Determing heatsink

[CorySCline]

Hello all! I have taken all the advice that has been presented to me and I understand that I need to to a lot of reading and learning before attempting the project I want to do. I have put a lot of time into reading over the datasheets and application notes provided by national and have narrowed down what I want to do to a more precise goal. I have decided on running a + and - 35V to an 8 Ohm load with RMS of 50 Watts in a mono application with 1 transformer, 1 rectifier, and 1 speaker. With that in mind, I have further attempted a heat sink calculation. I have attached a document (mostly to preserve formatting), in which I outline my math. If possible I would like yall to verify that I did it correctly

Possible discrepancy:
The values for JC and CS I was unsure on and this could have completely thrown me off. By unsure I mean the following:

I know JC to be = to 1 C/W and CS to be = .2 C/W (When using recommended thermal compound). In the calculation should I use 1 and .2 or 1(50) and .2(50)?

I used the former in my attached calculations.

[wwenze]

Another method of calculation -

Tjmax is 150ºC, ambient is 28ºC, 122 degrees of temp difference to transfer Pdmax of 31.02W, so total thermal resistance is 3.933ºC/W. Which is Rθjc + Rθcs + Rθsa.

So Rθsa = 3.933 - 1 - 0.2 = 2.73ºC/W.

...which matches the calculation in the .doc.

However, some real-life considerations:

At 2.73ºC/W and 31.02W, your heatsink will be 85ºC above ambient temperature, or 113ºC absolute if ambient is 28ºC. The capacitors around the heatsink may not like it.

Also, if the heatsink is external, 113ºC is dangerous to be left exposed. If it is internal, ambient temperature will not be 28ºC. You will have to consider the thermal resistance between air inside the case and air outside.

I would set the temperature that I want the heatsink to be running at and select the appropriate heatsink. I don't like anything above 80ºC and if it can be accessed by fingers I want it at a temperature that would not cause pain by accidental touching. But that's just me.

[CorySCline]

How did you come up with the 85 Degrees above? I cant find the formula you used.

[CorySCline]

Nevermind, I was overthinking it lol. To anyone who may have this similar question, multiply the 2.73 by the 31.01 (SA * PDMAX)

I have done your suggestion and used the PDMAX of 31.02 and a Total Desired Temperature of 80 Degrees Celsius. Which is a 52 Degree Above Ambient.. so X * 31.02 = 52 means I should look for a heat sink rated at max 1.68 C/W...Just to be sure, I am correct in understanding that The 1.68 I am determining is the degrees per watt of power that the heat sink will hold on to?

[!]

85C rise is 2.73'C/W * 31W = 84.63C (rounded up)

Do you expect to run at PDMax continuously? Actual/average PD depends on person (volume set, music, speaker SPL, room area, etc. For example if you think your average PD is going to be closer to 1/2 PDMax, and if your heatsink is mounted outside the chassis (or on the wall of it) or if inside, has vent holes/slits/etc below and above it, you can probably get away with using something roughly 4"L x 2"W x 1"D or larger with a typical extruded, vertically oriented fin design - except as wwenze mentioned, it'd get a bit too hot to comfortably touch if mounted externally. If the chassis can accept something larger, cut one down only enough that it fits.

While the calculations are important for a mass manufacturer trying to save pennies on the least heatsink that will do the job, specifying several thousands from a heatsink manufacturer, when you DIY you more often find it's a matter of the biggest *cost effective* thing you can fit in the chassis you want to use, whether that be something you already have, can buy online, or dig out of an old computer or metal scrap yard... or you might be going for a certain "look" to the project if you'll be showing it off.

If you didn't care about noise you could even use a little old socket 7 heatsink with a 50mm fan on it but then you have the noise, dust, eventual fan failure, and another power feed for the fan when a bit larger heatsink removes these factors... but it'll do on a bench.

[CorySCline]

Originally Posted by !

While the calculations are important for a mass manufacturer trying to save pennies on the least heatsink that will do the job, specifying several thousands from a heatsink manufacturer, when you DIY you more often find it's a matter of the biggest *cost effective* thing you can fit in the chassis you want to use, whether that be something you already have, can buy online, or dig out of an old computer or metal scrap yard... or you might be going for a certain "look" to the project if you'll be showing it off.

Thanks for the encouragement that exactness is not required in this case ...However I wanted to do it properly so that I may learn for future use .

That being said, I have decided on
http://search.digikey.com/scripts/Dk...me=294-1036-ND

Being only $2.50 and having a resistance of .5 C/W should keep it under 50 C over ambient.

[wwenze]

I find it hard that 2 x 3cm heatsink you linked to above is 0.5ºC/W. So checked it I did, and yup, it should be 17 according to this datasheet:

http://dkc3.digikey.com/PDF/US2011/P2591.pdf

As a rough guide, a Pentium 4/Socket A/other equivalently-hot CPU's stock heatsink is around 0.5ºC/W with fan.

[CorySCline]

Originally Posted by wwenze

I find it hard that 2 x 3cm heatsink you linked to above is 0.5ºC/W. So checked it I did, and yup, it should be 17 according to this datasheet:

http://dkc3.digikey.com/PDF/US2011/P2591.pdf

As a rough guide, a Pentium 4/Socket A/other equivalently-hot CPU's stock heatsink is around 0.5ºC/W with fan.

Datasheet: http://www.ctscorp.com/CHN/component...heets/2-16.pdf

I are confused lol

[CorySCline]

Wait....digikeys specs show it as .5 C/W @ Natural, but the products datasheet shows it with a doing that at 1000FPM with dissipation.....******* digikey lol

[CorySCline]

However, I am not finding anything reasonably priced on digikey. Suggestions? Lookin for a 1.8 C/W or less that isnt 4 feet long lol