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UCC256403: output overvoltage because of frequency drop

Ralf Hoffmann
Prodigy 40 points
Part Number: UCC256403

I have designed an LLC resonance converter with a UCC256403 and now I have the following problem.

   ( Key data:
     Uin: 220V < 275V < 320V
     Uout : 24V / 720W
     fnenn: 100kHz )

Unfortunately, the converter only works at higher outputs; at lower output powers, the switching frequency drops depending on the supply voltage.
This also causes the output voltage to rise to unacceptable values.
In the two diagrams I have illustrated this with four different supply voltages.

If I am interpreting the data sheet correctly, the UCC256403 should cover the entire frequency range up to burst mode independently.


Does anyone have any idea which parameter could be responsible for my problem or what I should check?


Thanks in advance !

  • 0
    TI__Guru 70515 points

    Hello,

    It sounds like your transformer turns ratio and your Lr/Lm ratio may not be correct in your design.

    There is an excel tool that you can use to double check your design that you can find at the following link.

    https://www.ti.com/tool/download/SLUC675

    Regards,

  • 0 in reply to Mike O'
    Prodigy 40 points

    Hello,
    Thanks for the quick reply.
    I have calculated the data of the transformer myself and the supplier of the transformer independently.
    When entering the data into the above pecified tool, everything seems to be ok.

    However, I do not understand why the UCC256403 neither increases the switching frequency nor switch to burst mode
    even at low supply voltage when output power decreases.
    (According to information from the transformer manufacturer, burst mode should not even be necessary.)

    Which reasons would prevent an increase in switching frequency, especially as the primary current is visibly inductive?

  • 0 in reply to Ralf Hoffmann
    TI__Guru 70515 points

    Hello,

    Do you have a schematic that I could review?

    Regards,

  • 0 in reply to Mike O'
    Prodigy 40 points

    Hello,

    in the meantime, I have reduced the lower resistance RLL/SS from 56k to 39 k and can now regulate down to almost minimum power. (RLL/SS_Upper=100k)
    Only at the highest input voltage the output voltage increases slightly at minimum load, although the switching frequency can still be higher according to the transformer manufacturer.


    Confidence is rising :-)

    Do you perhaps know a description for the optimum design of the BMTL/BMTH ratio?
    I can't really make heads or tails of the data sheet ... :-(

  • 0 in reply to Ralf Hoffmann
    TI__Guru 70515 points

    Hello,

    There is an application note sluaal2 that is a trouble shooting common issues with this device.  There is a section on the information you have requested.

    This information should help improve your designs behavior.

    https://www.ti.com/lit/an/sluaal2/sluaal2.pdf

    Regards,

  • 0 in reply to Mike O'
    Prodigy 40 points

    Hello,
    I have downloaded many applications, but this one was not among them.


    Thank you very much !