How to choose power electronics in a solar inverter

How to choose power electronics in a solar inverter

The global inverter market is growing. Manufacturers in this field are striving to improve system efficiency to achieve leadership. The intelligent design of the inverter system and the use of power semiconductors with the most advanced technology are the keys to achieving high efficiency.

Introduction

The application fields of photovoltaic systems are becoming more and more extensive. Mobile systems, in particular, benefit from solar energy without spending a penny. At the same time, due to the rising cost of conventional electricity, it is very attractive for household applications. The energy efficiency of the batteries themselves and the inverters that connect them to the public grid or distributed power sources are key to the success of this technology. Today, advanced inverters with a maximum output power of 5kW have a two-level topology.

Each group is connected to its own power conditioner, which is then connected to a common DC bus. Power regulators enable solar cells to operate at maximum efficiency. The inverter generates an AC voltage that is fed into the mains supply. Note that the power grid shown in Figure 1 is a dummy circuit that can be used with any inverter topology, plus a mains transformer and an output filter to prevent DC components from entering the mains.

However, there are some systems that do not use transformers, depending on the legal background of the country where the inverter is sold. The purpose of countries that allow transformers not to be used is to improve system efficiency, because transformers cause efficiency to drop by 1 to 2 percentage points. on the other hand,

The inverter must avoid DC components and require the current to be less than 5mA. Although this was difficult to do, we managed to achieve it in order to gain greater efficiency. Table 1 gives the contribution of each stage to system loss, system size and system cost.

It is easy to see that transformers are a major contributor to system losses and costs. However, the use of transformers is mandatory in many countries and, therefore, it is not considered within the scope of loss reduction considerations. The size and cost of the output filter, which attenuates the current ripple produced by the output inverter stage, is inversely proportional to the inverter switching frequency. The higher the switching frequency, the smaller and cheaper the filter. However, this relationship is compromised by the relationship between switching frequency and switching losses in the hard switching regime - the higher the switching frequency, the greater the losses and therefore the lower the efficiency. The switching frequency from 16kHz to 20kHz can meet the requirements of the inverter due to its lower audio noise and higher efficiency. Therefore, power circuits still need to be further studied.

The following will compare the advantages of several semiconductor technologies suitable for these two levels.

Power semiconductors for DC/AC boost converters

DC/DC converters operate at switching frequencies of 100kHz or above. The converter operates in continuous mode, which means that the current in the boost inductor produces a continuous waveform under nominal conditions. When the transistor is off, the transistor charges the inductor while the diode acts as a freewheeling diode. This means that the diode can actively turn off when the transistor turns on again. The figure below shows the typical reverse recovery characteristics of commonly used silicon diodes.

The reverse recovery characteristics of silicon diodes produce higher losses in both the boost transistor and the corresponding diode. Silicon carbide diodes do not have this problem. Just due to capacitance, a diode instantaneous negative current is generated, which is caused by the junction capacitance charge of the diode. Silicon carbide diodes can greatly reduce the turn-on losses of transistors and turn-off losses of diodes. They can also reduce electromagnetic interference because the waveform is very smooth and there is no oscillation.

In the past, many processes have been reported to avoid losses caused by the reverse recovery characteristics of diodes, such as zero-voltage switching and zero-current switching. All of this significantly increases component count and system complexity, often resulting in reduced stability. It is particularly worth mentioning that even in the hard switching state, by using silicon carbide Schottky diodes, the same efficiency of soft switching can be achieved with a minimum of components.

High switching frequencies also require high-performance boost transistors. The introduction of super junction transistors (such as CoolMOS) has brought hope for further reducing the on-resistance per unit area RDS(on) of MOSFETs.

It can be easily seen that compared with the standard process, the RDS(on) per unit area is about 4 to 5 times lower than CoolMOS. This means that CoolMOS achieves the lowest absolute on-resistance value in a standard package. This results in lowest conduction losses and highest efficiency. The RDS(on) per unit area of ​​the CoolMOS process shows better linearity. When the voltage is 600V, the advantages of CoolMOS are obvious, and if the voltage is higher, its advantages will increase. Currently, the highest voltage level is 800V.

Many studies have shown that using silicon carbide diodes and super junction MOSFETs such as CoolMOS is better than using standard MOSFET and diode processes

Power semiconductors for inverters

The output inverter is connected to the DC bus and the grid. Typically, the switching frequency is not as high as that of a DC/DC converter. The output converter must handle the sum of the currents produced by all group converters. Insulated gate bipolar transistors (IGBTs) are ideal devices for use in this inverter. Figure 5 shows two cross-sections of the IGBT process.

Both processes use wafer thinning processes to reduce conduction losses and switching losses caused by too thick a substrate. The standard process and TrenchStop process are non-epitaxial IGBT processes that do not use transistor growth processes because the cost of such processes is high because the blocking voltage is determined based on the thickness of the growing crystal.

In the off state, the standard NPT unit forms a triangular electric field inside the semiconductor. All blocking voltage is absorbed by the n-region of the substrate (depending on its thickness) so that the electric field drops to 0 before entering the collector region. The thickness of the 600V chip is 120mm and the thickness of the 1200V chip is 170mm. The saturation voltage has a positive temperature coefficient, simplifying parallel use.

The TrenchStop process is a combination of advanced trench gate and fieldstop concepts, which can further reduce conduction losses. The Trench gate process provides higher channel width, thereby reducing channel resistance. The ndoped field stop layer performs only one task: suppressing the electric field with extremely low off-state voltage values. This creates conditions for designing an almost horizontal distribution of the electric field in the n substrate layer. This shows that the resistance of the material is very low, so the voltage drop during the conduction process is very low. The advantages of the electric field stop layer can be realized by further reducing the thickness of the chip, thereby achieving all the above advantages. Parallel connection can also be achieved using the TrenchStop process.

For all three processes, the power rating of the transistors used remains constant. This means that the current of the device when the voltage is 600V is twice that of the device when the voltage is 1200V. In other words, one 50A/600V device is equivalent to two 25A/1200V devices.

Compared with 1200V devices, 6

The 00V TrenchStop process can reduce switching and conduction losses by 50%. Therefore, it is important for the entire system to use the excellent performance of the 600V process as much as possible. The 1200V TrenchStop process is further optimized for low conduction losses. Therefore, whether the Fast process or the TrenchStop product family has better performance depends on the switching frequency.

IGBT usually also requires a freewheeling diode to enable freewheeling. This is a specially optimized version of the EmCon process. It is optimized for the 15kHz switching frequency of 600V series devices. It used to be thought that freewheeling diodes must have a very low conduction voltage to achieve the lowest total losses. Depending on the application requirements, other optimizations can be performed to achieve lower total losses in the diode and IGBT. This shows that in applications with IGBTs and diodes with frequencies around 16kHz, a higher forward voltage drop is more appropriate to achieve low switching losses.

Therefore, the diode itself is already a good choice. In addition, it also reduces the switching losses of the IGBT during the turn-on process. The considerations in Part 2 above apply here as well. Using optimized EmCon diodes reduces losses by around 1W, which is an advantage. Note that as the load angle approaches 1, switching losses become the dominant losses because the diodes only conduct during the output inverter dead time.

  in conclusion

Power semiconductor devices require different characteristics to achieve maximum efficiency in solar inverter applications. The emergence of new processes, such as silicon carbide semiconductor diodes or TrenchStop IGBT, are helping people achieve this goal. Of course, achieving this requires optimizing not just the individual components, but also the way in which they function together. This results in minimal losses and maximum efficiency, two of the most important parameters for a solar inverter.