Remove heat from a DPAK how to?? - Page 1
Forum section: electronics / beginners

author: rakeshm55
author_rank: Regular Contributor
timestamp: April 19, 2019, 10:32:24 am
content: Hi,i plan to use SUD50P06-15 P- Channel Mosfet for one of my project. It is a DPAK package.Heat Sink (Chassis) used in my system will be connected to ground. How remove heat out of Pchannel Mosfet??Its thermal PAD is connected to drain. So connecting to board ground not possible.Since DPAK is SMD what is the standard practice for DPAK devices.

author: Gyro
author_rank: Super Contributor
timestamp: April 19, 2019, 10:46:22 am
content: The standard practice is to solder it down to a PCB with a decent area of copper to act as a heatsink. If it's being used for switching (cleanly) then the low Rds makes that sufficient normally.

author: rakeshm55
author_rank: Regular Contributor
timestamp: April 19, 2019, 11:10:19 am
content: It is used as a highside switch with 6A of continuous current. Expect a dissipation of over 1W.Considering junction to Ambient 65C/W . A temperature rise of 65C above ambient. Onboard ambient can go upto 75C So junction temperature of 140C is anticipated.I guess it is too close to max thermal limit.

author: max_torque
author_rank: Super Contributor
timestamp: April 19, 2019, 11:12:41 am
content: What i do is to use a multilayer pcb, solder the drain to the isolated top layer pour, and via that with lots of vias, to Layer 3, where the same pour is repeated. Then on the bottom layer and layer 2 i put a chassis ground referenced copper pour, again via'd together and make all those pours overlap, the amount they overlap and the total area depends on the total heat flux.That way the heat comes out of the dpak into top, spreads via the vias () into layer 3, and then the heat can move sideways through the (relatively) thin pcb material, which has a poor, but not terrible thermal impedance into the layers that are thermally coupled to the chassis. You do have to watch out if the device is switching fast, as those overlapping layers ARE capacitively coupled!Alternatively, you sink the heat into the top layer of the pcb only, and find space for a solder / bolt on heat sink, that is large enough to reject the heat into the air inside the enclosure, and then that heat gets rejected to the enclosure walls. The problem with this approach is static air is a terrible conductor, and hence for any significant power level you might need a fan blown heat sink

author: Gyro
author_rank: Super Contributor
timestamp: April 19, 2019, 11:17:35 am
content: Yes, you do need to be careful with SMD FETs, many of the parameters are fictitious (stated at 25'C Tj or pulse).Although Junction to Ambient is 65'C/W, Junction to Case is <1'C/W. It's not unreasonable to be able to dissipate 1W on a PCB. Common practice these days is to pick a lower Rds-on part rather than resort to a heatshink if things get tight.

author: aandrew
author_rank: Frequent Contributor
timestamp: April 19, 2019, 05:08:54 pm
content: Quote from: Gyro on April 19, 2019, 11:17:35 amThis is something that's always caused me a lot of confusion as an engineer. How to correctly apply these ratings to come up with an overall solution. 1C/W to case means the case temp would rise 1C with 1W dissipated. However how does that play with junction-to-ambient? How do you end up calculating the junction temp given all of these ratings?

author: Gyro
author_rank: Super Contributor
timestamp: April 19, 2019, 06:10:01 pm
content: Yes, thermal designisa tricky one. While the Rjc is small and reasonably well specified, it's rather more difficult to calculate the Heatsink to ambient - particularly when that Heatsink is a PCB.Aluminium heatsinks normally have a 'C/W figure and you can use equations to estimate thermal resistance of PCB based on copper weight and board thickness but these are guides at best as they depend on orientation, available ventilation slots, convection airflow, fan cooling etc.Ultimately it comes down to testing and allowing a safety margin. For instance, you can strap an aluminium cased resistor to a heatsink in the proposed case, dissipate the maximum expected power and measure the temperature rise. A bit more tricky but doable with a resistor soldered to a piece of PCB of the same size and orientation, even isolating a patch of copper the same area as the proposed flood.

author: MadScientist
author_rank: Frequent Contributor
timestamp: April 19, 2019, 06:58:53 pm
content: Thermal resistance models are just like electrical resistance and are added in series and reduced in parallel etcThe key determinant is φjc which is made up of φjc + φca ( case to ambient ) , now some very coarse rules apply, the case to pcb thermal resistance can be ignored and the rough rule I use is 500 W/C for 1cm2 of double sided pcb connected via an array of thermal via . The component φja is effectively ignored as it’s a thermal resistance in parallel with a much lower die to base thermal resistanceIe φca = 500 for 1cm2 , or 50 for 10cm2 etc.With that you can now do a rough calculation of the temp rise over ambient using a pcb as the heat sink , for single sided , use 1000C/W for 1cm2.Note these are very rough figuresSo a double sided board board with a largely unbroken copper planes of 80 x100mm on both side would have a case to ambient of around 6.25 W/C or 13 W/C for a single sided plane , assuming the case was connected directly to that plane and was in the centre of the planeOften in double sided boards the SMD is mounted on a small pcb pad ( ie DPAK ) connected via thermal visa to a ground plane , such vias have around 250-300 C/W thermal resistance and each one is in parallel , so say you had 16 under the Dpak , that would add circa 20 C/W which would make the total for the ground plane underneath around 30-35 C/W for 80 x100mmObviously there are complications where the thermal plane under the DPAK can’t be electrically connected to the underlying ground plane etcThat size plane could comfortably handle 1W with room for a rise in ambient. Obviously this assumes one heat source ( multiple sources can be approximation by addition ) a 1.5 to 2x safety margin is advisable and of course these figures are open air , static cooling , 1 ounce copperThis is very back of a napkin stuff and there’s can be serious discrepancies in real lifeA very good AP note ishttp://www.ti.com/lit/an/snva419c/snva419c.pdfDave

author: Doctorandus_P
author_rank: Super Contributor
timestamp: April 19, 2019, 09:46:03 pm
content: If you do a picture search for "dpak heatsink" you get some ideas. Some of them are heatsinks that can be placed by P&P machines on the PCB of the dpak.https://duckduckgo.com/?q=dpak+heatsink&iar=images&iax=images&ia=imagesI've also seen another way with copper studs.The idea is to design a big hole in the PCB under the DPAK, and then solder the copper stud with thread in the hole for close proximity to your coponent. Then some kind of heatsink can be screwed on the copper stud.There are also small copper heatsinks that can be directly soldered to the opposite side of the PCB and depend on a lot of thermal via's for the heat transfer through the PCB. But 1W is not much heat dissipation and I think that just an enlarged copper area on the PCB might be enough.For more heat dissipation sometimes TO220's are mounted with the tab up. This is especially done with for example motor controllers where multiple power transistors are used. For example for a 3-phase driver you need 6 power transistors and if you mount them as 2 rows of 3 with a bit of room between them, then you can use 4 screws between the TO220's to screw a single heatsink over those 6 transistors.

author: kjr18
author_rank: Regular Contributor
timestamp: April 21, 2019, 10:38:49 am
content: There was somewhere in Dave's mailbag solution for this. I forgot what name is this, but basically it was a metal peg with screw hole that you could attach a heatsink to it. This video had some fancy heatsink for high power led on thumbnail.Found it:https://temproducts.wordpress.com/Mailbag episode:

author: mvs
author_rank: Frequent Contributor
timestamp: April 21, 2019, 11:24:01 am
content: Quote from: rakeshm55 on April 19, 2019, 11:10:19 amMOSFETs are quite cheap now, so it might be a better option just to put two in parallel. It will eliminate 50% of heat and distribute remaining 50% over 2 packages.