heat sinking onto diecast boxes

[Zero999]

Whoops, in the picture I didn't solder the tabs to the copper bus bar but screwed them to it. The insulator is between the bus bar and the case. I did make another straight LM317 PSU with the regulator soldered to the bus bar. I don't have a picture though.

That is a large box, I'm going as small as i can. Given that I'd have to pot the thing anyway and that metal cases are much more expensive it looks cheaper to get an open potting box and just use cheap small heatsinks leaving the fins to stick out of the potting compound. I think you can get aluminium cases with fins on them that are open one side and you just screw any TO220 packages to the bottom (fined end) and pot the things up leaving a flat bottom but i bet they are not cheap and I've never seen them for sale, but have seen them in automotive stuff.

How big is the box?

Potting it in a normal plastic box with the heatsink sticking out does sound like a good idea. You can also buy special potting which is more thermally conductive but it's probably more expensive.

If you sand it or get it painted matte black it will dissipate a lot more heat by itself (no fan) than if you just leave it as shiny metal.

Yes that's what I did, although the finish wasn't as good as I'd hoped for. I probably didn't sand it enough.

[lewis]

30A at 12mR is almost 11W (ignoring switching losses) which is far too much. You'd struggle to dissipate that in a plastic potting box.

If it's a low voltage application, try one of these

One of these http://www.irf.com/product-info/datasheets/data/auirfs8409-7P.pdf for example will reduce your dissipation down to about 0.8W which is MUCH easier to get into a small plastic box. The extra you spend on the fet will be saved many times over by not having to bugger about with the heatsink/potting.

[Psi]

30A at 12mR is almost 11W (ignoring switching losses) which is far too much. You'd struggle to dissipate that in a plastic potting box.

One of these http://www.irf.com/product-info/datasheets/data/auirfs8409-7P.pdf

Damn! 0.55mR is a crazy low Rdson.   I didn't think you could get them below about 8mR.


Yeah, Simon should use that, 0.8W is trivial.

[digsys]

  www.irf.com/product-info/datasheets/data/auirfs8409-7P.pdf 

Bugger. I thought the IRFS3004-7Ps were low at 0.9mR !! and same package ! Bonus !! ... ordered a tube to try.
Gate capacitance goes up from 9,130pF > 13,975pF ..

[Zero999]

Yes, the lower R-on MOSFETs tend to have a higher capacitance.

I've found the PSU I was talking about above. Soldering the device directly to the copper bus bar, then mounting it to the box with the insulator in-between helps reduce the thermal resistance.

[con-f-use]

I like your designs. Good work!