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From: Hammy on 28 Jul 2010 11:52
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.

>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

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.

>The thermal resistance from junction to ambient Rth (j-a) consists of
>the thermal resistance from junction to case Rth (j-c) in series with
>the thermal resistance from case to ambient Rth (c-a). The thermal
>resistance from junction to case can be found from the transistor data
>sheet.
>
>Assuming that the Rth (j-c) is 0.20 C/W (what kind of package is this
>good?), thus Rth (c-a) would be 0.23 C/W, since 0.20+0.23=0.43 C/W
>
>>Would it be possible to get 300 to 400W total dissipation if I
>>parallel 2 to 4 FETS on the same heatsink? Or would I have to buy 2 or
>>three more of the large heatsink?
>
>Putting two transistors on the same heatsink will effectively divide
>the Rth(j-c) by two, but it does not affect Rth (c-a) thus Rth (j-a)=
>0.20/2+0.23=0.33 C/W. Thus P=(110-50)/0.33=182 W.
>
>With 4 transistors on the same small heatsink (assuming the extra
>transistors do not disturb the air flow) Rth (j-a)=0.20/4+0.23=0.28
>C/W and hence P=214 W.
>
I see so I would need two of those sinks with 2 to 3 FETS per.
Thanks for the detailed explanation.
From: Hammy on 28 Jul 2010 11:57
On Wed, 28 Jul 2010 14:57:56 +0200, Uwe Hercksen
<hercksen(a)mew.uni-erlangen.de> wrote:

>
>
>Hammy schrieb:
>>
>> 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.
>>
>> Would it be possible to get 300 to 400W total dissipation if I
>> parallel 2 to 4 FETS on the same heatsink? Or would I have to buy 2 or
>> three more of the large heatsink?
>
>Hello,
>
>if your calculation is right, two of those fets mounted on two of those
>heat sinks will dissipate 140 W each and 280 W together.
>Why do you think that one heatsink of the same size will be enough for
>300 to 400 W?
>
>Bye

Actually my suspicions were correct. See Paul and Johns post below.

If each transistor isnt carrying the full power I figured it must have
some impact on the thermals. I just wasnt sure how much.
From: John Larkin on 28 Jul 2010 12:07
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.
>
>>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
>
>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.


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.

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.

John

From: John Larkin on 28 Jul 2010 12:11
On Wed, 28 Jul 2010 23:17:53 +1000, Grant <omg(a)grrr.id.au> wrote:

>On Wed, 28 Jul 2010 07:48:39 -0400, Hammy <spam(a)spam.com> wrote:
>
>>
>>
>>When mosfets are sharing a heatsink how does this effect the total
>>power handling capability of the fets.
>>
>>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.
>>
>>Would it be possible to get 300 to 400W total dissipation if I
>>parallel 2 to 4 FETS on the same heatsink? Or would I have to buy 2 or
>>three more of the large heatsink?
>>
>>The mosfets are operating in linear mode it's for a variable
>>electronic load.
>>
>>To avoid thermal runaway in a fet due to Vgs( th) differences between
>>devices is it best to use dedicated opamps per fet or large source
>>resistors? I've read several papers but thought I'd ask here for
>>someone who has maybe done something similar.
>
>
>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 and could easily handle 400W, with an 8 x 0R33,
>50W metal pack resistors on other side of heatsink. Two parts of the
>heatsink mated to form an 80mm by 200mm tunnel that one bolted a fan
>to, I used a 90mm fan via adapter.
>
>0R22 source resistor too small to balance FETs better than 100%, so
>I selected best match eight from batch of 20, expensive and still
>not a good performer for intended use, okay for manual operation.

Shifting some of the heat away from silicon and into power resistors
is a good deal, when conditions allow, which means serious resistance
in the source and/or drain. I like these:

ftp://jjlarkin.lmi.net/Welwyn.JPG

An opamp per fet is a good way to balance dissipation accurately.

John


From: Paul Keinanen on 28 Jul 2010 14:10
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.

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