Design of fast charging circuit for lithium battery of handheld device based on RFID

1 Introduction

Radio Frequency Identification (RFID) technology, as a high-tech and information standardization foundation for rapid, real-time and accurate information collection and processing, has been recognized by the world as one of the ten most important technologies of this century, and has broad application prospects in various industries such as production, retail, logistics, and transportation. Radio Frequency Identification 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 systems 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 easy-to-carry and easy-to-use characteristics. However, most of the 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.

2 Boost circuit

The supply voltage of a single lithium battery is 3.7V, and the working voltage of the RFID reader is 5V, so a boost circuit is needed for the RFID handheld device.

2.1 Basic principle of boost circuit

The principle of the commonly used Boost 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 retention characteristics of the inductor, the current flowing through the inductor will not change to 0 instantly, but slowly change from the value when the charging is completed 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.

2.2 Design of boost circuit

The boost circuit uses the RT9266B high-efficiency DC-DC boost chip of Richtek Technology. RT9266B has the characteristics of low power consumption, low static 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, controls the output voltage of the boost circuit using feedback resistors R1 and R2, and controls the conduction and cutoff of the NMOS tube using the internal PWM controller of the RT9266B 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 3.7V 2000mAh polymer lithium battery is boosted to 5V using this boost circuit, the output voltage ripple is only 40mV, and the maximum output current can reach 500mA.

3 Charging Circuit

3.1 Basic Principles of Lithium Battery Charging Circuit

The charging process of lithium batteries can be divided into three stages: pre-charging, constant current charging, and constant voltage charging. When the voltage of the lithium battery is lower than the minimum charging voltage, it first enters the pre-charging stage, charging the battery with a small current (usually 10% of the standard current) until the battery voltage reaches the minimum charging voltage. The pre-charging at this stage can prevent the lithium battery from being damaged by direct high current constant current charging after over-discharge. When the battery voltage is higher than the minimum charging voltage, the charging enters the constant current charging stage. Usually the constant current charging current is 0.5C (C is the capacity of the lithium battery). When the voltage of the lithium battery reaches the standard voltage, it enters the constant voltage charging state, and the charging current continues to decrease until the current decreases to 100mA.

From Figure 2, it can be seen that 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.

3 Charging Circuit

3.1 Basic Principles of Lithium Battery Charging Circuit

The charging process of lithium battery can be divided into three stages: pre-charging, constant current charging and constant voltage charging. When the voltage of lithium battery is lower than the minimum charging voltage, it first enters the pre-charging stage, charging the battery with a small current (usually 10% of the standard current) until the battery voltage reaches the minimum charging voltage. The pre-charging at this stage can prevent the lithium battery from being damaged by directly charging with a large current constant current after over-discharge. When the battery voltage is higher than the minimum charging voltage, the charging enters the constant current charging stage. Usually the constant current charging current is 0.5C (C is the capacity of the lithium battery). When the voltage of the lithium battery reaches the standard voltage, it enters the constant voltage charging state, and the charging current continues to decrease until the current decreases to about 100mA, and the charging is completed.

3.2 Design of Lithium Battery Charging Circuit

The schematic diagram of the lithium battery charging circuit is shown in Figure 3, which is implemented using TI's bq2057. The bq2057 series is an advanced lithium battery charging management chip suitable for charging single-cell (4.1V or 4.2V) or dual-cell (8.2V or 8.4V) lithium-ion and lithium-polymer batteries. BQ2057 can dynamically compensate for the internal resistance of the lithium battery pack to reduce the charging time; with optional battery temperature monitoring, the battery pack temperature sensor is used to continuously detect the battery temperature. When the battery temperature exceeds the set range, BQ2057 shuts down the battery charging; the internal integrated constant voltage and constant current device has high/low-side current sensing and programmable charging current. The charging status recognition can be realized by the output LED indicator or the interface with the main controller. It has the characteristics of automatic recharging, minimum current termination charging, low power sleep, and high voltage accuracy (better than ±1%). The charger peripheral circuit designed using this chip is relatively simple and is very suitable for the compact design needs of portable electronic products.

This circuit adjusts the frequency of the PWM wave output at the CC terminal through the induction resistor R5 at both ends of SNS and COMP to control the conduction and cutoff of the Q1 transistor, thereby realizing the control of the maximum charging current.

The circuit has been tested to charge a 3.7V 2000mAh lithium polymer battery. The maximum charging current can reach 810mA and the battery can be fully charged in 3 hours. The charging data is shown in Table 1:

From the table above, we can see that when the charging circuit shows full, the measured battery voltage is 4.12V, which is 0.5V different from the standard voltage of 4.2V. The reason for the error is that during the charging process, the charging current of the lithium battery fluctuates. When the current is lower than a certain threshold momentarily, bq2057 considers that charging is complete and shuts off the charging circuit.

4 Conclusion

This paper designs a fast charging circuit for lithium batteries of RFID handheld devices. Experimental data show that the lithium battery charging and boosting circuit designed using RT9266B and bq2057 can meet the needs of practical applications. In addition, the package size of the two chips is small and the peripheral circuit is simple, which is very suitable for power management of handheld devices.

Innovation of this paper: This paper designs a fast charging circuit for lithium batteries of RFID handheld devices, which can quickly charge the lithium batteries of RFID handheld devices.