You may have heard of “EMI suppression” or “RF antenna” sheets, which are thin, flexible magnetic shields. Maybe they have been seen laying around the laboratory, typically black or gray in color, but sometimes they’re exotic and silver-looking and appear to have come off the mysterious aircraft that crashed in Roswell, New Mexico in 1947. You may have physically touched them, and/or maybe actually used them, but do we really know how they work or if the best material was used?
It all starts with magnetic materials. Those thin, flexible magnetic sheets are utilized in a variety of applications: to suppress unwanted EMI signals (for both radiated and susceptibility); for magnetic-field (H) shaping and directionality in NFC and RFID applications; to provide shielding and coil optimization in both magnetic induction and magnetic resonance wireless power applications; to lessen eddy-current losses of other resonance applications; to being used as an ESD protection device; and more.
许多供应商拥有各种材料，这些材料通过不同的数据表和数据曲线记录。但是，如果没有材料科学或丰富经验的深刻背景，选择合适的材料就会成为一个挑战。每个应用程序可能具有不同的关键参数集，例如需要减弱哪些频率或需要最低损失的频率？
磁性板配有金属粉末，铁氧体，聚合物和金属化的材料，每种供应商的材料都是独一无二的。每个供应商还可以以各种方式具有其材料优势和目标应用程序的数据，因此有时会使预期的用户留出许多问题。
Magnetic shields are considered by many as a “band-aid” for EMI suppression applications; that is, only used, maybe temporarily as a quick fix when all other solutions have been exhausted. The fact is, there are times when traditional solutions don’t work or would take too long to implement and that the “band-aid” may be the most cost-effective solution. There are also applications where magnetic shields must be used.
This article aims to help educate the reader and make the selection process easier and more efficient specifically for EMI suppression issues (due to limited space).
其背后的基本面
For EMI suppression (i.e. attenuating unwanted signals as compared to NFC/RFID applications where optimizing wanted signals and frequencies is key), the following equation can be used to help understand:
µ = µ' - jµ”（1）
其中µ'是材料的渗透性（因此与电感相关），并且µ”与材料的损失（电阻）有关，并且是材料晶粒水平的频率和相移的结果。超过频率，这两个参数可能会（将）根据材料而更改。
For EMI suppression applications:
µ’=较高的产率导致通过H场遏制或吸收获得更好的屏蔽性能
µ” = higher yields lead to better noise suppression/attenuation through material losses
Permeability defines the ability of a material to support the formation of a magnetic field within itself and contain magnetic flux. Therefore, containing more magnetic flux enables the higher µ’ material to keep more of the unwanted radiated magnetic field within the magnetic material and away from sensitive areas. When µ” is at higher values, a more resistive path is created for any magnetic field passing through the material, and it allows for the unwanted magnetic field noise to be absorbed and converted to heat. Each unique material composition, density and inner grain size and shape will impact both µ’ and µ”.
The quality factor (Q) of the material, a figure of merit that describes the potential to store energy relative to the loss within the material, is related to both µ’ and µ”and is given by:
Q = µ’/µ” (2)
For EMI suppression applications, one typically selects a low-Q material that’s lossy at the problem frequencies and thus increases the attenuation at those frequencies. This contrasts with NFC/RFID and other similar resonant low-power communication systems, where the emphasis is on increasing Q. Therefore, it’s critical to obtain the µ’, µ” curve to select the best material. Examples of material curves are shown in图1。
从这些曲线一组可以看出，材料的行为从低损失，在1 MHz范围的高Q到高损耗（µ”），从2-至4-开始开始，低Q（因此高衰减）MHz范围，最多继续2到3-GHz范围。通常，不需要这种广泛的衰减；因此，用户可以专注于在每个问题频率上选择最佳材料。
E & H Fields
Another key area are the impacts of the magnetic (H) and electric (E) field strengths. The magnetic field strength for an inductor (a common EMI generator in switch-mode power supplies—SMPS—and just one of many equations) is given as:
H∝N×1 (3)
其中n =电源磁体内使用的线圈模式的转弯数，而i =通过线圈（a）的电流。
Typically, the number of turns within an inductor isn’t provided by suppliers, as is commonly done for transformers. Thus, the user doesn’t always know the true H field strength. For power applications that use higher currents, these create both stronger predictable H fields and stronger, potentially spurious H fields. In turn, it creates more magnetic flux (density given as “B” in a typical B-H curve) subjected upon the magnetic shield.
The stronger the magnetic field, the better the shielding needs to be. This can be achieved by either increasing µ’ in Equation 1 to contain more magnetic flux or by increasing the physical thickness of the magnetic shield. This relationship is given in the following figure of merit:
µ’ × t (4)
where µ’ = permeability and t = thickness of the magnetic sheet.
In图1, the curve in red, labeled as IFL16, which has the highest attenuation, also possesses the highest permeability (µ’ = 220). This enables the material to better contain the flux energy. The performance impact of shield thickness is highlighted inFigures 2a and 2b。
InFigures 2a and 2b, “-200,” “-100,” “-050,” and “-025” are the respective thicknesses in microns (µm) so that, for example, “-100” is 100-µm thickness. Additional thickness also means more magnetic material mass is present and can support higher levels of magnetic flux. In certain EMI suppression applications, attention needs to be paid to the magnetic-flux-density (Bs) value of the material. One must ensure that the magnetic material doesn’t saturate and lose its shielding effectiveness through diminished attenuation performance and thus allow some level of the noise energy to pass through.
对于E场，可以将电场强度指定为：
e = f/q = kq/d2= v/d（5）
其中e =电场（矢量）强度（n/c）;f =电力（n）;Q =电荷量（C）;k =静电（库仑）常数（8.99 x 109Nm2/C2); D = distance between charge source and point of reference (m); and V = voltage (V) across distance (D).
It’s then apparent that the higher the voltage, the stronger the electrical-field strength. One of the key applications for EMI suppression sheets is to reduce the voltage induced on other components in proximity to the voltage source.
Why two sets of attenuation curves inFigures 2a and 2b？在试图抑制（保护）噪声的应用中，从PCB，FPC，功能区电缆或其他信号路径介质上产生的噪声，将磁性板直接应用于这些导电路径的顶部。
所示的曲线Figure 2aare measuring the impedance (/Z/) of the transmission line. Since adding a magnetic shield increases the inductance (L), the inductive reactance (XL=2πfl）随着频率的增加而增加，并成为整体阻抗的较大部分。因此，衰减一直在增加，直到1和2 GHz之间的共振频率，如图所示Figure 2afor the red curve (labeled IFL10M-200). This is an alternative option to placing a choke inductor, ferrite bead, or line filter directly in the signal path, which adds direct-current (dc) resistance that may not be desirable.
所示的接近衰减值Figure 2bare those of a “shield” application. The magnetic sheets are used to block and/or absorb unwanted EMI signals and provide isolation. This approach doesn’t need to be directly adjacent to the area needing protection.
Absorption vs. Reflection
当对EMI抑制屏蔽厚度上有物理限制时，需要决定是否牺牲屏蔽性能（使用较薄的屏蔽），考虑其他具有更好性能的材料，或更改为其他屏蔽方法（例如，使用金属盖来屏蔽）。
如上所述，使用较薄的盾牌将减少衰减，并允许更高水平的不需要的EMI噪声通过。这是一个问题吗？这取决于噪声能量的频率和水平。减轻更多DB可能是通过调节限制与不合规的差异。
允许更多噪声能量通过的另一个问题是，是否在磁场的另一侧有金属 - 能量可以在金属表面上引入涡流损失并产生热问题。对于SMPS Power应用程序尤其如此。这些不仅处于基本切换频率和谐波上，而且还处于FET（开关）打开/关闭响铃时频率更高的数量级。
For newer power switch technologies used in much higher frequency and voltage (and E field) applications (think GaN and SiC switches), the potential ringing is at a much higher level, and for a longer period. When saturation isn’t an issue, but higher attenuation is still needed, then the designer would need to focus more on higher loss material (higher µ”) rather than higher µ’.
Frequency has a major impact on the material to be used, and how it needs to be used. InFigure 2b，所有绘制的材料在1 GHz或附近完全失去了其衰减特性。如果EMI噪声问题高于1 GHz，则可能被迫使用混合材料或其他一些专业材料来实现高频噪声的衰减。在1 GHz以下，设计师需要查看提供的数据（或类似的数据）图1和2。
成本也成为一个问题，需要考虑。显然，材料越厚，成本越高。当进入出色的表现材料时，也可能是这种情况。高端，异国，混合材料都需要价格溢价。有时，用户别无选择，只能使用最佳性能材料，而成本成为较少的问题。
金属罐和杂种
替代方法也是一种可能性。金属“罐”是一种在电路板和/或具有高EMI易感性的组件或可能是噪声发生器的电路板和/或组件上放置的常见噪声抑制技术。尽管某些不需要的噪音被金属吸收，但实际上很大一部分反映了。从那里到达的地方需要更彻底的分析（和文章空间）才能充分理解。
磁性板的主要优点是，它们实际上吸收了噪声能量并在热量方面将其从电路和/或环境中删除，而金属罐无法做到。因此，对于将内部噪声产生组件（辐射噪声）放在内部噪声中的金属型应用程序，在金属内部添加磁性EMI抑制可以进一步抑制EMI噪声。
在某些情况下，只有足够的厚度可将EMI噪声降低到所需的水平，仅使用磁性屏蔽。在这种情况下，混合材料是一个不错的选择。这些是背面具有金属层的磁性板。该方法通过磁层（入射在金属背部，然后是反射信号）中产生两次EMI噪声。这对衰减具有两倍的影响，以及具有金属层创建的法拉第的“笼子”的优势，这也增加了衰减。金属罐和铁氧体层显示在图3a和3b。
In a typical SMPS, noise can be generated not only by the aforementioned inductor and switches, but also the IC, capacitors, and other circuit components. With just a metal cover (can), the noise energy generated is free to be reflected off the metal and subsequently re-introduced back into the circuitry, and it can add to the original noise source. Adding a magnetic shield to the metal shield assists in completing a better EMI suppression solution. In this case, both µ’ and µ” need to be considered and will be dictated by magnetic shield thickness.
There are cases when the individual values of both µ’ and µ” for low-Q material are important. One example is in wireless magnetic-resonance applications where EMI suppression is required, and stable inductance is critical. If a high µ’ EMI suppression material is used to achieve better shielding performance, and is close the power coil, due to magnetic coupling, the permeability of the shielding material canincreasethe inductance value of the resonance coil. As a result, it may shift the resonance frequency and decrease performance. In this scenario, having a low µ’ value but a much higher µ” would be desired.
Another similar resonant power application is when a metal enclosure is used to prevent EMI emissions. If the magnetic field (flux) goes into the metal, this will cause a decrease of magnetic flux lines in the field and减少电感最终使共振频率更高。在这种情况下，用户应靶向更高的µ'材料，以限制对所需的磁场的影响，而不是通过高µ”值来衰减该场。
现实世界应用
A Huawei P20 Pro handset was tested. This handset incorporates a metal shield that protects the address and data bus lines controlling the phone’s display(Fig. 4a)。黄色概述的区域显示了现有金属屏蔽的大小和位置。
选择了测试区域(图4 b)and with the existing solution removed, a baseline was established using a near-field EMI tester. It was then compared to:
- The existing solution
- 混合铁氧体 +铜溶液
- A permalloy material ungrounded solution
- 接地的薄胶材料溶液
The measured E-field (dBµV) level results are shown in图5a至5e。
从这些数据中，一些关键点变得明显。首先，磁性材料确实会影响可测量的E场值，从而影响衰减水平。其次，拥有金属层的确有助于衰减EMI噪声，无论是使用铁氧体还是粘合色磁性材料。最后，能够进一步磨碎盾牌可以改善屏蔽性能。
The permalloy shields used inFigures 5d and 5e, were substantially thinner, in the range of 20 µm, than all the other approaches which were in the 50- to 100-µm range. Additionally, this data is for a static frequency. When one views the attenuation over frequency, then it becomes even more obvious that many factors impact EMI noise suppression. Therefore, the choice of the magnetic material needs to be optimized for each unique application. This is shown for the two permalloy solutions in图6。
In this specific case, the noise suppression is greatly improved in the 55- to 80-MHz range. By simply grounding the magnetic shield, there’s roughly a 5-dBµV noise improvement in this range. At the peak noise frequency, around 22 MHz, there is a 2-dBµV improvement. More steps should be taken to improve on this if the compliance standard requires additional attenuation. Not shown is that the no-shield and existing-shield values ofFigures 5a and 5bhad their 67.6 dBµV and 59.0 dBµV respective noise peaks also occur at 22 MHz, which have been reduced down to 52.9 dBµV utilizing a permalloy material grounded shield.
Temperature Impact
在温度、磁垫的行为erial may change. For automotive, industrial, and other similar high-temperature applications, special consideration needs to be given to the magnetic shield’s properties. Three areas of importance are µ’, Bs, and the Curie temperature (Tc). µ’ tends to slowly increase with temperature. However, the magnetic-flux-density value tends to decrease with temperature, which will impact how much magnetic flux can be contained within the magnetic material. If the Bs value is substantially reduced, then EMI suppression performance will be altered.
如果在高环境环境中使用磁性屏蔽，则TC可能是至关重要的。总屏蔽温度将是由于衰减过程中吸收而引起的环境加上自温度的上升。如果超过库丽温度，则可以永久丢失材料的磁性，并且性能将受到严重影响。
Each material’s temperature dependency is unique, and the user will need to obtain attenuation versus temperature information for any magnetic-sheet candidate based on the actual temperature environment. The user also needs to ensure the material’s operating temperature is high enough to ensure the actual temperature doesn’t approach the Curie temperature.
Other Considerations
Many times, designers prefer to put the EMI suppression solution right over or directly attached to some portion of the circuitry. This has been a “norm” for magnetic sheets as they’re typically made of non-conductive materials (e.g. ferrite) or have a non-conductive surface. With metal cans, there has always been a clearly understood “stay clear” area and height-clearance requirement.
Now with specialty hybrid or metallized magnetic sheets, the designer must give attention to ensuring that electrical shorts aren’t created by placing the EMI suppression sheet, with its conductive surface, and bridging across various components and causing failures. Many suppliers will use a non-conductive resin layer or offer alternative magnetic-shield layer stack-ups to ensure no conductive surface is exposed（图7a和7b）。
如果设计师需要与盾牌进行电气接触，则可以选择这种方法，尤其是用于接地配置。如果要直接将盾牌放在组件的顶部或放置在金属罐的底部，则在上面讨论的添加的EMI抑制屏蔽层可能没有足够的间隙。因此，侮辱性的层盾牌可用于提供保护。
Conclusion
每个应用程序的EMI问题都是唯一的。上次有效的方法可能对下一个设计不起作用。有这么多不同的磁性材料可供选择，了解EMI问题的频率和噪声水平至关重要，并且了解材料的参数如何影响噪声抑制以符合调节限制。
Key areas addressed were:
- Guidelines of choosing a lossy material rather than high Q material typically used for NFC/RFID applications
- Both a material’s µ’ and µ” values can be the key element in determining the right material.
- 屏蔽材料，厚度和金属化层的影响
- In the real-world example provided, it was shown that using a ferrite + metal layer hybrid material improves attenuation and even more improvement can be realized by grounding the shield.
- 通常，具有金属化层将提供一定程度的改善衰减，并且随着频率接近GHz范围而变得更加有益。
永久磁铁,直流磁通,can pre-saturate thin magnetic shields. Therefore, if the end product contains them, they can reduce performance if not kept some distance away. Permittivity, an important magnetic material parameter, wasn’t addressed due to this being more critical for antenna performance rather than EMI suppression, but still impacts EMI shielding performance. The author hopes that, as a minimum, the reader now has a better understanding of magnetic material behavior and a better starting point in combatting their EMI woes.
阅读更多这样的文章TechXchange:研究EMI，EMC和噪音
Chris T. Burket is Senior Marketing Engineer forTDK Corporation of America。
