Description
This article provides some basic knowledge and selection points for popular science magnetic beads.
About the magnetic
bead The magnetic bead is an inductive EMI suppression filter, which is very similar to the inductor, and the most commonly used ferrite bead (Ferrite Bead) now.
Chip Ferrite Beads
The unit of the magnetic bead is ohm, and depending on the model, it can suppress noise of several MHz to several GHz, and is often used on signal lines and power lines (used in series).
Magnetic beads are very different from capacitors and inductors in filtering noise. Capacitors mainly provide a ground impedance path (blocking direct traffic), while inductors reflect noise (blocking traffic and direct traffic). How magnetic beads filter out noise will be mentioned below .
What are the similarities and differences between magnetic beads and inductors?
The symbols of magnetic beads and inductors in the circuit are the same, but they are different devices. The unit of magnetic beads is ohm (Ω), and the unit of inductance is Henry (H).
The magnetic beads are composed of oxygen magnets, the inductance is composed of a magnetic core and a coil, the magnetic beads convert the AC signal into heat energy, and the inductance stores the AC and releases it slowly, so the inductance is energy storage, and the magnetic bead is energy conversion (consumption ) device.
We all know that there are two ways of EMI, radiation and conduction, magnetic beads and inductors can solve EMC and EMI problems, but the emphasis is different.
Magnetic beads mainly solve the problem of radiation interference. Magnetic beads are often used on signal lines. Certain high-frequency circuits such as RF, oscillator circuits, and DDR SDRAM need to add magnetic beads to the power input part.
Inductors mainly solve the problem of conduction interference. High-frequency inductors are mainly used in low- and medium-frequency filter circuits, RF matching, etc., and power inductors are mainly used in DC-DC circuits.
What are the main parameters of magnetic beads?
① Impedance |Z|: It is customary in the industry that the impedance of magnetic beads is determined by the impedance at 100MHz, so we often see 100R@100MHz in the SPEC of magnetic beads, which means that at 100MHz frequency, the impedance of magnetic beads is 100R, The greater the impedance, the better the effect of suppressing noise. Some high-frequency magnetic beads give impedance at a frequency of 1GHz.
Impedance Z Frequency Characteristics
One thing to note, see the figure above, at 100MHz, the impedance of the bead is not the largest, so when selecting the model, you can choose the model with the largest impedance near the frequency point according to the noise frequency point.
100MHz is just a standard in a sense, just a standard, and I personally think it doesn't have much practical significance, because in practical applications, as the frequency changes, the impedance of the magnetic beads will change accordingly.
Therefore, it is rare to use magnetic beads for single-point grounding, and the frequency range of noise needs to be evaluated in advance.
② DC resistance DCR: refers to the resistance value of the magnetic beads when the DC current passes through the magnetic beads. Generally, the smaller the DCR, the better, and the smaller the attenuation of useful signals.
③ Rated Current: Refers to the maximum current allowed when the magnetic beads work normally.
What is the DC overlap characteristic of magnetic beads?
We all know that when different DC voltages are applied to the capacitor, the capacitor capacity will change (you can refer to the previously shared article: Who moved my capacitor capacity?)
Beads also have the same DC superposition characteristics, and chip ferrite beads are an inductor that uses ferrite. Therefore, when a large current is passed, special attention needs to be paid to the performance change due to magnetic saturation.
Example of DC superposition characteristics of chip ferrite beads
It can be seen from the above figure that when the current passing through the magnetic bead increases, its impedance will decrease, and the decrease in impedance means that the performance of suppressing noise will deteriorate. When the current decreases, the impedance will increase again, and the performance will recover.
Therefore, when selecting a model, we must consider the two parameters of rated current and impedance.
How to understand the equivalent model of magnetic beads
I wrote the equivalent model of resistors, capacitors and inductors before "Seeing is not believing!" Practical Equivalent Models for Resistors, Capacitors, and Inductors》
The equivalent model of the magnetic bead can be simplified as an inductor connected in series with a resistor. Of course, there will be DCR and parasitic capacitance parameters, which will not be explained here.
Simplified equivalent circuit of a ferrite bead
Both inductance and resistance are functions of frequency, so the impedance of the magnetic bead is Z=R+JWL. As shown in the figure below, the frequency corresponding to the arrow is called the crossover frequency, and some are called the corner frequency.
The |Z|, R, X impedance curves of magnetic beads
Less than the crossover frequency, Z and XL are almost coincident, and the magnetic bead is inductive at this time, with small inductance, and the noise is reflected at this time; when the crossover frequency is greater than the crossover frequency, the Z and R curves are almost coincident, and the magnetic bead is resistive at this time. A large resistor that absorbs noise and converts it into heat.
The greater the crossover frequency, the wider the frequency band in which the magnetic bead presents inductance, the weaker the ability to absorb low frequencies, and the stronger the ability to absorb high frequencies. The smaller the crossover frequency, the narrower the frequency band of the magnetic bead showing inductance, the stronger the ability to absorb low frequencies, and the weaker the ability to absorb high frequencies.
Precautions for magnetic bead selection
① The frequency band of useful signal and noise is analyzed to be larger than the crossover frequency, so that the magnetic beads can absorb noise instead of reflecting noise; the frequency of the signal is smaller than the crossover frequency, so as to prevent the signal from being attenuated.
② Consider the size of DCR In the DC circuit, prevent the DCR from being too large and cause signal attenuation. For example, when a 5V 500mA power supply passes through a DCR=1R bead, the voltage will attenuate by 500mA*1R=0.5V.
In general, the larger the AC impedance, the better the noise filtering, but the DCR will also be larger, which will attenuate the useful signal, so this is a trade-off process.
③ Consider the size of the rated current. Don’t blindly choose low-rated-current ferrite beads in order to save costs. Considering the DC superimposed characteristics, as the current increases, the impedance decreases and the noise-absorbing performance will also decrease. For example, the operating current of a certain circuit is below 300mA most of the time, and the maximum will reach 400mA, but it is rare. At this time, you can choose a magnetic bead with a rated current of 400mA.