What is capacitor
Capacitors are components that have the ability to store electrical energy. This energy is stored in an electrostatic field which is created by electrical charges accumulating on conducting plates placed across an electrical potential and separated by an insulating medium. Capacitance as a value is dependent upon the distance between the conducting plates, the dielectric constant of the insulating medium, and the common area of the conducting plates.
The unit of capacitance is the FARAD A farad of capacitance is a LOT of capacitance. We generally deal in microfarads (mfd.) or nanofarad (nf) or picofarads (pf)
A film type capacitor made from this simple approach would create a large component. To prevent the waste of space the plates are “rolled” together, with an extra layer of dielectric to prevent the plates from touching, which results in a higher capacitance and smaller size.
Film Capacitors come in two standard designs, a film-foil combination where plastic films are interleaved with metal foils that serve as electrodes, or as a Metalized construction where film has aluminum or zinc deposited in it to serve as the electrodes.
There are two basic types of “rolled” construction film- foil capacitors.
One is extended foil type, where foil is extended beyond the dielectric and the connection is made from the extended portion of foil, which are sprayed by metal and then tinned copper wire is soldered or welded. We call such capacitors as Non Inductive type film capacitors.
The other type of “rolled” construction is where connection is made by inserting tinned copper wire and welding it on the metal foil. Such capacitors are called Inductive type film capacitors. The distance that the electrode foil is indented from the edge of the dielectric is the margin. This margin provides some protection against arc-over and short circuit.
Metalized film construction is different from film-foil in that the foil is replaced with a very thin metallic film which is vacuum deposited on the dielectric. The thickness of the deposited layer is negligible to the foil it replaced, thereby saving more size. All metallised film capacitors are Non Inductive type.
Final encasement types of film capacitors include dipped epoxy, tape wrapped and epoxy endfilled and in box potted with the epoxy .
This may please be noted that capacitors in dipped epoxy or in box do not make any difference in the electrical parameters. Both meets same IEC specifications. The only difference in box type capacitor is, they have fixed dimensions with close tolerance and appearance is good.
Plastic film capacitors are the most versatile of all capacitors. The capacitors have a reputation for outstanding electrical characteristics in severe environmental conditions, high reliability, and stability in long life applications. Being non-polar gives these capacitors predictability in DC or AC applications as well as being stable over a broad temperature range. They have the lowest Dissipation Factor (DF) and Equivalent Series Resistance (ESR) of all capacitor types.
Metallised vs Film Foil Capacitors
Metalized film capacitors show three advantages where smaller applications are needed due to their smaller size and lower weight. Metalized capacitors also have a built in “healing” property where in the case of voltage stress the film will clear itself and heal the point of damage. Due to this “healing” property most metalized film capacitors are tolerant of over voltage.
Some film capacitors are “precleared” during manufacturing to blow away pin-holes and weak spots in the dielectric. Metallized film capacitors tend to have a higher ESR than film-foil, however, and somewhat lower insulation resistance. Film-foil capacitors are typically 50-150 % larger in volume than their metallized-film equivalent. Other variations include foil layers separated by metallized layers to get reduced ESR yet still allow for self-healing, and even foil and metallizing in the same electrode layer. In most cases, the idea behind these various constructions is to get the good self-healing of a metallized-film capacitor with the low ESR/high current capacity of film/foil.
Aluminum is the most common metal used as the metallization or foil material, as it is hard to beat for cost and reliability.
Circuits which require high currents may need a film-foil combination capacitor. The heavy foil of this combination offers low electrical resistance and excellent thermal dissipation preventing internal hot spots and optimizing the circuit efficiency.
Impregnated capacitors are recommended for high voltage circuits where corona may be a problem. Wax is suitable for a lower temperature application. For higher temperature application oil is a suitable impregnant . These capacitors can be either metalized or film-foil combination. Also, series wound units are used to prevent corona.
The most commonly used plastics for film capacitors are polyester, and polypropylene.
Polyester is probably the most popular of the film capacitors, at least for board-level applications. Actually, polyester is a generic term for a class of similar polymers, the one used in polyester capacitors being polyethylene terephthalate. Some people call it Mylar capacitors and some as PET. Low cost, small size and the ability to do many things well enough makes it a good choice for many non critical applications. High dissipation factor means it is best used in DC or relatively low-frequency/low-current pulse and AC power applications. Poor temperature drift, dielectric absorption, and leakage relegate it to non-critical analog circuit applications. Polyester has a high temperature drift but can be found layered with polypropylene to flatten the temperature curve (the two go in opposite directions). Polyester capacitors are available to 125C.
Polypropylene (PP) capacitors are available in a wide range of sizes and voltages, and are used in a wide variety of circuits. PP has a very low dissipation factor over it´s entire temperature range and over a wide frequency range. This makes polypropylene capacitors popular for high-frequency, high-current applications like switching power supplies.
While PPs very low dissipation factor has made it the only viable material for many high-power AC applications, its self-healing properties, critical for reliable high voltage operation, are only fair, impregnation helps make up for these deficiencies however.