Wireless charging solution for wearable devices

　　Wearable devices are rapidly emerging as an important market segment for electronic components. A key requirement for these devices is convenience, not only in the ability to access data on moving objects but also in ensuring the battery lasts a full day each day.

　　If users have to plug in devices to charge overnight, sometimes it's very likely they forget to charge and only wake up to find the device will be unusable for the rest of the day. Wireless charging offers a more convenient way to charge electronic devices. For wireless charging, simply place the electronic device on the charging pad, without inserting a micro USB or similar cable into the device to be charged, and users can place the pad within easy reach. If the wireless charging system is properly designed, a single charging pad can charge multiple devices simultaneously, eliminating the need to charge each one individually and making it easier for users to carry the pad and devices when going out.

　　Now, the convenience of wireless charging is no longer limited to wearable devices. This technology has long been widely used in electronic toothbrushes, even scaling up proportionally to charge electric vehicle batteries.

　　The basic working principle of inductive charging is the same as that of a power transformer. The induction coil in the charging pad generates an alternating electromagnetic field, which is then received by the coil of the device to be charged and converted back into useful current. Similar to traditional transformers, basic inductive charging also requires two coils to be close together for high efficiency. Otherwise, the resistance in the primary coil will generate considerable cumulative losses.

　　Resonant inductive coupling by generating two coils can improve long-distance energy transmission efficiency. Specifically, by combining inductor and capacitor loads, these two coils are tuned to produce resonance at the same frequency. Under these resonant conditions, a large amount of electrical energy can be transmitted from one coil to another coil several times its diameter.

　　Figure 1: Load modulation is used to encode data during transformer coupling.

　　The coil circuit's Q value can be adjusted to build a relatively strong magnetic field after multiple cycles. The energy carried in this oscillation signal is higher than the energy fed to the coil at any given time. Because the secondary coil can receive part of this oscillating magnetic field and convert it, the output electrical energy is higher than that of traditional transformers. Using tuned capacitors to achieve resonance can eliminate stray and magnetizing inductances in the emitter, fundamentally reducing the resistance loss of the coil winding, which is typically 10 to 100 times the induced loss.

　　To achieve a Q value higher than traditional transformers, coils are usually designed with solenoids, which also helps minimize skin effects. Typically, dielectric loss can be minimized by using small dielectric constant inductors or relying solely on air.

　　In practice, the coil is not always tuned to an exact resonant frequency. As long as the secondary coil intercepts a certain amount of magnetic field lines, the loosely coupled system can transmit electrical energy. Achieving tighter coupling through more precise coil matching can provide higher electrical energy, but for coils designed to operate simultaneously under resonant conditions, maintaining tight coupling between them is impossible. These circuits can be designed to operate only under detuned modulator conditions, where the resonant frequencies of the receiver and transmitter differ slightly.

　　Unfortunately, tightly coupled coils are also easily affected by alignment, and for consumer applications where users simply want to place the device on a charging pad to successfully charge without considering the best placement or placement, this is a problem. Therefore, the transmitter used for charging can use multiple coils. This increases design complexity but offers more freedom in location selection. Coil overlap is not required, simplifying assembly during production, although coil overlap increases density and allows for more freedom in receiver placement.

　　To successfully charge different devices with a single transmitter, certain standards must be adopted. Currently, there are two main standards in use. The Powermat system is a standard advocated by the Alliance for Wireless Power, designed around loosely coupled systems based on a single transmitter coil. The Wireless Power Consortium's Qi system allows for a variety of configurations, including simultaneous loose and tightly coupled operations. Most current transmitters use multi-coil tightly coupled configurations.

　　These two standards also consider energy management to ensure the charging pad only works when the device is charging. For example, the Qi system uses a communication protocol to relay signals on the coil to check for the presence of a device and whether it supports the Qi system. According to this standard, the transmitter can change the switching frequency on the coil within the range of 110 kHz to 205 kHz, serving as the main control mechanism for power delivery.

　　Under the Qi standard, simple modulation of the load is performed using coil voltage to send data to devices on the other side of the air gap. Communication from the secondary coil uses different dual-phase, bit coding schemes, with a constant operating frequency of 2 kHz and an additional start bit before every 8-bit data transmission. After transmitting data, parity checks and stop bits are used.

　　Figure 2: Dual-phase encoding enables binary data transmission capability.

　　A large amount of control data can be transmitted. The most commonly used types of control data packets include: signal strength, control error, terminal power requirements, and rectifier power levels. Signal strength helps adjust the device's position on the charging pad, and when used with visible or audible signals, it guides users to move along the charging pad until the signal strength is high enough to indicate good current power delivery.

　　The control error data packet can indicate the degree of error between the input voltage observed from the receiving coil and the required input voltage. Transmitters typically use control circuits to adjust the voltage applied to their coils. If there is a large error, the frequency of these error packets is set to a larger value. Every 32 ms, a packet is sent until the error drops below the threshold. From this perspective, these packets are sent every 250 ms. Control error data packets are very helpful for adjusting power delivery. Under light load conditions, receivers may require a higher voltage to overcome current transients—for example, waking wearable devices from sleep states. When the load current is large, portable devices may require a lower voltage to avoid power loss on the LDO regulator.

　　When the device is fully charged or an internal fault that may damage the battery is detected, it will send a request to stop power transmission. Power delivery is also controlled through rectified power supply information. This relays and forwards the portion of power received by the wearable device at its rectifier circuit output. The transmitter uses this information to determine the coupling frequency and also determines whether the receiver has reached its maximum power limit. Every 350 ms to 1800 ms, the transmitter uses gaps without data packets to determine whether the device on the charging pad has been removed. Rectifier power supply information also helps detect foreign objects.

　　Chipsets supporting the Qi protocol and controlling power delivery have already been launched. For example, Toshiba has launched TB6865AFG devices for transmitters. This highly integrated component includes an ARM Cortex-M3 processor running customer code and a PWM controller supporting external H-bridge circuits (for power delivery). According to the Qi standard, the controller can control power for up to two devices and supports foreign object detection.

　　The bq51013 device is a Texas Instruments product designed for secondary side, capable of AC/DC power conversion, rectification, and digital control functions required to send commands to transmitters. All devices in the bq5101x series use a low-resistance synchronous rectifier, LDO, and voltage and current loop controllers.

　　In addition to controllers, manufacturers also offer off-the-shelf coils that support the Qi protocol standard, which are designed to serve as transmitters, receivers, or both. For example, Abracon's AWCCA-50N50 series supports both transmitter and receiver applications. The coil diameter is slightly less than 50 mm and has strong anti-magnetic resistance, protecting the electronic components inside the device. These designs offer a Q factor selectable in the range of 70 or 160, with DC resistance around 20 mΩ or 70 mΩ in these two cases.

　　For smaller wearable devices, TDK has launched WR303050 coils and reduced their package size to 30 x 30 mm with a thickness of only 1 mm. At room temperature, the DC resistance is 0.41 Ω.

　　To enhance flexibility, Vishay Dale's IWAS-3827 offers a choice with rectangular rather than square substrates, measuring 38 mm in length and 27 mm in width. This coil is 1 mm thick, DC resistance is 0.18 Ω, and the typical Q value is 30.

　　Figure 3: AVishay Dale coil for wireless power supply.

　　To provide a more integrated solution, TDK's TMx-66-2M7 and TMx-58-2M7 can be packaged together with a TI receiver chip, achieving a package device with a total length of 66 mm and a thickness of just 1 mm.

　　Other optional wireless charging devices include various WPCC and WE-WPCC series wireless charging coils provided by Würth Electronics. These coils come in both transmitter and receiver configurations, with rated currents ranging from 0.8 to 13 A and a variety of sizes to meet various application requirements. We can use the Würth/TI Wireless Power Demo Kit (760308) to demonstrate the concept and benefits of wireless charging, which uses Würth transmitter and receiver coils.

　　As ecosystems around protocols like Qi expand, we can expect more integrated solutions to simplify design work and create simpler charging methods for wearable devices.