Selection Tips for Power Electronics in Solar Inverters

Introduction

The application fields of solar photovoltaic systems are becoming more and more extensive. Mobile systems, in particular, benefit from solar power without spending a penny. At the same time, due to the rising cost of conventional electricity, solar energy is very attractive for domestic applications. The energy efficiency of the solar cells themselves and of the solar inverters that connect the solar cells to the public grid or distributed power is key to the success of this technology. Today, advanced solar inverters with a maximum output of 5kW have a two-level topology. Figure 1 shows a multi-group configuration of such solar inverters.

Selection Tips for Power Electronics in Solar Inverters

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

However, there are also systems that do not use a transformer, depending on the legal context of the country where the solar inverter is sold. The purpose of allowing countries that do not use transformers 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 the DC component, requiring a current of less than 5mA. Although it was difficult to do, we managed to achieve it for greater efficiency. Table 1 shows the contribution of each stage to system loss, system size, and system cost.

Selection Tips for Power Electronics in Solar Inverters

It is easy to see that the transformer is a major contributor to system losses and costs. However, transformers are required in many countries, so it is not a consideration for reducing losses. The current ripple produced by the output inverter stage is attenuated by an output filter whose size and cost are inversely proportional to the inverter switching frequency. The higher the switching frequency, the smaller and cheaper the filter size. However, this relationship creates a trade-off with the relationship between switching frequency and switching losses in the hard transition state—the higher the switching frequency, the greater the losses, and therefore the lower the efficiency. From 16kHz to 20kHz switching frequency, due to lower audio noise and higher efficiency, it can meet the requirements of solar inverters. Therefore, the power circuit remains to be further studied.

The following will compare the advantages of several semiconductor technologies applicable to these two stages.

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 at rated conditions. When the Transistor is off, the transistor can charge the inductor while the diode acts as a freewheeling diode. That is, when the transistor turns on again, the diode can actively turn off. The figure below shows the typical reverse recovery characteristics of commonly used silicon diodes (black and red curves in Figure 2).

Selection Tips for Power Electronics in Solar Inverters

The reverse recovery characteristics of silicon diodes produce high losses in both the boost transistor and the corresponding diode. SiC diodes do not have this problem (as shown by the blue curve in Figure 2). It’s just that there is a momentary negative current in the diode due to the capacitance, which is caused by the junction capacitance charge of the diode. Silicon carbide diodes can greatly reduce the turn-on losses of transistors and the turn-off losses of diodes, and also reduce electromagnetic interference because the waveform is very smooth and there is no oscillation.

In the past, many techniques 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 these greatly increase the number of components and complexity of the system, often resulting in reduced stability. In particular, it is worth mentioning that the same efficiency as soft switching can be achieved with a minimum of components even in the hard switching state by using SiC Schottky diodes.

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, as shown in Figure 3.

Selection Tips for Power Electronics in Solar Inverters

It is easy to see that the RDS(on) per unit area is about 4 to 5 times lower than that of CoolMOS compared to the standard process. This means that in a standard package, CoolMOS achieves the lowest absolute on-resistance value. This results in the 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 advantage of CoolMOS is obvious, if the voltage is higher, its advantage will increase. Currently, the highest voltage level is 800V.

Several studies have shown that using SiC diodes and superjunction MOSFETs such as CoolMOS is superior to a solution using standard MOSFET and diode processes (shown in Figure 4).

Selection Tips for Power Electronics in Solar Inverters

Power Semiconductors for Inverters

The output inverter connects 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 bank converters. Insulated gate bipolar transistors (IGBTs) are ideal devices for use in this inverter. Figure 5 shows two cross-sections of the IGBT process.

Selection Tips for Power Electronics in Solar Inverters

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

In the off state, standard NPT cells create a triangular electric field inside the semiconductor.All blocking voltages are absorbed by the n-region of the substrate

Absorb (depending on its thickness) so that the electric field drops to 0 before entering the collector area. 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 to further reduce conduction losses. The Trench gate process provides higher channel width, which reduces channel resistance. The ndoped field stop layer performs only one task: suppressing the electric field with extremely low off-state voltage values. This makes it possible to design an almost horizontal distribution of the electric field in the n-substrate layer. This means that the resistance of the material is very low, so the voltage drop during conduction is low. The advantages of the field stop layer can be exploited by further reducing the thickness of the chip to achieve all of the above advantages. Paralleling is also possible using the TrenchStop process.

Table 2 gives a comparison of IGBTs with blocking voltages of 600V and 1200V. For all three processes, the power ratings of the transistors used remain constant. This means that the current of the device at 600V is twice that of the device at 1200V. That is, a 50A/600V device is equivalent to two 25A/1200V devices.

Selection Tips for Power Electronics in Solar Inverters

As can be seen from the table above, the 600V TrenchStop process can reduce switching and conduction losses by 50% compared to the 1200V device. Therefore, it is important for the whole 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, which of the Fast process or the TrenchStop product family has better performance depends on the switching frequency.

IGBTs also typically require a freewheeling diode to enable them to freewheel, which is a specially optimized version of the EmCon process. It is optimized for the 15kHz switching frequency of the 600V series devices. In the past, it was believed that a freewheeling diode must have a very low on-voltage to achieve the lowest total losses. Other optimizations can be made based on application requirements to achieve lower overall losses in diodes and IGBTs. This shows that in IGBT and diode applications at frequencies around 16kHz, a higher forward voltage drop is more appropriate to achieve low switching losses.

This is illustrated in Figure 6 (600V series). The left column represents the losses of the EmCon diode in the TrenchStop IGBT and EmCon3 process. The right column represents the losses of the TrenchStop IGBT and diode optimized for low conduction losses (called the Emcon2 process). The same diode in the right column is used in conjunction with an IGBT using Infineon’s Fast process (600V). The yellow and orange parts of the bar graph represent the conduction and switching losses of the IGBT, respectively. The dark blue and light blue parts are the conduction and switching losses of the diode, respectively.

It is easy to see that at a switching frequency of 16kHz, the cosine of the load angle is 0.7 and the rated current, the Emcon3 diode has higher losses during conduction (dark blue), but results in better switching performance . So, diodes are already a good choice on their own at this point. In addition, it reduces the switching losses of the IGBT during turn-on. The considerations from Section 2 above apply here as well. Using an optimized EmCon diode can reduce losses by around 1W, which is an advantage. Note that when the load angle is close to 1, the switching losses will become the dominant losses, since the diodes conduct only during the dead time of the output inverter.

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 IGBTs, is helping people achieve this goal. Of course, achieving this requires optimizing not only the individual devices, but also the way in which these devices work together. This results in minimum losses and maximum efficiency, two of the most important metrics for solar inverters.

By Frank Wolfgang, Consumer Power Technology Marketing, Automotive, Industrial and Diversified Electronics Markets, Infineon Technologies

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