Phase-Shifted Square Wave Modulation Method for Isolated Modular Multilevel DC/DC Converter

This invention provides a phase-shifted square wave modulation method for isolated modular multi-level DC/DC (IM2DC) converters. In the present invention, one square wave based modulation waveform with the same frequency and magnitude is applied to each cell of an isolated modular multi-level DC/DC converter and compared with a triangular carrier waveform to generate the gate signals. With the phase-shifted angle of the carrier waves, higher equivalent switching frequency can be achieved. Both full-bridge (FB) and half-bridge (HB) cells are allowed as the single cell.

This technology can be implemented to reduce the DC inductor size due to higher equivalent switching frequency. In addition, the required capacitor energy can be reduced, which decreases the capacitor size since they are dedicated to smooth the high switching frequency ripples only. Moreover, a high efficient power transfer capability can be achieved with the square wave compared to conventional sinusoidal waveforms.

In addition, this invention proposes a novel phase-shifted square wave modulation technique aiming at reducing passive components and devices sizes for single-phase and three-phase IM2DC applications in HVDC/MVDC systems. In various embodiments a square wave based modulation waveform is applied to each cell of IM2DC and compared to the phase-shifted carrier waveforms to generate device gate signals. Thus, higher equivalent switching frequency will be achieved and square wave based arm and AC link waveforms will be generated. The power flow of IM2DC is controlled by a phase shift angle of the square modulation waveforms between HVS and LVS. Compared to the conventional phase-shift sinusoidal method, the converter cell capacitors can be reduced significantly since they are required to smooth out the high switching frequency ripple components only. In addition, lower TDR can be achieved due to the higher power transferring capability of square waves. Both proposed method and quasi-two-level modulation can achieve low TDR and small cell capacitor size, however, the present invention can allow smaller DC inductors due to the multi-cell phase-shifted characteristics.