Precautions for using chip ferrite beads and measures for solving magnetic saturation

A sheet-like ferrite bead is an inductor using a ferrite. Therefore, when a large current is passed, special attention needs to be paid to the performance change due to magnetic saturation.

Fig. 1 is an example of a change in impedance value when a current passes through a sheet-like ferrite bead.

Fig.1 Example of DC overlap characteristics of chip ferrite beads

For this reason, when a large current passes through the sheet-like ferrite bead, the impedance value decreases, so if it is used at a position where a large current passes, the intended effect may not be achieved. In this case, it is necessary to consider selecting a component that can withstand the rated current sufficiently, or selecting a component with a higher impedance at an initial stage.

However, the magnetic saturation of the ferrite bead only occurs when a large current passes, and if the current drops, the performance is restored, so that there may be no problem in a circuit that only increases the current instantaneously. (Of course, there may be cases where the magnetic saturation of the moment occurs.) Because it is difficult to trace the impact in detail, I think it is best to use the actual machine evaluation to determine that the ferrite bead is not operating normally. It is subjected to a process of suspected magnetic saturation.

Ferrite magnetic beads cause overshoot and undershoot of signal waveforms

If ferrite beads are used in digital circuits, overshoot and undershoot in the waveform may occur. (figure 2)

Figure 2 Example of overshoot and undershoot caused by flaky ferrite beads

Since the inductance of the ferrite bead and the capacitance of other parts of the circuit (the input capacitance of the IC, etc.) cause resonance, this phenomenon easily occurs on the ferrite bead with a steep impedance curve. In order to improve this situation, a damping resistor can be placed in series with the ferrite bead. Since the resonance energy can be absorbed by the resistance by adding the damping resistor, the resonance becomes small, thereby reducing overshoot and undershoot. (image 3)

Figure 3 Example of improvement with damping resistor

However, since the use of a damper resistor causes this part of the voltage to drop, it is necessary to pay attention to whether the signal peak is low or not. The characteristic of the ferrite bead is that the steeper the impedance curve is, the smaller the internal consumption is, the more likely the resonance is, and the smoother the impedance curve is, the less likely the resonance is to occur. Therefore, do not use ferrite beads with too steep impedance curve. Prevent overshoot and undershoot.