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Old 12th April 2002, 07:36 PM   #11
Gabster is offline Gabster  Canada
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With about a inch square, 0.75 still valid in Cin^2/W.

Since the IRFP244 FET can opererate with a junction temperature up to 150C, I guess that 55C at the junction is not critical.

Thanks for helping, now I can buy these heatsink with peace of mind.

ciao,
Gabriel
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Old 13th April 2002, 02:59 AM   #12
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Somethings up with the program if it's telling you that you'll end up with a junction temp of 55C..........

if i've got my facts correct,

25W per device
each device has it's own heatsink
each heatsink is rated 0.82C/W for 100C rise .... about 1.134C/W for our puposes
and we need a electrical insulator in there too.

So,

Ambient = 25C
25 x 1.134 = 28.35C (heatsink)
25 x 0.75 = 18.75C (electrical insulator + 2 layers of grease)
25 x 0.83 = 20.75C (case to junction)

So, Junction temp should be up around 95C for the outter devices and probably about 110C for the inner devices....

Now, looking @ the datasheet,

@Tj = 110C, irfp244's are rated @ 75W and 215V
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Old 13th April 2002, 03:24 PM   #13
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Alright, I had misunderstood the graph. This is only simulating the heatsink...55C (58C at 23C ambient), is the temperature at the middle of the heatsink not at the junction of the FET. Still, the temperature of the heatsink itself will be pretty acceptable and this is what I was trying to figure out.

Looking at the written results, the simulator is stating (for 23C ambient) 97.4C exterior (case) and 118.2C interior (junction). Is 118.2C too much (acceptable) for the FET (max. 150C). I know it will influence the FET's life, but is it critical? What would be the ideal temperature range?

To me, it seems that the FET will be untouchable...almost dangerous.

regards,
Gabriel
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Old 14th April 2002, 02:47 AM   #14
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That's probably going to work out too high as the internal rows of fets will be in an ambient of about 50C and that's assuming that the ambient in the room never exceeds about 25C while operating the amps.

assuming best case situation, if all fets had case temp of 100C and junction temp of 120C,

IRFP244 are rated @ 60W and 210V.

more likely is that the outer rows of fets are close to the above temps and the inner rows are 30C hotter. case temp of 130C and junction temp of 150C,

IRFP244 are rated @ 24W and 200V.

If that is the case, those devices will fail very quickly.
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Old 14th April 2002, 04:34 AM   #15
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The point is therefore to improve the thermal conductivity between the FET and the heatsink. There's actually two variables: (1) the two layers of grease and (2) the insulator.

1) Digikey sell grease with 0.05C/W per 0.001" layer.
http://www.wakefield.com/pdf/Accessories.pdf

2) Digikey also sells mica insulators:
"These precision stamped MICA Insulators provide good thermal conductivity (.009 watts per square inch per
C per inch in thickness". These are 0.003" thick. Thus 0.33Cin/W~0.33C/W

This brings down the thermal resistance for the insulation from 0.75 to 0.43; 0.43x25W=10.8C which is not really enough; still 85C at the case and 100C at the junction.

Is there better insulator/grease out there? I really need the best to solve this problem.

Furthermore, one unknown at this moment is the heat dissipation done by the casing and supporting aluminum bars. I'm quite sure this is not neglectible. Combined with a massive venting (for the inner rows of heatsinks) I guess I could give it a try, build the thing and run a real life simulation. This will be the only way to have the real answers.

Once again, what would be the ideal temperature range? Is 100C at the junction acceptable?

best regards,
Gabriel
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Old 14th April 2002, 04:38 AM   #16
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100C junction temp is fine.

also, dont count on achieving started thermal resistance of grease + mica + grease as it is highly dependant on mounting pressure which for mosfets is pretty low.
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Old 14th April 2002, 10:38 AM   #17
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Quote:
Originally posted by Gabster
The point is therefore to improve the thermal conductivity between the FET and the heatsink. There's actually two variables: (1) the two layers of grease and (2) the insulator.

1) Digikey sell grease with 0.05C/W per 0.001" layer.
http://www.wakefield.com/pdf/Accessories.pdf

2) Digikey also sells mica insulators:
"These precision stamped MICA Insulators provide good thermal conductivity (.009 watts per square inch per
C per inch in thickness". These are 0.003" thick. Thus 0.33Cin/W~0.33C/W

This brings down the thermal resistance for the insulation from 0.75 to 0.43; 0.43x25W=10.8C which is not really enough; still 85C at the case and 100C at the junction.

Is there better insulator/grease out there? I really need the best to solve this problem.

Furthermore, one unknown at this moment is the heat dissipation done by the casing and supporting aluminum bars. I'm quite sure this is not neglectible. Combined with a massive venting (for the inner rows of heatsinks) I guess I could give it a try, build the thing and run a real life simulation. This will be the only way to have the real answers.

Once again, what would be the ideal temperature range? Is 100C at the junction acceptable?

best regards,
Gabriel
Yep,

Fischer Elektronik KAP-218 kapton insulators.... 0.07 C/W ! No need for grease as this is flexible stuff.

See: www.fischerelektronik.de probably other companies make it too.

http://www.fischerelektronik.de/fisc...Seiten/I12.pdf

Edwin
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Old 14th April 2002, 12:46 PM   #18
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kapton is a great insulator but it wont approach that rated thermal resistance given the mounting pressure we use .... also, although it is flexible, it has poor gap filling ability so grease will still be required.

best most practical insulator is mica.
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Old 14th April 2002, 02:00 PM   #19
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Quote:
Originally posted by AudioFreak
kapton is a great insulator but it wont approach that rated thermal resistance given the mounting pressure we use .... also, although it is flexible, it has poor gap filling ability so grease will still be required.

best most practical insulator is mica.
High Performance Kapton-Isolierscheiben
very low thermal resistance
optimised thermal conductivity
best mechanical characteristics
Polyimide carrier foil with silicone-free
thermal conductive THERMAPHASE
layer, completely coated on both sides
clean processing, no abrasion of
the coating
stacked foils do not stick together
good resistance against cleaning agents
no cold flow
low pressure force necessary,
thus particularly suited for spring-fixing
of semiconductors
cutouts and special versions according
to customers requirements

So LOW pressure force... spring-fixing !

Edwin
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Old 14th April 2002, 03:00 PM   #20
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that actually sounds pretty good.

i dont particularly like phase change materials but hey it might be worth a shot.
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