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Old 17th November 2004, 06:52 PM   #1
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Default interface resistance

HI everybody,
this is my first post to this forum.
I wish to build a class A ampli with IRPF150 mosfet (in TO-247AC case) to polarize with 2.5-3 A.
Now I am dimensioning the heatsink and I wish know thermal resistance case to sink.
IRPF150 datasheet suggests a Rcs=0.24C/W for "flat, greased surface".
What kind of grease will I buy?
Is this the best interface material to optimize heat dissipation?
When I purchased the mosfets they give me also any grey little sheet. How can I utilize them and what is their thermal resistance?
Is TO-247AC thermal resistance similar to TOP-3?

thanks for your answer and sorry my bad English!


ciao
antonio
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Old 17th November 2004, 08:18 PM   #2
AndrewT is offline AndrewT  Scotland
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Hi
T03p is nearly the same as TO247. The grease should be thermal, the suppliers will probably stock a variety. I've seen reference to silicon drying out but I cannot confirm.
Rcs for an insulator varies on which material you use - from 0.4 to 1 degree/watt. Some insulators are dry fit, they are more reliable, cleaner but often more resistance than mica and thermal grease. Thin Kapton is another option, 6thou has too much resistance imo. try to find 1thou.
regards Andrew T.
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Old 17th November 2004, 08:19 PM   #3
Mr Evil is offline Mr Evil  United Kingdom
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Silicone thermal paste is the norm, being cheap. Better is something like Arctic Silver, which can gain an order of magnitude improvement.

The interface resistance depends a lot on how much pressure you use to clamp the devices down, and how evenly it is applied. Putting a bolt through the hole in the plastic package is the worst. Clamping them down with a flat bar across the device is much better. Doing that you can get down lower than 0.24C/W.

If you need to electrically insulate the devices from the heatsink then the thermal resistance will be somewhat higher.
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Old 18th November 2004, 01:53 AM   #4
anatech is offline anatech  Canada
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Hi alphadelta,
Clamp using even pressure, but not too much pressure. Make sure the heatsink is smooth and not cupped or bowed. Screws are fine as long as you follow the mounting guidlines. Basically, snug but not real tight. A lockwasher will help here. Don't try to get close to the thermal limits of your heatsink and you will be fine. With heatsinks, too big is far better than a little small.
-Chris
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