Coupled Inductors
Coupled inductors share a magnetic path and influence each other. Coupled inductors are often used as transformers to step up or step down voltage or provide isolated feedback. These are also used in applications where mutual inductance is required.
Multilayer Inductors
Multilayer inductors have layers of coiled wire that are wound around a central core. Adding additional layers of coiled wire to an inductor increases the inductance, and it increases the capacitance between the wires. These inductors trade off higher inductance for a lower maximum operating frequency.
Molded Inductors
Inductors that are molded into a plastic or ceramic housing are known as molded inductors. Generally, these inductors have a cylindrical or bar form factor and can be found with several types of winding options.
Power Inductors
Power inductors are available in a variety of form factors and power levels. These inductors include everything from surface mount inductors that can handle a few amps to through-hole and chassis mount power inductors that can handle tens to hundreds of amps. Because power inductors are subjected to large amounts of current, these tend to generate large magnetic fields. To prevent these magnetic fields from inducing noise in other parts of the circuit, magnetically shielded inductors should be used if possible.
RF Inductors
High-frequency inductors, also called radio frequency (RF) inductors, are designed to operate at high frequencies. These inductors often have a higher resistance and lower current rating. Most RF inductors have an air core rather than a ferrite or other inductance-boosting core material. This is due to the increase in losses when those core materials are used to reduce the operating frequency of the inductor. Because of the operating frequency of the inductor, it’s important to mitigate against several sources of loss — whether it’s from the skin effect, proximity effect, or parasitic capacitance. The skin and proximity effects increase the resistance of an inductor. Several techniques reduce these losses, including honeycomb and spider web coils to reduce parasitic capacitance. Additionally, litz wires are often used to reduce the skin effect.
Chokes
A choke is an inductor that blocks high-frequency pulses while letting lower frequency pulses through. The name comes from the choking off or blocking of high-frequency signals. There are two classes of chokes:
Power and audio frequency chokes typically have an iron core to increase inductance and make more effective filters.RF chokes use iron powder or ferrite beads combined with complex winding patterns to reduce parasitic capacitance and operate effectively at high frequencies. Higher frequency chokes use non-magnetic or air cores.
Surface Mount Inductors
The push for smaller and more mobile devices has led to the explosion in options for surface mount inductors. Surface mount inductors are often used in DC-DC converters, EMI filtering, energy storage, and other applications. The small size and footprint make surface mount inductors an essential element in the mobile and portable electronic designer’s toolbox. Surface mount inductors are available with and without magnetic shielding, with current capabilities in excess of 10 amps, and with low losses. Surface mount inductors often use an iron or ferrite core or special winding techniques to optimize the performance of the inductor. This also helps maintain a small footprint and form factor.
Types of Inductor Cores
The core material of an inductor plays a large role in the performance of an inductor. The core material directly affects the inductance of the inductor. It determines the maximum operating frequency, as well as the current capacity of the inductor.
Air cores have higher frequency operation due to no core losses but have a lower inductance.Iron cores have low resistance with high inductance. Core losses, eddy currents, magnetic saturation, and hysteresis limit the operating frequency and current.Ferrite cores have non-conductive ceramic material for higher frequency operation. Magnetic saturation limits the current capacity.Toroidal cores are cores shaped like donuts that reduce radiated EMI and provide high inductance.Laminated cores have high inductance with lower hysteresis and eddy current losses.
