Flux band - a conductive loop, effectively a short-circuited turn, wrapped closely around the outside of an electronic transformer to reduce the effect of the fringing, leakage and stray flux, which might cause electromagnetic interference to the surrounding devices and circuits. ¹⁾

In audio applications also referred to as hum band.²⁾

Flux band around a ferrite transformer

^{S. Zurek, E-Magnetica.pl, CC-BY-4.0}

Flux band around a PFC inductor (power factor correction)

^{S. Zurek, E-Magnetica.pl, CC-BY-4.0}

The flux band couples electromagnetically to the stray flux, generates opposing currents and partially reduces radiated emissions. But because it is located on the outside of the returning limbs of the magnetic core it does not impact on the main operation of the transformer.

The flux band can be connected to ground or one of the windings (e.g. primary) to help in reduction of electrostatically coupled interference.

The presence of a conductive flux band might compromise the creepage and clearance distances, which are a safety related issue. Hence the correct dimensions must be designed to take those factors into account.

• Stray flux