Electronics > Projects, Designs, and Technical Stuff
Switching Noise
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engrguy42:

--- Quote from: TimNJ on May 29, 2020, 02:54:48 pm ---The loop area of your probe is acting as a nice antenna for the switching noise radiating off of the power supply and the common-mode noise currents flowing on the output cables. The common-mode noise currents cause a changing magnetic field in the vicinity of your power supply, which in turn get converted to a differential mode voltage that shows up on your oscilloscope input.

When power supply manufacturers measure ripple, usually they use a "ripple probe", that is a probe with a very small enclosed loop area loop area. The best way to do this is to use a probe tip BNC socket which allows you to plug the oscilloscope probe directly into a coaxial connector which is then wired to the power supply output. Manufacturers typically also specify a small capacitance at the end of the cable (10uF-100uF + 0.1uF ceramic). Most power supply manufacturers also use a 20MHz bandwidth limit on the oscilloscope. To further reduce EMI pickup, make sure to use a 1x probe.

With this setup, you have a more realistic idea about the power supply's actual ripple. Of course, even with the "real" ripple of the power supply, near-field radiated emissions can still cause system problems. As you saw with your accidental antenna probe, it can be quite easy to pick up noise, and that noise is real. If you have a system with lots of high sensitivity (high impedance) nodes and/or large PCB loop areas, then radiated switching noise can still be a problem for you.

--- End quote ---

Thanks, but is there a fix? I'm far less concerned with accurate measurement than I am with minimizing the switching noise in the first place. I assumed some well-placed ferrite cores on the power supply wires might do the trick, since this is MHz-range switching noise. Anyone know of a fix? I assume this is fairly common in workbench situations where guys have 126 different pieces of equipment stacked on top of each other....  :D
TimNJ:
I guess the point I was trying to make is: I think it's mostly your measurement. The switching noise you see on your scope is not necessarily what the powered system actually "sees". Yes, the power supply is emitting a bunch of HF junk, but it doesn't necessarily mean that it causes issues. (Rather, it doesn't mean that it actually gets picked up by the system and is converted into the large peak-to-peak value that you saw.) Also, if the power supply is from a real, reputable manufacturer then it should have underwent 3rd party EMI testing that proves compliance with international EMI requirements. These requirements set levels for acceptable levels of EMI. If you want to cut down on the radiated emissions, then you are definitely on the right path with adding ferrites and shields.
David Hess:
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.
engrguy42:

--- 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?
Jay_Diddy_B:
Hi,

If you share a few pictures of the power supply it may help.

Jay_Diddy_B
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