another thermal thing

That not only cools the chip, it keeps the source lead from melting.

--

John Larkin         Highland Technology, Inc 
picosecond timing   precision measurement  

jlarkin att highlandtechnology dott com 
http://www.highlandtechnology.com

I bought some copper foil from an arts-n-crafts store. I think it's .010", or maybe .005".

James

Since spreading is such a pain, you might consider the power SO-8 packages. They're about 0.8C/W and small, so you can sprinkle them around and effect the spreading by distributing the dissipation, without needing copper magic.

Cheers, James Arthur

Call some local roofing companies. Copper flashing is about .025 inch thick.

That is a fun idea! I wonder when it would pay. George H.

D2PAK 15.9 x 10.7 mm 0.4K/W SO-8 6.3 x 5.2mm 0.8K/W

The cross-section of copper available for spreading heat horizontally will always be at great disadvantage to the copper cross-section of a via field moving heat vertically through a thin board, UNLESS, perhaps, you touch the tabs themselves of densely-packed D-PAKs, and use those for the horizontal heat path.

Cheers, James

I was looking at pnp's in these SOT89 pac's. The bottom is basically one big collector pad. here's one.

(Perhaps not the best... not many in stock at DK...) Impressive numbers though, I assume fet's come in something similar, but I haven't looked. The lower voltage parts seem to have lower (thermal) resistance. (more doping in the collector.. more charge and heat carriers?)

George H.

Am 20.10.2015 um 05:52 schrieb John Larkin:

There are also heatsinks for D2PAK:

Won't be good for 40 Watts but it helps a bit.

Cheers,

Robert

Zetex makes some nice discretes.

I did a little experiment on a SOT89, actually a depletion fet, and decided it was OK at three watts dissipation soldered to a modest copper pour.

The depletion fets make great capacitor bleeders, constant current down to almost zero volts. Add an LED for extra fun.

I actually need to dissipate 500 watts, or a kilowatt with some margin. The choices are a lot of pick-and-place dpaks, or a half dozen isolated-drain TO247 type parts, with mounting hardware and silicone grease and hand soldering and all that.

IXYS makes some nice looking isolated mosfets, some rated over 500 watts. I'd never push them that hard. IR set the gold standard for lying about mosfet ratings, and then everyone else had to follow.

I take it diverting some of that dissipation into R wire is not practical for you.

NT

That part/package has Rtheta(jc) of 8C/W, (Love those low Vcesat BJTs, BTW) an order of magnitude higher than the super-duper SO-8's.

I should clarify that I wasn't talking literally about SO-8 (they're awful), but about the various thermally-enhanced variants, like this one:

Here's another cool customer:

They're built on copper slabs.

I don't know if they're better for John. There are many degrees of freedom yet undefined.

Cheers, James Arthur

Oh Dang, my mistake. I was looking at the numbers upside down.

Yeah JL said he has to get rid of 0.5-1 kW! That's way over anything I've even thought about. (I could turn it into a personal heater for the winter months :^)

George H.

Expensive water heater.

I love winter, just not all the cold weather. --James "Yogi" Arthur :-)

(For Phil, New York must seem like Canadian Florida!)

Cheers, James Arthur

Have you considered a Peltier Thermoelectric unit. You can find them pretty inexpensive from most any electronics suppliers.

)

Nah, I'm not from the frozen moose pasture, I'm from the Canadian Riviera. It gets a good 15 degrees colder here in the NYC 'burbs than it does in Van couver, but it's nice and sunny. It's actually sunny and 75 degrees here, s o I'm writing proposals in the hammock.

Cheers

Phil Hobbs

Peltiers are useless for cooling transistors, which run quite happily at we ll over 100C. In fact, the T_Jmax of your average power transistor is highe r than that of almost any Peltier. Peltiers are also inefficient, so the di ssipation (and hence the heat sink volume) would go way up. Not to mention the cost of such a huge array of Peltiers, their mechanical and thermal fra gility, and the inconvenience of mounting them.

Other than that, they're great. ;)

Cheers

Phil Hobbs

(who happily uses small Peltiers a fair amount, usually near room temperatu re)