Abstract:
A protection circuit is disclosed that issues an instruction to blow a fuse connected to a power source in response to detection of an abnormality of a voltage of the power source. The protection circuit includes a time control unit that detects the voltage of the power source and controls the length of time from the detection of the abnormality of the voltage of the power source to the issue of the instruction to blow the fuse in accordance with the detected voltage.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention generally relates to a protection circuit and a protection method, and particularly relates to a protection circuit and a protection method that issue an instruction to blow a fuse connected to a battery in response to detection of an abnormality of a voltage of the battery.  
         [0003]     2. Description of the Related Art  
         [0004]     There have been protection circuits that disconnect circuits from batteries by blowing fuses upon detection of an abnormality so as to protect the circuits. Examples of such protection circuits include an overcharge protection circuit adapted to prevent a battery from overcharging.  
         [0005]      FIG. 5  shows a block diagram of an example of a battery protection system  100 .  
         [0006]     The battery protection system  100  comprises batteries  111 - 1  through  111 - 4 , a protection IC  112 , a fuse device  113 , a charger  114 , first through fourth resistances R, first through fourth capacitors C, a delay capacitor Ct, and a transistor M.  
         [0007]     The batteries  111 - 1  through  111 - 4  are connected in series, with a positive terminal of the battery  111 - 1  connected to a terminal T+via the fuse device  113 , and a negative terminal of the battery  111 - 4  connected to a terminal T−.  
         [0008]     A connection point between the battery  111 - 1  and the fuse device  113  is connected to the protection IC  112  via the first resistance R. The first, second, and third capacitors C are connected between a detection terminal Ts 1  and a detection terminal Ts 2 , the detection terminal Ts 2  and a detection terminal Ts 3 , and the detection terminal Ts 3  and a detection terminal Ts 4  of the protection IC  112 , respectively. The battery  111 - 1  is connected between the detection terminals Ts 1  and Ts 2  via an integration circuit including the first and second resistances R and the first capacitor C. The battery  111 - 2  is connected between the detection terminals Ts 2  and Ts 3  via an integration circuit including the second and third resistances R and the second capacitor C. The battery  111 - 3  is connected between the detection terminals Ts 3  and Ts 4  via an integration circuit including the third and fourth resistances R and the third capacitor C. The battery  111 - 4  is connected between the detection terminals Ts 4  and Ts 5  via an integration circuit including the fourth resistance R and the fourth capacitor C.  
         [0009]     A terminal Tct of the protection IC  112  is connected to the delay capacitor Ct, while an output terminal Tout of the protection IC  112  is connected to a gate of the transistor M. The protection IC  112  detects overcharge of the individual batteries  111 - 1  through  111 - 4  by detecting voltages at each terminal of the batteries  111 - 1  through  111 - 4 . In the event of detection of overcharge of any of the batteries  111 - 1  through  111 - 4 , the protection IC  112  inverts the output of the output terminal Tout with a delay of a specified delay time determined by the delay capacitor Tct. The transistor M turns on in response to the inversion of the output of the output terminal Tout.  
         [0010]     The transistor M includes a source and a back gate connected to the terminal T−, and a drain connected to the fuse device  113 . The fuse device  113  comprises fuses F 1  and F 2 , and heaters H 1  and H 2 . The fuses F 1  and F 2  are connected in series between the terminal T+ and the batteries  111 - 1  through  111 - 4 . The heaters H 1  and H 2  are connected parallel to each other to form a parallel circuit, which is connected in series between the drain of the transistor M and a connection point between the fuses F 1  and F 2 . The heater H 1  is arranged to face the fuse F 1 , while the heater H 2  is arranged to face the fuse F 2 .  
         [0011]     The charger  114 , for example, is connected between the terminal T+ and the terminal T−. The charger  114  serves to charge the batteries  111 - 1  through  111 - 4 . If the protection IC  112  is overcharged, a current is applied to the fuse device  113  to blow the fuses F 1  and F 2  arranged inside the fuse device  113 . In this way, the batteries  111 - 1  through  111 - 4  are disconnected from the charger  114  for protection.  
         [0012]     In this type of protection circuit, a supply voltage to a heater varies depending on the number of batteries connected. Accordingly, a heating value of the heater varies, resulting in a variation of time taken to blow a fuse.  
       SUMMARY OF THE INVENTION  
       [0013]     According to one aspect of the present invention, there is provided a protection circuit that makes constant the length of time from detection of an abnormality to blowing of a fuse.  
         [0014]     According to another aspect of the present invention, there is provided a protection circuit that issues an instruction to blow a fuse connected to a power source in response to a detection of an abnormality of a voltage of the power source, the circuit including a time control unit that detects the voltage of the power source, and controls the length of time from the detection of the abnormality of the voltage of the power source to the issue of the instruction to blow the fuse in accordance with the detected voltage.  
         [0015]     In the aforesaid protection circuit, the time control unit preferably includes a voltage detection unit that detects the voltage of the power source, and a delay unit that controls the length of time from the detection of the abnormality of the voltage of the power source to the issue of the instruction to blow the fuse in accordance with the voltage of the power source detected by the voltage detection unit.  
         [0016]     The aforesaid delay unit preferably controls the length of time from the detection of the abnormality of the voltage of the power source to the issue of the instruction to blow the fuse in accordance with the detected voltage such that the length of time from the detection of the abnormality of the voltage of the power source to the blowing of the fuse is constant regardless of the detected voltage.  
         [0017]     In the aforesaid protection circuit, the power source may include a battery.  
         [0018]     According to still another aspect of the present invention, there is provided a protection method that protects a power source by blowing a fuse connected to the power source in response to a detection of an abnormality of a voltage of the power source, the method comprising a step of detecting the voltage of the power source, and a step of controlling the length of time from the detection of the abnormality of the voltage of the power source to the blowing of the fuse in accordance with the detected voltage so as to be constant regardless of the detected voltage.  
         [0019]     According to the above-described aspects of the present invention, since the length of time from detecting the abnormality of the voltage of the power source to issuing the instruction to blow the fuse is controlled in accordance with the detected voltage of the power source, the length of time from detecting the abnormality to blowing the fuse is constant regardless of the detected voltage. Therefore, the quality of products using the above-described protection circuit or the protection method can be improved. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0020]      FIG. 1  is a schematic diagram showing a configuration of a battery protection system according to an embodiment of the present invention;  
         [0021]      FIG. 2  is a block diagram of a protection IC;  
         [0022]      FIG. 3  is a waveform diagram of a protection IC;  
         [0023]      FIG. 4  is a waveform diagram of a protection IC; and  
         [0024]      FIG. 5  is a schematic diagram showing a configuration of an example of a related-art battery protection system; 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0025]     [System Configuration] 
         [0026]      FIG. 1  is a schematic diagram showing a configuration of a battery protection system  200  according to an embodiment of the present invention.  
         [0027]     The battery protection system  200  of this embodiment includes a protection IC  212 , which is different from the protection IC  112  of the battery protection system  100 . The protection IC  212  of this embodiment detects a series voltage of batteries  111 - 1  through  111 - 4 , and controls the length of time from detecting overcharge to instructing a fuse device  113  to blow fuses F 1  and F 2  in accordance with the detected series voltage such that the length of time from detecting the overcharge to blowing the fuses F 1  and F 2  is constant regardless of the detected series voltage.  
         [0028]     [Protection IC  212 ] 
         [0029]      FIG. 2  is a block diagram of the protection IC  212 .  
         [0030]     The protection IC  212  of this embodiment serves as a protection circuit to prevent the four batteries  111 - 1  through  111 - 4  from overcharging. The protection IC  212  comprises overcharge detectors  221 - 1  through  221 - 4 , an OR gate  222 , a delay circuit  223 , an output controller  224 , and an inverter  225 .  
         [0031]     The overcharge detector  221 - 1  is connected between a terminal Ts 1  and a terminal Ts 2  and configured to detect overcharge of the battery  111 - 1  by detecting a voltage between the terminal Ts 1  and the terminal Ts 2 . The overcharge detector  221 - 1  comprises resistances R 11  and R 12 , a current source  231 , a zener diode Dz, and a comparator  232 . The resistances R 11  and R 12  are connected in series between the terminal Ts 1  and the terminal Ts 2  so as to divide the voltage between the terminal Ts 1  and the terminal Ts 2  and output the divided voltage from a connection point between the resistance R 11  and the resistance R 12 . The connection point between the resistance R 11  and the resistance R 12  is connected to a non-inverting input terminal of the comparator  232 . The current source  231  and the zener diode Dz are connected in series between the terminal Ts 1  and the terminal Ts 2  so as to generate a reference voltage from the voltage between the terminal Ts 1  and the terminal Ts 2 . The reference voltage is output from a connection point between the current source  231  and the zener diode Dz. The reference voltage output from the connection point between the current source  231  and the zener diode Dz is supplied to an inverting input terminal of the comparator  232 . The comparator  232  compares the detected voltage at the connection point between the resistance R 11  and the resistance R 12  with the reference voltage at the connection point between the current source  231  and the zener diode Dz. If the detected voltage is lower than the reference voltage, the output of the comparator  232  is set to low level. If otherwise the detected voltage is higher than the reference voltage, the output of the comparator  232  is set to high level. The output of the comparator  232  is supplied to the OR gate  222 .  
         [0032]     The overcharge detector  221 - 2  is connected between the terminal Ts 2  and a terminal Ts 3  and configured to detect overcharge of the battery  111 - 2  by detecting a voltage between the terminal Ts 2  and the terminal Ts 3 . The overcharge detector  221 - 2  has the same construction as the overcharge detector  221 - 1 . The output of the overcharge detector  221 - 2  is supplied to the OR gate  222 .  
         [0033]     The overcharge detector  221 - 3  is connected between the terminal Ts 3  and a terminal Ts 4  and configured to detect overcharge of the battery  111 - 3  by detecting a voltage between the terminal Ts 3  and the terminal Ts 4 . The overcharge detector  221 - 3  has the same construction as the overcharge detector  221 - 1 . The output of the overcharge detector  221 - 3  is supplied to the OR gate  222 . The overcharge detector  221 - 4  is connected between the terminal Ts 4  and a terminal Ts 5  and configured to detect overcharge of the battery  111 - 4  by detecting a voltage between the terminal Ts 4  and the terminal Ts 5 . The overcharge detector  221 - 4  has the same construction as the overcharge detector  221 - 1 . The output of the overcharge detector  221 - 4  is supplied to the OR gate  222 .  
         [0034]     The OR gate  222  outputs a logical OR of the outputs of the overcharge detectors  221 - 1  through  221 - 4 . The output of the OR gate  222  is supplied to the delay circuit  223  and the output controller  224 .  
         [0035]     [Delay Circuit  223 ] 
         [0036]     The delay circuit  223  is configured to detect the series voltage of the four batteries  111 - 1  through  111 - 4  so as to control a delay time in accordance with the detected series voltage. The delay circuit  223  comprises a battery voltage detector  241 , an oscillator  242 , and a counter  243 .  
         [0037]     [Battery Voltage Detector  241 ] 
         [0038]     The battery voltage detector  241  comprises resistances R 21  through R 24 , current sources  251  through  253 , zener diodes Dz 11  through Dz 13 , comparators  254  through  256 , and is configured to detect the series voltage of the four batteries  111 - 1  through  111 - 4 .  
         [0039]     The resistances R 21  through R 24  are connected in series between the terminal Ts 1  and the terminal Ts 5  so as to divide a voltage between the terminal Ts 1  and the terminal Ts 5 , i.e., a sum of voltages produced by the four batteries  111 - 1  through  111 - 4 . A voltage at a connection point between the resistance R 21  and the resistance R 22  is supplied to a non-inverting input terminal of the comparator  254 . A voltage at a connection point between the resistance R 22  and the resistance R 23  is supplied to a non-inverting input terminal of the comparator  255 . A voltage at a connection point between the resistance R 23  and the resistance R 24  is supplied to a non-inverting input terminal of the comparator  256 .  
         [0040]     The current sources  251  through  253  and the zener diodes Dz 11  through Dz 13  are alternately connected in series between the terminal Ts 1  and the terminal Ts 5 . A first reference voltage Vref is generated at a connection point between the current source  251  and the zener diode Dz 11 . The first reference voltage Vref generated at the connection point between the current source  251  and the zener diode Dz 11  is supplied to an inverting input terminal of the comparator  254 .  
         [0041]     A second reference voltage  2 ×Vref is generated at a connection point between the current source  252  and the zener diode Dz 12 . The second reference voltage  2 ×Vref generated at the connection point between the current source  252  and the zener diode Dz 12  is supplied to an inverting input terminal of the comparator  255 .  
         [0042]     A third reference voltage  3 ×Vref is generated at a connection point between the current source  253  and the zener diode Dz 13 . The third reference voltage  3 ×Vref generated at the connection point between the current source  253  and the zener diode Dz 13  is supplied to an inverting input terminal of the comparator  256 .  
         [0043]     If the series voltage of the four batteries  111 - 1  through  111 - 4  is high enough to make the voltage at the connection point between the resistance R 23  and the resistance R 24  higher than the third reference voltage, the outputs of all the comparators  254  through  256  are set to high level. On the other hand, if the series voltage of the four batteries  111 - 1  through  111 - 4  is reduced and therefore the voltage at the connection point between the resistance R 23  and the resistance R 24  falls below the third reference voltage, the output of the comparator  254  is set to low level although the outputs of the comparators  255  and  256  remain at the high level.  
         [0044]     If the series voltage of the four batteries  111 - 1  through  111 - 4  is further reduced and therefore the voltage at the connection point between the resistance R 22  and the resistance R 23  falls below the second reference voltage, the outputs of the comparators  254  and  255  are set to low level although the output of the comparator  256  remains at the high level. If the series voltage of the four batteries  111 - 1  through  111 - 4  is further reduced and therefore the voltage at the connection point between the resistance R 21  and the resistance R 22  falls below the first reference voltage, the outputs of all the comparators  254  through  256  are set to low level.  
         [0045]     As such, the series voltage of the four batteries  111 - 1  through  111 - 4  is detected based on the outputs of the comparators  254  through  256 . The outputs of the comparators  254  through  256  are supplied to the counter  243 .  
         [0046]     [Counter  243 ] 
         [0047]     The counter  243  counts down the count pulse, which may be, for example, the oscillation pulse of the oscillator  242  with a frequency divided based on signals from the comparators  254  through  256 . The counter  243  starts a countdown from a count value preset by a delay terminal Tcd after the output of the OR gate  222  is set to high level.  
         [0048]     The frequency of the count pulse is divided to have: long cycles when the outputs of all the comparators  254  through  256  are high level; medium cycles when the outputs of the comparators  254  and  255  are high level; and short cycles when the output of only the comparator  254  is high level. The counter  243  switches its output to high level when the count reaches 0. It is therefore possible to increase the delay time from the point when the output of the OR gate  222  is switched to high level in response to detection of overcharge of any of the batteries  111 - 1  through  111 - 4  to the point when the instruction to blow the fuses F 1  and F 2  is issued as the series voltage of the batteries  111 - 1  through  111 - 4  increases, and to reduce the delay time as the series voltage of the batteries  111 - 1  through  111 - 4  decreases.  
         [0049]     The output of the counter  243  is supplied to the output controller  224 . The output controller  224  inverts its output from high level to low level when the output of the counter  234  is switched to high level. The output of the output controller  224  is supplied to the inverter  225 . The inverter  225  inverts the output of the output controller  224 , and outputs the inverted output from an output terminal Tout.  
         [0050]      FIGS. 3 and 4  illustrate waveform diagrams of the protection IC  212 . The diagram of  FIG. 3  shows a waveform produced when the series voltage of the batteries  111 - 1  through  111 - 4  is low. On the other hand, the diagram of  FIG. 4  shows a waveform produced when the series voltage of the batteries  111 - 1  through  111 - 4  is high. In the waveform diagrams of  FIGS. 3 and 4 , (A) indicates a voltage VDD between the terminal Ts 1  and the terminal Ts 5 ; (B) indicates the count value of the counter  243 ; (C) indicates an output voltage of the output terminal Tout; and (D) indicates a voltage of a terminal T+.  
         [0051]     The following describes operations performed when the series voltage of the batteries  111 - 1  through  111 - 4  is low during charging of the batteries  111 - 1  through  111 - 4  with reference to  FIG. 3 . Upon detection of overcharge at time t 1 , the counter  243  starts a countdown of the count pulse. When the count reaches 0 at time t 2 , the output of the counter  243  is set to high level. In response, the output of the output controller  224  is set to low level, and the output of the output terminal Tout is set to high level. When the output of the output terminal Tout is set to high level, a transistor M is turned on. Thus, a current is applied to heaters H 1  and H 2  to start heating the fuses F 1  and F 2 . When a heating temperature of the heaters H 1  and H 2  reaches a melting temperature of the fuses F 1  and F 2  at t 3 , the fuses F 1  and F 2  are blown.  
         [0052]     In this case, since the series voltage of the batteries  111 - 1  through  111 - 4  is low, a voltage applied to the heaters H 1  and H 2  is low. The time that the heaters H 1  and H 2  take to reach the melting temperature is therefore relatively long. Accordingly, in the length of time T 0  from detecting the overcharge to blowing the fuses F 1  and F 2 , a delay time Tc is short while a heating time Th is long.  
         [0053]     The following are operations performed when the series voltage of the batteries  111 - 1  through  111 - 4  is high with reference to  FIG. 4 . Upon detection of overcharge at time t 11 , the counter  243  starts a countdown of the count pulse. When the count reaches 0 at time t 12 , the output of the counter  243  is set to high level. In response, the output of the output controller  224  is set to low level, and the output of the output terminal Tout is set to high level. When the output of the output terminal Tout is set to high level, the transistor M is turned on. Thus, a current is applied to the heaters H 1  and H 2  to start heating the fuses F 1  and F 2 . When the heating temperature of the heaters H 1  and H 2  reaches the melting temperature of the fuses F 1  and F 2  at tl 3 , the fuses F 1  and F 2  are blown.  
         [0054]     In this case, since the series voltage of the batteries  111 - 1  through  111 - 4  is high, the voltage applied to the heaters H 1  and H 2  is high. The time that the heaters H 1  and H 2  take to reach the melting temperature is therefore relatively short. Accordingly, in the length of time T 0  from detecting the overcharge to blowing the fuses F 1  and F 2 , the delay time Tc is long while the heating time Th is short.  
         [0055]     According to the embodiment described above, the delay time Tc can be controlled such that the length of time T 0  from detecting the overcharge to blowing the fuses F 1  and F 2  is constant. Therefore, the quality of products using the protection circuit  212  can be improved.  
         [0056]     The present application is based on Japanese Priority Application No. 2005-049479 filed on Feb. 24, 2005, with the Japanese Patent Office, the entire contents of which are hereby incorporated by reference.