Thermal "socks" for MOSFETs?

[iXod]

Never seen this arrangement of heatsink thermal pads:

http://imgur.com/a/ioVUr

What are the little "socks" that slip over the MOSFETs? And the aluminum shield between the sock and the large thermal pad?

Maybe modern replacement MOSFETs will have lower on-resistance and run cooler so I don't have to chase down new socks (these deteriorated during disassembly).

This is the control voltage supply SMPS in a '95-vintage bindery machine (sorts & staples).

Thanks.

[T3sl4co1l]

That's for creepage -- putting a lot of distance (and insulation) between high voltage (mains) and the heatsink (ground).

The shield tab is for, well, shielding.  I recommend leaving it in.  At worst, the unshielded EMI causes internal interference, skewing output voltages and frying the equipment (or interfering with other systems inside, causing malfunction).  At best, EMI emissions are increased slightly, and you (or your customer?) comes back later, with a C&D notice from the FCC (or other gov't organization).

So, you can use an over-sized insulator, of whatever material, or the "sock" and tubular kinds are still around.

Tim

[b_force]

Seen these 'socks' a lot. I found them very horrible, they break pretty easily if you try to repair stuff.
Definitely not something I would rely on for safety creepage

[Kilrah]

Just took apart my first device that had that a week ago. Have also torn the first one I removed when it was a lot less resistant than I'd have thought, then was careful removing the others and got them out in one piece.

[iXod]

Just took apart my first device that had that a week ago. Have also torn the first one I removed when it was a lot less resistant than I'd have thought, then was careful removing the others and got them out in one piece.

Are you repairing this? Where did you source replacement socks?

Thanks.

[batteksystem]

You still need the isolation, and the heat sink.
Probably you can get around with a simpler ceramic or silver mica insulation layer.
Also, if I were you, I will not replace with a different part number, especially in an SMPS.
If you really need the high thermal performance, get some aluminum nitride insulation pads. These things are AWESOME.

The ideal characteristics will be:

1. High dielectric strength
2. Non-flammable
3. Mechanically strong
4. Not brittle
5. Cheap

[Kilrah]

Are you repairing this? Where did you source replacement socks?

I only ripped a bit of it since I stopped pulling as soon as I saw it tear and it's on the front side so it'll still be good enough to reuse, I'll just fit it on a secondary side part instead of the primary one it was on for good measure.