Product Information: Aluminum Electrolytic Capacitor Technical Notes/Capacitor, Power Supply Units RUBYCON CORPORATION

Product Information: Electrolytic Capacitor

Aluminum Electrolytic Capacitor Technical Notes

1. General

1-1 Basic Construction and Structure

     Basic construction of aluminum electrolytic capacitor is shown in Fig. 1.
     Aluminum electrolytic capacitors consist of anode aluminum foil formed with aluminum oxide film on the surface to function as the dielectric. The cathode aluminum foil functions as a collector, and the liquid electrolyte functions as the real cathode. The electrolyte is impregnated onto a separator (spacer) paper between both foils.
     An aluminum oxide film, which is formed through anodization (generally referred to as "forming") of aluminum foil in an appropriate electrolyte. The oxide film is very thin and its thickness is in proportion to the voltage applied.

Fig. 1

Fig. 1 Basic construction of an aluminum electrolytic capacitor

·Anode: Substrate of anode aluminum foil
·Cathode: The true cathode is electrolyte
·Dielectric: Aluminum oxide film formed on the surface of anode foil
·Cathode Foil: Electrically connects electrolyte to external terminal. The cathode foil does not require a forming process to form oxide film. Rather, it is covered with a natural oxide film on the surface due to the reaction of aluminum with oxygen in the air after etching. It is said that this natural oxide film has the withstanding voltage of approximately 1 to 2 volts.
·Spacer Paper: Preventing physical contact between anode and cathode foil is essential for electrical isolation and is necessary to store electrolyte.

     The oxide film on the anode foil withstands a DC voltage only when the capacitor is charged as positive polarity to the aluminum substrate and negative to the electrolyte. If the capacitor is charged with reversed polarity, it will lose withstanding voltage property in a few seconds. This phenomenon is called “The Valve Effect”, which is the reason why aluminum electrolytic capacitors have a polarity. If both electrode aluminum foils have a formed oxide film, then the capacitor will be a non-polarized.
     Various papers report the mechanism for "Valve Effect" of aluminum in which the predominant "Hydrogen Ion Theory" is explained hereunder. When the system, including aluminum foil with anodic oxide film and electrolyte are charged so that the electrolyte is at the positive side and the metal at the negative side, the hydrogen ions gathered on the surface of the oxide film pass through the film to reach the boundary between the metal and the film and convert into hydrogen gas through discharge. Bubbles of hydrogen gas peel the oxide film off the aluminum substrate with expanding force so that electric current flows after penetration of electrolyte. On the contrary, when the system is charged with reversed polarity, negative ions with much larger diameter gather on the surface of the film. However, the film maintains voltage because such negative ions are unable to pass through the film due to their larger diameter.

     As shown in Fig. 2, an aluminum electrolytic capacitor element has a cylindrical structure in which anode foil, cathode foil and separator paper are wound with electrode terminals.

Fig. 2

Fig. 2 Structure of aluminum electrolytic capacitor element

     An aluminum electrolytic capacitor is manufactured by impregnating the capacitor element with an electrolyte and enclosing it with an aluminum case and sealing materials. The type of terminal and sealant structure are different for each product type. Basic structures are shown in Fig. 3. SMD (Surface Mount) types have a shape in which the lead wires are processed and a seat plate is attached so as to accommodate surface mounting. Snap-in type have a tab connected to the sealing plate with snap-in terminal instead of lead wire, and then sealed by the sealing plate.

Fig. 3

Fig. 3 Basic structure of aluminum electrolytic capacitor

1-2 Material Composition

«Electrode foil»
     For electrode foil, high purity foil (generally 99% or more) with a thickness of 20µm to 120µm.
In order to obtain a large electrostatic capacitance, an electrochemical roughening treatment is applied. This process is called etching which increases the electrode foil surface area. The shape of the pits formed by this etching process is selected by considering area efficiency. Porous pit shape by AC etching method for low voltage capacitors and straight pit shape by DC etching method for high voltage capacitor is selected, respectively (Photo 1).

Low voltage foil

Low voltage foil

High voltage foil (Replica)

High voltage foil (Replica)

Photo 1 Cross section of aluminum etched foil

     Anodic oxidation treatment is applied for etched foil to form aluminum oxide dielectric layer on the foil surface to obtain target withstand voltage (Photo 2).

Etched foil

Etched foil

Formed foil

Formed foil

Photo 2 Surface of high voltage foil

«Separator paper»
     Separator paper is mainly composed of natural cellulose fibers and its thickness is generally from 20µm to 90µm. Paper thickness and type are selected according to product impedance and rated voltage. High density and thick paper tends to be used for products with a high rated voltage, low density paper is selected for low impedance products. Photo 3 shows enlarged photograph of separator paper for low and high voltage. Low voltage separator is made of relatively thin and round shaped fibers for the purpose of low impedance (low ESR). In contrast, High voltage separator is made of flattened fibers to maintain high withstand voltage.

Low voltage separator

Low voltage separator

High voltage separator

High voltage separator

Photo 3 Surface of separator paper

     Electrolyte is a solution with ionic substance dissolved in solvent. It is an important material composition because its characteristics greatly affect withstand voltage, temperature and frequency characteristics, along with the life of the product. We select the optimum electrolyte according to the rated voltage, operating temperature range, and other required characteristics.
     Electrolyte is also responsible for repairing defective areas of the anode dielectric layer. This repair performance is a unique feature of electrolytic capacitors not found in other capacitors such as ceramic and film.

«Case and Sealing Material»
     In order to prevent the dry out and leakage of electrolyte, an airtight seal is necessary. This is accomplished by the aluminum case and sealing material. In addition, a safety vent (explosion-proof valve) is placed in the case or sealing material in order to cope with internal pressure rise due to substantial gas generation under abnormal conditions.
     Insulative rubber or resin is used for sealing material as it also serves to prevent short circuit between external electrode terminals or case / external electrode terminals.

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