Research and design of the solar charging system based on FPGA Keywords:BUCK-BOOST,photovoltaic,MPPT,Weighted Variable-Step Voltage-Hysteresis Perturbation&ObservationAbstract:This article introduces an efficient solar charging system based on FPGA. The charging circuit composed of BUCK-BOOST switch converter, realizes the function of maximum power point tracking(MPPT) of solar cells by using high speed controller-FPGA and a new MPPT technology-Weighted Variable -Step Voltage -Hysteresis Perturbation&Observation(WVSVH P&O) method. This paper explains the design by its circuit configuration and control system, and simulates it in the simulink. The simulation results show that, this topology is able to achieve the purpose of charging efficiently with solar cells; The control strategy used here, compared with the conventional MPPT methods, shows its superiority of fast and efficient.0 Introduction After entering the 21st century, how to develop the economy sustainable under the double restrictions of the limited energy and environmental protection, has became a enormous historical topic human faced with. The deficient of conventional energy and the destruction of environment from fossil energy are forcing people to develop and use green new energy on a large scale. Among the numerous new energy, solar has been developed vigorously by human for its abundant reserves, wide distribution, clean, economy, high availability, etc1. Charging technology combined with photovoltaic technology is in a rapidly developing stage , and has made great progress already. But charging efficiency of charger is still not ideal, how to improve utilization of solar cells by using MPPT technology reasonable is still a technical problem in this area. Switching power supply becomes the direction of development of future power supply for its small size, light weight, high efficiency, low calorific, stable performance. At the time of choosing the structure of the charging circuit, noticed that the requirements of the relationship between input voltage and output voltage of BUCK converter and BOOST converter,limit their application.In order to make the system meet the requirements of photovoltaic charging application, taking system costs account as well, instead of choosing the BUCK converter and BOOST converter segmented running-parallelly circuit structure, select the BUCK-BOOST converter and a new MPPT technology -WVSVH P&O method to achieve the goal of charging energy storage devices efficiently by tracking the maximum power point of solar cells quickly and accurately23.1 Circuit configuration: The efficient solar charging system designed in this research is mainly composed of solar cells, BUCK - BOOST converter, FPGA, energy storage load. The structure of the circuit configuration is shown in Figure 1.Figure 1 Main components of charging system Solar cells used in this experiment are tested under the conditions: 25, AM1.5, 1000W/m2. Its parameter information: Open circuit voltage 22.41V; Short circuit current 0.62A; Peak voltage 17.9V; Peak current 0.56A; Maximum power 12.1W. The type of switching power supply used in this design is BUCK-BOOST. For, BUCK converter can only work properly in applications that its input voltage is higher than its output voltage; BOOST converter can only work properly in applications that its output voltage is higher than its input voltage. In order to make the system able to achieve high charging efficient during the whole charging process, BUCK-BOOST converter which can not only work in the BUCK state but also work in the BOOST state becomes the choice of this design. The FPGA controller used in this design is Cyclone EP2C8Q208C8N from Altera. By using the concurrent statement processing capability of FPGA, the running speed of the control system can be highly improved, control effect becomes more accurate and efficient. In addition, advanced custom integration brings great potential to reduce system costs. There are kinds of energy storage devices, the common electrical energy storage device is battery. But here, choose the emerging energy storage device with high power density characteristicSuper capacitor, to verify if the charging system based on BUCK-BOOST converter has the ability of charging efficient during the whole process by experiment of charging a 1F super capacitor from 0V to 40V.2 System control As this is a charging system combined with PV technology, by analyzing and studying MPPT technology with practical applications on the basis of understanding the basic characteristics of solar cells can formulate the control strategy legitimately.2.1 Characteristics of solar cells Solar cells are non-linear power. Under the given external environment, output current and output power of solar cells will change non-linearly with the change of its output voltage. The output characteristic curve is shown in Figu