I have a PCB with some TO3P packages which is usually clamped into an Aluminium chassis for heat sinking. I'm pushing it a bit, and without the chassis, on the bench in open air the transistors hit upwards of
200'C, so I thought I'd better find a heatsink. Something with about
50mm x 50mm flat mounting surface would fit.
Long time gone since I bought a heatsink, so I look at Farnell.
What's all this 'ceramic heat spreader' stuff?
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Does this work? Who snuck this in while I wasn't looking?
Alumina is not a very good heat conductor. Those thin things don't look like very good spreaders. I bet their theta numbers are fudged, specifically by applying heat uniformly across the surface, which is not how a "spreader" works.
I use aluminum pin-fin heat sinks as spreaders for FPGAs and such. The pins are useless, but the baseplate does suck heat out the central chip hot-spot and spread it around. Aluminum conducts heat 5 or so times better than alumina.
Everybody lies about heat sinks.
--
John Larkin Highland Technology, Inc
lunatic fringe electronics
Simpler alumina plate shapes have been used with transistor clip fastener (and physical retainer) detail pre-punched. I've never seen it with sigificant 3rd dimensional detail.
If it physically attaches to the heat source alone, I see no advantage to going to the added expense of using an electrical insulator material, unless it's a special safety or EMC issue.
Nobody over here got the memo. (They'd have burst out laughing if they had.)
Pretty amusing from the folks that gave us Featherstonehaugh, Cholmondeley, and Pontefract. ;) (*)
Jibbing at 'sodder', forsooth.
Cheers
Phil Hobbs
(For non-Commonwealth leftpondians: these are pronounced roughly "Fanshaw", "Chumley", and "Pumfrey".)
PS: It's weird how Aussies think that references to their own primitive plumbing makes a clever way to insult Americans.
--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC
Optics, Electro-optics, Photonics, Analog Electronics
160 North State Road #203
Briarcliff Manor NY 10510
hobbs at electrooptical dot net
http://electrooptical.net
How many watts is the TO3P package dissipating and what's your target operating temperature? You can use those numbers to calculate how many C/W the heat sink will need to handle. That will give you the approximate size of the heat sink.
How much vertical room do you have for the added heat sink? Any air flow?
Since you didn't bother specifying the transistor device number, I looked up a variety of TO3P packages devices and found that the highest operating temperature mentioned is 150C. At 200C, you transistor should have died from self immolation.
Are you trying to clamp that to the top of the TO3P package? That's all plastic and won't conduct much heat out of the package. It needs to go under the TO3P package or flip the TO3P package over so that the metal is on top.
TO3P package
Actually, I hadn't seen anything like that either, probably because it's a lousy idea. If the heat sink were made from Aluminum Nitride, it has a chance of working. But this one is Aluminum Oxide (alumina), so forget it.
--
Jeff Liebermann jeffl@cruzio.com
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558
** Using the chassis is a time honoured and perfectly good method when you have a good few watts to get rid of.
** A heatsink must be exposed to the outside air to be effective - or else fan blown with outside air. Small heatsinks attached to PCBs heat up everything around them and make the environment toxic to electro caps and other heat sensitive parts.
** Those things need fan cooling to work and even then not terribly well.
The fact the material is an electrical insulator is the only real novelty - you will still need to use a thermal compound to exclude air and get good coupling to the "spreader" from the device.
FYI:
I see increasing use of Beryllium Oxide as a heat spreader and insulator for TO220 devices. Used in rectangular strips about 2mm thick it provide massive insulation and conducts heat better than Aluminium.
The TO220 pak is then clamped to the heatsink, a far better way than using 3mm bolts and plastic bushes that crush over time.
Just to clarify, the transistors are 'upside down'. I just need to operate outside the chassis for debugging purposes, and was looking for a heatsink when I came across the ceramic type and wondered.
In fact, the device works at an ambient of 180'C and these transistors will then be above 200'C, so I'm not too worried about them, but running cooler pleases Mr Arrhenius and they last longer.
The specified operating temperature was passed with a cheery wave, sometimes you just have to.
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