Inductors are two-terminal components that are utilized for filtering, timing, and power electronics applications. They store energy in the form of magnetic fields while current is flowing. When an electromotive force is induced, inductors also have the ability to oppose a change in current. An inductor can also work like an open circuit for AC signals and as a short circuit for DC signals. It is important to note that the unit of inductance is Henry (H).
The four primary factors that affect the amount of inductance of an inductor is the number of turns in the coil, coil area, coil length, and core material. When their magnetic core is made of a ferromagnetic material, there is a higher level of inductance. Inductors with a magnetic core, on the other hand, have losses in the form of hysteresis and eddy currents.
Parameters To Consider
Beyond such factors, there are a few other parameters that one should keep in mind when selecting an inductor for their applications.
The Q Factor
The Q factor, or Quality Factor, is the ratio of an inductor’s reactance to effective resistance and is often determined by frequency. In particular, it impacts the sharpness of the center frequency in an LC circuit.
Self Resonant Frequency (SRF)
The SRF is the frequency where the inductor stops operating as an inductor. For example, in RF circuits, self resonant frequency must be chosen so that it does not exceed the operating frequency of the circuit. When the inductance is zero at the SRF, the Q factor is also zero.
Saturation current refers to the DC current that causes the inductance to drop to a certain value. The inductance typically drops since the core can only store a regulated amount of magnetic flux density. It is worth noting that saturation current is related to the inductor’s magnetic properties, while rated current is related to the physical properties and describes the maximum DC current that can be passed to the inductor.
DC Resistance (DCR)
DCR is the inherent resistance in the metal conductor of the inductor, and it can resemble a resistor in a series with the inductor. In DC-DC converters, DC resistance is important as the resistance leads to I2R losses that reduce the overall efficiency.
Tolerance is defined as the variation in the inductance value of an actual inductor when compared with specific values on a data sheet. This can result in unwanted frequency changes in an RF filter.
Different Types of Inductors
There are numerous inductor types, some of which are given different names based on their operation. For instance, inductors may also be referred to as a coil or reactor, and if they are used for blocking or decoupling higher frequencies, they are called “chokes.” In this section, we will outline common inductors, those of which are air-core inductors, ferrite core inductors, and iron core inductors.
Air-core inductors lack a core, have a high Q factor, and exhibit low losses. Since they do not have a core, a large number of coils are necessitated to gain the same level of inductance as their counterparts, resulting in an increase of size. They find use in high frequency applications like resonant circuits where low inductance values are optimal.
Ferrite Core Inductors
Ferrite core inductors are made of a ferromagnetic material that exhibits high permeability. They are known for their higher inductances when compared to other types and also have low losses. These types of inductors find use in noise filters, high quality transformers, and DC-DC converters.
Iron Core Inductors
Iron core inductors provide very high inductance values as a result of the iron core. They have the capacity to handle high power, but are limited in terms of frequency values. As such, they are usually found in low frequency applications like audio equipment.
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