ECAS:Polymer Capacitor Basics-Part 1

Introduction

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.

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Figure 1. Main Types of Capacitors

Certificates of the International Standard (IEC60384-14).


Certificates of the International Standard (IEC60384-14).

What Is a Polymer Capacitor?

There are many other types of capacitors, such as film capacitors and niobium capacitors, but here we will describe polymer capacitors, a type of capacitor produced by Murata among others.
In both tantalum electrolytic capacitors and aluminum electrolytic capacitors, a polymer capacitor is a type of electrolytic capacitor in which a conductive polymer is used as the cathode. In a polymer-type aluminum electrolytic capacitor, the anode is made of aluminum foil and the cathode is made of a conductive polymer. In a polymer-type tantalum electrolytic capacitor, the anode is made of the metal tantalum and the cathode is made of a conductive polymer. Figure 2 shows an example of this structure.

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Figure 2. Example of Structure of Conductive Polymer Aluminum Capacitor


In conventional electrolytic capacitors, an electrolyte (electrolytic solution) or manganese dioxide (MnO2) was used as the cathode. Using a conductive polymer instead provides many advantages, making it possible to achieve a lower equivalent series resistance (ESR), more stable thermal characteristics, improved safety, and longer service life. As can be seen in Fig. 1, polymer capacitors have lower ESR than conventional electrolytic capacitors.
Note that the type of valve metal used for the anode basically determines the type of dielectric, and this in turn determines the dielectric constant and DC bias characteristics, as well as the acoustic noise characteristics. Thus, a wide variety of characteristics can be obtained by combining anodes, cathodes, and dielectrics made of different materials. Each has its own strong and weak points, and these must be kept in mind when selecting components as part of the circuit design process.

The ECAS Series

Aluminum electrolytic capacitors can be broadly divided into three types, based on the cathode material and the structure. Murata’s aluminum capacitors are all-solid multilayer polymer aluminum capacitors (the ECAS series)(Fig. 3). Other varieties of aluminum capacitors include can-type wrapped aluminum capacitors that use either an electrolyte or a polymer as the cathode. What sets ECAS series capacitors apart are the high conductivity of the conductive polymer used as the cathode and the multilayer (laminated) structure of the aluminum elements. These make possible the lowest ESR obtainable among electrolytic capacitors. ECAS series capacitors also achieve large capacitances, and the capacitance remains stable when DC voltage is applied due to the lack of DC bias. Thus, the major features of the ECAS series are low ESR, high capacitance, and stable capacitance.

Figure 3. Example of Structure of ECAS Series Capacitor

Figure 4. ECAS Series Capacitor lineup



Comparison of Capacitor Characteristics

We will compare the characteristics of different types of capacitors, bringing in related data where relevant. Figure 5 is a comparative table of capacitor characteristics. Aluminum electrolytic capacitors (can type) using a conventional electrolyte and tantalum electrolytic capacitors using manganese dioxide (MnO2) are comparatively inexpensive, but they are inferior to polymer capacitors in their frequency characteristics, temperature characteristics, service life, and reliability. Murata’s ECAS series multilayer polymer aluminum capacitors have a relatively smaller lineup of product versions than other polymer capacitors, but their frequency characteristics, in particular, are superior.

(symbol) ◎: excellent, ○: good, △:  normal, ×: bad
Figure 5. Comparison of Capacitor Characteristics
 
Figure 6. Comparison of ESR and Capacitance @each Al capacitor (2V/330uF)