# A Novel Secondary Side Series LCD Forward Converter with High Efficiency and Magnetic Reset
## 1 Introduction
### Composition and Principle of a Circuit  
The schematic diagram of a novel secondary side series LCD forward converter is shown in Figure 1. Its circuit structure adds an LCD excitation energy transfer path (composed of D\({}_{3}\), D\({}_{4}\), \(C_{2}\), and \(L_{2}\)) to the secondary side of the traditional forward converter. The proposed converter consists of a power switch S, two inductors (forward inductor \(L_{1}\) and auxiliary inductor \(L_{2}\)), two capacitors (an output filter capacitor \(C_{1}\) and an additional capacitor \(C_{2}\)), four diodes (D\({}_{1}\)-D\({}_{4}\)), a transformer T (consists of magnetizing inductor \(L_{\rm m}\) and primary and secondary windings \(W_{1}\) and \(W_{2}\)), and one resistive load \(R_{L}\). The magnetizing current coupled to \(W_{2}\) charges \(C_{2}\) through D\({}_{3}\). Then, the capacitor \(C_{2}\) exchanges energy with the inductor \(L_{2}\) and the excitation energy is stored in the inductor \(L_{2}\). Finally, the inductor \(L_{2}\) transfers energy to the load through D\({}_{3}\), thus realizing the reliable magnetic reset of the transformer. To simplify the analysis of the operating principle of the novel secondary side series LCD forward converter, the following assumptions were made. (1) The switch, diodes, inductors, and capacitors were considered to be ideal. (2) The transformer leakage inductance was minimal, and the energy loss caused by the leakage inductance was not considered. (3) The output voltage was assumed to be constant. The magnetic reset circuit of the proposed converter can effectively transfer excitation energy to the load, improving the conversion efficiency of the converter. The working process was as follows. During the switch-on period, the input voltage \(V_{i}\) was applied across the primary winding of the transformer. The energy was transferred to the load through the secondary winding of the transformer and \(i_{L1}\) increased linearly. At the same time, due to the conduction of D\({}_{2}\), the capacitor \(C_{2}\) exchanged energy with the inductor \(L_{2}\). When the voltage across \(C_{2}\) dropped to zero and the switch S was still on, diode D\({}_{4}\) was naturally turned on. Since diode D\({}_{2}\) remained on, \(C_{2}\) and \(L_{2}\) were short-circuited, and the current flow through \(L_{2}\) remained unchanged. The forward energy continued to be supplied to the load through D\({}_{2}\) and \(L_{1}\), and \(i_{L1}\) continued to increase until S was turned off and the stage ended. At this stage, the zero-voltage natural conduction of diode D\({}_{4}\) was realized. During the switch-off period, D\({}_{3}\) was forward biased and D\({}_{4}\) was reverse biased. The excitation energy was released to the capacitor \(C_{2}\) via D\({}_{3}\), the voltage across \(C_{2}\) gradually increased from the minimum value, and the excitation energy was delivered to the capacitor \(C_{2}\), thus ensuring the reliable magnetic reset of the transformer core. Meanwhile, the inductor \(L_{1}\) transferred energy to the load through D\({}_{1}\), and \(i_{L1}\) gradually decreased. Since diodes D\({}_{1}\) and D\({}_{3}\) were in on state, \(i_{L2}\) remained unchanged until \(i_{L1}\) decreased to be equal to \(i_{L2}\) and diode D\({}_{1}\) was naturally turned off, thus realizing the zero-current turn-off of  
Figure 1: Circuit diagram of the proposed converter. \(\rm D_{1}\). After that, if \(\rm D_{3}\) still remained conductive, \(i_{L1}\) and \(i_{L2}\) continued to flow through \(\rm D_{3}\) until the switch was turned on, \(i_{L1}\) and \(i_{L2}\) decreased to the minimum value, the voltage across \(C_{2}\) reached its maximum value, and the excitation current decreased to the minimum value. If the excitation current decreased to zero before the switch was turned on, all the excitation energy of the transformer was released to \(C_{2}\), and the voltage across \(C_{2}\) increased to the positive the maximum value.