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From: Tim Williams on 28 Jul 2010 15:10
"Paul Keinanen" <keinanen(a)sci.fi> wrote in message news:lks056tu4e4tj4bnvoj8s128rm1f7lglhg(a)4ax.com...
> Just wondering, how such narrow channels will behave in the presence
> of dust :-).

Fine. My processor gets a bit dusty, I blow it out every so often. The CPU temperature is identical, before and after...

It's probably the first bit of dust that's the worst, the part that you can't un-stick from the metal surface without thorough cleaning. Like biofouling in aqueous systems, you try cleaning it off but the worst part is the hardest to clean.

Tim

--
Deep Friar: a very philosophical monk.
Website: http://webpages.charter.net/dawill/tmoranwms
From: John Larkin on 28 Jul 2010 16:55
On Wed, 28 Jul 2010 21:10:38 +0300, Paul Keinanen <keinanen(a)sci.fi>
wrote:

>On Wed, 28 Jul 2010 09:07:35 -0700, John Larkin
><jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote:
>
>>On Wed, 28 Jul 2010 11:52:43 -0400, Hammy <spam(a)spam.com> wrote:
>>
>>>On Wed, 28 Jul 2010 16:49:30 +0300, Paul Keinanen <keinanen(a)sci.fi>
>>>wrote:
>>>
>>>>On Wed, 28 Jul 2010 07:48:39 -0400, Hammy <spam(a)spam.com> wrote:
>>>>
>>>>>
>>>>>For example if I calculated an allowable 140W dissipation for a single
>>>>>fet mounted on a large 150mm x 100mm for a max ambient of 50C and max
>>>>>junction of 110C With a fan 1 x 42CFM.
>
>That air flow is about 20 l/s or about 24 g/s of air, thus the air
>will be heated with 4 C for each 100 W dissipated. If there are
>multiple transistors in the direction of air flow, the last
>transistors will receive warmer air than the first transistor.
>
>>>
>>>>The total thermal resistance from junction to ambient is
>>>>(110-50)/140=0.43 C/W, which sound very optimistic :-).
>>>
>>>They spec a thermal restiance of 0.08�C/W using the fan for this
>>>890SP-01500-A-100 Heatsink .
>>>
>>>http://www.farnell.com/datasheets/35754.pdf
>
>Looking at the picture, it appears that the fin and the air channel
>are both about 1.5 mm wide.
>
>Just wondering, how such narrow channels will behave in the presence
>of dust :-).
>
>If the channels are as wide as the fins, the combined equivalent
>channel would be about 25 x 50 mm or 0.125 dm�, thus with 20 dm�/s air
>flow, the air speed would be 16 m/s, which would be quite noisy.
>
>>>
>>>This is unfamiliar teritory for me I've never had to dissapate a large
>>>amount of power, so is that realistic? ;-)
>>>
>>>I'm using these FQA24N50 which has a RJC = 0.43 C/W
>>>and RCS = 0.24 C/W. TO-3PN package PD @ 25C is 290W.
>>>
>>>I see I forgot to add RCS.
>
>This depends of the mounting.
>
>Anyway Rth j-s would be at least 0.67 C/W and with Rth s-a 0.05 C/W
>and the total resistance Rth j-a 0.72 C/W.
>
>With 50 C ambient and 110 C junction, the maximum dissipation is 83 W.
>
>With two transistors 0.67/2+0.05=0.39 C/W, P=156 W (78 W/transistor).
>With four transistors 0.67/4+0.05=0.22 C/W, P=276 W (69 W/transistor).
>
>>It would be safer to use three or four fets on that heatsink to spread
>>out the heat and reduce the effective Tc-s. Keep the dissipation below
>>80 watts per, maybe.
>
>80 W/transistor would indeed be the maximum.
>
>>Also blast the air *into* the fins, not just around them. That sink
>>has high fin density, so air will prefer to flow around the fins
>>instead of between them.
>
>Or install the fan in the middle blowing downwards and cover the
>remaining area, so that the hot air escapes from the ends of the fins.
>Install the transistors on both sides of the fan, so that the input
>air temperature is the same.

I've got good results from the configuration you describe, putting the
fan in the middle of the heat sink, blowing directly down into the
fins, with air squirting out both ends. This is called "impingement
cooling" I think. The fets in the middle have a less air velocity but
better lateral spreading effect, and the fets on the ends get a lot of
air velocity, so it works out pretty well to distribute the fets
fairly evenly, just avoiding the very ends of the heatsink.

I have this crackpot theory that the back pressure caused by the air
scraping the fins should reduce the fan air volume to about half of
the fan's zero-back-pressure flow spec. Sort of an impedance matching
thing.

The other important thing is to keep Tc-s low by having the heatsink
be really flat, and not use insulators.

If you're really compulsive, use an opamp per fet to really control
the currents, and tune the gains to equalize real-world fet
temperatures.

John


From: Hammy on 28 Jul 2010 17:41
On Wed, 28 Jul 2010 13:55:05 -0700, John Larkin
<jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote:

[SNIP]
>
>I've got good results from the configuration you describe, putting the
>fan in the middle of the heat sink, blowing directly down into the
>fins, with air squirting out both ends. This is called "impingement
>cooling" I think. The fets in the middle have a less air velocity but
>better lateral spreading effect, and the fets on the ends get a lot of
>air velocity, so it works out pretty well to distribute the fets
>fairly evenly, just avoiding the very ends of the heatsink.
>
>I have this crackpot theory that the back pressure caused by the air
>scraping the fins should reduce the fan air volume to about half of
>the fan's zero-back-pressure flow spec. Sort of an impedance matching
>thing.
>
>The other important thing is to keep Tc-s low by having the heatsink
>be really flat, and not use insulators.

I'm not going to be using insulators. I might use two fans mounted
directly overtop.

>If you're really compulsive, use an opamp per fet to really control
>the currents, and tune the gains to equalize real-world fet
>temperatures.

Thats what I was thinking. I've been looking at Newark for a decent
quad op-amp but the pickings is pretty slim. They just have old 324'S
or similiar.

>John
>
From: Hammy on 28 Jul 2010 17:48
On Wed, 28 Jul 2010 21:10:38 +0300, Paul Keinanen <keinanen(a)sci.fi>
wrote:

[snip]
>>>http://www.farnell.com/datasheets/35754.pdf
>
>Looking at the picture, it appears that the fin and the air channel
>are both about 1.5 mm wide.
>
>Just wondering, how such narrow channels will behave in the presence
>of dust :-).

The P4 Heatsink for my PC is the same and dust does stick, it doesnt
even come off with compressed air. I have to use cue tips to rub it
off.
>If the channels are as wide as the fins, the combined equivalent
>channel would be about 25 x 50 mm or 0.125 dm�, thus with 20 dm�/s air
>flow, the air speed would be 16 m/s, which would be quite noisy.
>
>>>
>>>This is unfamiliar teritory for me I've never had to dissapate a large
>>>amount of power, so is that realistic? ;-)
>>>
>>>I'm using these FQA24N50 which has a RJC = 0.43 C/W
>>>and RCS = 0.24 C/W. TO-3PN package PD @ 25C is 290W.
>>>
>>>I see I forgot to add RCS.
>
>This depends of the mounting.
I'm not useing any insulators the heatsink will be live.

>Anyway Rth j-s would be at least 0.67 C/W and with Rth s-a 0.05 C/W
>and the total resistance Rth j-a 0.72 C/W.
>
>With 50 C ambient and 110 C junction, the maximum dissipation is 83 W.
>
>With two transistors 0.67/2+0.05=0.39 C/W, P=156 W (78 W/transistor).
>With four transistors 0.67/4+0.05=0.22 C/W, P=276 W (69 W/transistor).
>
>>It would be safer to use three or four fets on that heatsink to spread
>>out the heat and reduce the effective Tc-s. Keep the dissipation below
>>80 watts per, maybe.
>
>80 W/transistor would indeed be the maximum.

>>Also blast the air *into* the fins, not just around them. That sink
>>has high fin density, so air will prefer to flow around the fins
>>instead of between them.
>
>Or install the fan in the middle blowing downwards and cover the
>remaining area, so that the hot air escapes from the ends of the fins.
>Install the transistors on both sides of the fan, so that the input
>air temperature is the same.

Its going to be one of those trial and errors to see where the fan
performs the best. I may use two.

Thanks for the examples. :-)
From: Grant on 28 Jul 2010 19:27
On Wed, 28 Jul 2010 09:01:51 -0700, Fred Abse <excretatauris(a)invalid.invalid> wrote:

>On Wed, 28 Jul 2010 23:17:53 +1000, Grant wrote:
>
>> I put 8 x TO220 FETs direct (non-insulated) on flat aluminium plate, 3mm
>> thick by 40mm to spread heat onto one side of 80mm square fancooled
>> heatsink rated 0.3'C/W
>
>Assuming that you mean that you bolted, or otherwise clamped 8 transistors
>onto a thin aluminum plate, which you then attached to an aluminum
>heatsink, how's that going to "spread the heat"?

Lowered the MOSFET tab temperature, compared to bolted directly to
the heatsink. Problem is that some of the nice looking heatsinks
don't conduct heat very well.
>
>If I understand correctly what you did, that's going to make the
>situation worse than attaching the transistors directly to the heatsink.
>Extra thermal resistance at the interface between the plate and the sink.

No, a heat spreader improves things, oddly enough, by spreading the
heat. I started with insulated tabs, no go at all :( This is where
practical or empirical knowledge will beat your calculations hands
down.

While I can't tell you what the different alloys were, there is a
difference in colour and chip forming on drilling the aluminium plate
and the heatsink material I was working with.

And, if you look up difference in pure aluminium vs the various
common alloys, there's a great difference in thermal resistance.

On top of all that, I work with materials easy to find, I'd really
enjoy being able to mill out a heatsink from chosen material as I
recall Jim T. doing for his fan cooled power resistor.

I didn't want to use oil-cooled or water cooled because they're too
messy for indoors. Boiling water is very efficient for wasting a
lot of energy.

If one is game, hooking up a fully controlled bridge and pumping
the energy back to the mains is even better :) But the potential
fault currents stop me going there.

Diesel trains dump their motion into heating air, seems it's the
best method that scales.

Grant.
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