How much do you know about electromagnetic shielding?

Original: Engineers look at marine engineering

No matter what electronic product is, EMC is always a problem that it needs to face. EMC is Electromagnetic Compatibility, or electromagnetic compatibility. EMC is divided into EMS (electromagnetic susceptibility) electromagnetic immunity and EMI (Electromagnetic interference) electromagnetic interference. It is to evaluate the stability of the product itself, and the other is to evaluate the external noise level of the product, both of which are important indicators of product quality. This article takes a mobile phone as an example to introduce the basic principles of EMC, electrostatic surge and common solutions, which will help guide Engineer PCB layout and solve practical EMC problems.

There are interferences in everything, and there are interferences to prevent interference. There are three major directions to solve the EMC problem. Around these three major directions, a lot of solutions can be imagined. Born gossip, gossip is ever-changing. These three major directions are the interference source, the interference propagation path, and the interference receiver.

There is no love for no reason in the world, and there is no hate for no reason. Let’s start with the source of interference. According to the interference propagation path, there are two kinds of interference, spatial and conducted, and different paths have different interference sources.

Spatial interference can be divided into three categories, namely electrostatic coupling, electromagnetic coupling and electromagnetic coupling of radio waves. The noise generated by the interference source affects the receiving circuit through these three spatial paths, thereby causing system abnormalities.

1. Electrostatic coupling

Electrostatic coupling is sensitive to electric field. Generally, the voltage is large and the current is small. The simplified model is as follows: capacitive coupling is used between the interferer and the victim. Noise, this is electrostatic interference. If the victim’s impedance is high, the resulting interference will also be large, which is one of the reasons why high-impedance circuits are more likely to pick up noise.

So what are the means of alleviating the interference caused by electrostatic coupling?

Increase spacing: Reduce interference by reducing coupling capacitance.
Shorten the coupling length: Reducing the length of the parallel parts of the two traces is equivalent to reducing the parallel capacitance, thereby reducing the interference caused by the coupling capacitance.
Electrostatic shielding: metal grounding shielding, blocking interferers and victims.
Reduce the interference source voltage.
Filter at the source of the interferer.

2. Magnetic field coupling

If there is love, there will be hate. If there is capacitance, there will be inductance. The two are dual devices. Electromagnetic coupling is based on inductive coupling. The voltage is small and the current is large. When the wire flows through the current, a magnetic field will be generated, and the magnetic field will act on the victim line through mutual inductance. , resulting in interference, which is magnetic field coupling.

So what are the means to alleviate the interference caused by electromagnetic coupling?

Increase the spacing: reduce the interference by reducing the mutual inductance.
Shorten the coupling length and cross the traces vertically: Reducing the length of the parallel part of the two traces is equivalent to reducing the mutual inductance.
Electromagnetic shielding: The magnetic field is blocked by the eddy current of the metal plate, which can be ungrounded. If the metal plate is used for reflow, it is grounded.
Reduce the interference source current.
Filter at the source of the interferer.

3. Electromagnetic coupling and antenna

Electrostatic coupling and magnetic field coupling are sensitive to distance and belong to close-range interference. Increasing the distance can greatly reduce the interference. However, the interference of radio waves belongs to long-distance interference and is not very sensitive to distance.

Antennas can generate radio waves. Antennas can be divided into dipole antennas and loop antennas. As shown in the figure below, these antennas can both transmit signals and receive signals (pick up noise). Therefore, as a transmitting antenna, we need to Try to avoid interference from the antenna; for the victim device, try to avoid useless antennas in the internal design, resulting in picking up radio wave interference.

Dipole antennas are sensitive to voltage and loop antennas are sensitive to current.

4. Dipole Antenna

For dipoles, radio waves are more likely to occur when the length is 1/2 wavelength. For example, for a 750MHz signal, the speed of light after being launched into the air is 3*10^8 m/s, the wavelength is 400mm, and the wavelength Half of the antenna is 200mm, so if the antenna length is less than 200mm, it will help reduce interference;

λ=C/f, λ: wavelength; C: velocity; f: frequency

Adding an LC filter in front of the antenna can not only suppress high-frequency harmonics to reduce EMI, but also do impedance matching to achieve the best transmit power. We should also avoid separate wire ends when routing, which may become transmitting or receiving antennas.

5. Loop Antenna

Many magnetic field detection devices based on Faraday’s law of electromagnetic induction use a probe coil to pick up the magnetic field. The loop antenna can both transmit and receive radio waves, reducing the area of ​​the transmitting loop antenna is one of the effective ways to reduce interference. In our PCB layout, we should also shorten the trace length to avoid the formation of a loop antenna for transmission or reception.

6. Near Field and Far Field

Near field and far field are a pair of very important concepts, which play an important role in guiding us to optimize EMC.

The dividing line between the near field and the far field is d=λ/2π, where λ is the wavelength. When the antenna distance is less than d, it is the near field, and when it is greater than d, it is the far field.

The electric field is stronger in the near-field range of the dipole, and the electric field decays faster with distance.

The magnetic field of the loop antenna is stronger in the near field range, and the magnetic field decays faster with distance.

But whether it is a dipole antenna or a loop antenna, in the far-field range, the electric and magnetic fields decay at the same rate with distance, and the wave impedance at this time is 377Ω, which is an important parameter that will be used later.

7. Space conduction noise suppression

The noise suppression methods of electrostatic coupling and electromagnetic coupling have been introduced in the previous article, and I will not repeat them here. This part introduces the suppression of radio wave interference by shielding materials, also called electromagnetic shielding.

The shielding effect can be approximated by SE = R + A, where R is the reflection loss and A is the attenuation loss.

The reflection loss R is to use the impedance mismatch to reflect the noise to suppress the interference, which is very related to the impedance. Remember the 377Ω mentioned above? Will be used in a while.

The attenuation loss is to use the high-frequency skin effect to attenuate electromagnetic waves, which is related to the shielding material and frequency.

As mentioned above, the far-field wave impedance is 377Ω, and the shielding material such as copper plate is a high-conductivity material, and its impedance is very, very small. At 10MHZ, the inherent impedance of copper is only about 1mΩ, a difference of 300,000 times, and the impedance of iron is also very high. Small, the far-field wave impedance is very different from the impedance of the shielding material, resulting in reflection, so just looking at the reflection coefficient, the attenuation effect of 100dB can be achieved.

If a higher conductivity material is used, there will be more reflection losses and better shielding. With thicker material, there are more attenuation losses and better shielding.

The skin depth is a parameter to evaluate the strength of the skin effect. For different materials of the same size, the material with a smaller skin depth has a stronger attenuation of interference and a better effect of suppressing interference.

The picture below is the focus of this section!

Iron has a lower electrical conductivity than copper, but a higher magnetic permeability than copper. At the same frequency, iron has a smaller skin depth than copper, that is, due to the high permeability of iron, the attenuation loss is larger, and the anti-interference effect caused by the attenuation loss is better. The higher the frequency, the smaller the skin depth, so good shielding can be achieved even with very thin metal materials at high frequencies.

but!

If the frequency is very low, the skin depth is very large, and a very thick shielding material is required to suppress low-frequency interference. In this case, the shielding effect of materials such as iron with high magnetic permeability is better.

High-frequency interference shields the electric field, using thinner materials;

The low-frequency interference shields the magnetic field, and the thick glue material is used.

This article is comprehensively compiled from the popular science department of Xinhuanet, Science Popularization China-Scientific Principles Made Easy
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