DC/AC power converter

• US 6,021,052 A
• Filed: 09/22/1997
• Issued: 02/01/2000
• Est. Priority Date: 09/22/1997
• Status: Expired due to Term

• Current Assignee: Schneider Electric Solar Inverters Usa, Inc. (Schneider Electric SE)
• Original Assignee: Statpower Technologies Partnership
• Inventors: Manness, Douglas Kenneth, Unger, Thomas Michael, MacCallum, Haakon
• Primary Examiner(s): Wong, Peter S.
• Assistant Examiner(s): Vu, Bao Q.
• Application Number:
  US08/934,994
• Time in Patent Office: 862 Days
• Field of Search: 363/26, 363/37, 363/127, 363/97, 363/17, 363/64, 363/71, 363/24, 363/25, 363/40, 363/41, 363/131, 363/133
• US Class Current: 363/26
• CPC Class Codes: H02J 9/062 for AC powered loads
• CPC Class Codes: H02M 3/33584 Bidirectional converters
• CPC Class Codes: H02M 3/3372 of the parallel type
• CPC Class Codes: H02M 7/4807 having a high frequency intermediate AC stage

Abstract

A DC link power converter apparatus including a first DC to DC converter, a DC to AC converter and a load balancing storage element. The first DC to DC converter has first and second DC ports and the DC to AC converter has a third DC port and has an AC port. The first DC port is connectable to a DC source and the second DC port is connected to the third DC port. The AC port is connectable to an AC load. The load balancing energy storage element is connected to the second DC port for decoupling the DC to DC converter from the DC to AC converter by supplying energy to the third DC port when a voltage at the second DC port is tending to decrease and for storing energy received from the second DC port when a voltage at the third port is tending to increase.

Claims

1. A power converter apparatus comprising:

a) a first DC to DC converter having a first DC port connectable to a DC sources and a second DC port, said first DC to DC converter being controllable as a current fed transformer isolated boost step up converter to transfer energy from said first DC port to said second DC port;

b) a DC to AC converter having a third DC port and an AC port, said third DC port being connected to said second DC port and said AC port being connectable to an AC load; and

c) a load balancing energy storage element connected to said second DC port for decoupling said DC to DC converter from said DC to AC converter by supplying energy to said third DC port when a voltage at said second DC port is tending to decrease and for storing energy received from said second DC port when a voltage at said third port is tending to increase.

2. An apparatus as claimed in claim 1 wherein said first DC to DC converter is controllable as a voltage fed transformer isolated buck step down converter for transferring energy from said second DC port to said first DC port and wherein said DC to AC converter is bidirectional.

3. An apparatus as claimed in claim 2 further including a DC bus connecting said second and third DC ports together for transferring energy between said DC to DC converter and said DC to AC converter, said load balancing energy storage element decoupling said DC to DC converter from said DC to AC converter by supplying energy to said DC bus when a voltage at said DC bus is tending to decrease and for storing energy received from said DC bus when a voltage at said DC bus is tending to increase.

4. An apparatus as claimed in claim 3 where in said load balancing energy storage element includes a capacitor.

5. An apparatus as claimed in claim 3 wherein said DC to DC converter includes a first power control circuit and said DC to AC converter includes a second power control circuit, said first and second power control circuits operating independently of each other.

6. An apparatus as claimed in claim 3 wherein said first DC to DC converter includes:

a) a centre-tapped transformer having a centre-tapped primary winding having a number of turns, first and second primary winding terminals and a centre-tap terminal and a secondary winding having first and second secondary winding terminals;

b) an input inductor connected between said centre-tap terminal and a first terminal of said first DC port; and

c) first and second switching elements connected between said first and second primary winding terminals respectively and a second terminal of said first DC port.

7. An apparatus as claimed in claim 6 wherein said first and second switching elements are unidirectional.

8. An apparatus as claimed in claim 6 wherein said first and second switching elements include transistors.

9. An apparatus as claimed in claim 6 wherein said DC to DC converter includes a first inverter mode control circuit for controlling power flow from said first DC port to said DC bus, and a first charge mode control circuit for controlling power flow from said DC bus to said first DC port.

10. An apparatus as claimed in claim 6 wherein said DC to DC converter includes a first inverter mode control circuit for controlling power flow in said DC to DC converter.

11. An apparatus as claimed in claim 10 wherein said first inverter mode control circuit is operable to control said DC to DC converter as a push-pull, current-fed transformer-isolated boost converter.

12. An apparatus as claimed in claim 10 wherein said first inverter mode control circuit includes:

a) a current control loop circuit for generating a current feedback signal responsive to current at said first DC port;

b) a voltage control loop circuit for generating a current command signal in response to DC bus voltage; and

c) a first switching control circuit for producing switching control signals in response to said current feedback signal and said current command signal for controlling switching of said first and second switching elements.

13. An apparatus as claimed in claim 12 wherein said voltage control loop circuit includes:

a) a DC bus voltage feedback signal generator for generating a DC bus voltage feedback signal indicative of said DC bus voltage; and

b) a non-linear voltage loop compensation network responsive to said DC bus feedback signal for producing said current command signal, said non-linear voltage loop compensation network having a relatively slow response in changing said current command signal when said DC bus voltage feedback signal is changing relatively slowly and said non-linear voltage loop compensation network having a relatively fast response in changing said current command signal when said DC bus voltage feedback signal is changing relatively quickly.

14. An apparatus as claimed in claim 13 wherein said voltage control loop circuit includes:

a) a DC bus voltage reference source for producing a DC bus voltage reference signal; and

b) a first subtracter for subtracting said DC bus voltage reference signal from said DC bus voltage feedback signal to produce a first voltage error signal, said voltage error signal being communicated to said non-linear voltage loop compensation network, and wherein said non-linear voltage loop compensation network produces said current command signal in response to said voltage error signal.

15. An apparatus as claimed in claim 14 wherein said non-linear voltage loop compensation network includes a first low pass filter and at least one diode in parallel with said first low pass filter, said diode being operable to forward conduct when said voltage error signal changes at a rate at which an instantaneous voltage drop across said low pass filter exceeds a pre-defined value.

16. An apparatus as claimed in claim 12 wherein said first switching control circuit includes:

a) a second subtracter for subtracting said current feedback signal from said current command signal to produce a first current error signal;

b) a second low pass filter for filtering said first current error signal to produce a duty cycle command signal; and

c) a first pulse width modulator for producing first and second switching signals for placing said first and second switching elements respectively in conducting and non-conducting modes for periods of time dependent on said duty cycle command signal.

17. An apparatus as claimed in claim 16 wherein said first pulse width modulator includes first and second waveform generators for generating first and second waveforms 180 degrees out of phase with each other, said first and second waveforms including at least an on state and an off state and having a first duty cycle, said first and second waveforms being supplied to said first and second switching elements respectively to place said switching elements in conducting and non-conducting modes according to said on and off states respectively, of said first and second waveforms respectively.

18. An apparatus as claimed in claim 10 wherein said first inverter mode control circuit has a first circuit portion connected to said primary winding and a second circuit portion connected to said DC bus, and a first isolator for isolating said first circuit portion from said second circuit portion while providing for communication between said first and second circuit portions.

19. An apparatus as claimed in claim 18 wherein said first isolator includes an optical isolator.

20. An apparatus as claimed in claim 6 further including at least a first clamping circuit for controlling the voltage across at least one of said first and second switching elements.

21. An apparatus as claimed in claim 20 wherein said first clamping circuit includes a second DC to DC converter connected between said at least one of said first and second switching elements and said first terminal of said DC port.

22. An apparatus as claimed in claim 6 wherein said DC to DC converter includes a first charge mode control circuit for controlling power flow from said DC bus to said first DC port.

23. An apparatus as claimed in claim 22 wherein said first and second switching elements include first and second low voltage MOSFET transistors having first and second antiparallel diodes respectively.

24. An apparatus as claimed in claim 23 wherein said first charge mode control circuit is operable to control said DC to DC converter as a full-bridge voltage fed transformer-isolated buck step-down converter.

25. An apparatus as claimed in claim 6 further including third, fourth, fifth and sixth switching elements connected to said secondary winding in a full bridge topology.

26. An apparatus as claimed in claim 25 wherein said first DC to DC converter includes a second switching control circuit in communication with said third, fourth, fifth and sixth switching elements for controlling conduction of said third, fourth, fifth and sixth switching elements to produce a high frequency voltage waveform across said secondary winding.

27. An apparatus as claimed in claim 26 wherein said second switching control circuit includes a switching signal generator for generating third, fourth, fifth and sixth control signals responsive to voltage at said first DC port and current at said second DC port, said third, fourth, fifth and sixth control signals being operable to place said third, fourth, fifth and sixth switching elements in conducting and non-conducting states.

28. An apparatus as claimed in claim 27 wherein said control signal generator includes:

a) a bridge current command signal generator for producing a bridge current command signal in response to voltage and current at said first DC port;

b) a bridge current feedback signal generator for producing a bridge current feedback signal in response to current drawn at said second DC port;

c) a third subtracter for subtracting said bridge current feedback signal from said bridge current command signal to produce a duty cycle command signal; and

d) a second pulse width modulator responsive to said duty cycle command signal for producing said third, fourth, fifth and sixth switching signals operable to control switching of said third, fourth, fifth and sixth switching elements.

29. An apparatus as claimed in claim 28 wherein said bridge current feedback signal generator includes:

a) first and second current sense signal generators for generating first and second current sense signals representing current flow in said fourth and sixth switching elements respectively;

b) a summer for summing said first and second current sense signals to produce a raw bridge current feedback signal; and

c) a third low pass filter for filtering said raw bridge current feedback signal to produce said bridge current feedback signal.

30. An apparatus as claimed in claim 29 wherein said bridge current command signal generator includes:

a) a current error signal generator for generating a second current error signal in response to current flow at said first DC port;

b) a voltage error signal generator for generating a second voltage error signal in response to voltage at said first DC port;

c) a signal selector having first and second inputs for receiving said second current error signal and said second voltage error signal and an output for producing a lesser output signal responsive to the lesser of said second current error signal and said second voltage error signal;

d) a second isolator having an input electrically connected to said first DC port, for receiving said lesser output signal and an output electrically connected to said DC bus for providing said bridge current command signal to said third subtracter, said input and output being electrically isolated from each other.

31. An apparatus as claimed in claim 30 wherein said current error signal generator includes:

a) a charging current signal generator for generating a charging current signal indicative of current at said first DC port;

b) a fourth low pass filter for filtering said charging current signal to produce a filtered charging current signal;

c) a charge current command signal generator for generating a charging current command signal indicative of a desired charging current; and

d) a fourth subtracter for subtracting said filtered charging current signal from said charge current command signal to produce said second current error signal.

32. An apparatus as claimed in claim 30 wherein said voltage error signal generator includes:

a) a charging voltage signal generator for generating a charging voltage signal indicative of voltage at said first DC port;

b) a charging voltage reference signal generator for generating a charging voltage reference signal indicative of a desired charging voltage;

c) a fifth subtracter for subtracting said charging voltage signal from said charging voltage reference signal to produce a raw voltage error signal; and

d) a second non-linear voltage loop compensation network for filtering said raw voltage error signal to produce said second voltage error signal.

33. An apparatus as claimed in claim 32 wherein said second non-linear voltage loop compensation network includes a fifth low pass filter.

34. An apparatus as claimed in claim 32 wherein said second non-linear voltage loop compensation network has a relatively slow loop response when said raw voltage error signal is changing relatively slowly and a relatively fast loop response when said raw voltage error signal is changing relatively quickly.

35. An apparatus as claimed in claim 34 wherein said second non-linear voltage loop compensation network is operable to provide a slow response when said raw voltage error signal is changing relatively slowly and a fast response when said raw voltage error signal is changing relatively quickly.

36. An apparatus as claimed in claim 3 wherein said DC to AC converter includes:

a) a full bridge converter having an input connected to said DC bus and a bridge output for providing a pulse width modulated waveform; and

b) an AC output low pass filter having an input connected to said bridge output for receiving said pulse width modulated waveform and a filter output, said filter output acting as said AC port.

37. An apparatus as claimed in claim 36 wherein said full bridge converter includes seventh, eighth, ninth and tenth switching elements connected to said DC bus in a full bridge topology.

38. An apparatus as claimed in claim 37 wherein said DC to AC converter includes:

a) an AC current sense signal generator for generating a AC current sense signal responsive to current at said bridge output;

b) a DC bus voltage sense signal generator for generating a DC bus voltage sense signal responsive to voltage at said DC bus;

c) an AC output voltage sense signal generator for generating an output voltage sense signal responsive to voltage at said bridge output;

d) an inverter mode current command signal generator for generating an inverter mode current command signal in response to said AC output voltage sense signal; and

e) a third switching control circuit for producing switching signals for controlling said seventh, eighth, ninth and tenth switching elements in response to said inverter mode current command signal.

39. An apparatus as claimed in claim 37 wherein said DC to AC converter includes:

a) a DC bus voltage sense signal generator for generating a DC bus voltage sense signal responsive to voltage at said DC bus;

b) a DC bus voltage reference signal generator for generating a DC bus voltage reference signal;

c) a current waveform signal generator for generating a desired current waveform signal indicative of the desired current at said bridge output;

d) a charge mode current command signal generator for generating a charge mode command signal in response to said DC bus voltage sense signal, said DC bus voltage reference signal and said desired current waveform signal; and

e) a third switching control circuit for producing switching signals for controlling said seventh, eighth, ninth and tenth switching elements in response to said charge mode command signal.

40. An apparatus as claimed in claim 37 wherein said DC to AC converter includes:

a) a DC bus voltage sense signal generator for generating a DC bus voltage sense signal responsive to voltage at said DC bus;

b) a DC bus voltage reference signal generator for generating a DC bus voltage reference signal;

c) a current waveform signal generator for generating a desired current waveform signal indicative of the desired current at said bridge output;

d) an AC current sense signal generator for generating an AC current sense signal responsive to current at said bridge output;

e) an AC output voltage sense signal generator for generating an AC output voltage sense signal responsive to voltage at said bridge output;

f) a charge mode current command signal generator for generating a charge mode command signal in response to said DC bus voltage sense signal, said DC bus voltage reference signal and said desired current waveform signal;

g) an inverter mode current command signal generator for generating an inverter mode current command signal in response to said DC bus voltage sense signal;

h) a third switching control circuit for producing switching signals for controlling said seventh, eighth, ninth and tenth switching elements in response to said AC current sense signal, said DC bus voltage reference signal and at least one of said charge mode command signal and said inverter mode current command signal; and

i) a selector in communication with said charge mode current command signal generator, said inverter mode current command signal generator, and said third switching signal control circuit for selectively providing said inverter mode current command signal or said charge mode command signal to said third switching control circuit.

41. An apparatus as claimed in claim 40 wherein said inverter mode current command signal generator includes a DC offset correction circuit for varying said inverter mode current command signal in response to DC offset voltage at said bridge output.

42. An apparatus as claimed in claim 41 wherein said DC offset correction circuit includes:

a) a sixth low pass filter for producing a filtered DC offset signal in response to said AC output voltage sense signal;

b) a sinewave reference signal generator for generating a sinewave reference signal;

c) a sixth subtracter for subtracting said filtered DC offset signal from said sinewave reference signal to produce an output voltage command signal; and

d) a seventh subtracter for subtracting said AC output voltage sense signal from said output voltage command signal to produce said current command signal.

43. An apparatus as claimed in claim 40 wherein said charge mode current command signal generator includes:

a) an eighth subtracter for subtracting said DC bus voltage sense signal from said DC bus voltage reference signal to produce an average current signal; and

b) a multiplier for multiplying said average current signal by said desired current waveform signal to produce said charge mode command signal.

44. An apparatus as claimed in claim 40 wherein said third switching control circuit includes:

a) a third duty cycle command signal generator for generating a third duty cycle command signal indicative of a desired duty cycle of said seventh, eighth, ninth and tenth switching elements;

b) a third pulse width modulator for producing a width-modulated pulse stream in response to said third duty cycle command signal; and

c) a gate drive decoder circuit for generating switching element control signals for controlling said seventh, eighth, ninth and tenth switching elements in response to said width-modulated pulse stream.

45. An apparatus as claimed in claim 44 wherein said gate drive decoder circuit includes a programmable array logic device.

46. An apparatus as claimed in claim 44 wherein said third switching control circuit includes a magnetization compensation circuit for compensating said third duty cycle command signal in response to the magnitude of current at said bridge output to compensate for magnetization effects in said AC output low pass filter.

47. An apparatus as claimed in claim 46 wherein said magnetization compensation circuit includes a variable gain amplifier in communication with said third pulse width modulator for amplifying said third duty cycle command signal to provide an amplified third duty cycle command signal to said third pulse width modulator, said variable gain amplifier having a gain dependent upon said AC current sense a signal.

48. An apparatus as claimed in claim 40 wherein said third switching control circuit is operable to switch said seventh and tenth switching elements in unison, between a conducting state and a non-conducting state and is operable to switch said eighth and ninth switching elements in unison, between a conducting state, and a non-conducting state such that a dead time is provided when switching said seventh and tenth switching elements from a conducting state to a non-conducting state and when switching said eighth and ninth switching elements between said conducting state and said non-conducting state, said deadtime being a period during which each of said seventh, eighth, ninth and tenth switching elements is in a non-conducting state.

49. An apparatus as claimed in claim 40 wherein said DC to AC converter includes a snubber circuit connected between said bridge output and said DC bus.

50. An apparatus as claimed in claim 49 wherein said snubber circuit includes:

a) eleventh and twelfth switching elements connected to said bridge output;

b) second and third inductors connected to said eleventh and twelfth switching elements respectively and connected to each other; and

c) a diode connected to said second and third inductors and said DC bus.

51. An apparatus as claimed in claim 50 wherein said third switching control circuit is operable to control said eleventh and twelfth switching elements in response to said DC bus voltage sense signal and said AC current signal.

52. An apparatus as claimed in claim 51 wherein said third switching control circuit produces eleventh and twelfth switching signals for controlling said eleventh and twelfth switching elements to place said snubber circuit in communication with said bridge output for a period of time dependent upon current at the AC port and DC bus voltage.

53. An apparatus as claimed in claim 52 wherein said third switching control circuit is operable to vary said period of time in response to said DC bus voltage sense signal and said AC current sense signal.

54. A method of transferring power between a DC port and an AC port, the method including:

a) selectively operating a DC to DC converter in at least one of a current fed transformer isolated boost mode and a voltage fed transformer isolated buck mode to transfer energy between said DC port and a DC bus;

b) selectively operating a DC to AC converter in at least one of an invert mode and a charge mode to transfer energy between said DC bus and said AC port; and

c) supplying energy to said DC bus from a load balancing energy storage element when a voltage at said DC bus is tending to decrease and storing energy from said DC bus in said energy storage element when a voltage at said DC bus is tending to increase.

55. A method as claimed in claim 54 further including the step of independently controlling the transfer of power between said DC port and said DC bus and the transfer of power between said DC bus and said AC port.

56. A method as claimed in claim 55 further including:

a) generating a current feedback signal responsive to current at said DC port;

b) generating a current command signal in response to DC bus voltage; and

c) producing switching control signals in response to said current feedback signal and said current command signal for controlling switching of first and second switching elements in said DC to DC converter.

57. A method as claimed in claim 56 further including the steps of:

a) generating a DC bus voltage feedback signal indicative of said DC bus voltage; and

b) producing said current command signal with a non-linear voltage loop compensation network having a relatively slow response in changing said current command signal when said DC bus voltage changes relatively slowly and a relatively fast response in changing said current command signal when said DC bus voltage changes relatively quickly.

58. A method as claimed in claim 57 further including:

a) producing said DC bus voltage reference signal; and

b) subtracting said DC bus voltage reference signal from a DC bus voltage feedback signal to produce a first voltage error signal, and communicating said first voltage error signal to said non-linear voltage loop compensation network to produce said current command signal.

59. A method as claimed in claim 58 further including the step of rendering a diode connected between an input and an output of a low pass filter operable to forward conduct when said first voltage error signal changes at a rate at which an instantaneous voltage drop across said low pass filter exceeds a pre-defined value.

60. A method as claimed in claim 59 further including the steps of:

a) subtracting said current feedback signal from said current command signal to produce a first current error signal;

b) filtering said first current error signal with a second low pass filter to produce a first duty cycle command signal; and

c) producing first and second switching signals for placing said first and second switching elements respectively in conducting and non-conducting modes for periods of time dependent on said first duty cycle command signal.

61. A method as claimed in claim 55 further including controlling switching of third, fourth, fifth and sixth switching elements connected to a secondary winding of said DC to DC converter, to produce a high frequency voltage waveform across said secondary winding.

62. A method as claimed in claim 61 further including the steps of:

a) producing a bridge current command signal in response to voltage and current at said first DC port;

b) producing a bridge current feedback signal in response to current drawn at said DC bus;

c) subtracting said bridge current feedback signal from said bridge current command signal to produce a second duty cycle command signal; and

d) producing third, fourth, fifth and sixth switching signals operable to control switching of said third, fourth, fifth and sixth switching elements in response to said second duty cycle command signal.

63. A method as claimed in claim 62 further including:

a) generating current sense signals representing current flow in said fourth and sixth switching elements respectively;

b) summing said first and second current sense signals to produce a raw bridge current feedback signal; and

c) filtering said raw bridge current feedback signal to produce said bridge current feedback signal.

64. A method as claimed in claim 63 further including:

a) generating a second current error signal in response to current flow at said DC port;

b) generating a second voltage error signal in response to voltage at said DC port;

c) producing a lesser output signal responsive to the lesser of said second current error signal and said second voltage error signal; and

d) isolating said bridge current command signal prior to subtracting said bridge current feedback signal.

65. A method as claimed in claim 64 further including:

a) generating a charging current signal indicative of current at said DC port;

b) filtering said charging current signal to produce a filtered charging current signal;

c) generating a charging current command signal indicative of a desired charging current; and

d) subtracting said filtered charging current signal from said charging current command signal to produce said second current error signal.

66. A method as claimed in claim 65 further including:

a) generating a charging voltage signal indicative of voltage at said DC port;

b) generating a charging voltage reference signal indicative of a desired charging voltage;

c) subtracting said charging voltage signal from said charging voltage reference signal to produce a raw voltage error signal; and

d) filtering said raw voltage error signal with a second non-linear voltage loop compensation network to produce said second voltage error signal.

67. A method as claimed in claim 66 further including the step of providing a relatively slow loop response in said second non-linear voltage loop compensation network when said raw voltage error signal changes relatively slowly and a relatively fast loop response when said raw voltage error signal changes relatively quickly.

68. A method as claimed in claim 54 further including:

a) generating a DC bus voltage sense signal responsive to voltage at said DC bus;

b) generating a DC bus voltage reference signal;

c) generating a desired current waveform indicative of the desired current at said DC port;

d) generating an AC current sense signal responsive to current at said AC port;

e) generating an AC output voltage sense signal responsive to voltage at said AC port;

f) generating a charge mode command signal in response to said DC bus voltage sense signal, said DC bus voltage reference signal and said desired current waveform signal;

g) generating an inverter mode current command signal in response to said AC output voltage sense signal; and

h) selectively producing switching signals for controlling seventh, eighth, ninth and tenth switching elements in said DC to AC converter in response to said AC current sense signal, said DC bus voltage reference signal and at least one of said charge mode command signal and said inverter mode current command signal.

69. A method as claimed in claim 68 further including varying said inverter mode current command signal in response to DC offset voltage at a bridge output of a converter of said DC to AC converter.

70. A method as claimed in claim 69 further including the step of:

a) producing a filtered DC offset signal in response to said AC output voltage sense signal;

b) generating a sinewave reference signal;

c) subtracting said filtered DC offset signal from said sinewave reference signal to produce an output voltage command signal; and

d) subtracting said AC output voltage sense signal from said output voltage command signal to produce said inverter mode current command signal.

71. A method as claimed in claim 70 further including the step of:

a) subtracting said DC bus voltage sense signal from said DC bus voltage reference signal to produce an average current signal; and

b) multiplying said average current signal by said current waveform signal to produce said charge mode current command signal.

72. A method as claimed in claim 71 further including:

a) generating a third duty cycle command signal indicative of a desired duty cycle of said seventh, eighth, ninth and tenth switching elements;

b) producing a width-modulated pulse stream in response to said third duty cycle command signal; and

c) generating switching element control signals for controlling said seventh, eighth, ninth and tenth switching elements in response to said width-modulated pulse stream.

73. A method as claimed in claim 72 further including the step of compensating said third duty cycle command signal in response to the magnitude of current at said AC port to compensate for magnetization effects in said DC to AC converter.

74. A method as claimed in claim 73 further including the step of providing an amplified third duty cycle command signal to a third pulse width modulator, with an amplification dependent upon said AC current sense signal.

75. A method as claimed in claim 74 further including the step of switching said seventh and tenth switching elements in unison, between a conducting state and a non-conducting state and switching said eighth and ninth switching elements in unison, between a conducting state, and a non-conducting state such that a deadtime is provided when switching said seventh and tenth switching elements from a conducting state to a non-conducting state and when switching said eighth and ninth switching elements between said conducting state and said non-conducting state, said deadtime being a period during which each of said seventh, eighth, ninth and tenth switching elements is in a non-conducting state.

76. A method as claimed in claim 75 further including the step of producing eleventh and twelfth switching signals for controlling eleventh and twelfth switching elements to place a snubber circuit in communication with said bridge output for a period of time dependent upon current at the AC port and DC bus voltage.

77. A method as claimed in claim 76 further including the step of varying said period of time in response to said DC bus voltage sense signal and said AC current sense signal.

78. A power converter apparatus comprising:

a) first and second simultaneously operable DC to DC converters connectable to a common DC source, said first and second DC to DC converters each having first and second DC ports, said second DC ports being connected together;

b) a DC bus connected to said second DC ports;

c) a first DC to AC converter having a third DC port connected to said DC bus and a first AC port connectable to an AC source, said first DC to AC converter being bi-directional and operable in an AC to DC conversion mode;

d) a second DC to AC converter having a fourth DC port and a second AC port, said fourth DC port being connected to said DC bus and said second AC port being connected to an AC load; and

e) a load balancing energy storage element connected to said DC bus for decoupling said first and second DC to DC converters and said second DC to AC converter from said first DC to AC converter by supplying energy to said DC bus when a voltage at said DC bus is tending to decrease and for storing energy received from said DC bus when a voltage at said DC bus is tending to increase.

79. An apparatus as claimed in claim 78 wherein said first and second DC to AC converters are bi-directional.

80. An apparatus as claimed in claim 78 wherein said first and second DC to DC converters are bi-directional.

81. An apparatus as claimed in claim 78 wherein said first and second DC to DC converters include first and second current control loop circuits respectively for controlling the flow of power through said first and second DC to DC converters respectively.

82. An apparatus as claimed in claim 81 further including a DC to DC converter control circuit for providing current command signals to said first and second current control loop circuits respectively for apportioning contributions of energy to said load balancing energy storage element between said first and second DC to DC converters.

83. An apparatus as claimed in claim 78 wherein said first DC to AC converter includes a third current control loop circuit for controlling the flow of power through said first DC to AC converter.

84. An apparatus as claimed in claim 81 further including a DC to AC converter control circuit for providing a current command signal to said third current control loop circuit for controlling the flow of power through said first DC to AC converter.

85. An apparatus as claimed in claim 78 further including a fourth current control loop circuit for controlling the flow of power through said second DC to AC converter.

86. An apparatus as claimed in claim 85 further including a DC to AC converter control circuit for providing a current command signal to said fourth current control loop circuit for controlling the flow of power through said second DC to AC converter.

87. An apparatus as claimed in claim 78 wherein said first and second DC to DC converters are bi-directional and wherein said first converter and second DC to AC converters are bi-directional.

88. A power converter apparatus comprising:

a) first and second simultaneously operable DC to DC converters connectable to a common DC source, said first and second DC to DC converters each having first and second DC ports, said second DC ports being connected together;

b) a DC bus connected to said second DC ports;

c) first and second DC to AC converters each having third DC ports connected to said DC bus respectively and respective first AC ports for supplying power to first and second AC loads respectively; and

d) a load balancing energy storage element connected to said DC bus for decoupling said first and second DC to DC converters from said first and second DC to AC converters by supplying energy to said DC bus when a voltage at said DC bus is tending to decrease and for storing energy received from said DC bus when a voltage at said DC bus is tending to increase.

89. An apparatus as claimed in claim 88 wherein said first and second DC to DC converters include first and second current control loop circuits respectively for controlling the flow of power through said first and second DC to DC converters respectively.

90. An apparatus as claimed in claim 89 further including a DC to DC converter control circuit for providing current command signals to said first and second current control loop circuits respectively for apportioning contributions of energy to said load balancing energy storage element between said first and second DC to DC converters.

91. An apparatus as claimed in claim 88 wherein said first DC to AC converter includes a third current control loop circuit for controlling the flow of power through said first DC to AC converter.

92. An apparatus as claimed in claim 91 further including a first DC to AC converter control circuit for providing a current command signal to said third current control loop circuit for controlling the flow of power through said first DC to AC converter.

93. An apparatus as claimed in claim 92 wherein said second DC to AC converter includes a fourth current control loop circuit for controlling the flow of power through said second DC to AC converter.

94. An apparatus as claimed in claim 93 further including a second DC to AC converter control circuit for providing a current command signal to said fourth current control loop circuit for controlling the flow of power through said second DC to AC converter.

95. An apparatus as claimed in claim 94 wherein said first and second DC to AC converter control circuits control said first and second DC to AC converters such that said first and second DC to AC converters produce first and second AC waveforms respectively, said first and second AC waveforms being out of phase with each other.

96. An apparatus as claimed in claim 95 further including a reference signal generator for generating first and second AC waveform reference signals, said first and second AC waveform reference signals being provided to said first and second DC to AC converter control circuits respectively.

97. An apparatus as claimed in claim 88 wherein said first and second DC to DC converters are bi-directional and wherein said first and second DC to AC converters are bi-directional.

98. An apparatus as claimed in claim 88 wherein said first and second DC to AC converters include third and fourth current control loop circuits respectively for controlling the flow of power through said first and second DC to AC converters.

99. An apparatus as claimed in claim 98 further including a first DC to AC converter control circuit for providing third and fourth current command signals to said third and fourth current control loop circuits respectively for controlling the flow of power through said first and second DC to AC converters.

100. A power converter apparatus comprising:

a) first and second simultaneously operable DC to DC converters connectable to first and second DC sources respectively, said first and second DC to DC converters each having first and second DC ports, said second DC ports being connected together;

b) a DC bus connected to said second DC ports;

c) first and second DC to AC converters each having a third DC ports connected to said DC bus respectively and respective first AC ports for supplying power to first and second AC loads respectively; and

d) a load balancing energy storage element connected to said DC bus for decoupling said first and second DC to DC converters from said first and second DC to AC converters by supplying energy to said DC bus when a voltage at said DC bus is tending to decrease and for storing energy received from said DC bus when a voltage at said DC bus is tending to increase.

101. An apparatus as claimed in claim 100 wherein said first and second DC to DC converters include first and second current control loop circuits respectively for controlling the flow of power through said first and second DC to DC converters respectively.

102. An apparatus as claimed in claim 101 further including first and second DC to DC converter control circuits for providing first and second current command signals to said first and second current control loop circuits respectively for independently controlling power flow through said first and second DC to DC converters.

103. An apparatus as claimed in claim 100 wherein said first DC to AC converter includes a third current control loop circuit for controlling the flow of power through said first DC to AC converter.

104. An apparatus as claimed in claim 103 further including a first DC to AC converter control circuit for providing a current command signal to said third current control loop circuit for controlling the flow of power through said first DC to AC converter.

105. An apparatus as claimed in claim 104 wherein said second DC to AC converter includes a fourth current control loop circuit for controlling the flow of power through said second DC to AC converter.

106. An apparatus as claimed in claim 105 further including a second DC to AC converter control circuit for providing a current command signal to said fourth current control loop circuit for controlling the flow of power through said second DC to AC converter.

107. An apparatus as claimed in claim 106 wherein said first and second DC to AC converter control circuits control said first and second DC to AC converters such that said first and second DC to AC converters produce first and second AC waveforms respectively, said first and second AC waveforms being out of phase with each other.

108. An apparatus as claimed in claim 107 further including a reference signal generator for generating first and second AC waveform reference signals, said first and second AC waveform reference signals being provided to said first and second DC to AC converter control circuits respectively.

109. An apparatus as claimed in claim 100 wherein said first and second DC to DC converters are bi-directional and wherein said first and second DC to AC converters are bi-directional.

110. An apparatus as claimed in claim 100 wherein said first and second DC to AC converters include third and fourth current control loop circuits respectively for controlling the flow of power through said first and second DC to AC converters.

111. An apparatus as claimed in claim 110 further including a first DC to AC converter control circuit for providing third and fourth current command signals to said third and fourth current control loop circuits respectively for controlling the flow of power through said first and second DC to AC converters.