[SI-LIST] Re: Split gnd planes - for/against?
- From: "Matthias Mansfeld" <m.mansfeld@xxxxxxxxxxxxxxxxxxxxxx>
- To: si-list@xxxxxxxxxxxxx
- Date: Wed, 08 Apr 2009 00:38:01 +0200
On 7 Apr 2009 at 12:41, Lee Ritchey wrote:
> Steve,
>
> Several people responded with claims of real cases. Those are the
> ones that should be advanced to support the claims made.
>
OK... a real case. Power electronics without split planes is often a
no-go.
I make layouts for power inverters, motor controls etc. for nearly a
decade now.
There seems to be no big difference between SI and EMC. Power designs
that work thanks of good internal SI don't make much EMC trouble, on
the other hand a design with severe EMC flaws will primarily disturb
itself.
Real case: a laser diode power supply (step down converter, about 200
kHz from 20V DC down to 3Volts, 120 Amps diode current, board size
about a half palm).
There is electrically one ground, but must be split to separate the
high, switched currents (and back current paths) from control stuff.
Controller IC claims to need the star point under its belly.
The first example that did work but not yet perfectly (6 layers,
planes split as good as possible but by design and lack of layer
space instead of a star "point" a rather large region under the
controller IC which joins power ground (with 120 Amps switched return
currents) and digital control ground and analog (current and other
sensing stuff ground). That was still not enough separation against
some switching spikes galvanically coupled to the sensitive stuff.
Design changed, 2nd shot is currently at our prototype shop. 2
additional layers with completely separate planes for the different
grounds and combined only on a group of 3 adjacent vias. I'll report
whether this was enough to make it run flawless.
Regards
Matthias Mansfeld
--
Matthias Mansfeld Elektronik
* Printed Circuit Board Design and Assembly
Neithardtstr. 3, D-85540 Haar, GERMANY
Phone: +49-89-4620 0937, Fax: +49-89-4620 0938
E-Mail: m.mansfeld@xxxxxxxxxxxxxxxxxxxxxx
Internet: http://www.mansfeld-elektronik.de
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