# An Active-Clamp Forward Inverter Featuring Soft Switching and Electrical Isolation
## 5 Experimental Results  
According to the foregoing analysis, a DSP-based prototype of the proposed inverter is built as an example for illustration. Refer to the circuit diagram shown in Figure 7, the electrical specifications are listed in Table 1.  
Considering that the input voltage is 48 V and the peak output voltage \(V_{o,p}\) is 156 V, the transformer turns ratio (\(n\)) is selected as 10. From the transfer function of the forward converter, the duty ratio can be calculated as:  
\[d_{m1}\ =\ \frac{V_{o,p}}{n\times V_{in}}\ =\ 0.325. \tag{32}\]  
The boundary inductance \(L_{B}\) of \(L_{1}\) in boundary conduction mode (BCM) can be derived as:  
\[L_{B}\ =\ \frac{V_{o,p}\times(1-d_{m1})}{\text{I}_{o}\times 2f_{s}}, \tag{33}\]  
The inductor current is selected to operate in BCM under 20% load current and the peak of the output voltage. Therefore, the boundary inductance \(L_{B}\) can be calculated from Equation (33) to be \begin{table}
\begin{tabular}{c c} \hline \hline
**Electrical Specifications** \\ \hline Typical input voltage, \(V_{in}\) & 48 V \\ Output voltage, \(v_{o}\) & 110 Vrms \\ Line frequency, \(f_{ac}\) & 60 Hz \\ Switching frequency, \(f_{s}\) & 20 kHz \\ Output power, \(P_{o}\) & 400 W \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Electrical specifications of the proposed inverter. Figure 8: The block diagram of the control circuit of the PV grid-connection power system. Figure 7: The circuit diagram of a grid-connection power system using the proposed active-clamp forward inverter. 2.57 mH. This boundary value is the minimum of output inductor \(L_{1}\), so the inductance of 3 mH is selected in the implementation example. In this design, the magnetizing inductor \(L_{m1}\) of the transformer is 835 mH. From the operation principles mentioned before, the condition of that main switch \(S_{m1}\) achieves ZVS is:  
\[Z_{2}\geq\frac{V_{in}}{\left|i_{Lr1}(t_{6})\right|}, \tag{34}\]  
where \(Z_{2}~{}=~{}\sqrt{\frac{L_{r1}}{C_{k1}}}\). From Equation (23), the current \(i_{Lr1}\) (\(t_{6}\)) can be obtained as:  
\[i_{Lr1}(t_{6})~{}=~{}\frac{d_{m1}V_{in}}{2L_{m}f_{s}}. \tag{35}\]  
By combining Equations (34) and (35), the minimum value of the leakage inductance \(L_{r1}\) can be calculated as follows:  
\[L_{r1}\geq\frac{4f_{s}^{2}\times C_{k1}\times L_{m1}^{2}}{d_{m1}^{2}}~{}=~{}3.38(\mu H). \tag{36}\]  
Choose 4 mH as the inductance of the leakage inductor \(L_{r1}\). Besides, the active-clamp forward inverter uses the auxiliary switch \(S_{a1}\) and the clamp capacitor \(C_{c1}\) to reset the residual magnetizing flux of the transformer. When the resonance of \(C_{c1}\), \(L_{r1}\) and \(L_{m1}\) occurs, the small value of \(C_{c1}\) will shorten the resonant period and induce voltage spikes. As a result, the voltage stresses of the clamp capacitor \(C_{c1}\), the main switch \(S_{m1}\), and the auxiliary switch \(S_{a1}\) are increased. Therefore, the resonant period should be designed to be greater than ten times the main switch turn-off time, that is:  
\[2\pi\times\sqrt{(L_{r1}+L_{m1})\times C_{c1}}\geq 10\times(1-d_{m1})T_{S}. \tag{37}\]  
From Equation (37), the capacitance of \(C_{c1}\) can be calculated as:  
\[C_{c1}\geq\frac{100\times\left(1-d_{m1}\right)^{2}}{\left(L_{r1}+L_{m1}\right) \times\left(2\pi f_{s}\right)^{2}}~{}=~{}3.44(\mu F). \tag{38}\]  
Choose 4 mF as the capacitance of the clamp capacitor \(C_{c1}\). According to the previous design and calculations, the selected component parameters are summarized in Table 2. The experimental results described below will be used to verify the feasibility of the proposed circuit architecture and the correctness of the theoretical analysis.