Evaluation of photovoltaic inverter topology and power device standards-EEWORLD

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For the design of traditional power electronic devices, we usually measure their cost performance by how much money per kilowatt. However, for the design of photovoltaic inverters , the pursuit of maximum power is only secondary, and the maximization of European efficiency is the most important. Because for photovoltaic inverters, not only the increase in maximum output power can be converted into economic benefits, but also the improvement of European efficiency can be more obvious. The definition of European efficiency is different from what we usually call average efficiency or maximum efficiency. It fully considers the changes in sunlight intensity and more accurately describes the performance of photovoltaic inverters. European efficiency is obtained by adding up the efficiencies under different load conditions according to different proportions, among which the efficiency of half load accounts for the largest component (Figure 1).

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Figure 1: European efficiency calculation ratio

Therefore, in order to improve the efficiency of photovoltaic inverters, it is not enough to simply reduce the loss at rated load. The efficiency under different load conditions must also be improved. The economic benefits brought about by the improvement of efficiency can also be easily calculated. For example, taking a photovoltaic inverter with a rated power of 3kW as an example, according to the current market cost estimate, the installation cost of photovoltaic power generation per kilowatt is about 4,000 euros [2]. This means that every 1% improvement in the efficiency of the photovoltaic inverter can save 120 euros. The economic benefits brought about by improving the efficiency of photovoltaic inverters are obvious. "No matter the cost" to pursue higher efficiency has become the current trend of photovoltaic inverter development.

Selection of power devices

In the design of general inverters, IGBT is the most commonly used device, considering the cost-effectiveness factor. Because of the nonlinear characteristics of the IGBT conduction voltage drop, the IGBT conduction voltage drop does not increase significantly with the increase of current. This ensures that the inverter can still maintain low losses and high efficiency under maximum load conditions. However, for photovoltaic inverters, this characteristic of IGBT has become a disadvantage. Because the European efficiency is mainly related to the efficiency of the inverter under different light load conditions. Under light load, the on-state voltage drop of the IGBT does not drop significantly, which reduces the European efficiency of the inverter. On the contrary, the on-state voltage drop of the MOSFET is linear, and it has a lower on-state voltage drop under light load conditions. In addition, considering its excellent dynamic characteristics and high-frequency working ability, MOSFET has become the first choice for photovoltaic inverters. In addition, considering the huge economic returns after improving the European efficiency, the latest relatively expensive devices, such as SiC diodes, are also being used more and more in the design of photovoltaic inverters. SiC Schottky diodes can significantly reduce the conduction loss of the switch tube and reduce electromagnetic interference.

Design goals of photovoltaic inverters

For transformerless photovoltaic inverters, its main design goals are:

· Track the maximum power point of the solar cell input voltage to obtain the maximum input power
· Pursue the maximum European efficiency of the photovoltaic inverter
· Low electromagnetic interference

In order to obtain the maximum input power, the circuit must have the function of automatically adjusting the input voltage according to different sunlight conditions. The maximum power point is generally around 70% of the open-loop voltage. Of course, this is also related to the characteristics of the specific photovoltaic cells used. The typical circuit is realized through a boost circuit. Then the DC power is inverted into a grid-connectable sinusoidal AC power through an inverter. Introduction to the topology of

single-phase transformerless photovoltaic inverters The choice of topology is related to the rated output power of the photovoltaic inverter. For photovoltaic inverters below 4kw, a topology with a DC bus not exceeding 500V and a single-phase output is usually selected.

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Figure 2: Functional diagram of a single-phase transformerless photovoltaic inverter

This function (Figure 2) can be realized by the following schematic diagram (Figure 3).

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Figure 3: Schematic diagram of a single-phase transformerless photovoltaic inverter

The boost circuit achieves maximum power point tracking by adjusting the input voltage. The H-bridge inverter converts DC power into sinusoidal AC power and injects it into the grid. The IGBT of the upper half bridge acts as a polarity controller and works at 50HZ, thereby reducing the total loss and the output electromagnetic interference of the inverter. The IGBT or MOSFET of the lower half bridge performs PWM high-frequency switching. In order to minimize the size of the boost inductor and the output filter , the switching frequency is required to be as high as possible, such as 16KHz. We recommend using power modules to design photovoltaic inverters, because integrating all the devices on the topology of Figure 3 into one module can provide the following advantages: simple installation and reliability . Short R&D design cycle, which can bring products to market faster. Better electrical performance. For the design of the module, we must ensure: 1. Low inductance design of the DC bus loop. To achieve this goal, we must reduce the parasitic inductance inside and outside the module at the same time. In order to reduce the parasitic inductance inside the module, the binding wires, pin layout and internal routing inside the module must be optimized. In order to reduce the external parasitic inductance of the module, we must ensure that the positive and negative ends of the DC bus of the Boost circuit and the inverter bridge circuit are as close as possible while meeting the safety distance. 2. Configure a dedicated drive pin for the fast switch tube During the switching process of the switch tube, the parasitic inductance of the binding wire will cause the drive voltage to decrease. This will lead to an increase in switching losses and even oscillation of the switching waveform. Inside the module, by configuring a dedicated drive pin for each switch tube (directly drawn from the chip), it can be ensured that no large current flows in the drive loop, thereby ensuring the stability and reliability of the drive loop. This solution can only be achieved by power modules at present, and single-tube IGBTs cannot do it. Figure 4 shows Vincotech's latest photovoltaic inverter dedicated module flowSOL-BI (P896-E01), which integrates the advantages mentioned above:

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Figure 4: flowSOL-BI – boost circuit and full-bridge inverter circuit

Technical parameters:
Boost circuit consists of MOSFET (600V/45mΩ) and SiC diode
The bypass diode is mainly used to bypass the Boost circuit when the input exceeds the rated load, thereby improving the overall efficiency of the inverter.
The upper half of the H-bridge circuit consists of 75A/600V IGBT and SiC diode, and the lower half consists of MOSFET (600V/45mΩ)
Integrated temperature detection resistor

Efficiency calculation of flowSOL0-BI, a dedicated module for single-phase transformerless photovoltaic inverter

Here we mainly consider the loss of power semiconductors, and the loss of other passive devices, such as Boost inductors and output filter inductors, is not included. Based

on the relevant parameters of this circuit, the simulation results are as follows:

Conditions:

Pin=2kW
fPWM = 16kHz
VPV-nominal = 300V
VDC = 400V

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Figure 5: Boost circuit efficiency simulation result EE=99.6%

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Figure 6: flowSOL-BI inverter circuit efficiency simulation results – EE=99.2%
Standard IGBT full bridge – EE=97.2% (dashed line)

According to the simulation results, we can see that the efficiency of the module hardly decreases with the decrease of load. The total European efficiency of the module (Boost+Inverter) can reach 98.8%. Even with the loss of passive devices, the total efficiency of the photovoltaic inverter can still reach 98%. The dotted line in Figure 6 shows the efficiency change of the inverter using conventional power devices. It can be clearly seen that at low load, the inverter efficiency drops rapidly.

Introduction to the topology of three-phase transformerless photovoltaic inverter

High-power photovoltaic inverters require the use of more photovoltaic battery groups and three-phase inverter outputs (Figure 7), and the maximum DC bus voltage will reach 1000V.

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Figure 7: Functional diagram of a three-phase transformerless photovoltaic inverter

The standard application here is to use a three-phase full-bridge circuit. Considering that the DC bus voltage will reach 1000V, the switching device must use 1200V. As we know, the switching speed of 1200V power devices will be much slower than that of 600V devices, which will increase losses and affect efficiency. For this application, a better alternative is to use a center point clamped (NPC=neutral point clamped) topology (Figure 8). In this way, 600V devices can be used to replace 1200V devices.

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Figure 8: Schematic diagram of a three-phase transformerless NPC photovoltaic inverter

In order to minimize the parasitic inductance in the loop, it is best to integrate the symmetrical dual-Boost circuit and the NPC inverter bridge into one module.

Dual-Boost module technical parameters (Figure 9):

The dual-Boost circuit is composed of MOSFET (600V/45 mΩ) and SiC diode.
The bypass diode is mainly used to bypass the Boost circuit when the input exceeds the rated load, thereby improving the overall efficiency of the inverter.
The module integrates a temperature detection resistor

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Technical parameters of NPC inverter bridge module (Figure 10):

The middle switching link is composed of 75A/600V IGBT and fast recovery diode. The
upper and lower high-frequency switching links are composed of MOSFET (600V/45 mΩ).
The center point clamping diode is composed of SiC diode.
The module has an integrated temperature detection resistor.

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Figure 10: flowSOL-NPI – NPC inverter bridge

For this topology, the design requirements for the module are basically similar to the single-phase inverter module mentioned above. The only thing that needs extra attention is that both the dual-Boost circuit and the NPC inverter bridge must ensure the low inductance design between DC+, DC- and the center point. With these two modules, it is easy to design a higher power output photovoltaic inverter. For example, using two dual-Boost circuits in parallel and a three-phase NPC inverter bridge can get a high-efficiency 10kW photovoltaic inverter. Moreover, the pin design of these two modules fully considers the needs of parallel connection, and parallel use is very convenient.

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Figure 11: Layout of dual boost modules in parallel and three-phase NPC inverter output module

For PV inverters with a DC bus voltage of 1000V, the NPC topology inverter is the most efficient on the market. Figure 12 compares the efficiency of an NPC module (MOSFET+IGBT) and a 1200V IGBT half-bridge module.

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Figure 12: Comparison of NPC inverter bridge output efficiency (solid line) and half-bridge inverter efficiency (dashed line)

According to the simulation results, the efficiency of the NPC inverter can reach 99.2%, while the efficiency of the latter is only 96.4%. The advantages of the NPC topology are obvious.

Introduction to the design ideas of the next generation photovoltaic inverter topology

At present, the hybrid H-bridge (MOSFET + IGBT) topology has achieved a high efficiency level. The next generation of photovoltaic inverters will focus on the following performance improvements:

further improvement of efficiency
Reactive power compensation
Efficient bidirectional conversion mode

Single-phase photovoltaic inverter topology

For single-phase photovoltaic inverters, we first discuss how to further improve the efficiency of the hybrid H-bridge topology (as shown in Figure 13).

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Figure 13: Development of PV inverters – hybrid

In Figure 13, the switching frequency of the upper bridge arm IGBT is generally set to the grid frequency (e.g., 50Hz), while the lower bridge arm MOSFET operates at a higher switching frequency, such as 16kHz, to achieve a sinusoidal output. Simulation shows that this inverter topology can achieve an efficiency of 99.2% at a rated power output of 2kW. Due to the slow speed of the built-in diode of the MOSFET, the MOSFET cannot be used in the upper bridge arm.

Since the IGBT of the upper bridge arm operates at a switching frequency of 50Hz, it is actually not necessary to filter this branch. Therefore, the circuit topology is optimized to obtain the emitter open circuit topology shown in Figure 14. The advantage of this topology is that only the branch with high-frequency current passing through has a filter inductor, thereby reducing the loss of the output filter circuit.

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Figure 14 Improved transformerless upper bridge arm open emitter topology

Currently, Vincotech has a standard open emitter IGBT module product, model flowSOL0-BI open E (P896-E02), as shown in Figure 15:

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Figure 15: flowSOL0-BI-open E (P896-E02)

Technical parameters: The

boost circuit is composed of MOSFET (600V/45 mΩ) and SiC diode.
The bypass diode is mainly used to bypass the Boost circuit when the input exceeds the rated load, thereby improving the overall efficiency of the inverter.
The upper arm of the H bridge uses IGBT (600V/75A) and SiC diode, and the lower arm uses MOSFET (600V/45 mΩ).
The module has an internal integrated temperature detection resistor

. Let's analyze the emitter open circuit topology shown in Figure 14. When the MOSFET of the lower arm is working, the diode connected in anti-parallel with the IGBT of the upper arm does not work due to the effect of the filter inductor. In this way, MOSFET can also be used in the upper arm to improve the efficiency of the inverter when the load is light. The simulation results show that at a rated power output of 2kW, the European efficiency of this photovoltaic inverter can be increased by 0.2%, so that the efficiency reaches 99.4%. In actual applications, this topology will improve efficiency more, because the simulation results are obtained under the assumption that the chip junction temperature is 125℃, but due to the large size of MOSFET and the fact that photovoltaic inverters often work under light load, the MOSFET chip junction temperature is much lower than 125℃, so the on-resistance RDS-on of MOSFET in actual operation will be lower than the value in simulation, and the loss will be smaller accordingly. How to solve the problem of reactive power? The only way to handle reactive power with this circuit topology is to use FRED-FET, but the on-resistance RDS-on of these devices is usually very high. Another disadvantage is that its reverse recovery characteristics are poor, which affects the performance during reactive compensation and bidirectional conversion. However, in some special applications, if reactive power must be used to measure line impedance or protect certain components, the topology shown in Figure 16 will meet the above requirements.

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Figure 16: All-MOSFET topology for reactive loads

The topology shown in Figure 16 allows pure reactive loads, which can improve reactive compensation to the grid and also meet bidirectional power flow, such as efficient battery charging. If SiC Schottky diodes are applied, this circuit topology will achieve higher efficiency levels.

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Figure 17: European efficiency of different topologies at 2kW rated power

Three-phase photovoltaic inverter topology

Similar improvements can be made to the three-phase photovoltaic inverter of NPC topology.

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Figure 18: Three-level inverter

Taking one phase as an example, at a rated output of 2kW, the three-level inverter (Figure 18) can achieve 99.2% efficiency. With a slight modification, this topology can achieve reactive power flow.

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Figure 19: NPC topology inverter capable of reactive power output

After adding a 1200V diode between the output and the DC bus, this topology can output reactive power. It can also be used as a high-efficiency bidirectional inverter to achieve reverse energy conversion. In order to reduce losses, D3 and D4 are recommended to use SiC diodes. However, since the price of 1200V SiC is too high, the following topology will be a better choice.

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Figure 20: NPC topology inverter for reactive power output (adding 2 SiC diodes and 4 Si diodes)

This topology uses only two 600V SiC diodes (D4, D6). D3 and D5 use fast Si diodes, and D7 and D8 use small Si diodes to prevent overvoltage damage to the SiC diodes. Is it possible to use MOSFETs here? The answer is yes, provided that the body diode of the MOSFET is bypassed. This can be achieved by separating the output terminals of the upper and lower half bridges and matching them with their own filter inductors.

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Figure 21: NPC topology inverter using MOSFET to achieve reactive power output

The circuit topology of Figure 21 can improve efficiency at light loads.

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Figure 22: Efficiency comparison of all-MOSFET solution and hybrid solution at rated power of 2kW

Its European efficiency can be increased from 99.2% to 99.4%. Reactive power is achieved by the 1200V fast diode path. When selecting diodes, it is recommended to use SiC diodes, which can achieve higher efficiency during reverse conversion. Or as shown in Figure 23, D4 and D6 use 600V SiC diodes, and the other four use fast recovery Si diodes.

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Figure 23: NPC inverter topology using 2 SiC diodes, 4 Si diodes and separate outputs

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