When to Use Litz WireMay 01, 2015 by Kevin G. McGivern
This article discusses the best practices of when to use Litz Wire in design process for inductors, transformer, and/or systems.
Throughout the design process for anything from inductors, transformers, and systems the need to optimize efficiency and minimize heat rise is a concern. One of the key decisions that need to be made is what type of wire to use.
The choices of wire are solid, stranded, foil, and Litz wire. An important fact to keep in mind is that when you move down 3 AWG wire sizes the area of the wire decreases to one half of the original wire area. For example, a Rubadue S8NC133ECC-10 wire has an area of 8.37mm2, and a Rubadue S11NC133ECC-7 has an area of 4.17mm2.
What is Stranded Wire?
Stranded wire is a number of small wires twisted together to form a larger gauge wire. To a point, this works well – except when the operating frequency is at a level where too many small wires are required.
What is Litz Wire?
The term Litz wire is derived from the German term: litzendraht – which means woven wire. Small gauge wire is literally woven to form an equivalent wire area to a larger gauge. The area of the Litz wire is within 5% of the equivalent solid wire.
Why is there a need for this? As the frequency is increased in a wire the depth at which the current flows decreases. This is known as the skin effect. For those who are not familiar with the concept of current flow through a wire - here is an analogy that may be useful. When the speed of water increases as it flows through a pipe the water will form a spiral as it flows. This spiral causes the water to follow the curvature of the pipe – decreasing the amount of water flowing through the center of the pipe. A similar event occurs with electrons as they flow through a conductor.
DCR and ACR Resistances
There are two types of resistance that are associated with a conductor: DC Resistance (DCR) and AC Resistance. Ideally, the ACR and the DCR are equal, or the ACR is less than the DCR. When the skin-depth is less than the radius of the wire the ACR is substantially higher than the DCR.
The calculation of the DCR is quite simple: R=V/I. Where R is the resistance of the conductor, V is the voltage impressed across the conductor, and I represents the current through the conductor. The wire industry has standard tables for the ρ, (Ω/M), for the standard sizes of wire that are manufactured.
ACR is more complicated. The first step is to determine the skin depth, δ, for the frequency, or one can refer to a standard table that lists the maximum frequency for a given wire size. To determine the ACR for the wire simply subtract the area of the wire used by the skin depth from the area of the wire being used. This is done by determining the area of the cylinder created by the skin depth.
The new wire area is now: ANew= π*σ2. From one of the industry standard tables, the ACR can be determined by finding the wire with approximately the same area as the calculated area and then finding the ρ. The resistance of the conductor is then calculated by R=L*ρ while a specific resistor calculator is used for passive components dealing primarily resistance. Where R is the resistance of the conductor, L is the length of the conductor, and ρ is the resistivity of the wire material. If the ACR is more than 1.5 times the DCR, then a smaller wire must be used. Otherwise, the frequency losses in the wire will result in a large thermal loss.
The optimal solution for this is the use of smaller wires in parallel to form an equivalent wire. There are several advantages to this.
The most notable advantage is that both the DCR and the ACR are inversely proportional to the number of wires used to form the equivalent wire. When Litz wire is used in the windings of a coil the proximity effect is minimized. When multiple Filars are used in a coil the field is not evenly distributed between the wires.
The use of foil is good for high current and high-frequency applications. However, it is limited in the number of turns that can be used to create a coil by the height of the winding area of the coil former and the core. The decrease in the proximity effect increases the efficiency of the coil by decreasing the fringing of the field between the windings. Since the wires are woven together in Litz wire, the field is homogeneous throughout the wire. An added advantage is the flexibility increases dramatically. Litz wire can be readily formed into shapes that increase the manufacturability of the end product. The added flexibility makes routing or winding wires improves the manufacturability of the system.
The single drawback to Litz wire is the packing factor. This is determined by how many wires are used to create the Litz wire and how tightly they can be packed to form the wire. Thermally, the mass of metal used is equivalent to that of the same solid wire. There is an advantage in that the surface area of a Litz wire is greater than that of the equivalent solid wire. The increase in diameter is offset by the increase in flexibility of the new conductor.
As with any wire, insulation is critical. There are multiple insulation systems that are available. Of particular interest are the triple-insulated wires that are available. This allows optimum insulation that maintains the flexibility of the wire. Single insulation performs the task adequately. However, in order to obtain the same insulation levels as a triple-insulated wire, a single thick layer of insulation has to be drawn over the wire. This reduces the flexibility of the wire. Rubadue uses some of the best insulation materials found in their triple insulation process.
These include Tefzel® ETFE, TCA, FEP, and PFA. Whether the wire is being used in a harness, to form a coil, or as a lead the triple insulation allows the designer to use a wire that will meet both the frequency criteria for the system as well as the insulation requirements of the higher voltages that today’s systems require. This is available on individual wires as well as the various types of Litz wire.
The controls used during the insulation process provide consistent insulation on the wires that will allow the end-user full confidence, considering the system will handle the operating voltages and peak voltages that will be occurring throughout the life of the system. When these are used in mission-critical applications the confidence that the safety factor used by the engineer will be consistently met throughout the life expectancy of the system and will minimize the field failures and maximize the uptime of the equipment. The availability of these different materials allows the designer to pick the material that best fits within the insulation system that they are using.
The gains from utilizing Litz wire will increase efficiency as well as the manufacturability of the system. When this is viewed from a “Design For Excellence” point of view, this meets the Efficiency, Reliability, Economic, Repeatability, and the Manufacturability criteria.
About the Author
Kevin McGivern works as the President at McGivern Technologies LLC since July 2014 where he is working with power supply design, magnetic device design, high voltage isolation, partial discharge abatement, high voltage switching, and educational seminars. He holds a Master's Degree in Electrical Engineering at the Western New England University. He also holds a Doctorate Degree in Electrical Engineering at the Atlantic International University.