Abstract:
An improved over-voltage protection circuit for a motor vehicle electrical system includes an over-voltage responsive circuit for momentarily disconnecting the vehicle storage battery and alternator from vehicle electrical loads, and an auxiliary storage battery for supplying a safe operating voltage to the electrical loads during the momentary disconnection. The over-voltage responsive circuit includes a MOSFET device that couples the vehicle storage battery and alternator to the electrical loads and auxiliary storage battery, and a voltage responsive circuit that turns the MOSFET device off to decouple the electrical loads and auxiliary storage battery from the vehicle storage battery and alternator so long as an over-voltage condition is detected.

Description:
TECHNICAL FIELD 
     This invention relates to a motor vehicle electrical system, and more particularly to a high voltage protection circuit that isolates an electrical load from damage and/or power interruption during over-voltage conditions. 
     BACKGROUND OF THE INVENTION 
     It is well known that motor vehicle electrical systems are subject to over-voltage under a number of different conditions. For example, jump-starting the engine with an excessive supply voltage subjects the electrical loads to the excessive voltage as well. Also, the ignition voltage is subject to transient surges during so-called load dump events when the vehicle storage battery is momentarily or permanently disconnected from an engine-driven alternator. In such case, the alternator output voltage can rise well above the nominal charging voltage before the voltage regulator can remove the alternator field winding excitation. For this reason, many load devices are provided with over-voltage protection circuits that either block high voltages or shut down when the supply voltage rises above an over-voltage threshold. This is obviously an undesirable condition since it adds to the cost of the individual load devices, and in the case of a shut-down, renders the load device inoperative until a reset occurs. Accordingly, what is needed is an over-voltage protection apparatus that protects electrical load devices from damage and/or power interruption during both improper jump-starting and load dump conditions. 
     SUMMARY OF THE PRESENT INVENTION 
     The present invention is directed to an improved over-voltage protection circuit for a motor vehicle electrical system, including an over-voltage responsive circuit for momentarily disconnecting the vehicle storage battery and alternator from vehicle electrical loads, and an auxiliary storable battery for supplying a safe operating voltage to the electrical loads during the momentary disconnection. The over-voltage responsive circuit includes a MOSFET device that couples the vehicle storage battery and alternator to the electrical loads and auxiliary storage battery, and a voltage responsive circuit that turns the MOSFET device off to decouple the electrical loads and auxiliary storable battery from the vehicle storable battery and alternator so long as an over-voltage condition is detected. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a circuit diagram of a motor vehicle electrical system including an auxiliary storage battery and over-voltage protection circuit according to this invention. 
     FIG. 2 is a circuit diagram of the over-voltage protection circuit of FIG.  1 . 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to FIG. 1, the reference numeral  10  generally designates a motor vehicle electrical system according to this invention, including an engine  10 , an alternator  12  rotatably driven by engine  10  via a belt and pulley arrangement  14 , a voltage regulator  16 , a storage battery  18 , an ignition switch  20 , a fuse  22 , an over-voltage protection circuit  24 , an electrical load  26  and an auxiliary storage battery  28 . A common ground connection is provided for the alternator  12 , voltage regulator  16 , storage battery  18 , over-voltage protection circuit  24 , electrical load  26  and auxiliary storage battery  28 , as shown. The auxiliary storage battery  28  is directly coupled to the electrical load  26  via line  30 , while the main storage battery  18  is coupled to the load  26  via ignition switch  20 , fuse  22  and over-voltage protection circuit  24  via lines  32 ,  34 . The output terminal of alternator  12  is connected to the main storage battery  18  via line  36 , and the voltage responsive input of voltage regulator  16  is coupled to line  36  via line  38 . The voltage regulator  16  compares the sensed voltage to an internal reference voltage, and supplies current to a field winding (not shown) of alternator  12  for maintaining the voltage on line  36  substantially equal to the reference voltage. A driver manipulated ignition key closes the ignition switch  20  so that during engine operation, the alternator  12  supplies charging current to both main and auxiliary storage batteries  18  and  28 , as well as to the electrical load  26 . 
     The potential for over-voltages on line  36  occurs for at least two different reasons: excessive jump-start voltage, and load dump events. Excessive jump-start voltage usually occurs when a 24-volt source is used to jump-start a vehicle having a 12-volt electrical system, since both 12-volt and 24-volt electrical systems are utilized in production vehicles. For example, if a 24-volt source is connected in parallel with main storage battery  18 , line  32  jumps to 24-volts as soon as ignition switch  20  is closed. If the line  34  also jumped to 24-volts, the load  26  could be damaged if not adequately protected, or subject to automatic over-voltage shut-down. Load dump events, on the other hand, occur during engine operation when the storage battery  18  is momentarily or permanently disconnected from line  36 . This can occur due to a loose battery cable or an intermittent internal battery connection, or due to physical intrusion in a vehicle collision, for example. In this case, the alternator output voltage on line  36  can rise well above the regulator reference voltage before voltage regulator  16  can scale back the alternator field winding excitation. As with the case of excessive jump-start voltage, transient over-voltage due to load dump events is potentially damaging to the electrical load  26 , and may trigger an automatic over-voltage shut-down, resulting in loss of function. This is especially problematic when the electrical load  26  is a safety or emergency device that functions to trigger an emergency signal or communication in the event of a detected crash event, since the crash event may involve disconnection of the storage battery  18  from alternator  12 . 
     The over-voltage protection circuit  24 , shown in detail in FIG. 2, operates in the event of a specified over-voltage on line  32  to effectively decouple lines  32  and  34 , isolating the load  26  and auxiliary storage battery  28  from the alternator  12  and main storage battery  18 . In such event, the auxiliary battery  28  provides power to load  26  via line  30 , and the load  26  is protected from over-voltage damage, and loss of function due to automatic shut-down is prevented. Referring to FIG. 2, the over-voltage protection circuit  24  includes a P-channel MOSFET  50  coupling line  32  to line  34  through its source-to-drain circuit, a pull-down resistor  52  connected between the MOSFET (ate terminal  54  and ground for normally biasing the MOSFET  50  to a conductive state, and a voltage responsive circuit  56  for biasing MOSFET  50  to a non-conductive state when the voltage with respect to ground on line  32  exceeds a predefined threshold. The voltage responsive circuit  56  includes a zener diode  58  and resistor  60  coupled in series between line  32  and ground potential, a NPN transistor  62  having its base coupled to a junction  64  between zener diode  58  and resistor  60 , and a PNP transistor  66  having its emitter-collector circuit coupled across the gate-to-source circuit of MOSFET  50 . The zener diode  58  is ordinarily reverse biased, and the resistor  60  maintains the transistors  62  and  66  in non-conductive states. However, when the voltage on line  32  exceeds the breakdown voltage of zener diode  58  (which may be 18 volts, for example), a current path is established through the resistor  60 , and the resulting voltage at junction  64  biases transistor  62  to a conductive state. This establishes a current path through the emitter-base circuit of transistor  66 , the collector-emitter circuit of transistor  62  and resistor  68 , biasing transistor  66  conductive to place a low impedance path between the gate and source terminals of MOSFET  50  to bias MOSFET  50  to a non-conductive state. Finally, the over-voltage protection circuit  24  also includes a second zener diode  70  connected between line  32  and ground potential for limiting the peak voltage applied to the aforementioned circuit elements, and for establishing a low impedance path through fuse  22  and battery  18  in the event that a reverse polarity is applied to battery  18  during jump starting; for example, zener diode  70  may have a breakdown voltage of approximately 35-volts. 
     In normal operation, the alternator  12  and/or battery  18  supply power to the electrical load  26  (and charging current to auxiliary battery  28 ) via the source-to-drain circuit of MOSFET  50 , and the transistors  62  and  66  of over-voltage protection circuit  24  are biased off. If it becomes necessary to jump-start the engine  10 , and the battery cables are inadvertently reversed, a short-circuit current flows through the diode  70 , blowing the fuse  22  to protect the electrical system  10 . If the cables are properly routed, but the voltage of the jumping battery is too high (24-volts, for example), the resulting reverse current through zener diode  58  will bias transistors  62  and  66  on as explained above, quickly biasing MOSFET  50  off to isolate the load  26  and auxiliary battery  28  from line  32 . In this case, the load  28  never sees a voltage higher than the breakdown voltage of zener diode  58  (which may be 18-volts, as mentioned above), and power is continuously supplied to load  26  by virtue of the auxiliary battery  28 . As soon as the jumping voltage is removed, the over-voltage protection circuit  24  reverts to its normal state, with the MOSFET  50  coupling line  32  to line  34 . A similar effect is achieved if the engine  10  is running and a load dump event occurs; that is, the voltage responsive circuit  56  biases MOSFET nonconductive when the voltage with respect to ground on line  32  exceeds the breakdown voltage of zener diode  58 , to isolate the load  26  and auxiliary battery  28  from line  32 . Additionally, zener diode  70  conducts when the load dump voltage exceeds its breakdown voltage (which may be 35-volts, as mentioned above) to limit the voltage applied to MOSFET  50  and other circuit elements of over-voltage protection circuit  24 . In no event is the load  26  exposed to a source voltage in excess of the breakdown voltage of zener diode  58 ; as a result, the load  26  is protected from damage due to over-voltage, and experiences no loss of function due to automatic over-voltage shut-down. 
     In summary, the over-voltage protection apparatus of the present invention provides a simple and effective expedient for protecting electrical load devices from damage due to over-voltage and for ensuring continuous functionality of the load devices during over-voltage conditions. While described in reference to the illustrated embodiment, it is expected that various modifications in addition to those mentioned above will occur to persons skilled in the art. For this reason, it should be understood that protection circuits incorporating such modifications may fall within the scope of this invention, which is defined by the appended claims.