Sector winding - a type of magnetically-coupled winding in which the coils are physically separated by placing them on different parts of insulated surface.

Sector winding (with separation ensured by a 3 mm thick blank PCB) in a common-mode choke, for safety creepage and clearance requirements

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

Such type of winding is typically employed in common-mode chokes used for filtering in power supplies fed directly from mains voltage, or other circuit containing dangerous voltages.

For safety reasons, there are international standards which require not only galvanic separation, but also a minimum distance between the separated conductors (so-called clearance and creepage) so that greater margin of safety is attained. For such applications the bifilar windings cannot be used.¹⁾

In the image shown, the clearance between the windings it ensured by the manufacturer by placing a blank PCB-like material (no copper tracks) which separates the enamelled wires with at least 3 mm distance. The spacer is secured in place by using adhesive (grey blobs).

The red toroid is the magnetic core, covered with suitable insulation, such as powder coating. The core is glued to the base, the wires are soldered directly to the pins, which provide the electrical connection points to the PCB (green tracks visible underneath).

• Bifilar winding
• Common-mode choke