Current controlled active flywheel type neutral point clamped inverter

Claim 2

A power inverter for providing at a load an AC signal having a desired positive voltage during a positive cycle of operation and a desired negative voltage during a negative cycle of operation, comprising: a DC power source having a positive, a negative and a neutral terminal; first switching means connected between said positive terminal and an output terminal for positively biasing said output terminal when said first switching means is selectively enabled to obtain the desired positive voltage in the AC signal; second switching means connected between said negative terminal and said output terminal for negatively biasing said output terminal when said second switching means is selectively enabled to obtain the desired negative voltage in the AC signal; third switching means connected between said neutral terminal and said output terminal for providing a current path from said neutral terminal to said output terminal when enabled; fourth switching means connected between said neutral terminal and said output terminal for providing a current path from said output terminal to said neutral terminal when enabled; current sensing means connected between said output terminal and the load which can sense a current direction therebetween; and controller means connected to said current sensing means and to said third and fourth switching means for selectively enabling and disabling said third and fourth switching means in accordance with said current direction so as to minimize harmonics of said output.

Claim 3

The power inverter as claimed in claim 1, wherein said controller means is further connected to said first and second switching means for selectively enabling and disabling said first and second switching means to obtain a desired output voltage, said controller means further selectively enabling and disabling said third and fourth switching means in accordance with the enablement and disablement of said first and second switching means.

Claim 4

The power inverter as claimed in claim 1, wherein said controller means selectively enables and disables said third switching means when said sensed current direction is from said output terminal to the load if said second switching means is disabled and selectively enables and disables said fourth switching means when said current direction is from the load to said output terminal if said first switching means is disabled.

Claim 5

The power inverter as claimed in claim 3, wherein said controller means further enables said third switching means during the positive cycle of operation and enables said fourth switching means during the negative cycle of operation.

Claim 6

The power inverter as claimed in claim 4, wherein the positive and negative cycles of operation overlap such that the said third and said fourth switching means are both enabled for a time of inverter operation.

Claim 7

The power inverter as claimed in claim 1, wherein said first, second, third and fourth switching means are power transistors.

Claim 8

The power inverter as claimed in claim 6, wherein said first and second transistors are npn transistors, said first transistor having a collector connected to said positive terminal, an emitter connected to said output terminal and a base connected to said controller means, said second transistor having an collector connected to said output terminal, an emitter connected to said negative terminal and a base connected to said controller means.

Claim 9

The power inverter as claimed in claim 7, further comprising: a first diode having an anode connected to said neutral terminal and a cathode connected to a collector of said third transistor, said third transistor having an emitter connected to said output terminal and a base connected to said controller means; and a second diode having an anode connected to said output terminal and a cathode connected to a collector of said fourth transistor, said fourth transistor having an emitter connected to said neutral terminal and a base connected to said controller means.

Claim 10

The power inverter as claimed in claim 8, further comprising: a diode connected between said output terminal and said positive terminal for providing a current path from said output terminal to said positive terminal; and a diode connected between said negative terminal and said output terminal for providing a current path from said negative terminal to said output terminal.

Claim 11

The power inverter as claimed in claim 4, wherein said controller means comprises: a state machine which generates a first and a second control signal for indicating the positive cycle of operation and the negative cycle of operation, respectively, and which generates a third control signal which indicates when said second switching means is disabled during the negative cycle of operation and a fourth control signal which indicates when said first switching means is disabled during the positive cycle of operation; means to AND a signal from said current sensing means which indicates a current flow from said output terminal to the load with said fourth control signal to form a first output; means to OR said first output with said second control signal to form a second output which controls said fourth switching means; means to AND a signal from said current sensing means which indicates a current flow from the load to the output terminal with said third control signal to form a third output; and means to OR said third output with said first control signal to form a fourth output which controls said third switching means.

Claim 12

The power inverter as claimed in claim 10, wherein said third and fourth control signals overlap.

Claim 13

The power inverter as claimed in claim 10, wherein said state machine generates a fifth and a sixth control signal for selectively enabling and disabling said first and said second switching means, respectively.

Claim 14

A power inverter for providing at a load an AC signal having a desired positive voltage during a positive cycle of operation and a desired negative voltage during a negative cycle of operation, comprising: a DC power source having a positive, a negative and a neutral terminal; first switching means connected between said positive terminal and an output terminal for positively biasing said output terminal when said first switching means is selectively enabled to obtain the desired positive voltage in the AC signal; second switching means connected between said negative terminal and said output terminal for negatively biasing said output terminal when said second switching means is selectively enabled to obtain the desired negative voltage in the AC signal; third switching means connected between said neutral terminal and said output terminal for providing a current path from said neutral terminal to said output terminal when enabled; fourth switching means connected between said neutral terminal and said output terminal for providing a current path from said output terminal to said neutral terminal when enabled; current sensing means connected between said output terminal and the load for sensing current direction therebetween; and controller means connected to said current sensing means and to said third and fourth switching means for selectively enabling and disabling said third switching means when said current means detects a current flow from said output terminal to the load if said second switching means is disabled and for selectively enabling said fourth switching means when said current sensing means detects a current flow from the load to said output terminal if said first switching means is disabled.

Claim 15

A power inverter for providing at a load an AC signal having a desired positive voltage during a positive cycle of operation and a desired negative voltage during a negative cycle of operation, comprising: a DC power source having a positive, a negative and a neutral terminal; first switching means connected between said positive terminal and an output terminal for positively biasing said output terminal when said first switching means is selectively enabled to obtain the desired positive voltage in the AC signal; second switching means connected between said negative terminal and said output terminal for negatively biasing said output terminal when said second switching means is selectively enabled to obtain the desired negative voltage in the AC signal; third switching means connected between said neutral terminal and said output terminal for providing a current path from said neutral terminal to said output terminal when enabled; fourth switching means connected between said neutral terminal and said output terminal for providing a current path from said output terminal to said neutral terminal when enabled; current sensing means connected between said output terminal and the load for sensing current direction therebetween; and controller means connected to said current sensing means and to said third and fourth switching means for selectively enabling and disabling said third switching means when said current means detects a current flow from said output terminal to the load if said second switching means is disabled and for selectively enabling said fourth switching means when said current sensing means detects a current flow from the load to said output terminal if said first switching means is disabled.

Claim 16

The power inverter as claimed in claim 13, wherein said controller means further enables said third switching means during the positive cycle of operation and enables said fourth switching means during the negative cycle of operation.

Claim 17

The power inverter as claimed in claim 14, wherein the positive and negative cycles of operation overlap such that the said third and said fourth switching means are both enabled for a time of inverter operation.

Claim 18

The power inverter as claimed in claim 13, wherein said first, second, third and fourth switching means are power transistors.

Claim 19

The power inverter as claimed in claim 16, wherein said first and second transistors are npn transistors, said first transistor having a collector connected to said positive terminal, an emitter connected to said output terminal and a base connected to said controller means, said second transistor having an collector connected to said output terminal, an emitter connected to said negative terminal and a base connected to said controller means.

Claim 20

The power inverter as claimed in claim 17, further comprising: a first diode having an anode connected to said neutral terminal and a cathode connected to a collector of said third transistor, said third transistor having an emitter connected to said output terminal and a base connected to said controller means; and a second diode having an anode connected to said output terminal and a cathode connected to a collector of said fourth transistor, said fourth transistor having an emitter connected to said neutral terminal and a base connected to said controller means.

Claim 21

The power inverter as claimed in claim 18, further comprising: a diode connected between said output terminal and said positive terminal for providing a current path from said output terminal to said positive terminal; and a diode connected between said negative terminal and said output terminal for providing a current path from said negative terminal to said output terminal.

Claim 22

The power inverter as claimed in claim 16, wherein said controller means comprises: a state machine which generates a first and a second control signal for indicating the positive cycle of operation and the negative cycle of operation, respectively, and which generates a third control signal which indicates when said second switching means is disabled during the negative cycle of operation and a fourth control signal which indicates when said first switching means is disabled during the positive cycle of operation; means to AND a signal from said current sensing means which indicates a current flow from said output terminal to the load with said fourth control signal to form a first output; means to OR said first output with said second control signal to form a second output which controls said fourth switching means; means to AND a signal from said current sensing means which indicates a current flow from the load to the output terminal with said third control signal to form a third output; and means to OR said third output with said first control signal to form a fourth output which controls said third switching means.

Claim 23

The power inverter as claimed in claim 20, wherein said third and fourth control signals overlap.

Claim 24

The power inverter as claimed in claim 20, wherein said state machine generates a fifth and a sixth control signal for selectively enabling and disabling said first and said second switching means, respectively.

Claim 27

The method as claimed in claim 23, further comprising the step of: enabling the third switch during the positive cycle of operation; enabling the fourth switch during the negative cycle of operation.