A. Temperature coefficient (capacity-temperature characteristic):

The high K medium, which moves the Curie point spike to near room temperature by adjusting the formula, exhibits a very high dielectric constant at 25°C, but at the same time, the K value changes very much whether it is heating or cooling. The low-K medium, on the other hand, has a formulation system that allows the Curie spike to be depressed and broadened, thus exhibiting better stability as desired.

The temperature coefficient (T.C.) of Class I porcelain is expressed in ppm/℃, while Class II porcelain is expressed in% ΔC. The method of measuring the temperature coefficient is to place the chip capacitor sample in a temperature-controlled temperature laboratory or "T.C." laboratory, and accurately read the capacitance at different temperatures (usually -55°C, 25°C, and 125°C). Obviously, precision fixtures and test instruments become very important, especially when measuring small capacitance, its ppm/℃ value is very small, and the change of capacity is often much less than 1 picofaram compared with the reference value. Due to the presence of de-aging, attention must be paid to the measurement of class II media with height K. If the de-aged sample is measured during the heating process, the T.C. result must be wrong; therefore, the T.C. measurement must be performed at least one hour after the capacitor is de-aged.

The following expression can be used to calculate the temperature coefficient of Class I medium in any given temperature range, in ppm/℃:

T.C.(ppm/℃) = [(C₂-C₁) / C₁(T₂-T₁)]106

Here: C₁= T₁Capacitance under

C₂= T₂Capacitance under

and t₂> T₁

Example: The capacitance measurement of a sample is as follows:

-55℃,1997 pF

25℃,2000 pF

125℃,2004 pF

The slope of T.C. in the range of -55 ℃ to 25 ℃ is:

T.C. = [(2000-1997)10⁶] / 1997[25-(-55)] = 18.7 pps/℃

The T.C. slope from 25°C to 125°C is:

T.C. = [(2004-2000)10⁶] / 2000(125-25) = 20.0 ppm/℃

The temperature coefficient of class II media is expressed as a percentage change in the reference value of room temperature, which is several orders of magnitude larger than that of linear media.

　　T.C.( %) = (C₂-C₁) / C1

Here: C₁= T₁Capacitance under

C₂= T₂Capacitance under

B. Classification of media

Class I media because of its use of non-ferroelectric (electricity) formula, to TiO.₂It is the main component (dielectric constant is less than 150), so it has the most stable performance. by adding small amounts of other (ferroelectric) oxides, such as CaTiO₃或SrTiO₃The "extended" temperature compensation ceramics can exhibit an approximately linear temperature coefficient, and the dielectric constant increases to 500. Both types of media are suitable for capacitors with high stability requirements in the circuit, that is, the dielectric constant has no aging or negligible aging, low loss (DF<0.001, or DF<0.002 for extended T.C. media), The change in capacity or dielectric loss with voltage or frequency is zero or negligible, and the linear temperature characteristic does not exceed the specified tolerance.

The use of the "letter-number-letter" code form to represent the temperature coefficient of Class I ceramics has been widely used and is adopted by the Electronic Industries Association (EIA) standard 198.

The most commonly used class I medium in chip capacitors is COG, with a temperature coefficient of 0 ppm/℃ 30 ppm/℃, that is, NPO (negative-positive-zero) in MIL standard, which has a very flat temperature coefficient.

The actual measured temperature coefficient is not a perfect linear relationship, but it is acceptable as long as its value does not exceed the tolerance range specified by the last letter of the EIA code:

For example:

C0G -0 ppm/℃ ± 30 ppm/℃

S2L -330 ppm/℃ ± 500 ppm/℃

U2j-750 ppm/℃ ± 120 ppm/℃

M7g-100 ppm/℃ ± 30 ppm/℃

Class II media are composed of ferroelectrics. The dielectric constant of this type of medium is much higher than that of Class I medium, but its performance is less stable with temperature, voltage, frequency and time. Due to the diversification of ferroelectric ceramic properties, it is necessary to divide such media into two subclasses based on temperature characteristics.

　　"Stable Medium K" Class II Porcelain, Based on 25°C, the maximum temperature coefficient is ± 15% in the range of -55°C to 125°C. The dielectric constant of this medium is between 600 and 4000, which is consistent with the properties of X7R in EIA.

　　"High K" Class II Porcelain, the temperature coefficient exceeds the level of X7R. The dielectric constant of this high K medium is as high as 4000~18000, but the temperature coefficient curve is very steep, because the Curie point moves to near room temperature, and the maximum dielectric constant appears.

The most common medium-K material used for chip capacitor fabrication is X7R (ΔC max ± 15% within − 55 ° C. to 125 ° C.). For high K materials, Z5U (ΔC maximum + 22% ~ -56% within +10 ℃ to +85 ℃) and Y5V (ΔC maximum + 22% ~ -82% within -30 ℃ to +85 ℃) are most commonly used.