Litz wire consists of multiple individually insulated strands woven or braided together in a precise geometric pattern[cite: 1]. This construction is engineered to minimize high-frequency AC losses caused by the skin effect and proximity effect, making it an essential component in high-frequency custom transformers, resonant inductors, and wireless power transfer coils[cite: 1].
Understanding the skin effect
The skin effect is the tendency of high-frequency alternating current ($AC$) to distribute itself unevenly within a conductor, forcing current density to concentrate near the outer surface (the "skin")[cite: 1]. This reduces the effective cross-sectional area of the conductor, leading to a significant increase in $AC$ resistance ($R_{\text{ac}}$) and power losses[cite: 1].
The depth to which the current penetrates is known as the skin depth ($\delta$)[cite: 1]. For copper at 70°C, it can be approximated using the formula[cite: 1]:
$$\delta = \frac{66}{\sqrt{f}}$$Where $\delta$ is the skin depth in millimeters ($\text{mm}$) and $f$ is the operating frequency in Hertz ($\text{Hz}$)[cite: 1].
Skin depth at typical frequencies
| Frequency ($f$) | Skin depth ($\delta$) | Max recommended strand gauge |
|---|---|---|
| 10 kHz | 0.660 mm | AWG 22 (0.644 mm) |
| 50 kHz | 0.295 mm | AWG 28 (0.321 mm) |
| 100 kHz | 0.209 mm | AWG 32 (0.202 mm) |
| 250 kHz | 0.132 mm | AWG 36 (0.127 mm) |
| 500 kHz | 0.093 mm | AWG 38 (0.101 mm) |
| 1 MHz | 0.066 mm | AWG 40 (0.079 mm) |
For operating frequencies extending into mega-Hertz ranges, individual strand requirements cross into the ultra-fine domain (down to 0.010 mm); see our detailed breakdown on the mechanical physics of ultra-fine wire processing.
The proximity effect in custom transformers
While the skin effect looks at an isolated conductor, the proximity effect occurs when multiple current-carrying conductors are packed closely together, such as in transformer windings[cite: 1]. The magnetic field generated by adjacent wires distorts the current distribution within each strand, causing severe current crowding and localized overheating[cite: 1].
In multi-layer transformer windings, the proximity effect is often the dominant source of high-frequency loss, causing the $AC$-to-$DC$ resistance ratio ($R_{\text{ac}}/R_{\text{dc}}$) to spike exponentially if improper wire configurations are chosen[cite: 1]. These losses increase thermal dissipation (see thermal dissipation & power loss calculations) and shift the component's self-resonant frequency (SRF) by altering distributed capacitance and effective coil resistance[cite: 1].
Litz wire construction styles
Litz wire is classified into different types based on how the strands are bundled, twisted, and insulated[cite: 1]. Detailed bundle parameters are documented in the High-Frequency Litz Wire Construction Matrix[cite: 1]:
| Type | Construction | Best suited for |
|---|---|---|
| Type 1 | Single bunching of insulated strands | Low to medium frequencies, simple inductors |
| Type 2 | Bundles of Type 1 twisted together | Medium power transformers, high frequency |
| Type 3 | Braided or woven configuration | High-power, high-frequency switch-mode transformers |
| Type 4 | Bundles served with nylon or silk textile yarn | Applications requiring extra mechanical protection |
Engineering framework for litz wire selection
To choose the right litz wire for custom transformer configurations, implement this systematic approach[cite: 1]:
- Determine operating frequency ($f$): Identify the fundamental switching frequency and major harmonics of your design[cite: 1].
- Calculate maximum strand diameter ($d_{\text{max}}$): Ensure that the individual strand diameter is less than or equal to the skin depth ($d \le \delta$)[cite: 1]. For high efficiency, select a diameter matching $d \approx 0.5\delta \dots 0.8\delta$ using standard metric wire gauges[cite: 1].
- Calculate required total copper area ($A_{\text{total}}$): Determine the overall conductor cross-section based on your continuous current ($I_{\text{rms}}$) and target current density (typically $3\text{--}5\,\text{A/mm}^2$)[cite: 1]. High peak currents must also be evaluated to prevent core saturation in high-current inductors.
- Calculate required number of strands ($N$): Divide total copper area by individual strand area[cite: 1]: $$N = \frac{A_{\text{total}}}{A_{\text{strand}}}$$
- Evaluate winding window constraints: Factor in the litz packing density and outer diameter expansion due to individual strand insulation coatings and outer serving jackets[cite: 1]. (For techniques on optimizing spatial fill within narrow envelopes, see maximizing copper fill factor in miniature coils)[cite: 1].
Manufacturing & processing considerations
- Termination and soldering: Polyurethane-insulated strands (IEC 60317-11 / Grade 1–2) can be directly soldered using a high-temperature solder bath ($380\text{--}430^\circ\text{C}$), which melts away the enamel layer cleanly[cite: 1]. For higher thermal classes (e.g., polyimide coatings), mechanical or chemical stripping is required before crimping or welding[cite: 1].
- Bending limits: Avoid tight bends that can crush individual internal strands, causing voltage breakdowns and unequal current distribution across bundles[cite: 1].
- Fill factor limits: Litz wire has a lower copper fill factor ($0.50\text{--}0.65$) than solid magnet wire due to air gaps between bundles[cite: 1]. Account for this reduced volume allocation during early bobbin and winding window geometries (compare with orthocyclic vs. wild winding density limits)[cite: 1].
- Layer lead-out routing: High-frequency litz coils often benefit from an alpha winding structure to ensure symmetrical lead exit points on the outer diameter without internal crossover strands[cite: 1].
- Self-bonding variants: For air-core coils and bobbinless coil architectures, litz wire with an outer backlack adhesive layer can be thermally or chemically bonded (see self-bonding activation methods)[cite: 1].
For detailed strand tables, outer diameter multipliers, core loss dynamics, and serving options, refer to the High-Frequency Litz Wire Construction Matrix and the Core Physics & Permeability Matrix[cite: 1].