Catalogs
Specifications in catalogs may be subject to change without notice. For more details of precautions and guidelines for aluminum electrolytic capacitors, please refer to Engineering Bulletin No. 634A.
Structure of Aluminum Electrolytic Capacitors
The aluminum electrolytic capacitor contains an internal element of an anode foil, a cathode foil and paper separator rolled together, impregnated with an electrolyte, then attached to external terminals connecting the tabs with the anode or the cathode foils and sealed in a can case.
Among various types of capacitors, an aluminum electrolytic capacitor offers large CV to volume and features low cost. The capacitance [C] of aluminum electrolytic capacitors, as well as other capacitors, is expressed by the following equation:
C = 8.855 x 10-8 x εS/d (µF)
Where:
ε = Dielectric constant
S = Surface area of dielectric (cm2)
d = Thickness of dielectric (cm)
This equation shows that the capacitance increases in proportion as the dielectric constant becomes high, its surface area becomes large and the thickness of dielectric becomes thin, In aluminum electrolytic capacitors the dielectric constant of an aluminum oxide (Al2O3) layer is 8 to 10, which is not as high as compared with the other types of capacitors. However, the dielectric layer of the aluminum oxide is extremely thin (about 15Å per volt) and the surface area is very large. An electrochemical formed electrode foil makes the dielectric on the etched surface of aluminum electrode foil. Electrochemical etching creates 20 to 100 times more surface area as plain foil. Therefore, an aluminum electrolytic capacitor can offer a large capacitance compared with other types. Primary of Composition Material
Anode aluminum foil:
First, the etching process is carried out electromechanically with a chloride solution which dissolves metal and increases the surface area of the foil; forming a dense network like innumerable microscopic channels. Secondly, the formation process is carried out with a solution such as ammonium borate which forms the aluminum oxide layer (Al2O3) as a dielectric at a thickness of about 1.1 to 1.5nm/volt. The process needs to charge more the rated voltage into the foil.
Cathode aluminum foil:
As in the first manufacturing process of the positive foil, the cathode foil requires etching process. Generally, it does not require the formation process; therefore, the natural oxide layer of Al2O3, which gives a characteristic dielectric voltage of 1.0 volts, is formed.
Electrolyte and separator:
In a non-solid aluminum electrolytic capacitor, the electrolyte, an electrically conductive liquid, functions as a true cathode by contacting the dielectric oxide layer. Accordingly, the "cathode foil" serves as an electrical connection between the electrolyte and terminal. The separator functions to retain the electrolyte and prevent the anode and cathode foils from short-circuiting.
Can case and sealing materials:
The foils and separator are wound into a cylinder to make an internal element, which is impregnated with the electrolyte, inserted into an aluminum can case and sealed. During the service life of a capacitor, electrolyte slowly and naturally vaporizes under electrochemical reaction on the boundary of the aluminum foils. The gas will increase the pressure inside the case and finally cause the pressure relief vent to open or the sealing materials to bulge. The sealing material functions not only to prevent electrolyte from drying out but also to allow the gas to escape out of the can case in a controlled manner.
The Equivalent Circuit:
As the equivalent circuit of an aluminum electrolytic capacitor is shown below, it forms a capacitance, a series resistance, an inductance and a parallel resistance.
RESR = Equivalent series resistance (ESR)
RLC = Resistance due to leakage current
C = Capacitance
LESL = Equivalent series inductance
From a material composition point of view, the equivalent
circuit is subdivided as following:
Can, Cca = Capacitance due to anode and cathode foils R = Resistance of electrolyte and separator
Ran, Rca = Internal resistance of oxide layer on anode & Cathode foils
Dan, Dca = Diode effects due to oxide layer on anode & Cathode foils
Lan, Lca = Inductance due to anode & cathode terminals