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LM5116 Failure at medium designed load.

Ian Boddy
Intellectual 280 points

Other Parts Discussed in Thread: LM5116

Hi All,

I have built the circuit in the diagram attached and have got some good results up to around 3A.

The designed parameters are:

  • 40VDC nominal supply voltage (~36V UVLO)
  • 24VDC output
  • ~10A rated output maximum

The circuit works fine as already stated up to around 3A but beyond this I have noticed on turn-on I get a failure which results in an incorrect voltage output. This has happenned on two occasions now and I notcie the resistance across the output (with no load attached) ends up in the 4-5Ohm region?? The 4-5Ohm short seems to be (through basic removal of components) originating at the LM5116 itself??

To remedy this problem each time I have had to replace the LM5116 (U1) and the low side FET (Q2). Though I believe on investigation Q2 could have stayed as is.

I have assumed up to this point that the switching speed of Q1 and Q2 (in the order of 10ns max tr and tf) is so quick that the di/dt is causing ringing above the rated voltage of the two FETs (100V). I also assumed this was ultimatly leading to the LM5116 failing due to the 100V limit of the SW pin.

I have sourced some slower switching FETs (Internation Rectifier IRFR4615PbF, tr of 33ns and tf of 20ns) and am planning on swapping them in (with another replacement LM5116) and testing again.

The failure is fairly benign in that there are no pops or explosions to be observed. Just no output.

Am I following the correct assumption here or is there something I have overlooked during the design. I can provide screenshots of the layout for reference but the majority follows the LM5116-EVAL PCB in terms of layout and grounding and the snubber traces are as short as possible around Q2.

ANY help would be greatly appreciated as always :)

  • 0
    Intellectual 280 points

    Hi All,

    Just some extra information which isnt in the previous post:

    • Switching frequency of around 230KHz
    • Original design did use VCCX but this has been disabled
    • soft start time around 1 second
    • hiccup timer 50ms

    I have fitted some of the other FETs (IRFR4615PbF) to the PCB and left the rest as is. I can post some waveforms etc... of on open and light loads for reference if required?

    again...ANY help is much appreciated.

  • 0 in reply to Ian Boddy
    TI__Guru 68581 points

    Hi

    If you share the layout and the waveforms when the output was not well regulated, it will be helpfull a lot.

    Regards,

  • 0 in reply to EricLee
    Intellectual 280 points

    Hi Eric,

    I am currently soak testing the PCB now and will share my findings once the testing is complete,

    Just an FYI I fitted the different FETs (International Rectifier IRFR4615) and the PCB now works well!!

    On open load the waveform is stable and the device is clearly taking advantage of diode emulation mode as it can be seen on the SW node waveform. I have taken some samples of the SW Node waveform at intervals up to and including 6.35A so far. The output is well regulated and the PCB is doing a good job of drawing the heat away from the major components and into the air.

    My main concern at the moment is the amplitude of the switching spikes I can see, so far the negative-going spike peaks at around -25V and the positive spike is around 60V (both for a duration of around 5-10ns), the snubber across Q2 (which is now 2.2nf and 7R5) works well and gets rid of the positive spike within 2 periods of the ringing and the negative spike damps away to around -2.5V straight away (Note this is at 6.35A).

    I know the datasheet mentions the electrical limits of the SW pin are -3V to 100V but in context is that for DC rather than these VERY brief periods??

    I have projected (very basic linear assumption) that the positive spike will be under 100V at my 10A design limit, although I may be de-rating the PCB to something more manageable like 8A.

    Any comments feel free to ask, I will try to get some waveforms posted once my testing is finished.

  • 0 in reply to Ian Boddy
    Intellectual 280 points

    Please see the following SW Node waveform for when the device is running at 6.35A (40Vin, 24Vout).

    Yellow = Vin, Green = Vout, Blue = SW Node.

    And some zoomed in views of the negative and positive spikes:

  • 0 in reply to Ian Boddy
    TI__Guru 68581 points

    Hi

     The SW abs max of -3V is DC

    Regards,

  • 0 in reply to EricLee
    Intellectual 280 points

    Hi Eric,

    Thanks for the reply, I am already aware of the -3V DC rating of the SW pin but is there any information regarding transient ratings?

    I have seen a Ti application note (SLVA494A) which explains the ratings for a similar device family but not for the LM5116.

    Best Regards,

  • 0 in reply to Ian Boddy
    TI__Guru**** 191445 points

    I can't comment specifically for LM5116, but I can tell you this is a common concern.  All buck converters will exhibit this negative undershoot to some degree.  Proper component placement and  PCB routing can help to minimize the effect.