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cooling SMC triacs in D2PAK?

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max_torque:
I'm looking at a device that drives some single phase AC loads at various currents between 0.5A and 16A, and i plan to use Triacs as the load control devices.

Most triacs come in through hole packages, like the classic TO-220, which then need specific heat sinking. I had an idea to use SMC packages, such as d2pak, and arrange the triacs as a low side switch and to have a common neutral for all the devices, connected to the A2 terminal which is the tab of that package for most triacs.  With suitable via fields, i plan to sink the heat out the tab of the d2pak into a large top copper layer, and then through the pcb on the vias, and into a large copper pour on the back side, and then finally into a large(ish) heat sink screwed to the back of the pcb, probably using a thin thermal gasket for compliance. In this manner i heat sink all my devices together, and the heat sink is at Neutral potential, which would normally be close to Earth potential.   The entire device is inside a "finger proof" enclosure, so the heat sink cannot be touched, but it's better to be at Neutral potential, as that avoids issues with electrically isolation etc.

Being a low side switch, the loads are of course powered "all the time" on the high side, ie connected to Live, however the output sockets i am using are finger safe, and shuttered (and inside the overally unit, and only accessable via tools) and i have a non dynamically switching relay as a high side "safety" switch, which can turn off all the Hide side feeds to the devices (This is to prevent the process the loads are driving from going out of control should one of the low side triacs fail shorted.


Can anyone see a flaw in the plan?    :-/O

MagicSmoker:
Without all the thermal vias and external heatsink the thermal resistance from junction to ambient for a single D2Pak device is about 40-45C/W so this may or may not fly depending on the total losses and how well your additional heatsinking measures work. Likely you'll have to actually build and test one to find out.

Otherwise, "low-side" switching with a triac is fine.

EDIT - I just noticed the 16A max - that's almost certainly not going to be doable unless you can get R_theta[j-a] down to less than 5C/W.

T3sl4co1l:
Figure about 5W tops for a D2PAK with reasonably thorough heatsinking.  Maybe 2W for not much effort (a few in^2 copper pours, 2oz).  Maybe 10-20W with heroic effort (heavy copper, filled vias, low ambient temp e.g. water cooling).  So, whatever that comes out to, from the datasheet Pd spec, or whatever you can fill in for it with (Irms * Vf max? Don't forget switching losses too, if it's phase controlling).

Thermal vias with an insulator pad on the backside is a good way to do it, I've done it many times myself.  You can even fill the vias with solder, but take care that they don't end up proud of the bottom surface, or use a thick enough pad to clear them.

If you can ground the heatsink, that's even better and alleviates some of the finger safety precautions of the design.  If the mechanicals are fine as-is, obviously you don't have anything to gain this way, but just FYI.

Tim

max_torque:
Triac is T3035H

https://www.st.com/resource/en/datasheet/t3035h.pdf

nominal max rating is 30A


the datasheet number (page 3) (caution, number might be cobblers as it's a datasheet number.....) is 0.8 degC per watt junction to case.

16A is worst case dynamic, steady state load is more like 12A (load is 3kW resistive heater, so some "cold inrush" does occur for a short period), again according to the datasheet that's a worst case heating of 12 watts (page 4, figure 1)

Max ambient is 60 degC, so to stay below 150 degC junction (Tmax) i need an overall thermal impedance of 7.5 degC/watt.

The pcb this is all mounted too is pretty large (330mm x 120mm) and the back side is pretty much all available for heat sinking.

Sounds do able to me?  (The enclosure does include a fan and is well vented as well)

MagicSmoker:
Okay, so Vtm for this triac is 1.55V max (I assumed 1.4, which is a "typical" value, and an ambient of 50C) so for a 12A load you would have to get rid of 16.8 to 18.6W of heat, and if you went ahead with a 90C allowed rise (really not advisable) you would still need to get R_theta[j-a] to 4.8C/W or less... This just doesn't seem realistic to me.

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