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

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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PARAMETERS

There are a number of performance issues to consider with inductors. One of them is skin effect, that occurs because the resistance of a wire to high frequency current is higher than its resistance to dc current. Radio frequency alternating current doesn't penetrate far into the body of a conductor, but travels along its surface. So, in a solid wire, most of the cross-sectional area of the wire is not used to conduct the current, that flows in a narrow annulus on the surface. This effect increases the resistance of the wire in the coil, which may already have a relatively high resistance due to its length and small diameter.

The resistance of the wires also increases because of the proximity effect, which occurs in parallel wires that lie close to each other. The individual magnetic field of adjacent turns induces eddy currents in the wire of the coil, which causes the current in the conductor to be concentrated in a thin strip on the side near the adjacent wire. Like the skin effect, this reduces the equivalent cross-sectional area of the wire conducting current, thus increasing its resistance.

Another characteristic affecting inductor performance is parasitic capacitance, which is the capacitance between individual wire turns of the coil. While parasitic capacitance and doesn't cause losses in the wire, it can adversely change the coil's behavior. That is because each turn of the coil is at a slightly different potential, so the electric field between neighboring turns stores charge on the wire. Thus, the coil acts as if it has a capacitor in parallel with it.

WINDINGS

Honeycomb coils are multilayer types wound in patterns with successive turns that are not parallel, but crisscrossed at an angle.

Litz wire consists of several smaller wire strands that carry the current. Unlike ordinary stranded wire, the strands are insulated from each other, to prevent skin effect from forcing the current to the surface, and braided together. The braid pattern insures that each wire strand spends the same amount of its length on the outside of the braid, so skin effect distributes the current equally between the strands, resulting in a larger cross sectional conduction area than an equivalent single wire.

API DELEVAN INDUCTORS

API Delevan offers a wide range of inductors to meet various performance and application requirements. The HTPT66 series of high temperature power toroids (Fig. 1) are intended for high operating temperature environments in switching power supplies, as output chokes, in EMI/RFI filtering, and dc-dc converters. They exhibit temperature stability, excellent saturation current characteristics, and are available in custom electrical configurations. Standard inductances are 0.390µH to 100µH with current ratings as high as 18.3 A dc. The components operate within a temperature range of -55°C to +200°C, and are available with either tin-lead or lead-free termination finish compliant with EU Directive 2002/95/EC and applicable amendments.

Three new series of shielded surface mount power inductors available include the SP1812, SP1210 and SP1008 (Fig. 2). Recognizing the needs of design engineers, these power inductors are sized to industry standard packages for ease of circuit board layout. Inductance for series SP1008 range from 0.27µH to 100µH and max current ratings from 1.07A dc to 0.102A dc; series SP1210 range from 0.47µH to 390µH and max current ratings from 1.44A dc to 0.080A DC; series SP1812 range from 1.0µH to 390µH and max current ratings from 1.58A DC to 0.136A dc. All three inductors operate between -55°C and +125°C and are available with either traditional tin-lead or lead free, tin-silver-copper termination finishes.

API Delevan also offers the DC1050 choke, shown in Fig. 3, for high power circuit filtering in power amplifiers, power supplies, and for power line interference suppression as well as voltage regulation. These leaded inductors exhibit minimal dc resistance, and have superior current capabilities in power applications. Inductances for the DC1050 are available from 15µH to 4700µH, with current ratings as high as 19.9 A direct current. The parts operate over a temperature range of -55°C to +125°C, and -55°C to +80°C at full rated current. DC1050 inductors are available with either a tin-lead or a lead-free termination finish that complies with EU Directive 2002/95/EC and applicable amendments for lead-free assemblies.

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TESTING

API Delevan's inductors are tested in accordance with MIL-PRF-27, MIL-PRF-39010, MIL-PRF-15305, MIL-PRF-83446, and MIL-STD-981 and MIL-STD-202. These MIL specs are used as reference guidelines for the type of test, method, and requirement. These guidelines can be adjusted, based on design, application, or market need. All testing and methods for characterizing the inductive components are as performed by API Delevan. Test fixtures are available on special order. The factory recommends test equipment correlation sampling on applications requiring low inductance (≤0.10µH) and tight tolerance (≤2%) values.

API DELEVAN

API DELEVAN provides custom products designed to meet the highest quality standards as evidenced by the accreditation of its MIL-PRF-39010, “R” failure rate. The company focuses on the four pillars required by the engineering community:

Prototype Services — Solutions offered in hours or days, not weeks or months.
Smallest Size — Today's solutions, whether components or vertically integrated designs, must meet the needs of miniaturization.
Highest Performance — Better electrical & mechanical performance than other manufacturers.
Lowest Cost — Highest quality and lowest cost by adhering to daily Kaizen and Lean Manufacturing activities, producing one piece flow while simultaneously eliminating waste.

Engineer-to-engineer solutions for custom designs, and insistence on listening to, and working with,customers for the on-going success of this very important element of our business. The relationships have been long-standing and mutually rewarding ones.

An Established Reliability Program has accumulated over 250 million hours of life testing. In addition to making available an engineering staff with over six decades of accrued experience, API maintains a modern environmental lab, testing capabilities and a R&D department. API Delevan believes value comes not just with quality, but also with resourcefulness.