Author Topic: Switching Noise  (Read 5491 times)

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Offline engrguy42Topic starter

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Switching Noise
« on: May 28, 2020, 07:01:45 pm »
Sorting thru some old boxes and found an old PC power supply. Was about to trash it and decided to hook up the scope and see if it was still breathing.

Attached is the AC coupled trace on the 12V output with about an amp of load on it. Wow, that's some switching noise...maybe 5v peak-to-peak.

Anyone know what the culprit might be? Not really interested in a science project right now, but if it's one of those "oh yeah, that's obviously xxxxx" I might tear it open and replace it.

Thanks. 
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Offline Benta

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Re: Switching Noise
« Reply #1 on: May 28, 2020, 07:12:58 pm »
One thing to know about old PC power supplies: they need a minimum load on the main output (+5 V) to work correctly. Try with 2 A.

After that, your measurements may look different.

 

Offline engrguy42Topic starter

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Re: Switching Noise
« Reply #2 on: May 28, 2020, 07:18:25 pm »
One thing to know about old PC power supplies: they need a minimum load on the main output (+5 V) to work correctly. Try with 2 A.

After that, your measurements may look different.

Thanks. Yeah, tried almost 2A on the 5V and no difference.
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Offline jemangedeslolos

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Re: Switching Noise
« Reply #3 on: May 28, 2020, 07:19:32 pm »
How did you probe it?
 

Offline engrguy42Topic starter

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Re: Switching Noise
« Reply #4 on: May 28, 2020, 07:22:49 pm »
How did you probe it?

Just a differential probe across the load resistor
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Offline thm_w

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Re: Switching Noise
« Reply #5 on: May 28, 2020, 08:18:33 pm »
I think this is some spikes are typical, from what I remember measuring. There is a huge amount of capacitance on the motherboard and various other filtering.
Try adding a big poly cap across measurement point and see if it changes much.

You can find some reviewers that measure the ripple under load, for example:
https://www.tomshardware.com/reviews/bitfenix-bf450g-power-supply,5614-9.html
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Offline engrguy42Topic starter

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Re: Switching Noise
« Reply #6 on: May 28, 2020, 09:09:43 pm »
I think this is some spikes are typical, from what I remember measuring. There is a huge amount of capacitance on the motherboard and various other filtering.
Try adding a big poly cap across measurement point and see if it changes much.

You can find some reviewers that measure the ripple under load, for example:
https://www.tomshardware.com/reviews/bitfenix-bf450g-power-supply,5614-9.html

Thanks, but 5 volts peak to peak on a 12v output is WAY out of line  :D

Anyway, I called my little buddy "Bucky the Buck Converter" (only like $3, sitting in the drawer doing nothing) and he did a very nice job of cleaning things up.
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Offline T3sl4co1l

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Re: Switching Noise
« Reply #7 on: May 28, 2020, 09:26:41 pm »
That's in the MHz.  As likely common mode as anything.  What do you measure probing ground to ground?

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Offline thm_w

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Re: Switching Noise
« Reply #8 on: May 28, 2020, 09:39:56 pm »
Thanks, but 5 volts peak to peak on a 12v output is WAY out of line  :D

Anyway, I called my little buddy "Bucky the Buck Converter" (only like $3, sitting in the drawer doing nothing) and he did a very nice job of cleaning things up.

Yeah you are right, grabbed a random PSU and its more like 500mV peak to peak, not 5V.

Maybe try with a normal probe instead of a differential, just make sure ground wire goes to black.
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Offline engrguy42Topic starter

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Re: Switching Noise
« Reply #9 on: May 28, 2020, 09:50:23 pm »
Hey, wait a minute...

That same trace occurs with a normal (not differential) probe plugged into the scope, and power supply turned on, but the probe itself is connected to NOTHING !!!

It's induced by ZOMBIES!!!!  :scared:
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Offline jmelson

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Re: Switching Noise
« Reply #10 on: May 29, 2020, 02:11:34 am »
Hey, wait a minute...

That same trace occurs with a normal (not differential) probe plugged into the scope, and power supply turned on, but the probe itself is connected to NOTHING !!!
Welcome to the wonderful world of working with power switching circuits.  Huge amounts or radiated magnetic and electromagnetic fields, pulses on the power mains, it gets into everything.  And, a lot of scopes don't have enough filtering on the mains input to keep it out of their sensitive internal circuits.  I bet a good old Tek scope would at least show less of this noise.

Jon
 
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Offline engrguy42Topic starter

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Re: Switching Noise
« Reply #11 on: May 29, 2020, 10:22:28 am »
Okay, well I didn't want to do it, but I guess I have to. Rolling out the big guns on this one.

Just ordered some ferrite cores to snap over the wires. BAM !! Now we'll see who's boss.  :D
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Offline engrguy42Topic starter

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Re: Switching Noise
« Reply #12 on: May 29, 2020, 11:43:13 am »
Hey, wait a minute...

That same trace occurs with a normal (not differential) probe plugged into the scope, and power supply turned on, but the probe itself is connected to NOTHING !!!
Welcome to the wonderful world of working with power switching circuits.  Huge amounts or radiated magnetic and electromagnetic fields, pulses on the power mains, it gets into everything.  And, a lot of scopes don't have enough filtering on the mains input to keep it out of their sensitive internal circuits.  I bet a good old Tek scope would at least show less of this noise.

Jon

With the probe lead totally disconnected from the scope (unscrewed from the BNC) the trace is flat. But with the lead connected at the BNC but not connected to any devices, the same noise appears on the trace. Which implies to me that it's common mode noise generated by the power supply and being induced on the lead (even with the ground lead disconnected, BTW). Unless the BNC connector on the scope has some internal mechanism to zero the trace when nothing connected?

So it seems the scope goodness (ie, the "old scopes are awesome" fanboyism  :D ) is kinda irrelevant isn't it, and it's more about cleaning up the emitted noise from the power supply? Which is why I'm going the ferrite core route.

EDIT: BTW, if I move the offending power supply from the bench down onto the floor (maybe 5 feet away), with the scope lead plugged into the scope but the probe connected to nothing, the noise on the trace drops down to almost nothing.   

 
« Last Edit: May 29, 2020, 11:58:42 am by engrguy42 »
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Offline engrguy42Topic starter

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Re: Switching Noise
« Reply #13 on: May 29, 2020, 12:18:16 pm »
Another data point...

Covering the power supply in a tin foil hat (aluminum foil) cuts the peak switching noise viewed on the scope (with a lead connected to no devices) from around 1 volt peak to about 0.25 volts peak.

Yes, I actually got a roll of Reynolds Wrap and covered the power supply.  :-+

So yeah, a tin foil hat is sometimes necessary.  :D

Anyone know where I can buy a Faraday cage?  :D
« Last Edit: May 29, 2020, 12:32:52 pm by engrguy42 »
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Offline TimNJ

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Re: Switching Noise
« Reply #14 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.
 

Offline engrguy42Topic starter

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Re: Switching Noise
« Reply #15 on: May 29, 2020, 03:19:01 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.

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
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Offline TimNJ

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Re: Switching Noise
« Reply #16 on: May 29, 2020, 03:47:31 pm »
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.
« Last Edit: May 29, 2020, 03:57:19 pm by TimNJ »
 

Offline David Hess

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Re: Switching Noise
« Reply #17 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.
« Last Edit: May 30, 2020, 01:31:25 am by David Hess »
 

Offline engrguy42Topic starter

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Re: Switching Noise
« Reply #18 on: May 30, 2020, 11:46:26 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.

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?
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Offline Jay_Diddy_B

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Re: Switching Noise
« Reply #19 on: May 30, 2020, 11:49:31 am »
Hi,

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

Jay_Diddy_B
 

Offline engrguy42Topic starter

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Re: Switching Noise
« Reply #20 on: May 30, 2020, 11:56:47 am »
Like I say it's basically an ATX computer power supply. I checked inside and it's got a lot of PFC components. I think it came off an HP desktop, so it should be reasonably well designed.

But the image of the noise I posted looks to be a very typical SMPS output waveform, with switching noise plus ripple. The only issue is why the switching noise peaks are so freakin' high and not filtered out. I'm assuming there's a standard circuit in these things whose job is to filter out those switching peaks. Once I have that figured out I can focus on it and see if there's a bad component. 
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Offline engrguy42Topic starter

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Re: Switching Noise
« Reply #21 on: May 30, 2020, 12:06:38 pm »
Anyway, I was looking at an Analog Devices paper on switching noise for these SMPS's, and it says:

"Switching transition noise is typically in the range of 10 MHz to 300 MHz. It is much higher in frequency than the switching frequency of a switching regulator. For attenuation of this noise on the output of the power supply, an LC filter, commonly used to reduce the output ripple voltage, may not be the right choice. Ferrite beads are much better suited to attenuate such high frequencies."

It looks like the noise I'm seeing is in the 40MHz range, which matches what the paper says. And as I mentioned before, and apparently this paper confirms, I ordered some ferrite beads the other day to see if that helps. Or maybe there's a dead C in an internal LC filter circuit...

EDIT: Cool...I just checked and my ferrite cores are arriving today.  :-+  It's an assortment of like 20 different sizes, so I guess I'll be experimenting today.  :D
« Last Edit: May 30, 2020, 12:16:07 pm by engrguy42 »
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Offline T3sl4co1l

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Re: Switching Noise
« Reply #22 on: May 30, 2020, 12:23:31 pm »
Well, looks like a main switching edge coupling through...  Whether it's a two-switch forward or flyback, or traditional half bridge I'm not sure.

Check primary and secondary side grounding, and by that I mean RF grounding, capacitors and all that.  Missing or failed Y caps would be a possibility.  Can always put it through a line filter if it doesn't seem to be anything onboard.

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Offline Jay_Diddy_B

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Re: Switching Noise
« Reply #23 on: May 30, 2020, 12:25:46 pm »
Hi,
A photograph of the pcb would really help the discussion.

You describe the this an 'old' pc power supply. The techniques have changed over the years.

I have seen cases where the interference suppression components have been removed, to save cost, probably after the power supply past FCC testing.

You could have dried up output capacitors,
You could have damaged X or Y caps.


Like I say a few good photos will help a lot.

I have several decades of SMPS experience.

Regards,
Jay_Diddy_B
 

Offline engrguy42Topic starter

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Re: Switching Noise
« Reply #24 on: May 30, 2020, 12:50:26 pm »
Thanks. Here's the power supply guts. Didn't see any empty "important component belongs here" type spaces. And lots of PFC yumminess on the top there...
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