Capacitor types

This article is about commercial discrete capacitors as customary components for use in electronic equipment. For the physical phenomenon, see Capacitance. For the explanation of the units of measure of capacitance, see Farad.

Some different capacitors for electronic equipment

Capacitors are manufactured in many styles, forms, dimensions, and from a large variety of materials. They all contain at least two electrical conductors, called plates, separated by an insulating layer (dielectric). Capacitors are widely used as parts of electrical circuits in many common electrical devices.

Capacitors, together with resistors and inductors, belong to the group of passive components in electronic equipment. Small capacitors are used in electronic devices to couple signals between stages of amplifiers, as components of electric filters and tuned circuits, or as parts of power supply systems to smooth rectified current. Larger capacitors are used for energy storage in such applications as strobe lights, as parts of some types of electric motors, or for power factor correction in AC power distribution systems. Standard capacitors have a fixed value of capacitance, but adjustable capacitors are frequently used in tuned circuits. Different types are used depending on required capacitance, working voltage, current handling capacity, and other properties.

While, in absolute figures, the most commonly manufactured capacitors are integrated into dynamic random-access memory, flash memory, and other device chips, this article covers the discrete components.

General characteristics

Conventional construction

A dielectric material is placed between two conducting plates (electrodes), each of area A and with a separation of d.

A conventional capacitor stores electric energy as static electricity by charge separation in an electric field between two electrode plates. The charge carriers are typically electrons. The amount of charge stored per unit voltage is essentially a function of the size of the plates, the plate material's properties, the properties of the dielectric material placed between the plates, and the separation distance (i.e. dielectric thickness). The potential difference between the plates is limited by the properties of the dielectric material and the separation distance.

Nearly all conventional industrial capacitors except some special types such as "feed-through capacitors", are constructed as "plate capacitors" even if their electrodes and the dielectric between are wound or rolled. The capacitance C of a plate capacitor is

$C={\frac {\varepsilon A}{d}}$.

The capacitance increases with the area A of the plates and with the permittivity ε of the dielectric material, and decreases with the plate separation distance d. The capacitance is therefore greatest in devices made from materials with a high permittivity, large plate area, and small distance between plates.

Electrochemical construction

Schematic of double layer capacitor.

1. IHP Inner Helmholtz Layer
2. OHP Outer Helmholtz Layer
3. Diffuse layer
4. Solvated ions
5. Specifically adsorptive ions (Pseudocapacitance)
6. Solvent molecule

Another type – the electrochemical capacitor – makes use of two other storage principles to store electric energy. In contrast to ceramic, film, and electrolytic capacitors, supercapacitors (also known as electrical double-layer capacitors (EDLC) or ultracapacitors) do not have a conventional dielectric. The capacitance value of an electrochemical capacitor is determined by two high-capacity storage principles. These principles are:

• electrostatic storage within Helmholtz double layers achieved on the phase interface between the surface of the electrodes and the electrolyte (double-layer capacitance); and
• electrochemical storage achieved by a faradaic electron charge-transfer by specifically absorbed ions with redox reactions (pseudocapacitance). Unlike batteries, in these reactions, the ions simply cling to the atomic structure of an electrode without making or breaking chemical bonds, and no or negligibly small chemical modifications are involved in charge/discharge.

The ratio of the storage resulting from each principle can vary greatly, depending on electrode design and electrolyte composition. Pseudocapacitance can increase the capacitance value by as much as an order of magnitude over that of the double-layer by itself.

Classification

Capacitors are divided into two mechanical groups: Fixed-capacitance devices with a constant capacitance and variable capacitors. Variable capacitors are made as trimmers, that are either adjusted only during circuit calibration, or a device tunable during operation of the electronic instrument.

The most common group is the fixed capacitors. Many are named based on the type of dielectric. For a systematic classification these characteristics cannot be used, because one of the oldest, the electrolytic capacitor, is named instead by its cathode construction. So the most-used names are simply historical.

The most common kinds of capacitors are:

• Ceramic capacitors have a ceramic dielectric.
• Film and paper capacitors are named for their dielectrics.
• Aluminum, tantalum and niobium electrolytic capacitors are named after the material used as the anode and the construction of the cathode (electrolyte).
• Polymer capacitors are aluminum, tantalum or niobium electrolytic capacitors with conductive polymer as electrolyte.
• Supercapacitor is the family name for:
  • Double-layer capacitors were named for the physical phenomenon of the Helmholtz double-layer.
  • Pseudocapacitors were named for their ability to store electric energy electro-chemically with reversible faradaic charge-transfer.
  • Hybrid capacitors combine double-layer and pseudocapacitors to increase power density.
• Silver mica, glass, silicon, air-gap and vacuum capacitors are named for their dielectric.

Overview over the most commonly used fixed capacitors in electronic equipment

In addition to the above shown capacitor types, which derived their name from historical development, there are many individual capacitors that have been named based on their application. They include:

• Power capacitors, motor capacitors, DC-link capacitors, suppression capacitors, audio crossover capacitors, lighting ballast capacitors, snubber capacitors, coupling, decoupling or bypassing capacitors.

Often, more than one capacitor family is employed for these applications, e.g. interference suppression can use ceramic capacitors or film capacitors.

Other kinds of capacitors are discussed in the #Special capacitors section.

Dielectrics

Charge storage principles of different capacitor types and their inherent voltage progression

The most common dielectrics are:

• Ceramics
• Plastic films
• Oxide layer on metal (aluminum, tantalum, niobium)
• Natural materials like mica, glass, paper, air, SF₆, vacuum

All of them store their electrical charge statically within an electric field between two (parallel) electrodes.

Beneath these conventional capacitors a family of electrochemical capacitors called supercapacitors was developed. Supercapacitors do not have a conventional dielectric. They store their electrical charge statically in Helmholtz double-layers and faradaically at the surface of electrodes

• with static double-layer capacitance in a double-layer capacitor and
• with pseudocapacitance (faradaic charge transfer) in a pseudocapacitor
• or with both storage principles together in hybrid capacitors.

The most important material parameters of the different dielectrics used and the approximate Helmholtz-layer thickness are given in the table below.

Key parameters

Capacitor style	Dielectric	Relative Permittivity at 1 kHz	Maximum/realized dielectric strength (Volt/μm)	Minimum thickness of the dielectric (μm)
Ceramic capacitors, Class 1	paraelectric	12 to 40	< 100 (?)	1
Ceramic capacitors, Class 2	ferroelectric	200 to 14,000	< 35	0.5
Film capacitors	Polypropylene ( PP)	2.2	650 / 450	1.9 to 3.0
Film capacitors	Polyethylene terephthalate, Polyester (PET)	3.3	580 / 280	0.7 to 0.9
Film capacitors	Polyphenylene sulfide (PPS)	3.0	470 / 220	1.2
Film capacitors	Polyethylene naphthalate (PEN)	3.0	500 / 300	0.9 to 1.4
Film capacitors	Polytetrafluoroethylene (PTFE)	2.0	450 (?) / 250	5.5
Paper capacitors	Paper	3.5 to 5.5	60	5 to 10
Aluminum electrolytic capacitors	Aluminium oxide Al2O3	9.6	710	< 0.01 (6.3 V) < 0.8 (450 V)
Tantalum electrolytic capacitors	Tantalum pentoxide Ta2O5	26	625	< 0.01 (6.3 V) < 0.08 (40 V)
Niobium electrolytic capacitors	Niobium pentoxide, Nb2O5	42	455	< 0.01 (6.3 V) < 0.10 (40 V)
Supercapacitors Double-layer capacitors	Helmholtz double-layer	–	5000	< 0.001 (2.7 V)
Vacuum capacitors	Vacuum	1	40	–
Air gap capacitors	Air	1	3.3	–
Glass capacitors	Glass	5 to 10	450	–
Mica capacitors	Mica	5 to 8	118	4 to 50

The capacitor's plate area can be adapted to the wanted capacitance value. The permittivity and the dielectric thickness are the determining parameters for capacitors. Ease of processing is also crucial. Thin, mechanically flexible sheets can be wrapped or stacked easily, yielding large designs with high capacitance values. Razor-thin metallized sintered ceramic layers covered with metallized electrodes however, offer the best conditions for the miniaturization of circuits with SMD styles.

The figures in the table above gives the explanation for some simple facts:

• Supercapacitors have the highest capacitance density because of their special charge storage principles.
• Electrolytic capacitors have lesser capacitance density than supercapacitors but the highest capacitance density of conventional capacitors due to the thin dielectric.
• Ceramic capacitors class 2 have higher capacitance values in a given case than class 1 capacitors because of their higher permittivity.
• Film capacitors with their different plastic film material have a small spread in the dimensions for a given capacitance/voltage value of a film capacitor because the minimum dielectric film thickness differs between the different film materials.

Capacitance ranges vs. voltage ranges of different capacitor types

Capacitance and voltage range

Capacitance ranges from picofarads to more than hundreds of farads. Voltage ratings can reach 100 kilovolts. In general, capacitance and voltage correlate with physical size and cost.

Capacitor volumetric efficiency increased from 1970 to 2005 (click image to enlarge)

Miniaturization

As in other areas of electronics, volumetric efficiency measures the performance of electronic function per unit volume. For capacitors, the volumetric efficiency is measured with the "CV product", calculated by multiplying the capacitance (C) by the maximum voltage rating (V), divided by the volume. From 1970 to 2005, volumetric efficiencies improved dramatically.

• Miniaturizing of capacitors
• 
  Stacked paper capacitor from 1923 for noise decoupling (blocking) in telegraph lines
• 
  Wound metallized paper capacitor from the early 1930s in hardpaper case, capacitance value specified in "cm" in the cgs system; 5,000 cm corresponds to 0.0056 μF.
• 
  Folded wet aluminum electrolytic capacitor, Bell System 1929, view onto the folded anode, which was mounted in a squared housing (not shown) filled with liquid electrolyte
• 
  Two 8 μF, 525 V wound wet aluminum electrolytic capacitors in paper housing sealed with tar out of a 1930s radio.

Comparing the three main capacitor types it shows, that a broad range of overlapping functions for many general-purpose and industrial applications exists in electronic equipment.

Overlapping range of the applications

These individual capacitors can perform their application independent of their affiliation to an above shown capacitor type, so that an overlapping range of applications between the different capacitor types exists.

Types and styles

Ceramic capacitors

Construction of a Multi-Layer Ceramic Capacitor (MLCC)

Main article: Ceramic capacitor

A ceramic capacitor is a non-polarized fixed capacitor made out of two or more alternating layers of ceramic and metal in which the ceramic material acts as the dielectric and the metal acts as the electrodes. The ceramic material is a mixture of finely ground granules of paraelectric or ferroelectric materials, modified by mixed oxides that are necessary to achieve the capacitor's desired characteristics. The electrical behavior of the ceramic material is divided into two stability classes:

1. Class 1 ceramic capacitors with high stability and low losses compensating the influence of temperature in resonant circuit application. Common EIA/IEC code abbreviations are C0G/NP0, P2G/N150, R2G/N220, U2J/N750 etc.
2. Class 2 ceramic capacitors with high volumetric efficiency for buffer, by-pass and coupling applications Common EIA/IEC code abbreviations are: X7R/2XI, Z5U/E26, Y5V/2F4, X7S/2C1, etc.

The plasticity of ceramic raw material works well for many special applications and enables a diversity of styles, shapes and dimensional spread of ceramic capacitors. The smallest discrete capacitor, for instance, is a "01005" chip capacitor with the dimension of only 0.4 mm × 0.2 mm.

The construction of ceramic multilayer capacitors with mostly alternating layers results in single capacitors connected in parallel. This configuration increases capacitance and decreases losses and parasitic inductances. Ceramic capacitors are well-suited for high frequencies and high current pulse loads.

Because the thickness of the ceramic dielectric layer can be easily controlled and produced by the desired application voltage, ceramic capacitors are available with rated voltages up to the 30 kV range.

Some ceramic capacitors of special shapes and styles are used as capacitors for special applications, including RFI/EMI suppression capacitors for connection to supply mains, also known as safety capacitors, X2Y and three-terminal capacitors for bypassing and decoupling applications, feed-through capacitors for noise suppression by low-pass filters and ceramic power capacitors for transmitters and HF applications.

• Diverse styles of ceramic capacitors
• 
  Multi-layer ceramic capacitors (MLCC chips) for SMD mounting
• 
  Ceramic X2Y decoupling capacitors
• 
  Ceramic EMI suppression capacitors for connection to the supply mains (safety capacitor)
• 
  High voltage ceramic power capacitor

Film capacitors

Main article: Film capacitor

Three examples of different film capacitor configurations for increasing surge current ratings

Film capacitors or plastic film capacitors are non-polarized capacitors with an insulating plastic film as the dielectric. The dielectric films are drawn to a thin layer, provided with metallic electrodes and wound into a cylindrical winding. The electrodes of film capacitors may be metallized aluminum or zinc, applied on one or both sides of the plastic film, resulting in metallized film capacitors or a separate metallic foil overlying the film, called film/foil capacitors.

Metallized film capacitors offer self-healing properties. Dielectric breakdowns or shorts between the electrodes do not destroy the component. The metallized construction makes it possible to produce wound capacitors with larger capacitance values (up to 100 μF and larger) in smaller cases than within film/foil construction.

Film/foil capacitors or metal foil capacitors use two plastic films as the dielectric. Each film is covered with a thin metal foil, mostly aluminium, to form the electrodes. The advantage of this construction is the ease of connecting the metal foil electrodes, along with an excellent current pulse strength.

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

The plastic films used as the dielectric for film capacitors are polypropylene (PP), polyester (PET), polyphenylene sulfide (PPS), polyethylene naphthalate (PEN), and polytetrafluoroethylene (PTFE). Polypropylene has a market share of about 50% and polyester with about 40% are the most used film materials. The other 10% use the other materials, including PPS and paper, with roughly 3% each.

Characteristics of plastic film materials for film capacitors

		Film material, abbreviated codes
Film characteristics		PET	PEN	PPS	PP
Relative permittivity at 1 kHz		3.3	3.0	3.0	2.2
Minimum film thickness (μm)		0.7–0.9	0.9–1.4	1.2	2.4–3.0
Moisture absorption (%)		low	0.4	0.05	<0.1
Dielectric strength (V/μm)		580	500	470	650
Commercial realized voltage proof (V/μm)		280	300	220	400
DC voltage range (V)		50–1,000	16–250	16–100	40–2,000
Capacitance range		100 pF–22 μF	100 pF–1 μF	100 pF–0.47 μF	100 pF–10 μF
Application temperature range (°C)		−55 to +125 /+150	−55 to +150	−55 to +150	−55 to +105
C/C0 versus temperature range (%)		±5	±5	±1.5	±2.5
Dissipation factor (•10−4)
	at 1 kHz	50–200	42–80	2–15	0.5–5
	at 10 kHz	110–150	54–150	2.5–25	2–8
	at 100 kHz	170–300	120–300	12–60	2–25
	at 1 MHz	200–350	–	18–70	4–40
Time constant RInsul•C (s)	at 25 °C	≥10,000	≥10,000	≥10,000	≥100,000
	at 85 °C	1,000	1,000	1,000	10,000
Dielectric absorption (%)		0.2–0.5	1–1.2	0.05–0.1	0.01–0.1
Specific capacitance (nF•V/mm3)		400	250	140	50

Some film capacitors of special shapes and styles are used as capacitors for special applications, including RFI/EMI suppression capacitors for connection to the supply mains, also known as safety capacitors, snubber capacitors for very high surge currents, motor run capacitors and AC capacitors for motor-run applications.

• High pulse current load is the most important feature of film capacitors so many of the available styles have special terminations for high currents
• 
  Radial style (single ended) for through-hole solder mounting on printed circuit boards
• 
  SMD style for printed circuit board surface mounting, with metallized contacts on two opposite edges
• 
  Radial style with heavy-duty solder terminals for snubber applications and high surge pulse loads
• 
  Heavy-duty snubber capacitor with screw terminals

Power film capacitors

MKV power capacitor, double-sided metallized paper (field-free mechanical carrier of the electrodes), polypropylene film (dielectric), windings impregnated with insulating oil

A related type is the power film capacitor. The materials and construction techniques used for large power film capacitors mostly are similar to those of ordinary film capacitors. However, capacitors with high to very high power ratings for applications in power systems and electrical installations are often classified separately, for historical reasons. The standardization of ordinary film capacitors is oriented on electrical and mechanical parameters. The standardization of power capacitors by contrast emphasizes the safety of personnel and equipment, as given by the local regulating authority.

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 blurred. Historically, the boundary between these two families was approximately at a reactive power of 200 volt-amperes.

Film power capacitors mostly use polypropylene film as the dielectric. Other types include metallized paper capacitors (MP capacitors) and mixed dielectric film capacitors with polypropylene dielectrics. MP capacitors serve for cost applications and as field-free carrier electrodes (soggy foil capacitors) for high AC or high current pulse loads. Windings can be filled with an insulating oil or with epoxy resin to reduce air bubbles, thereby preventing short circuits.

They find use as converters to change voltage, current or frequency, to store or deliver abruptly electric energy or to improve the power factor. The rated voltage range of these capacitors is from approximately 120 V AC (capacitive lighting ballasts) to 100 kV.

• Power film capacitors 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, Germany
• 
  75MVAR substation capacitor bank at 150 kV

Electrolytic capacitors

Main article: Electrolytic capacitor

Electrolytic capacitors diversification

Electrolytic capacitors have a metallic anode covered with an oxidized layer used as dielectric. The second electrode is a non-solid (wet) or solid electrolyte. Electrolytic capacitors are polarized. Three families are available, categorized according to their dielectric.

• Aluminum electrolytic capacitors with aluminum oxide as dielectric
• Tantalum electrolytic capacitors with tantalum pentoxide as dielectric
• Niobium electrolytic capacitors with niobium pentoxide as dielectric.

The anode is highly roughened to increase the surface area. This and the relatively high permittivity of the oxide layer gives these capacitors very high capacitance per unit volume compared with film- or ceramic capacitors.

The permittivity of tantalum pentoxide is approximately three times higher than aluminium oxide, producing significantly smaller components. However, permittivity determines only the dimensions. Electrical parameters, especially conductivity, are established by the electrolyte's material and composition. Three general types of electrolytes are used:

• non solid (wet, liquid)—conductivity approximately 10 mS/cm and are the lowest cost
• solid manganese oxide—conductivity approximately 100 mS/cm offer high quality and stability
• solid conductive polymer (Polypyrrole or PEDOT:PSS)—conductivity approximately 100...500 S/cm, offer ESR values as low as <10 mΩ

Internal losses of electrolytic capacitors, prevailing used for decoupling and buffering applications, are determined by the kind of electrolyte.

Benchmarks of the different types of electrolytic capacitors

Anode material	Electrolyte	Capacitance range (μF)	Max. rated voltage at 85 °C (V)	Upper categorie temperature (°C)	Specific ripple current (mA/mm3) 1)
Aluminum (roughened foil)	non solid, e.g. Ethylene glycol, DMF, DMA, GBL	0.1–2,700,000	600	150	0.05–2.0
	solid, Manganese dioxide (MnO2	0.1–1,500	40	175	0.5–2.5
	solid conductive polymer (e.g. PEDOT:PSS)	10–1,500	250	125	10–30
Tantalum (roughened foil)	non solid Sulfuric acid	0.1–1,000	630	125	–
Tantalum (sintered)	non solid sulfuric acid	0.1–15,000	150	200	–
	solid Manganese dioxide (MnO2	0.1–3,300	125	150	1.5–15
	solid conductive polymer (e.g. PEDOT:PSS)	10–1,500	35	125	10–30
Niobium or niobium oxide (sintered)	solid Manganese dioxide (MnO2	1–1,500	10	125	5–20
1) Ripple current at 100 kHz and 85 °C / volumen (nominal dimensions)

The large capacitance per unit volume of electrolytic capacitors make them valuable in relatively high-current and low-frequency electrical circuits, e.g. in power supply filters for decoupling unwanted AC components from DC power connections or as coupling capacitors in audio amplifiers, for passing or bypassing low-frequency signals and storing large amounts of energy. The relatively high capacitance value of an electrolytic capacitor combined with the very low ESR of the polymer electrolyte of polymer capacitors, especially in SMD styles, makes them a competitor to MLC chip capacitors in personal computer power supplies.

Bipolar aluminum electrolytic capacitors (also called Non-Polarized capacitors) contain two anodized aluminum foils, behaving like two capacitors connected in series opposition.

Electrolytic capacitors for special applications include motor start capacitors, flashlight capacitors and audio frequency capacitors.

• Schematic representation
• 
  Schematic representation of the structure of a wound aluminum electrolytic capacitor with non solid (liquid) electrolyte
• 
  Schematic representation of the structure of a sintered tantalum electrolytic capacitor with solid electrolyte and the cathode contacting layers

• Aluminum, tantalum and niobium electrolytic capacitors
• 
  Axial, radial (single ended) and V-chip styles of aluminum electrolytic capacitors
• 
  Snap-in style of aluminum electrolytic capacitors for power applications
• 
  SMD style for surface mounting of aluminum electrolytic capacitors with polymer electrolyte
• 
  Tantalum electrolytic chip capacitors for surface mounting

Supercapacitors

Main article: Supercapacitor

Hierarchical classification of supercapacitors and related types

Ragone chart showing power density vs. energy density of various capacitors and batteries