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Registered Member #63
Joined: Thu Feb 09 2006, 06:18AM
Location:
Posts: 1425
Just working with more high frequency coils (this time ~8MHz), I've suddenly come to appreciate the pretty large thermal impedance of silicone pads.
Running the coil for about two minutes, I started to hear a clicking and bubbling noise. Not sure if it was the MOSFET, a mica capacitor, or alligator leads, I left the contraption on to see where it was coming from. Amazingly, the plastic insulating washer (between my screw and my MOSFET tab) was melting.
(I'm also amazed at how resilient the IRF730 was... I'm still using the same device, though I wouldn't be surprised if it's no longer reliable, considering its terrible thermal history).
I sanded the heatsink nice and clean, applied some thermal grease, and I haven't had any more problems with the MOSFET. Now he reaches a proper equilibrium, at the expense of some stray RF and making the heatsink a live deathtrap.
Why do I mention this? I'm sure lots of people habitually always use silicone insulating pads, whether they're needed or not. I guess it's a good habit, BUT, if you're running a component 'on the edge'... it's worth eliminating that extra thermal impedance.
Registered Member #56
Joined: Thu Feb 09 2006, 05:02AM
Location: Southern Califorina, USA
Posts: 2445
I have been having some similar problems (and have a ton more in the mail) with my lasers. With my 50w bars there is about 150w of power going through a 1" square area (and it isn't even distributed evenly across the block, it is hotter near the front) and I need to keep the diode at 25c. Even flat copper/copper surfaces with decent quality silver-filled thermal past I have seen 10F across the laser and the mount The solution is having pure metal all of the way, using indium foil between the surfaces. The calcs show that I should be able to keep it under 2F across teh joint, but I can't test that until I get the copper plates from John. If it comes to it I can even solder them together (just flux the surfaces, put in low melting alloy foil, and drop the whole mess on a hotplate at 100c) and then I should have a very reasonable temp drop, considering that the diode/copper contact is only .1cm^2, and it does fine as long as there aren't any voids in the solder.
Registered Member #15
Joined: Thu Feb 02 2006, 01:11PM
Location:
Posts: 3068
wrote ...
Why do I mention this? I'm sure lots of people habitually always use silicone insulating pads, whether they're needed or not. I guess it's a good habit, BUT, if you're running a component 'on the edge'... it's worth eliminating that extra thermal impedance.
Depends on the situation. Yes, for maximum performance you would definitely use thermal grease or a grafoil type pad. Of course, this oftens means you need to isolate the heatsink, so its a trade-off between thermal performance, and complexity etc....
Registered Member #72
Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659
If you need an isolated heatsink, and if you are pushing the temperature drop, then use the maximum area of insulator you can, using a heat spreader between the device and the insulator, this keeps the insulator drop to a minimum.
Bear in mind that brass, though cheap and extremely easy to work with, is about one order of magnitude less thermally conductive than copper or alli. So if you are really pushed, mount your device, with thin void-filling squdgey gunk, directly to copper.
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Thermal impedance: appreciating metal-to-metal contact
The solution is having pure metal all of the way, using indium foil between the surfaces. The calcs show that I should be able to keep it under 2F across teh joint, but I can't test that until I get the copper plates from John. If it comes to it I can even solder them together (just flux the surfaces, put in low melting alloy foil, and drop the whole mess on a hotplate at 100c) and then I should have a very reasonable temp drop, considering that the diode/copper contact is only .1cm^2, and it does fine as long as there aren't any voids in the solder. 
