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Old 8th April 2003, 10:12 AM   #1
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Default Isolating each Mos-Fet?

I'm building Anthonys sym-amp but after reading a thread about different method of mechanical design(dont remember wich..) I want to change....

Is there any problem to mount all IRFP240 on one copper plate and all IRFP9240 on another, so that they are electricly connected on each plate, and then isolate the two plates and mount them to the heatsink? Im hopin this will better the thermal resistance. I really dont see any problems with this, or is there (oscillating??)? What do you think?

Thanx!

Richard
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Old 8th April 2003, 10:27 AM   #2
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That should be ok but I wouldnt count on getting lower Rth because you've then got an extra thermal joint.
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Old 8th April 2003, 11:40 AM   #3
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Richard, I guess your cooling is not a big problem. Adding copper won't add much besides the coolness factor. It's nothing wrong with mounting transistors on isolated heatsinks but the basic rule is to keep it compact. I would recommend to have grounded heatsinks and isolated MOSFET's.

As Audiofreak says, try to have as few thermal junctions as possible. The best thing is actually to mount the transistors directly on the heatsink (if it's possible).
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Old 8th April 2003, 11:43 PM   #4
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Default Re: Isolating each Mos-Fet?

Quote:
Originally posted by richeros
I'm building Anthonys sym-amp but after reading a thread about different method of mechanical design(dont remember wich..) I want to change....

Is there any problem to mount all IRFP240 on one copper plate and all IRFP9240 on another, so that they are electricly connected on each plate, and then isolate the two plates and mount them to the heatsink? Im hopin this will better the thermal resistance. I really dont see any problems with this, or is there (oscillating??)? What do you think?

Thanx!

Richard
Gonna do the same thing as you describe for the Holton
Av800 I want to prototype. Gonna use 3/16" thick copper
plate and mount each mosfet to the copper with no
insulation, just white goo. Then isolate the copper plate
(mica?) to the aluminum heatsink. If you search the archives,
there is a good thread about this. 1/2 the people seemed
to like the idea, the other 1/2 didn't. heheh

In my case, my copper plate is 3/16" thick x 1.5" width
and the length is about 10".. The heatsink plate is 1/4" thick
aluminum.

Doesn't hurt to experiment.

Also, I wonder if the copper plate will be a good +V and -V
for the transistors since the tab = drain if I'm not mistaken?

I might try this stuff instead of generic white thermal compound..
http://www.arcticsilver.com/as3.htm

Anyone use this before ?
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Old 9th April 2003, 12:14 AM   #5
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i've used it on my CPUs HSF. if you want info on it you may wanna go to any computer hardware forum, overclockers are always trying to get the extra degree of cooling.

as i recall, hardocp.com did a review:

http://www.hardocp.com/article.html?art=MTQ0

however it only goes up to AS2, not AS3.
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Old 9th April 2003, 03:50 AM   #6
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Default Flat chat. or, On the level.

I have done this twice and my experience is - too get good results the copper and heatsink surfaces really have to be FLAT. The extruded aluminium heatsink I used had a bit of a bow in it; no big deal for small contact areas but when you span a much greater width with your copper plate there is a far greater chance of only marginal contact.

There are much better heat conducting materials than mica available nowadays from the likes of Bergquist, Chomerics and the like. The tradeoff in different grades of materials offered is voltage rating vs thermal resistance. You don't need much of a voltage rating so this should work out well.
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Old 9th April 2003, 04:03 AM   #7
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Default tip

just a tip i tryed arcticsilver on eletronics if your heat zinc is grounded it will short......

conducts electricity..... allso shorts out athlon's
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Old 9th April 2003, 04:28 AM   #8
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hmm, that's odd. my athlon is very much fine with the AS3. i need to put the kt266/333 cooling mod back on. i had cpu temps 2 degrees BELOW ambient at idle! full load was the same. i've used AS3 for a TBird and a Palimino and had no ill effects.
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Old 9th April 2003, 04:35 AM   #9
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Default athlon

ya works good on athlon's if thin my customers put it on thick


doo

a fet hmm u take your chances
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Old 9th April 2003, 04:49 AM   #10
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Default oxide

peter may be still selling these thay work great and you dont need to make the fet super tight for it to work good..hmm the name i think was ceramic oxide. no goo needed
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