Best form of D2PAK heatsinking - Page 1
Forum section: electronics / projects

author: pyrohaz
author_rank: Regular Contributor
timestamp: September 05, 2015, 12:24:37 am
content: Hi all,I'm designing a BLDC controller and I'll be using D2PAK MOSFETs for the 3 phase bridge. The reason for this controller is for me to learn the different methods of controlling BLDC motors, hopefully finally incorporating my implementations into an electric vehicle of mine (currently using stock chinese BLDC controllers). I'm not expecting TOO much heat to be generated by the MOSFETs as I'm hoping to be working at power levels <200W with a supply voltage of 24V.I have relatively large planes connected to the MOSFETs which should help sink some heat but I'm considering adding some heatsinks to aid in taking the heat out. I've attached an image of the PCB. One of my main aims was for it to fit on a 5x10cm board so I could get it manufactured relatively cheap by Smart Prototyping/iTead Studio.I'm currently considering getting a 50x50mm heatsink with some thermal conductive double sided tape and sticking one large heatsink across all the MOSFETs. My current choice of heatsink is:http://goo.gl/vyfSYJ(eBay link). Is there anything to advise against this? Obviously it increases risk of short circuits but other than that, I can't see any major problems. I'm going to assume however that the black plastic casing of the MOSFETs is relatively thermally insulating, reducing the efficiency of this setup. Would individual mini heatsinks for each MOSFET be a more efficient method, like:http://goo.gl/5vhWqU(eBay link), they're pretty perfectly sized.If this isn't a particularly worthy thing to do on the top side, would it be worth sticking these mini heatsinks on the bottom of the PCB?For reference, I'll be using IRFS3806PBF MOSFETs.Edit:I uploaded an old picture initiall, sorry.

author: georges80
author_rank: Frequent Contributor
timestamp: September 05, 2015, 12:38:59 am
content: The plastic casing is a relatively poor thermal path versus the metal solder tab. I personally would put a heatsink on the back of the PCB using thermal pad material (adhesive or 'tacky' + mechanical attachment). Put lots of vias on the solder pad of each FET, small diameter works better to minimise solder wicking.Work through your numbers to get an idea of the power dissipation that each FET will see and then determine heatsink requirements based on that. You may also need some airflow, depending on thermal losses, ambient operating temperature and heatsink surface area.cheers,george.

author: ajb
author_rank: Super Contributor
timestamp: September 05, 2015, 03:10:26 am
content: This PCB Via calculator will calculate thermal resistance:http://circuitcalculator.com/wordpress/2006/03/12/pcb-via-calculator/You can see how many vias it will take to get a lower thermal resistance through the PCB than through the case.If you can cram enough vias under each device, I'd consider removing all solder mask from the back side and use something like Laird Tgard K52 between the PCB and heatsink. It's a kapton film coated with a ceramic PCM, so it's very thin but with high dielectric strength. You'll need to make sure that the rear is dead flat with no solder pillows on the back side of the thermal vias.In any case you'll need to make sure you design enough clamping force into the assembly to get good contact across all devices--Maybe a clamping bar across each pair of devices with a screw in the middle, or a grid of screws across the whole heatsink. This would be even more of a problem if you tried to do a heatsink on top of the FETs.

author: Psi
author_rank: Super Contributor
timestamp: September 05, 2015, 03:18:46 am
content: You'd get away with 1 or two of those with just large PCB area for a heatsink.But 6 of them is going to need a heatsink as you say.

author: pyrohaz
author_rank: Regular Contributor
timestamp: September 06, 2015, 01:43:23 am
content: Quote from: georges80 on September 05, 2015, 12:38:59 amQuote from: ajb on September 05, 2015, 03:10:26 amI've now decided to remove the solder mask on the backside and attach the heatsinks directly to that! Thanks for the advice. Also, that via calculator is really useful, thanks!Quote from: Psi on September 05, 2015, 03:18:46 amYeh, I've got a couple of DPAK regulators on board but since they're supplying such a small amount of current (they're linear regulators too shock horror!), I'm using a large PCB area to heatsink them. I think I'll purchase a top heatsink, along with 6 mini bottom heatsinks. I'll leave the top heatsink off initially and add it if the heat emission seems significant and hope it makes a difference! Thankfully, eBay sell 40mm fans which are pretty convenient for the 50mm heatsinks.Thanks all for the replies!

author: matseng
author_rank: Frequent Contributor
timestamp: September 06, 2015, 02:23:09 am
content: Don't forget to order 0.8mm (or even 0.6mm if your fab has the same price for it) PCB instead of the regular 1.6mm. That will reduce the thermal resistance both through the FR4 material itself and through the vias.

author: rob77
author_rank: Super Contributor
timestamp: September 06, 2015, 06:34:47 am
content: i used a metal core pcb mounted to a big heatsink in one of my projects , but it had a very serious limitation - one sided pcb, but on the other hand you get a possibility to dissipate significant power levels

author: Gyro
author_rank: Super Contributor
timestamp: September 06, 2015, 09:04:29 am
content: You can get PCB heatsinks that solder to the top side copper around the device to increase cooling, see the datasheet PDF for application images:http://uk.farnell.com/aavid-thermalloy/573400d00000g/heat-sink-pcb-copper-to-268/dp/1651787

author: nctnico
author_rank: Super Contributor
timestamp: September 06, 2015, 12:05:58 pm
content: The SMT heatsinks are an option as well. I have used them before. The biggest problem is that they make it hard to replace a mosfet.

author: calexanian
author_rank: Super Contributor
timestamp: September 07, 2015, 05:58:07 am
content: This is actually becoming a bigger and bigger problem. I have yet to see a cost effective mas marketable solution for large dissipation surface mount devices. Harrumph Harrumph.https://youtu.be/JN99jshaQbY

author: Jeroen3
author_rank: Super Contributor
timestamp: September 07, 2015, 06:06:18 am
content: When looking at model aircraft brushless motor controllers you'll see they typically use a stacked pcb design.-Heatshrink-PCB with controller-Aluminum plate-PCB with fets-Aluminum plate-HeatshrinkCheaper ones often use only 1 aluminum layer, or low power version use none.

author: free_electron
author_rank: Super Contributor
timestamp: September 07, 2015, 06:08:26 am
content: Metalcore board, or board with copper coins in it.

author: rs20
author_rank: Super Contributor
timestamp: September 07, 2015, 06:08:55 am
content: Quote from: Gyro on September 06, 2015, 09:04:29 amThe limitation with these is that 1oz copper has a thermal resistance of 70 K/W per square, so you need to get the heatsinkreallyclose to the device or else you'll lose tens of K/W to the copper plane connecting the two. A similar concern exists for putting thermal vias next to, rather than under the device: after after 2 rows of vias, the effectiveness of the third and subsequent rows is absolutely negligible.To be fair, those particular heatsinks appear to cling very closely to the parts underneath, so it looks like they'd perform well. My point is more generally, 1oz copper has a thermal resistance of 70 K/W per square -- think about that when next relying on copper plane to dissipate heat!

author: Kjelt
author_rank: Super Contributor
timestamp: September 07, 2015, 01:17:28 pm
content: If you have the via's under the MOSFET it self, then make sure you have more then enough paste under the MOSFET before reflowing.I had in the past some problems that a lot of paste run through the via's to the other side, filling all via's (thats good) but little paste left for the thermal connection of the MOSFET. So check and doublecheck with prototyping before going to production.An alternative (but expensive) to individual heatsinks is to use one big heatsink for the entire pcb and put an electrically insulating but thermally conducting material between it such as Bergquist SIL-PAD.