Capacitor charge balancing technique for a three-level PWM power converter

A method for operating a power converter which includes an inverter that converts a DC voltage across first and second supply buses into an alternating voltage waveform at an output terminal to which a load can be connected, wherein the inverter is connected to a neutral node formed between first and second capacitors connected in series between the first and second supply buses, said method comprising:

• operating the inverter to selectively couple the first supply bus, the second supply bus and the neutral node to the output terminal to produce the alternating voltage waveform formed by a sequence of output pulses having positive, zero and negative voltage levels;
• deriving an imbalance compensation coefficient from a difference in voltages between the neutral node and each supply bus and from a direction of current between the neutral node and the inverter;
• and
• adjusting a width of the output pulses during consecutive half-cycles of a modulating waveform in response to the imbalance compensation coefficient.

A method for operating a power converter which includes an inverter that converts a DC voltage across first and second supply buses into an alternating voltage waveform at an output terminal for application to a load, wherein the inverter is connected to a neutral node formed between first and second capacitors connected in series between the first and second supply buses, said method comprising:

• generating a carrier waveform;
• generating a modulating waveform which oscillates;
• deriving an imbalance compensation coefficient from a difference in voltages across the first and second capacitors and a direction of current between the neutral node and the inverter;
• adjusting the modulating waveform in response to the imbalance compensation coefficient to form an adjusted waveform;
• comparing the adjusted waveform to the carrier waveform and in response thereto producing a set of control signals;
• and
• operating the inverter, using the set of control signals, to selectively couple the first and second supply buses and the neutral node to the output terminal thereby producing the alternating voltage waveform which is formed by a sequence of output pulses having positive, zero and negative voltage levels.