Battery protector

• US 5,581,170 A
• Filed: 08/08/1995
• Issued: 12/03/1996
• Est. Priority Date: 12/12/1994
• Status: Expired due to Term

• Current Assignee: Unitrode Corp. (Texas Instruments, Inc.)
• Original Assignee: Unitrode Corp. (Texas Instruments, Inc.)
• Inventors: Wofford, Larry, McClure, Winthrop H., Mammano, Robert A., Price, Burt L.
• Primary Examiner(s): Wong, Peter S.
• Assistant Examiner(s): SHIN, KYUNG E
• Application Number:
  US08/512,309
• Time in Patent Office: 483 Days
• Field of Search: 320/48, 320/15, 320/17, 320/18, 320/25-26, 320/5-6, 320/2, 320/13, 320/14, 320/29, 320/30, 320/39, 320/40, 320/54, 320/55, 327/429, 327/419, 327/425, 327/427
• US Class Current: 320/116
• CPC Class Codes: H02J 7/0019 using switched or multiplexed charge circuits

Abstract

A battery protector for providing overvoltage and undervoltage protection to one or more series-connected cells. The battery protector includes a voltage detection and comparison circuit, providing an overvoltage signal indicative of whether the voltage across any of the cells is greater than an overvoltage threshold level and an undervoltage signal indicative of whether the voltage across any of the cells is less than an undervoltage threshold level, and a switch connected in series with the current path between the cells and a charger and/or load. In one embodiment, the switch is a four-terminal FET and a bias control circuit is provided for selectively connecting the body region of the FET to ensure that current does not flow through the parasitic FET diodes. A second control circuit responsive to the overvoltage and undervoltage signals controls conduction of the switch, causing the switch to close if any of the following conditions occur: (1) the overvoltage and undervoltage signals do not indicate an overvoltage or undervoltage condition, respectively; (2) the overvoltage signal indicates an overvoltage condition, but the cells are attempting to discharge; or (3) the undervoltage signal indicates an undervoltage condition, but the charger is attempting to charge the cells.

Claims

1. A battery protection device for protecting at least one battery cell adapted for being charged by a battery charger and for delivering power to a load, said battery protection device comprising:

a voltage detection and comparison circuit having a plurality of input terminals adapted for coupling across said at least one cell and providing an overvoltage signal indicative of whether or not the voltage across one of said at least one cell is greater than an overvoltage threshold level or an undervoltage signal indicative of whether or not the voltage across said one cell is less than an undervoltage threshold level; and

a current carrying FET having a first terminal adapted for connection to said at least one cell, a second terminal adapted for connection to said charger and said load, a gate terminal responsive to said overvoltage signal or said undervoltage signal, and a back gate terminal adapted for selective connection to either said first or second terminal of said current carrying FET to prevent current flow through a body diode of said current carrying FET.

2. The battery protection device recited in claim 1 wherein said voltage detection and comparison circuit provides said overvoltage signal and said undervoltage signal and said gate terminal of said current carrying FET is responsive to said overvoltage and said undervoltage signals.

3. The battery protection device recited in claim 1 wherein said battery protection device is an integrated circuit having a substrate and the anode of said body diode of said current carrying FET is provided by said substrate, and wherein said battery protector further comprises a first control switch coupled between said first terminal of said current carrying FET and said substrate and a second control switch coupled between said second terminal of said current carrying FET and said substrate.

4. The battery protection device recited in claim 3 wherein said first control switch is closed when said battery charger is attempting to charge said at least one cell and wherein said second control switch is closed when said at least one cell is attempting to discharge.

5. The battery protection device recited in claim 1 further comprising a control circuit, coupled between said voltage detection and comparison circuit and said gate terminal of said current carrying FET, said control circuit being responsive to said overvoltage signal and said undervoltage signal for controlling said current carrying FET to selectively connect and disconnect said first and second terminals of said current carrying FET.

6. The battery protection device recited in claim 5 wherein said control circuit provides a logic signal to said gate terminal of said current carrying FET in a first logic state for interconnecting said first and second terminals of said current carrying FET when (1) said overvoltage signal is not indicative of an overvoltage condition and said undervoltage signal is not indicative of an undervoltage condition;

(2) said overvoltage signal is indicative of an overvoltage condition on one of said at least one cell and said at least one cell is attempting to discharge;

or (3) said undervoltage signal is indicative of an undervoltage condition on said cell and said battery charger is attempting to charge said at least one cell.

7. The battery protection device recited in claim 1 wherein said device protects at least two battery cells and said voltage detection and comparison circuit comprises:

a first multiplexer having a plurality of input terminals connected to terminals of each of said at least two cells and an output terminal to which the voltage across a selected one of said at least two cells is connected;

a second multiplexer having a plurality of input terminals, each receiving a respective threshold voltage, and an output terminal to which a selected on e of said plurality of threshold voltages is connected; and

a comparator circuit coupled to said output terminal of said first multiplexer and said output terminal of said second multiplexer for comparing the voltage across said selected one of said at least two cells to said selected threshold voltage.

8. The battery protection device recited in claim 1 wherein said device protects at least two battery cells and said voltage detection and comparison circuit comprises:

a level shifter coupled to a first one of said at least two cells for shifting the voltage across said first cell to be referenced to the same potential as a second one of said at least two cells;

a comparator circuit coupled to said level shifter for comparing said shifted voltage to a first threshold level to provide a first intermediate overvoltage signal, for comparing said voltage across said second cell to said first threshold level to provide a second intermediate overvoltage signal, for comparing said shifted voltage across said first cell to a second threshold level to provide a first intermediate undervoltage signal, and for comparing said voltage across said second cell to said second threshold level to provide a second intermediate undervoltage signal; and

a latch circuit comprising a first latch which generates said overvoltage signal in response to said first and second intermediate overvoltage signals and maintaining the overvoltage signal in a logic state indicative of an overvoltage condition until the voltage across each of the cells is less than a third threshold level and a second latch which generates said undervoltage signal in response to said first and second intermediate undervoltage signals and maintains the undervoltage signal in a logic state indicative of an undervoltage condition until the voltage across each of said cells is greater than a fourth threshold level.

9. The battery protection device recited in claim 1 wherein said device protects at least two battery cells and said voltage detection and comparison circuit comprises:

a plurality of differential amplifiers, each one having a pair of input terminals coupled to a terminal of a corresponding one of said at least two cells and an output terminal; and

a network coupled to said output terminal of said plurality of differential amplifiers for providing a high cell voltage signal proportional to the highest voltage across any of said at least two cells and a low cell voltage signal proportional to the lowest voltage across any of said at least two cells.

10. The battery protection device recited in claim 9 wherein said network comprises:

a first plurality of diodes, each one having an anode connected to said output terminal of a corresponding one of said plurality of differential amplifiers and a cathode connected to a high voltage circuit node at a voltage proportional to one diode drop below said high cell voltage signal; and

a second plurality of diodes, each one having a cathode connected to said output terminal of a corresponding one of said plurality of differential amplifiers and an anode connected to a low voltage circuit node at a voltage proportional to one diode drop above said low cell voltage signal.

11. An integrated circuit for protecting at least one battery cell from overvoltage and undervoltage conditions, said circuit comprising:

a four terminal FET having a first terminal adapted for coupling to said at least one cell, a second terminal adapted for coupling to a charger for charging said at least one cell and to a load powered by said at least one cell, a gate terminal, and a back gate terminal;

a voltage detection and comparison circuit for detecting the voltage across said at least one cell to provide an overvoltage signal indicative of whether or not said voltage across one of said at least one cell is greater than an overvoltage threshold voltage and an undervoltage signal indicative of whether or not said voltage across said one cell is less than an undervoltage threshold voltage;

a first control circuit for generating a gate drive signal for coupling to said gate terminal of said FET in response to said overvoltage signal and said undervoltage signal; and

a second control circuit responsive to the voltage across said first and second terminals of said FET for generating a body control signal for selectively connecting said back gate terminal to said first terminal or to said second terminal.

12. The integrated circuit recited in claim 11 wherein said back gate terminal is formed by a substrate of said integrated circuit.

13. The integrated circuit recited in claim 12 wherein said FET conducts:

when (1) said overvoltage signal is not indicative of an overvoltage condition and said undervoltage signal is not indicative of an undervoltage condition;

(2) said overvoltage signal is indicative of an overvoltage condition on one of said at least one cell and said at least one cell is attempting to discharge;

or (3) said undervoltage signal is indicative of an undervoltage condition on said one cell and said battery charger is attempting to charge said at least one cell.

14. The integrated circuit recited in claim 11 wherein said voltage detection and comparison circuit comprises a comparator for comparing said voltage across said at least one cell with said first threshold level to provide said overvoltage signal and for comparing said voltage across said at least one cell with said second threshold level to provide said undervoltage signal.

15. A battery protection device for protecting at least one battery cell from an overvoltage condition or an undervoltage condition, comprising:

a voltage detection and comparison circuit connected to said at least one cell for measuring the voltage across said at least one cell and providing an overvoltage signal indicative of whether or not said voltage across one of said at least one cell is greater than an overvoltage threshold level and an undervoltage signal indicative of whether or not said voltage across said one cell is less than an undervoltage threshold level;

a control circuit connected to said voltage detection and comparison circuit for generating an activation signal in response to said overvoltage signal and said undervoltage signal; and

a switch having a first terminal coupled to said at least one cell, a second terminal adapted for coupled to said charger and said load, and an activation terminal responsive to said activation signal, wherein said switch is adapted for being in a closed position;

(1) when said overvoltage signal is not indicative of an overvoltage condition and said undervoltage signal is not indicative of an undervoltage condition;

(2) when said overvoltage signal is indicative of an overvoltage condition on one of said at least one cell and said at least one cell is attempting to discharge;

or (3) when said undervoltage signal is indicative of an undervoltage condition on said one cell and said battery charger is attempting to charge said at least one cell.

16. The battery protection device recited in claim 15 wherein said switch is a FET and further comprises a fourth terminal connected to a body region of said FET, wherein said fourth terminal is selectively connectable to said first terminal or to said second terminal of said switch to prevent current form flowing through a body diode of said FET.

17. The battery protection device recited in claim 16 wherein said device protects at least two battery cells and said voltage detection and comparison circuit comprises:

a first multiplexer having a plurality of input terminals connected to terminals of each of said at least two battery cells and an output terminal to which the voltage across a selected one of said at least two cells is connected;

a second multiplexer having a plurality of input terminals, each receiving a respective threshold voltage, and an output terminal to which a selected one of said plurality of threshold voltages is connected; and

a comparator circuit coupled to said output terminal of said first multiplexer and said output terminal of said second multiplexer for comparing the voltage across said selected one of said at least two cells to said selected threshold voltage.

18. The battery protection device recited in claim 15 wherein said device protects at least two battery cells and said voltage detection and comparison circuit comprises:

a level shifter coupled to a first one of said at least two battery cells for shifting the voltage across said first cell to be referenced to the same potential as a second one of said at least two battery cells;

a comparator circuit having a pair of input terminals connected to said level shifter for comparing the shifted voltage to a first threshold level to provide a first intermediate overvoltage signal, for comparing said voltage across said second cell to said first threshold level to provide a second intermediate overvoltage signal, for comparing the shifted voltage to a second threshold level to provide a first intermediate undervoltage signal, and for comparing said voltage across said second cell to said second threshold level to provide a second intermediate undervoltage signal; and

a latch circuit comprising a first latch which generates said overvoltage signal in response to said first and second intermediate overvoltage signals and maintaining the overvoltage signal in a logic state indicative of an overvoltage condition until the voltage across each of the cells is less than a third threshold level and a second latch which generates said undervoltage signal in response to said first and second intermediate undervoltage signals and maintains the undervoltage signal in a logic state indicative of an undervoltage condition until the voltage across each of said cells is greater than a fourth threshold level.

19. The battery protection device recited in claim 15 wherein said device protects at least two battery cells and voltage detection and comparison circuit comprises:

a plurality of differential amplifiers, each one having a pair of input terminals coupled to a terminal of a corresponding one of said at least two cells and an output terminal; and

a diode network coupled to said output terminal of said plurality of differential amplifiers for providing a high cell voltage signal proportional to the highest voltage across any of said plurality of cells and a low cell voltage signal proportional to the lowest voltage across any of said plurality of cells.

20. The battery protection device recited in claim 19 wherein said network comprises:

a first plurality of diodes, each one having an anode connected to said output terminal of a corresponding one of said plurality of differential amplifiers and a cathode connected to a high voltage circuit node at a voltage proportional to one diode drop below said high cell voltage signal; and

a second plurality of diodes, each one having a cathode connected to said output terminal of a corresponding one of said plurality of differential amplifiers and an anode connected to a low voltage circuit node at a voltage proportional to one diode drop above said low cell voltage signal.