Designing for very hot environments
JRoark
Posts: 1,215
in Propeller 1
I’m contemplating a P1 design that will be operating basically forever at 80 mhz in a very hot environment with all 8 cores well utilized. The question is do I go with the SMD version or the DIP? There are no tight constraints on board dimensions so either package would work. I can also attach a heatsink or use the metal case, but the enclosure will be tightly sealed so no fans. Which package would work best for heat rejection? I’m leaning toward DIP but I’d like to hear from someone who has maybe already walked this road.
Comments
Here's a document that might be helpful:
https://www.ti.com/lit/an/spra953c/spra953c.pdf
-Phil
Based on that read it looks like I’ll be using the SMD version and mount the board upside down with a heat spreading copper plane below (above, really). That would seem to make the best of a marginal thermal environment.
What exactly is the "very hot temperature" that are you anticipating? What is the size of the PCB? How large is the internal size of the enclosure? Are both the internal temperature and external enclosure temperatures going to be the same? ie. A static condition and/or a heat soak condition?
BTW - The TI report is very good and a great starting point. Follow it closely!!!
The problem comes when there is a high demand for nitrogen somewhere downstream. This causes high flow rates thru the heat exchangers causing the purge nitrogen to get quite hot. And wet. The wetness gets mostly handled by an inertial separator, but the heat from the motor and the purge gas combines to make it challenging. The outside of the doghouse runs about 60c, but my gizmo has to be completely contained in its own enclosure within the doghouse, so there wont be much of a temperature gradient internally.
-Phil
Its like a chain. Every process depends on other processes to make it run. If the nitrogen stops, the purge is lost to the entire site. Not good. If the hydrogen stops, the catalytic flares stop which means those processes that need venting have to stop. So their on-site nitrogen farm keeps a few zillion cubic feet of nitrogen in cryo storage as a back-up so they can do an orderly shutdown. They also keep reserve natural gas (as LNG) in storage as a temporary hydrogen replacement for the same reason. Its all just amazing to me. It is totally outside of my usual comfort zone.
That is indeed brutal!!
What are you doing that you need 80 MHz?
Normally parts get de-rated, or run at lower rates, for higher temperatures; that is unless you want to be at the mercy of "Uncle Murphy". :P
P1 can function at 190C. https://youtube.com/watch?v=EjkXokgcBZw
There is no penalty for failure. This is an experiment. My stuff is totally passive and has no control of the machine. Control gets handled by the vetted, megabuck process controllers. I just get to watch for some things that nobody "upstairs" wants to admit are happening. The entire point of this exercise is to cause some institutional panic in the "it's fine / it'll cost too much / take too long / you can't do that / we don't have enough data to justify that" crowd. Basically the engineer knows that it can be done and wants to show them what a small effort it took. Once that happens, they'll bring in the big boys and my stuff will go into the bin. But it's going to take many months of data gathering to get there.
I’m thinking a smaller internal box to house the electronics with insulation between the two boxes. That way your electronics will probably remain cool even with the outside heat.
"There is no penalty for failure. This is an experiment. My stuff is totally passive and has no control of the machine"
Still, careful selection of components and giving everything a wide operational margin for temperature and electrical noise, will ensure some degree of success. A look-see at automotive and aircraft instrumentation is probably a good start IMHO.
Surely equilibrium will eventually be reached and the electronics will be the same temperature as the environment?
-Phil
There is some great insulation available which would prevent the inner box from heating due to the outside area.
Yes, In theory, but that package is EOL.
If that is momentary enough, you could add layered ‘Russian Doll’ thermal zones and some thermal inertia to the inner most zone, so you get a thermal LPF effect.
As mentioned, get good caps, and a top rated Automotive EEPROM, and regulator is going to need some care around their thermal shutdown points.
Multilayer PCBs are quite cheap, and a good way to spread all the heat about, and so lower the peak temperatures. Throw as much copper at it as you can.
What is the 3v3 derived from ? - A switching regulator may be better than LDO, and companies like Infineon have Automotive SMPS parts.
Not 'prevent' but 'slow down'.
As it sounds like the electronics will be subjected to these high temperatures for long periods then they will eventually reach the same temperature as their surroundings.
Basic thermodynamics. Heat flows from hot to cold.
Yeah, forget insulation, forget cooling, let the prop1 run at ambient. It isn't beyond what the prop1 can handle.
The power supply might need some attention though.
It turns out we completely missed a very basic trick. There are two nitrogen supplies. A hot, variable temperature low pressure supply that runs about 50 PSI (max), and a high pressure supply that stays between 150 and 200 PSI. Probably 95% of the facility purge systems use the low pressure side, and this is what we were looking at using too. But that high pressure port is ideal for our uses. Why? Because when you drop a compressed gas from high to low pressure, it expands... AND ABSORBS HEAT.
Duh. (Smacks head)
The magic solution was to put a needle valve downstream of a flowmeter on the high pressure line. With the flowmeter at 5-7 CFM, the worst case outlet temperature after the oriface is about 140F. Most of the time it is about 20 degrees cooler. A short neoprene hose brings the cold, low-pressure gas right into the case with the Prop. Problem solved. And because the moisture has been largely removed from the stream by the site compressor/aftercooler, the nitrogen is pretty dry.
So we are trying this approach for a bit to see how it works in the field.
Amazing what you can miss when you are fixated on a problem!
If this test setup runs without errors, Phase 2 begins with a custom board and some real monitoring.