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Polarity of inductors

Stan Zurek, Polarity of inductors, Encyclopedia Magnetica,
reviewed by Tom Taylor, 2022-09-09

Polarity of inductors - a topic relevant in some cases of inductors used at high frequencies, such as in switch-mode power supplies, in which the polarity of inductor placement can have a significant impact on the amount of radiated electromagnetic field, thus directly affecting EMC of a given device.1)2)

Typical polarised inductor in SMPS boost converter; inductor polarity is marked with the semi-circle printed on the top of this 47 uH inductor boost_converter_with_polarised_inductor_magnetica.jpg
Typical non-polarised unshielded inductor 10 uH (there is no marking of the terminal polarity) inductor_10uh_magnetica.jpg
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Non-polarised inductors

Magnetic field distribution is the same regardless the polarity of current in the inductor

Inductors are devices which store instantaneous energy in magnetic field. Winding polarity is obviously critical for magnetically coupled inductors (between the coupled windings), but for ordinary single (uncoupled) inductors, polarity of excitation or connection to the rest of the circuit does not affect the energy storage capability, so most inductors do not have the polarity marked, as it is irrelevant.

For such simple inductors, reversing the polarity of current simply reverses the direction of magnetic field associated with it, but the shape of the distribution of magnetic field in the space around the inductor remains the same, and thus the magnetic field generated by, and radiated away from such inductor is not affected.3)4)5)

However, it is possible that this magnetic field can be inductively coupled to other parts of the circuit and some asymmetry of performance can occur, for example by coupling into a circuit with a diode (so rectification could take place). But these effects are only of secondary importance and typically can be ignored, especially with miniature SMD components.

Non-polarised SMD inductor (L6) typical_smd_l_and_c.jpg
Non-polarised unshielded inductor
Non-polarised air-core inductor air_cored_inductor_magnetica.jpg

Polarised inductors

There are electronic circuits in which the polarity of inductors matters for EMC reasons, due to the way the inductors are designed and manufactured as well as the way they are energised.

One such case is switch-mode power supplies (SMPS), in which the current through the inductor is rapidly switched between on/off (or some intermediate stages of a ramp), as for example in a boost converter. One terminal of the inductor is connected to a node with a stable voltage (e.g. connected to a energy-storing capacitor), which makes this node “quiet” in the sense that it does not radiate significant electromagnetic field.

But the other terminal is connected to a node which is rapidly switched (from kHz to MHz, depending on design) thus ending up at ground potential (transistor switched on), and the boosted output voltage (transistor switched off) due to flyback action.

In a boost converter, one terminal of the inductor is connected to the “quiet” node (green arrow), and the other to “noisy” switched node (red arrow); start of winding should be connected to the “noisy” node (as indicated by the dot next to the inductor)

Therefore, the voltage and thus the electric field associated with that terminal changes rapidly, and thus radiates electromagnetic field. According to Maxwell's equations, changing electric field is equivalent to changing electric current, and therefore either of them generates radiating electromagnetic field.

Ampère's circuital law $$ \text{curl } \mathbf{B} = \mu_0 · \mathbf{J} + \mu_0 · \epsilon_0 · \frac {\partial \mathbf{E}}{\partial t}$$
where: B - magnetic flux density (T), J - current density (A/m2), E - electric field (V/m), μ0 - permeability of free space (H/m), ε0 - permittivity of free space (F/m)
Polarity of winding in a multi-layer inductor: red - “noisy” voltage, orange/yellow - medium voltage, green - “quiet” voltage

An inductor can be designed and manufactured so that the winding has several layers. If such inductor is connected with the noisy voltage applied to the end of the winding, then the outside turns behave as the source of electromagnetic field, and there is no attenuation.

The magnetic core made out of soft ferrite has low conductivity, and in the worst case it can be treated as an electrical insulator, which does not attenuate the electric field at all.

On the other hand, if the “noisy” voltage is connected to the start of the winding (e.g. marked with a dot), so that the noisy voltage is on the inside of the winding, and the quiet voltage is on the outside, then the quiet outside turns act as an electrostatic shield and significantly attenuate the electromagnetic field radiated away.

Flux band on a transformer transformer_flux_band_magnetica.jpg

This “self-shielding” effect is in some sense similar to the concept of the flux band, in which a layer of conductor wraps around the whole inductor or transformer.

The main task of a flux band is to attenuate the stray flux, but of course if such band is connected to ground then it also performs as an electrostatic shield, attenuating also the electrostatic component of electromagnetic field, so that the total EMC emissions are reduced.

For the inductor with multiple layers shown below (with blue markings), reversing its placement in the boost converter reduced the conducted emissions by around 15 dB, as shown in the graphs below, which can be a difference between failing or passing the EMC limits for a given product.

On this shielded inductor winding polarity is marked with the printed semi-circle which denotes the start of the winding; the outer ferrite was cracked to reveal the multi-layer winding underneath polarised_inductor_47uh_magnetica.jpg
Conducted emissions from a boost switch-mode power supply with recommended (left, green arrow) and reversed polarity (right, red arrow) of the main inductor by T. Taylor and S. Zurek CC-BY-SA-4.0

It should be noted that this self-shielding effect can be achieved only for multi-layer windings, because with a single layer there are no turns on the “outside” of such coil.

Non-polarised shielded inductor which does not have a “multi-layer” winding inductor_1uh_magnetica.jpg

Inductors biased by magnet

There are also several patents describing inductors, in which the magnetic cores are “biased” by a permanent magnet.6)7) For a unipolar excitation (as in a boost converter), the size of the magnetic core is dictated by the available flux swing, which is limited by the level of magnetic saturation.

By adding the magnet, the core can be pre-biased into a “negative” range, and thus the positive excitation can extend over a wider range of the flux swing. Obviously, for such an arrangement the polarity of the applied current is important, because reversed connection would lead to premature saturation (at much lower current).

However, such inductors are used extremely rarely in industry, because of the difficulties with assembly. Additionally, very high magnetic field generated by permanent magnets can permanently damage the magnetic properties of a soft ferrite.8)

Worse still, the permanent magnet in an arrangement as shown below, would see full flux swing and thus have excessive eddy currents generated in its volume. This would significantly reduce the energy efficiency of such converter, and therefore it is not a straightforward practical solution which could be used in a general case.

Inductor with a magnet, with increased B swing without magnetic saturation by United States Patent and Trademark Office, Public Domain (with modifications by S. Zurek)

See also


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polarity_of_inductors.txt · Last modified: 2023/09/05 14:29 by stan_zurek

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