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Wireless Charging Technology | How does Wireless Charge Work, Application and Standards


The wireless charging technology is mainly realized by the principles of magnetic resonance, electromagnetic induction coil and radio wave, and is a charging technology developed today. The wireless charging technology can free people from the complicated wired charging, avoiding the trouble caused by forgetting to charge the device or carrying too many charging lines and adapters. Today's wireless charging technology is still in the experimental stage, I believe that wireless charging technology will be popularized in our daily life in the near future.

This Video Dicuss the Truth about Wireless Charging in the term of  the Historical Process, Principles and Related Applications.



I Historical Process

II The Overview of Wireless

Charging Technology

2.1 Definition of Wireless Charge

2.2 Working Principle

2.3 Wireless Charging Standards

III Technology Applications and Status

3.1 Consumer Electronics Area

3.2 Electric Vehicle Area

3.3 Biomedicine

3.4 Space Power Generation

I Historical Process

Transmitting power wirelessly is a very old technology that dates back to the 19th century when humans began to have electricity. At that time, there were two ideas for the transmission of electricity. One is the cable group represented by Edison, that is, the cable is used for long-distance transmission of electricity. This solution is mature and reliable. The disadvantage is huge engineering volume and high cost. Another is Nikola Tesla, the inventor of the world's first AC generator, the wireless transmission method proposed at the end of the 19th century. At the time, Tesla envisioned the use of electromagnetic induction to allow low-loss transmission of electrical energy using the earth and sky ionosphere as medium. The experiment is said to have been successful but was suspended due to political and economic factors. The wireless transmission technology was later used only in the field of electrical signal transmission, that is, the exchange of information, and the long-distance energy transmission has never been put into practical use, although it is completely feasible in physics.

It was not until a hundred years later that this situation changed. In many low-power household appliances such as electric toothbrushes and razors, we have seen the application of non-contact wireless charging technology, which brings considerable convenience to users. With the practical use of passive RFID tags and the development of wireless network technology, wireless power supply experiments such as lighting up the light bulbs are also widely reported, which has ignited the infinite ambiguity of "wireless" future life. The scientific community has spared no effort to work in this direction.

In May 2001, the International Wireless Power Transmission Technology Conference was held in Reunion Island, France on the Indian Ocean. G. Pignolet of the French National Centre for Scientific Research conducted a public experiment: he used microwave technology to transmit electricity wirelessly, and finally lit a 200-watt bulb 40 meters away. Later, according to the article, the 10 kW experimental microwave power transmission device built on the island in 2003 has begun to provide point-to-point wireless power supply to the Grand-Bassin, which is close to 1km, at a frequency of 2.45GHz.

By the end of 2006, there were reports that MIT had a breakthrough in wireless power transmission technology. The wireless power supply unit, led by the research team led by physics professor Marin Soljash, can light 60-watt bulbs 2.1 meters away, with an energy efficiency of 40%. The related content was published in the "ScienceExpress" online magazine on June 7, 2007. And this "air-point bulb" experiment has aroused great attention from the major media in Europe, America and the world. Later, Joshua R. Smith of Intel Seattle Lab conducted an improvement study on this result and increased the power efficiency to 75% (within 1 meter). Such efficiency is good enough for devices such as laptops, smartphones, and tablets, and Intel also demonstrated this at the August 2008 Information Technology Summit.

However, compared to high-power power transmission, low-power wireless charging technology is more practical, and smartphones that require frequent charging are the biggest beneficiaries of this technology. We took the lead in seeing the commercial-grade wireless charging technology on the Nokia Lumia 920 smartphone. At the same time, a large number of mobile phone manufacturers and peripheral manufacturers followed up, and the wireless charging technology for smart phones went into the eve of the night.

II The Overview of Wireless Charging Technology

2.1 Definition of Wireless Charge

Wireless charging, also known as inductive charging, non-contact inductive charging, is a device that uses near-field sensing (or inductive coupling) to transfer energy from a power supply device (charger) to a powered device. The device uses the received energy to charge the battery and simultaneously for its own operation. Since the charger and the electric device transmit energy by inductive coupling, there is no wire connection between the two, so the charger and the electric device can be exposed without conductive contacts.

2.2 Working Principle

2.2.1 Electromagnetic Induction

Most of the wireless charging technologies we see today use electromagnetic induction technology, and we can think of this technology as a separate transformer. We know that the widely used transformer now consists of a magnetic core and two coils (primary coil, secondary coil); when an alternating voltage is applied across the primary coil, an alternating magnetic field is generated in the core to induce an alternating voltage of the same frequency on the secondary coil, and the electrical energy is transmitted from the input circuit to the output circuit. If the coil at the transmitting end and the coil at the receiving end are placed in two separate devices, a magnetic field is generated when power is input to the transmitting coil. When the magnetic field senses the coil at the receiving end, a current is generated, and we construct a radio energy transmission system.

Electromagnetic Induction 

The main drawback of this system is that the magnetic field rapidly decreases with increasing distance and generally works only in the range of a few millimeters to 10 centimeters. In addition, the energy is divergent in all directions, so the induced current is much smaller than the input current, and the energy efficiency is not high. But for objects that come into close contact, there is no problem. The first wireless charging product to take advantage of this principle was the electric toothbrush: Since the electric toothbrush is often in contact with water, the contactless charging method is adopted, which makes the charging contact point not exposed, enhances the waterproofness of the product, and facilitates the overall cleaning. There is a coil in each of the charging socket and the toothbrush. When the toothbrush is placed on the charging stand, there is magnetic coupling. The principle of electromagnetic induction is used to transmit power, and the induced voltage can be rectified to charge the rechargeable battery inside the toothbrush.

This working principle is completely feasible in smartphones, and Apple, LG, Panasonic, etc. are developing their own wireless chargers. Moreover, this system has high requirements for the matching of the coils, so a movable coil, a multi-coil, and a magnet attraction method are introduced.

2.2.2 Electromagnetic Resonance

When the oscillating circuit is in a non-ideal state and has a resistance, the resistance heats up and becomes a damped oscillation; when there is an applied periodic electromotive force in the oscillating circuit, it will become forced oscillation; when the frequency of the applied electromotive force is the same as the natural frequency w of the free oscillation of the circuit, the amplitude reaches a maximum value, which is called electromagnetic resonance.

Electromagnetic Resonance


Compared with electromagnetic induction, magnetic resonance technology has a certain degree of tolerance in terms of distance, it can support wireless charging of several centimeters to several meters, and it is more flexible in use. Magnetic resonance also uses two perfectly matched coils. When one coil is energized, a magnetic field is generated. The other coil resonates and generates current to illuminate or charge the device. In addition to being far away, the magnetic resonance method can simultaneously charge multiple devices, and there is no strict limit on the position of the device. Its flexibility of use ranks first in various technologies. And in terms of transmission efficiency, the magnetic resonance method can reach 40% to 60%, although it is relatively low, but it has not entered any commercialization.

 Fujitsu demonstrated the magnetic resonance system in 2010. In the demonstration, it successfully illuminated two bulbs within a distance of 15 cm, which has good practical value. Except for Fujitsu, Nagano Japan Wireless, Sony, Qualcomm, and WiTricity have all adopted this technology to develop their own wireless charging solutions. WiTricity's application area is wireless charging for electric vehicles.

 Electromagnetic Resonance

2.2.3 Electric Field Coupling

The "electric field coupling" wireless power supply system developed by Murata Manufacturing Co., Ltd. is a minority. The system also includes the Japanese Bamboo Works. The electric field coupling method is different from the "electromagnetic induction" and "magnetic resonance", and its transmission medium is not a magnetic field but an electric field.

Electric Field Coupling 

Compared with the traditional electromagnetic induction type, the electric field coupling method has three major advantages: The position of the device during charging has a certain degree of freedom; the electrode can be made thin and easier to embed; the temperature of the electrode does not rise significantly, which is also advantageous for embedding. First of all, in terms of position, although its distance can not reach the length of several meters like magnetic resonance, it is also free in the horizontal direction, and the user can charge the terminal freely on the charging stand.

The second characteristic of the electric field coupling method is that the electrode can be made very thin, for example, it can use copper foil or aluminum foil with a thickness of only 5 microns. In addition, the shape and material of the material are not required, and the transparent electrode and the film electrode can be used. Any other unconventional shape can be used except for the square shape. These characteristics determine that the electric field coupling technology can be easily integrated into thin, demanding smartphone products, which is the most significant advantage of this technology over other solutions. Obviously, if the electric field coupling technology is adopted, the smartphone manufacturers have a very loose degree of freedom in designing the products, and will not suffer from the elbow in the design of the charging module.

The third advantage is that the temperature of the electrode part does not rise. One of the problems that plagues wireless charging technology is that the temperature at the time of charging is high, which causes the battery pack close to the electrode or coil to be deteriorated by heat, thereby affecting the life of the battery. The electric field coupling method does not have such a problem, and the temperature of the electrode portion does not rise, so the internal design does not have to be too deliberate. The non-heating of the electrode part is mainly due to the increase of the voltage. For example, when the voltage is raised to about 1.5 kV during charging, the current intensity flowing through the electrode is only a few milliamperes, and the heat generation of the electrode can be controlled very well.  However, the fly in the ointment is that the power supply circuit of the power transmission module and the power receiving module will still generate a certain amount of heat, which generally causes the internal temperature to increase by about 10 to 20 ° C, but the circuit system can be placed at a remote location to avoid affecting the internal battery.

Murata has successfully developed 5 watt and 10 watt rechargeable products and is committed to miniaturization. And they also plans to launch small products to the market from this year, and in the future, it will move toward 50 watts, 100 watts and other high-power products.

2.2.4 Microwave Resonance

Microwave resonance is another mature wireless charging method, and its power transmission is similar to that of the old-fashioned ore radio. The ore radio itself does not have a DC power source. It uses an antenna to receive the carrier from the station and, after detection, produces an audio current in the handset. The principle of microwave transmission computer is to transmit power to a remote receiving device in the form of microwave or laser, and then use it after rectification, modulation, etc.

Powercast, the representative company that uses this principle, said that its finally developed miniature high-efficiency receiving circuit can capture the radio wave energy rebounded from the wall and maintain a stable DC voltage while adjusting with the load. With a transmitter mounted on the wall plug and a “mosquito” receiver that can be installed in any low-voltage product, the Powecast solution converts radio waves into direct current and charges the batteries of different electronic devices in a range of about 1m.

2.3 Wireless Charging Standards

2.3.1 Qi Standard

Qi is a wireless charging standard introduced by the Wireless Power Consortium(WPC) that is the world’s first standardization organization to promote wireless charging technology, with two features of convenience and versatility. First of all, different brands of products, as long as there is a Qi logo, you can use Qi wireless charger to charge. Secondly, it overcomes the technical bottleneck of wireless charging "universality". In the near future, mobile phones, cameras, computers and other products can be charged with Qi wireless charger, providing a possibility for large-scale application of wireless charging.

The mainstream wireless charging technology in the market mainly adopts three methods: electromagnetic induction, radio wave, and resonance, and Qi adopts the most mainstream electromagnetic induction technology. In terms of technology applications, Chinese companies have stood at the forefront of the wireless charging industry. It is reported that Qi's application products in China are mainly mobile phones, which is the first stage, and will be developed into digital products of different categories or higher power in the future.

2.3.2 Power Matters Alliance Standard

The Power Matters Alliance standard was initiated by Duracell Powermat, which is a joint venture between P&G and wireless charging technology company Powermat. In addition, Powermat is a support member of the Alliance for Wireless Power (A4WP) standard.

There are already three companies, AT&T, Google and Starbucks, joining the PMA Alliance (short for Power Matters Alliance). The PMA Alliance is committed to creating wireless power standards for mobile and electronic devices that comply with IEEE Association standards and is a leader in wireless charging.

Duracell Powermat has introduced a WiCC charging card using the Power Matters Alliance standard. The WiCC is larger than the SD card, and is internally embedded with components such as coils and electrodes for electromagnetic induction type non-contact charging. The thin card can be used by plugging it into the existing smartphone battery, which makes it easy for many portable terminals to support contactless charging.

2.3.3 A4WP Standard

A4WP is the abbreviation of the Alliance for Wireless Power standard, which was created by the wireless charging alliance created by Qualcomm, South Korea's Samsung and Powermat. The alliance also includes members such as Ever Win Industries, Gill Industries, Peiker Acustic and SK Telecom, with the goal of setting technical standards for wireless charging devices for electronic products, including portable electronics and electric vehicles.

III Technology Application and Status

3.1 Consumer Electronics Area

With the maturity of electromagnetic induction wireless charging technology, the first consumer electronics products that use wireless charging technology are electric toothbrushes. Since the electric toothbrush is often in contact with water, the contactless charging method is adopted, which makes the charging contact point not exposed, enhances the waterproofness of the product, and facilitates the overall cleaning. 

In addition to electric toothbrushes, consumer electronics products that use wireless charging technology on a large scale are mobile phones. The earliest mobile phone manufacturers using wireless charging technology were Palm. Later, mobile phone manufacturers such as Sharp, Nokia, Sony Ericsson and others followed suit.


Consumer Electronics

Early wireless charging technology was not efficient. Take electric toothbrush as an example. Some models have a built-in 700 mAh battery, which takes more than 4 hours to fully charge. And the battery capacity of smartphones in the past was generally above 1500 mAh, which means that it takes more than 8 hours to fully charge. The lower charging efficiency and possible compatibility risks have always restricted wireless charging technology in the field of smart phones, and this situation has not improved until the first international standard "Qi" for wireless charging in the world was introduced.

3.2 Electric Vehicle Area

At present, the wireless charging technology of electric vehicles mainly adopts electromagnetic induction type and magnetic field resonance type. For example, Volvo uses the road for wireless charging. In Sweden, the Volvo Group, the Swedish power company Alstom, and the Swedish Energy Agency are working together to test the use of roads to charge electric vehicles. By laying two power lines on the road, the electric vehicles can be supplied with electricity. The core of this technology is that the car has to be equipped with a collector, the collector is connected to the cable on the road, and it is charged by DC. The car does not have to be in the center of the cable, but it must be more than 60 kilometers per hour.

 Electric Vehicle

3.3 Biomedicine

Artificial organs, kidneys, liver; hearing aids, pacemakers, etc. Cardiovascular, tumor and other diseases can be treated by implanting microdevices into the human body. At this time, the implant is charged by wireless charging technology to avoid implanting the wires in the body. This contactless power transfer greatly improves the quality of life of patients undergoing transplant surgery because it is more comfortable and there is no risk of contamination. The picture below shows Stanford University and the medical device developed in May 2014 that only the size of the rice is implantable and wirelessly charged.

medical device

 3.5 Space Power Generation

The satellite solar power station is used to send materials such as solar panels or solar concentrators to a geostationary orbit of 35,800 km above the equator with a launch vehicle or space shuttle. The sun's rays in space do not have the influence of the Earth's atmosphere, and the radiant energy is very stable. It is an inexhaustible clean energy. And 99% of the time in the year is daytime, and its utilization efficiency is 6-15 times higher than that on the ground. If you use solar panels to convert sunlight directly into electricity, or use solar concentrators to converge sunlight as a heat source, you can generate electricity like a surface thermal power plant. The generated electric energy is supplied to the microwave source or the laser, and then the high-power electromagnetic beam is transmitted to the ground by the wireless power transmission technology, and the received microwave energy is converted into direct current through the rectifier, and is supplied to the user by the variable and power distribution facilities.

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