Skin Effect in Coils

Skin effect forces AC current toward the conductor surface as frequency rises, effective cross-section shrinks and AC resistance rises above DC value. Skin depth δ ≈ √(ρ/(πfμ)) sets the scale — at 100 kHz in copper, δ is on the order of 0.2 mm, so solid wire thicker than that wastes interior copper for current.

Coil designers feel skin effect in every multi-kW spiral unless Litz or thin foil is used. Ignoring it predicts optimistic efficiency and cold coils on paper that overheat in the warehouse lane.

01Mitigation

• Litz wire with strand diameter below δ (with bundle optimization).
• Thin copper foil or PCB traces near skin depth scale.
• Parallel conductors with appropriate spacing (careful with proximity).

02Frequency planning

Raising WPT frequency shrinks skin depth — magnetics shrink but conductor loss rises unless conductor technology upgrades. When changing frequency on an existing coil geometry, recompute Rac before assuming the same turn count works.

03Measurement

• Compare Rdc and Rac on production samples — ratio tracks process drift.
• Temperature rise test correlates with Rac better than Rdc.
• Impedance analyzer with four-wire fixture on coil terminals.