# An Active-Clamp Forward Inverter Featuring Soft Switching and Electrical Isolation
## 2 Circuit Configuration  
As mentioned previously, the popular PV grid-connection systems need a line-frequency transformer or a boost converter to step-up output voltage. Their system block diagrams are shown in Figures 1 and 2, respectively. In order to improve efficiency and reduce cost, the pseudo-dc-link inverters are proposed, as shown in Figure 3. However, these inverters are non-isolated, which may not avoid the leakage current of the PV panel. Therefore, an active-clamp forward inverter featuring soft switching and electrical isolation is proposed in this paper.  
Figure 1: PV grid-connection systems with a line-frequency transformer.  
The circuit diagram of the proposed inverter is shown in Figure 4. The active-clamp forward converter with zero voltage switching is used to generate the single-polarity rectified sine wave, and the full-bridge unfolding circuit with line-frequency switching switches the output polarity. The power stage of the active-clamp forward converter mainly consists of the main power switch \(S_{m1}\) (including MOSFET \(Q_{m1}\), body diode \(D_{m1}\), and parasitic capacitor \(C_{d81}\)), the real transformer \(T_{x1}\) (including magnetizing inductor \(L_{m1}\) and ideal transformer with turn ratio 1:\(n\)), the forward diode \(D_{1}\), the freewheel diode \(D_{2}\), the inductor \(L_{1}\), and the output capacitor \(C_{\rm f}\). The auxiliary switch \(S_{a1}\) (including MOSFET \(Q_{a1}\) and body diode \(D_{a1}\)) and the clamp capacitor \(C_{c1}\) are used for resetting the residual magnetic flux of the transformer. The energy of the magnetizing inductor \(L_{m1}\) and the clamp capacitor \(C_{c1}\) are alternately transferred. Except for effectively recovering the energy of residual magnetic to the input power source, both the switches \(S_{m1}\) and \(S_{a1}\) can achieve ZVS characteristics to reduce switching losses. In addition, the full-bridge unfolding circuit consists of the power switches \(S_{1}\), \(S_{2}\), \(S_{3}\) and \(S_{4}\), and the output filter is formed of the inductor \(L_{\rm o}\) and the capacitor \(C_{\rm o}\). The power switches \(S_{1}\), \(S_{2}\), \(S_{3}\) and \(S_{4}\) are only switching with line frequency, so that there is almost no switching loss.  
The traditional voltage source inverter is the buck type. In order to obtain a DC input voltage higher than the peak voltage of utility-line, an additional boost converter is necessarily adopted, resulting in low system conversion efficiency. Since the active-clamp forward inverter has both step-up and step-down functions, the boost converter is no longer required. The proposed inverter is suitable for the application with low input voltage and wide voltage variation. In addition, because partial  
Figure 4: The circuit configuration of the proposed active-clamp forward inverter.  
Figure 3: PV grid-connection systems with the pseudo-dc-link structure.  
Figure 2: PV grid-connection systems with the two-stage cascade structure.  
energy can be directly transmitted to the output during the main switch \(S_{m1}\) turning-on, the proposed inverter can effectively improve the conversion efficiency. Since only two switches \(S_{m1}\) and \(S_{a1}\) are switching with high frequency, the switching loss can be greatly reduced to improve system efficiency.