Film/Foil Construction

The advantage of this construction is the easy contactability of the metal foil electrodes and the good pulse strength.

A breakdown in the dielectric film of a F capacitor leads to an irreversible short circuit and thus, to failure.

To avoid breakdowns caused by weak spots in the dielectric, the insulating film chosen is always thicker than theoretically required by the values which are determined from the specific breakdown strength of the material. Films of less than 4 µm are not used for F capacitors because of their high proportion of weak points.
The necessity for thicker insulating film has an unfavorable effect on the size and the material used. In order to achieve a particular capacitance with thicker insulating film, the length of the band also has to be increased by the same amount. Thicker insulating film therefore squares the volume of the winding element.

A weak spot occurs, when depressions meet on the upper and lower surfaces of the film. The dielectric must then be at least thick enough to have just the required breakdown strength.

Advantage: High pulse loading capacity due to good contact of the terminating wires to the metal foil electrodes.

WIMA types with film/foil construction