How to choose SMD magnetic beads correctly
The raw material ferrite of the chip magnetic bead is an iron-magnesium alloy or an iron-nickel alloy. This material has a high magnetic permeability, which can be the smallest capacitance between the coil windings of the inductance in the case of high frequency and high resistance. Ferrite materials are typically used at high frequencies because at low frequencies they are predominantly inductive, resulting in very little loss on the wire. At high frequencies, they are predominantly reactive and vary with frequency. In practical applications, ferrite materials are used as high-frequency attenuators for radio frequency circuits.
Chip magnetic beads have high resistivity and permeability, and its equivalent circuit is a DCR resistor in series with an inductor and a capacitor and a resistor in parallel. DCR is a constant value, but the last three components are functions of frequency, that is to say, their inductive reactance, capacitive reactance and impedance will change with frequency. Of course, their resistance, inductance and capacitance are very little. Magnetic beads have better high-frequency filtering characteristics than ordinary inductors, and show resistance at high frequencies, so they can maintain a high impedance in a relatively wide frequency range, thereby improving the effect of FM filtering.
The following points should be paid attention to when placing magnetic beads:
1. What is the frequency range of the unwanted signal;
2. Who is the noise source?
3. How much noise attenuation is required;
4. What are the environmental conditions (temperature, DC voltage, structural strength);
5. What is the circuit and load impedance;
6. Whether there is space to place magnetic beads on the pcb board;
The specifications provided by the chip bead manufacturers generally reflect the characteristic curve of frequency and impedance. Generally, 100MHz is used as the standard. For example, when the frequency of 100MHz is used, the impedance of the bead is equivalent to 1000 ohms. For the frequency band we want to filter, it is necessary to select the larger the impedance of the magnetic beads, the better, usually the impedance above 600 ohms is selected. In addition, when choosing SMD magnetic beads, it is necessary to pay attention to the flux of the magnetic beads. Generally, it needs to be derated by 80%. When using it in the power supply circuit, the influence of DC impedance on the voltage drop should be considered.