Selection and Application of Magnetic Beads

Since the ferrite bead is used in the circuit to increase the high frequency loss without introducing DC loss, and is small in size and easy to be mounted on the lead wires or wires of the interval, the suppression effect on the noise signal above 1 MHz is very obvious, so Decoupling, filtering, and suppression of parasitic oscillations in high-frequency circuits. In particular, it is effective to eliminate the sudden change of current caused by the switching device inside the circuit and the high frequency noise interference of the filter power line or other wire lead-in circuit. Low-impedance power supply circuits, resonant circuits, Class C power amplifiers, and thyristor switching circuits are all very effective using ferrite beads for filtering. Ferrite magnetic beads can be generally divided into two types, resistive and inductive, which can be selected according to needs when used. The impedance of a single magnetic bead is generally ten to several hundred ohms. If a single amount of attenuation is insufficient, a plurality of magnetic beads can be used in series, but usually three or more effects are not significantly increased [7]. FIG. 3 shows a high frequency LC filter circuit constructed by using two inductive ferrite beads, which can effectively absorb the oscillating signal generated by the high frequency oscillator without breaking into the load and without reducing the load. DC voltage on.

Magnetic Beads Application Case

Magnetic Beads Application Case

Since any transmission line inevitably has lead resistance, lead inductance and stray capacitance, a standard pulse signal is prone to overshoot and ringing after a long transmission line. A large number of experiments have shown that the lead resistance can reduce the average amplitude of the pulse, while the presence of lead inductance and stray capacitance is the root cause of the overshoot and ringing. Under the same condition that the pulse front rise time is the same, the larger the lead inductance, the more serious the overshoot and ringing phenomenon. The larger the stray capacitance, the longer the rise time of the waveform, and the increase of the lead resistance will make the pulse The amplitude is reduced. In practical circuits, series resistors can be used to reduce and suppress overshoot and ringing.

Ferrite suppression components are also widely used in printed circuit boards, power lines, and data lines. If ferrite beads are added to the input end of the power line of the printed board, high frequency interference can be filtered out. Ferrite magnetic rings or magnetic beads are designed to suppress high-frequency interference and spike interference on signal lines and power lines. They also have the ability to absorb electrostatic discharge pulse interference. The numerical value of the two components is proportional to the length of the magnetic beads, and the length of the magnetic beads has a significant effect on the suppression effect. The longer the length of the magnetic beads, the better the suppression effect.

Selection and Application of Magnetic Beads

Selection and Application of Magnetic Beads

The ordinary filter is composed of a lossless reactance element, and its role in the line is to reflect the stop band 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 source impedance, a portion of the energy is reflected back to the source, causing an increase in the level of interference. In order to solve this drawback, a ferrite magnetic ring or a magnetic bead sleeve can be used on the incoming line of the filter, and the high frequency component is converted into heat loss by the eddy current loss of the high frequency signal by the magnetic ring or the magnetic bead. Therefore, the magnetic ring and the magnetic beads actually absorb the high-frequency components, so they are sometimes referred to as absorption filters.

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

When the EMI absorption magnetic ring/bead suppresses the differential mode interference, the current value through it is proportional to its volume, the two are out of regulation, causing saturation, which reduces the performance of the component; when suppressing common mode interference, the two wires of the power supply (positive and negative) At the same time, through a magnetic ring, the effective signal is the differential mode signal, the EMI absorption magnetic ring/magnetic bead has no influence on it, and the common mode signal shows a large inductance. A better method of using the magnetic ring is to repeatedly circulate the wire of the magnetic ring that passes through to increase the inductance. According to its suppression principle of electromagnetic interference, its suppression can be reasonably used.

The ferrite suppression element should be installed close to the source of the interference. For the input/output circuit, it should be as close as possible to the entrance and exit of the shield case. For the absorption filter composed of the ferrite magnetic ring and the magnetic beads, in addition to the use of high magnetic permeability consumable materials, it is also necessary to pay attention to its application. Their resistance to high-frequency components in the line is about ten to several hundred ohms, so its role in high-impedance circuits is not obvious. Instead, in low-impedance circuits (such as power distribution, power or RF circuits) Use will be very effective.

Magnetic Beads Application Case

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