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WEBENCH® Tools/UCC28740: Output current much lower than expected when testing with electronic load

Bryan Grant1
Intellectual 370 points

Part Number: UCC28740

Tool/software: WEBENCH® Design Tools

Hi TI,

I am designing a custom circuit for the UCC28740 that takes universal AC input and outputs 35.7VDC@0.6A. However, when I connect an electronic load to the output, the voltage is constant under very light loads <0.06A, but drops afterwards with a maximum power output of only ~2W. What would be some reasons why the circuit would not be providing the designed level of current? The main thing that is different is that in order to fit the required turn ratios onto the bobbin I proportionally lowered the number of turns of each coil, which would have changed their inductance values.

Thanks for any help,

Bryan Grant

  • 0
    TI__Mastermind 40580 points

    Hello Bryan,

    Thank you for your interest in the UCC28740 flyback controller.

    One possible reason for premature power limit would be that the current sense resistor Rcs is improperly designed.

    However, based on your change of turns on the transformer design, I think the most likely reason for a maximum power of ~2W is that the transformer is probably saturating.  By reducing the number of primary turns you are allowing the maximum flux density to increase inversely proportionally for the same peak primary current (Ippk).  Moreover, the reduction in inductance requires an increase in Ippk to deliver the same energy per cycle required for a given load at a given frequency.  I believe that with higher Ippk and lower Np, your peak flux density Bpk is exceeding the Bmax rating of the core material even at room temperature.

    I recommend that you consult with a magnetics design house to obtain the proper design and construction of the transformer to meet your system requirements.

    Regards,
    Ulrich

  • 0 in reply to Ulrich Goerke
    Intellectual 370 points

    Hi Ulrich,

    Thanks for the response. Is there a magnetics design house that you would recommend?

    Would it also be possible to lower Rcs to increase the Ippk as long as the reduced primary inductance is adequate to supply that amount?

    Thanks a lot for the help,

    Bryan Grant

  • 0 in reply to Bryan Grant1
    TI__Mastermind 40580 points

    Hello Bryan,

    Reducing Rcs to increase Ippk will not work.  When a transformer saturates, you are already getting more Ippk than the transformer can handle.  The transformer needs the proper number of turns (based on core area) to prevent saturation and overheating.  The overall system needs the proper primary inductance and Ippk to meet the performance targets for the converter.  These parameters are calculated by the UCC28740 Design Calculator Tool (www.ti.com/.../sluc487 ) or following the datasheet procedure.  Significant deviation from the tool's recommendations usually results in the inability to achieve one or more of your design targets. 

    Within TI, we often use Renco Electronics (Florida) and Wurth Electronics Midcom (South Dakota) to design various magnetics that we use, although they are not the only suppliers.  Just the first few that come to my mind.  You can search for their websites and/or search for other suppliers.  I found that the search phrase "custom transformer suppliers" seems to produce better search results and follow-on related search options than using other phrases. 

    Regards,
    Ulrich