Heatsinks and compound for VVR

[dynaman]

Seems like most of the VVR problems I've read about are heat related. I've never worked with devices like transistors, so I'm hoping y'all could help me out with keeping the MOSFET cool.

I'm gonna use Dana's VVR with a 4XEL84 Rocket build. Amp is going in to a painfully thin Bud aluminum chassis. I'll prolly have to relocate the MOSFET and figured I should also add a heatsink to the top of the chassis directly over the MOSFET.

I dug up some old heatsinks in my junk boxes and had a few questions.

1) Any reason why I cant "stack" a little one on top of a bigger one?

2) Compound? I'd imagine it's pretty important to spread between the actual sink and chassis, no?

3) Would it help to use compound between the MOSFET, insulator and chassis? I'm leery of using any sort of goop that could be electrically conductive.

Please and thanks!!

[ToneMerc]

personally i wouldn't stack two

device>insulator>grease>chassis

[Lonely Raven]

personally i wouldn't stack two

device>insulator>grease>chassis

I agree with the second part, but not sure about the first.

Having worked extensively with heat sinks and water cooling with computers, I learned a bit about thermal transfer.

You can add a heat sink directly under the MOSFET on the other side of the chassis. Stacking may or may not be wasting your time for several reasons. Personally, I'd just bypass a few of those reasons by using a really good, low profile heat sink vs. stacking heatsinks. Any computer store should have cheap ones for anything from video cards to RAM to processors. A better one would have more fins per inch, and a great one might have a copper base (copper transfers heat faster and more evenly than aluminum).

You would go MOSFET > Insulator > Grease > Chassis > Grease > Heatsink.

Heatsink Grease:

More is not better. You want it as thin as possible. The grease isn't a magic heat sink; it's purpose is to give you a thermally conductive barrier that forces all the air out from between the two objects you want to thermally couple. So put a dab on and smear it as thin as possible, no air bubbles between the device and the chassis, nor the chassis and the heat sink. I sometimes even use a razor blade like a micro putty knife to spread the grease thinly.

Many heat sink greases are conductive as the better ones often use silver particles (silver is on of the best heat conductors). So be careful with how and where you spread the grease.

I hope that helped some. If I ever get off my butt and add a VVR to an amp, I'll take photos of the process I use, but what I wrote above pretty much sums it up.

[Cliff Schecht]

Raven covered the using as thin a layer as possible of grease and this is important for getting the best transfer of heat from your device to your heatsink.

If you're interested you could dig up more information on how to properly calculate the amount of heat your MOSFET will be dissipating and how big of a heatsink you need. Any MOSFET datasheet will specify the max degC/W that the device can dissipate and you have to stay well below this max for reliable operation. You then calculate what the junction temperature is with your configuration (i.e. how much power you are putting through the FET) and how much your heatsink can pull out. I'll see if I can dig up more on this later, if somebody else doesn't beat me to it, as I have a test from 4-7 that I am not looking forward to!

[dynaman]

Ok, I'll be very careful with whatever grease I use so it doesn't short anything out.

Yeah, I realize that using grease and multiple sinks is hardly the most efficient way of dissipating heat. I'm just trying to use what I have on hand. The Bud chassis is only .050" thick. I was just thinking that a little extra meat on top of the chassis would help "localize" the heat and send a bit up into the air instead of just relying on the thin chassis.

Thanks for the replies.

[TUBEDUDE]

The silastic rubber insulators are more thermally efficient than the grease. Less messy also. The grease ruins clothing. The insulators are available in TO-220 and. TO-3 sizes from newark, allied and digikey.

[jaysg]

There are thermal pads for the TO-247 package. I used that and an Aavid sticky BGA type heatsink on the topside of my Liverpool.

http://www.mouser.com/catalog/catalogUSD/643/2058.pdf

[Masco]

I've been using these oxide pads: http://www.mouser.com/Search/ProductDet ... ey532-4180

I heard somewhere that the heat sink compound compresses and becomes less effective..?

Anyway, I've installed dozens of these oxide pads with zero failures.

[Lonely Raven]

I've been using these oxide pads: http://www.mouser.com/Search/ProductDet ... ey532-4180

I heard somewhere that the heat sink compound compresses and becomes less effective..?

Anyway, I've installed dozens of these oxide pads with zero failures.

Heat Sink compound is supposed to compress. It's purpose is to make sure there are no voids (air is a great insulator) between the two materials you want to thermally couple.

Where you have failures is where the cheaper grease/pads/compounds dry out and no longer keep the air out. The better compounds should stay viscus pretty much forever. I have a computer I put an early silver compound on 7-8 years ago...when I pulled the heat sink off, the compound was almost as greasy as the day I put it in. Hardly any change at all.

[Structo]

When I was into hot rodding PC's and overclocking the CPU's I used the Arctic Silver heat sink compound.
Probably what Raven is alluding to.

It performed well but there were some warnings from the community that if you were not careful you could short something out because the compound was electrically conductive due to the silver content.

I think that the problem was that a few end users did not follow instructions and used way too much.

As stated, a very thin, even layer is all that is needed.
I too used a razor blade to smooth out the layer of compound to a very thin and even layer.

It is used to only fill the imperfections in the metal of the two mating surfaces so that good heat transfer can take place.
Some guys even went as far as to lap (sand and polish) their computer CPU metal heat sink surfaces to try to eliminate all peaks and valleys in the metal surfaces. (I did as well)

The only thing I would be concerned with in using some of these TO-220 heat sinks is that many times the tab on the device is connected to the positive side of the device so it must be insulated from chassis ground.

In those cases I believe the device usually comes with isolating washers that fit in the hole of the tab so that the tab does not come into contact with the mounting screw and to ground.

The thermal pads have shown to be almost as good at thermal transfer as dielectric grease in some studies I have read so for less mess and fuss, I think those pads are a good way to go.