Little thermal thing

Two-ounce copper and closer placement.

It trashes your thermal forcing rejection. If your bandwidth goes down by 20x, so does your forcing rejection at low frequencies, unless you're prepared to do fancy lead-lag things that are liable to become unstable if you have a void in your thermal paste, for instance.

I have a current product that uses an 0603 thermistor mounted to a baby board in intimate contact with the cold plate. Crucially, the thermistor is on the same side of the board as the cold plate, and shares a common ground plane. The lateral clearance is about 0.040". (I could use a few in series if the cold plate were large enough to worry about gradients.)

Its thermal response is dramatically faster than a glass bead thermistor and silver epoxy, and besides, it's way way cheaper and more repeatable.

Cheers

Phil Hobbs

OK I assume that 'thermal forcing rejection' is the same or related to how much gain (delta(power)/delta(temp)) I can use. It's certainly true that with the faster common ground plane I can crank up the gain by a factor of ~20 also. (There's a V^2 term in there... I'm using the term 'gain' a bit loosely.)

So my response is trashed (reduced) at all frequency ranges... But if there are no big temperature or load changes so what?

Nice, I've always used the glass beads shoved into a hole with thermal goop. I never thought about the response time of the bead/ goop.

George H.

The forcing rejection depends on the loop gain, just like the load regulation of a linear voltage regulator.

Then you don't really need a temperature controller. ;)

If your system has a bandwidth of 0.01 Hz when it could have 0.2 Hz, for instance, the rejection of thermal forcing from somebody opening the lab door (say a 100s event) is reduced from 26 dB to essentially zero.

I was a bit worried about the low thermal diffusivity and high thermal mass of the glass epoxy smearing out the response, but with a short path in 1-oz copper it doesn't seem to be a problem, and it sure is easier to build and to repair. Use lock washers to make sure the thermal contact stays good.

Cheers

Phil Hobbs

Ugh! Yes, the thermal conductivity of fiberglass laminate is REALLY low. So, the 1 Oz copper has at least 20 X more conductivity than the substrate. One possible fix is to run a copper plane on the back side of the board.

Jon

Right, I'm going to spin some more boards and try that. (ground plane on back side.)

It is a rather shockingly bad time constant without that little piece of copper.

GH.

If I understand your geometry (and I probably don't) you could put the heater and the sensor on opposite sides of a thin board with a lot of copper on each. That would maximize the amount of FR4 heat conduction and minimize the heater-to-sensor tau.

The overall board tau depends on the mass of the rig and the heat flow out to the world. That could be slow even if the board were solid diamond.

Yeah that might work, a 'ground' plane on each side. It could be a little smaller then too.. that would help.

George H.