Unexplained interference in PSU test lab - Page 1
Forum section: electronics / projects

author: Phaedrus
author_rank: Frequent Contributor
timestamp: December 11, 2013, 11:33:17 pm
content: Hi guys,I'm having some weird issues in my lab. I do a lot of ATX power supply testing, and part of this testing involves measuring output ripple and noise. The problem is, I'm getting anomalously high readings, typically 30-50mV higher than comparable labs on the exact same power supplies. The actual ripple waveform is the same magnitude, +/-5mV, when measured with V-bars, but I'm getting large spikes which throw the automated reading off, and using the V-bars is too time-consuming when I have to make 30-40 measurements.Here is the test setup:Tektronix TDS3014C DPOFluke 87V DMMBK Precision 8512 1200W load2x BK Precision 8540 150W loadsExtech 380820 Power AnalyzerCustom test fixtureI thought the issue was the test fixture, where the power supply cables, DC loads, and scope probes attach to the PCB. I replaced the SMA connectors for the probes, I replaced the 0.1uF ceramic caps and 10uF electrolytic caps demanded by Intel testing procedures (and upgraded from Taicon caps to equivalent NCCs), and cleaned all connectors with isopropyl. No significant change, for quite a bit of effort.Using different probes or scope channels makes no difference. I'm considering if I might be picking up switching noise from the DC loads, or possibly conductive EMI from the mains. But I'm not sure how I could test for that. I'm kind of at a loss here.

author: minime72706
author_rank: Frequent Contributor
timestamp: December 11, 2013, 11:48:14 pm
content: What do the spikes look like? Are they periodic and always roughly the same size? Whether it's periodic or not, how often does it seem to occur? How wide are they?

author: Phaedrus
author_rank: Frequent Contributor
timestamp: December 11, 2013, 11:53:43 pm
content: I'll post a scope shot.

author: Phaedrus
author_rank: Frequent Contributor
timestamp: December 12, 2013, 12:02:21 am
content: The weird thing is, it matches up well with the peak of the waveform. But that spike shouldn't exist! We had an identical unit with the same spike, and when tested in another lab the spike wasn't there.This unit at this load (can't give you details, sorry) is supposed to be at ~40mVpk-pk ripple on the +12V rail. V-bars on the actual waveform show ~38mV. But I've got an 80mV spike.Scope is at 20MHz, 1M impedance, Sample mode, normal trigger, etc. Using 1x passive probes with SMA connectors.

author: notsob
author_rank: Frequent Contributor
timestamp: December 12, 2013, 12:13:15 am
content: Try turning your lights off

author: Galaxyrise
author_rank: Frequent Contributor
timestamp: December 12, 2013, 12:22:36 am
content: Quote from: Phaedrus on December 12, 2013, 12:02:21 amHow tough is it to move things between labs? If you could take all your equipment to the other lab and try replacing each part of the test rig one by one you should be able to isolate it to either a piece of equipment or the lab itself.

author: Phaedrus
author_rank: Frequent Contributor
timestamp: December 12, 2013, 12:26:11 am
content: By other lab I mean Europe. And lights make no difference.

author: TheBay
author_rank: Unknown
timestamp: December 12, 2013, 12:45:17 am
content: Sounds like some interference on mains, do you have homeplugs, energy saving light bulbs, fluorescent lighting....Have you scoped mains power?

author: Rufus
author_rank: Super Contributor
timestamp: December 12, 2013, 02:34:26 am
content: Quote from: Phaedrus on December 12, 2013, 12:02:21 amLined up with ripple indicates it is from a switching transient in the PSU. If it isn't on the output then the scope is seeing some common mode signal with respect to the scope mains earth input (or other ground connections?).Checking with a portable scope (no earth) would quickly tell you something if you have access to one.I would look at and experiment with the mains supply to the PSU, scope, and other test equipment. Try clip on ferrite cores on the scope mains lead and possibly probe lead they shouldn't make any difference but if they do you gained a clue.

author: Unknown
author_rank: Unknown
timestamp: December 12, 2013, 01:17:32 pm
content: Sounds like a common-mode signal picked up by the probe (although a coaxial SMA connection should have a very small loop area) indeed. Any chance you can cut one of the traces going to the test point and short the test point near the end of the trace? If you still see a signal with a shorted scope probe, then you definitely have a common-mode signal that may not exist in the actual circuit. Bob Johnson and Doug Smith have written on this topic, I'm sure some of it is available online.What kind of probe is this? A common high-impedance 1x probe is unlikely to have 20 MHz of usable bandwidth at reasonable test point impedances.

author: Phaedrus
author_rank: Frequent Contributor
timestamp: December 12, 2013, 11:57:16 pm
content: I checked with different sockets, with other devices in the building turned off, I tried power through the Power Analyzer (w/ isolation) and straight from the wall socket. Even tried a cheap ferrite bead, for all that's worth. No result. It's not on the AC side, as far as I can tell.It's either the scope/probes, the test fixture, or interference from the DC loads.

author: KJDS
author_rank: Super Contributor
timestamp: December 13, 2013, 07:50:52 am
content: Does the burst of noise always appear at the top of the general ripple?

author: AndyC_772
author_rank: Super Contributor
timestamp: December 13, 2013, 08:00:07 am
content: Can you post a picture of your test fixture, showing exactly how the connectors, probes and any other components are physically laid out? I agree with alm, it's almost certainly a probing problem, and it can probably be fixed with changes to how the various parts of the system are connected together.

author: ajc
author_rank: Contributor
timestamp: December 13, 2013, 08:33:58 am
content: I had a similar issue in my lab, but turned out to be some small flourescent lamps I used above my lab desk. Might also be worth checking / confirming you have a good earth connection in your lab. (another issue I found in my case)Out of interest, how are you grounding your probes? Any chance you are picking up the signal in the ground of the probe?

author: notsob
author_rank: Frequent Contributor
timestamp: December 13, 2013, 08:46:13 am
content: are you in an isolated building or do you have co-tenants around youi'e the interference could be coming from above-below or in a room to the side of you.

author: AndyC_772
author_rank: Super Contributor
timestamp: December 13, 2013, 09:01:17 am
content: If the spikes are in sync with the switching waveform, then one way or the other, they're coming from the PSU.A spike at the peak of a waveform, where the output voltage stops ramping one way and starts ramping in the other direction, is the absolutely classic manifestation of switching noise. It's either very poor PSU design (switching noise couples capacitively across the output inductor, and is not properly filtered by the PSU's output caps), or more likely, it's common mode noise which is made visible by lead inductance in the scope probe's ground.It can't possibly be "external" interference, it's something inherent in the system of PSU + test setup + scope.

author: Phaedrus
author_rank: Frequent Contributor
timestamp: December 13, 2013, 04:55:06 pm
content: The PSU design isn't the issue; like I said, these units have been tested in another lab and they don't have this problem. It's a high-end, high-wattage LLC resonant full bridge converter with interleaved PFC, designed by a very respectable firm. >90% efficiency at 50% load, very tight voltage regulation, etc. And very low ripple, when measured by anyone except me.

author: Phaedrus
author_rank: Frequent Contributor
timestamp: December 13, 2013, 04:58:47 pm
content: Could a voltage potential between scope ground and PSU ground cause this issue?

author: Phaedrus
author_rank: Frequent Contributor
timestamp: December 13, 2013, 05:06:36 pm
content: Also worth noting that the amplitude of this spike scales fairly linearly with load on the power supply, and it exists when testing other power supplies as well. At 0W load there's no spike. At 500W load there's a ~50mV spike. At >1000W load there's an 80mV spike.

author: AndyC_772
author_rank: Super Contributor
timestamp: December 13, 2013, 05:16:10 pm
content: Quote from: AndyC_772 on December 13, 2013, 08:00:07 amJust reiterating...

author: tld
author_rank: Regular Contributor
timestamp: December 13, 2013, 05:25:35 pm
content: Any chance the noise-difference between sites could be due to different mains-impedance?(Asking because I'm not sure, more than anything else)

author: Phaedrus
author_rank: Frequent Contributor
timestamp: December 13, 2013, 05:42:14 pm
content: Here you are:http://imgur.com/48QrdB2,WzoRpwn,OsSYmTx,msSUPkI,WEo9I8m,sG6XBy3,xBq6OaQ,WUYSWWmI did significant amounts of work on the test fixture, so forgive the flux residue. It's my #1 suspect as well (though I want to rule out other things). It had damage to the bottom, including a couple of severed traces, and a couple of damaged connectors on top.What I did:Removed all SMA connectors and replaced the ones we actively useReplaced ceramic 0.1uF capsReplaced electrolytic 10uF capsRepaired damaged traces with tinned copper wireAdded tinned copper wire and solder to some high-current traces to increase thermal mass and reduce impedancePut board on standoffsI can't see anything I did adding noise like this.

author: KJDS
author_rank: Super Contributor
timestamp: December 13, 2013, 06:07:15 pm
content: If you double the length of those SMA leads, then I'll put money that the length of ringing on that sharp pulse will double.Have a close measure of the ringing frequency. Work out what length lead that would correspond to and then look at your current lead lengths.Please note the above is conjecture, not fact, but it's fairly educated conjecture.

author: AndyC_772
author_rank: Super Contributor
timestamp: December 13, 2013, 06:24:38 pm
content: Remove the electrolytics on the back of the test PCB and replace them with leadless ceramic caps. Ripple and noise on a PSU are almost always measured across a decoupling cap, rather that just at the end of a long wire as you have now.A small (say, 100nF) ceramic cap won't have any effect on the magnitude of the 'real' ripple, but it will help suppress the unwanted high frequency switching spikes. If you're concerned about affecting the measurement accuracy, bear in mind that any PC motherboard will have many ceramic caps on it anyway, so you're actually testing a scenario which is more representative of a real use case, not less.There's an argument that in fact there's little point in having effective high frequency decoupling in a PSU that's designed to be connected to its load over long wires. The only really good reason is EMC. If a load requires low noise at high frequencies, it will always need to have effective, local decoupling anyway, and a decoupling cap at the opposite end of a power cable is useless.

author: fcb
author_rank: Super Contributor
timestamp: December 13, 2013, 06:25:03 pm
content: Are you testing at 110V and the EU lab at 230V?

author: Phaedrus
author_rank: Frequent Contributor
timestamp: December 13, 2013, 06:41:24 pm
content: EU lab has a Chroma AC source, and they test at both 230VAC and 115VAC.

author: Phaedrus
author_rank: Frequent Contributor
timestamp: December 13, 2013, 10:09:54 pm
content: The testing spec calls for both a 10uF and a 100nF capacitor at each test point. Turns out I had used 10nF by mistake. I put in 100nF caps and the transient went down! But is still there, and is showing even on known-good power supplies.

author: KJDS
author_rank: Super Contributor
timestamp: December 13, 2013, 11:35:45 pm
content: Are you using the scope in high impedance or 50 ohm mode?

author: Phaedrus
author_rank: Frequent Contributor
timestamp: December 13, 2013, 11:36:41 pm
content: 1 Meg.

author: Jay_Diddy_B
author_rank: Super Contributor
timestamp: December 13, 2013, 11:49:22 pm
content: Hi,Have you tried with the power supply under test plugged directly into an outlet?I don't know if your Extech AC power supply is either making a contributing to the noise or increasing the impedance in the ground lead.I like to use this set up for measuring ripple and noise. Since we a dealing with RF, this circuit is matched for 50 Ohms.Here is a picture of my test cable and the DC block. I use a 1uF capacitor because a standard (RF) DC block has too high a corner frequency.I do a check by connecting the signal and ground to the ground on the power supply output. I should get no signal, if I do there is common mode noise between the scope and power supply grounds.Jay_Diddy_B

author: Phaedrus
author_rank: Frequent Contributor
timestamp: December 13, 2013, 11:51:31 pm
content: Hi Jay,Switching between the Extech and various wall outlets was among the first things I tried. Unfortunately, this doesn't seem to be as simple an issue as mains interference.

author: Jay_Diddy_B
author_rank: Super Contributor
timestamp: December 13, 2013, 11:59:10 pm
content: Hi,I wasn't think that it was mains interference. I was think that the Extech AC source might be a switching, class D, amplifier type device, and contributing to the noise.Jay_Diddy_B

author: Phaedrus
author_rank: Frequent Contributor
timestamp: December 14, 2013, 12:33:46 am
content: Fixed it!Changed a number of things, but the biggest difference was decreasing the length of the electrolytic capacitor leads. Shortened each of them by 5-10mm (for a total length of ~10mm) and the ripple instantly dropped by 30mV and the spike almost disappeared. My readings now approximately agree with our Europe lab.Thanks all! Helped a lot!

author: juanfermed
author_rank: Regular Contributor
timestamp: December 14, 2013, 01:45:11 am
content: Great you fixed it, but does that mean the capacitor leads were picking up some kind of noise? or the inductance of the leads was causing that spike to appear? What was the cause of the problem ultimately? I am a total noob, cant guess. Thanks!

author: Phaedrus
author_rank: Frequent Contributor
timestamp: December 14, 2013, 05:00:24 pm
content: I think it was inductance in the leads. Why else would it coincide so perfectly with the peaks in the waveform? I'll need to make a spice model or something...