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Old 11th April 2006, 11:26 PM   #1
ttan98 is offline ttan98  Australia
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Default Mounting Mosfet on Heat Sink

I am sure most of you must have experience with mounting Class A mosfet on the heatsink. The way it is mounted is of utmost important in order to transfer heat generated to the heat sink.

a. I assume all the mosfets must have a ceramics insulation between the heatsink and mosfet. Both sides of the insulation must be coated with thermal paste. Computer heatsink paste suitable?

b. How do I ensure when screwing the mosfet on the heatsink, the mosfet is flat?

c. I believe I need an insulation screw slot(to hold the scew) to isolate the mosfet backpak from the heatsink.

Can those who have more inputs to this please comment.
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Old 12th April 2006, 01:47 AM   #2
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I think this is a great solution for a variety of reasons. I think the most important is that it keeps pressure on the chip through thermal cycles. You can use a sil pad underneath the FET. No goo is needed in the case of Silpads.

the clip...
http://www.mouser.com/catalog/625/1481.pdf

THERMAL PADS
http://www.mouser.com/?Ne=101&handle...=360&Ns=SField
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Old 12th April 2006, 06:58 AM   #3
JCM is offline JCM  United States
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Quote:
a. I assume all the mosfets must have a ceramics insulation between the heatsink and mosfet. Both sides of the insulation must be coated with thermal paste. Computer heatsink paste suitable?
Yes apply thermal paste to both sides. Usually the insulator is mica. I dunno about computer grease / paste - but I would think it would be the same as 'standard'.

>b. How do I ensure when screwing the mosfet on the heatsink, the mosfet >is flat?

If the heatsink is flat, the mosfet will lay on it flat - nothing difficult.

>c. I believe I need an insulation screw slot(to hold the scew) to isolate the >mosfet backpak from the heatsink.

That sleeve / insulated bushing IS needed for TO-220 cases - but NOT for TO-247cases. If the mounting screw makes contact with a surface of the case which is electrically connnected to one of the leads, yes, use a bushing.

A fairly good-sized washer might be used on the face of the transistor to make sure the tightening force is applied evenly. Sometimes a flat bar is used for one or more transistor.

Some threads about insulators and sil-pads, etc. :
here

and here

A little more info

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Old 12th April 2006, 02:37 PM   #4
ttan98 is offline ttan98  Australia
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Many thanks to chipco and particularly to JCM........
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Old 13th April 2006, 09:15 PM   #5
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For to-247 devices a thermal pad works great. Here's a link picturing how my Aleph30 mosfets were mounted using a #6 socket cap screw and thermal pad.

http://www.diyaudio.com/forums/showt...719#post870719

I used Bergquist pad's from Digikey that worked well:

http://www.digikey.com/scripts/DkSea...311093&Site=US

-David
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Old 13th April 2006, 10:52 PM   #6
ttan98 is offline ttan98  Australia
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dw8083,

Thermal pad will work but it is better or worse than mica with thermal paste(gool)? that is the question.. What is the best way to transfer heat away from the heat sink.

You see many computer DIY's remove their thermal pad between the CPU and heatsink and replace it with thermal paste. This way the transfer of heat is more efficient.

During amp testing period it is cleaner and easier to use thermal pad. For final installation, remove the thermal pad and replace it with mica and thermal paste. The latter has better heat transfer..If you think otherwise let me know.
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Old 14th April 2006, 11:18 AM   #7
Bensen is offline Bensen  Belgium
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Here is my solution for mounting FET's on a heatsink. It worked very easily. All the FET's are pushed to the heatsink with only two M6 bolts. This way is also better than using one bolt per FET, because now you push to the total surface of the semiconductor and not to a small round area.

I'm going crazy, please help, Pic's

Finished my solid state plate amp

Ben
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Old 14th April 2006, 12:27 PM   #8
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If the heatsink is anodized and free from deep scratches on the surface where the mosfet touches, then you can dispense of the silpads, and mount the mosfet directly on the heatsink, with paste. This lowers the thermal resistance by a factor of 5 or so.
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Old 14th April 2006, 07:52 PM   #9
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Quote:
Originally posted by Lars Clausen
If the heatsink is anodized and free from deep scratches on the surface where the mosfet touches, then you can dispense of the silpads, and mount the mosfet directly on the heatsink, with paste. This lowers the thermal resistance by a factor of 5 or so.
That works well - I do it on the F3 with the power JFET, where
the drain voltage is only a few volts above ground.
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Old 14th April 2006, 08:56 PM   #10
jleaman is offline jleaman  Belgium
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Quote:
Originally posted by Lars Clausen
If the heatsink is anodized and free from deep scratches on the surface where the mosfet touches, then you can dispense of the silpads, and mount the mosfet directly on the heatsink, with paste. This lowers the thermal resistance by a factor of 5 or so.

Are you sure that won't cause a short ?

I use the ceramic pad's. WOrk well and will never ever cause a short
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