Jack:
Automotive is a whole 'nother world.
As an example, do a google on "Load Dump" :)
(former automotive guy and glad to be out of it)
--
George Patrick
Tektronix, Inc.
Central Engineering, Engineering Design Services
P.O. Box 500, M/S 39-512
Beaverton, OR 97077-0001
* 503-627-5272 (voice) * 503-627-5587 (fax)
http://www.tektronix.com http://www.pcb-designer.com
"Off-Grid and Proud of it!"
-----Original Message-----
From: DesignerCouncil [mailto:[log in to unmask]] On Behalf Of Jack
C. Olson
Sent: Thursday, March 16, 2006 13:09
To: [log in to unmask]
Subject: Re: [DC] Walls of Air? or Pillars of Metal?
Greetings,
Maybe the reason for the wide variety of responses
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
designing a small 6-layer board about the size
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.
The part that seems easy to me is the request
to pour copper top and bottom and stitch the
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
for me; this is just an interesting subject that I
don't know enough about.
Jack (aka "the new guy")
At 07:45 PM 3/15/2006, Jack C. Olson wrote:
I have never thought about this until now,
but I'm confused about something...
Many times I have been asked to put a slot
in planes to isolate different areas of a PCB
from each other, almost as part of the floor-
planning. So in other words a power supply
area might be isolated from a digital processor
area by putting an air gap between them, or
maybe between primary and secondary of a
transfomer, or maybe the 3mm clearance
you put under an opto-isolator, right?
on the other hand...
Recently I have been asked to start building
"via fences" between areas, like stitching
ground planes together, and even been asked
to start putting a line of vias all around the
perimeter of the board, too. And flooding ALL
layers with copper and adding MORE vias to
stitch THEM together. In the past I have
used via stitching a lot, but it was for things
like guard traces along RF lines (1.67GHz
cellular signals) and occasionally along the
edges of shields. but not like this.
so...
Today a discussion came up whether to use
air gaps or via stitching, and I had a total brain
freeze. In my mind they both had the same
reason for existence, keeping things from
interfering with each other. But of course in
reality they are the exact opposite of each
other; one method adds conductors and the
other removes them.
Now I can't seem to grasp what I am missing
about this.
There's noise in my head.
So can someone please help me understand
when to choose one over the other, if at all?
(and hopefully the answer will be shorter than
the question?)
Jack
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