A note on Capacitors

Why does capacitors get heated?

Normally, flow of current is prevented by the dielectric that is present between the capacitive plates. The main reason is that dielectric is an insulator. The permitivity for the dielectric material defines 

the charge concentration of the material. To put the definition simple, permittivity of the material is the ability to store field. While permittivity of vaccum is 1, all the other dielectric materials have 

permittivity more than 1 which means they can concentrate some amount of energy. So, there definitely would be some leakage current flowing through capacitors. Higher the leakage current more the power loss and capacitor gets heated up. This again depends on temperature, pressure and frequency of operation.

Capacitors are rated for a particular voltage. What happens if we apply more than that voltage?

General rule of thumb is that capacitor voltage must be rated 2.5 times of the voltage applied in a particular application. However, many designers use capacitors near to their voltage rating. If voltage 

exceeds beyond the voltage rating, dielectric breakdown occurs and capacitor gets damaged. The main reason is that when over voltage is applied, there shall be more field strength created in the insulator causing more current to flow.

How can a small capacitor sizes be achieved?

For a given capacitance value, the only way is to use a dielectric material of very high relative permittivity. 

On what factors does the capacitor storage depends on?

Ideally, the capacitor should retain the charge forever. However, the dielectric has some leakage which causes the current to flow and slowly the capacitor discharges.

What is RC Time constant?

The time required to charge a capacitor to 63% of it's full capacity or the time required to discharge a capacitor to 37% of it's initial charge value is known as Time  constant. Time constant is calculated 

by product of Resistance and Capacitance.

What are the various issues that a improper capacitor selection can lead to?

More leakage through the capacitor can lead to excessive power dissipation, instability and output oscillation in the case of power regulators, increase in the ripple and hence noise in the circuit if proper filtering capacitor is not used, a normal capacitor in case of X and Y type requirement can lead to an EMI/EMC failure. 

What are the different dielectrics used in capacitors?

Ceramic (X7R,C0G,X5R,.)

Plastic Film

Polyester

AlO2 (Aluminium electrolytic)

MnO2 (Tantalum capacitors)

Mica (mainly in RF applications)

What is ferroelectric hysteresis loss in capacitor?

Dielectric materials also exhibit hysteresis similar to magnetic materials. This is a condition where the dielectric material exhibits saturation and this can lead to losses. This is commonly observed in 

ceramic materials.