Ceramic capacitor


Ceramic capacitor

In electronics, a ceramic capacitor is a capacitor constructed of alternating layers of metal and ceramic, with the ceramic material acting as the dielectric. The temperature coefficient depends on whether the dielectric is Class 1 or Class 2. A ceramic capacitor (especially the class 2) often has high dissipation factor, high frequency coefficient of dissipation.

Contents

Construction

Ceramic capacitors

A ceramic capacitor is a two-terminal non-polar device. The classical ceramic capacitor is the "disc capacitor". This device pre-dates the transistor and was used extensively in vacuum-tube equipment (e.g., radio receivers) from about 1930 through the 1950s, and in discrete transistor equipment from the 1950s through the 1980s. As of 2007, ceramic disc capacitors are in widespread use in electronic equipment, providing high capacity and small size at low price compared to other low value capacitor types.

Ceramic capacitors come in various shapes and styles, including:

  • disc, resin coated, with through-hole leads
  • multilayer rectangular block, surface mount
  • bare leadless disc, sits in a slot in the PCB and is soldered in place, used for UHF applications
  • tube shape, not popular now

Classes of ceramic capacitors

Three classes of ceramic capacitors are commonly available:[1] [2]

Class I capacitors: accurate, temperature-compensating capacitors. They are the most stable over voltage, temperature, and to some extent, frequency. They also have the lowest losses. On the other hand, they have the lowest volumetric efficiency. A typical class I capacitor will have a temperature coefficient of 30 ppm/°C. This will typically be fairly linear with temperature. These also allow for high Q filters—a typical class I capacitor will have a dissipation factor of 0.15%. Very high accuracy (~1%) class I capacitors are available (typical ones will be 5% or 10%). The highest accuracy class 1 capacitors are designated C0G or NP0.

Class II capacitors: better volumetric efficiency, but lower accuracy and stability. A typical class II capacitor may change capacitance by 15% over a −55 °C to 85 °C temperature range. A typical class II capacitor will have a dissipation factor of 2.5%. It will have average to poor accuracy (from 10% down to +20/-80%).

Class III capacitors: high volumetric efficiency, but poor accuracy and stability. A typical class III capacitor will change capacitance by -22% to +56% over a temperature range of 10 °C to 55 °C. It will have a dissipation factor of 4%. It will have fairly poor accuracy (commonly, 20%, or +80/-20%). These are typically used for decoupling or in other power supply applications.

At one point, Class IV capacitors were also available, with worse electrical characteristics than Class III, but even better volumetric efficiency. They are now rather rare and considered obsolete, as modern multilayer ceramics can offer better performance in a compact package.

These correspond roughly to low K, medium K, and high K. Note that none of the classes are "better" than any others—the relative performance depends on application. Class I capacitors are physically larger than class III capacitors, and for bypassing and other non-filtering applications, the accuracy, stability, and loss factor may be unimportant, while cost and volumetric efficiency may be. As such, Class I capacitors are primarily used in filtering applications, where the main competition is from film capacitors in low frequency applications, and more esoteric capacitors in RF applications. Class III capacitors are typically used in power supply applications. Traditionally, they had no competition in this niche, as they were limited to small sizes. As ceramic technology has improved, ceramic capacitors are now commonly available in values of up to 100 µF, and they are increasingly starting to compete with electrolytic capacitors, where ceramics offer much better electrical performance at prices that, while still much higher than electrolytic, are becoming increasingly reasonable as the technology improves.

Coding

There is a three digit code printed on a ceramic capacitor specifying its value. The first two digits are the two significant figures and the third digit is a base 10 multiplier. The value is given in picofarads (pF). A letter suffix indicates the tolerance[1]:

C ± 0.25 pF M ±20%
D ± 0.5 pF P +100 −0%
J ± 5% Y −20 +50%
K ±10% Z −20 + 80%

Example: a label of "104K" indicates 10×104 pF = 100,000 pF = 100 nF = 0.1 µF ±10%

There is also an EIA three character code that indicates temperature coefficient. For non-temperature-compensating capacitor, the code consists of three letters. The first character is a letter that gives the low-end operating temperature. The second is a digit gives the high-end operating temperature. The final letter gives capacitance change over that temperature range:

Letter (low temp) Digit (high temp) Letter (change)
X= −55 °C (−67 °F) 2= +45 °C (+113 °F) D= ±3.3%
Y= −30 °C (−22 °F) 4= +65 °C (+149 °F) E= ±4.7%
Z= +10 °C (+50 °F) 5= +85 °C (+185 °F) F= ±7.5%
6=+105 °C (+221 °F) P= ±10%
7=+125 °C (+257 °F) R= ±15%
8=+150 °C (+302 °F) S= ±22%
T= +22 to −33%
U= +22 to −56%
V= +22 to −82%

For instance, a Z5U capacitor will operate from +10 °C to +85 °C with a capacitance change of at most +22% to −56%. An X7R capacitor will operate from −55 °C to +125 °C with a capacitance change of at most ±15%.

Temperature-compensated capacitors use a different EIA code. Here, the first letter gives the significant figure of the change in capacitance over temperature in ppm/°C. The second character gives the multiplier. The third character gives the maximum error from that in ppm/°C. All ratings are from 25 to 85 °C:

Significant Figure Multiplier Tolerance
C: 0.0 0: -1 G: ±30
B: 0.3 1: -10 H: ±60
L: 0.8 2: -100 J: ±120
A: 0.9 3: -1000 K: ±250
M: 1.0 4: +1 L: ±500
P: 1.5 6: +10 M: ±1000
R: 2.2 7: +100 N: ±2500
S: 3.3 8: +1000
T: 4.7
V: 5.6
U: 7.5

For instance, a C0G will have 0 drift, with an error of ±30 ppm/°C, while a P3K will have −1500 ppm/°C drift, with a maximum error of ±250 ppm/°C.

Note that in addition to the EIA capacitor codes, there are industry capacitor codes and military capacitor codes.

HF use

Ceramic capacitors are suitable for moderately high-frequency work (into the high hundreds of megahertz range, or, with great care, into the low gigahertz range), as modern ceramic caps are fairly non-inductive compared to the other major classes of capacitors (film and electrolytic). Capacitor technologies with higher self-resonant frequencies tend to be expensive and esoteric (typically, mica or glass capacitors).

Sample self-resonant frequencies for one set of C0G and one set of X7R ceramic capacitors are:

10 pF 100 pF 1 nF 10 nF 100 nF 1 µF
C0G (Class 1) 1550 MHz 460 MHz 160 MHz 55 MHz
X7R (Class 2) 190 MHz 56 MHz 22 MHz 10 MHz

Tantalum capacitor replacement use

Multilayer ceramic capacitors are increasingly used to replace tantalum and low capacitance aluminum electrolytic capacitors in applications such as bypass or high frequency switching power supplies as their cost, reliability and size becomes competitive. In many applications, their low ESR allows the use of a lower nominal capacitance value.[3]

Microphony

Some ceramic capacitors are slightly microphonic.

See also

  • Tape casting

References


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