Abstract:
The present invention is to provide a power supply control apparatus which can connect a ground to a suitable electric potential when the ground is disconnected. The power supply control apparatus includes a control circuit having a switch element and a switch control unit, and a load. One terminal of load is connected to a direct-current power supply through the switch element, and the other terminal is connected to a ground electric potential. The switch control unit has a ground terminal connected to the ground electric potential and outputting a ground current flowing toward the ground electric potential. The control circuit includes a bypass device having a load side bypass system for passing the ground current to the ground electric potential through the load when connection between the ground terminal and the ground electric potential is disconnected. The load includes impedance for changing the electric potential of ground terminal into an electric potential in which the switch control unit stably acts when the ground current flows.

Description:
The priority application Number Japan Patent Application No. 2007-230215 upon which this patent application is based is hereby incorporated by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a power supply control apparatus for controlling power supply connection and power supply disconnection to a load. 
     2. Description of the Related Art 
     In an electronic circuit formed from discrete semiconductor parts, IC (Integrated Circuit) and so on, the electronic circuit is designed based on connecting a power supply and a ground to a normal electric potential. For example, when only the power supply of electronic circuit is connected to a electric potential of power supply and the ground is nowhere connected (that is, non-connection condition of ground), action of this electronic circuit becomes unstable. Specially, in an electric circuit mounted on a power supply control apparatus, when an unexpected high-current by the unstable action flows into the electronic circuit, elements of electronic circuit are destroyed. As a result, there is a possibility that the electronic circuit becomes a dangerous state such as ignition. Therefore, in the patent document 1, an earth connecting mechanism for avoiding the above unsteady action is proposed. 
     According to the patent document 1, when a power feeding side connector and a power incoming side connector are connected, an earth terminal is connected before a power supply terminal is connected. Thereby, in an electronic circuit arranged after the power incoming side connector, the non-connection state of ground is not caused. Furthermore, the power supply and the earth are connected by a pair of connector. Therefore, when the connector comes off, the power supply and the earth come off at the same time. Thereby, in an electronic circuit connecting a power supply and an earth to each separate connecter, the earth side connector holds a connected state. In addition, the non-connection state of ground is not caused. By using the above mentioned earth connecting mechanism of electric connector, generation of non-connection state of ground is prevented, and unsteady action state of electric circuit is avoided. 
     Patent Documents 1 
     
         
         Japan published patent application H08-78095 
       
    
     SUMMARY OF THE INVENTION 
     Objects to be Solved 
     However, the above mentioned earth connecting mechanism of the patent document 1 prevents a non-connection state of ground which is caused when an electric connector is connected and disconnected. Therefore, the earth connecting mechanism can not prevent non-connection state of ground which is caused when an earth wire connected to an earth terminal of power feeding side connector is disconnected. Furthermore, as a result, in this case, destruction of circuit element or ignition of apparatus may occur. 
     Consequently, the present invention provides a power supply control apparatus which can connect a ground of electronic circuit to a suitable electric potential when a ground connection of electric circuit is disconnected. 
     How to Attain the Object of the Present Invention 
     According to a first aspect of the present invention, a power supply control apparatus includes a control circuit and a load. The control circuit has a switch element opening and closing a circuit by a switch signal and a switch control unit outputting the switch signal. The load connects one terminal to a DC (direct-current) power supply through the switch element and connecting another terminal to a ground electric potential. The switch control unit has a ground terminal connected to the ground electric potential and outputting a ground current flowing toward the ground electric potential. The control circuit has a bypass device including a load side bypass system for applying the ground current to the ground electric potential through the load when connection between the ground terminal and the ground electric potential is disconnected. The load has impedance for changing the electric potential of the ground terminal into an electric potential in which the switch control unit stably acts when the ground current flows. 
     According to a second aspect of the present invention, the control circuit has a control signal wire connected to the ground electric potential when the switch element closes. The bypass device has a signal side bypass system for applying the ground current to the ground electric potential through the control signal wire when the connection between the ground terminal and the ground electric potential is disconnected and the switch element closes. The load side bypass system applies the ground current to the ground electric potential through the load when the connection between the ground terminal and the ground electric potential is disconnected and the switch element opens 
     According to a third aspect of the present invention, the bypass device has a reverse current preventive system. The reverse current preventive system always closes a circuit so as to apply a current to the load when the bypass device is connected to the load side bypass system in series and the DC power supply is normally connected, and the reverse current preventive system always opens the circuit so as to prevent the current from flowing into the load side bypass system when the DC power supply is connected in reverse. 
     Effect of the Invention 
     According to the invention, when connection between a ground terminal of switch control unit and a ground electric potential is disconnected, the ground terminal of switch control unit is connected to an electric potential in which the switch control unit stably acts. Thereby, a non-connection state is not caused in a control circuit. Furthermore, it is possible to prevent unsteady action of control circuit. Therefore, destruction of circuit element or ignition of apparatus can be prevented. 
     According to the invention, the ground terminal of switch control unit and the ground electric potential are connected through a control signal wire when a switch element closes. That is, the ground terminal of switch control unit is connected to the ground electric potential through a control signal wire when a DC (direct-current) power supply is connected to a load. Thereby, although the electric potential increases by connecting the DC power supply to the load, the ground terminal of switch control unit can be connected to the ground electric potential through the control signal wire. Therefore, it is securely possible to prevent unstable action of control circuit. As a result, destruction of circuit element or ignition of apparatus can be avoided. 
     According to the invention, the bypass device has a reverse current preventive system. Thereby, when the DC power supply is mistakenly connected in reverse, a short path of DC power supply is not formed by virtue of the bypass device. Therefore, the power supply control apparatus of the present invention can prevent damage of apparatus by short, and can provide a more safe power supply control apparatus. 
     The above and other objects and features of this invention will become more apparent from the following description taken in conjunction with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a circuit diagram showing first embodiment of a power supply control apparatus of the present invention; 
         FIG. 2  illustrates that earth connection is disconnected in point P at the power supply control apparatus of  FIG. 1 ; 
         FIG. 3  is a circuit diagram showing second embodiment of the power supply control apparatus of the present invention; 
         FIG. 4  illustrate that earth connection is disconnected in point P at the power supply control apparatus of  FIG. 3 ; 
         FIG. 5  is a chart showing flow of voltage in each part when the earth connection of point P is disconnected in the power supply control apparatus of  FIG. 3 ; and 
         FIG. 6  illustrates action when a DC (Direct-current) power is connected in reverse at the power supply control apparatus of  FIG. 3 . 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     In the following, first embodiment of the present invention is explained with reference to  FIGS. 1 and 2 . 
     As shown in  FIG. 1 , a power supply control apparatus  1  is mounted on a vehicle, and controls a power supply against a load of heating wire by a control signal outputted from an electronic control unit. The power supply control apparatus  1  includes a control circuit  10  and a heating wire  20  connected to the control circuit  10 . 
     The control circuit  10  controls connection and disconnection of DC (direct-current) power supply  4  against the heating wire  20 . The control circuit  10  has a FET (Field-Effect Transistor)  11 , a switch control unit  12 , and a bypass device  15 . In addition, the control circuit  10  has a power supply terminal  10   a , a load terminal  10   b , a signal terminal  10   c , and an earth terminal  10   d . The power supply terminal  10   a  is connected to the DC power supply  4  including a power supply voltage VCC. The load terminal  10   b  is connected to the heating wire  20 . The signal terminal  10   c  is connected to a signal wire  32  connecting the control circuit  10  with an electronic control unit  30 . The earth terminal  10   d  is connected to a ground electric potential  5   a.    
     The FET  11  corresponds to a switch element of claims. For example, the FET  11  is applied with an existing N-channel power MOSFET for power supply purpose. A gate terminal (G) is connected to a switch signal terminal  12   b , a drain terminal (D) is connected to the power supply terminal  10   a , and a source terminal (S) is connected to the load terminal  10   b , respectively. According to a switch signal S 1  outputted from the switch control unit, the FET  11  switches electrical signals ON/OFF (that is, closed circuit/opened circuit). Thereby, the heating wire  20  is connected or disconnected to the DC power supply  4 . 
     The switch control unit  12  is formed from semiconductor components or IC (Integrated Circuit) such as an operational amplifier or transistor and so on. The switch control unit  12  generates the switch signal S 1 , and controls ON/OFF of FET  11 . Furthermore, the switch control unit  12  has a ground terminal  12   a , a switch signal terminal  12   b , a power supply terminal  12   c  of switch control unit  12  (hereafter, the power supply terminal  12   c ), and an input terminal of request signal for connecting with the power supply  12   d  (hereafter, the input terminal  12   d ). The ground terminal  12   a  is connected to the earth terminal  10   d  through a common ground  18 . The switch signal terminal  12   b  is connected to the gate (G) of FET  11 . The power supply terminal  12   c  is connected to the power supply terminal  10   a . The input terminal  12   d  is connected to the signal terminal  10   c  through a control signal wire  6 . 
     when the power supply voltage VCC (for example, 14V) is connected to the power supply terminal  12   c , and a ground electric potential VG (for example, 0V) is connected to the ground terminal  12   a , the switch control unit  12  is normally powered. However, in a case that a voltage (that is, electric potential) of the power supply terminal  12   c  or a voltage of the ground terminal  12   a  is in the allowable range (for example, less than ±1.0V), it is possible to work all right. In addition, when the switch control unit  12  works, a ground current I outputted from the ground terminal  12   a  is equal to or lower than 20 mA. 
     The control signal wire  6  is arranged between the signal terminal  10   c  of control circuit  10  and the request signal input terminal  12   d  of switch control unit  12 , and connected with the signal wire  32  through the signal terminal  10   c . The control signal wire  6  inputs a request signal for connecting with the power supply S 2  (hereafter, request signal S 2 ), which is outputted from a later-described electronic control unit  30  toward the power supply control apparatus  1  (that is, control circuit  10 ), into the input terminal  12   d  of switch control unit  12 . In the embodiments of the present invention, the control signal wire  6  is connected to the switch control unit  12  but not limited thereto. The control signal wire  6  can be connected to various parts becoming the ground electric potential VG when the FET  11  is OFF. 
     The bypass device  15  has a load side diode  15   a . The load side diode  15   a  corresponds to a load side bypass system of claims. An anode terminal of diode  15   a  is connected to common ground  18  (that is, ground terminal  12   a ), and a cathode terminal of diode  15   a  is connected to the load terminal  10   b , respectively. In the embodiments of the present invention, the FET  11 , the switch control unit  12  and the load side diode  15   a  are separate parts, but not limited thereto. Those parts can be integrated with IC, and be applied with the control circuit  10 . 
     The heating wire  20  corresponds to load of claims. For example, the heating wire  20  is arranged at rear window of vehicle. The heating wire  20  heats the rear window, and demists. One terminal  20   a  of heating wire  20  is connected to the DC power supply  4  through the FET  11 , and the other terminal  20   b  of heating wire  20  is connected to the ground electric potential  5   b . Furthermore, as the heating wire  20  increases calorific value, a low resistance value (that is, impedance), for example resistance of about 0.7Ω, is used. The heating wire  20  is used in the embodiments of the present invention, but not limited thereto. For example, headlights of vehicle can be used. That is to say, any parts can be used if a voltage of the ground terminal  12   a  is in the range of allowable voltage for working VR (that is, voltage in which the switch control unit  12  works stably) when the ground current I flows. 
     The DC power supply  4  is formed with a battery mounted on a vehicle or a DC (Direct-Current)/DC-converter converting a voltage of battery and a direct-current voltage different from the voltage of battery. In addition, the DC power supply  4  is connected to the control circuit  10  and the heating wire  20 , and provides the power supply voltage VCC to them. A negative terminal of DC power supply  4  is connected to a ground electric potential  5   c.    
     The electronic control unit  30  is mounted on a vehicle, and controls headlights, an air conditioner, and heating wires of rear wind and so on. According to operation of user, the electric control unit  30  requests connection or disconnection of power supply to the heating wire  20  in the power supply control apparatus  1 . The electric control unit  30  has a control unit (not shown), a transistor  31 , and the signal wire  32  connected to the transistor  31 . 
     The transistor  31  is a NPN (Negative-Positive-Negative) transistor for small signal. A base terminal of transistor  31  is connected to not shown control unit, a collector terminal of transistor  31  is connected to the signal wire  32 , and an emitter terminal of transistor  31  is connected to a ground electric potential  5   d.    
     The signal wire  32  connects electrically the electric control unit  30  and the control circuit  10 , and transmits the request signal S 2 . In particular, the signal wire  32  is formed with a coated conductive wire, which is arranged between the collector terminal of transistor  31  and the signal terminal  10   c  of control circuit  10 , and a connector. Furthermore, the signal wire  32  is connected to a power supply voltage of electric control unit  30  through a pull-up resistor (not shown). When the transistor  31  is ON, the transistor  31  is connected to the ground electric potential  5   d . Thereby, the signal wire  32  becomes the ground electric potential VG. On the other hand, when the transistor  31  is OFF, the signal wire  32  is connected to the power supply voltage of electric control unit  30  through the pull-up resistor. Thereby, the signal wire  32  becomes an electric potential of power supply voltage of electric control unit  30  (for example, 14V). That is to say, an electric potential of signal wire  32  becomes the request signal S 2 . In the embodiments of the present invention, the transistor  31  is used for switching electric potential of signal wire  32 , but not limited thereto. For example, a mechanical switch and so on can be used. That is, any parts can be applied if the signal wire  32  can be connected to the ground electric potential VG when connection of power supply is requested. 
     The request signal S 2  requests the power supply connection and disconnection toward the heating wire  20 , and is transmitted from the electronic control unit  30  toward the control circuit  10 . When the power supply connection is requested, the request signal S 2  becomes L-Level (that is, ground electric potential VG). On the other hand, when the disconnection of power supply is requested, the request signal S 2  becomes H-Level (that is, electric potential of power supply voltage of electronic control unit  30 ). 
     Depending on the request signal S 2 , the switch signal S 1  becomes the power supply voltage VCC or the ground electric potential VG. In particular, when the request signal S 2  is the H-Level, the switch signal S 1  becomes the ground electric potential VG. Thereby, the FET  11  becomes OFF. On the other hand, when the request signal S 2  is the L-Level, the switch signal S 1  becomes the power supply voltage VCC. Thereby, the FET  11  becomes ON. 
     Next, in the above power supply control apparatus  1 , action when connection between the ground terminal  12   a  of switch control unit  12  and the ground electric potential  5   a  is disconnected is explained by referring to  FIG. 2  as an example. 
     In the power supply control apparatus  1 , in a case that the DC power supply  4  is not connected to the heating wire  20  (that is, when FET  11  is OFF), when connection between the earth terminal  10   d  and the ground electric potential  5   a  is disconnected by disconnecting in a point “P” shown in  FIG. 2 , an electric potential of terminal  20   a  of heating wire  20  becomes the ground electric potential VG. Thereby, the ground current I of switch control unit  12  flows along a current pathway I 1  from the heating wire  20  toward the ground electric potential  5   b  through the load side diode  15   a . A resistance value of heating wire  20  is 0.7Ω, and the ground current I of switch control unit  12  is at the most 20 mA. Thereby, an electric potential of terminal  20   a  of heating wire  20  becomes 0.014V at a maximum. As a result, the electric potential of ground terminal  12   a  in the switch control unit  12  is kept within the allowable voltage for working VR (0V±1.0V) even if considering voltage drop of load side diode  15   a  in a forward direction (generally, about 0.6V). Therefore, the control circuit  10  continues normal action, and maintains condition of power supply disconnection of heating wire  20 . 
     As mentioned above, according to embodiments of the present invention, when the connection between the ground terminal  12   a  of switch control unit  12  and the ground electric potential  5   a  is disconnected, the ground terminal  12   a  of switch control unit  12  is connected to an electric potential in a allowable range for working. Thereby, non-connection condition of ground is not caused in the control circuit  10 . Furthermore, it is possible to prevent unstable action of control circuit  10 . Therefore, destruction of circuit element or ignition of apparatus can be prevented. 
     Next, second embodiment of the present invention is explained with reference to  FIGS. 3 to 6 . 
     As shown in  FIG. 3 , a power supply control apparatus  2  is mounted on a vehicle, and controls a power supply against a load of heating wire by a control signal outputted from an electronic control unit. The power supply control apparatus  2  includes the control circuit  10  and the heating wire  20  connected the control circuit  10 . 
     The control circuit  10  controls connection and disconnection of DC (direct-current) power supply  4  against the heating wire  20 . The control circuit  10  has the FET  11 , the switch control unit  12 , and a bypass device  151 . In this second embodiment, the same portions are referred to the same reference signs as the first embodiment except the bypass device  151 . 
     The bypass device  151  has the load side diode  15   a , a signal side diode  15   b , a FET (Field-Effect Transistor)  15   c , and fixed resistors R 1  and R 2 . 
     The load side diode  15   a  corresponds to a load side bypass system of claims. An anode terminal of load side diode  15   a  is connected to a source terminal (S) of FET  15   c , and a cathode terminal of load side diode  15   a  is connected to the load terminal  10   b , respectively. 
     The signal side diode  15   b  corresponds to a signal side bypass system of claims. An anode terminal of signal side diode  15   b  is connected to the source terminal (S) of FET  15   c , and a cathode terminal of signal side diode  15   b  is connected to the control signal wire  6 , respectively. 
     The FET  15   c  corresponds to a reverse current preventive system of claims. For example, the FET  15   c  is applied with an existing N-channel MOSFET which can pass several tens mA (milliampere) of current. A gate terminal (G) of FET  15   c  is connected to a power supply voltage VCC through the fixed resistor R 1  about 20 kΩ to 100 kΩ. A source terminal (S) of FET  15   c  is connected to the power supply voltage VCC through the fixed resistor R 2  about 20 kΩ to 100 kΩ. A drain terminal (D) of FET  15   c  is connected to the common ground  18  of control circuit  10  (that is, the ground terminal  12   a ). When the DC power supply  4  is normally connected, the FET  11  is always ON (that is, closed circuit). On the other hand, when the DC power supply  4  is connected in reverse, the FET  11  is always OFF (that is, opened circuit). 
     Next, in the above power supply control apparatus  2 , action when connection between the ground terminal  12   a  of switch control unit  12  and the ground electric potential  5   a  is disconnected is explained by referring to  FIGS. 4 and 5  as an example. 
     In the power supply control apparatus  2 , in a case that the DC power supply  4  is not connected to the heating wire  20  (that is, when request signal S 2  is H-Level), when connection between the earth terminal  10   b  and the ground electric potential  5   a  is not connected by disconnecting in a point “P” shown in  FIG. 4 , the electric potential of terminal  20   a  of heating wire  20  becomes the ground electric potential VG. Thereby, the ground current I of switch control unit  12  flows along a current pathway I 21  from the heating wire  20  toward the ground electric potential  5   b  through the load side diode  15   a . A resistance value of heating wire  20  is 0.7Ω, and the ground current I of switch control unit  12  is at the most 20 mA. Thereby, an electric potential of terminal  20   a  of heating wire  20  becomes 0.014V at a maximum. As a result, the electric potential of ground terminal  12   a  in the switch control unit  12  is kept within the allowable range of voltage for acting VR (0V±1.0V) even if considering voltage drop of load side diode  15   a  in a forward direction (generally, about 0.6V). Therefore, the control circuit  10  continues normal action, and maintains condition of power supply disconnection of heating wire  20 . 
     When the power supply connection is requested from the electronic control unit  30  (that is, the request signal S 2  is L-Level), the heating wire  20  is connected to the DC power supply  4 . As a result, the electric potential of terminal  20   a  of heating wire  20  becomes the power supply voltage VCC. Therefore, as shown in  FIG. 4 , the ground current I of switch control unit flows toward a lower electric potential. That is, the ground current I of switch control unit flows along a current pathway I 22  from the control signal wire  6  toward the ground electric potential  5   d  through the signal side diode  15   d . In this time, the electric potential of ground terminal  12   a  of switch control unit  12  is kept within the allowable voltage for working VR even if considering voltage drop of signal side diode  15   b  in a forward direction (generally, about 0.6V). Therefore, the control circuit  10  continues normal action, and maintains condition of power supply disconnection of heating wire  20 . 
     On the other hand, when power supply disconnection is requested from the electronic control unit  30  (that is, the request signal S 2  is the H-Level), the connection between the heating wire  20  and the DC power supply  4  is disconnected. As a result, the electric potential of terminal  20   a  of heating wire  20  returns to the ground electric potential VG again. Therefore, the ground current I of switch control unit  12  flows toward a lower electric potential. That is, the ground current I of switch control unit flows along a current pathway I 21  from the heating wire  20  toward the ground electric potential  5   b  through the load side diode  15   a  again. As described above, the electric potential of terminal  12   a  of switch control unit  12  is kept within the allowable voltage for working VR. Therefore, the control circuit  10  continues normal action, and maintains condition of power supply disconnection of heating wire  20 . 
     Next, in the above mentioned power supply control apparatus  2 , action (function) when the DC power supply  4  is connected in reverse is explained with reference to  FIG. 6 . 
     In the power supply control apparatus  2 , when the DC power supply  4  is normally connected, a positive supply voltage is applied between the gate terminal (G) of FET  15   c  and the source terminal (S). Thereby, the FET  15   c  becomes normally ON. However, as shown in  FIG. 6 , when the DC power supply  4  is connected in reverse, there is not so much of a difference in electric potential between the gate terminal (G) and the source terminal (s). Therefore, the FET  15   c  is always OFF. For this reason, a short pathway I 31  of DC power supply  4  is disconnected by the FET  15   c , and generation of pathway which shorts the DC power supply  4  is avoided. 
     As described above, in the embodiments, when the DC power supply  4  is disconnected to the heating wire  20 , the ground terminal  12   a  of switch control unit  12  is connected to the ground electric potential  5   b  through the heating wire  20 . On the other hand, when the DC power supply  4  is connected to the heating wire  20 , the ground terminal  12   a  of switch control unit  12  is connected to the ground electric potential  5   d  through the control signal wire  6 . Thereby, according to action condition of control circuit  10 , the ground terminal  12   a  of switch control unit  12  can be connected to an electric potential near to a ground electric potential. Consequently, it is possible to prevent unstable action of control circuit  10  more securely. Furthermore, destruction of circuit element or ignition of apparatus can be prevented. 
     In addition, when the DC power supply  4  is mistakenly connected in reverse by the FET  15   c , the short pathway I 13  of DC power supply  4  is not formed by virtue of the bypass device  151 . Therefore, damage of power supply control apparatus  2  by short can be prevented. Furthermore, more safety power supply control apparatus can be provided. 
     While, in the embodiment, the present invention is described, it is not limited thereto. Various change and modifications can be made with the scope of the present invention.