High-voltage capacitors suit mission-critical military and aerospace applications

Reconstituted mica paper capacitors often find use in mission-critical military and aerospace applications. When thinking about the testing and design of any high-voltage capacitors, design engineers should consider the two operations as separate but intertwined concepts, with reliability as the goal for both.

Joe Moxley, Custom Electronics Inc -- EDN, September 23, 2010

“Mission critical” is one of those phrases engineers and executives in every industry have adopted. It’s often used to describe applications that are vital to a particular business; if that element becomes unavailable, the organization loses time, money, or both. When these mission-critical devices go down, companies and their customers might experience quite a bit of annoyance. When the military or aerospace companies say “mission critical,” however, the term takes on a different meaning. In these industries, application failure can result in breaches in national security or even fatalities.

The military and aerospace fields are two areas in which RMPCs (reconstituted mica paper capacitors) are used for numerous critical applications, such as 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.

Reconstituted mica paper capacitors for durability and longevity

Among the unique benefits of RMPCs is the fact that these capacitors are not position-sensitive, nor do they have a polarity, although the outside foil can be identified for those who find it necessary. For example, some circuit designers like to know which terminal is connected to the outside foil in the capacitor winding. By knowing what the outside foil lead is, they can arrange the capacitor connections in a way that reduces the potential voltage stresses on the outer layers of the capacitor and its adjacent components or assembly structure. In addition, some designers like to connect their low-voltage connection to the outside foil or "marked" lead. Make sure your RCMP supplier can accommodate these requests.

Additionally, RMPCs are solid state and contain no liquid to contaminate the electronic device or its surroundings, making them extremely durable to physical shocks and vibration. RMPCs are also 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% maximum drift at −65°C from nominal capacitance reading to 5% maximum drift at 175°C and a temperature coefficient of less than ±500 ppm/°C for the −65 to +125°C operating temperature range.

Furthermore, RMPCs can be subjected to a simple harmonic motion with an amplitude of 0.06 in. and with a varied frequency from 10 to 55 Hz in each of three mutually perpendicular directions for two hours without damage. Tests have shown that these capacitors can also survive 100,000 Gs of acceleration. Thermal cycling and thermal shock from −65 to +125°C will likewise cause no damage. Capacitance can range from 50 pF to 5 µF with voltage ratings from 1- to 75-kV dc 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 RMPCs is naturally resistant to radiation, with an approximate loss of voltage/charge of absorbed dose at 0.12% 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.

Core considerations for RMPC design

Testing cannot transform a capacitor that has been designed outside safe design limits, and a well-designed capacitor inadequately tested is equally unreliable. These two operations should be viewed as separate but intertwined concepts with absolute availability as the goal for both.

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. Note that any change of the components or processes used to manufacture the capacitors would require another set of testing-to-failure data.

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 QMS (quality-management system) for the aerospace industry, as well as ISO 9000:2008. Both provide excellent business models for effective and efficient control of a manufacturing system.

Accelerated life testing is essential for high-voltage capacitors. This is the practice of subjecting a test object to a higher-than-normal stress level to obtain failures much earlier than would normally occur at a lower stress level. Usually, two or more stress levels are used. The resulting failures can then be plotted to predict when failures will occur at a lower stress. When designing to get the maximum benefits from any capacitor, it is essential to design with a full understanding of how the component will be used. Determining what stresses it will see and how long it must survive should be the first part of the process. It is important that all stress factors are taken into account. For example, temperature and voltage stress levels are two common factors in determining capacitor life that almost all designs share. Designers should also ensure that the shape parameter obtained in the accelerated testing using the Wiebull distribution function is a close fit to the shape parameter one would obtain when using the operating stress level. Accelerated life testing can then provide a probable confidence level with a percentage of assurance that the capacitor will survive for a dependable length of time while at specific stress levels.

Tailoring RMPCs to end-user specifications

After capacitors complete design and testing processes, engineers can tailor certain elements to best meet the needs of intended end users. For example, the customer usually specifies the capacitor packaging based on the application. If the capacitor is to be left exposed to atmospheric conditions, then several options are available. Tape-wrapped and end-filled, fiberglass/plastic/metal potting form, or epoxy molded units offer varying levels of environmental protection, structural strength, and mounting options. If the final application entails potting the capacitor into an assembly, then a bare section design offers the best economy and smallest size. The choice of potting or casting materials, whether epoxy, silicone, polyurethane, or some other polymer, needs to be appropriate for the capacitor’s end use. The glass transition temperature, rated temperatures, Shore hardness, dielectric strength, volume and surface resistance, water absorption, shrinkage, and thermal conductivity are some of the criteria that need to be addressed. The design of mold or potting forms should avoid sharp edges, sharp points, or projections and allow for adequate coating of the capacitor pack. Fiberglass cloth may be used to wrap the capacitor pack to remove sharp edges and add structural strength in some applications. Some materials require that they be transferred to the mold under vacuum for best effects, and most require a heat cure for optimum performance.

The typical dielectric stress voltage is 200% of the rated voltage up to voltage ratings of 8-kV dc, at which point the stress is reduced linearly to 110% at greater than 45-kV dc. Capacitance is again checked after soldering, assembly, and burn-in operations. Capacitance, df, and dielectric stress are once more 100% tested at the completion of packaging options. Other customer-specified testing might include, but is not limited to burn-in voltage/temperature and length of time, corona inception and extinction voltages, capacitance, df, ESR, IR at various temperatures, thermo cycling/shock, shock and vibration, moisture resistance, fungus resistance, solderability, x-ray, and resistance to various chemicals. At a minimum, all capacitors should be tested 100% for capacitance, dissipation factor, and dielectric stress at the bare single element stage to ensure that all of the infant mortalities have been weeded out.

Designs of 1300V per mil are the industry standard for the dielectric stress between foils associated with RMPCs, with 26V per mil for the margin area. These stress levels will meet the requirements for 100,000+ hours of reliable use. Different winding techniques, such as straight winding with embedded foil, series winding with embedded foil, and extended foil, can be used to maximize desire characteristics. The number and location of tabs (terminals) are calculated to obtain the best current, voltage, and induction performance. Many times, capacitor requirements are such that multiple capacitors must be connected in parallel, series, or a combination of parallel/series connections to achieve the desired capacitance/voltage rating.

Other problems associated with these specialized applications are that the design requirements can be at extremes. Life can be as short as a one-pulse discharge for munitions systems to greater than 100×10⁸ pulses for aircraft ignition systems. Voltage can range from 1-kV dc to more than 100-kV dc, with an unending variety of wave forms that need to be studied for their effects in the circuit. Temperatures can range from the severe cold of outer space to the intense temperatures encountered in deep-well logging. Other design criteria that need to be considered are atmospheric pressures, thermal shocks/cycles, physical shock/vibrations, and radiation. There might be other customer requirements that need to be met, such as peak current, RMS current, inductance, impedance, ESR, humidity, special terminals, operating at reduced pressures, sheds, or corona shields. These are the major concerns that need to be addressed early on. Some compromises by the end user may be needed, either allowing more room or reducing voltages/stresses so that they fall within allowable limits as defined by the destructive testing.

There are numerous other questions engineers must ask as part of the process of tailoring capacitors to individual use, including the following:

• Will the capacitor design accept the desired electrical charge safely?
• Will it hold the desired charge for the required amount of time with an adequate margin of time?
• Will the amperage applied cause internal heating, and if so, how much and how will it be handled?
• Will the capacitor be in operation with partial discharges/corona?

These are all questions to be discussed with end users so that RMPCs can meet the absolute requirements for reliability.

What about pricing? Because each RMPC is custom-designed, the cost depends on the shape and design. The larger and more complicated the design, the greater the expense. The cost can fluctuate from as low as $5 to $3,000.

In some industries, “mission critical” is something of a hyperbole. Failed applications might lead to financial losses or minor inconveniences. In some industries, though, the need for stability and durability is truly essential. RMPCs provide the required reliability for these industries while still allowing for end-user customization. When downtime could result in serious security breaches or even loss of life, RMPCs are an engineer’s best choice.

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