| Electronics > Projects, Designs, and Technical Stuff |
| Switching Noise |
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| engrguy42:
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 |
| T3sl4co1l:
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 |
| engrguy42:
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. |
| engrguy42:
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 |
| T3sl4co1l:
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 |
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