Research On Topology And Control For Isolated High Step-up Forward-Flyback Converterts

High step-up DC/DC converter is the core part of high voltage switching power supply, which has been widely used in many fields such as aviation and aerospace. But the behavior of high step-up DC/DC converter is significiantly different from the generic ones, as the large turns ratio and the processing structure of high voltage transformer would aggravate its nonideality. And the nonideality would dramatically affect the operation of the converter. To meet the ever increasing application requirements for high step-up special switching power supply, through research on the operating principle and control strategy, voltage-multiplier forward-flyback converters, active-clamped forward-flyback converters and resonant forward-flyback converters have been conducted based on pulse width modulation technology. Research problems and results are listed as follows.Firstly, the characters of flyback converters which are operated in continous conduction mode(CCM) and in discontinuous mode(DCM) are analyzed and compared, and it comes to the conclusion that there is no direct correlation between the output voltage gain of flybacks in DCM and the transformer turn ratio. In order to reduce the switch stress and the transformer insulation strength, a novel hard-switching voltage-multiplier forward-flyback converter(VMFFC) is proposed in this paper based on conventional voltage-multiplier rectifier. A method of virtual capacitor is adopted to analyze this converter behavior and the critical condition at the boundary between DCM and CCM and its output voltage gains have been deduced. The results reveal that: the output voltage gain of VMFFCs in DCM largely depends on its characteristic parameter. The gain will be very large when the value of its characteristic parameter is small. This reduces the requirements of the transformer turn ratio and decreases its turns and size. Meanwhile VMFFCs is preferable for working in much wide input voltage range since they have both forward and flyback process. The dynamic model of VMFFCs has been constructed by the moving average method and the compensator design has been implemented. A laboratory prototype has been built based on the analysis. The validity of analysis for this circuit is verified by experimental results.Secondly, the influence of the leakage inductance on the performance of the converters is discussed from the physical model of transformers. An active-clamped circuit has been adapted into voltage-multiplier forward-flyback converters to suppress the surge voltage, and therefore it forms an active-clamped orward-flyback converter which has different merits. Based on the analysis of the principle, the output-input voltage gain in steady state has been derived. It reaches the stress for the primary switches and the secondary diodes. A compound circuit combining the merits of the voltage multiplier and flyback converter has been presented in order to further reduce voltage stress. Its operation and prominent characteristics are described, and simulation circuit has been constructed to verify the results.A resonant forward-flyback converter(RFFC) has been presented in this paper to alleviate the long reverse recovery time of high-voltage diodes. This converter achieves not only ZVS for all the switches but also ZCS for all the diodes. The problem about the contradiction between the high-frequecy switching and the long reverse recovery time of high-voltage diodes has been solved. And the performance of the converter is greatly improved. By optimal design of parameters for this converter, the leakage inductance of the transformer would resonate with the capcitor in secondary side. Based on detailed analysis for its operation principle, the output voltage gain, the stress of resonant capacitor and reset capacitor have been deduced. A experimental circuit has been built and the results have been validated. And then, a high-order VMs is integrated into RFFCs to reduce voltage stress of the devices, and its operating priciple and characteristic are analyzed.Finally, a large-signal feedforward control method has been proposed to upgrade the dynamic response of output voltage when input voltage varies. And the input voltage feedforward control circuit has been conducted. The main design process is comprised of calculating the parameters of elements, modeling this converter and designing the control circuit. The average model of this converter is derived by the average equivalent circuit method. A two-type compensator is designed to maintain the crossover frequency and the phase margin. Furthermore, a laboratory prototype has been constructed and the experiment results have verified the validity of the conclusions.