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The Millett Hybrid MOSFET-MAXed - Output Stage

Heat Sink Mounting

Questions have come up from time to time about how to properly mount the MOSFET-MAX output MOSFETs and voltage regulator to the heat sinks. While it's not rocket science, perhaps a bit of an explanation may help.

You need a 4-40 screw, preferrably a flat washer (3a) under the screw head to keep from damaging the transistor underneath, then after the heat sink - a flat washer (3b), lock washer (important), and the nut.

The only question then becomes what to use as the insulator (1) between the transistor and the heat sink. If the transistor is a true TO-220 - meaning it has a metal tab - then it needs a shoulder washer (2) to keep the screw from making contact between the transistor and the heat sink. If not a TO-220, a shoulder washer won't fit anyway - so don't use it.

The choice of pad almost becomes one of convenience. Mica is the cheapest by far, but needs grease. Thermafilm has better heat transfer than Mica, but costs more and still needs grease. Thermasil is the one that does not need grease, and often comes with an adhesive back, making it very convenient. You line up the hole with the heat sink hole and press down - very similar to applying a stamp these days (I remember when you had to always lick. ;-) ).

You may need to apply a bit more torque on tightening the screw assembly - so as to make the pad material "flow" into the joint - but other than that, easy as pie. Don't use grease with a Thermasil pad - you're wasting the heat transfer effect of the material.

This diagram and parts list legend at left, and complete explanations for mounting, selecting, and using heat sinks is in the Aavid catalog. It's available for free download on their website - 2007 Standard Products Catalog. It's also listed as a hot link reference to every Aavid heat sink at Mouser and DigiKey if you click on "data sheet." Wakefield also has a similar catalog.

Remember that there are competing products for all of the mounting kit parts. Screws, washers and nuts may be purchased at almost any hardware store. Shoulder washers are a littler rarer, but most electronics suppliers have them. Even the Thermasil pads have competing mfrs - Bergquist at DigiKey is a good example.

Thanks to forum user "pinkfloyd4ever" for prompting me to add this info to the MAX website.

file last changed:Sunday, September 7, 2008 7:00:00 AM
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