Copper tape provides low AC resistance and good thermal contact, suitable for high-current, low-impedance paths. Multi-strand/Litz wire reduces skin effect and AC loss at high frequency but requires complex winding. Selection depends on current, frequency, thermal limits, and space. Audio and LED OEMs balance efficiency, EMI, and manufacturability.

Copper Foil (Thin, Wide Ribbon):

  • Advantages:

    • Excellent for High Current, Low Turns Windings: Provides a large cross-sectional area with minimal number of turns.

    • Excellent Heat Dissipation: Large surface area to volume ratio.

    • Low DC Resistance ($R_{dc}$): For a given cross-section.

    • Good Mechanical Stability.

    • Can Create Very Low Leakage Inductance when wound in a “sandwich” structure with another foil winding.

  • Disadvantages:

    • Poor High-Frequency Performance for its Thickness: If the foil thickness is greater than the skin depth ($\delta$), the center of the foil is unused, leading to high $R_{ac}/R_{dc}$ ratio. Must use foil thinner than $\delta$ at the operating frequency.

    • Less Flexible, harder to wind with many turns.

    • Higher Inter-Winding Capacitance due to large parallel plate areas facing each other.

Litz Wire (Multiple Insulated Thin Strands Twisted/Bunched Together):

  • Advantages:

    • Optimal High-Frequency Performance: Each strand is thinner than the skin depth. The twisting transposes the strands, ensuring each one occupies all positions in the bundle over a length, equalizing the flux linkage and minimizing both skin and proximity effect losses. This gives the lowest possible $R_{ac}$ at high frequencies.

    • Flexible, easy to wind.

  • Disadvantages:

    • High Cost: More complex manufacturing process.

    • Lower Fill Factor: The insulation on each strand and the twisting reduce the amount of actual copper in a given winding window area.

    • Higher $R_{dc}$ for a Given Cross-Section: Compared to a solid wire of equal total area, due to the same fill factor issue.

    • Can be Bulky for high-current applications requiring large cross-sections.

Handbook’s Selection Guide:

  • Use Foil for: Low/medium frequency, very high current secondaries, or where precise control of leakage inductance and thermal performance is key.

  • Use Litz Wire for: High-frequency (tens of kHz to MHz) windings where minimizing $R_{ac}$ is the paramount concern, especially in resonant converters or high-Q inductors.

  • Use Single Solid Round Wire for: Low-frequency or DC applications, or very low-cost designs where high-frequency loss is not critical.