Pursuit of a better capacitor: Testing, materials, and design considerations

  • 05-Apr-2011 12:28 EDT
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Engineers must have a complete understanding of the application and the environments in which capacitors will function. This information—along with details regarding wire routing, operating voltages, and shape parameters—lets designers safeguard capacitors against negative scenarios.

Capacitors enable numerous mission-critical applications in the aerospace industry. Experience proves the benefits of these components for uses ranging from jet ignition to power supply systems. However, evolving design challenges mean that capacitors must adapt if they are to continue fulfilling their reputation for durability and longevity.

It’s no longer enough to design a capacitor that will encompass all its required electrical parameters. It must also be small enough to fit within the diminishing size of today’s electrical devices and resilient enough to survive the most extreme environmental conditions. Aerospace engineers evaluating capacitors must keep in mind several elements as they assess design options.

Accelerated life testing and statistical software can predict capacitor life with a certain probability at known voltage stress and temperatures. When engineers subject capacitors to higher than normal stress levels to obtain failures much earlier than would normally occur at a lower stress level, they can generally predict when failures will happen in live applications.

For such efforts to be successful, though, engineers must have a complete understanding of the application and the environments in which it will function. This information—along with details regarding wire routing, operating voltages, and shape parameters—lets designers safeguard capacitors against negative scenarios.

There are several options for capacitor packaging in the circuit. The best selection depends upon the particular application and use scenarios. For example, metal enclosures offer greater protection against harsh environments and provide the strongest means of mounting.

Another popular choice is epoxy molding, which can provide better electrical insulation and can be easily molded into intricate shapes for custom fit in confined spaces. Preformed fiberglass or plastic forms deliver consistent sizes and ease of mounting, while tape-wrapped and end-filled capacitors offer moderate environmental protection in a small, less expensive package.

Bare capacitor sections are also available, but engineers who use them must take particular care to defend against contaminants and physical damage prior to final assembly within the application.

With potting components, too, design engineers face numerous options that affect electrical, thermal, and physical properties. Silicones can withstand temperatures beyond 200°C, while epoxies offer voltage ratings of 4000 V/mil and more.

Engineers should consider additional characteristics of potting materials, including shore rating, heat transfer coefficients, shrinkage, moisture absorption, resistance to other chemicals, surface resistance, and thermal cycling ability.

Finally, the actual method used to encapsulate the capacitor is important, as well. Engineers can choose from the many popular two-component systems and either use a vacuum pressure type impregnation or use fiberglass or plastic potting forms with an open pour encapsulation. As with the above considerations, individual application needs are the paramount concern in creating a customized capacitor that meets the most stringent electrical, environmental, and physical requirements.

As aerospace manufacturers pursue a smaller, more reliable, longer-lasting, and more cost-effective capacitor, they should consider multiple issues. Testing, material choices, and packaging all count, but the individual needs of particular aerospace applications is the supreme consideration.

This article was written for Aerospace Engineering by Joe Moxley, Senior Engineer, Custom Electronics Inc.

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