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Reconstituted mica-paper capacitors (RMPCs), a refinement of mica-based capacitors that have been around for over a century, were first used in the ground-surveillance radar of the supersonic F-111 fighter bomber in the mid-1960s. RMPCs’ reliability and long life offsets a somewhat higher cost versus other capacitors, especially when a capacitor failure can cost millions of dollars in downtime for repairs and possibly lead to fatalities or breaches in national security. But to get the most out of RMPCs, especially custom-made ones, engineers must understand several issues and consult with experts in these matters.
RMPC advantages
RMPCs offer stable and durable performance for a wide variety of high-voltage (1 to 50-kV) circuits. They typically exhibit –3% maximum drift at –55°C from nominal capacitance, +5% maximum drift at 175°C, and temperature coefficients of less than ±500 ppm/°C in the –55 to 125°C operating range. RMPCs are more stable than ceramic capacitors (Type X7R have drifts of ±15% for the same temperature range) and most film capacitors (polyester capacitors have drifts of –6 to 15% over the same temperature range). RMPCs are not position sensitive and do not have a polarity. In other words, the capacitor does not have an anode (usually negative) or cathode (usually positive) terminal and can be put in a circuit without regard as to which terminal is which. But the outside of the capacitor can be marked or identified, usually by a black dot, so it can be connected to the ground side of the circuit for noise suppression when necessary.
RMPC are solid state and contain no liquid that might contaminate an electronic device or its surroundings, as can happen with oil-filled capacitors. Being solid state makes them extremely durable to physical shocks and vibration. Tests show that RMPCs can survive 100,000 gs of acceleration. They can be subjected to a simple harmonic motion with an amplitude of 0.06 in. and a frequency varying from 10 to 55 Hz in each of three mutually perpendicular directions for 2 hr without damage. Thermal cycling and 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 kVdc. Voltages can climb even higher with special designs. Ac voltages up to 20 kV present no problem to RMPCs and they can have corona-free (no partial-discharge) ratings. Peak current is usually limited by the discharge circuit’s inductance, not the dielectric’s rise-time effects. Mica used in RMPCs naturally resists radiation with an approximate loss of voltage or charge of 0.12% per krad. (One radiation absorbed dose or rad will cause 0.01 joule of energy to be absorbed by a kilogram of mass.) This trait is especially useful for satellite and space applications. Some RMPCs have been working in space despite exposure to solar and cosmic radiation for 34 years and counting.
RMPCs usually have a standard 1,300 V/mil of dielectric stress between foils with 26 V/mil for the margin area. These stress levels meet requirements for 100,000 hr of use.
Different winding techniques such as straight winding with embedded foil and series winding with embedded or extended foil can improve performance or reduce costs. For example, straight winding RMPCs for low-voltage applications (less than 8 kVdc) are the most economical option due to low labor and material costs. But series winding for high-voltage applications (greater than 8 kVdc) can also cut cost because fewer bare sections are required.
And many times, requirements are so complicated that several capacitors must be connected in parallel, series, or a combination of the two to get just the right capacitance and voltage ratings.
RMPCs can also take a variety of shapes. In many instances, users can get them in any shape they want as long as the RMPC has the required volume for capacitance and voltage rating. For example, if a capacitor required 6 in.3 of space for a specified capacitance and voltage rating, it can have any set of dimensions that add up to a final form with 6 in.3
Testing and designing RMPCs
Engineers should consider the design and testing of RMPCs as two separate but intertwined tasks — both with the goal of utmost reliability. Remember: No amount or kind of testing can transform a capacitor designed outside safe limits into a reliable one, and a well-designed capacitor with inadequate testing is as undependable as a poorly designed one.
To define an RMPC’s operational design limits, engineers need to use accelerated life and destructive testing. Many software packages predict capacitor life and confidence levels based on results from accelerated life testing. These results determine an RMPC’s maximum operating volts per mil of stress (1 mil = 0.001 in.) on the dielectric and the maximum operating temperature it can withstand. These can be combined to come up with a confidence level that it will survive for a specific length of time.
Good engineers can take these test results to design the smallest and most-reliable RMPC for an application. This method can also be used to design an RMPC that has targeted characteristics, including operational life.
During manufacturing, testing at critical points, such as after winding, impregnation, and section assembly, weeds out early failures due to material defects or operator errors. The number of failures at these test points should be small, less than 5%. In lots with failures exceeding this percentage, engineers should investigate to determine the root cause and whether the lot really meets design requirements. Final testing establishes that design requirements have been met, production processes are under control, and all defects have been rejected.
Keep in mind that voltage stress levels are different for ac and dc operations and are also affected by temperature during testing. Determine corona-inception and extinction voltages as volts per mil stress at this stage as well. Other areas that need to be defined are dissipation factor, IR, ESR, and temperature coefficient and drift between rated temperature extremes (See Glossary). And any changes in the components or processes used to manufacture the capacitors would require another set of testing-to-failure data.
Once safe operating parameters are established, designers can apply them to building RMPCs for an application. While maintaining the specified size, the following hurdles have to be overcome:
• The RMPC must safely accept the desired electrical charge. If the capacitor can handle the dielectric-withstand voltage, it will also accept the desired electrical charge.
• It must hold a desired charge for a required amount of time that includes a safety margin. This means it must pass burn-in testing. Remember: burn-in testing voltages are usually calculated as midway between rated voltage and dielectric-withstand voltage, and testing is done with the capacitor exposed to its the highest rated operational temperature for 24 to 48 hr (or more for critical applications).
• It must handle any internal heating that arises during operation. This means the Irms and duty rate need to be specified by the customer or the customer must supply a current wave form so the designer can determine the Irms.
• Self-heating can be calculated from the equivalent-series resistance of the capacitor, which is directly related to its dissipation factor, typically 0.3%. The equation for equivalent-series resistance is:
ESR = df/2 × π × f × c
where df = dissipation factor, f = frequency, and c = capacitance. Designers need to make sure self-heating does not put the RMPC over its rated maximum temperature. Otherwise, cool the device with forced air circulation, oil baths, or heat sinks.
• If the application calls for it, an RMPC might need to operate with partial discharges or corona. Fortunately, reconstituted mica paper naturally resists dielectric erosion caused by partial discharges and corona. But they should be avoided if possible. After all, corona-free operation extends an RMPC’s life.
There are other issues that need to be addressed early in the design process. For example, users might need to compromise by either allowing more room for a larger capacitor or reduce voltages so the RMPC falls within limits defined by destructive testing. And specialized aerospace and defense applications often have performance requirements that can fall at the extremes. For example, capacitor life might be as short as a one-pulse discharge for a weapon to more than 1,000 pulses for aircraft ignitions. Voltage can range from 1 to more than 100 kVdc, with an unending variety of waveforms that need to be studied for their effects on the circuit. Temperatures can go from the severe cold of outer space to the intense heat encountered in deep-well logging. Other design criteria that need to be considered are pressures, humidity, thermal shocks and their cycles, physical shock and vibrations, and radiation.
And customers may have other performance requirements such as peak current, rms current, inductance, impedance, and ESR. There can also be a need for special terminals such as solder lugs, threaded inserts, studs, or high voltage wire and connectors). Some applications need a bit of added hardware such as sheds or corona shields. Sheds are grooves cut perpendicular to the RMPC’s terminals that extend the surface area between terminals to help prevent tracking. Corona shields are terminals with all rounded or radius surfaces that eliminates sharp points on which coronas can form.
Packaging
Customers usually specify RMPC packaging based on the application. If it must be potted into an assembly, then a bare section offers the best economy and smallest size. If the RMPC will be exposed to atmospheric conditions, users and designers have a range of options. For example, tape-wrapped and end-filled, fiberglass, plastic, metal potting form, and epoxy-molded RMPCs offer varying levels of environmental protection, structural strength, and mounting options.
But there are some guidelines. The choice of potting or casting materials — whether epoxy, silicone, polyurethane, or some other polymer — need to be compatible with the end use. For instance, an epoxy compound rated by the manufacturer for use up to 125°C should not be used to encase an RMPC mounted directly onto a jet engine where temperatures can reach 180°C. The design of mold or potting forms should avoid sharp edges, points, and projections, and allow for adequate coating of the capacitor pack. If there are points and questions about strength, wrapping the RMPC in fiberglass cloth removes sharp edges and adds structural strength.
As a minimum, all finished RMPCs should be tested for capacitance and dissipation factor, and pass a dielectric stress test at the bare-section stage to ensure all defects have been eliminated. The typical dielectric stress-test voltage is double the rated voltage up to 8 kVdc, at which point stress declines linearly to 110% at ratings over 45 kVdc.
RMPC manufacturers should recheck capacitance after soldering, assembly, and burn-in. Capacitance, dissipation factors, and dielectric stress are once more 100% tested after packaging. Other customer-specified testing might include burn-in voltage, temperature, and length of time, corona inception and extinction voltages, capacitance, moisture and fungus resistance, solderability, X-ray tolerance, and resistance to various chemicals.
The future of RMPCs remains bright as the military, aerospace, and defense markets continue to require durable and high-temperature capacitors tailored specifically for critical functions.Capacitance glossary Burn-in test: Applying voltage to a capacitor at a set temperature for a given length of time to weed out defects. Capacitor pack: An assembly of bare sections. Corona: An electrical discharge accompanied by ionization of surrounding atmosphere. df: Dissipation factor: Rate of power loss in a dissipative system. Duty rate: Percentage of time power is applied to a capacitor. Dielectric-withstand voltage (DWV): A higher-than-rated voltage applied to a component as a test to ensure it will accept the rated voltage. Equivalent-series resistance (ESR): All the internal resistance of a component at a given frequency treated as a single resistance at one point. Glass-transition temperature: The temperature at which a material changes from “glassy” to “rubbery.” Insulation resistance (IR): The resistance after applied voltage for a set time. Irms: Amps root means square or average current. Mold: A die or hollow form for shaping capacitors. Usually comprised of the electrical components and a fluid or plastic filling that takes the shape of the die. Partial discharge (PD): A localized dielectric breakdown of a small portion of the electrical insulation. Shore hardness: A material’s resistance to permanent indentation. Surface resistivity: The resistance between two opposite sides of a square. It is independent of the size of the square or its dimensional units. Tc: The relative change in capacitance when temperature changes by 1°K. Thermal conductivity (Is): A measure of a material’s ability to conduct heat Volume resistivity: The electrical resistance between opposite faces of a 1-cm cube of insulating material, commonly expressed in ohm-centimeters. Water absorption: The ability of a dielectric material to absorb water. |
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