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LM2734: Schottky failure around LM2734XMK

jb nouvel
Prodigy 120 points
Part Number: LM2734
Other Parts Discussed in Thread: LMR50410

Hello,

we did this design around a LM2734XMK*. 15V or 6V4 input to 5V output.

Thousands were produced.
There is an unusual failure rate. Rare cases. But higher rate than expected.

The external schottky SS12 diode blows up without apparent reason.
Note that it seems to be a 'slow death' as the PCB area around is cooked when this happens.

PCB layout is very compact, designed 'by the book', as per datasheet, also using webench.

Temperature environment around 35 to 55degc.

Light load on the converter.

High speed scope probes were used to check for ringing, voltage spikes... all is very clean.

New diode was measured using a component tester. Reverse voltage confirmed to be much highr than the spec.

Applied strong over heat to components : no effect.

One particularity is that the input voltage +15V_PSA1 dynamically changes from
6.5V to 15V and back.
Can this be an issue?

Overall a very simple schematics, causing us a lot of troubles.

Hope you can shed some light.
Thanks,
Jean-Baptiste

  • 0
    TI__Mastermind 26710 points

    Hi Jean-Baptiste,

    Thanks for the detailed information. Is it possible to share schematic, BOM, layout for this design? It would be essential so I can lookup part numbers and their datasheets and also look at the net names and reference designators you are referring to.

    Thanks,

    Amod

  • 0 in reply to Amod Vaze
    Prodigy 120 points

    file removed

  • 0 in reply to jb nouvel
    Prodigy 120 points

    Hello,

    I just attached the schematics to the post.

    Diodes PN are on the sch.

    Thick film résistors.

    Ceramic caps.

    All smt.

    Inductor is Coilcraft XAL4030-472ME

    I haven't said but the board has conformal coating.

    Many thanks,

    Jean-Baptiste

  • 0 in reply to jb nouvel
    TI__Mastermind 26710 points

    Hi Jean-Baptiste,

    The design looks ok to me. Some questions and comments:

    Does replacing with new diode make the board work again? How long before the diode was blown? Any other boards that have same failure? what is the current drawn at output?

    It will be good to check

    a. peak voltage on SW node - steady state and during 6.5 to 15V transitions - is there a chance of VIN surge in some situation?
    b. peak current through diode - average current is 1A - larger currents over time may reduce life.
    c. dV/dt on SW node can exceed the diode spec.

    Diode manufacturer can help identify any selection issues pertaining to above points and more.

    Thanks,

    Amod

  • 0 in reply to Amod Vaze
    Prodigy 120 points

    file removed

  • 0 in reply to jb nouvel
    Prodigy 120 points

    Hello Amod,

    see attached the document that provides answers/screenshots to your answers.

    Many thanks,

    Jean-Baptiste

  • 0 in reply to jb nouvel
    TI__Mastermind 26710 points

    Hi Jean-Baptiste,

    The steady state waveforms look quite stable and clean. I do not see any obvious issues - also this looks like an issue that happened as a one time event or over time (like you had pointed out). So, short term testing may not reveal the issue easily. Perhaps using a diode with more margin on voltage/current/temp/dv/dt is an option. One final thought is regarding the 6.5V to 15V transition - is that transition very fast and is that slew rate enough to cause any dv/dt or SW node or current violations? This is something you can check in addition.

    Thanks!

    Amod

  • 0 in reply to Amod Vaze
    Prodigy 120 points
    Hello Amod,
    sorry for the slow response: holiday season time off.
    We did check transitions already - they are pretty slow.
    Triggering on the scope thousand times, and checking the waveforms. Nothing.
    We now have gone for much higher spec diode.
    It looks ok but it will not prove anything until mass production.
    We also tried much lower spec diodes, to try to make it fail. No success there.
    We are now considering a drastic change, that is to go to a synchonous rectifier
    and fortunately TI has a pin2pin compatible device LMR50410YQDBRQ1 !
    If it is a quality issue on the diode we will now be safe.
    Any comment on this?
    Many thanks,
    Jean-Baptiste
  • 0 in reply to jb nouvel
    TI__Mastermind 26710 points

    Hi Jean-Baptiste,

    No problem - I was out on vacation too. Wish you a happy new year!  Yes, the LMR50410 does seem p2p compatible here and is also a newly released device. Also, going the synchronous rectifier route might be better efficiency wise. Since this is a wider VIN device (36V) and falls under a different product expert, I am requesting them to review and provide any further comments.

    Thank you,

    Amod

  • 0 in reply to Amod Vaze
    Prodigy 120 points

    Good - thanks.

    Happy new year to you too!

    Jean-Baptiste

  • 0 in reply to jb nouvel
    TI__Expert 8130 points

    Hello Jean-Baptiste,

    Happy New Year!
    I looked over this thread and as you mentioned, fortunately, we have a pin-to-pin compatibility.

    The device covers all of the specs required (as informed) with max output current of 1-A, so there shouldn't be any problems. Plus, going the synchronous rectifier route should provide better efficiency. 
    Please make sure to select the appropriate components as recommended in the datasheet.

    Thanks,
    Alejandro