Research On Soft-switching Push-pull Forward Converter And Balance Strategy Of Converters In Parallel

For push-pull forward converter, since there are many advantages such as reducing the voltage spikes of power transistors, improving the efficiency of magnetic core and optimizing the volume of input filter, it has been widely applied in low-voltage high-current situation. However, the turn-off voltage spikes of transistors could not be eliminated in hard switching mode for conventional PPFC, which results in severe electromagnetic interference(EMI). The switching losses are increased and high-frequency DC-DC converter can be limited. In high power applications, multi-module parallel converters can effectively reduce the capacity of monomer and improve system reliability. However, due to inconsistent parameters among the modules, the converters can be easily damaged because of the imbalance on the output energy. In order to solve these problems mentioned above, the improvement of push-pull forward convertertopological structure and the optimization of energy balance hybrid control strategy for converters in parallel are studied in this paper.A novel zero-voltage-witching(ZVS) push-pull forward converter topology is proposed to overcome the disadvantages the traditional push-pull forward converter. After analyzing the operation principle of conventional push-pull forward converter and the switching process of the power transistors, the reason for high turn-off voltage spikes of transistors is identified. On the basis of these analyses above, we propose a novel zero-voltage-switching(ZVS) push-pull forward converter topology, and analyze its operational model. Furthermore, the voltage and current in the main branch of switch mode are calculated and the condition of achieving the zero-voltage switching(ZVS) is deduced. Besides, SABER simulation circuit is built to verify the validity of the new topology. The simulation results show that the new topology can achieve ZVS and eliminate the turn-off voltage as well as current spikes of power transistors. It can also increase the boost capacity and reduce the input current ripple by compared with the traditional push-pull forward converter.The novel ZVS push-pull forward converters are working in parallel to achieve high-power DC electric energy output. In order to ensure output energy balance of multi monomers in parallel, we propose a hybrid control strategy including battery state of charge(SOC) balance and current sharing for parallel converters. And the control process is analyzed in detail. After building small-signal modeling of novel ZVS push-pull forward converter, the transfer function of controlled object can be derived and the balance controller is designed based on the condition of system stability. The simulation results demonstrate the feasibility of energy balance control strategy and the validity of the energy balance controller design.For the sake of verifying the merits of the proposed novel ZVS push-pull forward converter and effectiveness of the energy balance hybrid control strategy, the hardware test platform is established, which includes the main circuit, drive and signal detection circuit, etc. Experimental results indicate that the novel ZVS push-pull forward converter can eliminate the turn-off voltage spikes of power transistors, reduce electromagnetic interference,and improve the ability of boosting voltage as well as the conversion efficiency. For multi-module parallel converters, current sharing strategy is introduced to ensure the accuracy of current balance and meet the requirements of the dynamic response speed. Besides, the system output voltage can maintains constant.