Is this a good way measuring the temperature of a heatsink?

[technix]

This is about the revision to my DIY constant current load. I need some way to measure the temperature of my IRL540N pass element that has a heatsink on it. I wonder if this is a good way doing it: bolting a TO-220 temperature sensor onto the same heatsink as the power element.

The intended assembly has a extruded aluminum heatsink sandwiched between two TO-220 packages. One of the two is my IRL540N pass element, and the other side is a TC74 digital temperature sensor. Both packages has a Sil-pad in between them as electrical isolation and thermal interface material. A single M3 bolt runs through the threaded hole on the heatsink holding the MOSFET in place with a plastic washer, and on the other side, with another plastic washer, a nut fastens the temperature sensor to the heatsink.

Is this a good idea trying to take temperature measurements of the power MOSFET? Also for a constant power load is thermal throttling a good replacement for protective thermal cut-off?

[OM222O]

If you have the option to change your heat sink to something that mounts similar to CPU coolers (meaning uses 4 screws on sides of a square rather than directly bolting to the TO-220 tab), you can use a through hole thermistor (most of them are small enough to fit through the hole, but check that the diameter is smaller than 3mm before purchasing). this will be a lot cheaper because thermistors are common as mud, it also removes the sil pads and gives you more space on the PCB (given your heat sink is small enough).

for better heat dissipation I can also recommend to solder the package with the tab side up, so it's directly contacting the heat sink. something similar to this video (skip to 8:45):


however if your project is final and you have purchased heat sinks in bulk, this makes little to no sense to change your entire design. the best approach would be to buy TO-220 mountable thermistors (again they are mot likely electrically isolated for the most part, but check your specific part before purchasing) similar to these:


they should still be cheaper than a digital chip. I hope that helps.

[technix]

If you have the option to change your heat sink to something that mounts similar to CPU coolers (meaning uses 4 screws on sides of a square rather than directly bolting to the TO-220 tab), you can use a through hole thermistor (most of them are small enough to fit through the hole, but check that the diameter is smaller than 3mm before purchasing). this will be a lot cheaper because thermistors are common as mud, it also removes the sil pads and gives you more space on the PCB (given your heat sink is small enough).

I had the idea of using a PPAK SO-8 power device and put a spring load heatsink on top of it, but the attachment of thermal sensor stumped me and I went back to vertical TO-220. I am not getting how the whole assembly comes together in this stack you are suggesting.

[T3sl4co1l]

For SMT, just put down a chip thermistor.   Use a goopy thermal pad that conforms to the board profile, so you get good thermal transfer to copper pours around components, as well as whatever you can get through their tops.

Where heatsinks aren't required, it's not unreasonable to put a thermistor nearby.  I have a device which dumps excess power into a couple TVS diodes; the copper pours surround a chip thermistor.  This heats it up pretty quickly (a few seconds).  It does well enough that I can mash the "fire" button and the peak TVS temperature stays within ratings.

Tim

[OM222O]

If you have the option to change your heat sink to something that mounts similar to CPU coolers (meaning uses 4 screws on sides of a square rather than directly bolting to the TO-220 tab), you can use a through hole thermistor (most of them are small enough to fit through the hole, but check that the diameter is smaller than 3mm before purchasing). this will be a lot cheaper because thermistors are common as mud, it also removes the sil pads and gives you more space on the PCB (given your heat sink is small enough).

I had the idea of using a PPAK SO-8 power device and put a spring load heatsink on top of it, but the attachment of thermal sensor stumped me and I went back to vertical TO-220. I am not getting how the whole assembly comes together in this stack you are suggesting.

here is the basic idea:


the green part is the PCB, the yellow thing is the through hole themistor and the black/metalic part is the TO-220 package (not to scale, this is just a quick and dirty CAD model). then you add enough thermal paste and put on the heatsink like the video I posted earlier. this should remove any confusion.

also SO-8 packages are very small, not intended for power applications. I recommend D2PAK package for those applications. what is the power levels you're thinking about?

[technix]

also SO-8 packages are very small, not intended for power applications. I recommend D2PAK package for those applications. what is the power levels you're thinking about?

The package I am using is not quite actually SO-8 though, it is a 5-pin PPAK package. The load is specified at 30V, 5A, 50W or 90 degrees Celsius whichever reaches first.