heat sinking onto diecast boxes - Page 1
Forum section: electronics / projects

author: Simon
author_rank: Unknown
timestamp: February 05, 2014, 12:16:16 pm
content: I'm working on a project that needs to do some power switching so need to heat sink a mosfet and what better way than onto the aluminium case the thing is in. But what are the surfaces of die cast boxes like ? are they generally smooth enough for heat sinking ? Perhaps I should consider a plastic box with a heat sink sticking out of it ?

author: Rerouter
author_rank: Super Contributor
timestamp: February 05, 2014, 12:43:04 pm
content: it depends on the manufacturer, from the ones i have experienced over a short distance they are relatively smooth but can curve and be warped slightly over the length of the side, just remember if not insulated can be a way to blow up a mosfet with ESD,

author: Psi
author_rank: Super Contributor
timestamp: February 05, 2014, 12:55:04 pm
content: Use a thermal pad, it will both insulate and provide some padding to make up for any non-smoothness in the metal.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 05, 2014, 01:46:01 pm
content: Don't expect to dissipate more than, oh, five watts in a reasonably sized (say 4" / 10cm) box, or more than 20W in a given component (in any size). Sil-pads are pretty awful, and you'll need a fairly pliable kind (= less conductive?) if the surface is uneven as mentioned.If you have access to a mill, you can of course get the surface cut as flat as you like.Tim

author: con-f-use
author_rank: Unknown
timestamp: February 05, 2014, 01:53:38 pm
content: If you're not worried about isolation, you can polish/grind the surface. For maximal conduction you can use silver thermal compound.

author: Simon
author_rank: Unknown
timestamp: February 05, 2014, 01:59:17 pm
content: The aim is to save on costs so if I have to start getting them milled it will be cheaper to replace the lid with some punched sheet metal.

author: HAL-42b
author_rank: Frequent Contributor
timestamp: February 05, 2014, 02:21:52 pm
content: Sil-pads seem like the way to go. Also mounting the mosfet to a L shaped bracket (bent sheet aluminium) and mounting that to the case may work also.

author: Simon
author_rank: Unknown
timestamp: February 05, 2014, 02:24:26 pm
content: well I'm also looking for minimum space and parts, I have easily have a box lid/base punched with flanges for fixing it down and folded ends so that there is something in the air to dump heat, it should not get too hot but with nothing it will build up fast.

author: qno
author_rank: Frequent Contributor
timestamp: February 05, 2014, 04:07:23 pm
content: How many watts do you need to dissipate?

author: Simon
author_rank: Unknown
timestamp: February 05, 2014, 04:53:24 pm
content: I'm not sure as of yet, I'm going to use an IRFZ48V to drive a 30A fan with 500 Hz PWM power drive, I'll have to experiment but I'd like to at least get the mosfet "connected" to the outside world to help loose the heat and not overheat the enclosure. I am also considering separating the "power Drive" as i already have the signal generator solution sorted for brush-less fans that don't need power driving.

author: fcb
author_rank: Super Contributor
timestamp: February 05, 2014, 05:14:34 pm
content: Diecast (eddystone etc..) boxes tend to have smooth walls (otherwise they would get stuck in the mold tools), so shouldn't be an issue.The main problem is that they tend to be thin walls, this limits the thermal resistance somewhat.As for insulation - probably look at bergquist gap pad, I have had a lot of success with this stuff, and it's available in some pretty thick formulations which can help. The other thing I do now (for high dissipation) is not to bolt power devices down to heatsinks, but to arrange either a spring clip or a bar to press down on the device.

author: Simon
author_rank: Unknown
timestamp: February 05, 2014, 05:20:30 pm
content: I think the average die cast box will work. It will need isolating anyway as I don't need the drain of the mosfet connected to a negative chassis as that will just run my fan all of the time. Of course I'll need a back e.m.f. diode too that might as well go in the case with the mosfet.

author: SeanB
author_rank: Super Contributor
timestamp: February 05, 2014, 05:55:49 pm
content: So a small aluminium block with the 2 devices mounted on one side with thin silicone insulators and on the other side a similat larger pad to hold it to the box surface probably using 2 screws. This allows heat spread to the box, and gives a nice step to allow the devices to be on the edge of a PCB making mounting of the board easier. You mount the devices and do a quick test before mounting the whole board and block in the box. No messy pastes either.

author: Simon
author_rank: Unknown
timestamp: February 05, 2014, 06:13:18 pm
content: given that I'm switching up to 30A I think it will be wires all around

author: SeanB
author_rank: Super Contributor
timestamp: February 05, 2014, 06:36:49 pm
content: Much easier to use a PCB with well stitched traces top and bottom and wide as well leading to a set of screw terminals soldered to the board. Then a plug and socket or a simple cable connection to the driver board will make assembly so much easier and make assembly errors so much less likely. As well it will not stress the power devices with lead flexing and breakage every time the case is opened along with giving strain relief.

author: Simon
author_rank: Unknown
timestamp: February 05, 2014, 06:39:47 pm
content: try calculating the trace width for 30A....... it is wider than the TO220 package pin pitch

author: Christopher
author_rank: Frequent Contributor
timestamp: February 05, 2014, 06:39:54 pm
content: Check the material used on the box, I came into this the otherweek with an Eddystone box which was a zink alloy. A few tests proved it to be crap at dissipating 25W..Quick way to check: Grab a ~10R TO220 Resistor (£5 on Farnell) and mount it to the case, it will dissipate all the power you put through it in a similar way to a Fet would I'd put double the power through the resistor you'd be expecting the mosfet to dissipate in steady-state. Attach a Thermocouple or three, datalog the temps and powers, now you have a Thermal Resistance value.Quote from: Simon on February 05, 2014, 06:39:47 pmUse 2Oz copper, 2 layers, loads of thermal vias? If 2 Oz copper is a prob solder wire strands to the board (not good for high-volume)

author: Simon
author_rank: Unknown
timestamp: February 05, 2014, 06:43:05 pm
content: yes diecast is crap as material so yes I'd need to do thorough testing. Plan is it won't make too much heat with a 12mR mosfet channel but then only testing can verify switching losses and of course there is the back e.m.f. diode that will only be realistic with the real load.

author: SeanB
author_rank: Super Contributor
timestamp: February 05, 2014, 06:44:02 pm
content: Quote from: Simon on February 05, 2014, 06:39:47 pmThat is why I said double sided...... plenty of space then if you use top and bottom and make a wide trace immediately after the device. The small bit near the device will be fine as the lead will heatsink it, and a wide trace top and bottom with a few vias will handle the current, They can be wide away from the device, and you only need 3 power handling wires in any case so can use pretty much all of the board for them. As well you can put the driver supply filtering on there as well, along with the fuses and transient suppression.

author: Simon
author_rank: Unknown
timestamp: February 05, 2014, 06:46:17 pm
content: yes but i have a negative wire in to the source and a wire out from the drain to the drain, why bother putting than on the PCB and almost doubling the size of the PCB and making more heat. There may be larger versions going up to 70A with 2-3 mosfets in parallel, there will be no getting away from just wiring it up even if stiff copper (busses" are used).

author: Neilm
author_rank: Super Contributor
timestamp: February 05, 2014, 07:31:49 pm
content: I've used a diecast box to dissipate about 20W (from memory from about 12 years ago). The biggest issue I had was the people assembling forgetting to install or incorrectly installing the SIL pads resulting in the FETs shorting out and blowing up.

author: Simon
author_rank: Unknown
timestamp: February 06, 2014, 08:26:34 am
content: What I'm thinking now is to use a plastic potting box and use small heat sinks like the ones I stock and pot them in with the fins sticking out of the potting, this will be cheaper and faster to assemble

author: Zero999
author_rank: Super Contributor
timestamp: February 08, 2014, 08:58:32 pm
content: Quote from: Simon on February 05, 2014, 12:16:16 pmI did this awhile ago on a bipolar tracking LM317/337 PSU.I achieved best results by soldering the regulator tabs to pieces of copper bus bar and putting the insulator between the copper and the case. I can't ever remember the case becoming too hot to handle, even when the power dissipation was about 40W.

author: Simon
author_rank: Unknown
timestamp: February 08, 2014, 09:04:04 pm
content: 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.

author: scientist
author_rank: Frequent Contributor
timestamp: February 08, 2014, 09:10:53 pm
content: 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.

author: Zero999
author_rank: Super Contributor
timestamp: February 08, 2014, 09:23:07 pm
content: 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.Quote from: Simon on February 08, 2014, 09:04:04 pmHow 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.Quote from: scientist on February 08, 2014, 09:10:53 pmYes that's what I did, although the finish wasn't as good as I'd hoped for. I probably didn't sand it enough.

author: lewis
author_rank: Frequent Contributor
timestamp: February 08, 2014, 09:24:24 pm
content: 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 oftheseOne of thesehttp://www.irf.com/product-info/datasheets/data/auirfs8409-7P.pdffor 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.

author: Psi
author_rank: Super Contributor
timestamp: February 08, 2014, 10:12:39 pm
content: Quote from: lewis on February 08, 2014, 09:24:24 pmDamn! 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.

author: digsys
author_rank: Unknown
timestamp: February 08, 2014, 10:53:01 pm
content: Quote from: lewisBugger. 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 ..

author: Zero999
author_rank: Super Contributor
timestamp: February 09, 2014, 11:32:45 am
content: 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.

author: con-f-use
author_rank: Unknown
timestamp: February 09, 2014, 07:04:55 pm
content: I like your designs. Good work!