Summary of Dealing with Noise Using Inductors

1. Dealing with Noise Using Inductors

• ・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

2. Impedance Characteristics of Inductors and Ferrite Beads

• ・Ferrite beads are classified as inductors, but their frequency-impedance characteristics differ from those of general 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.
• ・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.
• ・Ferrite beads easily reach saturation due to a direct current, and saturation causes the inductance to decline, so that the resonance point shifts to higher frequencies and the filter characteristics change. Hence appropriate caution is required.

3. Noise Countermeasures Using Winding-type Inductors: Constituting Filters

• ・Filters based on general inductors can be selected with a wide range of inductance values.
• ・In the low-frequency range, Πfilters that use inductors 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.

4. Noise Countermeasures Using Ferrite Beads: Converting Noise into Heat

• ・Ferrite beads have low Q values, and so are an effective means of dealing with noise over a relatively broad range of frequencies.
• ・Ferrite beads also basically function as a low-pass filter in the low frequency range. But 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.
• ・If the reactance declines below the point at which the reactance crosses the resistance component curve, the ferrite bead functions as a resistor, and serves to convert noise into heat.
• ・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.
• ・Functioning as a resistor to convert noise into heat is a major difference with filters that use a winding-type inductor.
• ・At still higher frequencies, ferrite beads function as high-pass filters, similarly to winding-type inductors.

5. Common Mode Filters

• ・Strictly speaking, a common mode filter is not an inductor, but it is a magnetic component that is vital as a noise countermeasure.
• ・In a common mode filter, there are two windings on a single core, in a structure that is equivalent to two inductors merged together.
• ・A common mode filter is used to remove common mode noise by utilizing self-induction action to hinder (choke) the flow of a common mode current.
• ・A common mode filter passes a differential mode current without passing a common mode current.

6. Dealing with Noise Using Common Mode Filter

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• ・When a common mode filter is used as an input filter to a switching power supply, a filter with a split-winding structure having a high differential mode impedance is used.
• ・This type of filter is generally sold as a common mode filter for power supply lines.
• ・An effect in attenuating differential mode noise can also be expected, but because the differential mode impedance is extremely low at frequencies between several hundred kHz and several MHz or so, use in conjunction with a filter for differential mode noise such as a Πfilter is standard.

7. Points to Be Noted Relating to Crosstalk

• ・Depending on the board wiring layout, crosstalk may detract from the efficacy of filters.
• ・The crosstalk means that noise coupling between close wirings due to stray capacitances and mutual inductances.
• ・If the wiring after the filter is close to unfiltered wiring that includes noise, noise coupling due to crosstalk results in a reduced filtering effect.
• ・As one countermeasure, a layout that is kept distant from lines containing noise can hold noise coupling to a minimum.

8. Points to Be Noted Relating Noise from GND Lines

• ・Depending on the way in which capacitors positioned before and after an inductor used in a Πfilter are grounded, noise flowing from ground may occur.
• ・As one countermeasure, in order to prevent direct noise propagation, connections to the GND plane can be made through vias to effectively utilize the parasitic inductance of the vias.