Class A heatsinking, or heatsinking in general!

[NUTTTR]

Hi All,

I'm wanting someone to "clear" up the rather differing opinions on the net about heatsinking, etc....
I was looking at
http://www.conradheatsinks.com/products/double_f.html
the last one on that page, it's PERFECT - what i want!
However, @ 0.25c/w @ 80deg above ambient, it doesn't seem so good to me...
What i've got is MJ21193's/94's that have a a junction of 0.7c/w.... PLUS mica washer and heatsink compound (people quote anywhere from 0.4c/w up to 1.2c/w) as the "rating"... So 'best' case scenario is 0.7+0.4+0.25 = 1.35c/w... Surely that can't be right? I'd assume that it would be better than that....
I WAS looking to run class A up to about 150w... - say 140 and have the last little bit as a/b - this means i have to get rid of a fair bit of heat, if i have a heatsink that has 0.05c/w+0.7c/w+0.4c/w still equals 1.05c/w...... which still puts my heatsink's above maximum temp... It's NEVER negative temp here, often 30+!! So that just doesn't work! I'm using 4 pairs of outputs per channel (i will jump to 5 pairs if it means better cooling!)... So that heatsink is excellent for the job, i could use 2 of them, but then it's the same price as another heatsink that i was looking at (which has an unknown c/w till we test it!). But even with 0c/w it's still not possible to run that much power through the heatsink! Maybe i'm misreading it, or miscalculating the values? The 'other' heatsink works out to about 333mmx500mm per channel, so it's BIG, but would require mounting brackets, etc for to-3's, however this has them already on there so looks good to me (and would work out at half the price) someone advise please!
Thanks heaps all
Aaron

[AndrewT]

Hi,
your heatsink formula is correct, FOR ONE transistor disspating all the power.
Build yourself a calculation table to insert values into
total power, Heatsink Rsa,Td=Pt*Rsa
device power, device Rcs,Td=Pd*Rcs
device power, device Rjc,Td=Pd*Rjc
choose a Ta to suit your location, season(summer?) & adjacent heat sources 25C to 40C.
Now start adding up temperature differences(Td) to ambient
to give heatsink(Ts), case(Tc), junction(Tj).
hint Ts max 50C to 70C depending on risk of burning?
Tc determines your derating factor from datasheets
Tj max 100C to 125C depending on reliability.
BTW you may need to get two or three of those 0.25C/W sinks per channel to meet your requirements.
regards Andrew T.

[pinkmouse]

This post by NP comes to mind.

[AKSA]

You should be considering using forced air cooling because there are too many heatsinks.

At 150W dissipation per channel, you would need two 300 x 150mm Conrad sinks per channel, each around 0.3C/watt. At 75W dissipated per heatsink, this is quite acceptable for four devices per rail suitably placed across the ledge or backing as it's technically only about a 25C rise above ambient....

The better option is to use two Conrad MF18 series, which are designed for forced air circulation. End on, fins facing each other, you have a 120mm x 120mm window ideally suited to a fan of the same dimensions, operating via a plenum chamber to cut turbulent flow and noise. You can then dissipate up to 175W per sink - 350W for the two - with only a measured 27C rise above ambient using a 48V fan operating at half voltage. You won't hear it beyond about five feet if the fan is insulated and the plenum set up properly (use a Papst for less than 20dbA of noise). I've done this work, checked all the power ratings, and it's very satisfactory, compact, and quiet as a churchmouse. And two MF18-151.5 are much cheaper than four MF30-151.5!!

Cheers,

Hugh

[NUTTTR]

Thanks for the formula Andrew i'll have to work it out!

Hugh - thanks, i HAD thought of that, i'm TRYING to avoid using fan's, but it looks like it's the only choice... My amp has 3 channels in it, so there would have to be at least 2 fans i'd guess, the amp is able to be ANYTHING up to 500x500x500, that's why i've got so much heatsink area to work with!!!
I could use 3 MF18's (151) and then have fans on each, in a sealed case (with only entry and exit for air, which was planned to be at the base of the case, so the toroids, etc get cooled as well...)
However, how do i mount to-3 devices to the MF18's? Do i have to get it "made" that way, or do i get something made up that allows me to bolt them to it? That's one of the things that's been getting me about the other ones, they already have everything there ready to go (flange)...?

Thanks for the help so far!
Aaron

[Circlotron]

With a bit of care you might be able to run live heatsinks and dispense with the mica washer, especially if your output stage is an emitter follower meaning the collectors and therefore the heatsinks don't flap up and down with the signal.

A fan will improve the efficacy of a heatsink by 5:1 or even 6:1.
90% of the improvement occurs by the time the fan is running at 75% of rated voltage. Increasing from 100% to 200% rated fan voltage makes barely 5% further improvement.

[NUTTTR]

Hmmmm, well fair enough
I was going to keep the mica washers, only because oneday i'll stick a screwdriver in there or something i'm sure and i'll forget! haha... Typical!
Next task - how to mount TO-3's to that heatsink?

[jackinnj]

here's a link to Thermalloy's "Thermal Resistance Tool"
http://www.aavidthermalloy.com/technical/thermal.shtml
-- for ambient use 32 LFM. In my 2x120 Watt Bridged Amp I use a pair of 11" x4" heat sinks with 10 fins (each fin is 1.3" high) -- the two heat sinks are joined together with aluminum so that they allow a tunnel in which a computer fan directs traffic. The heat sinks stay at about 106 degrees Fahrenheit.

if you run a device too close to its maximum the mean-time-between failure jumps.

[NUTTTR]

Hmmm....
I'm very much liking the idea of running fan forced now, the capability is much much better, nothing like normal air!
I'd like them to run as cool as i can, i can run 2 heatsinks together, but the 3rd one will be on it's own, which makes me wonder if i should make them all the same design, etc, not really sure how a 12x12cm fan will work with a 12x6 heatsink though...
Aaron

[jackinnj]

the other thing is that you don't have to run the fan at 12V -- it will run much more quietly (but push less air) at a lower voltage -- so get a cooking thermometer (those metal ones they sell at Bed n' Bath are pretty acurate and they go to over 220 degrees F) -- measure temperature on the heat sink with the fan at 12Volts, then reduce the voltage, allow the system to settle for a couple minutes anda take the temperature again.