Patent Publication Number: US-7224562-B2

Title: Short-circuit protective circuit

Description:
FIELD OF THE INVENTION 
   The invention relates to a short-circuit protective circuit for a constant-voltage power-supply circuit provided with a plurality of power-supply lines. 
   BACKGROUND OF THE INVENTION 
   As for a constant-voltage power-supply circuit, a voltage of a power-supply line is controlled to maintain a constant voltage by connecting a regulator to the power-supply line, wherein if a load circuit is short-circuited at an output side of the regulator owing to some causes, a voltage drop occurs, so that a large current flows into the regulator, causing the breakage of the regulator owing to overheat of the regulator. 
   To cope with such a problem, a short-circuit protective circuit is provided in the constant-voltage power-supply circuit. For example, JP 5-252649A discloses potential detection means for detecting whether the difference between an input voltage and an output voltage of a regulator exceeds a prescribed voltage wherein the output of the regulator is prohibited by controlling an output control input terminal of the regulator. Further, JP 9-34569A discloses that an output voltage is fed back to keep a transistor ON so that a regulator IC keeps its operation to maintain a stabilized output wherein if a load circuit of the output terminal is short-circuited, the voltage is not fed back to the transistor and as a result, the supply of output from the regulator IC is blocked. Still further, JP 2002-358130A discloses the function of a control circuit for turning on and off the output of a regulator IC by detecting an output voltage from an output terminal of the regulator IC. 
   All of the prior art references set forth above use the regulator provided with the output control terminal for a constant-voltage power-supply circuit, wherein if a voltage at an output side of the regulator drops owing to a short circuit, the voltage is fed back so as to input a stop signal to the output control terminal, thereby stopping the regulator. However, if such a circuit configuration is used for a power-supply circuit provided with a plurality of power-supply lines, the regulator has to be disposed for each power-supply line, resulting in the increase of cost. Further, even if one power-supply line is short-circuited to stop the supply of power, the remaining power-supply lines continue to supply power so that there is a possibility that the short-circuited load circuit influences upon the load circuits connected to the remaining power-supply lines. If the supply of power continues, the waste of power consumption is unavoidable. 
   SUMMARY OF THE INVENTION 
   The invention provides a short-circuit protective circuit for a constant-voltage power-supply circuit. The constant-voltage power-supply circuit can realize a short-circuit protection with a simple circuit configuration in a power-supply circuit provided with a plurality of power-supply lines. 
   The short-circuit protective circuit comprises: 1) a regulator having an input terminal connected to a specific power-supply line among a plurality of power-supply lines, an output terminal for a constant voltage output and an output control terminal for turning on or off the output of the regulator, 2) a plurality of constant-voltage circuits connected to each power-supply line other than the specific power-supply line, each of the constant-voltage circuits supplying a constant voltage for each power-supply line based on the constant voltage supplied from the output terminal of the regulator, 3) a plurality of voltage detection circuits connected to each power-supply line other than the specific power-supply line for detecting a voltage drop at an output side thereof and outputting a detection signal, and 4) an operation circuit connected to the output control terminal of the regulator for stopping the output voltages of the constant-voltage circuits all at once, by stopping the output voltage from the regulator in response to the detection signal outputted from the voltage detection circuit. 
   With the circuit configuration set forth above, if a voltage drop occurs at any of the power-supply lines, output voltages of all the power-supply lines are stopped, thereby realizing the short-circuit protection. Since the regulator is provided on the specific power-supply line and constant-voltage circuits are provided on the other power-supply lines for supplying constant voltages for the other power-supply lines based on the constant voltage outputted from the regulator, if the output of the regulator is stopped, all the power-supply lines are stopped at output thereof all at once. Accordingly, the constant-voltage circuit at each power-supply line has a function to provide ON/OFF control on the voltage of each power-supply line. 
   Further, if the voltage detection circuit provided on each power-supply line detects a voltage drop at the output side of the constant voltage circuits, thereby detecting the voltage drop at either power-supply line, the operation circuit sends out a signal to the output control terminal of the regulator to stop the output of the regulator. As a result, if a voltage drop occurs at any of the power-supply lines owing to a short-circuit, the operation circuit stops the output of the regulator. 
   As mentioned above, a circuit configuration for implementing ON/OFF control over the constant-voltage circuits provided on each power-supply line is not necessary, but a circuit configuration capable of stopping the output of the regulator alone is enough to implement the short-circuit protection, thereby making the circuit configuration simpler and reducing the cost of components. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a circuit configuration relating to an embodiment of the invention. 
   

   PREFERRED EMBODIMENT OF THE INVENTION 
   An embodiment of the invention will be described in detail. Since the working example described hereinafter is a preferred concrete example for the invention, it is technically limited, but the invention is not limited only to the working example unless the description expressly limits the invention to the working example. 
     FIG. 1  is a circuit configuration related to the embodiment of the invention. In  FIG. 1 , voltages V 1  to V 3  are supplied from a power-supply circuit, not shown, through power-supply lines. 
   The power-supply line through which the voltage V 1  (hereinafter referred to as the power-supply line of the voltage V 1 ) is supplied is connected to the input terminal (IN) of the regulator  1 . The regulator  1  has an output terminal (OUT) for outputting a predetermined constant voltage, a ground terminal (GND) to be grounded, an output control terminal (ON/OFF) for turning on or off the output of the regulator  1 , and the input terminal (IN), wherein the input and output terminals (IN,OUT) of the regulator  1  are connected to capacitors C 1  and C 2 , respectively, the ground terminal (GND) is grounded as it is, and the output control terminal (ON/OFF) is grounded via a resistor R 1  and connected to the power-supply line at the input side of the regulator  1  via a resistor R 2 . Further, the output terminal (OUT) of the regulator  1  is connected to an output line L 1  through which a constant voltage V 1 ′ is outputted. 
   A constant-voltage circuit  2  is connected to the power-supply line through which the voltage V 2  is supplied. The constant-voltage circuit  2  has a control transistor Tr 20 , the emitter of which is connected to the power-supply line through which the Voltage V 2  is supplied and a collector of which is connected to an output line L 2 . 
   The constant-voltage circuit  2  comprises a pair of transistors Tr 21  and Tr 22 , each having the same characteristics, emitters of which are connected to each other and grounded via a resistor R 20 . This circuit operates such that a current flowing to common emitter terminals thereof becomes constant and functions like a differential amplifier. 
   A base terminal of the transistor Tr 21  is connected between resistors R 21  and R 22  wherein the resistor R 21  is connected to the output line L 1  and the resistor R 22  is grounded. Accordingly, a voltage which is obtained by dividing the constant voltage V 1 ′ by the resistors R 21  and R 22  is applied to the base terminal of the transistor Tr 21  as a reference voltage. A collector terminal of the transistor Tr 21  is connected to a base terminal of the control transistor Tr 20 . 
   A base terminal of the transistor Tr 22  is connected between resistors R 23  and R 24  wherein the resistor R 23  is connected to the output line L 2  and the resistor R 24  is grounded. Accordingly, a voltage which is obtained by dividing the constant voltage V 2 ′ by the resistors R 23  and R 24  is applied to the base terminal of the transistor Tr 22  as a detection voltage. A collector terminal of the transistor Tr 22  is connected to the power-supply line at the input side of the control transistor Tr 20 . A capacitor C 20  is connected to the output line L 2  in parallel with the resistor R 23 . 
   A constant-voltage circuit  3  is connected to the power-supply line through which the voltage V 3  is supplied, and the circuit configuration thereof is the same as that of the constant-voltage circuit  2 . An output line L 3  is connected to a control transistor Tr 30  and a constant voltage V 3 ′ is outputted through the output line L 3 . A base terminal of the transistor Tr 31  is connected between resistors R 31  and R 32  wherein the resistor R 31  is connected to the resistors R 21  and R 22 , and the resistor R 32  is grounded. Accordingly, a voltage which is obtained by dividing the voltage applied to the transistor Tr 21  by the resistors R 31  and R 32  is applied to the base terminal of the transistor Tr 31 . 
   An operation circuit  4  is connected to the output control terminal (ON/OFF) of the regulator  1 , and a voltage detection circuit  5  which is connected to the output lines L 2  and L 3  is connected to the operation circuit  4 . The operation circuit  4  has a transistor Tr 40  a collector of which is connected to the output control terminal (ON/OFF) of the regulator  1 , and an emitter of which is grounded. A resistor R 40  and a capacitor C 40  are connected to the base terminal of the transistor Tr 40 , wherein the resistor R 40  is connected to the voltage detection circuit  5  and the capacitor C 40  is grounded. 
   A circuit configuration of the voltage detection circuit  5  is as follows. The voltage detection circuit  5  comprises a transistor Tr 50  and a diode D 50  that are connected between the power-supply line at the input side of the regulator  1  and the resistor R 40  in series with each other, a transistor Tr 51  and a diode D 51  that are connected in parallel with the transistor Tr 50  and the diode D 50 , a Zener diode D 52  through which a base terminal of the transistor Tr 50  is connected to the output line L 2 , and a Zener diode D 53  through which a base terminal of the transistor Tr 51  is connected to the output line L 3 . 
   An operation of the short-circuit protective circuit having the above-mentioned circuit configuration will be described as follows. When the voltages V 1  to V 3  are supplied from the power-supply circuit, not shown, to each power-supply line, the voltage V 1  is first supplied to the input terminal (IN) and the output control terminal (ON/OFF) of the regulator  1 , thereby turning on the regulator  1 . Accordingly, the constant voltage V 1 ′ is outputted from the output terminal (OUT) of the regulator  1  to the output line L 1 . 
   A voltage that is obtained by dividing the constant voltage V 1 ′ by the resistors R 21  and R 22  is applied to the base terminal of the transistor Tr 21  as a reference voltage so that the transistor Tr 21  is turned on and the control transistor Tr 20  is brought into conduction, thereby outputting a voltage V 2 ′ to the output line L 2 . A detection voltage obtained by dividing the voltage V 2 ′ by the resistors R 23  and R 24  is applied to the base terminal of the transistor Tr 22 . 
   As mentioned above, since the transistors Tr 21  and Tr 22  operate in such a manner that the emitters of the transistors are connected to each other so as to flow a prescribed current, if the voltage V 2 ′ varies, the detection voltage applied to the base of the transistor Tr 22  also varies, and alters the current to the emitter so that the current to the emitter of the transistor Tr 21  varies in response to the change of the current in a manner that the sum of both currents remains constant. 
   For example, if the detection voltage increases, the current to the emitter of the transistor Tr 22  increases, and in response thereto, the current to the emitter of the transistor Tr 21  decreases. Accordingly, the detection voltage is controlled such that the current from the base terminal of the control transistor Tr 20  to the collector of the transistor Tr 21  decreases, and a collector voltage of the control transistor Tr 20  decreases. 
   In such a manner, if the detection voltage applied to the bases of the transistor Tr 22  varies, in response thereto, the voltage is controlled such that the collector voltage of the control transistor Tr 20  varies so as to allow the voltage V 2 ′ to remain constant. Accordingly, the circuit operates such that the reference voltage applied to the base terminal of the transistor Tr 21  and the detection voltage applied to the base terminal of the transistor Tr 22  remain constant. Since the reference voltage is maintained at a prescribed voltage level based on the output voltage from the regulator  1 , the level of the voltage V 2 ′ is set such that the detection voltage of the base of the transistor Tr 22  remains constant as the reference voltage. Accordingly, the voltage V 2 ′ is outputted as the constant voltage that is set based on the reference voltage, and the resistors R 23  and R 24 . The capacitor C 20  bypasses the variation of the voltage V 2 ′ to reflect it on the base of the transistor Tr 22 . 
   Like the constant-voltage circuit  2 , the constant-voltage circuit  3  outputs the voltage V 3 ′ as a constant voltage that is set based on the reference voltage applied to the base of the transistor Tr 31 , and the resistors R 33  and R 34 . 
   The voltage detection circuit  5  operates such that if a potential between both ends of the Zener diode D 52 , namely, the potential between the voltage at the input side of the regulator  1  and the voltage V 2 ′ of the output line L 2 , exceeds a breakdown voltage of the Zener diode D 52 , the Zener diode D 52  is brought into conduction, and a voltage is applied to the base of the transistor Tr 50 , thereby turning on the transistor Tr 50 . 
   When the transistor Tr 50  is turned on, a voltage is applied to the base of the transistor Tr 40  through the diode D 50  and the resistor R 40  so that the transistor Tr 40  is turned on. When the transistor Tr 40  is turned on, the output control terminal (ON/OFF) of the regulator  1  becomes a ground voltage, and it is turned off. 
   When the output control terminal (ON/OFF) of the regulator  1  is turned off, the output therefrom is stopped so that the output voltage of the output line L 1  is stopped, and the reference voltage applied to the transistors Tr 21  and Tr 31  is also stopped, thereby stopping the output voltages from the constant-voltage circuits  2  and  3  all at once. 
   Accordingly, if the output line L 2  is short-circuited for any reason to bring about a voltage drop, the Zener diode D 52  is brought into conduction that is detected by the voltage detection circuit  5 . As a result, the transistor Tr 50  is turned on to output the detection signal to the operation circuit  4 , and the transistor Tr 40  of the operation circuit  4  is also turned on, thereby stopping the outputs of the output lines L 1  to L 3  all at once. Therefore, it is possible to prevent a trouble involved in the voltage drop at the output line L 2  from influencing upon other circuits even at the very minimum level. 
   The voltage detection circuit  5  has the Zener diode D 53  for detecting the voltage drop of the output line L 3  in the same manner as the output line L 2 . Even in the case where a voltage drop occurs at the output line L 3 , the voltage detection circuit  5  outputs a detection signal to the operation circuit  4  in the same manner as the case where a voltage drop occurs at the output line L 2 , thereby stopping the outputs of all the output lines. 
   The diodes D 50  and D 51  are connected to each other to prevent reverse-current, while the resistor R 40  and the capacitor C 40  operate as a time constant circuit so as not to turn on the transistor Tr 40  in response to the detection signal from the voltage detection circuit  5  when started. 
   As mentioned above, if a voltage drop occurs at any of the output lines, outputs of all the output lines are stopped. That is, if a voltage drop occurs at the output line L 1 , it directly influences upon the output terminal of the regulator  1  so that the output of the regulator  1  is stopped by the known short-circuit protective function incorporated in the regulator  1 , thereby stopping the outputs of all the output lines. If a voltage drop occurs at the output line L 1  or L 2 , the detection signal is outputted from the voltage detection circuit  5 , thereby having the operation circuit  4  to stop the output of the regulator  1 , and hence the outputs of all the output lines are stopped. 
   If the breakdown voltages of the Zener diodes D 52  and D 53  are appropriately adjusted, the short-circuit protective circuit can cope with a voltage drop that influences upon other circuits in addition to the case of short circuit. In addition, a detection voltage may be fixed individually for each output line by setting breakdown voltages of the Zener diodes connected to each output line at different values. Furthermore, even if the regulator is restarted, the output thereof is stopped unless the voltage drop of the output line is dissolved, and it does not return to an ordinary state, thereby enhancing safety. 
   The disclosure of Japanese Patent Application No. 2003-432161 including specification, claims, and drawings, is incorporated herein by reference.