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LM5175: Schottky diode and RC snubber on low-side mosfet

d_zero
Genius 4795 points
Part Number: LM5175

Dear *,

in this EVM https://www.ti.com/lit/ug/snvu440a/snvu440a.pdf?ts=1588749837292 the low-side mosfet has D2 schottky  diode  and RC snubber.

Can you tell how is the diode and snubber selected and calculated ?

Do you have an App-note regarding this diode and how does this improves efficiency?

Best Regards.

  • 0
    TI__Mastermind 21720 points

    Hello d_zero,

    Thanks for reaching out iwth your question and for considering the LM5175 in your design.

    Adding the schottky diode helps to reduce the voltage drop across the MOSFET during the dead-time between the two switches. If the schottky diode is not populated the body diode of the MOSFET will conduct. Typically the forward voltage drop of a body diode can be 1V or higher. The schottky diode will have a lower voltage drop increasing the efficiency. The dead time of the LM5175 is 55ns

    The snubber can be calculated in a number of different ways. One of the easiest ones is presented in the this blog. I suggest using this technique.

    Thanks,

    Garrett

  • 0 in reply to Garrett Roecker
    Genius 4795 points

    Hi Garrett,

    thank for the replay.

    1)

    Regarding the shottky diode, so lower the voltage drop is better, what about the current rating of the diode?

    If the  DCDC is rated to 2A current output how to choose diode regarding this current?

    Also is the any concerns regarding the capacitance of the shottky diode?

    2) I will try the snubber calculation.

    Best Regards.

  • 0 in reply to d_zero
    TI__Mastermind 21720 points

    D_zero,

    The average current rating for the diode can be relatively small as it will only be conducting for about ~55ns (dead time) each cycle. To find the true voltage drop look at the curve for the instantaneous forward voltage vs current. The current ill just be the peak inductor current.

    Please let me know if you have any questions.

    Thanks,

    Garrett