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HIGH-TEMPERATURE ELECTRONIC DESIGN

Part 1: What Are Your Options?
by George Novacek

Start ý A Little Theory ý Size and Type ý Silicon Bipolar Transistors ý Silicon MOSFETs ý Six of Oneý ý SOI Technology ý Other Operation Problems ý Sources and PDF

In a recent article I wrote, "Thermal Considerations in Electronic Design" (Circuit Cellar Online, September 2000), I discussed several aspects of thermal management in electronic circuit design. I then recognized that specifications allow operation of the majority of semiconductor devices up to 85ýC, with a handful reaching 125ýC, and a few going as high as 170ýC T_JMAX. That is usually the maximum junction temperature at zero power dissipation. When those devicesý power dissipation is derated with the real loads in mind, the specification upper temperature limit becomes nothing more than the survival temperature.

For a long time it has been known that electronics, including the plain vanilla silicon devices available off the shelf, can operate well above the usually accepted maximum 125ýC, but there remain considerable technical problems and, of course, risk. In this article, Iýll look at state-of-the-art, high-temperature electronics and see what design options you have if operation at an elevated temperature is the preferred (or necessary) option.

Why would you want to operate at elevated, often way out of spec, temperatures? Because in harsh environments, systems inevitably consist of transducers, connected via long wires to their electronic controllers placed in more suitable locations. This results in increased electromagnetic and radio frequency interference susceptibility and a high level of measurement noise, numerous other design tradeoffs notwithstanding. Having to tailor system architecture to operate outside the controlled environment because of the electronic componentsý limitations often renders less than optimum designs. Consider a few examples where electronic systems are commonly used and their typical operating temperature ranges are well above the established upper limit.

In the automotive engine department, operating temperatures will routinely range between ý40ýC (ý40ýF) and 165ýC (329ýF). Components installed in wheels, such as those belonging to a braking system, will see the upper operating temperature hit a balmy 250ýC (482ýF), and the temperatures within the engine combustion chamber can reach as high as 1000ýC (1832ýF) without much effort. Aerospace components, such as those installed inside jet engines or "smart skins," may be routinely exposed between 300ýC (572ýF) and 600ýC (1112ýF). Even common industrial processes, nuclear reactor monitoring, or the humble consumer electronics found in places such as microwave ovens would benefit if they could operate inside zones where temperatures of hundreds of degrees are normal. Expect 250ýC (482ýF) in communications equipment, 500ýC (932ýF) in microwave ovens and 550ýC (1022ýF) in nuclear reactors.

If you could make electronics operate at those elevated temperatures, your designs would be simpler. With the electronics integrated within the controlled mechanical structures and without long wire interfaces and separate packaging, more robust, economical, and reliable operation could be achieved.

Are you concerned with operation at low temperatures? Probably not. Semiconductors will generally operate better at low temperatures, although you must make sure that circuits such as oscillators and switching power supplies start up even with the low-temperature-reduced gain. The reliability is significantly improved. Some devices, namely low-noise amplifiers, are cooled in specialized applications to reduce thermal noise. It is mainly the mechanics (such as packaging) and some passive devices (typically electrolytic capacitors) that present engineering challenges at low temperatures. However, if everything else fails, maintaining the internal temperature at some minimum level through the use of a heater is a fairly straightforward engineering task.

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