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UCG28826: Turn ratio configuration (TR pin) and average current limit

G Godde
Prodigy 10 points
Part Number: UCG28826

Tool/software:

Hello,

I have a question regarding the turn ratio (TR pin) configuration. What is the use of this pin if we are already using one opto to feedback voltage? Lets say the transformer has turn ratio of 4 or a turn ratio of 10... how to setup this pin?

Maybe related question: There is a peak current limit (IPK pin) but no average current limit, right? If so, the output current limit need to be implemented by turning on the opto with a custom circuit placed on the secondary side, right?

Note: In our design, we don't need the full power of the chip (20W is enough), but has interest in the 500KHz capability.

Thank you,

  • 0
    TI__Prodigy 430 points

    Hi Godde,
    The function of TR pin is to set transformer turns ratio based upon the reflected voltage across the primary side of the transformer and will be used to define output OVP of the converter. The turns ratio (Nps) of the transformer can be low, equal or higher than turns ratio given in section "7.3.6 TR - Turns Ratio" (https://www.ti.com/lit/ds/symlink/ucg28826.pdf?ts=1748487113850&ref_url=https%253A%252F%252Fwww.ti.com%252Fproduct%252FUCG28826), in case where turns ratio of converter is low or high than TR given in datasheet, the TR pin will be set as per required output OVP.
    And answer to your second question, the output current limit is set based upon the device in-built feature of two-level overpower protection with LPS. 

    I hope this help to clarify your doubt and if you have any query further, please reach back to us. 

    Thanks 

    Regards

    Anita

  • 0 in reply to Anita Yadav
    Prodigy 10 points

    Thank you for the details.

    I understand now the TR pin is only for use with the 25V output OVP. Is this "TR" naming a standard at TI? If not, maybe it could simply be renamed OVP pin to be more clear.

    So if the transformer turn do not match the TR pin, the output OVP may be lower or higher without any other consequences. Since our design is for 12V output, that should be ok even with a turn ratio of 4 as long as TR pin is set to minimum (6).

    In our cases we need to implement a separate current sense/limit because the LPS is way too high and will burn the transformer. It would have been more convenient to have a power setting resistor with a wider range.

    Some feedbacks, if any use.

    Best Regards,