Split VS Common ground plane for switching converters - Page 1
Forum section: electronics / beginners

author: NaxFM
author_rank: Regular Contributor
timestamp: August 12, 2023, 03:44:01 pm
content: At work we need to develop a board dedicated to the measurement of the characteristics of 4 switching dc/dc converters. We have to measure different parameters (switching frequency, voltage accuracy, ocp, ovp and many more) and they must all share a common ground.Now we are debating wether is it better to have a big common ground plane shared between all the converters or split the ground planes and connecting them with a thin trace.There are many opinions and the info we find are quite controversial. Some say it's better to have one ground plane and other that it's better to have multiple ones.Of course the goal is to avoid the noise from one converter to influence the measurement of another.What is your opinion? Have you ever had a direct comparison between common and split ground plane?

author: Terry Bites
author_rank: Super Contributor
timestamp: August 12, 2023, 09:39:04 pm
content: In my opinion, it would be better to use split planes that connect them back to a single ground reference point. Currents take the path of least impedance and all other paths to a lesser degree. Controlling those paths is a better option than hoping a single ground plane will have a low enough impedance to swallow all the switching currents without creating unwanted potentials. That applies equally to the input supply rail. Each switcher will have local circulating currents and magnetic fields, can you be sure that they will not interact more strongly across an unbroken plane? It goes without saying that unclean grounding will increase emi and degrade regulation. I'd be watching out for power currents flowing along the same path as regulator ref and sense grounds.There is likely no right answer, it will depend on the application. What's right for the RF team may be irrelevant to the power team. I’d do some experiments to be sure.RF spooks hate (or love) split planes because the can become nice dipole radiators. Given that the wavelength of the switching frequency is very long compared to a slot in your ground plane it’s going to be an incredibly inefficient radiator. So, should you worry?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 12, 2023, 10:08:13 pm
content: Four converters simultaneously, or one at a time..?Are they literally just isolated converters for unspecified purpose? Or is this on a main board say for FPGA/SoC local supplies or something? Or what?Nothing about the parameters listed is in any way affected by plane structure, as aside from Fsw, they're DC measurements (and Fsw is low to modererate frequency, and doesn't require any signal quality to transmit).You've not made any kind of case for this, even being relevant, frankly -- let alone, rising to the level of needing the consideration of split planes.Split planes are like surgeries. Used carelessly, they result in permanent impairment or death of the patient outright. Used sparingly, in very carefully prescribed and considered circumstances, they can improve patient outcomes.Like surgery, split planes are almost never the answer. Placement can be improved, differential sensing used, shielding added, etc.Most of all, it's jarring to see a split-plane topic posted in the "beginner" section.If you have to ask, the answer is no!Tim

author: Smokey
author_rank: Super Contributor
timestamp: August 13, 2023, 07:48:12 am
content: Quote from: NaxFM on August 12, 2023, 03:44:01 pmthin trace?Have a look at the LM25148 datasheet layout section:https://www.ti.com/lit/ds/symlink/lm25148-q1.pdfThey are isolating the power supply control and switching stuff, but there are big connection points at the input and output of the planes....

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 13, 2023, 12:13:30 pm
content: Note that the above layout commits the sin of connecting to opposite sides of an island -- guaranteeing whatever ground loop current spans that region, is also carried by the surrounding circuit. Vin and Vout need to be routed to the same side, and carried on the same isthmus, for the peninsula to do its job. Needless to say, any connections to that header carry full common mode (ground loop voltage); to be quiet, they would have to be individually filtered and/or CM-choked (whatever is appropriate for their signal bandwidth) on their way across the ground slot, or the isthmus extended so they can cross over it as well.Tim

author: wraper
author_rank: Unknown
timestamp: August 13, 2023, 12:18:56 pm
content: It's an example of extremely poor datasheets/app notes. Yes, many of them are an utter garbage despite coming from large reputable companies. This literally makes not sense, it does not even really avoid ground loops, it just creates antennas for no good reason.Quote from: Smokey on August 13, 2023, 07:48:12 am

author: wraper
author_rank: Unknown
timestamp: August 13, 2023, 01:57:32 pm
content: Suggest watching at 30 minutes about app notes.Quote from: DavidAlfa on August 12, 2023, 04:07:48 pm

author: Smokey
author_rank: Super Contributor
timestamp: August 14, 2023, 02:18:27 am
content: Part of this is that it's so hard to quantify the actual differences between layouts for anyone that hasn't spent a ton of time in the EMC world and doesn't have the equipment and facilities (and budget).Take that TI app note:Say I have examples of both layouts working on the bench. What would you expect the functional differences to be between a solid plane and the split plate as shown in the app note, and how would a normal person in their non-professionally-EMI equipped lab be able to quantify that? What would be the minimum equipment required to do those measurements consistently and reliably and how many dB are we talking?

author: Smokey
author_rank: Super Contributor
timestamp: August 20, 2023, 11:37:33 pm
content: .... anyone?

author: wraper
author_rank: Unknown
timestamp: August 21, 2023, 12:01:21 am
content: The simple answer is that if you don't really know you need to split ground planes, and don't have an equipment and knowledge to verify impact, you should never split it. Not splitting ground plane is the safest option and splitting it is only advantageous in niche applications with extremely low noise required, particularly at low frequencies, like in audio equipment. Without really knowing what you're doing, splitting ground plane will be a detriment in 99% of cases. It may be OK either way, but cases when the result is not OK will be overwhelmingly prevalent with split GND. And anecdotally I've dealt with enough problems in very expensive commercial equipment caused by split GND, and needed to try all sorts of mumbo jumbo with reconnecting split GND planes to make that equipment behave in long term. It's no fun fixing equipment that operates 24/7 and IC SCR latch-up problem randomly re-appears after a month to a year and power cycle makes it go away. Replacing the circuit board does not help and thus eventually you start pulling hair out of your butt trying to fight that.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 21, 2023, 01:32:53 am
content: Quote from: Smokey on August 14, 2023, 02:18:27 amFunctional? In what application? In and of itself? Probably nothing. Maybe the voltage tolerance or line/load regulation would be slightly different (this would have to be measured against the same part(s) soldered into the board, or matching parts chosen, or by statistical sampling sets of each version). Mostly the difference would be in common mode noise, and how that interacts with whatever system it's integrated into.You're asking some conflicting things: understand that, a "normal person" (no, I won't take thatthatliterally; take an average EE, perhaps), that "hasn't spent a ton of time in the EMC world", will not be very well prepared to do much testing at all, even with all the equipment in the world. (Like, what are all these buttons on the VNA anyway? Who the hell is Smith and why is he a chart?)We would also need some metric on which to gauge the testing. How reasonable is the test -- is the test setup representative of some end application, or typical use-case? (This might depend on standards, it might be highly subjective, or it might be stipulated by the client.) How consistent is the testing, perhaps -- will that person be able to set up the same test the same way on multiple occasions, and obtain equal (within some reasonable margin, a few dB perhaps) results each time? How about when using different equipment each time: is the equipment calibrated and adjusted properly for the test setup?Conversely, a knowledgeable operator would know to ask the questions necessary to assemble a well-defined setup, and consult whatever relevant standards or intuition might apply in pursuit of that. For best results, suitable equipment will be needed of course (test chamber, ground plane if applicable, networks, antennas and other coupling devices, attenuators, calibration, and a test program to run on the machine*), but if a more representative measure is acceptable, it could be as simple as some LISNs on a ground plane with scope or spec measuring the output, with peaks corresponding to ambient noise (such as broadcast radio picked up from free space, in lieu of a test chamber) ignored, or perhaps filled in through alternative methods (perhaps better filtering of cables entering the area, or changing its orientation).*For example, typical susceptibility tests are done at stepped frequencies, pretty closely spaced; close enough that you likely wouldn't avoid notice if your unit has some strong and sharp resonance somewhere. Likewise for emissions; which is done with a couple detector methods (usually average and quasi-peak, with peak often handy as a faster, worst-case stand-in for QP, which is slower to measure), with given receiver bandwidth (9 or 100kHz usually).Even looser, perhaps one could simply probe voltage across the board (literally between connectors); a dubious method as it's path-dependent (the fields around the SMPS affect wires through that space), but perhaps by wiggling things around a bit, one finds a certain minimum noise level that can't be canceled out by merely moving wires around. And perhaps even without LISNs, one might check input, output and CM noise levels, knowing that there are resonances in the system (due to the lack of impedance stabilization; literally, the impedance are unstable, with peaks and dips all over), and by checking the waveform itself for apparent ringing, related to networks composed of wiring, connected (PSU or other aux. equipment), and onboard elements, one builds a picture of what the overall performance is like....But I think you can see, all of these methods incur greater and greater assumptions, demanding greater and greater knowledge of how signals move through wires and space, and how networks behave when combined together; and how to determine what aspects of the system are responsible for those network behaviors (like cable inductance or board capacitance).There is no simple answer, and there cannot be a simple answer -- EMC is a holistic subject, that demands comprehensive knowledge of the system under test (at least to such depth and breadth as the precision of measurement, and extent of networks across the system, entails), in order to be able to work with it. We use ground planes and filter networks (of lossy type, like LISNs, but also snubbers, ferrite beads, and so on) because they are easy to apply, and easy to reason about -- you don't have to think about what's on the far side of the plane, it's a solid surface, and has minimal voltage drop across it. Difficult to reason about, are extended, highly coupled systems -- consider a mesh of connectors between various boxes in free space, fields could be going every which way, including into the air and into neighboring (but not adjacently wired) loops.It's not a subject that can be studied through a brief question-answer format (as you have more-or-less designed your comments in this thread). Or to say perhaps, a Socratic method. This is problematic, because there is far more context around a given question, than an asker will even think to mention; and each answer (say, standing by itself in isolation, in response to the given question, ala the Stack format) has to be exceptionally long to even begin to touch on the nuance of all that surrounding information -- and indeed, at best grasping at (and at worst, chiding for) that omitted context and supporting information; likewise a series of such questions are highly redundant because so much of the "well, figure out all the other stuff first" kinds of parts end up equivalent.And all that contextual stuff probably starts to sound like a lot of noise, to the unfamiliar ear.The most clear answer to any one EMC question can only be: take the time to learn everything you can about the subject. We can help you along, but we can't feed you a river of knowledge; you must absorb that yourself through diligent work.It's an imposing topic, and a complex one to be sure, but I hardly think it's insurmountable. Some people certainly can grasp it. Many struggle to. Still many others are, somewhere between ignorant to it (i.e. just not familiar with it at all, their capability is untested), or demonstrably incapable of grasping it (at least from what effort they've put in / been exposed to).I'm far from a professionally trained teacher; I have no idea if some topics are inherently unteachable, to some people, or truly anything and everything is universal, given enough work (even if perhaps delivered at the right time; or perhaps truly, in general, at any age). In any case, it seems likely this is one such topic where, at the very least, it is more difficult, and will take more conscious effort to pursue.I don't know how you learn best -- if you can speak more to that effect, the meta, how you seem to learn in general, how you feel about new things, how you like to approach topics, and what actually works -- perhaps we can offer some comments to that effect?For my part, I feel comfortable with EMC, having spent many years reflecting on it -- and far fewer actually testing things, to be perfectly honest (for which one might rightfully temper that "comfort" with a bit more skepticism..!). I can say I'm formally trained in fields (physics and EE undergrad level); though it's been a while since I worked a raw fields problem of any depth, I'll admit. That quantitative reasoning went far to develop and strengthen my intuitive understanding of the subject; and several projects I've worked have developed it further (and more experimentally, i.e. in regards to real product testing and design).Also for my part, I struggle to describe these topics; I can hold all the relevant facts in my mind, at least for a moderately complex but still well-behaved system, but language itself lacks the expressiveness to convey anywhere near the complete cluster of facts necessary to describe a given setup. At least, short of writing endless paragraphs (which, you'll notice, happens often). Which of course succumbs to the inevitability of the reader not paying attention, skimming past relevant facts, etc. (It takes two to communicate; if I can't be concise enough for a given reader, I'm not going to get read by that reader.) Conversely, when experts discuss on the topic, they share much common ground: underlying assumptions that can be brought up and acknowledged with relative ease; but which seem to the unfamiliar observer to be unimportant details, or esoteric, disconnected information.I can at least say that, I have literally never been checked on such meta topics (theory of mind, teaching approach, etc.) before -- at least, not that I can recall, on this forum, in regards to this or most other EE subjects -- so in that sense I get to blame all of you for my chronically poor communication, if that's the case with respect to any given reader of my posts. But that would be demanding an awful lot. And, when one simply isn't even aware of meta-topics like this, one can hardly bring them up as a topic of discussion. Well, here I am trying again, perhaps someone will bite, this time?......But like, seriously guys, is this all just noise? Does any of this make sense? Is this helping anyone, and if so, can you express how? Perhaps others can become aware of these ways of understanding, perhaps we can all polish our talents for learning here?Idunno.tl;dr:I would ask -- what do you really want to learn from this conversation here? If you are lacking critical dependencies, no amount of questioning on a superseding topic will cure that deficiency. Try not to ask perfectly formed, reasonable questions; I mean, do communicate clearly, and use reason, but I mean more like: ask about things that are on the margin of your knowledge.Like, for my part, understanding RF port theory completely turned my mind from the casual "oh that's a bunch of RF black magic" line of thinking (which, I think, I never really accepted as a true statement, but in lieu of anyone discussing the topic in any kind of detail, any further understanding wasn't going to happen on its own), to a far more trivial matter -- mind, I say "trivial" but intuition always feels that way, and I don't say this to elevate myself, simply that intuition is how *I* picked it up, and perhaps you'll always have a more conscious or mechanical approach (say, piece out the equivalent circuits and work through the impedances manually); I don't know.Ports, I would consider a core prerequisite; without which, LISNs for example don't make any sense -- well maybe you have some idea that you should have a resistance into the filters going in/out of some piece of equipment, but you won't appreciate how that matters to transmission lines (cables), and common mode over the ground plane.And the fact that a ground plane is also a reference plane, meaning we can make a point-like measurement upon it -- and what dimensionality even means, what pointlike (a group of connections which can be analyzed without regard to speed of light) or linear (transmission line) or 3D (full fields) situations mean; and why this is important to our test equipment (aha, all test equipment is connected via cables -- 1D transmission lines; which terminate at a reference plane -- a point -- the input connector!).And then perhaps you'll appreciate when a plane isn't a plane (say, because it's full of traces and vias and holes, with an SMPS stuck inside of it?--), and why it matters how we measure around something, and why we might generalize a measurement, like to stick some EUT over a GND plane and measure CM and DM signals into and out of it...I think most EEs will have bits and pieces of this sort of understanding, but it's the totality of them all together, the synergy, that composes an understanding of RF, or EMC; the information connecting across the topic is as broad and highly-coupled as the systems they often study!Tim

author: Marco
author_rank: Super Contributor
timestamp: August 21, 2023, 05:56:36 am
content: Quote from: NaxFM on August 12, 2023, 03:44:01 pmMost of this discussion seems to be using the question as an excuse to discuss grounding in general again rather than for this oddbal application.This doesn't seem necessarily a practical circuit, but more a testbed which compromises isolation for easy testing. Which would have multiple probes with multiple ground clips attached at the same time for each converter? Which would explain the "must" sharing of ground. If so I think star would still work fine, regardless of multiple ground clips but I could be wrong.