How much do you know about magnetic beads 

 

What are magnetic beads

Magnetic beads are specially designed to suppress high-frequency noise and spike interference on signal lines and power lines, and also have the ability to absorb electrostatic pulses. Magnetic beads are used to absorb ultra-high frequency signals, such as some RF circuits, PLL, oscillator circuits, including ultra-high frequency memory circuits (DDR SDRAM, RAMBUS, etc.), which need to be added to the power input part, and the inductance is a kind of storage. Energy components, used in LC oscillator circuits, filter circuits for medium and low frequencies, etc., and their application frequency range rarely exceeds 50MHZ. Magnetic beads have high resistivity and permeability, which is equivalent to a series connection of resistance and inductance, but both resistance and inductance vary with frequency.

 

The function of magnetic beads

The function of the magnetic beads is mainly to eliminate the RF noise existing in the transmission line structure (circuit), the RF energy is the AC sine wave component superimposed on the DC transmission level, and the DC component is the required useful signal. To remove this unwanted signal energy, chip beads are used to act as high frequency resistors (attenuators).

Magnetic beads have high resistivity and permeability, which is equivalent to the series connection of resistance and inductance, but the resistance value and inductance value change with frequency. It has better high-frequency filtering characteristics than ordinary inductors, and it is resistive at high frequencies, so it can maintain a high impedance in a fairly wide frequency range, thereby improving the FM filtering effect.

As power supply filtering, inductors can be used. The circuit symbol of the magnetic bead is the inductance, but it can be seen from the model that the magnetic bead is used. In terms of circuit function, the principle of magnetic beads and inductors is the same, but the frequency characteristics are different.

The DC component is the wanted useful signal, while the radio frequency RF energy is unwanted electromagnetic interference transmitted and radiated along the line (EMI). To remove these unwanted signal energy, chip beads are used to act as high frequency resistors (attenuators), which allow DC signals to pass through and filter out AC signals. Usually high frequency signals are above 30MHz, however, low frequency signals are also affected by chip beads. Magnetic beads have high resistivity and permeability, which are equivalent to series connection of resistance and inductance. As long as the wire runs through it in the circuit. The high-frequency current is dissipated as heat in it, and its equivalent circuit is an inductor and a resistor in series, and the value of both components is proportional to the length of the bead. Some magnetic beads have multiple holes, and passing wires through them can increase the impedance of the component (the square of the number of times passing through the beads). Ferrite beads can be used not only to filter out high frequency noise in power supply circuits (can be used for DC and AC output), but also widely used in other circuits

 

The principle of magnetic beads

The main raw material of magnetic beads is ferrite. Ferrite is a ferrimagnetic material with a cubic lattice structure. The ferrite material is an iron-magnesium alloy or an iron-nickel alloy. Its manufacturing process and mechanical properties are similar to those of ceramics, and its color is gray-black. One type of core often used in EMI filters is ferrite material, and many manufacturers offer ferrite materials specifically for EMI suppression. This material is characterized by very large high-frequency loss and high magnetic permeability, which can minimize the capacitance generated between the coil windings of the inductor in the case of high frequency and high resistance. For ferrites used to suppress electromagnetic interference, the most important performance parameters are permeability μ and saturation magnetic flux density Bs. The permeability μ can be expressed as a complex number, with the real part constituting the inductance and the imaginary part representing losses, which increase with frequency. Therefore, its equivalent circuit is a series circuit consisting of an inductor L and a resistor R, both of which are functions of frequency. When a wire is passed through such a ferrite core, the impedance of the formed inductor increases in form with increasing 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 inductance. At low frequencies, R is very small, and the magnetic 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, and at this time the magnetic The loss of the core is small, and the whole device is a low-loss, high-Q inductor, which is easy to cause resonance. Therefore, in the low frequency band, sometimes the phenomenon of increased interference after the use of ferrite beads may occur.

In the high frequency band, the impedance is composed of resistance components. As the frequency increases, the magnetic permeability of the magnetic core decreases, resulting in a decrease in the inductance of the inductance and a decrease in the inductive reactance component. However, at this time, the loss of the magnetic core increases, and the resistance component increases. , resulting in an increase in the total impedance, when high-frequency signals pass through the ferrite, the electromagnetic interference is absorbed and converted into heat and dissipated.

Ferrite suppression components are widely used on printed circuit boards, power and data lines. If a ferrite suppression element is added to the inlet end of the power line of the printed board, high-frequency interference can be filtered out. Ferrite rings or beads are specially designed to suppress high frequency interference and spike interference on signal lines and power lines, and it also has the ability to absorb electrostatic discharge pulse interference.

The numerical value of the two components is proportional to the length of the magnetic bead, and the length of the magnetic bead has a significant influence on the suppression effect. The longer the length of the magnetic bead, the better the suppression effect.

As power supply filtering, inductors can be used. The circuit symbol of the magnetic bead is the inductance, but it can be seen from the model that the magnetic bead is used. In terms of circuit function, the magnetic bead and the inductance are the same in principle, but the frequency characteristics are different. The magnetic bead is composed of oxygen magnets. The magnetic beads have a great hindering effect on high-frequency signals. The general specification is 100 ohms/100mmHZ, and its resistance is much smaller than the inductance at low frequencies. Ferrite Bead is an anti-jamming component that has developed rapidly. It is cheap, easy to use, and has a remarkable effect of filtering out high-frequency noise.

 

Magnetic bead selection

1. The unit of magnetic beads is ohms, not Henrys, so pay special attention to this. Because the unit of the magnetic bead is nominal according to the impedance it produces at a certain frequency, the unit of impedance is also ohm. The characteristic curve of frequency and impedance is generally provided on the DATASHEET of the magnetic bead, which is generally based on 100MHz, such as 600R@100MHz, which means that the impedance of the magnetic bead is equivalent to 600 ohms at a frequency of 100MHz.

2. Ordinary filters are composed of lossless reactive components. Its function in the line is to reflect the stopband frequency back to the signal source, so this type of filter is also called a reflection filter. When the reflection filter does not match the impedance of the signal source, a portion of the energy is reflected back to the signal source, resulting in an increase in the interference level. In order to solve this drawback, a ferrite magnetic ring or magnetic bead sleeve can be used on the incoming line of the filter, and the eddy current loss of the high-frequency signal by the ferrite ring or magnetic bead can be used to convert the high-frequency component into heat loss. Therefore, the magnetic ring and the magnetic beads actually absorb high-frequency components, so they are sometimes called absorption filters.

Different ferrite suppression components have different optimal suppression frequency ranges. Generally, the higher the permeability, the lower the frequency of suppression. In addition, the larger the volume of the ferrite, the better the inhibitory effect. When the volume is constant, the long and thin shape is better than the short and thick shape, and the smaller the inner diameter is, the better the inhibitory effect is. However, in the case of DC or AC bias current, there is still the problem of ferrite saturation. The larger the cross section of the suppression element, the less likely it is to be saturated, and the greater the bias current that can be tolerated.

When the EMI absorbing magnetic ring/magnetic bead suppresses differential mode interference, the current value passing through it is proportional to its volume, and the imbalance between the two causes saturation, which reduces the performance of the components; when suppressing common mode interference, connect the two wires of the power supply (positive and negative) Passing through a magnetic ring at the same time, the effective signal is a differential mode signal, and the EMI absorption magnetic ring/magnetic bead has no effect on it, while for the common mode signal, it will show a large inductance. Another good method in the use of the magnetic ring is to repeatedly wind the wire passing through the magnetic ring several times to increase the inductance. According to its principle of suppressing electromagnetic interference, its suppressing effect can be used reasonably. Ferrite suppression components should be installed close to the source of interference. For input/output circuits, it should be as close as possible to the inlet and outlet of the shielding case. For the absorption filter composed of ferrite magnetic ring and magnetic beads, in addition to the use of consumable materials with high magnetic permeability, attention should also be paid to its application. The resistance they present to high-frequency components in the line is about ten to several hundred Ω, so its effect is not obvious in high-impedance circuits, on the contrary, in low-impedance circuits (such as power distribution, power supply or RF circuits) Use will be very effective.

The above are some related introductions to magnetic beads. If you have any questions, please feel free to leave a message. If you want to know more, please follow us and see you in the next issue.