In high-frequency power electronics and electromagnetic design, operating frequencies are routinely increased to achieve greater power density, tighter miniaturization, and higher coercive forces. However, higher frequencies create severe eddy current losses inside solid copper conductors due to two primary electromagnetic phenomena:
- Skin Effect: Alternating currents create opposing internal magnetic fields that force electrons toward the outer surface of the conductor. As frequency rises, the center of a solid wire becomes largely non-conductive, drastically reducing its effective cross-sectional area and causing alternating current resistance ($R_{AC}$) and thermal losses to climb.
- Proximity Effect: Conductors situated within the alternating magnetic fields generated by neighboring wires experience localized current crowding, creating substantial additional losses across coil windings.
Litz wire mitigates both loss mechanisms. By accurately calculating the conductor’s skin depth ($\delta$) at the operating frequency, engineers specify individual strand radii smaller than that skin depth. Multiple individually insulated, enamel-coated strands are then precisely transposed so that every strand occupies every radial position in the bundle equally. This equalizes strand inductance, forces uniform current distribution across the entire conductor cross-section, and suppresses circulating currents.
Modern Litz wire designs overcome historical tradeoffs regarding cost and packing efficiency:
- High Fill Factors: Advanced shaping techniques produce compacted square and rectangular profiles that achieve up to 85% copper fill factor (as low as 15% area loss), maximizing slot utilization in compact winding windows.
- Higher Q Factor: In resonant topologies—such as induction heating coils and wireless power transfer (WPT) systems—minimizing high-frequency series resistance ($R$) directly elevates the coil Quality Factor ($Q = \frac{X_L}{R}$), maximizing power transfer efficiency.
- Insulation Customization: Outer jackets and insulation barriers—including extruded fluoropolymers, wrapped dielectric tapes, and textile serves—are tailored to meet specific breakdown voltages, thermal ratings, and cooling requirements.
Key Technical Takeaways
- Skin Depth Sizing: Individual strand radii are selected to be smaller than the calculated skin depth at the target operating frequency.
- True Geometric Transposition: Strands are woven so each occupies internal and external positions equally, equalizing strand inductance and current sharing.
- Proximity Effect Reduction: Individually insulated strands prevent current crowding between adjacent turns in multi-layer windings.
- Compact Form Factors: Compacting Litz bundles into square or rectangular shapes yields up to 85% copper fill factor within winding windows.
- Quality Factor Optimization: Reducing series AC resistance maximizes the Quality Factor ($Q = \frac{X_L}{R}$), which is critical for efficient wireless power transfer and induction heating.
- Total Cost of Ownership: Because Litz wire utilizes copper far more efficiently at high frequencies, overall conductor volume and core size can often be reduced, lowering raw material costs.
Episode Transcript – Selecting Litz Wire for Increased Efficiency
The Push for High-Frequency Efficiency
Welcome to Selecting Litz Wire to Increase Energy Efficiency.
In today’s markets, higher energy efficiency and lower cost of ownership with no loss in performance is the goal. Litz wire can help achieve this in many magnetic devices and applications. Those applications include high-performance motors, high-current high-frequency inductors, induction heating equipment, high-frequency transformers, and wireless power transfer.
In the recent past, the emphasis has been on how to make subcomponents more and more affordable, with too little emphasis on built-to-last and low cost of energy ownership. Sometimes considerable losses were considered tolerable. But as we continue to realize, resources and energy are best used by making high-quality, energy-efficient devices.
Skin Effect and Proximity Effect Explained
In many devices, designers are choosing to raise the frequency to take advantage of higher coercive forces and denser fields. This is to maintain or increase power in a smaller form. The result can be increased eddy current losses in both skin and proximity effect. These losses that are caused by the current flowing on a smaller and smaller section of the perimeter of the conductor as the frequency rises are becoming more and more critical.
This is due to the increased demands of miniaturization, while maintaining or increasing performance that was previously mentioned, as well as the demand for increased overall energy efficiency in all devices.
Skin effect is due to the reaction between fields set up in different parts of the conductor. In a straight wire, the current traveling near the surface causes the induced currents to flow in the opposite direction in the center of the conductor. The greater the frequency, the stronger the effect, until the current occupies only a small fraction of the conductor, or only the skin layer of the conductor. Skin effect is more pronounced in low-resistivity materials, and conductors of magnetic materials show an exaggerated increase in resistance with frequency.
Proximity effect, on the other hand, happens when a conductor is located in a varying magnetic field that is generated by a neighboring source. This can increase losses in windings considerably and will rise in magnitude if techniques are not employed to minimize these losses.
Strand Sizing, Skin Depth, and Transposition
Since we can accurately calculate skin depth, we can select a strand that has a radius that is smaller than that calculated skin depth, then combine several strands to build to the copper area needed. If you’re changing from an existing device, then this may be less copper area than was needed previously, since your device will now have a greater efficiency. This may lower cost, not raise it, since less copper and core material may be needed.
The strands are twisted in a manner so that every strand eventually comes to the surface of the bundle. This ensures that each strand in the bundle is exposed to the same magnetic field. To prevent the current from flowing on the surface of the overall bundle, each strand has a very thin layer of film. The final result is a Litz wire that effectively utilizes the entire cross-section of the conductor at frequency, or stated another way, the inductance of each strand is equal to every other strand.
Formed Shapes, Compaction, and 85% Fill Factor
At this stage, the final form of the Litz needs to be considered. Many don’t realize that square and rectangular forms can be readily manufactured that more readily utilize the area in a winding window, or that have an aspect that will help with the desired field generation. The final Litz can also be tailored for the cooling and voltage standoff required by utilizing a myriad of insulation types. These include extruded plastics, tapes, and textiles.
Before we examine an example for a specific device, let’s review some of the drawbacks cited in the past, such as cost and fill factor. Advances in forming and shapes of wire have allowed much higher fill factors than some of the classic articles cite. NEWT has a catalog that demonstrates some typical solutions but can’t convey every option here. When designs have been prepared directly for customer applications, we have seen 85% copper fill or as low as 15% area loss, or lower in some cases. Manufacturing techniques for magnet wire and Litz have advanced as well, lowering costs of these products.
Induction Coils, High Q Factor, and Wireless Charging
In induction-type coils, high Q is important to maximize the efficiency and transferred power. Using Litz wire optimized around the frequency of operation of the system, the series resistance can be reduced, thereby increasing Q, defined as $X_L$ over $R$. This results in great efficiency and transferred power.
Why go to wireless electric charging of devices if we are not going to retain as much of the generated energy as possible? Not using a high-Q coil would be like standing a few feet away from your car with a gas hose and seeing how much you could get in the tank.
Design Evolution and Summary
In conclusion, if you are considering increasing the efficiency, reducing the size, or increasing the output of your permanent magnet device, induction coil, power inductor, or AC power interconnect, then Litz deserves a review.
The method that has been discussed here, in the New England Wire Technologies catalog, and on the New England Wire Technologies website, is based on the ideal case for an isolated transmission line. Magnetics design is currently evolving at a fast pace, and many other references are available for selecting Litz for specific devices and topologies. Please see the attached references or contact New England Wire Technologies for our latest information on design references.
Thank you for watching Selecting Litz Wire to Increase Energy Efficiency.
