The Behavior of Electromagnetic Radiation of Power Inductors in Power Management

One of the key components of DC-DC converters, which are widely used in power management applications, is the inductor. Usually, when engineers talk about inductors, they focus on their electrical performance characteristics, such as RDC, RAC, and core losses. But what about their electromagnetic (EM) radiation characteristics?

While often overlooked, the behavior of electromagnetic radiation of power inductors is an important consideration in power management. In this blog series, we uncover the vast and intricate topic of EM radiation.

Let’s begin with a brief introduction to how power inductors are affected by EM radiation.

Power Inductors and Electromagnetic (EM) Radiation

Power inductors in switch mode power supplies (SMPS) can be made of various core materials and different types of windings (coils). Inductors can also be classified into three types:

  • Unshielded
  • Semi-shielded
  • Shielded

Different types of inductors have advantageous and disadvantageous characteristics that permit or limit their range of application.

Due to the switching action in SMPS, AC voltage/current is produced over the inductor. Since an inductor can, in effect, operate as a transmitting loop antenna, the electromagnetic radiation depends on a number of factors, including:

  • Source properties (such as core material)
  • Shielding material
  • Orientation of the start of the winding

Electromagnetic radiation of an inductor in the low frequency spectrum range (100 kHz to 30 MHz) — which is caused by the switching frequency and harmonics — is dependent on both whether the inductor is shielded and the winding properties. On the other hand, in the high frequency spectrum range (30 MHz to 1 GHz) — where emissions are caused by ringing frequencies and their harmonics — the electromagnetic radiation is more dependent on the shielding characteristics of the core material, switching frequency, and transitions of the switching converter.

How Electromagnetic Radiation Works

The inherent design and operation of an inductor in DC/DC converters leads to unfavorable attributes, comparable to those of a loop antenna. The AC voltage and current in the inductor produces electric (E-field) and magnetic (H-field) fields, which propagate away from the source at right angles to one another.

Near to the loop antenna (source), the characteristics of the fields (E and H) are determined by the behavior of the source characteristics (switching frequency, transitions). However, far from the source, the properties of the field are determined by the medium through which it propagates. These separate yet interconnected phenomena can therefore be divided into two regions: the near-field and the far-field (as shown in Figure 2). The area within λ /2π of the source is defined as the near-field, whereas anything further than this is defined as far-field radiation.

In the near-field, the E- and H-fields must be considered separately, as the ratio between the two fields is not constant. The ratio E H ൗ is also called wave impedance. However, in the far-field, these fields combine to form a plane wave. That is why individual electric (E) and magnetic (H) fields are only discussed in the context of near-field. If the source has high current and low voltage, the magnetic field is said to be dominant, whereas if the source has low current and high voltage, the electric is said to be dominant.

For a loop antenna, the magnetic field near the source is high, resulting in a low wave impedance near the antenna. As the distance from the source increases, the magnetic field declines, simultaneously generating an electric field perpendicular to the direction of the H-field. The magnetic field attenuates at a rate of ( 1 r ൗ ) 3, and the electric field attenuates at a rate of ( 1 r ൗ ) 2, when moving away from the source where r is the distance. For a straight wire antenna, the wave impedance is high, which is why the electric field is dominant near the source and the attenuation characteristics are entirely opposite to that of loop antenna.

Now you have a bit more background into electromagnetic radiation and how it pertains to power inductors.

In our next blog post, we’ll discuss EM radiation specifically for unshielded, semi-shielded, and shielded inductors — plus how EM radiation changes according to different influences. Stay tuned to learn more!

If you’re looking for a reliable inductor for your power management application, view our selection of power inductors. We always offer free samples and quotes!

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