Noise Countermeasure Basics (IV): Noise Filter Ferrite Beads

This time we will introduce representative noise suppression components. The first is chip ferrite beads, which are ferrite bead inductors processed into an SMD (surface mount device) shape.

Figure 1 shows an example of the appearance of a lead-type ferrite bead inductor. It has a simple structure with a wire running through the ferrite. Although it is not wound like a normal coil, when current passes through the wire, magnetic induction is generated in the magnetic core, so the ferrite bead acts as an inductor. In addition, since ferrite material has a large loss at high frequencies, the current at high frequencies can be filtered in the ferrite, which can effectively absorb noise.

Chip ferrite beads are ferrite core inductors made into chip-type products. Figure 2 shows its most representative structure. The three-dimensional coil structure is achieved by forming a conductor coil between ferrite layers and calcining them through integrated processing.

In addition to the sheet shape, the internal structure of the lead-type bead inductor is processed into a coil structure to obtain a higher impedance value than the lead-type bead inductor with only one wire passing through it. (There are actually sheet-type ferrite beads with this structure that only has a wire passing through it). This structure is basically the same as the sheet-type multilayer inductor, but the difference from the inductor is that the ferrite material is more suitable for noise countermeasures. Figure 3 is an example of the impedance frequency characteristics of the sheet-type ferrite bead. As can be seen from the figure, its impedance also increases as the frequency increases, which is basically the same as the inductor. It can be seen that it can act as a low-frequency filter when connected in series in the circuit. In the impedance value (Z) of ordinary inductors, the reactance component (X) is mainly, while the sheet-type bead uses ferrite materials with large losses in the high-frequency range, so the resistance component (R) is mainly in the high-frequency band. Since the reactance component does not lose and the resistance component does lose, the sheet-type bead has better performance in absorbing noise energy than ordinary inductors, and the noise suppression effect is also better.

＜Different impedance curves can be selected according to the application＞

Generally speaking, the specifications of chip ferrite beads are determined by the impedance value at 100MHz. However, there are many different products with the same impedance value because their curve shapes are different. Figure 4 shows one example. Both BLM18AG601SN1 and BLM18BD601SN1 are chip ferrite beads with an impedance value of 600Ω at 100MHz, but from their impedance waveforms, it can be seen that compared with the relatively stable rising curve of BLM18AG601SN1, the curve of BLM18BD601SN1 shows a rapid rising trend.

Since the impedance of the chip ferrite beads with a relatively stable rising curve begins to increase in the low-frequency range, it can suppress noise in a wide range from low frequency to high frequency, but when the signal frequency is high, the signal frequency may also be attenuated. In contrast, the impedance of the beads with a rapidly rising curve only increases in the high-frequency range, so it can achieve noise suppression in the high-frequency area without affecting the signal. It can be seen that when selecting chip ferrite beads, the frequency of the signal and the noise frequency to be suppressed should be considered.

＜Improvement of high-frequency impedance through internal structure improvement＞

From the impedance frequency characteristics of the chip ferrite beads introduced in Figure 3, it can be seen that the impedance value begins to decrease at the range of 400-500MHz. This is due to the influence of the structure of the chip ferrite beads. Generally speaking, the impedance of the inductor will gradually increase as the frequency rises. However, as shown in Figure 5, there are relatively close parts near the beginning (entrance) and the end (exit) of the winding inside the ordinary chip ferrite bead. Since electrostatic coupling occurs in this part (a state of micro-capacitance exists), when high-frequency current passes through this part, it will not be easily affected by the inductor impedance. Since the higher the frequency, the easier it is for the electrostatic coupling part to pass, the lower the impedance displayed at higher frequencies.

To solve this problem, it is necessary to improve the structure that brings the winding start and end closer together. Figure 6 shows an example of how the internal structure of a sheet ferrite bead has been adjusted to improve high-frequency characteristics. Normally, the axis of the conductor coil of a sheet ferrite bead is vertical (i.e., longitudinally wound), but the axis of the conductor coil of a bead with improved high-frequency characteristics is horizontal. By making this change, the distance between the coil winding start and the end is increased, and the starting frequency at which the impedance begins to decrease is greatly increased.

In addition to the above, chip ferrite beads are also available in a variety of specifications, such as large current type and miniaturized type, from which users can select the product that best suits their application.