# A Novel Secondary Side Series LCD Forward Converter with High Efficiency and Magnetic Reset
## 1 Introduction  
Power converters are widely used in various instruments and electrical and electronic equipment. Currently, converters have a trend towards higher efficiency, higher power density, and higher reliability [1; 2; 3; 4; 5]. Depending on the circuit structure, the power converter can be divided into a non-isolation or an isolation structure. In common power supply systems, an isolation converter is usually selected. An important part of an isolated converter, the magnetic element can store energy and realize the input-output isolation, which is an essential factor affecting the electrical performance of the converter [6; 7]. Common isolated converter topologies mainly include a forward converter and a flyback converter. However, the output power of the flyback converter is greatly limited and its efficiency is not high, so it is mostly used in low-power applications [8; 9]. Compared to a flyback converter, the power of a forward converter is not limited by the energy storage capacity of the transformer. It exhibits a high working reliability and a relatively simple structure, and has been widely used in small and medium power fields. Although forward converters have many advantages, there are still many problems that still need to be solved. The forward converter has no magnetic reset function, and hence, it is likely to cause magnetic core saturation and other problems [10]. Magnetic saturation causes the current flowing through the switch to increase sharply, which greatly limits the promotion of the forward converter. Therefore, it is necessary to consider adding specific magnetic reset circuits to avoid magnetic saturation. In order to further promote the application of the forward converter, it is necessary to solve the problem of magnetic resetand improve the energy utilization rate. Therefore, it is essential to study new magnetic reset methods. To solve the problem of magnetic reset, an auxiliary winding reset mode was studied in [11], which returned energy to the input terminal. However, its additional auxiliary windings complicated the structure and design of the transformer and had a small duty cycle range. The switch was required to withstand higher voltages, making it difficult to achieve a large power output. In [12; 13; 14; 15; 16; 17], a primary-side RCD magnetic reset mode was studied. The reset mode comprised a simple structure and the range of the duty ratio was greater than 0.5. However, the voltage stress of the switch was large and most of the energy was consumed on the clamping resistor, reducing the efficiency and increasing the difficulty of the heat dissipation design. In [18; 19], an active clamp reset circuit was proposed. The advantage of this reset circuit was that it achieved the soft switching of the main and auxiliary switches, thus reducing circuit losses. However, its driving circuit was complex, and when the duty cycle was greater than 0.5, the voltage stress of the switch exceeded twice the input voltage. The reset circuits of the forward converter were located at the primary side of the transformer, and the excitation energy stored in the transformer during the conduction of the switch was either fed back to the input terminal or consumed. This part of the excitation energy was not fully utilized, which did not improve the efficiency of the converter. Based on the above mentioned research, it was necessary to continuously improve the reset mode. The new reset method considers placing the reset circuit on the secondary side of the transformer to improve the energy utilization rate and achieve the goal of continuously optimizing the performance of the forward converter. In [20; 21], a four-diode forward-flyback converter was proposed, which realized the transfer of the excitation energy to the load and improved the conversion efficiency of the transformer. However, four diodes were used in the circuit, which increased the cost and circuit loss of the converter.