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LM5148: Webench Power Designer vs. LM5148_LM25148_quickstart_calculator

Thomas Sørensen
Prodigy 110 points
Part Number: LM5148
Other Parts Discussed in Thread: LM25148

Tool/software:

Hi,

I have tried to do a design in Webench and thesame design in the spreadsheet for the LM5148.

Vin = 30V

Vout = 12V

Iout = 3A.

I just seem to see that there are a big difference in the loss of the Control Mosfet. The spreadsheet reports 1.2W and the Webench reports 168mW.

LM5148_LM25148_quickstart_calculator_A4_12V_Main.zip

  • 0
    TI__Mastermind 40040 points

    Hello Thomas, 

    If you would like me to take a closer look, please share the WB design also.  Did you confirm that WB is modeling the exact same top and bottom MOSFET as you have used in the spreadsheet?  If not, the results will differ.  the losses the calculator is report is mainly AC losses.  as the I^2*R losses are minimal.  That means things like Qg, Qgd in HS FET and Qrr for the LS FET will affect losses being reported.

    Hope this helps.

    David.

  • 0 in reply to David Baba
    TI__Guru 73752 points

    The high-side FET loss is high as the Qrr is 134nC. Prr = Vin*Ffsw*Qrr.

    PS: check the entry for the low-side FET Qgs, seems incorrect.

    At a higher level, why use a controller for 3A? Use a converter (with integrated FETs) - much easier solution.

    Regards,

    Tim

  • 0 in reply to Timothy Hegarty
    Prodigy 110 points

    Hello Tim,

    Thank you for the answer. Yes there was a typo, it should have been 5.5nC, but did not make any difference.
    I think I have to read up on why Qrr on the bottom FET affect the top FET losses?

    But changing the value, I can see it has a huge significance on the top FET loss.

    Yes it is overkill to use this controller for this specify task, I only considered it because I already have it another place in the design.

  • 0 in reply to David Baba
    Prodigy 110 points

    Hi David,

    No the FETs are not the same. I could not find these in the list on the WEBENCH.
    But I changed the bottom FET in my spreadsheet to the same used in WEBENCH and that decreased the losses significantly. 
    I think I have to read up on some of these parameters. Usually there is a big focus on Rds_on and Qgs etc, but can see the revserse recovery parameters of the body diode of the bottom FET have an big a impact.

    Thank you.

  • +1 in reply to Thomas Sørensen
    TI__Mastermind 40040 points

    Hello Thomas,

    As discussed, AC losses are what are causing a huge impact and differences in the calculated results and WB model.

    Looks like you are moving in the right direction with losses starting to match.  Let us know what further assistance you need here.  

    Thanks,

    David.