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TESTING 1, 2

Part 2: Standards: Prepping Your Prototype
by George Novacek

Start • Drainage • Temperature • Humidity • Shocks and Vibrations • Explosions • The Nasties • Sources

TEMPERATURE

The second parts of Sections 4 and 5 deal with operating temperature and temperature variations. Here’s where life gets interesting.

What you need to worry about are the high and low operating-temperature extremes. With avionic equipment, these temperatures range from –55ýC (–67ýF) to +70ýC (158ýF) but need to extend to low and high survival temperatures, most commonly set at –55ýC and +85ýC (185ýF). Components must remain operational even during a temperature shock, when the temperature changes at a rate of 5ýC/min. or greater. And, don’t forget that equipment with forced cooling is tested under conditions in which cooling fails.

To pass the tests, equipment must operate without a glitch at maximum power dissipation throughout the operating-temperature range. In fact, we want to ensure that no component’s internal temperature exceeds its derated maximum even at the highest operating temperature.

We do not need the system to operate at the survival temperature, although in many instances operating and survival temperatures are the same. But, here is where the real trial by fire is hidden. Down-powered equipment is soaked at the survival temperatures until all its parts have stabilized at those temperatures, and then it is powered up. At that instant, the men are separated from boys. Oscillators and switching power supplies either start up or sit idly before eventually giving up the ghost.

The problem is quite clear. Few electronic components are specified for this temperature range, especially at the low end. Military components, specified for –55ýC to +125ýC operation, not only cost many times more than the industrial- or extended-temperature-grade components, but they are fast disappearing from the market. Check the COTS sidebar to see what to do.

To ensure that a design works—assuming the maximum junction temperatures are derated to get the needed reliability—you have to consider the temperature dependency of analog and digital circuits because they manifest themselves differently. Thanks to modern microelectronics, the temperature dependency of analog circuits is no longer too difficult to bring under control by careful design. In the end, it is often reduced to making sure that parts are judiciously derated, power supplies start up properly, and references are stable, either inherently or through internal or external compensation.

As a rule, you should use DC coupling of circuits only when DC operation is needed. Some sensors require temperature compensation, which the vendors often recommend. It’s a good idea to stick to the recommended circuit. As an extreme measure, it may be necessary to measure the temperature by a separate circuit and apply compensation through software using a look-up table or a compensating algorithm.

Problems with temperature in digital circuits typically involve making sure that timing characteristics are sufficiently derated for operating margins throughout the temperature range. Present-day CAE (computer-aided engineering) tools are a great help in simulating circuit behavior under many different conditions.

In the end, the results of what I call (with a little disdain) "virtual design" are only as good as the mathematical model fed into the computer. Until someone can convince me that the model is 100% accurate, I’ll stick my breadboard into an environmental chamber, like the one pictured in Photo 1, and see the results for myself.

(Click here for photo)

Photo 1—This is the torture chamber for temperature, vibration, and humidity testing. Notice the slip table in front of the chamber. This, in conjunction with the cylindrical driver under the chamber, provides vibrations in all three axes.

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