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Self-healing Process in Metallized Capacitors
|Even the best
plastic films, like ceramic materials, are not free from
However, in the case of metallized film capacitors it is
possible to eliminate these faults by applying a much
higher voltage than the rated voltage. This process is
known as self-healing and practically
makes a "zero defect dielectric" possible.
Figure 1: Schematic representation of the self-healing
Figure 2: Isolated area after the self-healing process
self-healing process is started by an electric breakdown,
which takes about 10-8 secs. In the breakdown
channel, the dielectric is transformed into a highly
compressed plasma which is pushed out of the channel and
presses the dielectric layers apart (figure 1).
In the spreading plasma,
discharging continues over the metal electrodes.
Temperatures of approximately 6000 K occur and insulated
areas are formed around the original failure spot (figure
2). This self-healing process takes a few µsec and the
discharging in the plasma has already ceased before a
greater loss of voltage takes place. This quick
extinction of the plasma is necessary to avoid further
damage to the dielectric layer next to the point of
The pressure between the layers must not be too great, so
that the plasma can spread out from the breakdown channel
quickly. Large parts of the plasma get into areas of low
The flawless course of the
self-healing process depends on the thickness of
the metallization, on the chemical
composition and on the rate of the
applied voltage; here, apart from the chemical
composition, the production conditions have to provide
the prerequisites for optimum self-healing.
behaviour as a quality standard.