Aluminum Electrolytic Capacitors

TECHNICAL NOTE1 General Description of Aluminum ElectrolyticCapacitors1-1 The Principle of CapacitorThe principle of capacitor can be presented by theprinciple drawing as in Fig.1-1.When a voltage is applied between the metal electrodesplaced opposite on both surfaces of a dielectric,electric charge can be stored proportional to thevoltage.QCVQ : Quantity of electricity (C)V : Voltage (V)C : Capacitance (F)StALUMINUM ELECTROLYTICCAPACITORSAnode foilAnodic oxide filmElectrolyte(True cathode)1-2 Equivalent Circuit of the CapacitorThe electrical equivalent circuit of the aluminum electrolyticcapacitor is as presented in Fig. 2.D1Fig.1-2Separator sheetimpregnated with electrolyteCathode foil(Virtually, electrode forcathode extraction)Natural air oxide film®R1C1R2C2LR3Fig.1-1C. called the capacitance of capacitor, is expressed bythe following expression with the electrode area S[m 2 ],the electrode spacing t [m] and the dielectric constantof dielectric “”:SC[F]0—— t0 : Dielectric constant in vacuum (8.8510 12 F/m)Table 1Dielectric Dielectric Constant Dielectric Dielectric ConstantAluminum oxide filmMylarMica7 to 83.26 to 8Porcelain (ceramic)PolystyreneTantalum oxide film10 to 1202.510 to 20L L0 2The dielectric constant of an aluminum oxide film is7 to 8. Larger capacitances can be obtained by enlargingthe electrode area S or reducing t.Table 1-1 shows the dielectric constants of typical dielectricsused in the capacitor. In many cases, capacitornames are determined by the dielectric material used,for example, aluminum electrolytic capacitor, tantalumcapacitor, etc.Although the aluminum electrolytic capacitor is small,it has a large capacitance. It is because the electrodearea is roughened by electrochemical etching, enlargingthe electrode area and also because the dielectricis very thin.The schematic cross section of the aluminum electrolyticcapacitor is as in Fig.1-2.R1 : Resistance of terminal and electrodeR2 : Resistances of anodic oxide film and electrolyteR3 : Insulation resistance because of defective anodicoxide filmD1 : Oxide semiconductor of anode foilC1 : Capacity of anode foilC2 : Capacity of cathode foilL : Inductance caused by terminals, electrodes, etc.2 About the Life of an Aluminum ElectrolyticCapacitor2-1 Estimation of life with minimal ripple current(negligible).Generally, the life of an aluminum electrolytic capacitoris closely related with its ambient temperature and thelife will be approximately the same as the one obtainedby Arrhenius’ equation.T0T——10 )………………(1)Where L : Life at temperature T)L0 : Life at temperature T0The effects to the life by derating of the applied voltageetc. are neglected because they are small comparedto that by the temperature.CAT.No.2010/2011E