Aluminium Capacitor Components

The aluminium electrolytic capacitor is one of the mainstays of the capacitor industry being used in vast quantities both as leaded devices and in SMD forms. They offer the highest levels of capacitance for a given volume and are usually one of the lowest cost types. They have a high operating temperature range and a very wide operational frequency spectrum and therefore are often seen in applications such as power supplies or switching-mode power converters as they smooth DC voltages and reduce noise at higher frequencies. They are also commonly found in 50/60 Hz power supplies for decoupling purposes.

An aluminum electrolytic capacitor consists of an anode aluminum foil, a cathode aluminum foil and a liquid electrolyte that connects the two and forms the dielectric. It is this property that allows for a large capacitance value in a small package with low impedance values compared to other dielectrics such as Mylar or mica and even lower than supercapacitors.

The anode aluminum foil is etched, roughened and then a layer of electrically insulating aluminium oxide Al2O3 is formed on it using an applied current. This process is called anodic oxidation and this gives the aluminium anode its characteristic granular structure that can absorb a lot of energy. The anode is then connected to the internal circuit via a metal terminal.

After the anode is connected the cathode aluminum foil is etched and a thin layer of aluminium oxide is formed as well. The cathode is connected to the grounding terminal and acts as a collector. The electrolyte is an ionically conductive liquid that connects the two foils and serves as the true cathode. It is also responsible for allowing the capacitor to self-heal; when a local weak point in the dielectric develops the leakage current flowing through it increases the thickness of the dielectric at that location, thus improving its resistance.

Another key feature of the aluminium electrolytic capacitor is that it is a polarized device. This means that the correct polarity of DC voltage must be applied in order to operate it safely. Attempting to operate it with AC voltage or the wrong polarity will lead to short circuit and destroy the capacitor.

Due to their excellent performance and relatively low price aluminum electrolytic capacitors are a staple for many applications such as power supplies and switched-mode power converters, in DC-to-DC conversion and energy storage. They are also popular in automotive electronics as input and output decoupling capacitors, in industrial controls and in uninterruptible power supplies.

When comparing the ESR of various manufacturers of aluminium electrolytic capacitors it should be noted that the differences are relatively minor. The bigger difference lies in the capacitance and voltage rating of the individual capacitors. Changing the packaging (size) of a capacitor usually does not significantly change the ESR, however it will affect its ripple current and heat resistance. The voltage rating of the capacitor determines how much power it can handle; higher voltage ratings require more robust and larger sized packages.