Tool/software:
Niklas,
Problem solved!
As you had rightly suggested, I had to play with the compensation networks R and C values.
Initially, I thought I can make use of the bode plot in the spreadsheet simulator, but none of the values I tried worked.
I originally had 11.3K for R and 22 nF for C.
Changing the C to 1uF and 10uF made no difference.
I also tried R values of 1K and 100K, which made things worse.
Finally, a 10 uF and 6K gave me a significant improvement over what I had, and while not zero ripple (as discerned by the camera/human eye), it was adequate for my purpose.
Thank you once again for your suggestion.
Btw, while debugging, I accidentally dropped a screwdriver, and shorted something out, and one board no longer works. While I can scrap this board as I have all my other boards working fine, I am curious where I should look for a defective part that I can then replace.
For the discussion that follows, kindly refer to the schematic on the bottom of pg 1 of the LM5155 data sheet, reproduced below.
I did look at the Gate drive of the external FET to make sure the 5155 is alive. While I do see the drive, the scope is having a hard time triggering on it to show a periodic waveform. On a good board, I see it readily; however, the gate drive shape compares, so I think the 5155 on the bad board should be OK. I did an in-circuit measurement of the assorted filter caps on both good and bad boards, and they look OK.
On the drain side of the exernal FET on the bad board, the waveform does look different.
Would you suggest replacing the FET and/or diode?
Thank you,
Ajai
