Research On Two-mode Buck-Push-Pull Topology Applied To Multi-module System

In applications such as high voltage energy harvesting and flexible DC transmission,the input voltage is high and the input voltage range varies widely,and conventional topology is difficult to apply in such applications.A common solution is to use input-series-output-parallel(ISOP)flyback converter.However,the flyback converter is less efficiency and sustain high voltage stress when input voltage varies in a wide range especially in the high input voltage applications.The flyback modules connected in series to reduce the voltage stress in each module is not suitable for wide input voltage range applications.For the case of wide input voltage range,the literature proposes the use of the two-mode Buck-Boost scheme,but since the converter input and output need to be electrically isolated in a multi-module system,it cannot be used in an ISOP system.Therefore,it is necessary to study isolated converters in wide input voltage range applications.This dissertation presents a two-mode Buck-Push-Pull topology and its operating principle and operating mode is analyzed and designed.For applications with a wide range of input voltages,the two-mode Buck-Push-Pull topology automatically selects the appropriate operating mode based on the input voltage to operate efficiency over the entire input voltage range.In addition,this paper models and analyzes the two-mode Buck-Push-Pull topology.After modeled results,the two-mode Buck-Push-Pull topology is a second-order model in the entire input voltage range,which is convenient for controller designing.Compared with the cascaded system,the solution has a natural advantage in terms of control effect.In this paper,in the two-mode Buck-Push-Pull topology,by adding an auxiliary switch network,the primary side switch of the transformer achieves zero voltage switching and the diode in the secondary side of the transformer achieves zero current switching,which reduces voltage and current stress and increase efficiency.Then this paper models and analyzes the system of two-mode Buck-Push-Pull topology used in the ISOP system.In the ISOP system,due to the input voltage sharing problem,this paper compares and analyzes the causes of the unbalanced input voltage and the control scheme of the input voltage sharing.Then the reason for input voltage sharing loop cannot be used in the extreme load conditions is analyzed.The problem was solved by adding an auxiliary switching network.Then,this paper studies the input voltage sharing step response based on small single model and large single model separately.This dissertation uses the digital control chip TMS320F28335 on the experiments.The resolution bits of the AD module and PWM module is analyzed and designed to avoid limit cycle oscillation.Finally,the small-signal model of the two-mode Buck-Push-Pull is designed and discrete to the z domain.The feasibility of the two-mode Buck-Push-Pull circuit proposed in this paper is verified by simulation and experiments.Simulations and experiments verify that the two-mode Buck-Push-Pull topology can achieve zero-voltage switching in the primary switch of the transformer after selecting an appropriate auxiliary switch network,and the secondary side rectifier diode of the transformer achieves zero current switching,effectively reducing power devices.The voltage and current stress,and after choosing suitable controller parameters,the circuit can smoothly switch between the boost mode and the buck mode,widen the input voltage range that the converter can sustain,and make the circuit can be used in the wide input voltage range and high voltage applications.