Electromagnetic waves are composed of electric fields (E-fields) and magnetic fields (H-fields). Electromagnetic interference (EMI) shielding is commonly used to protect electrical components from electrically dominant waves (E-field). However, there are instances where components need to be shielded from magnetically dominant waves (H-field).
There are two types of magnetic shields: lossy shields and flux-entrapment shields. Lossy shields are used at high frequencies when the E-field is dominant, but there is still an H-field present. They work by inducing eddy currents within the material that flow in closed circular paths perpendicular to the larger external magnetic field, canceling out the H-field. The more commonly used shields for magnetic shielding purposes are flux-entrapment shields. These shields are made of a high permeability material, such as Mumetal, and surround the protected area from the magnetic source. The magnetic flux lines enter the shielding material instead of passing into the protected space. This article will discuss the design of flux-entrapment shields.
Enclosure Design Guidance for Magnetic Shielding Applications
When designing a magnetic shield, it is critical to provide a complete path for the magnetic field lines to prevent inadvertent interference. Additionally, it is important to use shields with rounded corners rather than sharp edges that contain gaps. Magnetic flux lines have difficulty turning 90 degrees, so overlapping joints should be used to provide magnetic continuity. In general, the simpler the shape, the easier it will be for the magnetic field lines to flow throughout the material.
The effectiveness of magnetic shielding can be expressed in terms of shielding factor (SF). For a spherical shell of magnetic shielding placed in a uniform magnetic field (Bo), SF can be calculated as:
SF = Bo/B = 2/3 µ (d/r)
where B is the shielded magnetic field, µ is the relative permeability of the shell material, r is the inter radius of the sphere shell, and d is the thickness of the shell.
In general, the greater the permeability and thickness of the shield material, the better the magnetic performance. Additionally, the smaller the effective radius of a shield, or relative distance from the component being shielded, the greater the performance. Therefore, it is important to ensure that the shield tightly envelops the space being shielded. Saturation can become an issue with high-strength magnetic fields, and nesting multiple shields is a popular method for protecting against saturation and increasing the level of attenuation achieved. When using multiple enclosures, the innermost shield should be made of a lower permeability, higher saturation material to reduce the strength of the magnetic field that encounters the higher permeability, lower saturation material of the outermost shield.
If you have any additional questions regarding your EMI shielding needs, please contact Leader Tech, and one of our Applications Engineers will be happy to assist.
Tong, Xingcun Colin. 2008. Advanced Materials and Design for Electromagnetic Interference Shielding. Boca Raton, FL: CRC Press.