OUTPUT VOLTAGE RIPPLE CONTROL FOR A DC-DC POWER CONVERTER

• US 20110273152A1
• Filed: 05/05/2011
• Published: 11/10/2011
• Est. Priority Date: 05/06/2010
• Status: Active Grant

• Current Assignee: Xandex, Inc.
• Original Assignee: Xandex, Inc.
• Inventors: Weir, Steven P.
• Granted Patent: US 8,916,764 B2
• Time in Patent Office: Days
• US Class Current: 323/282
• CPC Class Codes: H02M 1/143 using compensating arrangements for reducing noise from the supply in transmission systems H04B15/005
• CPC Class Codes: H02M 3/158 including plural semiconductor devices as final control devices for a single load

Abstract

An output ripple voltage average amplitude of a switch mode DC-DC converter is dynamically maintained. The converter has a switch and an output filter. By varying a switching period (T_PERIOD) of the switch, V_RIPPLE is maintained at a substantially constant value over a first range of converter input voltages and a second range of switch duty cycles. Where the output filter includes an inductor having inductance (L) and a capacitor having capacitance (C) the average amplitude of V_RIPPLE is dynamically maintained by varying T_PERIOD with respect to switch duty cycle (D) and input voltage (V_IN) so as to approximately satisfy the following relationship: T_PERIOD=(V_RIPPLE*8*L*C)^0.5/(V_IN*(D−D²))^0.5.

Claims

1. A method, comprising:

dynamically maintaining an output ripple voltage (V_RIPPLE) average amplitude of a switch mode DC-DC converter, said converter configured to be disposed between a power source and a load, said converter comprising a switch and an output filter, by varying 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.

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, wherein the converter is a buck converter

7. The method of claim 1, wherein the output filter comprises an inductor having inductance (L) and a capacitor having capacitance (C) and the average amplitude of V_RIPPLE is dynamically maintained by varying T_PERIOD with respect to switch duty cycle (D) and input voltage (V_IN) so as to approximately satisfy the following relationship:

T_PERIOD=(V_RIPPLE*8*L*C)^0.5*(V_IN*(D−

D²))^0.5

8. The method of claim 7, further comprising:

precompiling and storing, in a respective look up table, normalized values;

TABLE₁(D)=K₁/(D−

D²)^0.5,

and

TABLE₂(V_IN)=K₂/(V_IN^0.5) wherein

the average amplitude of V_RIPPLE is dynamically maintained by varying T_PERIOD so as to approximately satisfy the following relationship;

T_PERIOD=K₃*(TABLE₁(D))*(TABLE₂(V_IN)).

9. The method of claim 8, wherein a value of K₃ is selected so that units of T_PERIOD scale directly into a digital count value.

10. The method of claim 8, wherein a value of K₃ is selected so that units of T_PERIOD scale into a digital-to-analog converter command word value representative of an interval threshold for a mixed-signal PWM ramp generator.

11. An apparatus, comprising:

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

12. The apparatus of claim 11, wherein the ripple voltage average amplitude is controlled to be less than a selected level.

13. The apparatus of claim 12, 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.

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

15. The apparatus of claim 11, wherein the converter is a buck converter

16. The apparatus of claim 11, wherein the output filter comprises an inductor having inductance (L) and a capacitor having capacitance (C) and the average amplitude of V_RIPPLE is dynamically maintained by varying T_PERIOD with respect to switch duty cycle (D) and input voltage (V_IN) so as to approximately satisfy the following relationship:

T_PERIOD=(V_RIPPLE*8*L*C)^0.5*(V_IN*(D−

D²))<sup>−

0.5</sup>

17. The apparatus of claim 11, wherein the first range of converter input voltages is 5 volts to 50 volts.

18. The apparatus of claim 11, wherein the first range of switch duty cycles is 10% to 100%.

19. The apparatus of claim 11, wherein the converter is a boost converter.

20. The apparatus of claim 11, wherein the converter is a buck-boost converter.

21. 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.

22. The PV system of claim 21, wherein the average amplitude of the ripple voltage is controlled to be less than a selected level.

23. The PV system of claim 22, wherein the selected level limits ripple induced power harvest degradation of unbuffered PV substrings in mixed string configurations to a predefined worst-case limit.

24. The PV system of claim 21 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.

25. The PV system of claim 21, 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.

26. The PV system of claim 25, wherein the power converter is at least one of a central inverter and a battery charge controller.

27. The PV system of claim 25, 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.