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**11**

*Islamabad, Pakistan*

Hamood Ur Rahman

*College of Electrical and Mechanical Engineering*

*National University of Sciences and Technology (NUST)*

**Plasma Based Dry Release of MEMS Devices**

Microelectromechanical Systems (MEMS) are constructed to achieve a certain engineering function or functions by electromechanical or electrochemical means. Very intricate moveable structures can be fabricated using the sacrificial layer (Madou, 2002). These moveable suspended structures are micro-bridges fabricated using the sacrificial layer. These structures can be cantilever beam bridges fixed at one end or membrane bridges fixed at both ends. In the field of RF MEMS these two types of bridges are used to fabricate the

In a fabrication process, the final release of a MEMS device is the most crucial step. Surface micromachining process relies on both wet and dry etching techniques. Wet etching has been widely used for pattern delineation. In wet etching, liquid etchant dissolves away the exposed film and attacks isotropically, resulting in loss of pattern definition due to undercutting and rounding of film features. This chapter suggests a solution to the problem of stiction by avoiding the wet release and in the absence of Critical Point Dryer (CPD). A dry release technique is presented for the RF MEMS structures, that combines the removal of sacrificial layer through wet etching and its substitution with standard photoresist. After coating, this photoresist acts as a supporting layer under the structure and rejects the structure to attack the substrate (Orpana & Korhonen, 1991). During this complete process wafers are not allowed to dry at any moment of time, otherwise structures may be permanently bonded with the substrate. The supporting layer is removed by oxygen plasma using the Reactive Ion Etching (RIE). In the dry etching, residual waste is off concern which effect the reliability of the MEMS structures. Motivation for this unique process was that some left over residues were observed after the single step or traditional RIE process. Secondly, this process was more cost effective as compared to a wet release CPD technique using CO2 dryer. The process not only produced

A serious limitation of suspended MEMS structures is that they tend to deflect through stress gradient or surface tension induced by trapped liquids during the final rinsing and drying step. Problems like stiction and bridge collapse are associated with producing a free standing structure. The stiction is described as a process of bonding the top and bottom

**1. Introduction**

series or shunt switches.

less residual waste but achieved a clean dry release.

**2. Need for dry etching**


**2. Searching**

Searching is a method for finding certain things in given data/files which are of vital importance. There are different categories of search methods: basic and advanced, like one using trees and digital key transformations, including balanced trees, hashing binary search trees, and digital search trees, and trying different methods appropriate for different types of files. These methods in turn are related to each other and possess much resemblance with sorting methods.

### **3. Sorting**

Sorting is a method of rearranging files in the order that are covered in some depth due to their fundamental importance. A large variety of methods were developed, described, and compared. Algorithms including priority queues, selection, merging, and several related problems are created. Some of these algorithms are used as the basis for other more complex algorithms.

#### **4. String processing**

String processing algorithms include a range of methods for dealing with (long) sequences of characters. String searching basically leads to pattern matching which in turn leads to parsing. File compression techniques and cryptology are also part of advanced string processing applications.

### **5. Geometric algorithms**

Geometric algorithms encompass an assortment of procedures for resolving problems by involving points and lines along with other simple geometric objects. Moreover, it also finds application of ideas from several mathematical disciplines like algebra, combinatorics, topology, and differential geometry. During the last two decades, most of the geometric applications like CAD/CAM, computer graphics, VLSI design, molecular biology, robotics, GIS, spatial databases, sensor

networks, machine learning, and scientific computing become the motivation for computational geometry to evolve as a full-fledged discipline of theoretical computer science. Some typical high-end applications of geometric algorithms includes: optimal airspace design, air traffic controller's traffic balancing and automatic sectorization of airspace, wireless sensor networks design, analysis of 2D electrophoresis gels, prediction of resilience, and recovery of damage in neural networks.

## **6. Graph algorithms**

Graphs algorithms are the natural way to understand connection between the linked data and thus reveal the relationships in data. Exploring and tracking these interlinking connections divulge new insights and influence and facilitate to analyze each data point as part of a bigger picture. Graph algorithms are useful in a variety of complex difficult structures and reveal difficult-to-find patterns ranging from finding bottlenecks, susceptibilities to detect communities, fraud rings, improving machine learning predictions to predict the spread of disease or ideas, and thus enable us to leverage relationship within data to devise intelligent solution to enhance the effectiveness of machine learning models. Graph algorithms types such as exact or approximated, static or dynamic, distributed or centralized, deterministic or randomized, and matching and network flow, minimal spanning tree, and shortest path are some of the general approaches developed for searching in the graph.

### **7. Genetic algorithms**

A genetic algorithm (GA) is a search-based heuristic algorithm used for solving optimization problems in machine learning that is based on genetics and natural selection. GA is a subset of evolutionary algorithms which is based on behavior of chromosomes and their genetic structures which uses evolutionary generational cycle starting from initialization, fitness assignment, selection, reproduction, replacement, and termination, to yield high-quality solutions. GA makes use of various operations which enhance or replace the population to deliver a betterquality suitable solution and get inspiration from evolution and natural selection. Through the process of natural selection, organisms regulate to augment their likelihoods for endurance in a given situation. Eventually, incompetent elements perish from the population, to be replaced by successful-solution descendants. Apart from the applications in multimodal optimization, DNA analysis, design of aircraft, and genetic algorithms are found to be efficient and cost-effective plan for tricky traveling salesman problem. It does not need derivative information and possess excellent parallel capabilities which refine and optimize the solution to the multiobjective problems, and discrete and continuous functions. The idea behind genetic algorithms is extremely alluring.

#### **8. Neural networks**

Neural networks are basically inspired by the biological nervous system or neural networks in the brain and basically parallel computing devices. It is one of the most emerging areas in data science which revolutionizes and eventually leads to tremendous growth of artificial intelligence, machine learning, and deep learning and basically designed to recognize patterns and extracting features that can

#### *Introductory Chapter: Search Algorithm: Essence of Optimization DOI: http://dx.doi.org/10.5772/intechopen.104844*

be fed to other relevant algorithms for further classifications and clustering. The elementary computational unit of a neural network is a neuron also called as node, and each node or neuron is linked with certain weight. This weight is assigned as per the relative importance of that particular neuron or node. Neural networks are simply weighted digraphs with neurons acting as vertices, and weights on edges denote the connection strength of the pair. Each node or neuron collects values or weights from other neurons and accordingly computes the output.

Gradient Decent, Conjugate Gradient, Newton's Method, and Quasi-Newton Method are some of the popular optimization algorithms which find applications in context with neural network. Speed and memory footprint of all these algorithms may vary, but an ultimate objective is to accomplish various intricate computational task faster than the traditional systems.

In neural network, training of the neurons is accomplished by appropriately modifying connection strength in response to training data. Most apt application of neural networks is forecasting and classification of applications, such as gene predication, optical character recognition, and stock market time series prediction.

Having discussed about all these different types of algorithms, one thing which is most apparent that, irrespective of types of algorithms, searching lies at the heart of all. Rather, it is the intrinsic part of all algorithms as it finds application one way or other, and it is one of the first things any algorithm designer should try in the quest for efficiency. Basically, sorting is directly based on searching, wherein we use to compare, i.e., search for specific pattern of string for comparison, and then rearrange those strings based on desired sorting pattern of ascending or descending. Searching can be used to illustrate most algorithm design paradigms. Data structure techniques, divide-and-conquer, randomization, and incremental construction all lead to popular sorting algorithms. Binary search and its variants are the essential divide-and-conquer algorithms. Depth-first and breadth-first search provide mechanisms to visit each edge and vertex of the given graph. Strategy represents the pursuit for the big picture, the framework around which we construct our path to the goal. Strategies are used to win the minor skirmishes we must fight along the way. They prove the basis of most simple and efficient graph algorithms. Simulated annealing is a simple and effective technique to efficiently obtain good but not optimal solutions to combinatorial search problems. Combinatorial search, improved with tree pruning techniques, can then be used to find the more optimal solution of small optimization problems. Ingenious pruning procedures can speed up combinatorial search to a remarkable extent. Proper trimming will have a greater impact on search time than any other factor. Historically, computers have spent more time searching and sorting than doing anything else all put together. A quarter of all mainframe cycles are spent in searching and sorting data. Although it is unclear whether this remains true on smaller computers, but still searching and sorting remain the most ubiquitous combinatorial algorithm problem in practice.

An important key to algorithm design is to use searching/sorting as a basic building block, because once a set of items is sorted, many other problems become easy. Consider the few of the applications like:

• **Closest pair** – In a given a set of *n* different numbers, suppose we are interested in finding the pairs of numbers having minimum or no deviation or minimal difference between them. One way to find the desired pairs in quickest possible way is to arrange these numbers in sorted order either in ascending or descending order. Once sorted, the closet pair of numbers shall spatially reside next to each other. Thus, merely a linear scan through the sorted list will complete the job.

Thus, searching/sorting is the most thoroughly studied problem in computer science. Literally, loads of diverse algorithms are known, most of which have some advantages over all other algorithms in certain circumstances. Most of the stimulating concepts used in the design of algorithms appear in the context of searching/ sorting such as data structures, divide-and-conquer, and randomized algorithms.

#### *Introductory Chapter: Search Algorithm: Essence of Optimization DOI: http://dx.doi.org/10.5772/intechopen.104844*

Searching/sorting is a natural laboratory for studying basic algorithm design paradigms, since many useful techniques lead to interesting searching/sorting algorithms. Searching/sorting thus becomes the basic building block around which many other algorithms are built. By understanding searching thoroughly and devising effective searching techniques, we can obtain an amazing amount of power to solve other problems and thus will become the essence for optimizations.


**1. Introduction**

CeO2 has been used as catalysts and additives due to its high oxygen storage capacity and moderate basicity [1]. The inherent nature of CeO2 makes it attractive for chemical reactions involving oxygen removal and addition. The transformation of renewable energy to fuel gases often employs CeO2 in the process. The addition of CeO2 to catalysts suppresses the carbon deposition on the catalysts during the steam-reforming reactions of CH4 and tar to form syngas [2,3]. CeO2 also improves the resistance of Ni catalysts against H2S during methanation reaction [4] and reforming reaction [3,5] by accelerating the removal of sulfur from the surface of active species. Chemical looping reaction and thermochemical reaction also proceed over CeO2 [6–9]. Despite its various applications, it is preferable to reduce the total consumption of CeO2. The supply and price of CeO2 are volatile. Further, the environmental load during the mining and processing of rare earth elements is higher than those of base metals [10,11]. It is desired to use less CeO2 to make the catalysts, as well as the process, sustainable.

Various techniques have been employed to reduce the consumption of CeO2 by controlling its size and shape [12–14]. Fine CeO2 nanoparticles of high surface areas are prepared by surfactant-assisted methods [15,16]. The size of CeO2 particles also influences the oxygen mobility on the CeO2 surface [17,18]. The lattice constant of CeO2 increases as the size of nanoparticles decreased, leading oxygen vacancies to be more stable than on the coarse CeO2 particles [13,19,20]. The oxygen mobility on the CeO2 surface was also influenced by the plane indices of the surface. Among the low-index surfaces, CeO2(110) exhibits the highest reducibility while CeO2(111) is the least reducible [21–23]. The difference was attributed to the surface energy of the surface [24,25]. CeO2(111) is more stable than CeO2(110) and CeO2(100) because it is the non-polar, close-packed surface of CeO2. Meanwhile, polar CeO2(100) and less-densely packed CeO2(110) surfaces underwent reconstruction of their surface, forming facets containing reduced CeO2-x surface [12]. Such reconstruction of the

surface leads them to be more reducible than CeO2(111). Reportedly, nanorod composed of CeO2(110) surface and cubic CeO2 nanoparticles composed of CeO2(100) surface exhibited higher catalytic activity than conventional CeO2 nanoparticles mostly composed of CeO2(111) surface due to higher oxygen mobility on CeO2(110) and CeO2(100) surfaces at low temperatures [21–23]. However, such self-standing CeO2 particles sinter to ~20 m2/g or less, corresponding to 50 nm in diameter, after calcined at high temperatures >800 °C. Both nanorods and cubic CeO2 nanoparticles transform into conventional CeO2 nanoparticles after being exposed to such high temperatures. Al2O3 supports are widely employed to improve the thermal stability of CeO2 nanoparticles. CeO2 nanoparticles on Al2O3 remain ~10 nm in diameter after the heat treatment at >800 °C under the oxidizing atmosphere [26,27]. Contrary, CeO2 and Al2O3 react to form CeAlO3 and other oxides under reducing conditions [28–30]. Such reactions cause structural changes in the catalysts and sintering of CeO2 nanoparticles. High-temperature tolerant CeO2 nanoparticles are necessary to further reduce the consumption of CeO2 under reducing conditions.

Some studies employ MgO as a support to disperse CeO2 nanoparticles [31–34]. The melting point of MgO is ~2850 ◦C, which is higher than that of Al2O3 supports [31]. MgO is also known for its stability under reducing conditions [35]. Further, CeO2 nanoparticles are stabilized on MgO supports without forming any composite oxides [31–34,36]. Partial oxidation of CH4 was demonstrated over CeO2/MgO catalysts, suggesting their stability under the reducing conditions [34,37]. However, the average diameter of CeO2 increased to >5 nm after the calcination at 800 ◦C. Techniques to stabilize finer CeO2 nanoparticles are necessary to further reduce the consumption of CeO2.

MgO support of large surface area would contribute to stabilized CeO2 nanoparticles. Meanwhile, the stability of MgO depends on the atmosphere to which MgO is subjected [38–42]; relative surface energies of low-index surfaces of MgO vary depending on the humidity. Polar close-packed MgO(111) surface is stabilized by hydroxylation independent of the humidity [38]. On the contrary, non-polar MgO(100) is terminated by OH only under humid conditions since the OH-termination does not contribute to the stabilization as it is on the polar surface. Under humid conditions, therefore, OH-terminated MgO(111) is more stable than OH-terminated or pristine MgO(100). Meanwhile, pristine MgO(100) is more stable than OH-terminated MgO(111) under dry conditions [38]. Further, the sintering of MgO is accelerated in the presence of water vapor [39–42]. These reports suggest that the morphology of MgO depends on the humidity of the atmosphere under which the MgO is prepared. These studies imply that the humidity can influence the catalytic activity of the catalysts containing MgO. However, no research assessed the effect of humidity on the catalytic activity and morphology of CeO2/MgO.

This research demonstrates that CeO2/MgO catalysts of fine CeO2 nanoparticles by controlling the atmosphere during the preparation. Calcination in dry air realized CeO2 nanoparticles smaller than 3 nm in diameter even after heating at 800 °C. The prepared CeO2/MgO catalyst outperformed pure CeO2 for dry reforming reaction although the mass ratio of CeO2 in CeO2/MgO was less than 1/5 of pure CeO2.

### **2. Materials and Methods**


**1. Introduction**

Cucumber (*Cucumis sativus* L.) is cultivated throughout the world because of its ability to thrive in both temperate and tropical conditions. As a result, fresh cucumbers are available throughout the year [1]. Historically, cucumber is one of the oldest cultivated crops and believed to be a native of the Asia continent [2]; parts of China with temperate climates and parts of southern regions of India with tropical climates. At present, cucumber is the fourth most widely cultivated vegetable crop in the world (after tomatoes, onions and cabbage) and China is by far the world's largest producer of it [1].

*C. sativus* belongs to the gourd family of Cucurbitaceous, which also includes cantaloupes, squash, pumpkins, melon and water melon [3, 4]. *C. sativus*is a tender creeping vine and hairy leaves with 3 to 5 pointed lobes. In general, it bears roughly cylindrical fruits and may be as large as 60 cm long and 10 cm in diameter with dark-green skin, crispy moisture rich flesh, and small edible seeds concentrated at its core [5]. There are close to 100 varieties, but common ones include the English, garden, Persian, mini, and lemon. Cucumbers are mainly eaten in the unripe green form when they taste sweet, have crunchy texture, and unique flavor. Thus, they are best-harvested young, tender and just short of reaching maturity. On getting to full maturity, the cucumber skin becomes tougher and turns yellow-white with an accompanied bitter and sour [1, 5].

Relative to other vegetables that have been widely researched and reported on, cucumber seems to have received little interest probably because of seemingly lack of concentrated levels of well-known bioactive compounds presents in garlic, onion, tomatoes and the likes. Generally, cucumbers are consumed because of their refreshing quality, in part due to their very high water content and crunchy texture. Studies have shown that cucumbers contain lignans, vitamin K, cucurbitacins and their derivatives (triterpenoids), flavonoids (apigenin, luteolin, quercetin, and kaempferol), antioxidants (beta carotene and vitamin C) and B vitamins among other trace elements and minerals [6–8]. With a 95% level of water content saturated with naturally-occurring nutrients and trace elements meshed in high dietary fiber, cucumbers are beginning to gain attention in therapeutic, culinary and cosmetic uses.


**Chapter 3**

maintenance and self-recovery

Maintenance Management

engineering systems. In: Proceedings of the 8th World Congress on Engineering Asset Management (WCEAM 2013) & the 3rd International Conference on Utility Management & Safety (ICUMAS); 2013. pp.1509-1520. DOI 10.1007/978-3-319-09507-3\_128

Singapore: World Scientific Publishing

[25] Zaslavsky G. Chaos, fractional kinetics, and anomalous transport. Physics Reports. 2002;371(6):461-580

[26] Feller W. An Introduction to Probability Theory and its Applications. Vol. 2. Hoboken, New Jersey: John

Wiley & Sons Inc; 1991

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and System Health Management (PHM). Vol. 33. 2013. DOI: 10.3303/

[20] Kirillov A, Kirillov S, Pecht M. Remote calculating PHM cluster: The first results. In: Proceedings of the 4th IEEE Conference on Prognostics and System Health Management (PHM). Vol. 33. 2013. DOI: 10.3303/CET1333045

[21] Iakimkin V, Kirillov A, Kirillov S. Preprocessing of the observed data and the recognition of the hidden signs of fault. In: Proceedings of the 4th IEEE Conference on Prognostics and System Health Management (PHM). Vol. 33. 2013. DOI: 10.3303/CET1333012

[22] Kirillov A, Kirillov S, Pecht M. The calculating PHM cluster: CH&P mathematical models and algorithms of early prognosis of failure. In: Proceedings of the Prognostics and System Health Management Conference (PHM- Beijing). 2012. DOI: 10.1109/PHM.2012.6228771

[23] Iakimkin V, Kirillov A, Kirillov S. Distributed pre-processing of telemetry for mobile engineering objects. In: Proceedings of the 8th World Congress on Engineering Asset Management (WCEAM 2013) & the 3rd International Conference on Utility Management & Safety (ICUMAS). pp. 463-472. DOI: 10.1007/978-3-319-09507-3\_40

[24] Kleinert H. Path Integrals in

Physics, and Financial Markets.

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CET1333057


**2. Operation principles**

*Fiber Optics - From Fundamentals to Industrial Applications*

more than one DOCSIS channels to a single user.

coherent optical transmission, can be exploited.

population.

3.1, it is expected that HFC networks will continue to dominate the broadband access market in the United States, delivering fastest access speed to the broadest

The continuous envelope and high peak-to-average power ratio (PAPR) of OFDM signals, on the other hand, make them vulnerable to noise and nonlinear impairments in analog HFC networks [22–24]. Combined with demanding carrierto-noise ratio (CNR) requirements of high order modulations (>4096QAM), it is difficult to support DOCSIS 3.1 signals by legacy analog fiber links [15]. In this paper, we for the first time demonstrate the digitization of DOCSIS 3.1 signals to enable the upgrade of fiber distribution networks from analog to digital, so mature digital fiber technologies, e.g., intensity modulation/direct detection (IM/DD) and

To enable digital transmission of DOCSIS 3.1 signals, a digitization interface, i.e., analog-to-digital converter (ADC), is needed in the hub to digitize the analog signals into bits, and a digital-to-analog converter (DAC) is needed in the fiber node to retrieve the analog waveforms from digital bits for the following transmission over coaxial cable plant. Different from conventional Nyquist AD/DA that uses Nyquist sampling rates, such as common public radio interface (CPRI) in MFH networks [25], which has quantization noise evenly distributed in the frequency domain and needs many quantization bits, delta-sigma ADC features high sampling rate but only a few (one or two) quantization bits, and most importantly, it utilizes a noise shaping technique to push the quantization noise out of the signal band, so that signal and noise are separated in the frequency domain, and the in-band CNR of digitized signals can be optimized [26–29]. Moreover, a simplified DAC design based on low-cost passive filters can be used in the fiber node, which filters out the desired signals, eliminates the out-of-band noise, and at the same time, retrieves the analog waveforms. In the hub, a high-speed delta-sigma ADC is shared by multiple fiber nodes; whereas in each fiber node, only a low-cost passive filter is needed to filter out the desired signal and convert it to the analog waveform. Since there are more fiber nodes than hubs, especially given the fact that fiber node number is continuing to grow due to node segmentation and fiber deep migration, replacing Nyquist DAC with a low-cost passive filter can significantly reduce the cost and

Delta-sigma digitization has found wide applications in power amplifiers [30–32], RF transmitters [33–37] and receivers [38–42], visible light communications [43, 44], radio-over-fiber (RoF) [45–47], and MFH networks [48–50]. In Ref. [48-50], we first demonstrated delta-sigma digitization as a new digitization

As a fifth-generation broadband access technology, DOCSIS 3.1 specifications are being commercialized at a historically rapid pace to support ultra-high-resolution videos (4 K/8 K), mobile backhaul/fronthaul (MBH/MFH), and other emerging applications enabled by virtual reality and internet of things [10–14]. DOCSIS 3.1 specifications involve enhancement in both physical and MAC layers, which transform the physical layer signal from single-carrier QAM (SC-QAM) to orthogonal frequency division multiplexing (OFDM), for increased data rate, improved spectral efficiency, and flexible resource allocation. It provides up to 10 Gb/s downstream and 1.8 Gb/s upstream capacities to each subscriber [15, 16]. With subcarrier spacing of 25 or 50 kHz, DOCSIS 3.1 specifications support downstream channel bandwidths 24–192 MHz, and upstream channel bandwidths 6.4–96 MHz [17–19]. Moreover, higher order modulations up to 4096QAM were adopted with optional support of 8192 and 16384QAM [10, 15]. Similar to the LTE carrier aggregation in MFH networks [20, 21], DOCSIS 3.1 specifications support channel bonding to designate

**70**

complexity of fiber nodes.

The architecture of a HFC network is shown in **Figure 1**. Due to their similarity, a C-RAN architecture is also presented for comparison. In **Figure 1(b)**, the network segment from service gateway (S-GW) or mobile management entity (MME) to

**Figure 1.** *Architecture of HFC network and C-RAN: (a) HFC network and (b) C-RAN.*

baseband unit (BBU) is defined as mobile backhaul (MBH), which transmits the control and payload bits of LTE signals in digital baseband. The digital bits are received by BBUs, which synthesizes OFDM modulation and generates analog waveform of LTE signals. The network segment from BBU to remote radio heads (RRH) is defined as mobile fronthaul (MFH), where the LTE signals are transmitted over fiber in either analog waveform using RoF technology or digital waveform using CPRI digitization interface. The last segment of C-RAN involves the wireless transmission from the RRHs to mobile users, where LTE signals are transmitted in their analog waveforms via air interface.

Similarly, HFC networks in **Figure 1(a)** can also be divided into three segments, i.e., the core network segment from headend to hub, the fiber distribution network from hub to fiber nodes, and the coaxial cable plant from fiber nodes to cable modems (CMs). Similar to MBH, the segment from headend to hub transmits net bit information; similar to MFH, the fiber distribution segment from hub to fiber node is supported by either analog or digital fiber technologies, e.g., C-RAN uses RoF technology to deliver analog mobile signals; HFC uses analog fiber links to deliver analog DOCSIS/video signals; C-RAN uses CPRI as a Nyquist digitization interface; HFC has a similar interface called baseband digital forward or return (BDF/BDR). The last segment from fiber node to CMs is also similar to the wireless segment of C-RAN, where both DOCSIS and LTE signals are transmitted in their analog waveform over coaxial cable or air interface, respectively. Different analog or digital implementations of the fiber distribution segment are shown in **Figure 2**.


4

**Table 2.** Main multilevel inverters advantages.

The development of MLI has been marked by the progress of semiconductor materials technology (IGBT, MOSFET, etc.) and the evident evolution of digital processors (microprocessors, DSP and FPGA). In this sense, in [36], an interesting investigation is presented, and the most important conclusion could be that the use of 4H-SiC constitutes one of the most important aspects that enabled the current development of power converters. Hence, the multilevel inverters are rapidly emerging as a promising alternative in photovoltaic systems for high-power/medium-voltage DC/AC conversion. Within multilevel inverters, the Multilevel Voltage-Source Inverter (MVSI) inverter has got attention for a better quality power supply. In MVSI, the amplitude of the output voltage generally is less than the input, in these cases, the inverter behaves as a buck converter. Therefore, an intermediate boost stage is generally required since, in RES applications, the panel voltage is low. Nevertheless, by including this stage, the complexity of the system control increases, and the efficiency decreases.

There is a close compromise between THD, inverter output levels and filter size. When the levels increases, THD and filter size decrease, but a higher number of components is required. Nowadays, an important challenge is to design schemes with a reduced number of components. Authors in [37] introduce a novel topology, which seeks to reduce the number of switches as shown in Figure 4. This scheme has three power supplies and ten semiconductors to deliver fifteen output levels.

**Figure 4.** Multilevel topology proposed in [37].

The authors propose that the maximum output voltage by using this configuration is:

$$V\_{o,max} = V\_{dc} + 2V\_{dc} = 7V\_{dc} \tag{1}$$

where: *Vdc* is the input power supply and *Vo*,*max* is the output voltage in the load.

The document presented in [38] summarizes the main schemes that make it possible to reduce the number of elements. The work concludes that the reduction of components causes the use of more expensive devices, by raising the voltage rating of semiconductor circuit breakers. Other aspects, such as the increase in the number of energy sources and more complex control schemes are also pointed out. The above approach shows the compromise that exists between the number of energy levels that the converter delivers and the complexity of its control, presenting a proportional relationship between both variables [39].

The grid-connected PV systems must comply with certain standards such as VDE 0126-1-1, which regulates the maximum allowed of leakage current in the system. Leakage current flows when the terminals have high-frequency voltage transitions.This systems generally use transformers to ensure the isolation of the PV system. Hence, it is avoided the appearance of leakage current between the stray capacitances of the panel and the grid [40], causing EMI problems, increased harmonic distortion and possible damage to health. The use of an isolation stage transformer increases as the weight, cost and volume of the system. In addition, this element causes losses in efficiency of around 3%. The newer topologies seek to eliminate this element. The main advantages are higher efficiency, adequate power density, and lower cost [41]. Also, the performance of the control would be affected according to the winding settings [42]. For this reason, transformerless PV inverters capture the interest of the scientific community [43].

The European Network Code "Requirements for Generators" or VDE-AR-N 4105 is aimed at low voltage systems. The code establishes the grid connection standards for generation systems in Germany. The main parameters to monitor are the capabilities for frequency stabilization and the provision of reactive power. A comparative summary of the aforementioned standards is presented in Table 3. In [44], the principal regulations that the grid-connected systems must comply with are summarized.

**Table 3.** German code VDE comparison [45].

#### **3. Neutral Point Clamped Based Topologies**

In NPC inverters, multiple DC sources are generated by dividing the input bus voltage using a capacitors bank as shown in Figure 5a. The topology is recognized as one of the most popular schemes among MLI [46]. Table 4 shows the allowed, potentially destructive, and destructive switching patterns that could be implemented in the basic structure of the Three-Level NPC (3L-NPC) inverters. The NPC converters are mainly employed in high and medium-power range [47,48]. This DC/AC converter have low dv/dt, low THD [49] and can remove common-mode current making it attractive for PV applications [50]. There are certain high-power applications in which NPC inverters allow a higher DC-link voltage and also avoid the series connection of semiconductors in the same branch [51]. This devices have a large number of clamping diodes, unbalance problems in the DC-bus capacitors and a non-uniform distribution of losses in the switches. In standard operating conditions the values of the capacitors must have a value similar voltage. In [52], a new PWM modulation strategy is proposed to control the output voltage and balance the DC bus capacitors for converters.

An alternative to the traditional NPC topology is the Active-NPC (ANPC) scheme. This variant arises to eliminate the previously mentioned disadvantages. In ANPC design, instead of using clamping diodes, bidirectional semiconductors are used, as shown in Figure 5b. In both schemes (NPC and ANPC), it is possible to reduce the leakage current by connecting the middle point of the DC-bus to the grid ground. In this way, the value of *DC* + or *DC* − will depend on the sign of the output current. Therefore, the stray capacitance voltage remains constant and no leakage current arises. The conduction losses can be reduced by using different paths in the zero state. Currently, certain modifications of the traditional PWM have been implemented in ANPC topologies to achieve correct operation of the inverter, reducing conduction losses and achieving a correct balance of the capacitors.

**Figure 5.** Comparison of basic configurations: (**a**) 3L-NPC and (**b**) 3L-ANPC.

**Table 4.** Switching states 3L-NPC.

The techniques for eliminating the leakage current in PV inverters are grouped into two categories. The first introduces one switch to isolate the grid from the panels in freewheeling times. The second category maintains a neutral connection from the grid to the midpoint of the input capacitors and

ensures low-voltage variations. Considering an NPC inverter, the common-mode voltage is defined from Equation (2). From the mathematical point of view the reduction of the leakage current is achieved when there is no variation in the terms *Vcm*−*dm* and *Vcm*. Figure 6 shows a simplified electrical diagram, where the influence of the above terms in the emergence of leakage current is observed.

$$V\_{cm} = \frac{V\_{1n} - V\_{2n}}{2} \tag{2}$$

$$V\_{dm} = V\_{1n} - V\_{2n} \tag{3}$$

where: the common voltage and the differential voltage are defined as *Vcm* and *Vdm*, the voltage at the inverter output at (1) and (2) with respect to the neutral point (n) is defined as *V*1*n* and *V*2*n* respectively.

By considering Equations (2) and (3), *V*1*n* and *V*2*n* can be presented as:

$$V\_{1n} = \frac{V\_{cm} + V\_{dm}}{2} \tag{4}$$

$$V\_{2n} = \frac{V\_{cn} - V\_{dm}}{2} \tag{5}$$

$$V\_{cm-dm} = -\frac{V\_{dm}}{2} \tag{6}$$

where: the relationship between the common mode and the differential mode is determined by *Vcm*−*dm*.

**Figure 6.** Common-mode represented based on simplified electrical circuit diagram.

The control strategy in this type of converter is divided into current loop based controllers and Direct Power Control (DPC). A good dynamic inverter response as well as a simple control scheme are two characteristics present in the DPC technique. The main disadvantage of this control strategy is the presence of a variable-switching frequency. Today this technique is combined with others such as Space Vector Modulation (SVM) [53,54] and MPC to achieve a fixed-switching frequency [55–58]. Control schemes should also include an appropriate modulation strategy, with particular emphasis on capacitor balancing. In this way, safe operation of the switches is achieved, avoiding over-voltage conditions. Table 5 summarizes the works on the modulation techniques for NPC inverters. There are three factors that must be considered for the selection of the modulation strategy [59]:

**Table 5.** Summary of recent works on modulation strategies in NPC inverters.

Most of the methods [68] that reduce the common-mode base their principle on selecting the vectors corresponding to Common-Mode Voltage (CMV) lower or zero without considering the oscillation of the neutral point voltage. In [69], a novel virtual SVM where a zero NP current average and a low CMV in one control cycle is achieved. Martinez et al. in [70], present a comparative analysis on different modulation techniques used in PV inverters. The results of the work throw certain conclusions that are interesting:

An analysis of the lifetime of inverters for photovoltaic applications is carried out in [71], where an NPC based topology and a T-type inverter are compared. The authors conclude that inverters based on the NPC topology have a longer lifetime than T-type inverters. This conclusion exposes the durability and the use of this type of inverter in RES applications [72,73]. In this sense, authors in [74] summarize a group of inverter topologies used in RE applications, highlighting the presence of a low leakage current in each of them. Ma et al. in [72] propose a new PWM strategy for ANPC topologies, the scheme is illustrated in Figure 7. The cited work present a modulation strategy based on an adjustable losses distribution that offers excellent performance and an increase the efficiency of the topology of the 97%. The switching pattern is presented in Table 6.

**Figure 7.** ANPC Half Bridge proposed in [72].

**Table 6.** Switches states of adjustable losses distribution of ANPC Half-Bridge inverter proposed in [72].

Wang et al. in [75] propose a grid-connected 6S-5L-ANPC inverter. The topology reduces the number of switches since eight switches are generally used. This advantage reduces conduction and switching losses. An important comparison with traditional ANPC topologies considering the stress of semiconductor devices, the switching frequency, the switching losses, the conduction losses and the system volume is presented. In PV applications, special attention should be paid to THD. Therefore, the authors select the phase disposition PWM scheme as modulation strategy. This method directly affects the balance of flying capacitors. The proposal achieves a correct balance of the capacitors, also using a selection method to limit its voltage ripple. Figure 8 present the aforementioned topology and Table 7 shows the switching states of the inverter. In total, there are eight possible states. One of the most important results is a THD of 1.6%. The authors also present the equations for sizing capacitors in active and reactive power conditions. Equation (7) establishes the capacitor value under the condition of unit power factor and Equation (8) under reactive power condition.

$$C\_{fc} = \frac{I\_{pk}}{2\Delta V\_{fc} f\_s M} \tag{7}$$

$$\mathcal{C}\_{fc} = \frac{\sum\_{n=1}^{N} \Delta Q\_{fc}}{\Delta V\_{fc}} = \frac{2MI\_{pk}}{\Delta V\_{fc} f\_s} \sum\_{n=1}^{\frac{qf\_s}{2\pi f\_{Line}}} \sin^2(n \frac{f\_{Line}}{f\_s}) 2\pi \tag{8}$$

where: *fLine* represents the line, *fs* is the switching frequency frequency, *Ipk* is the peak value of the output, Δ*Vf c* is the voltage drop, Δ*Qf c* is the electric charge, the modulation index is defined as *M* and *N* is the number of switching cycles.

**Figure 8.** Proposed ANPC inverter topology in [75].

**Table 7.** Operation modes of the proposed topology in [75].

#### **4. Flying Capacitor Based Topologies**

The FC concept was first introduced in 1992; this type of inverter uses different capacitors to deliver various levels of power at the converter output [76]. The topology benefits include attractive properties in different power ranges, however they are more suitable for medium-voltage applications. Another advantage of topology is the possibility of using natural self-balancing. Furthermore, it has an equitable distribution of voltage stress between switches [77]. Also, as in the case of NPC, a single source can be used to generate multiple voltage levels.In commercial applications, the use of FC with more than three output levels are more common than the NPC alternative [78]. The presented topology is generally not used in PV applications. The scheme is more suitable for use in electric vehicles. However, it was decided to include it in work, since it is part of the most widely used voltage-source topologies. Figure 9 illustrates a Five-Level FC (5L-FC) inverter and Table 8 the operation modes are shown.

**Figure 9.** Basic configuration of 5-level inverter based on FC.

Despite the advantages mentioned, FC inverters have certain limitations that are addressed in current works. For example, capacitor banks reduce the life of the system, and sometimes the balance of floating capacitors can be complex [79]. The problem of capacitor voltage balance is the main limitation of the use of the FC topology. Consequently, the scheme has not been generally used in PV applications. In the last decade, investigations related have been reported, for example, in [80], using a *DA*-*DB* duty cycle mismatch measurement between two groups of Three-Level Flying Capacitor (3L-FC) topology switches to control the system without any additional detection. Table 9 summarizes several works between the years 2015–2019.

In [86] a novel converter is proposed, which has certain advantages, for example, reduced voltage stress on semiconductors, a wide voltage gain and a common grounded scheme. These characteristics can be commonly found in this type of inverter, however, FC-based converters are not generally used in RES, since they require a large number of input capacitors that increase the complexity of the techniques for balancing them [87].

THD is reduced with more energy levels at the inverter output. In the case of FC, requires a large number of capacitors. A derivation of this configuration has been presented in [88], where cross-connecting capacitors have achieved a higher voltage level at the output through additional switches.Another of the biggest challenges in this topology is to provide the necessary energy to activate the large number of switches that the scheme has. Ye et al. in [89] present the comparison of five methods that reduce space and increase the efficiency of gate drive power supply circuits. Also, the operation of a multilevel FC inverter where an additional circuit is provided to avoid the defective cell, if it exists, is presented in [90]. However, in this topology, the elements have to be oversized to operate at full-power level when the failure of one of the cells is detected.

The FC design must have several considerations. Various design methodologies are found in the bibliography. For example, authors in [91] propose a methodology based on harmonic representation of the switching functions. The advantage of the proposed methodology lies in the possibility of being extrapolated to any FC-based scheme. Currently the uses of the treated scheme are very varied. There are certain applications in which a DC-bus is used due to voltage variations. Large capacitors are connected in parallel to the bus to avoid such voltage variations. Some of this applications are back-to-back converters, Power Factor Compensators (PFC), and uninterruptible power supply. The FC topology is chosen as the infinite virtual capacitor converter, which is a nonlinear capacitor where the voltage dependence of the load has a flat region and the voltage remains constant [92].

## **5. Cascaded Based Topologies**

The CMLI integrates multiple H-bridge schemes to generate a multilevel voltage [93]. The scheme has certain advantages compared to NPC and FC topologies, for example, they do not employ clamping diodes, in addition, a greater number of energy sources making it more suitable for specific applications such as electric vehicle [94] and PV applications [95]. Another advantage of the CMLI scheme is that, if any device fails in the bridge, the converter will continue to operate although it will deliver less energy. Therefore, this configuration is, to some extent, fault-tolerant. Also, its modularity and smaller filter size make it more attractive for high and medium-power PV applications. Figure 10 present a basic configuration of the CMLI topology, and Table 10 shows the switching pattern for the five output levels.

**Figure 10.** 5L-CMLI basic configuration.

**Table 10.** 5L-CMLI switching table.

Despite the aforementioned advantages, the topology has certain limitations. The main disadvantage is the use of isolated DC sources for each H-bridge. This problem was solved in the FC and NPC topology, but the voltage adjustment of the capacitors is complex [96]. Also, during partial shading the energy captured by the system is reduced. In certain investigations have presented various studies about the partial shading of PV modules, but most of the schemes are complex designs that generally cause a decrease in the efficiency of the system and an increase in the cost of the inverter [97,98].

The selection of the controller in CMLI depends on the topology and the application. Each controller has favorable characteristics in certain systems, ranging from less complexity to a desired dynamic response. Various types of controllers are used with the scheme discussed. The most widely used are PR controller and PI. When used LC or LCL filters using traditional controllers such as PI is not appropriate because it does not completely eliminate the steady state error [99]. Control systems that employ proportional-resonant controllers eliminate steady-state error. These controllers provide infinite gain in resonance frequency. Equation (9) defines the ideal PR control. In [100] a new technique to synchronize MLI with the grid using PR controller is shown. In the presented design, the control scheme has a lower error between the real power and the reference compared to the PI controller.

$$G(S) = k\_P + \frac{k\_i S}{S^2 + w\_o^2} \tag{9}$$

where: *wo* is the fundamental frequency (grid frequency), *kp* and *ki* represent proportional and resonant gains respectively.

Grid-connected systems can be classified according to the Maximum Power Point Tracking (MPPT) method used. The two classifications are centralized or distributed. The distributed technique reports better efficiency in the literature, but is more complex and has a larger volume than the centralized MPPT methods [101]. In the case of cascade inverters, the implementation of a distributed method requires a large number of sensors and considerably increases the cost of the system. Figure 11 illustrates the most common architectures in the distributed MPPT method. In micro-inverters of the Figure 11a, the energy generated by the different modules is injected directly into the grid. In front-end DC optimizers presented in Figure 11b, the converters perform the MPPT separately. Its output is connected in series; thus, the power that is injected into the grid is the sum of each module. If a module has low efficiency, it does not affect the rest of the converters, since each module provides its power separately.

Authors in [102] present a low cost and straightforward distributed MPPT method for energy optimizers in CMLI-based photovoltaic systems using front-end DC optimizers. In [103] a simplified feedforward distributed MPPT method for grid-connected CMLI is presented. The authors use the method as a *"... superior solution for PV system grid integration due to its simple implementation, signal stage power conversion, no added complexity with increasing the number of connected modules, and it eliminates the need for individual control loop for each module..."* It is important to mention that most conventional techniques do not achieve a distributed MPPT, which decreases the efficiency of the system. Goetz et al. in [104] propose a modular Double Cascading H-bridge (*CHB*2) topology.

The scheme reduces the output of the inverter filter and achieves a fast dynamic response. The MPPT is carried out in each module, which incorporates a battery for energy storage. The use of batteries in PV applications is avoided since these elements are highly polluting, then they break with the concept of clean energy. One of the essential advantages of the topology is the possibility of being extended to *CHB*2 circuits in general.

**Figure 11.** Distributed MPPT architectures: (**a**) micro-inverters and (**b**) front-end DC optimizers.

A brief bibliographic review shows the use of topology as a fault-tolerance scheme. In photovoltaic applications, faults distort the output voltage, degrading the power supplied. A fault diagnosis scheme must detect the problem in the shortest possible time to avoid serious failures in the system, and each design requires its strategy. A common problem is considering that the system is in open circuit to monitor its status [105]. Shao et al. in [106], present a technique for detecting faults using Sliding Mode Observer (SMO) able to locate the fault element in the system. The authors in [107] present a detailed review of various faults in photovoltaic systems. The work identifies the main faults as line-line, earth, arc, shadow and others, and proposes its protection strategy. Various strategies are employed for fault detection in PV systems, using standard protection devices or offline/real-time testing of PV systems. Table 11 shows a summary of typical failure cases and respective protection/detection devices.

**Table 11.** Typical fault occurrences and respective protection/detection devices present in [107].

Note: Over Current Protection Devices (OCPDs), Ground Fault Detection and Interruption (GFDI) fuses/Ground Fault Protection Devices (GFPDs), Arc Fault Circuit Interrupters (AFCIs), Residual Current Monitoring Device (RCD), Insulation Monitoring Device (IMD), Earth Capacitance Measurement (ECM).

Currently, there is a trend towards the combined use of PV energy and batteries as a storage medium and certain studies analyze its feasibility. For example, authors in [108] conducts extensive research concluded in 2019, in Finland. The work considers the profitability of BESS investments between the years 2018 and 2035. The authors conclude that, these systems would not be profitable for RES applications at present, although a decrease in costs is estimated from 1270 to 1370 euros/kWh in 2018 to 830–930 euros/kWh in 2035. However, sometimes an uninterrupted flow of energy is required and the use of batteries is essential due to PV power intermittent nature. The BESS output is controllable and the system can be treated as a controllable load [109].

The grid-connected schemes in this typology, as in the previous ones, still have certain challenges. Decreasing the leakage current in transformerless systems is one of the main aspects to consider. In this regard, some research has been carried out, such as [110]. In this work an analysis of the behavior of the leakage current of the different modes of operation of the basic structure CMLI is presented. The analysis considers the common-mode inductor in each switching state and can be simpler if pole voltages are used. Also, the authors propose two schemes of suppression of the leakage current. The first solution uses low-capacitance common-mode capacitors and stray capacitors as part of the output filter. It is included that this solution is suitable for inverters operated at high-frequencies. Solution two is appropriate when the converter uses a switching frequency of less than 1.5 kHz.

Sonti et al. in [111] present a PWM technique to eliminate or reduce leakage current in CMLI-based schemes. The work integrates the applied MPPT and PWM algorithm. In this way, it is possible to reduce the high-frequency transitions of voltage and the CMV. Figure 12 shows the proposed architecture and Table 12 present the switching states. The switches *Sw*4, *Sw*5 and *Sw*6, *Sw*7 operate in a complementary way. Hence, there are three pairs of switches [*Sw*1, *Sw*2, *Sw*3], [*Sw*4, *Sw*5] and [*Sw*6, *Sw*7]. Modulation proposal isolates PV array and grid during freewheeling states, operating similarly to an H5 topology. The authors achieve leakage current reduction with the presence of low-frequency transitions in the PV terminal voltage.

**Figure 12.** Five-level CMLI proposed in [111].

**Table 12.** Operation mode of the topology proposed in [111].