Abstract:
A generator field regulator circuit communicates with a field winding of a Lundell machine and includes a first transistor. A second transistor has a source that communicates with a drain of the first transistor and a drain that communicates with one end of the field winding. A free-wheeling diode communicates with the drain of the first transistor and a source of the second transistor. An overvoltage detection circuit modulates a gate of the second transistor. A pulse width modulation (PWM) circuit modulates a gate of the first transistor to maintain generator rectified voltage at a nominal level. The overvoltage detection circuit outputs an overvoltage shutdown signal to the PWM circuit during a full-load dump mode to turn off the first transistor, which turns off the second transistor. The overvoltage shutdown signal is disabled when a terminal voltage of a generator falls below a predetermined voltage.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention relates to electric power generators, and more particularly load dump transient voltage control systems for power generators of vehicles.  
         BACKGROUND OF THE INVENTION  
         [0002]    Vehicles with internal combustion engines typically use a Lundell machine as a primary electrical power source. The Lundell machine provides a rugged and economical solution for electrical power demand in vehicles.  
           [0003]    Referring now to FIG. 1, a vehicle electrical generator  10  with a field regulator  14  is shown. The field regulator  14  is also known as a single quadrant chopper because field current and voltage are either positive or zero at any given time. The generator  10  also includes a Lundell alternator  18  (also known as a claw pole alternator), which is a wound-field, 3-phase synchronous machine. The output of the Lundell alternator  18  communicates with a 3-phase avalanche bridge rectifier  22  to produce DC power.  
           [0004]    The field regulator  14  generates an error signal based on a difference between a terminal voltage (across B+ and B−) and a reference V ref . The field regulator  14  includes a pulse width modulator (PWM) controller  26  that modulates (PWM) a power transistor Q 1 , which is in series with a field winding  30 . A duty cycle of the transistor Q 1  depends on a field current that is required to maintain an output voltage at a desired level for a given speed and load condition. Diode DF provides a freewheeling path for the field current when the transistor Q 1  is switched off.  
           [0005]    Referring now to FIG. 2, a test circuit  40  for evaluating transient performance of the vehicle electrical generator  10  is shown. The generator  10  is connected to battery  44  and a load  48 . The battery  44  can be a 42V battery and the generator  10  can be a 42V generator, although other voltage levels are contemplated. Switches S g , S b , and S l  are used for disconnecting the generator  10 , the battery  44  and/or the load  48 .  
           [0006]    In a full load dump, the generator  10  (supplying rated current) is suddenly disconnected from the battery  44  and most of the load  48 . The full load dump produces a severe transient on the power bus as is shown in FIGS. 3A and 3B. With a normal rectifier, the peak transient voltage V peak  can reach levels several times the nominal generator voltage V nom . The transient duration T load-dump  may last several hundred milliseconds prior to dropping below a maximum specified generator voltage V max .  
           [0007]    Avalanche rectifiers clamp the peak transient generator voltage V peak  to acceptable levels by absorbing generator output power until a magnetic field decays to lower levels. A duration of the load dump transient T load-dump  is primarily dependent on a field time constant in a single quadrant regulator. The field current decays at its natural rate while freewheeling through the diode DF when the transistor Q 1  is turned off due to over-voltage.  
           [0008]    The load dump energy absorbed by the avalanche rectifiers in higher power generators may require multiple devices in parallel or much larger devices to assure reliable operation. These devices significantly increase the cost, size and mass of the alternator, which is undesirable for vehicle applications.  
         SUMMARY OF THE INVENTION  
         [0009]    A generator field regulator circuit according to the present invention communicates with a field winding of a Lundell machine and includes a first transistor. A second transistor has a source that communicates with a drain of the first transistor and a drain that communicates with one end of the field winding. A free-wheeling diode communicates with the drain of the first transistor and a source of the second transistor.  
           [0010]    In other features, an overvoltage detection circuit modulates a gate of the second transistor. A pulse width modulation (PWM) circuit modulates a gate of the first transistor to maintain generator rectified voltage at a nominal level. The overvoltage detection circuit outputs an overvoltage shutdown signal to the PWM circuit during a full-load dump mode to turn off the first transistor and the second transistor. The overvoltage shutdown signal is disabled when a terminal voltage of a generator falls below a predetermined voltage.  
           [0011]    In still other features, first and second diodes communicate with a drain of the second transistor and set a clamp voltage of the second transistor when the overvoltage shutdown signal is enabled.  
           [0012]    In yet other features, the generator further includes a Lundell machine that includes the field winding and a 3-phase avalanche rectifier that communicates with the Lundell machine. The gate of the second transistor communicates with a cathode of a third diode and one end of a first capacitor. The third diode and the first capacitor keep the second transistor on during normal operation. The overvoltage shutdown signal exceeds a maximum operating voltage of the generator. The second transistor and the overvoltage detection circuit are maintained at a lower temperature than an ambient temperature of other components of the generator.  
           [0013]    Further areas of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples, while indicating the preferred embodiment of the invention, are intended for purposes of illustration only and are not intended to limit the scope of the invention. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0014]    The present invention will become more fully understood from the detailed description and the accompanying drawings, wherein:  
         [0015]    [0015]FIG. 1 is a functional block diagram of an electric power generator for a vehicle according to the prior art;  
         [0016]    [0016]FIG. 2 is a functional block diagram illustrating a test circuit connected to the electric power generator of FIG. 1;  
         [0017]    [0017]FIGS. 3A and 3B are waveforms illustrating current and voltage, respectively, during a full load dump;  
         [0018]    [0018]FIG. 4 is an electric power generator for a vehicle according to the present invention;  
         [0019]    [0019]FIG. 5 is an exemplary implementation of the electric power generator of FIG. 4; and  
         [0020]    [0020]FIGS. 6 and 7 are waveforms comparing the operation of the electric power generators of FIGS. 1 and 4. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0021]    The following description of the preferred embodiment(s) is merely exemplary in nature and is in no way intended to limit the invention, its application, or uses. For purposes of clarity, the same reference numbers will be used in the drawings to identify similar elements.  
         [0022]    The present invention provides a control system and method for a generator that achieves fast and efficient control of generator output voltage during load switch-off. The present invention also reduces a duration of a load dump transient T load-dump  and dissipated energy during the load dump transient. The present invention also reduces the size of overvoltage transient suppression devices and improves the reliability of the generator.  
         [0023]    Referring now to FIG. 4, a generator  100  includes a field regulator  104  according to the present invention that is capable of fast turn-off of the field current during load dump transients. The generator  100  further includes the Lundell alternator  18 , the 3-phase avalanche bridge rectifier  22 , and a transistor Q 2 . A drain of the transistor  02  is connected to a F n  terminal of the field winding  30 . A power source is connected to the source of transistor Q 1  and to the cathode of the diode DF.  
         [0024]    The breakdown voltage of the transistors Q 1 , Q 2  and the diode DF are greater than or equal to the rated voltage V peak  of the generator  100 . Under normal operating conditions, the transistor Q 1  is turned on and off by an output of a PWM controller  110  while the transistor Q 2  is on. During a load dump, a terminal voltage of the generator  100  exceeds an overvoltage refence (V ovref ). During the load dump, a fast field controller or overvoltage detection circuit  120  transmits a shutdown signal (V ovshdn ) to the PWM controller  110  to turn-off the transistor Q 1  and the transistor Q 2 . The transistor Q 2  acts as an avalanche diode in series with the field winding  30 . The diode DF provides a path for the energy stored in the field winding  30  to decay.  
         [0025]    The field winding  30  is subjected to a reverse voltage that is equal to the avalanche breakdown voltage of the transistor Q 2 . The field current decays to zero much more rapidly and reduces the duration of the load dump transient. After the field current is reduced to zero, additional time elapses before the terminal voltage reaches the nominal voltage V nom  level due to eddy currents in the rotor of the Lundell alternator  18 . When the terminal voltage falls below V max , the V ovshdn  signal is returned to an inactive state. The PWM controller  110  turns on the transistors Q 1  and Q 2 .  
         [0026]    Referring now to FIG. 5, an exemplary implementation of the above control logic and overvoltage control for the generator during a load dump transient is shown. B+ and B− terminals of the field regulator are connected to the battery/generator positive and negative terminals, respectively. A junction of resistors R 1  and R 5  provides a scaled down voltage signal V FDBK , which is proportional to the generator terminal voltage. Capacitor C 1  provides high frequency noise filtering for V FDBK .  
         [0027]    A junction of resistors R 3  and R 6  provides another scaled down voltage signal OV_LIM, which is proportional to the generator terminal voltage that is used for triggering the over-voltage shutdown circuit. A linear regulator  120  includes resistor R 2 , diode D 9 , transistor Q 3 , resistor R 4 , and capacitor C 2 , which provide a bias voltage V CC  to the PWM controller  110 . The PWM controller  110  can be a standard PWM control IC UC1526AN made by Texas Instruments, although other controllers can be used.  
         [0028]    Resistors R 7  and R 8  set a reference voltage for an error amplifier within the PWM controller  110  to control the generator terminal voltage during steady state conditions. The V FDBK  signal is connected to the error amplifier. Resistor R 9  and capacitor C 4  provide proportional-integral (PI) compensator for the error amplifier in the PWM controller  110  closed loop control.  
         [0029]    An over-current sense input CS+ is connected to the OV_LIM signal, which is calibrated using resistor R 17  to generate the shutdown signal SHDN! at a preset generator terminal voltage. Resistors R 16  and capacitor C 9  set a PWM frequency of the regulator to approximately 350 Hz. The 2-phase outputs of the PWM controller  110  (OUTA, OUTB) are combined using diodes D 6 , D 7  and resistor R 12  to achieve a 0 to 95% maximum duty cycle PWM signal, which drives the gates of the transistors Q 1  and Q 2 . Under normal operating conditions, the error amplifier adjusts the PWM duty cycle to control the average current through the field terminals F p −F n , which matches the generator terminal voltage to the reference voltage.  
         [0030]    A drive scheme according to the invention maintains the transistor Q 2  continuously on under normal operating conditions while transistor Q 1  is switched on and off with the PWM signal. When the PWM signal goes high for the first time, the transistor Q 1  is turned on via resistor R 15 . With a few microseconds delay, the transistor Q 2  is turned on via diode D 5 , resistor R 10 , capacitor C 6 , and the transistor Q 1 . When the transistor Q 1  turns off at the end of on time of the PWM signal, the voltage at the junction of drain of the transistor Q 1  and the source of the transistor Q 2  rises above that of the F p  or B+ terminal, reverse biasing the diode D 5  and forward biasing the diode DF. The gate-source charge required to maintain the transistor Q 2  on is now supplied by the capacitor C 6 , which is sized appropriately. Thus, the field current free-wheels through the transistor Q 2  and the diode DF until the transistor Q 1  is turned on again when the diode DF turns off, and the capacitor C 6  is recharged.  
         [0031]    During a full load-dump, when the rectified generator voltage exceeds the pre-set overvoltage limit, the SHDN! signal goes low. As a result, the transistor Q 1  is turned off by returning the PWM output to the low state. The transistor Q 2  is turned off by discharging the capacitor C 6 , via resistors R 10 , R 11  and the transistors Q 4 , Q 1  (body diode).  
         [0032]    The field current charges the drain-source capacitance of the transistor Q 2  until it exceeds the breakdown voltage of the device. The field current decays rapidly through the transistor Q 2  (operating as a zener diode) and the diode DF. When the field current reaches a level low enough for the generator terminal voltage to drop below the overvoltage limit (by an amount dictated by the hysteresis built into the comparator), the SHDN! signal returns to the high state. The transistor Q 5  is turned on and the transistor Q 4  is turned off. When the generator terminal voltage falls below the reference level, due to decreasing field current, the error amplifier generates the PWM pulses again to modulate the transistor Q 1  and to keep the transistor Q 2  on, maintaining the rectified generator voltage at the pre-set level.  
         [0033]    The breakdown voltage of the transistor Q 2  dictates the fall time of the field current, the load-dump over-voltage duration, and the energy dissipated in the avalanche rectifier bridge  22  at the generator output. The reverse voltage applied for fast field current reduction can be actively controlled using an active clamp circuit including diodes D 2 , D 3  between the drain of the transistor Q 2  and the junction of the resistors R 10 , R 11 . The applied reverse voltage across field terminals with the active clamp circuit will be the sum of the zener voltage of the diode D 3 , the forward drop of the diode D 2 , the threshold voltage of the transistor Q 2 , and the forward drop of the diode DF (i.e., V z     —     D3 +V f     —     D2 +V th     —     Q2 +V f     —     DF ). By applying a higher reverse voltage than the generator rectified voltage, the field current can be reduced faster, thereby reducing the over-voltage duration during full-load dump.  
         [0034]    Referring now to FIG. 6, simulation results of the generator load dump response (load current (I_gen(A)), output voltage (V_gen(V)), field voltage (V_field(V)), and field current (I_field(A))) for the conventional field controller is shown at  150  and for the field controller  104  according to the present invention is shown at  160 . Referring now to FIG. 7, simulation results during generator load dump for the conventional field controller is shown at  170  and for the field controller  104  according to the invention is shown at  180 . The peak current (I_zener(A)), total power (P_zener(W)) and energy dissipated (E_zener(Joules)) in the avalanche rectifier at the output of the generator are shown. The load dump over-voltage duration and the energy dissipated in the avalanche rectifier are reduced by more than a factor of 4 with the field controller  104  according to the present invention.  
         [0035]    The reduction in energy dissipated in the avalanche rectifiers during load dump results in lower junction temperature rise, which reduces possible failure due to overheating. Lower energy, lower cost diodes can be used or the reliability of the generator can be improved significantly against load dump failures. Another advantage of this scheme is to provide redundancy in the field control system. When a shorted transistor Q 1  occurs (that might result in a battery over-voltage), the transistor Q 2  can still interrupt the field current and prevent battery damage.  
         [0036]    Those skilled in the art can now appreciate from the foregoing description that the broad teachings of the present invention can be implemented in a variety of forms. Therefore, while this invention has been described in connection with particular examples thereof, the true scope of the invention should not be so limited since other modifications will become apparent to the skilled practitioner upon a study of the drawings, the specification and the following claims.