EMI board-to-board decoupling : ferrite or inductor? - Page 1
Forum section: electronics / projects

author: Halban
author_rank: Contributor
timestamp: July 01, 2021, 09:41:22 am
content: Hello everybody!I'm starting an audio project and I'm already thinking about FCC/CE and EMI emissions. I want to learn that scienceMy project uses 2 PCB : one for the HMI and the other is the motherboard.On the motherboard I have multiple PSU (12V, 3V3, 4V, ...). Some of these need to be carried on the HMI board, thanks to a ribbon cable, and the current can be high (up to 2A, powering for example PWM leds). So I guess that will create strong EMI. I learned that a good design need to be decoupled (with a LC filter) the power supply, on each boards.Attached you can find my +5V 2A switching power supply. The datasheet advised to use ferrites (L2 = 742792510, L5 = 7427932) for EMI improvement.My question is : on the other PCB, how to decouple this power? Is it better to use a ferrite (eg the same as L5), or an inductor (eg the reference TYA40126R8M-10) ?Thanks a lot for your help!Alban

author: Halban
author_rank: Contributor
timestamp: July 01, 2021, 09:47:37 am
content: PS: I have the same question for the +12V that uses a linear power supply (image attached). It's powering an OLED screen. Is an inductor better than a ferrite?

author: Andreas
author_rank: Super Contributor
timestamp: July 01, 2021, 08:19:56 pm
content: Hmm,in which frequency range do you expect the EMI.The ferrites are for frequencies above 100 MHz.According to data sheet they have ~100 Ohms AC impedance above 100 MHzwhich is low against the impedance of "free air" (377 Ohms)So the main effect is to improve the behaviour of the decoupling capacitors.A 6.8uH inductor also is a typical "UKW" choke.So more for the FM (100 MHz) frequency range.A inductor is always a compromise of size, current capability and price.And usually it is effective only over a very small frequency range.If your PWM is at much lower frequencies you will have to use much larger inductors.with best regardsAndreas

author: jonpaul
author_rank: Super Contributor
timestamp: July 02, 2021, 05:02:12 am
content: Bonjour a tous:1/ EMI filter design depends on the attenuation needed, bands of expected interference source and band the filtered side device is sensitive to, as well as the Zsource, Zload, etc.Has the OP considered all of these specifications?2/ The topologies used are often LC, RC, Pi, Tee, etc.3/ Beads are series inductors with intentionally lossy cores, and appear as a series R above a certain F. There are a wide range of ferrite materials and bead types to cover ranges from MHz ...GHz.we used the multi hold cylinders and the two hold oval balun types with good results, from TDK and Fair-Rite.4/ We have seen filters with beads, series L or low resistance R and no series L or bead, just bypass caps.5/ Regardless of the filter effectiveness, the filter is only going to attenuate conducted normal mode (or differential mode) noise and not common mode noise.6/ The boards and PSU can still radiate noise board-to-board AFTER the filter.7/ Professional, Aviation, MIL equipment uses shields, box within a box and feedthru filter and caps for best wideband noise attenuation.8/ Please consult the many fine reference texts on EMI filter design, for the engineering of EMI filters.For the application shown, I doubt if there will be much different in performance from no series..series L and a well chosen bead.9/ Any series L will self resonate at some frequency, so the inductance is only effective below the Fres.Hope this proves useful and interesting,Kind Regards,Jon(I had designed and mfg SMPS, HV PS and electronic ballasts about 45 years)

author: jonpaul
author_rank: Super Contributor
timestamp: July 02, 2021, 06:35:57 am
content: Hello again: Your Schematics have wrong topology, Vcc>>FilterCap shunt>>series L or bead>>IC Vcc pin.It should be Vcc>>>>Bead or L>>Shunt filter CIts an L-C filter and the bead in series with the IC is WRONG.Finally use a lytic in parallel with a 100 nF mon for best HF filtering.For these tiny currents I feel the series L or bead is not needed.Jon

author: Halban
author_rank: Contributor
timestamp: July 02, 2021, 03:09:32 pm
content: Thanks a lot for all these very interesting messages!I expect the EMI frequency range to be between 10MHz and 90MHz. So I guess I can use an inductor on the other board (the HMI PCB) as the frequencies are low? Then if I have strong EMI I can try different L values.(the circuit and its components references is the one advised by the manufacturer, so maybe it's better to no change it and to add filter on the other board side)--Thanks for the wrong topology warning, I will place the bead before the capacitor !What is the best technology for the 100 nF ? Film capacitors ?

author: jonpaul
author_rank: Super Contributor
timestamp: July 02, 2021, 03:22:59 pm
content: Cher Monsieur Halban, BRAVO pour tes efforts analog, et audio.a few moments please....1/ The EMI environment determines the filtering requirements, eg easy in linear PSU analog box, worse inside a PC with 500W SMPS, and really bad at a Broadcast transmitter hut.REALLY bad in industrial environments like factories.2/ What is the worse SORUCE of the EMI and what is the most sensitive RECEIEVER of the EMI? Is the Coupling via Conducted or radiated?3/ Most audio and digital audio boards use simple caps and NO EMI filters or even series L. A 10 ohm R is often fine in place of the L or beads.4/ You can use Faraday metal shields to reduce the EMI.5/ To learn more re EMI filters and electromagnetic shielding, suggest to read some classic books.6/ Look at some fine digital audio boards to see examples of good design.7/Copying the IC manufacturers app note or eval board, will result in repeating their mistakes....8/ 100 nF use an monolythic ceramic cap, film is not needed. We use AVX 25 ..35VBon chance,JonPS: Alors, je penses que nous habitons le meme pays:Vous habite près ou loin de Paris?VIVE LA FRANCE!

author: Halban
author_rank: Contributor
timestamp: July 05, 2021, 01:59:52 pm
content: Hello Jon,Effectivement, et j'habite dans Paris même, enchantéThanks again for your advices. I will buy a EMI book to go furtherAnd I will add the L footprints together with R footprints so I will maybe be able to see the effect with my spectrum analyser and some near filed probes.

author: jonpaul
author_rank: Super Contributor
timestamp: July 05, 2021, 05:45:22 pm
content: rebonjour Monsieur, je pars Paris plus tot, peut-être nous pouvons parlerons ou RDV...For audio/digital audio the interboard filter is the least problem, metal shields, PCB ground plane design and PSU isolation are the more critical for low noise high res digital audio.EMI ref books: Many are very old and costlyhttps://www.amazon.com/Electromagnetic-Compossibility-Electrical-Computer-Engineering/dp/0824718879dont have a list or photos at the moment.Can send better later in July.Bon chance,Jon

author: Halban
author_rank: Contributor
timestamp: July 09, 2021, 08:19:52 am
content: Hello ! Indeed we can meet, that could be interesting!Before that I will finish my PCB design, and then do some tests, I also ordered the french book "Guide pratique de la CEM" by Alain Charoy

author: Faringdon
author_rank: Super Contributor
timestamp: July 10, 2021, 04:24:18 pm
content: Are your PCBs all contained within a metal enclosure? If so, then you may be able to get away with very minimal LC filtering at the inputs/outputs of the SMPS…because the metal enclosure will stop radiated emissions from causing a big problem.So the first thing that anyone needs , so they can help you, is whether or not your product is contained within a metal enclosure?Also, it is needed to know what is your primary power source? Is it mains electricity, or is it battery?The AC input filter of the offline SMPS will be key to getting your product to pass EMC standards.If your product is supplied by a battery, then you don’t need to worry too much about high frequency conducted emissions…..because nobody else is likely to be using your battery for power……..so you are in your own “world”, and can do what you like regarding conducted emissions……but if you have high frequencys in your power current, then it will radiate from the circuit loops in your PCB layout…so filter it out……and have tight PCB layout.But , especially if you have a plastic (non metal ) case…….then you will need , preferably , common mode chokes at the input and output of your SMPS. Or you can use ferrite beads in the go and return. Common mode noise is what will cause your radiated emissions from 30MHz right up to 1GHz. Ferrite beads usually suffer drop in common mode attenuation when they have DC current levels in them, and this is where CM chokes come to the fore, because they keep their attenuation in such cases.But yes, you need to be sure your SMPS ‘s have diff mode L,C and common mode filters at their inputs and outputs.Remember also it’s the cables that will act as antennas….(as well as PCB traces, but not so much). So add eg a common mode choke , or ferrite beads at cable exits.Basically also, PCB layout of your smps will be important…..reduce switching power current loop areas. You need also enough diff mode filtration at the input to your smps’s in order that the input current is not too “spikey”…because “spikey” current will cause radiated emc problems…so LC filter it smooth.Remember also that an SMPS input filter should look like a low impedance to the SMPS. So your last filter component, just upstream of an SMPS, should be a capacitor…and preferably relatively big......---------------So anyway, if your product is mains powered, and has a metal enclosure, then you will have to solve the riddle of the “earthing” issue as attached. (choose either 1,2,3, or 4) That is, you need to Earth your secondary, but how you do it can affect the radiated emissions. You also need to connect your chassis to earth, but must not do so in a way which will cause radiated emissions.…..----------------------------------Also please find a PCB layout document….because bad PCB layout of SMPS will cause bad emc problems----------------------.................Also, in an SMPS , you need to identify the switching node, and minimise it in area, and shield it behind a plane of quiet node copper if possible, as the switching node will radiate emissions.Also, with high frequency common mode chokes for 100MHz plus, you need to use NiZn ferrite, and not MnZn, because MnZn is no good for really high frequencies.Best is to use common mode chokes that actually show you a graph of attenuation vs frequency in the datasheet

author: Halban
author_rank: Contributor
timestamp: July 12, 2021, 11:41:29 am
content: Hello, thank you very much for adding more advices!The enclosure will maybe not be made in metal. The AC/DC power source is a very popular external 15V 1A PSU from Meanwell.On my circuit 15V power input, I've added a filter that you can find attached. Does it look like good to you?

author: vad
author_rank: Frequent Contributor
timestamp: July 12, 2021, 12:27:12 pm
content: I would drop C20 and C45. They don’t improve impedance at high-frequency much, but can potentially cause HF ringing. C18 and C20 and their ESLs form LC oscillator. Same goes for C37 and C45 pair. Use low ESL 100nF capacitors (e.g. in wide 1210 package) for C18 and C37 instead.

author: Halban
author_rank: Contributor
timestamp: July 12, 2021, 04:07:33 pm
content: Thanks for this tip!I've tried to source some low-ESL 100n capacitors, but I didn't find this category and much information on the internet...Do you have a recommended part number?

author: vad
author_rank: Frequent Contributor
timestamp: July 12, 2021, 05:01:07 pm
content: Simply go for 1210 package from known manufacture. As a rule of thumb, all other things equal (dielectric, capacitance, voltage rating), 1210 has lower ESL than 0805, 0805 has lower ESL than 1206, 1206 is better than 0603.PS. Nothing wrong with C3225X7R2E104M200AA or 12101C104MAT2A.

author: Faringdon
author_rank: Super Contributor
timestamp: July 12, 2021, 08:30:03 pm
content: You have a front end filter in your meanwell, so that will take care of things for you.By the way, will your ground be earthed?...if so that will mean more noise, as you are earthing the secondary of an offline isolated smps.But basically you just need enough diff mode filtration to smooth out your current waveform.....and enough really high frequency common mode choking and/or ferrite beads in order to limit radiated emissions.You may even need to diff mode filter those DCDC's (i think you said you have some?)I mean if anything i guess you have overdone it with your shown filter, and yes as Vad says you can get filter resonance causing problems....but to be honest, this mightnt be such a bad problem for you.....if it is..put in a shunt rc to damp the rail. high frequency caps are good for reducing the area of switching power current loops.So anyway, you can make a cheap common mode filter by using a ferrite bead in go and return..but common mode chokes will tend to generally give you more CM attenuation.Basically, the key thing for you is radiated emissions to be reduced....and that will need small NiZn type filter chokes, and possibly shielding.Remember common mode chokes with distanced coils are better at high frequencies.........'cause they have less interwinding capacitance.You say your enclosure is plastic.....though there will be no chance of passing radiated emissions test if your circuit is in a plastic enclosure.It is one of the "Industrial secrets" of electronics product development that nobody will tell you...that is, that no product that contains SMPS , and wires/cables, will ever pass radiated emissions testing without being in a metal enclosure.....(it might be a "lattice" metal enclosure)..though a carbon fibre enclosure also does a good job of screening.

author: jonpaul
author_rank: Super Contributor
timestamp: July 13, 2021, 06:27:22 am
content: Rebonjour, High end audio, studio, broadcast, DIY, please precice the quality of the audio, what SNR/Resolution expected?In general audio powered by a switcher is not great.If the device is low power, preamp, EQ, etc consider a linear PSU, with a toroid, NOT a cheap Chinese switcher.What is the power required?Just a thought,Jon

author: Halban
author_rank: Contributor
timestamp: July 13, 2021, 08:46:15 am
content: Hi everybody, thanks again for all you tips!- in fact, it's not really an "audio" product that synthesize or record/play sounds, but it's more like a midi file player that produces no sounds. It's a 100% digital system, that embed a STM32F7, a 143MHz SDRAM, USB host and device, screens, SD card, lots of leds PWM-driven. So I guess that all these HF will produce EMI.- ground will not be earthed- the system will consume up to 1A @15VMaybe the above 15V PSU filter is overkill, same for the DCDC filters. But I wanted to try them on my first PCB so I can easily see if they are doing a difference. But maybe as they are, they can cause more issues?

author: jonpaul
author_rank: Super Contributor
timestamp: July 13, 2021, 11:13:47 am
content: The filters will do nothing if the device works and the PSU is approved and complies with all EMI and safety standards.Jon

author: Faringdon
author_rank: Super Contributor
timestamp: July 13, 2021, 05:13:35 pm
content: Too help with radiated emissions, you can shield the 5V 2A SMPS.Put a metal shield over it, and shield under it with ground plane.You can actually buy PCB fittable metal shields, which solder to your PCB, they are available in different sizes.Tight decoupling either sie of the SMPS with ceramic caps immediately nxt to smps in and out...and common mode chokes (only small) at in and out of smps....eg ACM4520 or similar.Enough diff mode inductance in input/output filter to smooth the crags off the current...thats all.And your cable exits/entracen to pcb....put common mdoe choke there......use shielded cable also........connect the shield to your cct ground...use the correct sheild....eg if you have high frequency comms cable, then shield it with some sheild that wont cause too much losses.What cables do you have coming/going off your pcb...what signals are in them?You take the ribble cable as GND/SIG/GND/SIG2/GND/SIG3....eg nice tight loopsRemember the cables coming/going to your pcb will be nightmare antennas......radiated emissions nightmare....so shielded cable.....even torroids and you take the cable through the torroid par example