What is a polymer electrolytic capacitor?

Polymer electrolytic capacitors can be used to replace aluminum electrolytic capacitors. There are two basic types of polymer capacitors. Aluminum polymer electrolytic capacitors have a solid dielectric, which means they cannot dry out. Hybrid polymer electrolytic capacitors have both a solid and a liquid dielectric. They combine the advantages of aluminum electrolytics and aluminum polymer (solid dielectric) electrolytic capacitors.

Both polymer capacitors offer advantages over the aluminum electrolytic capacitor, including lower equivalent series resistance (ESR), which reduces self-heating effects, and a higher maximum frequency of operation. However, they are more expensive than aluminum electrolytic capacitors. Like aluminum electrolytic capacitors, polymer capacitors are polarized.

Operation of polymer electrolytic capacitors

Both solid and hybrid polymer-based capacitors offer a performance edge over conventional aluminum electrolytic (including ceramic and film capacitors) when it comes to electrical characteristics, stability, longevity, reliability, safety and life cycle cost.

Polymer capacitors come in four main varieties, including the hybrid. Each type has different electrolytic and electrode materials, packaging and application targets. The various polymer and hybrid capacitors have distinct advantages in terms of their ideal voltages, frequency characteristics, environmental conditions and other application requirements.

Layered polymer aluminum capacitors

Layered polymer aluminum capacitors use conductive polymer as the electrolyte and have an aluminum cathode. Depending on the specific model, these capacitors cover a voltage range from 2 V to 35 V and offer capacitances between 2.2 µF and 560 µF. The distinguishing electrical characteristic of these polymer capacitors is their extremely low equivalent series resistance (ESR). For example, some of these polymer capacitors have ESR values as low as 3 mΩ.

Illustration of a layered polymer aluminum capacitors. (Image: Panasonic.)

Wound polymer aluminum capacitors

Wound polymer aluminum capacitors are also based on conductive polymers and aluminum, but they have a wound foil structure. Wound polymer capacitors cover a wider range of voltages and capacitance values than other types of polymer capacitors. Voltages extend from 2.5 V to 100 V, while capacitances run from 3.3 µF to 2700 µF. Like layered polymer capacitors, the wound style has extremely low ESR values. Some have ESR values below 5 mΩ.

Illustration of wound polymer aluminum capacitors. (Image: Panasonic.)

Polymer tantalum capacitors

Polymer tantalum capacitors employ a conductive polymer as the electrolyte and have a tantalum cathode. They span voltages from 2 V to 35 V and capacitances from 3.9 µF  to 1500 µF. They also have a low ESR, with some capacitors exhibiting ESR values as low as 5 mΩ.

Illustration of polymer tantalum capacitors. (Image: Panasonic.)

Polymer hybrid aluminum capacitors

Polymer hybrid aluminum capacitors use a combination of a liquid and conductive polymer to serve as the electrolyte, and aluminum as the cathode. This approach is the best of both worlds; the polymer offers high conductivity and a correspondingly low ESR. The liquid portion of the electrolyte, meanwhile, can withstand high voltages and provide higher capacitance ratings due to its large effective surface area. The hybrid capacitors offer a voltage range from 25 V to 80 V and capacitances between 10 µF and 330 µF. With 20 mΩ to 120 mΩ ESR values, hybrids are larger than other types of polymer capacitors.

Illustration of polymer hybrid aluminum capacitors. (Image: Panasonic.)

Advantages of polymer capacitors

There are many useful benefits of polymer capacitors, including desirable specs such as:

Frequency limits

All four types of polymer capacitors have a wide operating frequency range that allows them to be used at frequencies up to 500 kHz. In contrast, aluminum electrolytic capacitors are limited to operating frequencies up to only 100 kHz. Their low ESR values mean the impedance near the series resonant point is low, which also improves the usable frequency range provided by polymer capacitors.

Dissipation Factor (DF) and Q

Polymer electrolytes have a lower dissipation factor (DF) at lower frequencies. The dissipation factor is the ratio of the ESR to the capacitive reactance at the test frequency, and is the reciprocal of the quality factor (Q).

Ripple current rating and temperature

Polymer capacitors allow higher ripple current ratings up to six times greater than those of an aluminum electrolytic capacitor. Polymer capacitors also offer stability over temperature. Their capacitance stays very close to its 25^o C value. In contrast, an aluminum electrolytic capacitor can have its capacitance change up to 30% over its temperature range.

Lifetime, useful life, and endurance

Solid polymer capacitors have a long life that increases by a factor of 10 for each reduction of operating temperature by 20^o C. The life of an aluminum electrolytic capacitor will only increase by a factor of four for that same 20^o C reduction of operating temperature. In addition to lifetime, manufacturers also specify useful life and endurance, though note that the definition of endurance is not uniform among manufacturers.

Applications of polymer capacitors

Polymer capacitors can be used as substitutes or replacements for aluminum electrolytic capacitors. Consequently, they may be used in DC links for input and output filtering, DC power supply filtering (smoothing), DC/DC converters and filtering with lower-frequency switching speeds, energy storage, low-frequency bypassing and coupling in amplifiers with a signal chain operating under 500 kHz.

Due to their low ESR, polymer capacitors are used in applications which allow for a large ripple current such as buck, boost and buck-boost DC/DC converters, which hold the voltage at the capacitor relatively constant but produce a high ripple current. Using a polymer capacitor with low ESR is preferable in these cases, both to improve power efficiency and to increase safety in cases of overload and overheating.

Comparison of aluminum electrolytic and polymer hybrid aluminum electrolytic capacitors

In the high-frequency band, an aluminum electrolytic capacitor can be replaced with hybrid electrolytic capacitor with less capacitance.

Frequency characteristics of aluminum versus hybrid capacitors. (Image: Panasonic.)

A hybrid electrolytic capacitor’s ESR is stable from low temperatures to high temperatures, while aluminum electrolytic capacitors are very sensitive to temperature.

Temperature characteristics of aluminum versus hybrid capacitors. (Image: Panasonic.)

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What is a paper capacitor?

Paper capacitors use paper as their dielectric. These are non-polarized capacitors that are often selected for high voltage and high current applications. They are used in power systems as coupling, decoupling and bypass capacitors and as energy reservoirs in automotive audio power amplifiers.

For an explanation of these terms, read: The engineer’s capacitor glossary: All terms and acronyms defined.

Operation of paper capacitors

Paper capacitors have two conductive plates separated by a paper dielectric. These electronic components are typically impregnated with wax or oil to eliminate air spaces between the cellulose fibers. Some are rated to operate 50,000 hours, which at 24/7 operation equates to about 5.7 years.

Illustration of a paper capacitor manufactured by Kemet. (Image: Kemet.)

Capacitance values for paper capacitors range from about 0.001 µF to 10 µF, and maximum voltage ratings are less than 2000 V. These capacitors are susceptible to moisture, which can reduce their breakdown voltage. Metalized paper capacitors are available.

Applications of paper capacitors

Paper capacitor technology is considered vintage. Old radios and vacuum tube guitar amplifiers often used paper capacitors.

Alternatives to paper capacitors

Newer technology metal film capacitors or ceramic capacitors are superior alternatives to paper capacitors.

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What is a niobium electrolytic capacitor?

Niobium electrolytic capacitors are one of a few types of electrolytic capacitors you should consider for your electronic circuit. Aluminum electrolytic capacitors offer great volumetric efficiency (capacitance compared to size), are relatively inexpensive and are readily available. Tantalum electrolytic capacitors offer some improvements over aluminum electrolytic capacitors, but are more expensive and can have longer lead times. When tantalum capacitors are not available, niobium capacitors are the next choice.

Niobium is a sister metal to tantalum and shares many chemical characteristics. Niobium electrolytic capacitors offer a few disadvantages and several advantages when compared to tantalum electrolytic capacitors.

Illustration of SMD niobium electrolytic capacitors. (Image: Wikimedia / Elcap.)

Niobium capacitor technology appeared on the market in 2002 when Vishay offered early sampling and announced preproduction. When compared to tantalum, niobium is limited in its maximum rated voltage, lower volumetric efficiency, incompatibility with low equivalent series resistance (ESR) polymer electrodes and a limited range of values. Several vendors keep this technology as a niche line of the tantalum capacitor industry today. But some advantages of niobium electrolytic capacitors—including safety, reliability and lower cost—are worth closer examination and consideration.

Comparison of niobium and tantalum electrolytic capacitors

Tantalum capacitors are often used in power supply filter circuits where a large bulk capacitance is needed. Power supply filter applications can result in increases in peak voltage or current. A current surge, a large ripple current or an overvoltage spike can cause a tantalum capacitor to fail. When its dielectric breaks down, the heat generated by a large current flowing through a defect site will produce dielectric destruction. There will be a chemical reaction that can produce flaming.

The niobium capacitor design is very different. During a dielectric breakdown event, the temperature rise is significantly lower than for tantalum. The niobium oxide layer tends to grow at elevated temperatures resulting in a “self-arresting” feature. As a result, niobium capacitors reduce the ignition failure mode by 95% compared to tantalum capacitors. For this reason, niobium oxide capacitors are regarded to be one of the safest available capacitor technologies.

The below table compares several characteristics of niobium and tantalum oxide layers.

Applications of niobium electrolytic capacitors

Niobium electrolytic capacitors are regarded as substitutes for tantalum electrolytic capacitors, and their applications are similar. However, in low voltage (3.3 V or 5 V) applications, niobium capacitors are preferred because of their “non-burn” failure mode. In contrast, tantalum capacitors can become short circuits when they fail, which can produce arcs or flames.

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What is a multi-layer organic (MLO) capacitor?

Multilayer organic (MLO) capacitors are polymer-based capacitors that use high-conductivity interconnects in a multi-layer fashion. They are constructed like multilayer ceramic chip (MLCC) capacitors. As for the name, in chemistry a material is classified as organic if it contains at least one carbon atom, regardless of its source.

Depiction of a multilayer organic capacitor. (Image: Kyocera AVX.)

MLO capacitor technology is relatively new, as Kyocera AVX debuted the MLO capacitor in 2012. The MLO capacitor is a microwave device and satisfies a niche market.

“These new capacitors represent a paradigm shift from traditional ceramic and thin film passive SMD components,” writes Kyocera AVX in a general information sheet on its patented technology.

How MLO capacitors work

MLO capacitors exhibit low losses over a wide frequency range, and therefore have a high quality factor (Q) and a low dissipation factor (DF). They have a low equivalent series resistance (ESR), a high series resonant frequency (SRF), a tight tolerance, and they possess an extremely low dielectric absorption (on the order of 0.0015%). These devices can operate at microwave (GHz) frequencies.

The capacitance of MLO capacitors ranges from 0.1 to 2.5 pF ± 0.02 pF with voltages ratings from 50 to 500 VDC. They are available in a 0603-case size with a 100% tin finish. MLO capacitors features stable NP0 characteristics and an operating temperature range of -55 °C to +125 °C. MLO capacitors are expansion matched to FR4 printed circuit board material for improved reliability.

Applications of multilayer organic capacitors

Target applications for MLO capacitors include RF power amplifiers, low-noise amplifiers, filter networks, magnetic resonance imaging (MRI) systems, satellites, GPS systems, military and commercial radar, and instrumentation.

Their low dielectric absorption (DA) makes MLO capacitors ideal for sample and hold (S/H) circuits.

Alternatives to MLO capacitors

MLO capacitor technology is new and unique, and at present there are very few other comparable options.

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What is a mica capacitor?

As a dielectric, mica provides capacitors with stable, highly accurate capacitance values. Mica capacitors exhibit low losses, which means they have a high quality factor (Q) and low dissipation factor (DF).

For an explanation of these terms, read: The engineer’s capacitor glossary: All terms and acronyms defined.

Mica capacitors can withstand high voltages, operate at high temperatures and have low leakage current. Because mica capacitors have a very small inductive characteristic and low losses, they are often used in radio frequency (RF) circuits. Silver is used to form mica capacitor plates. Other metals, like copper and aluminum, have been used, but do not perform as well.

Capacitance and voltage rating of silver mica capacitors

Silver mica capacitors offer tight tolerances from ±0.05% to ±5%. It is difficult to manufacture silver mica capacitors with large capacitance values, and they run from 0.5 pF to a few nanofarads. Typical capacitance values range from 1 pF to 91,000 pF, while voltage ratings range from 50 V to 2500 V.

Silver mica capacitors with a capacitance of 1 nF. (Image: Wikimedia / Mataresephotos.)

The design of a silver mica capacitor does not permit internal air gaps. The entire assembly is sealed hermetically from the environment. Consequently, it is protected from the effects of air and humidity. This promotes long term stability. The average temperature coefficient for silver mica capacitors is ±50 ppm/^°C.

Applications of mica capacitors

Silver mica capacitors are used in high-frequency RF tuned circuits such as those found in filters, oscillators and power amplifiers. In filters, the tolerances and low losses (high Qs) of silver mica capacitors result in precise and predictable tuned-circuit performance. These same benefits promote excellent RF oscillator operation in terms of frequency stability and very small drift with temperature. Their high voltage rating makes their application in RF power amplifier coupling and decoupling attractive despite their relatively high cost. Silver mica capacitors can have very large dv/dt ratings (e.g., 100, 000 V/µs) which promotes their use as snubbers in pulse applications.

Alternatives to silver mica capacitors

As passive electrical components go, silver mica capacitors tend to be expensive. In low power RF applications, a good replacement for silver mica capacitors is ceramic capacitors. If small capacitance tolerances, low losses and a low temperature coefficient are needed, Class I ceramic capacitors can be used. These ceramic capacitors have characteristics like silver mica capacitors, but at a fraction of the price.

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What is a glass capacitor?

Glass capacitors use ultra-thin, high-purity glass as their dielectric. The glass dielectric is extremely stable, has negligible leakage and works well over a wide frequency range. One big advantage of glass capacitors is their ability to perform at high temperatures (up to a maximum of 200^o C).

Glass quartz dielectric trimmer capacitors. (Image: Knowles Precision Devices.)

In nearly all aspects glass capacitors are the most environmentally resistant of capacitor types. Glass capacitors are also resistant to the effects of nuclear radiation.

How glass capacitors work

The capacitance range of glass capacitors is limited compared to other capacitor types, ranging from a few picofarads to a few nanofarads. However, large voltage values are available, and voltage does not affect the capacitance. Glass capacitors have a zero-voltage coefficient over an operating range from -75° C to 200° C. Glass capacitors are like multilayer ceramic chip (MLCC) capacitors in that they are manufactured in a stacked form.

Glass capacitor packaging is somewhat bulky compared to other technologies. However, they demonstrate very high stability such as no corrosion or degradation and no micro fractures. Glass capacitors demonstrate a zero-aging rate, high quality factor (Q) and large RF current capability. The dielectric absorption is very low, from 0.01% to 0.5%. Glass capacitors have a typical temperature coefficient of ±50 ppm/°C.

Applications of glass capacitors

Given the high temperature ability of glass capacitors, they serve many applications in the areas of aerospace, electric and hybrid electric vehicles, DC power transmission and pulsed-power systems.

Alternatives to glass capacitors

Mica capacitors are like glass capacitors in terms of capacitance values, voltage and temperature ratings. Mica capacitors have a high Q and perform well in RF applications. Axial leaded mica capacitors can have capacitance values as large as 3 µF and voltage ratings as large as 5 kV. While 200° C operation is possible for mica capacitors, 175° C is the usual limit.

The below table compares glass and mica capacitors.

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The engineer’s complete guide to capacitors explains it all. Consider it your go-to reference on the many different types of capacitors (from the popular electrolytic capacitor to the specialized multilayer organic (MLO) capacitor) and how to select the right capacitor for all the most common circuit applications.

If you’re unfamiliar with any term used throughout this guide, you’ll find a clear explanation in this alphabetized glossary.

Aluminum electrolytic capacitors

Aluminum electrolytic capacitors are polarized. They have an anode electrode (+) made of pure aluminum foil, an electrolyte that acts as the cathode and a thin insulating layer of aluminum oxide that acts as the dielectric. Electrolytic capacitors have a higher charge capability (Q = CV) per unit volume than ceramic capacitors or film capacitors. Rather than using charge, the capacitance voltage product (CV) is used. Applications of aluminum electrolytic capacitors include audio, automotive, bypass, decoupling, filtering and motor starting capacitors.

Aluminum polymer capacitors

Aluminum polymer (electrolytic) capacitors are a polarized capacitor type that are based on an aluminum electrode with an aluminum oxide dielectric. These capacitors use a conductive polymer (solid) material instead of the traditional fluid electrolytes employed in aluminum electrolytic capacitors. This gives aluminum polymer capacitors a longer shelf life and a reduced ESR. Compared to aluminum electrolytic capacitors, aluminum polymer capacitors have an improved electrical performance, but with a greater sensitivity to operating environment. These capacitors are more expensive than aluminum electrolytic capacitors.

Aluminum polymer hybrid capacitors

Aluminum polymer hybrid capacitors use a combination of a liquid and conductive polymer to serve as the electrolyte and aluminum as the cathode. This approach is the best of both worlds: The polymer offers high conductivity and a correspondingly low ESR. The liquid portion of the electrolyte, meanwhile, can withstand high voltages and provide higher capacitance ratings due to its large effective surface area.

Buffer capacitor

A buffer capacitor is placed in parallel with electrical contacts to provide arc suppression.

Bypass capacitor

A bypass capacitor often provides a low-impedance path to ground. It can be used to keep noise out of a load. It can also be used to shunt a signal around a gain-setting resistor to modify a circuit’s voltage gain.

Capacitor networks/arrays

Capacitor networks, or arrays, contain two or more capacitors. The capacitors may be isolated from one another or have one terminal tied to a common bus. These devices can have either single or multiple capacitor values. Dielectric materials include ceramic, metallized polymer or metallized polypropylene. Capacitance values range from 10 pF to 80 µF with tolerances typically ranging from ±5% to ±20%. Maximum voltage ratings range from 6.3 V to 440 V. Capacitor arrays are available in SMT, through-hole and chassis-mount.

Coupling capacitor

A coupling capacitor is used to provide a low-impedance path to connect a signal from one point to another.

DC leakage current (DCL)

The DC leakage current specified at a given voltage and temperature.

Decoupling capacitor

Decoupling capacitors are usually connected between the DC power supply (e.g., V_CC) and ground.

Dielectrics and more

Capacitors are usually classified primarily by their dielectric material, but there can be more. For example, ceramic capacitors use various ceramic materials as their dielectric. However, there are two major classifications: Class 1 and Class 2. Further, there are subgroups under Class 2: X5R, X6R and X7R. Aluminum electrolytic capacitors are formed using aluminum electrodes and an electrolyte solution.

One common distinction is between electrolytic and non-electrolytic capacitor types. Electrolytic capacitors use a dielectric material that is formed in-place electrochemically by oxidizing the surface of the electrode material, whereas non-electrolytic (often called “electrostatic” capacitors) use dielectric materials that are generally formed through various mechanical processes and are not a chemical derivative of the electrode material itself.

Electrolytic capacitors offer high capacitance per unit volume, are polarized, low cost, high-loss and exhibit poor parameter stability. In contrast, non-electrolytic device types tend to be bulky for their ratings, are non-polar, relatively expensive, low-loss and (with exceptions) exhibit fair to excellent parameter stability.

Dielectric Withstand Voltage (DWV)

This is the maximum voltage that can be applied to a capacitor without producing voltage breakdown in its dielectric.

Dissipation Factor (DF)

The dissipation factor is associated with capacitors. It is the reciprocal of Q (DF = 1/Q).

Electric Double Layer Capacitors (EDLC)

This is a more descriptive name for supercapacitors.

Electrolytic capacitors

See “Dielectrics and more”.

Electrostatic capacitors

See “Dielectrics and more”.

EMI/EMC capacitor

These are capacitors used to prevent electromagnetic interference (EMI) and to implement electromagnetic control (EMC). Feedthrough capacitors are used commonly for this purpose. Ceramic and film capacitors are the fundamental types used for Class X and Class Y capacitors. Ceramic capacitors provide higher capacitor values in a smaller volume.

Energy storage capacitor

Capacitors store potential energy in an electric field. This is true in low-energy signal processing applications as well as large-energy backup systems in power grids and in renewable energy systems.

Equivalent Series Inductance (ESL)

This represents any inductance associated with the capacitor lead connections and the dielectric. See image for “Equivalent Series Resistance (ESR)”.

Equivalent Series Resistance (ESR)

This represents the resistive losses in the capacitor lead connections and plates. It is also taken to be the minimum resistance when the series resonant circuit (ESR, C_nominal, and ESL) is at resonance. See also “Equivalent Series Inductance (ESL)”.

Capacitor model (equivalent circuit).

Feedback capacitors

Capacitors are used to form negative feedback in op amp integrators. Feedback capacitors are also incorporated to limit the frequency of an op amp amplifier to a value below that determined by its gain-bandwidth product. In both cases the capacitors should have low leakage current and have adequate precision.

Filtering

Low-pass, high-pass, band-pass and band-reject filters can be either implemented using only passive or passive devices with active devices (such as op amps). These realizations usually involve relatively low energy levels. In power supply applications, large-valued filter capacitors are used to smooth out the pulsating DC produced by the rectifier stage. (See “Power conditioning”.) They are also found across the inputs and outputs of DC links.

Frequency compensation capacitors

Capacitors in conjunction with resistors are used to modify the phase shift and/or amplitude of a transfer function as a function of frequency to provide an adequate phase margin. The phase margin controls the stability of a system (for example, freedom from oscillation) and establishes its dynamic response (for example, overshoot, undershoot and settling time).

Gain-bandwidth product

The gain-bandwidth product is a constant (f_T) when an amplifier system is rolling off with frequency at a slope of -20dB/decade. In an amplifier circuit the closed-loop corner frequency (f_H) is f_H = f_T/A_v where A_V is the closed-loop voltage gain.

Layered polymer aluminum capacitors

These capacitors use conductive polymer as the electrolyte and have an aluminum cathode. Some polymer capacitors have ESR values as low as 3 mΩ.

Leakage resistance

This is a large resistance in parallel with the nominal capacitance. Ideally it is infinite. See the capacitor model in “Equivalent Series Resistance (ESR)”.

Lifetime

Many capacitors have strong wear mechanisms that limit their useful life. A lifetime specification provides an indication of a capacitor’s expected service life under specified operating conditions. Definitions of service life vary. One common definition is the length of service under specified conditions (which usually are near rated maximum values) within which 50% of fielded devices can be expected to fail. Some service life specifications are more stringent, while others may be more lenient.

Microphony

Microphony is the tendency to respond to acoustical vibrations (20 Hz to 20 kHz) or to convert vibrations to acoustical noise. Ceramic capacitors exhibit this effect.

Motor starting capacitors

In the case of a single-phase source induction motor, a rotating magnetic field is not produced inherently. Consequently, the motor contains a start winding in addition to its main winding. An initial rotating magnetic field is developed using the start winding with a series-connected start capacitor. The current following through the start winding (with the capacitor) produces a 90-degree phase angle difference (ideally) compared to the current flowing through the main winding.

Due to this phase angle difference, a resultant rotating stator magnetic field is produced which will rotate the shaft in the desired direction. A centrifugal switch is attached in series with the start capacitor. When the motor reaches sufficient speed, the centrifugal switch opens to disconnect the capacitor and the start winding. The motor starting capacitor is usually a non-polarized electrolytic (see “Dielectrics and more”.) In single-phase motor applications, capacitors with values above 70 µF are starting capacitors.

Niobium oxide capacitors

Niobium oxide capacitors are a type of polarized (electrolytic) capacitor incorporating oxides of niobium as anode and dielectric materials alongside a manganese oxide cathode system. Developed in response to a tantalum shortage, their properties and behaviors are like conventional tantalum manganese dioxide (Ta-MnO₂) capacitors, with a much narrower range of available capacitance and voltage values (limited to less than 10 V) and a greatly reduced likelihood of device failure resulting in ignition. Consequently, niobium oxide capacitors are regarded to be the safest capacitor technology.

Power factor correction capacitors

A typical AC power system can be modeled using a lumped resistor, a lumped inductor and a capacitor. These elements will be in parallel across the AC voltage source. Capacitive current is phase shifted 180^o from inductive current. Consequently, capacitive current can be used to cancel inductive current. The goal is to make the equivalent AC power load as purely resistive as possible. Modern polypropylene film power capacitors are state of the art for power factor correction. High-temperature operation may require glass capacitors.

Power conditioning capacitors

Power conditioning capacitors are connected in parallel with the DC power supply. The power conditioning capacitors hold the DC power supply level during brief AC power line interruptions and insure the minimum instantaneous voltage is large enough to avoid voltage regulator dropout.

Pulsed power capacitors

Pulsed power capacitors are energy discharge capacitors designed to provide high peak discharge current, high energy density, low inductance and low equivalent series resistance.

Quality Factor (Q)

The definition of the quality factor (Q) of a capacitor is given below:

Definition of quality factor (Q).

Relative permittivity

Relative permittivity measures a material’s capability to permit the establishment of an electric field, relative to that of a vacuum. It is also known as the material’s dielectric constant.

Resonant circuits capacitors

Resonant (tuned) circuits usually provide filtering and frequency selection in RF applications. These applications require capacitors that provide precision and stability. Class 1 (NPO/COG) ceramic capacitors and silver mica capacitors are often used.

Ripple current rating

The ripple current rating of a capacitor indicates the maximum AC current the capacitor should experience. Ripple current results in self-heating due to the capacitor’s ohmic and dielectric losses. The amount of current flow a given device can tolerate is finite and is influenced by environmental conditions.

Run capacitors

In single-phase motor applications, capacitors with values above 70 µF are starting capacitors. Run capacitors (typically 3 to 70 µF) are designed for continuous duty and are energized the entire time the motor is running.

Safety capacitors

Safety capacitors are placed across the AC power line to suppress electromagnetic interference (EMI) and high-frequency radio frequency interference (RFI). Should they fail, these capacitors are designed to fail in a safe mode, which means their failure will not lead to personal injury or equipment damage.

Safety rating

Safety capacitors are given an alphanumeric safety rating, such as X1, X2, Y1 and Y2, according to regulatory standards like IEC 60384-14. X certified devices are not expected to pose a shock hazard. X class capacitors connect line to line (for example, hot to neutral) and are designed to fail as short circuits, which causes the overcurrent protective device to open. Y class capacitors are certified for applications that may pose a shock hazard. They are connected from line to ground (for example, hot to ground or neutral to ground) and fail as open circuits.

The number in a rating like X1 indicates a level of tolerance to surge voltages, as specified in the applicable regulatory standard. Devices may also carry multiple safety ratings, indicating their certification for use in different circumstances. For example, a capacitor with an X1Y2 safety rating may be used in applications requiring an X1 rating as well as those requiring a Y2 rating.

Self-healing

Self-healing is the ability of a metallized capacitor to clear a fault area where a momentary short occurs due to dielectric breakdown under an over-voltage condition.

Series Resonant Frequency (SRF)

The model for a capacitor includes its nominal capacitance, equivalent series inductance (ESL) and its equivalent series resistance (ESR). See the image in “Equivalent Series Resistance (ESR)”.

The series resonant frequency (SRF) occurs when the impedance is at its minimum ESR value and the capacitive reactance is canceled by the ESL. The series resonant frequency is defined as:

Silicon capacitors

Silicon (and thin-film) capacitors are fabricated using integrated circuit and solid-state device techniques. The extreme precision and quality control methods produce capacitors that are nearly ideal in terms of parameter stability. Silicon capacitors compete with ceramic capacitors but tend to be more expensive.

Snubber capacitors

A simple snubber circuit consists of a capacitor in series with a small-valued resistor. Some also incorporate a switching diode to minimize losses. The purpose of the snubber circuit is to slow the rate of rise in the voltage (dv/dt) across a solid-state switch. Snubbers are used to absorb energy to eliminate the voltage spikes and ringing caused by a switch opening under inductive loads. Polypropylene film capacitors are self-healing and often used in snubber circuits.

Supercapacitors

Supercapacitors (also called ultra capacitors or electric double layer capacitors) are specially designed capacitors that possess extremely large values of capacitance (such as 12,000 F). They can be recharged very quickly and are used primarily for energy storage.

Tantalum polymer capacitors

Tantalum polymer capacitors are dry tantalum polarized capacitors. A conductive polymer anode material is used instead of the manganese dioxide found in other dry tantalum devices. While more expensive, these outperform traditional tantalum electrolytic capacitors and have a more benign failure mode.

Temperature range

The operating temperature range is the range of temperatures for which a capacitor has been qualified for use. The storage temperature range is the range of temperatures for which a capacitor in a non-active state will not experience damage or irreversible parameter shifts. Low temperatures below the specified storage temperature range could result in mechanical damage and ultimate device failure.

Thin-film capacitors

Thin-film capacitors are two-pad SMT devices available in package sizes ranging from 0201 to 1210. The devices are composed of thin-film layers deposited on a substrate and separated by a dielectric. Available capacitance values range from 0.05 pF to 1500 pF. Voltage ratings run from 2.5 V to 100 V. Tolerances can be as low as ±0.01 pF for small-capacitance units and as large as ±20% for larger capacitance values.

Tolerance

A capacitor’s tolerance describes the limits of deviation from its nominal capacitance value under specified test conditions—particularly the AC test voltage and its frequency. In general, tolerance figures specify the steady-state deviation from the nominal value due to variability in manufacturing. The deviation from the nominal value is also affected by device operation over its specified operating temperature range. It should be noted that test conditions (temperature, frequency, amplitude and DC bias value of test voltage, among others) frequently have a strong influence on observed device parameters.

Voltage ratings

Capacitor voltage ratings indicate the maximum voltage that can be applied. The test conditions should be reviewed carefully. The context of the rating is significant; in some cases, it may indicate the maximum safe working voltage, while in others it may be like a semiconductor’s absolute maximum rating and an appropriate de-rating factor should be applied.

Volumetric efficiency

A measure of a capacitor’s capacitance relative to its physical size.

Wound polymer aluminum capacitors

These capacitors are also based on conductive polymers and aluminum, but they have a wound foil structure. Wound polymer capacitors cover a wider range of voltages and capacitance values than other types of polymer capacitors. Some have ESR values below 5 mΩ.

WVDC

Working DC voltage, usually applied to polarized (electrolytic) capacitors.

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What is a film capacitor?

Film capacitors are used in many applications because of their stability, low inductance and low cost. They can also tolerate overvoltage surges. There are several types of film capacitors including polyester film, metallized film, polypropylene film, polycarbonate film, polytetrafluoroethylene (PTFE, sometimes branded as Teflon) film and polystyrene film.

How film capacitors work

Like all capacitors, metallized film capacitors incorporate metal plates separated by a dielectric. Film capacitors are also known as plastic film, polymer film, or film dielectric capacitors. Film capacitors are inexpensive and come with a nearly limitless shelf life. The film capacitor uses a thin dielectric material with the other side of the capacitor metalized. Depending on the application, the film capacitor is rolled into thin films. The general voltage range of these capacitors is from 50 V to 2 kV.

A variety of plastic film capacitors. (Image: Wikimedia/Elcap.)

Metallized film capacitors are not affected strongly by DC bias. Their volumetric efficiency is not as great as that for multilayer ceramic chip (MLCC) capacitors or electrolytic capacitors. These capacitors (as well as ceramics) are used in safety applications for EMI/RFI reduction and safe failure modes.

Capacitor self-healing

Self-healing is the ability of a metallized capacitor to clear a fault area where a momentary short occurs due to dielectric breakdown under an over-voltage event.

Metallized film capacitors have self-healing properties, while discrete foil electrode capacitors do not. Polypropylene film/foil capacitors are commonly used as snubber capacitors in low pulse applications. In comparison, polypropylene metallized film capacitors and double-sided metallized film capacitors have a self-healing property, and they are suitable for use in low pulse and medium pulse applications. These two types of capacitors are suitable for protecting various switching devices including thyristors, FETs and IGBT modules.

Applications of film capacitors

Metallized film capacitors find their way into a wide variety of applications, with different dielectrics suited to different use cases as detailed in the table below:

Film capacitors are used in electromagnetic interference (EMI) suppression and as safety capacitors (Classes X and Y). While ceramic capacitors offer better dv/dt capabilities, film capacitors are good (with a maximum value of 2200 V/µs) making them suited for use in snubber circuits. Film capacitors also have low equivalent series resistance (ESR), low equivalent self-inductance (ESL) and can tolerate large peak currents. Snubber capacitors in series with a small resistor are placed across solid-state switching devices. Snubbers protect the devices from voltage spikes and reduce the rate of rise (dv/dt) across them. A large dv/dt across a solid-state switch can cause it to turn on and power control will be lost. Snubbers are used widely in power electronics applications.

In fluorescent light ballasts, film capacitors are used to provide power factor correction to counter the choke inductance. Lighting ballasts are employed to provide proper operating conditions (including startup) of fluorescent lights. Older ballasts used an inductor (choke) but provided a poor power factor. Current ballast designs employ switched power supplies that rely on film capacitors to improve the power factor.

Power film capacitors are used in radar, pulsed laser, defibrillator and x-ray equipment. Low-power applications of film capacitors include coupling, decoupling, bypassing and filtering. In high power applications, power film capacitors can be rated to handle thousands of volts.

Polystyrene is an important metal film capacitor. It has a low dielectric absorption (DA) characteristic which makes it a great choice for sample-and-hold and peak detector applications. Polycarbonate capacitors provide a wide temperature range of operation (-55 °C to 125 °C).

Alternatives to film capacitors

Film capacitors are interchangeable with Class 1 (NPO/COG) ceramic capacitors in some applications (see the comparison table in The engineer’s guide to ceramic capacitors). Power film capacitors are large and unique and cannot be replaced with ceramic devices.

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What is a feedthrough capacitor?

Feedthrough capacitors use ceramic as a dielectric but are designed in such a way they are not just “simple” capacitors. They also exhibit coaxial cable properties. A feedthrough capacitor is a ceramic tube coated with a metal layer, forming two “plates” with one in the inside and the other on the outside. It is called a feedthrough capacitor because the porcelain tube is empty.

A feedthrough capacitor. (Image: Tusonix.)

A feedthrough capacitor is essentially a three-terminal capacitor. However, when compared with an ordinary three-terminal capacitor, it has less ground inductance. It has virtually no lead inductance since it is mounted directly on a metal chassis panel. Further, the metal plate’s input and output terminals are isolated, which eliminates high-frequency coupling. The primary function of feedthrough capacitors is to eliminate electromagnetic interference (EMI).

Types of feedthrough filters

A feedthrough capacitor acts like a low-pass filter and is used to filter out EMI. It attenuates the EMI conducted on the power line(s) or on a signal input line. This reduces the possibility of external EMI disturbing proper equipment operation. Feedthrough capacitors can also be used to attenuate any EMI generated by that equipment on its output lines. This falls under the category of electromagnetic control (EMC).

Some feedthrough capacitors are used in assemblies that also include inductors. This permits the use of the various filter arrangements such C-type filters, LC-type filters, Pi-type filters and T-type filters (see below image).

Feedthrough capacitor filters. (Image: Author.)

It is important to note these are distributed capacitances and inductances as found in transmission lines. They are not discrete (lumped) reactances. Feedthrough capacitors can form a variety of RF filter configurations. These are sometimes called “feedthrough filters.” Because these tube-type filters have transmission line characteristics, no significant self-resonance will occur even as high as 10 GHz.

The C-type filter is a three-terminal device. It has input and output terminals, and its bushing makes a connection to ground. It is designed to attenuate high-frequency noise (EMI). Generally, these filters are used with high impedance sources and loads.

The LC- (and CL-), Pi-, and T-type filters serve as low-pass filters, but can also provide impedance matching. These feedthrough devices are classified by their number of reactive (energy-storage) elements. The C-type filter is first order, the LC-type filter is second order, and the Pi-type and T-type are third order. In the case of a low-pass filter, the roll off is -20n dB/decade, where n is the filter order. Consequently, a first order filter rolls off at -20 dB/decade, a second-order filter rolls of at -40 dB/decade, and a third-order filter rolls off at -60 dB/decade. As the roll off becomes steeper, the filter comes closer to being ideal.

LC-type filters are feedthrough filters which include an inductor to supplement the action of the capacitor. These filters are often used in circuits with low-impedance sources and high-impedance loads, and vice versa. The inductor is oriented to connect to the low-impedance source. (By convention, if the LC is reversed to CL, the inductor is then connected to the load.)

The Pi-type filter has three reactive elements. This filter interfaces well with low-impedance sources and low-impedance loads. Because it is of the third order, it provides better high frequency performance than the C-type and LC-type filters.

The T-type filter exhibits high impedance from either input end. Like the Pi-type filter, it is not used as widely.

Feedthrough capacitor filter examples. (Image: Tusonix.)

Applications of feedthrough capacitors

Feedthrough capacitor filters are used in commercial, military and space applications. They are applied to medical equipment, rocket and missile launch systems, and radar and communication systems.

In typical (discrete component) interference filters, the effective filtering range runs from a few kilohertz to tens of megahertz. In contrast, the required filtering range for radio frequency interference (RFI) is from a few kilohertz to a gigahertz or more. Ordinary capacitors cannot filter out RFI successfully. Real (regular) capacitors possess a series equivalent inductance and a series equivalent resistance. The capacitance and the equivalent inductance will produce a series resonant frequency. The impedance of the capacitor is at its minimum at the series resonant frequency. As the noise frequency increases further, the capacitor will have a net inductive reactance that increases with further increases in frequency. Its bypassing to ground will become less effective. Parasitic capacitance between the leads serves as a coupling path, which further reduces the filtering effect.

In contrast, a feedthrough capacitor provides superior high-frequency filtering. The feedthrough capacitor has a very small parasitic inductance, a very low bypass impedance, and (because of its isolation mounting) it eliminates coupling between its input and output.

Alternatives to feedthrough capacitors

For simple, noncritical filtering, discrete filter circuitry can be used. However, for harsh EMI environments or critical EMC requirements, it is better to use a feedthrough filter. Note that feedthrough filters are also available in SMD packages. Not all feedthrough filters must have bushings and be wall mounted. It is important to understand the EMI/EMC requirements and discuss them with applications engineers.

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What is a ceramic capacitor?

Ceramic capacitors are used widely. Ceramic capacitors are non-polarized and have a good frequency response because they offer a low equivalent series resistance (ESR) and a low equivalent series inductance (ESL). Small capacitance values can withstand voltages as large as 1 kV. Depending on temperature range, temperature drift and tolerance, ceramic capacitors have two active classes: Class 1 and Class 2.

A ceramic disc capacitor. (Image: Wikimedia / Elcap.)

Ceramic capacitors are available in disc packages with radial leads. Surface mount multilayer ceramic chip (MLCC) capacitors are very popular. The stacking of very thin layers permits MLCC capacitors to provide relatively large values of capacitance at lower voltages. For example, AVX offers a military CDR 25 style MLCC that can possess 0.470 µF maximum at 50 V or a maximum of 0.150 µF at 100 V. Bare leadless disc ceramic capacitors are available for microwave applications.

Illustration of a multilayer ceramic chip (MLCC) capacitor. (Image: Wikimedia / Elcap.)

Class 1 ceramic capacitors

Class 1 ceramic capacitors are accurate and provide temperature compensation inherently. They are the most stable in terms of temperature sensitivity and drift, and they have the lowest losses. Class 1 ceramic capacitors are well suited for resonant circuit applications where stability is critical or where a well-defined temperature coefficient is needed. Consequently, they are used in applications that require a measure of precision, like timers and oscillators. Temperature coefficients are expressed using notation like the following:

• N200 means a negative temperature coefficient of 200 ppm/^oC
• P100 means a positive temperature coefficient of 100 ppm/^oC
• NP0 means the temperature coefficient is 0 ppm/^oC

The Electronics Industry Alliance (EIA) uses the notation COG instead of NP0. Class 1 ceramic capacitors are NP0.

Class 2 ceramic capacitors

Class 2 ceramic capacitors have improved volumetric efficiency, meaning that larger values of capacitance are available with a relatively smaller physical size. However, their tolerance is wider and they are not as stable as Class 1 ceramic capacitors. The ceramic dielectric is characterized by a non-linear change in capacitance over its temperature range. These capacitors are usually selected for use in less critical coupling, decoupling, and bypass applications. They are very susceptible to aging effects.

Class 2 ceramic capacitors have several dielectric names. The more popular groups are X7R, X6R and X5R.

Class 2 ceramic capacitors exhibit microphony. The dielectric has a piezoelectric characteristic. If the capacitor experiences mechanical vibrations, they can be transformed into electrical signals—like a microphone. The reverse effect can also occur. The varying electric field between the plates will cause them to move like a loudspeaker, and this can generate an audible sound. Sensitive low-level signal processing systems use only Class 1 ceramic capacitors to avoid the microphony effect.

Applications of ceramic capacitors

Class 1 ceramic capacitors perform well in applications that require precision like oscillators, timers and analog-to-digital converters. Class 2 ceramic capacitors are the usual choice for non-critical decoupling, coupling and bypassing applications.

Alternatives to ceramic capacitors

Metallized film capacitors are an alternative to ceramic capacitors. The below table describes the differences between these two options.

*Note that PPS is notation for polyphenylene sulfide (PPS) film capacitors. On a data sheet description, you might see “stacked metallized PPS film chip capacitor” for an SMD package.

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