Abstract:
A circuit for controlling an ignition coil is provided. The circuit includes a first transistor, second transistor, and a capacitor. The first transistor is connected in electrical series between the ignition coil and a voltage reference. The capacitor is connected between the ignition coil and a control input of the first transistor. The second transistor is configured to selectively connect the capacitor to the voltage reference.

Description:
BACKGROUND 
     1. Field of the Invention 
     The present invention generally relates to a circuit for driving an ignition coil. 
     2. Description of Related Art 
     In a spark ignited internal combustion engine, ignition coils provide the voltage required for electrical current to jump across a spark plug gap. The spark ignites an air-fuel mixture in the engine cylinder causing combustion. A switch, also referred to as a coil driver, is used on the primary side of the ignition coil to control the charge and discharge cycles of the ignition coil. 
     A typical ignition system is illustrated in  FIG. 1 . The system includes an ignition coil  210  having a primary side  216  and a secondary side  218 . The positive terminals of the primary side  216  and secondary side  218  of the ignition coil  210  are connected to a power source  212 . The negative terminal of the primary side is connected to a switching transistor  214 . The switching transistor  214  is connected between the ignition coil  210  and an electrical ground  220 . The negative terminal of the secondary side  218  is connected to a spark plug  222 . The spark plug  222  is connected between the ignition coil  210  and an electrical ground  220 . 
       FIG. 2  illustrates the voltage and current profiles at various points within the prior art system. The profile of the control signal provided to the switching transistor  214  is identified by reference numeral  224 , while the current flowing through the primary side  216  of the ignition coil  210  is denoted by reference numeral  226 . In addition, a profile of the primary coil voltage signal, as seen on the collector of transistor  214 , is denoted by reference numeral  228 . In a typical charge and discharge cycle the switching transistor  214  is turned on, charging the ignition coil  210  for a specified dwell period or to a specified charge current; and then the switching transistor  214  is turned off, allowing the secondary side  218  of the ignition coil  210  to discharge stored energy across the spark plug gap. 
     One problem is that the sharp turn-on during the charging cycle causes an oscillation on the secondary side  218  of the ignition coil  210 .  FIG. 3  illustrates the dwell command signal  224 , the dwell current  226 , low-side voltage  228 , and the undesirable secondary voltage oscillation  230 . The switching transistor  214  starts out in the off-state with the negative terminal of the primary side  216  equal to the battery voltage. After the switching transistor  214  is turned on, the transistor quickly transits through its linear range into the saturated on-state with very large voltage change across the primary side  216  of the ignition coil  210 . The resulting secondary voltage  230  during turn-on is a large oscillation magnitude that decays in time. If the oscillation magnitude exceeds a tolerable level, an unintended spark event can occur across the spark plug gap, resulting in premature combustion. One way to control the magnitude of the secondary oscillations is adding constraints in design of the ignition coil  210 . These constraints, however, result in poor coil performance. 
     In view of the above, it is apparent that there exists a need for an improved circuit for driving an ignition coil. 
     SUMMARY 
     In satisfying the above need, as well as overcoming the enumerated drawbacks and other limitations of the related art, the present invention provides an improved circuit for driving an ignition coil. 
     Generally, the circuit includes a pair of transistors, and a capacitor. The first transistor is connected in electrical series between the ignition coil and a voltage reference. The capacitor is connected between the ignition coil and a control input of the first transistor. The second transistor is configured to selectively connect the capacitor to the voltage reference. 
     A third transistor and a resistor are connected in electrical series between the control input of the first transistor and the voltage reference. The third transistor is configured to selectively connect the control input of the first transistor to the voltage reference. In addition, a current source is in electrical communication with the control input of the first transistor through a diode. 
     In another aspect of the present invention, a diode is connected between the capacitor and the voltage reference. 
     In yet another aspect of the present invention, the circuit includes a fourth transistor connected between the capacitor and the control input of the first transistor. This fourth transistor is configured to selectively connect the capacitor to the control input of the first transistor. 
     Further objects, features and advantages of this invention will become readily apparent to persons skilled in the art after a review of the following description, with reference to the drawings and claims that are appended to and form a part of this specification. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic view of a conventional spark ignition system; 
         FIG. 2  is a graph illustrating the timing of various voltage profiles for the spark ignition system of  FIG. 1 ; 
         FIG. 3  is a graph illustrating oscillation in the secondary voltage resulting from the conventional spark plug system; 
         FIG. 4  is a schematic view of a circuit for driving an ignition system in accordance with the present invention; and 
         FIG. 5  is a graph illustrating the timing of the switching voltage profile for the circuit illustrated in  FIG. 4 . 
     
    
    
     DETAILED DESCRIPTION 
     Referring now to  FIG. 4 , a system embodying the principles of the present invention is illustrated therein and designated at  10 . As its primary components, the system  10  includes a switching circuit  12  and an ignition coil  14 . 
     The switching circuit  12  receives power from a power source  16  through a current source  18 . The power source  16  is also connected to one of the primary terminals of the ignition coil  14 . The switching circuit  12  also receives a control signal at input node  22 . The switching circuit  12  is connected to the other terminal of the primary side of the ignition coil  14  and controls the ignition coil  14  based on the control signal provided to input node  22 . The secondary side of the ignition coil  14  has one terminal connected to electrical ground and the other terminal connected to a spark plug  20 . Based on the control of the switching circuit  12 , the ignition coil  14  generates a voltage to fire the spark plug  20 . 
     The switching circuit  12  itself includes a transistor  24  and a capacitor  28 . The transistor  24  is shown as an IGBT transistor, however, other transistors are also contemplated. Resistor  26  is indicative of the internal gate-emitter resistance of an IGBT transistor. The collector of transistor  24  is connected to the primary side of the ignition coil  14 . The emitter of transistor  24  is connected to an electrical ground  25 . The capacitor  28  is connected to the collector of transistor  24  and is in electrical communication with the gate of transistor  24  through diode  30 . As such, capacitor  28  acts as a Miller effect capacitor. The anode of diode  30  is connected to both the current source  18  and capacitor  28 . The cathode of diode  30  is connected to the gate of transistor  24 . Also connected to the gate of transistor  24  is resistor  32 . Resistor  32  is selectively in communication with electrical ground  25  through transistor  34 . Transistor  34  shown as an NPN bipolar transistor, however, other commonly known transistors are contemplated herein. As such, the collector of transistor  34  is connected to resistor  32  and the emitter of transistor  34  is connected to electrical ground  25 . Transistor  34  receives the control signal from node  22  through resistor  36 , thereby selectively providing a path from resistor  32  to electrical ground  25 . 
     In addition, the control signal provided to input node  22  is provided to transistor  40  through resistor  38 . Resistor  38  is connected to the control input of transistor  40 . Transistor  40  is shown as an NPN bipolar transistor, however, other common transistors may be readily substituted. The base of transistor  40  is connected to resistor  38  to selectively connect capacitor  28  to electrical ground  25 . As such, the collector of transistor  40  is connected to capacitor  28  and the emitter of transistor  40  is connected to an electrical ground  25 . Further, diode  42  is connected between capacitor  28  and electrical ground with the anode of diode  42  connected to electrical ground and the cathode of diode  42  connected to capacitor  28 . 
     For the circuit described, the gate voltage vs. gate charge characteristics of the transistor  24  are illustrated in  FIG. 5 . Over region  52 , the collector current is zero, and the gate impedance of transistor  24  is defined by the relationship C g1 =Q1/N g1 g. In region  54 , the enhancement gate region, collector current is turned on at V g1 , Q1 and the collector current of transistor  24  sharply increases with increasing collector to emitter voltage. The gate capacitance, in region  54 , is defined by the relationship C g2 =(Q2−Q1)/(V g2 −V g1 ). In region  56 , the gate is fully enhanced. The collector current of transistor  24  slightly increases with increasing collector to emitter voltage increase. The gate capacitance of transistor  24  is defined by the relationship C g3 =(Q3−Q2)/V g3 −V g2 ). 
     At the point between region  52  and region  54 , the collector current is turned on. Due to the inductive load represented by the primary winding of the ignition coil  14 , the current begins to increase at a rate given by
 
Δ i/Δt=V coil/ L coil,
 
where Δi/Δt is the rate of change of current, Vcoil is the voltage across the coil (approx. equal to VIGN, the voltage provided by power source  16 ) and Lcoil is the primary inductance. When the collector voltage of transistor  24  quickly drops to near zero, the rate of change of voltage across the primary winding changes very fast. The fast transient primary voltage change may trigger a large enough secondary voltage to cause forward ignition at the beginning of the dwell period. The configuration in  FIG. 4  slows the initial voltage transient across the coil primary preventing ignition.
 
     When transistor  24  is off transistors  40  and  38  are fully on, their collector voltages are almost at ground potential. Current from the current source  18  is shunted away from the gate of transistor  24  by transistor  40 , while transistor  34  clamps the gate to ground through resistor  32 . The collector of transistor  24  is at VIGN and capacitor  28  is charged to VIGN. 
     When transistor  24  is on, transistors  40  and  34  are turned off and the current from current source  18  is channeled into the gate of transistor  24  which will charge to voltage V g1 . When the gate voltage is greater than V g1 , transistor  24  begins to conduct and the collector voltage starts to change negatively. The negative voltage drop begins to discharge capacitor  28  and the discharge current flows from the current source  18  through the collector of transistor  24  and to ground. The current from the current source  18  will be reduced by the Miller feedback current which will limit the rate of change of collector voltage. The Miller feedback current is equal to C m (ΔV ce /Δt), where C m  is the capacitance of capacitor  28 , V ce  is the collector-emitter voltage of transistor  24 , and t is time. A dynamic equilibrium is established whereby the portion of the current charging the gate of transistor  24  and causing the collector voltage to drop balances the Miller feedback current. By this process, the current turn-on rate is precisely regulated and the voltage drop across the primary winding of the ignition coil  14  is slowed. Accordingly, the secondary voltage, induced by the changing primary voltage, is reduced so that forward firing is prevented. 
     When transistor  24  changes from fully-on to off, transistors  40  and  34  are turned on again. Transistor  40  will clamp the current from current source  18  to ground and transistor  34  will discharge the gate of transistor  24  through the resistor  32 . Therefore, the collector current is quickly turned off. Since the spark plug does not yet represent any reflected load on the primary, the collector voltage of transistor  24  will change positively at a high rate and large amplitude (fly-back). Transistor  40  not only clamps the current from current source  18  to ground but also the larger capacitive current caused by the fly-back voltage (ΔV ce /Δt). Without the clamping, the Miller feedback would slow the fly-back rate of change and the fly-back amplitude preventing the evolution of the high secondary voltage (up to 35 kV). 
     The primary fly-back voltage peaks at several hundreds volts and the peak marks the beginning of sparking. The spark current puts a heavy load across the secondary of the ignition coil  14  and the secondary voltage quickly drops down to about 800V which is the corona voltage during the combustion. Due to the discharge path provided by diode  42 , capacitor  28  then discharges from a high positive voltage converging to VIGN. 
     Diode  30  ensures that transistor  34  clamps the gate of transistor  24  to ground. Clamping of the gate is needed if the fly-back voltage exceeds the threshold voltage of the Zener diodes integrated within transistor  24 . In that case, transistor  24  is turned on again limiting the collector voltage to approximately the Zener voltage and the bias current of the Zener diode will have a path to ground through the resistor  32 . 
     As a person skilled in the art will readily appreciate, the above description is meant as an illustration of implementation of the principles this invention. This description is not intended to limit the scope or application of this invention in that the invention is susceptible to modification, variation and change, without departing from spirit of this invention, as defined in the following claims.