Sinking heat from SMD power silicon into a heat sink (complications!) - Page 1
Forum section: electronics / projects

author: max_torque
author_rank: Super Contributor
timestamp: April 28, 2014, 04:47:34 pm
content: Hi all,I have a project that will need to shift around 30W (continuous) from 8 SMD DPAK smart power switches into a heat sink.Ok, sounds easy doesn't it. Er, not so fast here is the complication: The heat sink is designed for through hole TO-220 devices and so is orientated PERPENDICULAR to the PCB! DOH!This is the enclosure (a Cinch MODice SE with intergral heat sink plates)datasheet:http://www.cinch.com/pdfs/1219678243-5810130041S_ICE30_rev_D.pdfIt takes a single PCB horizontally mounted at the bottom of the enclosure (slides in from the front with the connector/closure plateThe heat sinks are down each side, and vertical, and are designed to take vertical TO-220 devices, and there is a spring metal clip available to keep those devices pressed hard up against those heat sinks.Unfortunately, the smart power switches i want to use (well really "need" to use) are SMD only. So, the usual trick, of SMD devices on one side of pcb, and an array of solder filled vias to move heat through to the otherside of the pcb and an attached heat sink, really won't work (or, more accurately, i can't work out how to make it work in this case)I have had ideas about machining an internal alluminum heat transfer plate, that sucks up heat from the pcb, and moves it laterally across to the enclosures vertical heat sinks, but then i can't work out how to ensure all those parts are in good close contact for decent low thermal gradient?Any one done anything similar to this? Looking for inspiration here! ;-)(am considering using non heatsinked enclosure option, and just cutting hole in bottom and bonding in my own horizontal heat sink, but that is a bit of a pita. Device is low volume (say around 50units) but i want to make it as easy to assemble as possible)

author: SeanB
author_rank: Super Contributor
timestamp: April 28, 2014, 05:05:05 pm
content: Get a strip of copper or brass sheet cut to sze, and with a L shape so that you can solder the short side to the board either under the devices or nect to them, with the longer side clipped to the heatsink. That way you can use the clip, and if the plate is too thin add a simple metal bar to provide the correct thickness, and use a silpad for insulation between the brass and the heatsink.

author: max_torque
author_rank: Super Contributor
timestamp: April 28, 2014, 05:33:40 pm
content: interesting idea, i wonder on the trade off between the thickness of the sheet (to provide sufficient thermal transfer area) and it's flexibility (to ensure it is held tightly against the vertical heatsinks in the case?

author: Dago
author_rank: Frequent Contributor
timestamp: April 28, 2014, 05:49:34 pm
content: First question is what is dissipating 8W and can you reduce it somehow? Change linear power supplies to switchmode, use lower RDSon FETs etc.

author: max_torque
author_rank: Super Contributor
timestamp: April 28, 2014, 06:29:36 pm
content: The power dissipation is coming from a bunch (smart high side switches, that are switching 8 separate DC buses on and off (via CAN control). Unfortunately i don't have the ability to change how much current is being sourced (that depends on the load, rather than my distribution device). I am planning to use Infineon Profet power devices (BTS50055-1TMA) which have a maximum RDS(on) of 17mOhm. However, each channel is designed to supply 20amps continuously so that's 6.8W per device for a total of 54.4W. However, there is no foreseable situation where the loads all demand peak continuous current simultaneously, and current best estimation suggests no more than approx 50% of that total load is likely to be continuously demanded, hence the ~30W dissipation required.....

author: Dago
author_rank: Frequent Contributor
timestamp: April 28, 2014, 06:52:49 pm
content: Quote from: max_torque on April 28, 2014, 06:29:36 pmWith a quick look you can get N-channel FETs with under 1mOhm resistance. Meaning it would cut your worst case power dissipation to under 6% of you current value.For examplehttp://www.digikey.fi/product-detail/en/TPHR9003NL,L1Q/TPHR9003NLL1QCT-ND/4563093has a worst case RDSon of 1.4mOhm but there's load of 'em.

author: max_torque
author_rank: Super Contributor
timestamp: April 28, 2014, 06:56:12 pm
content: Indeed you can, but by the time you have added the cost and complexity of the necessary charge pump (to get a high side output) and separate current sensing and diagnostics/ overcurrent / over temperature limitation (all of which are included in those "smart" high side switches) my device would simply by too expensive and complex!At the moment, with the average load being "containable" i think it's just a matter of getting the "heat out of the box" so to speak!

author: SeanB
author_rank: Super Contributor
timestamp: April 28, 2014, 07:03:14 pm
content: Looks like the copper tab will be the best bet, or get a heatsink block that you can screw to the back of the board, like in the Agilent unit Dave just tore down, that used a heatsink on the back of the board with a whole lot of solder filled vias to conduct the heat to it via thick conformal silpad. It worked there, and as well you can use the block to mount the board to the case as well, along with the internal rails as support.

author: Dago
author_rank: Frequent Contributor
timestamp: April 28, 2014, 07:21:56 pm
content: Quote from: max_torque on April 28, 2014, 06:56:12 pmYou can get high-side driver chips for peanuts (example:http://www.digikey.fi/product-detail/en/FAN7361MX/FAN7361MXCT-ND/820888) and all the protection features would be quite simple to implement with a microcontroller (you'd prolly need just one temperature sensor for the whole thing, most likely the microcontroller would have an integrated one) or even some premade thermostat IC. Current monitoring (if it's really needed) would be simple to implement with a high-side current monitor such ashttp://www.digikey.fi/product-detail/en/ZXCT1050E5TA/ZXCT1050E5TR-ND/1767183I'd say you'd spend less time implementing it this way than thinking about how to dissipate that large amount of power

author: SeanB
author_rank: Super Contributor
timestamp: April 28, 2014, 07:32:18 pm
content: Don't think those high side drivers will survive an automotive environment without extra protection and they may not survive what an automotive power bus will give them without a lot of extra protective devices. The Infineon devices are designed for pretty much just connecting to the battery bus with minimal extra components. Basically a fuse and a transient clamp is all you need.

author: max_torque
author_rank: Super Contributor
timestamp: April 28, 2014, 07:36:15 pm
content: Whilst i would completely agree with you for a two channel similar device, trying to do everything that those smart high side switches do across 8 channels would be both onerous, time consuming, complex and require significant validation at a product level. By leveraging Infineons hard work and knowledge, i can remove a SIGNIFICANT amount of work from my (low volume) design, and crucially, without significant work i can show that to the people who will be accepting/using my device.I like the idea of some sort of internal heat distribution plate i just can't quite envisage what it will look like yet, and how to mount it, with sufficient even pressure to the integral heatsinks?

author: max_torque
author_rank: Super Contributor
timestamp: April 28, 2014, 07:39:10 pm
content: Quote from: SeanB on April 28, 2014, 07:32:18 pmIndeed, if you look at the datasheet, the ProFET smart switches are pretty tough little things, specifically designed for the application i have in mind!And they include all this:Features• Overload protection• Current limitation• Short circuit protection• Over temperature protection• Over voltage protection (including load dump)• Clamp of negative voltage at output• Fast deenergizing of inductive loads 1)• Low ohmic inverse current operation• Reversave? (Reverse battery protection)• Diagnostic feedback with load current sense• Open load detection via current sense• Loss of Vbb protection 2)• Electrostatic discharge (ESD) protection• Green product (RoHS compliant)• AEC qualifiedProduct SummaryOvervoltage protection Vbb(AZ) 62 VOutput clamp VON(CL) 42 VOperating voltage Vbb(on) 5.0 ... 34 VOn-state resistance RON 6.0 m?Load current (ISO) IL(ISO) 70 AShort circuit current limitation IL(SC) 130 Awhich is a fair chunk of work to do with descretes and mosfets or similar imo!

author: SeanB
author_rank: Super Contributor
timestamp: April 28, 2014, 07:48:47 pm
content: Simple way is to mount the devices on one side of the board with vias through with extra solder to allow them to fill, and then have a large copper plane on the bottom connected to 12V ( as it is common for all the devices and you need the broad trace anyway) with a Berquist Silpad of around 1mm thick to allow it to contact an aluminium block that is screwed to the board with around 3 screws through the board ( leave clearance of 5mm around the screw holes and have an insulating nylon washer on the top as well along with a washer, probably a M4 screw threaded about half way through the aluminium slug) to give good thermal contact. for the bottom a much thicker thermal pad to provide the interface will probably do, as it will conform to the irregular gap between the slug and the box. If the lid is removable on the box you can have a milled area and a thinner pad if you control the board mount, but the thick pad will handle most of the heat with only a minimal thermal resistance increase. You probably will want the spreader to be somewhat T shaped so the thick pad has a large heat transfer area. No screws for there, the pad itself should provide enough pressure and damping for vibration as is.

author: max_torque
author_rank: Super Contributor
timestamp: April 28, 2014, 08:30:06 pm
content: The problem is the exisiting heat sinks in that Cinch enclosure are vertical and on each side, meaning the heat sink "slug" has to be L shaped, and there will be an issue with tollerances in trying to get a good contact on each side of those heatsinks.I am currently leaning towards using an enclosure without any heat sinks, and milling out a large rectangular hole and bonding in my own heat sink, which allows me much better control of the heat path, with less thermal gradient / junctions

author: Monkeh
author_rank: Super Contributor
timestamp: April 29, 2014, 12:51:32 am
content: Infineon do though-hole 'smart switches', some of which have significantly lower on resistance. Why are you stuck with the one part in their range whichisn'tsuitable for your application?

author: max_torque
author_rank: Super Contributor
timestamp: April 29, 2014, 06:57:35 pm
content: Quote from: Monkeh on April 29, 2014, 12:51:32 amSo far, i have no been able to find suitable through hole devices that have1) a suitable current capabilty2) the ratiometric current sensing output3) don't force you to use the "tab" of the device as a terminal

author: peufeu
author_rank: Contributor
timestamp: April 29, 2014, 07:27:31 pm
content: If your heat sink likes TO220 devices, would it be possible to make a little PCB, mounted vertically, screwed on the heat sink, carrying your power devices, and connected to the main board with a right angle connector (blade, for example) ?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: April 29, 2014, 08:29:15 pm
content: Doesn't sound very "Smart" if you neglected to find parts suitable for your mechanical constraints.If you must, SMTs can be heatsunk by piling thermal vias underneath and around the footprint, and clamping the PCB against a heatsink, plus Sil-pad or whatever. Obviously, you can't have any components on the bottom side of the board. You'll need heat spreaders, which can be bent heavy aluminum sheet, or milled barstock. Or copper for that matter.Tim

author: Monkeh
author_rank: Super Contributor
timestamp: April 29, 2014, 09:43:06 pm
content: Quote from: max_torque on April 29, 2014, 06:57:35 pmBTS555, from a skim. It does not 'force' you to use the tab, merely recommends it..

author: Kjelt
author_rank: Super Contributor
timestamp: April 30, 2014, 07:27:48 am
content: To "bridge" any gaps between components and heatsink you can use "heat conductive gel foils" they are electrically isolating but thermally conducting.There are a few companies in the world that make them, Fisher elektronic is one of them.

author: max_torque
author_rank: Super Contributor
timestamp: April 30, 2014, 09:06:24 pm
content: The BTS555 looks like a possible candidate, but i am rather put off by the cost (around UK £8 in lower volumes) and the fact it is rather an overkill in terms of current capability (which means a low "sensitivity" of the integral current output pin)I suspect using just 1 small pin, rather than the tab to power the device would also at least half it capability, which means i would be paying quite a lot for a device i can't use to it's potential! DOH!I think a re-engineer of the non heat sink enclosures (which are cheaper) with an additional custom "cold sink" mounted through the bottom of the enclosure is looking like the best bet, meaning i can use cheap and optimum for my design SMC Profets, and still get the heat out of the case etc ;-)

author: Monkeh
author_rank: Super Contributor
timestamp: April 30, 2014, 09:13:06 pm
content: Quote from: max_torque on April 30, 2014, 09:06:24 pmThat is because the on resistance is suitably low to allow it.. Which makes your life a lot easier.

author: Kjelt
author_rank: Super Contributor
timestamp: April 30, 2014, 11:09:34 pm
content: Quote from: 3roomlab on April 30, 2014, 09:32:08 pmI suspect they are probably the same.Sil-pad is the name from Bergquist one of the other manufacturers that make these, they are available in many thicknesses and can be cut to match your profile.The thicker ones feel like bubblegum elastic the thin ones 0,1mm are very fragile.

author: max_torque
author_rank: Super Contributor
timestamp: May 01, 2014, 11:22:32 am
content: Quote from: Monkeh on April 30, 2014, 09:13:06 pmIndeed it might, but i don't think something like 30W is an extravagant amount of heat to disipate as long as the heat sinking is done right, and with the smaller devices being half the cost, that's quite a big reduction in the BOM cost?Hmm, decisions, decisions........ ;-)Those SIL pads and heat transfer materials look quite good, certainly will have to use something similar between the bottom of the pcb and what ever heat sink ends up being in there!

author: Dago
author_rank: Frequent Contributor
timestamp: May 01, 2014, 11:48:01 am
content: Quote from: max_torque on May 01, 2014, 11:22:32 am30W is a massive amount of heat for a small box thats supposed to be heated passively.

author: max_torque
author_rank: Super Contributor
timestamp: May 01, 2014, 12:20:31 pm
content: 30w inside a plastic box, with no heat route is indeed a bad idea.30w, inside a plastic box, with a large alluminium heat slug direct to the outside of the box, and then sunk into around 300kg or ally/steel enclosure at a max of 45degC seems less of an issue to me.........