Abstract:
The invention provides a power source circuit which is capable of preventing a current from being caused to flow from a battery charger to a secondary battery in the over-charge detection state. In order to cut off a current from a parasitic diode of a transistor having a gate electrode to which a signal to release the over-current state is inputted from a switching-device-controlling logic circuit, a charge and discharge controlling circuit includes a diode connected between the transistor and a current detecting circuit, thereby preventing a current from being caused to flow from the battery charger to the secondary battery.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to a power source circuit which is capable of monitoring a voltage of a secondary battery to control charge and discharge.  
           [0003]    2. Description of the Related Art  
           [0004]    [0004]FIG. 3 is a block diagram, partly in circuit diagram, showing schematically a configuration of a conventional power source circuit.  
           [0005]    A negative external terminal  313  to which either a battery charger  310  or a load  311  is to be connected, is connected to a negative electrode of a secondary battery  301 . A positive external terminal  312  to which the battery charger  310  or the load  311  is to be connected, is connected to a positive electrode of the secondary battery  301  through a switching device (P-channel MOS FET)  302  and a switching device (P-channel MOS FET)  303  connected in series. In addition, a charge and discharge controlling circuit  316  is connected in parallel with the secondary battery  301 . The charge and discharge controlling circuit  316  has the function of detecting a voltage of the second battery  301  and a voltage developed across the switching devices  302  and  303 .  
           [0006]    That is, the charge and discharge controlling circuit  316  has the function of detecting the state in which the battery voltage of the secondary battery  301  is lower than a predetermined voltage value to turn OFF the switching device  302 . Then, this state is referred to as “the over-discharge detection state” when applicable. In addition, the circuit  316  has the function of detecting the state in which the voltage developed across a VDD terminal  314  and a terminal  309  for detecting the battery charger connection and the over-current, is larger than a predetermined voltage value to turn OFF the switching device  302 . Then, this state is referred to as “the over-current detection state” when applicable. Also, the circuit  316  has the function of detecting the state in which the battery voltage of the secondary battery  301  is higher than a predetermined voltage value to turn OFF the switching device  303 . Then, this state is referred to as “the over-charge detection state” when applicable.  
           [0007]    Now, the description will hereinbelow be given with respect to the operation in the case of the over-charge state. The battery charger  310  is connected between the positive external terminal  312  and the negative external terminal  313  to charge the secondary battery  301  with the electric charges, thereby providing the state in which the voltage developed across the VDD terminal  314  and a VSS terminal  315  is higher than a predetermined voltage value. At the time when this state has been provided, an over-charge and over-discharge detecting circuit  317  sends an over-charge detection signal to a logic circuit  318  for controlling switching devices. In response to the over-charge detection signal, the logic circuit  318  for controlling switching devices sends a signal to turn OFF the switching device  302  through a charge control terminal  304 .  
           [0008]    Next, the description will hereinbelow be given with respect to the operation in the case of the over-discharge state. The load  311  is connected between the positive external terminal  312  and the negative external terminal  313  to discharge the secondary battery  301 , thereby providing the state in which the voltage developed across the VDD terminal  314  and the VSS terminal  315  is lower than the predetermined voltage value. At the time when this state has been provided, the over-charge and over-discharge detecting circuit  317  sends an over-discharge detection signal to the logic circuit  318  for controlling switching devices. In response to the over-discharge detection signal, the logic circuit  318  for controlling switching devices sends a signal to turn OFF the switching device  303  through a discharge control terminal  305 .  
           [0009]    Finally, the description will hereinbelow be given with respect to the operation in the case of the over-current state. At the time when the terminal voltage developed across the VDD terminal  314  and the terminal  309  for detecting the battery charger connection and the over-current, has become larger than a predetermined voltage value, a circuit  319  for detecting the battery charger connection and the over-current sends an over-current detection signal to the logic circuit  318  for controlling switching devices. In response to the over-current detection signal, the logic circuit  318  for controlling switching devices sends a signal to turn OFF the switching device  303  through the discharge control terminal  305 . In addition, when the load  311  is connected to provide the over-current state, the terminal  309  for detecting the battery charger connection and the over-current is pulled down. However, at the time when the load  311  between the positive external terminal  312  and the negative external terminal  313  is released, a signal to turn ON the P-channel MOS FET  306  is sent from the logic circuit  318  for controlling switching devices to pull up the terminal  309  for detecting the battery charger connection and the over-current to VDD, thereby carrying out the release of the over-current state.  
           [0010]    Here, when the battery charger  310  is connected in the over-charge detection state, even if the switching device  303  is turned OFF, the current is caused to flow from the battery charger  310  to the secondary battery  301  through a current path  320  passing through the positive external terminal  312 , the terminal  309  for detecting the battery charger connection and the over-current, and a parasitic diode  308  of the P-channel MOS FET  306  so that the battery  301  is charged with the electric charges. For this reason, a resistor  307  is provided in order to limit an amount of current which is caused to flow from the battery charger  310  to the secondary battery  301 .  
           [0011]    As described above, in the conventional power source circuit, the resistor  307  is provided in order to limit an amount of current which is caused to flow from the battery charger  310  to the secondary battery  301 . However, since the current is caused to flow through the resistor, it is impossible to cut off perfectly the current path.  
         SUMMARY OF THE INVENTION  
         [0012]    In the light of the foregoing, the present invention has been made in order to solve the above-mentioned problems associated with the prior art, and it is therefore an object of the present invention to prevent a current from being caused to flow into a secondary battery by providing a diode instead of employing a resistor having a fixed magnitude of resistance.  
           [0013]    Another object of the present invention is to provide a power source circuit having a current limit function of adjusting an amount of current of a secondary battery, the power source circuit comprising: a negative external terminal through which one of a battery charger and a load is connected to a negative electrode side of the secondary battery; a positive external terminal through which one of the battery charger and the load is connected to a positive electrode side of the secondary battery via a switching device connected in series with the positive electrode side of the secondary battery; and a charge and discharge controlling circuit connected in parallel with the secondary battery, wherein the charge and discharge controlling circuit includes: a voltage detecting circuit for detecting a voltage of the secondary battery to output a signal; a switching-device-controlling logic circuit for in response to the signal from the voltage detecting circuit, controlling the switching device; a current detecting circuit for outputting a signal to control the switching-device-controlling logic circuit on the basis of a value of the electric potential difference between a voltage of the positive electrode of the secondary battery and a voltage of the positive external terminal; a transistor having a gate electrode to which a signal having the information that exhibits the over-current state is inputted from the switching-device-controlling logic circuit; and a diode connected between the transistor and the current detecting circuit for cutting off a current from a parasitic diode of the transistor. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0014]    The above and other objects and advantages of the present invention will become clear from the following description with reference to the accompanying drawings, wherein:  
         [0015]    [0015]FIG. 1 is a block diagram, partly in circuit diagram, showing a configuration of a power source circuit according to an embodiment of the present invention;  
         [0016]    [0016]FIG. 2 is a block diagram, partly in circuit diagram, showing a configuration of a power source circuit according to another embodiment of the present invention; and  
         [0017]    [0017]FIG. 3 is a block diagram, partly in circuit diagram, showing a configuration of a conventional power source circuit. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0018]    The preferred embodiments of the present invention will hereinafter be described in detail with reference to the accompanying drawings.  
         [0019]    [0019]FIG. 1 shows a power source circuit according to an embodiment of the present invention in which each of switching devices  102  and  103  for controlling charge and discharge is a P-channel MOS FET.  
         [0020]    In a power source circuit  100 , a positive external terminal  112  to which either a battery charger  110  or a load  111  is to be connected, is connected to a positive electrode of a secondary battery  101 , and the battery charger  110  or the load  111  is connected to the positive electrode of the secondary battery  101  through the switching device  102  and the switching device  103  connected in series. In addition, a charge and discharge controlling circuit  116  is connected in parallel with the secondary battery  101 . The charge and discharge controlling circuit  116  has the function of detecting a voltage of the secondary battery  101  and a voltage developed across the switching devices  102  and  103 .  
         [0021]    The operation of the power circuit of the embodiment is similar to that of the conventional power source circuit.  
         [0022]    Next, the operation of the power source circuit of the embodiment will hereinbelow be described in detail.  
         [0023]    First of all, the description will now be given with respect to the operation in the case of the over-charge state. The battery charger  110  is connected between the positive external terminal  112  and the negative external terminal  113  to charge the secondary battery  101  with the electric charges, thereby providing the state in which the voltage developed across a VDD terminal  114  and a VSS terminal  115  is higher than a predetermined voltage value. At the time when this state has been provided, a circuit  117  for detecting over-charge and over-discharge sends an over-charge detection signal to a logic circuit  118  for controlling switching devices. In response to the over-charge detection signal, the logic circuit  118  for controlling switching devices sends a signal to turn OFF the switching device  102  through a charge control terminal  104 .  
         [0024]    Next, the description will hereinbelow be given with respect to the operation in the case of the over-discharge state. The load  111  is connected between the positive external terminal  112  and the negative external terminal  113  to discharge the secondary battery  101 , thereby providing the state in which the voltage developed across the VDD terminal  114  and the VSS terminal  115  is lower than the predetermined voltage value. At the time when this state has been provided, the circuit  117  for detecting over-charge and over-discharge sends an over-discharge detection signal to the logic circuit  118  for controlling switching devices. In response to the over-discharge detection signal, the logic circuit  118  for controlling switching devices sends a signal to turn OFF the switching device  103  through a discharge control terminal  105 .  
         [0025]    Finally, the description will hereinbelow be given with respect to the operation in the case of the over-current state. At the time when the terminal voltage developed across the VDD terminal  114  and the terminal  109  for detecting the battery charger connection and the over-current, has become larger than a predetermined voltage value, a circuit  119  for detecting the battery charger connection and the over-current sends an over-current detection signal to the logic circuit  118  for controlling switching devices. In response to the over-current detection signal, the logic circuit  118  for controlling switching devices sends a signal to turn OFF the switching device  103  through the discharge control terminal  105 . In addition, when the load  111  is connected to provide the over-current state, the terminal  109  for detecting the battery charger connection and the over-current is pulled down. However, at the time when the load  111  between the positive external terminal  112  and the negative external terminal  113  is released, a signal to turn ON the P-channel MOS FET  106  is sent from the logic circuit  118  for controlling switching devices to pull up the terminal  109  for detecting the battery charger connection and the over-current to VDD to carry out the release of the over-current state.  
         [0026]    Here, a diode  108  is provided in the position of the resistor  307  shown in FIG. 3 instead thereof. As a result, it is possible to cut off perfectly the current path  320  shown in FIG. 3 through which the current is caused to flow from the positive external terminal  112  to the secondary battery  101  via the terminal  109  for detecting the battery charger connection and the over-current and the parasitic diode  107  of the P-channel MOS FET  106  even when the battery charge  110  is connected in the over-current detection state. As a result, the current is prevented from being caused to flow into the secondary battery  101  so that the secondary battery is prevented from being charged with the electric charges in the over-charge detection state.  
         [0027]    [0027]FIG. 2 shows a power source circuit according to another embodiment of the present invention in which the switching device for controlling charge and discharge is an N-channel MOS FET. The case where the switching device for controlling charge and discharge is an N-channel MOS FET can also be described similarly to the case where the switching device for controlling charge and discharge is the P-channel MOS FET.  
         [0028]    Since at the time when a battery charger  210  is connected between a positive external terminal  212  and a negative external terminal  213  in the over-charge detection state, a current is caused to flow from a VSS terminal  215  to a battery charger  210  via a parasitic diode  207 , a diode  208  is disposed as shown in FIG. 2. As a result, it is possible to cut off a current path passing through a secondary battery  201  and the battery charger  210 , and hence the secondary battery is prevented from being charged with the electric charges in the over-charge state.  
         [0029]    As set forth hereinabove, according to the present invention, the diode is disposed in the position between the terminal for detecting the battery charger connection and the over-current and the P-channel MOS FET, i.e., in the position where the resistor  307  shown in FIG. 3 is disposed, whereby it is possible to prevent a current from being caused to flow from the battery charger to the secondary battery.  
         [0030]    While the present invention has been particularly shown and described with reference to the preferred embodiments, it will be understood that the various changes and modifications will occur to those skilled in the art without departing from the scope and true spirit of the invention. The scope of the invention is therefore to be determined solely by the appended claims.