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

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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