Design of fast charging circuit module for handheld device based on RFID

　　Radio Frequency Identification (RFID) technology, as a high-tech technology for rapid, real-time and accurate information collection and processing and the basis of information standardization, has been recognized by the world as one of the top ten important technologies of this century, and has broad application prospects in various industries such as production, retail, logistics, and transportation. RFID technology has gradually become an indispensable technical tool and means for enterprises to improve the level of logistics supply chain management, reduce costs, informatize enterprise management, participate in the international economic cycle, and enhance competitiveness.

　　The implementation of logistics supply chain management system based on RFID technology requires various RFID reading and writing devices. Handheld RFID reading and writing devices occupy a large market in logistics applications due to their portability and ease of use. However, most handheld RFID reading and writing devices on the market now have high power consumption. In order to extend their working time, they need to be powered by large-capacity lithium batteries. How to provide a method for fast charging of lithium batteries is a problem that this article needs to explore. This article designs a DC-DC conversion circuit that meets the power consumption requirements of RFID handheld devices, as well as the corresponding lithium battery fast charging circuit.

　　Basic principle of boost circuit: The principle of commonly used boost circuit is shown in the literature. The working process of this circuit to achieve boost can be divided into two stages: charging process and discharging process. The first stage is the charging process: when the transistor Q1 is turned on, the inductor is charged, and the equivalent circuit is shown in Figure 1 (a). The power supply charges the inductor, and the diode prevents the capacitor from discharging to the ground. Since the input is direct current, the current on the inductor first increases linearly at a certain ratio, which is related to the size of the inductor. As the inductor current increases, a large amount of energy is stored in the inductor.

　　The second stage is the discharge process: when the transistor Q1 is turned off, the inductor discharges, and the equivalent circuit is shown in Figure 2 (b). When the transistor Q1 changes from on to off, due to the current holding characteristics of the inductor, the current flowing through the inductor will not become 0 instantly, but slowly change from the value at the time of charging to 0. The original path has been disconnected, so the inductor can only discharge through the new circuit, that is, the inductor starts to charge the capacitor, and the voltage across the capacitor increases. At this time, the capacitor voltage can reach a value higher than the input voltage.

　　 Design of boost circuit: The boost circuit uses the RT9266B high-efficiency DC-DC boost chip from Richtek Technology. The RT9266B has the characteristics of low power consumption, low quiescent current, high conversion efficiency, and simple peripheral circuits. The chip has an adaptive PWM control loop, error amplifier, comparator, etc. Through an external feedback circuit, the output voltage can be set to any required amplitude with high voltage accuracy. The circuit diagram is shown in Figure 2.

　　As shown in Figure 2, the boost circuit stores energy through an external 10uH inductor, uses feedback resistors R1 and R2 to control the output voltage of the boost circuit, and uses the internal PWM controller of the RT9266B to control the conduction and cutoff of the NMOS tube to control the output current of the boost circuit. Since the chip has an adaptive PWM controller inside, it can adapt to a larger load variation range. When the boost circuit is used to boost a 3.7V 2000mAh polymer lithium battery to 5V, the output voltage ripple is only 40mV, and the maximum output current can reach 500mA.