Do I need shielded chokes for noise suppression - Page 1
Forum section: electronics / projects

author: Simon
author_rank: Unknown
timestamp: November 18, 2018, 09:53:12 pm
content: I am working on a project where we have common mode noise and differential mode noise. So i will use common mode inductors and single inductors. Do these inductors need to be shielded considering we have to undergo radiated noise tests? the system is inside a metal box so i suspect most of it is being conducted out rather than any radiated stuff getting out.

author: nctnico
author_rank: Super Contributor
timestamp: November 19, 2018, 12:21:38 am
content: No. But just make sure the noise isn't coupled into the wires through the ground plane.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: November 19, 2018, 02:21:10 am
content: Doing what, and how?EMC is a holistic study. It is impossible to say anything without full details of the system in question.Also, mind that ferrite beads saturate at fairly low currents, so you may not get the impedance you were expecting, using one on a power line. Better to use a CMC, or a real inductor.Tim

author: coppercone2
author_rank: Super Contributor
timestamp: November 19, 2018, 02:22:28 am
content: well if your gonna slap some parts in it I figure your gonna get better performance from a shielded choke the only thing it will have is like slightly higher parasitic coupling to ground/shield but its probably going to be almost nothing. I think you could take it into account if your doing a cable drive budget with how many pF it can drive but I am not sure actually, since its energy loss from the charge pump usually.The good precise old HP instruments I have all use shielded chokes.

author: Simon
author_rank: Unknown
timestamp: November 19, 2018, 07:36:40 am
content: Quote from: T3sl4co1l on November 19, 2018, 02:21:10 amI am looking at this one:http://katalog.we-online.com/pbs/datasheet/7448680140.pdfI have a problem in the 80-100MHz range. Because my boss is an idiot he thinks he can batch test multiple projects for EMC which means I just get a result back of a failure on the standard test and have no idea where the noise is actually coming from.I was thinking of using that part as a differential choke as well putting the coils in parallel or would series be better?

author: coppercone2
author_rank: Super Contributor
timestamp: November 19, 2018, 01:43:45 pm
content: that's pretty high. When I did these tests I was a bit worried about the wire past 30MHz (I thought it would be good to do a conducted scan to 100) but I was told not to really worry about it, 90 is really up there as far as conduction goes.Are you doing the military test with a LISN that goes up to 100MHz? Did your boss really hook up a bunch of products in a EMC test chamber to a single power strip?? That's insane. Try just varying a few assemblies (i.e .disconnect the fan or pump), remeasure and you will see how chaotic the spectrum is.If its a power rail or AC do you have room in your chassis to put a classic big-ass inline EMI filter (like one of the chassis mount one) as a test then progressively downgrade it until you find the thing that does the job as far as cost and size? If it almost works then put a common mode ferrite on the cable at the right frequency range with a few loops. Some datasheets provide information about what happens if you line up their EMI filters in series too, but its sparse and you have severe diminishing results. Like a small one for the entry cable (integrated into the fuses and cable jack) and then a larger one in the chassis for instance.How are you installing that choke when experimenting in the lab? I recommend solder shrink sleaves and making pre-made cables to make it a job you can do in 1 days rent.

author: Simon
author_rank: Unknown
timestamp: November 19, 2018, 02:03:03 pm
content: yea it's full military standard test, we go up to 1GHz I think but after 100MHz the signals drop off and the test allows for more noise anyway. 30-100MHz is usually the bug bear.No the testing was done one device at a time or the test house cannot certify anything but in one day several devices are set up and tested one after the other and pass/fail. What i need to know is how this particular device fails and to have some RFI investigation done so despite the test I am still flying blind as I never see the test or get to ask for anything specific.

author: Simon
author_rank: Unknown
timestamp: November 19, 2018, 02:04:50 pm
content: Quote from: coppercone2 on November 19, 2018, 01:43:45 pmThis choke is being designed onto a new PCB

author: T3sl4co1l
author_rank: Super Contributor
timestamp: November 19, 2018, 06:33:03 pm
content: A choke doesn't tell me anything about where you want to put it!Tim

author: Simon
author_rank: Unknown
timestamp: November 19, 2018, 07:00:19 pm
content: Quote from: T3sl4co1l on November 19, 2018, 06:33:03 pmhttp://katalog.we-online.com/pbs/datasheet/7448680140.pdf

author: T3sl4co1l
author_rank: Super Contributor
timestamp: November 19, 2018, 07:22:02 pm
content: Quote from: Simon on November 19, 2018, 07:00:19 pmYeah I saw that... what were you intending to do with it?This datasheet says exactlyzeroaboutyour application.If you just throw it in wherever, it may well make things worse. I have no idea!Tim

author: Simon
author_rank: Unknown
timestamp: November 19, 2018, 07:27:48 pm
content: Well I am 10-15dB over the limit on 80-105MHz, we believe the noise to be common mode.

author: nctnico
author_rank: Super Contributor
timestamp: November 19, 2018, 08:06:34 pm
content: Quote from: Simon on November 19, 2018, 07:00:19 pmThis seems to be a mains choke. Due to the air gap it will have both common and differential mode suppression. But as Tim suggests you have to put it in the right place.

author: Simon
author_rank: Unknown
timestamp: November 19, 2018, 08:10:53 pm
content: oh so that graph has both characteristics at the same time. That helps as I also need some differential filtering as well.I am supplying DC, and to the outside world anything but DC will be detectable.

author: coppercone2
author_rank: Super Contributor
timestamp: November 19, 2018, 08:48:23 pm
content: Are you measuring this noise with a 100MHz LISN or are you measuring it with the 30-somethinghz antenna in the room?I never saw a LISN that went past 400MHz.

author: coppercone2
author_rank: Super Contributor
timestamp: November 19, 2018, 08:49:54 pm
content: When I did superposition of the curves that were supplied to me with chokes/filters from their datasheets I got pretty close results actually (for tighter grade of test). My theoretical ended up working with a few minor tweaks and some help and it passed.Check the stupid stuff like paint masking, chassis fit, tightness, washers, etc.If you are just doing a PCB its different ,if you have a chassis its worth to bring a bag candidate filters and stuff to test.

author: Simon
author_rank: Unknown
timestamp: November 19, 2018, 08:54:47 pm
content: Radiated and conducted tests are carried out, we failed radiated.

author: nctnico
author_rank: Super Contributor
timestamp: November 19, 2018, 10:35:23 pm
content: In that case you'll need to figure out which wires carry the offending frequencies.

author: Simon
author_rank: Unknown
timestamp: November 20, 2018, 07:48:57 am
content: Well I have some fans that are nasty little buggers and I control their speed based on the input to be constant (read tacho adjust speed input). They generate all of this noise being shitty little brush-less fans which I have recomended we don't use but despite being right on two accounts already they still won't listen. They are also not very durable so I am having to regulate the power supply to not exceed 28V (my infamous low dropout regulator/oscillator). I don't know if the noise simply passed through my voltage regulator or if that in turn oscillated with the fan supply line. My new regulator will have significant damping as a reaction time of 1ms is fine and I don't want it strumming away at 90MHz in time with something in the load and passing that out of the box.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: November 20, 2018, 07:53:39 am
content: It's.... fans?Did you try ferrite beads on their leads?Tim

author: Simon
author_rank: Unknown
timestamp: November 20, 2018, 08:00:11 am
content: Quote from: T3sl4co1l on November 20, 2018, 07:53:39 amNo I'm shoving a filter PCB right up the back of them so that the wire connector is essentially now a PCB connector so that I get some bypassing as close as possible to the power inputs to stop the supply leads radiating all over the place.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: November 20, 2018, 08:35:00 am
content: That's a good plan.That works best on metal shell, brushed motors (I'm a bit incredulous that a "brushless" fan can be as noisy?), but I suppose it'll still help here.But that wouldn't be with the 16A choke, would it? Or are these somebig fuckingfans, not just little cooling fans?!Tim

author: Simon
author_rank: Unknown
timestamp: November 20, 2018, 09:09:54 am
content: The fans are small, yes they are a horrible brush-less design probably because they are so compact they hardly have room for bypass capacitors. They are so bad that with any amount of filtering starting them up at full supply voltage with no soft start means that they crowbar the power supply and drop out and then repeat, sitting there oscillating (mechanically). So now my voltage regulator has to be controlled by the µC to bring the voltage up slowly, they are a fucking pain but I have to use them.I was planning to put two of those inductors in parallel on my power input to the ECU/voltage regulator as I need 20A constant draw and then further filtering right up the back of the motors themselves. I could put those on or I could use what I already have, I need to look at specs and probably go through the Wurth website to see what they have in the 8-12A range that might be better than the TDK thing I used that has very little to say about it. The filter PCB screws to the back of the metal motor body with the screws going onto chassis ground pads on the PCB so that return to chassis is as short as possible.

author: Simon
author_rank: Unknown
timestamp: November 20, 2018, 09:22:23 am
content: On the motor filter board i used these:https://product.tdk.com/info/en/catalog/datasheets/cmf_commercial_power_acm1513_en.pdfbut I have no idea how to relate the impedance to the dB on the wurth part

author: coppercone2
author_rank: Super Contributor
timestamp: November 20, 2018, 05:03:26 pm
content: I have seen a choke attached to a fan in a 80s spectrum analyzer but i always thought it was brushed lol, maybe notFans and pumps suck ass in terms of emi. Also you end up needing to design qhat i call cheap trash interface circuits.So you control the fan speed by hacking the tacho?

author: Simon
author_rank: Unknown
timestamp: November 20, 2018, 06:09:28 pm
content: The fan has a 0-5V input with a pullupand an open collector tacho, so the fans are not closed loop controlled (because they are shit) so with a change in supply voltage or other conditions the speed varies. So I do the speed control loop in my ECU. But the fan drive electronics is noisy.