Abstract:
Drive current stabilization is achieved through the management of a drive current. The drive current may include a control current that is provided to a control terminal of a switch, a current limit input current that is provided to a current limit circuit associated with the switch and a stabilization current. The switch carries a load current responsive to a control signal on the control terminal. The magnitude of the control current is monitored and a magnitude of the stabilization current is increased responsive to a decrease in the magnitude of the control current to substantially maintain a magnitude of the drive current.

Description:
TECHNICAL FIELD  
       [0001]     The present invention is generally directed to a technique for drive current stabilization and, more specifically, to a technique for drive current stabilization of an automotive ignition system.  
       BACKGROUND OF THE INVENTION  
       [0002]     Frequently, modern automotive ignition systems have controlled an ignition coil current by modulating a control terminal, e.g., a gate, of a switching device, e.g., an insulated-gate bipolar transistor (IGBT), which provides a current path for a primary winding of an ignition coil. In such automotive ignition systems, it has generally been desirable for the current in the primary winding of the ignition coil to increase as quickly as possible, limited by an impedance of the primary winding, to a predetermined desired coil current limit level. Further, when the coil current limit level has been reached, it has generally been desirable for the coil current to smoothly transition to a steady-state value, with minimal oscillation during the transition.  
         [0003]     In a typical automotive ignition system, when an IGBT is used as the switching device, a designed current of approximately 500 uA has been used to quickly charge a gate capacitance of the IGBT and raise an IGBT gate voltage above a turn-on gate threshold of the IGBT. However, when the IGBT gate capacitance is charged to a maximum voltage level, as typically determined by an ignition control integrated circuit, the gate voltage can be maintained with significantly less current than the current initially required to quickly charge the IGBT gate capacitance. After the gate is fully charged, the lower IGBT gate current requirement continues while the primary winding current is increasing to the desired coil current limit level. When the coil current limit level is reached, the IGBT gate voltage is reduced in an attempt to maintain a constant primary winding current.  
         [0004]     In a typical automotive ignition system, the decrease in the IGBT gate voltage has been achieved through the use of a closed-loop feedback circuit, i.e., a gate control current limit circuit. When the IGBT gate voltage is reduced, a gate drive current source that is providing the IGBT gate charging current has generally increased its output current due to changes in the current source bias conditions. Thus, in order to reduce the IGBT gate voltage, the gate control current limit circuit has been required to sink the additional current. As mentioned above, in operation, the input current draw is at a maximum during the initial charging of the IGBT gate and subsequently reduces during the time that the primary winding current is increasing to the current limit level. Finally, the current draw again increases when the gate control current limit circuit reduces the IGBT gate voltage to control the primary winding current.  
         [0005]     In input-powered automotive ignition systems, a supply current is provided from an associated control unit, through a series resistor that is either internal or external to the ignition control integrated circuit. Unfortunately, the fluctuation in the supply current provided by the control unit, through the series resistor, causes a proportional voltage fluctuation to the gate drive current source. This voltage fluctuation, under some input conditions, causes the gate control current limit circuit to repeatedly change from an open-loop condition (IGBT fully on) to a closed-loop condition (IGBT gate voltage controlled). This voltage oscillation, in turn, causes an undesired oscillation in the primary winding current.  
         [0006]     What is needed is a drive current stabilization circuit that substantially maintains a constant current output from a current source that is driving a control terminal of a switching device that controls a primary winding current of an ignition coil, irrespective of the state of a current limit control loop.  
       SUMMARY OF THE INVENTION  
       [0007]     The present invention is generally directed to a technique for drive current stabilization. According to one embodiment of the present invention, a drive current is received that includes a control current that is provided to a control terminal of a switch, a current limit input current that is provided to a current limit circuit associated with the switch and a stabilization current. The switch carries a load current responsive to the magnitude of a control signal on the control terminal. The magnitude of the control current is monitored and the magnitude of the stabilization current is increased responsive to a decrease in the magnitude of the control current to substantially maintain the magnitude of the drive current.  
         [0008]     According to another aspect of the present invention, the magnitude of the stabilization current is reduced responsive to an increase in the magnitude of the current limit input current to substantially maintain the magnitude of the drive current. According to a different aspect of the present invention, the switch is one of an insulated-gate bipolar transistor (IGBT) or a field-effect transistor (FET).  
         [0009]     These and other features, advantages and objects of the present invention will be further understood and appreciated by those skilled in the art by reference to the following specification, claims and appended drawings. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0010]     The present invention will now be described, by way of example, with reference to the accompanying drawings, in which:  
         [0011]      FIG. 1  is an electrical diagram, in block and schematic form, of an exemplary automotive ignition system that implements a drive current stabilization circuit constructed according to one embodiment of the present invention;  
         [0012]      FIGS. 2A-2B  are graphs depicting waveforms of a primary winding current of an ignition coil for a prior art automotive ignition system and an automotive ignition system that implements a drive current stabilization circuit constructed according to one embodiment of the present invention, respectively;  
         [0013]      FIGS. 3A-3B  are graphs depicting waveforms of a drive current for a prior art automotive ignition system and an automotive ignition system that implements a drive current stabilization circuit constructed according to one embodiment of the present invention, respectively;  
         [0014]      FIGS. 4A-4B  are graphs depicting waveforms of a supply voltage for a prior art automotive ignition system and an automotive ignition system that implements a drive current stabilization circuit constructed according to one embodiment of the present invention, respectively; and  
         [0015]      FIG. 5  is an electrical schematic of a relevant portion of an exemplary automotive ignition system that implements a drive current stabilization circuit constructed according to one embodiment of the present invention. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0016]     According to one embodiment of the present invention, a drive current stabilization circuit for an automotive ignition system is disclosed that maintains a constant current output from a current source that drives a control terminal, e.g., a gate, of a switch, e.g., a field-effect transistor (FET) or insulated-gate bipolar transistor (IGBT), that controls a current through a primary winding of an ignition coil. The present invention is generally applicable to drive current stabilization for automotive ignition systems that are input-powered, as well as automotive ignition systems that are battery-powered. Further, it is contemplated that the present invention is applicable to other environments where a switch is utilized to provide a current path for a coil or other load.  
         [0017]     As is shown in  FIG. 1 , an automotive ignition system  100  includes a primary winding Lp of an ignition coil that is coupled between a battery B+ and a switch S 1 , e.g., a FET or an IGBT. An output terminal, e.g., an emitter, of the switch S 1  is coupled to ground through a sense resistor Rs. A gate control current limit circuit  108  is coupled across the sense resistor Rs. The circuit  108  monitors a voltage developed across the resistor Rs to determine a magnitude of the current flowing through the primary winding Lp. The circuit  108  acts to limit the current through the primary winding Lp, when the current reaches a desired level. A control unit  102  provides an electronic spark timing (EST) signal V 1  to a gate drive current source  104  and a gate control current limit circuit  108 , via a resistor Rsource, e.g., a 470 Ohm resistor. The gate drive current source  104  provides a drive current I 1  to a drive current stabilization circuit  106 , constructed according to one embodiment of the present invention. The drive current stabilization circuit  106  provides a current limit input current I 3  to an input of the gate control current limit  108  and a control current I 2  to a gate of the switch S 1 . The drive current stabilization circuit  106 , as required, sinks a stabilization current I 4  to substantially maintain a magnitude of the drive current I 1 .  
         [0018]     As is discussed above, in prior automotive ignition systems, which have not included the drive current stabilization circuit  106 , a drive current I 1  has been split between a current limit input current I 3 , which was used by the gate control current limit circuit  108 , and a control current I 2 , which was used to charge the IGBT gate capacitance and turn on the switch S 1 . After the gate capacitance of the switch S 1  was charged, the current I 2  would cease flowing and, thus, the current I 1  would decrease. This reduction in current would then cause the voltage V 1  to increase. When the current through the sense resistor Rs had increased to a desired current limit level, the current I 3  would increase to reduce the IGBT gate voltage. The increase in the current I 3  would then cause an equal increase of the current I 1  and, thus, the voltage V 1  would decrease. It should be appreciated that this voltage change was generally undesirable as it can cause oscillation in the primary winding Lp current.  
         [0019]     However, in automotive ignition systems that implement the drive current stabilization circuit  106 , designed according to the present invention, when the current I 2  decreases, the stabilization current I 4  increases by an approximately equal amount. In this manner, the drive current I 1  remains substantially constant (e.g., within +5 percent) and, as such, the voltage V 1  also remains substantially constant. According to one embodiment, the gate control current limit circuit  108 , the drive current stabilization circuit  106  and the gate drive current source  104  are integrated within an ignition control integrated circuit  107 .  
         [0020]      FIGS. 2A, 3A  and  4 A depict exemplary waveforms of an ignition primary current, the drive current I 1  current and the V 1  voltage, respectively, as a function of time, for a prior art automotive ignition system.  FIGS. 2B, 3B  and  4 B depict exemplary waveforms of the ignition primary current, the drive current I 1  current and the V 1  voltage, respectively, as a function of time, for an automotive ignition system including a drive current stabilization circuit  106  constructed according to the present invention. As is evident from comparing the signals of  FIGS. 3A and 3B , the drive current I 1  is significantly more constant during IGBT gate capacitance charging, when the gate is fully charged and when the gate voltage is reduced by current limit control. As is also evident from comparing the signals of  FIGS. 4A and 4B , the voltage V 1  is substantially more constant, when the gate is fully charged, as well as when the gate voltage is reduced by the current limit control.  
         [0021]     With reference to  FIG. 5 , transistor level circuit implementation of the drive current stabilization circuit  106  is depicted in relationship to related components of an ignition control integrated circuit (IC) of an automotive ignition system. Transistors Q 100  to Q 109  and resistors R 100  to R 103  form a reference current generator known as a ‘Delta Vbe generator’. As is well known to one of ordinary skill in the art, the ‘Delta Vbe generator’ is a standard building block and has a reference current (Iref) defined by the following equation:  
       Iref   =       Vt   *     Ln   ⁡     (   N   )         Rdvbe         
 
 where Vt is the thermal voltage defined by k*T/q, k is Boltzman&#39;s constant, T is the temperature in degree Kelvin and q is the electronic charge; N is the ratio of the emitter areas used to develop the Delta VBE current, i.e., transistors Q 105  to Q 108 , and in the disclosed implementation N is set equal to 9; and Rdvbe is the resistance chosen to establish a magnitude of the reference current Iref and corresponds to the value of resistor R 102 . The reference current Iref is used to drive a current mirror rail, which drives other circuits necessary for operation of the ignition control integrated circuit (IC), along with the gate drive current. 
 
         [0022]     In one embodiment, due to the relative emitter areas of the transistor Q 100  and the transistor Q 3  and the values of the resistors R 100 , R 3  and R 4 , the gate drive current I 1  is approximately eight times the Iref current. As is shown, the gate charging drive current I 1  is provided from a collector of the transistor Q 3 . When the drive current I 1  is initially turned on, the switch S 1  gate voltage is low and rises as a gate capacitance of the switch S 1  is charged. The current I 1 , supplied from the collector of the transistor Q 3 , is used by the gate control current limit circuit  108  or to charge the gate capacitance of the switch S 1 . At this point, the current I 4  is approximately equal to zero as transistors Q 2  and Q 6  are turned off. The transistor Q 2  remains off as long as its emitter voltage is no more than approximately 0.6 Volts greater than its base voltage. The emitter voltage of the transistor Q 2  tracks the gate voltage of the switch S 1  and its base voltage is defined by the following equation: 
 
 Q 2 base voltage= V 1−[( I ref* R 100)+ Vbe  of the transistor  Q 100]
 
         [0023]     While the switch S 1  gate capacitance is charging, the base voltage of the transistor Q 2  is higher than its emitter voltage and, as such, the collector of the transistor Q 2  does not provide current to turn on the transistor Q 6 . It should be appreciated that the transistors Q 100 , Q 1 , Q 4  and Q 5  and resistors R 100 , R 1  and R 2  create two current mirrors that discharge and maintain a low state on the base of the transistor Q 6 , when the transistor Q 2  is off. These current mirrors are configured to create a current that is a reduced version of the reference current Iref. It is desirable that this current be relatively small, e.g., a few microamperes, which allows the drive current stabilization circuit to become active when the base voltage of the transistor Q 2  is only slightly below the emitter voltage of the transistor Q 2 . This occurs when the transistor Q 3  approaches saturation and the collector voltage of the transistor Q 3  approaches the base voltage of the transistor Q 3 . When the transistor Q 3  approaches saturation, its base current increases, thereby creating an additional voltage drop across the resistor R 3 , lowering the base voltage of the transistor Q 2  relative to its emitter voltage.  
         [0024]     It should be appreciated that if the transistor Q 3  is allowed to be driven deep into saturation, the overall current draw of the component will reduce as the current output of the transistor Q 3  decreases. However, according to the present invention, the transistor Q 2  begins to conduct current when the transistor Q 3  begins to saturate. When the current conducted by the transistor Q 2  overcomes the pull-down current of the transistor Q 5 , the transistor Q 6  begins to turn on and the voltage at the collector of the transistor Q 3  is maintained, which keeps the transistor Q 3  from being driven into hard saturation. This eliminates the undesired change in the collector current of the transistor Q 3 .  
         [0025]     The current I 2  that was previously charging the switch S 1  gate capacitance is now diverted to ground, via the transistor Q 6 . As such, the drive current I 1  remains substantially unchanged. When the ignition coil primary winding current limit is reached, the gate control current limit  108  increases the current I 3  to reduce the switch S 1  gate voltage. As this reduces the voltage at the emitter of the transistor Q 2 , the current flow from the collector of the transistor Q 2  is stopped and the transistor Q 6  is turned off, which ends the current I 4  flow. In this manner, the overall current I 1  is relatively unchanged since the current I 4  that was flowing through the transistor Q 6  is now used by the gate control current limit  108 .  
         [0026]     Accordingly, a drive current stabilization circuit has been described herein that enhances drive current stabilization, which reduces undesired oscillation in a current carried by a primary winding of an ignition coil of an automotive ignition system.  
         [0027]     The above description is considered that of the preferred embodiments only. Modifications of the invention will occur to those skilled in the art and to those who make or use the invention. Therefore, it is understood that the embodiments shown in the drawings and described above are merely for illustrative purposes and not intended to limit the scope of the invention, which is defined by the following claims as interpreted according to the principles of patent law, including the doctrine of equivalents.