Plastic film capacitor definition
Plastic film capacitor is a capacitor that uses plastic film as the dielectric and aluminum or zinc as the electrodes to store electric charge.
One of the most common and extensively used capacitors is the film capacitor. A film capacitor is defined as a capacitor that employs a thin plastic film as a dielectric. It is used in electrical and electronic equipment. This type of capacitor has some other names including, plastic film capacitor, polymer film capacitor, or film dielectric capacitors. It is also known as film cap and power film capacitor.
What is dielectric?
Dielectric is the insulating material placed between the electrodes of a capacitor. Dielectric materials are chosen based on their ability to allow electrostatic attraction and repulsion. Good dielectric materials are poor conductors of electricity. However, they allow the electrostatic field.
In plastic film capacitors, polyester, polypropylene, polyethylene terephthalate, and polyphenylene sulfide are commonly used as dielectrics.
Types of plastic film capacitors
Film capacitors are classified into two types:
Film-foil capacitors
Metallized film capacitors
Film-foil capacitors
The film-foil capacitor is made of two plastic films or sheets; each is layered with thin aluminum metal foil or sheet. The plastic sheets and aluminum sheets are then rolled in the form of a cylinder and wire leads are attached to the both ends of aluminum sheets. Polyester, polypropylene, polyethylene terephthalate, and polyphenylene sulfide are commonly used as dielectric in film capacitors. In plastic film capacitors, aluminum sheets acts as electrodes and plastic sheets acts as dielectric.
Metallized film capacitors
In metallized film capacitors, the aluminum sheet or foil is replaced by a layer of metal vacuum deposited on the film layer. The most commonly used metal layer is aluminum or zinc that is extremely thin.
The plastic film layers made of synthetic material act as dielectric and the aluminum layers act as electrodes. The major advantage of film dielectric capacitors over natural dielectric capacitors is that the plastic film is synthetic or artificial. Therefore, we can able to increase thickness and heat resistance of the dielectric. In other words, we can change the thickness and heat resistance of the plastic film capacitor.
Characteristics and Construction of Film Capacitors
Because of their exceptional features, film capacitors are frequently utilized. This capacitor is non-polarized, making it acceptable for AC signal and power applications. Film capacitors may be produced with extremely precise capacitance values, and they keep that value for longer than other capacitor kinds. This means that, in comparison to other capacitor types, such as the electrolytic capacitor, the aging process is often slower. Film capacitors have a long shelf life and are extremely dependable, with a very low average failure rate. They have a low ESR (Equivalent Series Resistance), low self-inductance (ESL), and low dissipation factors as a result.
They can tolerate voltages up to kilovolts and deliver extremely strong surge current bursts. There is a specific class of film capacitors known as power film capacitors that can tolerate reactive power in excess of 200 volt-amperes. Special screw-type terminals that can handle high currents may be used on these capacitors. Because power film capacitors must occasionally be replaced in the field, screw-type terminals have replaced soldered connectors. Their better electrical characteristics and stability, unfortunately, come at a cost. Because film capacitors are bigger than electrolytic capacitors, only a few SMT (Surface – Mount Technology) packages are available. They can also catch fire if they are overloaded, but this is a frequent symptom of other capacitor types.
Film capacitors are formed of a thin dielectric film that is metalized on one side or not at all. The film is very thin, with a thickness of less than one micrometer. The film is sliced into ribbons after it has been drawn to the proper thickness. The ribbon width is determined by the capacity of the capacitor being manufactured. Two film ribbons are twisted together to form a roll, which is then pressed into an oval shape to fit into a rectangular case. Because rectangular components take up less space on the printed circuit board, this is crucial. Each of the two electrodes is connected to one of the films to create electrodes. Using the self-healing feature of film capacitors, a voltage is provided to burn out any defects. The interior of the box is then hermetically sealed with silicon oil to protect the film roll from moisture and dipped in plastic.
Various Types of Film Capacitors
The plastic industry began to manufacture thinner and more durable goods soon after the first film capacitor was introduced. Over the years, several types of plastic film capacitors have been employed as dielectrics to suit various circuit applications. Some film capacitors have plastic films that are simply placed between aluminum foils, while others have the plastic film metalized through a process in which metal is deposited on the film itself. Film capacitors are generally divided into two types based on their construction. It’s worth noting that the classification is solely based on structure.
Film/Foil Capacitors
The film/foil capacitor, as the name implies, utilizes plastic films as a dielectric and is housed between two layers of aluminum foil electrodes. The metallic layers do not come into contact with each other since these interleaved layers are so organized. Inductive or non-inductive capacitors are available.
How does a Film Capacitor Work?
The film Capacitor operates on the same concept as a “General Purpose Capacitor”:
It holds the electric charge as well as the electric energy on the electrode. It is typically used in conjunction with an inductor to produce the LC oscillating circuit. The capacitor’s operating principle is that the electric charge will travel under the electric field when the conductor has a medium between them. Having a medium between them inhibits the flow of charge and causes the charge to collect on the conductor, resulting in charge accumulation.
Applications for a Film Capacitor
Because of the features like stability, cheap cost, and low inductance, film capacitor is utilized in a variety of applications, including:
Film capacitor is widely employed in circuits for high-frequency filtering, high-frequency bypass, and first or second-order filtering. Electronics, household appliances, telecommunications, electric power, electrified trains, hybrid cars, wind power, solar power, and many more sectors make extensive use of it.
As previously stated, the film capacitor has lower ESR and ESL values and a lower distortion factor. In addition, in terms of the aging time, the film capacitor tends to resist the wearing-out stage for the longest period. As a result, it is a superior option for high-voltage and high-frequency applications.
Film capacitors can also be employed in more traditional applications such as voltage smoothing capacitors, filters, and audio crossovers. It can be used to store energy and then release it in the form of a high-current pulse when required.
In film capacitors, plastic film is used to construct the dielectric and aluminum or zinc is used to construct the electrodes of the capacitor. Film capacitors are also known as plastic film capacitors or film dielectric capacitors. Plastic film capacitors are mainly used in circuits where low loss and high insulation resistance is required.
