Following the continuous development of wide bandgap (WBG) devices and multilevel converter technology, medium voltage active front ends are becoming promising in future high-power-density and high-power applications. However, the cost issue is one of the major drawbacks, which stops the WBG devices from being widely applied in high-power areas. This article proposes a silicon carbide (SiC) and silicon (Si) hybrid five-level unidirectional rectifier. It requires only four SiC MOSFETs with relatively low blocking voltage and four Si diodes. Meanwhile, by adding snubber capacitors, all the Si devices are with low-speed switching, and the voltage stresses of fast SiC MOSFETs are minimized. In this article, operational analysis and carrier-based phase-disposition pulsewidth modulation scheme for this circuit are discussed in detail. The capacitor voltage balancing and unity power factor are both realized. Simulation and scaled-down experimental results are demonstrated to verify the proposed rectifier. Furthermore, the comparison of the hybrid five-level rectifiers is given to show the advantages of the proposed rectifier in terms of voltage stress, efficiency, and cost.

In this article, the output voltage control strategy of modular multilevel resonant dc converters (MMRDC) for medium-voltage wide-input-range applications is studied. First, the feasibility and influence of the switching frequency regulation and modulation index regulation are investigated in detail. It is found that the switching frequency choice has a significant effect on the voltage stress of arm inductors and the transformer. Then, based on these considerations, the modulation index regulation plus switching frequency regulation based design and control optimization is proposed. With the proposed design and control method, the switching frequency range for adapting to a wide input range is further reduced, especially under light load operating conditions. Besides, the high voltage stress on arm inductors is avoided and the high voltage dv/dt on the transformer is also eliminated. Finally, a laboratory prototype with 8-16 kV input and 375 V/60 kW output has been built and the experimental validation under a wide input range has been done. The efficiency over 96.4% under the wide input range proved the effectiveness of the optimized control methods.

Compared with full-SiC mosfets converters, the hybrid utilization of Si/SiC converters is an alternative approach to achieve the tradeoff between performance and cost. A systematic method is proposed to generate 2-SiC and 4-SiC hybrid three-level active neutral-point-clamped (3L-ANPC) inverter topologies from two types of switching cells based on the arrangement of freewheeling paths. The 2-SiC hybrid 3L-APNC inverter and the 4-SiC hybrid 3L-ANPC inverter are analyzed in detail with switching states and modulation strategies given. The power losses are quantitatively compared between the 2-SiC hybrid 3L-ANPC inverter and the 4-SiC hybrid 3L-ANPC inverter. The junction operating temperatures of active switches in different hybrid 3L-ANPC inverters are also estimated. With the same specifications and switching device parameters, the maximum output power of the 4-SiC hybrid 3L-ANPC inverter with modulation strategy I is almost 1.47 times than that of the 2-SiC hybrid 3L-ANPC inverter. A universal prototype for the full-SiC scheme, the full-IGBT scheme, and different Si/SiC hybrid schemes is built to evaluate these topologies at conversion efficiency and thermal characteristics. Experimental results and analysis show that the full-SiC 3L-ANPC inverter has the highest efficiency, whereas the 2-SiC hybrid 3L-ANPC inverter has the best cost performance. Further, the 4-SiC hybrid 3L-ANPC inverter with modulation strategy I has better thermal balance characteristic than that of the 2-SiC hybrid 3L-ANPC inverter, which shows significant superiority in high power density applications.
© 1986-2012 IEEE.

Medium-voltage high-speed drive is promising in applications, such as the centrifugal compressors and electrified transportation. However, it raises lots of design challenges for the converters, which drive the motors. A SiC and Si hybrid ANPC converter is developed to address these challenges. In this article, the essence of this hybrid structure is investigated to clearly show the derivation process of the hybrid active neutral-point-clamped (ANPC) converter and inspire future research on these hybrid topologies. A dedicated space vector modulation (SVM) scheme is proposed for the hybrid ANPC converter, which enables the three-phase hybrid ANPC converter operating with SiC mosfets in high frequency and Si IGBTs in fundamental frequency. A neutral point (NP) voltage balancing scheme is added to the SVM scheme to balance the NP voltage imbalance under low-frequency modulation index. All these efforts make the hybrid ANPC converter become a high power density and fairly low-cost solution. A 400 Hz fundamental frequency, 3.6 kHz switching frequency, medium-voltage high-speed drive system is assumed and tested by the scaled-down experimental results. The effectiveness of the proposed SVM and NP voltage balancing method have both been verified.
© 1986-2012 IEEE.

Higher-voltage-standard and higher-power-rating aerospace power systems are being investigated intensively in the aerospace industry to address challenges in terms of improving emissions, fuel economy, and also cost. Multilevel converter topologies become attractive because of their higher efficiency under high-voltage and high-switching-frequency conditions. In this paper, an asymmetrical-voltage-level back-to-back multilevel converter is proposed, which consists of a five-level (5L) rectifier stage and a three-level (3L) inverter stage. Based on the comparison, such an asymmetrical back-to-back structure can achieve high efficiency and minimize the converter weight on both rectifier and inverter sides. A compact triple-surface-mounted heatsink structure is designed to realize high density and manufacturable thermal management. This topology and structure are evaluated with a full-rating prototype. According to the evaluation, the achieved power density is 2.61 kVA/kg, which is 30% higher than that of traditional solutions. The efficiency at the rated power of the back-to-back system is 95.8%.