Filtering noise on power inputs - Page 1
Forum section: electronics / beginners

author: Simon
author_rank: Unknown
timestamp: May 16, 2017, 09:14:39 am
content: I need to filter out noise cause by a SMPS so that it does not propagate up the power input cable. Would I be right in using a pi filter ? I don't think a common mode choke would help here.

author: capt bullshot
author_rank: Super Contributor
timestamp: May 16, 2017, 09:33:48 am
content: Well, it depends ...SMPS usually produce output ripple - you can filter that using a pi filter or similar L/C (don't forget damping)SMPS usually produce common mode noise - you can filter that using a common mode choke (remember that most common mode chokes requires an impedance from its outputs to "clean" ground to achieve damping at typical SMPS switching frequencies)You can fight higher frequency components with the usual lossy ferrite stuff, but they won't work well at the fundamental switching frequency, so you should first investigate the kind of noise to be filtered.

author: Rerouter
author_rank: Super Contributor
timestamp: May 16, 2017, 09:35:56 am
content: Ok, to begin, what frequency(ies), what amplitude is the current noise, what mode of noise is it? (common, single ended),What amplitude does it need to be reduced to, to meet your spec,What kind of resistance can you afford in line with your supply?

author: Simon
author_rank: Unknown
timestamp: May 16, 2017, 09:44:26 am
content: I have an upstream wiring loom that I am trying to not turn into an areal. I have 100KHz spikes on the line. I don't understand how a common mode choke will help because these are current draw spikes aren't they ? they go up the positive and back down the negative so surely a common mode choke just lest the diff noise through ?

author: capt bullshot
author_rank: Super Contributor
timestamp: May 16, 2017, 10:04:45 am
content: With 100kHz spikes, I'd agree they are differential mode on a typical SMPS. Place some L/C filtering at the output of the supply.Otherwise (I don't know the term "wiring loom" in this context, and I suppose you meant aerial, not areal?), if the power and its return conductor are placed close together or twisted pair, this 100kHz ripple wouldn't be a concern in terms of radiating noise.Edit: understood "wiring loom" by using dict.leo.org. So if power and its return is in a bundle of wires, it should be close enough to not excessively radiate the 100kHz either way.

author: Simon
author_rank: Unknown
timestamp: May 16, 2017, 10:07:40 am
content: My problem is the wire that our customer will use to power the system and under which we have no control could become an aerial. So I need to apply some input filtering

author: capt bullshot
author_rank: Super Contributor
timestamp: May 16, 2017, 10:17:47 am
content: OK, customers wanting to use unspecified wiring is the engineers worst nightmare, I have to admit ...Still I wouldn't bother too much about the 100kHz ripple, beeing more concerned about the higher frequencies, and common mode noise.I you suspect the customer laying out a loop antenna by placing power and return far apart, then a pi filter or L/C filter surely will help to suppress the 100kHz ripple.In many other cases these big clamp-on lossy ferrites help to filter the common mode noise. Every SMPS emits plenty of common mode noise that may turn a perfectly arranged twisted pair power/return construction into an aerial - this happens at frequencies around and above 1MHz (not at the fundamental 100kHz switching frequency).If the system allows, grounding the SMPS output to its safety ground input may helpEdit: sorry, didn't read correctly - you're talking about your SMPS _input_ that may turn your customers wiring into an antenna?Input filtering to eliminate the reflected ripple is quite the same as I wrote above, just the other way round :-) Some kind of Pi filter will help you to dampen this, or just increase your input capacitor. Input ripple also won't radiate if the conductors are reasonably close together.But common mode noise may be more a concern, for the same reasons as above.

author: tszaboo
author_rank: Super Contributor
timestamp: May 16, 2017, 10:21:49 am
content: Please dont "assume" the kind of noise you have. It can be common mode, differential mode or bit of both. It can change if the powered device is plugged in with data cable to something else. Measure it. I actually expect it to be common mode, SMPS usually does that with the stay capacitances.

author: Simon
author_rank: Unknown
timestamp: May 16, 2017, 10:58:41 am
content: So we have a metal box with gear in it an a device with a SMPS, that obviously due to it's switching generates noise upstream and gulps of current are drawn down a pair of wires that could be long. Only this device is powered on these wires so are current going in on positive must be coming out on negative ? so a common mode choke will just help that happen. This is in a vehicle and the vehicle is not used as a power return.

author: tszaboo
author_rank: Super Contributor
timestamp: May 16, 2017, 11:21:08 am
content: No. Current only goes like that, if you put your DC glasses on. If you put your AC glasses on, current will flow into a capacitor. You have a capacitor between the ground (earth) and the metal box. Current will flow into that.I also think you have trouble with the term "common mode". It does not refer to current directions, it refers to the method of the noise. If you have a reference plan (like a GND plane on a PCB) and one signal that is the only time, when you can say that it is not common mode noise. You have two signals, positive, negative and the GND. Did you measure, if there is noise between the negative and the GND?

author: capt bullshot
author_rank: Super Contributor
timestamp: May 16, 2017, 11:28:29 am
content: Okay, I think I got your problem.I'd like to call the "gulps of current" "reflected input ripple current". This is nothing unusual and happens with quite every SMPS. Usually, you have at least an input filter capacitor to deal with that. An additional L/C will reduce this ripple. And yes, the current going in on the positive end shall be returned on the negative end. Assuming a reasonable wiring (not forming a loop antenna), this ripple won't do much harm (in terms of radiated energy). In reality, part of the input current gets diverted by the switching action through stray (or placed on purpose) capacitances to the chassis or the output side of the SMPS or both. And yes, a common mode choke places a high impedance into this parasitic path, so this unwanted current flow is reduced. You must ensure that there isn't any other low impedance return path for DC (like a grounded chassis connected to input ground), this will render the common mode choke useless. Say, from a DC view all current must flow into the circuit through one winding of the choke and all current must return through the other winding - the sum of both must be zero (at least for DC).BTW: too much reflected input ripple may harm the power source (by placing additional ripple current onto its output capacitor that it might not be rated for) - but that depends on your system, or may cause unwanted ripple voltage on the supply rail. So input filtering on a SMPS to reduce the input ripple is always a good thing

author: Simon
author_rank: Unknown
timestamp: May 16, 2017, 11:40:26 am
content: From the power input connector through to the SMPS it is all isolated from ground (chassis) The power supply is a battery on the vehicle.

author: capt bullshot
author_rank: Super Contributor
timestamp: May 16, 2017, 11:55:21 am
content: That's fine so far. If your device is low power (less than a few amps), I wouldn't be concerned about the input ripple. Maybe you won't need any additional filtering at all, if your device is a "dead end" (there are no other inputs or outputs). If there are other connections, where cables or other devices may be attached, these may form an aerial together with the power input wires.I'd be more concerned by the vehicles requirements on robustness for your SMPS.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 16, 2017, 09:24:55 pm
content: At the exit of the SMPS, add a filter on each line, against chassis ground.This filters both lines with respect to each other, and filters both lines with respect to ground: controlling common mode.The filters should be designed for nominal impedance (50 to 100 ohms, say), probably using a voltage-source input (single terminated) filter prototype, and probably having termination or impedance equalization added to the output (an R+C to ground, and a series R||L to the output wire).If you know the differential mode noise is low, you can join the inductors from the two wires onto a common core, and now the DC current cancels out so the core can be much smaller (it's just a regular CMC, now).If the SMPS does not have a metal chassis to ground to, find the nearest "ground" on it that acts the same way, i.e., as a ground and shield and cage. (Note this may be the input side ground!)Tim

author: Simon
author_rank: Unknown
timestamp: May 17, 2017, 06:34:44 am
content: I think connecting anything to ground is a no, no as it is the vehicle chassis is a Faraday cage. once you allow it to carry currents you have lost that line of defence as it becomes another conductor. So I connect my stuff to ground, someone else does the same and because one of us it drawing a large current our powers and grounds are not at the same potential, now you can have a couple of volts across the vehicle body: new aerial, or that is how I see it anyway, and I'm still not allowed to connect anything to ground/chassis.

author: tszaboo
author_rank: Super Contributor
timestamp: May 17, 2017, 11:34:12 am
content: Quote from: Simon on May 17, 2017, 06:34:44 amWow, this is shocking. Every car, the battery negative terminal is connected to the chassis. A bunch of electronic devices dont even have a negative wire going to it. Ever counted how many wires go to a spark plug? If I have to jumpstart my toyota, the manual said, to connect the positive to this special terminal in the fusebox, the negative wherever I see exposed metal. Of course it is going to conduct, and of course you have noise problems if you ignore the basics.

author: Simon
author_rank: Unknown
timestamp: May 17, 2017, 11:44:24 am
content: The chassis is not a negative conductor, it is a screen. I don't make these rules and not everything works on car rules. What they do on the engine I don't know or care. I am not allowed to use chassis as ground. Everything has a ground/negative wire. So any conducted emissions due to spiky current draw would ave nothing to do with the chassis.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 17, 2017, 08:22:06 pm
content: Quote from: Simon on May 17, 2017, 06:34:44 amNo. I defined what I meant by "ground". Please read again, carefully. I specifically noted that it should not be vehicle chassis, but the power supply chassis (or whatever internal connection serves the same purpose).Tim

author: Simon
author_rank: Unknown
timestamp: May 17, 2017, 08:53:15 pm
content: So essentially an LC low pass filter on input and output.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 17, 2017, 09:16:14 pm
content: Yes. Filter the input and output (+ and -) with respect to "ground", where "ground" is just a point connection that's handy to terminate the filters on.It is important that it's a "point", or "pointy" enough, topologically speaking (i.e., a ground plane), so that there is no RF voltage difference between the wires, and the points the filters are terminated into.Tim

author: Simon
author_rank: Unknown
timestamp: May 17, 2017, 09:19:58 pm
content: Well the nearest access I have is the input wire. It will be very board as close as possible.Sent from my Moto G (4) using Tapatalk

author: cvanc
author_rank: Frequent Contributor
timestamp: May 17, 2017, 11:35:28 pm
content: I work with equipment that has a Corcom-style line filter installed backwards, to help prevent internally generated noise from "backing out" of the AC power input. Is that what you mean? Perhaps this might help?Good luck with your project.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 18, 2017, 04:18:07 am
content: In a 50 ohm system -- which is what filters are usually quantified by -- it doesn't matter. s12 = s21 in a linear passive filter.For a source or load at other impedances, the attenuation depends on orientation (because neither side is necessarily matched anymore).So, it depends on what the offender is like, impedance-wise.The CM impedance of a typical SMPS is fairly high, because the noise source is capacitor coupled. For that, you'd have the filter's Y caps facing the PSU. OTOH, add some Y1 caps to GND, and the impedance drops sharply at HF, so you'd want the CMC side facing.Or maybe the other way around, because your offending frequency is in the middle, where the impedance is flippy-floppy (because inductance and capacitance, okay?).Tim

author: Simon
author_rank: Unknown
timestamp: May 18, 2017, 06:42:38 am
content: I'm thinking of making up a pi type filter. This is 24+V DC into a 10V step-down converter. Every time the SMPS chip switches it generates noise upstream that couples into anything in the harness. I don't know what the 10V out is that also powers other small devices in the system that are connected to the ECU circuit.So as I see it the SMPS is wanting to take short sharp gulps of current every time the internal mosfet switches on. So with a low ESR capacitor close to the input this would supply that initial demand that otherwise sends shock waves up the input wiring as the wiring inductance is unhappy with a sudden demand for current. Putting an inductor in line with the input will of course exacerbate this behaviour but keep it localised in the inductor and not over my 2m loom plus the length the customer attaches to it. In a sense the inductor I insert will for a voltage divider with the loom inductance as far as noise voltage induced in the loom goes bearing in mind the whole thing is in a metal box so irradiation is not a problem at this point. A further capacitor in parallel completing the pi filter will of course offer a further buffer to large current demands that still make it through the inductor.I don't know what the output looks like and need to have it on the bench and apply some input filtering before I over analyse it but for sure some filtering won't hurt and as we are only talking 250mA all round here the parts are small and cheap and I can afford to be paranoid.My loom is being rebuilt with twisted pairs wherever possibly required but hopefully after power filtering this will just be a nicety and I'm having to remake it anyway due to other changes.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 18, 2017, 10:00:53 am
content: It's not about transient current demands, or energy balance or whatever -- the capacitors onboard the PSU handle that. They are made large enough so that the voltage change is small (say, <5% of total), and therefore the energy change is small as well.After that first cap (or several in parallel, as the case may be), it's just AC small signal filtering: the reactive power is very small indeed.(That's not to say an energy balance iswrong, which it most certainly is not -- but it's not the most productive viewpoint, or analysis method, when you're talking about microscopic fractions of the total energy being switched.)It also doesn't address common mode noise (when applicable), which is due to induced voltage or current between wires that should otherwise be "common", or "isolated", as the case may be. The induction is usually from the switching edges, not just the sloshing-about of bulk energy. (Two different aspects of the same thing -- the high frequency "edge" transients don't change much with load, while the energy-sloshing clearly does.)Tim

author: Simon
author_rank: Unknown
timestamp: May 18, 2017, 10:45:16 am
content: Well what I am trying to deal with is what will be conducted up the loom to the vehicle fuse board and battery transmitting to all in sundry in the process. As it is if the can cable is not twisted it will induce enough noise to cause problems.Is there any value in splitting the inductors in two and putting half on the negative?Sent from my Moto G (4) using Tapatalk

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 18, 2017, 10:25:25 pm
content: Yes, filtering the negatives addresses common mode.Technically, for N leads, N-1 of them need filtering, and the remaining one then happens to be "ground" both inside and out. That works particularly well when the "ground" is a Faraday cage around the device. If it's not shielded, then filtering all N still helps.Tim

author: Simon
author_rank: Unknown
timestamp: May 19, 2017, 06:54:47 am
content: So I also have a higher power device that would be nice to filter. The maker have given me a nice big toroid that I can get 200uH of inductance on. Now he is talking just normal pi filter but if I do two windings one for neg and one for pos on the same core would this help ? obviously they need to be coupled in a non common mode choke way as I am trying to kill off the current spikes not help them through. I don't have room for two of these so it's two coils on one core or just a pi filter.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 20, 2017, 01:53:17 am
content: Is the toroid ferrite or powdered iron?Tim

author: Simon
author_rank: Unknown
timestamp: May 20, 2017, 07:38:33 am
content: Ferrite I think, it's certainly very solid, how can I tell the difference, it'a painted green and teal coloured on side, I don't know if that is some sort of colour coding for the material.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 21, 2017, 01:01:28 am
content: Check the listings:http://www.micrometals.com/materials_index.html(Also compare to pictures of the core types, so you can check if your color perception is right. The green and blue are usually somewhat pastel, which may be throwing off your perception.)Sounds like #52 powdered iron.I don't know of any bi-color ferrites, but it could be whatever, and painted whatever...Tim

author: Simon
author_rank: Unknown
timestamp: May 21, 2017, 06:17:20 am
content: It's a toroid (going to make some nice beefy 40A 47uH inductors with them) Yes pastel colour green and a spray of pastelly dark green or light teal on one face. They were supplied by the guy that supplied the load I am making the filter for so I'd trust his judgement as he preferred I wind my own with the materials he sent than use off the shelf stuff.

author: Simon
author_rank: Unknown
timestamp: May 22, 2017, 06:55:31 am
content: I seem to remember them being sent to me with 52 written on one of them so I assume that was the material type.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 22, 2017, 07:06:40 am
content: Should be okay then.If you have enough, you can use one for each connection, independently, and not worry about coupling or anything.The permeability is rather low to use as common mode chokes, so that's out.Differential mode chokes are good, but they don't filter common mode very well (gee, y'think?..), which is often the bigger offender.Here's a T300-26D with some wire on it:#52 is only slightly better than #26. Both are about as good for inductors as lead is for making bells...That means, this particular circuit has to run at a fairly low frequency, and use a lot of extra inductance, to avoid burning up the core. But for noise filtering, you won't have to worry about heating, and the loss can be an advantage, honestly (free dampening!).Tim

author: Simon
author_rank: Unknown
timestamp: May 22, 2017, 07:18:00 am
content: I think the fundamental operation of this device is 20KHz or there abouts. It already has a lot of input capacitance and I'll be adding more so it should not be too harsh, this device may actually be quieter than the SMPS in the ECU.