What Is a Wireless Charger?

With the continuous improvement of power equipment's requirements for power supply quality, safety, reliability, convenience, immediacy, special occasions, and special geographical environments, the contact power transmission method can no longer meet the actual needs. ^[1]

A wireless charger is a device that uses the principle of electromagnetic induction for charging. Its principle is similar to that of a transformer. By placing a coil on each of the transmitting and receiving ends, the transmitting end coil sends electromagnetic signals to the outside under the action of electricity, and the receiving end coil receives electromagnetic Signal and convert the electromagnetic signal into electric current, so as to achieve the purpose of wireless charging. Wireless charging technology is a special power supply method. It does not require a power line, it relies on electromagnetic wave propagation, and then converts the electromagnetic wave energy into electrical energy, and finally realizes wireless charging. ^[2]

Chinese name: Wireless charger
Foreign name: wireless charger
Features: No traditional charging power cord

Principle: Electromagnetic induction
development of: Intelligent
Standard: Qi Standard

Introduction to wireless charger

New types of electronic products, especially portable electronic products such as digital cameras, mobile phones, and tablet computers, are used more and more in people's work and life, and the accompanying chargers also use traditional wired chargers. However, the compatibility and versatility of the wired charger are relatively poor, and it is inconvenient for users to carry and charge. At the same time, waste disposal increases environmental pollution. Therefore, it is urgent to provide users with more reliable, convenient, convenient and timely charging equipment. The development of wireless technology has made it possible to transmit radio power, and the research and development of wireless chargers will also meet user requirements. ^[3]

Figure 1 Model of wireless charging system There will be three main forms of battery charging in the future: visual charging, smart charging and wireless charging. In 2016, there were only 138 members of the International Wireless Charging Alliance (WPC), but this year WPC has more than 200 technology companies, such as Apple, Samsung, HTC, Huawei, Lenovo, Xiaomi, Nokia, Sony and many other mainstream mobile phone manufacturers. Generally, a Qi wireless charger has a flat surface, called a charging pad, on which mobile devices can be charged. Qi specifies three different methods for coil calibration, namely: guided positioning (magnetic attraction) (as shown in Figure 1 (a)), free positioning (moving coil) (as shown in Figure 1 (b)), and free positioning (Coil array) (as shown in Figure 1 (c)), these three methods are briefly explained below. ^[4]

First, guided positioning (magnetic attraction): that is, one-to-one fixed position charging is used to guide the placed charging device and achieve precise calibration. The advantages of this calibration method are simple, but it requires a series of magnets in the charging device. Attracting materials, therefore, eddy current-related power losses (and thus temperature increases) will be attracted to magnetic attractors. Second, free positioning (with a movable primary coil): It is also one-to-one positioning and charging device charging. This method requires a mechanically movable primary coil, which is tuned to be coupled to the position of the charging device. However, Movable mechanical components often reduce system reliability. In addition, motor control of the primary coil is complex and expensive for multiple devices that are charged. Third, free positioning (coil array): allows multiple devices to be charged without considering their positions. Compared with the above two methods, this calibration method uses more expensive and more complex winding structures and control electronic elements as At the cost of providing more user-friendliness. ^[4]

Research status of wireless chargers

Wireless charging breaks the way that electrical energy transmission can only rely on direct contact transmission of wires. It belongs to non-contact transmission and can avoid problems such as contact sparks, sliding wear, and explosion shock that may be caused by contact electrical energy transmission. There are three main types of wireless energy transmission: electromagnetic induction, electromagnetic resonance, and electromagnetic radiation. Electromagnetic induction is currently the most commonly used method of wireless energy transmission. Its technology has been mass-produced, its production cost is lower than other technologies, and it has passed security and shopping mall verification. Currently, there are three major alliances dedicated to the development and standardization of wireless charging technologies. These three alliances are Alliance for Wireless Power (A4WP), Power Matters Alliance (PAM), and Wireless Power Consortium (WPC). Qi standard is the "wireless charging" standard introduced by WPC, which adopts the most mainstream electromagnetic induction charging technology at present. Qi standards are targeted at portable electronics such as cameras, video and music players, toys, personal care, and mobile phones. At present, the research and design of low-power wireless chargers are mainly aimed at wireless charging on mobile phones. All of them are based on TI's BQ500211 special-purpose chips. Some low-power terminals also use special-purpose integrated chips. It can save development time, but in the long run, it is not conducive to cost reduction and later expansion and upgrade. ^[5]

Although wireless charging technology has been developed to some extent, there are still some difficult technical problems in the development process. First, the charging efficiency is not high. Once the distance is a little far away, the charging efficiency will decrease sharply, and a lot of time and resources will be wasted to complete the charging, so the use is of little significance. Second, safety issues during charging. High-power wireless charging equipment will generate a large amount of electromagnetic radiation, which will cause certain adverse effects on physical health, and will also have interference effects on aircraft and communications. Third, practical aspects. The current wireless charging technology can only be achieved by fixing it at a fixed point, which is not convenient and is not practical. Fourth, the price is expensive. Because the wireless charging technology is still in the initial R & D application stage and the cost of research is high, the price of its R & D products is relatively high. ^[6]

How the wireless charger works

Wireless charger electromagnetic induction

This is the most common way of working with wireless chargers. It uses the principle of electromagnetic induction to generate current through electromagnetic induction between the primary and secondary coils, thereby achieving the transfer of energy in the space range. The implementation has been promoted by the Wireless Charging Alliance. ^[2]

Wireless charger radio wave

Radio waves are a mature wireless charging method for wireless chargers at this stage. Its working principle is to use miniature and efficient receiving circuits to capture radio waves in space, and then convert electromagnetic energy into stable electrical energy. At present, companies have announced that they can wirelessly charge electronic devices that are smaller than a cell phone a few meters away. ^[2]

Figure 2 Wireless charger

Wireless charger electromagnetic resonance

This is a wireless charging technology that is still in the research and development stage. This technology is mainly researched by a team led by a professor of physics at the Massachusetts Institute of Technology. Intel engineers use this technology as a basis to achieve a distance of about 1 A 60-watt light bulb is lit around a meter and has a transmission efficiency of 75%. Intel engineers say the next goal will be to use this wireless charging technology to charge retrofit laptops. However, in order to achieve this goal, it is also necessary to solve the interference and influence of electromagnetic fields on other components in the computer. ^[2]

Wireless charger using standard

In order to enable different brands of products to share the same charger and improve the versatility of wireless chargers, the Wireless Power Consortium (hereinafter referred to as the "Union"), the first standardization organization in the world to promote wireless charging technology, launched the " "Wireless charging" standard, under the Wireless Charging Alliance (WPC) standard, the power consumption of wireless transmission is only 0 ~ 5W. Systems that meet this standard range use inductive coupling between 2 planar coils to transfer power from the power transmitter to the power receiver. The distance between the primary and secondary coils is generally 5mm. The output voltage is regulated by a global digital control loop. At this time, the power receiver will communicate with the power transmitter and generate power consumption. The communication is a one-way communication from a power receiver to a power transmitter through backscatter modulation. In backscatter modulation, the power receiver adjusts the load, thereby changing the current consumption of the power transmitter. These current changes are monitored and demodulated into the information required by the two devices to work together. Communication protocols include analog, digital ping, identification, configuration, and power transmission. ^[3]

The typical startup sequence when a power receiver is placed on a power transmitter is as follows: ^[3]

(1) An analog ping from the power transmitter detects the presence of the object.

(2) The digital ping from the power transmitter is a longer version of the analog ping and allows the power receiver time to reply with a signal strength packet. If the signal strength packet is valid, the power transmitter will keep the coil energized and proceed to the next step.

Figure 3 Wireless charger

(3) In the identification and configuration phase, the power receiver will send some data packets to identify it and provide configuration and setting information to the power transmitter.

(4) In the power transmission phase, the power receiver sends a control error packet to the power transmitter to increase or decrease the power. During normal operation, control error packets are sent every about 250ms, and every 32ms during large signal changes. In addition, during normal operation, the power transmitter sends a power packet every 5s.

(5) In order to terminate the power transmission, the power receiver sends a "stop charging" message or does not communicate within 1.25s, so that the power transmitter enters a low power consumption state. Under the Qi standard, mobile phones, cameras, computers and other products can be charged with Qi wireless chargers, and large-scale wireless charging will become possible.

Wireless charger deployment strategy

Existing literature addresses wireless charger deployment strategies in four different scenarios:

(1) The point configuration deals with the deployment of static chargers to support static devices with wireless energy, such as Chiu ^{[7] to} minimize the number of chargers. Through theoretical analysis and numerical simulation methods, two centralized greedy algorithms are used to solve network charging coverage. Demand

(2) The goal of path configuration is to deploy a static charger in the travel path of the mobile device to charge the mobile device (such as for human wearable or implantable sensors), such as Liao ^[8] , to maximize the survival rate, Through theoretical analysis and system-level simulation, a centralized heuristic greedy algorithm is used to solve the limitation of the number of chargers;

(3) The multi-hop configuration determines the location of the static charger in the static network, where the devices can also have wireless energy transmission functions and can share energy with each other, such as Rault ^[9] , which minimizes the number of chargers, and uses numerical simulation methods The centralized solution based on hybrid ILP solves the network coverage requirements and limits the maximum number of energy transmission paths;

(4) Landmark configuration involves two steps: selecting the landmarks to visit in turn for mobile chargers and clustering landmarks as a group to deploy mobile chargers. The location of the landmark is that the parked charger provides concurrent charging for multiple static devices in the vicinity, such as Erol-Kantarci ^[10] , which minimizes the configuration of the landmark and maximizes the energy transmitted to the node with a higher priority. The simulation method uses a centralized solution based on ILP to solve the problem of the need for full energy replenishment and the limited capacity of the charger, as well as maximize the number of landmarks, the power requirements of the nodes with limited transmission range and high priority, and the charger The problem of limited capacity.