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　　Analysis of IGBT and MOSFET technology in solar inverters

　　The development of inverter technology is a requirement for solar energy applications. This article introduces the principle and architecture of solar inverters, focusing on IGBT and MOSFET technology. The realization of intelligent control is the key to the development of solar inverter technology.

　　1. Solar energy requirements for inverters

　　Converting solar radiation into electrical energy through solar photovoltaic technology is the most effective and promising renewable energy technology on the market. Nowadays, ordinary solar photovoltaic systems are composed of many closely connected solar panels. These battery panels are first connected in series, and then different series battery groups are connected in parallel to form a battery array.

　　At present, my country's photovoltaic power generation system is mainly a DC system, that is, the electric energy generated by the solar cell is used to charge the battery, and the battery directly supplies power to the load. For example, the solar household lighting system and the microwave station power supply system far away from the power grid are used more in northwest my country. DC system. This type of system has a simple structure and low cost. However, due to the different load DC voltages (such as 12V, 24V, 48V, etc.), it is difficult to achieve system standardization and compatibility, especially for civil power. Since most of them are AC loads, DC power It is difficult for photovoltaic power supplies to enter the market as commodities. Photovoltaic power generation will eventually realize grid-connected operation, which requires the adoption of a mature market model. In the future, AC photovoltaic power generation systems will surely become the mainstream of photovoltaic power generation.

　　A solar inverter is a power electronic circuit that can convert the DC voltage of solar panels into AC voltage to drive AC loads such as household appliances, lighting, and motor tools. It is a key component of the entire solar power generation system. The inverter has two basic functions: on the one hand, it is to connect the current to the grid for DC/AC conversion, and on the other hand, it is to find the best operating point to optimize the efficiency of the solar photovoltaic system. For specific solar radiation, temperature and battery type, the solar photovoltaic system has a unique optimal voltage and current, so that the photovoltaic system can produce the maximum energy. Therefore, inverters must meet the following basic requirements in solar applications:

　　1. Requires high efficiency. Since the current price of solar cells is relatively high, in order to maximize the use of solar cells and improve system efficiency, it is necessary to find ways to improve the efficiency of the inverter.

　　2. Requires high reliability. At present, photovoltaic power generation systems are mainly used in remote areas, and many power stations are unattended and unmaintained. This requires the inverter to have a reasonable circuit structure, strict component selection, and requires the inverter to have various protection functions, such as input DC Reverse polarity protection, AC output short circuit protection, overheating, overload protection, etc.

　　3. The DC input voltage is required to have a wide adaptive range. Since 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 changes with the remaining capacity and internal resistance of the battery. Fluctuates with changes, especially when the battery ages and its terminal voltage changes in a large range. For example, the terminal voltage of a 12V battery can vary between 10V and 16V, which requires the inverter to operate at a larger DC input voltage. Ensure normal operation within the range and ensure the stability of the AC output voltage.

　　4. In medium and large-capacity photovoltaic power generation systems, the output of the inverter power supply should be a sine wave with less distortion. This is because in medium and large-capacity systems, if square wave power supply is used, the output will contain more harmonic components, and high-order harmonics will produce additional losses. The loads of many photovoltaic power generation systems are communication or instrument equipment. These The equipment has high requirements for the quality of the power grid. When medium and large-capacity photovoltaic power generation systems are connected to the grid, in order to avoid power pollution with the public grid, the inverter is also required to output a sine wave current.

　　2. Principle and structure of solar inverter

　　Usually, the process of converting AC power into DC power is called rectification. Phase-controlled rectification is the most common AC-DC conversion process; and the process of converting DC power into AC power is called inversion, which is the reverse process of rectification. . In the inverter circuit, according to the different properties of the load, the inverter is divided into active inverter and passive inverter. If the AC side of the circuit is connected to the AC power supply, the DC power is converted into AC power with the same frequency as the AC power supply and sent back to the power grid through DC-AC conversion, which is called active inversion. The corresponding device is called an active inverter, and a phase-controlled rectifier with a control angle greater than 90° is a common active inverter. The circuit that converts DC power into AC power and directly supplies power to non-power loads is called a passive inverter circuit, also known as a frequency converter.

　　Inverter types include separately excited inverters, self-excited inverters, and pulse width modulation (PWM) inverters. Among them, the separately excited inverter requires an external AC voltage source to provide the rectified voltage to the thyristor. Separately excited inverters are mainly used in high-power grid connection situations; for photovoltaic power generation systems with power below 1MW, self-excited inverters are mainly used. The self-excited inverter does not require an external AC voltage source. The rectified voltage is provided by a part of the energy storage component of the inverter (such as a capacitor) or by increasing the resistance value of the rectifier valve to be turned off (such as a MOSFET or IGBT). A self-excited inverter whose output voltage is pulse modulated is called a pulse inverter. This kind of inverter reduces the harmonic content of voltage and current by increasing the number of pulse switching times in a cycle; the harmonic content is proportional to the number of pulse switching times. At present, there are two main output control modes of grid-connected inverters: voltage control mode and current control mode. The principle of the voltage control mode is to use the output voltage as the controlled quantity, and the system outputs a voltage signal with the same frequency and phase as the grid voltage. The entire system is equivalent to a controlled voltage source with very small internal resistance; the principle of the current control mode is Taking the output inductor current as the controlled target, the system outputs a current signal with the same frequency and phase as the grid voltage. The entire system is equivalent to a controlled current source with a large internal resistance.

　　Currently, there are many topologies for solar inverters, the most common ones are half-bridge, full-bridge and Heric (Sunways patented) inverters for single-phase, and six-pulse bridge and neutral-point clamp for three-phase Bit (NPC) inverter. The typical architecture of a solar inverter generally uses a full-bridge topology with four switches, as shown in Figure 1.

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