Discussion:
Do you put ground planes under inductors?
(too old to reply)
Bob Paddock
2014-07-22 13:52:59 UTC
Permalink
Raw Message
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.

Comments?
Stuart Brorson
2014-07-22 14:16:16 UTC
Permalink
Raw Message
This is really a question for the signal integrity guys over on
SI-List. However, I'll take a bite, keeping in mind that I am not an
expert.

I'd go with the ground plane since it acts as a shield, and makes
layout easy. It's always important to have a return path for
currents running around the board, and putting holes/slots in a ground
plane can interrupt return paths in unpredictable ways.

However, I would make it a point to clear other components away from
the inductor on both sides of the board. As you say, if the
time-varying magnetic flux induces currents in the ground plane, any
component on the opposite side of the board from the inductor might
feel the effects of the induced current. In particular, a ground
plane with non-zero impedance will experience potential drops
underneath the inductor, so you don't want components living there.

The opinion of a non-expert,

Stuart
Post by Bob Paddock
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.
Comments?
John Doty
2014-07-23 12:51:16 UTC
Permalink
Raw Message
Post by Stuart Brorson
I'd go with the ground plane since it acts as a shield, and makes
layout easy. It's always important to have a return path for
currents running around the board, and putting holes/slots in a ground
plane can interrupt return paths in unpredictable ways.
However, I would make it a point to clear other components away from
the inductor on both sides of the board. As you say, if the
time-varying magnetic flux induces currents in the ground plane, any
component on the opposite side of the board from the inductor might
feel the effects of the induced current. In particular, a ground
plane with non-zero impedance will experience potential drops
underneath the inductor, so you don't want components living there.
Generally good advice. There are a number of special cases.

For shielded inductors, including self-shielded geometries like toroids and pot cores, the field leakage is small. Treat these like any other component: generally you’ll want ground plane underneath them.

For unshielded coils, it depends on the orientation and purpose of the coil. It’s good to start from the point of view of an infinite perfectly conducting ground plane without holes. The field cannot penetrate such a plane. In this case, we may use the method of images.

If the coil axis is perpendicular to the plane, the image coil has its magnetic axis in the opposite direction, canceling the coil’s dipole moment. Since a quadrupole field falls off faster with distance than a dipole field, this helps confine the field to the vicinity of the inductor. The cost is a strong eddy current in the plane beneath the coil, reducing the coil’s inductance and Q.

If the coil axis is parallel to the plane, the image coil has its magnetic axis in the same direction, reinforcing the coil’s dipole moment. This can increase the field away from the coil by as much as a factor of two. Because the plane intercepts less magnetic flux, the eddy currents are smaller and the reduction in inductance and Q is less than for the perpendicular case.

Removing ground and power planes below the coil reduces these effects. It also allows some of the field to leak to the far side of the board. Remember that routing signal or power traces across any sort of gap in your ground plane is usually a bad idea.


John Doty Noqsi Aerospace, Ltd.
http://www.noqsi.com/
jpd-***@public.gmane.org

DJ Delorie
2014-07-22 15:48:10 UTC
Permalink
Raw Message
I typically keep the ground away from switchers, both to avoid
inducing ground current and to avoid stray capacitance. I also keep
it out from under sensitive crystal oscillators.
Gabriel Paubert
2014-07-22 17:33:43 UTC
Permalink
Raw Message
Post by Bob Paddock
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
Loading...