Output voltage ripple control for a DC-DC power converter

1. A method, comprising:

dynamically maintaining a ripple voltage (V RIPPLE ) average amplitude of a switch mode DC-DC buck converter output voltage (V OUT ), said converter configured to be disposed between a power source and a load, said converter comprising a switch and an output filter, by:

receiving, at a control module of the converter, a value for V OUT and for input voltage (V IN ) to the converter; and

varying, with the control module, a switching period (T PERIOD ) of the switch so as to maintain V RIPPLE at a substantially constant value over a first range of converter input voltages and a second range of switch duty cycles by varying T PERIOD with respect to V OUT and V IN so as to approximately satisfy the following relationship: T PERIOD =((V RIPPLE *8*L*C)/(V OUT −V OUT 2 /V IN )) 0.5 .

2. The method of claim 1 , wherein the ripple voltage average amplitude is controlled to be less than a selected level.

3. The method of claim 2 , wherein the selected level is 5% peak to peak of at least one of a nominal converter input voltage a nominal converter output voltage.

4. The method of claim 2 , wherein the selected level is 0.5 V peak to peak.

5. The method of claim 1 , wherein the power source is a photovoltaic substring.

6. The method of claim 1 , further comprising:

precompiling and storing, within the control module, in a respective look up table, normalized values:

TABLE 1 ( D )= K 1 /( D−D 2 ) 0.5 , and TABLE 2 ( V IN )= K 2 /( V IN 0.5 ) wherein:

D approximately satisfies the relationship: V OUT =D*V IN , and

the average amplitude of V RIPPLE is dynamically maintained by varying T PERIOD so as to approximately satisfy the following relationship:

T PERIOD =K 3 *(TABLE 1 ( D ))*(TABLE 2 ( V IN )).

7. The method of claim 6 , wherein a value of K 3 is selected so that units of T PERIOD scale directly into a digital count value.

8. An apparatus, comprising:

a switch mode DC-DC buck converter comprising a control module, a switch, and an output filter, the converter configured to be disposed between a power source and a load, wherein

the output filter includes an inductor having inductance (L) and a capacitor having capacitance (C);

the control module is configured to receive a value for converter output voltage (V OUT ) and for input voltage (V IN ) to the converter, and to vary a switching (T PERIOD ) of the switch so as to dynamically maintain an average amplitude of V OUT ripple voltage (V RIPPLE ) at a substantially constant value over a first range of converter input voltages and a second range of switch duty cycles by varying T PERIOD with respect to V OUT and V IN so as to approximately satisfy the following relationship: T PERIOD =((V RIPPLE *8*L*C)/(V OUT −V OUT 2 /V IN )) 0.5 .

9. The apparatus of claim 8 , wherein the ripple voltage average amplitude is controlled to be less than a selected level.

10. The apparatus of claim 9 , wherein the selected level is at least one of: 5% peak to peak of a nominal converter input voltage, 5% peak to peak of a nominal converter output voltage, and 0.5V peak to peak.

11. The apparatus of claim 8 , wherein the power source is a photovoltaic substring.

12. The apparatus of claim 8 , wherein the first range of converter input voltages is 5 volts to 50 volts.

13. The apparatus of claim 8 , wherein the first range of switch duty cycles is 10% to 100%.

14. A photovoltaic (PV) system, comprising a plurality of PV substrings, wherein

each of a first subset of said PV substrings is buffered by a respective switch mode DC-DC converter, and at least one PV substring is unbuffered, at least one converter comprising a control module, a switch, and an output filter, and configured to deliver an output voltage to the load, the output voltage characterized by a ripple voltage (V RIPPLE ), wherein

the control module is configured to vary a switching period (T PERIOD ) of the switch so as to dynamically maintain an average amplitude of V RIPPLE at a substantially constant value over a first range of converter input voltages and a second range of switch duty cycles; and

a switching frequency (1/T PEROID ) is lowest when a PV substring buffered b the DC-DC converter is subject to approximately similar illumination level as other PV substrings in the system.

15. The PV system of claim 14 , wherein the average amplitude of the ripple voltage is controlled to be less than a selected level.

16. The PV system of claim 14 , wherein a switching frequency (1/T PERIOD ) is lowest when a PV substring buffered by the DC-DC converter is subject to approximately similar illumination level as other PV substrings in the system.

17. The PV system of claim 14 , further comprising a load connected to the plurality of PV substrings, the load comprising a power converter, wherein the switching frequency is varied so as to improve maximum power point tracking by the power converter by introducing an artificial slope in a power transfer function.

18. The PV system of claim 17 , wherein the power converter is at least one of a central inverter and a battery charge controller.

19. The PV system of claim 17 , wherein the DC-DC converter power transfer function is monotonic, nonlinear, and exhibits a higher initial slope for ratios of string current to DC-DC converter input current marginally in excess of 1:1, and a decreasing slope for higher ratios.