Heatsink question

How thick is that 25mm flange that attaches to the main heatsink? How thick is the main heatsink?

The heat conduit is of the same thickness throughout - 4.5mm, including the flange that attaches to the main HS. The section of the heatsink covered by the flange is 4.7mm thick.The main heatsink is a readymade one of extruded (not cast) finned blackened aluminium, made by a company that's been in the business for at least 30 years. The heatsink I'm going to use is rated at 2 deg C/watt.

The two transistors together are expected to dissipate 10-20W total in normal use, with occasional 30W peaks. A bit marginal, which is why I'm concerned about the additional thermal resistance introduced by the indirect mounting. At least the HS will be mounted outside the cabinet in free air.

Personally I would not do this... 4.5 is a bit on the thin side, but without doing the computations I can't tell you what the actual result would be.

Personally I think you should reconsider your design...

Messy calculation. To grossly simplify, assume an aluminum alloy with thermal conductivity around 200 w/m-K or so. A sheet of that 4.5 mm thick will have theta of about 1 k/w per square. Treating the two transistors as a heat source at one end of your flange, and assuming the other end is well connected to the heat sink, one guesses net one square effective area, so the flange adds about 1 k/w.

Add a little more, 0.5 k/w per transistor maybe, to compensate for the hot-spot effect (thermal spreading resistance) of the relatively small TO-220 footprint on the relatively thin metal. So, 1.5 maybe.

This thermal stuff is distributed and follows diffusion math, so the only way to get accurate numbers is to do a serious finite-element analysis. It's easier to prototype it and measure.

I've used that geometry a few times. The flange bolts to a pc board and to the enclosure or a heatsink, and it doubles as a pc board mechanical support.

John

Thanks. That's the kind of guesstimate I was looking for and it about agrees with mine.

OK.

Please also read my reply to PeteD.

It may come to that. If possible, I'd rather change the HS and flange dimensions than the mounting method.

My problem is that I live in a remote corner town of India from where getting the stuff I need is not as simple as, say, ordering from RS in the US. The 4.5mm Al piece is from a building materials shop and the heatsink is to be cut from two 2-ft bars I bought during a visit to one of the larger cities some 15 years ago and which I've been using sparingly since.

What I can do is 1) look for a thicker flange *and* 2) use a longer length heatsink from the bar, though using a longer HS would require some modification of the enclosure. Or, I could add a small fan, which will also need some modification.

Your problem is that the 'flange' (the piece you bolt on to the heat sink and mount the transistors to) won't conduct sufficient heat to teh main heatsink.

Here's one I did a while back.

ftp://jjlarkin.lmi.net/Heatsink.JPG

The bracket is a stock extruded angle, hard anodized, and the transistors are mounted with thermally conductive epoxy. They seldom fail, so the epoxy isn't a problem. If I had to do this one again, I'd use silicone grease and mounting clips maybe.

John