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WEBENCH® Tools: Results mismatch between MOSFET Power Loss Tool and WEBENCH

Scott Brenneman
Genius 3265 points
Other Parts Discussed in Thread: CSD17575Q3, TPS40304, CSD16340Q3

Tool/software: WEBENCH® Design Tools

Hello,

I created a design using WEBENCH, a buck converter based on the TPS40304.  The tool selected specific NEXFETs (CSD16340Q3 and CSD17575Q3) for the high- and low-side switches, and the output report included power dissipation plots for those FETs.  I then went to the MOSFET Power Loss Tool, selected the same IC, the same inductor value and the same FETs and got dramatically different power loss plots compared to those from WEBENCH.  The Power Loss Tool gave numbers which were more than 2x higher, a non-trivial difference.  Which tool is more accurate?  

Thanks,

Scott

  • 0
    Genius 3265 points
    I should also note, for completeness, that in the both tools there were the same input and output voltages, output current and frequency.
  • 0 in reply to Scott Brenneman
    TI__Genius 14455 points
    Hi Scott, We are looking into this.

    Regards,
    Gerold
  • 0 in reply to Scott Brenneman
    TI__Prodigy 540 points

    Hi Scott,

         Can you please let me know the test conditions for your designs? Please give the following values;  

    VinMin = ?

    VinMax = ?

    Vout_Usr = ?

    Iout_Usr = ?

    Fsw = ?

    Ta = ?

      Also, please give the parameters you are using in the MOSFET Power Loss Tool (shown in below screenshot);

    Thanks,

    Hadi

  • 0 in reply to Hadi Ebrahimi
    Genius 3265 points

    VIN range is 3.0 to 14.0 V.

    VOUT is 2.3 V

    IOUT is 14.0 A

    Fsw = 600 kHz

    Ta = 30 C, or whatever the default is.  I can send you the WEBENCH report if you want.  In fact, I'll try to attach it here.webench_design_1090318_129_911506025.pdf

    I've attached a screenshot of the MOSFET tool setup.  Also, side comment: when I select CSD17575 as the low-side FET and click Submit, the tool selects the CSD16408 instead.

  • 0 in reply to Scott Brenneman
    TI__Prodigy 540 points

    Hi Scott,

       Thanks for the information. I investigated into this issue, and observed the mismatch in the calculated power losses for high-side and low-side FETs. I summarized the root cause of the mismatch as followings;

    1- Due to some infrastructure differences between WEBENCH and MOSFET Power Loss Tool, the used equations are somewhat different in these two. 

    2- WEBENCH model uses more complex equations to calculate effective Rds(on) based on FET's thermal resistance and junction temperature. However, MOSFET Power Loss Tool uses simplistic approach using a typical value for FETs Rds(on) which is independent of junction temperature and thermal resistance. I observed the effective Rds(on) for your given example is almost twice than that of being used in MOSFET Power Loss Tool. Since, your load current is 14A, a larger conduction loss mismatch to be expected. As you reduce the load current, the mismatch in calculated loss between two tools will also decreases.

    Since, WEBENCH model is more complex and take thermal effect into account, I would recommend you rely on WEBENCH tool for more accurate data. Of course, MOSFET Power Loss Tool becomes more favorable if simplicity and ease of use is the priority.

    Please let me know if you have any questions or feedback.

    Regards,

    Hadi

  • 0 in reply to Hadi Ebrahimi
    Genius 3265 points

    That's just the info I needed -- thank you!

    Scott