I have seen designs where there is a cutout in the groundplane under the power inductor in e.g. a buck converter. Inductor is nominally a "shielded" one.
I assume it is to prevent some kind of "shorted turn" effect? What do you think?
Might a continuous plane be better? It could help to shield any field leakage and reduce emissions and circuit noise.
I am asking generally, but say 1A, 500kHz.
Thanks,
-- John Devereux
Chip makers like Linear Tech often provide excellent information in their data sheets and app notes. I seem to recall the idea is to minimize injected currents into the ground plane to reduce noise, but it has been a while since I've done any SMPS design.
-- Rick
That's not a lot of power.
A poorly shielded inductor could squirt some grould-loop voltages into the ground plane, which might matter in some systems. I doubt that any "shorted turn" effects would be significant.
We mix switchers and low-level stuff, but we keep them as far apart as possible, and don't cut the ground plane under inductors.
Take a candidate inductor and measure it free-space, then up against a piece of copperclad. See if anything changes.
I saw the shorted turn effect with an unshielded open ended inductor used in a resonant circuit. The ground plane under the inductor reduced the Q from ~35 to ~20.
I'm not sure about SMPS... I guess it depends on how much of the loss is in the inductor.
George H.
You think it might be removed to reduce induced noise currents? I was thinking the other way.
Me too so far.
I can try that, good idea!
Yay, an excuse to fire up the old 4192A :)
-- John Devereux
if it is a magnetically shielded inductor, then it should not matter
if it is an open magnetic circuit inductor, then it could matter.
Mark
On Wed, 4 Nov 2015 14:48:32 -0800 (PST), snipped-for-privacy@yahoo.com Gave us:
More likely to maintain minimum creepage/gap between circuit segments.
Pot cores... no effect. Toroidal... no effect. Therefore it must be for protection rules/reasons.
People take great liberties with the word "shielded", but most such surface-mount inductors leak more field out the top than the bottom.
-- John Larkin Highland Technology, Inc picosecond timing precision measurement jlarkin att highlandtechnology dott com http://www.highlandtechnology.com
On Thursday, 5 November 2015 10:11:46 UTC+11, DecadentLinuxUserNumeroUno w rote:
:the
oPot cores do have some flux leakage around the gaps left to let the wires c ome in and out, and there's a - small - current loop between the pins that terminate the windings.
Non-progressively wound toroids don't have any leakage fields, but the usua l way to wind a toroid, with the winding progressing around the toroid, cre ates a single turn in the plane of the toroid, which will leak flux into an adjacent ground plane, and induce a circulating current.
-- Bill Sloman, Sydney
Not likely.
It's most likely an old wives tales. Some inductors aren't closed magnetically (or aren't done well) so there is some chance of inducing a current in any metal under the core. One of the engineers where I work does things like this but I find there's more danger in compromising the ground plane than any current induced in ground. The only place I delete planes is where it's specified by the part's manufacturer.
If it does, I think generally I *want* it do to that, so that the metal acts to confine the field.
That's what I was thinking...
The other thing is whether to do the thing with a separate island of ground for the local switcher return currents, and connect this island to the main circuit ground plane at a single point.
[ SMPS Parts] ----------------- local GND----------------''---------------------- main GND
-- John Devereux
Yep, hence my wording "nominally sheilded". It is easy to probe the leakage field around the system, with a small wire loop on the end of a coax and a scope. (Someone suggested a SMT inductor instead of the loop, might have been you).
-- John Devereux
On Thu, 05 Nov 2015 07:56:48 +0000, John Devereux Gave us:
Copper does not contain a magnetic field.
A faraday cage does for AC generated emissions, but merely attenuates it.
mu metal is needed to "steer" a magnetic field.
A copper plane on a PC board? No.
Again, it is likely to gain a proper "nearest distance" thing, and could keep stray currents from being injected into a power or ground plane. Note that traces are likely managed around such elements as well, and said ground plane exclusion probably has a lot of vias ringing the area as well.
Yes it does at high frequencies. I have made measurements, quite fun and not too hard to do actually. Small coil on the end of a 50ohm coax going into a sensitive scope. Trigger the scope from the SMPS, wave the coil around the board. You can see the signal, and also the field direction (by orienting the coil differently).
At 50Hz, sure, but a copper plane makes a big difference at 500kHz+.
-- John Devereux
te:
er the
do
.come in and out, and there's a - small - current loop between the pins tha t terminate the windings.
ual way to wind a toroid, with the winding progressing around the toroid, c reates a single turn in the plane of the toroid, which will leak flux into an adjacent ground plane, and induce a circulating current.
At the frequencies and performance in use today it very well could be a pre caution against proximity effect and the associated loss.
A lot of the surfmount "shielded" inductors are drum (H-shaped) ferrite cores, wound and then dropped into a ferrite tube. That makes a ring-shaped air gap at both ends and allows lots of field leakage. That's symmetric, except that the board end sometimes has a plastic base which, with the solder joint, spaces the part a bit above the board. I tried one, sitting on a PCB with a layer 2 ground plane. It lost a couple per cent of its free-space inductance on the board in its normal mounting position, maybe 5% if pushed onto the board upside down.
I think there are surface-mount power inductors with better shielding. Pot cores and toroids are more expensive than shielded drum cores. Unshielded drums leak huge fields.
We don't usually worry about it. On the other hand, it's really convenient to buy little potted dc/dc converters, cheap and all done. They sit above the board and usually have toroidal inductors inside. That can rip a lot of parts off your BOM. I sometimes use them even when I don't need the isolation.
It sure can block an AC field.
The familiar old slug-tuned inductor-in-a-can that used to dominate radios, had no other barrier to flux leakage downward, and it'd be a good idea to give it a groundplane. For a power inductor, though, any nearby conductive plane will change the leakage flux (and that can have deleterious effects, if the internal flux geometry changes as a result). I'd leave out the copper pour unless the manufacturer suggests it.
So, the reason to put more copper in: block leakage flux (i.e. shielding) The reasons to put less copper in: energy lost to heating, ground plane induced voltages, and interference with 'nominal' magnetization in the core
Copper isn't going to do much to confine a magnetic field.
I always do that, with the input and output capacitors, and switches connected to the island. Note that the ground plane isn't compromised at all.
Nearby copper changes the ringing in discontinuous mode by quite a bit. Maybe two semicircles of copper dampens a certain unwanted frequency.
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