Wireless inductive charging in low-power wearable designs

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Wireless technology can provide a convenient way to charge batteries in wearable devices that lack charging ports for design or aesthetic reasons. In the past, using wireless charging methods required custom RF design and expertise in electromagnetic induction theory. However, designers can now implement wireless inductive charging technology in low-power wearable designs using standard, commercially available parts from manufacturers such as Freescale Semiconductors, TDK, Texas Instruments, and Toshiba. Wireless power can be traced back to the early 1800s, when Michael Faraday described the ability of a conductor to generate an electromotive force through electromagnetic induction in a magnetic field. In the late 19th century, Nikola Tesla put Faraday's law of electromagnetic induction into practice, using magnetically coupled resonant technology to wirelessly light a lamp in his New York City laboratory. Today, the principles of electromagnetic induction power a variety of RFID tags, contactless smart cards, and kitchen stoves, and provide the technical basis for wireless chargers for electric toothbrushes, smartphones, and emerging wearable devices such as the Apple Watch. In fact, wireless charging technology is an attractive solution for wearable devices where charging ports are inconvenient, take up too much space, or are simply too obtrusive. In addition, by eliminating the wired charging port, designers of wearable products also eliminate the possibility of contamination and water ingress, thereby improving the reliability of the entire product. These devices replace the charging port with a power receiving coil that is safely built into the wearable product's casing. In electrodynamic induction, applying current to a coil generates an electromagnetic field that induces current in a second coil next to it. In fact, the alignment and distance between the two coils are critical to achieving high efficiency. In consumer applications, wireless charging practices that require precise positioning usually provide guides to help users align the mobile unit to a specified location on the base unit. In contrast, so-called free-positioning wireless chargers usually have multiple coils in the base unit, which responds to feedback from the remote unit to power the appropriate coil. Communication Channels Communication plays a key role in both guided and free-positioning wireless charging systems. During transmitter operation, the receiver transmits a packet of data back to the transmitter by modulating the load on the receiver antenna. In turn, the transmitter demodulates the reflected load to reconstruct the packet (Figure 1). Image of a typical wireless charging system

Figure 1: A typical wireless charging system consists of a power transmitting base station and a power receiving receiver, using electromagnetic coupling principles for power transfer and communication. Both wireless charging systems use data from the receiver to manage the power of the transmitter. During operation, the transmitter unit responds to error data from the receiver to increase or decrease the power sent to the transmitting coil as needed. The free positioning system uses the same general method to select the optimal position of the coil relative to the remote device. Designers can use these communication paths not only for control signals, but also to transmit application data back to the transmitter. Although information bandwidth is limited, it is sufficient for communication of device authentication, device status, and sensor data collected by remote devices. The combination of power conditioning, control, and communication functions translates into circuit designs with complex power and control logic requirements (Figure 2). However, for designers, semiconductor manufacturers offer many solutions that address these and other needs. Wireless charging system image Wireless charging system image

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Figure 2: Wireless charging systems can quickly increase in complexity to meet diverse requirements for energy transfer optimization and communication. (Source: Texas Instruments) Standard solutions The availability of standardized wireless charging solutions builds on the growing acceptance of industry standards that define the basic requirements for wireless charging protocols. While the standard interface is intended to enable interoperability between user mobile devices and base stations from different vendors, it is based on two wireless charging technologies—inductive charging and resonant charging. Inductive charging requires strict alignment of the transmitter and receiver, but is generally more efficient than resonant charging. Resonant charging, on the other hand, is less critical to alignment and distance between the transmitter and receiver, and can charge multiple devices simultaneously. Industry standards groups including the Wireless Power Consortium (WPC), the Power Merchant Alliance (PMA), and the Alliance for Wireless Power (A4WP) are currently in the early stages of collaboration to develop interoperability features. Although initially designed with a broad range of consumer applications in mind, these standard approaches have become the foundation for wireless charging solutions for wearable devices. For example, while the WPC’s Qi standard typically uses a larger A11 50mm transmit coil, designers can use a smaller coil with less resistance to avoid excessive power losses, resulting in better performance. For example, the 30 mm diameter TDK WR303050 has a 0.41 Ω DC resistance, a form factor and power transfer level more in line with the requirements of many wearable devices. When it comes to power control for wireless charging, devices such as the Toshiba TB6865FG and TB6860WBG complement the standards-based functionality of existing parts. Like other products of this type, Toshiba ICs integrate a wide range of features needed to simplify design, requiring only a small number of external components to support wireless charging systems that comply with the WPC Qi standard (Figure 3). Toshiba TB6865FG transmitter and TB6860WBG receiver images

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Figure 3: Devices such as the Toshiba TB6865FG transmitter and TB6860WBG receiver integrate the multiple functions needed to simplify the implementation of standards-based wireless charging systems. (Source: Toshiba) The TB6860WBG receiver combines modulation and control circuitry with rectifier power harvesting, a built-in high-performance DC to DC converter, configurable lithium battery charger circuitry, and protection functions. Its TB6865FG power transmitter integrates an MCU and extensive analog functions, including PWM circuitry, switch control, onboard filters, and pre-driver circuitry. The TB6865FG can control two sets of coils separately, allowing users to charge two mobile devices simultaneously. Freescale Semiconductor builds its Qi-based MWCT1000 and MWCT1101 transmitters around the 32-bit 56800EX core. The processor is designed to provide MCU functionality as well as DSP processing power, enabling a wide range of functions while consuming less than 30 mA in active mode.The device requires only 30 mW of standby power to perform its function of detecting nearby receivers. During power transfer, the Freescale device can achieve an efficiency of more than 75%. In addition to the MWCT1000 and MWCT1101, Freescale has also introduced the MWCT1001A and MWCT1003A for automotive applications. Texas Instruments has introduced a number of devices in its BQ50xxx transmitter and BQ51xxx receiver families. While the BQ51221 supports both WPC and PMA standards, most of the devices in TI's receiver family are designed to comply with the WPC Qi standard. Among these Qi-compliant devices, TI's portfolio includes 5 W receivers with regulated output levels of 5 V (BQ51013A and BQ51013B), 7 V (BQ51010B), and 8 V (BQ51020 and BQ51021). Other members of the family, including the BQ51050B (4.2 V output) and BQ51051B (4.35 V), integrate lithium-ion battery chargers—providing a comprehensive power management solution for wearable devices. Designed for low-power applications, TI's BQ51003 is a 2.5 W receiver ideal for wearables. By integrating the BQ51003 with a low-power linear charger, such as TI's BQ25100, designers can implement a complete wireless charging receiver subsystem with integrated lithium-ion battery management. For lithium-ion battery charging, the BQ25100 can accurately control fast-charge currents as low as 10 mA or as high as 250 mA, and can accurately terminate charging as low as 1 mA to support small lithium-ion coin-type batteries. On the transmitter side, Texas Instruments’ BQ500211A and BQ500212A offer full Qi standard functionality, including the ability to continuously monitor the effectiveness of ongoing power transfers to provide foreign object detection (FOD) and parasitic metal object detection (PMOD). In addition to providing FOD and PMOD capabilities, the BQ500410 supports free-positioning designs with three-coil transmitter arrays. For low-power transmitter designs, the BQ500210 is capable of operating with supply currents as low as 8 mA. Summary For wearable devices, wireless charging technology meets the need for compact solutions and eliminates concerns about the size and reliability of wired charging ports. In the past, adopting wireless power methods required expertise in electromagnetic theory and RF design techniques. Today, designers can easily implement wireless charging capabilities in extremely small wearable devices using off-the-shelf IC parts.

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Why can't I see any information about charging range and distance?