Abstract:
A control circuit is provided including a trigger circuit having a capacitor connected between a pulse generator and a second switching element, and a latch circuit connected via a diode, between a junction of a load with a first switching element and the control terminal of the second switching element, wherein when the pulse generator outputs a drive signal, both of the first switching element and the second switching element are turned on and the second switching element is held in the ON state through the latch circuit by a voltage at the junction. When the drive signal is stopped, both of the first switching element and the second switching element are turned off; meanwhile if the load is short-circuited, the first switching element and the second switching element are turned off through the latch circuit by the voltage at the junction.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an electronic control device for controlling vehicular equipment, mounted on an automobile, for example. 
     2. Description of the Related Art 
     Until now, an electronic control device has been disclosed that includes a current detection resister and a short-circuit protection circuit for detecting a short (short circuit) of a load based on a voltage generated by a current flowing through the current detection resister, so as to protect a load drive unit against an overcurrent, wherein when a current flowing through a drive circuit including the load and a switching element is detected exceeding a predetermined value, the current flowing through the circuit is limited or shut off. (For example, refer to Patent Document 1.)
     Patent Document 1: Japanese Laid-open Patent No. H3-106114   

     The conventional device disclosed in Patent Document 1, in order to limit or shut off the current flowing through the load drive unit when the current is detected exceeding a predetermined value, incorporates the current detection resister and the short-circuit protection circuit that detects the load short-circuiting, based on the voltage generated by the current flowing through the current detection resister, so as to protect the load drive unit against an overcurrent. However, the current detection resister needs to be connected in series with the load drive unit, and the resister not only wastes energy but also generates heat in itself, thereby leading to heating up the electronic control device as a whole. Moreover, in order to detect a large current, a resister having an appropriate power rating needs to be chosen, which in turn leads to an increase in the element size. Furthermore, when a short circuit is detected, the load drive unit needs to be held in a shut-off state; therefore, there has been a problem in that an extra latch circuit and the like need to be provided, leading to an increase in the circuit size. 
     SUMMARY OF THE INVENTION 
     The present invention has been made to resolve the foregoing problems with the conventional device, and aims at providing an electronic control device that enables protection against a short circuit with a simple circuit at low cost without using any current detection resister. 
     An electronic control device according to the present invention comprises: a first switching element for driving a load, connected in series with the load between a power source and ground; a second switching element for taking ON/OFF control of the first switching element, connected to the control terminal of the first switching element; a pulse generator for controlling the second switching element; and a control circuit including a trigger circuit having a capacitor connected between the pulse generator and the second switching element, and a latch circuit connected via a diode, between a first junction that is a connecting point of the load with the first switching element and the control terminal of the second switching element; wherein when the pulse generator outputs a drive signal, the control circuit turns on through the trigger circuit the second switching element for a predetermined time, thereby also turning on the first switching element so as to drive the load, and in addition, holds the second switching element in the ON state through the latch circuit by a voltage generated at the first junction, whereas when the pulse generator stops outputting the drive signal, the control circuit turns off both of the first switching element and the second switching element; meanwhile if the load is short-circuited, the control circuit turns off the second switching element through the latch circuit by the voltage generated at the first junction even when the pulse generator is outputting the drive signal, thereby also turning off the first switching element. 
     According to the present invention, an effect of obtaining an electronic control device can be brought about in which the current detection resister is not needed, its protection circuit is simplified, and as a result, protection against a short circuit is enabled at low cost. 
     The foregoing and other object, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a circuit configuration diagram for an electronic control device according to Embodiment 1 of the present invention; 
         FIG. 2  is a timing chart for explaining the operation of the electronic control device according to Embodiment 1 of the invention; and 
         FIG. 3  is a circuit configuration diagram for an electronic control device according to Embodiment 2 of the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Embodiment 1 
       FIG. 1  is a circuit diagram for an electronic control device according to Embodiment 1 of the present invention. In  FIG. 1 , a load  4  and a load drive transistor  11 , which is a first switching element, are connected in series with each other between a power source terminal  1  and ground  2 ; the load is energized or shut off with the load drive transistor  11  switched over between ON and OFF. The collector or drain of the load drive transistor  11  is connected to the load  4 , and the emitter or source thereof to ground  2 . 
     Firstly in the normal circuit operation, when a voltage from a pulse generator  3  changes from LOW to HIGH, a base current is supplied to a transistor  21 , which is a second switching element, for a predetermined time through a resister  22  and a capacitor  23  for triggering. The transistor  21  is turned on, and the load drive transistor is thereby supplied with a base current or a drain voltage so as to be turned on. 
     When the load drive transistor  11  is turned on, a current flows through the load  4 , so that the load  4  begins its operation. At this moment, the voltage at the collector or drain of the load drive transistor  11  becomes almost the same as that at ground  2  due to a voltage drop by the resistance of the load  4 . Before the trigger capacitor  23  is fully charged up and the base current to the transistor is thereby stopped, the voltage at the collector or drain of the load drive transistor  11  becomes almost the same as that at ground  2 , whereby the base current to the transistor  21  continues to flow through a resister  24  and a diode  25 , which constitute a latch circuit, and as a result, the transistor  21  and the load drive transistor  11  each continue to be on and the current supplied to the load  4  is continuously held. 
     Next, when the voltage from the pulse generator  3  changes from HIGH to LOW, the transistor  21  is turned off with its base current shut off. Therefore, the load drive transistor  11  is also turned off, and the load  4  is no longer supplied with the current, so that the load  4  stops the operation. 
     At this moment, the voltage at the collector or drain of the load drive transistor  11  becomes the same as that at the power source terminal  1 ; however a current does not flow into the capacitor through the diode  25 , and the voltage across the capacitor  23  is held at the LOW voltage of the pulse generator  3 . The diode  25  serves not only for the external load not to affect charging and discharging of the trigger capacitor  23  but also for a dark current not to flow into the circuit when the electronic control device is not in operation. 
     Next, if both ends of the load  4  are short-circuited with each other due to any abnormality while the load is in operation, the voltage at the collector or drain of the load drive transistor  11  becomes the same as that at the power source terminal  1 ; therefore, the voltage at the cathode of the diode  25  is made higher than that at the anode thereof, whereby the base current to the transistor flowing through the latch circuit made up of the resister  24  and the diode  25  is shut off, so that the transistor  21  is held in the OFF state. 
     Next, the operation of the above-configured electronic control device according to Embodiment 1 of the present invention will be explained in more detail. 
       FIG. 2  is a timing chart for explaining the operation of the electronic control device according to Embodiment 1 of the invention. “A” in  FIG. 2  shows a voltage waveform from the pulse generator; a waveform of an input current to the trigger capacitor  23 ; “C”, a voltage waveform at the base of the transistor  21 ; “D”, a voltage waveform at the base or gate of the load drive transistor  11 ; and “E”, a voltage waveform at the collector or drain of the load drive transistor  11 . 
     In  FIG. 2 , since the trigger capacitor  23  is charged up at time t 1  by the HIGH voltage output from the pulse generator  3 , a current flows through the emitter and base of the transistor  21  for a predetermine time. The transistor  21  is turned on by the current flowing through the emitter and base thereof, which supplies the transistor  11  with a base current or a gate voltage, so that the transistor  11  is turned on and a current begins flowing through the load  4  accordingly. 
     During the period (1) between time t 1  and time t 2 , before the trigger capacitor  23  is fully charged up after time t 1  and the current flowing therethrough is thereby stopped, the load drive transistor  11  is turned on, whereby the base current from the transistor  21  flows, through the resister  24 , the diode  25 , and the collector and emitter of the load drive transistor  11 , to ground  2 , so that the transistor  21  is held in the ON state. The load drive transistor  11  is also held in the ON state, and the current supplied to the load  4  is thereby continuously held. At this moment, the base voltage at the transistor  21  becomes a voltage lower than the HIGH voltage of the pulse generator  3  by a voltage between the base and emitter of the transistor  21 , VBE. 
     At time t 2  when the period (1) ends, the voltage output from the pulse generator  3  changes from HIGH to LOW, and the base voltage at the transistor  21  also changes from HIGH to LOW, thereby turning off the transistor. At this moment, the electric charge stored in the trigger capacitor is discharged to the pulse generator  3  through the resister  22 ; therefore a negative input current flows for a predetermined time. 
     In addition, when the transistor  21  is turned off, the base current or the gate voltage to the load drive transistor  11  is shut off, thereby turning off the transistor, and the current to the load  4  is consequently shut off. 
     Next, at time t 3 , since the trigger capacitor  23  is charged up by the HIGH voltage output again from the pulse generator  3 , a current flows through the emitter and base of the transistor  21  for a predetermine time. The transistor  21  is turned on by the current flowing through the emitter and base thereof, which supplies the load drive transistor  11  with the base current or the gate voltage, so that the transistor  11  is turned on and a current begins flowing through the load  4  accordingly. 
     After time t 3 , before the trigger capacitor  23  is fully charged up and the current through the capacitor is thereby stopped, the load drive transistor  11  is turned on, whereby the base current from the transistor  21  flows, through the resister  24 , the diode  25 , and the collector and emitter of the load drive transistor  11 , to ground  2 , so that the transistor  21  is held in the ON state. The load drive transistor  11  is also held in the ON state, and the current flowing through the load  4  is continuously held. At this moment, the base voltage at the transistor  21  becomes the voltage lower than the HIGH voltage of the pulse generator by the voltage between the base and emitter of the transistor  21 , VBE. 
     Now, if both ends of the load  4  are short-circuited with each other due to any abnormality at time t 4 , for example, the collector or drain voltage of the transistor  11  becomes the same as the voltage at the power source terminal  1 ; therefore, the voltage at the cathode of the diode  25  becomes higher than that at the anode thereof. At this moment, the base voltage of the transistor  21  rises up to the HIGH voltage of the pulse generator  3  from the voltage lower than the HIGH voltage of the pulse generator by the voltage between the base and emitter of the transistor  21 , VBE, so that the transistor  21  is turned off. Moreover, the trigger capacitor  23  is charged up in the meantime; therefore a current flows through the emitter and base of the transistor  21  for a predetermined time. 
     When transistor  21  is turned off, the base or gate voltage of the load drive transistor  11  is changed from HIGH to LOW. The load drive transistor  11  is thereby turned off, and a short-circuit current through the load  4  is shut off. 
     This short-circuit state of both ends of the load  4  is assumed to continue after time t 4  due to any abnormality. 
     Next, at time t 5  when the period (2) ends, the voltage output from the pulse generator  3  changes from HIGH to LOW and the voltage at the transistor  21  base also changes from HIGH to LOW accordingly; however the transistor  21  is in the OFF state after the load  4  has been detected short-circuiting; therefore the transistor  21  is held in the OFF state. 
     At this moment, the electric charge stored in the trigger capacitor  23  is discharged to the pulse generator  3  through the resister  22 ; therefore a negative input current flows for a predetermined time. Moreover, since the transistor  21  is held in the OFF state, the base current or the gate voltage of the load drive transistor  11  is held shut off, and the load drive transistor  11  is thereby held in the OFF state, so that the current through the load  4  is held shut off. 
     Next, during the period (3) between time t 6  and time t 7 , the trigger capacitor  23  is again charged up at time t 6  by the HIGH voltage output from the pulse generator  3 , a current flows through the emitter and base of the transistor  21  for a predetermined time. During this time, the transistor  21  is turned on, thereby turning on the load drive transistor  11 ; however except for that time, the load drive transistor  11  is turned off. By setting short enough the predetermined time in which the load drive transistor  11  is turned on, the transistor  11  is protected against the short-circuiting of both ends of the load  4 . 
     As described above, the electronic control device according to Embodiment 1 of the present invention does not detect a short circuit by a voltage generated across the current detection resister but is characterized in that protection against the short circuit can be ensured by allowing and/or forbidding the base or gate of the load drive transistor to be conductive by a voltage between the collector and emitter or between the drain and source of the load drive transistor, which is the first switching element for driving the load. 
     Moreover, the device is characterized in that a transistor that is the second switching element for allowing and/or forbidding the first switching element to be conductive is incorporated in the trigger circuit that supplies a current for a predetermined time synchronized with the pulse generator, and functions of both outputting the trigger for driving the load and forbidding the first switching element to be conductive if the load is detected short-circuiting are included in a single circuit. 
     Therefore according to Embodiment 1 of the present invention, an electronic control device can be obtained in which any current detection resister is not needed, the protection circuit is simplified, and as a result, protection against a short circuit is enabled at low cost. 
     Embodiment 2 
       FIG. 3  is a circuit diagram for an electronic control device according to Embodiment 2 of the present invention. In this Embodiment 2, a load drive transistor  111  and a transistor  121  correspond to the first switching element and the second switching element, respectively. That is to say, in  FIG. 3 , the load  4  and the first transistor  111  that is the switching transistor for driving the load are connected in series with each other between the power source terminal  1  and ground  2 . The load  4  is energized or shut off with the load transistor  111  switched over between ON and OFF. The collector or drain of the load drive transistor  111  is connected to the load  4 , and the emitter or source thereof to the power source terminal  1 . 
     Firstly, when the voltage from the pulse generator  3  changes from LOW to HIGH in the normal operation, a transistor  131  is supplied with a base current through a resister  132 , so as to be turned on. When the transistor  131  is turned on, a base current is supplied for a predetermined time to the transistor  121  through a capacitor  123 , a resister  122 , a resister  112 , and the collector and emitter of the transistor  131 . Therefore, the transistor  121  is turned on, and the load drive transistor  111  is thereby supplied with a base current or a drain voltage so as to be turned on. When the load drive transistor  111  is turned on, a current flows through the load  4 , so that the load  4  begins its operation. At this moment, the voltage at the collector or drain of the load drive transistor  111  becomes almost the same as that at the power source terminal  1  due to the voltage drop by the resistance of the load  4 . 
     Before the trigger capacitor  123  is fully charged up and the base current to the transistor  121  is thereby stopped, the voltage at the collector or drain of the load drive transistor  111  becomes almost the same as that at the power source terminal  1 , whereby the base current to the transistor  121  continues to flow through a resister  124  and a diode  125 , both transistor  121  and load drive transistor  111  continue to be on, and as a result, the current supplied to the load  4  is continuously held. 
     Next, when the voltage from the pulse generator  3  changes from HIGH to LOW, the base current to the transistor  131  is shut off and the transistor  131  is turned off accordingly, whereby the base current to the transistor  121  is also shut off and the transistor  121  is turned off. Therefore, the load drive transistor  111  is also turned off and the load  4  is no longer supplied with the current, so that the load  4  stops the operation. At this moment, the voltage at the collector or drain of the load drive transistor  111  becomes the same as that at ground  2 ; a current does not flow through the diode  125 , and as a result, the voltage across the capacitor  123  is held at the LOW voltage of the pulse generator  3 . The diode  125  serves for the external load not to affect charging and discharging of the trigger capacitor  123 . 
     Next, if both ends of the load are short-circuited with each other due to any abnormality while the load  4  is in operation, the voltage at the collector or drain of the load drive transistor  111  becomes the same as that at ground  2 ; therefore, the voltage at the anode of the diode  125  becomes lower than that at the cathode thereof, whereby the base current flowing to the transistor  121  through the diode  125  and the resister  124  is shut off, and as a result, the transistor  121  is held in the OFF state. 
     As described above, the electronic control device according to Embodiment 2 of the present invention is characterized the same as that of Embodiment 1 as follows: 
     (1) The logical value (HIGH or LOW potential) at the collector or drain of the load drive transistor is reversed if a short (short circuit) of the load is detected, and by making use of this, the control circuit that turns on and off the load drive transistor, which is the first switching element connected between the load and the power source terminal, continuously holds in the OFF state the transistor, which is the second switching element provided on the base side of the load drive transistor, for allowing and/or forbidding the load drive transistor to be conductive, and thereby also holds the load drive transistor continuously in the OFF state, so as to enable protection against the short circuit to be ensured. 
     (2) Moreover, when turning on the load drive transistor, which is the first switching element for driving the load, the control circuit for turning on and off the transistor temporarily turns on the transistor by the trigger circuit that supplies a current for a predetermined time synchronized with the pulse generator. Once the load drive transistor is turned on, the logical value (HIGH or LOW potential) at the collector or drain of the load drive transistor is reversed, and by making use of this, the control circuit can hold the load drive transistor in the ON state. 
     (3) Moreover, the control circuit incorporates a transistor that is the second switching element for allowing and/or forbidding the load drive transistor to be conductive, as part of the trigger circuit that supplies the current to the load drive transistor, when it is turned on, for the predetermined time synchronized with the pulse generator, whereby the protection circuit can be simplified. 
     In the foregoing Embodiments 1 and 2, the explanation has been made on a case in which both ends of the load are short-circuited with each other; however, this invention is not limited to this, but can be applied to other cases in which a predetermined voltage is generated at the collector of the load drive transistor, and similar effects can be brought about in protecting the load drive transistor. 
     Various modifications and alterations of this invention will be apparent to those skilled in the art without departing from the scope and spirit of this invention, and it should be understood that this is not limited to the illustrative embodiments set forth herein.