Patent Publication Number: US-2009237085-A1

Title: Voltage detection circuit

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a voltage detection circuit for detecting a voltage of a battery. 
     2. Related Background Art 
     As lithium-ion secondary batteries and the like are repeatedly charged and discharged, their charged/discharged voltages with respect to the charged/discharged time may fluctuate. For charging/discharging a secondary battery, it is necessary to forbid the battery from being charged in excess of an upper limit voltage of charging and discharged below a lower limit voltage of discharging from the viewpoints of securing the durability and safety of the battery, whereby a circuit for detecting the voltage of the battery is indispensable. Known as an example of circuits for detecting the voltage is an assembled battery voltage detection apparatus  1  described in Patent Literature 1 (Japanese Patent Application Laid-Open No. 2006-078323). The assembled battery voltage detection apparatus  1 , which is of a so-called capacitor type, comprises input-side sampling switches S 1  to S 9 , flying capacitors C 1 , C 2 , and output-side sampling switches S 10  to S 12 . 
     SUMMARY OF THE INVENTION 
     However, such an assembled battery voltage detection apparatus  1  detects the voltage through the capacitors C 1 , C 2  by alternately turning on/off the input-side sampling switches S 1  to S 9  and output-side sampling switches S 10  to S 12 , and thus fails to detect the battery voltage in real time. Also, since the input-side sampling switches S 1  to S 9  and output-side sampling switches S 10  to S 12  are necessary, a voltage detection circuit constituting the assembled battery voltage detection apparatus  1  becomes complicated. 
     In view of such a problem, it is an object of the present invention to provide a voltage detection apparatus which can detect the voltage of each battery in real time with a simple structure. 
     For achieving the above-mentioned object, the voltage detection circuit in accordance with the present invention comprises a coil connected between positive and negative terminals of a battery, and a magnetoresistive device (MR device) magnetically coupled to the coil. 
     When a current flows from the battery to the coil in the voltage detection circuit in accordance with the present invention, a magnetic field corresponding to the voltage of the battery is generated in the coil. Since the magnetoresistive device (MR device) is magnetically coupled to the coil, the direction of magnetization of a free layer in the M device varies in response to the strength of the magnetic field generated in the coil, thereby changing magnetic resistance. This makes it possible to detect the voltage of the battery according to the change of magnetic resistance in the MR device. By employing a magnetic coupler scheme including the coil and MR device, the voltage detection circuit in accordance with the present invention can detect the voltage of the battery in real time without providing and alternately turning on/off input- and output-side switches as conventionally done. 
     Preferably, the voltage detection circuit of the present invention further comprises amplification means for amplifying a signal from the MR device. By amplifying the signal from the MR device by using the amplification means, changes in voltage of the battery can accurately be detected even when the change of magnetic resistance in the MR device is very weak. 
     The voltage detection circuit in accordance with the present invention can detect the voltage of the battery in real time with a simple structure. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagram schematically illustrating a voltage detection circuit  1 A in accordance with an embodiment of the present invention; 
         FIG. 2  is a schematic diagram for explaining operations of the voltage detection circuit  1 A of  FIG. 1 ; 
         FIG. 3  is a diagram schematically illustrating a voltage detection apparatus  50  using the voltage detection circuit  1 A; 
         FIG. 4  is a flowchart for explaining operations of the voltage detection apparatus  50 ; and 
         FIG. 5  is a flowchart for explaining operations of the voltage detection apparatus  50 . 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In the following, an embodiment which seems to be the best for carrying out the present invention will be explained in detail with reference to the accompanying drawings. The same or equivalent constituents will be referred to with the same signs while omitting their overlapping descriptions.  FIG. 1  is a diagram schematically illustrating a voltage detection circuit  1 A in accordance with an embodiment of the present invention.  FIG. 2  is a schematic diagram for explaining operations of the voltage detection circuit  1 A having a magnetic coupler M C .  FIG. 3  is a diagram schematically illustrating a voltage detection apparatus  50  using the voltage detection circuit  1 A.  FIGS. 4 and 5  are charts for explaining operations of the voltage detection apparatus  50 . 
     As illustrated in  FIG. 1 , the voltage detection circuit  1 A comprises a coil  5  connected between positive and negative terminals of a battery (secondary battery)  3  through input terminals T 1 , T 2 , a resistance R 0  connected in series to the coil  5  in order to limit a current I flowing into the coil  5 , a bridge circuit  7  including a magnetoresistive device (MR device) R M  magnetically coupled to the coil  5 , and a differential amplification circuit  9  for amplifying the difference between voltages V 1 , V 2  issued from two output terminals DO 1 , DO 2  of the bridge circuit  7 . The differential amplification circuit  9  functions as amplification means for amplifying signals from the MR device R M . 
     The coil  5  generates a magnetic field in proportion to the magnitude of the current I flowing therethrough. Therefore, a magnetic field corresponding to the voltage V (which is proportional to I) of the battery  3  can be obtained by the coil  5 . 
     The bridge circuit  7 , which is electrically insulated from the coil  5 , is constituted by first and second resistance series which are connected in parallel. Between a power supply potential Vcc and a ground potential GRD, the M device R M  and a resistance R 1  are connected in series in this order as the first resistance series, while resistances R 2  and R 3  are connected in series in this order as the second resistance series. The first output terminal DO 1  for outputting the voltage V 1  is provided at a junction between the MR device R M  and resistance R 1 , while the second output terminal DO 2  for outputting the voltage V 2  is provided at a junction between the resistances R 2  and R 3 . 
     The MR device R M , an example of which is a GMR (Giant Magneto-Resistive) device, is arranged such as to oppose the coil  5 . The MR device R M  is constituted by a free layer L F  which changes its direction of magnetization in response to an external magnetic field, a fixed layer L S  having a fixed direction of magnetization, and a nonmagnetic intermediate layer L M  interposed between the free layer L F  and fixed layer L S  (see  FIG. 2 ). In the MR device R M , the direction of magnetization of the free layer L F  varies under the influence of the magnetic field generated in the coil  5  in response to the voltage of the battery  3 . When the direction of magnetization of the free layer L F  varies, the resistance of the M device R M  changes, thereby altering the voltage V 1  issued from the first output terminal DO 1 . On the other hand, the voltage V 2  from the second output terminal DO 2  does not change. 
     The differential amplification circuit  9  is used for acquiring the difference between the voltage V 1  issued from first output terminal DO 1  and the voltage V 2  issued from the output terminal DO 2 . The differential amplification circuit  9  has an inverting input terminal connected to the output terminal DO 1  through a resistance R 4 , and a non-inverting circuit terminal connected to the output terminal DO 2  through a resistance R 5 . As a consequence, the voltage V 1  issued from the first output terminal DO 1  is fed to the inverting input terminal of the differential amplification circuit  9 , while the voltage V 2  issued from the second output terminal DO 2  is fed to the non-inverting input terminal of the differential amplification circuit  9 . Also, the inverting input terminal is connected to an output terminal through a feedback resistance R 7 , while the non-inverting input terminal is connected to the ground potential GRD through a resistance R 6 . 
     Assuming that R 4 =R 5  and R 6 =R 7 , and expressing the resistance values of the resistances by the same polarity sign for convenience, a voltage V AMP  issued from the output terminal of the differential amplification circuit  8  is given by (R 7 /R 4 )×(V 2 −V 1 ) in this embodiment. Through an output terminal T 3  of the voltage detection circuit  1 A, the voltage V AMP  is fed to a control section  61  which will be explained later. 
     Specific operations of the voltage detection circuit  1 A including the magnetic coupler M C  composed of the coil  5  and M device R M  will now be explained with reference to  FIG. 2 . In the MR device R M  in accordance with this embodiment, as illustrated in  FIG. 2 , the direction of magnetization of the fixed layer L S  is fixed to the Y direction, while the direction of magnetization that is a magnetization easy axis of the free layer L F  is oriented in the Z direction. The nonmagnetic intermediate layer L M  is interposed between the fixed layer L S  and free layer L F . The nonmagnetic intermediate layer L M  is made of a conductor such as Cu in this embodiment, but may be an insulator such as Al 2 O 3  or MgO as well. 
     As illustrated in  FIG. 2 , when the current I starts to flow in the arrowed direction, a magnetic field (B) (in the −Y direction in the vicinity of the free layer L F ) is generated in the coil  5 , whereby the magnetic resistance of the MR device R M  varies under the influence of the magnetic field. More specifically, as the current I flows, the direction of magnetization of the free layer L F  begins to change gradually to the −Y direction (direction opposite from that of magnetization of the fixed layer L S ) under the influence of the magnetic field generated in the coil  5 . Consequently, the resistance value of the MR device R M  increases in proportion to the voltage of the battery  3 . 
     As the magnetic resistance (resistance) of the M device R M  increases, the voltage V 1  issued from the output terminal D 01  of the bridge circuit  7  decreases. As the voltage V 1  from the output terminal DO 1  decreases, the voltage V AMP  [=(R 7 /R 4 )×(V 2 −V 1 )] from the differential amplification circuit  9  becomes greater. Since the voltage V AMP  from the differential amplification circuit  9  increases as the voltage of each battery  3  rises as in the foregoing, the voltage of the battery  3  can be detected when appropriately related to the voltage V AMP  from the differential amplification circuit  9 . 
       FIG. 3  is a diagram schematically illustrating an assembled battery voltage detection apparatus  50  using the voltage detection circuit  1 A. In this system, the voltage detection apparatus  50  comprises voltage detection circuits  1 A connected to respective batteries  3  constituting an assembled battery  33  between their positive and negative terminals through input terminals T 1  and T 2 , a charger section  71  for charging the batteries  3 , an electric motor (load)  81  for discharging the batteries  3 , and a control section  61  for controlling the ON/OFF of switches SW 1 , SW 2 . 
     In this embodiment, the assembled battery  33  is used, for example, as a power supply for the electric motor  81  in an HEV (Hybrid Electric Vehicle) using both an engine (not depicted) and the electric motor  81  as driving sources for running. 
     Through the switch SW 1 , the charger section  71  is connected to the positive electrode terminal of the battery  3  constituting one end of the assembled battery  33 . The negative electrode terminal of the battery  3  constituting the other end of the assembled battery  33  is connected to the ground potential GRD. 
     The electric motor  81  has one end connected to the ground potential GRD and the other end connected between the batteries  3  and switch SW 1  through the switch SW 2 . Arranged between the switch SW 2  and electric motor  81  is a switch SW 3  which can be turned on/off by a user of the HEV. 
     The control section  61  is one which receives voltage V AMP  outputs from the respective differential amplification circuits  9  of the voltage detection circuits  1 A in the batteries  3  and controls the ON/OFF of the switches SW 1  and SW 2  such that each of the batteries  3  neither exceeds an upper limit voltage V MAX  nor falls from a lower limit voltage V MIN . 
     Specifically, during charging, the control section  61  digitally converts the voltage V AMP  from the differential amplification circuit  9  of each voltage detection circuit  1 A, and compares the resulting digital voltage V DIG  with the upper limit voltage V MAX  that has been determined and fed beforehand. When the voltage V DIG  is not lower than the upper limit voltage V MAX  as a result of the comparison, the switch SW 1  is turned off, so as to terminate the charging. When the voltage V DIG  is lower than the upper limit voltage V MAX  while being a usually employed voltage, on the other hand, the switch SW 1  is kept in the ON state. 
     During discharging, on the other hand, the control section  61  digitally converts the voltage V AMP  from each differential amplification circuit  9 , and compares the resulting digital voltage V DIG  with the lower limit voltage V MIN  that has been determined and fed beforehand. When the voltage V DIG  is not higher than the lower limit voltage V MIN  as a result of the comparison, the switch SW 2  is turned off, so as to terminate the dischargeable state. When the voltage V DIG  is higher than the lower limit voltage V MIN  while being a usually employed voltage, on the other hand, the switch SW 2  is kept in the ON state. 
     The control section  61  in this embodiment also functions to control the switch SW 1 , SW 2  such that the voltage V DIG  falls within the upper and lower ends of a voltage range usually in use. 
     Operations of the voltage detection apparatus  50  will now be explained with reference to  FIGS. 4 and 5 . First, operations of the voltage detection apparatus  50  during charging will be explained with reference to  FIG. 4 . When charging of the assembled battery  33  is started by a trigger signal issued from the control section  61 , the switches SW 1  and SW 2  are turned off, so as to be initialized (S 201 ). Thereafter, the respective voltage detection circuits  1 A of the batteries  3  issue their voltages V AMP (S 202 ). The control section  61  converts the issued voltages V AMP  into respective digital voltages V DIG , which are then compared with the upper limit voltage V MAX  (S 203 ). When at least one of the voltages V DIG  is the upper limit voltage V MAX  or higher, the control section  61  stops charging (S 206 ). When all the voltages V DIG  are lower than the upper limit voltage V MAX , on the other hand, the control section  61  keeps the switch SW 1  in the ON state. When the switch SW 1  is kept in the ON state, the control section  61  further determines whether the condition (1) that the voltages are not higher than the upper end of the usually used voltage range is satisfied or not (S 205 ). As a result, the flow returns to S 202  when the condition (1) is satisfied, whereas the charging is terminated when the condition (1) is not satisfied. 
     Operations of the voltage detection apparatus  50  during discharging will now be explained with reference to  FIG. 5 . When discharging of the assembled battery  33  is started by a trigger signal issued from the control section  61 , the switches SW 1  and SW 2  are turned off, so as to be initialized (S 301 ). Thereafter, the respective voltage detection circuits  1 A of the batteries  3  issue their voltages V AMP  (S 302 ). The control section  61  converts the issued voltages V AMP  into respective digital voltages V DIG , which are then compared with the lower limit voltage V MIN  (S 303 ). When at least one of the voltages V DIG  is the lower limit voltage V MIN  or less, the control section  61  stops discharging (S 306 ). When all the voltages V DIG  are higher than the lower limit voltage V MIN , on the other hand, the control section  61  keeps the switch SW 2  in the ON state. When the switch SW 2  is kept in the ON state, the control section  61  further determines whether the condition (2) that the voltages are not lower than the lower end of the usually used voltage range is satisfied or not (S 305 ). As a result, the flow returns to S 302  when the condition (2) is satisfied, whereas the discharging is terminated when the condition (2) is not satisfied. 
     In the voltage detection circuit  1 A in accordance with this embodiment, the coil  5  is connected between the positive and negative terminals of the battery  3 . Therefore, a magnetic field corresponding to the voltage of the battery  3  is generated in the coil  5 . Also, since the MR device R M  is magnetically coupled to the coil  5 , the direction of magnetization of the free layer L F  in the MR device R M  varies in response to the strength of the magnetic field generated in the coil  5 , thereby changing the magnetic resistance. This makes it possible to detect the voltage of the battery  3  according to the change of magnetic resistance in the MR device R M . Thus having the magnetic coupler M C  constituted by the coil  5  and the MR device R M  magnetically coupled to the coil  5  can detect the voltage of the battery  3  in real time with a simple structure without providing and alternately turning on/off input- and output-side switches. 
     Feeding the differential amplification circuit  9  with the voltages V 1  and V 2  issued from the respective output terminals DO 1  and DO 2  between the resistances (R M , R 1 ; R 2 , R 3 ) in the first and second resistance series constituting the bridge circuit  7  and amplifying the difference between the voltages V 1  and V 2  can accurately detect changes in voltage of the battery  3  even when the change in resistance of the MR device R M  is very weak. 
     The voltage detection apparatus  50  in accordance with this embodiment also has the control section  61  for converting the respective voltages V AMP  issued from the voltage detection circuits  1 A into the digital voltages V DIG  and controlling the switches SW 1 , SW 2  such that the voltages V DIG  neither exceed the upper limit voltage V MAX  nor fall from the lower limit voltage V MIN . This can secure the durability and safety of the batteries  3  constituting the assembled battery  33 . Further, the control section  61  functions to control the switches SW 1 , SW 2  such that the voltages V DIG  fall within the upper and lower ends of the usually used voltage range in the HEV, and thus can secure the durability and safety of each battery  3  more effectively. 
     Without being restricted to the above-mentioned embodiment, the present invention can be modified in various ways. For example, though the control section  61  converts the voltage V AMP  issued from the voltage detection circuit  1 A into the digital voltage V DIG  and controls the switches SW 1 , SW 2  according to the voltage V DIG  that is digital data, the voltage V AMP  that is analog data may be fed into an analog comparator or the like without being digitally converted, so as to control the switches SW 1 , SW 2 . 
     Though a GMR device is used as the MR device R M  in this embodiment, a tunneling magnetoresistive (TMR) device, for example, may be used without being restricted to the above. 
     The system illustrated in  FIG. 3  comprises the charger section  71  to which a plurality of batteries  3  are connected through the first switch SW 1 , the load  81  to which the plurality of batteries  3  are connected through the second switch SW 2 , and the control section  61  for controlling the first and second switches SW 1 , SW 2  according to the results of detection from the individual voltage detection circuits  1 A, and thus can be utilized in electric cars and hybrid cars.