A Novel Modulation Technique for Pulsating DC Link Multistage Converter with Zero Voltage Transition Based on Different and Unrelated Switching Frequencies

A novel modulation technique is presented for an Isolated DC/AC multistage Converter, based on Pulsating DC Link principle. In this converter, the absence of a filter at the output of the first stage provides high reliability features, fast dynamic response and Zero Voltage Transitions (ZVT) of the secondary stage switches thanks to the zero voltage time intervals of the pulsating DC link. The proposed modulation techniques allows the use of two different switching frequencies for the two converter stages in such a way that each stage can be modulated with the best switching frequency to improve the efficiency, dynamic performances and converter size. Moreover, the modulation technique permits the optimization of the ratio between the switching frequencies of the two power stages which is fundamental to avoid that the pulsating DC link can compromise the Total Harmonic Distortion THD of the output voltage. An exhaustive description of the presented modulation logic is provided, highlighting the benefits due to the increase of the switching frequency of the first stage. The synchronization between the power stages is also used to guarantee the ZV commutations of the second stage, during the freewheeling phases of the first stage with consequent reduction of losses. Furthermore the effects of the predistortion of modulating signals on the distortion of output voltages are discussed and the simulations results are presented. The experimental results, obtained by the characterization of 20kW prototype, are presented and discussed to validate the features of the proposed modulation logic.