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Old 7th July 2004, 01:03 AM   #21
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I was wondering if nobody here has ever heard of Kapton insulators. This is a sort of plastics often used for voice coil formers on speakers. I just collected some thermal data from the German company "Fischer Elektronik", they manufacture insulators of a lot of materials. Given a TO-247 footprint, these are the thermal resistances I have found:

Mica: 0,4 K/W
Silicone: 0,4 - 0,96 K/W
Aluminum oxide: 0,3 K/W
Kapton: 0,07K/W

I use Kapton insulators for many years now, and I have a lot less problems then before.
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Old 7th July 2004, 01:08 AM   #22
anatech is offline anatech  Canada
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Yup, that's the yellow film commonly used in car amps. It seems to work fine as well. I seem to remember it rips easily, but is otherwise resistant to damage. It is also used with heatsink compound (grease).
With any film you want to make sure nothing sharp "pokes" through to ruin your day.
-Chris
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Old 7th July 2004, 01:15 AM   #23
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Nelson Pass said:
"I like to think of bead blasting as creating a fractal surface
which in fact has a larger surface area than described by
the square of it's boundary dimensions. Kind of like the coast
of England or the fabric of space at the sub-atomic scale."

No argument on that theory... I've just found by experience that better intimate contact is more efficient. Perhaps you can tell me why???


Damon Hill said:
"Another possibility is simply increasing the thickness of the
heatsinking anodizing and mounting the device directly"

This can easily be done by having the sinks hard coat anodized instead of a simple anodize. Any standard hard coating process will penetrate the metal a ten thousandth as well as build up another tenth on the surface, there are some other hard coat process that build up even more thickness up to .003 or so. Hard coating is also far more fade resistant , scratch resistant and very color stable over long time periods. Standard anodize even when properly sealed will eventually fade out to a bluish or bronze or some other color, partly on its own and partly from exposure to light. Hard coating anodize is one of the toughest coatings known to man and can be literally diamond hard. Normal dielectric strength of standard hard coating is about 2kv. So..... I have yet to have my Aleph 2's done and am seriously considering just hard coating the sinks and chassis parts black and throwing the insulators away. I haven't yet thought of a reason not to do it this way.

Mark
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Old 7th July 2004, 01:55 AM   #24
Magura is offline Magura  Denmark
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Quote:
Originally posted by Netlist

I recall reading something about thermal isolation that had the best of both worlds. Must have been an old Elector magazine.
It appeared to be some kind of silpad that had the ability to become softer when heat was applied. The principle was to burn in the transistor or mosfet or whatever had to be isolated and after a period of time turn on the screws to tighten the contact with the heatsink.

/Hugo

You would be talking about What Laird sell as Thermaphase. It is the as far as I know best solution, as its easy to deal with and gives you the goo effect. Laird offers an very interesting product that Ive been using for a while with great results, Thermaphase on Kapton film, It have all the properties one could ever ask for, as its easy to handle, dosnt flow all over the place, got low thermal resistance (better than any of the other easy solutions) and is cheap.

Magura
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Old 7th July 2004, 03:01 AM   #25
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The yellow / orange sheet in car amp is Kapton? Where to buy this kapton, in sheet or roll?
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Old 7th July 2004, 03:21 AM   #26
sam9 is offline sam9  United States
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You can buy Kapton tape from Digikey.

If you can find the page in their catalog where the SilPads are, the same company also makes Kapton based pad. The product name is somethinh like "K-10", anyway, it has a "K" in it. They are not cheap comparred to SilPads but since the tape doesn't deform much, the Kapton pads are probablt better far all but the flaest surfaces.

I've use all of these with no problem, but only in Class-B. Class-A people probably need to be more finicky.
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Old 7th July 2004, 04:58 AM   #27
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I am careful to deburr the heat sink when using silpads. A good sharp burr will even do a number on mica. I have also used aluminum oxide (alumina) washers in the past, but torque is really critical because they crack so easily. I wouldn't use them in a production environment without a precision torque wrench. I'm refurbishing one of my old homebrews right now, and I replaced all the alumina under the TO-220s with mica because the alumina were all cracked.

A good way to apply grease to an insulator or device is to smear some on a round toothpick (or dip it) and roll it on.A wooden Q-tip handle works well, too. This technique gives you just enough grease for a good thermal interface without having it squish out everywhere when you tighten down the hardware. BTW, I always called the stuff bird dung (insert other expletive as desired).
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Old 7th July 2004, 08:23 AM   #28
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Quote:
Originally posted by Andypairo


The problem is their availability (and probably cost).
Do you have a reasonably-priced source?

Cheers

Andrea
In Poland such pads are available without problems.
For example for transistor case TO218/TO247 ( IRFP044 for example) such pad which has 3mm thickness and has price about
0.9$ with tax for one unit if You take only one and 0.85$ with tax for one unit if You take 5 units. I utilized them in my Aleph3 in one transistor output version ( something like mini-Aleph) where IRFP044N issues over 40W.
With mica and silikon I had problems with case temperature of transistors. With mentioned pads none problems.

Regards
Jacek
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Old 7th July 2004, 09:06 AM   #29
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Quote:
Originally posted by HBarske
I was wondering if nobody here has ever heard of Kapton insulators. This is a sort of plastics often used for voice coil formers on speakers. I just collected some thermal data from the German company "Fischer Elektronik", they manufacture insulators of a lot of materials. Given a TO-247 footprint, these are the thermal resistances I have found:

Mica: 0,4 K/W
Silicone: 0,4 - 0,96 K/W
Aluminum oxide: 0,3 K/W
Kapton: 0,07K/W

I use Kapton insulators for many years now, and I have a lot less problems then before.
If we compare only thermal resistance there should be almost none differences between alumnium oxide and mica. But difference at least in my circuits is huge !. I think that very important is thermal conductivity. Alumnium oxide has about 24W(m*K) , best material as beryl oxide almost 201W(m*K), kapton and others significanty less.

Regards
Jacek
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Old 7th July 2004, 09:48 AM   #30
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Do you need to use paste with Kapton insulators? I got 50 TO247 insulators with one of the recent group buys from Fischer
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