thermal tips re expo converters:

[Harry Bissell]

Been thinking about the heated expo converters... here are some thoughts to
share: 
1) RE: How chips lose heat see National Semi LM-34/35 precision temperature
sensors... The plastic package is actually a thermal insulator, most of the
heat reaches the chip through the leads. This has been documented in some app
notes for this part. So, when you think about putting thermal insulation
(styrofoam) over the chip, also consider it Under the chip.
2) Following the same logic... rout a slot in the PCB around the chip, and
leave just enough room for the runs... This will make heat entering or leaving
the chip radiate through the air, which is a hell of a lot less effective that
conduction through the PCB.
3) And... Use tiny traces in foil, just enough to carry the current you need.
If you need more current (Gnd, power) use two tiny traces in parallel. This
will conduct heat less heat than one trace of 2X the size. Again, you want to
be less efficient in heat transfer. 
4)All of the above would give you the smallest thermal mass, (hence faster
thermal response. This is a good thing for a feedback loop. It might be hard
to layout the PCB like this, but probably worth the effort.
5) a last thought... When you glue styrofoam around the chip try not to touch
the circuit with the glue. Make the insulated volume as small as you can. The
glue, or any moisture that could possibly collect (when cold) might cause
leakage. Digi-Key carries some little plastic dip plugs, that have some little
plastic header covers available. These would be perfect for covering a single
(heated) chip. Look for em. (sorry no catalog at home... but they are in
there) 
Whaddya think?  ;-) Harry Bissell

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