Selection of circuit design magnetic beads on PCB

Author: Lao duo

Address: http://www.165v.com/165v/div/2007-6-27/686-1.htm

The reason for the use of chip magnetic beads and chip Inductance: whether the use of chip magnetic beads or chip inductance is still applied. Chip Inductor must be used in the resonant circuit. The use of chip magnetic beads is the best choice to eliminate unwanted EMI noise.

1.The unit of the magnetic beads is Ohm, not Hunter.Pay special attention to this. Because the unit of the magnetic beads is nominal according to the impedance produced at a certain frequency, the unit of impedance is also ohm. Datasheet of magnetic beads generally provides characteristic curves of frequency and impedance, which are generally based on MHz,For example, 1000r @ 100 MHz means that the impedance of the magnetic beads is equivalent to 600 ohm at a MHz frequency..

2. A common filter is composed of zero-loss reactive components. Its function in the line is to return the impedance band frequency back to the signal source. Therefore, this type of filter is also called a reflection filter. When the reflected filter does not match the signal source impedance, some of the energy will be reflected back to the signal source, resulting in an enhancement of the interference level. To solve this problem, the ferrite magnetic ring or magnetic beads can be used in the inlet of the filter, and the eddy current loss of high-frequency signals can be converted into heat loss by using the zihuan or magnetic beads.Therefore, magnetic rings and magnetic beads are used to absorb high-frequency components. Therefore, they are also called absorption filters.

different ferrite suppression elements have different 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 small, the longer and finer shapes are better than the shorter and coarse shapes, and the smaller the inner diameter, the better the inhibition effect. however, in the case of DC or AC Partial flow, there is still a problem of ferrite saturation. The larger the cross section of the suppression element, the less saturated it is, and the larger the tolerable partial flow. When EMI absorption magnetic ring/magnetic beads suppress differential mode interference, the current value is proportional to its volume. The imbalance between the two causes saturation, reducing the performance of components, the two wires (plus and minus) of the power supply pass through a magnetic ring at the same time. The effective signal is a differential mode signal, and EMI absorption is not affected by magnetic ring/magnetic beads, however, common-mode signals show a large amount of inductance. Another good way to use the magnetic ring is to repeat the wires that pass through the magnetic ring several times to increase the inductance. It can be used properly based on its suppression principle of electromagnetic interference.
ferrite suppression elements should be installed near the interference source. For the input/output circuit, try to be close to the inlet and outlet of the shielding shell. In addition to the consumable materials with high permeability, the absorption filter composed of the inner body magnetic ring and the magnetic beads should be used. Their resistance to the high-frequency components in the line is about 10 to several hundred Ω, so it does not play an obvious role in the high-impedance circuit, it will be very effective in Low-Impedance Circuits (such as power distribution, power supply, or RF circuits.

Iv. Conclusion
Ferrite is widely used in EMI control because it can degrade at a high frequency and make the low frequency pass through without any obstacles. Magnetic rings/magnetic beads used for EMI absorption can be made into various shapes and are widely used in various occasions. For example, the PCB can be added to the DC/DC module, data cable, and power cord. It absorbs high-frequency interference signals on the road, but does not generate new zero poles in the system, and does not damage the stability of the system. It is used in combination with the power filter, which can well supplement the shortcomings of the high-frequency end Performance of the filter and improve the filtering characteristics in the system.

Magnetic Beads are used to suppress high-frequency noise and Spike interference on signal lines and power cables. They also have the ability to absorb electrostatic pulses.

Magnetic Beads are used to absorb ultra-high frequency signals, such as RF circuits, PLL, oscillating circuits, and ultra-high frequency memory circuits (ddr sdram, Rambus, etc.). They must be added to the input part of the power supply, inductance is a type of energy storage component used in LC oscillator circuit, medium and low frequency filter circuit, and its application frequency rarely exceeds the error 50 MHz.

The function of magnetic beads is mainly to eliminate RF noise in transmission line structures (circuits). RF energy is an AC Sine Wave component superimposed on the DC transmission level. DC composition is a useful signal, the RF power is useless electromagnetic interference transmitted along the line and radiation (EMI ). To eliminate unnecessary signal energy, use a chip magnetic bead to act as a high-frequency resistor (the attenuation). The device allows the DC signal to pass through and filter out the AC signal. Generally, the high-frequency signal is above 30 MHz. However, the low-frequency signal is also affected by the chip magnetic beads.

The chip magnetic beads are composed of Soft Ferrite Materials and constitute a rock-shaped 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 the signal frequency. Advantages of chip magnetic beads: miniaturization and lightweight features high impedance in the RF noise frequency range, eliminating electromagnetic interference in transmission lines. Close the magnetic circuit structure to better eliminate the string winding of signals. Excellent magnetic shielding structure. Reduce the direct resistance to avoid excessive attenuation of useful signals. Significant high-frequency and impedance characteristics (better RF energy elimination ). Eliminate parasitic vibrations in high-frequency amplification circuits. It works effectively within the frequency range of several MHz to several hundred MHz.

Pay attention to the following points for proper selection of magnetic beads:
1. What is the frequency range of unwanted signals;
2. Who is the noise source;
3. How much noise is needed;
4. What are the environmental conditions (temperature, DC voltage, and structural strength );
5. What is the circuit and load impedance;
6. Whether there is space to place magnetic beads on PCB;

The first three pieces can be determined by observing the impedance frequency curve provided by the manufacturer. The three curves in the impedance curve are very important, namely resistance, inductive resistance and total impedance. The total impedance is described by zr22 π Fl () 2 +: = FL. Through this curve, choose the magnetic beads with the highest impedance within the desired frequency range and the signal attenuation below low frequency and DC as small as possible. The impedance of the chip magnetic beads is affected when the DC voltage is too high. In addition, if the operating temperature is too high or the external magnetic field is too large, the impedance of the magnetic beads will be adversely affected.

The reason for the use of chip magnetic beads and chip Inductance: whether the use of chip magnetic beads or chip inductance is still applied.Chip Inductor must be used in the resonant circuit. The use of chip magnetic beads is the best choice to eliminate unwanted EMI noise.

application of chip magnetic beads and Chip Inductor:
Chip Inductor: RF and wireless communication, information technology equipment, radar detector, automobile, cellular phone, pager, audio equipment, PDAs (Personal Digital Assistant), wireless remote control system, and low-voltage power supply module.
chip magnetic beads: Clock generation circuit, filtering between analog circuit and digital circuit, I/O input/output internal connectors (such as serial port, parallel port, keyboard, mouse, long distance Telecom, local Area Network (LAN), RF circuit and vulnerable logical equipment, filtering out high-frequency conduction interference, computers, machines, video recorders (VCRs) in the power supply circuit ), emi noise suppression in television systems and mobile phones.