Tech Briefs

These capacitors can be used in containers or housings of almost any geometric form and material of construction.

A novel high- temperature, high- voltage power electronics capacitor incorporates materials of construction and electrical components that have been initially designed as a segment of an integral electronics component, package, or system to be subjected to harsh or high- temperature environments. The capacitor can withstand operating temperatures in excess of 300°C, while maintaining a capacitance between a fraction of 1 to several μFs.

The single Wound Capacitor Element (WCE) is shown in roll form.
Capacitor-grade reconstituted mica papers are used as the dielectric material for the insulator/separator. The mica papers are self-supporting sheets comprised of thin, overlapping flakes or platelets of inorganic native muscovite. The separator sheets range in thickness from 0.0005" to 0.002" and are free of adhesives, binders, and coloring agents.

A high operating voltage of 1-3 kVDC required that a suitable impregnant/ operating environment be developed to insulate the electrode edges and fill and insulate the critical regions or microscopic voids between the mica platelets. Such voids normally constitute about 30% of the total volume of the separator material. The majority of the work was performed within a pressurized environment of CO2 gas (maintained at near ambient to about 405.2 kPa of pressure).

The capacitor-grade materials are arranged to provide a single wound capacitor element (WCE). A standard winding machine, with a take-up means of about 1" diameter, is used to collect a feed of two conductor layers, separated and insulated from each other and the take-up means, by positioning at least one layer of an insulator material between them. The thickness of the insulator and conductor can range from 10-50 μm and 5-10 μm, respectively. The insulator is also used as a support or foundation for the conductor layers.

A mica paper separator is positioned over the take-up means that comprises two layers of about 0.0009" mica paper and a single layer of about 0.0007" mica paper. A conductor layer of aluminum foil of about 0.0002" is applied over the separator layer and take-up means. A second separator is then positioned over the first conductor layer, which is comprised of two and one 0.0007" thickness of mica paper. Finally, a second conductor layer of 0.0002" foil is placed directly over the second insulator layer.

For this WCE, the separator layers are overlaid, in registration, on top of each other with the first conductor layer separating them. When the second conductor is positioned over the second separator layer, the first conductor is longitudinally overlapped by about 6.9 cm at the center. This staggered placement causes the edges or outer peripheries of the conductors to alternatingly extend about 0.38 cm beyond the edges of the corresponding separator layers on which they are positioned. The opposing edges or peripheries of these conductors lie buried between the separator layers. Thus, the first conductor layer will extend above and slightly beyond the left edge of the first separator layer, and the second conductor will extend above and beyond the right edge of the second separator layer. This procedure is repeated in multiple windings until all separators and conductors fed are consumed, and a final WCE is obtained.

This work was done by Lynn Mandelcorn, John Bowers, Eugene R. Danielson, Stephen R. Gurovich, and Kenneth C. Radford for the US Army RDECOM-TARDEC.

ARL-0050

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High-Voltage, High-Temperature Power Electronics Capacitor (reference ARL-0050) is currently available for download from the TSP library.

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