In terms of the Chinese market, the two major manufacturers, Apple and Samsung, have basically launched Wireless Charging handset models, which covers a large market. The transmitter is a very good market for China...
There are currently 3 different standards for wireless charging, but more than 90% of the market now uses the WPC Qi standard. One of the reasons why the market has grown so far today is that Apple announced in February last year that its iPhone 8/X will support wireless charging. Secondly, from the perspective of the transmitting end, the automotive industry is now very interested in wireless charging. About 100 models will use the front-loaded wireless charger this year. And now Qi wireless charging (receiving side output power) can reach 15W at the maximum, if such a large power is used to charge, mobile phone battery counts according to 3,000mAh, drive 40 minutes to be able to fill 60% of the mobile phone. Therefore, its practical value is very high relative to 5W. This will also further promote the development of the wireless charging industry.
According to market analysis agency IHS, the market for wireless charging receiver ICs will reach over 500 million units this year. The market is quite impressive. In fact, so far this year, 200 million mobile phones have been used for wireless charging, which is consistent with the data. In addition, for the Chinese market, Apple and Samsung, two major manufacturers, basically have wireless mobile phones, which covers a large market. The Transmitter (Tx) is a very good market for China - currently domestic Chinese companies produce a lot of transmitter products, including 15W, 5W and Apple's wireless fast charge products. In addition, last month, both Xiaomi and Huawei announced the built-in wireless charging flagship mobile phone, as well as supporting wireless chargers.
This is the "Wireless Charging and Fast Charging Technology Forum" jointly organized by ASPENCORE's "EET", "Electronic Technology Design (EDN)" and "International Electronic Business (ESMC)" China Edition. The good news about wireless charging was heard in the speech of Rui Liu, senior director of IDT product definition and system application.
His topic is “Wireless Charging: Transmitter and Receiver Design and Trade-offsâ€, which describes the principles of various wireless charging technologies, the main parameters to be considered when designing transmitters and receivers, and the practical application. Some questions arrived. Full of information, the following and the readers share.
Three kinds of wireless charging technology principles and advantages and disadvantages
Electromagnetic induction (WPC Qi standard). The principle of electromagnetic induction is similar to that of a transformer, that is, there is a primary coil (transmitter coil) and a secondary coil (receiver coil), and energy is transmitted from the primary to the secondary through a magnetic field. Liu said: “The only difference from the transformer is that it was coupled through the air instead of the magnetic coupling. The previous Electric Toothbrush can be charged by plugging it in. This is similar to the truth, except that the magnetic core is three-dimensional. We are Plane.†The most critical issue of this technology is coupling, so that the efficiency is high.
Because it is based on the principle of transformers, so its power and frequency is not too high, such as WPC standard frequency is less than 205kHz. In addition, when designing the transmitter, consider the EMI and regulatory requirements in the area of ​​sale.
Electromagnetic resonance (A4WP standard). This standard frequency is relatively high. It is implemented using magnetic resonance. Liu said: "This is a bit like a radio - it needs a resonant circuit, tuned to that frequency to receive the tone, not tuned to receive the tone." That is, the magnetic resonance receiver should be transferred to it. The operating frequency is 6.78MHz. If this frequency is not adjusted, it will not receive power.
The A4WP's specification is more "elastic": First, it can be one-to-many, and second, it can be placed casually. In addition, it is relatively easy to communicate between the transmitter and receiver, using Bluetooth Low Energy (BLE) instead of in-band. Its disadvantage is that it uses a relatively large coil to achieve one-to-many, so there is a case where the magnetic field is not fully utilized and the energy is saturated. It is quite good that the current efficiency can be 50% (up to 55%). The second is high costs. Liu pointed out: "If you have DC-DC Power Supply in your seat, you can understand that the efficiency of 1M and 6M switching power supplies is completely different. It is difficult to figure out the efficiency of 6.78M, because 6.78M is a communication frequency band with frequency band requirements. The accuracy is higher, it must use a crystal oscillator to avoid interference, so the cost is relatively high.In addition, its coil is relatively large, there is magnetic field (ie EMI) radiation, and 6.78M has no effect on the human body, it is not yet good The conclusion."
Electric field coupling. At present, electric field products have not yet been seen in the market. Even if they are, they are transmitted in accordance with the principle of electric fields under extremely special circumstances. At present, electric fields do not have high-power products and there is no market.
How does wireless charging work?
The first is to have a transmitter, to have a DC power supply, and to use a half-bridge to generate a square wave. Then through an inductor and capacitor, from DC to AC. Once the alternating current is generated, it will generate a magnetic field through the inductor—especially the WPC's Qi standard, which uses the magnetic field to transmit energy.
Second, with this magnetic field, there must be a receiver to receive this energy. When the alternating magnetic field passes through the receiving coil, it will generate an alternating voltage. Then you need to use a full bridge (or half bridge, full bridge more) to rectify, to produce a relatively stable DC voltage. Once the loop is closed, there will be current transmission, which enables wireless charging.
Liu added: "Of course, the magnetic field below the transmitter coil is not needed. If there is a circuit board or other metal underneath, eddy current losses will occur. Similarly, the magnetic field above the receiver is not needed in the air. Therefore, no magnetic field is needed. In order to shield, this is why all magnetic coils are provided with a magnetically shielded material."
In addition, energy needs to be controlled. The demand side of energy is the receiver. The receiver needs to send a signal to tell the transmitter that the energy is insufficient, and then the transmitter increases or decreases the energy. Communication can use BLE, but now WPC uses the amplitude shift keying (ASK) (In-Band), that is, amplitude modulation mode, the amplitude modulation frequency is 2kHz; more than 100 kHz power through the carrier, amplitude modulation, you can add the signal Go in. This becomes a complete wireless charger.
Determine coil size and distance
The system can work with this step, but we also need to determine the coil size, as well as the distance. The following figure shows the magnetic field distribution of the transmitting coil. It can be seen from this that the magnetic field density is concentrated in the middle zone. The further away from the transmitter coil, the lower the magnetic field strength. Therefore, to be a transmitter with a relatively high efficiency, the receiver coil and the transmitter coil cannot be separated too far. At present, the Qi requirement is less than 8mm, usually 4~5mm. Second, the smaller the coil, the lower the efficiency. If the receiver's coil can fully enclose the transmitter's magnetic flux, then efficiency will increase.
Because it is the principle of using a transformer, it can be implemented using the transformer's mutual inductance principle. To solve the design instability, you can solve the following mutual inductance formula.
The coupling coefficient k is ideally 0.2 to 0.8, and the range of variation is large. The coupling to the middle is better. The smaller the coil is, the poorer the coupling is. Knowing the inductance of the two coils and the mutual inductance between them, the k value can be calculated.
Effects of longitudinal, lateral and angular displacement on coupling coefficient
In the figure below, the left side shows the coupling of the receiver and the transmitter coil, and the right side shows the relationship between the coupling coefficient k and the displacement d. The misalignment includes the vertical, horizontal, and angle conditions. Among them, longitudinal changes have a particularly large impact; lateral changes (for example, within 5 to 7 mm) have a relatively small effect. This curve also explains that a WPC-compliant receiver (such as a mobile phone) is placed on the charging board and moved 5mm from the center outside the center, which has no significant impact on efficiency; 7mm is relatively large. Liu said: “The distance between z is within 5mm and the xy/radius is not more than 5mm. The efficiency is better. How good is it? If the 5V input, 5V output, 5W, if the receiver coil AC resistance (ACR ) If it is less than 120mΩ or 150mΩ, then the efficiency can be 78~80%; if it is 12V input, 12V output, and 15W, the efficiency can be 87~88%, depending on the coil of the receiver. That is ACR." Why is the receiving coil? Because WPC requires very high transmitter coils. To design a transmitter, it must be done according to its reference design. In other words, there is basically not much room for change in transmitter design, and receiver coils do not have much regulation.
The green part in the figure below is a better curve. The vertical axis is efficiency, and the horizontal axis is the coil distance. When the coil size ratio or distance is too large, the efficiency will be worse.
Liu emphasized: “We basically use the transmitter coil as the standard. Why? Because the transmitter coil is controlled by the WPC, there is no way to change the coil. When doing Qi certification, you must tell which coil is used, which A transmitter reference design. The certification unit will check whether the transmitter passes or not according to the design criteria. If the inspection fails, it needs to be redesigned."
If we can meet the parameters in the green part of the figure, then the efficiency can be better (more than 70%).
Qi's limitation is that it cannot be far away. Liu pointed out: “When I discussed wireless charging with my customers and colleagues, we all had the wrong understanding that we thought that our wireless charging should be when I put my mobile phone in my pants pocket and walk in my house to charge it. This cannot be achieved in Qi's standards, but now people are studying long-distance wireless charging."
Power transmission efficiency optimization summary
The following summarizes how to achieve the maximum efficiency of power transmission for the Qi standard.
1. The receiver coil must not be too far away from the transmitter coil (control within 5mm is optimal);
2. The receiver coil should not be too small, but should be as large as possible with the transmitter coil (currently in the mobile phone receiver coil is to achieve diameter 35 ~ 54mm. The big advantage of the coil is that it is easy to pair with the transmitter, The disadvantage is that the cost is relatively high);
3. Use some alignment methods, such as visual (image), mechanical or magnet (such as wearable products). When it comes to magnets, there is one point to note: the permanent magnets are placed in the center of the transmitter and have a small impact on the transmission loss. They will reach 5 to 10%. Therefore, it is recommended that three magnets be used outside the coils.
4. Use Litz wire to reduce coil AC resistance as much as possible;
5. Design a suitable coil gain (output voltage/input voltage) to maintain optimum operating conditions.
Emphasize that the transmitter side, in DC-DC or even DC-AC, has an enable pin for self-protection. On the receiver side, the rectification is 4 MOSFETs (several hundred V) and there is no way to turn it off, otherwise power consumption is too high. Therefore, the self-protection of the receiver is very important, especially in the case of high power (above 5W) and high voltage (above 9V).
Qi's communication uses 2kHz/500μs ASK. Liu emphasized that wireless charging is a system with a receiver and a transmitter, the receiver is the master, and the transmitter is a slave. The transmitter needs the receiver to tell it how much power it needs. Therefore, communication must not be interrupted. Otherwise, problems will arise. If the receiver's self-protection does not work well, greater problems will arise.
ASK communication can be either voltage or current. At present, it is better to do this through voltage and current, which can improve reliability.
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