Abstract:
A circuit for protecting a power switch in the event of a reverse battery connection, wherein the power switch comprises a MOSFET having a body diode and the MOSFET is connected in series with a load, the circuit comprising terminals connected to the circuit and normally connectable to respective positive and negative power potentials of the battery, and the circuit comprising a switching circuit coupled to the terminals such that the switching circuit turns on the power switch in the event that the terminals are connected with a reverse battery connection.

Description:
CROSS REFERENCE TO RELATED APPLICATION  
       [0001]    The present application claims the priority and benefit of U.S. Provisional Application S.No. 60/477,420, filed Jun. 10, 2003, (IR-1852 PROV II (2-3558)) entitled IMPROVED METHOD TO REMOTELY SENSE THE TEMPERATURE OF A POWER SEMICONDUCTOR, IN P ARTICULAR OF THE POWER MOS DEVICE and U.S. Provisional Application S.No. 60/470,476, filed May 14, 2003 (IR-1851 ( 2-2286 )) entitled CURRENT SENSING DRIVER OPERABLE IN LINEAR AND SATURATED REGIONS, the entire disclosures of which are hereby incorporated by reference herein. 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0002]    The present invention relates to protection circuits for power switches, and in particular, to a reverse battery protection circuit for a power switch.  
           [0003]    Power switches are employed to switch electrical loads, for example, such loads as electrical motors. With reference to FIG. 1, a simplified prior art circuit showing a driver  30  driving a power switch  10  comprising a MOSFET is shown. The MOSFET switches the load  20 , which might comprise an electrical motor. In an automotive application, the load and power switch are provided in series across the battery connections. The MOSFET power switch  10  includes a body diode  10 A, which, in normal operation when the battery is connected as shown in FIG. 1, is back biased. However, as shown in FIG. 2, if the battery connections are inadvertently reversed, body diode  10 A will conduct, and because of its relatively high forward voltage drop, dissipate a substantial amount of power which can lead to destruction of the switching device  10 . For example, as shown in FIG. 1, when the battery is connected properly, the power dissipation in the semicondutor switch  10  is on the order of approximately 1 watt, assuming a current through the load of about 10 amps and an RDSon of the semiconductor switch of about 10 milli ohms. If the battery is improperly connected in a reverse battery connection, which may occur accidently upon improper installation of the battery or during a reverse jumping of a dead battery from a good battery, the power dissipated in the body diode of the switch, which will be forward biased as shown in FIG. 2, is approximately 6 watts considering that the forward voltage drop of the diode is 0.6 volts and the current remains at 10 amps. This six-fold power dissipation may destroy the switching device.  
         SUMMARY OF THE INVENTION  
         [0004]    It is accordingly an object of the present invention to provide a reverse battery protection circuit which will prevent damage to the semiconductor switching device if the battery is reverse connected.  
           [0005]    The objects of the present invention are achieved by a circuit for protecting a power switch in the event of a reverse battery connection, wherein the power switch comprises a MOSFET having a body diode and the MOSFET is connected in series with a load, the circuit comprising terminals connected to the circuit and normally connectable to respective positive and negative power potentials of the battery, and the circuit comprising a switching circuit coupled to the terminals such that the switching circuit turns on the power switch in the event that the terminals are connected with a reverse battery connection.  
           [0006]    The objects of the invention are also achieved by a method for protecting a power switch in the event of a reverse battery connection, wherein the power switch comprises a MOSFET having a body diode and the MOSFET is connected in series with a load, the method comprising providing terminals on a protective circuit normally connectable to respective positive and negative battery potentials of the battery; and providing a switching circuit coupled to the terminals such that the switching circuit turns on the power switch in the event that the terminals are connected with a reverse battery connection, thereby preventing the body diode from conducting.  
           [0007]    Other features and advantages of the present invention will become apparent from the following description of the invention which refers to the accompanying drawings. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWING(S)  
       [0008]    The invention will now be described in greater detail in the following detailed description with reference to the drawings in which:  
         [0009]    [0009]FIG. 1 shows a MOSFET power switching device according to the prior art connected to a load;  
         [0010]    [0010]FIG. 2 shows the power switching device of FIG. 1 connected to a load under the influence of a reverse battery connection;  
         [0011]    [0011]FIG. 3 shows a reverse battery protection circuit according to the invention;  
         [0012]    [0012]FIG. 4 shows a simplified reverse battery protection circuit according to the invention; and  
         [0013]    [0013]FIG. 5 shows a graph to explain the circuit of FIGS. 3 and 4. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0014]    With reference now to the drawings, FIG. 3 shows a reverse battery protection circuit according to the present invention. The power switch is again shown by reference numeral  10 . The body diode is indicated at  10 A. The load  20  is connected in series with the power switch.  
         [0015]    The present invention provides a circuit to turn on the power switch  10  in the event of a reverse battery connection. This will prevent the body diode  10 A from conducting and dissipating power. The load will be energized in the reverse current direction but this will not generally cause any damage because the body diode is not conducting, and the power dissipated in the switch  10  is the same power as would be dissipated in the switch  10  when the switch is on under proper battery connections.  
         [0016]    The basic reverse battery protection circuit is based around MOSFETS M 1  and M 2 .  
         [0017]    With reference to FIG. 3, terminal ST is connected to a logic ground, typically through a low resistance R 1 . The load  20  is connected to a power ground. VDD represents the voltage from the battery, typically 12-14 volts. Under a proper battery connection, VDD is at 12-14 volts and ST is at ground level. Under these circumstances, the gate of MOSFET M 3  connected to line  25  will be low and MOSFET M 3  will be off. MOSFET M 4  is on holding the gate of M 3  low. This will cause line  27  to be floating and the gate (line  25 ) of MOSFET M 1  to be low and thus MOSFET M 1  will be off. Also, a MOSFET M 2 , whose gate is connected to VDD, is turned on, ensuring the gate of MOSFET M 1  is low, and thus off. Accordingly, the driver  30  driving the power switch  10  will operate the power switch  10  in its normal mode to drive the load  20 .  
         [0018]    In addition, a charge pump  40  comprising a plurality of MOSFETS  40 A,  40 B,  40 C and  40 D will be off because transistor  50  is maintained off.  
         [0019]    Under a reverse battery connection, VDD will be at ground and ST will be at a positive voltage level, typically from 5-14 volts. As a result, line  25  will be high turning on MOSFET M 3 . MOSFET M 4  will be off. Accordingly, the gate of MOSFET M 1  will be high and line  27  will also be high because M 3  is on.  
         [0020]    Accordingly, M 1  is turned on causing the high signal on line  27  to be provided to the gate of power MOSFET switching device  10 , turning it on, thereby preventing the reverse battery connection from conducting through the body diode  10 A which cannot conduct when the power switch  10  is on. The reverse battery connection results in conduction through the load and the power MOSFET  10 , but the body diode  10 A is not conductive.  
         [0021]    In addition, a charge pump  40  may be provided comprising the transistors  40 A,  40 B,  40 C and  40 D and capacitors  50 A,  50 B,  50 C and  50 D. The charge pump is provided to avoid a voltage drop due to the bulk effect which may be too great a voltage drop at a low reverse battery connection to turn on the MOSFETs. The charge pump only operates during a reverse battery connection. When the battery is reverse connected, the transistor  50  allows charging of the charge pump between ST and VDD. During normal battery connection, the gate of M 1  is shorted by transistor M 2  to the source connection S 2  to prevent M 1  from turning on.  
         [0022]    In addition, a further transistor, bipolar transistor  70  and current source  60 , may be provided. There are provided to prevent the gate of the driver output from shorting to the source S 2 .  
         [0023]    If source S 2 , which is connected to the substrate of the driver  30 , is greater than about 0.6 V, then current will flow in the body diode of the output transistor (not shown) of the driver  30 . In this case, all n+ nodes of the driver transistor will be shorted to VDD by a parasitic bipolar transistor. In particular, this will cause the gate to be shorted to the source of the driver output. Current source  60  and bipolar transistor  70  are provided to avoid the condition that S 2 -VDD is greater than 0.5V. Thus, the gate of the driver is not shorted to the source. FIG. 5 shows this graphically. The voltage S 2  is prevented from rising to 0.6V by transistor  70  and current source  60 .  
         [0024]    [0024]FIG. 4 shows a simplified reverse battery protection circuit. It is basically the same as the circuit of FIG. 3 but eliminates the charge pump  40  as well as transistors M 3 , M 4  and  50 . Transistors M 3  and M 4  have been reduced to the resistor R 1 A and diodes D 1  to D 5 . Transistors M 1  and M 2  operate in the same was described with respect to FIG. 3. Thus, when the battery is properly polarized, transistor M 2  is turned on and transistor M 1  is off, thus allowing driver  30  to operate transistor  10  in a normal fashion. When the battery is reverse connected, transistor M 2  is off and M 1  is on and the gate of the transistor  10  is high, turning it on and preventing the body diode  10 A from conducting. Current source  60  and transistor  70  are provided for the same reasons discussed with respect to FIGS. 3 and 4. In the event of a reverse battery connection, transistor M 2  is turned off and transistor M 1  is turned on thus allowing current to flow through diodes D 1 -D 5  through transistor M 1  turning on transistor  10 , preventing reverse current flow through the body diode.  
         [0025]    Although the present invention has been described in relation to particular embodiments thereof, many other variations and modifications and other uses will become apparent to those skilled in the art. Therefore, the present invention should be limited not by the specific disclosure herein, but only by the appended claims.