response of both these blocks to be outside and not influence the frequency response of your TL431 design.

    At a high level, put your error amp and main compensation on the secondary side using a TL431. Then make sure the combined Bode plot of this error amp design, the opto-isolator transfer function, and the error amp around the UCC28950 combine to give the compensation recommendation in the datasheet or design calculator worksheet.
  • 0 in reply to Brian Johnson96
    Prodigy 115 points
    Thanks Brian,
    Yeah I think I figured out how to follow along step by step. What I think I'm going to do is follow along the pdf link and try to make the bode plots match the bode plots in the 28950 pdf and leave the internal op amp as a follower on the 28950. Do you think this would be a good idea? I think this may be what you were recommending in your post as well, but just to be sure, I ask for confirmation.

    Thanks,
    Ruben
  • 0 in reply to Ruben Iraheta
    TI__Expert 3200 points
    Hi Ruben,

    Yes, your thinking is correct on the strategy to design your compensation loop. I was trying to tell you the same but got a little wordy.
  • 0 in reply to Ruben Iraheta
    Prodigy 105 points
    Ruben, do you have any results with isolated feedback?