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Sub-harmonic Oscillation issue with LM5116MHX

Dennis Kong
Prodigy 145 points
Other Parts Discussed in Thread: LM5116

Hi,

I designed one of our power circuits with the help of WEBENCH Designer.

Vin: 24VDC

Vout: 12VDC/10A

Device: LM5116MHX/NOPB

WEBENCH Design#: 1104891/58

http://webench.ti.com/appinfo/webench/scripts/SD2.cgi?ID=164736::power::dkong@rheosense.com

I used  exactly the same components as specified in the BOM

When I got the board, the output voltage looked ok, When I probed the switching node, I expected nice 50% duty waveform as the simulation, but I could see 'sub-harmonic oscillation" and ringing as the pictures in the attached file. I tried different resistors for slope compensation, but it didn't work.

Could you take a look at my design and give me some clues to solve this problem, please?

Best regards,

DennisSchematic_DC-DC_Switchers(ver.C).pdfRheoSense_SubharmonicOscillation_LM5116.pdf

  • 0
    TI__Guru**** 191445 points
    It looks like you are in diode emulation mode (and you are because DEMB = GND in your schematic). That is a normal waveform for diode emulation mode. If you increase the load, does the waveform return to a square wave?
  • 0 in reply to JohnTucker
    TI__Guru 58981 points
    Hi Dennis
    I agree with John's idea. Also, please carefully select the trigger source and select rising edge trigger. Internal oscillator is aligned with HO turn-on / SW rising .
    Regards,
    EL
  • 0 in reply to JohnTucker
    Prodigy 145 points
    Hi John,Thank you for your prompt reply.
    You mean the wave is NOT of sub-harmonic oscillation, but normal? Then it will be great.
    I tested it up to 1A load and the result was the same, but I will test it with 5-6A load again. I also would like to add 10K resistor between DEMB and GND as mentioned in the datasheet to see if this makes different.
    My second concern is the large leading spike and ringing on the switch node. It directly affects GND as shown in the previous pictures. It looks as if there is some limitation to reduce it only by changing values of the snubber circuit. Do I have to add a small resistor in series between HO and the gate of high side MOSFET? I need your help about this.
    Thank you.Best regards,Dennis
  • 0 in reply to EricLee
    Prodigy 145 points

    Hi Eric,

    Thank you for your time and advice.

    I should have chosen the rising edge trigger of the oscilloscope as you pointed out. At that time, I probed here and there, and switched trigger source and mode back and forth... I will definitely choose the proper trigger source next time.

    Best regards,

    Dennis

  • 0 in reply to Dennis Kong
    TI__Guru**** 191445 points
    I will let Eric respond further. He is way more familiar with LM5116.
  • 0 in reply to JohnTucker
    TI__Guru 58981 points
    Hi Dennis

    There are some ways to improve SW node ringing.

    - 2~10ohm gate resistor at HO pin
    - Stronger snubber (Bigger Csnubber, smaller Rsnubber)
    - Select lower Qrr MOSFET on Q11.
    - Populate SMD type ceramic capacitor directly between the drain of the high-side MOSFET to the ground connection of the sense resistor
    - Re-layout.

    Regards,
    Eric Lee
  • 0 in reply to EricLee
    Prodigy 145 points

    Hi Eric,

    I will try all of your suggestions step by step except for the re-layout which I cannot do at this stage.

    Thank you so much for your support.

    Have a good weekend!

    Dennis