AndrewT said:
Ouch..I been dissed..
Does this site have any way of supporting an excel spreadsheet?
If so, I could whip up a 45 degree spread model so that anybody can plug in a die size, an insulator thickness, and come up with a rather close to reality number for thermal resistance junction to heatsink..
Cheers, John
AndrewT said:
I should be able to create the spreadsheet sometime next week.
I will start a new thread with it, and put it in this forum..
The mods can move it if they deem it better suited elsewhere.
Cheers, John
ps...I already have a spreadsheet in my archive notebook, it has the overall thermal resistance calculations as well as strain calculations for power cycling fatigue and strain caused by thermal cycling fatigue (reliability stuff for military hybrids)...but unfortunately, it was stored on both 3.5 and 5 inch floppy disks..the 3.5 disks are no longer readable (I have found many 3.5's have failed in that fashion), and I do not have access to a 5 inch drive.
I did some testing a month or so ago comparing temp rise of TO3's in an amplfier that used 4 devices per rail. I had silpads and mica with grease. I got slightly better heat transfer with the silpads. I've been using them for 25 years without trouble. That's why I bought about 5000 when I saw them at the surplus store.
Thick, really ?
I've used TO220 aluminium oxide insulators that were 4.5mm thick, that was 1989.
Regular thickness of available TO3 Al2O3 insulators here was 2.9mm, and still is.
The thinnest ones i know of are 1.5mm, but not in TO3 shape.
Silpads and silicon sheets have gone a long way too.
The first silicon sheets i was able to get my hands on was really expensive and performed much worse than mica/goop. Only good if there's no mica insulator in the required shape/size available.
A year ago i tried sheets from Bergquist, and they did better than mica.
And than there's Mica and Mica, thickness and quality varies a mile. I managed to get a bunch of them for Japanese MT200 devices, thin as a raisorblade and the brightest mica i've ever seen.
I've used Aavid Thermalloy TO-247 Aluminium Oxide insulators for KSA50 output devices, 2 insulators under each Sanken SA1216/SC2921.
Way lower thermal resistance than regular mica, check the Krell KSA50 builders wiki for details.
I've used TO220 aluminium oxide insulators that were 4.5mm thick, that was 1989.
Regular thickness of available TO3 Al2O3 insulators here was 2.9mm, and still is.
The thinnest ones i know of are 1.5mm, but not in TO3 shape.
Silpads and silicon sheets have gone a long way too.
The first silicon sheets i was able to get my hands on was really expensive and performed much worse than mica/goop. Only good if there's no mica insulator in the required shape/size available.
A year ago i tried sheets from Bergquist, and they did better than mica.
And than there's Mica and Mica, thickness and quality varies a mile. I managed to get a bunch of them for Japanese MT200 devices, thin as a raisorblade and the brightest mica i've ever seen.
I've used Aavid Thermalloy TO-247 Aluminium Oxide insulators for KSA50 output devices, 2 insulators under each Sanken SA1216/SC2921.
Way lower thermal resistance than regular mica, check the Krell KSA50 builders wiki for details.
....I have used 3mm AlO insulators for smps apps. In that case the 3mm
thickness is a good thing reducing the capacitance to the cooler.
The AlO insulators are excellent for heatsinking but requires grease to fill
all voids on the transistor and cooler. I try to always use spring clips to hold the semiconductors btw.
With the Silpads you don't need to mess with the grease and its easy to cut. try to cut the AlOs....
To confuse things even more. The best heat transfer insultor I have encountered
are the phase changing type. Made up from a waxlike compound that melts during heating.
If you are using paste don't use "coarser grit" than yuo need, the grease is only there to fill the voids.
This is where DIY excells, you can take your time spreading the paste even with
your fingertips.
Lay down the transistor / Insulator asbly take it away and see if there
are voids.
/ Mattias
thickness is a good thing reducing the capacitance to the cooler.
The AlO insulators are excellent for heatsinking but requires grease to fill
all voids on the transistor and cooler. I try to always use spring clips to hold the semiconductors btw.
With the Silpads you don't need to mess with the grease and its easy to cut. try to cut the AlOs....
To confuse things even more. The best heat transfer insultor I have encountered
are the phase changing type. Made up from a waxlike compound that melts during heating.
If you are using paste don't use "coarser grit" than yuo need, the grease is only there to fill the voids.
This is where DIY excells, you can take your time spreading the paste even with
your fingertips.
Lay down the transistor / Insulator asbly take it away and see if there
are voids.
/ Mattias
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