Mission-Critical Applications Require High-Voltage Capacitors to Meet Stability and Durability Requirements

Aug 1, 2010 12:00 PM
Joe Moxley, Senior Engineer , Custom Electronics, Inc., Oneonta, NY

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In any industry, there are applications whose failure could mean lost revenue and increased annoyance. In very few industries, application failure could result in breaches in national security or even fatalities. In such markets, the design of mission-critical applications demands the utmost care and forethought. The military and aerospace fields are two areas in which reconstituted mica paper capacitors are used for numerous critical applications. These include aerospace ignition systems, baggage x-ray systems, power supply filtering for military and commercial applications, energy storage with high current discharges for military and commercial detonation systems, voltage multipliers, traveling wave tubes, radar, particle accelerators, power utilities, exciters for industrial generators, down hole well logging, partial discharge detection and welding equipment. The failure of any of these applications could be devastating financially, militarily or fatally, and is therefore unacceptable.

Reconstituted mica paper capacitors (RMPCs) shown in Fig. 1 are more stable than ceramics or most film capacitors and offer a durable platform from which to work when a high-voltage capacitor is required. RMPCs have a -3 percent max drift at -65°C from nominal capacitance reading to +5 percent maximum drift at 175°C and a temperature coefficient of less than ±500 ppm/°C for -65°C to 125°C operating range. Among the other unique benefits of RMPCs are the fact that these capacitors are not position-sensitive, nor do they have a polarity, although the outside foil can be identified for those that find it necessary. Additionally, RMPCs are solid state and contain no liquid to contaminate the electronic device or its surroundings, enabling them to tolerate physical shock and vibration.

Tests have shown that these capacitors can survive 100,000 Gs of acceleration. They can be subjected to a simple harmonic motion with amplitude of 0.06 inches and with a variable frequency from 10 to 55 Hz in each of three mutually perpendicular directions for two hours without damage. Thermal cycling and thermal shock from -65°C to 125°C will likewise cause no damage. Capacitance can range from 50 pF to 5 µF with voltage ratings from 1 kVdc to 75 kVdc and more with special designs. AC voltages up to 20 kV present no problem and can also be found with corona-free (no partial discharge) ratings. Peak current is usually limited by the inductance of the discharge circuit and not by the rise time effects of the dielectric. The mica used in RMPC is naturally resistant to radiation with an approximate loss of voltage/charge of absorbed dose at 0.12 percent krad. RMPCs are also a well-tested option. For example, there are satellite and space applications in which some RMPCs have been in continuous use for more than 34 years.

Design Considerations

Design engineers must keep in mind two operations when considering the testing and design of any high-voltage capacitors. These two operations should be viewed as two separate, but intertwined concepts with absolute reliability as the goal for both. In other words, testing cannot transform a capacitor that has been designed outside safe design limits into a reliable one, and a well-designed capacitor inadequately tested is equally undependable. When it comes to RMPCs and their related engineering and design functions, the involvement of a recognized quality management system is essential. Aerospace Standard AS 9100 is the ultimate quality management system (QMS) for the aerospace industry, as well as ISO 9000:2008. Both provide excellent business models for effective and efficient control of a manufacturing system.

Keep in mind that the voltage stress levels are different for AC and DC operations and are also subject to the temperatures at which they are tested. Corona inception and extinction levels should be determined as volts per mil stress at this stage, as well. Other areas that need to be defined are dissipation factor, IR, ESR, temperature coefficient and drift between the rated temperature extremes. Not any change of the components or processes used to manufacture the capacitors would require another set of testing-to-failure data.

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