| The
film/foil construction is mainly used for
capacitors with smaller capacitance (100pF through
0.1μF).
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
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