Author Topic: Switching Noise  (Read 5489 times)

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Offline engrguy42Topic starter

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Re: Switching Noise
« Reply #25 on: May 30, 2020, 05:09:52 pm »
Well, FWIW, I tried wrapping some of my new ferrite cores around the +/- wires coming out of the power supply and it's pretty disappointing. Yeah, I know that it's a science project where you're supposed to match the core material to the frequency, blah, blah, blah, but I just grabbed some no-name (and no spec) cores off Amazon to see if they'd make a difference.

The only difference I noticed was when wrapping a larger core around both wires...it knocked the switching peak down by maybe 30%, but no affect on the ringing frequency or anything else. So I'm guessing this power supply is just emitting some nasty switching noise EMF and it's radiating into the room and getting picked up by the DUT circuit wires and my probes.

Hell, it's in a metal case and connected to ground, etc., so all I can figure is there's something amiss inside the case, like a bad cap or something.

Anyway, looks like this will go in the scrap bin, or maybe I'll get the energy to desolder some components and put them in my parts drawers.

But I know this will be nagging at me to figure out...  :D
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Offline T3sl4co1l

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Re: Switching Noise
« Reply #26 on: May 30, 2020, 05:12:17 pm »
Well no, not on the top in the photo, that's line filter, more or less a 5 pole line filter, plus some ferrite bead and extra ceramic caps on the mains entry, by the looks of it.  Definitely a PFC choke on the left though, the black core probably being Kool-Mu.  Not sure why there are so many devices on the heatsink, I'd expect four (1 MOSFET and 1 diode for PFC; 2 MOSFET for output, most likely 2-switch forward).

Most likely failure I think would be something missing from ground.  Measure the caps.  Check the secondary side grounding, it should be shorted to the case, possibly in multiple locations.

Could still be some kind of electrical failure, other than failed filter components; a transistor accidentally shorted to a heatsink, or insulation failure in the transformer maybe, could do that I suppose (if unlikely without also shitting the bed completely).

Tim
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Electronic design, from concept to prototype.
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Offline engrguy42Topic starter

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Re: Switching Noise
« Reply #27 on: May 31, 2020, 11:34:23 am »
Interesting...

I checked continuity between the power supply "ground" (black wires) and the power supply case. Nothing. I did the same with another ATX power supply. Same thing. I'm wondering if the secondaries are designed to be isolated, and maybe rely on the main PC ground when they plug in to the motherboard?

Also, I did a bit more testing. Scope turned on with a single test lead/probe connected to the BNC, but the probe left unconnected, and it's just resting on top of the power supply. The power supply was plugged in, but NOT turned on. Which means it was running in standby mode presumably.

And the attached is the noise I got. Of course it varied as I moved the probe around and more or less got induced in the leads. But wow, even in standby mode this thing is emitting that much RFI.

BTW, I also connected a wire between the black/ground wire and the case ground thru a ferrite core, and that made no difference.

So again we're back to the question: how do you filter something like this, and what could be going wrong here? Or is it normal?

EDIT: ...and if I add a 50 ohm resistor across the probe the noise goes away. And when I turn the PS on, the switching noise comes back. Is there something obvious and dumb I'm missing?

See, this is what I hate about this freakin' electronics stuff. All these weird zombie induced effects that suddenly appear then go away for no reason. What a PITA. You waste all this time trying to squash stuff that doesn't exist.     
« Last Edit: May 31, 2020, 12:01:15 pm by engrguy42 »
- The best engineers know enough to realize they don't know nuthin'...
- Those who agree with you can do no wrong. Those who disagree can do no right.
- I'm always amazed at how many people "already knew that" after you explain it to them in detail...
 

Offline engrguy42Topic starter

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Re: Switching Noise
« Reply #28 on: May 31, 2020, 12:43:19 pm »
Okay, so I re-did my measuring per the attached image. Added a ferrite core on the twisted leads coming from the ATX supply 12V, added a few capacitors across the 12V, and used one of those low noise ground attachments to the probe. And this seems to have significantly reduced the measured noise. Still the switching spikes are there, but they're down in the 100mV peak range.

So I'm guessing this is much closer to the actual value of peak switching noise...maybe 50-100mV peak.

Okay, I feel better now...  :D
- The best engineers know enough to realize they don't know nuthin'...
- Those who agree with you can do no wrong. Those who disagree can do no right.
- I'm always amazed at how many people "already knew that" after you explain it to them in detail...
 

Offline T3sl4co1l

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Re: Switching Noise
« Reply #29 on: May 31, 2020, 01:42:15 pm »
Capacitive coupling of high frequencies.  Which because the open circuit probe is such a high impedance (a few pF), doesn't take much.

How about this, clip 1k across the probe and measure:
- Probe GND to enclosure: probably nothing
- Probe tip to enclosure: probably nothing, or incidental noise
- Probe ground to secondary ground: some noise
- Probe tip to secondary ground: full noise

Hopefully 60Hz hum doesn't interfere with this test...

You're measuring the noise in otherwise-ordinary circumstances (probing circuits) because of the RF voltage drop along the ground clip and other unbalanced ground paths; and the CMRR of the probe cable itself might not be fantastic.

It seems the enclosure is a good enough ground, and the secondary leads are carrying CM switching noise.  The noise is a voltage generated between enclosure and secondary.  It probably has a high impedance (100s ohms, kohms?), so that it can be easily shunted to ground with some bypass caps to the enclosure, or direct grounding.  As long as you have (had?) it open, this is easy enough to add.

Ferrite beads will do little to nothing, because their impedance is modest (~100s ohms).  You need either a very high impedance (10s kohm+?), which isn't a practical value CMC at this frequency or amperage, or a low impedance shunting it back to the source.

Which also suggests additional filtering if desired; run the secondary through a CLC (CM) filter mounted to the enclosure.  The last C also gives a low impedance, so ferrite beads may be useful again, say if you need to dampen a bit of ringing elsewhere.

Tim
Seven Transistor Labs, LLC
Electronic design, from concept to prototype.
Bringing a project to life?  Send me a message!
 

Offline David Hess

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Re: Switching Noise
« Reply #30 on: May 31, 2020, 06:22:25 pm »
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.

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?

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.
 

Offline engrguy42Topic starter

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Re: Switching Noise
« Reply #31 on: May 31, 2020, 08:01:34 pm »
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.

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?

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.

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
- The best engineers know enough to realize they don't know nuthin'...
- Those who agree with you can do no wrong. Those who disagree can do no right.
- I'm always amazed at how many people "already knew that" after you explain it to them in detail...
 


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