SMD DPAK linear regulator heatsinking - Page 1
Forum section: electronics / projects

author: Jeroen3
author_rank: Super Contributor
timestamp: November 15, 2021, 07:28:15 pm
content: We've all been there. Needing to cool down a DPAK surface mount with a copper plane heatsink.But how much copper plane is enough?Many appnotes exist, for example these ones offer reproducible examples.Infineon AN-2021_02andAN201708_PL51_023that can give you some idea.If you want more, then you need to add those little heatsink things, and those are expensive.But then I thought, what if you surround the DPAK plane with SMD resistors? They are little ceramic plates that can act as heat radiator. And they cost almost nothing.Anyone tried something like that?Or is this just dumb?

author: wraper
author_rank: Unknown
timestamp: November 15, 2021, 07:37:54 pm
content: And why do you think that resistors will be better at radiating the heat than a simple copper polygon?

author: Alti
author_rank: Frequent Contributor
timestamp: November 15, 2021, 08:38:50 pm
content: If that is linear regulator and you are dissipating a lot of heat then this usually means you can put a bunch of resistors before the regulator. Then at low currents almost all energy is dissipated in DPAK and at full current half the energy is dissipated in DPAK and half in resistors.Quote

author: tooki
author_rank: Super Contributor
timestamp: November 15, 2021, 10:30:00 pm
content: Quote from: Jeroen3 on November 15, 2021, 07:28:15 pmI’ll leave it to you to answer the final question, but first think about this: the insulating tiles on the space shuttle were made of ceramic foam. “Glass” stovetops are made of vitroceramic material because it a) doesn't expand when hot, and b) stays cool just centimeters away from a burning-hot burner. Some ceramics are much better conductors of heat, but by and large they’re not particularly great. Meanwhile, copper is aspectacularlygood thermal conductor.The first document you link to has a table of thermal conductivity of many materials. Copper has a k of 401 W/mK. Aluminum metal is 237 W/mK. Aluminum nitride, the stuff the thermal jumpers are made from, is listed as 150 (the jumper datasheet says 177 W/mK). And alumina, the stuff SMD resistor bodies are made from? The table lists it as 45 W/mK.You’re suggesting covering your copper with little alumina tiles. Does that actually make sense??If you need to cool the chip more, use thermal vias to connect it to a plane on the other side of the board.If you really need to get heat out and can’t connect to the planes (because they’re different nets), get the SMD thermal jumpers, since they are electrically insulating but thermally conductive. But they’re not cheap. A heat sink is usually a better option.

author: Benta
author_rank: Super Contributor
timestamp: November 15, 2021, 11:27:37 pm
content: Convective cooling is always about thermal conductivity (=copper thickness) and surface, surface, surface.You are operating with a "material-to-air" interface. There's a reason that heat sinks have such fancy geometries: it's about conducting heat in the best way (=thickness) to the largest surface possible.I'd forget your resistor idea (or rather, I won't even comment it). Thermal via stitching to a large copper pad on the other side of the PCB is probably the best solution from a space/area point of view. Going to thicker copper cladding on the PCB might also help (better thermal conductivity to the edges of your cooling pad).

author: T3sl4co1l
author_rank: Super Contributor
timestamp: November 16, 2021, 12:30:37 am
content: You have two problems:1. Getting the heat out from the center hotspot.2. Dissipating it into the environment.#1 is most obvious when you have a small component with a large temp rise, even if the heatsinking area is otherwise adequate. Risk factors include small components (so, D2PAK > DPAK > SOT223 > SOT89, etc.), thin copper, lack of vias, insufficient thermal spoke size, etc.Note that PCB material has poor lateral conductivity, and very poor vertical (thru-plane) conductivity. So, copper pours absolutely help, and thermal vias help immensely at carrying heat to other layers. And, having thin layers of the stuff (say from outer copper to inner plane) rather than a huge thickness of it (like through a 2-layer board) is a big difference.#2 is just area. You need enough board area to do it passively, or you need to add heatsinks to increase it.The combination of both, limits the practical area for one device to dissipate into, even if you use solid copper on both sides, and thermal vias. The solution to this, actually involves Bessel functions (assuming a flat PCB, with circular source and outline, with uniform (linear) convective dissipation rate (units of e.g. K/(m^2 W) )), but the jist of it is, the active area (hot spot) is proportional to lateral conductivity over dissipation rate, with a typical case being 2 x 1oz copper doing about 5W at reasonable temp rise (say to a DPAK) in a 3-5cm radius around the footprint.By adding more material thickness (sure, including solder, chip resistors, jumpers, you name it), you increase the lateral spreading factor, and thus reduce the temp difference across the board -- the device temperature drops, while more board area is at a higher temperature than it would be otherwise, thus the dissipation increases. Which is to say, overall K/W drops.So, yes, alumina chips would help. They need to be comparable to the geometry in question though, and I think this would be where you encounter problems justifying it. A DPAK would be right at home among a field of 2512s, I'd guess -- one "course" of which should do a good job spreading out the heat, close in. But these aren't so cheap, and I think you'll start to appreciate the scope of the problem here; and doing it with smaller chips just incurs even higher assembly and part costs.You might do even better with solid metal chips or jumpers, but these tend not to be too cheap, and if you try and use them in the circuit as well (i.e. carrying load and doing useful things like current sense), they're probably in the wrong place (i.e. drain not source) or getting hot themselves, so maybe not such a great cooling path instead.There's also test probe targets, usually just a chit of metal with some sort of solderable plating (Sn/Au), in various sizes. Usually small round or square, but maybe there's rectangular or larger ones too? Not sure. Still pretty specialty, like the thick SMT jumper thingys, so, not as cheap as they really ought to be.Or you can always make your own, get some metal laser or waterjet cut, or punched. Use 'em fresh (Cu, tin plated steel), or get them plated for longer shelf life (hrm, I guess electroless Sn or NiAu?). Or while you're at it, make buses, brackets, etc. that fit into slots, screw holes, etc. for super current capacity or heat spreading. So, uh, of course the more special-cased you get, the less likely you are to find any such thing as a standard part at a distributor. Would be nice to see some basic shapes like squares and rectangles; various bent and oddly shaped things, jumpers of various cross section and length, stuff like that though: good luck.Tim

author: Jeroen3
author_rank: Super Contributor
timestamp: November 16, 2021, 07:09:48 am
content: Quote from: wraper on November 15, 2021, 07:37:54 pmI don't know. Just a guess. More stuff is more surface area right?Can't really find any blackbody emissivity numbers.Thermal vias are difficult, they require the board fab to inject special epoxy in the via before plating.The alternative being single sided untented vias with solder paste over them that flows in.From boards I've views with the thermal camera with components on a plane, after about 10mm the thermal gradient is finished. Perhaps the resistors will help increasing this gradient over a bigger surface area. Those thermal jumper look like they are intended for that purpose. They look like smd resistors that skipped the production line. Or are they really a different material?I guess I should just try it and make some reference fooprints for dpak with known thermal performances.Placing mechanical and Probe points I have considered. But since those are quite rare, and also expensive.Note that right now, the board what made me think up this question, is working fine with an 15x15mm top plane with via matrix as the infineon appnote on a 4 layer board.

author: tooki
author_rank: Super Contributor
timestamp: November 16, 2021, 07:14:18 pm
content: Thermal visa are typically just ordinary vias connecting two planes.