ANOTHER QUESTION: heatsink compound and mica film

[CraigB]

I have a question about the proper installation of a transistor that is bolted to a heatsink.  I understand that you want to use a very thin layer of thermal grease on the heatsink.  The transistor comes with a piece of mica film.  Is the grease supposed to be applied to both the heatsink and the metal back of the transistor, with the mica piece sandwiched in between then bolt the transistor to the heatsink?

Thanks!  Craig

[DummyLoad]

yes. be sure to insulate the tab from the heat sink - in most cases the tab is also the collector. usually, if you bought a mounting kit the insulating washers are included. sometimes i use nylon hardware for smaller devices, e.g., to-220 and to-226 package outlines.

[tubesornothing]

Yes, and after you install use your ohmmeter to check that the heatsink is truly isolated from the case.

[Shrapnel]

+100.  make sure that they are electrically isolated, as if that tab is shorted to the case, the whole thing is shorted and that's bad bad bad.

[sluckey]

I like to put the compound directly on the transistor and use a toothpick to spread it to a thin and even coat. Then put the mica insulator on the transistor, sliding it slightly to get good contact with the transistor. Then I put another thin and even coat of compound on the clean side of the mica insulator. I don't actually put any compound on the heatsink. In fact, I clean off any old compound. Now just install the 'buttered' transistor to the heatsink. Using this method produces good heat transfer and the cleanest install for me. There are plenty of other ways to do this though.

I like the white silicone paste compound best. It can be messy if you aren't careful.

[tubenit]

After reading this .................. I am wondering if I have installed my VVR correctly and safely?   It seems to be working fine & it sounds fantastic.

I did not use any compound and simply installed the clear film (mica?) between the mosfet and the aluminum chassis thinking that would isolate things OK?

Was this done correctly???   

If not, can you tell me what I need to do to make it right?

Using this picture can someone illustrate where the compound would go if you were using a heatsink (vs. an aluminum chassis) ?

I plan to install another one on my CBR which is cathode biased 5881's and so I want to get this done correctly and safely.

With respect, Tubenit

[PRR]

> It seems to be working fine

Here's the deal. Ideally you measure the different temperatures: transistor, heatsink, air. If the transistor is under 75 deg C (hurts the skin but does not raise blister), go worry about something else. If hotter, see if the bottleneck is transistor to chassis, or chassis to air.

BUT DO NOT stick your finger on a 400 Volt device!!

Put a teeny drop of water on the epoxy case, so it won't roll onto the leads. Cook for a few minutes. Depending on the amp and the VVR scheme, worst-case may be half-loud and working very hard. If the water does not boil, the transistor will probably last forever.

IR thermometers are handy for non-lethal temp checks. Just note that cheap ones "read" a large area. If you have a hot-spot, you have to get close.

If the transistor IS recklessly hot: compare transistor to heatsink. If these two temps are very different, you need "better contact". Flatten the heatsink surface. Get some torque on the screw (but do NOT cut the insulating washers!). Grease is a final step -after- all other details are optimized.

A plastic "TO3" flange on a sheet-metal chassis, if the device-chassis joint is nearly-right, the BIG problem is that sheetmetal won't carry much heat. Super-polishing and silver-greasing the transistor is not going to change that.

On a thin steel chassis, heat conduction is pretty poor. Your beefy aluminum can carry-away many Watts safely. Grease is probably not needed.

Grease IS needed when you have many-fin 1/4" HEAT SINK, and need to get full use of the expensive sink.

Grease is useful on modern PC CPUs with many-many fanned fins. The cooler can pull heat out fast enough that the metal-metal joint between CPU and sink becomes an impediment. Hot CPUs may run 10 deg C cooler with careful grease. OTOH, they can run 50 deg C hotter if you get a speck of crap between CPU and sink. Done that, had the instant shut-down. The crappy "clamps" on the current Intel CPU heatsinks are very prone to bad contact.

[CraigB]

The package on this has the mounting hole insulated from the drain, it appears, but it does come with a little nylon washer/insert.  As long as the drain, gate & source are insulated via the mica from the heatsink/ground, is it OK to omit the nylon washer?  The reason I ask is I don't have any hardware that tiny that can go through the hole of the washer.  Thanks!

Craig

[tubesornothing]

If the mounting is truly isolated from either the GDS then not a big deal - but I doubt it.  Especially if they provided an insulating shoulder washer (like in tubenits photo above).  I think you'll just have to head to the hardware store and grab a #4 nut and bolt.

[CraigB]

ton, I don't read continuity or even any resistance between the screw hole and G, D or S.  This is why I'm not understanding the purpose of the washer.  there's about a 1/16" ring that goes through the package to the back side or underside of the screw hole, and yes the underside plate that goes down on a heatsink is common to the drain.  Bolted down to the heatsink, I read no continuity or R to ground. 

Hmm, forget taking a chance.  But I'm pretty sure the bolt will need to be smaller than #4, it's really small.  Hopefully I can find something, but if not I'll use a nylon bolt and nut and crazy glue so it can't vibrate loose.  Thanks for the reply -Craig