Hysteretic current control method and an uninterruptible power supply using same

• US 6,724,596 B2
• Filed: 01/26/2001
• Issued: 04/20/2004
• Est. Priority Date: 01/26/2001
• Status: Expired due to Fees

• Current Assignee: Great Lakes Chemical Corporation (Lanxess AG)
• Original Assignee: Powerware Corporation (Eaton Corporation PLC.)
• Inventors: Wade, Joseph R.
• Primary Examiner(s): Jackson, Stephen W.
• Application Number:
  US09/770,480
• Publication Number:
  US 20020101751A1
• Time in Patent Office: 1,180 Days
• Field of Search: 361/18, 361/93.1, 361/58, 361/100, 361/115, 361/78, 361/90, 361/89
• US Class Current: 361/93.1
• CPC Class Codes: H02M 1/385 with means for correcting output voltage deviations introduced by the dead time
• CPC Class Codes: H02M 7/53803 with automatic control of output voltage or current

Abstract

Presented is a control system and method for minimizing the adverse affects resulting from conventional hysteretic current control of power inverters, and an uninterruptible power supply using same. The system of the present invention uses control circuitry for monitoring and classifying the PWM switching events commanded by conventional hysteretic current control and adjusting the dead time delay injected based on these PWM events. Preferably, the control circuitry uses a minimum delay dead time component to preclude the possibility of a shoot through condition from occurring, and also includes a frequency control component of the total dead time delay. In one embodiment this frequency control component may vary in proportion to the switching frequency commanded by the hysteretic current control loop. In an alternate embodiment, the PWM switching events are classified into differing modes of operation, during which a particular fixed frequency control dead time delay is selected. The control of the present invention may also use a maximum difference current control limit for the dead time delay to preclude excessive output current swings beyond the typical hysteretic current control limits.

Claims

1. A current control method for use with an inverter having at least a first and a second switching element controllable to construct an AC output waveform in accordance with a pulse-width modulated (PWM) control signal from a hysteretic control circuit, comprising the steps of:

monitoring a duration of each PWM event;

classifying each of the PWM events into one of at least two classifications;

comparing each of the PWM'"'"'s classification to a previous event classification;

selecting a dead time delay based on said step of comparing;

commanding off one of the switching elements; and

delaying for the selected dead time delay a commanding on of another of the switching elements after the commanding off of the one of the switching elements.

2. The method of claim 1, wherein the step of classifying comprises the step of classifying each of the PWM events as one of long duration and short duration.

3. The method of claim 2, wherein the step of selecting a dead time delay comprises the steps of:

selecting a minimum dead time delay; and

adding a frequency control dead time delay component when the step of comparing identifies two consecutive short events.

4. The method of claim 3 wherein the first switching element couples a positive rail to an output of the inverter and wherein the second switching element couples a negative rail to the output of the inverter, and wherein the step of commanding off one of the switching elements comprises the step of commanding off the first switching element, and wherein the step of delaying for the selected dead time delay a commanding on of another of the switching elements comprises the step of delaying for the selected dead time delay the commanding on of the second switching element.

5. The method of claim 4, wherein the step of commanding off one of the switching elements comprises the step of commanding off the second switching element, and wherein the step of delaying for the selected dead time delay a commanding on of another of the switching elements comprises the step of delaying for the minimum dead time delay the commanding on of the first switching element.

6. The method of claim 4, wherein transitions from the first switching element to the second switching element are delayed for the minimum dead time delay, and wherein transitions from the second switching element to the first switching element are delayed for the selected dead time delay.

7. The method of claim 1, wherein the step of selecting a dead time delay comprises the steps of:

selecting a first dead time when two consecutive events of a first classification are detected;

selecting a second dead time when two consecutive events of a second classification are detected; and

maintaining a previous selected dead time when two consecutive events are of different classifications.

8. The method of claim 7, further comprising the step of selecting a third dead time during an overcurrent condition.

9. A method of controlling a switching frequency of inverter switches for use with an inverter having at least a first and a second switching element controllable to construct an AC output waveform in accordance with a pulse-width modulated (PWM) control signal from a hysteretic control circuit, comprising the steps of:

monitoring a duration of each PMW event;

injecting a first dead time delay between transitions of the switching elements when the duration is greater than a preset time; and

injecting a second dead time delay between transitions of the switching elements when the duration is less than the preset time.

10. The method of claim 9, further comprising the steps of:

comparing the duration of a current PWM event to the duration of an immediately preceding PWM event;

maintaining the injection of the first dead time delay once selected by said step of injecting the first dead time delay until the duration of two consecutive PWM events is less than the preset time; and

maintaining the injection of the second dead time delay once selected by said step of injecting the second dead time delay until the duration of two consecutive PWM events is greater than the preset time.

11. The method of claim 9, wherein the step of injecting the first dead time delay comprises the step of injecting a minimum dead time delay of duration minimally sufficient to prevent a current shoot through condition.

12. The method of claim 11, wherein the step of injecting the second dead time delay comprises the step of injecting the minimum dead time delay plus a frequency control component of duration to cumulatively reduce excessive heating of the switching elements.

13. The method of claim 9, further comprising the steps of:

detecting an overcurrent condition; and

injecting a third dead time delay between transitions of the switching elements in response to said step of detecting.

14. A controller for use with an inverter having at least a first and a second switching element controllable to construct an output AC waveform for use by equipment coupled to an output thereof, comprising:

an inverter control circuit using hysteretic current control to generate a pulse-width modulated (PWM) inverter control signal to control transitions of the switching elements to construct the output AC waveform in response to a voltage and a current feedback from the inverter output, said PWM inverter control signal having PWM events defined by transitions thereof; and

a logic driver circuit coupled to receive said PWM inverter control signal, said logic circuit generating a first and a second drive signal to transition the first and second switching elements in response to said PWM inverter control signal; and

wherein said logic driver circuit monitors a duration of said PWM events and delays a generation of said first and said second drive signals for one of at least a first and a second dead time based on said duration of said PWM events.

15. The controller of claim 14, wherein said logic driver circuit generates a first dead time of duration minimally sufficient to prevent a shoot through condition from occurring, and a second dead time comprising said first dead time plus an additional frequency control delay of duration to cumulatively reduce a switching frequency governed by said PWM inverter control signal.

16. The controller of claim 14, wherein said hysteretic current control generates said PWM inverter control signal to maintain the current feedback within a maximum and a minimum current limit, and wherein said logic driver circuit uses said second dead time to delay generation of said first and said second drive signals in a manner to allow the current feedback to fall below said minimum current limit to reduce a switching frequency of the switching elements.

17. The controller of claim 14, wherein said hysteretic current control generates said PWM inverter control signal to maintain the current feedback within a current control band, and wherein said logic driver circuit uses said second dead time to delay generation of said first and said second drive signals in a manner to allow the current feedback to exceed said current control band to reduce a switching frequency of the switching elements.

18. The controller of claim 14, wherein said logic driver circuit classifies said duration of said PWM events into at least a first and a second class, and wherein the logic driver circuit delays a generation of said first and said second drive signals for said first dead time when said PWM events are of said first class, and for said second dead time when said PWM events are of said second class.

19. The controller of claim 18, wherein said logic driver circuit maintains a current selected dead time until at least two consecutive PWM events are classified as other than that required for said current selected dead time.

20. An uninterruptible power supply, comprising:

an inverter having a first input adapted to receive line AC power, a second input adapted to receive stored DC power, and an inverter output, the inverter having at least a first and a second switching element controllable to supply an AC voltage and current output on said inverter output from the line AC power and the stored DC power; and

a controller having a voltage and a current feedback input coupled to said inverter output, and a first and a second drive signal output operatively coupled to said first and said second switching elements to control construction of said AC voltage and current output, said controller using hysteretic current control to generated a pulse-width modulated (PWM) control signal, said controller further including a logic driver circuit operative to generated said first and said second drive signals in response to said PWM control signal; and

wherein said logic driver circuit monitors a duration of each pulse-width of said PWM control signal and inserts a dead time delay between transitions of said first and said second drive signal having a duration based on said duration of each pulse-width of said PWM control signal.

21. The uninterruptible power supply of claim 20, wherein said logic driver circuit generates a first dead time of duration minimally sufficient to prevent a shoot through condition from occurring, and a second dead time comprising said first dead time plus an additional frequency control delay of duration to cumulatively reduce a switching frequency governed by said PWM control signal, and wherein said logic driver circuit uses said first dead time when said duration of a pulse-width is greater than a predetermined threshold, and wherein said logic driver circuit uses said second dead time when said duration of a pulse-width is less than said predetermined threshold.

22. The uninterruptible power supply of claim 21, wherein said logic driver circuit maintains a currently selected dead time delay until at least two consecutive pulse-widths have a duration that dictate a different dead time delay.

23. The uninterruptible power supply of claim 21, wherein said logic driver inserts a third dead time delay when said controller detects an overcurrent condition.

24. A method of controlling a switching frequency of inverter switches for use with an inverter having at least a first and a second switching element controllable to construct an AC output waveform in accordance with a pulse-width modulated (PWM) control signal from a hysteretic control circuit, comprising the steps of monitoring a duration of each PMW event and injecting a dead time delay between transitions of the switching elements based on the duration of the PWM event.

25. A method of controlling a switching frequency of inverter switches for use with an inverter having at least a first and a second switching element controllable to construct an AC output waveform in accordance with a pulse-width modulated (PWM) control signal from a hysteretic control circuit, comprising the steps of monitoring a duration of each PMW event, classifying each PWM event, and injecting a dead time delay between transitions of the switching elements based on the classification of the PWM event.