2019.04.25

Points of this article

・Inductors used to deal with noise can be broadly divided into filters based on winding-type inductors and ferrite beads, which convert noise into heat.

・Relative to general inductors, ferrite beads have a high resistance component R and low Q value.

・Inductors in general can tolerate comparatively large DC superposition currents, and within this range, the impedance is not affected very much by the DC current.

・Ferrite beads tend to saturate easily when a DC current is passed, and saturation causes the inductance to fall and the resonance point to shift to higher frequencies.

・Filters based on general inductors can be selected with a wide range of inductance values.

・Ferrite beads have low Q values, and so are an effective means of dealing with noise over a relatively broad range of frequencies.

In the previous article, the basic characteristics of inductors were explained. This time, we explain actual noise countermeasures, while comparing these measures with ferrite beads, a counterpart of inductors, that are similarly often used to deal with noise.

When capacitors alone are insufficient to adequately eliminate noise, the use of inductors is considered. Inductors used as noise countermeasures can be broadly divided into two types.

①Winding-type inductors: Acting as filters

②Ferrite beads: Converting noise into heat

Before embarking on a discussion of noise countermeasures using inductors and ferrite beads, the basic characteristics of each are reviewed. Ferrite beads are classified as inductors, but their frequency-impedance characteristics differ from those of most inductors.

Compared with general inductors, ferrite beads have a high resistance component R and a low Q value. These characteristics can be utilized in noise elimination.

The direct current characteristics are also different.

Inductors generally can tolerate comparatively large DC superposition currents, and within this range the DC current does not have much of an effect on the impedance, with almost no change in the resonance point. In contrast, ferrite beads easily reach saturation due to a DC current, and saturation causes the inductance to fall and the resonance point to shift to higher frequencies. Consequently the filter characteristics change, and so due caution is necessary.

Now let us consider noise countermeasures using inductors and ferrite beads.

Here we explain π filters that use inductors. In the low-frequency range, such filters act as a low-pass filter based on an inductor and a capacitor. At higher frequencies the inductor behaves like a capacitance and the capacitor behaves as an inductor, so that the filter functions as a high-pass filter, and therefore there is no noise elimination effect.

Ferrite beads also basically function as a low-pass filter in the low frequency range. But as explained above, in this range ferrite beads are easily saturated due to a DC current, so that the inductance declines and the bead cannot eliminate noise in the targeted band.

Referring to the graph on the right side, there is a point at which the reactance declines and crosses the resistance component. If the band exceeds this point, called the cross point, the ferrite bead functions as a resistor, and serves to convert noise into heat. This is a major difference from filters that use winding-type inductors. In still higher-frequency regions, the ferrite bead functions as a high-pass filter, similarly to a winding-type inductor.

Because filters that use ferrite beads convert noise into heat in addition to shunting noise away, they can be expected to provide excellent noise elimination performance. However, attention must be paid to their DC bias current characteristics.

Downloadable materials, including lecture materials from ROHM-sponsored seminars and a selection guide for DC-DC converters, are now available.