Characteristics of ferrite anti-interference magnetic core
The function of ferrite anti-interference magnetic core is equivalent to a low-pass filter, which effectively solves the high-frequency interference suppression problem of power lines, signal lines, and connectors. It also has a series of advantages such as simple, convenient, and effective use, and small space occupation. Using ferrite anti-interference magnetic core to suppress electromagnetic interference (EMI) is an economical, simple, and effective method, and has been widely used in various military or civilian electronic devices such as computers.
Ferrite is a type of material that utilizes high conductivity magnetic materials to infiltrate and aggregate one or more other metals such as magnesium, zinc, nickel, etc. at 2000 ℃. In the low frequency range, the ferrite anti-interference magnetic core exhibits very low inductive impedance values, which does not affect the transmission of useful signals on data or signal lines. In the high-frequency range, starting from around 10MHz, the impedance increases, but the inductive reactance component remains very small, while the resistive component rapidly increases. When high-frequency energy passes through the magnetic material, the resistive component converts this energy into thermal energy and dissipates it. This forms a low-pass filter that greatly attenuates high-frequency noise signals, while ignoring the impedance of low-frequency useful signals without affecting the normal operation of the circuit.
Application and Performance Requirements of Soft Magnetic Ferrite Materials
For soft magnetic ferrite, magnetic permeability is usually desired μ I and resistivity ρ To be high, the coercivity Hc and loss Pc should be low. According to different usage, there are also different requirements for the Curie temperature, temperature stability, magnetic permeability reduction coefficient, specific loss coefficient, etc. of the material.
(1) Manganese zinc based ferrite materials are divided into two categories: high permeability ferrite and high-frequency low-power ferrite (also known as power ferrite). The main characteristic of manganese zinc high permeability ferrite is its particularly high magnetic permeability,
Usually μ Materials with i ≥ 5000 are called high permeability materials, and general requirements μ I ≥ 12000.
Manganese zinc high-frequency low-power ferrite, also known as power ferrite, requires high magnetic permeability (generally required) in terms of performance for power ferrite materials μ I ≥ 2000), high Curie temperature, high apparent density, high saturation magnetic induction intensity, and ultra-low core loss at high frequencies.
(2) Nickel zinc based ferrite materials have lower performance than MnZn based ferrite materials in the low-frequency range below 1MHz. However, above 1MHz, due to its porous properties and high resistivity, NiZn based ferrite materials have significantly better performance than MnZn based soft magnetic materials in high-frequency applications. Its resistivity ρ Up to 108 Ω • m, with low high-frequency loss, especially suitable for high-frequency 1-300MHz; Moreover, the Curie temperature of NiZn based materials is higher than that of MnZn, with Bs as high as 0.5T and Hc as low as 10A/m. It is suitable for various inductors, mid cycle transformers, filter coils, and chokes. Nickel zinc high-frequency ferrite materials have a wide bandwidth and low transmission loss, and are commonly used in high-frequency electromagnetic interference resistance and surface mount devices that integrate high-frequency power and anti-interference, as electromagnetic interference (EMI) and radio frequency interference (RFI) magnetic cores. Nickel zinc power ferrite can be made into RF broadband devices to achieve energy transmission and impedance transformation of RF signals within a wide frequency range. Its lower frequency limit is several thousand hertz, while the upper frequency limit can reach several thousand megahertz; The use of nickel zinc power ferrite materials in DC to DC converters can increase the frequency of switching power supplies and further reduce the volume and weight of electronic transformers.
Common magnetic rings - Generally, there are two types of magnetic rings on the connecting line: nickel zinc ferrite magnetic rings and manganese zinc ferrite magnetic rings, each of which plays a different role.
Manganese zinc ferrite has the characteristics of high magnetic permeability and high magnetic flux density, and has lower losses at frequencies below 1MHz.
Nickel zinc ferrite has characteristics such as extremely high impedance, low magnetic permeability of less than a few hundred, and low losses at frequencies above 1MHz. The magnetic permeability of manganese zinc ferrite ranges from thousands to tens of thousands, while nickel zinc ferrite ranges from hundreds to thousands. The higher the magnetic permeability of ferrite, the greater its impedance at low frequencies and the smaller its impedance at high frequencies. Therefore, when suppressing high-frequency interference, nickel zinc ferrite should be selected; On the contrary, manganese zinc ferrite is used. Alternatively, both manganese zinc and nickel zinc ferrite can be sheathed on the same bundle of cables, which can suppress interference in a wider frequency band. The larger the difference between the inner and outer diameters of the magnetic ring, the greater the longitudinal height, and the greater its impedance. However, the inner diameter of the magnetic ring must be tightly wrapped around the cable to avoid magnetic leakage. The installation position of the magnetic ring should be as close as possible to the interference source, that is, it should be close to the inlet and outlet of the cable.
At present, the most widely used soft magnetic ferrite materials are the spinel type manganese zinc series and nickel zinc series. From an application perspective, they can be divided into several types: high permeability, high-frequency high-power (also known as power ferrite), and electromagnetic interference resistant (EMI) ferrite.
Principles of using magnetic cores
(1) The magnetic ring length is relatively good.
(2) The closer the aperture of the magnetic core and the cable it passes through, the better.
(3) When there is low-frequency interference, it is recommended to wrap the cable around 2-3 turns. When there is high-frequency interference, it is not allowed to wrap the cable around (due to the presence of distributed capacitance), and a longer magnetic ring should be used.
Selection of dimensions for ferrite suppression components:
After selecting the material, generally speaking, the larger the volume of ferrite, the better the suppression effect. When the volume is constant, the impedance of long and thin shapes is greater than that of short and thick shapes, and the suppression effect is better. The larger the cross-sectional area of ferrite, the less susceptible it is to saturation and the greater the bias current it can withstand. The smaller the internal diameter of ferrite, the better the inhibitory effect.
The application of ferrite suppression components on circuit boards:
The EMI on the circuit board mainly comes from digital circuits with periodic switches, and its high-frequency current generates a common mode voltage drop between the power line and ground wire, causing common mode interference. To suppress this interference, we add a decoupling capacitor between the power line and the ground wire to short-circuit high-frequency noise, but the decoupling capacitor usually causes high-frequency resonance and new interference. To achieve this, adding ferrite suppression components to the I/O ports on the circuit board will effectively attenuate high-frequency noise.
Application of ferrite suppression components on power lines:
Adding ferrite suppression components to the I/O ports on the power line can suppress high-frequency interference transmission and mutual interference between the power supply and the circuit board. However, it should be noted that there may be DC bias when using ferrite components on the power line. The impedance and insertion loss of ferrite decrease with the increase of DC bias, but when the bias increases to a certain value, ferrite will experience saturation phenomenon.
