Cascaded switching power converter for coupling a photovoltaic energy source to power mains

• US 9,077,262 B2
• Filed: 04/28/2009
• Issued: 07/07/2015
• Est. Priority Date: 04/29/2008
• Status: Active Grant

• Current Assignee: Cirrus Logic Incorporated
• Original Assignee: Cirrus Logic Incorporated
• Inventors: Melanson, John L.
• Primary Examiner(s): Berhane, Adolf
• Assistant Examiner(s): Quddus, Nusrat
• Application Number:
  US12/933,990
• Publication Number:
  US 20110032734A1
• Time in Patent Office: 2,261 Days
• Field of Search: 315/291, 315/294, 315/293, 333/99, 333/17.2, 323/266, 323/268, 323/271, 323/282, 323/285, 323/286, 363/37, 363/17, 363/40, 363/41, 363/55, 363/56.02, 363/95, 363/96, 363/98, 363/131, 363/132, 363/97, 363/89
• US Class Current: 1/1
• CPC Class Codes: H02J 2300/24 of photovoltaic origin
• CPC Class Codes: H02J 3/381 Dispersed generators
• CPC Class Codes: H02J 3/40 Synchronising a generator for connection to a network or to another generator
• CPC Class Codes: H02M 1/14 Arrangements for reducing ripples from dc input or output
• CPC Class Codes: H02M 7/4807 having a high frequency intermediate AC stage
• CPC Class Codes: H02M 7/53832 in a push-pull arrangement
• CPC Class Codes: Y02E 10/56 Power conversion systems, e.g. maximum power point trackers

Abstract

A cascaded switching power converter for coupling a photovoltaic (PV) energy source to power mains provides a high-efficiency and a potentially simple control mechanism for AC solar energy conversion systems. The PV energy source charges a capacitive storage element through a DC-DC converter, and an inverter couples energy from the capacitive storage element to the mains supply. The DC-DC converter is controlled so that ripple present on the capacitive storage element due to current drawn by the inverter is not reflected at the input of the DC-DC converter, which is accomplished by varying the conversion ratio of the DC-DC converter with the ripple voltage present across the capacitor. The average voltage of the capacitor can also be increased with increases in the available power output from the PV energy source, so that a corresponding increase in power is transferred to the mains supply.

Claims

1. A power converter for providing an AC power output from a photovoltaic power source, the power converter comprising:

a capacitive storage element;

an inverter circuit having an input coupled to the capacitive storage element and an output providing the AC power output at a line frequency, wherein the inverter circuit draws charge from the capacitive storage element cyclically at half-periods of the line frequency and thereby causes a ripple variation in a voltage across the capacitive storage element; and

a DC-DC converter having an input coupled to an output of the photovoltaic power source, and an output coupled across the capacitive storage element, wherein a conversion ratio of the DC-DC converter is varied by controlling a switching circuit of the DC-DC converter such that the output of the photovoltaic power source is maintained at a substantially constant voltage, and wherein the conversion ratio of the DC-DC converter varies at the half-periods of the line frequency as the output of the DC-DC converter follows the ripple variation in the voltage across the capacitive storage element.

2. The power converter of claim 1, wherein the conversion ratio of the DC-DC converter is further controlled such that an average voltage across the capacitive storage element is controlled in conformity with a maximum power point of the photovoltaic power source, wherein as an available output power from the photovoltaic power source increases, an average level of the voltage across the capacitive storage element is increased.

3. The power converter of claim 2, further comprising a control circuit for perturbing the conversion ratio of the DC-DC converter at the start of a measurement interval to vary a voltage of the input of the DC-DC converter, and determining the maximum power point of the photovoltaic power source from a resulting change in an indication of an output current of the inverter.

4. The power converter of claim 3, wherein the indication of the output current of the inverter is a voltage across the capacitive storage element.

5. The power converter of claim 2, further comprising a control circuit for perturbing the voltage across the capacitive storage element at the start of a measurement interval, and measuring a resulting change in the conversion ratio of the DC-DC converter to determine the maximum power point of the photovoltaic power source.

6. The power converter of claim 1, wherein the conversion ratio of the DC-DC converter is controlled solely in conformity with a measured variation of the voltage across the capacitive storage element.

7. The power converter circuit of claim 1, wherein the control circuit controls a duty cycle of the switching circuit in conformity with the output of the comparison circuit.

8. The power converter circuit of claim 1, wherein the control circuit controls a switching frequency of the switching circuit in conformity with the output of the comparison circuit.

9. The power converter of claim 1, wherein the inverter adjusts a current of the AC power output to regulate an average energy stored in the capacitive storage element.

10. The power converter of claim 9, wherein a magnitude of the current of the AC power output is controlled solely in conformity with a measured voltage across the capacitive storage element.

11. The power converter of claim 1, wherein the DC-DC converter further comprises:

a reference voltage circuit for generating a reference voltage;

a comparison circuit having inputs coupled to the reference voltage circuit and the input of the DC-DC converter for comparing a voltage of the DC operating point of the photovoltaic power source to the reference voltage; and

a control circuit for controlling switching of the switching circuit of the DC-DC converter in conformity with an output of the comparison circuit, wherein the control circuit maintains the voltage of the DC operating point of the photovoltaic power source at a substantially constant voltage.

12. The power converter of claim 1, wherein the DC-DC converter further comprises a control circuit for controlling switching of the switching circuit of the DC-DC converter in conformity with the ripple variation in the voltage across the capacitive storage element.

13. An integrated circuit for controlling a power converter for providing an AC power output from a photovoltaic power source, the integrated circuit comprising:

a sensing circuit for sensing a voltage across a capacitive storage element used to transfer energy from an output of the photovoltaic power source to the AC power output; and

a control circuit for generating switching signals for controlling a DC-DC converter having an input coupled to the output of the photovoltaic power source, and an output coupled across the capacitive storage element, wherein the switching signals are controlled in conformity with an output of the sensing circuit.

14. The integrated circuit of claim 13, wherein the control circuit varies a conversion ratio of the DC-DC converter such that the output of the photovoltaic power source is maintained at a substantially constant DC operating point across the half-periods of the line frequency, whereby any ripple voltage across the capacitive storage element caused by a load is not reflected in the input impedance of the DC-DC converter.

15. The integrated circuit of claim 13, wherein the control circuit further comprises a circuit for controlling switching of an inverter circuit having an input coupled to the capacitive storage element and an output providing the AC power output.

16. A method of providing an AC power output from a photovoltaic power source, the method comprising:

generating an AC power output at a line frequency from a capacitive storage element using an inverter, wherein the inverter draws charge from the capacitive storage element cyclically at half-periods of the line frequency and thereby causes a ripple variation in a voltage across the capacitive storage element;

charging the capacitive storage element with an output of a DC-DC converter that draws power from the photovoltaic power source; and

varying a conversion ratio of the DC-DC converter by controlling a switching circuit of the DC-DC converter such that the output of the photovoltaic power source is maintained at a substantially constant voltage, and wherein the conversion ratio of the DC-DC converter varies at the half-periods of the line frequency as the output of the DC-DC converter follows the ripple variation in the voltage across the capacitive storage element.

17. The method of claim 16, wherein the varying further controls the conversion ratio of the DC-DC converter such that an average voltage across the capacitive storage element is set in conformity with a maximum power point of the photovoltaic power source, wherein as an available output power from the photovoltaic power source increases, an average level of the voltage across the capacitive storage element is increased.

18. The method of claim 17, further comprising:

perturbing the conversion ratio of the DC-DC converter to vary a voltage of the input of the DC-DC converter; and

determining the maximum power point of the photovoltaic power source from a resulting change in an indication of an output current of the inverter.

19. The method of claim 18, wherein the indication of the output current of the inverter is a voltage across the capacitive storage element.

20. The method of claim 17, further comprising:

perturbing the voltage across the capacitive storage element; and

measuring a resulting change in the conversion ratio of the DC-DC converter to determine the maximum power point of the photovoltaic power source.

21. The method of claim 16, wherein the varying is performed solely in conformity with a measured variation of the voltage across the capacitive storage element.

22. The method of claim 16, further comprising adjusting a current of the AC power output to regulate an average energy stored in the capacitive storage element.

23. The method of claim 22, further comprising measuring a voltage across the capacitive storage element, and wherein the adjusting is performed solely in conformity with a result of the measuring.

24. A method of providing an AC power output from a photovoltaic power source, the method comprising:

generating an AC power output at a line frequency from a capacitive storage element using an inverter;

charging the capacitive storage element with an output of a DC-DC converter that draws power from the photovoltaic power source;

measuring a voltage across the capacitive storage element; and

controlling an output current of the inverter in conformity with a result of the measuring.

25. The method of claim 24, further comprising controlling a conversion ratio of the DC-DC converter in conformity with the result of the measuring.

26. A power converter for providing an AC power output from a photovoltaic power source, the power converter comprising:

a capacitive storage element;

an inverter circuit having an input coupled to the capacitive storage element and an output providing the AC power output at a line frequency; and

a DC-DC converter having an input coupled to an output of the photovoltaic power source, and an output coupled across the capacitive storage element, wherein a conversion ratio of the DC-DC converter is varied by controlling a switching circuit of the DC-DC converter such that the output of the DC-DC converter has a high impedance permitting voltage variation due to an input current of the inverter, and such that the input of the DC-DC converter has a low impedance so that the output of the photovoltaic power source is maintained at a substantially constant DC voltage.

27. The power converter of claim 26, wherein the inverter adjusts a current of the AC power output to regulate an average energy stored in the capacitive storage element.

28. A method of providing an AC power output from a photovoltaic power source, the method comprising:

generating an AC power output at a line frequency from a capacitive storage element using an inverter, wherein the inverter draws charge from the capacitive storage element cyclically at half-periods of the line frequency and thereby causes a ripple variation in a voltage across the capacitive storage element;

charging the capacitive storage element with an output of a DC-DC converter that draws power from the photovoltaic power source; and

varying a conversion ratio of the DC-DC converter by controlling a switching circuit of the DC-DC converter such that the output of the DC-DC converter has a high impedance permitting voltage variation due to an input current of the inverter, and such that the input of the DC-DC converter has a low impedance so that the output of the photovoltaic power source is maintained at a substantially constant DC voltage.

29. The method of claim 28, further comprising adjusting a current of the AC power output to regulate an average energy stored in the capacitive storage element.