The topological structure diagram of the designed main circuit is shown in Figure 2, which is mainly divided into three parts: capacitor step-down conversion circuit, BUCK conversion charging circuit, and signal detection and feedback circuit. The 220V AC input on the power supply side passes through two groups of capacitor step-down converters, a BUCK circuit and other links, making the input current and input voltage in the same frequency and phase, which greatly improves the power factor. After regulation, the charging voltage required by the battery pack is obtained. The output voltage is divided by three resistors; the voltage across R5 serves as the output voltage sampling signal, and the voltage across R6 serves as the output current sampling signal. The sampling signals are sent to the control center as closed-loop control signals. In the repair charging mode, the duty cycle is adjusted according to the current feedback quantity to make Q1 and Q2 conduct and turn off cyclically, that is, Q1 conducts when Q2 turns off, and Q2 conducts when Q1 turns off, so that the battery is charged and discharged cyclically, thereby achieving the effect of online repair while charging the battery.
The crystal oscillator circuit and reset circuit module are shown in Figure 3 below. The crystal oscillator circuit is generated by the internal clock mode, with XTAL1 as the input terminal and XTAL2 as the output terminal. A crystal oscillator and trimming capacitors are connected across these two pins externally to form a stable self-oscillator, providing an accurate clock signal for the microcontroller. The reset circuit adopts automatic power-on reset: the capacitor C6 is charged by the Vcc power supply, and C6 is selected as 10 microfarads. The high-level duration on the RST pin is longer than the high-level time required for reset, ensuring reliable system reset. In addition, a manual reset setting is provided to prevent program runaway and chaos caused by unstable power-on.
The design adopts the circuit shown in Figure 4 to provide power. The single-phase power-frequency AC voltage is stepped down by a 220:9 power-frequency transformer, then rectified by a rectifier bridge circuit and filtered by a capacitor before being supplied to three groups of 7805 voltage stabilization modules. The voltage waveform after the capacitor filter still has relatively large ripples, and the microcontroller and switching power supply require stable waveform voltage for power supply. Therefore, three groups of 7805 voltage stabilization modules are designed to output in series. It features good working stability and strong anti-interference ability.
Since the upper and lower groups of the capacitor step-down converter correspond to the positive and negative half-cycles of the AC side respectively, the upper capacitor converter is controlled to turn on and the lower group to turn off during the positive half-cycle of the sine wave, and the lower capacitor converter is controlled to turn on and the upper group to turn off during the negative half-cycle of the sine wave, so a zero-crossing detection module is set up. Figure 5 shows the zero-crossing detection circuit. During the positive half-cycle of the sine wave, D1 conducts forward and the comparator outputs a high level; during the negative half-cycle of the sine wave, D2 conducts forward and the comparator outputs a low level. The conduction states of the upper and lower groups of the capacitor step-down converter are controlled according to the output signal of the comparator.
The display part adopts an LCD1602 liquid crystal display, which has the characteristics of small size, low power consumption and strong anti-interference ability. In use, only the ASCII codes of the data to be displayed need to be written into the internal display data RAM (DDRAM), and the control circuit can display the data on the screen. The circuit principle of its display part is shown in Figure 6 below, which can accurately display information such as the voltage, current, power percentage and charging status of lead-acid batteries during charging.
The charging object of the on-board charger in this design is a lead-acid battery pack composed of 4 batteries connected in series, with a capacity of 210 AH. According to the manufacturing process and specification parameters of lead-acid batteries, as well as the demand for battery repair during charging, the software flow is shown in Figure 7.
After the charger is connected to 220V AC voltage, it enters the initialization program to judge whether the positive and negative poles are connected correctly. If not, the corresponding coil of the relay is controlled to pull in through the adjustment module to correct the phase sequence, and then power supply starts. Before the charger outputs DC voltage, the microcontroller first detects the terminal voltage of the battery pack. If the terminal voltage of the battery pack is higher than the normal value and lower than 76V, the charger starts to work. At this stage, it adopts a voltage and current double closed-loop control mode to enter the constant-current charging state. When the terminal voltage of the battery pack exceeds 86V, it switches to the constant-voltage charging mode. At this stage, the duty cycle at the end of the previous stage is kept unchanged for constant-voltage charging of the battery. As the voltage of the battery pack rises during this stage, the charging current gradually decreases. When the difference in the internal charging current is detected to be less than 0.5A, it switches to pulse repair charging, and finally the power supply is disconnected when charging is completed.
