Forums

Groundplane under SMPS power inductor

Started by John Devereux November 4, 2015
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
On 11/4/2015 11:33 AM, John Devereux wrote:
> > 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.
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
On Wed, 04 Nov 2015 16:33:30 +0000, John Devereux
<john@devereux.me.uk> wrote:

> >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,
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.
On Wednesday, November 4, 2015 at 11:33:34 AM UTC-5, John Devereux wrote:
> 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?
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.
> > 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
John Larkin <jjlarkin@highlandtechnology.com> writes:

> On Wed, 04 Nov 2015 16:33:30 +0000, John Devereux > <john@devereux.me.uk> wrote: > >> >>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, > > 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.
You think it might be removed to reduce induced noise currents? I was thinking the other way.
> > 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.
Me too so far.
> Take a candidate inductor and measure it free-space, then up against a > piece of copperclad. See if anything changes.
I can try that, good idea! Yay, an excuse to fire up the old 4192A :) -- John Devereux
On Wednesday, November 4, 2015 at 12:55:50 PM UTC-5, John Devereux wrote:
> John Larkin <jjlarkin@highlandtechnology.com> writes: > > > On Wed, 04 Nov 2015 16:33:30 +0000, John Devereux > > <john@devereux.me.uk> wrote: > > > >> > >>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? > >> >
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), makolber@yahoo.com Gave us:

>On Wednesday, November 4, 2015 at 12:55:50 PM UTC-5, John Devereux wrote: >> John Larkin <jjlarkin@highlandtechnology.com> writes: >> >> > On Wed, 04 Nov 2015 16:33:30 +0000, John Devereux >> > <john@devereux.me.uk> wrote: >> > >> >> >> >>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? >> >> >> >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
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.
On Wed, 4 Nov 2015 14:48:32 -0800 (PST), makolber@yahoo.com wrote:

>On Wednesday, November 4, 2015 at 12:55:50 PM UTC-5, John Devereux wrote: >> John Larkin <jjlarkin@highlandtechnology.com> writes: >> >> > On Wed, 04 Nov 2015 16:33:30 +0000, John Devereux >> > <john@devereux.me.uk> wrote: >> > >> >> >> >>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? >> >> >> >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
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  wrote:
> On Wed, 4 Nov 2015 14:48:32 -0800 (PST), makolber@yahoo.com Gave us: > > >On Wednesday, November 4, 2015 at 12:55:50 PM UTC-5, John Devereux wrote: > >> John Larkin <jjlarkin@highlandtechnology.com> writes: > >> > >> > On Wed, 04 Nov 2015 16:33:30 +0000, John Devereux > >> > <john@devereux.me.uk> wrote: > >> > > >> >> > >> >>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? > >> >> > >> > >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 > > 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.
Pot cores do have some flux leakage around the gaps left to let the wires come 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 usual way to wind a toroid, with the winding progressing around the toroid, creates 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
On Wed, 04 Nov 2015 18:11:36 -0500, DecadentLinuxUserNumeroUno
<DLU1@DecadentLinuxUser.org> wrote:

>On Wed, 4 Nov 2015 14:48:32 -0800 (PST), makolber@yahoo.com Gave us: > >>On Wednesday, November 4, 2015 at 12:55:50 PM UTC-5, John Devereux wrote: >>> John Larkin <jjlarkin@highlandtechnology.com> writes: >>> >>> > On Wed, 04 Nov 2015 16:33:30 +0000, John Devereux >>> > <john@devereux.me.uk> wrote: >>> > >>> >> >>> >>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? >>> >> >>> >>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 > > More likely to maintain minimum creepage/gap between circuit segments.
Not likely.
> > Pot cores... no effect. Toroidal... no effect. Therefore it must >be for protection rules/reasons.
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.