Method and apparatus for achieving current balance in parallel connected switching devices

• US 5,338,994 A
• Filed: 06/04/1993
• Issued: 08/16/1994
• Est. Priority Date: 07/20/1989
• Status: Expired due to Fees

• Current Assignee: General Electric Company
• Original Assignee: General Electric Company
• Inventors: Lezan, Georges R. E., Walker, Loren H.
• Primary Examiner(s): Pellinen, A. D.
• Assistant Examiner(s): Elms, Richard T.
• Application Number:
  US08/071,375
• Time in Patent Office: 438 Days
• Field of Search: 307/55, 307/58, 307/60, 307/32, 307/18-29, 307/45, 307/52, 307/53, 307/64, 307/82, 307/85-87, 307/491, 307/633, 307/131, 307/645, 307/640, 307/242, 307/246, 307/33, 307/34, 307/38, 323/271, 323/272, 323/910, 363/70, 363/54, 363/72, 363/69, 363/71, 363/87, 363/96, 363/129, 363/137, 363/56, 363/57, 363/135, 363/138
• US Class Current: 307/86
• CPC Class Codes: H02M 1/088 for the simultaneous control of series or parallel connected semiconductor devices
• CPC Class Codes: H03K 17/125 in thyristor switches

Abstract

A scheme for maintaining current balance within a plurality of semiconductor devices, particularly thyristors and gate turn-off thyristors, which plurality is connected between two electrical power sources which devices are configured to be simultaneously conductive to share the current between the two sources, employs the scheme for maintaining the current within the several devices of the plurality equal. The scheme provides for the generation of individual current signals which represent the individual current magnitudes within the several devices. Further generated is an average current signal representing the average current within the plurality. A difference signal is generated with respect to each of the individual current signals which represents the difference between the average and the individual current signal. The difference signal is then employed to vary the time of application of the next control signal to the associated one of the devices to thereby minimize current imbalance within the plurality.

Claims

1. For transferring electrical power between two electrical power sources, a switching arrangement comprising:

a) at least one plurality of series connections each comprising a switching device and an inductor, the series connections being connected in parallel with the parallel combination being connected between said two power sources;

b) means to control the conductive state of said switching devices to effect periods of simultaneous current within each of the series connections alternating with periods of no current within said connections;

c) means to provide individual current signals representing the respective currents in each of the simultaneously conducting devices of said plurality;

d) means to provide an average current signal representing the average value of said individual current signals;

e) means to develop a difference signal as a function of the difference between the respective individual current signal and the average current signal;

f) means to integrate said difference signal to provide an integrated signal;

f) means to generate control signals operative to render conductive associated ones of said switching devices; and

,

g) means responsive to said integrated signal to vary the time, with respect to a predetermined norm of associated devices, of application of said control signal to the associated ones of said devices whereby differences in current value within said devices are reduced.

2. For transferring electrical power between two electrical power sources, a switching arrangement comprising:

a) at least one plurality of series connections each comprising a switching device and an inductor, the series connections being connected in parallel with the parallel combination being connected between said two power sources;

b) means to control the conductive state of said switching devices to effect periods of simultaneous current within each of the series connections alternating with periods of no current within said connections;

c) means to provide individual current signals representing the respective currents in each of the simultaneously conducting devices of said plurality;

d) means to provide an average current signal representing the average value of said individual current signals;

e) means to develop a difference signal, with respect to each of said individual current signals, as a function of the difference between the respective individual current signal and the average current signal;

f) means to generate control signals operative to render conductive associated ones of said switching devices; and

g) means responsive to the difference signal to vary the time, with respect to a predetermined norm of associated devices, of application of said control signal to the associated ones of said devices whereby differences in current value within said devices are reduced.

3. The invention in accordance with claim 2 wherein said means responsive to the difference signal includes timing means responsive to each of the difference signal to adjust, respectively in response thereto, the time of application of said control signals.

4. The invention in accordance with claim 3 wherein said timing means adjust the timing of said control signals in a manner to selectively lead or lag the time of the predetermined norm of the associated devices of developing said control signals.

5. The invention in accordance with claim 3 wherein said timing means provides a delay in providing said control signals with respect to the time of the predetermined norm of the associated devices of developing said control signals.

6. The invention in accordance with claim 3 further including means to generate a voltage signal representative of the voltage across said plurality of switching devices prior to their becoming conductive and gain means responsive to said voltage signal to vary the value of said difference signal prior to the application thereof to said timing means.

7. In a power switching system of the type having at least one leg connected between two power sources, each leg comprising a plurality of respective series connections being connected in parallel with each other, each of the series connections comprising a switching device and a respective inductor, each switching device being operable in successive control cycles so that the switching device is in a conductive state for a first time period of each control cycle and in a nonconductive state for a second time period of each control cycle to cause current to flow in the series connection during the first time period and to prevent current from flowing during the second time period, a method of controlling the switching devices in a respective leg to enhance current balance within the plurality of series connections and comprising the steps of:

a) selectively controlling the conductive state of each of the switching devices during each control cycle by applying respective control signals to each of the switching devices to effect substantially coincident respective first time periods of simultaneously current flow in all of the series connections alternating with respective second time periods of no current flow;

b) generating a respective individual current signal representative of current flow within each of the series connections during respective first time periods of simultaneous current flow;

c) generating an average current signal representative of the average current flow within the series connections based upon the individual current signals;

d) generating a respective difference signal as a function of the difference between each of the individual current signals and the average current signal; and

e) selectively adjusting relative timing of the application of respective control signals to respective switching devices during respective first time periods of current flow during a control cycle responsive to respective difference signals from a prior control cycle to thereby enhance current balance within the plurality of series connections of the respective leg of the power switching system.

8. The invention in accordance with claim 7 wherein the step of generating a difference signal involves the steps of:

a) combining the one of said individual current signals and said average current signal at a prescribed point in time to develop an error signal; and

,

b) integrating said error signal to achieve said difference signal.

9. The invention in accordance with claim 8 wherein said prescribed point in time is defined by delaying a predetermined period after the nominal time of initiating current conduction in said plurality.

10. The invention in accordance with claim 9 further including the steps of:

a) providing a voltage signal proportional to the forward voltage across said plurality; and

,

b) modifying the effect of said difference signal as a function of said voltage signal.

11. The invention in accordance with claim 8 further including the steps of:

a) providing a voltage signal proportional to the forward voltage across said plurality; and

,

b) modifying the effect of said difference signal as a function of said voltage signal.

12. The invention in accordance with claim 8 wherein said prescribed point in time is defined by;

a) determining that a forward voltage across said plurality exceeds a predetermined minimum and;

b) delaying a predetermined period after the determining step.

13. The invention in accordance with claim 12 further including the steps of:

a) providing a voltage signal proportional to the forward voltage across said plurality; and

,

b) modifying the effect of said difference signal as a function of said voltage signal.

14. The invention in accordance with claim 7 wherein the steps of generating a difference signal and adjusting of time are performed separately for each of the series connections of said plurality.

15. The invention in accordance with claim 7 wherein:

a) in response to a difference signal of substantially zero magnitude from a prior control cycle, the next control signal is applied to the associated one of said devices at a first point in time;

b) in response to a difference signal indicating a larger than average current from a prior control cycle, the next control signal is applied to the associated one of said devices at a point in time delayed from said first point in time; and

,

c) in response to a difference signal indicating a smaller than average current from a prior control cycle, the next control signal is applied at a point in time before said first point in time.

16. The invention in accordance with claim 15 wherein the amounts of time before and after said first point in time are proportional to the magnitude of the respective difference signals.

17. The invention in accordance with claim 7 further including the steps of:

a) providing a voltage signal proportional to the forward voltage across said plurality; and

,

b) modifying the effect of said difference signal as a function of said voltage signal.

18. The invention in accordance with claim 7 wherein only difference signals representing a larger than average current value from a prior control cycle are recognized and wherein said step of adjusting is effective only to individual ones of said devices associated with such difference signals and serves to delay in time the application of the next control signal from a normal time of applying said control signal.

19. The invention in accordance with claim 18 wherein the amount of delay in time is proportional to the value of said difference signal.

20. The invention in accordance with claim 7 wherein the step of generating a difference signal comprises:

a) integrating one of said individual current signals during a first time period of a first control cycle to provide a first integrated current signals to generate an error signal for said first control cycle; and

d) respectively integrating the error signal developed on successive control cycles for said plurality to generate said difference signal.

21. The invention in accordance with claim 20 wherein said steps of generating a difference signal and adjusting of time are performed separately for each of the devices of said plurality.

22. The invention in accordance with claim 20 further including the steps of:

a) providing a voltage signal proportional to the forward voltage across said plurality; and

,

b) modifying the effect of said difference signal as a function of said voltage signal.

23. For transferring electrical power between two electrical power sources, a switching arrangement comprising:

a) at least one plurality of series connections being connected in parallel with each other and with the parallel combination being connected between the two power sources, each switching device being operable in a conductive state, during which current flows in the series connection, and a nonconductive state, during which no current flows in the series connection;

b) means for controlling the conductive state of said switching devices in successive control cycles by application of respective control signals to said switching devices to effect substantially coincident first time periods of simultaneous current flow within all of the series connections alternating with second time periods of no current flow;

c) means for providing individual current signals representing the respective currents in each of the switching devices during respective first time periods of simultaneous current flow;

d) means for providing an average current signal representative of the average current flow within the series connections based upon said individual current signals;

e) means for providing a difference signal, as a function of the difference between an individual current signal and the average current signal; and

f) means cooperating with said control means for selectively adjusting relative timing of the application of respective control signals to respective switching devices during respective first time periods of current flow in a control cycle responsive to respective difference signals from a prior control cycle to thereby enhance current balance within said plurality of series connections.

24. The invention in accordance with claim 23 wherein said switching devices are thyristors.

25. The invention in accordance with claim 24 wherein said thyristors are the gate turn-off type.

26. The invention in accordance with claim 23 wherein said means for developing a difference signal includes adjustable delay means for determining the time of developing said difference signal.

27. The invention in accordance with claim 26 wherein said delay means includes a timer and a sample and hold circuit for each of said individual current signals and said average current signal.

28. The invention in accordance with claim 27 wherein said delay means is adjustable to provide an output to determine the time of applying the content of said sample and hold circuits.

29. The invention in accordance with claim 27 further including means to generate a voltage signal representative of the voltage across said plurality of switching devices prior to their becoming conductive and means to prevent the initiation of the operation of said delay means until said voltage signal reaches a predetermined magnitude.

30. The invention in accordance with claim 23 wherein said means for providing the difference signal comprises:

a) first integrating means responsive to the individual current signals during a control cycle for providing a first integrated current signal;

b) second integrating means responsive to said average current signal during said control cycle for providing a second integrated current signal; and

c) means to combine said first and second integrated signals for providing said difference signal as a function of a difference therebetween.

31. The invention in accordance with claim 30 wherein said means responsive to said difference signal includes means to integrate said difference signal to yield an integrated signal and wherein said integrated signal is employed to effect the time variation.

32. The invention in accordance with claim 31 wherein the difference signal is duplicated for each of the plurality of switching devices.

33. The invention in accordance with claim 30 wherein the difference signal is duplicated for each of the plurality of switching devices.