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Coupling Coefficient

Engineering reference on Coupling Coefficient for wireless power system design, integration, and deployment.

6 minArticle 02/16Resonant WPT

Coupling Coefficient — educational diagram — Fig.: Educational diagram for “Coupling Coefficient”.

The coupling coefficient k quantifies how strongly two coils share magnetic flux. For self-inductances L1 and L2 and mutual inductance M, k = M / √(L1 L2). Perfect coupling is k = 1; practical wireless docks often operate well below that.

01What moves k

Air gap: larger gap usually lowers k sharply.
Lateral / angular misalignment: reduces overlapping flux.
Coil size and geometry: DD, circular, and multi-coil arrays shape the k map.
Ferrite and shielding: guide flux and can raise useful coupling while containing leakage.

Coupling Coefficient — supporting diagram — Fig.: Supporting illustration for “Coupling Coefficient”.

02Design window, not a single number

A resonant charger is specified across a k range (best case, nominal dock, worst misalignment). Compensation and control must keep efficiency, voltage stress, and soft-switching margins acceptable across that window. Publishing only a peak k from a perfect lab alignment misleads fleet operators.