This article will analyze the factors that restrict wireless charging speed from the following four parts:
1, lithium battery charging principle
2, review the principle of wireless charging
3. How do mobile phones on the market enter the wireless fast charging?
4, how to improve the charging speed of mobile phones?
Reading tips:
The first part and the second part are comparative techniques;
The second part explains the root cause of determining the wireless charging speed, it is recommended to read;
The third and fourth parts are closer to the actual application, and everyone can choose to read first.
First, the principle of lithium battery charging
The front of the lithium battery usually has a battery charging chip (charger) that controls the charging speed of the battery.
What is a battery? Think of it as a large capacitor, and the current flowing into it determines its charging speed.
Lithium battery charging is divided into four phases:
1. Trickle charging: When the battery is very small and almost completely discharged, the phone is pre-charged. At this point, the battery charging chip will output a small current, usually 1/10 of normal charging. For example, if the normal charging current is 1A, the trickle charge can be 100mA.
2. Constant current charging: When the trickle charge is completed, the battery charging chip will increase the current output and make the current constant to continue the charging current. During the constant current charging phase, the battery is charged most of the time.
3. Constant voltage charging: When the battery is nearly full, the battery charging chip will have a constant output voltage. The magnitude of the charging current is determined by the internal resistance of the internal voltage of the battery, the internal voltage and the output voltage. Namely: charging current = (output voltage - internal voltage) / battery internal resistance.
4. Stop charging: There are two ways to stop charging: stop charging when the charging current is lower than a certain value in the constant voltage phase; or start timing from constant voltage charging and stop charging after two hours.
Most battery charging times are in the constant current charging phase. If recognized as a fast state of charge, the battery charging chip will output a large current; if recognized as a slow state of charge, the battery charging chip will output a small current. For safety reasons, when the internal temperature of the mobile phone is too high, the battery charging chip also outputs a small current to reduce the power loss inside the mobile phone.
Second, the principle of wireless charging review
On the mobile phone side, the wireless charging system will set the target voltage for the LDO's output, typically 5V or 9V, to power the battery charging chip.
The transmitter is used to control the input voltage of the LDO (VRECT) to ensure that the LDO input voltage is slightly higher than the output voltage, thus making the LDO's conversion efficiency the highest.
The battery charging chip actually controls the charging current for battery charging. The main considerations are two factors: 1. Does the phone enter a fast charging state? 2. Is the temperature too high?
The figure below shows a typical charging curve for a wireless charger to charge a Samsung mobile phone. Trickle charging is not resolved and will be omitted in many cases. When the temperature is too high, the battery charging chip will automatically reduce the charging current.
Question and answer
Q: It is often asked that the phone should not be so powerful, and the transmitter should be hard plugged in, so that the phone will charge faster?
A: No, it will slow down. Because when the transmitter is hard connected to the receiving end, the voltage of VRECT will rise, but the output voltage VOUT of the LDO will not change, the constant output current controlled by the battery charging chip will not change, and the efficiency of both LDOs will deteriorate. . Equivalently, the energy of the hard plug to the receiving end will be consumed due to the loss of the LDO, and the temperature of the mobile phone will rise. If it rises to a certain extent, it is more likely to cause the battery charging chip to reduce the output current.
Third, how do mobile phones on the market enter wireless fast charging?
Apple phone: Identify whether the wireless charging switch frequency is around 127.7kHz. If yes, please enter fast charging.
Samsung mobile phone: Samsung will send a command to TX to request fast charging. If TX tells Samsung mobile phones via FSK communication, I can support your fast charging, then both handshaking and enter fast charging. Weak encryption has been cracked, and Samsung also announced the agreement at the recent WPC meeting.
Xiaomi MIX2S: A powerful encryption algorithm that is almost impossible to crack. Support for Xiaomi fast charging requires a millet license. At present, only Xiaomi white wireless charger supports Xiaomi fast charge.
Huawei P20 Porsche Edition: The same as the Xiaomi MIX2S. At present, only Huawei Porsche wireless charger supports Huawei wireless fast charging.
LG V30: The official WPC website shows that it has passed EPP certification and supports the EPP Fast Charge Protocol, which means that EPP-certified transmitters can support fast charging.
Sony XZ2: Same as LG V30.
Voda 15W-EPP product NU1509 + NU1015
Vodafone has introduced a 15W solution that complies with the WPC EPP protocol, which uses the standard Apple fixed-frequency voltage-regulated fast-charging architecture to support fast charging of Apple Express, Samsung Fast Charge and EPP-certified handsets. The entire solution uses the third-generation wireless charging architecture SoC + SmartBridge. The advantage of this architecture is not only the increased integration and cost reduction, but also the intelligent full bridge can improve wireless by capturing more comprehensive information in the full bridge of the power supply. Charging performance.
Fourth, how to improve the charging speed of mobile phones?
Meet the conditions of mobile phone wireless fast charging;
Controlling the mobile terminal VRECT voltage to stabilize at a target value;
The adjustment of the fixed frequency voltage regulation mode is more linear and it is easier to satisfy this condition;
The steps of the inverter control mode that need to meet the frequency change are small enough that the VRECT will deviate significantly from the target voltage. Many inexpensive MCU solutions are at risk of inaccurate frequency changes, failure to pass authentication, and wireless charging of mobile phones.
The duty cycle adjustment scheme must also meet the adjustment accuracy of the duty cycle high enough, otherwise the VRECT will have large deviations and fluctuations from the target voltage, which is relatively high for the MCU. At the same time, due to the duty cycle scheme, it is difficult to adjust the duty cycle to a very low level, otherwise hard handoff and communication problems may occur. In low power transmission, the power transmitted by the transmitting end is greater than the demand value of the mobile phone, and the voltage of the VRECT is much larger than the target value.
Lower the surface temperature in contact with the phone;
The coil should not be placed on the surface of the emitter and isolated by air. Air is the best insulating material to prevent heat from passing through the coil to the surface of the transmitter housing.
Add a heat sink under the magnetic isolation material at the bottom of the coil to get the heat from the coil as quickly as possible.
Improve the efficiency of the TX transmitter.
Increase the convection of the air and the outside world, and accelerate the heat dissipation, such as adding an electric fan at the transmitting end.
Charge the phone when the ambient temperature is low or there is air convection. For example, clip the car's wireless charging stand to the air conditioning vents.
The second phase of the Woda Academy will stop here. Any questions can be posted directly at the bottom of the article or at a public number. You are welcome to discuss each other. Voda hopes to create a more open wireless charging development environment through Voda Academy to promote the development of wireless charging.
