Film Capacitor

 

What are film capacitors?

 

Film capacitors are capacitors which use a thin plastic film as the dielectric. This film is made extremely thin using a sophisticated film drawing process. Once the film is manufactured, it may be metalized or left untreated, depending on the needed properties of the capacitor. Electrodes are then added and the assembly is mounted into a case which protects it from environmental factors. They are used in many applications because of their stability, low inductance and low cost. There are many types of film capacitors, including polyester film, metalized film, polypropylene film, PTFE film and polystyrene film. The core difference between these capacitor types is the material used as the dielectric, and the proper dielectric must be chosen according to the application.

 

PTFE film capacitors, for example, are heat-resistant and used in aerospace and military technology, while metallized polyester film capacitors are used in applications that require long term stability at a relatively low. Cheaper plastics are used if cost is a bigger concern than performance.

 

Film capacitor definition

 

A film capacitor is a capacitor that uses a thin plastic film as the dielectric. They are relatively cheap, stable over time and have low self-inductance and ESR, while some film capacitors can withstand large reactive power values.

 

Characteristics

 

Film capacitors are widely used because of their superior characteristics. This capacitor type is not polarized, which makes them suitable for AC signal and power use.  Film capacitors can be made with very high precision capacitance values, and they retain that value longer than other capacitor types. This means that the aging process is generally slower than in other capacitor types, such as the electrolytic capacitor. Film capacitors have a long shelf and service life, and are very reliable, with a very low average failure rate. They have low ESR (Equivalent Series Resistance), low self-inductance (ESL), and as a result very low dissipation factors. They can be made to withstand voltages in the kilovolt range and can provide very high surge current pulses. A special class of film capacitors, which is called power film capacitors is available, and this class of film capacitors can withstand reactive power in excess of 200 volt-amperes. These capacitors may have special screw-type terminals which can withstand high currents. Screw-type terminals replace soldered joints because power film capacitors sometimes need to be changed in the field. Unfortunately, their superior electrical properties and stability come at a price. Film capacitors are bulkier than their electrolytic equivalents, which means that limited SMT (Surface – Mount Technology) packages are available. They can also burst into flames if overloaded, but this characteristic is somewhat common among different capacitor types.

 

Construction and properties

 

Film capacitors are made of a thin dielectric film which may or may not be metallized on one side. The film is extremely thin, with the thickness being under 1 µm. After the film is drawn to the desired thickness, the film is cut into ribbons. The width of the ribbons depends on the capacity of the capacitor being produced. Two ribbons of film are wound together into a roll, which is often pressed into an oval shape so that it can fit into a rectangular case. This is important because rectangular components save precious space on the printed circuit board. Electrodes are added by connecting each of the two electrodes to one of the films. A voltage is applied to burn out any imperfections using the self-healing property of film capacitors. The case is then sealed using silicon oil to protect the film roll against moisture, and dipped in plastic to hermetically seal the interior.

 

Typical film capacitors have capacitances ranging from below 1nF to 30µF. They can be made in voltage ratings as low as 50V, up to above 2kV. They can be manufactured for use in high-vibration automotive environments, high temperature environments and high-power applications. Film capacitors offer low losses and high efficiency while providing a long service life.

 

Applications for film capacitors

 

Power film capacitors are used in power electronics devices, phase shifters, X-ray flashes and pulsed lasers, while the low power variants are used as decoupling capacitors, filters and in A/D convertors. Other notable applications are safety capacitors, electromagnetic interference suppression, fluorescent light ballasts and snubber capacitors.

 

Lighting ballasts are used for proper starting and operation of fluorescent lights. When a ballast is faulty, the light will flicker or fail to start properly. Older ballasts used only an inductor, a solution which provides a poor power factor. New designs use a switched power supply which relies on film capacitors for power factor correction.

 

Snubber capacitors are protective devices which damp or “snub” inductive kickback voltage spikes. These circuits often use film capacitors because of their low self-inductance, high peak current and low ESR, which are all critical factors in a snubber design. Polypropylene film capacitors are most often used in this type of circuit. Snubbers are used in many areas of electronics, especially power electronics in devices such as DC-DC converters and others.

 

Film capacitors can also be used in a more conventional way as voltage smoothing capacitors, in filters, audio crossovers. They can be used to store energy and release it in a high-current pulse when needed. High-current electrical pulses are used to power pulsed lasers or generate lighting discharges.

 

Plastic film capacitors potted in rectangular casings, or dipped in epoxy lacquer coating (red color)

Film capacitors, plastic film capacitors, film dielectric capacitors, or polymer film capacitors, generically called “film caps” as well as power film capacitors, are electrical capacitors with an insulating plastic film as the dielectric, sometimes combined with paper as carrier of the electrodes.

 

The dielectric films, depending on the desired dielectric strength, are drawn in a special process to an extremely thin thickness, and are then provided with electrodes. The electrodes of film capacitors may be metallized aluminum or zinc applied directly to the surface of the plastic film, or a separate metallic foil. Two of these conductive layers are wound into a cylinder shaped winding, usually flattened to reduce mounting space requirements on a printed circuit board, or layered as multiple single layers stacked together, to form a capacitor body. Film capacitors, together with ceramic capacitors and electrolytic capacitors, are the most common capacitor types for use in electronic equipment, and are used in many AC and DC microelectronics and electronics circuits

 

A related component type is the power film capacitor. Although the materials and construction techniques used for large power film capacitors are very similar to those used for ordinary film capacitors, capacitors with high to very high power ratings for applications in power systems and electrical installations are often classified separately, for historical reasons. As modern electronic equipment gained the capacity to handle power levels that were previously the exclusive domain of "electrical power" components, the distinction between the "electronic" and "electrical" power ratings has become less distinct. In the past, the boundary between these two families was approximately at a reactive power of 200 volt-amperes, but modern power electronics can handle increasing amounts of power.

 

Polypropylene (PP) film capacitors

Polyester (PET) film capacitors

Polyethylene naphthalate (PEN) film capacitors

Polyphenylene sulfide (PPS) film capacitors

Polytetrafluoroethylene (PTFE) film capacitors

Polystyrene (PS) film capacitors

Polycarbonate (PC) film capacitors

Paper (film) capacitors (MP) and mixed film capacitors

Power film capacitors (DC Link capacitor, AC filter capacitor, snnuber capacitor)

 

 

A key advantage of modern film capacitor internal construction is direct contact to the electrodes on both ends of the winding. This contact keeps all current paths to the entire electrode very short. The setup behaves like a large number of individual capacitors connected in parallel, thus reducing the internal ohmic losses (ESR) and the parasitic inductance (ESL). The inherent geometry of film capacitor structure results in very low ohmic losses and a very low parasitic inductance, which makes them especially suitable for applications with very high surge currents (snubbers) and for AC power applications, or for applications at higher frequencies.

 

Another feature of film capacitors is the possibility of choosing different film materials for the dielectric layer to select for desirable electrical characteristics, such as stability, wide temperature range, or ability to withstand very high voltages. Polypropylene film capacitors are specified because of their low electrical losses and their nearly linear behavior over a very wide frequency range, for stability Class 1 applications in resonant circuits, comparable only with ceramic capacitors. For simple high frequency filter circuits, polyester capacitors offer low-cost solutions with excellent long-term stability, allowing replacement of more expensive tantalum electrolytic capacitors. The film/foil variants of plastic film capacitors are especially capable of handling high and very high current surges.

 

Power film capacitors

 

Power capacitors for higher power snubbing in a thyristor electronic control for HVDC transmission at Hydro-Québec fulfill the same snubber functions as film snubbers, but belong to the family of power capacitors

Power film capacitors with screw terminals for applications in power systems, electrical installations and plants

 

Power film capacitor for AC power factor correction (PFC), packaged in a cylindrical metal can

 

Power film capacitor in rectangular housing

 

One of several energy storage power film capacitor banks, for magnetic field generation at the Hadron-Electron Ring Accelerator (HERA), located on the DESY site in Hamburg

 

Capacitor bank with 75 MVA for power factor correction of 150 kV transmission lines

The relatively simple fabrication technique of winding gives film capacitors the possibility of attaining even very large sizes for applications in the high power range, as so-called "power capacitors". Although the materials and the construction of power capacitors are mostly similar to the smaller film capacitors, they are specified and marketed differently for historical reasons.

 

Power capacitors can be used for a wide variety of applications, even where extremely non-sinusoidal voltages and pulsed currents are present. Both AC and DC capacitors are available. AC capacitors serve as damping or snubbing capacitors when connected in series with a resistor, and are also specified for the damping of undesirable voltage spikes caused by the so-called charge carrier storage effect during switching of power semiconductors. In addition, AC capacitors are used in low-detuned or close-tuned filter circuits for filtering or absorbing harmonics. As pulse discharge capacitors, they are useful in applications with reversing voltages, such as in magnetizing equipment.

 

The scope of application for DC capacitors is similarly diverse. Smoothing capacitors are used to reduce the AC component of fluctuating DC voltage (such as in power supplies for radio and television transmitters), and for high voltage testing equipment, DC controllers, measurement and control technology and cascaded circuits for generation of high DC voltage. Supporting capacitors, DC-filter or buffer circuit capacitors are used for energy storage in intermediate DC circuits, such as in frequency converters for poly-phase drives, and transistor and thyristor power converters. They must be able to absorb and release very high currents within short periods, the peak values of currents being substantially greater than the RMS values.

 

Advantages

 

Polypropylene film capacitors can qualify for Class 1 applications

Very low dissipation factors (tan δ), high quality factors (Q) and low inductance values (ESL)

No microphonics compared with ceramic capacitors

Metallized construction has self-healing properties

High rated voltages, up to the range of kV possible

Much higher ripple current, compared with electrolytic capacitors

Much lower aging, compared with electrolytic capacitors

High and very high surge current pulses possible

 

For more film capacitors, please contact sales@anxoncap.com, Cixi AnXon Electronic Co., Ltd is the proven and reliable capacitor manufacturer and supplier in China.