Abstract:
A method for protecting voltage regulator driver circuitry during a short circuit condition of an alternator field coil includes passively detecting a drop in field coil voltage during an on-portion of a duty cycle of the field coil voltage, wherein the passive detection of the drop in field coil voltage signifies an interrupt event. Responsive to the interrupt event, a logical state of a driver enable control signal is changed so as to deactivate driver circuitry associated with a switching device used to pass field current through the field coil, wherein the driver circuitry, when deactivated, prevents the switching device from passing current regardless of the state of a pulse width modulation (PWM) control signal applied to the driver circuitry.

Description:
BACKGROUND 
       [0001]    The present invention relates generally to rotating electric machinery and, more particularly, to a method and system for protecting voltage regulator driver circuitry during a field coil short circuit condition. 
         [0002]    Generators are found in virtually every motor vehicle manufactured today. These generators, also referred to as alternators, produce electricity necessary to power a vehicle&#39;s electrical accessories, as well as to charge a vehicle&#39;s battery. Generators must also provide the capability to produce electricity in sufficient quantities so to power a vehicle&#39;s electrical system in a manner that is compatible with the vehicle&#39;s electrical components. The alternator or generator typically uses a voltage regulator to regulate the charging voltage and output current in order to provide consistent operation during varying loads that would otherwise create voltage drops and other operational problems. Presently, conventional vehicle charging systems may utilize a voltage regulator having either a discrete transistor or, alternatively, a custom integrated circuit known as an Application Specific Integrated Circuit (ASIC). 
         [0003]    Still other vehicle designs may also employ voltage regulators with advanced microprocessor functions that maintain a highly accurate regulated voltage produced by a generator. Microprocessor based regulators may also include advanced clock and memory circuits that store battery and power supply reference data, battery voltage and generator rotation speed, as well determine how much the battery is being charged and at what rate at any point in time. 
         [0004]    In operation of a vehicle alternator, it is possible that the field coil used to generate the magnetic field of the rotor portion of the alternator may become short-circuited. In such a case, the voltage regulator driver circuitry should be deactivated in order to discontinue the flow of field current through the driver devices until such time as the short circuit condition is cleared. Conventionally, such short circuit protection (when provided at all) involves use of a number of components, such as (for example) a small shunt resistance within the field coil path and an analog voltage comparator to determine whether the voltage across the shunt resistor exceeds a nominal voltage when the field coil is not short circuited. Accordingly, it would be desirable to be able to provide short circuit protection for voltage regulator driver circuitry in a manner that results in fewer hardware components and/or reduced component costs. 
       SUMMARY 
       [0005]    The foregoing discussed drawbacks and deficiencies of the prior art are overcome or alleviated by, in an exemplary embodiment, a method for protecting voltage regulator driver circuitry during a short circuit condition of an alternator field coil, including passively detecting a drop in field coil voltage during an on-portion of a duty cycle of the field coil voltage, wherein the passive detection of the drop in field coil voltage signifies an interrupt event; and responsive to the interrupt event, changing a logical state of a driver enable control signal so as to deactivate driver circuitry associated with a switching device used to pass field current through the field coil, wherein the driver circuitry, when deactivated, prevents the switching device from passing current regardless of the state of a pulse width modulation (PWM) control signal applied to the driver circuitry. 
         [0006]    In still another embodiment, a storage medium includes a computer readable computer program code for protecting voltage regulator driver circuitry during a short circuit condition of an alternator field coil, and instructions for causing a computer to implement a method. The method further includes passively detecting a drop in field coil voltage during an on-portion of a duty cycle of the field coil voltage, wherein the passive detection of the drop in field coil voltage signifies an interrupt event; and responsive to the interrupt event, changing a logical state of a driver enable control signal so as to deactivate driver circuitry associated with a switching device used to pass field current through the field coil, wherein the driver circuitry, when deactivated, prevents the switching device from passing current regardless of the state of a pulse width modulation (PWM) control signal applied to the driver circuitry. 
         [0007]    In still another embodiment, a voltage regulator for an electrical generator includes an electronic device configured to compare an output voltage of the generator to a desired set point voltage thereof, driver circuitry in communication with the electronic device, the driver circuitry configured to selectively activate and deactivate a switching device used to pass field current through a field coil, in response to a difference between the output voltage and the desired set point voltage; one or more components configured to passively detect a drop in field coil voltage during an on-portion of a duty cycle of the field coil voltage, wherein the passive detection of the drop in field coil voltage signifies an interrupt event; and the electronic device further configured to protect the driver circuitry and switching device during a field coil short circuit condition by changing a logical state of a driver enable control signal, responsive to the interrupt event, so as to deactivate the driver circuitry and prevent the switching device from passing current regardless of the state of a pulse width modulation (PWM) control signal applied to the driver circuitry. 
         [0008]    In still another embodiment, a vehicle charging system includes an alternator having one or more stator windings on a stationary portion thereof and a field coil on a rotatable portion thereof. A voltage regulator is configured to regulate an output voltage of the alternator through control of a field current through the field coil. The voltage regulator further includes an electronic device configured to compare an output voltage of the alternator to a desired set point voltage thereof; driver circuitry in communication with the electronic device, the driver circuitry configured to selectively activate and deactivate a switching device used to pass field current through the field coil; one or more components configured to passively detect a drop in field coil voltage during an on-portion of a duty cycle of the field coil voltage, wherein the passive detection of the drop in field coil voltage signifies an interrupt event; and the electronic device further configured to protect the driver circuitry and switching device during a field coil short circuit condition by changing a logical state of a driver enable control signal, responsive to the interrupt event, so as to deactivate the driver circuitry and prevent the switching device from passing current regardless of the state of a pulse width modulation (PWM) control signal applied to the driver circuitry. 
     
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]    Referring to the exemplary drawings wherein like elements are numbered alike in the several Figures: 
           [0010]      FIG. 1  is a schematic diagram of an exemplary vehicle charging system employing a microprocessor based voltage regulator, suitable for use in accordance with an embodiment of the invention; 
           [0011]      FIG. 2  is a more detailed schematic diagram of the voltage regulator shown in  FIG. 1 ; 
           [0012]      FIG. 3  is a more detailed schematic diagram of the voltage regulator of  FIGS. 1 and 2 , illustrating a method for protecting voltage regulator driver circuitry during a field coil short circuit condition, in accordance with an embodiment of the invention; and 
           [0013]      FIG. 4  is a waveform diagram depicting an exemplary operating scenario of the protection circuitry shown in  FIG. 3 . 
       
    
    
     DETAILED DESCRIPTION 
       [0014]    Disclosed herein is a method and system for protecting voltage regulator driver circuitry during a field coil short circuit condition. Briefly stated, a voltage regulator (e.g., microprocessor based) is configured with the capability of sensing a field coil short circuit condition through a simple (resistor/diode) combination of passive components, and thereby generating an interrupt signal that disables the driver circuitry associated with the field coil. Further, when implemented at least in part in software, the present techniques do not require more expensive hardware (e.g., differential amplifiers) configured within the ECM and/or voltage regulator. 
         [0015]    Referring initially to  FIG. 1 , there is shown a schematic diagram of an exemplary vehicle charging system  100  employing a microprocessor based voltage regulator, suitable for use in accordance with an embodiment of the invention. It should be appreciated that although  FIG. 1  depicts a vehicle charging system, the present embodiments are applicable to other types of regulated generator systems. A vehicle alternator  101  has a plurality of windings  102  (e.g., three-phase, delta configuration) in a stator portion thereof, and a field coil  104  in a rotor portion thereof. The alternating current (AC) voltage generated in the windings  102  is converted to a direct current (DC) voltage by a full-wave rectifier  106 , which in turn includes three diode-pairs configured in parallel. The DC output of the rectifier  106  is fed to the positive terminal of a vehicle battery  108 , wherein the magnitude of the output voltage is dependent upon the speed of the rotor and the amount of field current supplied to the field coil  104 . 
         [0016]    In certain alternator designs, the stator may actually include independent pairs of stator windings and an associated pair of rotor field coils to reduce noise in view of increased load escalation. However, for purposes of simplicity, only one set of stator windings and field coil is illustrated. It will also be appreciated that the windings  102  could alternatively be connected in a Y-configuration having a common neutral point. 
         [0017]    As further illustrated in  FIG. 1 , a voltage regulator  110  is utilized to regulate and control the magnitude of the output voltage generated by the alternator  101 , and thus control the (direct current) charging voltage applied to the battery  108  and associated vehicle loads (e.g., load  112  connected through switch  114 ). It does so by controlling the magnitude of field current supplied to field coil  104  through high-side alternator terminal “F+” shown in  FIG. 1 . Additional details concerning the generation of current through the field coil  104  by regulator  110  are discussed in further detail hereinafter. 
         [0018]    One skilled in the art may also recognize other standardized terminals associated with the alternator, including: the high-side battery output terminal “B+”, the phase voltage terminal “P” used to monitor the AC output voltage of the alternator; and the ground terminal “E” used to provide a ground connection for the alternator. An electronic control module  116  (ECM), which may represent the vehicle&#39;s main computer, receives a charge warning lamp signal through lamp terminal “L” of the regulator  110 , used to control a charge warning lamp  118  when ignition switch  120  is closed. The ECM  116  also receives a rotor switching signal through terminal “F m ”, indicative of the field current signal F+ applied to the field coil  104 . 
         [0019]    Referring now to  FIG. 2 , a more detailed schematic diagram of at least portions of the voltage regulator  110  of  FIG. 1  is illustrated. For purposes of simplification, various discrete electronic components (e.g., resistors, capacitors, etc.) of the regulator  110  are not depicted in  FIG. 2 . A microcontroller  122  having control logic code therein receives feedback of the alternator charging system voltage(s) in digital form through an internal analog-to-digital converter (ADC) configured therein. Based on a comparison between the sensed system voltage and a predetermined set operating voltage of the system, the microcontroller generates a PWM output signal (PWM_DC) that is coupled to a high-side driver  124 . The high-side driver  124  in turn provides a pulsed switching signal to the control terminal (e.g., gate) of transistor  126 . Based on the duty cycle of the pulsed signal, the on/off switching of transistor causes field current to intermittently flow through field coil  104 . During “off” periods of the duty cycle, energy within the field coil is dissipated through a flyback diode  128 . 
         [0020]    As indicated above, the regulator  110  attempts to maintain a predetermined charging system voltage level (set point). When the charging system voltage falls below this point, the regulator  110  increases the level of field current by increasing the duty cycle of the PWM_DC current. Conversely, when the charging system voltage increases above the system set point, the  110  decreases the level of field current by decreasing the duty cycle of the PWM_DC current. 
         [0021]    As further indicated above, it is possible for the field coil  104  to become short-circuited during operation of the alternator  101  due to, for example, the presence of metal shavings in the rotor. Accordingly,  FIG. 3  is a more detailed schematic diagram of the voltage regulator shown in  FIGS. 1 and 2 , illustrating a system and method for protecting voltage regulator driver circuitry during a field coil short circuit condition, in accordance with an embodiment of the invention. From a hardware perspective, a simple resistor/diode combination is used to passively detect a field coil short circuit condition, combined with the use of internal microprocessor software to generate a command that deactivates the high-side driver  124 . 
         [0022]    More specifically, a resistor R 1  is configured in series between the high-side alternator terminal F+ and an input pin of the microcontroller  122 , designated as “Interrupt” in  FIG. 3 . In addition, a diode D 1  is configured between the PWM output pin (PWM_DC) of the microcontroller  122  and the “Interrupt” input pin, wherein a forward biasing of the diode D 1  couples the voltage on the PWM_DC output pin to the “Interrupt” input pin of the microcontroller  122 . Under normal operating conditions, the output signal on PWM_DC has the same duty cycle, but opposite phase, as the output voltage of the field coil. A logical low signal applied to the input of the high-side driver  124  in turn drives the gate of an N-channel MOSFET device  126  high. The high-side driver  124  is thus “active low” in that the transistor  126  is rendered conductive when the output voltage of the PWM_DC pin is logical low (e.g., 0 volts), assuming that the high-side driver is actively enabled in the first place. In this regard, a “Driver Enable” output signal of the microcontroller  122  is coupled to the high side driver  124  through a resistor R 2 , which selectively activates or deactivates the high-side driver  124 , depending on whether or not normal operating conditions exist. 
         [0023]    So long as normal operating conditions exist, the voltage on the “Interrupt” input pin of the microcontroller  122  remains at logic high, and the internal logic and/or software of the microcontroller  122  maintains the “Driver Enable” output signal at active low. On the other hand, during a short circuit of the field coil  104  (indicated by the dashed line  130  in  FIG. 3 ), the voltage on the “Interrupt” input pin of the microcontroller  122  transitions to logic low due to the short. The internal logic and/or software of the microcontroller  122  detects a falling edge transition of the “Interrupt” pin voltage, and switches the “Driver Enable” output signal logic high, thereby disabling the high-side driver  124  and preventing any field current from flowing through the transistor  126  until such time as the short circuit condition is cleared. 
         [0024]      FIG. 4  is a waveform diagram depicting an exemplary operating scenario of the protection circuitry shown in  FIG. 3 . As is shown, the four waveforms depicted in  FIG. 4  are the PWM_DC output signal of the microcontroller  122 , the field output voltage at F+, the voltage of the “Interrupt” input pin of the microcontroller  122 , and the voltage of the “Driver Enable” output pin of the microcontroller  122 . Prior to time t 1 , the regulator is in a normal operating condition, in that there is no short circuit condition across the field coil  104 . During the “off” portions of the PWM_DC duty cycle, the field output voltage is high, which in turn keeps the voltage at the “Interrupt” pin at high. Moreover, during the “on” portions of the PWM_DC duty cycle (when the field output voltage is low), the voltage at the “Interrupt” pin at still maintained at logic high, notwithstanding the discharged voltage at F+, due to the combination of D 1  and R 1 . So long as the microcontroller  122  does not detect a falling transition of the logical high voltage at the “Interrupt” input pin, it will maintain the “Driver Enable” output pin at an active low logic level. 
         [0025]    However, at time t 1 , a short circuit condition now exists across the field coil  104 , so as to result in the field output voltage immediately falling to 0 volts. Because this coincides with the “off” portion of the duty cycle of PWM_DC, there is no signal voltage present on PWM_DC that would allow R 1  and D 1  from preventing the voltage at the “Interrupt” input pin from discharging to logic low. Consequently, the microcontroller  122  switches the “Driver Enable” signal from logic low to logic high, thereby deactivating the high-side driver  124 . 
         [0026]    Between time t 1  and t 2 , it will be noted that the next “on” portion of the PWM_DC duty cycle is reached. Correspondingly, the voltage on the “Interrupt” pin is at least temporarily restored to logic high. However, this brief rise is not yet enough information for the microcontroller  120  to determine whether the short circuit condition has been eliminated because the field coil voltage would nominally be 0 at this point in the duty cycle, even under normal conditions. Accordingly, assuming that the short circuit condition still exists by the next “off” portion of the PWM_DC duty cycle at time t 2 ,  FIG. 4  further illustrates a falling edge in input voltage of the “Interrupt” pin and a brief pulse in the output voltage at F+, coinciding with the transition of the PWM_DC signal to low. This represents a short circuit attempt at pulling down the “Interrupt” pin voltage through the resistor (R 1 ) coupling with PWM_DC. However, the voltage coupled to the “Interrupt” pin is quickly discharged through R 1  and the shorted out field coil  104 . Therefore, because the voltage at the “Interrupt” pin was not maintained during the “off” portion of the PWM_DC duty cycle, the microcontroller  120  does not restore the “Driver Enable” output pin to active low logic level at this point. 
         [0027]    It is then assumed that the short circuit condition is cleared by time t 3 , corresponding to the next “off” portion of the duty cycle of PWM_DC. At this point, the voltage at the “Interrupt” pin is still maintained high immediately after PWM_DC goes low, since the field coil is no longer shorted out and can prevent current from instantaneously discharging the “Interrupt” pin voltage. The microcontroller  122  therefore detects this condition and resets the “Driver Enable” signal back to active low so that the high-side driver  124  can turn on transistor  126 , passing current through the field coil  104  and generating the output voltage thereon. 
         [0028]    Although the exemplary method and system outlined above is depicted as being implemented in software within the microcontroller  122 , one skilled in the art will also appreciate that the logic can also be implemented through hardware configured within an ASIC type regulator, for instance. In view of the above, the present method embodiments may therefore take the form of computer or controller implemented processes and apparatuses for practicing those processes. The disclosure can also be embodied in the form of computer program code containing instructions embodied in tangible media, such as floppy diskettes, CD-ROMs, hard drives, or any other computer-readable storage medium, wherein, when the computer program code is loaded into and executed by a computer or controller, the computer becomes an apparatus for practicing the invention. 
         [0029]    While the invention has been described with reference to a preferred embodiment or embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the appended claims.