Noise-suppression sheets facilitate EMI-controlled designs


Any electronic product must pass applicable electromagnetic compatibility (EMC) tests before it can be placed in its intended market. Accepting that prevention is better than cure, it’s usually ideal to design for compliance from an early stage of development. Various approaches can be taken, from applying known best practices to using an EMC simulator, where available, and doing EMC pre-tests in-house or with a specialist partner.

Despite the best-laid plans, however, mandatory testing at an approved test house can present surprises. A solution is needed quickly; any significant redesign at such a late stage of development is expensive and causes delays. Typical approaches include placing additional low-pass filters, often using ferrite beads, at known trouble spots to reduce conducted interference or to introduce shielding to block radiated emissions and protect sensitive components.

As an alternative, composite magnetic materials are available as flexible sheets that can be trimmed and formed to block EMI signals at a specific location. These noise-suppression sheet (NSS) materials are available in various permeability ratings, which allow designers to attenuate interference in a given frequency band by selecting an appropriate value and suitable thickness. The material can be cut to an appropriate size and applied as a shield using self-adhesive backing.

The NSS materials are relatively easy to use and bypass the custom design and fabrication challenges typically encountered to produce a metal shield that must be bonded or screwed into place during final assembly. Perhaps less well known is that NSS can be formed around wires such as power lines as an elegant replacement for ferrite beads and cores by wrapping it around cables and easily secured using a convenient and production-friendly heatshrink sleeve (Figure 1).

diagram of NSS employed in designsFigure 1 Here is how engineers can apply NSS to attenuate EMI from components and cables. Source: KEMET

However, placing NSS at trouble spots as a tactical response when interference rears its head is just one of the many ways these materials can be used. NSS can also be a strategic ally when designed into the product at an early stage. In addition to helping ensure EMC compliance, it can also be used to enhance various aspects of the system’s performance, such as energy efficiency and electrostatic discharges (ESD) protection.

So, examining the structure and properties of NSS can help designers understand its versatility and support for a wide range of applications.

NSS composition and properties

NSS is a composite magnetic material made by blending micron-sized magnetic material powders in a polymer base (Figure 2).

diagram of NSS composite magnetic material structureFigure 2 NSS is a composite magnetic material. Source: KEMET

The material has complex permeability (μ) comprising two components, μ and μ. The value of μ determines how well the material can support magnetic flux, whereas μ expresses the noise absorption effectiveness.

Mathematically, it’s expressed as follow:

μ = μ – j μ

Here, μ and μ are analogous to inductive and resistive properties, respectively. As the signal frequency increases, μ reaches a threshold and begins to fall rapidly, while μ rises (Figure 3).

graph of NSS permeability ratingsFigure 3 The NSS materials have complex permeability ratings. Source: KEMET

By carefully controlling these properties, KEMET has created the Flex Suppressor NSS family, which features characteristics for attenuating noise signals and sustaining magnetic flux in various frequency bands from 1 MHz to 40 GHz (Figure 4). They are used in applications from consumer electronics and automotive infotainment to super-high frequency (SHF) equipment such as 5G infrastructure.

graph of NSS attenuation characteristicsFigure 4 The NSS materials can attenuate unwanted noise in various frequency ranges. Source: KEMET

NSS for noise absorption

Acknowledging that design for EMC should be considered properly at the beginning of any electronic design project, NSS can be considered as part of the solution from the outset. Moreover, in addition to preventing unwanted interactions with nearby equipment, it’s also important to prevent the system from interfering with itself.

Any system can contain numerous interference sources, such as reflection of signals from the inside of the casing or openings like the screen or speaker aperture, and noise radiated from ICs or cables. In a multi-board assembly, preventing crosstalk between substrates is also important. Placing filters in-circuit at multiple points, and introducing shielding to handle various noise signals can complicate the design and add to the bill of materials. Alternatively, applying one or several individual pieces of NSS can be faster and simpler. No board real-estate, grounding, or soldered components such as L-C filters are required.

Figure 5 shows NSS used for de-sensing receiver circuitry in wireless devices, such as mobiles, IoT nodes, and gateways, and remote controls to ensure reliable communication and optimal range. In this way, effective use of NSS can lower RF transmitter power requirements and ease receiver design, delivering advantages such as low power consumption, long battery life, and small size.

diagram shows how NSS improves de-sensing of an RF receiver for a laptopFigure 5 The use of NSS improves de-sensing of an RF receiver. Source: KEMET

Moreover, NSS can be applied as shown in Figure 6 to protect circuitry against ESD that can cause system components such as controllers and line drivers to malfunction.

diagram showing how NSS can abe used for protection against ESDFigure 6 NSS can also be used for protection against ESD. Source: KEMET

Optimizing μ for flux shaping

NSS formulas such as the Flex Suppressor EFW series have been optimized to boost electromagnetic coupling between transmitters and receivers (Figure 7). Designers can thus enhance the performance of wireless power transfer (WPT) systems to ensure faster charging and increased energy efficiency resulting in a lower cost of ownership.

diagram showing how NSS can boost WPT efficiencyFigure 7 Careful placement of NSS can boost WPT efficiency. Source: KEMET

Flex Suppressor can also be used effectively in RFID systems to improve coupling of the reader’s electromagnetic energy to activate nearby tags. Figure 8 shows how placing the NSS directly behind the reader’s antenna marshals the radiated energy that would otherwise be lost to strengthen the field in front of the antenna.

diagram of how NSS can optimize RFID reader performanceFigure 8 The NSS material tuned for 13.56 MHz can optimize RFID reader performance. Source: KEMET

As an example of the effect produced, using NSS material optimized for the 13.56-MHz frequency standardized in the ISO1444/1443 RFID specification, the distance from which the reader can activate the tag can be increased by 85 mm, or almost 300%, from 45 to 130 mm.

As demonstrated in the above design examples, NSS materials can be used effectively in many ways to realize device integration. Much more than simply an emergency add-on in the event of EMC failure, it can effectively support best practices in design for EMC and various signal integrity roles to improve system performance, particularly in power-conscious wireless devices.

By taking advantage of flux-shaping properties, designers can also utilize NSS to boost WPT efficiency and maximize RFID reader performance, ultimately delivering products to market that are compact, elegant, satisfying to use, and economical to own.

Patrik Kalbermatten is senior manager handling distribution promotion for magnetic, sensor, and actuator product management at KEMET Electronics Corp.

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