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LM51231-Q1: Possible Error In SNVA991, Table 2-2

Jerry Frank
Prodigy 10 points
Part Number: LM51231-Q1

Tool/software:

Hello,

I am trying to calculate losses for the synchronous MOSFET. In table 2-2, the body diode conduction loss uses a term of (Vload*Fsw)/2. This results in a very high calculated loss. I think the voltage should be the Vsd of the body diode at the calculated current. Using this number I get much closer to the value in my own quick calculation and the values supplied by WEBENCH and the https://www.ti.com/tool/SYNC-BOOST-FET-LOSS-CALC.

Could you please let me know which version is correct?

Thank you!

  • +1
    TI__Mastermind 19595 points

    Hello Jerry,

    Thanks for reaching out to us via e2e.

    Table 2-2 details out all the individual contributions to the overall losses.
    It assumes the following:
    - While the high-side FET is turned on, it is causing conduction losses during that time.
    - During that same time period there is NO current flowing through the high-side body-diode.

    I think you are referring to this equation:

    - There will be two SHORT periods of time before and after the channel of the high-side FET is turned on:
    td1, where the switch node voltage has already risen, but the channel of the high-side FET has not yet turned on
    td2, where the high-side FET has already turned off, but the switch node is still sitting at a high voltage.
    During these times the current will flow through the high-side body-diode.
    - These times, called td1 and td2 are very short (single digit or low double-digit nanoseconds).
    They are system related, so you will not find such a parameter specified in any datasheet (nor in that document itself).

    Please compare to the equation for the switching losses in the low-side FET:
    This looks the same, but in that case, the short times are the rise time and fall time.
    td1 and td2 are different from trise and tfall.

    Regarding the voltage in the equation for the body diode conduction losses, I would agree with you that the forward voltage drop of the body diode should be used instead of the output voltage.


    All information in this correspondence and in any related correspondence is provided “AS IS” and “with all faults” and is subject to TI’s Important Notice (www.ti.com/.../important-notice.shtml).

    Best regards
    Harry