Electronics > Projects, Designs, and Technical Stuff
cooling SMC triacs in D2PAK?
langwadt:
--- Quote from: T3sl4co1l on October 19, 2019, 10:25:46 pm ---Can easily get 20-30W through a TO-220 and thermal pad, up to maybe 50W with thin, conductive pad and spring clamp. Even with greased contact (live heatsink?), ..
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The triac mentioned even has a version with isolated (2500V rated) tap, at the cost of being 1.6C/W vs. 0.8C/W
max_torque:
All good points! :-+
Having a large hole through the pcb, and machining an alluminum block that sticks through that hole and allows the use of TO-220s direct to that block, and thence to the heat sink on the other side of the pcb is certainly doable. The current package, due to connectors, has 30mm of "free" space behind the pcb, with is what i'd like to utilise for the heat sink.
Looking at the overall power rejection number (45w or so) it's certainly going to need some forced convection, so i need to go back to the CAD and work out the best location for the fan, which may then drive the heat sink location etc
I'll report back when i understand the package envelope a bit better for round two :box:
T3sl4co1l:
--- Quote from: langwadt on October 19, 2019, 10:57:44 pm ---
--- Quote from: T3sl4co1l on October 19, 2019, 10:25:46 pm ---Can easily get 20-30W through a TO-220 and thermal pad, up to maybe 50W with thin, conductive pad and spring clamp. Even with greased contact (live heatsink?), ..
--- End quote ---
The triac mentioned even has a version with isolated (2500V rated) tap, at the cost of being 1.6C/W vs. 0.8C/W
--- End quote ---
Yup, which puts it in the thermal-pad class. Which is perfectly acceptable given the ratings (imagine that :) ).
A shame there aren't any affordable insulators with good conductivity. AlN is less toxic than BeO and almost as good, but it's still hard to find (by itself). Al2O3 and AlN are commonly used for DBC (used for packaged insulators, like the iso-tab TRIAC). Thin insulators (mica, polyimide) are atrocious and only work because they are so thin. Thermal pads need a resin or rubber filler so can't do much better (though there are some quite attractive formulations available, but again, for a price..).
Would be cool if a metal-slug process were feasible for PCB fab + reflow assembly. Could actually run D(n)PAKs at ratings. It's not at all impossible, but I'm not aware of any real production application..? (There's also heavy metal PCBs, probably made with a combination of milled bar and electroplated foil, but, that's into the deep end of "heroic effort"...)
--- Quote from: max_torque on October 20, 2019, 08:45:04 am ---Looking at the overall power rejection number (45w or so) it's certainly going to need some forced convection, so i need to go back to the CAD and work out the best location for the fan, which may then drive the heat sink location etc
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If saving heatsink cost or package size is valuable, you might consider MOS SSRs. Possibly discrete (hand made) at this power level. (Not sure offhand if any commercial units of comparable rating beat a TRIAC.) Or, to be perfectly honest: good old fashioned relays, though I'm guessing you already have reasons to avoid those, hence the TRIACs in the first place.
Tim
max_torque:
I had thought about a parallel Triac and relay, i have microprocessor control on this board, so i could use the triac to modulate, then switch the relay on when it "full on" which would remove most of the heat flux on the longer duration burns, but that's a lot of extra effort!
http://www.farnell.com/datasheets/1723975.pdf
There is that ^^ BTA26 which is available in a TOP-3 package, with an insulated tab and is 0.9 degC/W, and in that package has nice widely based (good clearance/creepage!) strong pins, which could make the "hang the device in space over a hole in the pcb" type afair a flyer?
It has a slightly lower peak current rating, but looking at the data sheet numbers that seems mostly an arbitrary limit, and it has slightly lower dv/dt capability, but i'll snubber it and it's driving a broadly resistive load, so i don't think that's an issue. Cost is about par with the previously mentioned triac. Vtm is, as you would expect, pretty much the same, so i'll still have 15w to deal with.
max_torque:
Ok, i think i am converging on a solution that stands some chance of doing the numbers !
1) Swapped to TOP-3 packaged Triacs, as they are chunky and easy to work with, and have a big tab and surface area. BTA26 from ST has insulated tab (2.5kV) so that helps make the heat sink interface nice and easy
2) Triacs now mount from backside of pcb, and are held (with spring clips) to a pair of heatsinks, also mounted to the pcb, but with little spacer to allow air to flow under the sinks, and for the sinks to clear the back of the pcb for isolation (might end up with plastic insulator in this gap, water jetted out to clear as necessary)
3) Using decent sized heatsinks, each with a pair of Triacs on, means max thermal load into each heatsink is 30 watts worst case
4) Powerful (ish, 16w rating) 240VAC fan sits with 3d printed duct on top next to the pcb stack, and therefore shoots it's air down behind the pcb, and over the heat sinks, ensuring a decent airflow and hopefully not too many stagnant areas
Fitting everything on two sides of the pcb, with the I/O connectors, the piggyback logic pcb and mounting into the assembly was quite a challenge! How people did this before decent 3d CAD i'll never know :-DD
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