Descriptions and applications for toroidal inductors and coils with a list of manufacturers and the many uses for toroidal inductors and coils
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Toroidal inductors and coils are passive electrical components that store energy in a magnetic field and resist changes in a current as electricity passes through them. They consist of insulated wires that are wound around a core that has the shape of a donut, a shape that creates a higher magnetic field and higher inductance since the magnetic field lines form closed loops that prevent the field from leaving the core material. The symmetry of toroidal cores prevents magnetic flux from escaping outside the core, referred to as leakage flux, a design that radiates less electromagnetic interference (EMI) than other core types.
The basis of a toroidal inductor or coil is its toroid, a shape that is commonly referred to as a donut or hollow circular ring. In order to associate it with a solenoid, toroids are called circular solenoids that are also wound to form a magnetic field. The magnetic field of a toroid is calculated using Ampere circuit law. When used as an electrical component, a toroid is wound with a number of turns of copper wire and is the foundation of a toroidal inductor and coil.
In many cases, toroidal inductors and toroidal coils are used interchangeably due to their two terminal electrical components, which help toroidal inductors and coils resist changes in current. Wire that is wrapped around the donut shape of the core helps produce a toroidal inductor’s magnetic field. When electric energy enters an inductor, it is captured and stored in the magnetic field.
The process of a toroidal inductor and toroidal coil is inductance, which is a filtering of ripple currents at the output. When there are high inductance values, ripple currents are lowered, the efficiency of an electric circuit improves, and there is a reduction in EMI. Inductance does not mean opposing current but is a factor that opposes change in current flow. Toroidal inductors and coils are combined with capacitors to create tuned LC circuits for transmitting and receiving radio and microwave frequency signals.
The magnetic field of a toroid is calculated through the use of Ampere’s circuit law, a concept that was developed by André-Marie Ampère, who experimented with forces that act on current carrying wires. Ampere is credited with founding the science of electrodynamics, which is known as electromagnetism. His work in electromagnetism took place at the same time as Micheal Faraday’s, who postulated Faraday’s law.
Ampere’s law states that magnetic fields are related to the electric current that is produced in them and are associated with a given current as long as the electric field does not change. Conductors carry currents and generate magnetic fields that surround wires. Toroidal inductors and coils are more efficient at retaining electromagnetic fields due to their circular configuration, which does not allow a magnetic field to escape.
Square cores have been used for many years as inductors but have been found to be inefficient due to the high amount of leakage flux that escapes outside the core. The symmetrical shape of toroidal inductor cores limits leakage flux guaranteeing the inductance will cause less electromagnetic interference (EMI) than that of square shaped inductors. The common use of toroidal inductors is in applications where large inductance is required, such as amplifiers, inverters, and different forms of power supplies. The high demand for toroidal inductors has necessitated the development of several different types to meet market needs.
The cores of toroidal inductors are made of magnetic materials with different levels of electrical resistivity, hysteresis, and magnetic permeability. Electrical resistivity is the opposite of conductivity and is the measure of a material's ability to resist the flow of current. Various materials have differing levels of resistivity, a factor that is used to compare a material's ability to conduct current. Materials with high resistivity are poor conductors. Toroidal inductors resist the flow of current and are used to smooth fluctuations in voltage or current in a circuit.
Hysteresis is a lag of magnetic flux density behind magnetic field strength, which is caused when the direction of current is reversed and a material gets demagnetized. Materials that show hysteresis are nonlinear and show a loop that is generated by measuring the magnetic flux coming from a material while changing the external magnetic field. The hysteresis loop provides a substance with retentivity and coercivity, with retentivity meaning the amount of magnetization present and coercivity being the amount of reverse external magnetizing field that is required to demagnetize a material.
Magnetic permeability is the ability of a material to form a magnetic field. If the poles of a material are positioned easily to the application of a magnetic field, the material has high magnetic permeability. The reverse is true if the poles do not react appropriately. The magnetic permeability of toroidal inductors varies in accordance with the materials used to make the core with permeabilities ranging between 750 micrometers (µ) and 15,000 µ.
The purpose of toroidal inductors is to allow low frequencies while maintaining large inductances in electronic circuits. They have carefully wound wires that are wound around the ring core made of different types of materials and have better inductance than solenoids with a core. The variations in toroidal inductors is due to their heavy use for a wider variety of applications.
Low loss toroidal inductors are high current carrying inductors that can be vertically mounted for optimum performance. They are highly efficient with low core loss and an air gap for high energy storage. The shielding of low loss toroidal inductors protects against magnetic radiation. Their low magnetostriction helps reduce any noise created by the inductor. They are used as EMI and RFI filters, converters, inverters, and amplifiers for the telecom and medical industries as well as other commercial and industrial uses.
High temperature toroidal inductors are made of high temperature materials that can withstand temperatures up to 200°C (392°F). Toroidal inductors manufactured without high temperature construction are severely affected by high temperatures and may be burned. According to Pierre Curie’s research, a Curie temperature rating is the temperature at which a material loses its magnetism. A rise in ambient temperature affects the magnetic field of a toroidal inductor, which affects its performance. Toroidal inductors made of the proper materials are able to endure high temperatures and provide exceptional performance.
High current toroidal inductors have high frequency magnetics with a broad inductance and current range. The range of inductance from HCTIs is from 10 µH up to 1000 µH with a current rating of 2.4 A up to 20 A. Much like all forms of toroidal inductors, high current toroidal inductors come in a wide range of models to fit different and unique applications. HCTI inductors have low stray EMI with high efficiency and low operating temperature. Vertical mounts of high current toroidal inductors take up less space but provide the same outstanding performance. They are suitable for energy storage when switching power supplies and can be used for PC board mounting.
Toroidal inductors, as with all forms of inductors, use magnetic flux modulation to measure electrical current due to having a core of permeable material. The main function of a toroidal inductor is to step down AC currents for easy measurement by dividing the current. The stepped down output is sent to ammeters or other instruments that monitor the current. They are known for high accuracy with their measurement range depending on the range they are designed to handle. It is for this reason that they are used in power generation and transmission applications.
Toroidal inductors are fluxgate current transducers, which use the magnetic flux of the core to measure electrical current. The primary winding is wound around the core. Current passing through the primary winding induces the magnetic field in the core. An excitation coil is wound around the core as well. When alternating current (AC) is applied, it generates a magnetic field that saturates and demagnetizes the magnetic core, creating a modulation in the core.
Fluxgate sensors produce a low distortion that represents the primary current in a low voltage AC signal that is processed by a measurement system. The accuracy and precision of current measurement is the main reason current sensing toroidal inductors are so widely used for applications.
The structure of a toroidal fluxgate inductor consists of a drive winding and sense winding with some forms having a feedback winding when the sensor operates in a closed loop. When there is a change in the flux in the drive core, the change in the flux is sensed by the sense winding.
Under normal conditions, toroidal inductors let DC current through while blocking high frequency AC signals. A toroidal DCR is the resistance by the inductor to signals with a frequency of 0 Hz and is a resistance wire component of an inductor. The DCR varies in accordance with the application for which a toroidal inductor is being used.
Toroidal inductors with high DCR have high losses that can damage circuit efficiency, which necessitates choosing inductors with a low DCR value, if possible. The DCR of inductors normally ranges from less than 1 Ohms up to 4 Ohms. In general, toroidal inductors have very low resistance to DC currents due to their low frequency but have high resistance to input that is high frequency.
Since DCR is such a low, insignificant part of a toroidal inductor, it is interesting that it is mentioned at all. The problem with DCR is due to one of its characteristics, which is to dissipate heat that reduces inductor efficiency. It is for this reason that the DCR factor in a toroidal inductor be kept low. To determine the level of DCR in a toroidal inductor, it can be measured using a multimeter.
Coupled toroidal inductors have two or more windings and can function in dc-dc converters by transferring energy from one winding to the other windings. As with all forms of toroidal inductors, coupled toroidal inductors come in a wide range of sizes, inductance values, and current ratings and are shielded from EMI interference. The ratio of windings can be equal or unequal with either type being easily available. Like other toroidal inductors, the effectiveness of coupled toroidal inductors is dependent on the core material and the arrangement of the windings.
SMD toroidal inductors are designed to be surface mounted in electronic applications. They are made with thin flat wires wound around the toroidal core. For increased protection, the windings are coated with a layer of epoxy or other protective materials. What differentiates SMD inductors from other forms of toroidal inductors is their compact size, which makes them ideal for use in electronics with limited space.
The small compact size of SMD toroidal inductors makes them easy to install, a factor that is important in electronic assembly operations. The wide range of sizes, values, types of cores of SMD toroidal inductors gives them their versatility, adaptability, and suitability for a variety of applications. In addition, the structure and design of SMD toroidal inductors makes them highly reliable and dependable. They can withstand the harshest of conditions, including high temperatures, extreme vibrations, and impacts.
Slotted toroidal inductors are a unique and unusual form of toroid that have a gap cut through a cross section of the toroid. There are applications where toroidal inductors are limited by excessive inductance, DC currents, or effects created by a ferrite core. Slotted toroidal inductors are designed to reduce the effects of DC bias. They minimize inductance when fewer turns are used due to flux density. The slot on a slotted toroidal inductor helps to achieve the specifications of the inductor within its saturation limits. The engineering of slotted toroidal inductors takes careful planning even though slotted toroidal inductors are preferred over bobbin types.
The mounting of toroidal inductors takes several forms depending on the requirements of an application with basic types of mountings being horizontal or vertical. There are an endless number of options regarding the mounting of toroid inductors, which are constantly growing and changing as new electronics are introduced. The most used and most common forms of mountings, along with horizontal and vertical, are through hole and surface mounting (SMD).
Horizontal Mountings – Horizontal mountings have toroidal inductors mounted on their sides with each toroid having its own points of termination, size, and shape. Leads are attached to the terminal of the mount by soldering.
Vertical Mountings – The use of vertical mountings is to save space, is commonly used on printed circuit boards, and is the most economical form of mounting. The materials used for vertical mountings are normally plastic, which allows for easy cleaning.
Through Hole Mounting – With through hole mountings, a toroidal inductor is connected directly to a circuit board by a lead through a hole in the circuit board. To ensure a secure hold, the toroidal inductor is held in place by being soldered. This particular mounting makes it possible to mount toroidal inductors vertically or horizontally, as needed.
Toroidal inductors are used in a wide variety of modern electronics because they are small, affordable, dependable, and flexible. The different constructions of toroidal inductors make it possible to use them to fulfill specific functions that are required by applications.
Toroidal inductors operate on the relationship between current and magnetic fields. The interrelationship between electric current and magnetic fields was discovered by Hans Christian Ørsted in 1820, when he noticed the needle on a compass next to a wire moved such that it was perpendicular to the wire. As electric current in a wire increases, the strength of the generated magnetic field increases. The coiling of wire around a toroid further increases the strength of the magnetic field and concentrates the field in the center of the coil.
Changes in the flow of the electric current, changes the magnetic field, which produces voltage that opposes the change in the electric current flow, known as electromotive force (EMF), a factor that is proportional to the rate of change in the current in the coil. As the electric current increases, the EMF opposes the increase with the reverse being true when the current decreases.
Inductance describes how a circuit element stores energy in a magnetic field and is measured in henry (H). It is represented by the letter L and defined as the ratio of the voltage across a circuit element to the rate of change in the current that moves through it. The created magnetic field in a wire due to the flow of current induces voltage in nearby conductive materials. In the case of a toroidal inductor, the conductive material is the coil of wire around the toroid core.
The core of a toroidal inductor serves as the inductor and is composed of a magnetic core and its coiled wires. The wound wires of the core are the factors of a toroidal inductor that generates the magnetic field that stores electrical charge. The high inductance of a toroidal inductor is due to the closed loop core that generates a strong magnetic field and the production of very low electromagnetic interference. The closed loop design also does not have gaps, a factor that prevents leakage.
One of the common materials used to produce toroidal cores is ferrite, which is a non-metallic, high resistive material with dielectric properties. Ferrite has high magnetic permeability with high frequencies. The three types of ferrite are permanent, soft, and microwave with soft ferrite being the most commonly used for toroidal inductors due to producing little or no residual magnetic field.
The quality factor, referred to as the Q factor, is a measure of how well an inductor stores and releases energy. The Q factor is important when choosing a toroidal inductor, especially when it comes to choosing a toroidal inductor for an application that requires high efficiency with low power loss. As with all aspects of an inductor, the Q factor is influenced by wire material, wire diameter, and the type of core material.
The resonant frequency of a toroidal inductor is the frequency an inductor exhibits maximum reactance and minimum impedance. It is the frequency at which a toroidal inductor’s reactance nullifies its resistance leading to pure resistivity impedance. Resonant frequency is the inductance of the coil, capacitance between the coils, and capacitance between the coils and conductive elements in the circuit. Toroidal inductors have increased reactance when operating above their resonant frequency. Operating a toroidal inductor above its resonant frequency can lead to decreased efficiency, heat dissipation, and damage to the inductor.
The temperature coefficient of a toroidal inductor is a measurement of how the coils change in relation to the temperature. As with several forms of electronics, the temperature of a toroidal inductor determines the applications where it can be used. The properties of the coil wire and core strongly influence the temperature coefficient. An increase in the temperature surrounding a toroidal inductor increases the resistance of the core and wire, which causes a decrease in inductance.
The saturation current is the maximum current that a toroidal inductor can handle before inductance decreases. When the core material becomes saturated, the strength of the magnetic field in the core reaches its maximum level, which causes a decrease in inductance. Inductors with large cores and a high number of wire turns can handle higher currents and are normally chosen for high current applications.
To determine which toroidal inductor to choose for an application, manufacturers provide data sheets that outline the saturation current, which is calculated or estimated in accordance with the core materials and the core’s geometry. As a general rule, it is important to choose a toroidal inductor with a saturation current that is higher than the maximum current that is expected from an application, which prevents saturation degradation.
Televisions, sound systems, and computers rely on inductors for their performance. Unfortunately, certain types of inductors produce EMI interference, flux leakage, and an assorted list of other problems. Toroidal inductors were introduced to overcome the problems presented by other forms of inductors. They have become highly popular due to being compact, lightweight, and having high efficiency.
There is a wide selection of materials that are used to manufacture the cores of toroidal inductors, which include ceramic, powder, and magnetic metals. Each of the various types of materials reacts differently, a factor that necessitates the selection of a core that fits the frequency and requirements of an application. The materials for a toroidal inductor are magnetic with varying levels of resistivity, hysteresis, and magnetic permeability. The efficiency and effectiveness of a toroidal inductor depends on the quality of the magnetic materials that make up the core.
Ferrite cores are the most common type of core. They are produced using metal oxide ceramic and iron oxide that is mixed with nickel, manganese, cobalt, copper, or zinc. Of the various combinations of materials, manganese zinc ferrite and nickel zinc ferrite cores are the most popular.
Manganese Zinc Ferrite (MnZn Ferrite) – Manganese zinc ferrite cores are soft magnetic ferrite with a spinel structure composed of iron, manganese, zinc oxides, and ceramic salts. They have a low coercivity and high permeability with a frequency range of 1kHz up to 10 kHz due to low magnetic loss at high frequencies. Since toroidal inductors change their magnetic field multiple times per second, MnZn ferrite cores are ideal for such conditions due to their low coercivity and low loss.
Powdered metal cores are made using grains of metal alloys that are combined with insulating materials. The mixture is placed under a great deal of pressure to achieve the required shape and density. Powder metal cores have high resistivity and have low hysteresis and eddy current losses. They have exceptional inductance stability for DC and AC conditions. Since powder metal cores are not pressed with a binder, they do not endure thermal aging.
Laminated iron alloy cores are made by rolling sheets of metal and pressing them together to achieve the proper toroidal shape. The shaped materials are placed in layers that are laminated with insulating material, a construction that reduces eddy currents. The alloying metals for laminated iron cores are nickel iron and silicon iron with nickel iron laminated cores being used for high frequency applications. Silicon iron laminated cores are used to minimize DC resistance and lower saturation distortion.
Taped wound toroidal cores are produced by special machines that wind insulated tape on a mandrel with controlled tension for a uniform cross section. Once the cores are wound, they are annealed in an atmosphere of hydrogen and nitrogen to develop magnetic characteristics. Since annealed cores are sensitive to mechanical stress, taped cores are housed in cases to protect them from the electrical windings and other potential problems. Cases are made from a variety of materials including plastics, phenolic, nylon, glass reinforced nylon, and aluminum with non-metallic cases being the most widely used.
The types of cores described above are a small sampling of the many cores that are used to produce toroidal inductors. Materials that have not been included are molypermalloy powder cores, nickel iron powder cores, Kool Mμ, various versions of Xflux, and gapped ferrite cores.
The winding or fill factor for a toroidal inductor is carefully calculated prior to winding wires around the core. The fill factor for toroidal inductor cores is the ratio of the conductor cross section and the area of the core. For toroidal inductor cores, the winding factor varies between 20% and 60% with most applications ranging between 35% and 40%.
The windings of a toroidal inductor are spread over the surface of the core to provide higher flux density since the rolling flux is in the same direction as the rolling direction of the core, which saves on volume and weight. Higher current density can flow through the wires of a toroidal inductor due to the distribution of the wires over the core that leads to efficient cooling of the windings.
The wire for most toroidal inductors is made from electrolytically refined copper that is coated with one to four layers of a polymer film insulation. The insulation of the wire acts as a flux. The Q factor for the wire is that the wire should be as large as possible and still be able to fit the turns on the core without overlapping around the inner circumference. Turns on the inside can touch or come very close to touching. A turn is counted when the wire passes through the center of the toroid.
There are several ways to wind copper wire around a toroidal core with hand winding being the simplest. As the diameter of wire increases, more tension is required to form the wire around the body of the core. Great care is necessary during the winding process to avoid stretching or breaking the wire or cracking the core.
The purpose of toroidal inductors is for energy efficiency when low frequency inductance is required. They are widely used due to their ability to prevent leakage flux and the likelihood of electromagnetic interference. The compact size of toroidal inductors makes them useful in amplifiers, inverters, and power supplies.
The assortment of applications where toroidal inductors are used include telecommunications, medical equipment, industrial controls, music instruments, ballasts, electronic brakes, refrigeration, electronic clutches, amplifiers, and air conditioners.
Toroidal inductors are used in different electronic circuits such as inverters, power supplies, and electronic equipment including computers, radios, televisions, and audio sound systems.
A common use for toroidal inductors is in switch mode power supplies, EMI sensitive circuits, and filters to block high frequency noise and interference from power supply lines to allow DC current to pass. Toroidal inductors are used with capacitors to remove frequencies from signals
One of the uses for toroidal coils is in high frequency transformers that convert AC current from one voltage to another. The shape of a toroidal inductor used in a toroidal transformer has higher inductance than typical solenoid inductors. The toroid shape enhances efficiency, has higher inductance, and can carry greater current. In addition, the toroidal shape limits resistance due to its larger diameter and fewer windings.
As with most toroidal inductors, toroidal transformers operate quietly with very few stray magnetic fields or EMIs. Their design and weight make them adaptable to any application. The toroidal design and shape increases efficiency due to its elimination of air gaps, allowing a transformer to operate at a higher Tesla measurement.
The main factor that separates toroidal inductors from other forms of inductors is their shape, which is a closed loop donut that enables a toroidal inductor to concentrate magnetic flux in the core. The primary function of a toroidal inductor is to resist change in the flow of current.
The wide use of toroidal inductors is due to how efficiently they perform. The many designs and compact sizes of toroidal inductors makes them flexible enough to fit into any space. In many cases, they are combined with a capacitor to increase efficient operation of an application.
| Advantages of Toroidal Inductors | |
|---|---|
| Advantage | |
| High Inductance | The toroidal shape of toroidal inductors creates a closed loop that results in higher inductance values. |
| Wide Frequency Range | The wide frequency range or toroidal inductors makes them flexible and adaptable for a variety of applications. |
| Low EMI | The low EMI emissions that are radiated by toroidal inductors prevents damage to nearby electric circuits and components |
| Compact Size | The toroidal design of toroidal inductors is compact and enables the inductors to be used in applications where size is z concern. |
| Light Weight | The fewer material used to produce toroidal inductors makes them lighter. |
| Strong Magnetic Field | The closed loop design produces a strong magnetic field. |
| Quieter | One of the outstanding characteristics of toroidal inductors is their quiet operation due to the lack of an air gap. |
| Short Windings | The short windings increase electrical performance, efficiency, and reduces distortion and fringing effects. |
| Equilibrium | The equilibrium of a toroidal inductor allows very small magnetic flux to escape from the core. |
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