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LM5175: Buck-Boost Region Instability

Jeff Tooley84
Prodigy 50 points
Other Parts Discussed in Thread: LM5175, CSD18531Q5A

Hi,

I have a confounding problem with the LM5175 that I am hoping someone in the community can help me with.

I started with the LM5175 demo board. For my application (Vin = 10V-28V, Vout = 24V) I used WebBench to identify the recommended components. Assembled these components onto the demo board. Everything looked stable across the input voltage range, up to 4A.

I then designed a board to incorporate this design (Rev. A). Everything looks just like it did on the demo board.

I had to spin the board (Rev. B), and as part of the redesign the power supply section had to be relocated, but is largely the same layout. Also, the redesign included a change in FET's to keep them cooler under max. load (from NXP PSMN011-60ML to CSD18531Q5A). 

Rev. B is now assembled, and at 4A, with the LM5175 in buck-boost region (most specifically around 21V-22V) the 24V output has a ~700mV, ~50kHz oscillation (switching frequency is set to 394kHz) riding on top of 24V. Further, when sweeping from 22V to 19V the 24V output will occasionally apparently shut down, falling to 12V, before the power supply recovers. 

Anyone have any thoughts on what could be causing this Rev. B behavior? I have tried green wires, part replacement, and additional caps everywhere I can see/think where the problem may lie. I am about to the point of hand assembling the 24V power supply from scratch on a bare PCB to see if the problem can be recreated (I have observed this issue on (3) machine assembled PCB's). 

  • 0
    TI__Genius 11490 points

    Hi Jeff,

    Thank you for using LM5175. I suggest you take a look at this article to understand some of the common troubleshooting tips related to this part:

    Controlling switch-node ringing in DC/DC converters

    Now, the FET change you did is an important one. I think LM5175 performance is best with FETs that have low QRR and mild reverse recovery characteristics. Excessive switch node ringing can result in certain detrimental effects as explained in the article. We often suggest using a zener clamp across the boot capacitors to protect against overvoltage due to ringing, esp with higher QRR FETs.

    One suggestion is to switch to a different FET in the same 5mm x 6mm package that your board currently has with lower QRR. One good example is BSC039N06 but , of course, there are many others from other vendors.

    Let me know if these tips don't solve your issue.

    Regards,

  • 0 in reply to Vijay86929
    Prodigy 50 points
    Hi Vijay,

    Thank you for the reply.

    I have run the FET change to ground (reviewed the FET datasheets, and reverted back to our Rev. A FET's). This did not affect problem.

    Also, I have tried the suggestions from the article you provided. None of them resolved our problem.

    From further testing I have been able to deduce that an extra 50uF of ceramic Cout eliminates the ~700mV, ~50kHz oscillations. However, the LM5175 will occasionally stop switching (and Vout collapse) for ~10ms while sweeping Vin from 19V to 22V.

    What do I need to look at on the board in order to drive to the root cause of the problem of why the LM5175 stops switching within this narrow Vin range?

    Best Regards,
    Jeff
  • 0 in reply to Jeff Tooley84
    Prodigy 50 points

    Hi Vijay,

    This LM5175 problem is an urgent issue for us.

    Would I be able to converse with either you or another TI engineer about this issue?

    Best Regards,

    Jeff

  • 0 in reply to Jeff Tooley84
    TI__Genius 11490 points

    Hi Jeff,

    I will have to see the schematic to understand you circuit. Also does the re-start issue happen with light load or only with full load.

    Regards

  • 0 in reply to Vijay86929
    Prodigy 50 points
    Hi Vijay,

    The re-start issue happens only at heavy (not full) load. We are using Rsense = 7mohm. Issue appears to only occur with load > 3A.

    Do you have an e-mail address and phone # so that we can discuss further getting an NDA in place to review the schematic?

    Regards,
    Jeff
  • 0 in reply to Jeff Tooley84
    TI__Genius 11490 points
    Hi Jeff,

    I think you will probably need to contact your local TI person for NDA etc.

    Regards
  • 0 in reply to Vijay86929
    Prodigy 50 points

    Hi Vijay,

    Do you have contact information for my local TI person? I am in central CT.

    Regards,

    Jeff

  • 0 in reply to Jeff Tooley84
    TI__Genius 11490 points
    Hi Jeff,

    I have contacted some people to see the procedure for dealing with confidentiality issue on e2e.