How to Solve EMI and EMC with Beads and Inductors

- Nov 20, 2017 -

Inductance coil is mainly used for EMI suppression of low-frequency interference signals, and beads mainly for high-frequency interference signal EMI suppression, therefore, for a wide band interference signal EMI suppression, you must also use a number of different nature Inductor will be effective.


Magnetic beads and inductors in solving EMI and EMC What is the difference between the role of each have their own characteristics, the effect is not to use beads will be better?

Beads are specifically designed to suppress high frequency noise and spikes on signal lines, power lines, and the ability to absorb electrostatic impulses. Beads are used to absorb UHF signals. Some RF circuits, PLLs, oscillating circuits, DDR memory circuits (DDRSDRAM, RAMBUS, etc.) need to add beads to the power input. The inductor is a kind of energy storage Components, used in the LC oscillator circuit, the low-frequency filter circuit, the application of the frequency range of rarely over 50MHZ. Beads have high resistivity and permeability, equivalent to the resistance and inductance in series, but the resistance and inductance Values vary with frequency.

The function of the magnetic beads is mainly to eliminate the RF noise present in the transmission line structure. The RF energy is the AC sine wave component superimposed on the DC transmission level. The DC component is the desired signal, while the RF RF energy is useless. The electromagnetic interference is transmitted and radiated (EMI) along the line. To eliminate these unwanted signal energies, use chip beads to act as a high frequency resistor (attenuator) that allows the DC signal to pass and filter out the AC signal. Usually high-frequency signal is above 30MHz, however, the low-frequency signal is also affected by the chip beads. Magnetic beads have a high resistivity and permeability, which is equivalent to the resistance and inductance in series, but the resistance and inductance values vary with frequency. He has better high-frequency filtering characteristics than ordinary inductors, showing resistivity at high frequency, so it can maintain a high impedance over a wide frequency range, thus improving the FM filtering effect.

Beads can be equivalent to an inductor, but the equivalent inductance and inductance coil is different, the biggest difference between the magnetic beads and the inductor is that the inductance coil has a distributed capacitance. Therefore, the inductance coil is equivalent to an inductance in parallel with a distributed capacitance.

Inductors (inductors) and transformers are made of insulated wires (eg enamelled wire, yarn covered wire, etc.) around the electromagnetic induction component, is also commonly used in electronic circuits, one of the components, related products such as common mode filters . When a current is passed through the coil, a magnetic field is generated around the coil. When the coil current changes, the magnetic field around it also produces a corresponding change, the magnetic field changes can make the coil itself induced electromotive force (electromotive force used to represent the ideal power supply active element voltage), which is self-inductance. When two inductive coils are close to each other, the change of the magnetic field of one inductive coil will affect the other inductive coil. This effect is mutual inductance. The size of the mutual inductance depends on the degree of coupling of the inductance of the inductance coil and the two inductance coils. Components made using this principle are called mutual inductor.

In theory, it is necessary to suppress the conducted interference signal to suppress the inductance of the inductor as large as possible. However, for the inductor, the larger the inductance is, the larger the distributed capacitance of the inductor is and the more the two cancel each other out.

The impedance of the inductor increases initially with increasing frequency, but as its impedance increases to its maximum, the impedance decreases rapidly with increasing frequency, due to the parallel distributed capacitance. Where the impedance increases to a maximum, it is where the distributed capacitance of the inductor coil resonates in parallel with the equivalent inductance. If we still want to further increase the inhibitory frequency, then our last choice of the inductor coil is its minimum limit, only 1 lap or less than 1 lap. Beads, that is, wearable inductors, is a number of turns less than 1 coil of the inductor. However, the wear-through inductance than the single-coil inductance coil distribution of small times several times to several times, therefore, through-core inductance than the single coil inductance coil operating frequency higher.

Inductance through the heart inductance is generally relatively small, about a few micro-Heng to tens of micro-Heng between the amount of inductance and the size of the core-wire inductance and the length of the wire, as well as cross-sectional area of magnetic beads are related, but Magnetic beads with the largest relationship between the amount of magnetic beads have to count the relative permeability.


In addition, when the core inductor operating frequency is high, there will be eddy current in the bead body, which is equivalent to the permeability of the core inductance to be reduced, this time, we generally use the effective permeability. The effective permeability is the relative permeability of the bead below a certain operating frequency. However, due to the working frequency of the beads are only a range, so in practical applications, the average permeability.

At low frequencies, magnetic beads generally have large relative permeability (greater than 100), but their effective permeability at high frequencies is only a fraction or even a fraction of the relative permeability. Therefore, the magnetic beads also have the problem of cut-off frequency, the so-called cut-off frequency, is the effective magnetic permeability of the beads down to close to 1 when the operating frequency fc, when the magnetic beads have lost an inductive effect. The general cutoff frequency of magnetic beads fc are between 30 ~ 300MHz, the cut-off frequency of the material with the beads, the higher the magnetic permeability of the core material, the lower the cut-off frequency fc but because low-frequency core material vortex Loss is relatively large. Users in the design of the circuit may require the core material provider to provide core operating frequency and effective permeability of the test data, or through the inductor at different operating frequencies below the curve.

Another use of magnetic beads is used to make electromagnetic shielding, shielding effect of its electromagnetic shielding effect is better than shielded wire, which is generally noticed. Its use is to allow a pair of wires through the middle of the bead, then when current flows through the two wires, the magnetic field generated will be mostly concentrated in the bead body, the magnetic field will not radiate outward; the magnetic field in the magnetic The eddy current is generated in the bead body. The direction of the eddy current power line is opposite to the direction of the power line on the conductor surface, which can cancel each other. Therefore, the bead also shields the electric field, that is, the bead has a strong shielding effect on the electromagnetic field in the conductor .

The advantage of using magnetic beads for electromagnetic shielding is that beads are not grounded and can eliminate the need for grounding of shielded wires. Magnetic beads as electromagnetic shielding, for two-wire, it is equivalent to a line in the common-mode inductance, the common-mode interference signal has a strong inhibitory effect.

Thus, the inductor is mainly used for EMI suppression of low-frequency interference signals, and the beads are mainly for EMI suppression of high-frequency interference signals. Therefore, it is necessary to adopt more EMI suppression for a wideband interference signal. A different type of inductance will be effective. In addition, the common mode conducted interference signal EMI suppression, but also pay attention to suppress the inductor and Y capacitor connection location. Y capacitor and suppression inductance as close as possible to the input of the power supply, that is, the position of the power outlet, and the high-frequency inductance should be as close as possible to the Y capacitor, and the Y capacitor should be as close as possible to the ground connected to the earth This is only valid for EMI suppression.

Article from NdFeB Industry Network

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