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I have selected the T2535-800G TRIAC (similar to the BTA24-800CWRRG that is currently in use in my system) and I want to use this heatsink V-1102-SMD/A-L (DigiKey part #: A10751-ND). Would the heatsink work for my application if the highest current is 10 amps at 110V? I use a custom PID-controlled pulse skipping modulation to control two TRIACs that control AC heating elements. I use the BTA24 devices currently and want to switch everything to surface mount. Would the heatsink I specified above work with the SMT TRIACs at 10 amps at 110V? They will be actively cooled with PID-controlled fans and I will use the ISOTMP35 devices to sense the temperature of the two SMT TRIACs to make sure they do not overheat or get anywhere near the breakdown temperature. Unfortunately, I have not had much exposure to thermal design and therefore do not trust my math in something this critical. If you could help me out it would be greatly appreciated.
The math 'aint usually the hard part, so much as properly interpreting the documentation so as to have proper numbers to math on… Pre-cooked discussion on the topic is available in the posts linked below.
Some relevant snippets from the datasheets are below.
The FET figure 1 indicates that at your specified 10A maximum, the triac can be expected to burn ~10watts.
That amount of thermal power (modeled as a current flow) has to go from the semiconductor device to the outside of the semiconductor package (modeled as a resistance, RΘJA) then laterally through the copper mounting pad to the heat sink’s mouting point, then from the heat sink’s own thermal resistance into the ambient air.
Unfortunately there’s some missing information here:
How much thermal resistance to expect from that lateral transfer through the copper pad
What sort of air velocity the fans mentioned are going to offer
What you specify as the maximum permitted ambient temperature for your system
Part 1’s kinda tough. For sake of discussion, let’s spitball that figure as 2°C/W
Part 2: Let’s assume an air velocity of 500 FPM. Per the heatsink’s chart, that should make the heatsink’s effective thermal resistance about 2.5°C/W.
Part 3: Let’s assume you call it 50°C.
Total thermal resistance is then expected to be about 1.4+2+2.5=5.9°C/W
Multiply that by your thermal load: 10W*5.9°C/W=59°C. This is how much you would expect the semiconductor junction inside the chip to increase in temperature above what you’re cooling it with. Since we stipulated that to be 50°C, under these conditions and assumptions we’d expect the silicon in the chip to max out at about 109°C, which is a comfortable distance from the 150°C max mentioned in the datasheet.
Now, let’s contextualize that a bit:
That thermal resistance figure across the copper pad is an educated WAG. Reality will differ.
The reality of your airspeed situation will also differ.
We’re ignoring effects of direct cooling of the package from this forced airflow. That should end up working in one’s favor.
If reliability/longevity are concerns, one probably doesn’t want to visit that 150°C mark on the regular, and certainly not play peek-a-boo with it. Differences in thermal expansion coefficients put stress on the bond wires within a part and degrade the thermal contact of the silicon to the outside of the package. If your application is of a sort that’s going to be cycling between full-on and off every few minutes, targeting a lower maximum temperature is probably wise.
In summary, we could probably say the proposed approach is plausible, but needful of validation.
Do be mindful of how/where you plant those temp sensors mentioned. Under your max conditions the junction’s already going to be 14° hotter than the copper pad, so if you were to have a bit of copper between your triac and the sensor with a bunch of cool air flowing over the works… Well, it’s not hard to imagine getting measurements that are on the order of ~30° low.
Thank you very much for the detailed response. Maybe I should have given a little background on the project. I currently use the FA-T220-38E heatsink and the BTA24-800CWRG, but the combination is physically too large. I also have observed that the highest temperature while running is 119 degrees Fahrenheit, using a thermistor (bolted directly to the tab of the TRIAC) in the current design that is running with these mentioned components and with a fan running over the heatsink. In the original post, I forgot to add that it is 110V “AC” at 10 amps maximum. The initial on-time peaks at about 119 degrees Fahrenheit but settles to about 80-90 degrees Fahrenheit after the PID control kicks in and then runs around that temperature while the machine runs. The fan I currently use has PWM control and a tach output to monitor the fan speed and control the fan. The maximum speed is 10000 RPM. I would have to take the top cover off and remove the fan to give you a specific part number of the fan but it is a good high-quality high-speed fan that moves a lot of air.
My goal is to place the heatsink directly over the SMT TRIAC, place the ISOTMP35 temperature sensor pads almost touching the tab of the TRIAC, and place the fan directly over the TRIAC and heatsink. I think this should give me the best possible solution. Can you recommend a good quality fan that would work for something like this that has PWM and TACH and is high speed with high airflow and small size? Please let me know.
Sorry, I didn’t reply sooner and didn’t specify the type of fan in greater detail. I will need a 12V high-speed fan with PWM control and TACH output. This said fan needs to have a high RPM and high airflow and ideally be a little larger than the heatsink specified in the OP. By the way, do you think SMT TRIAC will handle the 10 A load given that I can keep the device cool? Please let me know.
I apologize I must have misset the 12 volts when I was reading, the link below would be all the 12 Volt DC fans that we carry that have PWN, a tech and have an RMP rating of higher then 10,000 RPMs. And you can shorten the list by selecting the size you need. 12VDC DC Brushless Fans (BLDC) | Fans | Electronic Components Distributor DigiKey
As for if the SMT Triac will handle a 10-amp load, it looks like @rick_1976 answered question above.
