Polymer Aluminum Electrolytic CapacitorsWhat is a polymer aluminum electrolytic capacitor?

If you take a look at the main board of an electronic device such as a personal computer, you’re likely to see some of the six types of capacitors shown below (Fig. 1).
Common types of capacitors include tantalum electrolytic capacitors (MnO2 type and polymer type), aluminum electrolytic capacitors (electrolyte can type, polymer can type, and chip type), and MLCC.

There are various other types of capacitors, such as film capacitors and niobium capacitors, but here we will describe aluminum electrolytic capacitors (polymer aluminum electrolytic capacitors), a type of capacitor produced by Murata Manufacturing, among others.

An aluminum electrolytic capacitor is a polarized capacitor that uses aluminum oxide film as a dielectric.
These capacitors are also classified according to whether the electrolyte is liquid or solid.

< Structure >

・Aluminum foil is used for both the anode part and cathode part

　Anode

Conductor (aluminum) with a high-purity of 99.9% or more

　Cathode

Liquid or solid electrolyte closely adhered to the dielectric

・Dielectric

Oxide film adhered to the surface (aluminum: Al₂O₃)

The capacitance of a capacitor increases the wider the electrode area, the higher the dielectric constant, and the narrower the distance between the electrodes.

Aluminum is a special metal with a rectifying effect called a valve metal. Other metals with the performance of valve metal include tantalum and niobium. The dielectric constant of oxide film (Al₂O₃), which is a dielectric formed by anodic oxidation on the aluminum surface, is approximately 7 to 10. That means it is not necessarily higher than the dielectrics used in other capacitors. Nevertheless, forming a porous structure with a surface area expanded up to 350-fold by etching the aluminum surface with electrolysis realizes a large-capacity capacitor. Moreover, the withstanding voltage (rated voltage) of a capacitor depends on the thickness of the dielectric. On the other hand, the thicker the dielectric, the smaller the capacitance. Aluminum is characterized by a thin dielectric per 1 V of approximately 1.3 nm. Accordingly, it is suitable for designing high withstanding voltage and large-capacity capacitors.

Aluminum electrolytic capacitors are also classified by whether the electrolyte is a liquid or solid. Aluminum electrolytic capacitors with a liquid electrolyte are only the can type, which can hermetically seal in the liquid electrolyte (electrolytic solution). They are characterized by a large capacity and high withstand voltage.
Meanwhile, aluminum electrolytic capacitors with a solid electrolyte do not need to be hermetically sealed because they are a solid (conductive polymer). Therefore, there is a resin-molded surface-mounted type in addition to a can type. They are characterized by a large capacity and low product height. Furthermore, there is no risk of volatilization because it is a solid electrolyte. This means they are characterized by a long service life compared to capacitors with liquid electrolytes.

Conductive polymer capacitors refer to electrolytic capacitors that use a conductive polymer for the cathode.
This is the same for tantalum electrolytic capacitors and aluminum electrolytic capacitors. However, aluminum electrolytic capacitors use aluminum foil for the anode and conductive polymer for the cathode.
On the other hand, conductive polymer-type tantalum electrolytic capacitors use tantalum metal for the anode and conductive polymer for the cathode.
Fig. 3-1 shows a schematic diagram of a polymer aluminum electrolytic capacitor.

Conventional-type electrolytic capacitors use a liquid electrolyte (electrolytic solution) or MnO2 for the cathode. If we use conductive polymer for the cathode in place of those to make a conductive polymer capacitor, we can realize the following: 1. low ESR, 2. stable temperature characteristics, 3. improved safety, and 4. longer service life. (Refer to Fig. 1 and Fig. 3-2 for 1. low ESR.)

In addition, the type of dielectric is basically determined by the type of valve metal used for the anode in the case of electrolytic capacitors. The dielectric constant, DC bias characteristics, and acoustic noise characteristics are then determined by that. In this way, we can obtain various characteristics by combining multiple types of materials for the anode, cathode, and dielectric. There are strong areas and weak areas depending on the material. Therefore, electronic device designers need to use them as appropriate when designing circuits.

Murata's polymer aluminum electrolytic capacitors (the ECAS series) has the feature of low ESR, low impedance, and large scale. Furthermore, there is no DC bias characteristic of capacitance, and since temperature characteristics are also stable, it excels in the Ripple absorption, smoothness, and the transient response performance.

Therefore, it is suitable for smoothing of input-output current of various power supply circuits, and the backup use over the load change of the CPU circumference. This contributes to reduction of the number of parts, or reduction of substrate area.