From: John Larkin
Subject: Re: TO-220 on a big aluminum sheet
Date: Fri, 03 Jan 2003 19:19:56 -0800
Organization: Posted via Supernews, http://www.supernews.com
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On Fri, 3 Jan 2003 14:30:45 -0500, Keith R. Williams
>In article ,
>> On Fri, 03 Jan 2003 04:39:57 GMT, Spehro Pefhany
>> >On Thu, 02 Jan 2003 15:57:10 -0800, the renowned John Larkin
>> > wrote:
>> >>see a.b.s.e.
>> >The nomograms in the GE SCR manual for doing this sort of thing are
>> >pretty messy looking. Thanks for the info.
>> >Best regards,
>> >Spehro Pefhany
>> The other cool thing to try would be to plot theta versus plate area;
>> but too much work. It's a shame that thermal modelling software is so
>Why not use Spice or some other circuit simulator? The thermal
>gurus I worked with 25 years ago at IBM used IBM's circuit
>simulator program (ASTAP) for some pretty sophisticated thermal
>modeling of multi-chip packages, even entire mainframes.
Spice is hard to use for distributed (diffusion model) situations with
geometry issues. A real thermal modeling program would chop the sheet
up into hundreds (at least) of finite-element chunks, and do the
conduction+convection+radiation analysis of each until the thing
settles to equilibrium. If there is enough heat to cause significant
air motion, we add a fluid dynamics analysis on top of that.
I have used Spice to model simple thermal situations, especially for
transient (pulsed) heating. But you've got to chop up the metal into a
small number of idealized (isothermal, ie capacitor) blocks, and
that's not often very realistic.
Oh, incidentally, this is from one of my engineering notes files...
TRANSIENT THERMAL MODELLING
Analog circuit analysis techniques can be applied to thermal systems.
The following equivalences are accurate to about 10%...
1 amp 1 watt
1 farad 1 gram aluminum
1 volt 1 degree C
1 second 1 second
1 ohm 1 degC/watt
So, with appropriate unit scaling, Spice can be used to model simple