# A Novel Secondary Side Series LCD Forward Converter with High Efficiency and Magnetic Reset
## 1 Introduction
### Analysis of the Working Mode and Energy Transfer Process of the Converter  
Depending on whether the excitation current decreases to zero during the switch-off period, the excitation inductance can be divided into a continuous conduction mode (CCM) and a discontinuous conduction mode (DCM). At the same time, according to the working principle of the converter, when \(i_{L2}\) and \(i_{L1}\) are equal, they will jointly supply energy to the load. Therefore, through the analysis of the working principle of the converter, the inductors \(L_{1}\) and \(L_{2}\) can only work in CCM or DCM at the same time. To realize a high power transmission, improve the energy transmission efficiency, and reduce the switching loss, it is necessary to choose the best working mode of the converter. When \(L_{\rm m}\) works in the CCM, the secondary winding transfers energy to the capacitor \(C_{2}\) through \(\rm D_{3}\) during the switch-off period, and the voltage across \(C_{2}\) reaches the maximum value when the switch is turned on. Therefore, the voltage stress of the switch in this mode is much higher than the input voltage. When \(L_{\rm m}\) operates in the DCM, the secondary winding \(W_{2}\) delivers energy to the capacitor \(C_{2}\) via diode \(\rm D_{3}\) until the excitation current decreases to zero. The voltage across the capacitor \(C_{2}\) reaches the maximum value. After that, the capacitor \(C_{2}\) transfers energy to the load through \(L_{1}\), \(L_{2}\), and \(W_{2}\) until the voltage across \(C_{2}\) decreases lower than the output voltage. At this time, the voltage at both ends of the secondary winding \(W_{2}\) is changed to a positive direction voltage. Therefore, at the moment when the switch is turned on, the drain source of the switch is lower than the input voltage \(V_{i}\), and the proposed converter can achieve a low-voltage turn-on of the switch. Therefore, in order to reduce the switching loss, \(L_{\rm m}\) was selected to work in the DCM. At the same time, in order to achieve a large energy transmission with the proposed converter, it was necessary to ensure that one of the inductors, \(L_{2}\) or \(L_{1}\), operates in the CCM. Since \(L_{1}\) and \(L_{2}\) can only work in the CCM or DCM simultaneously, \(L_{1}\) and \(L_{2}\) were selected to work in the CCM. Therefore, it was concluded that the optimal working mode of the proposed converter was \(L_{\rm m}\)-DCM, \(L_{1}\)-CCM, and \(L_{2}\)-CCM. The waveform of the proposed converter operating in the optimal operating mode is shown in Figure 2. As shown in Figure 2, the working process of the converter can be divided into six stages in one cycle. The energy transmission characteristics of each stage were specifically analyzed as follows. Phase I [\(t_{0}\)-\(t_{1}\)]: At \(t_{0}\), the switch S was turned on, and the positive voltage across the secondary side, coupled from \(V_{1}\) through the transformer, was \(V_{1}/n\). At this time, \(\rm D_{2}\) was forward biased, while \(\rm D_{1}\), \(\rm D_{3}\), and \(\rm D_{4}\) were reverse biased. \(L_{1}\) was magnetized from the voltage of the secondary winding \(W_{2}\) of the transformer and \(i_{L1}\) increased linearly from the minimum value with a slope of \((V_{j}/n-V_{o})/L_{1}\). Due to the conduction of \(\rm D_{2}\), \(C_{2}\) and \(L_{2}\) resonated through \(\rm D_{2}\), and the capacitor \(C_{2}\) transferred energy to the inductor \(L_{2}\) until the voltage across \(C_{2}\) decreased to zero. Phase II [\(t_{1}\)-\(t_{2}\)]: At \(t_{2}\), after the voltage across \(C_{2}\) decreased to zero, diode \(\rm D_{4}\) was naturally turned on as diode \(\rm D_{2}\) continued to be turned on and \(i_{L2}\) remained unchanged. At the same time, the forward energy continued to be provided to the load and the inductor \(L_{1}\) through D\({}_{2}\), and \(i_{L1}\) continued to rise linearly. This mode was completed at \(t_{2}\) when \(i_{L1}\) reached the maximum value.