[SI-LIST] Re: Walls of Air? or Pillars of Metal?
- From: "Grasso, Charles" <Charles.Grasso@xxxxxxxxxxxx>
- To: <OLSON_JACK_C@xxxxxxx>
- Date: Thu, 16 Mar 2006 14:13:56 -0700
Jack - Your approach (if implemented properly) is very effective.
I think that your main concern is with proximity of the power supply and
the uP - Based on your details I would say that you will not see
coupling interference of the power supply onto the uP.=20
The power supply is a DC - DC converter - right? Low power - 15W or so?=20
What is more likely is that you'll have an emissions issue with the
powersupply itself than the circuit in general.=20
Best Regards
Charles Grasso
Senior Compliance Engineer
Echostar Communications Corp.
Tel: 303-706-5467
Fax: 303-799-6222
Cell: 303-204-2974
Pager/Short Message: 3032042974@xxxxxxxx
Email: charles.grasso@xxxxxxxxxxxx;
Email Alternate: chasgrasso@xxxxxxxx
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx]
On Behalf Of Jack C. Olson
Sent: Thursday, March 16, 2006 2:05 PM
To: OLSON_JACK_C@xxxxxxx
Cc: si-list@xxxxxxxxxxxxx; si-list-bounce@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: Walls of Air? or Pillars of Metal?
Greetings,
Maybe the reason for the wide variety of responses=20
I got (mostly private. Is this a touchy subject?)
was that the question was too general.
Let me just give you the scenario and then ask one
more question, ok?
This is a farily simple example: Let's say I am=20
designing a small 6-layer board about the size=20
of a pack of cigarettes, to be mounted in a plastic
box for an automotive application.
Besides the two connectors (one for input power
and to send the "result" back, and the other for
a bank of LEDs and switches) the board contains
a small low-power switching power supply and a
microprocessor.=20
The part that seems easy to me is the request
to pour copper top and bottom and stitch the=20
perimeter, because in my overly-simplistic mind
that creates a kind of "metal box" for the circuitry
both for immunity and radiating reasons
(If I am already making an invalid assumption
let me know, ok?)
but the other question is:
Should I do something between the power supply
and the microprocessor?
I'm not really asking anyone to design the board=20
for me; this is just an interesting subject that I
don't know enough about.
Jack (aka "the new guy")=20
"Jack C. Olson" <OLSON_JACK_C@xxxxxxx>=20
Sent by: si-list-bounce@xxxxxxxxxxxxx
03/16/2006 08:17 AM
Please respond to
OLSON_JACK_C@xxxxxxx
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[SI-LIST] Walls of Air? or Pillars of Metal?
Caterpillar: Confidential Green Retain Until: 04/15/2006
Retention=20
Category: G90 - General Matters/Administration
I just posted a question on the IPC
Designer's Council, and then suddenly
remembered si-list, which might be a
better place to ask this question.=20
So this is my first visit here, and sorry=20
for the cross-post.
-=3D-=3D-=3D-
I have never thought about this until now,=20
but I'm confused about something...=20
Many times I have been asked to put a slot=20
in planes to isolate different areas of a PCB=20
from each other, almost as part of the floor-=20
planning. So in other words a power supply=20
area might be isolated from a digital processor=20
area by putting an air gap between them, or=20
maybe between primary and secondary of a=20
transformer, or maybe the 3mm clearance=20
you put under an opto-isolator, right?=20
on the other hand...=20
Recently I have been asked to start building=20
"via fences" between areas, like stitching=20
ground planes together, and even been asked=20
to start putting a line of vias all around the=20
perimeter of the board, too. And flooding ALL=20
layers with copper and adding MORE vias to=20
stitch THEM together. In the past I have=20
used via stitching a lot, but it was for things=20
like guard traces along RF lines (1.67GHz=20
cellular signals) and occasionally along the=20
edges of shields. but not like this.=20
so...=20
Today a discussion came up whether to use=20
air gaps or via stitching, and I had a total brain=20
freeze. In my mind they both had the same=20
reason for existence, keeping things from=20
interfering with each other. But of course in=20
reality they are the exact opposite of each=20
other; one method adds conductors and the=20
other removes them.=20
Now I can't seem to grasp what I am missing=20
about this.=20
There's noise in my head.=20
So can someone please help me understand=20
when to choose one over the other, if at all?=20
(and hopefully the answer will be shorter than=20
the question?)=20
Jack=20
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