How to solve EMI and EMC with magnetic beads and inductors

    Magnetic beads are specially used to suppress high-frequency noise and spike interference on signal lines and power lines, and also have the ability to absorb electrostatic pulses. Magnetic beads are used to absorb ultra-high frequency signals. For example, some RF circuits, PLLs, oscillation circuits, including ultra-high frequency memory circuits (ddrsdram, Rambus, etc.) need to add magnetic beads to the power input part, while inductance is an energy storage element used in LC oscillation circuits, medium and low frequency filter circuits, etc. its application frequency range rarely exceeds 50MHz Magnetic beads have high resistivity and permeability, which is equivalent to the series connection of resistance and inductance, but the resistance and inductance change with frequency.

    The function of the magnetic beads is mainly to eliminate the RF noise existing in the transmission line structure (circuit). The RF energy is the AC sine wave component superimposed on the DC transmission level. The DC component is the required useful signal, while the RF energy is the useless electromagnetic interference transmitted and radiated along the line (EMI). To eliminate these unnecessary signal energies, chip magnetic beads are used to act as high-frequency resistors (attenuators), which allow DC signals to pass through and filter out AC signals. Generally, the high-frequency signal is above 30MHz, however, the low-frequency signal is also affected by the chip magnetic beads. Magnetic beads have high resistivity and permeability, which is equivalent to the series connection of resistance and inductance, but the resistance value and inductance value change with frequency. It has better high-frequency filtering characteristics than ordinary inductors, showing resistance at high frequencies, so it can maintain a high impedance in a relatively wide frequency range, thus improving the FM filtering effect.

    The magnetic bead can be equivalent to an inductance, but this equivalent inductance is different from the inductance coil. The biggest difference between the magnetic bead and the inductance coil is that the inductance coil has a distributed capacitance. Therefore, the inductance coil is equivalent to an inductance in parallel with a distributed capacitance. As shown in Fig. 1. In Fig. 1, LX is the equivalent inductance (ideal inductance) of the inductive coil, Rx is the equivalent resistance of the coil, and CX is the distributed capacitance of the inductance.

    Inductors (inductors) and transformers are electromagnetic induction components made by winding insulated wires (such as enameled wire, gauze wire, etc.), and are also one of the commonly used components in electronic circuits. Related products such as common mode filters. When there is current passing through the coil, a magnetic field will be generated around the coil. When the current in the coil changes, the magnetic field around it also changes correspondingly. This changed magnetic field can make the coil generate induced electromotive force (electromotive force is used to represent the terminal voltage of the ideal power source of the active element), which is self inductance. When two inductors are close to each other, the magnetic field change of one inductor will affect the other inductor, which is mutual inductance. The size of mutual inductance depends on the degree of coupling between the self inductance of the inductance coil and the two inductance coils. The components made by using this principle are called mutual inductors.

    Theoretically, to suppress the conducted interference signal, it is required that the larger the inductance of the suppressed inductance, the better. But for the inductance coil, the larger the inductance, the larger the distributed capacitance of the inductance coil, and the two effects will cancel each other.

    Fig. 2 is a graph showing the relationship between the impedance and frequency of an ordinary inductance coil. It can be seen from the graph that the impedance of an inductance coil initially increases with the increase of frequency, but when its impedance increases to the maximum value, the impedance decreases rapidly with the increase of frequency, which is due to the effect of parallel distributed capacitance. When the impedance increases to the maximum value, it is the place where the distributed capacitance of the inductance coil and the equivalent inductance produce parallel resonance. In the figure, L1, L2 and L3 show that the larger the inductance of the inductance coil, the lower the resonance frequency. It can be seen from Fig. 2 that if the interference signal with the frequency of 1MHz is to be suppressed, L3 is better than L1, because the inductance of L3 is more than ten times smaller than L1, so the cost of L3 is much lower than L1.

    If we want to further improve the suppression frequency, then the inductance coil we finally choose will have to be its minimum limit value, which is only 1 turn or less. The magnetic bead, that is, the through core inductance, is an inductance coil with less than one turn. However, the distributed capacitance of the through core inductance is several times to several tens of times smaller than that of the single coil inductance coil. Therefore, the operating frequency of the through core inductance is higher than that of the single coil inductance coil.

    The inductance of the through core inductor is generally small, ranging from a few microhenries to several tens of microhenries. The inductance is related to the size and length of the wire in the through core inductor and the cross-sectional area of the magnetic beads. However, the relative magnetic permeability of the magnetic beads is the most closely related to the inductance of the magnetic beads

    Fig. 3 and Fig. 4 are schematic diagrams of wire and through core inductance respectively. When calculating through core inductance, first calculate the inductance of a straight wire with circular section, and then multiply the calculation result by the relative permeability of magnetic beads to find the inductance of through core inductance.

    In addition, when the operating frequency of the through core inductor is very high, eddy currents will also be generated in the magnetic beads, which is equivalent to the decrease of the permeability of the through core inductor. At this time, we generally use the effective permeability. Effective permeability is the relative permeability of magnetic beads at a certain operating frequency. However, because the operating frequency of the magnetic beads is only a range, the average permeability is often used in practical applications.

    At low frequencies, the relative magnetic permeability of magnetic beads is generally large (greater than 100), but at high frequencies, its effective magnetic permeability is only a fraction of the relative magnetic permeability, or even a few tens of times. Therefore, the magnetic beads also have the problem of cutoff frequency, which is the working frequency fc when the effective permeability of the magnetic beads drops to close to 1. At this time, the magnetic beads have lost the role of an inductor. Generally, the cutoff frequency fc of the magnetic beads is between 30 and 300MHz, and the cutoff frequency is related to the material of the magnetic beads. Generally, the higher the permeability of the magnetic core material, the lower the cutoff frequency fc, because the eddy current loss of the low-frequency magnetic core material is relatively large. When designing the circuit, the user can ask the supplier of the magnetic core material to provide the test data of the working frequency and effective permeability of the magnetic core, or the curve diagram of the through core inductance under different working frequencies. Fig. 5 is a frequency graph of the through core inductance.

    Another use of magnetic beads is to do electromagnetic shielding. Its electromagnetic shielding effect is better than that of shielding wires, which is not noticed by ordinary people. The use method is to let a pair of wires pass through the middle of the magnetic beads, so when a current flows through the two wires, most of the magnetic field generated will be concentrated in the magnetic beads, and the magnetic field will not be radiated outward; Because the magnetic field will generate eddy currents in the magnetic beads, the direction of the power lines generated by the eddy currents is opposite to the direction of the power lines on the surface of the conductor, which can cancel each other. Therefore, the magnetic beads also have a shielding effect on the electric field, that is, the magnetic beads have a strong shielding effect on the electromagnetic field in the conductor.

    The advantage of using magnetic beads for electromagnetic shielding is that the magnetic beads do not need to be grounded, which can avoid the trouble of grounding the shielding wire. Using magnetic beads as electromagnetic shielding is also equivalent to connecting a common mode suppression inductance in the line for double wires, which has a strong suppression effect on common mode interference signals.

    It can be seen that the inductance coil is mainly used for EMI suppression of low-frequency interference signals, while the magnetic beads are mainly used for EMI suppression of high-frequency interference signals. Therefore, EMI suppression of an interference signal with a wide frequency band must be effective only when multiple inductors with different properties are used at the same time. In addition, for EMI suppression of common mode conducted interference signals, attention should be paid to the connection position between the suppression inductance and the Y capacitance. The Y capacitance and suppression inductance should be as close to the input end of the power supply as possible, that is, the position of the power socket, and the high-frequency inductance should be as close to the Y capacitance as possible, and the Y capacitance should be as close to the ground wire connected to the ground (the ground wire of the three-core power line), which is effective for EMI suppression.

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