Factors that Influence Electromagnetic Radiation of Power Inductors

As we discussed in our previous blog post, electromagnetic (EM) 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.

In this post, we will discuss EM radiation as it pertains to unshielded, semi-shielded, and shielded inductors. We will also cover how EM radiation changes according to different influences.

EM Radiation Behavior of Unshielded, Semi-Shielded, and Shielded Inductors

As engineers have become more mindful of electromagnetic radiation as a source of potential EMI, component manufacturers have responded by offering ranges of inductors 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. These are generally the worst offenders from an EMI perspective due to the unhindered propagation of EM fields. In semi-shielded inductors, magnetic materials are usually glued over the exposed windings with epoxy resin.

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 shielded inductors.

But, as most electrical 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 the overall project requirements. One of those restraints is inescapably 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 that cannot be compromised.

Würth Elektronik is one of the few companies to offer semi-shielded inductors that walk the fine line balancing 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 size 8040 ( 744 040 841 00) are presented in Figure 5 and compared with shielded inductor WE-PD size 7345 ( 744 777 10) and unshielded inductor WE-PD2 size 7850 ( 744 775 10).

EM Radiation due to the Influence of the Start of the Winding in an Inductor

One EMI consideration that can easy be overlooked is the orientation of the start of the coil winding. The “dot” on the inductor package represents the start of winding. 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 will be present on the outside winding layer, which can cause unacceptable levels of electric or capacitive coupling.

Magnetically shielded inductors are effective at shielding H-field dominant radiation, but they 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 higher 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 Würth Elektronik shielded inductor was measured with a WE-LHMI ( 744 373 680 22). The transistor of the converter was operating at 400 kHz, producing the fundamental resonance and subsequent harmonics. The spectrum clearly shows that emissions from the inductor re 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).

EM Radiation due to the Influence of Switching Transitions

There cannot be electromagnetic interference if 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.

Because of switch node ringing, fast transitions, and high switching frequency, it is necessary to choose an appropriate inductor for achieving EM compatibility. Typically, the ringing frequency is in the 100 to 200 MHz range. The effectiveness of attenuating emissions at these frequencies very much depends on the inductor’s properties, especially the core material and thickness.

The effect on the radiation of H and E-fields when changing the core material may be seen in Figure 12 and Figure 13. The DC-DC converter used for the testing is switching at 400 kHz, and the ringing frequency on the switch node is about 180 MHz. As demonstrated, an inductor with a NiZn core ( WE-PD 744 771 402 2) is much more successful at limiting H- and E-field radiation at higher switching frequencies than an inductor with MnZn core ( WE-HCF 744 363 022 0).

Clearly, EM radiation is an important consideration in terms of potential EMI and other issues pertaining to power management. That’s why it’s so crucial to select the right power inductor for your design.

In our next blog post, we’ll explain how shielding materials affect electromagnetic radiation and, by implication, the effectiveness of your power inductor. 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!