The basics of noise countermeasures have already been explained in the lessons up to lesson 9 and, as a sequel to this, in this lesson we will discuss the points to consider when using noise countermeasure parts and the actions to take when difficulties are encountered. In the first part we will talk about chip ferrite beads.
1. Superimposed DC characteristics of chip ferrite beads
As we mentioned in lesson 4, chip ferrite beads are types of inductors that use ferrites. For this reason, it is necessary to bear in mind that their performance will change depending on their magnetic saturation when high currents flow.
Figure 1 shows some examples of the changes occurring in the impedance values when currents are passed to chip ferrite beads.
As the figure shows, the impedance value decreases when high currents flow to the chip ferrite beads; this means that the expected results will not always be obtained when the beads are used in places where high currents flow. At times like these, it is necessary to select parts that have some leeway in their rated current or those with a high initial impedance or figure out some other inventive plan of action. However, magnetic saturation occurs in ferrite beads only when high currents flow, and once the current is reduced, the original performance of the beads is restored. What this means is that sometimes no problems will be posed in circuits where the current increases only momentarily (but keep in mind that the magnetic saturation in that moment may cause problems). It is hard to determine the specific effects of this so we suspect that magnetic saturation is to blame in cases where it is assumed that ferrite beads will not work well according to the evaluations undertaken in actual products.
2. Dealing with the overshoot and undershoot in the signal waveforms generated by ferrite beads
Overshoot and undershoot sometimes appear in the waveforms when ferrite beads are used in digital circuits (see figure 2).
This phenomenon is caused as a result of the resonance produced by the inductance of the ferrite beads and the static capacitance (input capacitance of the ICs, etc.) of the other parts of the circuits, so it tends to occur with ferrite beads that have a sharp impedance curve. One way to improve the situation is to insert damping resistors in series with the ferrite beads. Inserting damping resistors causes the resistors to absorb the energy of the resonance so the resonance is reduced and both the overshoot and undershoot are diminished (see figure 3).
However, since use of these damping resistors causes the voltage to drop in a corresponding manner, attention must be paid as to whether the reduced wave height of the signals is generating problems. The properties of ferrite beads are such that the sharper their impedance curve, the lower the internal loss and the more likely resonance will occur and also that, conversely, the more gentle the impedance curve, the less likely resonance will occur. Therefore, refraining from using ferrite beads with a curve that is sharper than necessary helps to prevent overshoot and undershoot.
In this lesson, we have talked about ferrite beads, and in the next lesson we will be examining the points to consider concerning chip 3-terminal capacitors.
Written by: Yasuhiro Mitsuya, Product Promotion Dept., Component Business Unit, Murata Manufacturing Co., Ltd.