Abstract:
A current control system for controlling current provided to a load includes a current sensor that senses a current to the load; a first power switch selectively enabled to supply power to the load and disabled to prevent power from being supplied to the load; and a control circuit. The control circuit includes a comparator that compares the sensed current with a commanded current to determine whether to enable or disable the first power switch, and a timer circuit that prevents the power switch from being enabled by the comparator more than once within a predetermined time period.

Description:
BACKGROUND 
       [0001]    The present invention is related to controlling current to loads, and in particular to a system and method for adaptively controlling current to inductive loads. 
         [0002]    Pulse width modulation (PWM) has been used extensively to control power loss in loads. PWM involves controlling current to the load through the use of power switches. The switch is enabled via the PWM signal at a predetermined frequency and duty cycle to control power dissipated in the load. Power supplied to the load is increased by increasing the duty cycle of the PWM signal. 
         [0003]    Using fixed frequency PWM to control current creates several drawbacks. Ripple currents are created in the load when current is enabled and disabled to the load. Ripple current is an unwanted alternating-current (AC) error current in the load. The amplitude of the ripple current increases as the frequency of the PWM is reduced. 
         [0004]    Electromagnetic interference (EMI) is also generated when switching power on and off to the load. The amount of EMI generated is proportional to the frequency of the PWM. The level of EMI is also affected by the turn-on and turn-off time of the switches. The shorter the turn-on and turn-off time of the switches, the greater the EMI generated. 
         [0005]    Power dissipation in the driver circuit is also a concern when switching power on and off to the load. As the PWM frequency is increased, the amount of power dissipation in the driver circuit is increased. This is due to the fact that very little power loss occurs when metal-oxide-semiconductor field-effect transistors (MOSFETs) are in an on or off state, but significant power loss occurs while the MOSFET is operating in the linear region during turn-on or turn-off. It is desirable to provide a current control system that minimizes the power dissipation, EMI generation, and ripple current generated by the driver circuit. 
       SUMMARY 
       [0006]    A current control system includes a current sensor, a power switch, and a control circuit. The control circuit includes a comparator and a timer circuit. The current sensor senses current supplied to a load. The power switch is controlled to selectively connect power to the load. The comparator compares the sensed current with a commanded current, and the timer circuit ensures the power switch is not enabled more than once within a predetermined time period. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0007]      FIG. 1  is a block diagram illustrating a system for controlling current to an inductive load according to an embodiment of the present invention. 
           [0008]      FIG. 2  is a flowchart illustrating a method for controlling current to an inductive load according to an embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0009]    The present invention is related to current control for inductive loads. A current control circuit is used to control a low-side switch in order to control power to an inductive load. The control circuit utilizes a comparator to compare the actual current through the load with a reference voltage indicative of a desired current through the load. If the comparator indicates that the current to the load is greater than the desired current, the low-side switch is turned off. If the comparator indicates that the current to the load is less than the desired current, the low-side switch is turned on to provide power to the load. A timer circuit is utilized to ensure that the low-side switch is not switched on multiple times within a predetermined period of time in order to minimize electromagnetic interference (EMI), ripple current in the load, and power dissipation in the low-side switch. In this way, the current control system can adapt to the needs of any inductive load. 
         [0010]      FIG. 1  is a block diagram illustrating system  10  for controlling current to inductive load  12  according to an embodiment of the present invention. System  10  includes high-side switch  14 , low-side switch  16 , power rail  18 , current sensor  20 , current command input  22 , power return  24 , current control circuit  26 , and high-side switch enable input  36 . Current control circuit  26  comprises comparator  28 , NOR gate  30 , latch  32 , and timer circuit  34 . High-side switch  14  and low-side switch  16  are implemented as any type of power switch, such as metal-oxide-semiconductor field-effect transistors (MOSFETs). Current sensor  20  is any device capable of sensing the current to inductive load  12  such as, for example, a shunt resistor. 
         [0011]    System  10  is utilized to control the current provided to inductive load  12 . Load  12  may be any type of inductive load such as, for example, a solenoid. When both high-side switch  14  and low-side switch  16  are enabled, power is provided to inductive load  12  from power rail  18 . Current control circuit  26  controls low-side switch  16 . High-side switch  14  is controlled externally by, for example, a microcontroller. During normal system operation, high-side switch  14  remains enabled. Although illustrated with both high-side switch  14  and low-side switch  16 , separate embodiments of system  10  may be implemented without the use of high-side switch  14  wherein power is provided to load  12  solely through the use of low-side switch  16 . 
         [0012]    Comparator  28  is used to compare a sensed current from current sensor  20  with a desired current. The desired current is represented by a voltage from current command input  22 . This voltage may be set, for example, by a microcontroller. The sensed current will also be represented by a voltage provided by current sensor  20 . The two voltages are provided to comparator  28 , which may be implemented by any well known comparator. An output of comparator  28  indicates whether or not the current through inductive load  12  is greater than the desired current. If the monitored current is greater than the desired current, comparator  28  outputs a logic high value voltage. If not, comparator  28  outputs a logic low value voltage. The output of comparator  28  is provided both to latch  32  and NOR gate  30 . In the embodiment shown in  FIG. 1 , latch  32  is implemented as a set-reset (SR) latch and is used to store the state of the output controlling low-side switch  16 . The output of comparator  28  is provided to the reset input of latch  32  and the output of NOR gate  30  is provided to the set input of latch  32 . In other embodiments, latch  32  may be implemented as any type of latch capable of storing a state of the output controlling low-side switch  16 . 
         [0013]    Low-side switch  16  is disabled when the monitored current is greater than the desired current. Comparator  28  generates a logic high output that is provided to the reset input of latch  32 . Because the output of comparator  28  is a logic high value, the output of NOR gate  30  will be a logic low value, and therefore the voltage provided to the set input of latch  32  will be a logic low value. Given the two inputs, the state of latch  32  will be reset, which will provide an output to disable low-side switch  16  and cut off power to load  12 . As illustrated, the inverted output of latch  32  is used to control low-side switch  16 . By only disabling power to the load when the monitored current is greater than the desired current, low-side switch  16  can remain enabled when power is initially provided to load  12  to more quickly ramp the current up to an operating level. The power dissipated through load  12  is also reduced from traditional PWM methods by disabling low-side switch  16  immediately after the monitored current becomes greater than the desired current. 
         [0014]    Low-side switch  16  is enabled when the current sensed by current sensor  20  is less than the desired current. When the sensed current is less than the desired current, the output of comparator  28  will be a logic low value. When the output of both timer circuit  34  and comparator  28  are at logic low values, the voltage to the set input of latch  32  will be at a logic high value, and the voltage to the reset input of latch  32  will be at a logic low value. This will set the state of latch  32  to a value indicative of enabling low-side switch  16 . The output of latch  32  is used to enable low-side switch  16 . 
         [0015]    Timer circuit  34  is utilized to ensure that low-side switch  16  is not enabled more than one time within a predetermined time period. By preventing low-side switch  16  from being enabled multiple times within a short time period, the EMI generated by system  10  is greatly reduced. The ripple current through load  12  and the power dissipation through low-side switch  16  are also greatly reduced in comparison to traditional PWM methods. The predetermined period of time for timer circuit  34  is any period of time selected to minimize EMI, ripple current, and power dissipation in system  10  such as, for example, approximately 100 microseconds. 
         [0016]    Timer circuit  34  is any timer circuit known in the art, such as a monostable  555  timer circuit. Timer circuit  34  is edge triggered and is reset each time low-side switch  16  is enabled. When the timer circuit is reset and begins counting, the output of timer circuit  34  is a logic high value and remains at the logic high value until timer  34  has reached the predetermined time period. This ensures that the output of NOR gate  30  cannot be a logic high value during the predetermined time period and therefore cannot enable low-side switch  16 . When timer circuit  34  reaches the end of the predetermined time period, the output of timer circuit  34  transitions to a logic low value and remains at the logic low value until it is reset. This allows low-side switch  16  to be enabled if the monitored current is less than the desired current. 
         [0017]      FIG. 2  is a flowchart illustrating a method  60  for controlling current to inductive load  12  according to an embodiment of the present invention. At step  62 , power is initially provided to load  12  by enabling low-side switch  16 . The current sensed by current sensor  20  will be less than the value indicated by current command input  22 . Timer circuit  34  begins counting. At step  64 , the monitored current is compared to the desired current using comparator  28 . If the monitored current is greater than the desired current, method  60  proceeds to step  66 . If the monitored current is less than the desired current, method  60  remains at step  64 . At step  66 , low-side switch  16  is disabled, cutting off power to load  12 . At step  68 , the monitored current is compared to the desired current using comparator  28 . If the monitored current is less than the desired current, method  60  proceeds to step  70 . If the monitored current is greater than the desired current, method  60  remains at step  68 . At step  70 , it is determined if timer circuit  34  has reached the predetermined time period. If it has, method  60  proceeds to step  72 . If it has not, method  60  returns to step  68 . At step  72 , low-side switch  16  is enabled and timer circuit  34  is reset. Method  60  returns to step  64 . Method  60  continues to loop for the duration of normal system operation. 
         [0018]    In this way, the present invention describes a system and method for controlling current to an inductive load. Although the present invention has been described with reference to preferred embodiments, workers skilled in the art will recognize that changes may be made in form and detail without departing from the spirit and scope of the invention.