With the extensive use of electronic equipment, interference is ubiquitous. Severe interference can cause electronic equipment to not work properly. Therefore, controlling interference sources and suppressing electromagnetic interference has become one of the main issues to consider when designing and applying electronic circuits. The use of shielding technology is an effective measure to resist interference, but for various situations of interference sources, if the same shielding measures are taken without analysis, not only will it not achieve satisfactory results, but it may even backfire due to improper shielding methods. This article analyzes the interference sources in detail, and then "prescribes the right medicine" and adopts appropriate shielding methods to suppress interference to the maximum extent, so as to improve the reliability of electronic equipment operation.
2 Electric field shielding
2.1 Theoretical Analysis
When the interference generated by the interference source appears in the form of voltage, there is capacitive electric field coupling between the interference source and the electronic equipment. In this case, the most effective anti-interference method is to implement electric field shielding. As shown in Figure 1 (a), the voltage of the interference source to the ground is Ux, the impedance of the electronic equipment to the ground is Zs, and the coupling capacitance between the two is C in the figure. Then the coupling interference voltage on the electronic equipment is:
Us＝jωCZsUn/(1+jωCZs) （1）
(a) Electric field shielding
(b) Metal shell shielding
Figure 1 Schematic diagram of electric field coupling and shielding
From formula (1), we can see that the magnitude of coupling interference is related to frequency. As the frequency increases, the interference increases. Therefore, the higher the frequency, the more necessary it is to use shielding, and the more obvious the shielding effect is.
If the interference source is shielded by a metal shell, as shown in Figure 1 (b), C1 is the capacitance between the interference source and the shielding shell, C2 is the capacitance between the electronic device and the shielding shell, and Zm is the impedance of the shielding shell to ground. The coupled interference voltage after shielding can be obtained as:
Us =ω2C1C2ZmZsUn/{(ω2C1C2ZmZsUn-1) -jω[(C1+C2)Zm+C2Zs]} (2)
2.2 Shielding measures
If the shielding shell is ideally grounded, that is, Zm＝0, then Us＝∞, and the coupling interference can be completely eliminated. In other words, the necessary condition for completely eliminating the above interference is that the shielding shell is well grounded. If the shielding shell is not grounded, then Zm＝∞, and C 1 >C, C 2 >C. Comparing equation (1) with equation (2), Us 2 >Us 1. It can be seen that the coupling interference after shielding cannot be suppressed, but becomes more serious.
Likewise, if the interference source is not shielded, but the electronic device is shielded, the result is similar to the shielding effect described above.
3 Magnetic field shielding
When the interference source appears in the form of current, the magnetic field generated by this current interferes with adjacent signals through mutual inductive coupling. An effective way to suppress this type of interference is to implement magnetic field shielding.
The first thing to pay attention to when shielding a magnetic field is the frequency of the interference source. This is because the shielding principle varies with the interference frequency. It involves the selection of shielding materials and the design and manufacture of the shielding shell. Without analysis, it is impossible to achieve the effect of suppressing interference.
3.1 Low frequency magnetic field shielding
3.1.1 Theoretical analysis
The low frequency referred to here is generally below 100kHz. As shown in Figure 2, assume two parallel wires 1 and 2 are close to each other. The magnetic field coupling interference of wire 1 to wire 2 is:
U 2 =jωMI 1 （3）
Figure 2 Magnetic coupling of wires
Where: M is the distributed mutual inductance between the two conductors, M = Φ/I 1 ; I 1 is the current flowing through conductor 1; Φ is the current; I 1 generates the cross-connected magnetic flux to conductor 2. In order to suppress magnetic field coupling interference, the distributed mutual inductance M should be minimized, that is, the cross-connected magnetic flux Φ between the interference source and the interfered circuit should be reduced.
3.1.2 Shielding measures
To shield this type of interference, it is recommended to use ferromagnetic materials with high magnetic permeability to make shielding shells to shield the interference source, so that the magnetic flux generated by the interference source can be guided to the ferromagnetic material, so as not to cross-connect with the interfered circuit. Similarly, the interfered circuit can also be shielded. Regarding the production of shielding shells, the following matters should be noted: the smaller the magnetic resistance Rm of the magnetic circuit of the selected material, the better Rm = L/μS (L is the length of the magnetic circuit; S is the cross-sectional area of the magnetic circuit; μ is the magnetic permeability). From the above formula, it can be seen that: iron, silicon steel sheets, Permalloy, etc. with high μ values are selected; when designing the shielding shell, the shell should be thick enough to increase S to achieve the purpose of increasing the shielding effect; no openings should be allowed in the direction perpendicular to the magnetic flux direction to avoid increasing the magnetic resistance; in order to better improve the shielding effect, multi-layer shielding is sometimes used, and attention should be paid to tightening the shielding shell during installation.
3.2 High frequency magnetic field shielding
3.2.1 Theoretical analysis
The shielding principle of high-frequency magnetic fields above 100kHz is to use the anti-magnetic field of eddy currents generated by electromagnetic induction on the surface of the shielding shell to achieve the purpose. The magnetic loss of the above-mentioned ferromagnetic materials is too large under high frequency conditions, which is not conducive to forming as large eddy currents as possible on the shielding shell, and cannot effectively eliminate the interference of high-frequency magnetic fields. Figure 3 is an equivalent circuit diagram of a shielding shell made of a good conductor shielding an electronic circuit.
Figure 3 Schematic diagram of electronic circuit shielding equivalent
In Figure 3, L is the inductance of the electronic circuit; M is the mutual inductance between the electronic circuit and the shielding shell; Ls is the inductance of the shielding shell; I is the current of the electronic circuit; and Rs is the resistance of the shielding shell. It can be concluded that the eddy current formed on the shielding shell is:
Is=jωMI/(Rs+jωLs) （4）
When the frequency is high, ωLs>>Rs, and Rs can be ignored. Then equation (4) can be simplified to
Is≈MI/Ls （5）
When the frequency is low, ωLs<<Rs, and ωLs can be ignored. Then equation (4) can be simplified to
Is≈jωMI/Rs （6）
3.2.2 Shielding measures
It can be seen from formula (4) that eddy current increases with increasing frequency, which means that conductive materials should be used for high-frequency magnetic field shielding.
It can be seen from formula (5) that in the high frequency band, the size of the eddy current is independent of the frequency, that is, after the eddy current increases to a certain extent with the increase of frequency, its shielding effect will no longer be enhanced if the frequency continues to increase.
It can be seen from formula (6) that in the low frequency band, ω is low, Is is small, and the shielding effect is poor; Rs is small, Is is large, the shielding effect is good, and the shielding loss is also small, which requires the shielding material to be a good conductor.
Due to the high-frequency skin effect, eddy currents only flow through a thin layer on the surface of the shielding shell. Therefore, when designing a high-frequency shielding shell, unlike a low-frequency shielding shell, it does not need to be made very thick, but only needs to ensure a certain mechanical strength, generally 0.2~0.8mm. For shielded wires, a multi-strand braided mesh is usually used because it has a larger surface area under the same volume.
4 Conclusion
In the use of shielding technology, in order to achieve good results, different methods should be adopted according to the actual situation of the interference source:
When the interference generated by the interference source appears in the form of voltage, electric field shielding should be adopted. The shielding shell is required to be well grounded, and the grounding resistance should be less than 2mΩ.
When the interference generated by the interference source appears in the form of electric current, magnetic field shielding should be adopted.
When the frequency of the interference source is lower than 100kHz, use ferromagnetic material with high magnetic permeability to make the shielding shell. The shielding shell should be as thick as possible, and care should be taken not to open in the direction perpendicular to the magnetic flux.
When the frequency of the interference source is higher than 100kHz, a good conductor material should be used to make the shielding shell. The thickness of the shell should only be considered to meet the mechanical strength requirements, and only 0.2~0.8mm is sufficient.
Through the above research and analysis, we can find the right remedy and handle the electromagnetic interference shielding problem with ease in the future.
References:
[1] Liu Wenyan, Zhou Xueping, Liu Hui, Modern Test System[M], Changsha: National University of Defense Technology Press, 1995
[2] Jiang Xianwang, Jia Ruilin, Zhao Yan, Electromagnetic shielding design of electronic instruments [J], Telemetry and Remote Control, 1997 (3), Vol.18 No.2: 28-32
[3] Zhang Jianjun, Song Shushan, Electromagnetic Interference Methods and Suppression Technology[J], Journal of Jinan Jiaotong College, 1998 (6), Vol.6 No.2: 25-29
[4] How to do electromagnetic shielding well [J], Today's Electronics, 2002 (8): 42-44
[5] Lai Zuwu, Electromagnetic Interference Protection and Electromagnetic Compatibility [M], Beijing: Atomic Energy Press, 1993
[6] Li Haisheng, Ye Ruizhen, A brief discussion on electromagnetic interference in measurement [J], Guangdong Electric Power, 2000 (4), Vol.13 No.2: 63-64
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