Talk about the function of magnetic beads in circuits

EMI The problem is one of the most noteworthy problems in electronic product design . The last one is about the common mode inductance in circuit design EMI Application scenario and working principle of the problem . Of course, in addition to the common mode inductance , There are also many devices that will be used in circuits to solve EMI The problem of . Among them, magnetic beads belong to one .

brief introduction
Magnetic beads belong to EMI Squelch components . It plays the role of eliminating interference noise in the circuit . It is equivalent to resistance, inductance and capacitance in series and parallel in the circuit . Its impedance characteristics will also change with the working frequency of the circuit .

function
As mentioned above, magnetic beads belong to EMI Squelch components . Generally speaking, it is the function of eliminating interference noise . But in detail , It is mainly used to eliminate signal lines 、 High frequency noise and spikes on the power line , At the same time, it also has the ability to absorb electrostatic pulses .

for example ,RF Energy is the AC component superimposed on the DC signal . However, the DC component is a useful part , and RF Energy belongs to the part of interference . He will radiate outward along the line , Affect other electronic devices . here , We add magnetic beads , It will greatly improve the noise generated by the circuit . Its function is to eliminate the high-frequency noise interference on the signal line .

field
On the mobile phone that Xiaobian is engaged in / For the tablet field , It is mainly used in the power input port of some devices ,Camera The main clock part of the circuit and some RF In circuit .

Magnetic bead equivalent model

R1 Is the core loss The impedance is R1
R2 Is the coil impedance The impedance is R2
L1 The magnetic bead has coil and inductance The impedance is jwL
C1 There is parasitic capacitance between coils The impedance is 1/jwc

So the total impedance can be used Z=R+jX To represent the
among R It is the real part ,jX It is the imaginary part .

Therefore, there are three curves in the following figure

SZ1005G121TF For a magnetic bead we used . You can see that its horizontal axis is frequency , The longitudinal axis is impedance .
So in different frequency bands , The displayed impedance value Z Completely different .

As you can see from the diagram , When X=0 when , Its impedance reaches the maximum .
When X The corresponding impedance is greater than 0 Corresponding frequency band , Its impedance begins to increase with the increase of frequency .
When X The corresponding impedance is less than 0 Corresponding frequency band , Its impedance begins to decrease with the increase of frequency .

In the use of , We often use its impedance characteristics to eliminate noise , That is, it consumes the interference energy in the circuit , So as to convert it into other forms of energy .

Parameters
This is the same as the frequency just now - Parameters of magnetic beads corresponding to impedance diagram and similar magnetic beads

You can see , It is mainly divided into impedance , frequency , DC resistance and the maximum current allowed to pass .

The key parameters

1. impedance
   The values displayed in the impedance part are that the magnetic beads work in 100MHz The impedance of . In some places, it also means 75Ω@100MHz form . The reason is the same . The greater the impedance , The better the noise suppression . So when choosing , Please be sure to look at the frequency - Impedance curve , Observe impedance Z The frequency corresponding to the highest point . That is, the frequency corresponding to the highest point is the frequency of noise in the circuit .

2. DC resistance DCR
   It refers to DC operation . The resistance value presented by the magnetic bead , commonly DCR The smaller the better. , The attenuation of useful signals is small .

3. Maximum working current
   It refers to when the magnetic bead works , The maximum current allowed .

The selection
The application of magnetic beads is mainly divided into two scenarios ：

1. The power side
2. The signal line

How to choose in the corresponding scene , Its frequency - Impedance curve is very important
This is applied to these two scenarios , The impedance of the magnetic bead we chose at that time - Frequency curve .

The power side

Signal line end

These two graphs Both are the two magnetic beads we use in our work .

You will find the impedance of the power supply - The frequency curve belongs to “ Short and fat ” Type of . It is characterized by low impedance in all frequency bands , It plays a certain filtering role .

However, the impedance of the magnetic beads at the end of the signal line - The frequency curve belongs to “ Lanky ” Type of . Because the interference signal on the signal line is often high-frequency , And the useful signal belongs to low frequency . So the figure above shows , The impedance of magnetic beads in low frequency band is very small , Therefore, the attenuation of useful signals is small . However, the impedance is large at high frequencies , It can perfectly consume high-frequency energy .

Last , There is also a special point

When magnetic beads are applied to the power supply , Please add a capacitor at the back end .

The magnetic bead is at the power end , Equivalent to the existence of inductance . When there is disturbing noise , It shows the characteristics of inductance . Absorb the noise of interference . When the noise on the line does not exist , It releases noise , When there is no bypass capacitor , It is likely to release noise to the load , Impact on load . When a bypass capacitor is connected , It can release the noise to the ground .