DC (battery) power input filtering / inductor selection - Page 1
Forum section: electronics / beginners

author: crypton
author_rank: Contributor
timestamp: June 01, 2017, 08:49:21 am
content: //Edited the subject which is now essentially How to choose correct inductor for the filtering.I dunno, am I overthinking this?« on: June 01, 2017, 06:49:21 PM »Quote« Edit on: June 08, 2017»Summary of some gathered info from this thread so far:-avcc-decoupling-inductor-or-ferrite topic- some useful info bits.-http://www.ti.com/lit/an/snva755/snva755.pdf- Appnote about the exact filtering at hand!Concerning the type of inductor:1) Mostly I've been suggested to useFerrite beadfor such filtering. BUT- What I have read so far is that Ferritebeadwould be a bad choice forhigh current, low frequency(500kHz fundamental)battery input filtering(expected max of 650mA @12V). Ferrite beads rated at that current will go into saturation and lose their inductance - thus filtering @500kHz is out of the window.- Also even on zero DC current, their impedance graphs go zero at 1MHz, meaning they are mostly meant for high frequency application.I am yet to see a ferrite bead that would be useful below MHz.MAYBE?But no definitive data below MHz.2) TI's appnote is usingan inductor with ferrite core. - increasing inductance with fewer turns and containing the field much more. Ok, lets see:- With ferrite core, dc bias current is still an issue. So I cannot just use2.2uH Multilayer Ferrite inductor, rated at lets say 800mA.So I would still use an inductor, but- Make adjustments so that I get enough inductance for the filtering even at high currents. f.i:(then ...A3R3W and ...M4R7W should be OK)Sure corner frequency goes higher but thats ok. Also DC resistance could be an issue.So am I correct to compensate for DC bias current? and not to use just ferrite beads?

author: SvanGool
author_rank: Regular Contributor
timestamp: June 01, 2017, 09:04:26 am
content: Looking at the circuit and the link to the component picture I would expect that L1 is a ferrite bead, especially because you have a higher frequent buck converter, then it is meant to keep your circuit frequencies out of the main input (conducted emission). Assuming that, I don't see a purpose for the diode although I would expect a reverse voltage protection diode in series with your fuse, also because the TVS is unidirectional then this would look like a fully normal automotive protection circuit.

author: crypton
author_rank: Contributor
timestamp: June 02, 2017, 08:29:17 am
content: Thanks for the answer. Indeed, looking for "automotive power filter", comes up with several similar circuits.But why ferritebead?I've since understood that using a ferrite bead for higher load currents is not effective at all (having filtering in mind).In my case I'd have to use approx 3A rated ferrite bead for to accomodate ferrite impedance for 625mA@12V dc bias current - OK doable.But other thought was that the LC filter has to have inductance and ferrite bead simply has not enough inductance at 500kHz to act well for filtering (without using large capacitors i guess).Or am I missing something?Other than that I think I'll go for the wirewound inductor as this has the least DC resistance (thus less load from the Lipo battery) but my question of interest was,IF I could use the multilayer oneand win in package size? And probably then diode is also needed?

author: SvanGool
author_rank: Regular Contributor
timestamp: June 02, 2017, 09:14:35 am
content: Because a ferrite bead has good impedance and DC resistance characteristics: the DC resistance of a TDK MPZ2012S221A(T) is 0.04 Ohm which is probably a lot lower than the fuse you are using and can cope with up to 3A of current. I think you are mixing up impedance and DC resistance.

author: crypton
author_rank: Contributor
timestamp: June 02, 2017, 12:12:43 pm
content: hmm, thanks. I'm not convinced though.I surely lack anykind of physical experience on this part and so rely mostly on others' experience and available application notes. Thus reading some notes and stackexchange post, which probably succeeds explaining the worries better than I am.http://www.analog.com/en/analog-dialogue/articles/ferrite-beads-demystified.htmlhttps://electronics.stackexchange.com/questions/115021/ferrite-bead-positionI haven't done much input protection and noise supression before, at least not in a measurable way. I have been moslty in the dark, things just work, wanted to implement my circuitry more properly this time and hopefully learn smth new.My issue is that if findferrite beaduseful mostlyabove MHz range, especially in their resistive state range, which usually is within 100MHz. My application has susceptible noise in thehundreds of kHz.So I need inductance in that frequency range (for kHz there isn't much) for the LC network to do its job. Ok, I can get low DC resistance, but no filtering from kHz noise. Yes?Add load current to the equation (>600mA @12V) and it gets worse (probably stays the same for sub MHz range, where nothing happens anyway).That is mostly why I don't see ferrite bead as a viable selection for this application, especially right on the main power (which will power everything else!) - I have currently no way to confirm this - only based on what I have read. Hopefully I haven't got it wrong...Maybe this clarifies my issues.

author: SvanGool
author_rank: Regular Contributor
timestamp: June 02, 2017, 04:04:27 pm
content: I gave you a part of my experience, you are free to make your own choices

author: crypton
author_rank: Contributor
timestamp: June 05, 2017, 09:09:44 am
content: Quote from: SvanGool on June 02, 2017, 04:04:27 pmYes, of course, I would gladly take this experience but I feel I didn't fully understand it.Like I don't understand WHY and HOW can be ferrite used in such application (as drawn in the first post)Okei, you say there are ferrites with such parameters but I haven't seen currently any chip ferrite bead that shows data below 1MHz thus I've concluded that chip ferrite beads, near these frequencies, are basically a jumper wire and provide no useful to filtering >500kHz (which is noise in my case).I would gladly use ferrite, to surpress higher freq. noise, but for me it seems it doesn't work from 500kHz.In relation to evb149 post:Quote from: evb149 on June 02, 2017, 04:47:48 pmI tried googling "low frequency ferrite bead" and DID come up with something:1)http://www.fair-rite.com/wpapers_anotes/ferrite-cores-for-low-frequency-emi-cable-suppression-2/Many of such searches were this toroidal types for cables. No go.2) Some chip form ferrites:https://www.lairdtech.com/product-categories/emc-components/ferrite-chip-beads/chip-beads-low-frequency?language=enBut again, no data to see on low frequency range as evb149 suggests. Although some have higher impedances starting from 1MHz, suggesting there is usable AC resistance below this range and could be useful, but none is shown exactly. See 35F0121-1SR-10 as an example.So what - I should Imagine, what values could be on my frequency of interest?Not to mention the bias current which can ruin the day.Quote from: evb149 on June 02, 2017, 04:47:48 pmFor this BLM series I only saw data on MHz range. Especially inthis catalog, page 23 f.i.. And again under MHz range the graphs are near zero.I tried calculating that for my application I would need 6.91 Ohm XL @500kHz (2.2uF) to make use of a ferrite bead. But I don't know how to transpose this to the 100MHz spec, that these ferrite beads use.

author: tatus1969
author_rank: Super Contributor
timestamp: June 05, 2017, 10:53:39 am
content: Quote from: crypton on June 02, 2017, 12:12:43 pmThere's one caveat here: the guys having made these application notes may have done it the same way, and if the original source of information is wrong, then that is copied over and over again. Power supply decoupling is such a story, along with the conception that adding some 100nFs + 10uFs to a digital circuit is always a good idea. There are countless others, and you'll find them frequently in datasheets and appnotes.My recommendation is to always go through the effort to understand the subject as deeply as possible, which enables you to sort the good from the bad.In this case: when I saw the circuit that you sketched, my first thought was: nice resonant circuit, that will filter all frequencies well, except one. And adding that diode is a rather helpless attempt to mitigate that. The correct solution was already given here: make the circuit dissipative. One way to do this is to replace the inductor with a ferrite, another one was discussed here recently:https://www.eevblog.com/forum/microcontrollers/avcc-decoupling-inductor-or-ferrite/msg1174716/

author: crypton
author_rank: Contributor
timestamp: June 07, 2017, 02:58:42 pm
content: Quote from: tatus1969 on June 05, 2017, 10:53:39 amWell, I meant notes regarding ferrite, it's characteristics vs inductor selection. Not about decoupling. But yes, I am aware that datasheets or appnotes can be buggy. But one has to start from somewhere...I come by this again and again and can't find anyone saying or showing that ferrite bead is a good solution filtering high current power paths (in my case 625mA @12V). Which I am trying to understand.My current understanding is that ferrite bead will saturate very quickly and filtering is out of the window. Thus I had one possible idea to add ~4x bigger current capable ferrite bead (~3A), to accomodate the saturation issue (have enough inductance at 625mA @12V).Quote from: tatus1969 on June 05, 2017, 10:53:39 amThanks for the thread. Got some useful insight along with many other stuff.Okei, here can be my culprit - "have dissipative circuit" - and I have totally missed seeing the thing as a tank circuit. Always saw that as LC, lowpass filter. Thanks!Thought about it - When with inductor I will amplify my noise at resonant and not attenuate much (or at all) on low frequencies, then ferrite does the opposite(?) - it attenuates only at the characteristic frequency (where it becomes resistive) and it's almost always around several MHz (since I haven't seen high current chip ferrite beads that show data under MHz range).That currently aside, I found an appnote (which I'm still reading and analyzing) regarding that exact filtering and it does have some points also brought out from the avcc-decoupling thread - concerning the needed dampening of the filter/LC to reduce the amplification on resonant - which seems to be the key issue here indeed (due to LC being a tank circuit).http://www.ti.com/lit/an/snva489c/snva489c.pdfORhttp://www.ti.com/lit/an/snva755/snva755.pdfwhich seems to give the same thing as a more concrete example based on one SMPS and thus one can see, what components have been used.There it only talks about inductors (that do have ferrite cores). But nothing about chip beads.btw I do use some ferrites on some RF/analog chips' power lines (which consume in order of tens of milliamps) which tend to be sensitive to ripples.

author: tatus1969
author_rank: Super Contributor
timestamp: June 08, 2017, 04:27:32 pm
content: Quote from: crypton on June 07, 2017, 02:58:42 pmQuote from: crypton on June 07, 2017, 02:58:42 pmQuote from: crypton on June 07, 2017, 02:58:42 pmFerrite beads work differently from inductors. They typically have only little inductance, they dissipate AC energy by utilizing the magnetic remanence instead. However, this is typically only effective at higher frequencies. You shoot at conducted or radiated EMI with them, and that only starts at 30MHz here in Europe.In your case, where you want to filter out lower frequencies, an inductor based PI filter as you proposed is working better, just add appropriate damping for the pole. If your requirement would be 100kHz ~ 1GHz for example, then you can combine LC and ferrite beads.