Understanding the EMI Suppression Device "Ferrite"
About ferrite
Ferrite is a material containing iron oxide and a certain amount of nickel, zinc, manganese and other elements, which has the characteristics of high permeability and low magnetic saturation induction intensity; When inductive devices cannot be used for high frequencies, ferrite beads or magnetic rings can be used, and ferrite devices and applications are shown in the figure below.
Ferrite beads are typically used at high frequencies, where they are reactive and frequency-dependent. At low frequency, the inductance is small and the line loss is small; Ferrite beads are high-frequency attenuators of RF energy.
These ferrite devices are absorption filters and consist of dissipative components. In the stopband, the lossy device absorbs the energy of the electromagnetic disturbance and converts it into heat loss, thus acting as a filter. Ferrite material is a widely used consumable device that can be used to form a low-pass filter.
Ferrite is a ferromagnetic material with a cubic lattice structure. Its manufacturing process and mechanical properties are similar to those of ceramics, and the color is black gray, so it is also known as black magnetic porcelain. The molecular structure of ferrite is MO· Fe2O3, where MO is a metal oxide, usually MnO or ZnO. In addition, according to different applicable frequency ranges, ferrites are divided into medium frequency band 20~150KHz, medium and high frequency band 100~500KHz and ultra-high frequency band 500~1000KHz.
Second, the characteristics of ferrite
Although the impedance of the inductor formed by the wire passing through the ferrite core increases with frequency, the mechanism is completely different at different frequencies.
As shown in the figure below, the impedance of a ferrite consists of its resistance R and its inductance X.
In the low frequency band, the impedance is made up of the inductance's inductive reactance. At this time, the permeability of the core is high, the inductance is large, the loss of the core is small, and the whole device is an inductor with low loss and high Q characteristics, which is easy to cause resonance. Therefore, in the low frequency band, there is sometimes an increase in interference.
In the high frequency band, the impedance is made up of resistive components. As the frequency increases, the permeability of the core decreases, the inductance of the inductor decreases, and the reactance component decreases. However, at this time, the loss of the core increases, and the resistance component increases, resulting in an increase in the total impedance.
When a high-frequency signal passes through the ferrite, the electromagnetic energy is dissipated in the form of heat.
The equivalent circuit of ferrite is different at low and high frequencies
It is an inductor at low frequencies and a resistor that varies with frequency at high frequencies.
There is an essential difference between an inductor and a resistor, the inductor itself does not consume energy, but only stores energy.
As a result, the inductor and the capacitor in the circuit form a resonant circuit, increasing the interference at certain frequencies. Resistors dissipate energy, which in turn substantially reduces interference.
When an electric current flows through the wire of the ferrite, a magnetic field is generated in the ferrite core, and when the strength of the magnetic field exceeds a certain value, the core is saturated, the permeability decreases sharply, and the inductance decreases. Therefore, when a large current flows through the filter, the low-frequency insertion loss of the filter changes. At high frequencies, the permeability of the core is already low, and at high frequencies, the loss characteristics of the main core work, and the current has little effect on the high-frequency characteristics of the filter.
In fact, ferrite beads are better explained by the parallel connection of inductance and resistance. At low frequencies, the inductor shorts the resistor; At high frequencies, the inductive reactance is so high that current can only flow through the resistor. Ferrite materials with different permeability are selected according to the frequency of interference suppression. The higher the permeability of the ferrite material, the greater the low-frequency impedance and the smaller the high-frequency impedance.
In addition, ferrite materials with high permeability generally have a higher dielectric constant, and when the conductor passes through, the parasitic capacitance formed is larger, which also reduces the high-frequency impedance.
3. Application of ferrite
Different applications have different requirements for the properties of the ferrite material and the shape of the ferrite core.
The application of ferrite is mainly in the following three aspects.
Low-level signal applications: The properties of the required ferrite material are determined by the permeability, which requires not only low losses in the ferrite core, but also good magnetic stability, i.e. it does not change much with time and temperature. Ferrite applications in this area include high-Q inductors, common-mode inductors and broadband, matched pulse transformers, radio receiving antennas, and active and passive antennas.
Power conversion and filtering: ferrite materials are required to have high magnetic flux density and low loss at operating frequency and temperature. Applications in this area include switching power supplies, magnetic amplifiers, DC-DC converters, power line filters, trigger coils, and transformers for switching power supplies.
Suppression of electromagnetic disturbances: The greatest influence on the performance of ferrite is the permeability of the ferrite material, which is directly proportional to the impedance of the ferrite core.
Ferrites generally suppress unwanted conduction or radiated signals in 3 ways.
First, the most common application is the use of ferrite cores directly on the leads of components or board-level circuitry. In this application, the ferrite core suppresses any parasitic oscillations, attenuation induction, and high-frequency unwanted signals transmitted to or connected to the component leads.
Secondly, the ferrite is used as an inductor to form a low-pass filter, which provides inductive and capacitive paths at low frequencies and large losses at higher frequencies.
Finally, less commonly used applications are the use of ferrite as an actual shield to isolate conductors, components, or circuits from scattered electromagnetic fields in the environment.
In the first two applications, the ferrite core suppresses conducted disturbance by eliminating or greatly attenuating the high-frequency currents of the source of electromagnetic disturbance.
Ferrites are used to provide high enough high frequency impedance to reduce high frequency currents. Theoretically, an ideal ferrite would provide high impedance in the high frequency band and zero impedance in all other frequency bands. But in fact, the impedance of the ferrite core is frequency-dependent, when the frequency is lower than 1MHz, its impedance is the lowest, for different ferrite materials, the highest impedance appears between 10~500MHz.