WHY METALLISED FILM?

Metallised plastic film capacitors have several inherent advantages which make them the standard plastic dielectric capacitors. Firstly, the vacuum deposited electrode (the metallising) is at least fifty times thinner than the alternative foil electrodes, resulting in more capacitance per unit volume and also lower cost. Metallised film exhibits the property of self - healing, which means that in the case of a small imperfection in the plastic film the increased current flow in the metallising at this point causes it to vapourise, isolating the fault and greatly extending the operating life of the component.

WHY FILM / FOIL?

Foil electrodes are used in high current and high dv / dt applications, where low ESR and low self heating are vital.

POLYCARBONATE is generally used for close tolerance (down to 0.25% and below) or high stability applications as it has high stability of capacitance with temperature. It is generally specified for demanding applications where ruggedness and reliability are important. Operating temperature is up to 125oC. Polycarbonate capacitors exhibit a high insulation resistance and hence low leakage current.

POLYESTER ( polyethylene terephthalate ) is the general purpose dielectric. It can be used for DC operation and low current AC applications. It is not generally used to give capacitance tolerances of less than 5%. Its upper temperature limit is 100oC. It has a higher permittivity than the three other materials used by MFD, and this coupled with its low cost ensures the best capacitance/price ratio of the plastic films. Polyester capacitors do not have as high an insulation resistance as polycarbonate or polypropylene devices.

POLYPROPYLENE is generally used for AC and pulse applications because of its low losses, and consequent low self heating. It can be used for general purpose DC work but its application is limited by its upper operating temperature of 85oC and its low permittivity and hence larger volume per unit capacitance. Polypropylene capacitors exhibit a high insulation resistance and hence low leakage current.

COMPARISON OF THE MAIN DIELECTRICS