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Litz wire is a particular type of multistrand wire or cable used in electronics to carry alternating current (AC) at radio frequency. The wire is designed to reduce losses due to the skin effect and proximity effect at frequencies up to about .
It consists of many thin wire strands, individually insulated and twisted or woven together, following one of several carefully prescribed patterns often involving several levels of bundle conductor (already-twisted wires are twisted together into small bundles, which are then twisted into larger bundles, etc.). The result of these winding patterns is to equalize the proportion of the overall length over which each strand is at the outside of the conductor. This has the effect of distributing the current equally among the wire strands, reducing the impedance.
Litz wire is used in high-Q for radio transmitters and radio receiver operating at low frequencies, induction heating equipment, and switching power supplies.
The term litz wire originates from Litzendraht ( Litze), German for See English translation of Litzendraht. See translation of Litze and translation of Draht . See also German translations of . or ., , and http://www.litz-wire.com/applications.html translate Litzendraht to .
The depth to which AC current penetrates in a conductor is determined by a parameter called the skin depth, which is the depth at which the current is reduced to 37% of its surface value. The skin depth decreases with frequency. At low frequencies, where the skin depth is larger than the diameter of the wire, the skin effect is negligible and the current distribution and resistance are virtually the same as in DC. As the frequency rises and the skin depth gets smaller than the wire diameter, skin effect becomes significant, the current is increasingly concentrated near the surface, and the resistance per unit length of the wire increases above its DC value. Some examples of skin depth in copper wire at different frequencies are:
Litz wire is another method, which employs a stranded wire with individually insulated conductors, forming a Bundle conductor. Each thin conductor is less than a skin-depth, so an individual strand does not suffer an appreciable skin effect loss. The strands must be insulated from each otherotherwise all the wires in the bundle would short together, behave like a single large wire, and still have skin effect problems. Furthermore, the strands cannot occupy the same radial position in the bundle over long distances: the electromagnetic effects that cause the skin effect would still disrupt conduction. The weaving or twisting pattern of the wires in the bundle is designed so that the individual strands are on the outside of the bundle for a distance where the electromagnetic field changes are smaller and the strand sees low resistance, and are on the inside of the bundle for a distance where the EM field changes are the strongest and the resistance is higher. If all the strands have a comparable electrical impedance, then current is distributed equally to every strand in the cable. This allows the interior of the litz wire to contribute to the overall conductivity of the bundle.
Another way to explain the benefit of litz braiding is as follows: the generated by current flowing in the strands are in directions such that they have a reduced tendency to generate an opposing electromagnetic field in the other strands. Thereby, for the wire as a whole, the skin effect and associated power losses when used in high-frequency applications are reduced. The ratio of distributed inductance to distributed resistance is increased, relative to a solid conductor, resulting in a higher Q factor at these frequencies.
At frequencies above about , the benefits become gradually offset by the effect of parasitic capacitance between the strands. At microwave frequencies, the skin depth is much smaller than the diameter of the strands, and the current that is forced through the inner strands induces strong Eddy current in the outer strands, which negates the benefits of litz wire to the point where it performs much worse than solid wire of the same diameter.
Litz wire has a higher impedance per unit cross-sectional area, but litz wires can be used at thicker cable sizes, hence reducing or maintaining cable impedance at higher frequencies. Skindepth, Litz wire, braided conductors and resistance, W8JI. Construction of litz wires usually involves extremely fine wires often available with a silver plate or solid silver. The individual strands often make use of a low-temperature lacquer coating that typically requires Silver soldering-Soldering iron temperatures to meltwhich is removed when making connections. The bundles of wires can also use silk outer insulation.
Litz wire is frequently found in power applications in frequencies ranging between lower tens to higher hundreds kilohertz, namely and transmitters of inductive chargers (e.g. the Qi standard). Multiple parallel twisted strands of enameled wires can be found also in transformers in some switching power supplies.
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