Bidirectional Current Control Circuit Suitable for Controlling the Charging and Discharging of Rechargeable Battery Cells

1. Field of the Invention

The present invention relates to current control circuits, and in particular, to bidirectional current control circuits suitable for use in controlling the charging and discharging of rechargeable battery cells.

2. Description of the Related Art

Circuits for controlling the bidirectional flow of current can be found in many applications. One increasingly common application is that of current control circuits for controlling the charging and discharging of rechargeable battery cells, including lithium and lithium-ion battery cells such as those used in portable computers.

As is well known, lithium and lithium-ion secondary (rechargeable) battery cells require protection from prolonged over-charging, over-discharging and over-current to prevent degradation of the performance of the cells, as well as to prevent the possibility of rupturing the case of the cells and potential combustion of the electrolyte within the cells. Power metal oxide semiconductor field effect transistor (MOSFET) switches in series with the current path for the cells are commonly used to disconnect the cells from chargers that fail to terminate the charging operation within specified voltage limits. Such MOSFET switches are typically driven from protection circuits internal to the battery pack which monitor the individual cell voltages for over-charge and over-discharge, as well as monitor the current for the battery pack for over-current in either direction.

In many applications, there are actually two power MOSFETs that are serially connected with the cells. Each MOSFET switch is turned on or off depending upon the type of failure. Normally, both MOSFETs are turned on and when a fault occurs, one of the MOSFETs is turned off by the protection circuit. The MOSFET that is turned off blocks current flow in the battery pack in the same direction that the current was flowing when the fault occurred.

Normally, recovery of such protection circuits requires some form of external circuitry for monitoring the operating environment of the battery pack (e.g., temperature, etc.) or the terminal voltages on the protection MOSFETs. It would be. desirable, however, to avoid the need for such external circuitry and provide for self-recovery of the protection circuit following a return to a safe operating condition.

A bidirectional current control circuit in accordance with the present invention provides for rapid recovery from fault conditions whereby a portion of the circuit had been disabled in response to an over-current condition. The presence of a safe current level flowing opposite to the direction of the current flow in which an over-current condition occurred is used to identify that a safe operating condition has been restored. Accordingly, external circuitry for monitoring the operating environment (such as temperature, etc.) or terminal voltages is not required.

In accordance with one embodiment of the present invention, a bidirectional current control circuit suitable for use in controlling the charging and discharging of rechargeable battery cells includes a current control circuit, a current detection circuit and a switch controller. The current control circuit includes: an on state which provides a bidirectional current conductivity; a first partially off state which provides a first unidirectional current conductivity in a first direction; and a second partially off state which provides a second unidirectional current conductivity in a second direction which is opposite to the first direction. The current detection circuit is coupled to the current control circuit and is configured to detect a magnitude and a direction of a current being conducted through the current control circuit. The switch controller is coupled to the current control circuit and the current detection circuit and is configured to: maintain the current control circuit in the on state when the current magnitude is less than first and second maximum currents; switch the current control circuit from the on state to the first partially off state when the current direction is in the second direction and the current magnitude is greater than the second maximum current; switch the current control circuit from the first partially off state to the on state when the current direction changes from the second direction to the first direction and the current magnitude is greater than a first minimum current; switch the current control circuit from the on state to the second partially off state when the current direction is in the first direction and the current magnitude is greater than the first maximum current; and switch the current control circuit from the second partially off state to the on state when the current direction changes from the first direction to the second direction and the current magnitude is greater than a second minimum current.

In accordance with another embodiment of the present invention, a bidirectional current control circuit suitable for use in controlling the charging and discharging of rechargeable battery cells includes first and second switch circuits, a current detection circuit and a switch controller. The first switch circuit includes a first on state which provides a first bidirectional current conductivity and includes a first off state which provides a first unidirectional current conductivity in a first direction. The second switch circuit is coupled in series with the first switch circuit, includes a second on state which provides a second bidirectional current conductivity and includes a second off state which provides a second unidirectional current conductivity in a second direction which is opposite to the first direction. The current detection circuit is coupled to the first and second switch circuits and is configured to detect a magnitude and a direction of a current being conducted through the first and second switch circuits. The switch controller is coupled to the first and second switch circuits and the current detection circuit and is configured to: maintain the first and second switch circuits in the first and second on states when the current magnitude is less than first and second maximum currents; switch the first switch circuit from the first on state to the first off state when the current direction is in the second direction and the current magnitude is greater than the second maximum current; switch the first switch circuit from the first off state to the first on state when the current direction changes from the second direction to the first direction and the current magnitude is greater than a first minimum current; switch the second switch circuit from the second on state to the second off state when the current direction is in the first direction and the current magnitude is greater than the first maximum current; and switch the second switch circuit from the second off state to the second on state when the current direction changes from the first direction to the second direction and the current magnitude is greater than a second minimum current.

In accordance with still another embodiment of the present invention, a bidirectional current control circuit suitable for use in controlling the charging and discharging of rechargeable battery cells includes first and second switch circuits, a current detection circuit and a switch controller. The first switch circuit is configured to receive a first control signal with enable and disable states and in accordance therewith provide a first bidirectional current conductivity and a first unidirectional current conductivity in a first direction, respectively. The second switch circuit is coupled in series with the first switch circuit and is configured to receive a second control signal with enable and disable states and in accordance therewith provide a second bidirectional current conductivity and a second unidirectional current conductivity in a second direction, respectively, such that the second direction is opposite to the first direction. The current detection circuit is coupled in series with the first and second switch circuits and is configured to detect a magnitude and a direction of a current being conducted through the first and second switch circuits and in accordance therewith provide a current detection signal which identifies the current magnitude and the current direction. The switch controller is coupled to the first and second switch circuits and the current detection circuit and is configured to receive the current detection signal and in accordance therewith provide the first and second control signals. The first and second control signals are maintained in the first and second control signal enable states when the current magnitude is less than first and second maximum currents. The first control signal: changes from the enable state to the disable state when the current direction is in the second direction and the current magnitude is greater than the second maximum current; and changes from the disable state to the enable state when the current direction changes from the second direction to the first direction and the current magnitude is greater than a first minimum current. The second control signal: changes from the enable state to the disable state when the current direction is in the first direction and the current magnitude is greater than the first maximum current; and changes from the disable state to the enable state when the current direction changes from the first direction to the second direction and the current magnitude is greater than a second minimum current.

In accordance with yet another embodiment of the present invention, a method of providing a bidirectional current control suitable for use in controlling the charging and discharging of rechargeable battery cells includes the steps of: detecting a magnitude and a direction of a current being conducted through a series circuit; maintaining the series circuit in an on state which provides a bidirectional current conductivity when the current magnitude is less than first and second maximum currents; switching the series circuit from the on state to a first partially off state which provides a first unidirectional current conductivity in a first direction when the current direction is in the second direction and the current magnitude is greater than the second maximum current; switching the series circuit from the first partially off state to the on state when the current direction changes from the second direction to the first direction and the current magnitude is greater than a first minimum current; switching the series circuit from the on state to a second partially off state which provides a second unidirectional current conductivity in a second direction which is opposite to the first direction when the current direction is in the first direction and the current magnitude is greater than the first maximum current; and switching the series circuit from the second partially off state to the on state when the current direction changes from the first direction to the second direction and the current magnitude is greater than a second minimum current.

In accordance with still yet another embodiment of the present invention, a bidirectional current control circuit suitable for use in controlling the charging and discharging of rechargeable battery cells includes a current control circuit and a switch controller. The current control circuit is configured to selectively conduct a current and in accordance therewith provide a plurality of signals which together identify a voltage across the current control circuit due to the conduction of the current. Such voltage has a first polarity when the conduction of the current is in a first direction, and a second polarity which is opposite to the first polarity when the conduction of the current is in a second direction which is opposite to the first direction. The current control circuit includes an on state which provides a bidirectional current conductivity, a first partially off state which provides a first unidirectional current conductivity in the first direction, and a second partially off state which provides a second unidirectional current conductivity in the second direction. The switch controller is coupled to the current control circuit and is configured to: maintain the current control circuit in the on state when the voltage is less than first and second maximum voltages; switch the current control circuit from the on state to the first partially off state when the voltage has the second polarity and is greater than the second maximum voltage; switch the current control circuit from the first partially off state to the on state when the voltage polarity changes from the second polarity to the first polarity and the voltage is greater than a first minimum voltage; switch the current control circuit from the on state to the second partially off state when the voltage has the first polarity and is greater than the first maximum voltage; and switch the current control circuit from the second partially off state to the on state when the voltage polarity changes from the first polarity to the second polarity and the voltage is greater than a second minimum voltage.

In accordance with another embodiment of the present invention, a bidirectional current control circuit suitable for use in controlling the charging and discharging of rechargeable battery cells includes two switch circuits, two terminals and a switch controller. The first switch circuit has a first on state which provides a first bidirectional current conductivity and has a first off state which provides a first unidirectional current conductivity in a first direction. The second switch circuit is coupled in series with the first switch circuit and has a second on state which provides a second bidirectional current conductivity and has a second off state which provides a second unidirectional current conductivity in a second direction which is opposite to the first direction. The two terminals are coupled at opposing ends of the series coupling of the two switch circuits and are configured to provide a set of signals which together identify a voltage across the series coupling of the two switch circuits due to a conduction of a current by such switch circuits. Such voltage has a first polarity when the conduction of the current is in the first direction, and a second polarity which is opposite to the first polarity when the conduction of the current is in the second direction. The switch controller is coupled to the switch circuits and the terminals and is configured to: maintain the first and second switch circuits in the first and second on states when the voltage is less than first and second maximum voltages; switch the first switch circuit from the first on state to the first off state when the voltage has the second polarity and is greater than the second maximum voltage; switch the first switch circuit from the first off state to the first on state when the voltage polarity changes from the second polarity to the first polarity and the voltage is greater than a first minimum voltage; switch the second switch circuit from the second on state to the second off state when the voltage has the first polarity and is greater than the first maximum voltage; and switch the second switch circuit from the second off state to the second on state when the voltage polarity changes from the first polarity to the second polarity and the voltage is greater than a second minimum voltage.

In accordance with still another embodiment of the present invention, a bidirectional current control circuit suitable for use in controlling the charging and discharging of rechargeable battery cells includes two switch circuits, two terminals and a switch controller. The first switch circuit is configured to receive a first control signal with enable and disable states and in accordance therewith provide a first bidirectional current conductivity and a first unidirectional current conductivity in a first direction, respectively. The second switch circuit is coupled in series with the first switch circuit and is configured to receive a second control signal with enable and disable states and in accordance therewith provide a second bidirectional current conductivity and a second unidirectional current conductivity in a second direction, respectively, wherein the second direction is opposite to the first direction. The two terminals are coupled at opposing ends of the series coupling of the two switch circuits and are configured to provide a set of signals which together identify a voltage across the series coupling of the two switch circuits due to a conduction of a current by such switch circuits. Such voltage has a first polarity when the conduction of the current is in the first direction, and a second polarity which is opposite to the first polarity when the conduction of the current is in the second direction. The switch controller is coupled to the two switch circuits and the two terminals and is configured to receive the set of signals and in accordance therewith provide the first and second control signals, such that: the first and second control signals are maintained in the first and second control signal enable states when the voltage is less than first and second maximum voltages; the first control signal changes from the enable state to the disable state when the voltage has the second polarity and is greater than the second maximum voltage; the first control signal changes from the disable state to the enable state when the voltage polarity changes from the second polarity to the first polarity and the voltage is greater than a first minimum voltage; the second control signal changes from the enable state to the disable state when the voltage has the first polarity and is greater than the first maximum voltage; and the second control signal changes from the disable state to the enable state when the voltage polarity changes from the first polarity to the second polarity and the voltage is greater than a second minimum voltage.

In accordance with yet another embodiment of the present invention, a method of providing a bidirectional current control suitable for use in controlling the charging and discharging of rechargeable battery cells includes the steps of: detecting a magnitude and a polarity of a voltage across a series circuit; maintaining the series circuit in an on state which provides a bidirectional current conductivity when the voltage magnitude is less than first and second maximum voltages; switching the series circuit from the on state to a first partially off state which provides a first unidirectional current conductivity in a first direction when the voltage has the second polarity and the voltage magnitude is greater than the second maximum voltage; switching the series circuit from the first partially off state to the on state when the voltage polarity changes from the second polarity to the first polarity and the voltage magnitude is greater than a first minimum voltage; switching the series circuit from the on state to a second partially off state which provides a second unidirectional current conductivity in a second direction which is opposite to the first direction when the voltage has the first polarity and the voltage magnitude is greater than the first maximum voltage; and switching the series circuit from the second partially off state to the on state when the voltage polarity changes from the first polarity to the second polarity and the voltage magnitude is greater than a second minimum voltage.

These and other features and advantages of the present invention will be understood upon consideration of the following detailed description of the invention and the accompanying drawings.

FIG. 1 is a functional block diagram of a bidirectional current control circuit in accordance with one embodiment of the present invention.

FIG. 2 is a schematic and functional block diagram of a bidirectional current control circuit suitable for use in controlling the charging and discharging of rechargeable battery cells in accordance with one embodiment of the present invention.

FIG. 3 is a cross-sectional representation of one of the protection MOSFET devices in the circuit of FIG. 2.

FIG. 4 is a finctional block diagram of a bidirectional current control circuit in accordance with another embodiment of the present invention.

FIG. 5 is a schematic and functional block diagram of a bidirectional current control circuit suitable for use in controlling the charging and discharging of rechargeable battery cells in accordance with another embodiment of the present invention.

Referring to FIG. 1, a bidirectional current control circuit 10 in accordance with one embodiment of the present invention includes a current control circuit 12, a current detection circuit 14 and a switch controller 16, interconnected substantially as shown. Currents 11, 13, 15 flow in directions 1 and 2 into, out of and between the current control circuit 12 and current detection circuit 14, as well as between the current control circuit 12 and an external load (not shown) and/or power source (not shown). The current detection circuit 14 provides one or more current detection signals 17 to the switch controller 16 which, in turn, provides one or more control signals 19 to the current control circuit 12.

Referring to FIG. 2, one embodiment 110 of the current control circuit 10 of FIG. 1 is particularly suitable for use in controlling the charging and discharging of rechargeable battery cells 202. This control circuit 110 includes two drain-connected MOSFETs 152, 154 and a low value precision resistor 114, all connected in series with the battery cells 202. (Alternatively, the MOSFETs 152, 154 could be connected source-to-source.) Nodes 111 and 203 serve as the terminals for the battery pack containing the battery cells 202 and control circuit 110.

The current which is to be monitored during charging and discharging of the battery cells 202 flows in directions 1 and 2 through the MOSFETs 152, 154 and resistor 114 during charging and discharging, respectively. In both cases, a voltage is generated across the resistor 114 at nodes 113 and 115. This differential voltage provides two voltage signals 117a, 117b which together serve as a differential current detection signal for the switch controller 116. If these voltage signals 117a, 117b identify an over-current condition, i.e., the current-times-resistance voltage drop is higher than some predetermined threshold level caused by excessive charging or discharging current, the switch controller 116 turns off the appropriate MOSFET 152/154 with the control signal 119a/119c driving the gate terminal of such MOSFET 152/154.

For example, if an excessive current, as identified by the voltage signals 117a, 117b at the resistor 114, flows in direction 1 during charging the first MOSFET 152 is turned off by the switch controller 116 by making the voltage difference between voltage signals 119a and 119b equal to zero. Conversely, during an over-current condition when discharging the battery cells 202, the second MOSFET 154 is turned off by the switch controller 116 by making the voltage difference between voltages 119c and 117b equal to zero.

Even when otherwise turned off, however, each MOSFET 152, 154 nonetheless allows current to flow in the opposite direction through its respective body diode 162, 164, provided that the other MOSFET is still turned on. For example, if an over-current condition occurred during current flow in direction 1, thereby resulting in the first MOSFET 152 being turned off, a discharge current in direction 2 is still possible through the body diode 162 of such MOSFET 152 since the second MOSFET 154 is still turned on. This allows the battery cells 202 which should not accept any more charge to be prevented from accepting charge currents (direction 1) while nonetheless still allowing discharge currents (direction 2) to flow.

The presence of a discharge current (direction 2) following a fault condition and shutdown of the first MOSFET 152 during charge current flow (direction 1) indicates, due to the opposite current flow, that a safe operating condition has been restored. The resulting opposite voltage drop across the resistor 114 identifies this return to a safe operating condition to the switch controller 116 which, in turn, returns the previously turned off MOSFET 152 back to its conductive state by driving its gate control voltage signal 119a back to a sufficiently positive state (i.e., sufficiently beyond the threshold voltage V.sub.T of the MOSFET 152).

Hence, when each MOSFET 152, 154 is turned on, it is in an on state which provides bidirectional current conductivity. When each MOSFET 152, 154 is turned off, it is actually in a partially off state in the sense that a unidirectional current conductivity is provided due to the presence of a corresponding body diode 162, 164.

Referring to FIG. 3, the body diode 162 of the first MOSFET 152 is identified. Such a body diode is created when the body 252 and the source terminal are at substantially equal voltage potentials.

Referring to FIG. 4, a bidirectional current control circuit 30 in accordance with another embodiment of the present invention includes a current control circuit 12 and a switch controller 36, interconnected substantially as shown. As in the circuit 10 of FIG. 1, currents 11, 13 flow in directions 1 and 2 into and out of the current control circuit 12, as well as between the current control circuit 12 and an external load (not shown) and/or power source (not shown). The current control circuit 12 provides detection signals 37 to the switch controller 36 which, in turn, provides one or more control signals 19 to the current control circuit 12.

Referring to FIG. 5, one embodiment 310 of the current control circuit 30 of FIG. 4 is particularly suitable for use in controlling the charging and discharging of rechargeable battery cells 202. This control circuit 310 includes two drain-connected MOSFETs 152, 154 connected in series with the battery cells 202. (Alternatively, the MOSFETs 152, 154 could be connected source-to-source.) Nodes 311 and 203 serve as the terminals for the battery pack containing the battery cells 202 and control circuit 310.

The current which is to be monitored during charging and discharging of the battery cells 202 flows in directions 1 and 2 through the MOSFETs 152, 154 during charging and discharging, respectively. In both cases, a voltage is generated across the MOSFETs 152, 154 at nodes 311 and 313. This differential voltage provides two voltage signals 337a, 337b which together serve as a differential detection signal for the switch controller 336. If these voltage signals 337a, 337b identify an over-current condition, i.e., the voltage drop across the MOSFETs 152, 154 (the current multiplied by the sum of the drain-to-source resistances of the MOSFETs 152, 154) is higher than some predetermined threshold level caused by excessive charging or discharging current, the switch controller 336 turns off the appropriate MOSFET 152/154 with the control signal 119a/119c driving the gate terminal of such MOSFET 152/154.

For example, if an excessive current, as identified by the voltage 337a/337b across the MOSFETs 152, 154, flows in direction 1 during charging the first MOSFET 152 is turned off by the switch controller 336 by making the voltage difference between gate voltage 119a and source voltage 119b equal to zero. Conversely, during an over-current condition when discharging the battery cells 202, the second MOSFET 154 is turned off by the switch controller 336 by making the voltage difference between gate voltage 119c and source voltage 119d equal to zero. (The use of the signal lines connected between the switch controller 336 and nodes 311 and 313 are multiplexed in the sense that during its input, or voltage sensing, mode the switch controller 336 uses them to monitor the voltages 337a, 337b at the source terminals of the MOSFETs 152, 154, and during its output mode the switch controller 336 uses them to convey the control voltages 119a, 119b, 119c, 119d for the MOSFETs 152, 154.)

As discussed above, even when otherwise turned off, each MOSFET 152, 154 allows current to, flow in the opposite direction through its respective body diode 162, 164, provided that the other MOSFET is still turned on. For example, if an over-current condition occurred during current flow in direction 1, thereby resulting in the first MOSFET 152 being turned off, a discharge current in direction 2 is still possible through the body diode 162 of such MOSFET 152 since the second MOSFET 154 is still turned on. This allows the battery cells 202 which should not accept any more charge to be prevented from accepting charge currents (direction 1) while nonetheless still allowing discharge currents (direction 2) to flow.

The presence of a discharge current (direction 2) following a fault condition and shutdown of the first MOSFET 152 during charge current flow (direction 1) indicates, due to the opposite current flow, that a safe operating condition has been restored. The resulting opposite voltage drop across the associated body diode 162 identifies this return to a safe operating condition to the switch controller 336 which, in turn, returns the previously turned off MOSFET 152 back to its conductive state by driving its gate control voltage signal 119a back to a sufficiently positive state (i.e., sufficiently beyond the threshold voltage V.sub.T of the MOSFET 152). This return to a safe operating condition is recognized by the switch controller 336 because the voltage drop across the conductive body diode 162 is several tenths of a volt, as opposed to the voltage drop of several thousandths of a volt normally seen across a fully conductive (i.e., turned on) MOSFET with its correspondingly low drain-to-source resistance, and because the polarity of such voltage is now reversed from the original voltage which was present when the original over-current condition occurred.

The switch controllers 116, 336 can be implemented in accordance with any one of many techniques well known in the art for monitoring differential voltages and generating corresponding output voltages. Examples of circuits appropriate for use as the switch controller 336 can be found in U.S. Pat. No. 5,534,788, and in commonly assigned, co-pending U.S. pat. appplication Ser. No. 08/801,162, the disclosures of which are both incorporated herein by reference.

Various other modifications and alterations in the structure and method of operation of this invention will be apparent to those skilled in the art without departing from the scope and spirit of the invention. Although the invention has been described in connection with specific preferred embodiments, it should be understood that the invention as claimed should not be unduly limited to such specific embodiments. It is intended that the following claims define the scope of the present invention and that structures and methods within the scope of these claims and their equivalents be covered thereby.