Abstract:
A current regulator that controls a load current by selectively connecting the load to a high voltage source and a low voltage source. The switching is triggered by a command pulse. By switching between the high voltage source and the low voltage source, the regulator controls load current without generating emissions. The low voltage source also maintains the load current at a selected level without requiring switching.

Description:
REFERENCE TO RELATED APPLICATIONS  
       [0001]    This application claims the benefit of U.S. Provisional Appln. No. 60/353,659, filed Jan. 23, 2002. 
     
    
     
       TECHNICAL FIELD  
         [0002]    The present invention relates to current regulators, and more particularly to a load current regulator that maintains a load current at a selected level with a signal that switches between a high level and a low level.  
         BACKGROUND OF THE INVENTION  
         [0003]    Many systems and devices, such as fuel injectors, rely on current regulators for controlling operation. Currently known current regulators for fuel injection systems regulate load current through an injector coil load by chopping (e.g., by a pulse width modulated (PWM) signal) to maintain the load current at a desired average level. As is known in the art, an injector coil in the fuel injection system is driven by current that is regulated by the PWM signal. To increase the current to a reference level, the PWM signal powers the injector coil by turning on a switch, such as FET switch, to connect the coil to a voltage source (e.g., a 48 V source) that is high enough to ensure a fast current rise time. When the load current reaches a predetermined level (e.g., 20 A), the PWM signal turns the switch off, shutting off current to the injector coil and allowing the current to fall until it reaches a lower threshold. This process is repeated as needed, causing the voltage level connected to the injector coil to switch between 48V and 0V via the PWM signal.  
           [0004]    Regulators employing chopping, however, produce emissions each time the PWM signal transitions from high to low (e.g., on to off) and from low to high. Because regulators using PWM signals tend to transition frequently to obtain the desired average level, the generated emissions often reach undesirable levels that are difficult to reduce without affecting the operation of the regulator itself.  
           [0005]    There is a desire for a system that can regulate load current without producing undesirable radio frequency emissions.  
         SUMMARY OF THE INVENTION  
         [0006]    The present invention is directed to a current regulator that controls a load current with two voltage sources having different values. In one embodiment, a first, high voltage source powers the load current at the current&#39;s rising and falling edges and a second, low voltage powers the load current while it is maintained at a selected reference level. The switching is triggered by a command pulse and by load current feedback. Together, the two voltage sources regulate the load current in a similar manner as a PWM signal. By using two voltage sources rather than a PWM signal, the inventive system avoids generating emissions normally associated with PWM signals. Further, by providing a low voltage source to maintain the load current, the inventive current regulator maintains the load current without requiring excessive switching. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0007]    [0007]FIG. 1 is a block diagram illustrating one embodiment of the invention;  
         [0008]    [0008]FIG. 2 is a block diagram illustrating an embodiment of the invention in greater detail; and  
         [0009]    [0009]FIG. 3 is a timing diagram illustrating the operation of components shown in FIG. 1. 
     
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS  
       [0010]    [0010]FIGS. 1 and 2 illustrates a current regulator  100  according to one embodiment of the invention, and FIG. 3 is a timing diagram illustrating an operation sequence of components in the regulator  100 . FIG. 1 illustrates the broad concept of the invention, while FIG. 2 illustrates one embodiment of the invention in greater detail. The invention is generally directed to a system  100  that can control operation of any drive circuit  101  by controlling current through a load  102  with two voltage sources  104 ,  106  instead of a PWM signal. Although the example below focuses on a fuel injection system, the invention can be incorporated as a load current regulator for an inductive load in other contexts as well.  
         [0011]    In this embodiment, a command pulse from a microprocessor (not shown) enters the regulator  100  to control the load current flow through a load  102 . In this example, the load  102  is an injector coil that starts and stops fuel injection. The command pulse itself is controlled based on, for example, commands from an engine controller (not shown) that determine how much fuel is needed at any given instant.  
         [0012]    Instead of relying on a PWM signal to control the current through the load, the inventive current regulator  100  has a high voltage source  104  and a low voltage source  106  that can be alternately connected into the current regulator  100  via a switch  108 . The switch  108  will select one of the voltage sources  104 ,  106  at any given time based on the command pulse. In one embodiment, the high voltage source  104  has a value high enough to ensure that the current to the load  102  rises quickly to the reference level when it is selected, while the low voltage source  106  has a value to maintain the load current at a selected level. In general, the high voltage source  104  is selected at the rising edge of the command signal to raise the load current to the reference level and at the falling edge of the command signal to discharge the load current from the load  102  back to the high voltage source  104 . The low voltage source  106  is selected when the desired load current has been achieved to maintain the load current at the reference level.  
         [0013]    More particularly, with respect to FIGS. 1 through 3, the command pulse is sent to a rising edge one shot device  110 , which responds to a rising edge of the command pulse, and a falling edge one shot device  112 , which responds to a falling edge of the command pulse. When a rising edge in the command pulse triggers the rising edge one shot device  110 , the rising edge one shot device  110  sends an output through an OR gate  113  into a flip-flop  114 . The flip-flop  114  generates an output to any known control mechanism to cause the switch  108  to select the high voltage source  104 , as shown in FIG. 2.  
         [0014]    In this embodiment, high and low side switches  116   a ,  116   b  and high and low side shunts  118   a ,  118   b  connect the load  102  to the sources  104 ,  106 . The high side switch  116   a  is controlled by a level shifter  119 , which also receives the command pulse as an input. At this point, the command pulse turns on the low side switch  116   b  directly and turns on the high side switch  116   a  through the level shifter  119 .  
         [0015]    A comparator  120  monitors the load current and compares it with the reference level. In one embodiment, the comparator  120  is connected at the low side shunt  118   b . As shown in FIG. 3, the comparator  120  output remains high as long as the load current remains below the reference level. When the load current reaches the reference level, the comparator  120  output goes low, resetting the flip-flop  114 . When the flip-flop  114  is reset, the switch  108  is switched to select the low voltage source  106  to maintain the load current at the reference level.  
         [0016]    The switch  108  continues to select the low voltage source  106  until the command pulse switches from high to low. At that point, the falling edge of the command pulse triggers the falling edge one shot device  112 . The falling edge one shot device  112  sends an output through the OR gate  113  and sets the flip-flop  114 . The flip-flop  114  causes the switch  108  to select the high voltage source  104 .  
         [0017]    When the command pulse goes from high to low, the command pulse directly turns off the low side switch  116   b  and turns off the high side switch  116   a  via the level shifter  119 . This, in combination with connecting the high voltage source  104 , causes current to recirculate through the load  102  quickly and back into the high voltage source  104 , ensuring that the load current drops rapidly. To enable current recirculation, diodes  122   a ,  122   b  are connected on the high side and the low side, respectively, of the load  102 .  
         [0018]    Thus, by switching between the two voltage sources  112 ,  114 , the output of the regulator maintains the load current at a selected level without any chopping. Further, by switching a high voltage source and a low voltage source instead of simply connecting and disconnecting a high voltage source, the invention allows the load current to be maintained at a selected level without excessive switching; the low voltage source acts as a load current maintenance device.  
         [0019]    It should be understood that various alternatives to the embodiments of the invention described herein may be employed in practicing the invention. It is intended that the following claims define the scope of the invention and that the method and apparatus within the scope of these claims and their equivalents be covered thereby.