Model Naming Method and Structural Characteristics of Magnetic Beads

The full name of magnetic beads is ferrite magnetic bead filter (another is magnetic beads made of amorphous alloy magnetic materials). It is an anti-interference element with remarkable effect of filtering high-frequency noise. 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. Magnetic beads have high resistivity and permeability. They are equivalent to resistance and inductance in series, but both resistance and inductance change with frequency. Compared with ordinary inductors, it has better high-frequency filtering characteristics and shows resistance at high frequency, so it can maintain high impedance in a wide frequency range, so as to improve the effect of FM filtering. The circuit symbol of magnetic beads should not be drawn as inductance. It is suggested that the schematic identification and tag number should be different, so that the reader can easily see that magnetic beads are used.

01. Model naming method of magnetic beadsThe model of magnetic beads is generally composed of the following five parts: Part I: category, which is mostly represented by letters. Part II: size, which is represented by numbers (British system). Part III: material, which is represented by letters, where X represents small. Part IV: impedance, impedance at 100MHz Part V: packaging method, expressed in letters. For example, a certain type of magnetic beads is named as follows: ferrite chip beads (ordinary type) size: 1005 (0402) 1608 (0603) 2012 (0805) product specification naming method:It should be pointed out that at present, the naming method of magnetic bead model is different from each manufacturer, and there is no unified standard.

02. Structural characteristics of magnetic beads

Ferrite bead is an anti-interference component with rapid application and development. It is cheap, easy to use and has remarkable effect of filtering high-frequency noise. In the circuit, as long as the wire passes through it (what I use is like an ordinary resistor, the wire has passed through and glued, and there is also the form of surface mount). When the current in the conductor passes through, ferrite has little impedance to the low-frequency current, but has a great attenuation effect to the higher frequency current. The high-frequency current is dissipated in the form of heat. Its equivalent circuit is an inductor and a resistor in series. The values of the two components are proportional to the length of the magnetic bead. There are many kinds of magnetic beads. The manufacturer shall provide technical specifications, especially the curve of the relationship between impedance and frequency of magnetic beads. Some magnetic beads have multiple holes, which can increase the component impedance (the square of the times of passing through the magnetic beads) by passing through the wire. However, the increased noise suppression ability at high frequency cannot be as expected, and it is better to use more magnetic beads in series.

Ferrite is a magnetic material, which will produce magnetic saturation due to excessive current, and the permeability will decrease sharply. Magnetic beads specially designed in structure shall be used for high current filtering, and its heat dissipation measures shall be paid attention to. Ferrite magnetic beads can not only be used to filter high-frequency noise in power supply circuits (DC and AC output), but also be widely used in other circuits, and its volume can be made very small. Especially in digital circuits, because the pulse signal contains high-frequency harmonics, which is also the main source of high-frequency radiation of the circuit, it can play the role of magnetic beads in this occasion. Ferrite beads are also widely used in noise filtering of signal cables.

03. Magnetic bead unitThe unit of magnetic beads is Ohm, not hunter, which should be paid special attention to. 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 600R@100MHz At 100MHz, the impedance of the magnetic bead is equivalent to 600 ohms.04. Main characteristic parameters of magnetic bead

1. DC resistance (MOhm): the resistance value of the magnetic bead when DC current passes through the magnetic bead. 2. Rated current (MA): indicates the maximum allowable current when the magnetic bead works normally. 3. Impedance [Z] @ 100MHz (Ohm): here refers to AC impedance. 4. Impedance frequency characteristic: a curve describing the change of impedance value with frequency. 5. Resistance frequency characteristic: describe the curve of resistance value changing with frequency. 6. Inductive reactance frequency characteristic: describe the curve of inductive reactance changing with frequency. The figure below shows the characteristic parameters and frequency characteristic curve of magnetic beads of a manufacturer:

05. Classification and type selection of magnetic beads

1 According to the application of magnetic beads, magnetic beads can be roughly divided into ordinary type, high current type, peak type, etc. Ordinary type: ordinary magnetic beads are used for occasions where the current is not too large (generally less than 600mA) and there are no special requirements. Its DC resistance is generally a few tenths of ohms. It can effectively suppress and absorb electromagnetic interference and radio frequency interference of electronic equipment. Its impedance range is generally in the range of several to several thousand ohms. High current type: this type of magnetic bead is used in occasions requiring high current.

Because it is used in occasions requiring high current, its DC resistance must be very small, about one order of magnitude less than that of ordinary magnetic beads, and its impedance value is generally small. Spike type: the characteristic of this type of magnetic bead is that its impedance rises sharply in a certain frequency region, so that a higher attenuation effect can be obtained in a specific frequency region without affecting the signal. 2 Selection of magnetic beads magnetic beads are mainly used for EMI differential mode noise suppression. Its DC impedance is very small, but it has high impedance at high frequency. Generally speaking, 600r refers to the impedance value at 100MHz test frequency. Two aspects should be considered in the selection of magnetic beads: one is the noise interference in the circuit, and the other is the current to be passed.

To roughly understand the frequency and intensity of noise, the frequency impedance curves of different magnetic beads are different. Choose the one with higher magnetic bead impedance at the center frequency of noise. For those with large noise interference, choose those with higher impedance, but it is not that the higher the impedance, the better, because the higher the impedance, the higher the DCR and the greater the attenuation of useful signals. However, there is generally no clear calculation and selection standard, which mainly depends on the actual use effect. 120r-600r are commonly used. Then it depends on the passing current.

If it is used in the power line part, the model with larger rated current shall be selected, and if it is used in the signal line part, the requirements for rated current are generally not high. In addition, the larger the impedance of the magnetic bead is, the smaller the rated current is. The selection of magnetic beads should be carried out according to the actual situation. For example, for 3.3V and 300mA power supplies, it is required that 3.3V should not be lower than 3.0V, so the DC resistance DCR of magnetic beads should be less than 1R. In this case, 0.5r is generally selected to prevent parameter drift. For the noise suppression ability, if the noise of 100MHz and 300mvpp is required to reach the level of 50mvpp after passing through the magnetic beads, assuming that the load is 45 ohms, it should be selected 225R@100Mhz , how is 225r calculated for magnetic beads with DCR

The current flowing through the circuit also determines the magnetic beads with rated current, The next step is to determine the DCR (DC impedance) of the magnetic beads, and the allowable DCR range of the magnetic beads can be calculated according to the power supply range of the voltage of the later stage circuit. Packaging can be selected according to the situation. However, it should be noted that the impedance of the magnetic bead is a little different from that in the specification after you apply the voltage.   To correctly select magnetic beads, you must pay attention to the following points:   1. What is the frequency range of unwanted signals;   2. Who is the noise source;   3. How much noise attenuation is required;   4. What are the environmental conditions (temperature, DC voltage, structural strength);   5. Circuit and load impedance;.

06. Function of magnetic beads

The shape of magnetic bead is similar to that of inductance. Its main function is simply to be disturbed by power supply, signal noise and so on. In the low frequency band, the impedance consists of the inductive reactance of the inductor. R is very small at low frequencies. The permeability of the magnetic core is high, so the inductance is large, L plays a main role, electromagnetic interference is reflected and suppressed, and the loss of the magnetic core is small. The whole device is an inductance with low loss and high Q characteristics. This inductance is easy to cause resonance. Therefore, the interference may be enhanced after the use of ferrite beads in low frequency band. In the high frequency band, the impedance is composed of resistance component. With the increase of frequency, the permeability of the magnetic core decreases, resulting in the decrease of inductance and inductive reactance component. At this time, the loss of the magnetic core increases and the resistance component increases, resulting in an increase in the total impedance. When the high-frequency signal passes through the ferrite, the electromagnetic interference is absorbed and converted into heat energy. Ferrite magnetic beads are widely used in printed circuit boards. For example, magnetic beads (large magnetic rings) are sleeved at the inlet end of the power line of the printed circuit board to filter out high-frequency interference. 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. Inductance is an energy storage element, and magnetic beads are energy conversion (consumption) devices. Inductors are mostly used in power filter circuits, focusing on suppressing conductive interference; Magnetic beads are mostly used in signal circuits, mainly in EMI (electromagnetic compatibility). Magnetic beads are used to absorb UHF signals, such as in some RF circuits, PLL, oscillation circuits, circuits containing UHF memory, etc. Magnetic beads need to be added to the power input part.

07. Derating of magnetic beadsDerating criteria for magnetic devices: