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In EMC testing, how effective are beads and inductors in solving EMC and EMI

Magnetic beads and inductors are solving the differences between EMI and EMC levels, and what are their characteristics. Will the effect of applying magnetic beads be better? In principle, magnetic beads can be equivalent to an inductor, so in principle, EMI and EMC circuits of magnetic beads are equivalent to restraining inductance, mainly restraining high-frequency transmission electromagnetic interference.

Magnetic beads can be equivalent to inductance, but this equivalent inductance is different from inductance coil. The biggest difference between magnetic bead and inductance coil is that the inductance coil has distributed capacitance. Therefore, the inductance coil is equivalent to the parallel connection of inductance and capacitance. As shown in Figure 1. In Figure 1, the equivalent inductance of LX (ideal inductance), the equivalent resistance of RX coil, and the distributed capacitance of CX inductance.


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In theory, to suppress the transmission electromagnetic interference, it is required that the larger the inductance of the suppression inductor is, the better. However, for the inductive coil, the more the inductance is, the larger the distributed capacitance of the inductive coil is, and the effect of the two may resist each other.


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Figure 2 shows the relationship between impedance and frequency of common inductance coil. It can be seen from the figure that the impedance of inductance coil increases with the increase of frequency, but when its impedance increases to the maximum value, due to the effect of parallel capacitance, the impedance decreases rapidly with the increase of frequency. When the impedance increases to the highest value, the distributed capacitance of the inductance coil is equivalent to the inductance, causing parallel resonance. In the figure, L1>L2>L3. It can be seen that the more inductance of the inductive coil, the lower the resonant frequency. As can be seen from Figure 2, if the frequency is 1, MHz to suppress electromagnetic interference, L1 is better than L3. Since the inductance of L3 is more than 10 times smaller than L1, the cost of L3 is also much lower than L1.

If we want to further improve the suppression frequency, then the inductance coil we finally choose must be its minimum specified value, only 1 turn or less. The magnetic bead, namely the through-core inductor, is an inductive coil with less than one turn. However, the distributed capacitance of the through-core inductor is several times to dozens of times smaller than that of the single-coil inductor, so the output power of the through-core inductor is higher than that of the single-coil inductor.

The inductance of the through-the-core inductor is generally small, ranging from a few to dozens of micro-heters. The size of the inductance is related to the size and length of the wire in the through-core inductor and the cross section of the magnetic bead, but the relative permeability of the magnetic bead is the most relevant to the inductance of the magnetic bead. Figure 3. Figure 4 is the schematic diagram of the conductor and the through-core inductor. When calculating the through-core inductance, first calculate the inductance of the circular cross-section straight wire, and then multiply the relative permeability of the magnetic beads to calculate the inductance of the through-core inductance.


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In addition, when the output power of the through-core inductor is very high, there will be vortex in the magnetic bead, which is equivalent to reducing the permeability of the through-core inductor. At this point, we usually use a reasonable permeability. The reasonable permeability is the relative permeability of magnetic beads at a certain output power. However, because the working frequency of magnetic beads is only a category, the average permeability is often used in practical applications.

At low frequencies, the relative permeability of magnetic beads is very high (more than 100), but at high frequencies, the reasonable permeability is only a fraction or even a fraction of the relative permeability. Therefore, the magnetic beads also have cut-off frequency problems. The frequency is to reduce the effective permeability of the magnetic beads to the operating frequency fc when it is close to 1. At this time, the magnetic beads have lost the effect of inductance. Generally, the cut-off frequency fc of magnetic beads is between 30 and 300 MHz, and the cut-off frequency is related to the magnetic bead material. Generally, the higher the permeability of magnetic core material is, the higher the cut-off frequency fc is. On the contrary, the lower the eddy current loss of low-frequency magnetic core material is. When designing the circuit, the user can specify the magnetic core material supplier to conduct data test on the output power and reasonable permeability of the magnetic core, or the trend chart of the through-core inductance under each output power. Figure 5 shows the trend of the times of the through-core inductor.


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Another use of magnetic beads is to do electromagnetic shielding. Its electromagnetic shielding effect is better than that of shielded wire, which is not noticed by ordinary people. Its application method is to let a pair of wires pass through the middle of the magnetic bead. When the current flows through the two wires, most of the magnetic field is concentrated in the magnetic bead, and the magnetic field will not radiate; Because the magnetic field produces vortices in the magnetic beads, which cause the cable direction to be opposite to the cable direction on the conductor surface and resist each other, so the magnetic beads also have a shielding effect on the electric field, that is, the magnetic beads have a strong shielding effect on the magnetic 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 shield wire. Using magnetic beads as electromagnetic shielding is also equivalent to the common-mode suppression inductance in the connection line for double conductors, and has a strong suppression effect on the common-mode interference signal.

It can be seen that in the electromagnetic compatibility test, the inductance coil is mainly used for EMI suppression of low-frequency electromagnetic interference, and the magnetic bead is mainly used for EMI suppression of high-frequency interference signals. Therefore, the electromagnetic interference of a wide band is suppressed. EMI suppression must be conducted by simultaneously selecting a variety of inductors with different properties. In addition, EMI suppression should also be carried out for common mode transmission electromagnetic interference, and attention should be paid to restraining the connection between inductance and Y capacitance. The Y capacitance and suppression inductance should be as close as possible to the input end of the power supply, that is, the position of the power plug. The high-frequency inductance should be as close as possible to the Y capacitance. The Y capacitance should be as close as possible to the ground wire (the ground wire of the three-core power line). EMI suppression is effective.


keywords: EMC testEMCEMI
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