Charge balancing circuit

• US 6,806,686 B1
• Filed: 04/25/2003
• Issued: 10/19/2004
• Est. Priority Date: 04/25/2003
• Status: Expired due to Term

• Current Assignee: UCAP Power, Inc.
• Original Assignee: Maxwell Technologies Incorporated (Tesla Inc.)
• Inventors: Thrap, Guy C.
• Primary Examiner(s): Tso, Edward H.
• Application Number:
  US10/423,708
• Publication Number:
  US 20040212346A1
• Time in Patent Office: 543 Days
• Field of Search: 320/116, 320/118, 320/119, 320/121, 320/127, 320/128
• US Class Current: 320/118
• CPC Class Codes: H02J 7/0016 using shunting, discharge or bypass circuits
• CPC Class Codes: H02J 7/345 using capacitors as storage or buffering devices

Abstract

A charge balancing circuit is disclosed that is configured to provide charge balancing for a bank of series connected charge storage devices such as capacitors. One embodiment of the charge balancing circuit comprises a voltage divider, an operational amplifier, and a negative feedback resistor connected between every two capacitors. The circuit is configured to monitor the voltage in each of the capacitors and, if the voltage in one of the capacitors is higher than the other, the circuit transfers energy from the higher charged capacitor to the lower charged capacitor until the capacitors are balanced. A current limiting resistor can be included for limiting the output current of the operational amplifier to a safe value and for providing feedback information regarding the health of the capacitor. An additional gain stage can also be included for increasing the output current of the operational amplifier for banks of large charge storage devices such as capacitors.

Claims

1. A charge balancing circuit for providing charge balancing for at least two charge storage devices connected in series, the at least two charge storage devices capable of being charged by a supply voltage, the charge balancing circuit comprising:

a voltage divider for equally dividing the supply voltage across the at least two charge storage devices;

an operational amplifier having inputs and an output, the operational amplifier being connected at its output between each at least two charge storage devices and the operational amplifier being connected to the voltage divider such that the voltage divider provides a voltage to an input of the operational amplifier;

a negative, feedback resistor for providing feedback regarding a stored voltage in each of the at least two charge storage devices to an input of the operational amplifier;

wherein the operational amplifier is configured to produce an output current when the stored voltage of a first of the at least two charge storage devices is higher than the stored voltage of a second of the at least two charge storage devices, the output current causing energy stored in the first charge storage device to be transferred to the second charge storage device until the at least two charge storage devices are balanced.

2. The charge balancing circuit of claim 1 further comprising a current limiting resistor connected between the operational amplifier and the at least two charge storage devices, the current limiting resistor being configured to limit the output current of the operational amplifier to a safe value.

3. The charge balancing circuit of claim 1 further comprising a gain stage for increasing the output current of the operation amplifier.

4. The charge balancing circuit of claim 1 wherein the voltage divider further comprises two divider resistors of approximately the same value connected to the operational amplifier.

5. The charge balancing circuit of claim 4 wherein the two divider resistors are of a value high enough to minimize the quiescent current draw of the circuit.

6. The charge balancing circuit of claim 1 wherein the negative feedback resistor is configured to cancel any input bias current supplied to the operational amplifier by the voltage divider.

7. The charge balancing circuit of claim 5 wherein the negative feedback resistor has a value of approximately half the value of each of the two divider resistors such that the negative feedback resistor can effectively cancel any input bias current supplied to the operational amplifier.

8. The charge balancing circuit of claim 2 wherein the current limiting resistor has a value equal to the supply voltage divided by the maximum output current of the operational amplifier.

9. The charge balancing circuit of claim 2 wherein a voltage drop across the current limiting resistor can be used to provide information regarding the health of the at least two charge storage devices.

10. The charge balancing circuit of claim 3 wherein each gain stage comprises two transistors connected between the operational amplifier output and the at least two charge storage devices.

11. The charge balancing circuit of claim 10 wherein each of the two transistors includes a base, emitter and collector and wherein the bases and emitters of each of the two transistors are connected together.

12. The charge balancing circuit of claim 10 wherein the two transistors form a complementary symmetry emitter follower transistor pair.

13. The charge balancing circuit of claim 10 further comprising a current limiting resistor connected between the gain stage and the at least two charge storage devices, the current limiting resistor being configured to limit the output current of the gain stage to a safe value.

14. The charge balancing circuit of claim 13 wherein the current limiting resistor has a value based on the supply voltage, the saturation current of the two transistors, and the maximum current rating of the two transistors.

15. The charge balancing circuit of claim 1 wherein the at least two charge storage devices are capacitors.

16. A charge balancing circuit for providing charge balancing for n charge storage devices connected in series, wherein n is greater than or equal to 2, the n charge storage devices capable of being charged by a supply voltage, the charge balancing circuit comprising:

n-1 voltage dividers for equally dividing the supply voltage across the n charge storage devices;

n-1 operational amplifiers having inputs and an output, each operational amplifier being connected at its output between two charge storage devices and each operational amplifier being connected to one of the n-1 voltage dividers such that each voltage divider provides a voltage to an input of an operational amplifier,

n-1 negative feedback resistors, each negative feedback resistor connected to one of the n-1 operational amplifiers, the n-1 negative feedback resistors for providing feedback regarding stored voltage in the n charge storage devices to an input of one of the n-1 operational amplifiers;

wherein each operational amplifier is configured to produce an output current when the stored voltage at a first of the two charge storage devices to which it is connected is higher then the stored voltage at a second of the two charge storage devices to which it is connected, the output current causing energy stored in the first charge storage device to be transferred to the second charge storage device until the two charge storage devices are balanced.

17. The charge balancing circuit of claim 16 further comprising n-1 current limiting resistors, each current limiting resistor connected between an operational amplifier output and two charge storage devices, each current limiting resistor being configured to limit the output current of the operational amplifier to which it is connected to a safe value.

18. The charge balancing circuit of claim 16 further comprising n-1 gain stages, each gain stage configured to increase the output current of an operation amplifier.

19. The charge balancing circuit of claim 16 wherein each voltage divider further comprises two divider resistors of approximately the same value connected to one of the n-1 operational amplifiers.

20. The charge balancing circuit of claim 19 wherein the two divider resistors are of a value high enough to minimize the quiescent current draw of the circuit.

21. The charge balancing circuit of claim 16 wherein each negative feedback resistor is configured to cancel any input bias current supplied to the operational amplifier to which it is connected.

22. The charge balancing circuit of claim 20 wherein each negative feedback resistor has a value of approximately half the value of the two divider resistors such that the negative feedback resistor can effectively cancel any input bias current supplied to the operational amplifier to which it is connected.

23. The charge balancing circuit of claim 17 wherein a voltage drop across the n-1 current limiting resistors can be used to provide information regarding the health of the two charge storage devices to which each current limiting resistor is connected.

24. The charge balancing circuit of claim 18 wherein each gain stage comprises two transistors connected in parallel.

25. The charge balancing circuit of claim 24 wherein each of the two transistors includes a base, emitter and collector and wherein the bases and emitters of each of the two transistors are connected together.

26. The charge balancing circuit of claim 24 wherein the two transistors form a complementary symmetry emitter follower transistor pair.

27. The charge balancing circuit of claim 23 wherein the voltage drop across each current limiting resistor can be compared to an average voltage drop across the n-1 current liming resistors to determine the health of each of the n charge storage devices.

28. The charge balancing circuit of claim 24 further comprising n-1 current limiting resistors connected between each of the n-1 gain stages and two charge storage devices, the n-1 current limiting resistors being configured to limit the output current of the gain stage to a safe value.

29. The charge balancing circuit of claim 28 wherein each of the n-1 current limiting resistors has a value based on the supply voltage, the saturation current of the two transistors to which it is connected, and the maximum current rating of the two transistors to which it is connected.

30. The charge balancing circuit of claim 16 wherein the n charge storage devices are capacitors.