What is film capacitor?
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 power levels.
Overview of construction and features
Film capacitors are made out of two pieces of plastic film covered with metallic electrodes, wound into a cylindrical shaped winding, with terminals attached, and then encapsulated. In general, film capacitors are not polarized, so the two terminals are interchangeable. There are two different types of plastic film capacitors, made with two different electrode configurations:
Film/foil capacitors or metal foil capacitors are made with two plastic films as the dielectric. Each is layered with a thin metal foil, usually aluminum, as the electrodes. Advantages of this construction type are easy electrical connection to the metal foil electrodes, and its ability to handle high current surges.
Metallized film capacitors are made of two metallized films with plastic film as the dielectric. A very thin (~ 0.03 μm) vacuum-deposited aluminum metallization is applied to one or both sides to serve as electrodes. This configuration can have "self-healing" properties, in that dielectric breakdowns or short circuits between the electrodes do not necessarily lead to the destruction of the component. With this basic design, it is possible to make high quality products such as "zero defect" capacitors and to produce wound capacitors with larger capacitance values (up to 100 μF and larger) in smaller cases (high volumetric efficiency) compared to film/foil construction. However, a disadvantage of metallized construction is its limited current surge rating.
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.
Typical capacitance values of smaller film capacitors used in electronics start around 100 picofarads and extend upwards to microfarads.
Unique mechanical properties of plastic and paper films in some special configurations allow them to be used in capacitors of very large dimensions. The larger film capacitors are used as power capacitors in electrical power installations and plants, capable of withstanding very high power or very high applied voltages. The dielectric strength of these capacitors can reach into the four-digit voltage range.
The formula for capacitance (C) of a plate capacitor is:
C=ε *(A/d)(ε stands for dielectric permittivity; A for electrode surface area; and d for the distance between the electrodes).
According to the equation, a thinner dielectric or a larger electrode area both will increase the capacitance value, as will a dielectric material of higher permittivity.
Example manufacturing process
The following example describes a typical manufacturing process flow for wound metallized plastic film capacitors.
1.Film stretching and metallization — To increase the capacitance value of the capacitor, the plastic film is drawn using a special extrusion process of bi-axial stretching in longitudinal and transverse directions, as thin as is technically possible and as allowed by the desired breakdown voltage. The thickness of these films can be as little as 0.6 μm. In a suitable evaporation system and under high vacuum conditions (about 1015 to 1019 molecules of air per cubic meter) the plastic film is metallized with aluminum or zinc. It is then wound onto a so-called "mother roll" with a width of about 1 meter.
2.Film slitting — Next, the mother rolls are slit into small strips of plastic film in the required width according to the size of the capacitors being manufactured.
3.Winding — Two films are rolled together into a cylindrical winding. The two metallized films that make up a capacitor are wound slightly offset from each other, so that by the arrangement of the electrodes one edge of the metallization on each end of the winding extends out laterally.
4.Flattening — The winding is usually flattened into an oval shape by applying mechanical pressure. Because the cost of a printed circuit board is calculated per square millimeter, a smaller capacitor footprint reduces the overall cost of the circuit.
5.Application of metallic contact layer ("schoopage") — The projecting end electrodes are covered with a liquefied contact metal such as (tin, zinc or aluminum), which is sprayed with compressed air on both lateral ends of the winding. This metallizing process is named schoopage after Swiss engineer Max Schoop, who invented a combustion spray application for tin and lead.
6.Healing — The windings which are now electrically connected by the schoopage have to be "healed". This is done by applying a precisely calibrated voltage across the electrodes of the winding so that any existing defects will be "burned away" (see also "self-healing" below).
7.Impregnation — For increased protection of the capacitor against environmental influences, especially moisture, the winding is impregnated with an insulating fluid, such as silicone oil.
8.Attachment of terminals — The terminals of the capacitor are soldered or welded on the end metal contact layers of the schoopage.
9.Coating — After attaching the terminals, the capacitor body is potted into an external casing, or is dipped into a protective coating. For lowest production costs some film capacitors can be used "naked", without further coating of the winding.
10.Electrical final test — All capacitors (100%) should be tested for the most important electrical parameters, capacitance (C), dissipation factor (tan δ) and impedance (Z).
The production of wound film/metal foil capacitors with metal foil instead of metallized films is done in a very similar way.
As an alternative to the traditional wound construction of film capacitors, they can also be manufactured in a "stacked" configuration. For this version, the two metallized films representing the electrodes are wound on a much larger core with a diameter of more than 1 m. So-called multi-layer capacitors (MLP, Multilayer Polymer Capacitors) can be produced by sawing this large winding into many smaller single segments. The sawing causes defects on the collateral sides of the capacitors which are later burned out (self-healing) during the manufacturing process. Low-cost metallized plastic film capacitors for general purpose applications are produced in this manner. This technique is also used to produce capacitor "dice" for Surface Mount Device (SMD) packaged components.
Self-healing of metallized film capacitors
Metallized film capacitors have "self-healing" properties, which are not available from film/foil configurations. When sufficient voltage is applied, a point-defect short-circuit between the metallized electrodes vaporizes due to high arc temperature, since both the dielectric plastic material at the breakdown point and the metallized electrodes around the breakdown point are very thin (about 0.02 to 0.05 μm). The point-defect cause of the short-circuit is burned out, and the resulting vapor pressure blows the arc away, too. This process can complete in less than 10 μs, often without interrupting the useful operation of the afflicted capacitor.
This property of self-healing allows the use of a single-layer winding of metallized films without any additional protection against defects, and thereby leads to a reduction in the amount of the physical space required to achieve a given performance specification. In other words, the so-called "volumetric efficiency" of the capacitor is increased.
The self-healing capability of metallized films is used multiple times during the manufacturing process of metallized film capacitors. Typically, after slitting the metallized film to the desired width, any resulting defects can be burned out (healed) by applying a suitable voltage before winding. The same method is also used after the metallization of the contact surfaces ("schoopage") to remove any defects in the capacitor caused by the secondary metallization process.
The "pinholes" in the metallization caused by the self-healing arcs reduce the capacitance of the capacitor very slightly. However, the magnitude of this reduction is quite low; even with several thousand defects to be burned out, this reduction usually is much smaller than 1% of the total capacitance of the capacitor.
For larger film capacitors with very high standards for stability and long lifetime, such as snubber capacitors, the metallization can be made with a special fault isolation pattern. In the picture on the right hand side, such a metallization formed into a "T" pattern is shown. Each of these "T" patterns produces a deliberately narrowed cross-section in the conductive metallization. These restrictions work like microscopic fuses so that if a point-defect short-circuit between the electrodes occurs, the high current of the short only burns out the fuses around the fault. The affected sections are thus disconnected and isolated in a controlled manner, without any explosions surrounding a larger short-circuit arc. Therefore, the area affected is limited and the fault is gently controlled, significantly reducing internal damage to the capacitor, which can thus remain in service with only an infinitesimal reduction in capacitance.
In field installations of electrical power distribution equipment, capacitor bank fault tolerance is often improved by connecting multiple capacitors in parallel, each protected with an internal or external fuse. Should an individual capacitor develop an internal short, the resulting fault current (augmented by capacitive discharge from neighboring capacitors) blows the fuse, thus isolating the failed capacitor from the remaining devices. This technique is analogous to the "T metallization" technique described above, but operating at a larger physical scale. More-complex series and parallel arrangements of capacitor banks are also used to allow continuity of service despite individual capacitor failures at this larger scale.
Highly simplified cross-section diagram of self-healing, after a point-defect short-circuit between the metallized electrodes is burned out. Lower diagram shows top view of foil after burn-out of a point defect.
"T metallization" segmentation to isolate and reduce damage during the self-healing process
Internal structure to increase voltage ratings
Examples of partial metallization on one side of the metallized insulating film, to increase the voltage rating of film capacitors. This technique effectively forms multiple small capacitors, connected in series, to raise the effective breakdown voltage
The rated voltage of different film materials depends on factors such as the thickness of the film, the quality of the material (freedom from physical defects and chemical impurities), the ambient temperature, and frequency of operation, plus a safety margin against the breakdown voltage (dielectric strength). But to a first approximation, the voltage rating of a film capacitor depends primarily on the thickness of the plastic film. For example, with the minimum available film thickness of polyester film capacitors (about 0.7 μm), it is possible to produce capacitors with a rated voltage of 400 VDC. If higher voltages are needed, typically a thicker plastic film will be used. But the breakdown voltage for dielectric films is usually nonlinear. For thicknesses greater than about 5 mils, the breakdown voltage only increases approximately with the square-root of the film thickness. On the other hand, the capacitance decreases linearly with increased film thickness. For reasons of availability, storage and existing processing capabilities, it is desirable to achieve higher breakdown voltages while using existing available film materials. This can be achieved by a one-sided partial metallization of the insulating films in such a manner that an internal series connection of capacitors is produced. By using this series connection technique, the total breakdown voltage of the capacitor can be multiplied by an arbitrary factor, but the total capacitance is also reduced by the same factor.
The breakdown voltage can be increased by using one-sided partially metallized films, or the breakdown voltage of the capacitor can be increased by using double-sided metallized films. Double-sided metallized films also can be combined with internal series-connected capacitors by partial metallization. These multiple technique designs are especially used for high-reliability applications with polypropylene films.
Increasing the breakdown voltage of film capacitors by using double sided metallized foils
Internal structure to increase surge ratings
An important property of film capacitors is their ability to withstand high peak voltage or peak current surge pulses. This capability depends on all internal connections of the film capacitor withstanding the peak current loads up to the maximum specified temperature. The collateral contact layers (schoopage) with the electrodes can be a potential limitation of peak current carrying capacity.
The electrode layers are wound slightly offset from each other, so that the edges of the electrodes can be contacted using a face contacting method "schoopage" at the collateral end faces of the winding. This internal connection is ultimately made by multiple point-shaped contacts at the edge of the electrode, and can be modeled as a large number of individual capacitors all connected in parallel. The many individual resistance (ESR) and inductance (ESL) losses are connected in parallel, so that these total undesirable parasitic losses are minimized.
However, ohmic contact resistance heating is generated when peak current flows through these individual microscopic contacting points, which are critical areas for the overall internal resistance of the capacitor. If the current gets too high, "hot spots" can develop and cause burning of the contact areas.
A second limitation of the current-carrying capacity is caused by the ohmic bulk resistance of the electrodes themselves. For metallized film capacitors, which have layer thicknesses from 0.02 to 0.05 μm the current-carrying capacity is limited by these thin layers.
Shape-optimized metallization to increase the surge current rating
The surge current rating of film capacitors can be enhanced by various internal configurations. Because metallization is the cheapest way of producing electrodes, optimizing the shape of the electrodes is one way to minimize the internal resistance and to increase the current-carrying capacity. A slightly thicker metallization layer at the schoopage contact sides of the electrodes results in a lower overall contact resistance and increased surge current handling, without losing the self-healing properties throughout the remainder of the metallization.
Another technique to increase the surge current rating for film capacitors is a double-sided metallization. This can double the peak current rating. This design also halves the total self-inductance of the capacitor, because in effect, two inductors are connected in parallel, which allows less-unimpeded passage of faster pulses (higher so-called "dV/dt" rating).
The double-sided metallized film is electrostatically field-free because the electrodes have the same voltage potential on both sides of the film, and therefore does not contribute to the total capacitance of the capacitor. This film can therefore be made of a different and less expensive material. For example, a polypropylene film capacitor with double-sided metallization on a polyester film carrier makes the capacitor not only cheaper but also smaller, because the thinner polyester foil improves the volumetric efficiency of the capacitor. Film capacitors with a double-sided metallized film effectively have thicker electrodes for higher surge current handling, but still do retain their self-healing properties, in contrast to the film/foil capacitors.
The highest surge-current rated film capacitors are film/foil capacitors with a metal foil construction. These capacitors use thin metal foils, usually aluminum, as the electrodes overlying the polymer film. The advantage of this construction is the easy and robust connection of the metal foil electrodes. In this design, the contact resistance in the area of the schoopage is the lowest.
However, metal foil capacitors do not have self-healing properties. A breakdown in the dielectric film of a film/foil capacitor leads to an irreversible short circuit. To avoid breakdowns caused by weak spots in the dielectric, the insulating film chosen is always thicker than theoretically required by the specific breakdown voltage of the material. Films of less than 4 μm generally are not used for film/foil capacitors because of their excessively high numbers of point defects. Also. the metallic foils only can be produced down to about 25 μm in thickness. These tradeoffs make the film/foil capacitor the most robust but also the most expensive method for increasing surge current handling.
Three examples of different film capacitor configurations for increasing surge current ratings