New inverter optimizes photovoltaic system design

With the growing demand for green energy, the rapid development of solar power generation in recent years has attracted widespread attention from all sides. In 2009, for example, the total output value of the solar energy industry in the United States increased by 36% despite the overall economic downturn, and attracted venture capital of up to 1.4 billion US dollars. According to reliable reports from relevant parties, in the next three years, the annual demand for photovoltaic power generation systems in the world will maintain an annual growth rate of 30%. Such a high growth rate forecast is based on the following factors: the current excess production capacity has reduced the average manufacturing cost of photovoltaic systems by 25%; the installation price of photovoltaic systems is continuously declining; and government subsidies from countries and regions around the world. China has very rich solar energy resources, and the country's subsidy and support policies have been introduced in recent years. For example, the most influential Golden Sun Project proposes to provide 50% and 70% financial subsidies to photovoltaic grid-connected projects and off-grid photovoltaic power generation projects in areas without electricity, respectively. It can be believed that China's solar energy industry is now in a period of rapid development.

The design of power electronics plays a vital role in the overall performance of solar power generation systems. The highest conversion efficiency is always the primary factor that system design engineers consider. Since the efficiency of photovoltaic conversion panels is very low, usually not more than 20%, the conversion efficiency of solar inverters is crucial to reducing the total area of ​​solar panels and the total volume of the system. In addition, the power loss in the power conversion process directly leads to the temperature increase of the semiconductor wafer, so this part of the loss energy must be effectively dissipated through the heat sink. The temperature rise and thermal stress of the device during operation are important parameters that affect reliability. In other words, reducing power conversion losses not only saves energy, but also improves system reliability and reduces system volume and cost. This article will explain the design principles of solar inverters with the goal of improving energy efficiency, and introduce various new devices, new circuits and the latest control technologies that have emerged in the market.

Circuit topology

To convert the "raw electricity" (fluctuating DC voltage) output by solar panels into a constant and reliable sinusoidal AC mains, the implementation methods are usually divided into two architectures: single-stage conversion and two-stage conversion, also known as DC-free and DC-with chopping. The DC-DC chopper can keep the voltage on the input side of the inverter constant and adjustable, thereby achieving decoupling between voltage and power control. Sometimes it is also beneficial to the selection of power semiconductor devices and system cost optimization. However, there is no doubt that this additional conversion device is likely to have a negative impact on system efficiency, so more and more manufacturers are developing or evaluating single-stage conversion architectures, even though this will face more complex inverter control and potential higher device tolerance requirements. Among the new topologies, HERIC? and multi-level structures have attracted more attention in the industry and are expected to become the mainstream topology, especially when connected to the grid.

Figure 1: HERIC topology

Figure 2: Three-level clamping diode topology.

As shown in Figure 1, the structure of the HERIC inverter is to add a pair of diode series switches in anti-parallel as the output on the basis of the traditional single-phase inverter full bridge. The switching devices in the newly added circuit switch at the power frequency cycle speed, and there is no special requirement for the device speed. After applying appropriate phase control, this circuit can handle reactive power more effectively, thereby improving the efficiency of the entire system. It should be noted that this topology is still within the validity period of the patent obtained by Sunways, so it is more widely used in Europe than in Asia and North America.

The three-level diode clamped inverter is a new type of solar inverter circuit topology that has received special attention recently. It has been successfully used in high-voltage centralized solar power generation applications. Each bridge arm of the three-phase three-level circuit shown in Figure 2 is composed of four switches with anti-parallel diodes in series, and each phase has a diode phase arm connected across the main switches, and its midpoint is directly connected to the neutral point of the DC bus. This diode phase arm acts as a voltage clamp to ensure that when the circuit is working, the maximum voltage stress on each main switch device is half of the bus voltage. Due to this special topological structure, the three-level output has the advantages of low harmonics (AC output is closer to sine), low electromagnetic noise level, low voltage tolerance of the required switching devices, and expandable levels. When solar grid-connected power generation, it is particularly suitable for three-phase high-power and high-voltage occasions. In addition, multi-level circuits have significant significance for system cost savings. It is reflected in three aspects: First, practice has proved that the price of high-voltage semiconductor switching devices is twice that of low-voltage devices with the same current tolerance and half the voltage tolerance, so the device cost of the three-level circuit is lower; second, the harmonics of the output voltage are small, and the size of the required filter magnetic components is greatly reduced, thereby reducing the cost of the filtering equipment; finally, due to the increase in the number of open tubes, even in the pulse width modulation mode, some of the main switches of the three-level can be switched at a low frequency, so relatively economical switching devices can be used.

In recent years, the popularization and application of photovoltaic power generation in various countries have made considerable progress. As a key link in power conversion, power electronic converters play a vital role in the overall performance and reliability of photovoltaic systems. After briefly reviewing the development of the solar energy market in recent years, this article focuses on analyzing the design needs of solar inverters and expounds on the optimization principles of power semiconductor devices and circuit topology.

With the growing demand for green energy, the rapid development of solar power generation in recent years has attracted widespread attention from all sides. In 2009, for example, the total output value of the solar energy industry in the United States increased by 36% despite the overall economic downturn, and attracted venture capital of up to 1.4 billion US dollars. According to reliable reports from relevant parties, in the next three years, the annual demand for photovoltaic power generation systems in the world will maintain an annual growth rate of 30%. Such a high growth rate forecast is based on the following factors: the current excess production capacity has reduced the average manufacturing cost of photovoltaic systems by 25%; the installation price of photovoltaic systems is continuously declining; and government subsidies from countries and regions around the world. China has very rich solar energy resources, and the country's subsidy and support policies have been introduced in recent years. For example, the most influential Golden Sun Project proposes to provide 50% and 70% financial subsidies to photovoltaic grid-connected projects and off-grid photovoltaic power generation projects in areas without electricity, respectively. It can be believed that China's solar energy industry is now in a period of rapid development.

The design of power electronics plays a vital role in the overall performance of solar power generation systems. The highest conversion efficiency is always the primary factor that system design engineers consider. Since the efficiency of photovoltaic conversion panels is very low, usually not more than 20%, the conversion efficiency of solar inverters is crucial to reducing the total area of ​​solar panels and the total volume of the system. In addition, the power loss in the power conversion process directly leads to the temperature increase of the semiconductor wafer, so this part of the loss energy must be effectively dissipated through the heat sink. The temperature rise and thermal stress of the device during operation are important parameters that affect reliability. In other words, reducing power conversion losses not only saves energy, but also improves system reliability and reduces system volume and cost. This article will explain the design principles of solar inverters with the goal of improving energy efficiency, and introduce various new devices, new circuits and the latest control technologies that have emerged in the market.

Circuit topology

To convert the "raw electricity" (fluctuating DC voltage) output by solar panels into a constant and reliable sinusoidal AC mains, the implementation methods are usually divided into two architectures: single-stage conversion and two-stage conversion, also known as DC-free and DC-with chopping. The DC-DC chopper can keep the voltage on the input side of the inverter constant and adjustable, thereby achieving decoupling between voltage and power control. Sometimes it is also beneficial to the selection of power semiconductor devices and system cost optimization. However, there is no doubt that this additional conversion device is likely to have a negative impact on system efficiency, so more and more manufacturers are developing or evaluating single-stage conversion architectures, even though this will face more complex inverter control and potential higher device tolerance requirements. Among the new topologies, HERIC? and multi-level structures have attracted more attention in the industry and are expected to become the mainstream topology, especially when connected to the grid.

Figure 1: HERIC topology

Figure 2: Three-level clamping diode topology.

As shown in Figure 1, the structure of the HERIC inverter is to add a pair of diode series switches in anti-parallel as the output on the basis of the traditional single-phase inverter full bridge. The switching devices in the newly added circuit switch at the power frequency cycle speed, and there is no special requirement for the device speed. After applying appropriate phase control, this circuit can handle reactive power more effectively, thereby improving the efficiency of the entire system. It should be noted that this topology is still within the validity period of the patent obtained by Sunways, so it is more widely used in Europe than in Asia and North America.

The three-level diode clamped inverter is a new type of solar inverter circuit topology that has received special attention recently. It has been successfully used in high-voltage centralized solar power generation applications. Each bridge arm of the three-phase three-level circuit shown in Figure 2 is composed of four switches with anti-parallel diodes in series, and each phase has a diode phase arm connected across the main switches, and its midpoint is directly connected to the neutral point of the DC bus. This diode phase arm acts as a voltage clamp to ensure that when the circuit is working, the maximum voltage stress on each main switch device is half of the bus voltage. Due to this special topological structure, the three-level output has the advantages of low harmonics (AC output is closer to sine), low electromagnetic noise level, low voltage tolerance of the required switching devices, and expandable levels. When solar grid-connected power generation, it is particularly suitable for three-phase high-power and high-voltage occasions. In addition, multi-level circuits have significant significance for system cost savings. It is reflected in three aspects: First, practice has proved that the price of high-voltage semiconductor switching devices is twice that of low-voltage devices with the same current tolerance and half the voltage tolerance, so the device cost of the three-level circuit is lower; second, the harmonics of the output voltage are small, and the size of the required filter magnetic components is greatly reduced, thereby reducing the cost of the filtering equipment; finally, due to the increase in the number of open tubes, even in the pulse width modulation mode, some of the main switches of the three-level can be switched at a low frequency, so relatively economical switching devices can be used.

Common types and selection principles of power electronic devices

The power semiconductor devices used in the broad solar inverter (including the input DC chopper stage) are mainly MOSFET, IGBT, and Super Junction MOSFET. Among them, MOSFET is the fastest, but also the most expensive. In contrast, IGBT has a slower switching speed, but has a higher current density, so it is cheap and suitable for high current applications. Super junction MOSFET is between the two, a product with a compromise between performance and price, and is widely used in actual design. In general, the type of device to be selected depends on the cost, efficiency requirements and switching frequency. If hard switching is required to be above 100 kHz, generally only MOSFET can do it. In the lower frequency band such as 15 kHz, if there is no special efficiency requirement, IGBT is selected. The frequency between these depends on the customer's specific requirements for conversion efficiency and cost. As a pair of contradictions between system efficiency and cost, design engineers will compare the target system requirements based on their corresponding relationship to determine the component model that is closest to the system requirements. To help designers quantify the relationship between efficiency and device cost, Table 1 lists the power loss and price factors of three semiconductor switch devices. For ease of comparison, all parameters are normalized to the MOSFET case. Currently, there are no devices with a voltage exceeding 900V in the superjunction MOSFET process.

Table 1 Performance and price comparison of common switching devices (all figures are normalized to MOSFET)

In addition to the three most typical fully controlled switch devices mentioned above, there are also products based on new materials and new processes such as silicon carbide diodes and ESBT?. Their current prices are still relatively high and they are mainly used in occasions with special requirements for solar power generation efficiency. However, with the continuous improvement of production technology and the decline in device unit prices, these devices will gradually become mainstream products and even replace some of the above-mentioned devices.

The latest products from industry

Due to the huge scale and development potential of the solar power generation market, major semiconductor manufacturers around the world are competing to launch their own products to pursue the market. In recent years, various new devices and technologies for solar power conversion have emerged one after another. In the fierce market competition, Microsemi's solar products are unique for their advanced processes and application technologies.

Single-phase full-bridge hybrid module and three-level hybrid module

The hybrid single-phase full-bridge power module shown in Figure 3 is a product dedicated to solar single-phase inverter. With the unipolar modulation method, the two switch tubes of each bridge arm work in completely different switching frequency ranges. Taking the figure as an example, the upper tube always switches on and off at the power frequency, while the lower tube always operates at the pulse width modulation frequency. According to this working characteristic, the upper tube always uses a relatively cheap gate trench IGBT to optimize the conduction loss, while the lower tube can choose a non-punch-through (NPT) IGBT to reduce the switching loss. This topology not only guarantees the highest system conversion efficiency but also reduces the cost of the entire inverter equipment. Figure 4 shows the conversion efficiency curves of different device combinations to verify the superiority of this solar power module. It can be found that this hybrid device configuration can achieve a conversion efficiency of more than 98% under different loads.

Figure 3: Hybrid device solar inverter module.

Figure 4: Comparison of inverter efficiency with different device combinations.

In Microsemi's three-level inverter module, a hybrid device mechanism is also introduced. Its main purpose is to make full use of the fact that the switching frequency of the devices at the two ends is much higher than that of the two adjacent devices in the middle. Therefore, the APTCV60 series three-level module uses a structure of super junction MOSFETs at both ends and Trench IGBT in the middle to further improve efficiency.

ESBT

ESBT is a new type of high-voltage fast switching device for solar energy. It combines the advantages of IGBT and MOSFET. It not only has a higher voltage tolerance than MOSFET, but also has lower losses than fast IGBT devices. Microsemi's ESBT solar boost chopper module, which is about to be launched on the market, integrates silicon carbide diodes and ESBTs, and is aimed at ultra-high efficiency boost applications from 5 kW to 20 kW. Its voltage tolerance is 1200V, the saturation on-state voltage between the collector and emitter is very low (close to 1V), the optimized switching frequency is between 30 kHz and 40 kHz, and single-chip module or dual-chip module packaging can be selected. Experiments show that this power module reduces losses by 40% compared to the corresponding IGBT module on the market. According to the experimental results of the 6 kW reference design, the conversion efficiency of this module between 50% and full load is at least 0.6 percentage points higher than the fastest IGBT device. Therefore, before the price of silicon carbide fully controlled devices drops to an acceptable range, ESBT will be the only choice for ultra-high efficiency solar power conversion applications.

Figure 5: ESBT boost chopper module.

Bypass diode

The bypass diode at the front end of the solar inverter is not strictly speaking part of the inverter, but as part of the solar power generation equipment, it is also crucial to the operation of the inverter and the reliability of the entire system. Microsemi has launched two new products for this application: LX2400 and SFDS1045. LX2400 incorporates the latest heat dissipation packaging technology - CoolRUNTM process, which does not require a heat sink and has a temperature rise of less than 10? C when passing a current of 10A. The reliability design with the goal of 30 years of stable operation guarantees a leakage current of less than 100uA, a steady-state current capability of 20A, and a bidirectional lightning resistance function. Its biggest feature is the lowest temperature rise in the industry. SFDS1045 is a new generation of Schottky diodes and the thinnest bypass diode in the industry to date. It is only 0.74mm thick and is placed under the glass package, which is particularly suitable for direct application in solar panels. In addition, its unique flexible copper pins have satellite application-level reliability.

in conclusion

Improving conversion efficiency and reducing costs are long-term issues in solar inverter design and the biggest challenges facing engineering designers. This article discusses the design principles, typical topologies and selection methods of switching devices for centralized solar inverters based on how to design an optimized new generation of solar power conversion systems. It also explains how design engineers can use new technologies at all levels of devices, circuits and systems to optimize inverter system design. Practice has proved that many of Microsemi's related new products can optimize system performance from multiple aspects, providing efficient, reliable and economical system solutions for the solar inverter market.