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Original question:

TPS23756: TPS23756 Transformer Polarity/Phase Required?

TPS23756: Audible Noise issue PMP40514 Based PCB

eljeffo
Intellectual 560 points
Part Number: TPS23756
Other Parts Discussed in Thread: PMP40514

Hi TI Folks!

So I made a PCB modeled after the PMP40514 schematic and it functionally works.  However I have a high pitched audible whine noise emitting from L3.

Output voltage and current both seem to be fine.

I noticed this same effect with some cheap chinese POE modules, but didn't notice it with the Evaluation Kit.  On some cheap chinese modules I notice that I only get noise, but no ouput.

Is there a common reason for a high pitched whine to emit from the output inductor?  Or perhaps more specifically, is there anything I can do to eliminate the whine?

Thanks so much for your thoughts!

  • 0
    Intellectual 560 points

    Oh, I might have been so wrong... seems to be a ceramic Cap C24 on the flyback between the two mosfets is the most audible of all.  I tried two different capacitors, but I'm wondering if it was just poor capacitor choice, or if I have a misconfiguration in my circuit that's making the problem much worse than it needs to be.

  • 0 in reply to eljeffo
    Intellectual 560 points

    ah okay, I think just need to change the caps to ones that don't make as much noise.

    If there's anything I can do on the feedback/opto circuit that's worth looking into please let me know :)  I'm happy to learn more!

  • 0 in reply to eljeffo
    TI__Mastermind 21320 points

    Hello,

    Sorry to jump in after a few posts, would you say you resolved your issue? Is there something else we can assist you with?


    For the feedback circuit, we check the load transient (like from 0.1A-1A) and the gain/phase margin. To check the margins, use a bode machine. The following two papers provide some control theory and calculations:

    https://www.onsemi.com/pub/Collateral/TND352-D.PDF 

    If this post answers your question, please indicate so by marking this thread as resolved. Thank you.

     

    Regards, 

     

    Michael P.

    Applications Engineer

    Texas Instruments 

  • 0 in reply to Michael Pahl
    Intellectual 560 points

    Michael,

    Thanks for the PDF, I'll read into it.

    I have some new anti-noise capacitors ordered, so hopefully that will fix my issue.

    However, as always, there may be more than one problem.  Should be able to update you in a few days when my shipment arrives and we swap out some parts. 

  • 0 in reply to eljeffo
    Intellectual 560 points

    Michael,

    FYI I had the wrong value output inductor (way too low) which was the root cause of all the initial voltage whipping and capacitor noise issues.  Now I think it's just normal tuning (which I have yet to learn!)

    Thanks a bunch!

    Jeff