Hi pooge,
I used to be able to buy the mica ones from my Japanese parts supplier. He's gone now. If you can get flat stock of anything for this application, use it.
Hi j beede,
Luckily, I have never had this happen to me. The idea was abandoned, although some small manufacturers may try this out from time to time.
Hi Nordic,
If the amp runs as warm as any other amplifier does, I'd install mice insulators. I'm not a betting man by any means.
You have good instincts.
Hi norb,
If you did over-tighten your outputs, you may have also distorted the package. This may crack the die or loosen it from the heat spreader or case. If you see any warp in the transistor case, consider replacing them. I get failed repair jobs where this is the cause of the failure. Over-tightening semiconductors is extremely common amongst technicians. There are no classes on mechanical assembly or general workmanship available, and shops just want a young tech to churn stuff out.
-Chris
I used to be able to buy the mica ones from my Japanese parts supplier. He's gone now. If you can get flat stock of anything for this application, use it.
Hi j beede,
That's just about what it is. Proper heat sink compound works better.
This was tried in the 80s to save on weight. Units have died because they were put in "standard service position". The fluid ran to one side and ceased to transfer heat.
Luckily, I have never had this happen to me. The idea was abandoned, although some small manufacturers may try this out from time to time.Hi Nordic,
Yes, it does. The higher the temperature, the faster this happens.
I would if I were you. I recommend this be done for people I do service for.
They may be relying on the anodizing to insulate the parts. That's a bet I won't take. Not unless they run the heat sink at B+ and another at B- to eliminate some thermal resistance. That may leave the heat sink live though.
If the amp runs as warm as any other amplifier does, I'd install mice insulators. I'm not a betting man by any means.
You have good instincts.
Hi norb,
That means either the surface was not clean and smooth, or excessive mounting force was used. If you over-tighten parts on a heat sink, you can easily distort the surface of the heat sink as well. The surface can be pulled outwards and will need to be sanded down again. Many times you may see a small chamfer around holes to reduce the chances of material pulling up.
If you did over-tighten your outputs, you may have also distorted the package. This may crack the die or loosen it from the heat spreader or case. If you see any warp in the transistor case, consider replacing them. I get failed repair jobs where this is the cause of the failure. Over-tightening semiconductors is extremely common amongst technicians. There are no classes on mechanical assembly or general workmanship available, and shops just want a young tech to churn stuff out.
-Chris
Hi Chris,
Have you ever tried the Wakefield Kapton insulators? The ones I was thinking of are the 175-6 series described in this PDF file. Strangely, if you try to find the Wakefield accessories PDF from their site links, you reach a PDF that is missing key information on these insulators. I found the link above (which has complete information) by coincidence at newark.com.
Just curious to see if you've used them, and if so, what your thoughts are.
Have you ever tried the Wakefield Kapton insulators? The ones I was thinking of are the 175-6 series described in this PDF file. Strangely, if you try to find the Wakefield accessories PDF from their site links, you reach a PDF that is missing key information on these insulators. I found the link above (which has complete information) by coincidence at newark.com.
Just curious to see if you've used them, and if so, what your thoughts are.
Hi Andy,
No, I haven't tried these that I know of. I have used some pretty exotic insulators that are greaseless. There is one that comes from an unknown manufacturer that is supposed to be gap filling. Most are a one use product. In my opinion, if you really need to get the thermal resistance down that much, the part is dissipating too much power, use more devices. For an OEM, the labour saving would be the key. It is very difficult to get even distribution without contamination in a normal factory setting. You should see what I've pulled out of products that failed shortly after purchase. A greaseless insulator really helps in this regard.
Thank you very much for this information! I see that it even includes mounting torque!
Everyone should download this information!
I have used Deltabond 155. It's really good stuff. As a matter of fact, the last of mine has dried up. I will buy some more if I need it. It is beautiful stuff for increasing small heat sink area by adding another. Attaching bias compensation transistors to output bodies is a snap with this stuff.
I think that a hobbyist is better off with mica and grease. I only use thermal pads for prototypes and transistor testing and matching. Once everything is figured out, thermal compound and mica. This is a forgiving product that always produces good results. With the greaseless insulators, there is a tendency to reuse the product. The entire point is that most are single use - for good reason. Often, they are also difficult to completely remove from the heat sink surface, more so than grease is.
Humans are lazy.
-Chris
Edit: I use the 120 series compound in the 8 oz jar.
No, I haven't tried these that I know of. I have used some pretty exotic insulators that are greaseless. There is one that comes from an unknown manufacturer that is supposed to be gap filling. Most are a one use product. In my opinion, if you really need to get the thermal resistance down that much, the part is dissipating too much power, use more devices. For an OEM, the labour saving would be the key. It is very difficult to get even distribution without contamination in a normal factory setting. You should see what I've pulled out of products that failed shortly after purchase. A greaseless insulator really helps in this regard.
Thank you very much for this information! I see that it even includes mounting torque!
Everyone should download this information!
I have used Deltabond 155. It's really good stuff. As a matter of fact, the last of mine has dried up. I will buy some more if I need it. It is beautiful stuff for increasing small heat sink area by adding another. Attaching bias compensation transistors to output bodies is a snap with this stuff.
I think that a hobbyist is better off with mica and grease. I only use thermal pads for prototypes and transistor testing and matching. Once everything is figured out, thermal compound and mica. This is a forgiving product that always produces good results. With the greaseless insulators, there is a tendency to reuse the product. The entire point is that most are single use - for good reason. Often, they are also difficult to completely remove from the heat sink surface, more so than grease is.
Humans are lazy.
-Chris
Edit: I use the 120 series compound in the 8 oz jar.
Thanks. I was mainly considering the Kapton insulators for convenience reasons due to not having to mess around with grease. They appear to have the second-highest thermal resistance of the group. Looks like the 174 series insulators have the lowest thermal resistance of the bunch. They're probably the best bet. I'll use those with the 120 series compound as you suggest. Good insulators make a surprisingly big difference in junction temperature calculations! And it's a low-cost thing, so I'd like to do as well in this area as possible.
Hi Andy,
Your heat sink probably has more of an effect. That, the surface smoothness and proper mounting pressure. If you add airflow (proper orientation) or forced air, your junction temperature should really drop.
I suspect that below some thermal resistance figure, other things in the thermal path become more important.
Since you are looking at a sim, consider using two heat sinks with one transistor mounted on each. Set them up as a current source to pass the same current with the same voltage across them. Leave them running for a while until they reach thermal equilibrium, then measure the temperatures of each. A measure of temperature between the case and heat sink might illustrate this better if there is enough heat to be dissipated.
Your biggest problem with the test might be an unforeseen draft hitting one and not the other. after doing this simple test, you will have empirical data to compare your sim with.
-Chris
Your heat sink probably has more of an effect. That, the surface smoothness and proper mounting pressure. If you add airflow (proper orientation) or forced air, your junction temperature should really drop.
I suspect that below some thermal resistance figure, other things in the thermal path become more important.
Since you are looking at a sim, consider using two heat sinks with one transistor mounted on each. Set them up as a current source to pass the same current with the same voltage across them. Leave them running for a while until they reach thermal equilibrium, then measure the temperatures of each. A measure of temperature between the case and heat sink might illustrate this better if there is enough heat to be dissipated.
Your biggest problem with the test might be an unforeseen draft hitting one and not the other. after doing this simple test, you will have empirical data to compare your sim with.
-Chris
From Zhoufang is the only place I know. I may be interested in a few pieces.
http://www.diyaudio.com/forums/vend...therm-red-86-83-isolators-247-1-ea-stock.html
I have seen questions about availability of kerafol. It is available in sheets from conrad.de or conrad.nl in EU and pre-cut from zhoufang on this site.
Conrad Electronic - System Information
Conrad Electronic - System Information
I was planning on ordering Kerafol from Conrad in Holland when I came across another product in their online catalogue with an even higher (25 W/mK !!) heat transfer rate. I enclose the datasheet. Anyone know this stuff?
Price (at Conrad in Holland, incl. 19% VAT/GST) is 0,54 euro each, dropping to 0,38 each at qty 100.
Price (at Conrad in Holland, incl. 19% VAT/GST) is 0,54 euro each, dropping to 0,38 each at qty 100.
http://www.diyaudio.com/forums/pass-labs/37262-mica-goop-2.html#post1034968
Think thickness difference.
Think thickness difference.
Ah yes, I forgot that at 25W/mK, it is still nearfly a factor 10 lower than Aluminium. Thinner would be much better!
They actually have thinner versions (not as thin as Kerafol, though):
Keramische isoleerschijf TO247 in de Conrad online shop
If I remember it correctly, heat transfer through a solid slab is defined as
A = area of slab
t_1 and t_2 are temperatures on either side of the slab
b = thickness of slab
We know that the Kerafol material has k = 6,5 W/mK and is 0,25mm thick and the Quick Cool material has k = 25 W/mK but is 1,5mm thick.
Assume (for arguments sake) that A, t_1 and t_2 are constants.
While the Quick Cool pad is not quite as good as Kerafol (6,5 / 0,25 = 26 vs. 25 / 1,5 = 16,7), it seems to be better than a lot of the other options, including the cheaper Kerafol 70/50 of similar price (it has k = 1,4W/mK and is 0,25mm thick giving 5,6).
Warmtegeleidend folie 70/50 TO-247 in de Conrad online shop
They actually have thinner versions (not as thin as Kerafol, though):
Keramische isoleerschijf TO247 in de Conrad online shop
If I remember it correctly, heat transfer through a solid slab is defined as
q = ( k * A (t_1 - t_2 )) / b
where k = heat conductance constant of the slab material
A = area of slab
t_1 and t_2 are temperatures on either side of the slab
b = thickness of slab
We know that the Kerafol material has k = 6,5 W/mK and is 0,25mm thick and the Quick Cool material has k = 25 W/mK but is 1,5mm thick.
Assume (for arguments sake) that A, t_1 and t_2 are constants.
While the Quick Cool pad is not quite as good as Kerafol (6,5 / 0,25 = 26 vs. 25 / 1,5 = 16,7), it seems to be better than a lot of the other options, including the cheaper Kerafol 70/50 of similar price (it has k = 1,4W/mK and is 0,25mm thick giving 5,6).
Warmtegeleidend folie 70/50 TO-247 in de Conrad online shop
Had not seen the link before replying. Did not know Al2O3 had been discussed. Interesting.
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