Question on greasing mosfets

Aleph 5.
As this is my first project I beg your forgiveness for asking beginners questions.
I am unsure about how to apply heat sink grease on the mosfets. I have mica washer kits but I also got some rubber washer kits that are advertised as not needing any grease.The mica washers do not seem to be the same size as the rubber ones.They have the smaller rectangular shape of the to-220 although they seem to cover the metal part of the larger power mosfets as well. Is it better to use the mica with grease or the rubber without and why? Do you also need to use grease on the input mosfets on their small heat sinks as well or will the mica washers be enough since they isolate electrically. Finally, if you use mica washers do you put grease on both sides of the washer or only on one ?
 
Hello,

I think it's better to use the mica ones with grease on both sides cause the thermal resistance will be lower than that of the rubber (silicone?) ones.
You can also use some grease on the input transistors although I think that for the Aleph 5 you`ll only need a heatsink for the current source transistor. The input diff pair can live without one but it won`t do any harm if you want to use one. I wouldn´t use washers at all for the input and current source transistors, just see to it that the heatsinks don´t touch anything apart from the printed circuit board.

william
 

Nelson Pass

The one and only
Paid Member
2001-03-29 12:38 am
Don't even think of silicone insulators for to-3 or t0220
pakages. They shear easily, and I always get high failure
rates, do matter what I do. Micas are perfect for this,
and we use the Thermalloy white grease.

You can use the silicone for the to-247 and other plastic
packages where the screw hole is lined with the plastic.
These work just great without the mess caused by the
grease.
 
A couple of small points with mica washers and thermal grease.
Mica washers can be split to make them thinner and therfore conduct the heat better. Don't worry about the electrical breakdown of mica, you'll need them very thin to begin worrying about that.
Secondly, most people use way too much grease, remember it shouldn't form a layer seperating the mica and the metal, it should only fill in the air gaps due to the uneven surface contacts. Use it evenly and sparingly

Cheers, Adrian