# An Active-Clamp Forward Inverter Featuring Soft Switching and Electrical Isolation
## 5 Experimental Results
In this case, the inverter output current \(i_{o}\) is controlled to follow the frequency and the phase of the utility-line voltage \(v_{s}\). In this way, the power of the PV panel can be delivered to the load and the utility-line by the proposed active-clamp forward inverter. It can be clearly seen from Figure 13 that the output current is close to an ideal sine wave and in phase with the utility-line voltage. At this condition, the inverter outputs about 200 W of power, and utility-line provides approximately 200 W of power to meet the total power requirement of the load. It can prove the feasibility of grid-connection of the proposed inverter. In addition, the power factor, the THD and odd-order harmonics of the output current are measured and recorded in Table 4. Either the THD or each harmonic can meet the specification requirements, and the power factor reaches 0.994. This further confirms the feasibility of the circuit structure and control strategy mentioned in this paper. \begin{table}
\begin{tabular}{c c} \hline \hline \multicolumn{2}{c}{**Power Factor 0.994**} \\ \hline T.H.D. & 2.23\% \\
3 rd Harmonic & 1.92\% \\
5 th Harmonic & 0.83\% \\
7 th Harmonic & 0.65\% \\
9 th Harmonic & 0.34\% \\
11 th Harmonic & 0.21\% \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Measured power factor, T.H.D. and odd-order harmonics of the output current during grid-connection mode. Figure 12: Measured efficiency curve of the active-clamp forward inverter. Figure 13: Measured waveforms of the load current \(i_{L}\), the inverter output current \(i_{o}\), utility-line current \(i_{s}\) and utility-line voltage vs. during grid-connection operation. Many pseudo-dc-link inverters based on forward converters are proposed to improve conversion efficiency and to achieve electrical isolation [20; 21]. In order to compare the difference between them and the proposed forward inverter in this paper, a brief discussion is presented as follows according to (1) the reset circuit for residual magnetic flux, (2) the conversion efficiency, (3) the characteristic of switch, and (4) the circuit complexity. 1. Reset circuit for residual magnetic flux The forward inverter in Reference [20] needs a third winding in series with a diode to reset the residual magnetic flux. Therefore, the maximum duty ratio is limited, resulting in a reduction of boosting capacity. The dual-switch forward inverter in Reference [21] adds an additional power switch and a diode to reset the residual magnetic flux. Since the input current goes through these devices, the volume and cost are greatly increased. In the proposed active-clamp forward inverter, an auxiliary switch and a clamp capacitor are used to reset the residual magnetic flux through the resonance of the clamp capacitor and the magnetizing inductor, which can avoid the problems mentioned above. 2. Conversion efficiency According to the efficiency curve shown in Figure 12, the efficiency of the active-clamp forward inverter is up to 90.2% at 48 V input and 400 W output power. The measured efficiency of the forward inverter in Reference [20] is 89.5% at 45 V input and 110 W. Besides, the measured efficiency of the dual-switch forward inverter in Reference [21] is only 82% at full load. Therefore, it can be concluded that the proposed inverter has better conversion efficiency than other forward inverters. 3. Characteristic of switch In the proposed active-clamp forward inverter, both of the main switch and the auxiliary switch have the characteristic of ZVS turn-on. In addition to reducing switching losses, switching noise can also be reduced to avoid electromagnetic interference (EMI) problems. For the forward inverters proposed in References [20; 21], the power switches are hard switching, which may increase switching loss and cause serious EMI issues. 4. Circuit complexity In order to reduce the core size of the transformer, multiple converters with interleaved parallel connections are used in the forward inverter in Reference [20].