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LM5122: LM5122 Low Side FET Source-Drain Short.

jungho LEE
Prodigy 10 points
Part Number: LM5122
Other Parts Discussed in Thread: CSD17304Q3, CSD17577Q3A, CSD18543Q3A

Tool/software:

Hi.
I have some questions about LM5122.

Our Operating condition => 15~21V Battery, Output 24V, Current : 3.5A(margin)
Web-bench High Side FET : CSD17577Q3A / Low Side FET : CSD17304Q3

But you used alternative component due to margin.
High Side/Low Side FET : CSD18543Q3A


1st Question : Why did you use a different component in wab-bench??
->

2nd Question : If usd same componet, What problems might come up?
->

3rd Question : A failure has occurred.(Low side FET Source-Drain short). In what cases could such a defect happen?
                       Also, there is a 5A bead on the input side, and it was burnt. What could be the reason for the bead failure?"

->

please help me.

  • 0
    TI__Mastermind 29900 points

    Hi Jungho,

    Thanks for using the e2e forum.

    When I compare the MOSFETs suggested by webench and the alternative part, I see the Vds voltage rating changes from 30V to 60V.
    In this boost application, the highest voltage at the FETs should be VOUT, which is regulated to 24V.
    This would leave a margin of 25% with the 30V FETs from webench.

    Some degree of margin is highly recommended between VOUT and MOSFET rating, as there can be overshoots on the switch node which go above the 24V.
    The amplitude of the overshoots depends on the schematic and layout, which means it is possible that overshoots above 30V can occur.
    To reduce overshoots, we recommend to increase gate resistance or add RC snubbers to the switch node. Please measure the switch node waveform to confirm there are overshoots.

    The choice of MOSFET selection is how much overshoot is seen in the system and how much margin is desired to make sure they will not break during operation.
    If customer says 25% margin is sufficient and the overshoots are reduces with larger gate resistance, the is no problem with the FETs suggested by webench.
    If customer wants more margin, the alternative part with 60V rating would be better suited.
    There would even be the option of choosing a 40V rated FET, which would be a compromise.

    Best regards,
    Niklas

  • 0 in reply to Niklas Schwarz
    Prodigy 10 points

    Hi Niklas~~
    My 2nd Question is clear by your answer.

    But I wonder why you recommended something different. All recommended components are also rated for 30V.(1st Queastion)

    And due to the replacement component, could a shoot-through occur during high-side/low-side FET switching operation?

  • 0 in reply to jungho LEE
    TI__Mastermind 29900 points

    Hi Jungho,

    Thanks for the quick feedback.
    I think a 30V rated component is okay to recommend here. A higher rated component only makes sense if you see strong overshoots on the switch node which go close to the abs max ratings.

    You mentioned the 30V rated part is recommend by webench. Where was the 60V rated part recommendation coming from?

    A shoot-through is unlikely, independent of the selected MOSFET parts. The LM5122 regulator checks when the driver signal go low and adds dead-time between each switching operation to avoid shoot-throughs.

    Best regards,
    Niklas