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Get to know the magnetic beads quickly
2020-02-20
 
Ferrite beads are mainly used for EMI differential mode noise suppression. Ferrite beads have almost no impedance in the low frequency band, and only show high impedance with very wide bandwidth at high frequencies. Therefore, magnetic beads are usually selected when suppressing high frequency interference. Generally speaking, 600R refers to the impedance value at the 100MHZ test frequency.

In addition to paying attention to the three parameters of 100M impedance, DC impedance and rated current, you should also pay attention to the type of use of the magnetic beads. For example: high-frequency high-speed magnetic beads, power magnetic beads (high current), ordinary signal magnetic beads.

Ferrite beads are widely used in printed circuit board power lines and signal data lines. If ferrite beads are added to the entrance of the power cord of the printed board, high-frequency interference can be filtered.

Ferrite beads are used to suppress high-frequency interference and spike interference on signal lines and power lines. It also has the ability to absorb electrostatic discharge pulse interference.

In the signal interface, the magnetic beads mainly suppress the interference and external interference brought by the port, such as 232 interface, VGA interface, LCD interface, etc. Ferrite beads are also used to filter high-frequency noise in digital power circuits, such as crystal power , PLL power supply, etc. In digital circuits, because pulsed digital signals contain high-frequency harmonics, which are also the main source of high-frequency radiation in the circuit, they can be used to play the role of magnetic beads in such occasions, such as clock signal lines , Data address bus and so on.

There are two aspects to consider when choosing a magnetic bead: one is the situation of noise interference in the circuit, and the other is the amount of current that needs to be passed.

To roughly understand the frequency and intensity of noise, the frequency impedance curve of different magnetic beads is different, and the one with a higher impedance at the center frequency of the noise should be selected. The larger the noise interference, the higher the impedance should be selected, but the higher the impedance, the better, because the higher the impedance, the higher the DCR, and the greater the attenuation of the useful signal. However, there are generally no clear calculation and selection criteria. It mainly depends on the actual use effect. It is very common between 120R-600R.

Then it depends on the magnitude of the passing current. If it is used in the power line part, the model with a larger rated current should be selected. If it is used in the signal line part, the rated current is generally not high. In addition, the magnetic beads generally have a larger impedance and a smaller rated current. The selection of magnetic beads should be carried out according to the actual situation. For example, for 3.3V and 300mA power supply, if 3.3V cannot be lower than 3.0V, then the DC resistance of the magnetic beads DCR should be less than 1R. In this case, 0.5R is generally selected to prevent parameter drift. For the noise suppression ability, if it is required that for 100MHz and 300mVpp noise, the level of 50mVpp is reached after passing the magnetic beads, assuming a load of 45 ohms, then 225R @ 100MHz, DCR <1R magnetic beads, How did you figure it out?

250mV / (50mV / 45ohm) = 225ohm

First you need to know the frequency band of the noise you want to filter, then choose a suitable impedance in this frequency band (the actual size can be obtained through simulation, and the simulation model can be obtained from the manufacturer). The second step is to determine the circuit. The maximum current passed, and the current flowing through the circuit is determined, which also means that you have to choose a magnetic bead with a rated current. The next step is to determine the DCR (DC resistance) of the magnetic bead, which can be calculated according to the range of the voltage supply of the circuit in the next stage The range of DCR of the magnetic beads allowed. The package can be selected according to the situation. However, it should be noted that the impedance of the magnetic beads is a little different from the specifications after you apply the voltage.

To correctly select the magnetic beads, you must pay attention to the following points:

* What is the frequency range of unwanted signals (noise interference);
* Who is the source of the noise?
* How much noise attenuation is required;
* What are the environmental conditions (temperature, DC voltage, structural strength);
* What is the circuit and load impedance;
* Is there room to place magnetic beads on the PCB?
* The first three can be judged by observing the impedance frequency curve provided by the manufacturer.
* Notes on the use of magnetic beads:
* It is recommended to derate the rated current by 80%.
* Do not exceed the maximum working temperature.
* Beads attenuate noise while destroying the effective signal waveform. Therefore, choose ordinary or high Q-type beads according to the specific suppression frequency to avoid excessive attenuation of normal signals.
* The insertion loss of the magnetic beads is related to the line impedance. The higher the impedance, the smaller the insertion loss.
* When the step space is small and there are many magnetic beads, a magnetic bead row can be used.
 
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