One commonly-used capacitor type is the ceramic capacitor. Ceramic capacitors are non-polarized capacitors used in applications such as filtering, coupling, decoupling, and timing. The ceramic capacitor voltage rating is fairly high and is capable of handling both DC and AC voltage. However, ceramic capacitors are usually derated while employed in circuits. In this article, we will explore the reasoning for derating the ceramic capacitor voltage.
Ceramic capacitors are fixed, non-polarized capacitors in which the ceramic materials act as a dielectric. Ceramic capacitors consist of a metal layer and alternating layers with ceramics. During the manufacturing of ceramic capacitors, a metal is coated at either end of a thin ceramic dielectric material. Several of these ceramic layers are stacked together and are separated from each layer using more ceramic. The layers are connected by metal electrodes, which are taken out as terminal leads. Ceramic capacitor values vary from 1pF to about 1µF, with a working ceramic capacitor voltage rating of up to a few thousand volts. These capacitors are suitable for high-temperature applications.
There are two types of ceramic capacitors:
ㆍMultilayered Ceramic Capacitors (MLCC): Multilayered ceramic capacitors are mostly used in surface mount technology and, due to their smaller size, in EMI or RFI suppression systems. They are also employed as feed-through capacitors.
ㆍCeramic Disc Capacitors: In ceramic disc capacitors, the ceramic disc is coated with silver electrodes on both sides. Multiple layers of ceramic materials are included to enhance the capacitor. They are usually manufactured based on through-hole technology. The most common application of ceramic disc capacitors is as a safety capacitor in EMI suppression circuits.
There are three classifications of ceramic capacitors:
ㆍClass 1 Ceramic Capacitor: A class I ceramic capacitor uses ceramic materials that are not sensitive to temperature changes. Class I ceramic capacitors are commonly used in high-frequency circuits in TV and radio tuners, oscillators, and filters.
ㆍClass 2 Ceramic Capacitor: Ceramic materials derived from barium titanate (with a permittivity equal to 6000+) that are temperature sensitive are used in class 2 ceramic capacitors. They are suitable for coupling, bypass, and buffer applications.
ㆍClass 3 Ceramic Capacitor: Ceramic capacitors offer higher volumetric efficiency than class 2 ceramic capacitors. However, class 3 ceramic capacitors offer poor temperature stability, accuracy, and aging over time compared to their counterparts.
Here are three key characteristics of ceramic capacitors that engineers should keep in mind:
Accurate precision and tolerances: Ceramic capacitors exhibit stable capacitance values and stable performances.
Size: MLCC capacitors offer high packaging density and support circuit compactness.
High power and voltage withstanding capacity: Ceramic capacitors can handle high power and high voltages. Power ceramic capacitors are well-known for high voltage ratings ranging from 2kV to 100kV.
Ceramic Capacitor Voltage Rating
In ceramic capacitors, there are two conducting electrodes or plates separated by an insulating or dielectric material. Electrodes are placed closer to increase the capacitance with a thin layer of dielectric material. The dielectric material used possesses a breakdown voltage value. When the voltage applied across the capacitor plates exceeds the breakdown voltage value, the molecular structure of dielectric material changes and starts to conduct current through it. As the applied voltage of the capacitor crosses the breakdown value, it behaves like a resistor.
The voltage rating of a ceramic capacitor can be related to the dielectric strength or breakdown voltage of the dielectric material. The voltage rating of the ceramic capacitor gives the maximum safe potential difference that can be applied between the positive and negative capacitor plates. It is the voltage that a ceramic capacitor can handle safely without dielectric breakdown.
Ceramic capacitors rarely experience catastrophic failures, as they are often built with a large safety margin in voltage rating. However, the capacitance value of ceramic capacitors can decrease up to 90% at rated voltage. This phenomenon, especially when observed in under-applied DC voltages in multilayered ceramic capacitors, is called the voltage coefficient of capacitance (VCC).
To prevent VCC and to impart protection, derated ceramic capacitors are used in electronic circuits. The rule of thumb for derating is to select a ceramic capacitor with a voltage rating greater than or equal to two times the voltage to be applied across it in the application. That means, for example, if the actual capacitor voltage is 50V, select a capacitor rated for at least 100 V.
It is a common practice in electronic component selection to derate the ceramic capacitor voltage rating by 50% to prevent explosion as well as VCC. In any electronic circuit designed with ceramic capacitors, this trend should be followed.
ㆍThe two types of ceramic capacitors are multilayered ceramic capacitors (MLCC) and ceramic disc capacitors.
ㆍThe voltage rating of a ceramic capacitor gives the maximum safe potential difference that can be applied between the positive and negative capacitor plates.
ㆍIt is a common practice in electronic component selection to derate the ceramic capacitor voltage rating by 50% to prevent explosion as well as VCC.