Reliability Design Guide for High-Voltage Capacitors

Dec 1, 2003 12:00 PM
By Jim McCoskey, General Manager, Electronic Products Division, Reynolds Industries Inc., Los Angeles

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Over the last 40 years, a series of misconceptions regarding mica capacitor applications has led novice users to consistently over-derate wound or rolled mica/epoxy dielectric capacitors.

Mica, K₂A1₃(Si0₄)₃, a complex aluminum silicate in dielectric form, has been successfully used for many years as an integral part of high-voltage (2-kVdc to 50-kVdc) capacitor manufacturing, particularly in the 50-pF to 5-µF value range. The mineral term “mica” refers to a family of geological forms whose crystals exhibit a laminating-type structure, which allows it to be cleaved into thin sheets that are structurally strong, flexible, chemically inert and transparent. Mica has unrivaled physical and electrical properties in comparison to other capacitor dielectrics, especially ceramic. Mica also is extremely stable. Capacitance will change only -2% at -54°C, and to +3% at +125°C. Mica is an excellent insulator, and is resistant to high temperature, thermal shock, mechanical shock and vibration.

From Mica Mines to Paper

The mined mica is cleaved and baked-out to remove earth moisture and any combustible materials. A cleaning process using an acid follows the bake-out. The acid is then neutralized and the mica is rinsed with deionized water. Next, a pulverizing process converts the cleaved mica into microplatelets, typically in the 0.01-mm to 0.1-mm range. These platelets are then mixed with de-ionized water to become a pulp. The mica pulp is converted to a paperlike material using processes similar to bond paper manufacturing ranging in thickness from 0.018 mm to 0.05 mm (0.007 in. to 0.002 in.). Resulting from this process is a paperlike material that has no binders and is held together only by Van der Waal's forces.

The mica now can be wound, in combination with capacitor-grade aluminum foil, and then epoxy impregnated to become high reliability, reconstituted mica capacitors. A typical reconstituted mica capacitor is comprised of approximately 85% mica, 14% epoxy impregnant material and 1% capacitor-grade aluminum foil. These ratios can vary slightly with respect to capacitance range and voltage capability.

Reliability is directly related to electrical field stresses through the mica/epoxy dielectric structure. An assessment and derating, in volts per mil (0.001 in. or 0.025 mm), can be discerned from the industry standard, 1250 V to 1330 V per mil, operating, for greater than 100,000-hr mean- time-to-failure (MTTF) applications (see Fig. 1).

Fig. 1. Mica capacitor dielectric breakdown curve-voltage vs. time.

The dielectric stress, in volts per mil, for established reliability applications is selected to be below the dc corona (partial discharge) extinction voltage at the normal working voltage for the unit. This stress level is in the region of 1500 Vdc, with inception voltages in the area of 2000 Vdc for a well-made industry-standard capacitor. AC corona inception is approximately 700 Vac at 60 Hz, irrespective of the dc rating of the device.

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