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Date:	Tue, 22 Jul 2014 19:33:43 +0200
From:	Gabriel Paubert <paubert AT iram DOT es>
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Subject:	Re: [geda-user] Do you put ground planes under inductors?
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On Tue, Jul 22, 2014 at 09:52:59AM -0400, Bob Paddock wrote:
>  Filippo's question about inductor footprints raised a question in my
> mind about inductor layout placement.
> 
> I've read conflicting design advice on if a ground plane should be
> placed under the inductor used in switching power supplies.
> 
> One school of though says the ground plane acts as a shield, doesn't
> put slots into the plane to interfere with return currents etc., this
> seems to be the most common approach.  This seems to assume everything
> is perfect in the real world.
> 
> The other school of thought is not to put the plane under the inductor
> because the magnetic flux could introduce currents into the ground
> plane modulating 'ground', a type of ground pulling.  This seems to
> assume the real world is not perfect, ie. there are real world effects
> that we don't really want.

There are reall world effects, but it may aslo depend on the orientation 
of your inductor and of the distance of the ground plane: is it a multilayer
board or a 2 layer board?

I've never seen ill effects on switchmode power supplies, but I've never
built any with inductors have to handle more than 2A, so it's rather
on the low side (I built one fo 4A but is used two phases with an LT3510).

On the other hand I can say that for rather simple RF filters using
0603 inductors (LQW18A series), while it worked fine (maybe with a small
adjustment of some capacitor values) on a 2 layer PCB, it failed
spectacularly on a 6 layer board (layers 2 and 5 being ground planes)
and that in this case you have to remove ground around the inductor
on the corresponding ground plane layer.

	Gabriel

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