To understand how electrolytic capacitors are formed, advantages and their uses in applications, let’s take a review of what capacitor is ? and at what parameter capacitance of a capacitor depends.
What is “CAPACITOR “?
“ The capacitor can be defined as an electronic device used to store electronic charges in term of an electric field”
The capacitor is a passive component. At this point you are thinking what the term passive define? “ passive indicate those devices which are not capable to control current by mean of another electrical signal. ” (i.e, resistors, transformer, inductors, diodes and capacitor)
Two types of capacitors (i.e polar and non-polar capacitors). An electrolytic capacitor is a polar capacitor (i.e having polarity). In this article, we will be discussing polar capacitor (i.e aluminium electrolytic capacitor which is categorized as an electrolytic capacitor).
The two metallic plates used in capacitors (i.e anode and cathode) have an ability to store charges when the correct voltage polarity is applied. In parallel plate capacitor when the voltage is applied then the electric field in produced. This electric field is very high as a result the capacitance of a capacitor will decrease (because capacitance is inversely proportional to the electric field). The capacitance is the ability to store charges in 1V potential difference. Since in order to increase the capacitance of capacitor a dielectric is inserted between the plates of the capacitor, a dielectric is an electrically insulating material.
By introducing a dielectric the electric field is decreased, which result in the decrease of voltage and the capacitance will increase. Capacitance depends on three parameters (i.e, the area of the capacitor, a distance between the plates, and permittivity of the dielectric).
Electrolytic capacitors are used in wide variety of applications such as in power supplies, motherboards of TV and computers, more inert circuits, microcontroller boards, audio amplifiers, used for coupling and decoupling purpose etc. Electrolytic capacitors provide a very high value of capacitance because of the use of dielectric between the plates of the capacitor.
HOW BASICALLY ARE AN ELECTROLYTIC CAPACITORS FORMED?
An electrolytic capacitor comprises two plates (i.e, anode and cathode) made up of metal, the dielectric is formed on the anode plate by the process of anode oxidation, this process forms an insulating oxide layer (i.e, a dielectric for capacitor) on the anode plate (anode oxidation is an electrochemical process which is needed to make the metal durable and corrosion-resistant). Whereas the electrolyte (a liquid which is ionized ) which serves the role of the cathode. The thickness of this oxide layer depends on the maximum operating voltages of the capacitor in order to protect the dielectric from breakdown.
Three types of electrolytic capacitors in term of their metal plates:
- ALUMINIUM ELECTROLYTIC CAPACITOR.
- TANTALUM ELECTROLYTIC CAPACITOR.
- NIOBIUM ELECTROLYTIC CAPACITOR.
In this article, we will be discussing aluminium electrolytic capacitor.
ALUMINIUM ELECTROLYTIC CAPACITOR:
Aluminium electrolytic capacitors hold the main position among other types of capacitors. And can be used for wide variety of applications. The main advantages which make electrolytic capacitors more useful over other types of capacitors are a there very high value of capacitance, high reliability, and best performance.
CONSTRUCTION PRINCIPLES OF ALUMINIUM ELECTROLYTIC CAPACITORS:
Since an electrolytic capacitor consists of two aluminium foils (i.e, one foil serves a role of anode and other serves a role of the cathode), separated by a dielectric material (we have discussed earlier the role of dielectric ). At cathode side, there is an electrolyte solution (basically a liqid which is ionized) and a spacer paper.
An anode plate of an aluminium electrolytic capacitor is made up of aluminium (i.e, a metal) which is highly pure (i.e, 99.99% pure). The thickness of this plate is around 20~100um.
- ETCHING (ROUGHING):
Since the capacitance of a capacitor is also dependent on the surface area of the metallic plate. By the process of etching, the effective surface area of the capacitor is increased.
There are two basic processes of etching.
- The aluminium foil is subjected to a hydrochloric acid solution (also known as physical etching) .
- The aluminium foil which serves a role of the anode by the process of electrolysis is placed in the aqueous hydrochloric acid solution (which is basically an electrochemical process). At this point, you are thinking what is an electrolysis? In this process, an aluminium foil is placed into a liquid containing ions and then the liquid is subjected to electric current (i.e, either DC or either AC ).
The method of etching can be determined by the desired performance of a capacitor. After etching the chlorine ions on the foil’s surface can destroy the aluminium metal slowly by a chemical reaction as a result capacitor may be damaged. Therefore to protect the foil from damaging the aluminium foil is rinsed into the water. The picture below shows the surface area for low and high voltage foil after etching.
- FORMING (ANODE OXIDATION):
After etching the process of forming a dielectric. The oxide layer is formed on the etched aluminum foil, this oxide layer plays a role of a dielectric. The etched foil is immersed into an electrolytic solution (i.e, ammonium phosphate or ammonium boric acid) and subjected to a DC voltage. During this process, AL2O3 (i.e, a dielectric ) layer is formed on the aluminum foil. The thickness of this oxide layer is proportional to the applied voltage ( generally 1.4nm per volt).
The aluminum foil at the cathode side serves a role to provide electrical contact between the electrolyte and the external terminal. This aluminum is of low purity (i.e, 9.8%). This foil is also etched but it is not subjected to oxidation process as the anode aluminum foil. But it also has very thin oxide layer due to the reaction of aluminum surface with the air which occurs naturally. This natural oxide layer can withstand the voltage of about (1-2 V).
The electrolyte (i.e, a conductive liquid which is ionized) between the anode and the cathode foil basically plays the role of a cathode of the capacitor.
The basic requirement of an electrolyte is mentioned below:
- Electrically conductive
- When it comes in contact with the anode and the cathode foils then it must be chemically stable
- At very high temperature it has low volatility.
The cathode and the anode of the capacitor must be protected from short circuit (i.e direct contact). The spacer paper between the anode and cathode is made up of high purity absorbent to protect physical contact between anode and cathode (i.e to protect it from the short circuit).
The thickness of the spacer paper is dependent on the voltage rating of the capacitor. For 100V capacitor the thickness of spacer paper is between 35-75um. To protect the spacer paper from a breakdown at higher voltages. It is made thick that it can tolerate the rated voltage of the capacitor. One reason that capacitor exploid at higher voltages then the rated voltage is that when the voltage above the rated one is applied to the capacitor then the thickness of spacer paper (designed to tolerate the rated voltage of capacitor ) cannot tolerate the applied voltage and breakdown occurs (as a result spacer paper is damaged and the anode and cathode are short-circuited and the capacitor exploits.
Firstly the master roll of aluminuium foil is passed through a process of etching and the through a process of forming (i.e, a dielectric layer). After this, the anode and cathode foils are slitted from the master roll of a specified width and length. Then the anode and cathode foil are stitched with a lead and then the spacer paper (i.e, between anode and cathode), anode and cathode foils are winded together. After this, the wounded element is immersed into an electrolytic bath (i.e, a process of impregnation ) under low air pressure.
The electrolyte contains polyhydric alcohols such as ethylene glycol (i.e, plays a role of solvent) and ammonium salts (i.e, plays a role of solutes) to restore the damaged oxide layer (dielectric) and improve the performance and life of the capacitor. Then the impregnated element is assembled in which the impregnated element is attached with a rubber seal, and also placed into aluminium case and sealed with a rubber case.
Now the sealed capacitor is covered with a sleeve which is made up of PVC. This sleeve indicates the information of a capacitor. Since during the process of slitting and stitching, the oxide layer formed by the process of FORMING is possibly damaged, to make capacitor works properly it is necessary to restore the oxide film. In the process of AGING, the capacitor is applied with a DC voltage in high temperature in order to restore the oxide layer. This process makes the leakage current to be stable. Now the capacitor is finally tested to achieve the required criteria.