Abstract:
A power supply system contains two batteries and a load device which is connected to the batteries such that, upon failure of one of the batteries, the other takes over the supply of power. The batteries and the load device are connected to one another by a single ground line and arranged such that, upon an interruption between the interconnected grounds of the batteries and the ground of the load device, the load device is fed with the difference voltage of the batteries. A reliably operating power supply system is created which is operational by with only one ground line.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of priority based on German Application No. 19913131.7, filed on Mar. 23, 1999, which is incorporated by reference herein in its entirety. 
     BACKGROUND OF THE INVENTION 
     The invention relates to a power supply system with two batteries of different voltages and at least one load device which is connected in parallel with both batteries, such that if one battery fails, the other battery assures the supply of power. Such duplicate power supplies are gaining interest in areas such as automobiles, where it is necessary to assure operation of apparatus critical to safety. 
     FIG. 4 shows a block circuit diagram of a conventional power supply system. Voltage terminals  2  and  4  of an electronic control apparatus  6  are connected each through fuses  8  and  10 , respectively, to batteries  12  and  14 , respectively. Battery  12  in the example shown has a voltage of 12 V and battery  14  has a voltage of 42 V. Each voltage terminal leads to power supply centers  16  and  18 , respectively, which are matched to a particular battery voltage. Each power supply center is connected to an electronic control unit  20  whose output signal operates, through an output  22 , a switch  24  which is contained in a unit  26  to be controlled by the control apparatus  6 . 
     The power is supplied to the unit  26  via power supply lines  28  and  30 , each connected to one of the two batteries  12  and  14 , respectively, and connected by diodes  32  and  34 , respectively, to the input terminal of switch  24 . 
     A ground conductor  36  runs from the battery  14  to the ground connection of the control apparatus  6  and to the ground connection of the unit  26 . The ground of battery  12  is connected by an additional ground line, which can be the vehicle body itself, to the ground of the control apparatus  6 . 
     The operation of this conventional power supply system is well known in the art. If either of the two batteries  12  or  14  fails, the remaining battery supplies the power. The diodes  32  and  34  and/or the power supply centers provide assurance that current flows only from the operational battery to the unit  26  and prevents the occurrence of a short circuit discharge. 
     In order to reduce cost and weight, and to improve safety, it is desirable to eliminate the additional ground connection  36 , which is typically designed with a large cross section. 
     Additionally, the diagnosis of the two separate grounds can be difficult and is prone to errors. 
     A short circuit in the lines of connected units or sensors (not shown) should have as little effect as possible on the operation of the overall system. 
     Another conventional power supply system is disclosed in DE 196 02 171 A1. According to this power supply system, the power input of a voltage regulator is connected through a double diode to both a main line connected to the positive pole of a battery and to a secondary line which is connected through an ignition switch to the positive pole of the on-board network. A buffer condenser is connected to the secondary line and all grounds of the system are brought together. In the event of a voltage collapse on the main line, the power is supplied through the secondary line with current flowing from the buffer condenser. If the voltage on the secondary line collapses, power is supplied through the buffer condenser, which is of sufficient size to buffer the power input. 
     Yet another conventional power supply system is disclosed in DE 196 45 944 A1. This power supply system contains a controller for an on-board network with at least two batteries which serve to power various load devices. The controller is between the two batteries and contains a power supply center which is connected through diodes to the two batteries. The system has a common ground. The controller also has a bidirectional connection for a bus, such as a CAN bus. 
     In the event of interruptions of the ground connection between the batteries, failures, which are difficult to diagnose, can occur in the load device of the conventional power supply systems described above. 
     SUMMARY OF THE INVENTION 
     The present invention provides for a power supply system with improved operating reliability. 
     The present invention also provides a power supply that assures power is supplied in the event of a battery failure, and when the ground connections between the batteries on the one hand and the load on the other hand fail, the load is supplied with the voltage difference between the batteries. 
     With the power supply system according to the present invention, a supply of power is assured in the event of virtually all individual failures. If a failure occurs in the ground of the load device, the potential applied to this device is increased by the smaller battery voltage. The battery of weaker voltage is then charged by the supply current. 
     In the case of a motor vehicle, the ground terminals of both batteries can be common with the car body. Individual failures in power lines do not lead to a loss of function. With a simple voltage measurement or threshold circuit, the lines can be diagnosed with a trouble signal at which time the car can be taken to a service shop. 
     The invention is applicable wherever load devices are supplied with current by two batteries of different voltage, so that power will be supplied even if one battery fails or there is another problem. The invention is especially advantageous when used in motor vehicles where numerous safety-critical load devices are to be supplied with power with maximum reliability (e.g., electronic motor controls, airbags, etc.). 
     The present invention is achieved by providing a power supply system. The system comprises a first battery connected to a first line at a first voltage relative to ground; a second battery connected to a second line at a second voltage relative to ground; a load device having a third line adapted to be connected to one of the first and second lines and having a fourth line adapted to be connected to ground; and a set of diodes associated with the load device. The set of diodes includes a first diode permitting current flow from the first line to the third line when the first voltage is greater than the second voltage; a second diode permitting current flow from the second line to the third line when the second voltage is greater than the first voltage; a third diode permitting current flow from the fourth line to the second line when i) the first voltage is greater than the second voltage and ii) the fourth line is disconnected from ground; and a fourth diode permitting current flow from the fourth line to the first line when i) the second voltage is greater than the first voltage and ii) the fourth line is disconnected from ground. 
     The present invention is also achieved by providing a power supply system for connecting a first battery and a second battery to a load device. Each of the batteries has a positive terminal and a negative terminal, and the load device has an inlet and an outlet. The system comprises a first diode having a first anode adapted to be connected to the positive terminal of the first battery and a first cathode adapted to be connected to the input of the load device; a second diode having a second anode adapted to be connected to the positive terminal of the second battery and a second cathode adapted to be connected to the input of the load device; a third diode having a third anode adapted to be connected to the output of the load device and a third cathode adapted to be connected to the positive terminal of the first battery; and a fourth diode having a fourth anode adapted to be connected to the output of the load device and a fourth cathode adapted to be connected to the positive terminal of the second battery. 
     The present invention is further achieved by providing a method of supplying power from first and second batteries to a load device. The first battery has a first voltage relative to ground, the second battery has a second voltage relative to ground, and the load device has an inlet and outlet. The method comprises supplying a first current to the load device inlet from the first battery when the first voltage is greater than the second voltage; supplying a second current to the load device inlet from the second battery when the second voltage is greater than the first voltage; connecting the second battery in series between the output of the load device and the first battery when i) the first voltage is greater than the second voltage and ii) a common ground between the load device and the two batteries is broken; and connecting the second battery in series between the first battery and the input of the load device when i) the second voltage is greater than the first voltage and ii) the common ground is broken. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The accompanying drawings, which are incorporated herein and constitute part of this specification, illustrate presently preferred embodiments of the invention, and, together with the general description given above and the detailed description given below, serve to explain features of the invention. 
     FIG. 1 is a schematic diagram of a power supply system according to the present invention. 
     FIG. 2 is a current flow diagram of the circuit shown in FIG. 1, in the case of existing ground faults. 
     FIG. 3 is a current flow diagram of the present invention in the case a voltage loss in one battery and the ground faults of FIG.  2 . 
     FIG. 4 is a conventional power supply system. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 1 is a schematic diagram of a power supply system according to the present invention. Voltage terminals  2  and  4  of a control apparatus  6  are connected through fuses  8  and  10 , respectively, to the positive terminal of each of the two batteries  12  and  14 , respectively. The battery  12  in the example shown supplies a voltage of 12 V and battery  14  supplies a voltage of 42 V. In the control apparatus  6 , diodes  40  and  42  are connected to terminals  2  and  4 , respectively, so that the batteries  12  and  14  are connected in parallel at the input of a power supply center  44 . The diodes  40  and  42  ensure that battery  14  cannot discharge through battery  12 , and in the event of a fault in battery  14  (e.g., a voltage collapse due to complete discharge), battery  14  cannot discharge through battery  12 . 
     The power supply center can be a conventional switched mode power supply center that enables operation with supply voltages between, for example, 6 and 50 V. The power supply center  44  is followed by a control unit  20  which is energized by the power supply center with a supply voltage of 5 V. 
     A unit  26 , e.g., a servo motor, can be connected by lines  46  and  48  to supply lines  50  and  52 , respectively, coming from batteries  12  and  14 , respectively. Each of the supply lines  46  and  48  contains diodes  32  and  34 , respectively. 
     An additional load device, e.g., a sensor  58 , can also be connected to the supply lines  50  and  52  through lines  60  and  62 , respectively, in each of which is a diode  64  and  66 , respectively. 
     The ground connection of the entire system is made through a single ground line  70  that connects all grounds. 
     Connecting lines  72  and  74  are connected to the ground line  70  and to the supply lines  50  and  52 , respectively, on the sides of the diodes  40  and  42 , respectively, that are remote from the power supply center  44 . 
     In each of the lines  72  and  74  there is a diode  76  and  78 , respectively, which permits current to flow only from the ground line to the supply line. 
     Similarly, the ground of unit  26  is connected by connecting lines  80  and  82  with diodes  84  and  86 , respectively, to lines  46  and  48 , respectively. Also, the ground of the sensor  58  is connected by connecting lines  88  and  90  with the diodes  92  and  94  that connect them to lines  60  and  62 , respectively. 
     Switch means  96  and  98  (e.g., relays or power FET&#39;s), are connected to the output of the control unit  20  and can interrupt the lines  46  and  48  according to an output signal from the control unit  20 . 
     The operation of the system will now be described in detail. 
     As long as both batteries  12  and  14  are operative, power is supplied by the stronger battery  14  with a higher voltage. If battery  14  fails, battery  12  takes over supplying power. It should be noted that the unit  26  and the sensor  58  are sufficiently insensitive to voltage for their ability to operate. If desired, these load devices can have their own power supply center. 
     Also, as shown in FIG. 1, a signal line  100  (e.g., a CAN bus) can be used to connect the control apparatus  6  to another control apparatus. This signal line  100  is capacitance-coupled at  102  to an output, not shown, of the control apparatus  6 . 
     With the aid of FIG. 2 the operation of the system of FIG. 1 will be explained hereinbelow in the case of an interruption  104  of the ground line  70  between the batteries  12  and  14  and the control apparatus  6 , and between the control apparatus  6  and the sensor  58  at  106 . 
     As shown in FIG. 2, the switch means  96  and  98  are shown in the closed position, unlike FIG.  1 . 
     In the event of ground interruptions, the return current to the batteries can no longer pass through the ground line, but is passed through other current paths. These other current paths are drawn in heavy lines in FIG.  2 . 
     The current flows from the battery  14  through the supply line  50  and the diode  40  to the power supply center  44 . And from the ground of power supply center  44 , current flows through the connecting line  72  and the diode  76  through battery  12  to battery  14 . In this manner the voltage difference between the two batteries  12  and  14  is present at the power supply center  44 , namely 30 V in the example represented, battery  12  being charged by the current output of battery  14 . 
     Similarly, the current flows from the supply line  50  through line  48  to unit  26  and from its ground through the connecting line  82  with the diode  86  and line  46 , back to supply line  52 . 
     In like manner, the supply of power to the sensor  58  runs through line  60  and diode  66  and then through the connecting line  90  and the diode  92  to line  62 . 
     The raising of the potential of the individual devices by the voltage of battery  12  is without effect as long as the individual load devices are not connected electrically with additional load devices which are possibly at a different potential. If this is the case, as for example in the connection through the signal line  100 , the particular line is coupled differentially or capacitively, so that a secure signal transfer is possible regardless of the potential. The signal line  100  contains, for example, two counterpolarizing lines as a CAN bus. 
     If in the state of the circuit according to FIG. 2 the battery  14  drops out because it is exhausted, the result is a circuit as shown in FIG. 3 for feeding the power supply center. The current flows from the battery  12 , through the supply line  52  and the diode  42  to the power supply center  44 , and from the ground  70  for the power supply center  44  through the connecting line  74  and the diode  78  to the supply line  50 , where it flows as current for charging battery  14 , and to the ground terminal of battery  12 . Of course, this way of supplying power stops as soon as battery  14  is charged to the voltage of battery  12 . 
     The system described can be varied in many ways. The ground line  70  can be, for example, the motor vehicle&#39;s body to which the various apparatus and load devices are connected. 
     Several units can be connected to the control apparatus  6 , all of them connected to the batteries by lines switched in the control apparatus  6  like the lines  46  and  48 , etc. 
     While the present invention has been disclosed with reference to certain preferred embodiments, numerous modifications, alterations, and changes to the described embodiments are possible without departing from the sphere and scope of the present invention, as defined in the appended claims. Accordingly, it is intended that the present invention not be limited to the described embodiments, but that it have the full scope defined by the language of the following claims, and equivalents thereof.