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Capacitor

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DEPT. OF HEALTH, EDUCATION, AND
WELFARE PUBLIC HEALTH SERVICE
FOOD AND DRUG ADMINISTRATION
*ORA/ORO/DEIO/IB*
Date: 10/23/87 Number: 50
Related Program Areas:
Medical Devices, Radiological Health

_____________________________________________________

 

ITG SUBJECT: CAPACITOR

 

The purpose of this ITG is to acquaint the investigator with the capacitor. Only the basics will be discussed, since it is beyond the scope of this ITG to go into great detail. It is stressed that there is no single capacitor that out performs all others, as each capacitor is designed to perform a specific task. This ITG will explain capacitor operation theory, the various types of capacitors, physical and electrical specifications of capacitors, the failure modes of the various types, design considerations, and environmental effects.

 

Theory

 

Electrically, "capacitance" is present between any two adjacent conductors. A capacitor consists of two conductors, usually parallel metal plates, separated by a dielectric material or vacuum so as to store a large electrical charge in a small volume. Depending on proposed application the dielectric can be air, gas, paper, organic film, mica, glass, or ceramic. The operation of a capacitor is similar to blowing up a balloon and releasing the air from it. Imagine blowing up a balloon, pinching the air nozzle for a few seconds, and then releasing the air nozzle so that air may flow out. Similarly, a capacitor is charged (blown up) to some voltage (air pressure) by an AC or DC voltage source (air blower). Once the voltage source is removed the capacitor will hold the voltage for some time (pinching the air nozzle) and then it will begin to rid itself of the electricity (releasing the air nozzle). The rate at which the capacitor discharges is dependent on how much resistance the discharging current meets. The more resistance you have the slower the current will discharge from the capacitor. Thinking in terms of balloons, we can say that the tighter you pinch the air nozzle (resistance) the slower the air will flow out (current discharge). If a big piece of metal is put across the two capacitor terminals, the capacitor will instantaneously discharge and sparks will occur. This is due to the sudden flow of discharge current thru a neglible resistance. This phenomenon is similar to popping a balloon where the unresisted flow of air through the pinhole is so great that the balloon explodes.

 

The basic equations governing the operation of a capacitor are:

 

(1) Capacitance (C) = Charge (Q)    = ke A                         
                     -----------      --o-                                    
                     Voltage (V) d


Where C is in units of farads (f), Q is in coulombs (C), and V is in volts (V). A capacitor possesses one farad of capacitance if its potential is raised one volt when it receives a charge of one coulomb. On the right hand side of the equation, k is the dielectric constant (no units), e o is the premittivity of air (8.85 x 10 -1 2 f/cm), A is the area of one of the capacitor plates (cm 2), and d is the separation distance between the two plates (cm). Capacitance is most commonly expressed in 10 6 subdivisions called microfarads (uf).

 

(2) Energy (J) = 1/2 Capacitance (c) x Voltage 2 (V) = QV                                                           --                                                            2

 

where J is in units of watt-seconds or Joules.

 

Equation (1) shows that capacitance can be increased in several ways; by decreasing the voltage, obtaining a dielectric with higher k, increasing the capacitor plate area, or decreasing the distance between the capacitor plates. Equation (2) shows that the energy experiences its largest increase if the voltage is increased.

 

Capacitors are mainly used as energy storage devices; that is, they store electrical energy until the energy is required to enter the circuit which is using the capacitor. Capacitors are now widely used for keeping DC current from entering a part of a circuit (blocking), ridding a circuit of unwanted noise or distortion (filtering), combining desired frequencies to resonate in a circuit (coupling), and excluding certain frequencies from resonating in a circuit (bypassing).

 

Types

 

Capacitors generally come in two types; fixed and variable. Fixed capacitors are manufactured to possess a specific capacitance which cannot be changed and variable capacitors are manufactured to allow capacitance to be varied over a wide range.

 

Capacitors are also classified into two generic categories; electrostatic and electrolytic. Electrostatic capacitors are filled with dielectrics composed of a gas, liquid, solid, or combination of these. Electrolytic capacitors are characterized by a very thin metallic oxide dielectric film formed on the surface of one or more electrodes.

 

A. Fixed Capacitors

 

Ceramic Capacitors - These are a unique family of capacitors with dielectric constants ranging from 6-10,000. They can be easily manufactured to desired physical and electrical characteristics by applying ceramic chemistry. Ceramic capacitors are so widely used that they come in three classes. Class I ceramics are used for resonant circuits and high-frequency bypass and coupling. These capacitors have a wider temperature range compared to Class II and Class III capacitors. Class II ceramics are used where miniaturization is required for bypassing at radio frequencies, filtering, and interstage coupling. Class III ceramics are used where low-voltage coupling and bypassing in transistor circuits are necessary.

 

Vacuum Capacitors - These capacitors have the lowest possible dielectric constant and are limited to capacitances of 10 3 pf (10- 3 uf), can range up to 50 kv (50x10 3 volts), and can carry huge currents up to 100 amperes. Vacuum capacitors, are extremely useful because their lifetime, barring any particle contamination in the vacuum chamber, is indefinite.

 

Mica Capacitors - These capacitors find their use in such applications as high-frequency filtering, bypassing, blocking, buffering, coupling, and fixed tuning.

 

Metalized Paper and Film Dielectric Capacitors - The use of this class of capacitors is ideal where great amounts of heat will be present in a circuit. These capacitors possess a unique property called self-healing whereby they eliminate momentary short circuits induced in their dielectrics caused by surrounding circuit elements. Once the capacitor becomes too hot, the localized heat generated is sufficient to vaporize the thin electrode in the area of the possible breakdown. The ability to self-heal permits these capacitors to have higher voltage ratings for a given thickness.

 

Radio Frequency Interference (RFI) Capacitors - RFI capacitors are ideal for suppressing unwanted noise from electronic circuits. This minimizes the amount of noise passing from one stage of the circuit to another, thus improving overall circuit performance.

 

Film Capacitors - These capacitors are widely used where circuits will experience exposure to moisture. Their resistance to moisture penetration is, by far, superior. Film capacitors are applied in circuits requiring blocking, buffering, bypassing, coupling, tuning, and timing.

 

Electrolytic Capacitors - Electrolytic capacitors are very different from those previously mentioned in that electrolytics are usually polarized. This means that the polarity of the applied voltage must match the polarity of the capacitor or intense heating will occur and the capacitor will burn out. Electrolytics meet design needs for low-frequency filtering, long-term timing, coupling and decoupling, and certain bypass applications requiring high capacitance values and small volumes.

 

Other capacitors commonly used as fixed capacitors are air, glass, and paper types. These are the earliest capacitors to be used and they still find usage in general purpose cases.

 

B. Variable Capacitors

 

Variable capacitors, also called trimmers, are invaluable in the design of electronic equipment. Variable capacitors are generally employed to provide a range of capacitance and are commonly used in applications where exact capacitance values cannot be obtained using normal design procedures. These capacitors are usually constructed such that varying the capacitance is accomplished by adjusting the metal plates in the capacitor. Screws on these capacitors increase or decrease effective plate area thereby causing an increase or decrease in capacitance. (Inspection of Equation (1) shows this.) The most widely used trimmers are ceramic, glass, air, plastic and mica.

 

C. Special Capacitors

 

Feed-through Capacitors - These capacitors are used in cases where conventional capacitors are not effective for filtering at high radio frequencies. Feed-through capacitors are three terminal devices that do not exhibit the series-resonant characteristic of the conventional capacitor. This enables them to suppress radio-frequency interference over a wide range of frequencies and they are especially valuable in filtering power-supply and control-circuit wiring in shielded high-frequency equipment.

 

High-energy Storage Capacitors - These capacitors are constructed with oil-impregnated paper and/or film dielectrics. Their primary use is for pulse forming networks which employ voltages greater than 1000 volts. For slightly lower voltages special electrolytic capacitors can be used. Commutation Capacitors - These are constructed from oil-impregnated paper and film dielectrics. They are mainly used in triggering circuits since they are characterized by fast rise times (time it takes capacitor to rise from 10% to 90% of its maximum voltage) and high current transients and peak voltages associated with switching.

 

Packaging - Capacitors come in a wide variety of packaging styles. The most common styles are molded, glass-encased, chip, potted, coated, and Dual-In-Line Packaging (DIP). Molded capacitors are rectangular-chip capacitors that can be molded into radial or axial-lead rectangular packages or axial-lead cylindrical packages. Glass-encased capacitors can be single or multi-layered chips with axial leads attached sealed into a glass tube. These look a lot like molded capacitors. Chip capacitors are thin, flat rectangular capacitors without leads or body encasement so that they may be put into microelectronic circuits. Potted capacitors, in many ways, are synonymous with molded capacitors. The only difference is that potted capacitors are oven cured. Coated capacitors, more commonly known as dipped capacitors, come in rectangular and disk styles with radial leads and are dipped in liquid resin. Coated capacitors find great usage where exact dimensions can be compromised. DIP capacitors are single or multi-layered capacitors processed into integrated-circuit-type packages. Mica chips come in button styles. This package is composed of a stack of silvered-mica disks connected in parallel.

 

Figures 1, 2, and 3 show a few of the various types and packaging styles of capacitors. Figure 1A (image size 29KB) shows dipped- radial-lead capacitors (top row) and molded-axial-lead capacitors (bottom group); Figure 1B (image size 29KB) shows glass-encased- axial-lead capacitors (A), chip capacitors (B & C), molded-radial- lead capacitors (D), molded-axial-lead capacitors, and dipped-radial- lead capacitors (F); Figure 1C (image size 29KB) shows the various styles of feed-through capacitors; and Figure 1D (image size 29KB) shows dipped-radial-lead capacitors (top and bottom left), molded- axial-lead capacitors (bottom right group), button capacitors (Middle- middle group), and fixed terminal capacitors (top middle and top right). Figure 2A-C (image size 13KB) shows various types of trimmer capacitors. Figure 3 (image size 7KB) (Figure shows (a) mica; (b) glass; (c) ceramic; (d) general-purpose ceramic; (e) solid electrolyte tantalum; (f) foil tantalum; (g) feed-through button mica and ceramic; (h) general-purpose plastic film; and (i) general- purpose paper.

 

Physical and Electrical Specifications

 

There are numerous criteria which the designer uses to choose the capacitor that will best perform a specific task. Listed here are some of the most important specifications used in evaluating capacitor performance.

 

Dissipation Factor (DF) - This is a measure of loss in a capacitor. Sometimes this is interchanged with a measure of loss called the power factor (PF). Losses in large AC coil and paper capacitors are DF's while losses in most capacitors used in DC or low-level AC capacitors are PF's. Ideally current should lead voltage by 90 in a capacitor but due to manufacturing processes the current leads the voltage by some angle A. The DF = tan(90 -A) and PF = sin(90 -A). The lower the DF, the better the capacitor.

 

Equivalent Series Resistance (ESR) - In capacitors, this is defined as the AC resistance (R) of a capacitor expressing loss at a given frequency (f). The ESR is related to the PF by the relation:

 

               R =  PF   x 10 6                        
                   ---                        
                   2 fc

 

in units of ohms.

 

Insulation Resistance (IR) - This is the resistance across the terminals of a capacitor. IR is inversely proportional to capacitance and temperature so as capacitance (or temperature) increases the IR will decrease.

 

Dielectric Strength - This corresponds to the maximum voltage that a dielectric material can withstand without rupturing. Electrostatic capacitors are often specified by their dielectric withstanding voltage (DWV) and this is synonymous with dielectric strength. Dielectric strength is usually specified in volts per mil at constant temperature.

 

Dielectric Absorption - This is the property of an imperfect dielectric where all electric charges within the body of the material caused by an electric field are not returned to that field. Dielectric absorption is measured by determining the "reappearing voltage" which appears across a capacitor at some point in time after the capacitor has been fully discharged under short circuit conditions. It is expressed as the ratio of reappearing voltage to charging voltage.

 

Volumetric Efficiency - This is achieved by getting the most capacitance out of the smallest volume possible. The volume is a function of dielectric material used and the method of construction. Capacitors with high volumetric efficiency are the most applicable in most of the new integrated-circuit electronic-equipment designs.

 

Temperature Coefficient (TC) - TC is the change in capacitance per degree change in temperature. It may be positive, negative, or even zero and is expressed in parts per million per degree Celsius (ppm/ 0C). The equation that determines the TC is:

 

TC = C1-C 2 x 10 6         
     ------        
    (T 1-T 2)C 1

 

where C 1 and C 2 are the initial and final capacitances and T 1 and T 2 are the initial and final temperatures.

 

Voltage Ratings - There are two types of voltage ratings to consider when evaluating capacitor performance; DC and surge voltage and AC voltage. In the case of DC and surge voltage ratings, the thickness of the dielectric determines the maximum surge and DC voltages that may be applied. AC voltage ratings are usually specified for ceramic capacitors. This rating corresponds to the AC voltage required to make the sum of the given DC voltage and AC voltage less than the rated DC voltage.

 

In addition to these ratings there are certain types of electrolytic capacitors in which the applied voltage is of primary concern. Electrolytic capacitors are sensitive to the effects of voltage because they are highly polarized devices. Even if the applied voltage is less than the maximum voltage specified, the voltage drop across the ESR of the capacitor will shorten the capacitor's life expectancy through an accelerated effect of internal heating.

 

Current Ratings - Current ratings to consider are the leakage and ripple currents. Leakage current is the stray DC current of relatively small value which flows through the capacitor when voltage is applied across the terminals. Ripple current is the AC component of a unidirectional current. For electrolytic capacitors, there is also a maximum allowable charge and discharge current rating.

 

Frequency - Since there is an internal inductance in a capacitor there will be a resonant frequency. Depending on capacitor type, this frequency may or may not fall in a range that is a problem for the designer. This problem would arise because the designer would want the capacitor to block or minimize DC current, and at resonance the internal impedance is a minimum which causes maximum DC current.

 

Failure Modes

 

Electrolytic Capacitors - Most failures in electrolytic capacitors result from two cases; either the breakdown of the dielectric film due to low IR or the leakage of the electrolyte due to high IR. Dielectric breakdown is an electrochemical failure that is caused by improper chemical composition of dielectric material used in their manufacture. The addition of contaminants such as chloride is also a predominant factor in dielectric breakdown. Electrolyte leakage is a mechanical failure and is most commonly caused by insufficient compression seal, leakage at the weld on the bottom of the cylinder (in axial-lead devices), and leakage around the aluminum or tantalum terminals in plastic (molded) headers or seals. Other failure modes exist in the form of poor welds or pressure connections becoming open-circuits after a short shelf life or operating life.

 

Ceramic Capacitors - Most failures in ceramic capacitors are caused by encasement materials used to protect the capacitor and lead assembly from external environments. Other failures include electrical degradation and intermittent failures. Electrical degradation is caused by thermal expansion of encapsulants and moisture between the coating and capacitor section. Intermittent or open failures are caused by poor soldering techniques and terminal design that result in loose or detached leads.

 

Paper and Film Capacitors - Paper and film capacitors are subject to the same failure modes as electrolytic capacitors with the exception of electrolyte leakage. Seal leakage is common in poorly made oil-impregnated capacitors. Mechanical failures are caused by fracture of the electrode tab at the point of attachment to the electrode or to the external lead. Rough edges on foil electrodes cause early shorting, especially if the lower plate is thicker than the upper.

 

Design Considerations

 

The reliability of a capacitor is dependent upon the degree of success achieved in housing the capacitor element in a mechanically and environmentally secure enclosure. Capacitors with internal lead construction must be mechanically and electrically sound before the encasement is applied. Encapsulated dipped, or molded capacitors can not withstand dynamic environments such as high levels of shock and vibration. For mechanical integrity, metallurgical bonds and reinforcing materials should be used.

 

When considering which capacitor best performs a specific circuit task there are several options available. These options depend on the cost of the capacitor and the capacitor's physical and electrical properties with respect to the task it is about to perform. If precision is a must, then it is advised that mica, glass, ceramic and film (polystyrene) capacitors be employed. These capacitors possess exceptional capacitance stability with respect to temperature, voltage, frequency, and life. Circuits that will settle for semiprecision can use paper/plastic film capacitors (with foil or metalized dielectric) since they presently constitute a large portion of applications. If precision is of no importance whatsoever, then general purpose capacitors are recommended. These are the least expensive capacitors and they have good performance ratings. Where suppression of radio-frequency interference is required, RFI and feed-through capacitors are the best equipped. For heavy currents (60-40 Hz power supplies), paper or film dielectric capacitors should be used for suppression, and ceramic and button-mica high-frequency style capacitors are recommended for low currents. Ceramic chip capacitors are highest on the list for use in microelectronic circuits. These capacitors are electrically and physically the best suited for such purposes. If a capacitor needs to be used as a transmitter, then it is advised that gas, vacuum, or ceramic capacitors be employed. These capacitors possess the necessary high radio frequency (rf) power-handling capability, high rf current and voltage rating, low loss, low internal inductance, and very low ESR.

 

Environmental Effects

 

The effective operation of a capacitor is greatly dependent on the physical environment that will be surrounding it. Out of these many possible effects, those of primary concern with respect to medical devices are temperature, humidity, dynamics, pressure, and radiation.

 

Temperature - The maximum operating ambient temperature surrounding a capacitor in an application is of critical importance. As the ambient temperature changes, the dielectric constant and capacitance of most capacitors change. The useful service life of a capacitor decreases if it is subjected to high temperatures for great amounts of time. As the temperature of the environment which surrounds the capacitor rises, the capacitor should receive less than the rated applied peak voltage.

 

On the other end of the spectrum, cold temperatures can present problems as well. Electrolytic capacitors change their capacitance immensely within a few degrees once they are exposed to temperatures below 25 C. Aluminum electrolytics lose their capacitance at -55 C and tantalum loses about 20%. Equipment at low temperatures should be given time for the capacitance to rise once the equipment has been powered up.

 

Humidity (Moisture) - An important consideration in the application of a capacitor is making sure that no moisture penetrates the sealing of the capacitor case. The effects of humidity are parametric changes (especially IR), reduced service life, and serious failure due to gross moisture penetration. Most sensitive to moisture are the paper-dielectric nonhermetically-sealed capacitors. Moisture can easily penetrate into paper and can be trapped during manufacture, penetrate the capacitor during service life, or penetrate the capacitor once exposed to a moist environment.

 

Dynamic Environments - Dynamic environments can mechanically damage or destroy a capacitor. The main dynamic environments are in the form of shock, vibration, and acceleration. The movement of a capacitor assembly inside a case can cause capacitance fluctuations, electrode attachment failures, and dielectric and insulation failures. A capacitor's susceptibility to dynamic environments is dependent on its physical construction; the larger the complex elements in the capacitor, the lower the frequency of response of the elements.

 

Barometric Pressure - The pressure dictates the altitude at which a hermetically sealed capacitor can safely operate. This altitude is dependent on the design of the end-seal case-wall, the voltage at which the capacitor will be operated, and the type of impregnant used in the dielectric material. As the altitude increases, the dielectric strength across the end-seal will decrease. If the altitude is increased with barometric pressure reduced, then the pressure inside the capacitor will increase the mechanical stress on the case and seal until failure occurs.

 

Radiation - Radiation particles can degrade the electrical performance of capacitors. The principle cause of radiation-induced capacitor defects is dimensional changes in the interelectrode spacing. This change is due to gas evolution and swelling. Changes due to radiation are more pronounced in organic-dielectric capacitors. Capacitors using organic materials like polystyrene, polyethylene terephthalate, and polyethylene are less satisfactory in a radiation environment by nearly a factor of ten than those capacitors employing inorganic dielectrics. The electrolytic capacitors (aluminum and tantalum) are capable of extended radiation exposure with tantalum being more radiation resistant. Another defect from radiation occurs when the dielectric in the capacitor experiences a noticeable increase in its conductivity in an ionizing-radiation environment. This results in the very dangerous discharging of a charged capacitor.

 

References

 

  1. Chute, George M., Electronics in Industry. New York: McGraw-Hill Book Company, 1971.
  2. Fink, Donald G., ed., Electronics Engineers Handbook. New York: McGraw-Hill Book Company, 1975.
  3. Fink, Donald G., ed., Standard Handbook for Electrical Engineers. New York: McGraw-Hill Book Company, 1960.
  4. Fugiel, Max, Modern Microelectronics. New York: Research and Education Association, 1972.
  5. Harper, Charles A., ed., Handbook of Components for Electronics. New York: McGraw-Hill Book Company, 1977.

 

 

FIGURE 1 (1A, 1B, !C, !D) are TYPICAL CERAMIC (A-C) AND 

MICA (D) CAPACITORS

TYPICAL CERAMIC (A-C) AND MICA (D) CAPACITORS

 

 

FIGURE 2 (2A, 2B, 2C) ARE TYPICAL TRIMMER CAPACITORS

TYPICAL TRIMMER CAPACITORS

 

 

FIGURE 3 COMMON FIXED CAPACITORS

COMMON FIXED CAPACITORS

 

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