g. Ceramic capacitor:
The non – polarized type ceramic capacitors which are also known as ‘Disc capacitors’ are widely used these days. These are available in millions of varieties of cost and performance. The features of ceramic capacitor depend upon:
· Type of ceramic dielectric used in the capacitor which varies in the temperature coefficient.
· Dielectric losses.
The exact formulas of the different ceramics used in ceramic capacitors vary from one manufacturer to another. The common compounds such as titanium dioxide, strontium titanate, and barium titanateare the three main types available although other types such as leaded disc ceramic capacitors for through hole mounting which are resin coated, multilayer surface mount chip ceramic capacitors and microwave bare leadless disc ceramic capacitors that are designed to sit in a slot in the PCB and are soldered in place.
These are made by placing silver coated ceramic plateson two sides and assembled together to form the capacitor. The surface mount version consists of the ceramic dielectric in which a number of interleaved precious metal electrodes are contained. This structure gives rise to a high capacitance per unit volume. The inner electrodes are connected to the two terminations, either by silver palladium (AgPd) alloy in the ratio 65 : 35, or silver dipped with a barrier layer of plated nickel and finally covered with a layer of plated tin (NiSn).
The Electronics industries alliance (EIA) has broadly classified the ceramics used in these capacitors into 3 classes – class 1,class 2 and class 3.The lower is the class better are its overall characteristics but is on the cost of size. Each class defines the working temperature range, temperature drift, tolerance, etc. The typical values range from 10pF to 1uF. The capacitance values are labeled by three digit codes where the first two digits represent a number and the third digit is the multiplier digit.
For example: 103 means 10 * 103 pF which is 0.01uF
104 which is 10*104 pF which is 0.1uF
The tolerance is indicated by a letter like j=5%, K=10% and M=20%.
These capacitors are commonly used as a timing element in filter circuit and balancing oscillator circuits in radio frequency applications, coupling and decoupling networks.
The three ceramic classes decided by EIA are:
a. Class1 - Class 1 ceramic capacitors are the most stable forms of ceramic capacitor with respect to temperature. The common compounds used as the dielectrics are magnesium titanate for a positive temperature coefficient (PTC), or calcium titanate for capacitors with a negative temperature coefficient (NTC). Using combinations of these and other compounds it is possible to obtain a dielectric constant of between 5 and 150. They have an almost linear characteristic and their properties are almost independent of frequency within normal bounds. Temperature coefficients between +40 and -5000 ppm/C can be obtained.
Class 1 capacitors offer the best performance with respect to dissipation factor. A typical figure may be 0.15%. It is also possible to obtain very high accuracy (~1%) class 1 capacitors rather than the more usual 5% or 10% tolerance versions. The highest accuracy class 1 capacitors are designated C0G or NP0.
EIA has defined a set of codes in order to have a managed way of ceramic capacitor performance. The codes of class 1 and class 2 capacitors are different.
The class 1 codes are as follows:
· The first character is a letter which gives the significant figure of the change in capacitance over temperature in ppm/C.
· The second character is numeric and gives the multiplier.
· The third character is a letter and gives the maximum error in ppm/C.
One common example of class 1 capacitor is a C0G. This has 0 drift, with an error of 30PPM/C.
b. Class 2 - Class 2 capacitors are better in size, but have less accuracy and stability. As a result, they are normally used for decoupling, coupling and bypass applications where accuracy is not of prime importance. A typical class 2 capacitor may change capacitance by 15% or so over a -50C to +85C temperature range and it may have a dissipation factor of 2.5%. It will have average to poor accuracy (from 10% down to +20/-80%). However for many applications these figures would not present a problem.
The class 2 codes are as follows:
. The first character is a letter which gives the low-end operating temperature.
· The second is numeric which provides the high-end operating temperature.
· The third character is a letter which provides capacitance change over that temperature range.
The common examples of class 2 ceramic capacitors are:
· The X7R capacitor which operates from -55C to +125C with a capacitance change of up to 15%.
· The Z5U capacitor which operates from +10C to +85C with a capacitance change of up to +22% to -56%.
c. Class 3 - Class 3 ceramic capacitors are small in size with less accuracy, stability and low dissipation factor. This type of capacitors cannot withstand high voltages.
Barium titanate that has a dielectric constant about 1250 is used as the dielectric. A typical class 3 capacitor will change its capacitance by -22% to +50% over a temperature range of +10C to +55C. It may also have a dissipation factor of around 3 to 5%. It will have a fairly poor accuracy (commonly, 20%, or -20/+80%). Therefore, class 3 ceramic capacitors are typically used as decoupling or in other power supply applications where accuracy is not of prime importance. However, they must not be used in applications where spikes are present as they cannot withstand high voltages.
SMT ceramic capacitors are also available in standard packages which have following designations given in the below table.