Design of Aluminum Electrolytic Capacitors and General Features

Posted on 16 April 2019

As it is the case with all capacitors, an aluminum electrolytic capacitor comprises two electrically conductive material layers that are separated by a dielectric layer. One electrode (the anode) is an aluminum foil with an enlarged surface area. The oxide layer (Al2O3) that is formed on this is used as the dielectric. In contrast to other capacitors, the counter electrode (the cathode) of aluminum electrolytic capacitors is a conductive liquid, the operating electrolyte. A second aluminum foil, the so-called cathode foil, serves as a large-surfaced contact area for passing current to the operating electrolyte.

The anode of an aluminum electrolytic capacitor is an aluminum foil of extreme purity. The effective surface area of this foil is greatly enlarged (by a factor of up to 200) by electrochemical etching in order to achieve the maximum possible capacitance values. The type of etching pattern and the degree of etching is matched to the respective requirements by applying specific etching processes.

Etched foils enable very compact aluminum electrolytic capacitor dimensions to be achieved. They are used almost exclusively. The electrical characteristics of aluminum electrolytic capacitors with plain (not etched) foils are, in part, better, but these capacitors are considerably larger and are only used for special applications.

The dielectric layer of an aluminum electrolytic capacitor is created by anodic oxidation (forming) to generate an aluminum oxide layer on the foil. The layer thickness increases in proportion to the forming voltage at a rate of approximately 1.2 nm/V. Even for capacitors for very high voltages, layer thicknesses of less than 1µm are attained, thus enabling very small electrode spacing. This is one reason for the high volumetric efficiency achieved (e.g. in comparison to the minimum thickness of a paper dielectric, 6 to 8 µm). During the forming process the very fine pits of the etched foils will encrust partially in proportion to the forming voltage and thus also to the achieved layer thickness. Due to this effect, the final operating voltage range must already be taken into account when the foils are etched. The oxide layer constitutes a voltage-dependent resistance that causes the current to increase more steeply as the voltage increases. A characteristic curve as shown in figure 1 is obtained.

Figure 1. Current-voltage characteristics of an aluminum electrolytic capacitor

 

When the forming voltage VF is exceeded, the forming process starts and large amounts of gas and heat are generated. The same effect, yet on a smaller scale, can already be observed in the knee of the curve. In order to achieve a high degree of operating safety of the capacitor, the rated voltage VR  is defined as being on the quasi-linear part of the curve. As the capacitor is subjected to surge voltages VS for short periods only, this range lies between the rated voltage and the forming voltage. The difference between forming voltage and operating voltage, the so-called over-anodization, thus has a substantial effect on the operating reliability of the capacitor. High over-anodization offers the possibility of producing especially reliable capacitors designated as long-life grade “LL” capacitors to IEC 60384–1.

Since the electrolytic capacitors have a liquid as a cathode, they are also designated as “wet” or “non-solid” capacitors. The liquid has the advantage that it fills the fine etching pits, therefore optimally fitting into the anode structure. The two aluminum foils are separated by paper spacers. The paper serves various purposes - it serves as a container for the electrolyte, the electrolyte is stored in the pores of the absorbent paper, it acts as a spacer to prevent electric short-circuits, and ensures the required dielectric strength between the anode and cathode foils (see figure 2).

Figure 2: Winding construction of an aluminum electrolytic capacitor

 

 

For further information on aluminum electrolytic capacitors, please read the following articles:

Aluminum Electrolytic Capacitors - Overview and Key Applications

Equivalent Series Resistance (ESR) for Capacitors

Temperature Influence on Series Capacitance

Heat Dissipation and Cooling for Aluminum Capacitors

Mounting Positions of Aluminum Electrolytic Capacitors with Screw Terminals

 

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