In recent years, Modular Multilevel Converter (MMC) is an emerging and highly attractive multilevel converter topology for High Voltage Direct Current (HVDC) transmission and Flexible AC Transmission System (FACTS). The technology on how to balance the floating DC capacitors plays a key role in the new converters. A Fundamental Frequency Sorting Algorithm (FFSA) based on Carrier Phase Shift PWM (CPS-PWM) is proposed to balance the floating DC capacitors. The main idea is to change the corresponding relationship between the CPS-PWM carriers and the submodules according to the capacitor voltage increments during the previous fundamental period. It utilizes no current detection, avoids the excessive frequent sorting and saves calculating resources for the controller so that more submodules can be dealt with. Moreover, the proposed FFSA guarantees that all the switching frequencies of the submodules are strictly identical, which equals to the PWM carrier frequency. It is beneficial for the thermal design of the sub-modules and the lifetime of the power switches. Finally, simulation and experimental results verify the validity of the proposed balancing method.
With the background of power supply for the high precision apparatuses, an isolated high frequency DC-AC converter based on differential output structure is proposed. It is composed by two phase-shifted full bridges (PSFB) which share a common half bridge as the lagging leg on the primary side. Two current-doubler synchronous rectifiers (CDR) constitute differential output structure on the secondary side. The AC output is synthesized by the two PSFB-CDRs generating same DC bias and phase-reversed AC components. Compared to traditional solutions, the new circuit has the potential capability to obtain ultra-low distortion output voltage without using special control or compensations. Moreover, zero voltage switching (ZVS) is realized in the primary circuit within a wide load range, while zero current switching (ZCS) is achieved in the secondary circuit. Operation principles are analyzed and a 1kW prototype is built to validate the proposed scheme.Index Terms-Isolated high frequency DC-AC, differential output structure, ultra-low distortion, soft-switching.
an isolated inverter based on differential output is proposed. The inverter is composed by two phase-shifted full bridges (PSFB) converters. The two PSFBs share a common leg which works as the lagging one on the primary side. Two current-doubler synchronous rectifiers (CDR) are on the secondary side and constitute differential structure. With the same DC bias and phase-reversed AC components in the two PSFBs, an AC output voltage is acquired. Compared to traditional H-bridge inverters which claim high-quality outputs, the new configuration has the potential capability to obtain ultra-low distortion in output voltage without using any special control or compensations, because there is no duty cycle loss in each PSFB. Moreover, switches on the primary side can realize zero voltage switching (ZVS) within a wide load range, while the ones on the secondary side can achieve zero current switching (ZCS). Finally, simulation model and a 1kW prototype are built to verify the effectiveness of the proposed scheme.
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