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A capacitor is an electrical device that stores energy in the form of an electric field. It consists of two metal plates separated by a dielectric or non-conductive substance. Capacitor types are roughly divided into fixed capacitance and variable capacitance. The most important are capacitors with fixed capacitance, but there are also capacitors with variable capacitance, such as rotating or fine-tuning capacitors. Capacitors with fixed capacitance are divided into film capacitors, ceramic capacitors, electrolytic capacitors, and superconducting capacitors. This article briefly introduces the working principle, structure, and application characteristics of ceramic capacitors.
1. Polarity of Ceramic Capacitors
Ceramic capacitors are most commonly found in every electrical device, which uses ceramic materials as dielectrics. Ceramic capacitors are a non-polarized device, which means that they have no polarity. So it can be connected in any direction on the circuit board. Therefore, they are usually much safer than electrolytic capacitors. This is the symbol of a non-polarized capacitor given below. Many types of capacitors, such as tantalum bead capacitors, have no polarity.
2. Structure and Performance of Ceramic Capacitors
There are three main types of ceramic capacitors, but there are other types to choose from:
1. Leaded disc ceramic capacitors for through-hole mounting with resin coating.
2. Surface mount multilayer ceramic capacitors (MLC).
3. A special type of microwave bare lead-free disc ceramic capacitor designed to be mounted in a slot on a PCB.
Ceramic disc capacitors are made by coating both sides of a ceramic disc with silver contacts, as shown in the figure above. Ceramic disc capacitors have capacitance values of about 10pF to 100μF and a variety of voltage ratings, ranging from 16V to 15KV or even more.
To achieve higher capacitance, these devices can be made of multiple layers. MLCCs are made of a mixture of paraelectric and ferroelectric materials, or layered with metal contacts. After the layering process is completed, the device is placed at high temperature and the mixture is sintered, resulting in a ceramic material with the desired properties. Finally, the resulting capacitor consists of many smaller capacitors connected in parallel, which results in an increase in capacitance.
It is understood that MLCCs consist of more than 500 layers, with a minimum layer thickness of about 0.5 microns. As technology advances, the thickness of the layer will decrease and the capacitance will increase at the same volume.
Ceramic capacitor dielectrics vary from manufacturer to manufacturer, but common compounds include titanium dioxide, strontium titanate, and barium titanate.
Working principle of ceramic capacitors
3. Classification of ceramic capacitors
According to the different ceramic materials, they can be divided into low-frequency ceramic capacitors and high-frequency ceramic capacitors. According to the structural form, they can be divided into multiple types of capacitors such as disc capacitors, tubular capacitors, rectangular capacitors, sheet capacitors, and through-hole capacitors. Different categories of ceramic capacitors are defined according to the operating temperature range, temperature drift, and tolerance.
1. Class I ceramic capacitors
This type of capacitance is mainly related to temperature. These are the most stable capacitors and they have nearly linear characteristics.
The most common compounds used as dielectrics are:
A. Magnesium titanate with a positive temperature coefficient.
B. Calcium titanate for capacitors with a negative temperature coefficient.
2. Class II ceramic capacitors
This type of capacitor shows better performance in terms of volumetric efficiency, but this comes at the expense of lower accuracy and stability. Therefore, they are often used in decoupling, coupling, and bypass applications where accuracy is not important.
A. Temperature range: -50°C to +85°C
B. Dissipation factor: 2.5%
C. Accuracy: Poor
3. Class III ceramic capacitors
This type of ceramic capacitor has high volumetric efficiency, low accuracy, and low loss factor. It cannot withstand high voltages. The dielectric used is usually barium titanate.
A. Class III capacitors vary their capacitance by -22% to +50%
B. Temperature range is +10°C to +55°C.
C. Dissipation factor: 3 to 5%.
D. Has fairly poor accuracy (usually 20% or -20/+80%).
E. Typically used for decoupling or other power supply applications where accuracy is not an issue
