Analysis of Multistage DC/AC converter based on Pulsating DC link principle

Nowadays the different application fields of the Power Electronics and the industries are demanding incessant enhancements of the features of power converter as efficiency, reliability and power density. To answer to this request, the researchers are pushing the limits of these features thanks to the growing use of Wide band-gap (WBG) power devices, new improvements in the field of power magnetics and new topologies of converter. The aim of this thesis is present the results of complete analysis of new topology of insulated DC/AC converter, based on the Pulsating DC link principle. A preliminary study of the topologies used in the field of multistage isolated DC/AC converter is presented, focusing the attention on the solutions based on Fixed DC link and the new architectures based on topology innovations and particular design. In particular in the horizon of the novel and innovative DC/AC insulated topologies, the attention was focalized on the Pulsating DC link Converter (PDLC), reliable architecture featured by pulsating evolution of the DC link thanks to the elimination of the intermediate filter. A novel version of the PDLC topology is presented, highlighting the interesting feature of the Zero Voltage Transition (ZVT) of the VSI stage, thanks to an accurate use of the pulsating DC link. Moreover it has been demonstrated how the action of Active Clamp circuit is strictly necessary, to minimize the overvoltage problem on the DC link voltage and manage the overvoltage associated energy. Successively a complete analysis of the proposed topology during the energising phases is reported, showing the equivalent representation of the PDLC and the relative differential equation system. According to this results a multiparametric analysis of the converter is performed, using accurate LTspice simulations, for different combinations of influence parameters. Thanks to this analysis, the design guidelines for the main components of the converter, as transformer and Active Clamp circuit, have been defined. Regarding the modulation techniques for this topology, a complete overview of the different operating principles is presented as well as the review of the techniques presented in literature. The attention has been focused on the techniques based on the ZVT of the inverter stage, since they permit the reduction of the switching losses of this stage. Specifically two different modulation techniques based on the described principle are presented, showing the different operating principles and the relative implementations. The main difference between the two techniques is the relationship between the switching frequencies of the main power stages, that is a fundamental aspect in PDLC topology and influences the features of the modulation technique. To validate the analytical analysis and simulation results, a 30kW prototype of proposed PDLC topology has been implemented and the main details of the implementation are provided. To conclude the thesis, the prototype was fully characterized on resistive load and the principal results are presented, highlighting the ZVT of the Inverter stage and the results achieved with the two different modulation techniques.