In this paper, a photovoltaic charging controller with MPPT function is designed. The charging voltage and current of the battery are detected by single chip computer, and the working state of the charger is automatically switched, which effectively improves the life of the battery. When the illumination condition is insufficient, the voltage and current of the battery do not exceed the given value, and the MPPT function is turned on to ensure the maximum efficiency of the charging of the whole system. Matlab simulation shows that the design is feasible. Photovoltaic power generation technology can convert non-polluting and renewable solar energy into energy that can be easily stored, so people pay more and more attention to it in today’s global energy shortage and serious environmental pollution. However, photovoltaic power generation has been facing the problem of low conversion efficiency. Improving conversion efficiency is the focus of considerable attention in both civil and industrial fields. As the junction of photovoltaic power generation technology and storage battery, thermostatic element the photovoltaic charging controller plays a key role in improving conversion efficiency. However, the traditional battery charging technology can not track the maximum power point of photovoltaic cells, resulting in energy loss and low utilization efficiency. A photovoltaic charging controller with MPPT control is designed in this paper. On the basis of three-stage charging mode of storage battery, adding MPPT function and using conductance increment control strategy, the photovoltaic battery has the maximum power output when the light is insufficient, thus improving the power generation efficiency of photovoltaic battery and the service life of storage battery.
The charge and discharge control system of photovoltaic power generation is mainly composed of solar array, controller and storage battery. The controller shown in Figure 1 is mainly composed of DC-DC conversion circuit, detection circuit, driving circuit, microprocessor and so on. The charging current and battery voltage are obtained by AD sampling to control the charging state. At the same time, according to the state of the battery, to decide whether to carry out MPPT control, the microprocessor achieves the control goal by changing the duty cycle of the PWM. The parameters of solar cells selected in this design are as follows: peak power of 60W, open circuit voltage VOC = 21.3V, short circuit current ISC = 3.74A, maximum power point voltage VPM = 17.5V, maximum power point current IPM = 3.43A at T = 25 C, S = 1000W/m3.
The battery is lead-acid battery 12V-38AH. In order to meet the charging requirements of solar cells, buck BUCK converter is selected for DC-DC converter circuit, and its main circuit schematic diagram is shown in Figure 2. For the switch Q_1 of Buck DC-DC converter circuit, the driver circuit needs to be designed. The driver chip a3120 is used. The structure of the chip is shown in Figure 3. The chip is a photoelectric coupled MOSFET gate driver chip with a working voltage of 15-30V and a driving current of 2A on the output side. The chip uses photoelectric coupling to completely isolate the driving circuit from the control circuit. It is small in size, simple in structure and convenient in application. The PWM signal generated by the DSP is input to a3120 and output to the gate of the switch to drive the MOSFET, as shown in Figure 4. The parameters needed to be detected in the system are the voltage Ubat on the battery side, the charging current Ibat, the voltage Upv and the current Ipv at both ends of the photovoltaic cell. Voltage detection is relatively simple, through the selection of appropriate resistance value for voltage dividing can be no longer detailed. The detection of charging current I1 uses MAX471 current detection chip. MAX471 is a precise current sensing amplifier developed by MAXIM Company in the United States. It has built-in 35m precise sensor resistance. The upper and lower limits of the measurable current are ( 3A). The structure of MAX471 is shown in Fig. 5 and Fig. 6. OUT is connected with 2K 1% precise resistance at the output end to convert current into ground voltage output. The power characteristic curve of solar photovoltaic cells is a single peak function. The dP/dV=0 at the maximum power Vmax and the derivative of dP/dV at both ends of Vmax are not zero. If step=A*abs(dP/dV) is used as the step-size data in the admittance increment method, A will be larger at a long distance from the maximum power point and smaller at a close distance from the maximum power point.
Appropriate A can realize variable step tracking in the maximum power tracking process [3]. The flow chart of the algorithm is shown in Figure 7. In this design, an improved stage charging method is used to charge the battery.
The charging process is divided into three states, i.e. MPPT full charging state, constant charging state and floating charging state. The charging stage of the battery is judged by the charging voltage value of Ubat and the charging current value of Ibat. The control flow chart of the whole algorithm is shown in Fig. 8. According to the characteristics of battery parameters, the maximum charging current of the battery is 0.3C, and the maximum charging current is 11.4A. The charging state can be controlled by adjusting the turn-on and turn-off of the MOS transistor to meet the control requirements. The whole battery charging process is divided into the following three cases [4]. Constant charge state. When the charging current reaches 3.8A, the battery should be charged at constant current, and the charging current should not exceed 0.
1C. The whole process needs to monitor the charging voltage. Once the current reaches the set maximum, the constant voltage charging mode is turned on. Floating state. When the open-circuit voltage of the battery is Ubat > 14.2V, we think that the voltage range of the battery is normal. When the charging current is less than the cut-off current threshold, the charging circuit is cut off and the charging program is withdrawn. Solar cell model is built by using open circuit voltage, short circuit current, maximum power point voltage and maximum power point current of solar cell. Volt-ampere characteristic curve and power curve of solar cell are obtained. The simulation results are shown in figs. 9 and 10.
The simulation results show the correctness of the solar energy model. When the voltage and current of the storage battery exceed the set value, the MPPT function is turned on to detect the current and voltage at both ends of the solar cell, and the maximum power output is achieved by variable step size method. As shown in Figure 11, in this mode, the final voltage can be stabilized near the maximum power point. As shown in Figure 12, there are two independent PI control loops in the constant charge state, namely constant current and constant voltage. Firstly, the voltage at both ends of the battery is detected. If the voltage is lower than the threshold set by SWITCH switch, the current loop circuit is turned on and charges the battery with 10A constant current. With the charging going on, the voltage rises continuously. When the voltage exceeds the threshold, the voltage loop circuit is switched to charge the battery level with 13.2V constant voltage to achieve protection. The purpose of storage battery. The voltage, current and SOC curves of the whole charging process are shown in Figure 13. Voltage is in the rising stage, not reaching the limit value of 13.2V. At this time, it is in the constant current stage, and the current is basically stable at 3.8A. Through simulation, it can be verified that the design is improved on the basis of three-stage charging method, which protects the life of the battery. At the same time, according to the state of the battery, it can decide whether to turn on MPPT function to give full play to the maximum effect of photovoltaic cells.