Brief Analysis of Low-Power Wireless Power Technology (I)

With the development of science and technology, there are more and more portable electronic products. Modern wireless technology makes them lighter and easier to connect to the Internet. It seems that the digital age has rapidly developed into the wireless age. However, power cords have prevented us from entering this wireless age.

In real life, every electronic product has a matching charger. Billions of power adapters are designed, produced, used, manufactured, transported, and even recycled every year, which leads to a series of problems. They not only increase the cost of products, but also waste resources, are not conducive to environmental protection, and bring considerable confusion to users. Wireless power technology with unified standards can easily solve this series of problems.

1 Current Status of Wireless Power Technology

Wireless power technology is a new technology that uses radio technology to transmit electrical energy. Electrical energy can be transmitted to the load without contact. As early as the 1970s, quite a few people in the United States were studying this issue, but very few people in China were studying it. On December 17, 2008, the world's first Wireless Power Consortium (WPC) to promote the standardization of wireless charging technology was established by Convenient Power Limited, Fulton Innovation LLC, Logitech SA, National Semiconductor, Olympus, Philips, Sanyo, Sanfi Electronics and Texas Instruments. Later, Samsung and Nokia also joined the alliance. In January 2009, the first meeting of the Global Wireless Power Consortium was held in Hong Kong Science Park, and the latest global plan to improve the convenience of charging electronic products was launched. On July 23, 2010, the alliance formulated the Qi standard, determined the low-power (no more than 5W) technical specifications, defined the interface specifications, test procedures and minimum performance, and certified the first batch of products on August 14, 2010. On August 31, 2010, WPC officially introduced the Qi standard to China and began to promote it in China. After that, WPC successively held standard release activities in Hong Kong and many cities in Europe and the United States. Currently, the Qi standard has been supported and adopted by 126 industry-leading member companies of WPC, 30% of which are Chinese companies, including Haier, Huawei and Sangfei Communications.

2 Principles of Wireless Power Technology

Wireless power technology is a technology that uses radio to transmit electrical energy. It requires the highest possible transmission efficiency and the largest possible transmission power to meet the demand for electricity. Its research and application areas cover a wide range, and the transmission power varies greatly, ranging from tens of milliwatts for biological transplantation and tens of watts for small equipment to thousands of watts for electric vehicles or sports robots and megawatts for maglev train applications. There are currently three solutions: electromagnetic induction technology, radio wave technology, and electromagnetic resonance technology.

2.1 Electromagnetic induction technology

This technology is similar to the transformer technology commonly used in power systems. When an alternating current is passed through the primary side of the transformer, an induced electromotive force will be generated on the secondary side due to the principle of electromagnetic induction. If the secondary circuit is connected, an induced current will appear, and its direction follows Lenz's law, and its magnitude can be solved by Maxwell's electromagnetic theory. Relative to wireless power supply, the primary side of the transformer is equivalent to the power transmitting coil, and the secondary side is equivalent to the power receiving coil, so that the wireless transmission of electric energy from the transmitting coil to the receiving coil can be realized. This non-contact wireless power transmission method has low manufacturing cost, simple structure, reliable technology, and the transmission power can range from a few watts to hundreds of watts.

However, the transmission distance is less than 1cm, the charged product must be placed near the charger, and the charger must have the ability to identify the charged product, otherwise it will transmit energy to any nearby metal, causing it to heat up and create danger.

2.2 Electromagnetic resonance technology

This technology is based on the principle of electromagnetic resonance coupling. The two required resonant systems for transmission and reception can be made of induction coils respectively. By adjusting the transmission frequency to make the transmitter vibrate at a certain high frequency, it does not produce ordinary electromagnetic waves that are diffused everywhere, but forms an energy channel between the two coils. The natural frequency of the receiving end is the same as that of the transmitting end, so resonance occurs. With each resonance, more voltage will be generated in the receiving end sensor. After multiple resonances, enough energy will be gathered on the surface of the sensor, so that the receiving end receives energy in this non-radiative magnetic field, thereby completing the conversion of magnetic energy to electrical energy and realizing wireless transmission of electrical energy. This contactless wireless power transmission method can transmit power of up to several kilowatts and a transmission distance of up to 3~4 meters, but the required frequency must be protected. Transmission within a few meters requires a frequency of several MHz to several hundred MHz.

2.3 Radio wave technology

This technology uses microwaves or lasers to achieve remote transmission of electrical energy. The system consists of an electromagnetic wave generator, a transmitting antenna, a receiving antenna, a high-frequency electromagnetic wave rectifier, a substation and a wired power grid.

The electromagnetic wave generator is a microwave source or laser, which converts the electric energy transmitted by the power supply into high-power, high-frequency electromagnetic waves, which are fed to the transmitting antenna; the transmitting antenna sends out the electromagnetic waves; the receiving antenna collects the energy of the electromagnetic waves and inputs them into the high-frequency electromagnetic wave rectifier, and the generated high-voltage direct current is sent to the wired power grid after being inverted. This non-contact wireless power transmission method has a transmission distance of up to 10m, but the transmission power is small (up to 100mW) and the efficiency is low. A large amount of power sent by the transmitter radio wave is wasted in the form of radio waves.