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Sep 11 2018

Different Applications of Magnetic Beads and Inductors in the Circuits of EMI and EMC

Introduction

    What are the functions and characteristics of magnetic beads and inductors in solving the problems of EMI and EMC? First of all, pay attention to the difference between magnetic beads and inductors. 

Article CoreInductors and Magnetic BeadsPurposeAnalyze differences between magnetic beads and inductors. 
Article NameDifferent Applications of Magnetic Beads and Inductors in the Circuits of EMI and EMCCategorySemiconductor Information
ApplicationCircuits and ApplicationKeywordsEMI,EMC,Magnetic beads,Inductors


Catalogs

Ⅰ What Is Inductors
Ⅱ What Is Magnetic Beads
Ⅲ Difference of Inductors and Magnetic Beads
Ⅳ Summary

Inductors

    Inductance is an attribute of closed loop, which is mostly used in power filter circuit, while magnetic beads are mostly used in signal loop. For EMC, magnetic beads are mainly used to suppress electromagnetic radiation interference, while inductors are used to suppress conductive interference.


Magnetic Beads

    Magnetic beads, being used to absorb ultra-high frequency signals, are added into the input part of the power supply in some circuits, such as RF circuits, PLL, oscillation circuits, and circuits including ultra-high frequency memory(DDR SDRAM, RAMBUS, etc.). Both magnetic beads and inductors can be used to deal with EMC, EMI problems.


Difference of Inductors and Magnetic Beads

    In principle, the magnetic bead can be equivalent to an inductor, so the magnetic bead acts as a suppressing inductor in the EMI and EMC circuits, mainly suppressing the high-frequency conducted interference signal and suppressing the inductance. However, there is still a difference between the magnetic beads and inductors in principle, and the biggest difference is that the inductor has distributed capacitance. Therefore, a induction coil is equivalent to an inductor in parallel with a distributed capacitor. As shown in Figure 1, LX is the equivalent inductance (ideal inductance) of the inductor, RX is the equivalent resistance of the coil, and CX is the distributed capacitance of the inductor.

Equivalent Circuit Diagram of Induction Coils  

Fig. 1 Equivalent Circuit Diagram of Induction Coils

    Theoretically, to suppress the conducted interference signal, required to suppress the inductance of the inductor as much as possible. However, for the induction coil, the larger the inductance, the larger the distributed capacitance of the induction coil, and the effects of this two will cancel each other out.

The relationship between impedance and frequency of a common induction coil

Fig.2 The relationship between impedance and frequency of a common induction coil

     It can be seen from the figure that the impedance of the coil begins to increase with increasing frequency, but when its impedance increases to the maximum value, the impedance decreases rapidly as the frequency increases. This is because of the paralleling distributed capacitance. When the impedance increases to the maximum, it is where the distributed capacitance of the inductor and the equivalent inductance produce parallel resonance. In the figure, L1> L2> L3, it can be seen that the larger the inductance of the inductor, the lower the resonance frequency. As can be seen from Figure 2, if you want to suppress the interference signal with a frequency of 1MHZ, it is better to use L3 instead of L1, because the inductance of L3 is ten times smaller than L1, so the cost of L3 is much lower than L1.

     To further increase the suppression frequency, the last choosing inductor coil has to be its minimum, that is only 1 turn or less than 1 turn. A magnetic bead, that is, a through-core inductor, is an induction coil with less than one turn. However, the distributed capacitance of a through-core inductor is several times to dozens of times smaller than that of the single-turn inductor coil. Therefore, the through-core inductor has a higher operating frequency than the single-turn inductor.

     The inductance of the through-core inductor is generally small, ranging from a few mH to dozens of mH. The inductance is related to the size, length of the wires in the  through-core inductor and the cross-sectional area of the magnetic beads. However, the largest affecting factor is the relative permeability of the magnetic beads Uy. Fig. 3 and 4 are the principle diagrams of conductor and through-core inductor respectively. When calculating inductance, the inductance of a straight conductor with a circular cross-section should be calculated first, and then multiplied by the relative permeability of magnetic beads, finally, the inductance of the through-core inductor can be calculated. 

Inductance diagram of straight line with circular section

Fig. 3 Inductance Diagram of Straight Line with Circular Section 

magnetic bead inductance diagram 

Fig. 4 magnetic bead inductance diagram 

    In addition, when the operating frequency of the inductor is very high, eddy current will be generated in the magnetic beads, which equals to reduce the magnetic conductivity. We generally use effective conductivity at this condition, which is the relative permeability of a magnetic bead under a certain working frequency. However, because the working frequency of magnetic beads is only in a range, the average permeability is often used in practical applications.

     At low frequencies, the relative magnetic permeability of a typical magnetic bead is very large (greater than 100), but at high frequencies, its effective permeability is only a fraction of the relative ones, even a few tenths. Therefore, the magnetic beads also have a problem of the cutoff frequency. The so-called cutoff frequency is an operating frequency that reduces the effective magnetic permeability to close to 1, making magnetic beads not act as an inductor. Generally, the cutoff frequency of the magnetic beads is between 30 and 300 MHz, depending on the material of the magnetic bead. Generally, the higher the magnetic permeability of the magnetic core material, the lower the cutoff frequency, because the low-frequency magnetic core material has large loss of the eddy current. When designing the circuit, the designer can request the supplier to provide test data of the operating frequency and effective permeability, or a graph of the through-core inductor under different operating frequencies. Below figure 5 is the frequency curve of the through-core inductor.

穿心电感频率曲线图.jpg

Fig.5 Frequency curve of the through-core inductor

    Ordinary people would not notice that electromagnetic shielding, another usage of magnetic beads, whose effect is better than that of shielding wires. The method is to let a pair of wires pass through the magnetic beads. When a current flows from the two wires, the enerating magnetic field will be mostly concentrated in the magnetic beads, without radiating outward. Since the magnetic field generates eddy current in the magnetic bead, and the direction of the power line generating by eddy current is opposite to that of the power line on the onductor surface, they can cancel each other, so the magnetic bead also has a shielding effect on the electric field. In a word, the magnetic bead has a strong shielding effect on the electromagnetic field in the conductor.

    The advantage of electromagnetic shielding by magnetic beads is that the magnetic beads are not grounded. For two wires, using Magnetic beads as electromagnetic shielding is equivalent to connect a common mode suppression inductor on the line, which has a strong inhibitory effect on common mode interference signals.


Summary

    From the above, both the magnetic beads and inductors can play a role of suppression in EMC and EMI circuit, and they are mainly different in inhibiting fields. Inductors can no longer play the role of inductors in high-frequency resonance, and it is necessary to know the two ways of EMI, namely: radiation and conduction, which use different method. The former uses magnetic beads and the latter uses inductors. We should pay attention to the connection position between CMR inductor and capacitor Y. The series inductor between ground wire and other input and output lines is called CMR inductor. The most effective way to suppress conducting interference is connecting one end of CMR inductor to the ground wire (common end) of the machine and the other end is connected to a inductor Y, whose other end is connected to the earth.



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