lowish frequency EMC filtering - lowest ESR or highest Capacitance - Page 1
Forum section: electronics / projects

author: Simon
author_rank: Unknown
timestamp: March 19, 2020, 02:37:51 pm
content: I am looking to design a filter. The frequencies of concern are hundreds of kHz to a few MHz. The item clearly does not have much bulk capacitance. I have a 35mohm ESR 1500µF capacitor that is sizable and 2x 68µF 35mohm capacitors to give lower ESR even without a lot of capacitance.Problem is this is becoming quite bulky. I'd like to drop something. What do I go for, leave the big capacitance with pretty low ESR or the keeping 2x 68µF caps for half the ESR.Basically is 1500µF over the top and not helping but less than 35mOhms a better bet. This is a 5A fan motor.

author: Kleinstein
author_rank: Super Contributor
timestamp: March 19, 2020, 03:05:29 pm
content: Filtering needs more than just a low ESR capacitor. The usual filter needs at least also an resistor or inductor. The more practical filter is something like a T or Pi topology, so 2 caps and an inductor or the other way round.For the capacitor one may get better of combining 2 caps. An electrolytic cap for the lower frequencies and a relatively small ceramic one (with low ESR) for the higher ones. 1500 µF is not that much for 5 A current when it comes to the low frequency end (a few kHz).The inductor may be as bulky as the caps

author: Simon
author_rank: Unknown
timestamp: March 19, 2020, 03:36:29 pm
content: I am indeed putting in pi filters, it's a case of I have to have inductors, they are the big chunky things so one for common mode and one for differential. But the capacitors have more options to play with as like you say i can use several.I have a number of ceramics, they are not bulky items. I am looking first to the bulky items which are the large and medium caps. Basically if 1500µF is not going to help sat behind 0.035 ohms ESR then I can take that out and put a smaller one in. In the area of concern i have 2x 68µF and 1x 1500µF all 0.035R ESR so maybe I should just make that 3x 68µF and save some room while keeping the ESR.

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: March 19, 2020, 04:22:34 pm
content: I've had good results using Oscon capacitors for noise/EMC compared to regular aluminum electrolytics. You have to bench test as data sheets don't reveal everything.

author: Zero999
author_rank: Super Contributor
timestamp: March 19, 2020, 04:37:54 pm
content: One thing which is often forgotten is filters also often need resistors, as well as inductors, otherwise they can resonate at certain frequencies, making things worse, rather than better. Often discrete resistors aren't required, because the inductors act as resistors, at problem frequencies, but it's something to watch out for.

author: AndyC_772
author_rank: Super Contributor
timestamp: March 19, 2020, 04:46:34 pm
content: The trouble is it's hard to know the electrical equivalent circuit of the noise source. What's its ESR, and how does it vary with frequency? Chances are you'll never really know, so it's hard to model.Do you have a spectrum analyser, and could you use that to test out the effectiveness of any given filter design in your own lab before you spend money at an EMC facility?Bear in mind that the effectiveness of an EMC fiter is, more often than not, down to its layout and construction as much as it is the component values that are used. It's all too easy to demonstrate that a design "should" work, but the parasitics introduced when you build that design into a real filter mean it really doesn't work well.

author: Mr Evil
author_rank: Regular Contributor
timestamp: March 19, 2020, 05:34:24 pm
content: Quote from: Kleinstein on March 19, 2020, 03:05:29 pmBe careful doing this. The capacitance of the smaller one will resonate with the self-inductance of the larger one, and capacitors with low ESR are worse as they provide less damping. If you're unlucky, that will happen at exactly the frequency you're trying to filter out, giving worse results than no filtering at all.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 19, 2020, 06:31:31 pm
content: What does it connect between?Tim

author: Simon
author_rank: Unknown
timestamp: March 19, 2020, 08:08:11 pm
content: Quote from: T3sl4co1l on March 19, 2020, 06:31:31 pmA fan motor and it's supply.

author: Simon
author_rank: Unknown
timestamp: March 19, 2020, 08:20:56 pm
content: I popped some resistors in just in case.

author: Simon
author_rank: Unknown
timestamp: March 19, 2020, 08:22:25 pm
content: Quote from: Mr Evil on March 19, 2020, 05:34:24 pmIt's on the end of a piece of wire so there is inductance/resistance from the load to the filter. Basically, suck it and see. I keep warning them that until we do one and test it we have nothing to go on.

author: trobbins
author_rank: Frequent Contributor
timestamp: March 19, 2020, 10:39:35 pm
content: Is the filter aimed at an attenuation spec, or some other kind of benchmark performance, or just suck it and see?I don't see any identification of a supply - is it dc or ac, and what is it? We could presume you are describing electrolytic caps, and used in a polarised circuit, but there are other plausible presumptions.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 20, 2020, 08:21:19 am
content: Quote from: Simon on March 19, 2020, 08:08:11 pmYeh, what kinds? Is it filtering supply noise or brush noise or what?Tim

author: Simon
author_rank: Unknown
timestamp: March 20, 2020, 08:47:46 am
content: It's a 24V vehicle supply running a little 120W fan. We aim to stop the interference from the "noisy" fan motor going up to the supply.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 20, 2020, 09:32:48 am
content: Does that need to be at the fan, then, or where the cable comes into, I guess you have a box you're making that it goes to--? How long is the fan lead? What supply emissions level is required?Quote from: Simon on March 19, 2020, 08:22:25 pmWell heck, don't wait up, measure it yourself if you can! Get a ~5uH choke, 1uF coupling capacitor into 50 ohm scope or analyzer, and see what it does!Once you know the impedance, source intensity, and load intensity, you can design a filter.Tim

author: AndyC_772
author_rank: Super Contributor
timestamp: March 20, 2020, 09:51:43 am
content: On a 24V vehicle supply, every device on the bus should already be designed to cope with much, much worse than the noise from a fan motor. Have a look at the ISO pulse tests that apply, they're quite scary.You may not need a filter at all.

author: Simon
author_rank: Unknown
timestamp: March 20, 2020, 04:47:51 pm
content: Quote from: AndyC_772 on March 20, 2020, 09:51:43 amNo you don't understand. We supply the fan (with the rest of the equipment) and they fit it to a vehicle. They then expect it to pass the military EMC standards so a bit more stringent than your average auto stuff.

author: trobbins
author_rank: Frequent Contributor
timestamp: March 20, 2020, 08:33:31 pm
content: Why wasn't that info provided at the start of the thread?What standard, and what particular clause and outcome are you aiming for?

author: Simon
author_rank: Unknown
timestamp: March 20, 2020, 09:23:58 pm
content: Well I was not asking for specific details on how to design the filter but on the priciple of how various capacitor combinations might work. I had to be explicit as it was suggested that i did not need a filter.Def Stan 59-411 land class c or mil std 461

author: trobbins
author_rank: Frequent Contributor
timestamp: March 21, 2020, 03:33:48 pm
content: Have you done a pre-test with your widget/fan using LISN and spectrum analyser for conducted emissions from 30Hz to 10MHz, and compared against the compliance requirement levels for CE101 and CE102 in Mil-Std-461 ? I would have thought that would be your starting gun, from which you then started to design any additional filtering requirement.

author: Simon
author_rank: Unknown
timestamp: March 21, 2020, 04:44:17 pm
content: Yes which is why I am asking about the behavior of the electrolytics in the 100kHz to 3MHz range. That is where we cross the line.

author: Mr Evil
author_rank: Regular Contributor
timestamp: March 21, 2020, 05:21:06 pm
content: To answer the original question then, I would expect a 1500µF capacitor to be inductive at hundreds of kHz and up, so the 68µF capacitors are likely to be better. The more diigent manufacturers have charts of impedance vs. frequency in their datasheets, so you can see where they start to become ineffective.

author: Simon
author_rank: Unknown
timestamp: March 21, 2020, 05:31:56 pm
content: Why do you say the higher capacitor will be more inductive? Is this due to the sheer size of it so longer plates and paths for current to flow?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 21, 2020, 06:22:55 pm
content: Use the impedance curves, when you can find them, or estimate them from known parameters.A filter is an impedance network, doesn't really matter if the elements are strictly capacitive or inductive. No true capacitance exists in real life, after all. An inductor divider for instance is still a divider.Electrolytics are typically modestly capacitive at low frequencies, <Hz to ~kHz. Midband, ESR dominates. At high frequencies, ESL dominates (>100s kHz), and there's probably some higher level structure up there too (electrolyte physics? body stub resonance in the 100s MHz?).The capacitance is given. ESR is usually given, or impedance at 100kHz or something like that. Possibly it can be reconstructed from ripple current ratings, too (but I would suggest, just keep shopping to find one that is rated for impedance/ESR).ESL isn't usually given, but is typical for package size -- lead length dominates. 10s of nH.Knowing this, we can already approach some misconceptions --Quote from: Mr Evil on March 21, 2020, 05:21:06 pmThis may be true, but impedance magnitude is more important than phase. The 1500uF cap will have much lower ESR, and not much more ESL. If ESL is 10nH and you add a 1uH filter choke between a voltage source and the capacitor, the asymptotic attenuation at high frequencies will be around 0.01 / 1 or ~40dB. That's nothing to sneeze at.Even better, use multiple in parallel, with the traces routed such that signal enters from one end, goes through the capacitors as a ladder network of their own, and out the other side. This puts some trace inductance between capacitors, further dividing the high frequency response even though the trace inductance may be tiny (5 or 10nH) and puts their ESLs effectively in parallel. Well, better than parallel, because it makes a cascaded impedance divider (ladder network). Most times, this is a tiny or negligible impact (because ESR dominates, or ESL is a few times larger than the trace inductance), but it can be better at basically no cost, even if the advantage might be fractional dB.Where I need wideband filtering, I use a cascade of lowpass filters with increasing cutoffs. For example, 1uH into 100uF (electrolytic), then 0.22uH into 10uF (ceramic), then FB (might be ballpark 0.3uH || 30R near cutoff) into 0.1uF. This improves the shortcomings of each proceeding stage with the next, so the attenuation at very high frequencies can be essentially total.If the offender is at low frequencies, such lengths will not be necessary, and merely using appropriate capacitors will suffice. That may require larger or smaller values. Whatever works, shop around, do simulations.Note that we're mostly discussing differential filters. This hasn't been explicitly mentioned very often in this thread. Such a high attenuation filter isn't very useful, as common mode noise will quickly dominate, or radiation directly off the PCB. The solution is to address common mode in the same way you would address differential or normal mode noise, taking a ground reference plane (preferably a metallic enclosure, but a ground plane PCB will work in a pinch) and filtering with respect to that, using paired connections where necessary (e.g., data pairs where the differential bandwidth can't afford to be filtered so heavily, or beefy power supply lines for which a current compensated choke is cheaper).Tim

author: Vovk_Z
author_rank: Super Contributor
timestamp: March 21, 2020, 06:52:34 pm
content: Try CLC filter. One C can be bulk low-ESR aluminium cap (1500 uF is ok), other one can be ultra low ESR solid polimer cap >=47 uF.L - about 10 uH as starting point.

author: trobbins
author_rank: Frequent Contributor
timestamp: March 22, 2020, 01:04:25 am
content: Some more queries that imho go to scoping what kind of filter could be appropriate, given you are indicating using electrolytics and have an issue up in to the MHz range:Can you describe the AC current waveform and amplitude drawn from the 28VDC supply, or provide a spectrum plot?Can you identify the likely noise generation mechanism within your fan?Does the fan have any internal filtering devices, or any location to retrofit any filter related parts (eg. a terminal box)?How close can any outboard filter get to the fan power terminals ?Does the fan have a metal casing?Is it the aim to deploy an outboard retrofit filter, that is physically and electrically bonded to the fan?What is the temperature range that the filter could be subjected to?