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LM5146: LM5146 MOSFET selection and power loss

Neo.Wang
Intellectual 830 points
Part Number: LM5146

Tool/software:

Hi TI experts,

Can LM5146 work with logic level MOSFETs whose Vg(th) is as low as minimal 1.1V but provides very good Qoss and Qrr? Eg. Infineon MOSFET BSC070N10LS5. Any design consideration for this FET with LM5146?

In LM5146 Quickstart tool, why the Bottom FET Coss and Qoss loss are counted into Top FET?

Thanks

Neo

  • 0
    TI__Mastermind 49471 points

    Hello Neo 

    It is okay to use BSC070N10LS5 together with LM5146. If the gate threshold is too small, the MOSFET can be accidently turned on by a noise coupling.  

    The COSS, QOSS are charged/discharged during the high-side switch on/off transition. That's why. The low-side MOSFET does a kind of ZVT switching assuming the forward voltage drop of the low-side body diode  is negligible.  

    -EL  

  • 0 in reply to EricLee
    Intellectual 830 points

    Hi EL,

    Is there any good practice to eliminate the risk of accident turning on of Logic level MOSFET?

    In our schematic there is a 100kohm resistor between MOSFET gate and source, is this good to hold GATE low and not turning on accidently?

    Thanks

    Neo

  • 0 in reply to Neo.Wang
    TI__Mastermind 49471 points

    Hello Neo 

    Populating 100kOhm pull-down gate resistor is a good idea which helps preventing an accident turn-on when the device is not powered. 

    When the device is powered and the regulator is switching, populating PNP transistor helps preventing an unexpected turn-on by SW node dV/dT if a gate resistor is used.

    If the LO trace is directly connected to the MOSFET, connection from LO to the MOSFET must be as short as possible to reduce series parasitic inductance

    I will be out of office and I will not be able to reply this E2E post anymore.
    If you are still trying to resolve the issue, please don't reply on this post, but create a NEW post to get a help from other engineers quickly.
    -EL

    -EL 

  • 0 in reply to EricLee
    Intellectual 830 points

    Got it. Thanks.