Bifilar winding - this name applies to two different types of windings:

• magnetically coupled (typically two isolated conductors), with the most important feature being high magnetic coupling betwen them
• non-inductive (wound with two isolated conductors, but later joined electrically at some point), with the most imporatn feature being low value of total inductance of such configuration

Bifilar windings are made by winding two wires simultaneously.¹⁾ If more wires are used, then also the following names are applied:^{2) 3)}

• bifilar coil - 2 wires
• trifilar coil - 3 wires
• quadfilar or quadrifilar coil - 4 wires
• quintufilar coil - 5 wires, and so on.

Magnetically-coupled bifilar windings

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Non-inductive bifilar winding

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Common-mode choke with windings separated (also called sector winding), for safety creepage and clearance requireements

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Bifilar-wound common-mode choke soldered directly onto a PCB

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Magnetically coupled windings are used whenever there is a need to exchange energy or signal between different parts of an electromagnetic circuit. Hence, they are used in transformers, coupled inductors, electric motors and generators, etc.

Magnetic coupling is stronger for windings which are physically closer to each other. Therefore, multi-filar windings are used for reducing leakage inductance.

For this reason, certain windings are wound with multiple wires run together so that the wire-to-wire spacing can be a small as possible. This is important for example in pulse transformers and common-mode chokes.

However, requirements of galvanic isolation and safety do not allow use bifilar windings in all cases, and even for the same type of product a non-bifilar winding (sector winding) may be required.⁴⁾

Because of the proximity of conductors, a component with bifilar windings (and multi-filar) have increased parasitic capacitance (self-capacitance), which can become problematic at high frequency, do to self-resonance effect.

Planar coils for wireless charging (top and bottom sides) for low power application such as mobile phone

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Bifilar windings can be also used for different reasons, rather than improving magnetic coupling between the two (or more) wires.

One reason is lowering the power loss by reduction of the wire diameter so as to stay below the limit dictated by the skin effect. Using two (or more) wires fed simultaneously during the winding process produces a winding which is in some sense similar to the litz wire.

Yet another example is shown in the photograph for the wireless charging coils. A thickness of such coils is a limiting factor for applications such as mobile phones. In the particular solution shown in the image, two thinner wires are wound side-by-side so that the thickness can be reduced but the effective cross-section area of the wire can be retained.

Additionally, two thinner wires help reducing the losses due to skin effect, because wireless charging typically operates at relatively high frequency, e.g. in the region of 100 kHz.

Non-inductive winding is not used for electromagnetic energy transfer. A typical application for non-inductive winding is a wire-wound shunt resistor, used for current-to-voltage conversion (i.e. the voltage drop across a resistor is directly proportional to the current flowing through it).

Non-inductive bifilar winding

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An ideal shunt resistor should introduce only pure electrical resistance, without any parasitic inductance or parasitic capacitance because they affect the linearity of voltage drop with respect to frequency and harmonics.

But typically a significant lenght of wire is required in order to obtain the given value of resistance, and if the wire is wound on some former it has the shape of a coil and thus it might have a non-negligible value of inductance (and capacitance).

In order to reduce the inductive effect two wires can be wound together (bifilar) at the same time. Then if one end is joined electrically, then current is made to flow “clockwise” in one wire, and tracing back the same path “anticlockwise” in the second wire. The magnetic fields from both wires cancel out to a great extent and the total parasitic inductance is minimised.⁵⁾

A more complex way of achieving even lower parasitic inductance is the Ayrton-Perry winding, which is not bifilar and more difficult to make.

• Winding
• Non-inductive winding
• Magnetically-coupled windings
• Ayrton-Perry winding