What Are Inductors?

Inductors are passive electronic components that store energy in a magnetic field when current flows through them. A conductor wound into a coil, with or without a magnetic core, constitutes the basic inductor: the changing current induces a back-electromotive force that opposes the change, giving the component its characteristic property of inductance, measured in henrys. The relationship between voltage, current, and inductance is governed by Faraday's law of electromagnetic induction. Inductors are fundamental two-terminal components alongside resistors and capacitors, and they appear in circuits wherever energy storage, filtering, or impedance transformation is needed. Their practical design spans a wide range of physical scales, from nanometer-scale spiral inductors integrated into silicon for radio-frequency chips to meter-scale coils in power conditioning equipment.

Construction and Core Materials

An inductor's inductance depends on the coil geometry and the magnetic permeability of the material threading its magnetic circuit. Air-core inductors, wound without a surrounding magnetic material, are linear and free from core saturation but require many turns or large diameters to achieve high inductance values. Adding a ferromagnetic core multiplies the inductance by the relative permeability of the material. Ferrite, a ceramic iron oxide compound, combines high permeability with high electrical resistivity, which limits the eddy-current losses that would otherwise be severe at the switching frequencies of modern power converters. Research published in IEEE Transactions on Electromagnetic Compatibility on RF equivalent circuit modeling of ferrite-core inductors characterizes how ferrite cores behave across frequency, showing that the effective permeability and loss tangent vary significantly above a few megahertz, which must be accounted for in circuit models. Powder iron cores offer lower permeability but a distributed air gap that keeps inductance stable under high DC bias, making them preferred in output inductors for buck and boost converters where the current carries a large DC offset.

Core shape also matters. Toroidal cores confine nearly all the magnetic flux within the ring, minimizing radiated electromagnetic interference. E-core and pot-core assemblies allow the coil to be wound separately and snapped onto the core, simplifying manufacture. Coilcraft's application note comparing ferrite and powder-core inductors outlines the tradeoffs between the two dominant core families in terms of permeability, saturation current, and losses at switching frequencies.

Inductor Performance Parameters

Three parameters govern inductor selection for most applications. Inductance sets the energy storage capacity and the cutoff frequency of filters. Saturation current is the DC level at which the core begins to lose permeability, causing inductance to drop sharply and potentially driving a converter into instability. The quality factor Q relates stored energy to dissipated energy at a given frequency; a high-Q inductor introduces little resistance into a resonant circuit. Self-resonant frequency, determined by the distributed capacitance between the wire turns, sets the upper frequency limit above which the component behaves as a capacitor rather than an inductor. These parameters interact and must all be considered when choosing an inductor for a specific circuit.

Tunable Inductors and Resonant Circuits

In tunable circuits, the effective inductance can be varied by mechanically adjusting the position of a ferrite slug within the coil, changing the proportion of the coil that the core threads. This approach is used in variable oscillators and IF transformers in radio receivers. For integrated circuits and microwave applications, varactor-loaded transmission line inductors and switched inductor banks provide electronic tuning without mechanical parts. TE Connectivity's overview of inductor types categorizes surface-mount, through-hole, and chip inductor families and documents the range of inductance values, current ratings, and package sizes available from standard product lines.

Applications

Inductors have applications in a wide range of fields, including:

• Power supply filtering and energy storage in DC-DC converters
• Electrical ballasts for fluorescent and discharge lamps
• Radio-frequency tuned circuits and impedance matching networks
• Electromagnetic interference suppression on power and signal lines
• Inductive power transmission and wireless charging coils
• Sensor coils in inductive transducers and proximity detectors