Points of Discussion
- Definition and Overview
- Construction and Styles of Ceramic Capacitors
- Multilayer Ceramic Capacitors
- Ceramic Power Capacitors
- Electrical Characteristics
Definition and Overview
A capacitor is defined as a passive electrical device that stores electrical energy in an electric field. It is a two-terminal device.
A ceramic capacitor is a kind of capacitor where the ceramic powder utilized as the dielectric material.
Ceramic capacitors have a fixed value. It consists of more than two ceramic alternating layers and a metal layer, which acts as the capacitor’s electrode. The composition of the capacitor represents the electrical behaviour, and thus, they have different applications. There are two types of ceramic capacitors.
Class 1 type Ceramic Capacitor:
These capacitors provide higher stability and lower losses for applications in resonant circuits.
Class 2 type Ceramic Capacitor:
These capacitors provide higher volumetric efficiency for buffer, by-passing, and coupling applications.
Multilayer ceramic capacitors are the most used capacitors in electronic devices. That is why it is the most produced capacitor also (more than any capacitor). The range of products is about one trillion units per year!
Construction and Styles of Ceramic Capacitors
Ceramic capacitors are made up of a blend of excellent granules of paraelectric materials, accurately blended with other types of materials to accomplish the desired characteristics. Ground granules of ferroelectric materials can also be used for the blending. The ceramic is separated from the mixing and sintered at high temperatures.
Being one of the most popular types of capacitor, a ceramic capacitor has various styles and shapes. Some of them are discussed below.
- Multilayer Ceramic Chip Capacitor (MLCC): It is rectangular and utilized as surface mounting purpose.
- Ceramic Disc Capacitor (CDC): A single layer disc has a coat of resin. It has through-hole leads.
- Feedthrough Ceramic Capacitor (FCC): It is a tube-like capacitor whose inner metallization is contacted with a lead, external metallization for soldiering. It is used as a bypass-capacitor in high-frequency circuits.
- Ceramic Power Capacitor (CPC): This type of ceramic capacitor has a larger ceramic body, and it is specially designed for high voltage applications.
Multi-layer ceramic capacitor (MLCC)
It is constructed of individual capacitors, correspondingly placed one after another via the terminal surfaces. The primary material needed to build every single MLCC is the ground granules of paraelectric materials, which is further modified by adding some pre-determined additives. Ferroelectric materials can also serve the purpose, as mentioned earlier. Now, all these powdered materials are mixed equally. The manufacturer determines the composition of the blending and the size of the particles.
A thin ceramic foil is employed from a dust suspension with a suitable loose-leaf folder. The foil is then cut into equally sized sheets with metal paste. These sheets will be the electrodes for the capacitor. In a further automated process, the sheets are kept one after another in a required number of layers. They are also solidified by giving pressure. The capacitance value is also determined by relative permittivity, size, and the number of layers.
After the cutting process, the mixture is burnt out of the stacked layers. Now, a sintering process occurs at 1200 to 1450 degrees Celsius. It produces the final and main crystalline structure. The burning forms the desired dielectric properties. After the burning, cleaning, and metallization of both surfaces are done. The metallization process connects the ends and the inner electrode in parallel connections. The capacitor is also introduced with terminals in the metallization process.
The formula for the capacitance of an MLCC capacitor is based on a parallel plate capacitor’s procedure, which has several layers. It is given as follows.
C = (ε . n . A ) / d
Here, ε is the permittivity of the dielectric material. A stand for the electrode’s surface area, ‘n’ is the number of layers, and d is the distance between the electrodes.
A more considerable value of ‘A,’ i.e., more electrode surface area and a thinner dielectric, eventually increase the MLCC capacitor’s capacitance value. A material with higher permittivity does the same for MLCC capacitor.
The era of digitalization has increased the need for miniaturization. An MLCC miniaturization involves the reduction of dielectric thickness and simultaneously increase in the number of layers. There is no need to say, but the process requires enormous efforts and needs a lot of expertise.
In 1995, the minimum value of the thickness of the dielectric layer possible was nearly four micro-meters. As time flows, the thickness gradually decreases with the advancement of technologies. By 2005 the thickness measured was close to 1 micro-meter. And five years later, the consistency was measured as 0.5 micro-meter.
The reduction in these capacitors’ size is achieved by reducing the power grain size and making the layers thinner. Technological advancement has helped the manufacturer to control the process more precisely. That is why more numbers of layers are being stacked.
What is Ceramic Power Capacitors ?
Ceramic Power Capacitors
Ceramic capacitors used in very high power or high voltage applications are known as ceramic power capacitors.
The materials used for making a ceramic power capacitor are the same as materials used to make small ceramic capacitors. This type has applications in high voltage power systems, electrical transformers, and various electrical installations.
Previously the power variation part was held separately by the electrical power components. Now, the distinction between the ‘electronic’ and ‘electrical’ becomes less distinct. In previous times, the boundary in the middle of electrical and electronics was roughly at a reactive power of 200volt-amps. Modern time electronics can handle the excess energy.
Usually, power ceramic capacitors are made for a higher power value than 200 volt-amps. Ceramic power capacitors have a great range of diversity in their style. The good plasticity of raw ceramic material and the higher dielectric strength of ceramics provide a path for many applications and explain the diversity. These power capacitors have already spent decades in the market.
The production depends upon the requirement as the high stability low losses requirement leads to class 1 power capacitors’ production. Similarly, a condition in high volumetric efficiency leads to the production of class 2 ceramic power capacitors. Class 1 types of capacitors are generally used for resonant circuits, whereas class 2 types are used as circuit breakers, power distribution lines, and high voltage power supplies.
The size of the power capacitors can be considerable. Working inside a high-power application can generate lots of heat. That is why some particular types of ceramic power capacitors have water cooling facilities.
Series equivalent circuit
The circuit below specifies the model.
C is the capacitor’s capacitance; RESR is the equivalent series resistance, which takes into account all the ohmic losses. LESL is the equivalent series inductance and considered as the self-inductance of the capacitor. Bleak is the leakage resistance.
Capacitance, standard values, and tolerances
The allowed percentage deviation from the capacitance’s rated value is known as the tolerance of the capacitor. Particular applications can determine the required capacitance value.
A standard capacitor is considered as a storage component in electrical energy. Sometimes it is used as a resistive element in an AC circuit. An electrolytic capacitor is used as a decoupling capacitor in a course. It blocks the DC component of the signal with the help of the dielectric material.
ESR, Dissipation Factor, Quality Factor
Ceramic power capacitors suffer ohmic AC losses. The DC loss is known as ‘leakage current’ and is negligible for an AC specific purpose. The ohmic AC loss is non-linear in type and depends on frequency, humidity, temperature. There are two physical conditions behind the losses.
- The line losses occur due to internal supply line resistance. The connection resistance of the electrode-contact also has some influence to it.
- The dielectric loss occurs due to dielectric polarization.
ESR or equivalent Series Resistance is specified as the aggregate of total resistive losses of a capacitor. That can also be identified as dissipation factor (DF, tan δ) or as Quality Factor (Q) depending upon the requirement.
The dissipation factor is commonly used to specify class 2 capacitors. It is determined as the tangential value of reactance (Xc – XL).
The following formula represents it.
tan δ = ESR * ωC
Unlike class 2 capacitors, class 1 capacitors use quality factor (Q) for the specification. Quality factor (Q) is the reciprocal of the Dissipation Factor (DF).
Q = 1 / tan δ = f0 / B
B is the bandwidth, and f0 is the resonant frequency.