Analysis of shielding technology in electromagnetic compatibility (EMC)

Analysis of shielding technology in electromagnetic compatibility (EMC)


The shielding of electric field, magnetic field and electromagnetic field is actually different!
The shielding problem of magnetic field is a problem with both practical and theoretical significance. According to different conditions, the shielding of electromagnetic field can be divided into three cases: electrostatic shielding, static magneto-shielding and electromagnetic shielding. These three cases are qualitatively different and have internal connections and cannot be confused.

Electrostatic shielding

In the state of electrostatic equilibrium, whether it is a hollow conductor or a solid conductor; no matter how much the conductor itself is charged, or whether the conductor is in an external electric field, it must be an equipotential body, and its internal field strength is zero. This is the theoretical basis of electrostatic shielding. Because the electric field in a closed conductor shell has typical and practical significance, we take the electric field in a closed conductor shell as an example to discuss electrostatic shielding.

(i) The electric field inside a closed conductor shell is not affected by the charge or electric field outside the shell.

If there is no charged body inside the shell and there is a charge q outside the shell, electrostatic induction will charge the outer wall of the shell. When the electrostatic equilibrium is reached, there is no electric field inside the shell. This does not mean that the charge outside the shell does not generate an electric field inside the shell. Since the outer wall of the shell induces charges of opposite signs, the combined field strength generated by them and q at any point in the space inside the shell is zero. Therefore, the inside of the conductor shell will not be affected by the charge q or other electric field outside the shell. The induced charge on the outer wall of the shell plays an automatic adjustment role. If If the shell of the above-mentioned cavity conductor is grounded, the induced positive charge on the shell will flow into the ground along the grounding line. After the electrostatic balance, the cavity conductor is equipotential with the earth, and the field strength in the cavity is still zero. If there is a charge in the cavity, the cavity conductor is still equipotential with the ground, and there is no electric field in the conductor. At this time, there is an electric field in the cavity because there are induced charges of opposite signs on the inner wall of the cavity. This electric field is generated by the charges inside the shell, and the charges outside the shell still have no effect on the electric field inside the shell.

From the above discussion, it can be seen that the internal electric field of the closed conductor shell is not affected by the charges outside the shell regardless of whether it is grounded or not.

(II) The external electric field of a grounded closed conductor shell is not affected by the charge inside the shell.

If there is a charge q in the cavity inside the shell, due to electrostatic induction, the inner wall of the shell carries an equal amount of opposite-sign charge, and the outer wall of the shell carries an equal amount of same-sign charge. There is an electric field in the space outside the shell. This electric field can be said to be indirectly generated by the charge q inside the shell. It can also be said to be directly generated by the induced charge outside the shell. However, if the outer shell is grounded, the charge outside the shell will disappear, and the electric field generated by the charge q inside the shell and the induced charge on the inner wall outside the shell is zero. It can be seen that if the charge inside the shell is to have no effect on the electric field outside the shell, the outer shell must be grounded. This is different from the first case.

It should also be noted here:

① We say that grounding will eliminate the charge outside the shell, but it does not mean that the outer wall of the shell must be uncharged in any case. If there is a charged body outside the shell, the outer wall of the shell may still be charged, regardless of whether there is a charge inside the shell.

② In practical applications, the metal shell does not have to be strictly and completely closed. Using a metal mesh cover instead of a metal shell can also achieve a similar electrostatic shielding effect, although this shielding is not complete and thorough.

③ When the electrostatic balance is reached, there is no charge flowing in the grounding wire. However, if the charge inside the shielded shell changes with time, or the charge of the charged body near the shell changes with time, there will be current in the grounding wire. The shielding cover may also have residual charge, and the shielding effect will be incomplete and incomplete.

In short, whether the closed conductor shell is grounded or not, the internal electric field is not affected by the charge and electric field outside the shell; the electric field outside the grounded closed conductor shell is not affected by the charge inside the shell. This phenomenon is called electrostatic shielding. Electrostatic shielding has two meanings:

one is the practical meaning: the shielding prevents the instrument or working environment inside the metal conductor shell from being affected by the external electric field. It also does not affect the external electric field. In order to avoid interference, some electronic devices or measuring equipment must implement electrostatic shielding, such as grounded metal covers or denser metal mesh covers on indoor high-voltage equipment, and metal tube shells for electron tubes. Another example is the power transformer for full-wave rectification or bridge rectification, which is wrapped with metal sheets or a layer of enameled wire between the primary winding and the secondary winding and grounded to achieve a shielding effect. In high-voltage live operations, workers wear equalizing suits woven with metal wire or conductive fibers to shield the human body. In electrostatic experiments, there is a vertical electric field of about 100V/m near the earth. It is necessary to eliminate the influence of this electric field on the electric To study the movement of electrons under the action of gravity alone, eE
The second is theoretical significance: indirect verification of Coulomb's law. Gauss's theorem can be derived from Coulomb's law. If the inverse square exponent in Coulomb's law is not equal to 2, Gauss's theorem cannot be obtained. On the contrary, if Gauss's theorem is proved, the correctness of Coulomb's law is proved. According to Gauss's theorem, insulating metal The field strength inside the spherical shell should be zero, which is also the conclusion of electrostatic shielding. If an instrument is used to detect whether the shielding shell is charged or not, the correctness of Gauss's theorem can be determined based on the measurement results, which verifies the correctness of Coulomb's law. The latest experimental results were completed by Williams et al. in 1971, pointing out that in the formula

F=q1q2/r2±δ, δ<(2.7±3.1)×10-16,

which shows that within the experimental accuracy that can be achieved at this stage, the inverse square relationship of Coulomb's law is strictly established. From the perspective of practical application, we can think it is correct.

Static magnetic shielding

The static magnetic field is a steady current or permanent magnet. The magnetic field generated by the body. Static magnetic shielding uses ferromagnetic materials with high magnetic permeability μ to make a shield to shield the external magnetic field. It is similar to electrostatic shielding but different.

The principle of static magnetic shielding can be explained by the concept of magnetic circuit. If the ferromagnetic material is made into a loop with a cross section as shown in Figure 7, then in the external magnetic field, most of the magnetic field is concentrated in the ferromagnetic loop. This can be analyzed by taking the ferromagnetic material and the air in the cavity as a parallel magnetic circuit. Because the magnetic permeability of ferromagnetic materials is thousands of times greater than that of air, the magnetic resistance of the cavity is much greater than that of ferromagnetic materials. Most of the magnetic induction lines of the external magnetic field will pass along the wall of the ferromagnetic material, and the magnetic lines entering the cavity will be The flux is very small. In this way, the cavity shielded by ferromagnetic materials basically has no external magnetic field, thus achieving the purpose of static magnetic shielding. The higher the magnetic permeability of the material, the thicker the tube wall, and the more significant the shielding effect. Because ferromagnetic materials with high magnetic permeability such as soft iron, silicon steel, and Permalloy are often used as shielding layers, static magnetic shielding is also called ferromagnetic shielding.

Static magnetic shielding is widely used in electronic devices. For example, the leakage magnetic flux generated by transformers or other coils will affect the movement of electrons and affect the focusing of the electron beam in oscilloscopes or picture tubes. In order to improve the quality of instruments or products, the components that generate leakage magnetic flux must be subjected to static magnetic shielding. In watches, the outer cover of the movement is made of soft iron. The thin shell can play a magnetic shielding role.

As mentioned earlier, the effect of electrostatic shielding is very good. This is because the electrical conductivity of metal conductors is more than ten orders of magnitude greater than that of air, while the difference in magnetic permeability between ferromagnetic materials and air is only a few orders of magnitude, usually about several thousand times greater. Therefore, there is always some leakage in static magnetic shielding. In order to achieve better shielding effect, multi-layer shielding can be used to shield the residual magnetic flux leaking into the cavity again and again. Therefore, magnetic shielding with good effect is generally bulky. However, if you want to create an absolute "static magnetic vacuum", you can use the Meissner effect of superconductors. That is, a superconductor is placed in an external magnetic field, and the magnetic field in its body The induction intensity B is always zero. Superconductors are completely antimagnetic and have the most ideal static magneto-shielding effect, but they are not yet widely used.

Electromagnetic shielding

When the electromagnetic field propagates in a conductive medium, the amplitude of its field quantity (E and H) decays exponentially with the increase of distance. From the energy point of view, electromagnetic waves have energy loss when propagating in a conductive medium, so it manifests as a decrease in the amplitude of the field quantity. The field quantity on the surface of the conductor is the largest, and the deeper it goes into the conductor, the smaller the field quantity. This phenomenon is also called the skin effect. The skin effect can be used to prevent high-frequency electromagnetic waves from penetrating into good conductors to create electromagnetic shielding devices. It is more universal than electrostatic and static magneto-shielding.

Electromagnetic shielding is an effective means to suppress interference, enhance equipment reliability and improve product quality. Reasonable use of electromagnetic shielding can suppress interference from external high-frequency electromagnetic waves and avoid affecting other equipment as an interference source. For example, in a radio, a hollow aluminum shell is used to cover the outside of the coil to prevent it from being interfered by the external time-varying field and thus avoid noise. The same is true for audio feed lines using shielded wires. The oscilloscope is wrapped with iron sheets to prevent stray electromagnetic fields from affecting the scanning of electron beams. The high-frequency electromagnetic waves generated by the components or equipment inside the metal shielding shell cannot penetrate the metal shell and will not affect external equipment.

What material is used for electromagnetic shielding? Because electromagnetic waves decay quickly in good conductors, the thickness from the conductor surface to 1/e (about 36.8%) of the surface value is called the skin thickness (also known as the penetration depth), represented by d, and there is electromagnetic shielding. When the electromagnetic field propagates in a conductive medium, the amplitude of its field quantity (E and H) decays exponentially with increasing distance. From the energy point of view, electromagnetic waves lose energy when propagating in a conductive medium, so it manifests as a decrease in the amplitude of the field quantity. The field quantity is the largest on the surface of the conductor, and the deeper it goes into the conductor, the smaller the field quantity. This phenomenon is also called the skin effect. The skin effect can be used to prevent high-frequency electromagnetic waves from penetrating into good conductors and to create an electromagnetic shielding device. It is more universal than electrostatic and static magnetic shielding.

Electromagnetic shielding is an effective means to suppress interference, enhance equipment reliability and improve product quality. Reasonable use of electromagnetic shielding can suppress interference from external high-frequency electromagnetic waves and avoid affecting other devices as a source of interference. Equipment. For example, in a radio, a hollow aluminum shell is used to cover the outside of the coil to prevent it from being disturbed by the external time-varying field and thus avoid noise. The same is true for audio feed lines using shielded wires. The oscilloscope is wrapped with iron sheets to prevent stray electromagnetic fields from affecting the scanning of electron beams. The high-frequency electromagnetic waves generated by the components or equipment inside the metal shielding shell cannot penetrate the metal shell and will not affect external equipment.

What materials are used for electromagnetic shielding? Because electromagnetic waves decay quickly in good conductors, the thickness from the conductor surface to 1/e (about 36.8%) of the surface value is called skin thickness (also known as penetration depth), represented by d, where

μ and σ are the magnetic permeability and electrical conductivity of the shielding material, respectively. If the TV frequency f=100 MHz, for copper conductor (σ=5.8×107/?m, μ≈μo＝4π×10-7H/m), d=0.00667mm can be obtained. It can be seen that the electromagnetic shielding effect of good conductors is significant. If it is iron (σ＝107/?m), d=0.016mm. If it is aluminum (σ＝3.54×107/?m), d＝0.0085mm.

In order to obtain effective shielding, the thickness of the shielding layer must be close to the wavelength of the electromagnetic wave inside the shielding material (λ=2πd). For example, in a radio, if f＝500kHz, then in copper, d＝0.094mm(λ=0.59mm). In aluminum, d＝0.12mm(λ=0.75mm ). Therefore, using thinner copper or aluminum materials in the radio can achieve good shielding effects. Because the TV frequency is higher and the penetration depth is smaller, the required shielding layer thickness can be thinner. If the mechanical strength is considered, the necessary thickness must be maintained. At high frequencies, due to the large hysteresis loss and eddy current loss of ferromagnetic materials, the quality factor Q value of the resonant circuit decreases. Therefore, high magnetic permeability magnetic shielding is generally not used, but high conductivity materials are used for electromagnetic shielding. In electromagnetic materials, since skin current is eddy current, electromagnetic shielding is also called eddy current shielding.

At the industrial frequency (50Hz), d in copper = 9.45mm, and d in aluminum = 11.67mm. Obviously, the use of copper and aluminum is no longer suitable. If iron is used, d = 0.172 mm, ferromagnetic materials should be used at this time. Because the electromagnetic field attenuation in ferromagnetic materials is much greater than that in copper and aluminum. Because it is low frequency, there is no need to consider the Q value problem. It can be seen that under low frequency conditions, electromagnetic shielding is converted into static magnetic shielding. Electromagnetic shielding and electrostatic shielding have similarities and differences. The similarity is that both are made of metal materials with high conductivity; the difference is that electrostatic shielding can only eliminate capacitive coupling and prevent electrostatic induction, and the shielding must be grounded. Electromagnetic shielding is to make the electromagnetic field only penetrate a thin layer of the shielding body, and eliminate the interference of the electromagnetic field by eddy current. This shielding body does not need to be grounded. However, because the conductor used for electromagnetic shielding increases electrostatic coupling, it is better to ground it even if only electromagnetic shielding is performed, so that the electromagnetic shielding also plays the role of electrostatic shielding.