Output voltage ripple control for a DC-DC power converter

• US 8,916,764 B2
• Filed: 05/05/2011
• Issued: 12/23/2014
• Est. Priority Date: 05/06/2010
• Status: Active Grant

• Current Assignee: Xandex, Inc.
• Original Assignee: Xandex, Inc.
• Inventors: Weir, Steven P.
• Primary Examiner(s): Barnie, Rexford
• Assistant Examiner(s): Ortiz, Elim
• Application Number:
  US13/101,796
• Publication Number:
  US 20110273152A1
• Time in Patent Office: 1,328 Days
• Field of Search: 323/282, 232/222, 232/223, 232/271, 232/282, 232/299, 232/304, 136/244, 307/45, 307/71, 307/80
• US Class Current: 136/244
• 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))^.5.

Claims

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_OUT²/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;

TABLE₁(D)=K₁/(D−

D²)^0.5, and TABLE₂(V_IN)=K₂/(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=K₃*(TABLE₁(D))*(TABLE₂(V_IN)).

7. The method of claim 6, wherein a value of K₃is 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_OUT²/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.