Capacitor Basics

In principle a capacitor consists of an insulating dielectric between two electrodes. The capacitance (C) of a capacitor is given by 

The ongoing trends of miniaturization and cost reduction require an increase of capacitance per volume of a capacitor. According to the previous equation above this can be realized by increasing the surface (A) of the electrode, by decreasing the thickness (d) of the dielectric, and by choosing a dielectric material having a higher relative permittivity (s,). 

---

F r d ic Current. The double layer is a leaky capacitor because Faradaic current flows around it. This leaky nature can be represented by a voltage-dependent resistance placed in parallel and called the charge-transfer resistance. Basically, the electrochemical reaction at the electrode surface consists of four thermodynamically defined states, two each on either side of a transition state. These are (11) (/) oxidized species beyond the diffuse double layer and n electrons in the electrode and (2) oxidized species within the outer Helmholtz plane and n electrons in the electrode, on one side of the transition state and (J) reduced species within the outer Helmholtz plane and (4) reduced species beyond the diffuse double layer, on the other. 

On the analog side, tbe filter is often the conventional resistor-capacitor or RC filter. However, other possibihties exist. For example, one type of A/D converter is called an integrating A/D because the converter basically integrates the input signal over a fixed intei val of time. By making the intei val I/60th second, this approach provides excellent rejection of any 60-Hz electrical noise. 

The basic principle of these methods is to charge a capacitor up to the specified lest voltage and then discharge it through the coil under test. 

We have summarized in Table 25.2 the basic parameters to select the correct type of components for capacitor duty. 

To clarify the subject the basics and the behaviour of power capacitors in operation are also discussed. 

The major consumer of tantalum is the capacitor production industry. About 60% of the total amount of tantalum currently produced is in the form of fine, agglomerated high purity powder of capacitor grade. Tantalum capacitors have high volumetric efficiency and reliability. A basic description of tantalum capacitor technology is presented in overview [19]. 

A DEA is basically a compliant capacitor where an incompressible, yet highly deformable, dielectric elastomeric material is sandwiched between two complaint electrodes. The electrodes are designed to be able to comply with the deformations of the elastomer and are generally made of a conducting material such as a colloidal carbon in a polymer binder, graphite spray, thickened electrolyte solution, etc. Dielectric elastomer films can be fabricated by conventional... 

The basic defect of film capacitors is tfieir low value of specific electrostatic capacity. Therefore, such capacitors are practically useful only in the pico- and nanofarad range. For this reason, valiant attempts have been made in recent years to increase the specific capacity of capacitors per unit of mass, volume, and plate (electrode) surface area. 

Similarly, AVX has come up with improved IDC capacitors called LICA capacitors (low inductance chip array). They were developed in a joint effort between AVX and IBM. Their basic principle also remains the same—flux cancellation by opposite current flows. (See Figure 4-18.) They look and feel like regular IDCs (and need to be laid out similarly), but they have an improved internal electrode structure to further minimize ESL. See how the currents are forced inside the chip in Figure 4-19. 

The material properties of PS offer new ways of making electronic devices. For the manufacture of cold cathodes, for example, oxidized microporous polysilicon has been found to be a promising material. The application of basic semiconductor processing steps such as doping, oxidation and CVD to a macroporous material enable us to fabricate silicon-based capacitors of high specific capacitance. Both devices will be discussed below. 

At the heart of impedance analysis is the concept of an equivalent circuit. We assume that any cell (and its constituent phases, planes and layers) can be approximated to an array of electrical components. This array is termed the equivalent circuit , with a knowledge of its make-up being an extremely powetfitl simulation technique. Basically, we mentally dissect the cell or sample into resistors and capacitors, and then arrange them in such a way that the impedance behaviour in the Nyquist plot is reproduced exactly (see Section 10.2 below on electrochemical simulation). 

As noted above, electrochemical capacitors are close cousins to batteries. The simple circuit shown illustrates their basic operation. 

Electrolytes are ubiquitous and indispensable in all electrochemical devices, and their basic function is independent of the much diversified chemistries and applications of these devices. In this sense, the role of electrolytes in electrolytic cells, capacitors, fuel cells, or batteries would remain the same to serve as the medium for the transfer of charges, which are in the form of ions, between a pair of electrodes. The vast majority of the electrolytes are electrolytic solution-types that consist of salts (also called electrolyte solutes ) dissolved in solvents, either water (aqueous) or organic molecules (nonaqueous), and are in a liquid state in the service-temperature range. [Although nonaqueous has been used overwhelmingly in the literature, aprotic would be a more precise term. Either anhydrous ammonia or ethanol qualifies as a nonaqueous solvent but is unstable with lithium because of the active protons. Nevertheless, this review will conform to the convention and use nonaqueous in place of aprotic .]... 

Fig. 11.1. Two basic types of current ampliflers. (a) Feedback picoammeter. It consists of two components, an operational amplifier (op-amp) A, and a feedback resistor 1 fb- a typical value of the feedback resistor used in STM is 10 fl. The stray capacitance Cfb is an inevitable parasitic element in the circuit. In a careful design, Cfb 0.5 pF. The input capacitance Cm is also an inevitable parasitic element in the circuit. Those parasitic capacitors, the thermal noise of the feedback resistor, and the characteristics of the op-amp are the limiting factors to the performance of the picoammeter. (b) An electrometer used as a current amplifier (the shunt current amplifier). The voltage at the input resistance is amplified by the circuit, which consists of an op-amp and a pair of resistors R, and R2. The parasitic input capacitance Cm limits the frequency response, and the Johnson noise on Rm is the major source of noise. Also, the input resistance for this arrangement is large.

---