The Behavior of Electromagnetic Radiation of Power Inductors in Power Management

This article focuses on the Electromagnetic (EM) radiation behavior of power inductor(s) in DC-DC converters, which is dependent on several parameters such as ripple current, switching frequency, rise & fall time of a switching device, the core material and its frequency dependent complex permeability and suggests several design tips to mitigate these EMI effects.

Introduction

DC/DC converters are widely used in power management applications and the inductor is one of the key components (Figure 1). The usual focus is on electrical performance characteristics such as RDC, RAC and core losses. But, the EM radiation characteristics can often be overlooked.

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Figure 1: Inductors are a key component in DC-DC converters

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 and shielded. Different types of inductor have advantages and disadvantages 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, operate as a transmitting loop antenna; the electromagnetic radiation depends on numerous factors.

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 whether the inductor is shielded and the winding properties. Whereas, 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.

Electromagnetic (EM) Radiation

The inherent design and operation of inductor(s) in DC/DC converters leads to unfavorable attributes which are comparable to that 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 it propagates through. These separate yet interconnected phenomena can be divided into two regions; the near-field and the far-field (Figure 2).

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Figure 2: Phenomena can be divided into two regions; the near-field and the far-field

For a loop antenna, H-field near the source is high resulting in a low wave impedance near the antenna. As the distance from the source increases, the H-field declines simultaneously generating an E-field perpendicular to the direction of the H-field. The H-field attenuates at a rate of (1/r)³ and the E-field attenuates at a rate of  (1/r)² when moving away from the source where r is the distance.

EM radiation behavior of Unshielded, Semi-Shielded and Shielded Inductors

As we have seen in the previous section, radiation from inductors in DC/DC converters is no triviality, especially when considering the type and proximity of surrounding components and their susceptibility to magnetic coupling. As engineers have become more mindful of this source of potential EMI, component manufactures have responded by offering inductors (Figure 3) that are shielded and semi-shielded in addition to conventional unshielded inductors. Shielded inductors are manufactured to fully encapsulate the coil in a form of magnetic shielding. With unshielded inductors, the coil windings are typically exposed or otherwise magnetically not shielded. In semi-shielded inductors, magnetic materials are usually glued over the exposed windings with epoxy resin.

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Figure 3a: Inductors, shielded and semi-shielded in addition to conventional unshielded inductors (photo)

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Figure 3b: Inductors. shielded and semi-shielded in addition to conventional unshielded inductors (diagram)

Each type of inductor has its own advantages and disadvantages. The main advantage of the shielded inductor is its relatively low emissions when compared to semi-shielded or unshielded inductors (Figure 4).

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Figure 4:  Shielded inductors have relatively low emissions when compared to semi-shielded or unshielded inductors

But as most engineers know, a fine balancing act must be maintained when a new design is in development.

Increasing a desirable characteristic can often increase undesirable characteristics, which are ultimately restrained by project requirements. One of those restraints is unescapably size. Shielded inductors, when compared to the same Inductance value required of unshielded inductors and of the same dimensions, have lower DC-resistance and lower saturation. Naturally, this would direct the less experienced engineer to select an unshielded inductor, which is smaller and has higher saturation current capability. But this will ultimately lead to a myriad of electromagnetic interference and compatibility issues which cannot be compromised.

Wuerth Elektronik eiSos is one of the few companies to offer semi-shielded inductors, which achieve the fine balancing line between space requirements, electrical characteristics and EMI. Semi-shielded inductors are particularly suited to applications where components close to the inductor are not severely sensitive to radiation. The excellent saturation characteristics of the WE-LQS semi shielded inductor (74404084100) are presented (Figure 5) and compared with shielded inductor WE-PD (74477710) and unshielded inductor WE-PD2 (744775 10).

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Figure 5: Excellent saturation characteristics of the WE-LQS semi shielded inductor

EM Radiation due to the Influence of Start of the Winding

One EMI consideration that can easily be overlooked is the orientation of the start of the coil winding, represented by the 'dot' on the inductor package (Figure 6). It is important to connect the dotted end of the inductor closest to the switch node as this is the end that will undergo the most dV/dt and thus generate the most interference. In this way, the AC flux from switch node switching will be shielded by the outer windings. If the non-dot end is connected to the switch node, AC flux voltages are present on the outside winding layer which can cause unacceptable levels of electric or capacitive coupling.

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Figure 6a and 6b: It is important to connect the dotted end of the inductor closest to the switch node

Magnetically shielded inductors are effective at shielding H-field dominant radiation but may not be able to shield E-field dominant radiation in all conditions. Effective E-shielding depends on the material properties and complex permeability. The greater the thickness and permeability of the core material, the more effective the inductor will be at shielding the E-field.

As an example, the E-field emissions of a Wuerth Elektronik eiSos shielded inductor was measured with a WE-LHMI (74437368022). The DC-DC converter employed for testing operated at 400 kHz, producing the fundamental resonance and subsequent harmonics. The spectrum clearly shows that emissions from the inductor are up to 8 dB lower when the dotted end of the inductor is connected to the switch node (Figure 7). It is, therefore, highly recommended to use the inductor in the correct orientation. The H-field emissions, however, are unaffected by the change of orientation of the inductor (Figure 8).

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Figure 7:  Emissions from the inductor are up to 8 dB lower when the dotted end of the inductor is connected to the switch node

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Figure 8:  H-field emissions, however, are unaffected by the change of orientation of the inductor

EM radiation due to the Influence of Switching Transitions

There cannot be electromagnetic interference if either the source, medium or the victim is not present. As switching frequencies increase, DC/DC converters also employ faster rise and fall times of the switching device to keep switching losses low. But this creates steep switch-node transitions, accompanied by switch-node ringing and spikes (Figure 9). The resulting switch-node ringing and the inductor voltage waveform are also presented (Figure 10).

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Figure 9a and 9b:  DC/DC converters also employ faster rise and fall times of the switching device to keep switching losses low. But this creates steep switch-node transitions, accompanied by switch-node ringing and spikes

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Figure 10a and 10b:  The resulting switch-node ringing and the inductor voltage waveform are also presented

Because of switch node ringing, fast transitions and high switching frequency, it is necessary to choose an appropriate inductor for achieving electromagnetic compatibility. Typically, the ringing frequency is in the 100 to 200 MHz range. The effectiveness of attenuating emissions at these frequencies depends on the inductors properties, most notably, the core material (Figure 11) and thickness.

Usually, iron powder and metal alloy powder inductors have less E-field shielding effectiveness at frequencies above 1 MHz where MnZn and NiZn have better performance.

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Figure 11: The effectiveness of attenuating emissions at these frequencies depends on the inductors properties

The effect on the radiation of H and E-fields when changing the core material may be seen below (Figure 12 and Figure 13). The DC-DC converter used for the testing is switching at 400 kHz and ringing frequency on the switch node is about 180 MHz. As demonstrated, an inductor with a NiZn core (WE-PD 7447714022) is much more successful at limiting H and E-field radiation at higher switching frequencies than an Inductor with MnZn core (WE-HCF 7443630220).

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Figure 12:  The effect on the radiation of H and E-fields when changing the core material

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Figure 13:  The effect on the radiation of H and E-fields when changing the core material

Shielding

Every core material has inherent advantages and disadvantages which are particularly suited to a specific application. In these circumstances where a core material cannot be substituted, external solutions may be necessary to mitigate emissions. For example, inductors made of iron powder/metal alloy core material have excellent saturation characteristics and can be made in extremely small sizes but have limited shielding characteristics at frequencies above 1 MHz. Hence, to shield the emissions external shielding may be required to ensure electromagnetic compatibility. Metal and magnetic shielding solutions can be utilized based on the application.

Metal shielding materials are made of copper, aluminum, metal alloys and composite mixtures. Metal shielding usually consists of an enclosure that is attached over the source to reflect the noise. The thickness and type of material can be chosen based upon shielding effectiveness (Figure 14) and cost.

Interestingly, some iron powder inductor manufacturers are integrating a metal bridge on the top of the inductor to improve shielding performance. However, this approach is less advantageous as it may first appear as these inductors are not as flexible to design and emission requirements as they have a limited effect at a constrained range of switching frequencies and fixed source properties.

Alternatively, magnetic shielding can be achieved using magnetic materials or µ-metals and their effectiveness are dependent on material permeability, impedance and thickness. The material characteristics are similar to those shown in figure 10.

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Figure 14:a and 14b: Thickness and type of material chosen depends upon shielding effectiveness and cost.

Effect of Shielding in the near-field

As mentioned earlier, the switch node ringing frequency on one demo-board is ≈130 MHz and on the other demo-board it is ≈ 180 MHz. Since the advantages of iron powder and metal alloy inductors cannot be compromised most of the time, Wuerth Elektronik offers a huge variety of metal and µ-metal shielding materials such as copper tape, various composite metal shielding cabinets with and without vents, NiZn and ferrite plates, etc. These products offer flexible and adaptable solutions that are suitable for specific design restraints and can be selected for the particular range of frequencies where attenuation is required (Figure 14). For example, the effectiveness of metal shielding of a Würth Elektronik WE-LHMI iron powder inductor is demonstrated by a 10 dB reduction in E-field emissions (Figure 15).

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Figure 15: WE-LHMI iron powder inductor is demonstrated by a 10 dB reduction in E-field emissions

Effect of Shielding in the far-field

The effectiveness of shielding is not only limited to near field radiation. Notable reductions in far-field emissions can also be achieved using metal and ferrite solutions. The same demo-board was tested in the EMI chamber for far-field radiation. The value of enclosing an iron powder core inductor with a 1.5 mm thick aluminum shield is demonstrated below (Figure 16 and Figure 17) where the ringing frequency has been substantially reduced. In addition, attenuation was also notable over the whole frequency range including harmonics. Similarly, the addition of adding a 3 mm thick ferrite plate has a similar effect when placed on the iron powder inductor (Figure 18).

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Figure 16:  The value of enclosing an iron powder core inductor with a 1.5 mm thick aluminum shield is demonstrated  - compare to figure 18

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Figure 17:  The value of enclosing an iron powder core inductor with a 1.5 mm thick aluminum shield is demonstrated  - compare to figure 17

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Figure 18:  the addition of adding a 3 mm thick ferrite plate has a similar effect when placed on the iron powder inductor

Summary

EM radiation is an incredibly vast and intricate topic as seemingly small variations in any one parameter can influence emission sources and therefore the near-field and far-field characteristics. The characterization of the near-field can be a complicated and lengthy process because many experiments and observations are needed to fully comprehend and resolve EMI issues.

Additionally, the move towards high switching frequencies for higher power densities and better efficiency, as the availability of new technologies in MOSFETs becomes readily available (GaN, SiC), further necessitates consideration and control of emissions. When switching at higher frequencies the usual approach for power magnetics is no longer valid. Wuerth Elektronik completely understands the necessity to address this and is fully prepared to offer support with issues that arise due to changing technologies, designs and parameters.

Designing one specific inductor that is valid for a few conditions is not the WE way. A small change in the switching device can affect the source characteristics of the inductor dramatically. Specific components for a specific design for a specific application is required. For this reason, WE offers a wide variety of products together with unparalleled levels of support and technical assistance to ensure your device meets EMI standards.

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