Metallized polpropylene film capacitor Lifetime Evaluation

Plastic film capacitors come in several forms. Essentially they use a metallized polpropylene film to enable the electrodes to the created and spaced apart from one another.

The different types of metallized plastic film capacitor provide different properties, each suited to slightly different applications.

The values of metallized plastic film capacitors may range anywhere from several picofarads to a few microfarads dependent upon the actual type. Normally they are non-polar. In general they are good general-purpose capacitors that may be used for a variety of purposes, although their high frequency performance is not usually as good as that of the ceramic types.

What kind of Film capacitor construction？

There are two main formats for the construction of film capacitors. The actual construction type depends upon the dielectric material used and the requirements for the physical construction.

· Film foil:   This form of film capacitor has two metal foil electrodes that are separated by the plastic film. The terminals are typically connected to the end-faces of the electrodes by means of welding or soldering.

· Metallised film:   In this type of film capacitor the plastic film has a very thin layer of metallisation deposited onto the film. The thin metal layer is typically only 0.02 to 0.1µm thick. This is vacuum deposited onto the plastic film. My leaving a small area free of metallisation at either end, it is possible to connect the metallisation to one connection or the other of the film capacitor.

Some special types of construction may be used for film capacitors that are required for niche or specialist applications. These are not widely seen.

Metal film capacitor summary

The table below provides some of the salient features about some of the more widely used metallised plastic film capacitors that can be taken into consideration when designing circuits or replacing old components.

There are many different types of metallized film and plastic film capacitors that are available. Even though they use a similar form of technology, the variation of dielectrics means that they have a variety of performance parameters and careful selection of the type of dielectric is necessary to provide the optimum performance.

The plastic containers are not completely moisture tight—there is always some residual permeability in polymers. In the case of metallized films, this may lead to electrode corrosion when the capacitors are submitted to environmental conditions of high humidity. The electric-field stess in metallized film capacitors may be much larger than in film foil capacitors. This is obtained thanks to the ability of the electrodes to self-heal. If a breakdown occurs in the polymer, the current will increase through the defect and on the electrode near the defect. Close to the defect the current density will be big enough to evaporate the 100 nm metallic layer. If the capacitor is well designed, the phenomenon will stop when the diameter is large enough to insulate the defect and small enough not to damage the film. The electrode resistance (given in ohm/square) is the key parameter to define to achieve good self-healing behaviour, with Joule losses as small as possible. A thick metallized layer will present a lower resistance, but higher energies will be involved during the self-healing process, leading to greater damage.

What’s the capacitor failure mode?

 Capacitor failure modes Most of the metallized film capacitors fail because the capacitance drops below the required tolerance. This normally occurs after the expected lifetime given by the manufacturer. The capacitance drop is generally accompanied by an increase of the loss factor. From a general point of view, the causes of capacitor failures may occur because of bad design, bad processes, or inappropriate application conditions. During the design phase, the following causes may lead to failure: the dielectric film is too thin, insulation distances are too small, the metallization layer is too thick or too thin, or the conductor is the wrong size. During production, causes may include the following: poor mechanical tension control during the winding, bad drying (leaving too high a humidity content in the capacitor), or bad sealing. In application, the causes may be: higher voltages, EMI, lightning, higher temperature, or a high humidity environment。

A bad choice of the metallization resistance value, or poor metallization control during the film manufacturing process, leads to bad self-healing management, which may damage the dielectric film mechanically and produces heat which is transmitted locally to the next film layers. At this location the dielectric strength of the film drops and breakdown may occur. Consequently, chimneys of melted polypropylene may appear through the winding. The formed channel is conductive, inducing a drop in the insulation resistance and a leakage current that can generate enough heat to melt the polypropylene and increase the internal pressure of the capacitor. Along with bad metallization resistance, the final consequence can, in the worst case, lead to fire ignition or even a capacitor explosion.

Metallized polpropylene film capacitor’s Lifetime expectancy

 Lifetime expectancy The lifetime of a capacitor is the time to failure, where failure is defined as the lack of ability of a component to fulfil its specified function. The failure modes are classified into two main categories: ‘early failures’ and ‘wear out failures’, which are reflected in the curve known as the ‘bathtub’ curve (Fig. 5): at the beginning of the component’s existence, in its ‘infancy’, the failure rate is rapidly decreasing. These ‘youth’ failures are normally screened by routine tests performed by the manufacturer. They are due to design and process weaknesses which have not been detected by the design and process failure modes and effects analysis FMEA performed during the development. They are more probably due to production process variations or to changes in material quality. The process variations are due to tool wear, operator change, and lack of formation. This early failure mode is not taken into account by the Weibull model theory. In normal operation this failure process should not be observed in the field of applications. If it occurs, the capacitors are normally covered by manufacturer’s product warranty.