## 2 Architectures Analysis  
The main focus of this paper is to design a power optimizer architecture for PV string, which is interfaced with a centralized MPPT inverter. Three architectures, conventional, advanced, and proposed, are presented in Figure 1, where MPPT operation of the string is managed by a centralized inverter. Note that in such architectures, each DC-DC converter at module-level serves as a unit-level power optimizer to ensure that it maintains the common string current (\(I_{string}\)) under adverse mismatch effects, such as partial shading. The discussion about the operation and features of the architectures are as follows.  
Figure 1a displays the conventional architecture, in which a buck-boost converter is usually used for each module. In this architecture, each module-level converter is always active, and the common string current (\(I_{string}\)) is maintained at the output ports of converters. For instance, the lower current generation of module M2, due to partial shading, is mitigated at the output port of the converter, where the converter is set in buck-mode and duty cycle (\(D\)) is adjusted to make the output current equivalent to the string current. Other modules may operate at low \(D\) values. This architecture lacks the ability of a selective converter operation, which, in turn, reduces its efficiency, as the operations of all converters increase the power processing loss. However, the voltage stress on switching devices is lower, as the converters do not make the parallel connection with an inverter, meaning this architecture does not need to draw extra current from the inverter's DC-link node. Besides, the control operation is relatively simple since basic topology is used for each converter.  
Figure 1b shows the advanced architecture, in which each module-level converter is designed through an isolated topology [21]. In this architecture, all the modules are connected in series and common \(I_{string}\) is maintained at the PV modules side by providing extra current from the inverter DC-link node. For that, only isolated converters of shaded modules are made active. For instance, the low current at the node of shaded module M2 is lifted through its isolated converter, which takes an extra differential current from the inverter node and supplies it to the M2 node to support the \(I_{string}\). Simultaneously, the converters of modules M1 and M2 are dormant. Because of the selective operation of converters, the efficiency of the architecture is good. However, the complexity of the system is high because of the involvement of magnetic components in each converter. Another issue is the higher stress on switching devices, as all the converters make the parallel connection with the inverter's DC-link node. Moreover, the control complexity of the system is high. Another important aspect is that if all converters are operational,  
Figure 1: Merits and demerits of three power optimizer architectures for PV string interfaced with centralized inverter.  
the overall efficiency of this architecture becomes even lower than the basic architecture because isolated converters operate with higher power processing losses.