1 INTRODUCTION
Solar energy is a clean and efficient renewable energy. In sunny day, the roof of photovoltaic cells to solar energy into electric energy for people to use at night. Experts predict that in 2040, the global photovoltaic generating capacity of the world's total generating capacity will account for 26%, 2050 will become the pillar of the world's energy.
2 Maximum Power Point Tracking
The largest power point tracking (Maximum Power Point Tracking, referred to MPPT) system is a module by adjusting the electrical work, and photovoltaic panels to make more electricity output of the electrical system. Figure 1 show a photovoltaic cells with output power Pb ub output voltage and output current ib relationship. A map for the general controller photovoltaic cells working in 12 V, only 53 W output power at the point (General Power Point); B MPPT controller for photovoltaic cells to always work at maximum power point, when the output of up to 75 W The power points (the maximum power point).
Maximum Power Point mainly by the ambient temperature and the impact of strong sunlight. In strong sunlight remain unchanged, with increasing temperature, photovoltaic cells to reduce the open circuit voltage, the greatest power output decreased. When the temperature is unchanged, strong increase in sunlight, photovoltaic cells of the open circuit voltage will remain basically unchanged. Substantial increase in short-circuit current, maximum power output increased significantly. Circuit.
First, the calculation of consumption in the power of R1: 
Then, type (1) on both sides of the R1 derivation available:
By-(2) available, when r = R1 when, dP1/dR1 = 0, at P1 for the maximum.
As photovoltaic cell system by light, temperature, sunlight angle, and other factors, its output voltage ub, output current r ib internal resistance and also in the non-stop changes. Only the use of DC / DC converters to achieve load the dynamic changes, photovoltaic cells has always been to ensure maximum power output.
MPPT need timely and accurate sampling of the current battery charge voltage and charge current. Taken by multiplying the current charging power, with the power of a moment of the charge, compared to adjust the PWM duty cycle, so that photovoltaic cells always working at maximum power point. Figure 3 show specific control strategy.
3 MPPT hardware design
As the output of photovoltaic cells were non-linear, and very volatile, so the use of single-ended flyback converter. The converter from take-off and landing pressure converter and transformer deductions from isolation, can be simple and efficient delivery of DC output, 100 W power is widely used around the small switch in power. Controllers work in the current intermittent mode.
Figure 4 show MPPT to the hardware design principle. The microcontroller used MC68HC08SR12 microprocessors, the use of A / D sampling module power output current and output voltage, and then adjust PWM duty cycle, the ultimate realization of photovoltaic cells in the maximum power output. MPPT control strategy have a direct effect on the voltage and current sampling is accurate.
Figure 5 show a sampling voltage circuit. It uses Optocoupler PC817 and the three-regulator of TLA31 compatible. TLA3l the regulator is a programmable, when the output voltage transformer sub uout changes, Optocoupler subsequent changes in the output voltage, A / D will be sampling the current charge voltage.
Figure 6 show a current sampling circuit. Sampling by its resistance Rsam ends of the voltage sampling and the use of differential operational amplifier output to the MCU to enlarge the A / D sampling-to be the main circuit in the current value. As signals need accurate sampling, with the power and isolation, the use of linear Optocoupler HCNR200. In addition, the MCU and the surrounding circuit electricity directly through the battery isolation by PSA, the system without external power supply, is very convenient.
4 analysis software
Because of strong sunlight and temperature change is a slow process, the parameters do not have high real-time sampling, a sampling every few seconds to meet the requirements. Have interrupted the time interval can be adjusted, the shorter the initial, fast approaching the maximum power point; longer late to prevent the maximum power point system in the vicinity of oscillation. To prevent mistaken judgement system, each control, are conducted three times, three times as the result of consensus, the implementation of the corresponding control strategy, otherwise re-sampling, this will maximize the system to ensure the normal operation. Figure 7 showed the software to achieve MPPT process.
As SCM and switching power supply to work together, electromagnetic interference between the larger, and the A / D sampling requirements of precision. They need to use digital filtering software. Here a mean filter, through the average for several sampling methods to achieve the objective of removing interference.
The use of battery charging. To 12 V battery as an example, in charging voltage to 14.7 V maximum limit of voltage. To maintain the voltage to charge two to four hours reaching saturation. The charge current maximum rated capacity of not more than 25% (such as when a storage capacity of 100 A of the battery, the biggest charge current to 25 A). Fuses used to achieve hardware circuit overcurrent protection. Figure 8 diagram showing a software protection
5 results and prospects
Table 1 is a major component of the system and performance indicators. Figure 9 reflects the December 20, 2005 from 9:00 to 16:00 Power P, MPPT charge of the charger and maximum power point changes. December 20 the weather changes quickly, sometimes sunny, sometimes cloudy, sometimes dreary. MPPT can reach a maximum 50 W, the smallest only about 5 W, which reflects the power output of solar variability.
After testing, the MPPT charge of the solar power system reflects the volatility of output power, and can quickly track the maximum power point, the stability of the photovoltaic system work effectively played a crucial role. Strive to improve the efficiency and reliability, and further improve the MPPT algorithms, the next step is the focus of the study.