A circuit design of a position-free low-power wireless charging system

　　A complete wireless charging system consists of two parts: the TX end and the RX end. The structure of wireless charging is similar to a hollow transformer, and energy transmission is achieved through coil coupling. Usually the transmitting coil and its driving circuit are installed in a charging plate, and the receiving coil and its driving circuit are embedded in the device to be charged, such as a smartphone. The efficiency of energy transmission is related to factors such as the distance between the coils, the degree of coil alignment, the direction of the coil, the coil material, magnetic field shielding, impedance matching, transmission frequency and duty cycle. Among them, the distance and alignment between the coils have a great influence on the transmission efficiency.

　　Circuit principle: An array of three transmitting coils is used to expand the charging area in order to obtain better charging efficiency and experience. The BQ500410A enables the three transmitting coils in turn at a time interval of 400 ms, and enables the analog switch of the corresponding COMM feedback signal path at the same time. The BQ500410A will look for the strongest COMM feedback signal, and then drive the corresponding transmitting coil to work to obtain the best coil matching. Therefore, only one transmitting coil is working at the same time, and the other two transmitting coils are in standby mode. In order to reduce electromagnetic radiation, the wireless charging system also adds a ferrite magnetic isolation sheet on the back of the coils at both ends of the transmitter and receiver, so that the energy transmission area is limited to between the two magnetic isolation sheets, avoiding the radiation generated when the wireless charging system is working to interfere with smartphones or other devices.

　　The low power consumption of the system is achieved by adding TI's MSP430 low-power MCU to cooperate with BQ500410A. In order to achieve low power consumption, one of the most direct methods is to directly turn off the power supply when there is no load to completely shut down the BQ50041A. However, if this is done, the charging status, error status, operation mode and drive pin status information will be completely lost. After adding MSP430, BQ500410A can be shut down periodically to save power consumption, and its wake-up signal is provided by MSP430. At the same time, various status information is also saved by MSP430, and the LED status indicator is also driven by MSP430 instead of the original BQ500410A. In this way, although the complexity and cost of the system are increased, the standby power consumption is reduced from the original 300 mW to about 90 mW.