Inductor between ground plane and chassis ground - Page 1
Forum section: electronics / projects

author: dariotzvir
author_rank: Newbie
timestamp: January 13, 2022, 04:43:54 pm
content: Was checking a smart PDU (I guess thats the formal name) and noticed that all the mounting holes for the chassis were connected to the ground plane via an inductor, my guess that they are for filtering some EMI or nasty transient that could happen when you first connect he device since I have seen similar problems when working with a grounded chassis.The question is: Is it worth it or good practice or is it just for the sake of it, since I´m very new at the layout busniess.And for a bit of trivia: somebody messed up the list of the plugs jajajaja.

author: Leo Bodnar
author_rank: Frequent Contributor
timestamp: January 13, 2022, 06:40:23 pm
content: It's a ferrite bead. If you don't know (or can't explain exactly) what you are doing, connect the groundplane to chassis directly!

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 13, 2022, 07:49:19 pm
content: A direct chassis connection is good in a lot of cases, but some examples where it might fail: if the connections have too much stray inductance / line length, or there's too much distance between them, or the operating frequency is too high relative to those length scales: then there will be resonant modes, at relevant frequencies. These EM modes are analogous to the mechanical modes of the same system: that is, the board is fixed at the mount points (which carry current/force) and acts as a sprung plate inbetween (with voltage/displacement maximum in the ~middle between mounts). So, the boundary conditions (mount points) give 1/2 wave as the lowest mode, with bending shaped like a hammock. Similarly the E-field between PCB and chassis goes like the displacement of the hammock between its mounts. (But actually it's a sinusoidal envelope, not a catenary. So the higher modes have additional nodes between the mounts.)Since the board probably has significant width, there will actually be a mess of modes, as different sides bounce with/against each other, much as vibrations on a bowed plate (Chladni patterns). This doesn't help much with analysis (you pretty much have to simulate it, if you need to know the fields), but illustrates the analogy, and if nothing else, confirms that as long as you're working at lower frequencies, you don't have to worry about this very much.So, for spacing of say 10cm, the first mode will be on the order of 1GHz. A lot of things won't need to worry about that, so, yeah.So, when these modesareimportant, it may pay to dampen them by introducing loss to those mount points. Ferrite beads have the property of lossy inductance, usually being about as much resistance as inductance*. Downside, this allows some voltage drop between nearby board area and chassis (you'remaking a worse ground), but potentially less than would appear around the antinode (the midway area where the E field is strongest).*Which can be described in equivalent ways: Q = 1, X_L = R, Z ∝ sqrt(f). The rate of impedance change is necessarily linked with the phase angle of the impedance; R and X are not free variables, but linked by this condition (Kramers-Kronig relations).You can consider the situation with respect to number of ground points:For a single ground point, the board acts as a capacity hat over the connection's inductance (and various other modes at frequencies above there). If the ground point is in the middle, there's probably a dipole mode as well (opposite ends of the board move up and down in/out of phase with each other, while the ground point remains ground-ly). If at an end or corner of the board, then a 1/4 wave resonant mode where the ground point acts as ground, and the board acts like a whip antenna.For two ground points, if at opposite ends of a long board, it acts as a loop or folded dipole, with nodes at the ends (ground points) and antinode in the middle (1/2 wave mode). If one of the mounts is lossy, then it acts as a parallel termination for the 1/4 wave mode of the single-ground case, and series termination of the double-ground case.And so on up; for three points, there can be a width dimension between them, and there are modes for each edge of the triangle thus defined; as well as higher order, waveguide modes between the board and chassis. Four points, more sides, more pairs of points -- more sides -- that can have resonant modes; the frequencies go up (because there's less distance between any given pair of points) while the modes get more complicated. Note that there can be low-loss modes between a given pair of direct connections, while other modes are suppressed by other connections having damping (resistance or ferrite bead).Galvanic isolation is a separate issue. Note that all of this happens at very high frequencies, far removed from mains frequency or DC. If the circuit can't be direct-grounded because of isolation requirements at low frequency, then capacitors can be used to achieve adequate RF grounding.Hence why you see so many small (often blue) capacitors in power supplies and stuff.And why sometimes those have ferrite beads on their leads.So, predicting the application of these elements, not so straightforward. Knowing the lowest resonant modes, is a good start at least, and you an prioritize any connections you expect to need a lower impedance connection on.For example, in the pictured case, it might well be that a low impedance is preferred for that nearby connector, as it may be subject to ESD transients for instance, which should be quickly sunk to chassis. OTOH, if it's already sunk to ground (the metal shell sits inside a faceplate with EMI springs/gaskets?), it might be better to leave some impedance to the mounting hole, to dampen resonant modes between the connector/shell and PCB. There might be hard grounds elsewhere on the board, or paired with some damping themselves. (Like, maybe the diagonally opposite corner is damped, and the other two corners are hard tied or something.)Tim