Wide Voltage Range Operation of Isolated Modular Multilevel DC-DC Converter

Derived from the classical phase-shift control DC-DC converter, isolated modular multilevel (MML) DC-DC converter has two MMCs connected via a medium frequency AC transformer. Due to the high voltage handling ability, small transformer size, and natural ZVS soft switching this topology is attractive for medium-and high-voltage DC-DC conversion applications. However, in conventional modulation methods of the converter, when the amplitude of the input DC voltage is not matched with that of the output voltage, current stresses of the switches and the transferred reactive power in the transformer become higher, which limits the use of the converter in wide voltage range applications. Aimed to overcome this drawback, a novel modulation algorithm along with the arm voltage control strategy for the converter is proposed in this paper to guarantee high efficiency of the converter over a wide voltage range. Combined with the existing quasi two-level modulation technology this method can produce a trapezoidal voltage waveform with 2N+1 levels at the AC stage to reduce dv/dt stresses and voltage harmonics across the transformer. A converter prototype has been built and tested, and the experimental results verify the effectiveness of the proposed control strategy.