Capacitors

A capacitor store voltage between metal surfaces separated by a dielectric material. A dielectric material is an electrical insulator that can be polarized by an electric field. The capacitor capability to receive an electrical charge is measured in Farad (F). A capacitor opposes fast voltage changes and is often used for energy storage. When capacitors are connected in parallel, it is the same as increasing the size of the metal surfaces and therefor also the capacitance. The dielectric material of the capacitor has a maximum break down voltage (BVD). Capacitors are categorized by their dielectric materials, such as electrolytic, tantalum or polypropylene. In an electrolytic capacitor, one metal sheet prefers negative charges and the other positive charges. Therefor electrolytic capacitors can only be used in direct current circuits. The electrolytic capacitor terminals must be connected with the right polarity. The value of polarity depending capacitors is often measured in µF. Film capacitors and ceramic capacitors does not have polarity and can be used also in alternating current circuitry. Non-polarized capacitors typically have values measured in nF or pF.
 

Aluminum Electrolytic Capacitors

Aluminum electrolytic capacitors are used in for instance DC links (600 µF and 600 V), or as hold up capacitors. They are the dominating technology for values starting from 1000 µF. For increased life time, it is recommended to use electrolytic capacitors specified for 105 °C. When an electrolytic capacitor is used at the rated voltage, leakage current increase and the temperature increase. Therefor it is recommended to use electrolytic capacitors at 80% of the rated voltage. When exposed to an over voltage, the capacitor starts to boil and expand. A hold-up capacitor is sometimes used to avoid voltage drops in power down situations, when changing batteries for instance. Avionic standard such as DO-160 or MIL-STD-704 and the railway standard EN50155 require hold-up times of 50 ms or more. This requires large capacitors with high equivalent series resistance (ESR). If a small plastic film capacitor is connected in parallel with an electrolytic capacitor, the value of the larger capacitor may be decreased. In addition to their DC-DC modules, Gaïa Converter offers hold-up modules that makes it possible to use capacitors with higher rated voltage but lower values to reduce size.
 

Tantalum Electrolytic Capacitors

Tantalum as a dielectric material increase capacitance per volume, offers better high frequency characteristics and longer-term stability than aluminum electrolytic. On the other hand, tantalum is a conflict mineral so it may only be used when needed in defense-, space- and medical applications. Tantalum electrolytic capacitors are used in power supply filters and for micro processor couplings. Their low leakage current is also an advantage in sample-and-hold circuits with long holding time requirements.





 

Ceramic Capacitors

Ceramic dielectric materials resist higher voltages than other isolating materials such as plastic or alumina oxide. Ceramic capacitors are often chosen because they are small (down to 0201) and because of their low equivalent series resistance (ESR) over a wide frequency band. Low ESR at higher frequency results in less self-heating. Ceramic capacitors are available in values up to 100 µF but typically used up to 1 µF. Examples of dielectric material in multilayer ceramic capacitors (MLCC) are C48X, NP0 or X7R. NP0 is only available in values up to 1 nF and X7R is available from a few pF to tenths of µF. NP0 and X7R has low piezo effect which makes them possible to use in audio applications. In the input stage of a power supply, there is sometimes a so-called X capacitor in ceramic material between phase and neutral. The advantage is that the capacitor is small and cheap, but the disadvantage is that it can be damaged by voltage transients from the grid.
 

Plastic Film Capacitors

Plastic film capacitors typically have a capacitance between 10 pF up to 10 F. They can be divided in film capacitors and metallized film capacitors. Many different material and plastic films can be used as dielectric. The main dielectric materials used today are polypropylene and polyester. Manufacturers of plastic film capacitors continuously try to make them smaller, but they will never become as small as ceramic capacitors. Self-healing is one of the great advantages with metallized film capacitors. If a self-healing capacitor is used as parallel connected X capacitor on the input stage, it can resist transients from the grid. Self-healing typically takes 10 µs and happens when the metal close to the short circuit vaporize so that the capacitor can go back to normal operation.

 

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