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The Inductor

An Inductor is a passive electrical component consisting of a coil of wire which is designed to take advantage of the relationship between magentism and electricity as a result of an electric current passing through the coil.

In our tutorials about Electromagnetism we saw that when an electrical current flows through a wire conductor, a magnetic flux is developed around that conductor. This affect produces a relationship between the direction of the magnetic flux, which is circulating around the conductor, and the direction of the current flowing through the same conductor. This results in a relationship between current and magnetic flux direction called, “Fleming’s Right Hand Rule”.

But there is also another important property relating to a wound coil that also exists, which is that a secondary voltage is induced into the same coil by the movement of the magnetic flux as it opposes or resists any changes in the electrical current flowing it.

In its most basic form, an Inductor is nothing more than a coil of wire wound around a central core. For most coils the current, ( i ) flowing through the coil produces a magnetic flux, ( NΦ ) around it that is proportional to this flow of electrical current.

An Inductor, also called a choke, is another passive type electrical component consisting of a coil of wire designed to take advantage of this relationship by inducing a magnetic field in itself or within its core as a result of the current flowing through the wire coil. Forming a wire coil into an inductor results in a much stronger magnetic field than one that would be produced by a simple coil of wire.

Inductors are formed with wire tightly wrapped around a solid central core which can be either a straight cylindrical rod or a continuous loop or ring to concentrate their magnetic flux.

The current, i that flows through an inductor produces a magnetic flux that is proportional to it. But unlike a Capacitor which oppose a change of voltage across their plates, an inductor opposes the rate of change of current flowing through it due to the build up of self-induced energy within its magnetic field.

In other words, inductors resist or oppose changes of current but will easily pass a steady state DC current. This ability of an inductor to resist changes in current and which also relates current, i with its magnetic flux linkage, NΦ as a constant of proportionality is called Inductance which is given the symbol L with units of Henry, (H) after Joseph Henry.Because the Henry is a relatively large unit of inductance in its own right, for the smaller inductors sub-units of the Henry are used to denote its value.

Power inductor characteristics

There are complex trade-offs that engineers need to understand regarding power inductors’ characteristics and the parameters of how they are used.

This difficulty originates from the many characteristics of power inductors and their applications. These may include factors such as temperature and current magnitude.

To illustrate some of these factors, the inductance property of power inductors causes a decrease of inductance as the current increases. This is known as the DC superimposition characteristic. Temperature increases that result from a rise of current, affect changes in inductor core magnetic permeability and saturation magnetic flux density. Noise characteristics is also affected by the magnetic shield structure. DC resistance can also change with the same inductance value depending on the thickness and number of windings. This may cause affect how heat is generated.

Power inductors are normally categorized as wire-wound, thin-film and multilayer inductors. This is based on their design and production differences. Manufacturers often utilize magnets, ferrite or other metallic magnets as power inductor cores. Ferrite cores exhibit high inductance and a high magnetic permeability value, whereas metallic magnetic cores exhibit exceptional saturation magnetic flux density. This makes them ideal for larger current applications.

In addition, power inductors work with two main types of ranted currents: allowed current for DC superimposition, and allowed current for temperature rise.

The inductance of the power inductor core will drop when the core becomes magnetically saturated.

The maximum recommended current that should eb transmitted without reaching magnetic saturation is the same as the allowed current for DC superimposition. The current that is defined by the heat generation of the electrical resistance in the inductor’s windings is the allowed current for temperature rise. The rated current for the inductor is should be equal or less that these two types of allowed currents. For example, there may be a drop of 40 percent from the initial inductance value and a rise of temperature of 40℃ due to self-heat generation.

Each of these parameters are co-dependent with each other and very complex, making each power inductor unique and uniquely suited for different applications. Consequently, the selection of the right inductor for each application is critical to it success.

In addition to the application in which they will be placed, the size, cost and efficiency of DC-DC conversion should be considered when selecting the most appropriate power inductors for any application.

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