In three level inverter (TLI) technology, the neutral point clamped (NPC) inverter was the first multilevel topology widely implemented by the industry. The NPC inverter continues to be extensively used in high voltage and high power applications such as high voltage direct current (HVDC) power transmission. The topology is basically based on three phases, wherein each phase includes four serially connected switches and two diodes connected to neutral point or mass. By switching the four switches appropriately, a three level output on each phase is provided, namely the levels +V, 0, and −V of the input voltage are provided on the output of each phase. Through appropriate shift of the switching times in the different phases, the NPC inverter may deliver power of a direct current (DC) voltage source to a power grid (PG). Hence, the NPC inverter may be used to deliver electrical energy produced in a solar cell power plant into the public electric power system, for example. In present NPC inverters, Insulated Gate Bipolar Transistor (IGBT) switches are used, in particular in order to provide for small switching losses. Similarly, it is also known to use Metal Oxide Semiconductor Field-Effect Transistors (MOSFETs) or Junction Gate Field-Effect Transistors (JFETs).
In presently known NPC inverters, charge pump or bootstrap circuits are the circuits of choice for control of the switches. However, such circuits increase in particular the complexity of the NPC inverter.
USPTO Backgrounds
USPTO Backgrounds