Control of power converters having a parallel resonant commutation circuit
First Claim
1. An efficient power converter for converting an input voltage to a different frequency output voltage comprising a power converter circuit having a plurality of controlled power devices that are sequentially rendered conductive at a variable frequency, a parallel resonant commutation circuit including commutating inductance means and parallel commutating capacitor means for supplying a variable amount of reactive power to render said controlled power devices nonconductive, and constant turn-off time control means for computing the timing of firing signals to provide an approximately constant turn-off time for said controlled power devices, to thereby adjust the operating frequency, wherein said constant turn-off time control means comprises sensing means for sensing selected instantaneous power circuit parameters and analog computation circuit means for cyclically and instantaneously deriving an output signal for timing said firing signals, said sensing means and computation circuit means being operative to produce said output signal when the predicted time to passage through zero of the voltage of said commutating capacitor means after the firing of each incoming controlled power device is approximately equal to a predetermined constant turn-off time for a corresponding outgoing controlled power device, and said sensing means and computation circuit means comprises at least one potential sensing device for sensing the instantaneous voltage of said commutating capacitor means and at least one current sensing device for sensing a plurality of instantaneous power circuit currents indicative of the commutating capacitor means current after the firing of each incoming controlled power device, a burden resistor coupled to the outputs of said current and potential sensing devices and forming an analog circuit which generates a control signal, and means for comparing said control signal with a reference voltage and producing said output signal.
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Abstract
In a variable operating frequency inverter or other converter with a parallel resonant commutation circuit, the frequency changes with the load to control the reactive power available for commutation of the thyristors or other controlled power devices. A constant turn-off time control circuit automatically adjusts the operating frequency so that the reactive power for commutation is optimum and reliability is improved. The control circuit and method utilizes sensed instantaneous power circuit parameters from which the future state of the commutating capacitor voltage can be predicted, and a computation circuit for determining the timing of firing signals to provide an approximately constant turn-off time for a conducting device.
21 Citations
23 Claims
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1. An efficient power converter for converting an input voltage to a different frequency output voltage comprising a power converter circuit having a plurality of controlled power devices that are sequentially rendered conductive at a variable frequency, a parallel resonant commutation circuit including commutating inductance means and parallel commutating capacitor means for supplying a variable amount of reactive power to render said controlled power devices nonconductive, and constant turn-off time control means for computing the timing of firing signals to provide an approximately constant turn-off time for said controlled power devices, to thereby adjust the operating frequency, wherein said constant turn-off time control means comprises sensing means for sensing selected instantaneous power circuit parameters and analog computation circuit means for cyclically and instantaneously deriving an output signal for timing said firing signals, said sensing means and computation circuit means being operative to produce said output signal when the predicted time to passage through zero of the voltage of said commutating capacitor means after the firing of each incoming controlled power device is approximately equal to a predetermined constant turn-off time for a corresponding outgoing controlled power device, and said sensing means and computation circuit means comprises at least one potential sensing device for sensing the instantaneous voltage of said commutating capacitor means and at least one current sensing device for sensing a plurality of instantaneous power circuit currents indicative of the commutating capacitor means current after the firing of each incoming controlled power device, a burden resistor coupled to the outputs of said current and potential sensing devices and forming an analog circuit which generates a control signal, and means for comparing said control signal with a reference voltage and producing said output signal.
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2. A power converter according to claim 1 wherein said control signal passes through zero in each half cycle, and said comparing means comprises a comparator for generating said output signal upon the passage through zero of said control signal.
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3. A power converter according to claim 2 wherein said control signal has multiple zerO-crossings in each half cycle, and said computation circuit means further includes time delay means for inhibiting the generation of spurious output signals subsequent to the output signal at the initial zero-crossing.
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4. A power converter according to claim 1 wherein said potential sensing device is a potential transformer and said current sensing device is a current tansformer, and said burden resistor is connected between the terminals of the secondary winding of said current transformer and in series with the secondary winding of said potential transformer to thereby generate said control signal.
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5. A power converter according to claim 4 wherein said comparing means comprises a comparator for generating said output signal upon the initial zero-crossing of said control signal in each half cycle, and time delay means for inhibiting the generation of spurious output signals at subsequent zero-crossings of said control signal during a predetermined interval following the initial zero-crossing.
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6. An efficient inverter for converting a supply voltage to a high frequency output voltage comprising a power inverter circuit having at least a pair of thyristor devices that are alternately rendered conductive at a variable frequency, a parallel resonant commutation circuit including commutating inductance means and a parallel commutating capacitor for supplying a variable amount of reactive power to alternately render said thyristor devices nonconductive, and constant turn-off time control means comprising sensing means for sensing selected instantaneous power circuit parameters, and further comprising analog computation means for deriving on an instantaneous basis an output signal timed to produce a firing signal for an incoming thyristor device that results in an approximately constant turn-off time for an outgoing thyristor device, to thereby automatically adjust the inverter operating frequency, wherein said sensing means includes a potential sensing device for the commutating capacitor voltage and at least one current sensing device for sensing a plurality of power circuit currents indicative of the commutating capacitor current after the firing of said incoming thyristor device.
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7. An inverter according to claim 6 wherein the output of said current sensing device is applied across a burden resistor, and said burden resistor is connected in series with the output of said potential sensing device to continuously generate a cyclically varying control signal.
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8. An inverter according to claim 7 wherein said power inverter circuit further includes series current-limiting commutating inductance means, and the value of said burden resistor is adjusted to compensate for the resulting overlap in conduction of the incoming and outgoing thyristor device.
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9. An inverter according to claim 7 wherein said computation means includes means for comparing said control signal with a reference voltage and deriving said output signal at the initial zero-crossing in each half cycle.
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10. An inverter according to claim 9 wherein said computation means further includes time delay means for inhibiting the generation of spurious output signals at subsequent zero-crossings of said control signal during a predetermined interval following the initial zero-crossing.
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11. An inverter according to claim 6 wherein said current sensing device is connected to sense the commutating capacitor current and the current through said thyristor devices.
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12. An efficient inverter for converting a supply voltage to a high frequency output voltage comprising a power inverter circuit having at least a pair of thyristor devices that are alternately rendered conductive at a variable frequency, a parallel resonant commutation circuit including commutating inductance means and a parallel commutating capacitor for supplying a variable amount of reactive power to alternately render said thyristor devices nonconductive, and constant turn-off time control means comprising sensing means for sensing selected instantaneous power circuit parameters, and further comprising computation means for deriving an output signal timed to produce a firing signal for an incoming thyristor device that results in an approximately constant turn-off time for an outgoing thyristor device, to thereby automatically adjust the inverter operating frequency, wherein said sensing means includes a potential sensing device for the commutating capacitor voltage and at least one current sensing device for power circuit currents indicative of the commutating capacitor current after the firing of said incoming thyristor device, said power inverter circuit further includes a feedback power device associated with each thyristor device and connected to return feedback currents to the supply, and said current sensing device is connected to sense the current through said thyristor devices, the feedback current, the current through said commutating inductance means, and the load current.
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13. An inverter according to claim 12 wherein said potential sensing device is a potential transformer and said current sensing device is a current transformer, and said computation means includes a burden resistor connected between the terminals of the secondary winding of said current transformer, said burden resistor further being in series with the secondary winding of said potential transformer to thereby generate a cyclically varying control signal, and means for comparing said control signal with a reference signal and producing said output signal at the initial zero-crossing of said control signal in each half cycle.
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14. An inverter according to claim 13 wherein said power inverter circuit includes series current-limiting commutating inductance means, and the value of said burden resistor is adjusted to compensate for the resulting overlap in conduction of the incoming and outgoing thyristor device.
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15. An inverter according to claim 14 wherein said computation means further includes time delay means for inhibiting the generation of spurious output signals at subsequent zero-crossings of said control signal during a predetermined time interval following the initial zero-crossing.
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16. The method of controlling a power converter circuit having a plurality of controlled power devices that are sequentially rendered conductive at a variable frequency to convert an input voltage to a different frequency output voltage, and a parallel resonant commutation circuit including commutating inductance means and parallel commutating capacitor means for supplying a variable amount of reactive power to render said controlled power devices nonconductive, said method comprising the steps of sensing selected instantaneous power circuit parameters, including the commutating capacitor means voltage and a plurality of power circuit currents, from which the future state of said parallel commutating capacitor means can be predicted, computing from the sensed parameters on an instantaneous basis the timing of firing signals for said controlled power devices to provide an approximately constant turn-off time thereof, and supplying timed firing signals to said controlled power devices to automatically adjust the converter operating frequency.
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17. The control method according to claim 16 wherein the step of computing the timing of firing signals comprises continuously deriving from the sensed power circuit parameters a cyclically varying control signal, and comparing said control signal with a reference.
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18. The control method according to claim 17 wherein said control signal has multiple zero-crossings in each half cycle, and the comparing step includes generating an output signal upon the initial passage through zero of said control signal, inhibiting the production of spurious output signals at subsequent zero-crossings of said control signal during a predetermined time interval following the initial zero-crossIng, and utilizing said output to derive the timed firing signal.
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19. The method of controlling a power inverter circuit having at least a pair of thyristor devices that are alternately rendered conductive at a variable frequency to convert an input voltage to a different frequency output voltage, and a parallel resonant commutation circuit including commutating inductance means and a parallel commutating capacitor for supplying a variable amount of reactive power to alternately render said thyristor devices nonconductive, said method comprising the steps of continuously sensing selected instantaneous power circuit parameters from which the furture state of said commutating capacitor can be predicted, computing from the sensed parameters the timing of an output signal used to generate a firing signal for an incoming thyristor device that results in an approximately constant turn-off time for an outgoing thyristor device, and supplying timed firing signals to said thyristor devices to automatically adjust the inverter operating frequency, wherein the computing step comprises deriving from the sensed power circuit parameters a cyclically varying control signal, comparing said control signal to a reference, and generating said output signal when the instantaneous value of said control signal equals the reference, further wherein said control signal has multiple values equal to the reference, and the computing step further includes inhibiting the generation of spurious output signals for a time interval in each half cycle following the generation of a valid output signal.
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20. The method of controlling a power inverter circuit having at least a pair of thyristor devices that are alternately rendered conductive at a variable frequency to convert an input voltage to a different frequency output voltage, and a parallel resonant commutation circuit including commutating inductance means and a parallel commutating capacitor for supplying a variable amount of reactive power to alternately render said thyristor devices nonconductive, said method comprising the steps of continuously sensing selected instantaneous power circuit parameters from which the furture state of said commutating capacitor can be predicted, computing from the sensed parameters the timing of an output signal used to generate a firing signal for an incoming thyristor device that results in an approximately constant turn-off time for an outgoing thyristor device, and supplying timed firing signals to said thyristor devices to automatically adjust the inverter operating frequency, wherein the computing step comprises deriving from the sensed power circuit parameters a cyclically varying control signal, comparing the control signal to a zero voltage reference, generating said output signal at the initial zero-crossing of said control signal in each half cycle, and inhibiting the generation of spurious output signals for a predetermined time interval.
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21. An efficient power converter comprising a variable operating frequency power converter circuit having a plurality of controlled power devices that are sequentially rendered conductive at a variable frequency, a parallel resonant commutation circuit including commutating inductance means and parallel commutating capacitor means for supplying a variable amount of reactive power to render said controlled power devices nonconductive, and constant turn-off time control means for computing the timing of and generating firing signals to provide an approximately constant turn-off time for said controlled power devices at each operating frequency, said constant turn-off time control means comprising sensing means for continuously and simultaneously sensing at each instant a plurality of selected instantaneous power circuit voltage and current parameters from which the future state of said commutating capacitor means can be determined, and computation cirduit means for Cyclically and continuously computing the timing of each firing signal from the sensed instantaneous voltage and current parameters at each instant.
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22. An efficient power converter for converting an input voltage to a different frequency output voltage comprising a power converter circuit having a plurality of controlled power devices that are sequentially rendered conductive at a variable frequency, a parallel resonant commutation circuit including commutating inductance means and parallel commutating capacitor means for supplying a variable amount of reactive power to render said controlled power devices nonconductive, and constant turn-off time control means for deriving firing signals timed to provide an approximately constant turn-off time for said controlled power devices, to thereby adjust the operating frequency, wherein said constant turn-off time control means comprises sensing means for sensing selected instantaneous power circuit parameters and computation circuit means for cyclically deriving an output signal for timing said firing signals, said sensing means and computation circuit means comprises at least one potential sensing device for sensing the instantaneous voltage of said commutating capacitor means and at least one current sensing device for sensing at least one instantaneous power circuit current indicative of the commutating capacitor means current after the firing of each incoming controlled power device, a burden resistor coupled to the outputs of said current and potential sensing devices and forming an analog circuit which generates a control signal, and means for comparing said control signal with a reference voltage and producing said output signal, and said power converter circuit further includes series current-limiting commutating inductance means, and the value of said burden resistor is adjusted to compensate for the resulting overlap in conduction of an incoming and corresponding outgoing controlled power device.
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23. An efficient power converter for converting an input voltage to a different frequency output voltage comprising a power converter circuit having a plurality of controlled power devices that are sequentially rendered conductive at a variable frequency, a parallel resonant commutation circuit including commutating inductance means and parallel commutating capacitor means for supplying a variable amount of reactive power to render said controlled power devices nonconductive, and constant turn-off time control means for computing the timing of firing signals to provide an approximately constant turn-off time for said controlled power devices, to thereby adjust the operating frequency, wherein said constant turn-off time control means comprises sensing means for sensing selected instantaneous power circuit parameters and analog computation circuit means for cyclically and instantaneously deriving an output signal for timing said firing signals, said sensing means and computation circuit means being operative to produce said output signal when the predicted time to passage through zero of the voltage of said commutating capacitor means after the firing of each incoming controlled power device is approximately equal to a predetermined constant turn-off time for corresponding outgoing controlled power device, and said sensing means comprises a potential sensing device for sensing the instantaneous commutating capacitor means voltage and at least one current sensing device for sensing a plurality of instantaneous power circuit currents indicative of the commutating capacitor means current after the firing of each incoming controlled power device.
Specification