Strange ~50kHz unstable wave on the output of my SMPS - Page 1
Forum section: electronics / projects

author: Northy
author_rank: Regular Contributor
timestamp: September 22, 2020, 07:25:09 pm
content: I'm testing a SMPS design, but it's got a strange signal on the output.Spec of the supply:24V In from bench supply - the output of this looks ok when checked.17V Out1A OutFsw 1MHzI can see the 1MHz switching frequency, but it's superimposed on a ~50kHz signal that sort of goes up and down.I'm measuring with a 1X probe, with short wire wound ground probe.I've attached a picture of the output.Am I doing something stupid here? Is this really there?Thanks,G

author: Benta
author_rank: Super Contributor
timestamp: September 22, 2020, 08:04:45 pm
content: We're talking 20 mVpp here, so it could be anything. A CRT monitor next to the scope? Something else running in the vicinity?Rule out that kind of stuff first.

author: TimNJ
author_rank: Super Contributor
timestamp: September 23, 2020, 03:21:41 am
content: To add to Benta, do you have any fluorescent under bench lights?Most important idea is to keep the probe loop area minimized. Standard scope probe with long ground lead/clip is a bad idea. Use 1X probe attenuation to minimize pickup of radiated junk. Set the scope to 20MHz BW, not that that helps too much for your 50KHz issue. If available, use a probe tip to BNC adapter and run twisted pair to the output voltage test points. This minimizes probe loop area.If the signal still exists after all/most of the above is implemented, we can check if maybe it's control loop stability. For 1MHz switching frequency, it might be possible. Usually, control loop instability might be on the order of 1 to 10KHz, but newer designs may have higher loop bandwidth. Simple way to check loop stability is to run worst case dynamic loading and to check if the transient response looks well damped. You can use the same setup as the ripple measurement.

author: TimNJ
author_rank: Super Contributor
timestamp: September 23, 2020, 04:28:13 am
content: By the way, although not exactly relevant to your question, I should clarify that setting the scope's bandiwdth to 20MHz does not imply that the power supply is incapable of generating noise with content about 20MHz. Semiconductor ringing waveforms may have fundamentals between 10-50MHz on average, and with harmonic content into the 100's of MHz. It certainly can be conducted onto the output wires, but often time this high frequency noise gets exaggerated since it tends to radiate out from the DUT more easily. With an imperfect measurement setup, or a measurement performed very close to the switching circuitry, it's pretty easy for your probe to turn into an antenna.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 23, 2020, 04:57:13 am
content: That's peak detect (acq mode)..?Doesn't look particularly 50kHz-y, looks more chaotic. Is it compensated properly?Tim

author: Northy
author_rank: Regular Contributor
timestamp: September 23, 2020, 07:30:17 am
content: I'm using a 1X probe with short ground wire clip (not standard ground lead/clip).I tried moving the scope and scope lead away from anything else.I tried turning the light offI'll try a few more things today. I've got a couple of 12V batteries I can use instead of the bench supply.I'll look at the output transient response.Quote from: T3sl4co1l on September 23, 2020, 04:57:13 amI zoomed out to show how it seems to go up and down, if you zoom in there is a ~50kHz look to it.I'll see if I can get some more pictures.Thanks,G

author: Northy
author_rank: Regular Contributor
timestamp: September 23, 2020, 09:13:53 am
content: I turned everything off apart from the scope. I powered the input from two lead acid 12V batteries in series, still there.Here's some more pictures.It does kind of have a 50kHz feel to it. When I zoom the 1MHz switching frequency can be seen.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 23, 2020, 09:17:36 am
content: What does the compensation node look like?Tim

author: Northy
author_rank: Regular Contributor
timestamp: September 23, 2020, 10:12:23 am
content: The compensation node? Where would I be looking? The FB pin?Thanks,G

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 23, 2020, 10:18:24 am
content: No idea. Didn't mention IC, schematic, layout--Tim

author: Northy
author_rank: Regular Contributor
timestamp: September 23, 2020, 10:44:16 am
content: It's an MPS MP4572 evaluation board.I've changed:R7 = 0RR11 = 28kR5 = 2k05G

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 23, 2020, 12:04:00 pm
content: Ah okay.They... don't even have a datasheet. What's the point?Well, I guess you've made your evaluation, of their eval board (including recommended values), and it sucks...Tim

author: magic
author_rank: Super Contributor
timestamp: September 23, 2020, 12:31:12 pm
content: Maybe you have just made a discovery that bandgap references have noise?Assuming 0.8~1.2V reference in the chip, you run it at some 15~20x gain so 10mVpp corresponds to maybe a bit over 500µVpp which would be in the ballpark of 100µV RMS and maybe 300nV/rtHz over a 100kHz bandwidth. That's not even very much worse than regulators like LM317 or LM78.

author: Northy
author_rank: Regular Contributor
timestamp: September 23, 2020, 03:59:53 pm
content: I got rid of the scope probe and used a coax connection direct to the scope - same result.I thought I’d try something quickly, adding some big electrolytics to the output, and it seemed to work. 2 x 220uF seemed to get rid of the the odd ~50kHz ripple and get it down to ~8.4mVpk-pk of switching noise. Does this give any clues?G