# An Active-Clamp Forward Inverter Featuring Soft Switching and Electrical Isolation
## 1 Introduction  
Today, because photovoltaic (PV) energy is noiseless, pollution-free, non-radioactive and inexhaustible, PV grid-connection power systems are receiving increasing attention [1; 2; 3]. For the PV grid-connected systems, inverters are necessary to convert the DC voltage of the PV panel to an AC voltage, and then supply power in parallel with utility-line. Therefore, many experts have invested in the research of the circuit architecture and control strategy of inverters [4; 5; 6; 7]. The full-bridge voltage-source inverter is the popular topology in low-power PV grid-connected systems. However, due to its buck-typed conversion, a line-frequency transformer has to be inserted between the inverter and utility-line. This system structure greatly increases the volume, weight and cost of the system, and reduces the power density. Inserting a boost converter to step-up the output voltage of the PV panel is the alternative solution. The boost converter can not only regulate PV output voltage but also achieve maximum power point tracking (MPPT) [8; 9]. The main advantage of this two-stage cascade structure is the independent control of the power converters. It is easy to optimize the function of each stage, and the input voltage of the inverter is regulated. However, the disadvantages are the increase in the complexity of the system circuit, the increase in cost, and the reduction in reliability. The power loss due to multiple energy processing will reduce the overall system efficiency. The pseudo-dc-link inverters areproposed to improve these shortcomings [10; 11]. In these inverters, a DC converter is used to generate a single-polarity rectified sine wave, and a line-frequency bridge unfolding circuit is used to switch the polarity of the output voltage. Since there is only single energy processing, power loss can be reduced to improve efficiency. Among these high-efficiency inverters with pseudo-dc-link, the buck-boost type converters are widely used to implement non-isolated inverters for the photovoltaic grid-connected power system with wide-range input voltage fluctuation [12; 13; 14; 15]. Furthermore, in order to obtain higher boosting capacity and meet the safety requirement of electrical isolation, isolated DC-DC converters, such as flyback converter, are often used to design isolated pseudo-dc-link inverters [16; 17; 18; 19]. The flyback converter has the advantages of simple structure and easy control. However, because the transformer of the flyback converter needs to function as an inductor for energy storage, all energy needs to be temporarily stored in the transformer and then transferred to the output load, so its output power and efficiency are limited. In order to improve the output power and conversion efficiency of the inverter with electrical isolation, the forward inverter is proposed [20], where the forward converter is adopted to replace the flyback converter and to generate a rectified sine wave. However, since the forward converter needs a third winding of the transformer to reset its residual magnetic flux, the maximum duty ratio of the power switch is limited, thereby reducing the boosting capacity of the converter. The dual-switch forward inverter [21] adds additional power switches, diodes, and a secondary winding to reset the residual magnetic flux of the transformer. The transformer can be operated in both of the first and third quadrants, but its volume and total circuit cost are greatly increased. Therefore, an active-clamp forward inverter is proposed in this paper. By replacing the traditional reset winding with an auxiliary switch and a clamp capacitor, the residual magnetic flux can be reset through the resonance of the clamp capacitor and the magnetizing inductor. In addition to recovering the energy of the magnetizing inductor back to the input, the power switch can also be turned on with zero voltage switching (ZVS) to reduce switching losses and effectively increase the efficiency of the inverter.