Selection of Mutilayer Chip Bead

2025-02-16

Physical model of magnetic beads

Ferrite beads are a kind of anti-interference component that is currently developing rapidly. They are cheap, easy to use, and have a significant effect in filtering high-frequency noise. In the circuit, just pass the wire through it. When the current passes through the wire, the ferrite has almost no impedance to the low-frequency current, but it will have a greater attenuation effect on the higher-frequency current. The high-frequency current dissipates in the form of heat, and its equivalent circuit is an inductor and a resistor in series. The values of the two components are proportional to the length of the magnetic bead. There are many types of magnetic beads. Manufacturers should provide technical indicators, especially the curve of the relationship between the impedance and frequency of the magnetic beads. Some magnetic beads have multiple holes. Passing a wire through can increase the impedance of the component (the square of the number of times the magnetic bead is passed through), but the increased noise suppression ability at high frequencies may not be as much as expected, and it would be better to use more magnetic beads in series.

Ferrite is a magnetic material that will produce magnetic saturation and a sharp drop in magnetic permeability due to excessive current passing through it. High current filtering should use specially designed magnetic beads, and attention should also be paid to its heat dissipation measures. Ferrite beads can not only be used to filter high-frequency noise in power supply circuits (can be used for DC and AC output), but can also be widely used in other circuits, and their volume can be made very small. Especially in digital circuits, since pulse signals contain high-frequency high-order harmonics, which are also the main source of high-frequency radiation in circuits, the role of magnetic beads can be played in such occasions. Ferrite beads are also widely used in noise filtering of signal cables.

Magnetic beads are mainly used to suppress EMI differential mode noise. Their DC resistance (DCR) is very small and their high-frequency AC impedance is relatively large. The common 600Ω refers to the AC impedance value of the magnetic beads measured at a signal frequency of 100MHz. According to different application scenarios, magnetic beads can be divided into ordinary type, high current type and spike type.

1.Ordinary magnetic beads. Mainly used in scenarios with relatively small current (less than 600mA) and no special requirements. The DC resistance of ordinary magnetic beads generally does not exceed 1Ω, and the AC impedance range is generally between a few ohms and several thousand ohms. The main function is to suppress and absorb electromagnetic interference and radio frequency interference.

2.High current magnetic beads. Mainly used in high current scenarios, so the DC resistance needs to be strictly limited. The DC resistance generally does not exceed 0.1Ω, and the AC impedance is also smaller than that of ordinary magnetic beads.

3.Spike magnetic beads. Mainly used to attenuate signals in a specific frequency range to achieve frequency selection. Its AC impedance is very high in a certain frequency range and very low in other frequency ranges, showing a band-stop characteristic.

Main characteristic parameters of magnetic beads

1. DC resistance DCResistance (mohm): The resistance value of this magnetic bead when DC current passes through this magnetic bead.

2. Rated current RatedCurrent (mA): Indicates the maximum allowable current when the magnetic bead is working normally.

3. Impedance [Z] @ 100MHz (ohm): This refers to AC impedance.

4. Impedance-frequency characteristics: A curve that describes the change of impedance value with frequency.

5. Resistance-frequency characteristics: A curve that describes the change of resistance value with frequency

6. Inductive reactance-frequency characteristics: A curve that describes the change of inductive reactance with frequency.

Selection of ferrite beads

When selecting ferrite beads, two factors need to be considered: noise interference and current flow.

(1) In terms of noise interference, the frequency and intensity of the noise need to be considered. Different types of ferrite beads have different frequency impedance curves. When selecting, you should choose ferrite beads with higher impedance corresponding to the noise center frequency to better suppress noise. The greater the noise interference, the higher the impedance of the ferrite beads. However, high-impedance ferrite beads will also cause greater attenuation of useful signals, so the higher the impedance is, the better. The signal-to-noise ratio needs to be considered comprehensively. There is currently no clear calculation formula and selection standard. The selection needs to be based on actual results. Generally, ferrite beads with AC impedance between 120Ω and 600Ω are more commonly used. For example, for a 100MHz, 300mVpp noise, after passing through the ferrite beads, it is required to reach a level of 50mVpp. Assuming the load is 45Ω, then a ferrite bead with 225Ω@100MHz and DCR<1Ω should be selected. 225Ω is selected because (45Ω/50mV)*250mV=225Ω.

(2) The rated current should be considered for the current flow. The larger the DC resistance of the magnetic bead, the smaller the rated current. The selection should be based on the actual situation. For example, the output voltage of the system power supply is 3.3V, the rated current required for the power input of the load module is 300mA, and the input voltage required for the input power of the load module cannot be lower than 3.0V. The power input pin of the load module needs to select a magnetic bead with a DC resistance less than 1Ω, and then consider the derating design. Generally, a 0.5Ω magnetic bead is selected.

Therefore, when selecting magnetic beads, you can follow the following steps.

(1) First, you need to determine the frequency band of the noise to be filtered, and then select the appropriate AC impedance within this frequency band (the approximate impedance range can be obtained through simulation, and the simulation model can be consulted by the manufacturer).

(2) Determine the maximum current passing through the circuit. The current flowing through the circuit determines the rated current of the magnetic bead. After the rated current is determined, the DCR range of the magnetic bead can be calculated according to the voltage range required by the subsequent circuit.

(3) Select the package. The package can be selected according to the actual situation such as the layout and structure of the single board. However, it should be noted that the impedance of the magnetic bead after the voltage is applied is different from the impedance in the specification. To select the magnetic bead correctly, you must pay attention to the following points.

① What is the frequency range of the unwanted signal?

② What is the noise source?

③ How much noise attenuation is required?

④ What are the environmental conditions (temperature, DC voltage, structural strength)?

⑤ What are the circuit and load impedances?

Finally, you also need to pay attention to derating when selecting magnetic beads.

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