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Old 24th September 2013, 09:38 AM   #4361
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Quote:
Originally Posted by keantoken View Post
For this reason I've always been very interested in this idea:

Another way to insulate transistors from heatsinks

But now I've found the same can be done without using a heat spreader bar.
That's more or less what I did though my bar was bigger & differently shaped. From that thread, it looks like I could have used a much smaller bar .. but exact details are hidden by the fog of senility.
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Old 24th September 2013, 10:06 AM   #4362
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Originally Posted by AndrewT View Post
This has a substrate that is 3mil Kapton.
That instantly tells me it is a bad Thermal washer.
Please read more carefully. I wrote k52-1. One third the thickness.
What is better?

Best wishes
David
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Old 24th September 2013, 10:13 AM   #4363
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Originally Posted by Dave Zan View Post
Any one know a lower thermal resistance product?

Best wishes
David
Table 1 from EUVL's Linear Audio Vol 3 article.

Know your literature

Jan
Attached Images
File Type: jpg thermal.jpg (106.1 KB, 298 views)
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Old 24th September 2013, 10:28 AM   #4364
AndrewT is offline AndrewT  Scotland
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Quote:
Originally Posted by Dave Zan View Post
Please read more carefully. I wrote k52-1. One third the thickness.
What is better?

Best wishes
David
the 1mil and 2mil thickness Kapton will be better.
But still no better than 1mil mica.
Jan's post shows 2mil Kapton and 2mil mica for comparison.
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Old 24th September 2013, 12:08 PM   #4365
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Quote:
Originally Posted by jan.didden View Post
Table 1 from EUVL's Linear Audio Vol 3 article.
Know your literature
Thanks Jan.

Actually I DO know that one. I read the best literature
But I opened an incorrect folder and and posted the K52-1 when I meant to post the Keratherm. Mea Culpa.
So anyone can beat the Keratherm?

Best wishes
David
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Old 24th September 2013, 07:45 PM   #4366
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Quote:
Originally Posted by keantoken View Post
Another way to insulate transistors from heatsinks

But now I've found the same can be done without using a heat spreader bar.
Kean, can you tell us how you do this without a heat spreader bar?
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Old 24th September 2013, 09:16 PM   #4367
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All you need is a thin film to use as a spacer between the transistor and heatsink. You'll just have to look for thin films around the shop with a micrometer, unless you're willing to buy something online. I found that wax paper had a thickness of 1mil, but that's a pretty questionable material for heatsink interfacing. I never would have thought so, but heavy duty aluminum foil has a thickness of 0.75mil. So I cut out a very small foil washer to fit within the plastic ring around the screw hole in the TO-247 type package. Depending on the transistor package, it may be more convenient to put the foil in other places, for instance in strips along the top and bottom providing those parts are plastic.

I wouldn't try this with an uneven heatsink (or one with deformed screwholes), and I found most of my heatsinks weren't very even when held up to the light to inspect the gap. TO-247 would be most workable for deformed heatsinks since the top and bottom are plastic and it can be supported by foil strips around the edges rather than a washer around the screw.
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Old 24th September 2013, 09:53 PM   #4368
Waly is offline Waly  United Kingdom
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Quote:
Originally Posted by Dave Zan View Post
Anodize is not a cost problem in my case.
The elimination of lots of fiddly little washers would be nice.
But not if anodize is as problematic as Andrew and Toni think.
Not sure if you get the concept of "thick" or "hard" anodizing. That's much different from the usual aluminum coating, see the Anodizing Reference Guide. Oxide layers are orders of magnitude thicker and harder than type I or type II. If you can get such hard anodizing for cheap, then definitely use it - and don't forget to post some pictures. You can hope to get a 0.2 C/W improvement in the Rthc-r over the standard greased mica or thin kapton sheet. That would help with a 5-8 degrees case and junction temperature reduction.

Adding another pair to your existing n output devices pairs will lower the junction temperature with as much as P*(Rthj-c + Rthc-r)/(n+1). For say five pairs of output devices (n=5), adding another pair would help as much as a 0.15-0.2 C/W reduction in Rthc-r. That would be about as much as switching from washers to hard anodizing, and if you think this is worth the cost, then again, so be it. But I'm afraid you are as concerned about the heat sink washers as about RHPs in audio amplifiers .

Funny enough (but intuitive if you think it through) lowering the case to heat sink thermal resistance is much more efficient in keeping the junctions cooler for only one or two output pairs.
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Last edited by Waly; 24th September 2013 at 10:17 PM.
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Old 25th September 2013, 01:08 AM   #4369
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Quote:
Originally Posted by Waly View Post
Not sure if you get the concept of "thick" or "hard" anodizing. That's much different from the usual aluminum coating, see the Anodizing Reference Guide. Oxide layers are orders of magnitude thicker and harder than type I or type II. If you can get such hard anodizing for cheap, then definitely use it - and don't forget to post some pictures. You can hope to get a 0.2 C/W improvement in the Rthc-r over the standard greased mica or thin kapton sheet. That would help with a 5-8 degrees case and junction temperature reduction.
Thermal interface speed is important indeed.
Anyway, Tabor Extrusions makes heatsinks with a very thick glossy black oxide coating.
Drilling damages the coating and for example Onsemi outputs that have the plastic plug in the middle of the hole are okay; however, plain devices, such as LM1875 need the hole chamfered. To chamfer a device's mounting hole, you'd get a drill bit three times as big as the hole and drill into the hole on the back side of the device, but just a little bit, not all the way through. When the device is mounted, the little beveled will get filled full of thermal grease, preferably Arctic Ceramique or GC Waldham #44.
Also any sharp corners need sanded smooth prior to pasting and mounting.

Drilling from the fin side works a lot better because it doesn't make a little hill at the hole area, but drilling from the face side is problematic because the hole area gets taller.

The prospect of direct mounting onto oxide is more dodgy (less safe) than mica sheets due to the precision needed and that shipping the finished work may eventually cause contact, which would blow the fuses. It is hard to find the drill bits long enough to do it right.

Here is a chopped bit of a Tabor extrusion:
Aavid Thermalloy - 627253B06000 - Fans, Motors & Thermal Management - Heatsinks - Allied Electronics
Click the image to open in full size.
Probably enough for 160 watt Class AB amp (it would take two for stereo).
Using these to form the sides of the enclosure makes a funny size enclosure, 6" tall, but shallow at 10" deep. I wish they had 4" $24 sections, so much more convenient for me than the 6" $36 sections.
These can be bought from the manufacturer, Tabor, for much less than Aavid's $6 per inch retail price.
P.S.
Personally, I don't direct mount when the amplifier is in metal enclosures because mica is still safer. Even more important than thermal interface speed? Safety is more important (so I use mica). No free lunch.
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Last edited by danielwritesbac; 25th September 2013 at 01:23 AM.
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Old 25th October 2013, 04:26 PM   #4370
danny92 is offline danny92  Portugal
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Hi everyone and sorry for talking about a different matter,
I'm interested in making an amplifier using beta enhanced VAS + cascode, but I was warned by a friend, that said me that it was a discussion between Michael Kiwanuka and Bob Cordell, and it was proved that this topology is locally unstable. I've made the simulations following Michael Kiwanuka's instructions present in his .asc file and I've got the same results 192 degrees of phase shift at 0dB, I've changed this circuit to look a little more like the ones present in Bob's book and I have something between 170-175 at 0 dB which is really bad news
Removing the cascode I still have 166 degrees phase shift at 0 dB, which isn't good enough for me.
What do you think?
Beta enhanced VAS isn't that good?
I've made some experiments and I've noticed that adding base stopper resistors to the EFs seems to help reducing this phase shift problem, but could have other problems, the values should be calculated taking in account cpi(cbe) and cmiu(cbc), and then using 1/(2*pi*Cin*fmax), I've used the following fmax values, 10 MHz for the first EF, 2 MHz for the second, and 1 MHz for the output devices.
I Don't know if this solves the problem, but I think that maybe it's only encovering it. With 2SC/2SA as drivers it's almost impossible to have a low phase shift.
I will post the test circuits latter, and I will also test the effects with mosfets outputs.
Can anyone help me here?
I don't know almost anything about local stabillity.
Thank you very much for your attention,
Best regards,
Daniel

Last edited by danny92; 25th October 2013 at 04:28 PM.
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