Power inverter for generating voltage regulated sine wave replica

• US 5,373,433 A
• Filed: 05/05/1992
• Issued: 12/13/1994
• Est. Priority Date: 05/05/1992
• Status: Expired due to Term

• Current Assignee: Schneider Electric Solar Inverters Usa, Inc. (Schneider Electric SE)
• Original Assignee: Trace Engineering (Mission Critical Electronics LLC)
• Inventors: Thomas, Gregory H.
• Primary Examiner(s): Stephan, Steven L.
• Assistant Examiner(s): BERHANE, ADOLF D
• Application Number:
  US07/878,639
• Time in Patent Office: 952 Days
• Field of Search: 363/40, 363/43, 363/64, 363/95, 363/97, 363/98, 323/358, 323/359, 323/361
• US Class Current: 363/43
• CPC Class Codes: H02M 7/49 Combination of the output voltage waveforms of a plurality of converters

Abstract

A power inverter formed by several transformers having their secondary windings wired in series and their primary windings connected to respective switching bridges. The turns ratios of each of the primary windings vary from each other by a factor of 3 to provide good voltage resolution over a wide dynamic range. The switching bridges are controlled by a decoder and timing circuit which is, in turn, controlled by a microprocessor. The microprocessor closes the switches in the switching bridges in up to 27 different combinations to produce 27 different output voltages, thereby generating a relatively accurate replica of a sine wave. Voltage regulation is accomplished by sampling the A.C. voltage, comparing the magnitude of the A.C. voltage to a reference voltage, and adjusting the selection of output voltage values so that the A.C. voltage matches the reference voltage. The switching bridge may also be controlled to convert an A.C. voltage applied to the transformer secondaries to a D.C. voltage for battery charging purposes.

Claims

1. A power inverter for converting a D.C. voltage applied between first and second D.C. terminals to an A.C. voltage having a predetermined frequency and amplitude generated between first and second A.C. terminals, said inverter comprising:

a plurality of transformers, each having a primary transformer winding coupled to a secondary transformer winding by a respective turns ratio, the secondary transformer windings for all of said transformers being connected in series with each other between said A.C. terminals such that the voltage across said A.C. terminals is proportional to the sum of the products of the turns ratio of each of said transformers and the voltage applied thereto;

switch means connected between said D.C. terminals and the said primary transformer winding for each of said transformers, the switch means for each of said transformer windings being controllable by a respective set of control signals between an off state in which said primary transformer winding is shorted, a positive state in which said primary transformer winding is connected between said D.C. terminals in one direction, and a negative state in which said primary transformer winding is connected between said D.C. terminals in the opposite direction, thereby causing the voltages across the secondary transformer windings to be either positive, zero or negative, said switch means including a switching bridge formed by first and second pairs of series-connected semiconductor switches, each pair of said switches being connected in parallel between said first and second D.C. terminals, with said primary transformer winding being connected between the junctions between the semiconductor switches in said pairs, and wherein said control signals applied to said switch means during said positive state close the semiconductor switch in said first pair that is connected to said first D.C. terminal, close the semiconductor switch in said second pair that is connected to said second D.C. terminal, and open the remaining semiconductor switches, thereby causing current to flow through said primary transformer winding in one direction, and wherein said control signals applied to said switch means during said negative state close the semiconductor switch in said first pair that is connected to said second D.C. terminal, close the semiconductor switch in said second pair that is connected to said first D.C. terminal, and open the remaining semiconductor switches, thereby causing current to flow through said primary transformer winding in the opposite direction; and

control means connected to said switch means for generating said control signals and applying said control signals to said switch means, said control means generating 3^N combinations of said state control signals as a function of time, where N is the number of said primary transformer windings, said combinations of said control signals being generated to cause the A.C. signal generated across said A.C. terminals to approximate a predetermined waveform, said control means including decoder means for applying said control signals to each series-connected pair of said semiconductor switches, said decoder means generating control signals that cause both semiconductor switches in each pair to be off for a predetermined period each time said switch means are switched to a positive state or a negative state, thereby preventing both semiconductor switches in a pair to be simultaneously closed.

2. A power inverter for converting a D.C. voltage applied between first and second D.C. terminals to an A.C. voltage having a predetermined frequency and amplitude generated between first and second A.C. terminals, said inverter comprising:

a plurality of transformers, each of which includes a primary transformer winding coupled to a secondary transformer winding by a respective turns ratio, the secondary transformer windings for all of said transformers being connected in series between said A.C. terminals such that the voltage across said A.C. terminals is proportional to the sum of the products of the turns ratio of each of said transformers and the voltage applied thereto;

a switching bridge for each of said primary transformer windings, each of said switching bridges including first and second pairs of series-connected semiconductor switches, each pair of said switches being connected in parallel between said first and second D.C. terminals, with said primary transformer winding being connected between the junctions between the semiconductor switches in said pairs;

a control signal generator connected to each of said semiconductor switches, said control signal generator generating a first set of control signals during a first state, a second set of control signals during a second state, and a third set of control signals during a third state;

said first set of control signals closing the semiconductor switch in said first pair that is connected to said first D.C. terminal, closing the semiconductor switch in said second pair that is connected to said second D.C. terminal, and opening the remaining semiconductor switches, thereby causing current to flow through said primary transformer winding in one direction during said first state;

said second set of control signals closing the semiconductor switch in said first pair that is connected to said second D.C. terminal, closing the semiconductor switch in said second pair that is connected to said first D.C. terminal, and opening the remaining semiconductor switches, thereby causing current to flow through said primary transformer winding in the opposite direction in said second state; and

said third set of control signals closing the semiconductor switch in said first and second pair that are connected to said first D.C. terminal and opening the remaining semiconductor switches, thereby shorting the respective primary transformer windings;

said first, second, and third states for respective primary transformer windings being generated in a predetermined combination as a function of time so that an A.C. signal having a predetermined amplitude and waveform is generated between said A.C. terminals, said control signal generator including a microprocessor including a memory storing a lookup table having a plurality of sets of data, each set of data corresponding to a predetermined combination of control signal states for said primary transformer windings which either prevent current from flowing or cause current to flow through said primary transformer windings in a predetermined combination corresponding to a predetermined phase range of a sine wave, said microprocessor generating said control signals in a predetermined sequence corresponding to a sequence of said sets of data stored in said lookup table; and

an output voltage regulator controlling the voltage of said A.C. signal, said regulator comprising an analog-to-digital converter having an analog input to which a signal indicative of the voltage of said A.C. signal is coupled, said analog-to-digital converter having a digital output on which a digital word indicative of the voltage of said A.C. signal is generated, said digital output being connected to an input port of said microprocessor, and wherein said lookup table contains a plurality of data tables, each of which contains said plurality of sets of data corresponding to a sine wave having a respective amplitude, the data table from which said microprocessor chooses said sets of data being determined as a function of the digital word output from the digital output of said analog-to-digital converter whereby the voltage of said A.C. signal is regulated to a relatively constant value.

3. A power inverter for converting a D.C. voltage applied between first and second D.C. terminals on an A.C. voltage having a predetermined frequency and amplitude generated between first and second A.C. terminals, said inverter comprising:

a plurality of transformers, each of which includes a primary transformer winding coupled to a secondary transformer winding by a respective turns ratio, the secondary transformer windings for all of said transformers being connected in series between said A.C. terminals such that the voltage across said A.C. terminals is proportional to the sum of the products of the turns ratio of each of said transformers and the voltage applied thereto;

a switching bridge for each of said primary transformer windings, each of said switching bridges including first and second pairs of series-connected semiconductor switches, each pair of said switches being connected in parallel between said first and second D.C. terminals, with said primary transformer winding being connected between the junctions between the semiconductor switches in said pairs;

a control signal generator connected to each of said semiconductor switches, said control signal generator generating a first set of control signals during a first state, a second set of control signals during a second state, and a third set of control signals during a third state;

said first set of control signals closing the semiconductor switch in said first paper that is connected to said first D.C. terminal, closing the semiconductor switch in said second pair that is connected to said second D.C. terminal, and opening the remaining semiconductor switches, thereby causing current to flow through said primary transformer winding in one direction during said first state;

said second set of control signals closing the semiconductor switch in said first pair that is connected to said second D.C. terminal, closing the semiconductor switch in said second pair that is connected to said first D.C. terminal, and opening the remaining semiconductor switches, thereby causing current to flow through said primary transformer winding in the opposite direction in said second state; and

said third set of control signals closing the semiconductor switch in said first and second pair that are connected to said first D.C. terminal and opening the remaining semiconductor switches, thereby shorting the respective primary transformer windings;

said first, second and third states for respective primary transformer windings being generated in a predetermined combination as a function of time so that an A.C. signal having a predetermined amplitude and waveform is generated between said A.C. terminals, said control signal generator including a microprocessor including a memory storing a lookup table having a plurality of sets of data, each set of data corresponding to a predetermined combination of control signal states for said primary transformer windings which either prevent current from flowing or cause current to flow through said primary transformer windings in a predetermined combination corresponding to a predetermined phase range of a sine wave, said microprocessor generating said control signals in a predetermined sequence corresponding to a sequence of said sets of data stored in said lookup table; and

a battery charger circuit causing said inverter to function as a battery charger by converting an A.C. voltage applied between said first and second A.C. terminals to a D.C. voltage generated between first and second D.C. terminals, comprising a battery charger control circuit operatively connected to said A.C. terminals and to said switching bridge, said battery charger control circuit sampling said A.C. voltage to determine the frequency and phase of said A.C. voltage, said battery charger control circuit generating said first set of control signals when said A.C. voltage has one polarity and said second set of control signals when said A.C. voltage has the opposite polarity.