At present, the photovoltaic power generation system in China is mainly DC system, that is, the power from solar cells is charged to the battery, and the battery directly supplies power to the load. For example, the solar household lighting system in Northwest China and the power supply system of microwave station far away from the grid are DC systems. This kind of system has simple structure and low cost, but due to the different load DC voltage (such as 12V, 24V, 48V, etc.), it is difficult to realize the standardization and compatibility of the system, especially for civil power. Because most of the AC load, the photovoltaic power supply powered by DC power is difficult to enter the market as a commodity.

In addition, photovoltaic power generation will eventually achieve grid connected operation, which must adopt a mature market model. In the future, AC photovoltaic power generation system will become the mainstream of photovoltaic power generation. The requirements of photovoltaic power generation system for inverter power supply are AC power output photovoltaic power generation system, which is composed of photovoltaic array, charge and discharge controller, battery and inverter (grid connected power generation system can generally save battery), and inverter is the key component. Photovoltaic power generation system has high requirements for inverter

1. High efficiency is required. Due to the high price of solar cells, in order to maximize the use of solar cells and improve the efficiency of the system, we must try to improve the efficiency of the inverter.

2. High reliability is required. At present, photovoltaic power generation system is mainly used in remote areas, many power stations are unattended and maintained, which requires the inverter to have a reasonable circuit structure, strict screening of components, and requires the inverter to have a variety of protection functions, such as input DC polarity reverse protection, AC output short circuit protection, overheating, overload protection, etc.

3. The DC input voltage is required to have a wide range of adaptability. As the terminal voltage of the solar cell changes with the load and sunshine intensity, although the battery plays an important role in the voltage of the solar cell, the voltage of the battery fluctuates with the change of the residual capacity and internal resistance of the battery, especially when the battery is aging, the terminal voltage changes in a wide range, such as 12V storage The terminal voltage of the inverter can vary from 10V to 16V, which requires the inverter to work normally in a large range of DC input voltage and ensure the stability of AC output voltage.

4. In medium and large capacity photovoltaic power generation system, the output of inverter should be sine wave with less distortion. This is because in the medium and large capacity system, if the square wave power supply is used, the output will contain more harmonic components, and higher harmonics will produce additional losses. Many photovoltaic power generation systems are loaded with communication or instrument equipment, which have higher requirements for the quality of the power grid. When the medium and large capacity photovoltaic power generation systems are connected to the grid, in order to avoid power pollution with the public power grid The inverter is also required to output sine wave current. The inverter converts DC into AC. if the DC voltage is low, the standard AC voltage and frequency can be obtained by boosting the voltage through AC transformer. For large capacity inverters, due to the high DC bus voltage, the AC output can reach 220 V without transformer boost. For medium and small capacity inverters, due to the low DC voltage, such as 12 V and 24 V, the boost circuit must be designed. There are three kinds of medium and small capacity inverters: push-pull inverter circuit, full bridge inverter circuit and high-frequency boost inverter circuit. In push-pull circuit, the neutral plug of step-up transformer is connected to the positive power supply, and two power transistors work alternately to output AC power. Due to the common grounding of power transistors, the drive and control circuit is simple, and the transformer has a certain leakage inductance, which can be limited So the reliability of the circuit is improved. Its disadvantages are low utilization rate of transformer and poor ability of dynamic load. The full bridge inverter circuit overcomes the shortcomings of push-pull circuit. When the power transistor adjusts the output pulse width, the effective value of the output AC voltage changes accordingly. Because the circuit has freewheeling circuit, even for inductive load, the output voltage waveform will not be distorted. The disadvantage of this circuit is that the power transistors of the upper and lower bridge arms are not in common ground, so special driving circuit or isolated power supply must be used. In addition, in order to prevent the upper and lower bridge arms from conducting together, it is necessary to design a turn off and then turn on circuit, that is, the dead time must be set, and the circuit structure is complex.

The output of push-pull circuit and full bridge circuit must be equipped with step-up transformer. Because of the large volume, low efficiency and high price of step-up transformer, with the development of power electronics technology and microelectronics technology, high power density inversion can be realized by using high frequency step-up conversion technology. The front step-up circuit of this kind of inverter circuit adopts push-pull structure, but the working frequency is within 20 The step-up transformer is made of high-frequency magnetic core material, so it is small in size and light in weight. After high-frequency inverter, it turns into high-frequency AC through high-frequency transformer, and then gets high-voltage DC (generally above 300V) through high-frequency rectifier filter circuit, and then realizes inverter through power frequency inverter circuit. With this circuit structure, the power of the inverter is greatly increased, the no-load loss of the inverter is correspondingly reduced, and the efficiency is improved. The disadvantage of this circuit is that the circuit is complex and the reliability is lower than the above two circuits.