Bifilar Winding

Bifilar winding places two conductors side by side with equal turns, often wound together so magnetic fields from each cancel for common-mode current while reinforcing differential-mode flux. In WPT, bifilar techniques appear in compensation inductors, cancellation windings, and some coil structures aimed at reducing leakage inductance to a defined value.

When used in the main power coil, bifilar arrangements must preserve the intended coupling to the partner coil — not all bifilar patterns are equal; some target EMI cancellation on the cable exit instead.

01Applications

• Leakage-controlled transformers in multi-stage WPT converters.
• Cancellation windings to reduce far-field EMI from lead-out cables.
• Symmetric current paths in dual-half-bridge coil drives.

02Manufacturing

Twist pitch and spacing affect capacitance between bifilar partners — measure L and C on samples. Terminations must pair strands correctly; swapping one lead inverts the intended field. Litz bifilar bundles require vendor specification of twist direction.

03Pitfalls

• Accidental short between bifilar strands at a nick point.
• Unequal strand length in termination creating circulating current.
• Confusing bifilar cancellation with the main coupling winding role.