krw <krw@nowhere.com> writes:> 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.If it does, I think generally I *want* it do to that, so that the metal acts to confine the field.> 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.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> The > only place I delete planes is where it's specified by the part's > manufacturer.-- John Devereux
Groundplane under SMPS power inductor
Started by ●November 4, 2015
Reply by ●November 5, 20152015-11-05
Reply by ●November 5, 20152015-11-05
John Larkin <jjlarkin@highlandtechnology.com> writes:> 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.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
Reply by ●November 5, 20152015-11-05
On Thu, 05 Nov 2015 07:56:48 +0000, John Devereux <john@devereux.me.uk> Gave us:>If it does, I think generally I *want* it do to that, so that the metal >acts to confine the field.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.
Reply by ●November 5, 20152015-11-05
DecadentLinuxUserNumeroUno <DLU1@DecadentLinuxUser.org> writes:> On Thu, 05 Nov 2015 07:56:48 +0000, John Devereux <john@devereux.me.uk> > Gave us: > >>If it does, I think generally I *want* it do to that, so that the metal >>acts to confine the field. > > Copper does not contain a magnetic field.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).> 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.At 50Hz, sure, but a copper plane makes a big difference at 500kHz+.> 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.-- John Devereux
Reply by ●November 5, 20152015-11-05
On Wednesday, November 4, 2015 at 7:10:32 PM UTC-5, Bill Sloman wrote:> 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, SydneyAt the frequencies and performance in use today it very well could be a precaution against proximity effect and the associated loss.
Reply by ●November 5, 20152015-11-05
On Thu, 05 Nov 2015 07:59:36 +0000, John Devereux <john@devereux.me.uk> wrote:>John Larkin <jjlarkin@highlandtechnology.com> writes: > >> 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. > >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).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.
Reply by ●November 5, 20152015-11-05
On Thu, 05 Nov 2015 06:57:59 -0500, DecadentLinuxUserNumeroUno <DLU1@DecadentLinuxUser.org> wrote:>On Thu, 05 Nov 2015 07:56:48 +0000, John Devereux <john@devereux.me.uk> >Gave us: > >>If it does, I think generally I *want* it do to that, so that the metal >>acts to confine the field. > > Copper does not contain a magnetic field.It sure can block an AC field.
Reply by ●November 5, 20152015-11-05
On Wednesday, November 4, 2015 at 4:18:30 PM UTC-8, krw wrote:> On Wed, 04 Nov 2015 18:11:36 -0500, DecadentLinuxUserNumeroUno > <DLU1@DecadentLinuxUser.org> wrote:> >>> > 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.> > 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.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
Reply by ●November 5, 20152015-11-05
On Thu, 05 Nov 2015 07:56:48 +0000, John Devereux <john@devereux.me.uk> wrote:>krw <krw@nowhere.com> writes: > >> 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. > >If it does, I think generally I *want* it do to that, so that the metal >acts to confine the field.Copper isn't going to do much to confine a magnetic field.> > >> 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. > >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 GNDI 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.> > >> The >> only place I delete planes is where it's specified by the part's >> manufacturer.
Reply by ●November 5, 20152015-11-05
In article <87wptxu3l1.fsf@devereux.me.uk>, 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,Nearby copper changes the ringing in discontinuous mode by quite a bit. Maybe two semicircles of copper dampens a certain unwanted frequency. -- I will not see posts from astraweb, theremailer, dizum, or google because they host Usenet flooders.
