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WEBENCH® Tools/LM5176: Recalculate efficiency

Michael Balourdas
Expert 4745 points
Part Number: LM5176

Tool/software: WEBENCH® Design Tools

Hello,

I am trying to use LM5176 as a variable output buck. In Webench, I want to change the FB resistors and leave everything else the same and see the different efficiency estimates. But when I edit a resistor and click "Recalculate", the efficiency and other operating point numbers stay the same. Is there something else I need to do to see the new efficiency number?

Thanks,

Michael

  • 0
    TI__Intellectual 1710 points

    Hi Michael,

    Please note that in Webench, changing 1 Bom component manually will not modify other BOM elements. please be careful about using this approach. Having said, I was able to observe the changes in all relevant operating values when you change RFBT. I created a default design condition for LM5176. changed RFBT from 280k to 100k. efficiency increased about 0.1% and Vout actual went from 12V to 4.8V. If you want us to test a specific condition, please share your design. 

    Regards,

    Vishwanath

  • 0 in reply to Vishwanath Tigadi
    TI__Expert 4745 points

    Hi Vishwanath,

    Here is my design: https://webench.ti.com/appinfo/webench/scripts/SDP.cgi?ID=C380B4668CAE1799

    I tried your exact scenario: changing RFBT from 280k to 100k and I don't see a change in efficiency. I even tried changing it to 10k and still no change. Any idea why that might be happening?

    Thanks,

    Michael

  • 0 in reply to Michael Balourdas
    TI__Intellectual 1710 points

    Hi Michael,

    Firstly I verified the changes I had mentioned once again. they are working as expected.

    In the design you've shared, I changed Rfbt to 1 Ohm and efficiency still doesn't change. this is mainly due to low power loss in feedback path compared to output power. your design is at 20V 3.25A that is 65W. The power dissipation in feedback is in terms of uW. hence even if the efficiency is changing, it is changing in third decimal place or lower and hence it is not reflecting in Webench. 

    The best way to verify your change is to look at the operating value called Vout Actual. This is changing as expected.

    Regards,

    Vishwanath