Abstract:
A protection circuit for a primary battery pack provides the battery pack with reverse charge protection. The protection circuit features a metal oxide semiconductor field effect transistor (MOSFET) with a steady state source to drain voltage. The source to drain voltage is controlled through a feedback loop that includes an operational amplifier. The MOSFET is configured in series with one or more battery cells allowing current to flow from the cells and preventing current from flowing to the cells. The MOSFET provides reverse charge protection with a small forward voltage drop and a small reverse charge leakage current.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority to and the benefit of U.S. Provisional Application Ser. No. 60/908,354, filed on Mar. 27, 2007, and entitled BATTERY PROTECTION CIRCUIT FOR LITHIUM CARBON MONOFLUORIDE BATTERY. 
     
    
     FIELD OF INVENTION 
       [0002]    The present invention relates to battery protection circuits. More particularly, the invention relates to battery protection circuits for battery packs featuring lithium carbon monofluoride (Li—CFx) battery cells. 
       BACKGROUND OF THE INVENTION 
       [0003]    Battery packs provide electrical power for electrical loads. Primary battery packs feature battery cells that deliver power to electrical loads by irreversibly converting potential chemical energy into electrical energy. Secondary battery packs feature battery cells that reversibly convert chemical energy into electrical energy and may be “recharged” converting electrical energy back into potential chemical energy. Battery packs having one or more Li—CFx battery cells are not rechargeable and thus are primary battery packs. 
         [0004]    Li—CFx cells have high energy density, a long shelf life and are light in weight. This makes battery packs featuring Li—CFx cells ideal for many applications, including military applications. Primary battery packs with Li—CFx cells are often packaged to look and feel like the secondary battery packs used to drive the same or similar electrical loads. The similar packaging presents a concern that a battery pack with Li—CFx cells will be improperly placed into a secondary battery pack charger leading to cell leakage or an explosion. 
         [0005]    To protect the cells of a primary battery pack from receiving an inadvertent and potentially dangerous charge, a Schottky diode may be introduced. The Schottky diode is placed in series with cells and oriented to allow current (charge) to flow from the cells and prevent current from flowing to the cells. The Schottky diode, however, introduces an undesirable forward voltage drop of about 0.15 to 0.45 Volts. The forward voltage drop for Schottky diodes having desirable small reverse voltage leakage currents is especially high (often greater than 0.35 volts). This high Schottky diode forward voltage drop results in less cell voltage being delivered to the electrical load. 
         [0006]    The use of a Schottky diode is especially problematic for Li—CFx battery packs, particularly when used in harsh environments. Li—CFx battery packs may feature two cells of about 3.0 volts arranged in series to provide a 6.0 volt battery pack. The small number of cells (i.e., two) and the large cathode voltage delays characteristic of Li—CFx cells make it difficult to introduce a Schottky diode and meet many low-temperature operating requirements, such as MIL-PRF-49471B. 
         [0007]    Those skilled in the art will appreciate that there is a need for a protective circuit for primary battery packs that does not introduce a large forward voltage drop or introduce a large voltage delay. 
       SUMMARY OF THE INVENTION 
       [0008]    A reverse charge protection circuit features a metal oxide semiconductor field effect transistor (MOSFET). The MOSFET is configured to be connected in series with one or more battery cells of a primary battery pack. The MOSFET is biased through a feedback control loop having a control element. The control element drives the MOSFET source to drain voltage to a predetermined constant voltage by controlling the MOSFET gate voltage. The MOSFET operates principally in the ohmic region conducting current away from the battery cells and preventing current from flowing to the battery cells. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWING FIGURES 
         [0009]    A more complete understanding of the present invention may be derived by referring to the detailed description and claims when considered in connection with the drawing figures, where like reference numbers refer to similar elements throughout the figures, and: 
           [0010]      FIG. 1  is a diagram of an exemplary embodiment of a primary battery pack according to an embodiment of the present invention; 
           [0011]      FIG. 2  is a diagram of an exemplary embodiment of a battery protection circuit for battery cells of a primary battery pack featuring an N-channel MOSFET; and 
           [0012]      FIG. 3  is a diagram of an exemplary embodiment of a battery protection circuit for battery cells of a primary battery pack featuring a P-channel MOSFET. 
       
    
    
     DETAILED DESCRIPTION 
       [0013]    The following description is of exemplary embodiments of the invention only, and is not intended to limit the scope, applicability, or configuration of the invention in any way. Rather, the following description is intended to provide a convenient illustration for implementing various exemplary embodiments of the invention. As will become apparent, various changes may be made in the function and arrangement of the elements described in these embodiments without departing from the scope of the invention as set forth in the appended claims. 
         [0014]      FIG. 1  illustrates a battery pack  100  according to an exemplary embodiment of the present invention. The battery pack  100  comprises battery cells  102  electrically connected with a circuit  104 . 
         [0015]    The battery cells  102  comprise a first cell  106  and second cell  108  connected in series. 
         [0016]    The circuit  104  comprises a MOSFET  110  having a gate  112 , a drain  114  and a source  116 . The drain  114  is connected with a negative terminal  118  of a power supply  120 . The power supply  120  has a positive terminal  122  that is connected with the inverting input  124  of an operational amplifier  126 . The operational amplifier  126  has a non-inverting input  128  connected with the source  116  of the MOSFET  110  and an output  130  connected with the gate  112  of the MOSFET  110 . 
         [0017]    The circuit  104  has a positive input  134  connected with the first cell  106  and a negative input  136  connected with the second cell  108 . The positive input  134  is also connected with a positive power input  142  of the operational amplifier  126 . The circuit  104  also has a positive output  138  electrically equivalent to the positive input  134 , and a negative output  140 . The negative output  140  is connected with the negative power input  144  of the operational amplifier  126 . 
         [0018]    The circuit  104  is configured to prevent current from flowing to the battery cells  102  from the outputs  138 ,  140 . The MOSFET  110  performing the current gate-keeping function is an n-channel MOSFET configured with source  116  and drain  114  in communication with the battery cells  102 . The circuit  104  also features a control loop configured to provide a small source  116  to drain  114  voltage and a thus a small overall circuit  104  voltage drop between inputs  134 ,  136  and outputs  138 ,  140 . The circuit  104  is configured to function in four primary states: namely load attached, no load attached, charger connection, and forced discharge. 
         [0019]    In this embodiment of the battery pack  100  the battery cells  102  are comprised of two lithium carbon monofluoride (Li—CFx) battery cells  106 ,  108 . Alternate embodiments feature lithium manganese dioxide (Li—MnO 2 ) battery cells as well as other types of non-rechargeable battery cells. Alternate embodiments may also feature one or more battery cells  102 . The battery cells  102  may be configured in series, in parallel, or in a series and parallel topology. 
         [0020]    The circuit  104  operates in a quiescent state when outputs  138 ,  140  are open (i.e. no load is applied). In the quiescent state, the power supply  120  voltage determines the source  116  to drain  114  voltage of the MOSFET  110 . The MOSFET  110  is configured to operate in the ohmic region with the MOSFET  110  conducting the op-amp  126  quiescent supply current from the source  116  to the drain  114 . The MOSFET  110  source  116  to drain  114  voltage is controlled through a feedback loop. 
         [0021]    In this embodiment, the circuit  104  features an n-channel MOSFET  110 . In an exemplary embodiment, the MOSFET  110  is an International Rectifier IRF7470. Alternate embodiments feature bipolar junction transistors, p-channel MOSFETS as well as other types of transistors as would be known to one skilled in the art. 
         [0022]    The feedback loop is configured to operate with the power supply  120  supplying a reference voltage to the inverting input  124  of the operational amplifier  126 . In one embodiment, the reference voltage supplied is 100 mV greater than the MOSFET  110  drain  114  voltage. In another embodiment, the reference voltage supplied is from about 10 mV to about 200 mV volts greater than the MOSFET  110  drain  114  voltage. In addition, the reference voltage may be any voltage suitable to operate the MOSFET  110  in an ohmic state. The lower range of the reference voltage is dependent on the accuracy of the circuit  104 ; a more accurate the circuit can be operated with a lower reference voltage. The non-inverting input  128  of the operational amplifier  126  is fed with the source  116  voltage of the MOSFET  110 . 
         [0023]    In this embodiment the operational amplifier  126  is featured as a control element in the feedback control loop. Other embodiments feature other types of control elements, such as transistor circuits, microprocessors or the like. Moreover, any suitable control element as would be known to one skilled in art may be used. 
         [0024]    The operational amplifier  126  is configured to be powered by the battery cells  102 . The operational amplifier  126  compares the source  116  voltage with the reference voltage and outputs a control voltage. The control voltage of the operational amplifier  126  drives the gate  112  of the MOSFET  110  completing the feedback loop. 
         [0025]    In this embodiment the operational amplifier  126  and the MOSFET  110  are powered by the battery cells  102 . Alternate embodiments may feature an independent power source. In one embodiment, the operational amplifier  126  and the MOSFET  110  are both powered by an independent power source. In another embodiment, the operational amplifier  126  and the MOSFET  110  are powered by separate independent power sources. 
         [0026]    The feedback loop is configured to control the source  116  to drain  114  voltage of the MOSFET  110  in normal operation by driving the source  116  to drain  114  voltage to the power supply  120  voltage. The open circuit output  138 ,  140  voltage of the circuit is thus less than the battery cells  102  voltage by the voltage amount of the power supply. In one embodiment, the power supply  120  voltage is 100 mV, resulting in the open circuit output  138 ,  140  voltage of the circuit being 100 mV less than the battery cells  102  voltage. 
         [0027]    When a load is properly applied between the outputs  138 ,  140  of the circuit  104 , the feedback loop insures that the source  116  to drain  114  voltage of the MOSFET  110  remains 100 mV. The MOSFET  110  operates in an ohmic region conducting current from source  116  to drain  114 . Current flows from the battery cells  102  through the load (not shown), through the MOSFET  110  and back to the battery cells  102 . 
         [0028]    When the battery pack  100  is improperly placed in a battery charger (not shown), current will begin to flow from the circuit outputs  138 ,  140  toward the circuit inputs  134 ,  136  and toward the battery cells  102 . This will force the MOSFET  110  source  116  to drain  114  voltage to drop below the power supply  120  voltage with the operational amplifier  126  increasing the source  116  to drain  114  resistance until the MOSFET  110  is turned off. With the MOSFET  110  turned off, the MOSFET  110  will not conduct current, creating an open circuit  104  between the battery cells  102  and the battery charger and protecting the battery cells  102  from a charging current. 
         [0029]    When the battery pack  100  undergoes a forced discharge, the control loop will maintain a 100 mV source  116  to drain  114  voltage with the battery cells  102  discharging through the circuit  104 . When the battery cells  102  no longer have enough power to power the MOSFET  110 , forced current will flow through the MOSFET body diode. 
         [0030]      FIG. 2  illustrates a battery cell protection circuit  200  featuring an N-Channel MOSFET according to an embodiment of the present invention. The battery cell protection circuit  200  has a positive input  202  and a negative input  204 . The battery cell protection circuit  200  also has a positive output  206  and a negative output  208 . The positive input  202  is connected with the positive output  206  through a fuse  211 . 
         [0031]    A MOSFET  210  having a gate  212 , a drain  214  and a source  216  is configured with the source  216  connected to the negative output  208  of the battery protection circuit  200  and the drain  214  connected with the negative input  204  of the battery protection circuit  200 . The source  216  is also connected with a non-inverting input  218  of an operational amplifier  220 . The gate  212  is connected with the inverting input  222  of the operational amplifier  220  through capacitor C 4    224 . The output of the operational amplifier  226  is connected with the gate  212 . 
         [0032]    A voltage divider comprised of resistors R 1    228 , R 2    230 , and R 3    232  is configured in series and extends from the fuse  211  to the negative input  204  of the battery cell protection circuit  200 . The voltage divider is connected with the inverting input  222  of the operational amplifier  220  between resistors R 2    230  and R 3    232 . 
         [0033]    The operational amplifier  220  is configured with a positive power input  234  connected with the voltage divider between resistors R 1    228  and R 2    230 . The operational amplifier  220  also has a negative power input  236  connected with the source  216  of the MOSFET  210 . A Zener diode  238  and capacitor C 1    240  are configured in parallel with the positive power input  234  and negative power input  236  of the operational amplifier  220 . 
         [0034]    Capacitors, C 2    242  and C 3    243  are configured in series and extend from the positive power output  206  to the negative power output  208  of the battery cell protection circuit  200 . Capacitors C 2    242  and C 3    244  provide the battery cell protection circuit with electrostatic discharge protection. 
         [0035]    The battery cell protection circuit  200  is configured to protect primary battery cells from a charging current. The MOSFET  210  conducts current from the source  216  to drain  214  when a load is connected to the outputs  206 ,  208  of the battery protection circuit  200 . In addition, the MOSFET  210  is also configured to turn off when outputs  206 ,  208  are connected to a charger. When the battery cell protection circuit  200  is improperly placed in a battery charger (not shown), current will begin to flow from the circuit outputs  206 ,  208  toward the circuit inputs  202 ,  204  and toward the battery cells. This will force the MOSFET  210  source  216  to drain  214  voltage to drop and the operational amplifier  220  to increase the source  216  to drain  214  resistance until the MOSFET  210  is turned off. With the MOSFET  210  turned off, the MOSFET  210  will not conduct current, creating an open circuit between the battery cells and the battery charger and protecting the battery cells from a charging current. 
         [0036]    In this embodiment, the battery cell protection circuit  200  features an n-channel MOSFET  210 . Alternate embodiments may feature bipolar junction transistors, p-channel MOSFETS as well as other types of transistors as would be known to one skilled in the art. 
         [0037]    The voltage divider comprising the cell voltage of battery cells (Cellvoltage) and resistors R 1 , R 2  and R 3  determines the source  216  to drain  214  voltage of the MOSFET  210 . The voltage divider provides 
         [0000]    
       
         
           
             
               
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         [0000]    volts to the inverting input  222  of the operational amplifier  220 . In this exemplary embodiment, resistor R 1  is 47 KΩ, resistor R 2  is 10 MΩ and resistor R 3  is 100 KΩ. Thus 0.03 volts (30 mV) are provided to the inverting input  222  of the operational amplifier  220  when a 3 volt cell drives inputs  202 ,  204 . 
         [0038]    The non-inverting input  218  of the operational amplifier  220  is fed with the source  216  voltage of the MOSFET  210 . The output  226  of the operational amplifier  220  drives the gate  212  of the MOSFET  210 , forming a feedback loop. The operational amplifier  220  compares the source  216  voltage with the voltage at resistor R 3    232  and provides a control voltage to the gate  212  of the MOSFET  210 . The control voltage drives the source  216  to drain  214  voltage of the MOSFET  210  to the approximate voltage drop observed across resistor R 3    232 . 
         [0039]    Capacitor C 4    224  is configured between the output  226  of the operational amplifier and the inverting input  222  of the operational amplifier. Capacitor C 4  provides the feedback loop with stability by dampening the feedback response. 
         [0040]    In this embodiment, capacitor C 4  is 27 nF. In alternate embodiments C 4  may be larger or smaller, for example in the range of about 10 nF to about 100 nF. Moreover, capacitor C 4  may be of any capacitance which introduces effective damping of the feedback response. In another embodiment, a battery cell protection circuit  200  may not include capacitor C 4 . 
         [0041]    Zener diode  238  provides over-voltage protection for the operational amplifier  220 . When the Zener diode  238  reaches its breakdown voltage it will maintain the breakdown voltage across the diode and hold the supply voltage of operational amplifier  220  constant, protecting it from an over-voltage. In this embodiment the Zener diode has a breakdown voltage of 12 V. Other embodiments may feature Zener diodes with other breakdown voltages. The breakdown voltage is determined by the maximum allowable supply voltage of operational amplifier  220 . For example, the Zener diode may have a breakdown voltage in the range of about 6 volts to about 18 volts. In another embodiment, a battery cell protection circuit  200  may not include a Zener diode for over-voltage protection. 
         [0042]    Capacitor C 1    240  provides the battery cell protection circuit  200  with stability by sourcing operational amplifier  220  power current spikes that would not otherwise flow through resistor R 1 . The stability is provided when operational amplifier  220  draws more current than is available through R 1 , the additional current is supplied by a charged C 1 . In one embodiment, C 1    240  is larger than C 4    224  to maintain the circuit stability. 
         [0043]    In this embodiment, capacitors C 1 , C 2  and C 3  are each 100 nF. In alternate embodiments C 1  C 2  and C 3  may be larger or smaller, for example in the range of about 50 nF to about 1 μF. In another embodiment, a battery cell protection circuit  200  may not include capacitors C 1 , C 2 , or C 3  or any combinations of C 1 , C 2 , and/or C 3 . 
         [0044]      FIG. 3  illustrates a battery cell protection circuit  300  featuring a P-Channel MOSFET according to an embodiment of the present invention. The battery cell protection circuit  300  has a positive input  302  and a negative input  304 . The battery cell protection circuit  300  also has a positive output  306  and a negative output  308 . The positive input  302  is connected with the positive output  306  through a fuse  311 . 
         [0045]    A MOSFET  310  having a gate  312 , a drain  314  and a source  316  is configured with the source  316  connected to the positive output  306  of the battery protection circuit  300  and the drain  314  connected with the positive input  302  through the fuse  311 . The source  316  is also connected with a non-inverting input  318  of an operational amplifier  320 . The gate  312  is connected with an inverting input  322  of the operational amplifier  320  through capacitor C 4    324 . The output  326  of the operational amplifier  320  is connected with the gate  312 . 
         [0046]    A voltage divider comprised of resistors R 1    328 , R 2    330 , and R 3    332  is configured in series and extends from the fuse  311  to the negative input  304  of the battery cell protection circuit  300 . The voltage divider is connected with the inverting input  322  of the operational amplifier  320  between resistors R 2    330  and R 3    332 . 
         [0047]    The operational amplifier  320  is configured with a positive power input  334  connected with the source  316  of the MOSFET  310 . The operational amplifier  320  also has a negative power input  336  connected with the voltage divider between resistors R 1    328  and R 2    330 . A Zener diode  338  and capacitor C 1    340  are configured in parallel with the positive power input  334  and negative power input  336  of the operational amplifier  320 . 
         [0048]    Capacitors, C 2    342  and C 3    344  are configured in series and extend from the positive power output  306  to the negative power output  308  of the battery cell protection circuit  300 . 
         [0049]    The battery cell protection circuit  300  is configured to protect primary battery cells from a charging current. The MOSFET  310  conducts current from the drain  314  to the source  316  when no load or a proper load is connected to the outputs  306 ,  308  of the battery protection circuit  300 . The MOSFET  310  is also configured to turn off when outputs  306 ,  308  are connected to a charger. A charger at output  306 ,  308  creates a positive source  316  to drain  314  voltage which causes the op-amp to turn off the MOSFET  310 . 
         [0050]    In this embodiment, the battery cell protection circuit  300  features a p-channel MOSFET  310 . Alternate embodiments may feature bipolar junction transistors, n-channel MOSFETS as well as other types of transistors as would be known to one skilled in the art. 
         [0051]    The voltage divider comprising the cell voltage of the battery cells (Cellvoltage) and resistors R 1 , R 2  and R 3  determines the drain  314  to source  316  voltage of the MOSFET  310 . The voltage divider provides 
         [0000]    
       
         
           
             Cellvoltage 
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         [0000]    volts to the inverting input  322  of the operational amplifier  320 . In this exemplary embodiment, resistor R 1  is 47 KΩ, resistor R 2  is 10 MΩ and resistor R 3  is 100 KΩ. Thus, in accordance with an exemplary embodiment, (Cellvoltage −0.06) volts are provided to the inverting input  322  of the operational amplifier  320  when two 3 volt cells in series drive inputs  302 ,  304 . 
         [0052]    The non-inverting input  318  of the operational amplifier  320  is fed with the source  316  voltage of the MOSFET  310 . The output  326  of the operational amplifier  320  drives the gate  312  of the MOSFET  310  forming a feedback loop. The operational amplifier  320  compares the source  316  voltage with resistor R 3    332  voltage divider voltage and provides a control voltage to the gate  312  of the MOSFET  310 . The control voltage drives the drain  314  to source  316  voltage of the MOSFET  310  to the voltage drop observed across resistor R 3    332 . 
         [0053]    Capacitor C 4    324  is configured between the output  326  of the operational amplifier and the non-inverting input  322  of the operational amplifier. Capacitor C 4  provides the feedback loop with stability by dampening the feedback response. 
         [0054]    In accordance with one exemplary embodiment, capacitor C 4  is 27 nF. In alternate embodiments C 4  may be larger or smaller, for example in the range of about 10 nF to about 100 nF. Moreover, capacitor C 4  may be of any capacitance which introduces effective damping of the feedback response. In accordance with another exemplary embodiment, a battery cell protection circuit  300  may not include capacitor C 4 . 
         [0055]    Zener diode  338  provides over-voltage protection for the operational amplifier  320 . When the Zener diode  338  reaches its breakdown voltage it will maintain the breakdown voltage across the diode and hold the supply voltage of operational amplifier  320  constant, protecting it from an over-voltage. In this embodiment the Zener diode has a breakdown voltage of 12 V. Other embodiments may feature Zener diodes with other breakdown voltages. The breakdown voltage is determined by the maximum allowable supply voltage of operational amplifier  320 . For example, in accordance with one exemplary embodiment, a Zener diode may have a breakdown voltage in the range of about 6 volts to about 18 volts. In accordance with another exemplary embodiment, a battery cell protection circuit  300  may not include a Zener diode for over-voltage protection. 
         [0056]    Capacitor C 1    340  provides the battery cell protection circuit  300  with stability by sourcing operational amplifier  320  power current spikes that would not otherwise flow through resistor R 1    328 . The stability is provided when operational amplifier  320  draws more current than is available through R 1    328 , the additional current is supplied by a charged C 1    340 . In one embodiment, C 1    340  is larger than C 4    324  to maintain the circuit stability. 
         [0057]    Capacitors C 2    342  and C 3    344  are configured in series and extend from positive output  306  to negative output  308 . Capacitors C 2    342  and C 3    344  provide the battery cell protection circuit with electrostatic discharge protection. 
         [0058]    In accordance with one exemplary embodiment, capacitors C 1 , C 2  and C 3  are 100 nF. In other exemplary embodiments C 1  C 2  and C 3  may be larger or smaller, for example in the ranges of for example in the range of about 50 nF to about 1 μF. In accordance with still another exemplary embodiment, a battery cell protection circuit  300  may not include capacitors C 1 , C 2 , or C 3  or any combinations of C 1 , C 2 , and/or C 3 . 
         [0059]    In accordance with various embodiments of the instant invention, battery protection circuits using a FET have a low voltage drop, allowing more battery cell voltage to be delivered to a load and the load circuit to have a higher voltage. This higher voltage increases the power output of the battery, resulting in better low temperature operation and less voltage delay when compared to general battery protection circuits using a diode. Additionally, the voltage drop across the FET is more consistent over a temperature range than the voltage drop across a diode. 
         [0060]    Those skilled in the art will recognize there are many equivalent circuits to those disclosed with alternate circuit topologies, circuit elements, and element sizes and functions. The scope of the disclosed invention is not limited to the exemplary embodiments described but embraces all embodiments and equivalents recited in the claims. 
         [0061]    Finally, it should be understood that various principles of the invention have been described in illustrative embodiments only, and that many combinations and modifications of the above-described structures, arrangements, proportions, elements, materials and components, used in the practice of the invention, in addition to those not specifically described, may be varied and particularly adapted to specific users and their requirements without departing from those principles.