Any discussion of polypropylene film capacitors must cover polypropylene film/foil as well as metallized-film capacitors. There are a-c applications for which a metallized film capacitor cannot be used effectively. Deterioration of the connection between the film metallization and the metal end-spray can occur when metallized-film capacitors are subjected to extremely high peak currents. Increasing the metallized-film electrode length can help, but the use of film/foil capacitors may be a preferable option, particularly in the case of low microfarad values.

Polypropylene-film capacitors using foil electrodes are appropriate for non-sinusoidal applications with very high peak currents such as snubber applications. Rugged electrode terminations produce the ability to handle high peak currents. These capacitors can also handle higher average current as a result of low dielectric loss, lower electrode resistance, and greater heat conduction reducing potential hot spot problems.

The type 710P polypropylene-film/foil capacitor is qualified to MIL-C-55514/10 as MIL Style CFR 15. This document specifies the maximum allowable peak current capacity, expressed as dV/dt (Amps/µF), for each capacitor listed. These limits are an order of magnitude greater than those for equivalent metallized-film capacitor ratings (for example, 300 v/µs vs. 40 v/µs for 200-volt rated parts).

The performance characteristics (Figures 26, 27 and 28) of polypropylene-film/foil capacitors are also of interest because of the extremely high insulation resistance. In addition, the temperature coefficient of capacitance is negative and typically 150ppm/degrees C, a characteristic suitable for matching with positive TCC components where circuit stability is a requirement.

Correctly designed and manufactured polypropylene-film capacitors will be valuable additions to those film capacitors currently covered by active MIL specifications. The following considerations apply:

1. Excellent polypropylene film quality is available and a high reliability capacitor design is achievable.
2. Very low ESR may be obtained by combining the low loss characteristics of polypropylene film and optimized section geometry.
3. A properly designed polypropylene-film capacitor can be used to good advantage in high-frequency, high-current applications.
4. Performance characteristics are excellent, suggesting possible uses beyond the a-c applications mentioned here.
5. Polypropylene-film/foil capacitors are good choices for non-sinusoidal applications where the metallized capacitor may not effectively handle high peak current (dV/dt) requirements.

ACKNOWLEDGEMENT

The authors would like to thank Mr. Mark Rumler for obtaining much of the data used as a basis for this paper.

REFERENCES

"CAPACITOR FILMS’ Klaus Schubring, Kalle (Presented at CARTS 1984)

"A-C APPLCIAITON OF PLASTIC-FILM CAPACITORS", Jerold D. Kowalsky and Herbert L. Rice, Sprague electric Co. (Presented at CARTS 1983)

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