One Powerful Decade Update: Inductors Provide Important Functions in Power Electronic Systems

Jul 1, 2010 12:00 PM
Walter Wike Engineering Manager, API Delevan, East Aurora, NY

Inductors rely on the magnetic field that forms around a current-carrying conductor that tends to resist changes in the current. Electric current through the conductor creates a magnetic flux proportional to the current, that creates a corresponding change in magnetic flux that, in turn, by Faraday's Law. Faraday's Law generates an electromotive force (EMF) that opposes this change in current.

For example, a 1 Henry (H) inductor produces an EMF of 1 V when the current through the inductor changes at the rate of 1 A/S. The number of loops, the size of each loop, and the material it is wrapped around all affect the inductance. Magnetic flux linking these turns can be increased by coiling the conductor around a material with high permeability, such as iron.

An ideal inductor has inductance, but no resistance or capacitance, and does not dissipate or radiate energy. In contrast, a real inductor has a combination of inductance, resistance (due to the resistivity of the wire and losses in core material), and capacitance. At a frequency, usually higher than its working frequency, some real inductors behave as resonant circuits (due to their self capacitance), whereas at a different frequency the capacitive component of impedance may dominate.

Besides dissipating energy in wire resistance, magnetic core inductors may dissipate energy in the core due to hysteresis, and at high currents (bias currents) show gradual departure from ideal behavior due to nonlinearity caused by magnetic saturation. At higher frequencies, resistance and resistive losses in inductors grow due to skin effect in the inductor's winding wires. Core losses also contribute to inductor losses at higher frequencies.

Real-world inductors may act as antennas, radiating a part of energy processed into surrounding space and circuits, and accepting electromagnetic emissions from other circuits, taking part in electromagnetic interference, EMI. Therefore, real-world inductor applications deal with parasitic parameters, while impedance may be a minor importance.

In switch-mode power supplies, an inductor is an energy storage device. The inductor is energized for a specific fraction of the regulator's switching frequency, and de-energized for the remainder of the cycle. This energy transfer ratio may determine the input-voltage to output-voltage ratio.

INDUCTOR CONSTRUCTION

An inductor is usually constructed as a coil of conducting material, typically copper wire, wrapped around a core either of air or a ferromagnetic material. Core materials with a higher permeability than air increase the magnetic field and confine it closely to the inductor, thereby increasing the inductance. Low frequency inductors are constructed like transformers, with cores of electrical steel laminated to prevent eddy currents. ‘Soft’ ferrites are widely used for cores above audio frequencies, since they don't cause the large energy losses at high frequencies that ordinary iron alloys do.

Inductors come in many shapes. Most are constructed as enamel coated wire wrapped around a ferrite bobbin/toroid with wire exposed on the outside, while shielded types enclose the wire completely in ferrite.

Small inductors can be etched directly onto a printed circuit board by laying out the trace in a spiral pattern. Some of these planar inductors use a planar core. Small value inductors can also be packaged into ICs using the same processes used to make transistors.

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