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Operating MOSFETs and IGBTs in Near Vacuum and Cold. (-55 C)

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Patrick

Thu Feb 22 2018, 10:08PM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

Im running into some issues with MOSFETs at high altitude.

Supposedly the flight controllers work at 100,000 ft, but Ive been told discreet TO-220 case format MOSFETs dont survive. So we use solid state relays, and mechanical ones. CPU/chip based devices seem to work fine in little foam board boxes all the way up for 45 minutes at a time. I fail to see why a MOSFET would fail other than being improperly driven at the gate. the threshold voltage does rise by as much as 0.4v if I remember. But at -40 - 50 C they shouldn't be catastrophically burning out.

I didnt design and unfortunately dont have knowledge of the previous circuits that failed. though they took place in the 90's. when power MOSFETs may not have been so good.

Im thinking of vacuum + dry ice chamber experiments, but was met with skepticism as there has been 30+ years of debate in aviation and NASA and other agencies as to weather such testing is even valid.

im looking through data sheets and possible under-heating or over-heating mechanisms of failure and thermal runaway, or that the MOSFET death was a symptom a greater problem.

PDFs:

]sihf840.pdf[/file] a common modern IRF840. says -55C.

Keep in mind TTL and CMOS ICs as well as modern GPS, STM32 micro-controllers, GoPros, rc servos, high power RF transmitters and normal lithium ion and LiPo batteries all work fine.

Of course Im trying to understand how the 3 means of transport work, radiation, convection and convection in near vacuum.

if i recall the thermal resistance of the TO-220 package, regardless of transistor type, is 62 degrees C per Watt. So 3-5 seconds at 3 amps from a 15volt source through a common resistor could cuase a huge thermal inertia event in a vacuum with just a tab for a heat sink, depending on Rds.

And of course radiation causing latch-up.

...

Fri Feb 23 2018, 04:01AM

Registered Member #56 Joined: Thu Feb 09 2006, 05:02AM
Location: Southern Califorina, USA
Posts: 2445

I think that you are correct, as long as the device works at cold temperatures and is not being stressed mechanically (ie, suffering from poor heatsink design that puts stress on the leads, etc) it should work fine. One issue we ran into is that air at high altitude is much easier to ionize, so you need to be very careful about mid to high voltages. For voltages >1kv we ended up needing to put the device in a pressurized container to avoid excessive leakage.

Sulaiman

Fri Feb 23 2018, 08:33AM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

Patrick wrote ...

Im running into some issues with MOSFETs at high altitude.
...
Of course Im trying to understand how the 3 means of transport work, radiation, convection and convection in near vacuum.

if i recall the thermal resistance of the TO-220 package, regardless of transistor type, is 62 degrees C per Watt. So 3-5 seconds at 3 amps from a 15volt source through a common resistor could cuase a huge thermal inertia event in a vacuum with just a tab for a heat sink, depending on Rds.

And of course radiation causing latch-up.

I have no experience with high altitude / low pressure so I'm guessing ....

A Paschen curve for atmospheric gasses has a minimum breakdown voltage c300V at around 1 torr.cm
e.g. peak of Everest, 253 torr ... 300V could 'jump' 1/253 cm = 0.04 mm ... not a problem
at 90,000 ft, 14 torr, 300 V could 'jump' about 0.7 mm ... maybe a problem.
Probably c 30 km altitude for a 1 cm 'jump' ?

I suggest that you ignore thermal resistance to ambient via convection (62 C/W) and use conduction to either a large thermal mass, or a cooled surface.
For short flights you may be able to use phase-change cooling ?
(melt ice, boil water etc.)
(e.g. boil water = 2265 J/g ... boil off 1g water = 10W for 226 seconds.
If the water starts as mostly ice then even better.)
The heat sink would be 100 C, thermal resistance to heatsink = 1 C/W, Tj <=150 C ... Pmax = 50W ideally.
If you use rf power mosfets they are usually rated for Tj <= 200 C with low thermal resistance to heatsink.
(because I have stock I'd use MRF185, but others are similar)
RF mosfets come in a range of package sizes.
Maybe other TO220 devices have higher Tj max, ?

One failure mechanism could be thermal shock/differential expansion,
worst case would be to try to use the transistor near the end of the flight with no attached thermal mass.

P.S. if you do use the boiling of water to cool semiconductors then you could consider
routing the exhaust pipe to keep other vital components warm.
pvc tubing should be adequate, ptfe tubing more reliable and lighter.

EDIT: I forgot, as the pressure drops the b.p. of water drops, so much greater power dissipation is possible.
At about 14 mmHg pressure, ice sublimes, will you get that high (c90,000 ft.) ?
Peak of Mt.Everest c253 mmHg, water b.p. c62 C
================================================ ======
Just te re-iterate my brilliant idea ...
Add 1g ice to payload at lift-off, the additional weight will reduce to zero by end of flight.
To melt 1g ice = 1 x 333.55 = 333.55 J
Heat from 0 to 100 C = 1 x 100 x 4.18 = 418 J
Boil off = 1 x 2265 = 2265 J
Total = 3 kJ = 50W for 1 minute

If you are brave then put the water at 0 C in a SEALED container with pressure relief at 1 Atm. to enable the full 0 to 100 C rise ... or more.

DerAlbi

Fri Feb 23 2018, 01:28PM

Registered Member #2906 Joined: Sun Jun 06 2010, 02:20AM
Location: Dresden, Germany
Posts: 727

One issue with temperature is the thermal expansion of the copper backplate, the silicon chip and most important the plastic/epoxy-casing are all different.
The silicon-copper-interface itself is not that much of a problem, but the plastic is.
This is why for example a 2N3055 in the full metal TO-3 package has such extended temperature range (-65Â°C -> 200Â°C) .
Stress on the silicon does inpact electron movility, changes Vgsth and other parameters so that they become out of spec.
Diode junctions (like Base-Emitter) also have a high temperature coefficient. Operating points can shift so much, that some integrated circuits cant work anymore. Higher UCC helps sometimes.
Afaik, regarding to IGBTs, low temperatures lower (???) the Uce-max rating.

Hazmatt_(The Underdog)

Fri Feb 23 2018, 05:17PM

Registered Member #135 Joined: Sat Feb 11 2006, 12:06AM
Location: Anywhere is fine
Posts: 1735

Critical components in payloads are kept at constant temperatures with heater tapes and thermal strips, it wastes power but it keeps critical systems alive.

Boards can be conformal coated to reduce electrical stress, and if you have to, you can put everything into a solder-seamed can and backfill with dry Nitrogen to make hermetic systems.
The "trick" at that point is finding old stock hermetic feedthroughs from Surplus Sales of Nebraska or other sources like Ebay, Anritsu, Micro Mode, Pasternack, or Gilbert, and keeping the price down.

Patrick

Sat Feb 24 2018, 06:14PM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

And for active semi's that are prone to overheating, should I use heat pipes to a constant heat source like a temperature soaked aluminum block, or iron ? or is lead better? i guess aluminium isnt.

Hazmatt_(The Underdog)

Sun Feb 25 2018, 04:54PM

Registered Member #135 Joined: Sat Feb 11 2006, 12:06AM
Location: Anywhere is fine
Posts: 1735

If your active semis are overheating and you need a source of heat to keep everything alive, you bolt them to the housing and get a free heater out of the deal. ^^

If the housing has to go on a rocket, lead should be obvious that it is out of the question.
An Aluminum or Magnesium housing for your system would be preferred, and hermetic if doable.

Patrick

Sun Feb 25 2018, 07:32PM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

I was thinking lead or copper would be lighter than aluminum for the same heat capacity.

Im not opposed to magnesium in some applications, but on a rocket with electrical shorts, i just see my luck running out time after time.

hen918

Sun Feb 25 2018, 08:36PM

Registered Member #11591 Joined: Wed Mar 20 2013, 08:20PM
Location: UK
Posts: 556

Patrick wrote ...

I was thinking lead or copper would be lighter than aluminum for the same heat capacity.

...

apparently aluminium has the highest (apart from water)

Patrick

Mon Feb 26 2018, 02:32AM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

ok nevermind, blunder on my part.

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