Understanding the role of inductors in power electronics

Inductors are typically used as energy storage devices in switched-mode power devices to produce DC current. The inductor, which stores energy, supplies energy to the circuit to maintain current flow during “off” switching periods, thus enabling topographies where output voltage exceeds input voltage.

Due to the way they work—by not only altering electric field but the magnetic field around it—many people struggle to understand them.

What is an inductor?

An inductor is arguably the simplest of all electronic components. It’s a passive two-terminal electrical component that stores energy in a magnetic field when electric current flows through it. Typically, an inductor will consist of an insulated wire that’s wound into a coil, much like a resistor. This design was settled on following extensive trial and error methods that considered methods like Hanna curves and area-product.

When the current flowing through the coil changes, the time-varying magnetic field induces voltage in the conductor with a polarity which opposes the change in current that created it. As such, inductors oppose any changes in current that pass through them.

The induced magnetic field also induces an electrical property known as inductance—the ratio of voltage to the rate of change of current. Inductance quantifies how much energy an inductor is capable of storing.

Inductor design and key components

The design of an inductor is governed by electrical, mechanical, and thermal requirements of a given application. In general, it involves:

Selecting the core material

Deciding a core shape and size

Selecting a winding wire

The core material is an enamel-coated magnetic wire typically made of copper which is then coated in layers of insulating polymer material. The winding can form many shapes, including circular, rectangular foil, and square cross-section. A magnetic wire is chosen to confine and guide the magnetic fields, and this is insulated to prevent problems like short circuits and breakdowns.

The different types of inductor

Different applications require different types of inductor. In almost all cases, you’ll find that an inductor in a system is formed around a core material—generally iron or iron compounds—to support the creation of a strong magnetic field.

Iron core inductors

Iron is the classic and most recognizable magnetic material, making it the perfect choice for use in inductors. As above, iron in inductors takes the form of an iron core. They are typically used for low frequency line filtering due to their relatively large inductances. They are also used a lot in audio equipment. Inductors don’t always need to have an iron core, though.

Air core inductors

As the name suggests, air core inductors have no core—the core is open air. Since air has a low permeability, the inductance of air core inductors is very low. This means that the rate of current rise is relatively fast for an applied voltage, making them capable of handling high frequencies found in applications like RF circuits.

Ferrite core inductors

A ferrite is a ceramic material made by mixing and firing iron(III) oxide blended with small amounts of one or more additional metallic elements, such as nickel and zinc. When used in inductors, ferrite powder is mixed with an epoxy resin and molded to form a core around which a magnetic wire can be wound. Ferrite inductors are the most widely used type as it’s possible to finely control their permeability by tuning the ratio of ferrite to epoxy.

power inductors

A coil is a generic name for an electrode in the shape of a spiral. Among the different types of coils, there are coils called “inductors” which are used for electrical applications. Inductors can be further categorized into RF inductors used for signal processing, and power inductors for power supply lines. The power inductors discussed in this section form part of the voltage conversion circuit in a DC-DC converter or other device.Here we will explain the operation of a power inductor in a DC-DC converter. A power inductor is used in a step-up, step-down, or step-up/step-down circuit to convert a certain voltage to the required voltage. Among those different circuits, it is primarily used in a type of circuit called a “switching regulator.”

Figure 1-1 shows an example of a switching regulator step-down circuit.It uses an IC, power inductor, and capacitor to convert a DC input voltage to the required output voltage. The power inductor works with the capacitor to play the role of rectifying the rectangular wave output from the IC to a direct current.If either one of these components is missing, the output cannot be properly rectified.

What is a transformer?

A transformer is a device that transfers electric energy from one alternating-current circuit to one or more other circuits, either increasing (stepping up) or reducing (stepping down) the voltage.

Where are transformers used?

Transformers are employed for widely varying purposes. For example, a transformer is often used to reduce the voltage of conventional power circuits to operate low-voltage devices and to raise the voltage from electric generators so that electric power can be transmitted over long distances.

Why is the iron core of a transformer laminated?

The iron core of a transformer is laminated to reduce eddy currents. Eddy currents are the small currents that result from the changing magnetic field created by the alternating current in the first coil. They need to be minimized so they won’t disturb the flow of electricity from the primary coil to the secondary coil.

transformer, device that transfers electric energy from one alternating-current circuit to one or more other circuits, either increasing (stepping up) or reducing (stepping down) the voltage. Transformers are employed for widely varying purposes; e.g., to reduce the voltage of conventional power circuits to operate low-voltage devices, such as doorbells and toy electric trains, and to raise the voltage from electric generators so that electric power can be transmitted over long distances.

Transformers change voltage through electromagnetic induction; i.e., as the magnetic lines of force (flux lines) build up and collapse with the changes in current passing through the primary coil, current is induced in another coil, called the secondary. The secondary voltage is calculated by multiplying the primary voltage by the ratio of the number of turns in the secondary coil to the number of turns in the primary coil, a quantity called the turns ratio.

Operation

Current transformers are unique because they usually have only one winding. The primary is connected to the line load in series. When the primary has a high current rating, the primary winding may be a straight conductor that runs through the magnetic circuit's center. This straight conductor indicates a one-turn winding. 

When the primary has a low current rating, the primary winding may consist of several turns wrapped around the core. This provides the required flux on low-current applications or to compensate for line drop to a power meter.

The secondary consists of many turns of wire wrapped around a core. The number of turns is determined by the desired turns ratio of the current transformer. The primary current of a current transformer is not controlled by the secondary, as it would be in a two-winding potential transformer. The secondary of a current transformer cannot affect the current in the primary, as the load on the feeder determines the primary current.

When the primary circuit is energized, the secondary of a current transformer must never be left open. When the circuit is operational, the load on the secondary maintains low magnetizing currents and thus low turn-to-turn potentials. When the secondary becomes an open circuit, the magnetizing currents rise, and the current transformer acts as a step-up potential transformer. The voltage can rise to a destructive level and cause a short between the turns as the result of the degradation of the insulation. Therefore, a current transformer should always have its secondary shorted when not connected to an external load.

What is a High Voltage Transformer?

A high voltage transformer is a type of transformer that operates at a high voltage level. High voltage transformers are normally used in high voltage labs for testing purposes. These high voltage transformers are subjected to transient voltages and surges during their normal operation when the insulation under test breaks down.

To withstand these impulse voltages, the insulation of these transformers must be carefully designed. These are usually single-phase core-type transformers.

This type of transformer is generally oil immersed. Bakelite sheets are used for separating high tension and low tension windings. The high voltage transformers used for HV cable testing also need to supply sufficient electric current.

This can generate a lot of heat, and as such the cooling system of these transformers is very carefully designed. Special care also needs to be taken to ensure proper transformer voltage regulation.

For insulator testing purposes, the required current is very less but, while the insulator breaks down during testing, there would flow huge current through the transformer. To limit this current, a high resistance is connected in series with transformer.