High-Power High-Voltage Isolated DC-DC Converters using an NPC Balanced Capacitive Divider

A new isolated DC-DC converter is presented suited for high voltage applications. It is based on the series connection of NPC inverter cells, using semiconductor devices with lower voltage blocking capability than the total input voltage. The galvanic isolation is realized using multiple MF transformers, each designed for a fraction of the global power. The output comprises multiple diode rectifiers connected in series. The circuit uses a capacitive voltage divider at its input, allowing an active balancing of the intermediate levels according the principle of the current diverter. The balancing half bridges allow an N-1 mode of operation in case of a fault or disconnection of one of the series connected cells. The used 3-Level NPC topology for the half bridges allows in addition the continuous variability of the total voltage transformation ratio. The operation principle is verified by simulation, completed by a small scale experimental set-up. Three-and four-stage topologies are used in the study, but the principle can be extended to a high number of modules, making the topology suitable for very high voltage levels, where structures with redundancy (N-1) can be designed.