Polymer Capacitor Basics (Part 1): What Is a Polymer 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.
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.
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.
Main Applications of the ECAS Series
By adding the ECAS series to its line of multilayer ceramic capacitor (MLCC) products, Murata has further broadened the range of options available to customers. As electronic devices gain ever-more-sophisticated functionality, stricter voltage control is needed for the power lines of the CPU, etc. Maintaining voltage line stability sometimes requires large capacitance. In the past we would have suggested using multiple MLCCs, but now in many cases we can propose combining ECAS series capacitors with MLCCs in order to reduce both the quantity and cost of components.
Figure 4 shows a simplified circuit diagram of the power line for a CPU or FPGA.
The ECAS series delivers low ESR, low impedance, and stable capacitance, making it ideal for applications such as smoothing (eliminating ripple and high transient response) various types of power lines subject to large fluctuations in the current load.
Basically, ECAS series capacitors are almost always used in combination with MLCCs, and the ECAS series capacitors really show their value in applications involving suppression of voltage fluctuations (high-speed back-up). Polymer-type tantalum electrolytic capacitors and polymer-type aluminum electrolytic capacitors (can-type) are widely used to suppress voltage fluctuations, but ECAS series capacitors are even more ideally suited for such applications due to their low ESR and good balance between ESR and capacitance.
ECAS series capacitors are widely used in products such as PCs and peripherals (notebook PCs, servers, motherboards, multifunction printers, etc.), and digital AV products (LCD televisions, game consoles, set-top boxes, etc.).
If you are designing the power line for a CPU, we recommend the ECAS series.
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Produced by: Polymer Device Department, Murata Manufacturing Co., Ltd.