Inductors are available in different sizes and have different uses. Their sizes vary depending on the material used for their manufacture. The main classification is done as fixed and variable indicators. An inductor of some Henry’s can be in the shape of a simple resistor to a dumbbell. In color coding, a fixed inductor always has silver as the first color.
The core of the inductor is his heart. There are several types of inductors according to the main material used.
Types of Inductors
There are several types of inductors depending on the applications and their factors.
A commonly seen inductor with simple winding is an air-core inductor. It has nothing but air as its main ingredient. Non-magnetic materials such as plastics and ceramics are also used as main materials and they also come under these air-core inductors.
They are widely used in RF applications due to their low loss at high operating frequencies.
The main drawback of an air core inductor is that mechanical vibrations can affect its inductance.
These inductors have a ferromagnetic material, such as ferrite or iron, as the main material. The use of such core materials helps in increasing the inductance, due to their high magnetic permeability. Permeability measures the ability of a material to support the formation of a magnetic field within it.
Inductors that have a ferromagnetic core material suffer from core loss and energy loss at high frequencies. These inductors are used in manufacturing some types of transformers.
Iron Powder Core Inductors
The core of such inductors is composed of a mixture of iron grains with an organic binder such as epoxy resin etc.
The epoxy insulation coating on the iron particles minimizes eddy current losses in the core. Since the size of the particles determines the eddy flow in the core. The smaller the particle size, the less eddy current is induced.
The air gap is evenly distributed between the particles of the core thereby reducing the magnetic permeability of the core. Hence the saturation current of this core is relatively very high.
Iron cores are susceptible to core loss at high frequency. Thus, they are used for frequencies below 100 kHz. Due to their high saturation current, they are used mostly in chokes in high-power applications such as storage chokes, dimmer chokes, filter chokes, etc.
Iron powder is much cheaper which makes this type of main design more cost-efficient no matter the size.
Ferrite Core Inductors
This type of inductor uses a ferrite core. Ferrite is a material with high magnetic permeability made from a mixture of iron oxide (ferric oxide, Fe2O3) and a small percentage of other metals such as nickel, zinc, barium, etc.
There are two types of ferrites i.e. hard ferrite and soft ferrite.
- Hard ferrites are used in permanent magnets because they do not demagnetize very well. They are not used in inductors because of their high hysteresis loss.
- Soft ferrite magnetism is easily changed and is a good conductor of magnetic fields. Thus they are used in transformers and inductors.
Ferrite cores have very low electrical conductivity which reduces the eddy current in the core, resulting in very low eddy current loss at high frequency. Therefore they can be used in high-frequency applications.
The ferrite material is very cheap because it is composed almost of iron rust and is very resistant to corrosion.
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Ceramic Core Inductors
Ceramic is a non-magnetic material like air. The ceramic core is used to provide a shape for the coil and a structure for its terminals to sit on. Since it is a non-magnetic material, it has low magnetic permeability and low inductance. But it provides a reduction in the main disadvantages. It is available mostly in SMD packaging and is used in applications where low core loss, high Q, and low inductance are required.
These types of inductors have a toroidal core which is a circular ring or donut-shaped core. The core is made of ferromagnetic material.
The advantage of this circular core is that the magnetic field is contained within the core and there is very little magnetic flux leakage. Due to the low leakage flux, the magnetic field in the core is high. This increases the inductance of the toroidal core inductor and exceeds that of the rod or bar-shaped core inductors of similar material.
Another important aspect of toroidal cores is that the core emits less electromagnetic interference (EMI) than other inductors. This is why they are preferred in designing compact devices, where the components are very close to each other.
They are used in power supplies, control circuits, communication systems, medical devices, etc.
Laminated Steel Core Inductors
In these types of inductors, the core is laminated, which means that it is made up of a bunch of thin sheets that are stacked on top of each other in a tight form. The sheets are coated with insulation to increase their electrical resistance and to prevent eddy flow between them. Hence the eddy current loss in laminated core inductors is significantly reduced. They are used in high-power applications.
Bobbin/Drum Core Inductors
It is available mostly in SMD packaging and is used in applications where low core loss, high Q, and low inductance are required.
The coil is wound around the cylinder. The bobbin core does not provide a closed magnetic path; instead, the flux travels through the disc into the air gap and then enters the core through a second disc at the other end. This provides a large air gap for its magnetic field to store more energy. and hence increases the saturation current of the inductor. The inductor can withstand high peak currents without saturation but at the expense of electromagnetic interference (EMI) radiation.
There are two types of bobbin core inductors i.e. shielded and unshielded.
- Shielded bobbin core inductors have an additional layer on top of the winding to complete the flux path containing the magnetic field inside the core. These types of inductors have lower EMI due to reduced flux leakage and higher inductance due to increased magnetic permeability, but at the cost of a lower saturation current than non-shielded core inductors.
- An unshielded bobbin core inductor discussed above lacks a closed flow path and has high saturation current at the cost of low inductance and EMI.
Unshielded core inductors are cost-effective. They are used in power conversion applications where the peak current is large. They are available in axial, radial, and SMD packaging.
These inductors consist of several layers of wire wound on top of each other. Such inductors have a large inductance due to an increase in the number of turns of the winding.
Multi-layer inductors are available in SMD (Surface Mount Devices) packaging.
SMD multilayer inductors consist of several layers of conductive traces on top of each other separated by a ferrite material. These traces act as an inductor’s wire. However, as the number of turns of the coil increases, the parasitic capacitance also increases. This reduces the Q factor of the inductor which can be improved by using ceramic dielectric materials as losses occur at very high frequencies in the ferrite core.
Due to their compact SMD design, they are used in mobile communication devices.
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Thin Film Inductors
This type of inductor is designed on a substrate of thin ferrite or magnetic material. A conductive spiral-shaped trace of copper is placed on top of the substrate. The design allows stability and resistant to vibration.
Due to its high accuracy, performance, and compact size, it is used in mobile communication devices, wireless networks and power supplies, etc.
This type of inductor is coated with insulation such as molded plastic or ceramic, just like resistors.
The core is made of ferrite or phenolic material. The winding can be in various designs and is available in different shapes like axial, cylindrical and bar shapes. They are also available in SMD and THT. Their small size and light weight allow them to be used in PCBs (Printed Circuit Boards), mobile devices and computers, etc.
Couple inductors are made up of two windings around a common core.
The changing magnetic flux due to the first winding induces an emf in the second winding; This phenomenon is known as mutual inductance. Both windings are electrically isolated. Thus the coupled inductor provides electrical isolation between the two circuits. A transformer is a coupled inductor.
They have many applications depending on their winding. 1:1 winding ratio inductors are mostly used to increase electrical isolation or series inductance. The winding ratio of 1:N coupled inductors (which can increase or decrease voltage) is used in other energy conversion circuits such as flyback, SEPIC, ZETA, etc.
These inductors are specifically designed to withstand high currents without reaching the magnetic saturation region. To increase the saturation current rating, the magnetic field of the inductor is increased, causing EMI (electromagnetic interference).To reduce EMI, most power inductors are used with proper shielding. They are available in both SMD and through-hole packaging from a few amps to a few hundred amps.
Radio-Frequency RF Inductors
Inductors of this type are designed for high-frequency applications. A common inductor does not perform very well due to its high impedance and core losses at high frequency. Most of these losses are due to parasitic capacitance, skin effect, proximity effect and core loss (eddy current loss), etc.
The eddy current loss is directly proportional to the frequency. Thus, instead of using an air core inductor, it is eliminated by completely removing the core.
Whereas the parasitic capacitance is due to the potential difference between the turns of the winding which are in close proximity. This causes the inductor to self-resonate at a higher frequency. This is mitigated by maintaining some space between the wires to avoid parallel turns, and by wounding the coils in a spiderweb or basket weave (honeycomb) design.
The skin and proximity effect is caused by an increase in frequency which increases the resistance of the wire. This high frequency causes a skin effect where most of the current flow across the surface of the wire due to an increase in resistance inside the wire where less current flows. The proximity effect has the same result but occurs because the eddy current induced in the proximity between two wires forces it to flow across the surface of the wires. Composed of curved strips to increase surface area to reduce resistance due to these effects.
The choke is just a simple inductor but is specifically designed to block (knee) high-frequency signals. The impedance of the choke increases significantly with an increase in frequency. Therefore it blocks high AC current and allows DC and low-frequency AC current with some loss.
Inductors used as chokes are manufactured without using any impedance minimization techniques that are used to increase their Q-factor. The choke has a low Q-factor and is intentionally designed as we want its impedance to increase by increasing the frequency.
There are two types of chokes i.e. AF choke and RF choke.
- AF (audio frequency) chokes are used to block audio frequencies and allow only DC current.
- RF (radio-frequency) chokes are designed to block RF frequencies while allowing DC and audio frequencies.
These inductors are designed for variable inductance. This variable inductor is designed in more than one possible way.
The most common design of variable inductors is a movable ferrite core. Moving the core along the winding will increase or decrease the permeability which affects the inductance of the inductance. The core can be designed to slide or screw in or out of the coil.
Another method of variable inductor design is to increase or decrease the number of turns by means of a movable contact at the top of the winding. The conductors used in these windings have no insulation (so the core must be insulated), thus moving the contact over the turns will change the effective number of turns. Since the number of turns is directly proportional to the number of turns, the inductance varies accordingly. But the downside of such a method is that the contact shortens more than one turn thus increasing the losses in the windings. This problem can be solved by increasing the space between the individual turns and using a grove wheel as a contact. This type of variable inductor is known as a roller inductor.
The most efficient way is the use of a variometer. It provides the continuous change in inductance. The variometer is made up of two coils (one inside the other) connected in series in a 1:1 ratio. The mutual inductance between these two coils plays a full role in changing the total inductance. The inner coil can be rotated using a shaft that changes the direction of the magnetic field lines created by that coil.
When the magnetic fields are in the same direction it adds up and provides maximum inductance. When their directions are perpendicular to each other the inductance decreases. When they are exactly opposite each other the magnetic fields cancel each other out and the total inductance is minimal.