The reason for using chip beads and chip inductors: whether to use chip beads or chip inductors mainly depends on the application. SMD inductors are required in the resonant circuit, and SMD beads are used when unnecessary EMI noise needs to be eliminated.
1. The unit of the magnetic bead is ohm, not Hunter. This point must be paid special attention to. Because the unit of the magnetic bead is nominally based on the impedance it generates at a certain frequency, the unit of impedance is also ohms. The DATASHEET of the magnetic bead generally provides a characteristic curve of frequency and impedance, which is generally based on 100MHz, such as 1000R 100MHz, which means that the impedance of the magnetic bead is equivalent to 600 ohms at a frequency of 100MHz.
2. Ordinary filters are composed of lossless reactive components, and their role in the line is to reflect the stopband frequency back to the signal source, so this type of filter is also called a reflection filter. When the reflection filter does not match the impedance of the signal source, a part of the energy will be reflected back to the signal source, causing the interference level to increase. In order to solve this problem, a ferrite magnetic ring or magnetic bead sleeve can be used on the inlet of the filter to use the eddy current loss of the ring or magnetic bead on the high-frequency signal to convert the high-frequency component into heat loss. Therefore, the magnetic ring and the magnetic beads actually absorb high-frequency components, so they are sometimes called absorption filters.
The function of the magnetic beads is mainly to eliminate the RF noise present in the transmission line structure (circuit). RF energy is an AC sine wave component superimposed on the DC transmission level. The DC component is the required useful signal, while the radio frequency RF energy is useless. The electromagnetic interference is transmitted and radiated (EMI) along the line. To eliminate these unwanted signal energy, chip beads are used to play the role of high-frequency resistance (attenuator). This device allows DC signals to pass, while filtering out AC signals. Usually the high frequency signal is above 30MHz, however, the low frequency signal will also be affected by the chip beads.
The chip magnetic beads are composed of soft ferrite materials, forming a monolithic structure with high volume resistivity. Eddy current loss is inversely proportional to the resistivity of ferrite materials. Eddy current loss is proportional to the square of the signal frequency. The advantages of using chip beads: miniaturization and light weight have high impedance in the frequency range of radio frequency noise, eliminating electromagnetic interference in the transmission line. Closed magnetic circuit structure can better eliminate signal cross-winding. Excellent magnetic shielding structure. Reduce the DC resistance to avoid excessive attenuation of useful signals. Significant high-frequency characteristics and impedance characteristics (better elimination of RF energy). Eliminate parasitic oscillations in high-frequency amplifier circuits. Effectively work in the frequency range of several MHz to several hundred MHz. Some suggestions for choosing the core of magnetic beads correctly:
1. What is the frequency range of the unwanted signal and who is the noise source;
2. Is there space to place magnetic beads on the PCB board;
3. How much noise attenuation is needed;
4. What are the environmental conditions (temperature, DC voltage, structural strength);
5. What is the circuit and load impedance?