"Principle of magnetic beads
The main raw material of magnetic beads is ferrite. Ferrite is a kind of ferromagnetic material with cubic lattice structure. The ferrite material is iron magnesium alloy or iron nickel alloy. Its manufacturing process and mechanical properties are similar to ceramics, and its color is gray black. A kind of magnetic core often used in EMI filter is ferrite material. Many manufacturers provide ferrite materials specially used for EMI suppression. This material is characterized by very large high frequency loss and high permeability. It can be the capacitance generated between the coil windings of the inductor under the condition of high frequency and high resistance. For the ferrite used to suppress electromagnetic interference, the important performance parameter is permeability μ And saturated flux density BS. Permeability μ It can be expressed as a complex number, the real part constitutes the inductance, and the imaginary part represents the loss, which increases with the increase of frequency. Therefore, its equivalent circuit is a series circuit composed of inductance L and resistance R, both of which are functions of frequency. When the conductor passes through the ferrite core, the inductance impedance increases with the increase of frequency, but the mechanism is completely different at different frequencies.
In the low frequency band, the impedance is composed of the inductive reactance of the inductor. At low frequency, R is very small, the permeability of the magnetic core is high, so the inductance is large. L plays a major role, and the electromagnetic interference is reflected and suppressed. At this time, the loss of the magnetic core is small. The whole device is an inductor with low loss and high Q characteristics. This inductor is easy to cause resonance. Therefore, in the low frequency band, Sometimes, the interference may be enhanced after the use of ferrite beads. In the high-frequency band, the impedance is composed of resistance components. With the increase of frequency, the permeability of the magnetic core decreases, resulting in the decrease of inductance and inductive reactance components of the inductance. However, at this time, the loss of the magnetic core increases and the resistance component increases, resulting in the increase of the total impedance. When the high-frequency signal passes through ferrite, the electromagnetic interference is absorbed and converted into heat energy for dissipation.
Ferrite suppression elements are widely used in printed circuit boards, power lines and data lines. If a ferrite suppression element is added at the inlet of the power line of the printed board, the high-frequency interference can be filtered out. Ferrite magnetic rings or beads are specially used to suppress high-frequency interference and peak interference on signal lines and power lines. It also has the ability to absorb electrostatic discharge pulse interference.
The numerical value of the two elements is directly proportional to the length of the magnetic bead, and the length of the magnetic bead has a significant impact on the suppression effect. The longer the length of the magnetic bead, the better the suppression effect.
Selection of magnetic beads
1. The unit of magnetic bead is Ohm, not hunter, which should be paid special attention. Because the unit of magnetic bead is nominal according to the impedance it produces at a certain frequency, and the unit of impedance is also ohm. The characteristic curves of frequency and impedance are generally provided on the datasheet of the magnetic beads. Generally, 100MHz is used as the standard, such as 1000R@100MHz At 100MHz, the impedance of the magnetic bead is equivalent to 600 ohms.
2. Ordinary filter is composed of lossless reactance elements. Its function in the line is to reflect the stopband frequency back to the signal source, so this kind of filter is also called reflection filter. When the impedance of the reflection filter does not match the signal source, some energy will be reflected back to the signal source, resulting in the enhancement of the interference level. In order to solve this problem, ferrite magnetic ring or magnetic bead sleeve can be used on the inlet line of the filter to convert the high-frequency component into heat loss by using the eddy current loss of the ring or magnetic bead on the high-frequency signal. Therefore, magnetic rings and beads actually absorb high-frequency components, so they are sometimes called absorption filters.
3. Different ferrite suppression elements have different suppression frequency ranges. Generally, the higher the permeability, the lower the suppression frequency. In addition, the larger the volume of ferrite, the better the inhibition effect. When the volume is fixed, the long and thin shape has better inhibition effect than the short and thick shape, and the smaller the inner diameter, the better the inhibition effect. However, in the case of DC or AC bias current, there is also the problem of ferrite saturation. The larger the cross section of the suppression element, the less easy it is to saturate, and the greater the bias current it can bear.
4. When EMI absorbing magnetic ring / bead suppresses differential mode interference, its current value is directly proportional to its volume. The imbalance between them causes saturation and reduces the performance of the element; When suppressing common mode interference, two wires (positive and negative) of the power supply pass through a magnetic ring at the same time. The effective signal is the differential mode signal. The EMI absorption magnetic ring / bead has no effect on it, but it will show a large inductance for the common mode signal. Another better way to use the magnetic ring is to make the wires passing through the magnetic ring wind several times repeatedly to increase the inductance. According to its suppression principle of electromagnetic interference, its suppression effect can be used reasonably.
5. The ferrite suppression element shall be installed near the interference source. For the input / output circuit, it shall be as close as possible to the inlet and outlet of the shielding shell. For the absorption filter composed of ferrite magnetic ring and magnetic beads, we should not only select the consumable material with high permeability, but also pay attention to its application. Their resistance to high-frequency components in the line is about ten to hundreds of Ω, so its role in high impedance circuits is not obvious. On the contrary, it will be very effective in low impedance circuits (such as power distribution, power supply or RF circuits).
Difference between magnetic bead and inductance
Inductance is an energy storage element, and magnetic beads are energy conversion (consumption) devices. Inductors are mostly used in power filter circuits, focusing on restraining conductive interference; Magnetic beads are mostly used in signal circuits, mainly in EMI. Magnetic beads are used to absorb UHF signals. For example, some RF circuits, PLL, oscillation circuits and UHF memory circuits (DDR, SDRAM, Rambus, etc.), magnetic beads need to be added to the power input part, while inductance is an energy storage element, which is used in LC oscillation circuit, medium and low frequency filter circuit, and its application frequency range rarely exceeds 50MHz.
1. Chip inductor: inductive elements and EMI filter elements will be widely used in PCB circuit of electronic equipment. These components include chip inductors and chip magnetic beads. The following describes the characteristics of these two devices and analyzes their general and special applications. The advantages of surface mount components are small package size and can meet the requirements of actual space. In addition to different impedance value, current carrying capacity and other similar physical characteristics, other performance characteristics of through-hole connectors and surface mount devices are basically the same. When chip inductors are needed, they are required to realize the following two basic functions: circuit resonance and choke reactance. Resonance circuit includes resonance generation circuit, oscillation circuit, clock circuit, pulse circuit, waveform generation circuit and so on. The resonant circuit also includes a high-Q band-pass filter circuit. To make the circuit resonate, capacitance and inductance must exist in the circuit at the same time. Parasitic capacitance exists at both ends of the inductor, which is due to the fact that the ferrite body between the two electrodes of the device is equivalent to a capacitive medium. In the resonant circuit, the inductance must have high Q, narrow inductance deviation and stable temperature coefficient in order to meet the requirements of narrow band and low frequency temperature drift of the resonant circuit. High Q circuit has sharp resonance peak. Narrow inductance bias ensures that the resonance frequency deviation is as small as possible. The stable temperature coefficient ensures that the resonant frequency has stable temperature variation characteristics. The difference between the standard radial lead-out inductor, axial lead-out inductor and chip inductor is only that the package is different. The inductance structure includes a coil wound on a dielectric material (usually alumina ceramic material), or a hollow coil and a coil wound on a ferromagnetic material. In power applications, when used as a choke, the main parameters of inductance are DC resistance (DCR), rated current, and low Q value. When used as a filter, a wide bandwidth characteristic is desired, so the high-Q characteristic of the inductor is not required. Low DCR can guarantee the voltage drop. DCR is defined as the DC resistance of the component without AC signal.
2. Chip magnetic beads: the function of chip magnetic beads is mainly to eliminate the RF noise existing in the transmission line structure (PCB electricity). RF energy is the AC sine wave component superimposed on the DC transmission level. DC component is the required useful signal, while RF RF energy is useless. Electromagnetic interference is transmitted and radiated (EMI) along the line. To eliminate these unwanted signal energy, chip magnetic beads are used as high-frequency resistors (attenuators), which allow DC signals to pass through and filter AC signals. Usually, the high-frequency signal is more than 30MHz. However, the low-frequency signal will also be affected by chip magnetic beads. The chip magnetic beads are composed of soft ferrite materials, forming a monolith structure with high volume resistivity. Eddy current loss is inversely proportional to the resistivity of ferrite materials. Eddy current loss is proportional to the square of signal frequency.
Parameters of magnetic beads
Nominal value: because the unit of magnetic bead is nominal according to the impedance generated at a certain frequency, the unit of impedance is also ohm. Generally, 100MHz is the standard, such as 2012b601, which means that the impedance of magnetic bead is 600 ohm at 100MHz.
Rated current: rated current refers to the allowable current that can ensure the normal operation of the circuit.
What is the role of magnetic beads and inductors in solving EMI and EMC? First, let's look at the difference between magnetic beads and inductors. Inductance is an attribute of closed loop, which is mostly used in power filter loop, while magnetic beads are mainly used in signal loop and EMC countermeasures. Magnetic beads are mainly used to suppress electromagnetic radiation interference, while inductors focus on suppressing conductive interference. Magnetic beads are used to absorb UHF signals. For example, some RF circuits, PLL, oscillation circuits and circuits containing UHF memory (DDR, SDRAM, Rambus, etc.), magnetic beads need to be added to the power input part. Both can be used to deal with EMC and EMI problems.
What is the role of magnetic beads and inductors in solving EMI and EMC? First, let's look at the difference between magnetic beads and inductors. Inductance is an attribute of closed loop, which is mostly used in power filter loop, while magnetic beads are mainly used in signal loop and EMC countermeasures. Magnetic beads are mainly used to suppress electromagnetic radiation interference, while inductors focus on suppressing conductive interference. Magnetic beads are used to absorb UHF signals. For example, some RF circuits, PLL, oscillation circuits and circuits containing UHF memory (DDR, SDRAM, Rambus, etc.), magnetic beads need to be added to the power input part. Both can be used to deal with EMC and EMI problems.
The key of magnetic beads and inductors in EMI and EMC circuits is to suppress high-frequency conducted interference signals and inductors. However, from the perspective of principle, the magnetic bead can be equivalent to an inductance, which is still different. The difference is that the inductance coil has distributed capacitance. Therefore, the inductance coil is equivalent to an inductance connected in parallel with a distributed capacitor. As shown in Figure 1. In Figure 1, LX is the equivalent inductance (ideal inductance) of the inductance coil, Rx is the equivalent resistance of the coil, and CX is the distributed capacitance of the inductance.
Figure 1: equivalent circuit diagram of inductance coil.
Theoretically, to suppress the conducted interference signal, the larger the inductance of the suppression inductance is, the better. However, for the inductance coil, the larger the inductance is, the larger the distributed capacitance of the inductance coil is, and the effects of the two will cancel each other.
Figure 2: relationship between impedance and frequency of ordinary inductance coil.
Figure 2 shows the relationship between the impedance and frequency of an ordinary inductance coil. It can be seen from the figure that the impedance of the inductance coil increases with the increase of frequency at the beginning, but when its impedance increases to the 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 value, it is where the distributed capacitance of the inductance coil and the equivalent inductance produce parallel resonance. In the figure, L1 > L2 > L3, it can be seen that the greater the inductance of the inductance coil, the lower its resonant frequency. It can be seen from Figure 2 that if the interference signal with 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, the inductance coil we choose has to be its limit value, only 1 turn or less. Magnetic bead, namely through core inductance, is an inductive coil with less than 1 turn. However, the distributed capacitance of through core inductance is several times to dozens of times smaller than that of single coil inductance coil. Therefore, the working frequency of through core inductance is higher than that of single coil inductance coil.
The inductance of the through core inductor is generally small, ranging from a few micro Heng to dozens of micro Heng. 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 bead, but the relative permeability uy of the magnetic bead is also related to the inductance of the magnetic bead. Fig. 3 and Fig. 4 refer to the schematic diagrams of the wire and through core inductor respectively. When calculating the through core inductance, Firstly, the inductance of a circular cross-section straight wire should be calculated, and then the inductance of the through core inductance can be calculated by multiplying the calculation result by the relative permeability of the magnetic beads.
Figure 3: inductance diagram of circular section straight conductor.
Figure 4: magnetic bead through core inductance diagram.
In addition, when the working frequency of the through core inductor is very high, eddy currents will be generated in the magnetic beads, which is equivalent to the reduction 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 working frequency. However, because the working frequency of magnetic beads is only a range, the average permeability is often used in practical applications.
At low frequency, the relative permeability of magnetic beads is generally large (greater than 100), but at high frequency, its effective permeability is only a fraction or even a few tens of relative permeability. Therefore, the magnetic beads also have the problem of cut-off frequency. The so-called cut-off frequency is to reduce the effective permeability of the magnetic beads to the working frequency fc close to 1. At this time, the magnetic beads have lost the role of an inductance. Generally, the cut-off frequency fc of magnetic beads is between 30 ~ 300MHz. The cut-off frequency is related to the material of magnetic beads. Generally, the higher the permeability of magnetic core materials, the lower the cut-off frequency fc, because the eddy current loss of low-frequency magnetic core materials is relatively large. When designing the circuit, the user can ask the supplier of magnetic core materials to provide the test data of magnetic core working frequency and effective permeability, or the curve of through core inductance under different working frequencies. Fig. 5 is a frequency curve of through core inductance.
Figure 5: frequency curve of through core inductance.
Another use of magnetic beads is to do electromagnetic shielding. Its electromagnetic shielding effect is better than that of shielding wire, which is not paid much attention by ordinary people. The use method is to let a pair of wires pass through the middle of the magnetic beads. 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 radiate outward; Because the magnetic field will generate eddy current in the magnetic bead, the direction of the power line generated by the eddy current is exactly opposite to that of the power line on the surface of the conductor, which can offset each other. Therefore, the magnetic bead also has a shielding effect on the electric field, that is, the magnetic bead has 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 inductor in the line for double wires, which has a strong inhibitory effect on the common mode interference signal.
From the above, we can see that magnetic beads and inductors can inhibit EMC and EMI circuits, mainly due to the difference in inhibition, while inductors can no longer act as inductors after high-frequency resonance. First, we must understand the two ways of EMI, namely, radiation and conduction
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