Direction-Selective Parallel Module Structure for Cascaded Bridge and Modular Multilevel Converters with Minimum Transistor Count

Modular multilevel converters have developed into a door opener for a variety of energy applications. Enabling parallel connectivity between modules can offer several advantages to systems, including sensorless balancing, module-to-module energy exchange, reduced impedance, low-frequency operation down to dc output, and a larger effective capacitance. Existing module circuits, however, require many individual switches or offer only very limited parallel functionalities. We propose a new module topology, which we name the direction-selective parallel structure, which requires only a minimum of transistors. A combination of only four active switches and four diodes enables bidirectional equilibration, bipolar module output, and inter-module switched-capacitor features. The topology is highly attractive for existing converters working with cascaded H-bridge elements as the addition of only four diodes enables the key features of more complicated module topologies, particularly their superior balancing, such as sensorless estimator-free balancing, and module-to-module energy exchange. The diodes furthermore well match the insulated gate bipolar transistors implemented in most larger systems. We elaborate on the working principle of the direction-selective parallel module and demonstrate its operation.