Inductors: Theory & Practice
29 February 2020
Inductors are sometimes regarded as mysterious by electronics design engineers; consequently one needs to know how to select the correct ones.
In a world very much driven by microprocessors, ASICs, MCUs, GPUs & DSPs, passive components – such as inductors – are often regarded as somewhat mysterious. Many think of their use as a bit of a “black art” that is hard to fathom. However, as Mark Patrick, Technical Marketing Manager at Mouser Electronics tells us, they are very straightforward in terms of their construction.
Generally consisting of a simple coil of wire wound around a metallic former, they can perform a variety of important functions in modern circuits. Consequently, design engineers need to have a good understanding of how they work, as well as being able to select the correct ones for their particular application.
Resistance to change
While some inductors are air-cored, most use either an iron core or, more commonly, a ferrite one – which increases the strength of the magnetic field necessary for inductor operation. As current flow increases, so does the strength of the magnetic field and, when this current changes, the voltage across the inductor also changes, as noted in Michael Faraday’s law of induction. Known as a “back EMF”, this voltage creates a current flowing in the opposite direction, thereby opposing the original current flow – this effect is covered in detail by Lenz’s law.
In essence, an inductor opposes any change in the current flowing through it. This is a very useful property and can be employed in circuits to smooth out signals, such as the output stage of power systems. It is especially valuable in DC/DC converters, where inductors create a smooth DC output from the “chopped” switching waveform. Inductors are often used to provide analogue filtering in RF and audio applications too, or they can serve as simple suppressors to filter and suppress EMI in communication buses.
Know your parameters
Inductors are usually designated by the circuit descriptor “L” and the unit of inductance is the Henry, denoted by the symbol “H” (named after the 19th-century scientist Joseph Henry). One Henry constitutes the amount of inductance required to induce a back EMF of one Volt when the current through the inductor changes at the rate of one Ampere per second. In practical terms, a Henry is quite large, so inductor values are commonly found in the range of milliHenries (mH) to nanoHenries (nH).
The filtering response of an inductor will vary with frequency. Every inductor has a self-resonant frequency (SRF), which is the frequency at which it has the most pronounced filtering effect. Above this frequency, the inductor starts to behave more like a capacitor, thereby reducing its filtering effect – so it is necessary to select an SRF above the operating frequency of the system.
As inductors are formed from wire, there is a finite DC resistance (DCR) that increases with thinner wire and also with longer lengths of wire – which, in general, means that the DCR is loosely proportional to the inductance value. DCR causes system losses, in direct relation to the square of the current passing through the inductor. This can cause the inductor to become hot during operation – which is undesirable, especially in dense power supply applications where dissipation of excess heat presents a serious challenge.
Closely allied to the DCR resistance is the current rating – which is generally defined as the current at which the nominal inductance has fallen by a specified percentage, or a specified temperature increase in the inductor core. While both are valid measurements, there is no universally accepted industry-wide standard for the values of inductance or temperature change, often making comparison between devices from different manufacturers challenging.
The final parameter to be borne in mind is the “Q-factor” – or, to use its full name, the quality factor. Mathematically, this is the ratio of the inductance to the resistance at a given frequency, and it is seen by many as a measure of the efficiency of the inductor. The higher the number representing the Q-factor, the more closely the particular inductor component approximates a theoretically perfect inductor.
The physical format of the inductor is also a consideration. While many are available as surface-mount types for automated placement, larger types (such as those utilised in power applications) may still have leads – requiring an extra production step. Some inductors include a shield (which prevents electrical noise affecting other areas of the circuit) and, as all of the magnetic flux remains within the inductor, they are also more efficient (with a higher Q-factor).
Practical use of inductors
Even though most inductors are essentially wire wound around a ferrite core, different materials, wire types, form factors and mechanical constructions result in very different performances, making each inductor best suited to a particular application. RF inductors, for instance, are used in systems that require some form of wireless communication, and are generally designed to maintain the highest levels of signal integrity. Many RF inductors differ from general-purpose inductors in that they tend to use a non-conductive plastic coil former. In order to achieve the performance needed for RF applications, inductance tolerances are normally tight (±5%) and the construction method ensures that the winding pitch is constant along the length of the inductor. One of the newest types of fixed RF inductors is the 132 series from Coilcraft.
In some RF applications, inductors are used in tuned circuits and are required to be adjustable. This is achieved by inserting an aluminum screw into the coil that can be adjusted, so the overlap between the coil and the screw changes. These types of inductors may be unshielded, but are often available with a metal shielding can as well, such as with Coilcraft’s 164 and 165 series.
Electro-magnetic interference (EMI) is an issue for most design engineers these days, as systems are often required to meet certain regulatory approvals to ensure they will not interfere with the RF spectrum, or be affected by RF energy from other devices. At the simplest level, inductors for these applications may be a ferrite bead with a single turn of wire, or a two-part ferrite bead that can be clamped around a PCB trace. There are inductors designed specifically for EMI filtering, and these are generally small surface-mount units characterised by the frequency ranges they are intended for. Modern systems require high-speed communications, and devices such as the 1206 series common mode chokes are optimised for use in high-speed USB signal lines.
Many data communication applications require multiple lines to be filtered, so to save PCB space, multiple inductors are offered by some manufacturers in a single convenient package. The CCDLF series offers up to 10 windings in a single surface-mount package measuring just 9.14mm x 9.14mm.
Power is one of the main areas for inductors, and there are a number of specific applications within the power sphere, as well as more general filtering applications, that require their use. Certain power inductors such as the DO1608C series are optimised for modern high-volume consumer products, including notebook computers and mobile phone handsets. They offer a compelling combination of low DC resistance to reduce losses and curb heat generation, while supporting the high levels of energy storage needed to deal with ripple currents. Operating over a wide temperature range (from -40°C to +85°C) and with AEC-Q200 Grade 3 qualification, they can be deployed in automotive circuitry.
For higher-power applications, the SER80xx series offers a range of inductance values from 0.50 to 10µH in a compact 8.80mm x 8.80mm surface-mount package. The current rating can be in excess of 20A for these components, which means they are suitable for boost inductors in buck converters, etc. The Coilcraft datasheet conveniently specifies the saturation current (ISAT) for a 10%, 20% and 30% drop in nominal inductance, as well as for a 20°C and 40°C temperature rise – making it easier to compare with products from other manufacturers.
While theoretical calculation will arrive at an approximate value for an inductor, the final value is most often arrived at after building a prototype and substituting component values until the optimum performance is achieved. To ease this task, Mouser offers a number of inductor designer kits that contain multiple values of inductors so that prototypes can be rapidly debugged – representing a valuable addition to any prototyping lab.
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