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Posted on 20 February 2020

Classification of Film Capacitors

 

 

 

 

 

 

 

Film capacitors, also referred to as FK capacitors, are fixed capacitors with plastic film dielectrics. Film capacitors are grouped and designated according to the dielectric used since the dielectric in use has a great effect in determining film capacitor characteristics. Film capacitors are classified further in terms of film and foil arrangement, construction, and the types of terminals used.

Classification of film capacitors by dielectric

Short identification codes for the type of construction, describing the dielectric and the basic technology applied, are defined in standard DIN 41379.

The last character in the short code indicates the type of dielectric:

  • T → Polyethylene terephthalate (PET)
  • P → Polypropylene (PP)
  • N → Polyethylene naphthalate (PEN)

An M (M → Metallization) is prefixed to the short identification code of capacitors with metallized films.

Figure 1. Classification of film capacitors according to DIN 41 379

1) MFP and MFT capacitors are constructed using a combination of metal foils and metallized plastic films. They are not covered by DIN 41379.

Classification of film capacitors by film and foil arrangements

For a better understanding of the differences in the internal structure of capacitors, figure 2 shows some typical film and foil arrangements.

Figure 2. Examples of typical film and foil arrangements for film capacitors

Figure 3 shows the relation between various film and foil arrangements and the capacitor types.

Figure 3. Schematic film and foil arrangements of various capacitor types

Classification of film capacitors by construction

Film capacitors are produced using either winding methods or stacking methods.

Wound technology

In the conventional production process, the capacitors are made by individually rolling the metallized films or the film/foils into cylindrical rolls and then covering them with an insulating sleeve or coating.

Figure 4. Conventional production process in wound technology

In the MKT, MKP and MFP type series from EPCOS, the production range includes capacitors with space-saving flattened wound bodies with insulating coatings or inside plastic casings, as well as cylindrical wound capacitors. Flat windings are produced by compressing the cylindrical rolls before they are placed in casings, so that the casing form is optimally used.

Stacked-film technology

In stacked-film production technology, large rings of metallized film are wound onto core wheels with diameters up to 60 cm. In this way the "master capacitors" are produced under well defined and constant conditions.

Figure 5. Stacked film capacitor production technology

As a result, the capacitor production lots obtained when the rings are sawed apart to produce the actual stacked-film capacitor bodies are especially homogeneous.

  • The pulse handling capabilities of stacked-film capacitors are of a particular advantage. Each individual layer acts as a single capacitor of small capacitance, so any damage to the contacts due to overloading is restricted to the layers concerned, which might get disconnected. Therefore the total capacitance can decrease slightly, without further affecting the performance and reliability of the part.
  • Additionally, uncoated stacked-film capacitors (SilverCapTM) offer the special advantage that they can be adapted to customs design requirements. A wide range of sizes can be offered without considering case size standards or providing special tools for new cases.

Classification of film capacitors by finish and terminal

A variety of standard designs with corresponding types of terminals are available to suit different applications and operating environments.

Figure 6. Capacitor designs

 

For more information, please read:

Introduction to Capacitors

Calculating the Useful Life of Capacitors

EMC Filter as an Application for Film Capacitors

Film Capacitors for DC-Link

 

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