Material Considerations for High Frequency, High Power Capacitors

The objectives of this program were to develop a capacitor for high frequency, high power operation in the 10 to 40 kHz range at the 75 kVAR power level. The material selection required careful consideration with regard to heat dissipation and thermal stability for reliable service in space at continuous ac voltage. The primary consideration in the selection ion of a capacitor dielectric system normally is given to the solid dielectric material, since the characteristics of the solid determine the performance reliability, and the life of the capacitor. An important property of a solid dielectric is considered to be the dielectric loss, since the frequency of ac operation ion and the space environment direct attention to the thermal properties. The dielectric strength is also a key factor because the life will depend on the durability of the dielectric in regards to deterioration caused by partial discharges in the active volume and at the high-stress edges of the capacitor pad conductors. The temperature coefficient of dissipation factor should indicate self stabilization well below 125°C and the temperature coefficient of capacitance should be negative. An all-film long-life polypropylene capacitor was developed that functioned at the high current density of 125 A rms without supplementary need for cooling while operating in a vacuum atmosphere. The capacitor power density was approximately 24 kVAR/kg when in service at a frequency of 40 kHz. This paper describes the capacitor design and material considerations, for thermal stability and long-life performance reliability.