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LM5175: LM5175 Buck Boost Converter: WEBENCH-designed output capacitance not enough to stabilize PSU.

Jim Fuoco
Prodigy 220 points
Part Number: LM5175

I took a WEBENCH design for a 10-14 Vin, 12 V / 8 A out buck/boost converter controlled by  the LM5175. The output is extremely unstable without adding significantly more capacitance. The design specifies 9 x 22 uF (198 uF) capacitance. I can somewhat stabilize the output by adding another 1000 uF

Here is a snapshot of the schematic from the tool:

Here is my re-drawn schematic used for my PCB design:

Here are scope shots showing different loads & capacitance. Yellow = input to PCB. Blue = Output from PSU:

  • No additional caoacitor added. Minimal load (< 100 mA). Audible buzzing from inductor 
  • 1000 uF capacitor added. Minimal load ( <100 mA). No audible buzzing:
  • 1000 uF capacitor added. 20 W load. No audible buzzing:
  • 1000 uF capacitor added. 3.3 A load. No audible buzzing: 
  • 1000 uF capacitor added. 5 A load. no audible buzzing. Iitially unstable, then stabilizes:
  • 1000 uF capacitor added. 6.7 A load. no audible buzzing; unstable output:
  • 2200 uF capacitor added. 6.7 A load. no audible buzzing.
    • Unstable. 8 A fuse input burnt out. After shorting fuse with wire, PSU smoked and failed.

I'm curious why essentially 6x'ing the output capacitance solves the problem...

  1. is the WEBENCH design under specified?
  2. Do I have a schematic problem?
  3. Could counterfit components used by the PCBA manufacturer be the issue?
  4. I have a few more copies of the board I can test on; I'm not sure where to begin probing for other issues.
  5. Why does the 6.7 A load cause instability for the 1000uF fix?

Thanks in advance.

  • 0
    Prodigy 220 points

    It seems like M3 (low drive input FET) is failing; on another board I noticed smoke coming from the component.

  • 0 in reply to Jim Fuoco
    TI__Mastermind 21720 points
    Hi Jim,

    If the MOSFET is smoking it is most likely damaged. IT might have been damaged by over voltage if the the switch node is ringing above 30V.

    The output capacitance provided by WEBENCH seems to be on the low end. I would also suggest adding some electrolytic capacitors to provide some bulk capacitance. To double check the compensation loop of the components on the board please use the excel quick-start calculator below.
    www.ti.com/.../lm5175quickstart-calc

    Taking a quick look at the schematic I don't see any issue.

    -Garrett
  • 0 in reply to Garrett Roecker
    Prodigy 220 points

    Hi Garrett,

    Pleasure to meet you, and thanks for the advice.

    I played around the quick start calculator you linked to, a nice tool for sure. I have a couple questions:

    Given my I/O parameters, the minimum output capacitance is given as 33.3 uF. The design from the WEBENCH tool that I use gives 198 uF; roughly 6x which should already be over-provisioned.

    However above you mention that the WEBENCH tool may be under-provisioned. That agrees with my hands-on debugging, where adding another 1000uF stabilized the output.Now admittedly that may have been overkill, I can try smaller values and see what gives stability.

    I am curious what you saw that made you feel the 200 uF to be under-provisioned.

    I keep trying to plug in my compensation component values but it keeps crashing Excel, haha. Maybe because I am using Excel for Mac?