Characteristics and key selection criteria for various types of capacitors

Capacitors are essential energy storage devices in analog and digital electronic circuits. These devices can be used for timing, waveform generation and shaping, blocking DC, AC signal coupling, filtering and smoothing, and energy storage. Because of this wide range of uses, various capacitor types have emerged using pole materials, insulating dielectrics, and physical forms. For each of these capacitor types, a specific range of applications apply. The wide variety of products means that time needs to be spent classifying all of them to find the best choice for the design in terms of performance characteristics, reliability, lifetime, stability, and cost.

To properly match a capacitor to the intended circuit application, understanding each capacitor's characteristics is required. This understanding must cover the capacitor's electrical, physical, and economic characteristics.

This paper will describe the various types of capacitors, their characteristics, and key selection criteria. The text will illustrate the key differences and attributes of capacitors using products from Murata Electronics, KEMET, Cornell Dubilier Electronics, Panasonic Electronics Corporation, and AVX Corporation.

What is a capacitor?

A capacitor is an electronic device that stores energy in an internal electric field. It is a basic passive electronic component, just like resistors and inductors. All capacitors have the same basic structure, with two conductive pole plates separated by an insulator called a dielectric that can be polarized upon application of an electric field . The capacitance is proportional to the area A of the plates and inversely proportional to the distance d between the plates.

The first capacitor was the Leiden bottle, invented in 1745. The Leyden bottle was a glass bottle lined with metal foil on both the inner and outer surfaces and was originally used to store electrostatic charges. Benjamin Franklin used the Leyden bottle to prove that lightning was an electrical phenomenon, one of the first recorded applications.

Capacitor Structure

Capacitors are available in various physical mounting configurations, including axial, radial, and surface mount.

Axial capacitors are constructed with alternating metal foil, and dielectric layers or a double-sided metalized dielectric rolled into a cylindrical shape. Connection to the conductive pole plate can be achieved using inserted tabs or circular conductive end caps.

The radial type is usually constructed with alternating metal and dielectric layers stacked on top of each other. The metal layers are bridged together at the end. The radial and axial configurations are suitable for through-hole mounting.

Surface mount capacitors are also constructed with alternating conductive and dielectric layers. Tin Caps bridge the metal layers at each end to suit surface mounting.

Capacitor Circuit Model

The circuit model of a capacitor includes capacitive, inductive, and resistive components.

The circuit model of a capacitor contains a series of resistive elements representing the ohmic resistance of the conducting element and the dielectric resistance. This is called the equivalent or effective series resistance (ESR).

A dielectric effect occurs when an AC signal is applied to a capacitor. The AC voltage causes the polarization of the dielectric to change each cycle, resulting in internal heating. Dielectric heat generation is a function of the material and is measured by the dissipation factor of the dielectric. The dissipation factor (DF) is a function of the capacitance and ESR of the capacitor.

Due to capacitive reactance, the dissipation factor is frequency-dependent, dimensionless, and usually expressed as a percentage. The lower the dissipation factor, the less heat generation and thus the lower the losses.

The model has an inductance element called the effective or equivalent series inductance (ESL). It represents the lead and conductive path inductance. Series inductors and capacitors cause series resonance. Below the series resonance frequency, the device behaves primarily as a capacitor, and above the series resonance frequency, the device behaves more as an inductor. In many high-frequency applications, this series inductance can be a problem. Vendors can minimize inductance using layered structures, as shown in the radial and surface mount component configurations.

The parallel resistance represents the insulation resistance of the dielectric. The values for the various model components depend on the capacitor configuration and the construction material selected.

Ceramic Capacitors

This type of capacitor uses ceramic dielectrics. There are two types of ceramic capacitors: Type 1 and Type 2. Type 1 is based on paraneoplastic ceramics like titanium dioxide. These ceramic capacitors have high stability, good capacitance temperature coefficient, and low losses. Due to their inherent accuracy, these devices can be used in oscillators, filters, and other RF applications.

Class 2 ceramic capacitors use ceramic dielectrics based on ferroelectric materials such as barium titanate. Due to the high dielectric constants of these materials, Class 2 ceramic capacitors offer higher capacitance per volume than Class 1 capacitors but with lower accuracy and stability. They are used in bypass and coupling applications where absolute capacitance values are unimportant.

Murata Electronics' GCM1885C2A101JA16 is an example of a ceramic capacitor. This Class 1, 100 picofarads (pF) capacitor has a 5% tolerance and is rated at 100 V in a surface mount configuration. The capacitor is suitable for automotive use and is rated for temperatures from -55°C to +125°C.

Film capacitors use plastic film as the dielectric. The conductive pole plate can be either a foil layer or two thin metalized layers on each side of the plastic film. The plastic used for the dielectric determines the characteristics of the capacitor. Film capacitors are available in various forms.

Polypropylene (PP): These devices have particularly good tolerances and stability, with low ESR, ESL, and high rated breakdown voltage. Due to the temperature limitations of the dielectric, they can only be used as leaded devices. PP capacitors can be used in high power or high voltage circuits such as switch mode power supplies, ballast circuits, high-frequency discharge circuits, and audio systems where low ESR and ESL are essential for signal integrity.

Polyethylene Terephthalate (PET): Also known as polyester or polyester film capacitors, these are the most volumetrically efficient film capacitors due to their high dielectric constant. These capacitors are typically used as radial lead devices for general-purpose capacitor applications.

Polyphenylene sulfide (PPS): These capacitors are produced only as metalized film devices and have excellent temperature stability, making them suitable for use in circuits where good frequency stability is required.

An example of a PPS film capacitor is the ECH-U1H101JX5 from Panasonic Electronics Corporation. This 100pF device has a tolerance of 5% and is rated at 50V in a surface mount configuration. The operating temperature range is -55°C to 125°C and is suitable for general electronics applications.

Polyethylene naphthalate (PEN): Like PPS capacitors, these capacitors are available only in a metalized film design. They are high-temperature tolerant and can be used in surface mount configurations. Applications are focused on areas requiring high temperature and high voltage performance.

Polytetrafluoroethylene (PTFE) or Teflon capacitors are known for their high temperature and high voltage resistance. They are produced in metalized and metal foil configurations. PTFE capacitors are primarily used in applications that require exposure to high temperatures.

Electrolytic Capacitors

Electrolytic capacitors are known for their high capacitance value and high volumetric efficiency. This is achieved by using a liquid electrolyte as one of its pole plates. Aluminum electrolytic capacitors consist of four layers: an aluminum foil cathode; an electrolyte impregnated paper barrier layer; an aluminum anode chemically treated to form a fragile aluminum oxide layer; and finally, another paper barrier layer. These layers of material are then rolled up and placed in a sealed metal can.

Electrolytic capacitors are polarized, direct current (DC) devices, which means that the voltage must be applied to the specified positive and negative terminals. Although the case has a pressure relief diaphragm to control the response and minimize the possibility of damage, failure to connect electrolytic capacitors can lead to an explosive failure properly.

The main advantages of electrolytic capacitors are their high capacitance values, small size, and relatively low cost. These capacitance values have a wide tolerance range and a relatively high leakage current. The most common application for electrolytic capacitors is as filter capacitors in linear and switching power supplies.

An alternative to aluminum electrolytic capacitors is the aluminum polymer capacitor, which uses a solid polymer electrolyte instead of a liquid electrolyte. Aluminum polymer capacitors have a lower ESR and longer operating life than aluminum electrolytic capacitors. Like all electrolytic capacitors, they are polarized and are used in power supplies as filtering and decoupling capacitors.

Kemet's A758BG106M1EDAE070 is a 10µF, 25V radial lead aluminum polymer capacitor with longer life and higher stability over a wide temperature range. The device is suitable for industrial and commercial applications such as cell phone chargers and medical electronics.

Tantalum capacitors are another form of electrolytic capacitors. The device has a layer of tantalum oxide chemically formed on a tantalum foil. Its volumetric efficiency is better than aluminum electrolytic capacitors, but the maximum voltage level is usually lower. Compared to aluminum electrolytic capacitors, tantalum capacitors have a lower ESR and higher temperature tolerance, which means they can better withstand soldering processes.

Kemet's T350E106K016AT is a 10µF, 10% tolerance, 16V, radial lead tantalum capacitor. It offers small size, low leakage current, and low dissipation factor, making it suitable for filtering, bypassing, AC coupling, and timing applications.

The last electrolytic capacitor type is the niobium oxide electrolytic capacitor. Developed in response to a shortage of tantalum, niobium electrolytic capacitors use niobium and niobium pentoxide instead of tantalum as the electrolyte. Due to the high dielectric constant, its unit capacitance package size is smaller.

An example of a niobium oxide electrolytic capacitor is the NOJB106M010RWJ from AVX Corp., which is a 10µF, 20% tolerance, 10V capacitor on a surface mount configuration. Like tantalum electrolytic capacitors, it is used for filtering, bypassing, and AC coupling applications.

Mica Capacitors

Mica capacitors (mostly silver mica) are characterized by low capacitance tolerance (±1%), low capacitance temperature coefficient (typically 50ppm/°C), external dissipation factor, and low variation in capacitance with applied voltage. The device has tight tolerances and high stability for RF circuits. Mica dielectrics are sprayed with silver layers on both sides to provide a conductive surface. Mica is a stable mineral that does not interact with the most common electronic contaminants.

Cornell Dubilier Electronics MC12FD101J-F is a 100 pF, 5% tolerance, 500 V mica capacitor in a surface mount configuration. The device is used in RF applications such as MRI, mobile radio, power amplifiers, and oscillators. The rated operating temperature range is -55°C to 125°C.

Summary

Capacitors are an essential component in electronic design. Over the years, various types of capacitors have been developed with varying characteristics, and some capacitor technologies are only suitable for specific applications. It is worthwhile for designers to take the time to acquire knowledge of the various capacitor types, configurations, and specifications. Only then can they be sure to select the best device for the given application.