Electronics > Projects, Designs, and Technical Stuff

Switching Noise

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engrguy42:
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. 

engrguy42:
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

T3sl4co1l:
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.

Tim

Jay_Diddy_B:
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

engrguy42:
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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