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Passive Electronic Components Lecture 9

of 2428-May-2017

Capacitors 2Lecture Plan

1. Common types of capacitors2. Special types of capacitors3. Typical applications4. Selection guidelines

1. Common types of capacitors. The most commonly used today are MLCC, aluminum, and tantalum capacitors.

Ceramic capacitors. Single-Layer and Multilayer Ceramic Capacitors (MLCC) .

First HF electronic devices were built using mica capacitors. In 1940 a vitreous enamel was developed in DuPont to replace mica in capacitors. Manufacturing processes for single-layer and multilayer ceramic capacitors were developed in 1940s in USA. Common ingredients of ceramic dielectrics include titanium dioxide, barium titanate, and strontium titanate. The exact formulas for the various ceramics differ from one manufacturer to another. The EIA standard (Electronic Industries Alliance, USA) classifies the ceramics into four classes (1, 2, 3, 4), and into types within those classes. The lower is the class number, the better are the overall characteristics, but the larger is the size of the capacitor for a given capacitance. Types within each class define: temperature range [C], temperature coefficient of capacitance (TCC) nominal value [ppm/K], TCC tolerance [ppm/K]. Class 1 ceramic capacitors include the most temperature stable C0G capacitors (their old designation is NP0), and temperature compensating capacitors. They are characterized by low content of barium titanate.

EIA Codes For Temperature Slope of Class 1 Ceramic Capacitors.

Significant figure, ppm/K Multiplier Tolerance, ppm/Kat +25…+85C

C 0.0 0 -1 G ±30B 0.3 1 -10 H ±60L 0.8 2 -100 J ±120A 0.9 3 -1000 K ±250M 1.0 4 +1 L ±500P 1.5 6 +10 M ±1000R 2.2 7 +100 N ±2500S 3.3 8 +1000T 4.7V 5,6U 7.5

For example, C0G has TCC of 0.0±30 ppm/K in +25…+85C temperature range. P3K has TCC -1500 ±250 ppm/K.

Passive Electronic ComponentsLecture 9

of 24 In -55…+25C temperature range the negative tolerance is 1.25…4 times higher than the positive depending on the "Significant figure" value. For example, C0G is ±30 ppm/K at +25 …+85C, but -75…+30 ppm/K at -55 …+25C. The industry commonly uses an "N/P" designation system in spite of its obsolescence because it is more intuitive. "N" means a negative temperature coefficient, "P" - a positive coefficient. Manufacturers use the both designation systems in their catalogs.

Some EIA and industry codes for commonly available Class 1 capacitorsIndustry code P100 NP0 N030 N075 N150 N220 N330 N470 N750 N1500 N2200

EIA code M7G C0G B2G U1G P2G R2G S2H T2H U2J P3K R3L

C0G capacitors are characterized by low temperature drift (limited to ±30 ppm/K over most of their temperature range), insignificant aging effects, low voltage coefficient, almost frequency independent capacitance, low leakage, and low dissipation factor. The only problem with C0G capacitors is their size. 47 nF is about the largest through-hole capacitor, and 27 nF is about the largest chip capacitor. Dielectric absorption is typically mediocre. Temperature compensating capacitors are available in the range of +1000ppm/K…-5600ppm/K or P1000…N5600. Tolerance on the temperature slope can vary from ±30 ppm/K for the lower slopes (N30…N220) up to ±1000 ppm/K for N5600. Above N4200 the properties start to look more like Class 2 than Class 1. Both dissipation factor (DF) and dielectric constant go up as the temperature slope of Class 1 ceramics goes more negative, thanks to the additives. It means that for example N1000 is more size-efficient than C0G.

Advantages: Low and predictable TCC. Low dissipation factor (DF). Low Voltage Coefficient of Capacitance (VCC).

Shortcomings: Large size (low volumetric efficiency). Mediocre dielectric absorption.

Applications:As a rule of thumb, Class 1 ceramics can be used in many common digital and analog electronics applications including GHz range. The exceptions would be sample-and-hold circuits and integrators, where mediocre dielectric absorption may be a problem.

Class 2, 3, and 4 ceramic capacitors Class 2 and higher ceramic capacitors are typically based on ceramics with high content of barium titanate (Ba2TiO3).


of 24EIA codes for non-temperature-stable ceramic capacitors (Class 2 and Higher)

Low temp. limit, C: High temp. limit, C: Capacitance change over the rated temperature range, %:

X -55 4 +65 A ±1.0Y -30 5 +85 B ±1.5Z +10 6 +105 C ±2.2

7 +125 D ±3.38 +150 E ±4.79 +200 F ±7.5

P ±10R ±15S ± 22T +22 -33U +22 -56V +22 -82

X7R, Z5U, and Y5V are the most common types. For example, X7R is a capacitor that would be usable at -55…+125C and should change its capacitance no more than ±15% in this temperature range with respect to capacitance value at 25C. The capacitance of Class 2 and higher capacitors is affected by voltage. So the above temperature drift limits are valid only if low voltage is applied across capacitor.

Advantages: Small size. (Comparable with electrolytic capacitors). Lower ESR than in electrolytic capacitors.

Shortcomings: Very high Temperature Coefficient of Capacitance (TCC). High Voltage Coefficient of Capacitance (VCC). It is different for AC and DC. That is

why Class 2 and higher capacitors are normally characterized at no more than 1V AC or DC. VCC is more dependent on the ratio of applied voltage to maximum rated voltage than on the applied voltage itself. Therefore, the using of the highest rated voltage part of the given size is beneficial.

High voltage coefficient of dissipation factor. It is different for AC and DC. Dissipation factor rises at the frequencies above 1 MHz.

High frequency coefficient of capacitance. Commonly capacitance drops at the frequencies above 1 MHz.

Significant aging rate ( becomes lower as times goes by) because the original crystal structure slowly transforms to another one. It can be reversed by annealing of capacitor for several hours at +125…+150C. But after the heat treatment the aging process starts all over again.

High- ceramic capacitors show significant piezoelectric effect caused by barium titanate. The higher is the stronger is this effect. It may cause problems in low-level signals circuitry making it vulnerable to mechanical shock and vibration. It is the reason to use Class 1 ceramics in analog circuitry.

The table below shows what happens to capacitance when DC voltage is applied to various types of Class 2 capacitors.

Relative capacitance versus applied DC voltage U


of 24C0G X7R Z5U

U = 0.1 U rated 1.00 1.05 1.20U = 0.5 U rated 1.00 0.95 0.50U = 1.0 U rated 1.00 0.70 0.30

Applications. Value range 0.047…100 F. (In lower value range they are commonly replaced with C0G. For

higher values electrolytic capacitors are used). Coupling (DC blocking) and power supply bypassing.

For the best results: (a) use capacitors with the lowest ; (b) apply a voltage that is significantly lower that rated voltage). Multilayer Ceramic Capacitors (MLCC) The construction of ceramic capacitors has changed dramatically since its invention. Primarily a common form of ceramic capacitor was a ceramic disk. A metal layer was screened and fired on each side, and wires were soldered on. This construction is still good for pico- and nanofarad range of high-voltage capacitors. But with the decline of the vacuum tubes, demand for such parts has likewise declined. Today's electronics requires more capacitance at lower voltage and in a smaller size. Multilayer Ceramic Capacitor – MLCC matches these requirements and is manufactured now in huge volumes. MLCC manufacturing process is based on screen printing of very thin alternating layers of ceramic dielectric and silver-palladium electrodes using respective inks. The new tendency is using of base metals electrodes (BME) made of nickel or copper. The resulting “sandwich” is fired to make a finished capacitor. The dielectric layers may have thickness of 1.5…30 m. The end electrodes that connect the internal electrodes are formed by dipping of two face sides of the capacitor into respective ink followed by firing. It is silver ink if the internal electrodes are made of silver-palladium or copper ink if the internal electrodes are made of copper or nickel. Finally the terminals of the parts are usually nickel plated and solder-plated.

Leaded MLCC MLCC chip

There is the trade-off between metal electrode thickness and electrical performance. Thicker electrodes ensure lower ESR but the additional thickness also increases the mechanical stresses in electrode/dielectric interface and decrease volumetric efficiency. Consequently, thicker electrodes not only increase the cost of manufacturing by increasing the consumption of precious metals (silver, palladium), they also increase the level of internal defects (like cracks and delaminations), resulting in reduced product yields.


of 24 The thinner is the ceramic, the more is capacitance-per-volume or volumetric efficiency (it grows inversely to a square of ceramic thickness). The limiting factor is a voltage breakdown. At present, the top parameters of MLCCs are:

up to 350 ceramic dielectric layers, dielectric layer thickness - 1.5m (6.3V rated voltage), electrode thickness - 1.5m.

Capacitors manufactured by Yageo in 2016 (X5R, 6.3V)

Case Size EIADimensions, mmMax. cap., µF02010.60.3104021.00.51006031.60.82208052.01.254712063.21.6100

Special MLCCs Because of wide range of applications MLCCs have diverged into a number of special types:

Low profile (low height) capacitors for use in extremely thin products like smart cards, or to be mounted under a RAM IC for power supply bypassing.

High temperature operation capacitors. (At least +200C). High voltage capacitors. (Up to 10 kV for chip capacitors). Capacitors intended for gluing to PCB using conductive glue. Commonly they have silver-

palladium or gold terminals). Capacitors for microwave applications (lateral field capacitors). Low ESR capacitors (reverse-geometry MLCCs). Forming the terminal electrodes along the

long dimension of the device widens and shortens the current path between terminations, thus reducing inductance and resistance.

Chip capacitor arrays comprise 2 or 4 capacitors and contribute in reduction of mounting cost of circuit assembly. 4-capacitors array may be as small as 2.01.251.0 mm.

4-capacitors arrays: ceramic multilayer (left) and film (right)

The X2Y Circuit Architecture (Syfer Technology Ltd.) is a patented technology. It is characterized by presence of additional ground electrodes between the regular electrodes and works both as an a single X capacitor (line to line decoupler) and as two Y capacitors


of 24(line to ground filters) simultaneously. An application can use the X capacitor and the two Y capacitors together or separately in a circuit. The needs of the circuit designer determine the attachment or non-attachment of the appropriate corresponding ground terminations.

Advantages: Replaces 2 or 3 capacitors with one device Matched capacitance line to ground on both lines Low inductance due to cancellation effect Differential and common mode attenuation Reduces board area required for filtering Effects of temperature variation eliminated Effect of voltage variation eliminated Effect of ageing equal on both lines High current capability

Applications: Balanced lines Twisted pairs Broadband input/output filtering EMI Suppression

Capacitors constructed in X2Y Circuit Architecture have extremely low internal inductance readings. This minimal inductance characteristic is the result of the mutual cancellation of the opposing magnetic fields within the architecture's physical layers of grounds and electrodes.


of 24 |Z| Chip (AVX Corporation). It is discrete impedance-matching series

resistor/capacitor chip of 0603 size. The chip is used for line termination applications to achieve maximum signal integrity by eliminating reflections and reducing DC power consumption.

While resistor alone is still the prevalent method for matching impedance, the biggest problem with this method is that when the signal line is high, DC current is drawn continuously. If a capacitor is placed in a series with the resistor, the capacitor acts as a DC voltage/current block, allowing only certain AC signals to pass to ground. More precise resistor values are necessary to minimize reflections and reduce noise.

Sizes are 0603, 0805. Array 06034 is available. Capacitance values are 33, 47, 68, 100 and 150pF with a tolerance of ±20%, and the resistor values are 22, 33, 47, 51, 100 and 150 Ohms with a tolerance of ±10%. The DC rated voltage is 25V.

Aluminum capacitors. 1900 The capacitor using aluminum oxide as a dielectric material was revealed by Wheatstone and Duff Durrentet. However, it was not used practically.1908 Aluminum electrolytic capacitor using electrolytes was developed in General Electric (USA). Aluminum capacitors may have wet or solid electrolyte.

Wet electrolyte aluminum capacitors. Aluminum electrolytic capacitor consists of:

anode electrode (anode aluminum foil), cathode electrode (cathode aluminum foil plus electrolyte), dielectric - aluminum oxide layer formed on the anode foil surface that electrically

isolates the anode, electrolytic paper that mechanically separates the anode foil from the cathode foil but

does not prevent the ions in the electrolyte from moving between the foils.A very thin oxide layer formed by electrolytic oxidation (formation) of anode foil offers superior dielectric constant and has rectifying properties. The thickness of the grown oxide film is nearly proportional to the applied voltage (approximately 1.0…1.4 nm/V). When in contact with electrolyte, the oxide layer possesses an excellent forward direction insulation property. Together with large effective surface area attained by anode foil etching, a high capacitance in small sized capacitor becomes available. Maximal capacitance of aluminum capacitors with wet electrolyte is 1 Farad.


of 24 By etching the surface of aluminum foil, the effective area of the foil as compared to the initial area can be enlarged 80…100 times for low voltage capacitors and 30…40 times for middle and high voltage capacitors. Since the oxide layer has rectifying properties, a capacitor has polarity. If both the anode and cathode foils have an oxide layer, the capacitor will be bipolar (non-polar). The shortcoming of this design is two times reduction of capacitance (at the same dimensions of the foils) when compared to polar capacitor with oxide layer on the anode foil only. A 0.05…0.11 mm thick anode foil and a 0.02…0.05 mm thick cathode foil are facing each other and are interleaved with a paper and wound into a cylindrical shape. This is called a "capacitor element". At this stage a capacitor element has negligible capacitance because of thick dielectric gap between the electrodes (oxide + air + paper). But when this capacitor element is impregnated with liquid electrolyte the electrolyte electrically connects with a cathode foil and gets in touch with oxide dielectric layer. At that, the oxide layer appears to be the only dielectric layer between aluminum electrodes. A capacitor attains a high capacitance value. Electrolyte is functioning as a cathode. The basic characteristics required from an electrolyte are:

high electrical conductivity, forming property (healing of any flaws in the dielectric oxide of the anode foil), chemical stability with the anode and cathode foils, sealing materials, etc., superior impregnation characteristics, low vapor pressure.

The standard capacitance tolerance is 20% (M); however, capacitors with a capacitance tolerance of 10% (K) are also manufactured for special usage. The capacitance of aluminum electrolytic capacitors changes with temperature and frequency. The standard test conditions are 120Hz AC voltage and +20C ambient temperature.


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Solid electrolyte aluminum capacitors.

Conductivity of wet and solid electrolytesType of capacitor Type of electrolyte Conductivity,

mS/cmResistivity,

mmWet electrolytic Electrolytic solution 3 3,300Solid electrolytic Solid MnO2 30 330Solid polymer electrolytic Organic semiconductor (TCNQ complex salt) 300 33

Polymerized organic semiconductor 3,000 3.3

For comparison: resistivity of the best polymerized organic semiconductor (3.3 mm) is 2500 higher than resistivity of so called "highly resistive metal alloy" nichrome (0.0013 mm).

Construction of Vishay OS-CON capacitor


of 24The principal advantage of solid aluminum capacitor when compared with wet one is very low ESR. It permits for example filtering in a DC-DC converter with a fewer number of capacitors. Another advantage is its safety (ignition free, open circuit failure mode).

Tantalum capacitors . 1930 Wet Foil Tantalum Capacitor was manufactured by Fansteel Metallurgical Corporation.1955 A solid tantalum capacitor was developed in Bell Telephone Laboratories (USA).1956 Sprague Electric started first-to-market solid tantalum capacitor based on its own patent.

Foil, Wet Slug and Solid Tantalum Capacitors. Tantalum electrolytic capacitors are a step forward from aluminum capacitors. They come in a number of types with different advantages, but in general, they have smaller size, lower leakage, lower dissipation factor, lower ESR, more stable capacitance over temperature, and good service life. The first tantalum capacitors were manufactured from tantalum foil with a sulfuric acid electrolyte. Their construction resembles construction of aluminum capacitors. Foil tantalum capacitors are manufactured for use at up to 300V voltage and up to 125C temperature. They are made in both industrial and military styles but their usage is mostly military. The foil anode is a poor use of the expensive metal. Most of the modern tantalum capacitors are "slug" capacitors. Tantalum powder is sintered into a porous yet strong slug together with a tantalum wire. It constitutes the anode of the capacitor. The many small particles produce a very large surface area. A layer of tantalum pentoxide (Ta2O5 with 27) is grown over the slug surface to serve as dielectric. In one version of the capacitor, the electrolyte is gelled sulfuric acid. This is called a wet-slug capacitor. It must be well sealed. Wet-slug tantalums are the capacitor of choice for some applications, especially at very high temperatures (up to 200 °C). They are available with up to 125 V rated voltage. Tantalum wet slug capacitor (see picture below) consists of:

anode electrode (slug sintered from tantalum powder), cathode electrode (tantalum case plus gel electrolyte), dielectric - tantalum oxide layer formed on the anode electrode surface that electrically

isolates the anode,

The vast majority of modern tantalum capacitors are "dry-slug" or "solid" capacitors. They consist of:

anode electrode (slug sintered from tantalum powder), dielectric (tantalum oxide Ta2O5 layer formed on the anode electrode surface that

electrically isolates the anode), cathode electrode (MnO2 + graphite + silver paint).


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A layer of manganese dioxide (MnO2) is deposited over tantalum pentoxide Ta2O5 followed by a layer of colloidal graphite and a layer of silver paint. Manganese dioxide (MnO2) layer is formed by pyrolysis of manganese nitrate Mn(NO3)2. Porous anode "slug" with oxidized surface is dipped into an aqueous solution of manganese nitrate and after soaking is baked in an oven at approximately 250°C to produce a MnO2 coat over tantalum pentoxide dielectric. The simplified chemical equation of the pyrolysis is

Mn(NO3)2 MnO2 + 2NO 2

The graphite layer between manganese dioxide and external silver coating is used to prevent the silver layer coming into direct contact with the manganese dioxide. If this occurs a chemical reaction would take place

Ag + 2MnO2 AgO + Mn2O3

The silver would be oxidized to high resistivity silver oxide, and the manganese dioxide reduced to manganese (III) oxide, which has a high resistivity too.

With no liquid involved, solid tantalum capacitor can be sealed with just an epoxy dip, although the better ones may have a molded body like in the picture below.

Tantalum powder is characterized by “CV parameter” that indicates the product of capacitance and rated voltage that is provided by 1 g of tantalum powder. It is measured in [FV/g]. CV of modern powders approaches to 100,000. The powder is mixed with a suitable binder/lubricant to ensure that the particles will adhere to each other when pressed to form the anode. The powder is then compressed under high pressure around a Tantalum wire to make a Tantalum "slug". This is followed by sintering at high temperature (typically 1500…2000°C) under vacuum. This causes the individual particles to join together to form a sponge-like structure. This structure is of high mechanical strength and density, but is also highly porous giving a large internal surface area. The next stage is the production of the dielectric layer of tantalum pentoxide. This is produced by the electrochemical process of anodization.


of 24The slugs are dipped into a very weak solution of acid, for example phosphoric acid, at an elevated temperature, for example 85°C, and the voltage and current are controlled to form the pentoxide layer. The dielectric thickness is controlled by the voltage applied during the formation process. Initially the power supply is kept at a constant current until the required formation voltage has been reached. The thickness of the oxide layer depends on the formation voltage, which is typically 3-4 times higher than the rated voltage of the future capacitor. The oxide growth rate is 1.710-9 m/V. Suppose that the oxide for 25V capacitor has to be built. Its thickness will be 1.710-9 m/V × 25 V × 4 = 1.710-7 m. The electric field strength in this capacitor will be 25 V / 1.710-7 m ≈ 15 107 V/m. Tantalum pentoxide is remarkable dielectric. That makes it possible to construct capacitors with small dielectric thickness and therefore high capacity. However, the metal - metal oxide interface is rectifying. That is, in one direction it is a good insulator, and in the other direction it is a conductor. This is why tantalum capacitors are polar. Non-polar capacitor may be made by opposite (back-to-back) series connection of two polar capacitors. Tantalum capacitors are available with capacitance up to several hundreds µF in voltage ratings to about 100V and up to several thousands µF in low-voltage ratings (6…10V). Therefore, maximum capacitance of tantalum capacitors is significantly lower than that of aluminum capacitors.

Effective area. Consider for example a 10F 25V tantalum electrolytic capacitor. Let us calculate oxide film thickness. Suppose that N – formation ratio (N = 4); U = 25 V rated voltage; v = 1.710-9 m/V (Ti2O5 growth rate).

d=NUv=4⋅25⋅1. 7⋅10−9≈0 .2⋅10−6 (m ) .

Electrode surface area A may be evaluated supposing that = 27:

A=Cdεε0

=10⋅10−6⋅0. 2⋅10−6

8 . 85⋅10−12⋅27≈0 . 0084 (m2)≈8 . 4⋅103mm2

.

10F 25V conformal coated 595D Vishay tantalum capacitor has 7.13.22.5 (mm) dimensions. His external surface area is approximately

2× (7 . 1×3 . 2+2. 5×3. 2+7 . 1×2 .5 )≈97 mm2

Therefore, surface area of the porous anode 8.410-3 mm2 is about 100 times higher than the external surface area of the capacitor.

The Ta2O5 layer is prone to defects. The key to the solid tantalum capacitor's reliability is self-healing phenomenon of MnO2. If a flaw in the Ta2O5 layer develops, the leakage current will cause local heating in the MnO2 layer and will convert it at 450°C to significantly less conductive oxide Mn2O3 sealing off the flaw. The important condition of the self-healing is a current limiting. If the current reaches approximately 1A the temperature goes higher and at 500°C amorphous Ta2O5 dielectric turns into conductive crystalline form that results in further current increase. Finally a tantalum anode may be ignited. It burns very fast consuming oxygen that is released as a result of MnO2 decomposition. The dominant failure mode in this case is “short-circuit”. Maximal rated voltage in solid tantalum capacitors usually does not exceed 50V because the tantalum particle size limits the Ta2O5 thickness that can be grown. Maximal temperature rating is commonly 85…125C. A rarely mentioned characteristic of solid tantalums is their rapid decline in leakage current. When both aluminum and tantalum electrolytic capacitors are powered up, their leakage current starts high,


of 24but declines over time. For aluminums, it takes some minutes to decline the leakage to a stable value. In tantalum capacitors it occurs in seconds. Even though "dry-slug" tantalum capacitors are not wet-chemistry devices, they are still polarized. Anyhow, the failure of tantalum capacitors may not be detected in the beginning. Sometimes the failure in capacitor that was installed backwards may occur after more than a year of its use. One of the ways to decrease ESR of Ta capacitors is use of multi-anode construction (parallel connection of several pellets in the same package).

Conductive polymer in Ta capacitors. Tantalum resistivity is 13cm. MnO2 resistivity is approximately 2cm or 5 orders of magnitude more! Therefore MnO2 resistivity is the main part of ESR. Some years ago MnO2 was replaced by almost 100 times more conductive substance – conductive polymer. It possesses self-curing property too (decomposes at 280-300°C in the locality of dielectric breakdown). The shortcomings of these capacitors when compared with MnO2 devices are lower maximal working temperature (105°C versus 125°C) and higher leakage current 0.1CV (A) versus 0.01CV (A), where C – nominal capacitance, F, V – rated voltage, V.

Niobium and niobium oxide (NbO) capacitors (new products). They are very similar to Ta capacitor but have less weight, lower risk of ignition (NbO capacitors), and are expected to be cheaper. They are intended to compete with ceramic and Ta capacitors in low voltage (below 16V) range.

Film capacitors . Various polymer films are used as dielectric in the film capacitors: polyester, polypropylene, polycarbonate, polyphenylene sulfide, teflon. Film thickness may be as low as 0.9-1.2 m.

Polyester (mylar) capacitors are very popular, because they are inexpensive. Even though most of their characteristics are very good. But their capacitance varies drastically with temperature about the same as in aluminum electrolytic capacitors.

Polypropylene capacitors also have a very low dissipation factor and a good stability but lower operating temperature (up to +100°C). They are ideal for AC applications, high current and pulse applications.

Polycarbonate capacitors have a higher temperature range up to +125°C, low dissipation factor and a very low change of capacitance with temperature. Commonly they are used as precision capacitors.


of 24Polyphenylene sulphide (PPS) capacitors have a higher temperature range up to +150°C, low dissipation factor and a very low change of capacitance with temperature and presently being offered as replacement for polycarbonate capacitors.

Teflon capacitors have very low dielectric absorption, high insulation resistance, low dissipation factor, high temperature capability. But they are expensive. Commonly they are used as precision capacitors.

Film capacitors are subdivided into: (a) metallized, (b) foil (see picture below).

The electrodes in a foil (non-metallized) design are separate sheets of metal foil wound with interlayer sheets of dielectric film material. These foil electrodes extend alternately out of each end of the capacitor roll beyond the dielectric. This provides a mass of metallic material to which leads are attached by welding or soldering. Their disadvantage is short circuit failure mode. So they should not be used in across the line applications. In the metallized type of construction the foil electrodes are replaced with a thin layer of 99% pure aluminum vapor-deposited directly onto the dielectric film. It greatly reduces the physical size of the capacitor. Provisions are made, by means of masking, to provide each dielectric with a margin. The two sheets of metallized dielectric are positioned properly relative to a right and left margin and wound into a roll. Since the dielectric material itself presents at each end of the wound capacitor along with the metallization layer, direct attachment of the leads is not possible. The end terminations of the metallized capacitor are accomplished by application of a fine molten metal spray. The sprayed metal particles make contact with the electrode material and form metal layer on the face surfaces of wound element. The lead wires are attached to this metal spray layer. The metallized capacitor offers a distinct advantage over the non-metallized unit: self-healing feature. It results from the extreme thinness of the metallized electrode material. Whenever a flaw or weak spot in the dielectric results in a short condition, the dissipated heat vaporizes away the small pattern of the electrode film around the point of the short. As a result of the vaporization, the short condition is removed and the capacitor is again operational. This phenomenon is known as a "clearing" or self-healing process (see picture below).


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Advantages of a metallized capacitor in comparison with non-metallized capacitor: The self-healing feature. The volumetric efficiency is higher. Weight savings is in direct proportion to the volumetric efficiency. The cost in the higher capacitance ratings (above 0.1 F) is less. Failure mode is open circuit.

Disadvantages of a metallized capacitor in comparison with non-metallized capacitor: The dissipation factor is higher. The insulation resistance is lower. The maximum current limitation is lower. The maximum AC voltage-frequency capability is less.

Properties of film capacitors versus X7R and Z5U ceramic capacitors

Commonly upper rated temperature in film capacitors does not exceed 100°C. But in polyester, polycarbonate film capacitor it is +125°C, and in polyphenylene sulfide film capacitor it may reach +150°C.

Electric double-layer capacitor (EDLC).

EDLCs fill gap between aluminum electrolytic capacitors and galvanic batteries. They are capable of several hundred thousands charge – discharge cycles and can be combined in power modules by series and parallel connections. The components available on the market have a capacitance in 0.01F…


of 242700F range, an energy density of 4.9Wh/kg, and a power density of 4.5kW/kg at 2.5V voltage rating.

Possibility of carbon activation is 2000 m2/g. “Dielectric” thickness is 2…5 nm. Within the group of carbon-based EDLCs, two concepts can be distinguished:

aqueous systems with low ESR (1.2 cm at 300 K for 30 % sulfuric acid) and a maximum cell voltage of about 1.2 V,

organic systems with relatively high ESR (>40 cm at 300 K) and a maximum voltage of about 2.5 V. Surge voltage should not exceed 2.7V. Operation temperature should not exceed +65C.

EDSL capacitance depends on two parameters: Co and K. Co is a basic capacitance which is independent on the voltage while K is coefficient of linear dependence on the voltage:

C=C0+KU ;q=(C0+ KU )U =C0 U +KU 2 ;

i( t )=(C0+2K|U|)dUdt

;

E(U )=(C0+43

K|U|)U2

2.

EDLC sometimes are referred to by the trade names: “Supercapacitor”, Nippon Electric Company (NEC) and “Ultracapacitor” Pinnacle Research Institute (PRI).

Manufacturing of EDLC


of 24Examples of EDLC:

EPCOS B49300 UltraCap. Capacitance - 2700F. Voltage rating - 2.3V. Stored energy - 9522J. Maximal current - 200A. Weight - 650g. Volume - 0.59 liter.

Seiko Instruments Inc. Chip-type 3.2 x 2.5 x 0.9 mm. Capacity: 14mF (30 times more electric capacity compared with a similar sized tantalum capacitor). Charge voltage: 2.6V. Application: backup for memory, clock function and power management ICs for mobile-phones and most other portable electronic devices.

3. Typical applications of capacitors

BLOCKING: A capacitor is used to block or prevent DC voltage from circuit output. Toblock the DC voltage, the capacitor is placed in series with the load.The major characteristics for the capacitor are insulation resistance, ESR, and voltage rating.

COUPLING: A coupling capacitor is used to pass only the AC signal from one circuit stage to another. The capacitor is connected in series between the stages just like in blocking application.The major characteristics are insulation resistance, ESR, and voltage rating.

BYPASS (DECOUPLING): A bypass capacitor is used to keep the AC portion of an inputsignal from reaching a load. The capacitor is placed in parallel with the load to produce a low impedance path across the load.The impedance of the capacitor should be 10% of the input impedance of the load.


of 24The main characteristics for the capacitor are insulation resistance and ESR.

TIMING: A capacitor is used to store a charge until a specific amount of time has elapsed.The major capacitor characteristics are insulation resistance/leakage current, capacitancestability, ESR and dielectric absorption.

SAMPLE AND HOLD: The capacitor is used to store a charge until a sample is taken. The major capacitor characteristics are similar to a timing application.

FILTERING: A filter capacitor is used to smooth the DC pulses after rectification. Thecapacitors store charge and deliver it to the load when the rectified or pulsating DC voltagedecreases below the peak of the DC voltage signal.The major capacitor characteristics are capacitance, ESR, and ripple current rating.


of 24ENERGY STORAGE: Capacitors in energy storage applications are used to deliver a highenergy, short duration pulse of energy. Major capacitor characteristics in this application are: ESR, pulse rating and voltage rating.

EMI SUPPRESSION. Capacitors in EMI Suppression are used to suppress noise pulses thatcould damage the circuit elements. EMI filters are commonly capacitors or capacitor/resistornetworks mounted across the input of a device. Mains capacitors connected between two phases, or between a phase and neutral must comply with the X capacitor requirements of EN 60384-14 standard. Capacitors connected between the primary circuit and protective earth must comply with the Y capacitor requirements of EN 60384-14.

ARC SUPPRESSION (SNUBBER). A capacitor/resistor network is used to suppress arcsacross relay or switch contacts to prevent their damage.

POWER FACTOR CORRECTION. (Power Factor is the ratio of the active power to the apparent power). Capacitors are used to bring the phase angle of the circuit back to unity. Failure to do so will result in a penalty charged by the power companies:

P . F .= WV⋅A

.

W - Active Power, VA - Apparent Power

EDLC capacitor applications

All-weather quick start applications- The current car battery is geared up to meet peak power needs during engine startup even in coldest weather conditions that impair battery performance. EDLC can supply the seconds-long peak power unaffected by the weather and permit the battery to be downsized and its useful life extended.- The current catalytic converter in cars sends untreated exhaust gas into the environment for a few minutes until it is warmed up and begins functioning. EDLC can quickly pre-heat the catalytic


of 24converter and enable it to function immediately.

Load-leveling and Uninterruptible Power Systems (UPS)- Because the EDLC has significantly less energy storage capability than battery, it is not a viable substitute for the battery in Uninterruptible Power Systems as a long-term power source. However, as a short-term support for Uninterruptible Power Systems, its rapid response capability means that it can act as a temporary bridge until an alternative power source kicks in.- Moreover, the EDLC in the Uninterruptible Power System serves a load -leveling function by absorbing power surges and spikes and then releasing clean quality power essential for precision high-tech equipment. No-maintenance applications - Many buoys in sea lanes emit light during night-time using the energy captured from the sun and stored in the battery. But, the battery needs replacing every couple years, and the servicing of these widely scattered buoys is an expensive undertaking. The light buoys can be made practically maintenance-free if EDLC are used instead of batteries to store solar energy.- Construction sites and road hazards need to be warned of by lighted signs and markers at night-time. By using a solar panel and EDLC in the same housing, maintenance-free signs and markers can be produced. These can be quickly set up in field conditions whenever needed without going through expensive and time-consuming wiring process.

Peak pulse power applications - Unlike analog equipment that draws a steady current, a digital wireless communications device loads the battery with short, heavy current spike during its transmit mode. If an EDLC is added to the system, then it can take over the task of providing the intermittent pulse power while the battery functions only as a supplier of steady current. Users benefit from longer talk-time between charges and from the extension of battery-life.

Quick charge applications - EDLC can be charged in seconds whereas batteries require hours of charging time. Wireless power tools with an EDLC can be charged just before use without waiting time. Moving toys such as miniature racing cars are also applications that can benefit from quick charge properties of the EDLC.

Memory back-up applications - Already widely used in consumer electronics products, small-size EDLC protect user data and clock information from being lost during short-period power outages or, in case of portable devices, during replacement of batteries. For this use, the EDLC is better than the battery because it is cheaper and requires no replacing during the lifetime of the application device.

Automotive applications and Electric Vehicles - The use of EDLC for generative braking can greatly improve the fuel efficiency of cars under stop-and-go urban driving conditions. Only EDLC have both storage capacitance and high current handling capability to capture and store large amount of electrical energy generated by braking within a short time and to release it again for re-acceleration. The generative braking has the potential to be one of the biggest applications for large-size EDLC in the medium term.- EDLC can enhance the performance and competitiveness of an electric vehicle. It permits faster acceleration; extends the range by generative braking; and extends battery life by freeing it from stressful high power tasks.

Though some applications of EDLC such as memory back-up applications are already in widespread use, most applications described above are just in the beginning phase of being adopted. Moreover,


of 24EDLC are friendly to environment, help conserve energy, and enhance the performance and portability of consumer devices.

4. Selection guidelines There are following factors that have to be taken into consideration when selecting a capacitor:

1) Capacitance and capacitance tolerance.2) Capacitance change versus temperature.3) Capacitance change versus DC bias.4) Rated voltage.5) Acceptability of short circuit condition.6) Acceptability of acoustic noise.

Typical Characteristics of Different Types of Capacitors

Characteristics MLCC C0G

MLCCX7R

MLCCZ5U

Polystyrene film

Al Ta Double-layer

Capacitance range 1pF – 0.01F

1pF – 2.2F

1000pF - 27F

100pF – 0.03F

0.5F – 1F

0.1F – 1000F

5F – 3600F

Min. tolerance, % 0.5 5 20 0.5 20 5Standard tolerance, % 5 10 +80 -20 5 +100 -

1020 +30 -10

Rated voltage range, V 50 - 200

50 - 200

50 - 100

100-600 3-500 6-125 2.3

Temperature range, C -55…+200

-55…+125

+10…+85

-55…+70

-40…+105(wet)+125

(solid)

-55…+150

(solid)+175(wet)

-30…+70

D.F. at 1 kHz, % 0.1 2.5 3.0 0.1 8 8-24D.A., % 0.6 2.5 NA 0.05 NA NAFrequancy response9 – best2 – poorest

9 8 8 6 2 5

Polarity NP NP NP NP P P P

Applications Types of capacitor(in descending order of performance)

UHF applications MLCC C0GPrecision analog circuits Film, MLCC C0GPower line blocking and filtering Ta, MLCC Z5U, Al + MLCC;High temperature applications MLCC, TaHigh energy storage Double-layer, Al, Ta


of 24Capacitors' cost versus capacitance

-Impedance comparison between MLCC and electrolytic capacitors


of 24Performance comparison between MLCC and low ESR electrolytic capacitors

Lower ESR of the MLCC structure allows to employ devices with smaller capacitance ratings than the tantalum (or aluminum) alternative:

Tantalum capacitornom. value, µF

Nom. value of possibleMLCC alternative, µF

1 0.1 to 0.472.2 0.22 to 14.7 0.47 to 2.210 1 to 4.722 2.2 to 1047 4.7 to 22100 10 to 47220 22 to 100

Capacitor Technology CV Diagram(Source – AVX Technical Information ”Tantalum and niobium technology roadmap”)


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