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Construction and classification of film capacitors

Views:3     Author:Site Editor     Publish Time: 2020-03-24      Origin:Site

Plastic film capacitor is referred to as film capacitor or FK capacitor. It uses a plastic film as a dielectric.

The main characteristics of film capacitors are: non-polarity, high insulation resistance, excellent frequency characteristics (wide frequency response), and small dielectric loss. Because of these advantages, film capacitors are widely used in analog circuits. Especially in the part where the signal is connected, a capacitor with good frequency characteristics and extremely low dielectric loss must be used to ensure that the signal does not cause too much distortion during transmission. Among all plastic film capacitors, the characteristics of PP capacitors and PS capacitors are the most significant.

1. Basic structure

The internal structure of the film capacitor is mainly as follows: a metal foil (or a foil obtained by metallizing a plastic) is used as an electrode plate, and plastic is used as a dielectric. Obtained by winding or stacking process. The arrangement of foils and films is different, resulting in a variety of construction methods. Figure 1 is a typical schematic of a thin film capacitor. 

film capacitor

2. Basic classification

The main classification of film capacitors: dielectric, arrangement of film (dielectric) and foil (electrode plate), structure, line end method. 

Classification from dielectric materials:

From the perspective of application characteristics, the performance of key characteristics is still due to their different dielectrics. Dielectric film DIN 41379 classifies film capacitors as follows:

Type T: PET-Polyethylene terephthalate (or ester)

Type P: P P-Polypropylene

Type N: PEN-Polyethylene naphthalate

film capacitor2

                                                                                        


Capacitors prefixed with M are metallized films. MFP and MFT capacitors consist of metal foil and metallized plastic film, and are not within the scope of DIN 41379.

Types

Characteristic

Typical application

Polyester

(Polyethylene Terephthalate, or PET)

Made of metallized film or film / foil

High dielectric constant and high dielectric strength. Metallized polyester film has self-healing properties and higher capacity density.

Bypass, Coupling, Filtering


Polyester

(Polyethylene Naphthalate, PEN)

Made of metallized film

High temperature polyester. The high melting point makes these capacitors SMD-type and can be used in high-temperature environments.

General purpose


Polypropylene (PP)

Made of metallized film or film / foil

Low loss and low loss angle temperature coefficient, low dielectric absorption characteristics (that is, the capacity is less affected by humidity), high dielectric strength, high insulation resistance, and negative temperature coefficient of capacity.

Stabilized oscillators and filters, sample-and-hold circuits, pulse applications, AC applications, and main filtering


Polyphenylene sulfide (POLYPHENYLENE SULPHIDE, PPS)

Made of metallized film

Low loss, wide operating temperature range, low temperature coefficient, good stability.

Counters and filters.

Automotive and other high ambient temperature applications


Some typical characteristic constants of dielectrics:

film capacitor3

About metallization:

Metallized film is made by vacuum depositing a thin layer of metal on a plastic film as an electrode. The thickness of the electrode foil is omitted, and the volume per unit capacity of the capacitor is reduced. Therefore, metallized film capacitors are relatively easy to make small and large capacitors. The films used in metallized film capacitors include polyethylene, polypropylene, polycarbonate, and the like. For example, MKP capacitor is short for metailized polypropylene film capacitor; MKT is short for metailized polyester. In addition to the winding type, there are also laminated types. Metallized film capacitors are characterized by a Self Healing Action. That is to say, if a small part of the electrode causes a short circuit due to the fragile electrical boundary, the electrode metal surrounding the short circuit part will be restored by the larger area of melting and evaporation due to the electrostatic energy or short circuit current carried by the capacitor at that time. Insulation makes the capacitor return to its role again.

From structurally: 

film capacitor4

Rolled flat style (foot)

Rolling type (axial foot)

Stacked typestacked film

From the line end:

film capacitor5


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