thermal tips re expo converters:

[Steve Varner]

How small is small for the traces? Since I make my own PCBs from scratch, I usually have somewhat large traces.
They don't exactly look like those micro-miniature commercial PCBs. I also tin all my traces, so I guess I
wouldn't do this on the 3046 traces. Right?

SV


Harry Bissell wrote:

> 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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