With the continuous improvement of the requirements for power supply quality, safety, reliability, convenience, immediacy, special occasions, and special geographical environments, the contact-type power transmission method is increasingly unable to meet actual needs. Therefore, the development of a wireless power transmission system with the advantages of wireless transmission of electric energy, high transmission efficiency, small equipment size, ease of carrying and integration, will become one of the most attractive scientific research directions in the 21st century.
1. The advantages of wireless charging over wired charging
Compared with traditional charger methods, the biggest advantage of wireless charging technology is that it avoids the use of charging cables. After adopting wireless charging technology, different electronic devices can adopt a unified wireless charging standard, and only one charger is needed to charge different devices. Another advantage of wireless charging technology is non-contact. For electronic devices, the non-contact charging method does not require an exposed charging port, which can achieve a better waterproof design. The charger can also be embedded in the lamp base, table or even the bedside table to facilitate users to charge anytime and anywhere.
2. Technical principle of wireless charging scheme
The wireless power transmission technology was proposed by physicist Nikola Tesla in 1890. According to the different principles of power transmission, wireless power transmission can be divided into three methods: electromagnetic induction, electromagnetic resonance, and electromagnetic radiation. As early as the 1920s and 1930s, some scholars have begun to discuss the possibility of wireless power supply. The real breakthrough in wireless charging technology occurred in 2007, when the MIT research team successfully lit a light bulb 2 meters away. With the continuous development and improvement of wireless charging technology, there are currently two mainstream technical routes: electromagnetic induction wireless charging and electromagnetic resonance wireless charging.
2.1 Principle of electromagnetic induction wireless charging
The principle of electromagnetic induction wireless charging is the same as that of a transformer. There is a coil at the transmitting and receiving ends. The transmitting coil is connected to a wired power supply to generate an electromagnetic signal, and the receiving coil induces the electromagnetic signal at the transmitting end to generate current to charge the battery. The magnetic field of the wireless charging system is mainly transmitted through air, so the efficiency of magnetic field transmission is low. For the wireless charging efficiency of the system to reach more than 70%, the two coils need to be very close, and the coil sizes need to be similar, the positioning between the two should be accurate, and the technology can only support one-to-one charging.
Electromagnetic induction wireless power transmission systems usually use non-contact transformer coupling for wireless power transmission. That is, the tight coupling magnetic circuit of the transformer of the system is separated. The primary winding of the transformer is high-frequency alternating current. Through the electromagnetic induction of the primary and secondary windings, the electrical energy is transmitted to the secondary winding to supply power to the electrical equipment, and the electrical energy is Wireless transmission between power supply and electric equipment.
2.2 Principle of electromagnetic resonance wireless charging
The principle of the electromagnetic resonance scheme is the same as the principle of sound resonance. The primary and secondary coils transmit energy through magnetic resonance. The transmitting end oscillates at a specific resonant frequency, and the resonant frequency of the secondary coil is adjusted to be consistent with that of the primary coil to realize energy transfer. The charging distance of the electromagnetic resonance scheme can reach 45mm or more, the range is large, not limited to the planar structure, and one-to-many charging can be realized at the same time. The secondary coil of the receiving end can be configured in different sizes to adapt to different device powers.
Electromagnetic resonance wireless power transmission systems usually use two resonant bodies of the same frequency to generate strong mutual coupling, and use coils and plate capacitors placed at both ends to form a resonant circuit to achieve wireless energy transmission. In June 2007, Marin Soljasik, an assistant professor of physics at the Massachusetts Institute of Technology, and his research team conducted an experiment, energizing a coil with a diameter of 60 cm, and connecting it to another one at a distance of 1.9 m. The 60W bulb on the coil is lit. This experiment shows that the coils of the transmitting end and the receiving end form a magnetic resonance system. When the magnetic field oscillation frequency of the transmitting end is the same as the natural frequency of the receiving end coil, the receiving end will resonate, thus realizing the wireless transmission of energy. The electric energy consumed is only one millionth of the traditional electromagnetic induction power supply technology, and the effective transmission distance is tens of centimeters to several meters, so this transmission form is suitable for medium-range wireless power transmission.
2.3 Principle of electromagnetic radiation type wireless power transmission
The electromagnetic radiation type wireless power transmission system uses the microwave band for wireless transmission of electric energy. The power is generated by the power source, and the power frequency AC power is converted into microwaves through the microwave converter, and then sent to the space through the microwave transmitting antenna of the transmitting station, and then transmitted to the ground microwave receiving station. The received microwave is converted into the power frequency by the converter Alternating current for users. Microwaves are electromagnetic waves with a wavelength between radio waves and infrared rays. Due to their high frequency, they can pass through the ionosphere without reflection. The universe is ideal for microwave transmission with almost no energy loss. The loss when passing through the atmosphere is about 2%. Therefore, the electromagnetic radiation type wireless power transmission system has higher power transmission efficiency and is suitable for long-distance wireless power transmission.
Three, wireless charging technology standards
In order to promote the application of wireless charging technology, communication operators, semiconductor companies and high-tech companies have formed different wireless charging alliances and have introduced their own wireless charging technology standards.
Currently, the most mainstream wireless charging standard is the Qi standard launched by WPC (Wireless Power Consortium). The traditional Qi standard uses an electromagnetic induction scheme. The Qi standard has good versatility and convenience. From homes, cars, to airports, cafes, and offices, the Qi standard supports a wide range of scenarios. Generally, wireless chargers with the Qi standard logo can power electronic devices that support the Qi standard. In order to make up for the lack of electromagnetic induction solutions that can only be charged one-to-one, the latest Qi1.2 standard adds support for electromagnetic resonance solutions, enabling simultaneous charging of multiple devices. Airfuel, another wireless charging alliance, has also launched its wireless charging technology standards, including the PMA standard using electromagnetic induction schemes and the A4WP standard using electromagnetic resonance schemes. However, compared with the Qi standard, Airfuel is not technically mature enough and has not been well applied commercially.
Fourth, electromagnetic induction wireless charger design
The principle block diagram of a typical electromagnetic induction wireless power transmission system is shown in Figure 1.
The electromagnetic induction power transmission system is mainly composed of an energy conversion part, an energy transmission part and an energy reception part. The input AC power is rectified, filtered, and stabilized into DC power, and is inverted by a high-frequency inverter. The high-frequency alternating current generated by the inverter is input into the primary coil of the separate transformer and coupled with the secondary coil to produce Induces the electromotive force, and then supplies power to the load after high-frequency rectification and filtering. This text uses the electromagnetic induction type electric energy transmission principle to design the main hardware circuit of the wireless charger as shown in Figure 2.
4.1 Launch circuit design
The circuit is mainly composed of two parts: an oscillating signal generator and a resonant power amplifier. The NE555 is used to form a signal generator with an oscillation frequency of about 400kHz to provide an excitation signal for the power amplifier circuit; the resonant power amplifier is composed of an LC parallel resonant circuit and a switch tube Q1. When the resonant frequency of the frequency selective loop of the power amplifier is the same as the frequency of the excitation signal, the power amplifier resonates. At this time, the voltage and current in the coil reach the maximum value, thereby generating the largest alternating electromagnetic field.
4.2 Receiving circuit design
When the receiving coil is close to the transmitting coil, an induced electromotive force is generated in the receiving coil. When the resonant frequency of the receiving coil loop is the same as the resonant frequency of the transmitting coil, the induced voltage reaches the maximum value. When both the transmitting coil loop and the receiving coil loop are in resonance, the system has the best energy transmission efficiency.
4.3 Charging circuit design
The power management chip is used to design a charging circuit with 4 working modes: trickle current, constant current, overcharge and float charge. When the circuit is in constant current charging mode, the magnitude of the charging current is set by the current detection resistor connected between the CSP pin and the BAT pin; when the circuit is in overcharge and floating charge mode, the voltage divider network formed by external resistors is set Charging voltage. When the input voltage is too low, the charging circuit enters a sleep state. When the input voltage is greater than the starting voltage of 6V, the charging circuit starts to charge the battery. The voltage divider resistors R6 and R7 feed back the voltage of the battery terminal to the FB pin of the chip, and the chip determines which charging mode to enter according to the voltage value fed back from the FB pin. When the voltage of the FB pin is close to 3.6V, the chip works in an overcharged state. If the battery voltage is lower than 81.8% of the set overcharge voltage, the charging circuit automatically enters the trickle charging mode, and the charging current is 13% of the set constant current charging current. When the battery voltage is greater than 81.8% of the set overcharge voltage, the charging circuit enters the constant current charging mode. The charging voltage set by the charging circuit designed in this article is 5V, which can charge most electronic devices commonly used in life.
Five, shared wireless charger design scheme
When you are away from home, a dead cell phone will bring various inconveniences to people's lives. In order to solve the above mobile phone charging problem, it is necessary to design a shared wireless charger that is suitable for indoor public places such as restaurants, cafes, railway stations, airports, etc., and supports various mobile phones. The shared wireless charging design scheme mainly includes four modules: wireless charging module, user interaction module, central control module and remote server module. The function description of each module is as follows.
(1) Wireless charging module: The wireless charging module is mainly responsible for the realization of the charging function of the smart phone. There are currently two mainstream wireless charging technology routes: electromagnetic induction wireless charging and electromagnetic resonance wireless charging. Although the electromagnetic resonance wireless charging scheme can support one-to-many charging and a longer charging distance, the electromagnetic induction wireless charging scheme is more mature in technology and business. The Qi standard launched by the Wireless Power Consortium WPC (Wireless Power Consortium) is currently the most mainstream wireless charging standard. In the wireless charging module, we will also use a wireless transmitting coil that supports the Qi standard, so as to realize the charging support for all kinds of mobile phones.
(2) User interaction module: The user interaction module is mainly responsible for the realization of interactive functions with users. In order to allow users to use our wireless mobile phone chargers easily and quickly, we adopted the user login method of scanning the QR code on WeChat. Users can directly use WeChat account to log in to the small program we provide, enter the number of the wireless charger nearby, and get the permission to charge. After the user finishes charging, he can also pay the corresponding fees through WeChat Pay.
(3) Central control module: The central control module is mainly responsible for communicating with the remote server and controlling the opening and closing of the wireless charging function. After the remote server successfully authenticates the user's identity, it will send corresponding information to the central control module. After the central control module receives the charging start information from the remote server, it will turn on the wireless charging function. When the remote server sends a stop charging message, the central control module will turn off the wireless charging function.
(4) Remote server module: The remote server module is mainly responsible for services such as user identity authentication and fee calculation. After the user logs in with the WeChat account, the remote server will query the user's corresponding information in the database. When the user clicks the start and stop charging buttons in the WeChat applet, the remote server module will communicate with the central control module to turn on and off the wireless charging function. After the user is charged, the remote server will also calculate the corresponding fee.
The usage process of the shared wireless mobile phone charger is as follows: (1) The user scans the QR code on the desktop and logs in to the WeChat applet. (2) The user enters the wireless charger number around him, clicks the start charging button, and places the phone on the desktop to start charging. (3) After charging is completed, the user clicks the stop charging button. (4) According to the fee indicated in the mini program, the user uses WeChat Pay to complete the corresponding fee settlement.
to sum up
This article reviews the existing wireless charging technology routes and corresponding technical standards, and proposes a shared wireless mobile phone charger design. Our design plan combines Qi wireless charging standard and various functions of WeChat software, which can provide reference for future product design. A wireless charger is designed using the principle of electromagnetic induction wireless power transmission. Tests show that the charger can achieve high-efficiency transmission of electrical energy within a short distance. Moreover, the charging circuit of the charger has greater superiority and advancement compared with the circuit given in the literature.