## 4 Power Converter Topologies
### PV Stage (Unidirectional DC-DC Converters)  
In the PV stage, a unidirectional DC-DC converter is required that is capable of extracting the maximum power from each solar module in a PV array and at the same time maintaining the voltage at the output of the converter, which is connected to the DC bus of the system.  
During high temperatures and low levels of irradiation, the PV endures its main issues with performance, significantly reducing the power provided by the PV array [33]. When using non-isolated or transformerless converters, the use of series-connected PV modules is necessary to achieve the required voltage of the DC bus. However, as discussed in Section 2.2, these mismatch conditions become more problematic when series-connected PV modules are used. Many solutions are reported in the literature with the aim of addressing this problem.  
As reported by [PERSON] and [PERSON] [30], several maximum power point tracking (MPPT) techniques have been developed and implemented. The methods most used are perturb and observe, incremental conductance, fuzzy logic control, and hill climbing; however, this last technique is the most preferred, since it reduces oscillation in steady state, thus providing better tracking efficiency.  
On the other hand, methods such as artificial neural network or fuzzy logic control are more flexible when handling module mismatch or partial shading conditions according to [13; 59].  
The most used power converters for the PV stage are the standard non-isolated, boost, and buck-boost topologies. Although, there is also reported the use of Cuk and single-ended primary inductor converter (SEPIC) in the literature [13; 29; 42].  
To acquire a higher output voltage, the input voltage magnitude must be stepped up accordingly as shown in Figure 6a by the DC-DC boost converter [29]. On the other hand, the magnitude of the input voltage can either be lower or higher than the output voltage magnitude when we speak of the buck-boost topology, as indicated in Figure 6b. This converter is developed using a boost and buck basic topologies cascade connected. The Cuk converter is shown in Figure 6c; its work mimics the buck-boost converter. The common terminal of the input voltage is employed with reverse polarity so that the Cuk converter can step up or down input voltage. The SEPIC shown in Figure 6d, also has the same features of a buck-boost converter: possesses a non-inverting output polarity and step up and down the input voltage [29].  
Table 2 describes the main advantages, disadvantages, and applications of the aforementioned converters used in the PV stage.  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Ref.** & **DC-DC Topology** & **Advantages** & **Disadvantages** & **Applications** \\ \hline \multirow{8}{*}{[29; 42; 58]} & \multirow{8}{*}{Boost Converter} & \(\bullet\) & The reverse current is blocked by the free-wheeling diode & \(\bullet\) & MPP cannot be tracked under low & \multirow{4}{*}{\(\bullet\)} & Not suitable for high variability \\  & & \(\bullet\) & It is remarkably the most effective topology at any price & \(\bullet\) & It has a & environmental conditions \\  & & \(\bullet\) & Less ripple on output side & \(\bullet\) & It cannot achieve & \multirow{2}{*}{\(\bullet\)} & Low input voltage and \\  & & \(\bullet\) & Cheaper filter components & \(\bullet\) & values near the & \multirow{2}{*}{\(\bullet\)} & \\  & & \(\bullet\) & It has output & & modules \(\
abla_{\text{OC}}\) & \\ \cline{1-1}  & & & non-inverted polarity & & & \\ \hline \hline \end{tabular}
\end{table}
Table 2: Comparison of non-isolated DC–DC converters for the PV stage.  
Figure 6: Schematic diagram of non-isolated DC–DC converters. (**a**) Boost converter; (**b**) buck-boost converter; (**c**) Cuk converter; (**d**) single-ended primary-inductor converter (SEPIC) [29; 60].