Analysis of the Principle and Practical Operation of Wireless Charging

introduction

After several years of promotion and evolution, wireless charging technology has finally begun to attract people's attention. Wireless charging refers to the charging of battery-powered devices through wireless induction. It is precisely because of its convenience that consumers are willing to pay extra fees to purchase wireless charging related products; as a result, a large number of manufacturers are willing to invest in related product development and are optimistic about its business opportunities. Since this technology is quite new and each manufacturer has its own description of the technology, wireless charging, inductive power, contactless charging, and contactless charging all refer to the same technology. The distance is wireless from 1mm to several meters. The interaction between the power supply end and the power receiving end is called induction, so wireless charging is a broad term without specific specifications.

1. Simple in principle, difficult in practice

There are many methods for wireless charging developed in the experimental stage, but the only one that has the opportunity to be mass-produced and commercialized is coil induction. The principle of coil induction is very simple. It is a physical phenomenon discovered a hundred years ago, but for a long time in the past, such coil induction was only used in wound transformers. In the early days, it was discovered that the "E"-shaped iron core of the wound transformer can be connected to the mains after winding the wires and then closely connected to the mains, so that electricity can be transmitted by induction, but the induction effect disappears after the distance is slightly separated. This is because at 60Hz of the mains, the energy of electromagnetic wave transmission will decay rapidly as the distance increases. In today's applications, since the device itself needs to be packaged in a shell, the shell thickness of the transmitter and the receiver is at least 3mm. In the early development of electric toothbrush products, it was found that when the distance is increased, the operating frequency of the coil needs to be increased to allow the power to be transmitted farther; there is a characteristic in electromagnetic waves, that is, the higher the frequency of electromagnetic waves, the lower the energy attenuation after the longer the transmission distance. Later, RFID applications began to develop, mainly planning three frequency bands: LF low frequency (125~135KHz), HF high frequency (13.56MHz), and UHF ultra-high frequency (860~960MHz). These frequency bands also created the reference points for the frequency used in the design of the current wireless power system. As early as 10 years ago, wireless charging of electric toothbrushes was already on the market. At that time, the transmission power was small and the charging time was long. Considering the power consumption of current smart handheld devices, the charging energy at that time was not enough to use, so it has not been practical for 10 years. However, in the past few years, new technologies have been developed that can operate with higher "resonance" receiving efficiency. Since this technology is relatively new, there are many opinions from all walks of life, but they all have a very important feature, that is, there will be capacitors configured on the receiving coil to form a receiving antenna with frequency characteristics, which can obtain greater power transfer at a specific frequency. This part is different from the early electromagnetic induction. When the distance is increased, a good power transmission effect can still be obtained. The principle of resonance is very simple, just like a piano tuner who puts different amounts of water in a glass, and with precise tuning, a certain glass can be broken through resonance. However, other articles have not mentioned that if an ordinary person who has not been trained as a professional piano tuner may never be able to tune a frequency that can make a glass break! This is the principle that is simple, but the practice is difficult.

2. Easy to display, difficult to market

Electronic parts are like untuned pianos when they leave the factory. Pianos can produce high-quality sounds after being precisely tuned by professional tuners. When mass-produced, it may not be possible to have each product tuned by professionals before shipment for cost considerations. If each product needs to be corrected by professionals, it will be difficult because there are limited professionals. This is like many wireless charging products that have been released a long time ago. The products can perform perfectly at the press conference, but after a long wait, the products have not been launched yet? Just like the example just mentioned, wireless charging products must be precisely tuned to achieve a good resonance effect. Under such circumstances, mass production will become very difficult. Therefore, the design of the wireless charging system must first be able to self-adjust the resonance part, so as to solve the mass production problem. In 2008, INTEL released a product that can transmit electricity from two coils one meter away to light up a 60-watt light bulb. At that time, it also announced that the era of wireless power has arrived; but three years have passed and related products have not been launched. Think about it carefully. Even if such a strong electromagnetic energy has no effect on the human body, it will have a very strong lethality to the surrounding electrical products. The principle of wireless power system is the same as that of induction cooker. It transmits energy through electromagnetic waves, but the target is different. Induction cooker uses a frequency of about 50KHz. After the energy is emitted, it heats the pot for cooking. In the past, there was a video circulating on the Internet that showed a mobile phone placed on the surface of an induction cooker in operation. In a short time, the mobile phone burned. The principle is the same. The electromagnetic waves will pass through the shell of the mobile phone and directly heat the metal structure inside until it burns. As mentioned in the previous article, in order to extend the transmission distance, the transmission frequency must be increased. The frequency of the induction cooker is low and decays to below the safety limit after a few centimeters away. INTEL announced that the frequency must be increased to about 13MHz to transmit power at a distance of one meter. In this case, if there is a metal object between the coils, it will be heated and dangerous. During the performance, the staff can stand between the two coils without danger because there is little metal in the human body, so the temperature rise is limited. When the frequency of the electromagnetic wave is increased to more than 1GHz, it will directly heat the water molecules; this principle becomes a microwave oven. The water molecules are stirred by the electromagnetic waves and emit heat. Therefore, microwave ovens are different from induction cookers. They must be operated in a shielded body to avoid harming the human body. This part is different from the wireless communication products on the market because the energy gap is huge; the wireless power system needs to transmit power to the power receiving device, so high power transmission is required. After the wireless communication product receives the low-power signal, it amplifies the signal through the internal battery. Therefore, whether it is heating the metal at 13MHz or directly harming the human body above 1GHz, the wireless power must solve the safety problem when designing before it can be put on the market. This is why it is easy to demonstrate but difficult to put on the market.

3 Three major performance indicators - efficiency, safety, and power

Electric toothbrushes have been wirelessly charged for 10 years. At that time, due to the low power demand, there was no need to consider efficiency and safety. The conversion efficiency of early systems was only 20%-30%, and there was no safety mechanism and it would not identify the target and continuously supply power. Such a system is the same as a miniature induction cooker. Since the power is very small, the receiving demand is only about 0.1W. With only 20% conversion efficiency, 80% of the energy is converted into heat dissipation during transmission. It is inferred that the transmitter provides 0.5W of energy to the receiver, which is 0.1W of energy. The heat generated by 0.4W is limited and the temperature rise of the system is not obvious. The maximum output capacity of the system is also not large, that is, 0.5W, so placing metal foreign objects on the transmitter will not cause danger; but today's device demand is much higher than 0.1W. From the perspective of the popular smart phone, the receiving needs 5V-1A, that is, 5W of charging energy. If the system of the electric toothbrush is used for design, there will be great problems. The 5W demand of the receiving end has 20W of energy converted into heat dissipation with only 20% conversion efficiency. Such energy will generate huge heat energy and cause the system temperature to rise sharply. Under such calculations, the maximum output capacity of the system will be 25W. If there is no safety design, placing metal foreign objects on the transmitter may cause fire accidents. Therefore, the problems derived from the increase in power demand require a new design to complete wireless charging. Therefore, wireless charging, which appeared 10 years ago, is still under improvement. In order to achieve the target power, the newly designed system must first solve the problems of efficiency and safety.

4 High conversion efficiency relies on advanced specification parts and materials

Today’s wireless charging systems are all designed using resonance, and their architectures are roughly the same, with the following structures:

4.1 Transmitter has

A. DC power input

B. Frequency Generator

C. Switch to switch the power

D. Transmitting coil and capacitor resonant combination

4.2 The receiver has

A. Receiving coil and capacitor resonant combination

B. Rectifier

C. Filtering and voltage regulator

D. DC power output

In this architecture, every link from the transmitter 1. DC power input to the receiver D. DC power output is a key point of efficiency loss. In the power circuit, every component with impedance characteristics that the current passes through will lose some energy on it. The advancement of materials in recent years has also greatly increased the practicality of wireless charging. Among them, several advanced components are related to transmission efficiency in wireless charging systems. In order to achieve high conversion efficiency, these components and materials need to be combined.

A. Frequency generator: Currently, several companies have developed this part into IC for sale. It is a key component on the transmitter circuit board.

B. Switches for switching power: Most are made of MOSFET, and low on-resistance and high switching speed are the key points for selection.

C. Transmitter/receiver coil and capacitor resonant combination: This is a technology that has never appeared before. Since there are no rules to follow, it can only be achieved through continuous trial and error. In addition, in order to prevent excess energy from dissipating to other places, magnetic materials are placed on the non-sensing side of the coil. This type of material property also has new applications.

D. Rectifier: Since the operations on the coil are all high-frequency, high-voltage energy signals that need to be effectively converted into direct current before they can be used by the receiving device, most of them are currently composed of ultra-low VF Schottky diodes.

E. Filtering and voltage regulator: This part is difficult because the space of the receiving device is limited and it is difficult to miniaturize the design. Usually, high conversion efficiency circuits are configured with large passive components.

5. The most difficult part of design - safety

As mentioned earlier, wireless charging systems emit electromagnetic wave energy just like induction cookers. This has two major problems:

One is that when there is no target charging device on the transmitter, it still transmits energy, which will cause energy waste in a long time and does not conform to the current trend of energy saving. Another more serious problem is that when a metal foreign object is placed on the transmitter, the electromagnetic wave heats it; this situation may burn the device at the least, or cause a fire that endangers the life and property of the personnel. Therefore, if the wireless charging system is to be marketed, it must have an important function, namely "receiving end target identification". When the correct target is placed on the transmitter, it will start to send power, otherwise it will not send power. There are many ways to detect close-range devices, but there is a problem in the wireless charging system that it is impossible to use expensive parts to complete this function. Remember that the current design is just a charger, and if the cost is too high, the market will not accept this function.

There are currently two practical methods to accomplish this function: 1. Magnetic activation: Install a magnet on the receiving end, and when the transmitting end senses the magnetic force, it starts to send energy. This method is simple and effective because no one will accidentally put a magnet on the transmitter and let it burn. 2. Data transmission on the induction coil: This is currently considered the safest method. It is the same as the principle of RFID. It uses the power transmission in the two coils to transmit the data code together. This method is the safest and the most difficult to complete because there is high-energy power transmission on the induction coil, and it also includes the interference of system noise and load current changes. How to effectively transmit the data code is a big problem.

6. Variable power systems need to be based on data transmission mechanisms

An ideal system is to place different receivers on the wireless charging transmitter. The receivers can be different devices, from low-power headphones to high-power laptops, and they should be able to respond to different targets; but the power requirements of each receiving device are different, so the transmitter must be able to automatically adjust the power output. However, such a function must be based on the transmitter and receiver being able to transmit data codes to communicate, so how to use the coil of inductive power to transmit data codes is the key point of research and development. Several companies have invested in the development of this technology several years ago, and each company has different methods. The stability in practice also needs to be verified.

7. Wireless charging common standards are ideal but difficult to achieve

Currently, some companies are promoting wireless charging standards. The ideal standard is that it can be used across brands. This is a very idealistic goal. The so-called standard requires two parts to be regulated in order to operate; the first is to have a common resonant frequency. Power transmission needs to be transmitted through a preset resonant frequency. The frequency of the electromagnetic wave energy provided by the transmitter needs to be the resonant frequency of the receiver to obtain good conversion efficiency. The second is a standard data transmission code or other identification activation method. The transmitter needs to correspond to the correct receiver to start sending power. A common standard is indeed what the market expects. At present, the group that promotes the standardization of wireless charging has been operating for a long time, but there are still few products on the market. After a deep understanding of this part, some problems can be found. One part is that the standard is not yet complete, so that R&D personnel cannot successfully complete the product according to the specification; another problem is that the standard is not free. Before the product is launched, it is necessary to pay the royalties of the relevant patents. Therefore, a common standard is the trend of the future, but the actual application is not yet mature.

8 Three key components affect three industrial chains

As far as wireless charging products are concerned, there are three key components, including control circuit boards, induction coils, and magnetic materials.

Wireless charging is still in its infancy, and the market is expected to grow rapidly in the next two to three years, and will become one of the standard specifications for branded products in four years. The growth of this market will affect not only the electronics industry, but also the mechanical industry, as the induction coil requires the production of precision jigs, and the chemical industry, as the coil requires the use of magnetic materials with high-efficiency electromagnetic wave shielding capabilities. Therefore, the growth of a product can affect three industrial chains, because this product is not a product that has already existed in the past, but a brand new category and a brand new application. The relevant materials must be redeveloped and produced, and from a business perspective, this is also a new field that can be developed.