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NTC thermistor used in wireless charging

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 as the receiving power and stored in the battery of the device. The earliest application is not a mobile phone but an electric toothbrush, and the current application has become mature. It has applications in electric toothbrushes, mobile phones, razors, earphones, watches, sockets, car entertainment charging assistants, etc.

In the wireless charging circuit, the transmitter, under the control of the MCU, transforms a DC voltage into an alternating oscillation signal with an oscillation frequency of f through a chopping method, and transmits it through LC oscillation. (The MCU alternately controls the orderly on and off of the 4 mos tubes at the originating point according to the value of f). After receiving the f reading rate signal (energy wave) at the LC end of the receiving end (parameter design to f frequency harmonic connection), it becomes a pulsating DC voltage with the same direction after being rectified by the full bridge. After filtering, stabilized voltage and LDo become a stable standard DC voltage to charge the battery of the mobile terminal. The MCU at the receiving end monitors the entire charging status (parameters such as charging current, charging voltage and battery temperature) in real time. Prevent overvoltage, overcurrent, overcharge, and prevent explosion and fire caused by excessive temperature rise. The receiving end and the MCU simultaneously send the series of monitoring signal parameter values to the MCU at the transmitting end simultaneously. A temperature-detecting temperature-measuring NTC thermistor is embedded next to the charging circuit and the battery, and working with the MCU, it can track the instantaneous temperature change of the wireless charging operation.

The specific design ideas of the wireless charging circuit:

Design NTC thermistor circuit for wireless charging

1. High-power wireless charging circuit:

For high-power wireless charging circuits such as new energy buses and new energy loan trucks. Regardless of whether they use constant current or constant voltage charging methods, due to the high voltage and large current, a power NTC thermistor can be set at the entrance of the input voltage of the transmitter (position as NTC1 in the figure above). In order to prevent or weaken the harmful impact of the surge current on the 4 MOS tubes, to increase the life of the MOS tubes and reduce the failure rate during charging.

The pulsating high voltage direct current after full bridge rectification at the receiving end will often have a sudden large spike pulsating current. In this position (as shown in the above NTC2), properly designing and placing an NTC thermistor is also very beneficial to the 4 rectifier tubes and the rear LDO.

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 ≥35°C). An NTC temperature sensitive element is set near the MOS tube at the transmitting end and near the rectifier tube at the receiving end to form a temperature sensor system. It is used for real-time monitoring of temperature changes during the charging process at the transmitter and receiver, and is a necessary design for the thermal design of the charging system and the safety system.

2. Low-power wireless charging circuit:

According to the actual situation of the low-power charging circuit, the thermal design ideas and product safety requirements of the above-mentioned high-power charging circuit can be imitated. Properly increase or decrease the number of power NTC thermistors or temperature measurement NTC thermistors.

The wireless charging power NTC thermistor produced by YXAUN Electronics has the corresponding specifications of MF72 and MF73T-1. It is mainly used for the input voltage entrance of the transmitter and the back end of the full bridge rectifier to prevent or reduce the impact of surge current on circuits and devices; Each high-power wireless charger needs to match 1-2 MF72 or MF73T-1 power NTC thermistors. According to the power requirements, the film diameter ranges from ø3mm to ø40mm, and the maximum steady-state current can reach 80A. The product has excellent performance and reliable quality. All products have passed UL, TUV, CQC safety certification.

YXAUN Electronics produces wireless charging temperature measuring NTC thermistor, the corresponding specification is MF52B. Mainly used for the power output terminal (wireless charging socket), to measure the temperature of the coil in time and feedback the control terminal to distribute the charging power; Each wireless charging output terminal is matched with 1-2 MF52B temperature measuring NTC thermistors. Generally installed in the center of the induction coil, the thickness of the head generally cannot exceed the thickness of the coil, so this type of MF52B has strict requirements on the size of the product head. Generally, the diameter of the head is required to be ≤1.5mm, and some terminal designs even require it to be ≤1.0mm.

The MF52B product has high reliability, good consistency, and long service life. It can work normally in very harsh environments and has passed the automotive-grade AEC-Q200 test. At present, the high-precision temperature-measuring NTC thermistor produced by YAXUN Electronics has significant advantages such as high accuracy, high stability, high interchangeability, and low price, and is well received by customers and favored by the market.

The various NTC thermistors of YAXUN Electronics can be used in wireless charging besides. It has also been widely used in other fields such as medical equipment, automotive electronics, household appliances, industrial automation, etc.
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