Wireless charging is a process in which an alternating electric field is sent through a coil, and the coil on the back of the device is used to receive electrical energy and store it into the battery of the device. The earliest application is not a mobile phone but an electric toothbrush. At present, the application has become mature, and it is used in electric toothbrushes, mobile phones, shavers, headphones, watches, sockets, and car entertainment charging aids.
In the wireless charging circuit, under the control of the MCU, the transmitter converts a DC voltage into an alternating oscillation signal with an oscillation frequency of f by chopping (the MCU alternately controls the orderly on-off of the 4 mos tubes at the transmitter according to the value of f) , emitted by LC oscillation. After the LC end of the receiving end (parameter design is harmonically connected to the f frequency) after receiving the f reading rate signal (energy wave), it becomes a pulsating DC voltage with constant direction after full-bridge rectification, and then it is filtered, stabilized and LDo changed. The battery of the mobile terminal is charged into a stable standard DC voltage, and the MCU at the receiving end monitors the entire charging state (charging current, charging voltage and battery temperature and other parameters) in real time. Prevent overvoltage, overcurrent, overcharge, and prevent fire and explosion caused by high temperature rise. The receiving end and the MCU simultaneously send the series of monitoring signal parameter values to the MCU at the transmitting end. A temperature measurement NTC thermistor for temperature detection is embedded next to the charging circuit and the battery, and works together with the MCU to track the instantaneous temperature change of the wireless charging operation.
Specific design ideas of wireless charging circuit:
1. High-power wireless charging circuit:
For high-power wireless charging circuits such as new energy buses and new energy trucks, regardless of whether they use constant current or constant voltage charging, due to the high charging voltage and large charging current, it can be used at the entrance of the input voltage of the transmitter. A power-type NTC thermistor (located as NTC1 in the above figure) is set to prevent or weaken the harmful impact of the inrush current on the 4 MOS tubes, so as to increase the life of the MOS tubes and reduce the failure rate during charging.
In the pulsating high-voltage DC after full-bridge rectification at the receiving end, there will often be sudden large spikes and pulsating currents. In this position (as shown in NTC2 in the above figure), an NTC thermistor should be appropriately designed and placed to connect the four rectifier tubes and the rear end of the pulsating current. LDOs are also beneficial.
High-power charging to prevent the charging temperature from being too high (especially when the ambient temperature is already high, such as when the ambient temperature in summer is Greater than or equal to 35 degree ), set up an NTC temperature sensitive sensor near the MOS tube at the transmitter and near the rectifier at the receiver The temperature sensor system composed of components is used to monitor the temperature change of the charging process of the transmitting and receiving ends in real time, which is a necessary design for the thermal design safety system of the charging system.
2. Low-power wireless charging circuit:
According to the actual situation of the low-power charging circuit, the thermal design ideas of the high-power charging circuit and the requirements of the product safety level can be imitated, and the number of power-type NTC thermistors or temperature-measuring type NTC thermistors can be appropriately increased or decreased.
