At switching frequencies above 50 kHz, solid wire windings suffer from skin effect and proximity effect, causing winding resistance to skyrocket and efficiency to drop. Litz wire—made of many thin, individually insulated strands twisted together—solves both problems.
This guide explains litz wire construction, strand selection, and design rules for high-frequency transformers and inductors.
Shaanxi SiRui Technology Co., Ltd. uses litz wire in all high-frequency transformer and inductor designs above 50 kHz. Contact: sales@sirui-tech.com | Web: www.sirui-tech.com
1. The Problems Litz Wire Solves
Skin effect: at high frequency, current flows only near the conductor surface. Skin depth shrinks with frequency—~0.21 mm at 100 kHz, ~0.095 mm at 500 kHz. Any conductor thicker than 2× skin depth wastes copper.
Proximity effect: adjacent winding conductors create magnetic fields that push current into uneven regions, increasing AC resistance far beyond skin effect alone.
2. How Litz Wire Works
Litz wire bundles many thin strands (each thinner than skin depth), twisted and transposed so every strand takes turns at every position in the bundle. This equalizes current sharing and minimizes both skin and proximity loss.
3. Strand Diameter vs Frequency
| Frequency | Max Strand Diameter |
|---|---|
| 20 kHz | ≤ 0.4 mm |
| 50 kHz | ≤ 0.3 mm |
| 100 kHz | ≤ 0.2 mm |
| 200 kHz | ≤ 0.15 mm |
| 500 kHz | ≤ 0.08 mm |
| 1 MHz | ≤ 0.05 mm |
4. Design Tips
- Don’t oversize: too many strands = worse fill factor and thermal performance. Size for RMS current at 4–6 A/mm²
- Twist pitch: 10–20× strand diameter; too short adds circulating loss
- Termination: strip all strand insulation, solder or crimp properly—poor termination negates litz benefit
- Insulation: each strand enamel-insulated; bundle may have outer sleeve
5. When Litz Is Worth It
- Switching frequency > 20 kHz
- Winding loss dominates (high current, high frequency)
- Efficiency target 95%+
- Below 20 kHz, solid magnet wire is usually sufficient and cheaper
6. Sirui-Tech Litz Capability
- Litz winding: round and flat formats
- Strand sizes: 0.05–0.3 mm, custom stranding
- Applications: LLC transformers, PFC inductors, high-frequency chokes
Conclusion
Litz wire is essential for high-frequency magnetics above 20–50 kHz. Match strand diameter to skin depth, choose appropriate stranding, and ensure proper termination. The cost is paid back in reduced winding loss and higher efficiency.
Need litz-wire magnetics? Contact Sirui-Tech at sales@sirui-tech.com with your frequency and power level.