When designing a circuit that must operate at high temperatures, often a discrete solution is one of the few that can address all of the design criteria. Probably the most significant criterion is the ability to keep operating within specifications for the required number of hours. Other performance parameters will, of course, present lesser but still significant problems.
The engineer must realize that with high temperatures come big deratings. Items such as resistors and power transistors must generally be considerably oversized. This can further compound the issue. And once the design is complete, sources (suppliers) of high temperature parts must be found.
The amount of time-at-temperature spec will significantly affect parts selection. Typically, after a few thousand hours of operation at elevated temperatures, materials break down. Cracks or fractures will cause parts such as resistors, inductors, and capacitors to fail. In power supplies, ripple current ratings can derate significantly. To a lesser extent, the working voltage rating should be derated. Diode and transistor operation also changes at high temperature. MOSFET leakage goes up; diode break down voltage goes down.
There are a few good sources of high temperature parts. The purchasing of these parts can be more problematic at low- and mid-quantities (1 piece to around 10,000 pieces), meaning that one often has to rely on a distributor rather than an OEM for these parts.
OEMs such as Vishay offer many different styles of high temperature resistors as well as capacitors. Dearborn Electronics from Florida offers high temperature capacitors for a variety of applications. Some such as Vishay specialize in high volume applications for products, and so finding parts for low to mid volume applications may require cooperation from a stocking distributor such as TTI.
Distributors such as Trendsetter Electronics in Austin, Texas, specialize in high temperature parts, and often stock many of the more common items in prototype quantities. Other distributers that do not specialize in high temperature parts carry the more mundane devices. Even so, they stock a reasonable variety, and new offerings become available regularly. Do not overlook the Internet distributors, such as Digikey, Newark, Mouser, et al., as their parametric search engines are continually improving. Note also that these distributors do offer some devices suitable for high temperature applications.
Just because a part has passed a military qualification for 1000 hours does not mean it will run for 40k-hrs at the extended temperature. The part must be tested in the actual application to know for certain that it will work reliably. Military standards are rigorous, but may not cover the same thermal cycling, vibration, humidity, or other environmental parameters as the actual application.
The design engineer has to look at all the packaging materials and do the testing and qualification. Only then will he or she have confidence in a part for the application.
What are your experiences with high temperature parts?