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UCC28700: Reference design and its calculation

ArTzy
Genius 17695 points
Part Number: UCC28700

Hi Experts,

Seeking your assistance on this query:

I am looking at this schematic right now:
https://www.ti.com/lit/df/tidrbq6a/tidrbq6a.pdf

In the UCC28700 datasheet I try to get the Rs1 and Rs2 values.

In the datasheet I can see the formula, but I have some problems with the values of Rs1. Based on the reference design I sent you I get a very different value to Rs1.

The NPA in my calculations is 40/11, the Vin(run) is 230 I guess? (Maybe I am wrong here). In the reference design Rs1 is 75k, and I cannot figure out why. Is there something I'm missing?

For your assistance please. Thank you.

Regards,
Archie A.

  • 0
    TI__Guru* 86286 points

    Hello,

    The input voltage to this design is 85V to 265V RMS.  Vin(run) should be less than 230 V that may be why you are not getting the exact values.

    Also did you look up the transformer turns ratio that was used in the design?  The NPA turns ratio may be different than what your are calculating.

    Once you have the transformer turns ratios.  Based on RS1 and RS2 you should be able to calculate the Vin(run) voltage, VDD regulated voltage and output votlage.

    Regards,