Fig 1. Shown are test results for permeability and loss versus frequency of typical sintered-ferrite sheet material (3M’s EM-600).
Fig 2. A coil and associated flux field were modeled in three environments: coil in free space with a well-defined flux field (a), coil near metal surface with reduced flux field due to eddy current losses (b), and coil near metal with a FFDM between the coil and metal (c). The latter shows significantly improved flux-field performance.
Fig 3. The mobile device in a typical EMIC-WP system device uses FFDMs to optimize the receiving coil induction as it enters the primary coil’s flux field.
Demands continue unabated for efficient battery-recharging solutions, pushing designers to quickly come up with the latest and greatest. Enter wireless power charging, which is an emerging trend in the mobile-device area. In the near term, it will impact devices such as cell phones, laptops, and tablets; long-term goals include electric-vehicle batteries.
Wireless charging generally means that no wires are needed to carry power from a power source directly to the battery for charging. Several different methods accomplish this task, ranging from electromagnetic inductive coupling (EMIC) and electromagnetic resonance coupling to laser power transfer systems and others. EMIC is currently the most cost-effective and easily implemented approach. Electromagnetic resonance coupling is similar in nature, but becomes more complex in the implementation.
In its simplest form, EMIC sends a signal from point A (the primary coil) to point B (the pickup or receiving coil). Electrical current, which flows through the primary coil, produces an electromagnetic flux field radiating away from the coil (a signal). The receiving coil is within range of the primary coil so that the flux field of the primary coil intersects with the receiving coil. The flux field induces a flow of current in the receiving coil, and that power is used to wirelessly charge the mobile system’s battery.
EMIC Wireless Power
The basic design concept of an EMIC wireless-power (EMIC-WP) transfer or charging system is straightforward. The tough part of the design is achieving key design targets that make it acceptable for consumers as a viable mobile-device charging option.
Mobile EMIC-WP systems should provide aesthetic design (i.e., slim and sleek to fit consumer expectations of a mobile device and associated EMIC-WP station), light weight, reliability, efficient power transfer, and safety. If a design meets all of those factors, than EMIC-WP could well settle in as a de facto solution for all types of mobile devices.
Many factors go into designing an EMIC-WP system, such as the control and power electronics, coil designs, spacing considerations between primary and receiving coil, design frequencies of operation, etc. Designers must also be aware of several pitfalls.
For instance, power-transfer efficiency lingers at the top of the list of customer concerns. If the power transfer is significantly lower than that from a direct wired charger system, consumers will be less likely to consider the system’s added cost. Also, the device could be deemed less mobile if the EMIC-WP dimensions differ greatly on the receiver side (and for some, if the charging station is bulky). Thus, it may detract from its acceptance as a common-mode powering solution. Finally, consumer doubts regarding safety may arise if the EMIC-WP solution presents negative effects such as interference with another component’s electronic function or heating of nearby components during operation.
To combat these issues, designers make sure to incorporate flux-field directional materials (FFDMs) in their EMIC-WP systems. FFDMs, which interact with the flux field generated by the coils, ensures that the flux field generated through the EMIC-WP system achieves crucial design targets.
FFDM Permeability And Loss
Proper selection of an FFDM is based on EMIC-WP system performance goals. FFDMs are characterized by their ability to interact with the generated flux field at the selected EMIC-WP operating frequency.
Each FFDM type will interact with the specific flux field based on two characteristics that change according to the operating frequency:
• FFDM permeability: Permeability measures the degree the flux field can be coupled into an FFDM to redirect the flux field (i.e., to improve flux-field interaction with a coil, reduce eddy-current losses, provide flux field shielding). FFDMs with higher permeability values at the operating frequency are desirable for various reasons, such as meeting a desired application thickness, decreasing weight, or achieving a smaller XY dimensional area.
• FFDM loss: Loss measures reduction of flux-field strength during FFDM interaction. When the flux field passes through the FFDM, the FFDM may dissipate some percent of a flux field as a heat loss. A low loss is desired at the EMIC-WP system’s operating frequency. Loss (i.e., energy converted to heat) in a material, which is frequency-dependent, is generally associated with eddy-current generation in the FFDM, magnetic hysteresis losses, and ferromagnetic resonance interactions.
Another material that’s often associated with an FFDM is an electromagnetic absorber. It features relatively lower permeability and higher loss at a typically higher application frequency than that of an EMIC-WP system. Absorber materials, which reduce the flux-field strength of an electromagnetic-interference (EMI) signal, are used to achieve the highest loss possible at the frequency range of concern.
For example, an absorber often will be applied to reduce EMI noise from IC device or an EMI noisy power source. In mobile systems with many antennae or higher-frequency processors, an absorber minimizes EMI that otherwise would be collected by unintended antennae (Wi-Fi, a 4G antenna) or data flex. EMI also can reduce an antenna or data flex’s signal-to-noise performance, leading to higher data error rates or poor read distance from source, or impact a device’s EMI acceptance testing.
FFDMs come in three basic forms: sintered-ferrite (SF) sheets, composite-magnetic-filler (CMF) sheets (elastomer + magnetic fillers), and magnetic foils (MFs).
SF sheets vary in composition and performance characteristics of permeability and loss across a frequency range (Fig. 1). Typical SF materials include nickel zinc ferrite and manganese zinc ferrite. Selection of the SF type is based on application frequency, power-design efficiency, permeability, loss, minimum thickness, cost, and ease of use. These products are typically stiff and may be brittle and require protect films for protection, die cutting and ease of handling.
CMF sheets consist of an elastomer filled with magnetic fillers. The CMF type offers good flexibility and modest cost. These sheets generally exhibit lower permeability versus the better performing SF sheets, and can be useful options for certain EMIC-WP systems.
MFs offer the highest permeability potential and, thus, improved EMIC-WP performance. These products deliver a thin solution and can be stacked into multilayer solutions to optimize an EMIC-WP solution.
Each FFDM type varies in permeability and loss for a given frequency. In an EMIC-WP design, the material can be used individually or with each other to offer a solution to meet primary design characteristics. A design may place an FFDM underneath the coil and/or along the edges of the coil design.
When employed in a typical product, attributes of each of these material types vary widely. A comparison of their performance in an EMIC-WP system, and their optimum permeability/loss ratio at an EMIC-WP system’s operating frequency, brings this to light (see the table).
Optimize EMIC-WP With FFDM
FFDMs help designers achieve EMIC-WP system optimization due to its contributions toward:
• Aesthetic design: High-performance FFDMs (highest permeability, lowest loss at operating frequency) reduce thickness, allowing for thin form-factor design.
• Lighter weight: FFDMs can improve the coil efficiency, leading to use of a smaller coil that will ultimately limit the final design weight and size.
• Reliability: FFDMs help enhance the EMIC-WP system’s design robustness and improves reliability by limiting stray EMI fields and associated negative impacts, such as induction heating of other system components.
• Efficient power transfer: FFDMs can be used to focus the primary EMIC-WP coil’s flux field to more effectively couple with the receiver coil, increasing power-transfer efficiency. An FFDM design also helps improve charging times. Overall, well-designed EMIC-WP systems incorporating FFDMs can feature greater than 70% power-transfer efficiency and nearly the same time period for a device battery charge versus common mobile devices’ hardwired outlet chargers.
Public literature also points out the potential benefits of EMIC-WP systems when compared with common consumer hardwire-connected chargers. EMIC-WP systems with the ability to charge multiple devices and containing smart electronics that power down the system to a negligible standby power can offer greater power-usage savings over hardwired chargers plugged continuously into a power source.
Overall, real-world use of a hardwired wall charger versus an EMIC-WP system suggests that the EMIC-WP system is potentially at least a neutral power-consumption system or an improvement. FFDM selection and geometric implementation in the coil design are critical to meeting the power-transfer design goals.
• Safety: With multiple mobile devices, EMIC-WP systems can be considered safer than more common consumer alternatives. That’s because the EMIC-WP household will have fewer hardwired connections as well as fewer connect-disconnect actions.
FFDMs can dramatically affect EMIC-WP coil performance as shown by comparing three different situations: free space, the coil near a metal type structure, and FFDM between the coil and metal structure (Fig. 2). The modeling indicates how eddy-current losses can affect an energized coil if they’re near the metal surface without FFDMs. An FFDM refocuses the flux field and ensures proper flux-field management for the highest possible system performance.
In a typical configuration of an EMIC-WP system, the mobile device uses FFDMs to optimize the receiving coil induction as it enters the primary coil’s flux field (Fig. 3). The primary coil field is enhanced with the use of the FFDM to ensure a well-defined flux field and low loss into other parts of the assembly. The receiving coil’s FFDM optimizes the flux field across the coil to establish a high degree of inductive coupling.
The EMIC-WP system can be designed with singular or multiple coils to simplify device location on the surface of a primary coil(s) for an optimized charging cycle. FFDMs are used with both systems and can vary in implementation for permeability, thickness, multilayer design, combination of materials, geometric shapes, etc. All work to optimize the flux-field path characteristics and power-transfer efficiency.
Some FFDMs are useful in other mobile-device applications, such as near-field communication (NFC) or radio-frequency identification (RFID) applications. Unlike the EMIC-WP’s power flux field, NFC/RFID applications feature primary (sending) and receiving coils (or antenna) that transmit a data flux field. FFDMs can be used to improve the coil efficiencies and improve communication performance for distance and error rates.
FFDMs may also find homes in applications for EMI shielding of low-frequency magnetic noise generated by flowing electrical currents in many electronic devices. FFDM is able to interact with and redirect the radiating magnetic flux field generated by the flowing current, and protect other devices, system lines, or adjacent components from the flowing current magnetic flux field.