Electronics > Projects, Designs, and Technical Stuff
Switching Noise
David Hess:
--- Quote from: engrguy42 on May 30, 2020, 11:46:26 am ---
--- Quote from: David Hess on May 30, 2020, 01:29:22 am ---Even with a differential probe, it requires good technique to get meaningful results. I usually end up using direct differential coaxial connections which modern differential probes do not support, but old ones do.
Also beware that modern differential probes tend to fall out of calibration due to poor construction and of course lack service documentation. So expect poor common mode rejection unless you verify it yourself.
--- End quote ---
Apparently there are a lot of measurement experts out there... :D
But are there any SMPS experts? The real issue is why this power supply is emitting this switching noise. Anyone know what may have failed in order for all of this switching noise to occur?
--- End quote ---
If the measurement is not accurate, and this type of noise measurement is very difficult to make, then the switching noise may not exist. Your results are consistent with a poor measurement so I and others have suggested going back and verifying your measurement setup by doing things like shorting the differential inputs at the ground connection.
I would go further and measure the high frequency common mode rejection of the differential probe since that is a common (ahem) problem these days.
engrguy42:
--- Quote from: David Hess on May 31, 2020, 06:22:25 pm ---
--- Quote from: engrguy42 on May 30, 2020, 11:46:26 am ---
--- Quote from: David Hess on May 30, 2020, 01:29:22 am ---Even with a differential probe, it requires good technique to get meaningful results. I usually end up using direct differential coaxial connections which modern differential probes do not support, but old ones do.
Also beware that modern differential probes tend to fall out of calibration due to poor construction and of course lack service documentation. So expect poor common mode rejection unless you verify it yourself.
--- End quote ---
Apparently there are a lot of measurement experts out there... :D
But are there any SMPS experts? The real issue is why this power supply is emitting this switching noise. Anyone know what may have failed in order for all of this switching noise to occur?
--- End quote ---
If the measurement is not accurate, and this type of noise measurement is very difficult to make, then the switching noise may not exist. Your results are consistent with a poor measurement so I and others have suggested going back and verifying your measurement setup by doing things like shorting the differential inputs at the ground connection.
I would go further and measure the high frequency common mode rejection of the differential probe since that is a common (ahem) problem these days.
--- End quote ---
Thanks, but the goal here was to decide, fairly quickly, if the power supply should be tossed or if it's useful. If it was legitimately generating what I initially thought (around 1-2 volts switching peaks) it should be repaired or tossed. But at this point I've determined that, with some simple changes in how I'm measuring (as described in my last post), the actual number is closer to 0.05-0.1 volts peak. Which to me says it's fine. Whether it's off by 0.126 femtovolts is somewhat irrelevant to the overall goal here. As I said, this isn't worth a science project.
What I did learn was, to my surprise, the ability of these things to generate large amounts of induced switching noise, especially since they are in grounded metal cases with tons of internal filtering. And also that a well-placed ferrite core can have an impact (maybe 20% in this case), though relatively small. And those low noise ground attachments to the scope probes are a big help, as is using a 1x multiplier rather than x10. As is making sure all the wiring on the breadboard is tight. And also doing some trial and error with additional filter capacitors.
BTW, I also yanked another ATX supply and got similar results, so it further confirms the belief that this thing hasn't failed. Now I just need to figure out what the hell I'm gonna do with it... :D
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