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Old 15th October 2013, 08:24 PM   #1
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Default Heatsinks

As I guess a lot of people have done, I'm building an amplifier into a steel chassis.

The amplifier in question is a Pass F5 so things are expected to get a bit warm.

Now, I've got two huge heatsinks which should dissipate the heat.

My question is - how will the steel reduce the effectiveness of the heat transfer ?

i.e. I'll have Tj -Cs - Steel - Heatsink.

I know that I'd be better cutting two huge holes in the steel to mount the MOS-FETs directly onto the heatsinks but is this really necessary?
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Old 15th October 2013, 08:27 PM   #2
Johno is offline Johno  Australia
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Yes
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Old 15th October 2013, 08:37 PM   #3
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I was sure I had a reference around here showing thermal conductivity of various metals...
Memory tells me the steel would increase Tj-a by at least several C/W and probably more, it being considerably worse than copper and aluminum.
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Old 15th October 2013, 08:59 PM   #4
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Looks like I'll be cutting some mouse holes then.

Interesting how many Class A projects have been illustrated on here with that thin bit of steel between the active devices and the heatsinks, I've done it myself on cheap chip amp projects.

Last edited by KatieandDad; 15th October 2013 at 09:09 PM.
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Old 15th October 2013, 09:25 PM   #5
sreten is offline sreten  United Kingdom
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Hi,

It all depends on real physics, noting most transistors have steel tabs.
The physics indicate cutting holes in the steel sheet is pointless, why
you will never see it in a commercial amplifier. The layer is thin and
very spread so will likely have less effect than the insulator layer.

At the same time steel has a high thermal capacity, so although
long term it will reduce maximum heat flow, short term it will
increase it, and simply put the thicker the layers the output
trannies are attached to the better they are at spreading heat.

I'd say all in all its simply not necessary to cut any holes for
typical heatsinks but there is a point with very serious
heatsinks where it might make sense, usually doesn't.

I'd say most of the time its heat spreading ability is useful,
but that of course also depends on the heatsink design.

rgds, sreten.
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Old 15th October 2013, 09:27 PM   #6
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I thought decent semi's had copper tabs ?

Here is the almost complete F5 with it's heatsink - There are two of these.
Attached Images
File Type: jpg DSIR9603.JPG (209.0 KB, 438 views)

Last edited by KatieandDad; 15th October 2013 at 09:40 PM.
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Old 15th October 2013, 10:10 PM   #7
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Don't be diverted from protecting your junction temperature. Its an F5 you are building for goodness sake, not some relic with steel TO3's.

Every intervening material between junction and ambient adds 2 sets of thermal resistance. The wiser path is to minimise the thermal resistance. But I see your problem, its an ugly shaped heatsink to attach to the outside of a case, have you considered an alternative thats easier to mount?
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Old 15th October 2013, 10:53 PM   #8
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I disagree completely and absolutely with sreten on this one.

Steel is a lousy conductor of heat compared to aluminum.
The greater number of transitions between materials the worse the thermal conductivity.
The F5 runs in real Class A, so there is real heat, unlike a low bias Class AB or B.
One would like to maximize the transfer efficiency given the temperature inside the Mosfets.

Which is essentially what Johno just said.

I'd cut holes in the side of this steel chassis large enough to permit a very wide window, as much as 100% or better for the heatsink's rear side (assuming a flat back heatsink?).

The heatsink needs to have a clear path for airflow from below to above the fins, and (imho) an airpath on the back side as well.

The heatsink can easily be mounted using auxiliary hardware, parts as required. Whatever is required or usable.

You need fairly massive heatsinks for the F5.

Bigger if you go above the stock power rating of ~20watts per channel.

In general the cooler the devices can stay the better the thermal stability and the higher the SOA will be as well.

I'd mount the heatsinks, using the tabs already in the extrusion, by putting them on some
standoffs - physically float the heatsink off the chassis so there is easy airflow.

How about some pix of the chassis??
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Last edited by bear; 15th October 2013 at 10:59 PM.
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Old 15th October 2013, 11:42 PM   #9
sreten is offline sreten  United Kingdom
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Quote:
Originally Posted by bear View Post
I disagree completely and absolutely with sreten on this one.
Hi,

Well back it up with some real physics assuming typical steel
cased or tabbed devices, I know why steel is good enough.

You have completely missed the points I made earlier.
Which make a lot more sense than dogma.

It depends on the heatsink design whether a steel
layer is better or not, not just it must be worse.

rgds, sreten.
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Last edited by sreten; 15th October 2013 at 11:58 PM.
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Old 15th October 2013, 11:56 PM   #10
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The Pass amps are Class A, and require all the heatsink they can muster. I've even spent time lapping (with 1200 grit sandpaper) the heatsink tabs on MOSFETS (and mounting areas on the heatsinks--particularly around any drilling flash) to insure the best possible MOSFET to heatsink contact. That and the use of mica (and grease) or kerafoil will also be needed.

Do NOT mount the MOSFET on (or directly through) the steel chassis--significantly cuts down the efficiency of the heatsink, and your MOSFET temperature will show that.

If you MUST mount the heatsink to a steel chassis, first locate and mark the location of the semi's on the chassis. Then remove the semi's from the steel, and cut a 1-1/2 to 2 inch circular hole (use a hole saw or a Greenlee chassis punch. Replicate the locations of the semi's on the heatsink, and you'll be OK. In effect, you have the semi mounted on the heatsink, with the steel having a large hole in it, to allow you do do this. (Just remember you can't wire--and test--the amp, until you have the semi-steel-heatsink "sandwitch" assembled(!)
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