How to use magnetic beads and inductors to solve EMI and EMC

What is the difference between magnetic beads and inductors in solving EMI and EMC? What are their characteristics? Will the effect of using magnetic beads be better?

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 static pulses. Magnetic beads are used to absorb ultra-high frequency signals. Some RF circuits, PLL, oscillation circuits, and ultra-high frequency memory circuits (DDRSDRAM, RAMBUS, etc.) all require magnetic beads to be added to the power input part. Inductors are energy storage components used in LC oscillation circuits, medium and low frequency filter circuits, etc., and their application frequency range rarely exceeds 50MHZ. Magnetic beads have high resistivity and permeability, which is equivalent to resistors and inductors in series, but the resistance and inductance values ​​change with frequency.

The function of magnetic beads is mainly to eliminate RF noise existing in the transmission line structure (circuit). RF energy is an AC sine wave component superimposed on the DC transmission level. The DC component is a useful signal, while the RF energy is useless electromagnetic interference transmitted and radiated along the line (EMI). To eliminate these unwanted signal energies, chip magnetic beads are used to play the role of high-frequency resistors (attenuators). The device allows DC signals to pass through and filters out AC signals. Usually the high-frequency signal is above 30MHz, however, low-frequency signals will also be affected by chip magnetic beads. Magnetic beads have high resistivity and permeability. They are equivalent to resistors and inductors in series, but the resistance and inductance values ​​vary with frequency. They have better high-frequency filtering characteristics than ordinary inductors. They are resistive at high frequencies, so they can maintain high impedance in a fairly wide frequency range, thereby improving the FM filtering effect.

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

Inductors (inductor coils) and transformers are electromagnetic induction elements wound with insulated wires (such as enameled wires, yarn-covered wires, etc.), and are also one of the commonly used components in electronic circuits. Related products include common-mode filters, etc. When current passes through the coil, a magnetic field is generated around the coil. When the current in the coil changes, the magnetic field around it also changes accordingly. This changing magnetic field can cause the coil itself to generate an induced electromotive force (electromotive force is used to represent the terminal voltage of the ideal power supply of the active component), which is self-inductance. When two inductor coils are close to each other, the change in the magnetic field of one inductor coil will affect the other inductor coil, and this influence is mutual inductance. The size of the mutual inductance depends on the self-inductance of the inductor coil and the degree of coupling between the two inductor coils. The component made using this principle is called a mutual inductor.

Theoretically, to suppress conducted interference signals, the inductance of the suppression inductor should be as large as possible. However, for an inductor coil, the larger the inductance, the larger the distributed capacitance of the inductor coil, and the effects of the two will cancel each other out.

Figure 2 is a graph showing the relationship between the impedance and frequency of an ordinary inductor. It can be seen from the figure that the impedance of the inductor initially increases with the increase in frequency, but when its impedance increases to the maximum value, the impedance decreases rapidly with the increase in frequency. This is because of the effect of the parallel distributed capacitance. When the impedance increases to the maximum value, the distributed capacitance of the inductor and the equivalent inductance produce parallel resonance. In the figure, L1 > L2 > L3, from which it can be seen that the greater the inductance of the inductor, the lower its resonant frequency. As can be seen from Figure 2, if you want to suppress an interference signal with a frequency of 1MHz, it is better to choose L3 than L1, because the inductance of L3 is more than ten times smaller than that of L1, so the cost of L3 is also much lower than that of L1.

If we want to further increase the suppression frequency, then the inductor we finally choose will have to be its minimum limit value, which is only 1 turn or less. Magnetic beads, also known as through-hole inductors, are inductors with less than 1 turn. However, the distributed capacitance of through-hole inductors is several to dozens of times smaller than that of single-turn inductors. Therefore, through-hole inductors have a higher operating frequency than single-turn inductors.

The inductance of a through-hole inductor is generally small, ranging from a few microhenries to tens of microhenries. The inductance is related to the size and length of the wire in the through-hole inductor, as well as the cross-sectional area of ​​the magnetic bead. However, the relative magnetic permeability of the magnetic bead is the most important factor affecting the inductance of the magnetic bead.

Figure 3 and Figure 4 are schematic diagrams of guide wire and through-hole inductance respectively. When calculating through-hole inductance, we must first calculate the inductance of a straight conductor with a circular cross-section, and then multiply the result by the relative magnetic permeability of the magnetic bead to find the inductance of the through-hole inductance.

In addition, when the operating frequency of the through-hole inductor is very high, eddy currents will be generated in the magnetic bead, which is equivalent to reducing the magnetic permeability of the through-hole inductor. At this time, we generally use the effective magnetic permeability. The effective magnetic permeability is the relative magnetic permeability of the magnetic bead under a certain operating frequency. However, since the operating frequency of the magnetic bead is only a range, the average magnetic permeability is often used in practical applications.

At low frequencies, the relative magnetic permeability of general magnetic beads is very 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 percent. Therefore, magnetic beads also have the problem of cutoff frequency. The so-called cutoff frequency is the operating frequency fc when the effective magnetic permeability of the magnetic beads drops to close to 1. At this time, the magnetic beads have lost the function of an inductor. The cutoff frequency fc of general magnetic beads is between 30 and 300MHz. The high or low cutoff frequency is related to the material of the magnetic beads. Generally, the higher the magnetic 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 circuits, users can ask the supplier of magnetic core materials to provide test data on the operating frequency and effective magnetic permeability of the magnetic core, or a curve chart of the through-core inductor at different operating frequencies. Figure 5 is a frequency curve chart of the through-core inductor.

Another use of magnetic beads is for electromagnetic shielding. Its electromagnetic shielding effect is better than that of shielded wires, which is not noticed by most people. The method of use is to let a pair of wires pass through the middle of the magnetic beads. Then when current flows through the pair of wires, the magnetic field generated will be mostly concentrated in the magnetic beads, and the magnetic field will no longer radiate outward; because the magnetic field will generate eddy currents in the magnetic beads, the direction of the electric lines generated by the eddy currents is exactly opposite to the direction of the electric lines on the surface of the conductor, and they can cancel each other out. Therefore, magnetic beads also have a shielding effect on electric fields, that is, 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 requiring the shielded wire to be grounded. For dual-conductor wires, using magnetic beads as electromagnetic shielding is equivalent to connecting a common-mode suppression inductor in the line, which has a strong suppression effect on common-mode interference signals.

It can be seen that the inductor coil is mainly used to suppress EMI for low-frequency interference signals, while the magnetic bead is mainly used to suppress EMI for high-frequency interference signals. Therefore, to suppress EMI for a wide-band interference signal, multiple inductors with different properties must be used at the same time to be effective. In addition, to suppress EMI for common-mode conducted interference signals, attention should also be paid to the connection position of the suppression inductor and the Y capacitor. The Y capacitor and the suppression inductor should be as close as possible to the input end of the power supply, that is, the location of the power socket, and the high-frequency inductor should be as close as possible to the Y capacitor, and the Y capacitor should also be as close as possible to the ground wire connected to the earth (the ground wire of the three-core power cord). This is effective for EMI suppression.

appendix:

1. Calculation of the inductance of a straight conductor with a circular cross section and the inductance of a core conductor:

As shown in Figure 3, the inductance of a straight conductor with a circular cross section is:

[H] (1)

in:

L: Inductance of a straight conductor with a circular cross section [H]

: Cable length [m]

r: conductor radius [m]

: vacuum permeability,

[H/m]

Explain that this is

>> The calculation formula under the condition of r. When there is a magnetic bead outside the circular cross-section straight conductor, it is referred to as a magnetic bead. The inductance of the magnetic bead is times the inductance of the circular cross-section straight conductor, which is the relative magnetic permeability of the magnetic core.

,

The magnetic permeability of the magnetic core, also known as the absolute magnetic permeability, is a unitless constant that can be easily obtained through actual measurement.