## 4 Three-Port Converters with Partial Power Regulation  
### Two-Port Partial Power Converters  
Traditionally, a full power electronic converter (FPC) is connected to the PV module to meet the load requirements and to implement the MPPT algorithm [76]. The circuit configuration and power flow scheme are shown in Figure 8a and 8b, respectively. In a PV system employing an FPC, all PV power is transferred to the load through a DC-DC converter, reducing the power conversion efficiency. To improve the efficiency of the whole PV system, partial power converters (PPCs) are proposed. Only a fraction of the PV power is processed by the DC-DC converter to the load, which can significantly reduce the power  
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Topologies** & **Diodes** & **Inductors** & **Switches** & **Capacitors** & **Rated Power (W)** & **Efficiencies (\%)** \\ \hline
[18] & 0 & 1 & 2 & 3 & 15 & 95.4-99.68 \\
[70] & 5 & 2 & 3 & 4 & 100 & 91–94 \\
[71] & 2 & 2 & 2 & 2 & 100 & 85–94 \\
[72] & 5 & 1 & 4 & 3 & 50 & 87.8-93 \\
[73] & 1 & 2 & 4 & 4 & 1000 & 96–98 \\
[74] & 5 & 2 & 3 & 4 & 200 & 86–99 \\
[75] & 2 & 1 & 5 & 3 & 300 & 94.2–96.8 \\ \hline \hline \end{tabular}
\end{table}
Table 4: Comparison of non-isolated TPCs.  
losses and the volume [77; 78]. Figure 9a and 9b show the circuit configuration and power flow scheme of PPCs, respectively. As can be seen in these figures, an additional direct power flow path from the PV to the load (red lines) is used. Since the losses on the direct power path are negligible, the efficiency is improved, and the voltage and current pressures on the key components, such as inductors, switches, and diodes, are reduced [79].  
The basic idea of PPCs is that a connection between the PV panel and the load, as shown in Figure 10a (red arrow), is added to a FPC to reduce the power rating. As can be seen in Figure 10a, part of the PV power is transferred to the load through this path (red arrow) with almost 100% efficiency. Another part is processed through a full power converter, with efficiency \(\eta_{c}\), to the capacitor C (blue arrow) or returned to the capacitor \(C_{pv}\) connected to the PV panel (green arrow), depending on the circuit operation modes. When \(v_{c}\) is positive, the circuit is operated in the step-up mode, whereas the step-down mode is entered if \(v_{c}\) is negative. Figure 10b and 10c show two examples of step-up operation with buck-boost and flyback converters, respectively. Figure 10d and 10e illustrate two examples of step-down operation with boost and flyback converters, respectively. Points (a), (b), (c), and (d) in Figure 10b-e relate to the points in Figure 10a.  
In fact, PPCs are special connections of full power converters [80; 81; 82]. A variety of PPCs based on FPCs, such as CUK converters [83], SEPIC converters [84], and synchronous-rectification LLC resonant converters [85], have been proposed. Figure 11 shows the difference between full power boost converters and boost type PPCs as an example. Compared to full power boost converters (blue dashed line) with a power rating of 1, partial power boost converters have a reduced power rating range from 0 to 1, due to the special connection between the PV panel and the load (red dashed line) with almost 100% efficiency. The power rating in the partial power boost converter depends on the voltage ratio between \(v_{pv}\) and \(v_{L}\)[86]. Table 5 presents a comparison of PPCs.  
Figure 8: Full power converter. (**a**) Circuit configuration. (**b**) Power flow scheme.  
Figure 9: Partial power converter. (**a**) Circuit configuration. (**b**) Power flow scheme (red arrows indicate direct power flow).