[SI-LIST] Re: How to create a "fuse" in a PCB?
- From: Krunal Desai <movax@xxxxxxxxx>
- To: jeff.loyer@xxxxxxxxxx
- Date: Wed, 27 Jan 2016 10:29:52 -0800
On Wed, Jan 27, 2016 at 10:14 AM, Jeff Loyer <jeff.loyer@xxxxxxxxxx> wrote:
Can anyone share their experience(s) with designs that have failed due to
excessive heat caused by excessive current in their PDN? I’ve been reading
IPC-2152 and Doug Brooks’ articles and wonder if you could create a power
delivery shape that would provide adequate voltage to the load(s) and yet
heat excessively. From what I’ve read, the thermal conductivity of copper
and FR4 is such that, for any reasonable case, the heat generated would be
dissipated by adjacent copper and FR4. I suspect the active components
would have much more influence on the PCB temperature than the passive PDN
shapes, but don’t have any concrete information on that. Taken to its
extreme, I wonder what would happen if you had a 1-micron wide trace that
was only 1-micron long, with large planes on either side of that trace.
Its short length keeps it from being highly resistive, but the current
density is obviously very high in that 1-micron trace. While there may not
be much voltage drop, would the heat generated be dissipated by the
adjacent large copper shapes which have very low current density
(generating very little heat)?
Hi Jeff --
I've explored the idea in the past for one of our applications -- the
two primary analysis equations I used were from Preece and Onderdonk.
I believe tools like the Saturn PCB Toolkit implement Onderdonk's
equation to estimate fusing currents and est. burn times. I decided
not to go this route because of how dependent on manufacturing
tolerances this is, and in general the relative imprecise nature of
it.
It's a cool trick to pull-off, but for any real safety application a
real fuse is a must.
Krunal
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