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Jun 17 2020

Capacitor Basics: Capacitor Types

I Introduction

A capacitor is an electronic component composed of an insulator between two conductors, like a sandwich. We can understand it as a container that holds the electric charge. In actual capacitors, two conductors are filled with an insulating dielectric. There are numerous types of dielectrics, so the types of capacitors formed are also different. For example, according to dielectric materials, capacitors can be divided into gas dielectric capacitors, liquid dielectric capacitors, inorganic solid dielectric capacitors, and organic solid dielectric capacitors; according to polarity, they can be divided into polarized capacitors and non-polarized capacitors.

This article will introduce the various types of capacitors in detail and some additional basic knowledge of them, mainly explaining from the perspective of the manufacturing process and structure.

Capacitors: types, use and testing. 

Catalog

I Introduction

II The Basic Principle of Capacitors

III Film Capacitor

3.1 Metal Foil Film Capacitor

3.2 Metallized Film Capacitor

IV Electrolytic Capacitor

4.1 Aluminum Electrolytic Capacitors

4.2 Tantalum Electrolytic Capacitors

4.3 Niobium Electrolytic Capacitors

V Ceramic Capacitor

5.1 Ceramic Disc Capacitor

5.2 Multi-layer Ceramic Capacitor

5.3 Monolithic Capacitors

5.4 Classification of Ceramic Media

VI Supercapacitor

VII Fixed, Trimmer and Variable Capacitors

7.1 Mica Capacitor

7.2 Paper Capacitor

7.3 Trimmer Capacitor

7.4 Variable Capacitor

VIII Comparison of Polarized Capacitors and Non-polarized Capacitors

8.1 Medium

8.2 Performance

8.3 Capacity

8.4 Structure

8.5 Application Environments and Use

IX Axial and Radial Leaded Capacitors

X A Quiz About Capacitor Types

II The Basic Principle of Capacitors

Capacitors, along with inductors and resistors, are the three basic passive devices in electronics. The function of the capacitor is to store electrical energy in the form of electric field energy.

Taking the parallel plate capacitor as an example, we briefly introduce the basic principle of capacitance.

Parallel Plate Capacitor

Figure1. Parallel Plate Capacitor

As shown in the figure above, a DC voltage is applied to two metal plates that are close to each other and are parallel to each other (the dielectric between the plates). After stabilization, the metal plate connected to the positive electrode of the voltage will exhibit a certain amount of positive charge, while the metal plate connected to the negative electrode of the voltage will exhibit an equal amount of negative charge. In this way, an electrostatic field is formed between the two metal plates, so the capacitor stores electrical energy in the form of electric field energy, and the stored charge is Q.

The amount of charge stored in the capacitor Q is related to the voltage U and its own property (that is, the capacitance value C), that is, Q=U*C. According to the theoretical derivation, the capacitance formula of the parallel plate capacitor is as follows:

Formula

In this formula:

C is the capacitance value, the unit is F (Farad)

ε is the dielectric constant of the medium, F/m

S is the area of the metal flat plate, m²

d is the distance between metal plates, m

The ideal capacitor contains a dielectric, and there is no free charge, so it is impossible to produce charge movement, which is the current.  How does the ideal capacitor pass AC power?

AC Power

Voltage can form an electric field inside the capacitor, and alternating voltage will produce an alternating electric field. According to the law of full current in Maxwell's equations:

Formula

This means that either a current or a changing electric field can generate a magnetic field. Maxwell defines ε(∂E/∂t) as a displacement current, which is an equivalent current and represents the change of the electric field. (The current here represents the current density, or J)

Let the AC voltage change sinusoidally, ie:

Formula

The actual displacement current is equal to the current density times the area:

Formula

Therefore, the capacitive reactance of the capacitor is 1/ωC. When the frequency is high, the capacitive reactance will be very small, which means passing the high frequency.

DC Blocking

The DC voltage does not change with time, the displacement current ε(∂E/∂t) is 0, and the DC component cannot pass through.

The characteristics of actual capacitors are non-ideal and have some parasitic effects; therefore, a more complicated model is needed to represent the actual capacitors. The commonly used equivalent model is as follow:

Equivalent Model

Figure2. Equivalent Model

  • Since the medium is not absolutely insulated, there is a certain conductivity; therefore, any capacitor has a leakage current, expressed by the equivalent resistance Rleak;
  • The conductors and electrodes of the capacitor have a certain resistivity, and there is a certain dielectric loss of the dielectric; these losses are uniformly expressed as the equivalent series resistance ESR;
  • There is a certain inductance in the conductor of the capacitor, which has a greater impact at high frequencies, expressed as the equivalent series inductance ESL;

In addition, there is a certain hysteresis in any medium, that is, after the capacitor is quickly discharged, the voltage is suddenly disconnected, and the capacitor will recover part of the charge, which is represented by a series RC circuit(Related post: LC circuit).

  • Most of the time, the main concern is the ESR and ESL of the capacitor.

Quality Factor

As with inductors, the quality factor of the capacitor can be defined, which is the Q value, which is the ratio of the stored power of the capacitor to the power loss:

Qc=(1/ωC)/ESR

The Q value is a relatively important parameter for high-frequency capacitance.

Self-Resonance Frequency

Because of the existence of ESL, a resonant circuit is formed together with C, and its resonant frequency is the self-resonant frequency of the capacitor. Before the self-resonant frequency, the impedance of the capacitor becomes smaller as the frequency increases; after the self-resonant frequency, the impedance of the capacitor becomes smaller as the frequency increases, which is inductive. As shown in the following figure:

Self-Resonance Frequency

Figure3. Self-Resonance Frequency

According to the capacitance formula, in addition to the size of the capacitor, the size of the capacitance is related to the Permittivity of the dielectric. The performance of the dielectric affects that of the capacitor, and different media are suitable for different manufacturing processes.

Capacitors can be divided into three main categories according to manufacturing process:

  • Film Capacitor
  • Electrolytic Capacitor
  • Ceramic Capacitor

III Film Capacitor

Film capacitors are made by winding two plastic films with metal electrodes into a cylindrical shape, and finally encapsulated; because its medium is usually plastic material, also known as plastic film capacitors. Its internal structure is roughly as shown in the following figure:

The Structure of Film Capacitor

Figure4. The Structure of Film Capacitor

Film capacitors can be divided into two categories according to the manufacturing process of their electrodes:

3.1 Metal Foil Film Capacitor

For metal foil film capacitors, a thin metal foil, usually aluminum foil, is directly added to the plastic film as an electrode. This process is relatively simple, the electrode is easy to lead out, and can be applied to large current occasions.

3.2 Metallized Film Capacitor

Metalized film capacitors form a thin metal surface directly on the surface of the plastic film by vacuum deposition process as an electrode. Because the thickness of the electrode is very thin, it can be wound into a capacitor with a larger capacity. However, due to the thickness of the electrode, it is only suitable for small current applications.

Metallized Film Construction

Figure5. Metallized Film Construction

The metallized film capacitor has the function of self-repair, that is, if there is a breakdown point inside the capacitor, an avalanche effect will occur at the damaged place, and the vaporized metal will form a vaporized assembly surface at the damaged place, the short circuit disappears, and the damaged point is repaired . Therefore, the reliability of the metalized thin film capacitor is very high, and will not fail due to short circuit.

There are two winding methods for film capacitors:

  • Inductive winding method Before winding, the lead has been connected with the internal electrode.
  • After the non-inductive winding method, gold plating and other processes are used to connect the internal electrodes of the two end surfaces into one surface, so that a smaller ESL can be obtained, and the high frequency performance should be higher.

In addition, there is a laminated type non-inductive capacitor, the structure is similar to MLCC, the performance is better, and it is easy to make SMD package.

Winding Methods

Figure6. Winding Methods

The characteristic of the film capacitor is that it can achieve large capacity and high withstand voltage. However, due to process reasons, its size is difficult to be small, and it is usually used in strong electric circuits, such as the power electronics industry.

Winding Method

Figure7. Winding Methods

IV Electrolytic Capacitor

Electrolytic capacitors use metal as an anode, and form a metal oxide film on the surface as a medium, and then wet or solid electrolyte and metal as a cathode. Electrolytic capacitors are mostly polarized. If the metal on the cathode side also has an oxide film, it is a non-polarized electrolytic capacitor.

Depending on the metal used, there are three types of electrolytic capacitors:

4.1 Aluminum Electrolytic Capacitors

Aluminum electrolytic capacitors should be the most widely used electrolytic capacitors and the cheapest. Its basic structure is shown in the following figure:

The Structure of Aluminum Electrolytic Capacitor

Figure8. The Structure of Aluminum Electrolytic Capacitor

The manufacturing process of aluminum electrolytic capacitors is roughly as follows:

  • First, the aluminum foil will form a very rough surface by electroetching process, which increases the surface area of the electrode and can increase the capacitance;
  • The anode is oxidized by a chemical method to form an oxide layer as a medium;
  • Then, a layer of electrolytic paper is added between the anode aluminum foil and the cathode aluminum foil as a separator, and is pressed and wound;
  • Finally, fill the electrolyte, the electrolytic paper will absorb the electrolyte, and the package is molded.

Wet aluminum electrolytic capacitors using electrolyte are the most widely used, with the advantages of large capacitance, high rated voltage, and low cost. The disadvantages are also obvious, that is, shorter life, poor temperature characteristics, and larger ESR and ESL. For hardware development, it is necessary to avoid over-design. In the case of meeting performance requirements, cheap is the biggest advantage.

Recommendation: How to Test Aluminum Electrolytic Capacitors

4.2 Tantalum Electrolytic Capacitors

The most widely used tantalum electrolytic capacitor should use manganese dioxide as a solid electrolyte. The internal structure of the solid tantalum electrolytic capacitor is roughly as shown in the figure below:

The Internal Structure of the Solid Tantalum Electrolytic Capacitor

Figure9. The Internal Structure of the Solid Tantalum Electrolytic Capacitor

Compared with aluminum electrolytic capacitors, the dielectric constant of tantalum oxide (tantalum pentoxide) is much higher than that of aluminum oxide (aluminum oxide). With the same volume, the capacity of tantalum capacitors is larger than that of aluminum electrolytic capacitors. Tantalum capacitors have a longer life and more stable electrical performance.

The Internal Structure of the Solid Tantalum Electrolytic Capacitor

Figure10. The Internal Structure of the Solid Tantalum Electrolytic Capacitor

Tantalum capacitors also use conductive polymer as electrolyte, the structure is similar to the manganese dioxide tantalum capacitor in the above figure, which is to replace manganese dioxide with conductive polymer. Conductive polymers have higher conductivity than manganese dioxide, so ESR will be lower.

In addition, there are wet tantalum capacitors, which are characterized by super large capacity, high withstand voltage, and low DC leakage current, which are mainly used in military and aerospace fields.

Wet Tantalum Capacitors

Figure11. Wet Tantalum Capacitors

4.3 Niobium Electrolytic Capacitors

Niobium electrolytic capacitors are similar to tantalum electrolytic capacitors, in that niobium and its oxides replace tantalum. The dielectric constant of niobium oxide (niobium pentoxide) is higher than that of tantalum oxide (tantalum pentoxide). The performance of niobium capacitors is more stable and more reliable.

V Ceramic Capacitor

Ceramic capacitors use ceramic materials as dielectric materials. There are many types of ceramic materials with different dielectric constants and stability, which are suitable for different occasions.

Ceramic capacitors mainly include the following:

5.1 Ceramic Disc Capacitor

The main advantage of the ceramic capacitor is that it can withstand high voltage, and it is usually used as a safety capacitor, which can withstand 250V AC voltage. Its appearance and structure are shown below:

Ceramic Disc Capacitor

Figure12. The Structure of Ceramic Disc Capacitor

5.2 Multi-layer Ceramic Capacitor

Multi-layer ceramic capacitors, that is, MLCCs, chip multi-layer ceramic capacitors are currently the most widely used capacitor types in the world. Their standardized packaging and small size are suitable for automated high-density chip production.

The internal structure of the multilayer ceramic capacitor is shown below:

Internal Structure of Chip Multilayer Ceramic Capacitor

Figure13. Internal Structure of Chip Multilayer Ceramic Capacitor

5.3 Monolithic Capacitors

Because multilayer ceramics need to be sintered and porcelainized to form an integrated structure, the multilayer ceramic capacitors in lead packages are also called monolithic capacitors.

The structure of monolithic capacitors is that several ceramic film blanks are covered with electrode paddle material, and after being laminated, they are wound into an inseparable whole at a time, and the outside is encapsulated with resin.

Monolithic capacitors are a new type of capacitors with small volume, large capacity, high reliability and high temperature resistance. Low-frequency monolithic capacitors with high dielectric constant also have stable performance and are actively small.

5.4 Classification of Ceramic Media

According to EIA-198-1F-2002, ceramic media are mainly divided into four categories:

Class I: Ceramic medium with temperature compensation characteristics, the dielectric constant is mostly low, not more than 200. It is usually a paraelectric medium. Under temperature, frequency and bias voltage, the dielectric constant is relatively stable and the change is small. The loss is also very low, the dissipation factor is less than 0.01.

Coding of Class 1 Capacitors According to EIA Specification

Figure14. Coding of Class 1 Capacitors According to EIA Specification

The most stable and most used is the C0G capacitor, or NP0. NP0 is the code name for the IEC/EN 60384-1 standard as Negative Positive Zero, using N and P for Positive and Negative deviations.

Due to the low dielectric constant, the capacitance value of C0G capacitor is small and can be up to 0.1uF. The 0402 package usually has a maximum of 1000pF.

Class II, III: Among them, the temperature characteristic A-S belongs to Class II, and the dielectric constant is about several thousand. The temperature characteristic T-V belongs to Class III, and the dielectric constant can be as high as 20000. It can be seen that the performance of Class III is more unstable. According to the classification of IEC, both Class II and III belong to the second category, high dielectric constant media. For example, X5R and X7R are Class II capacitors, which are widely used in power supply decoupling, while Y5V belongs to Class III capacitors, and their performance is not stable.

EIA Coding of Class 2 and 3 Capacitors

Figure15. EIA Coding of Class 2 and 3 Capacitors

The capacitance value of Class II and III capacitors can be up to several hundred uF, but due to the high dielectric constant medium, most of them are ferroelectric medium (Ferroelectric), and the temperature stability is poor. In addition, the dielectric constant of ferroelectric media will decrease under DC bias voltage.

Class IV: The manufacturing process is different from the usual ceramic materials. The internal ceramic particles are all a thin oxide layer on the outside, and the core is a conductor. This type of capacitor has a large capacity but a small breakdown voltage. Due to the unstable performance and high loss of these capacitors, they have been basically eliminated.

VI Supercapacitor

Supercapacitor refers to a new type of energy storage device between a traditional capacitor and a rechargeable battery. There are two ways to store charge: EDLC and pseudocapacitance. It not only has the characteristics of rapid charge and discharge of the capacitor, but also has the energy storage characteristics of the battery.

The capacity of the super capacitor is particularly large. It can replace the battery as a power supply device, and can also be used in conjunction with the battery. Supercapacitors charge fast, can be fully charged and discharged, and can be charged to any desired voltage, as long as the rated voltage is not exceeded.

There are many applications of supercapacitors, for example, many cities in China have supercapacitor electric buses. There are also applications in some electronic products, such as some driving recorders, which can continue to supply power for several days.

Supercapacitors

Figure16. Supercapacitors

VII Fixed, Trimmer and Variable Capacitors

A capacitor with a fixed capacitance is called a fixed capacitor. According to the different media can be divided into ceramics, mica, paper, film, electrolysis. Having described film capacitors, electrolytic capacitors, and ceramic capacitors, let's look at the other two types of fixed capacitors.  

7.1 Mica Capacitor

Mica capacitor can be divided into foil type and silver type. Silver electroplating is very direct on mica sheet by vacuum evaporation or sintering method. Due to the elimination of air gap, the temperature coefficient is greatly reduced and the capacitance stability is higher than foil type. Mica capacitors are widely used in high frequency electrical appliances and can be used as standard capacitors.

The glaze capacitor is made of a special mixture with a concentration suitable for spraying into a film. The medium is then sintered with a silver layer electrode to form a "monolithic" structure. Glass glaze capacitor is comparable to mica capacitor in performance and can withstand various climates. It can generally work at 200℃ or higher, with rated working voltage up to 500 V and loss tan = 0.0005 ~ 0.008.

Silver Mica Capacitors

Figure17. Silver Mica Capacitors

7.2 Paper Capacitor

Paper capacitors are widely used in radio and electronic equipment. Generally, two aluminum foils are used as electrodes, which are separated by overlapping winding of capacitor paper with a thickness of 0.008 ~ 0.012 mm. Simple manufacturing process, low price, can obtain a large capacitance, generally below 0.25 F, but the capacity error is large and difficult to control, good quality is ±10%, loss (tan ≤ 0.015), temperature and frequency characteristic stability is poor.

The paper capacitors commonly used in the past are non-sealed, impregnated only with ground wax, paraffin wax and chlorinated diphenyl, etc., which are prone to aging and poor stability. They are easily affected by humidity, insulation resistance decreases after being affected by moisture, and atmospheric pressure also affects them. The paper capacitor whose core is sealed inside the metal or ceramic tube is of good quality and has little influence on the external climatic conditions. It can be normally used in the situation with the relative humidity up to 95 ~ 98 %.

The electrode of metallized paper capacitor uses vacuum evaporation to directly attach the metal to the capacitor paper, which is only about 1/4 of the volume of ordinary paper capacitor. Its main feature is its "self-recovery" function, that is, it can be "self-healing" after breakdown. It is an improved type of paper capacitor.

Oil - immersed capacitors have higher voltage than ordinary paper capacitors, good stability, suitable for high - voltage circuits.

Paper capacitors are intermediate frequency capacitors, which are generally used in low-frequency circuits and usually cannot be used in frequencies higher than 3 ~ 4 MHz.

Paper Capacitor

Figure18. Paper Capacitor

7.3 Trimmer Capacitor

Trimmer capacitors, also called semi-variable capacitors, have a capacitance that can be adjusted within a small range and fixed to a certain capacitance value after adjustment.

Ceramic trimmer capacitors are of high quality and small size, and can usually be divided into two types: round tube type and round chip type.

Trimmer capacitors for mica and polystyrene media are usually of spring-loaded structure, which is simple in structure but less stable.

The wire-wound porcelain trimmer capacitor is used to change the capacitance by removing the copper wire (external electrode), so the capacitance can only be reduced and is not suitable for repeated debugging.

7.4 Variable Capacitor

As the name implies, a variable capacitor means that the capacitance value can vary over a large range and can be determined to a certain value. Variable capacitors are divided into two forms: film medium and air medium. It is commonly used in coupling and tuning circuits, such as double capacitor, ceramic capacitor and so on.

VIII Comparison of Polarized Capacitors and Non-polarized Capacitors

8.1 Medium

What is the medium? To put it bluntly is the substance between the two plates of the capacitor. Most polarized capacitors use an electrolyte as the dielectric material. Generally, capacitors of the same volume have large polar capacitance. In addition, different electrolytic materials and processes produce polarized capacitors of the same volume. Furthermore, pressure resistance is also closely related to the use of dielectric materials. There are likewise many non-polarized capacitor dielectric materials, most of which use metal oxide film and polyester. Because the reversible or irreversible performance of the medium determines the use environment of polarized and non-polarized capacitors.

8.2 Performance

Performance is the requirement for use, and maximum demand is the requirement for use. If the metal oxide film capacitor is used for filtering in the power supply part of the TV, the capacitor capacity and withstand voltage required by the filtering must be achieved. Maybe only a power supply can be installed in the case. Therefore, only polarized capacitors can be utilized to filtering, and these capacitors are irreversible. In other words, the positive electrode must be connected to the high potential end, and the negative electrode must be connected to the low potential end.

Generally, the electrolytic capacitor is above 1 microfarad for coupling, decoupling, power supply filtering, etc. Non-polarized capacitors are mostly below 1 microfarad, participating in resonance, coupling, frequency selection, current limiting, etc. Of course, there are also large-capacity and high-pressure-resistant ones, which are mostly used for reactive power compensation of electric power, phase shifting of motors, and frequency shifting power supply. There are many types of non-polarized capacitors, so this article won’t go into details.

Classification of Capacitors

Figure19. Classification of Capacitors

8.3 Capacity

As mentioned earlier, the electrical media of the same volume are different, so the capacity is not equal.

8.4 Structure

In principle, any shape capacitors can be used in the environment without considering the tip discharge. The electrolytic capacitors (polarized capacitors) that are usually used are round, and the square ones are rarely utilized. The shape of non-polarized capacitors varies. Like tube shape, deformed rectangle, sheet shape, square shape,combined square shape and round shape, etc., see where it is used. Of course, there are invisible. Intangible here refers to distributed capacitance. The distributed capacitance must not be ignored in high-neck and intermediate-frequency devices.

8.5 Application Environments and Use

In the repair of home appliances, all of the above may be found. If you want to understand in a simple way, you have to find out by yourself.

Because of the relationship between its internal materials and construction, the capacity of polarized capacitors (such as aluminum electrolysis) can be very large, but its high-frequency characteristics are not good, so it is suitable for power supply filtering and other occasions, but there are also good high-frequency characteristics. Polarized capacitor-tantalum electrolysis, its price is relatively high;

Non-polarized capacitors are small in size, low in price, and satisfactory in high-frequency characteristics, but they are not suitable for large capacity. Like ceramic capacitors, monolithic capacitors, and polyethylene (CBB) capacitors, ceramic capacitors are generally used in high-frequency filtering and oscillation circuits.

Axial and Radial Type Construction

Figure20. Axial and Radial Type Construction

IX Axial and Radial Leaded Capacitors

One method of packaging capacitors is the lead structure.   

Axial capacitance refers to the capacitance of the two pole leads on the same axis. Generally, it is a non-inductive structure. It is made of metalized polyester film as the dielectric/electrode. The wire is tinned copper clad steel wire (or flexible wire), the outer layer is wrapped with polyester tape, and both ends are sealed with epoxy resin.

Axial Lead Structure

Figure21. Axial Lead Structure

Axial leads (the leads are on the same plane as the capacitor axis) are radial leads. The figure below shows an example of a radial lead. The lead is in the radial position of the capacitor. Critical dimensions are lead spacing "S", height "H", length "L" and thickness "P'. Because they are inserted on the printed circuit board rather than on the surface of the circuit board like surface mount components, axial And radial elements are collectively referred to as "plug-in elements".

Radial Lead Structure

Figure22. Radial Lead Structure

X A Quiz About Capacitor Types

Question:

The capacitors which use chemical reactions to store charge are called

A.ceramic capacitors

B.fixed capacitors

C.parallel plate capacitors

D.electrolytic capacitors

Answer:

D

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