Shielding Techniques on EMC

The importance of electromagnetic interference (EMI) shielding cannot be overstated, especially with the proliferation of electronic devices in our daily lives. As these devices become more integrated into our routines, understanding the challenges posed by electromagnetic waves (EMW) and the techniques to shield against them becomes crucial. This article delves into the various shielding methods, their applications, and their importance in maintaining EMC.

Key Takeaways:

EMC Shielding is essential for protecting sensitive electronic signals from external electromagnetic disturbances.
Electromagnetic Interference (EMI) can cause disruptions ranging from minor communication line disturbances to significant safety-critical signal interruptions.
Various materials and techniques can be employed for EMC Shielding, depending on the type of electronics and frequencies involved.
Traditional metals like copper and aluminium are commonly used for shielding.
Modern techniques involve the use of polymeric composites and modified metals.
Measurement techniques are essential to determine the effectiveness of shielding materials.
Local Shielding Techniques have been applied to modern devices like half-bridge inverter legs to reduce common-mode current noise.

Understanding Electromagnetic Interference (EMI)

What is EMI?

Electromagnetic interference (EMI) refers to the disruption caused when one electronic device interferes with another. This interference can manifest in various ways, from static on a radio to more severe disruptions, such as blackouts caused by lightning strikes on power grids.

Real-world Impacts of EMI

EMI isn’t just a theoretical concern. There have been numerous reports of blackouts caused by EMI in power grids due to lightning and disruptions to the functionality of satellites orbiting earth due to solar storms.

The Impact of EMI on Human Health and Devices

Prolonged exposure to EMWs can have adverse effects. Studies in Europe have shown that such exposure can affect infants’ brain development and, in some cases, lead to carcinogenic conditions. Moreover, medical devices such as cardiac pacemakers are known to malfunction due to EMI, which can be life-threatening.

What is EMC Shielding?

EMC Shielding refers to any method employed to protect a sensitive electronic signal from external electromagnetic disturbances. This includes preventing a potent signal from leaking out and causing interference with surrounding electronics. EMC stands for ElectroMagnetic Compatibility, indicating that a circuit has been designed to prevent disruption through shielding. Electromagnetic frequencies can influence many sensitive electronics, leading to various issues, from a simple hiss on a communication line to a complete disruption of a safety-critical signal. As such, it impacts a wide range of sectors, including consumer and industrial electronics, military, and emergency services.

How does EMC Shielding Work?

The primary objective of effective EMC Shielding is to prevent electromagnetic interference (EMI) or radio frequency interference (RFI) from affecting sensitive electronics. This protection is achieved using a metallic screen that absorbs the electromagnetic interference transmitted through the air. The shielding effect is based on the principle used in a Faraday cage. The metallic screen completely surrounds the sensitive or transmitting electronics, absorbing the transmitted signals and causing a current within the screen’s body. This current is then absorbed by a ground connection or a virtual ground plane, ensuring that the protected signal remains free of electromagnetic interference.

Materials Used for EMC Shielding

Several techniques and materials can be employed for EMC Shielding. The choice of materials depends on the type of electronics and the frequencies involved. Some common materials and methods include:

Metallic Foil or Plaited Braid: Used to shield equipment wires. Coaxial cables have this EMC shield built into their construction.
Board Level Shielding (BLS): Involves a PCB with a built-in ground plane and a metal box (shield can) placed over sensitive or transmitting elements, forming a Faraday cage arrangement.
Conductive Paints and Magnetic Materials: Used in environments where magnetic fields are below the 100Khz range.
Sheet Metal, Metal Foam, Conductive Plastics, and Mesh Metal Screening: Depending on the frequency, the shielding doesn’t have to be solid but can have regularly placed holes or even be wire fencing.

Modern Shielding Techniques

With the advancement of technology, there’s a push towards finding alternative materials for EMI shielding. Some of these include:

Polymeric Composites

These are new materials that are being researched for their potential in EMI shielding. They offer the advantage of being lightweight and moldable.

Modified Metals

By altering the properties of traditional metals, researchers aim to enhance their EMI shielding capabilities while addressing their inherent challenges.

Interaction of EMWs with Materials

When EMWs encounter a material, several interactions can occur:

Reflection

This is the most apparent interaction where EMWs bounce off the material. For effective reflection, there should be a significant difference in the impedance of the material and the medium of the EMWs.

Absorption

Here, the material takes in the EMWs, converting their energy into heat. This requires the presence of electric and/or magnetic dipoles within the material.

Multiple Reflections

In this interaction, EMWs reflect multiple times within the material, especially if the material is porous.

Measurement Techniques for EMI Shielding Effectiveness

To determine how effective a material is at shielding against EMI, various measurement techniques are employed:

Open Field Method

This method simulates real-world conditions, making it one of the most realistic methods for measuring EMI shielding effectiveness.

Shielded Box Method

Here, the transmitting and receiving antennas are placed on either side of the shielding material, with the receiving antenna enclosed in a sealed box.

Coaxial Transmission Line Method

Commonly used due to its repeatability, this method involves a sample holder where a reference and a load sample are placed for measurement.

Desired Properties in Shielding Materials

While the primary property sought from EMI shielding materials is their effectiveness, other properties are also desired based on the application:

Mechanical properties: How strong and durable is the material?
Optical properties: How does the material interact with light?
Permeability: How easily can substances pass through the material?
Morphological properties: What is the structure of the material?

Applications of EMC Shielding

EMC Shielding finds applications across various sectors:

Medical and Laboratory Equipment: Vital for disrupting and preventing signal interference in patient monitoring equipment, in-body medical devices like pacemakers, and more.
Data Protection: Prevents unauthorized access to data stored on RFID chips or embedded devices.
Military, Government, and Financial Systems: Used in combination with air-gapped systems to enhance existing security measures.

Conclusion

With the increasing reliance on electronic devices, the importance of EMI shielding continues to grow. While traditional metals remain popular due to their high effectiveness, research into alternative materials is ongoing. These new materials aim to address the shortcomings of metals while providing effective EMI shielding.

FAQ

What is the primary purpose of EMC Shielding?
- The main goal is to prevent electromagnetic interference (EMI) or radio frequency interference (RFI) from impacting sensitive electronics.
How does a Faraday cage work in EMC Shielding?
- A Faraday cage involves a metallic screen that completely surrounds the sensitive or transmitting electronics, absorbing the transmitted signals and causing a current within the screen’s body.
What materials are commonly used for EMC Shielding?
- Materials like metallic foil, plaited braid, conductive paints, magnetic materials, sheet metal, metal foam, and mesh metal screening are used based on the application’s requirements.

EMC/EMI Shielding Explained | Harwin

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References

Mathur, P., & Raman, S. (2020). Electromagnetic Interference (EMI): Measurement and Reduction Techniques.
Mariscotti, A. (2021). Critical Review of EMC Standards for the Measurement of Radiated Electromagnetic Emissions from Transit Line and Rolling Stock.
Dupo, V. B., Belista, F., de Sagun, V., Bugtai, N., Baldovino, R., Abad, A. C., & Dungao, J. (2019). Electromagnetic shielding to improve EMC of a robotic medical device.
Derkacz, P. B., Schanen, J., Jeannin, P., Chrzan, P., Musznicki, P., & Petit, M. (2022). EMI Mitigation of GaN Power Inverter Leg by Local Shielding Techniques.

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