Abstract:
A device to control the charging rate of an ignition coil for an internal combustion spark ignition engine. The device controls the turn-on rate of the primary coil by slew-rate limiting using switching devices and a Miller-effect capacitor in order to reduce secondary oscillation magnitudes originated by a sharp transition of the controlling switch.

Description:
BACKGROUND  
       [0001]     1. Field of the Invention  
         [0002]     The present invention generally relates to a circuit for driving a flyback transformer, also referred to as an ignition coil, of a spark ignited internal combustion engine. More specifically it is related to controlling the turn-on rate of the ignition coil.  
         [0003]     2. Description of Related Art  
         [0004]     In a spark ignited internal combustion engine, ignition coils provide the voltage required for electrical current to jump across a spark plug gap, igniting 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.  
         [0005]     A typical ignition system is illustrated in  FIG. 1 . The system includes an ignition coil  10  having a primary side  16  and a secondary side  18 . The positive terminals of the primary side  16  and secondary side  18  of the ignition coil  10  are connected to a power source  12 . The negative terminal of the primary side is connected to a switching transistor  14 . The switching transistor  14  is connected between the ignition-coil  10  and an electrical ground  20 . The negative terminal of the secondary side  18  is connected to a spark plug  22 . The spark plug  22  is connected between the ignition coil  10  and an electrical ground  20 .  
         [0006]      FIG. 2  illustrates the voltage and current profiles at various points within the system. The profile of the control signal provided to the switching transistor  14  is identified by reference numeral  24 . The current flowing through the primary side  16  of the ignition coil  10  is denoted by reference numeral  26 . In addition, a profile of the primary coil voltage signal, as seen on the collector of transistor  14 , is denoted by reference numeral  28 . In a typical charge and discharge cycle the switching transistor  14  is turned on, charging the ignition coil  10  for a specified dwell period or to a specified charge current; and then the switching transistor  14  is turned off, allowing the secondary side  18  of the ignition coil  10  to discharge stored energy across the spark plug gap.  
         [0007]     One problem is that the sharp turn-on during the charging cycle causes an oscillation on the secondary side  18  of the ignition coil  10 .  FIG. 3  illustrates the dwell command signal  24 , the dwell current  26 , the primary side (low-side) voltage  28 , and the undesirable secondary voltage oscillation  30 . The switching transistor  14  starts out in the off-state with the negative terminal of the primary side  16  equal to the battery voltage. After the switching transistor  14  is turned on, the transistor quickly transits through its linear range into the saturated on-state with a very large rate of voltage change across the primary side  16  of the ignition coil  10 . The resulting secondary voltage  30  during turn-on is a large oscillation magnitude that decays over 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  10  resulting in poor coil performance.  
         [0008]     In view of the above, it is apparent that there exists a need for an ignition coil driving circuit with an improved ignition control.  
       SUMMARY  
       [0009]     In satisfying the above need, as well as overcoming the enumerated drawbacks and other limitations of the related art, the present invention provides a circuit for driving an ignition coil with soft turn-on.  
         [0010]     Soft turn-on is another method for controlling the magnitude of the secondary oscillations. Soft turn-on includes shaping the turn-on rate during the dwell period. The circuit implements voltage slew-rate limiting across the ignition coil primary during the beginning of the dwell period; and disables the voltage slew-rate limiting at the end of the dwell period. A miller-effect capacitor is used for slew-rate limiting; along with transistors and diodes to switch the miller-effect capacitor into the circuit during turn-on and out of the circuit during turn-off.  
         [0011]     The circuit for driving the ignition coil includes a first transistor, a second transistor and a capacitor. The first transistor is connected in electrical series with the ignition coil and the second transistor is connected to the control input of the first transistor. A capacitor is connected between the first transistor&#39;s collector and a control input of the second transistor thereby functioning as a Miller-effect capacitor. The first and second transistors amplify the capacitive effect of the capacitor while both transistors are operating in the linear region. Since the slew-rate needs to be limited during turn-on, but cannot be limited during turn-off because of the effect on the secondary voltage and spark; additional circuitry must be added. A first diode couples the capacitor with the transistor control during turn-on and isolates it during turn-off. A second diode and resistor provide a discharge path for the capacitor during turn-off.  
         [0012]     A specific impedance is maintained at the gate of the main switching transistor during primary turn-off to maintain proper flyback voltage regulation. A zener diode and resistors provide a current path when the flyback voltage at the collector of first transistor exceeds a desired value, limiting the flyback voltage.  
         [0013]     With the proposed soft turn-on, the first transistor transits to its saturated state through the linear range at a significantly slower rate greatly reducing the rate of voltage change across the coil primary. The secondary voltage oscillation magnitude is greatly reduced.  
         [0014]     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  
       [0015]      FIG. 1  is a schematic view of a typical spark ignition system;  
         [0016]      FIG. 2  is a graph illustrating the timing of various voltage profiles for a typical spark ignition system;  
         [0017]      FIG. 3  is a graph illustrating undesirable oscillation in the secondary voltage resulting when soft turn-on is not implemented;  
         [0018]      FIG. 4  is a schematic view of a circuit for driving an ignition system in accordance with the present invention;  
         [0019]      FIG. 5  is a graph illustrating the timing of various voltage profiles for a circuit in accordance with the present invention; and  
         [0020]      FIG. 6  is a graph illustrating significantly reduced oscillation in the secondary voltage when soft turn-on is implemented.  
     
    
     DETAILED DESCRIPTION  
       [0021]     A system embodying the principles of the present invention is illustrated in  FIG. 4  and designated at  40 . The system  40  includes an ignition coil  42 , spark plug  54 , and a circuit  41  including a switching portion  49  and a rate limiting portion  51 .  
         [0022]     The switching portion  49  can be implemented on the high-side of the primary or on the low-side. A low-side driver implementation will be illustrated for discussion purposes; however the same principles can be applied to a high-side driver.  
         [0023]     Ignition coil  42  includes a primary side  44  and a secondary side  46 . The positive terminals of the primary side  44  and secondary side  46  are connected to a power source  48 , generally the automotive battery. The negative terminal of the primary side  44  of the ignition coil  42  is connected to a switching portion  49  of the circuit  41 . For the embodiment shown in  FIG. 4 , the switching portion  49  includes a first transistor  50  and a second transistor  58 . The first transistor  50  is shown as an IGBT (Insulated Gate Bipolar Transistor) and is the main switching transistor implemented on the low-side negative terminal of the primary side  44  of the ignition coil  42 . The first transistor  50  is configured to selectively connect the negative terminal of the ignition coil  42  to an electrical ground  52  allowing current to flow through the primary side  44  of the ignition coil  42 . When commanded, the first transistor turns on, allowing current to begin flowing through the primary side  44  of the ignition coil  42 . The rate of change of current ( di/   dt ) is established by the Voltage (V) across and inductance (L) of the primary side  44  of the ignition coil  42 ; and defined by the equation  
           ⅆ   i       ⅆ   t       =       V   L     .         
 
 The energy stored (E) due to the current (i) through and inductance (L) of the primary side  44  of the ignition coil  42  is defined by the equation E=i 2 *L. When the current (i) through the primary side  44  of the ignition coil  42  reaches a level such that enough energy (E) is stored, the command is turned off shutting off the first transistor  50 . When the first transistor  50  is shut off, the energy (E) stored in the primary side  44  of the ignition coil  42  is transferred to the secondary side  46  of the ignition coil  42 , resulting in a high voltage and current flow across the spark plug  54  gap. 
 
         [0024]     Zener diode  84  is connected between the gate and the collector of the first transistor  50 . Zener diode  84  may be the intrinsic diode of an IGBT package for the first transistor  50 . The Zener diode  84  limits the flyback voltage at the negative side of the primary side  44  of the ignition coil  42  by providing a current path through Zener diode  84 , and resistors  80  and  82  when the first transistor  50  is commanded off and the flyback voltage exceeds the desired level.  
         [0025]     The circuit  41  also has a rate limiting portion  51 . The rate limiting portion  51  is in electrical communication with the switching portion and is configured to limit the transition time as the switching portion  49  transitions from a non-conducting to a fully conducting mode. The rate limiting portion  51  includes a capacitor  56 . A first side of the capacitor  56  is connected to the collector of the first transistor  50 , while the second side of capacitor  56  is connected through diode  86  with the base of a second transistor  58 . The emitter of the second transistor  58  is connected to the gate of the first transistor  50  through resistor  82 , and to ground through resistor  80 . Since the gate of the first transistor  50  is very high impedance, the current flowing into this gate can be neglected. So that the current through the resistor  82  can also be considered to be zero; except when Zener diode  84  is conducting (immediately following the first transistor  50  shutting off and flyback Voltage is present in the primary side  44  of the ignition coil  42 ).  
         [0026]     The first transistor  50  is operated in three modes—cutoff, linear, and saturation. Waveforms illustrating the three states of the first transistor  50  are shown in  FIG. 5 . The first transistor  50  is operated in the cutoff mode in regions  106  and  108 , in linear mode in region  110  when slew-rate limiting is needed, and in saturation mode in regions  112  and  114 . The voltage of the dwell control pulse is denoted by reference numeral  100 . The voltage at the negative terminal of the primary side  44  of the ignition coil  42  is denoted by reference numeral  102 . The gate voltage of the first transistor  50  is denoted by reference numeral  104 . The collector voltage  102  of the first transistor  50 , or primary low-side voltage, starts out at the battery voltage when the dwell control  100  and gate voltage  104  of the first transistor  50  are both zero. When the dwell control  100  goes high, the gate voltage  104  of the first transistor  50  quickly increases to where the first transistor  50  just begins to conduct and enters the linear region. The gate voltage  104  of the first transistor  50  slowly increases through the linear range and the primary low-side voltage  102  linearly decreases until it reaches the saturation voltage of the first transistor  50 . Once the first transistor  50  reaches saturation, the gate voltage  104  increases, keeping the first transistor  50  in the saturation mode. At the end of the dwell control  100  pulse the gate voltage  104  goes to zero cutting off the first transistor  50  and the primary low-side voltage  102  quickly becomes very large due to the ignition coil inductive flyback. The magnitude of the flyback voltage is determined by the zener diode  84  and the resistors  80  and  82   
         [0027]     The capacitor  56  is known as a Miller-effect capacitor and is the key to be able to transit the gate voltage quickly from the off state to the start of the linear range, slowly through the linear range, and quickly again after saturation is reached. The Miller-effect is achieved by placing a capacitor between the input and output of an inverting amplifier; resulting in C Effective =C Miller * [1+K] where K is the gain of the inverting amplifier. Although the first transistor  50  is an inverting amplifier, capacitor  56 , the Miller-effect capacitor, cannot be applied directly from the gate to the collector. This is because the capacitor  56  needs to be switched out during turn-off when diode  84  needs to be biased correctly using resistor  80  and resistor  82  in order to regulate the coil&#39;s flyback voltage. Therefore, a second transistor  58  is added and capacitor  56  is place from the base of the second transistor  58  to the collector of the first transistor  50  resulting in the Miller-effect when, and only when, both the second transistor  58  and the first transistor  50  are operating in the linear range. Furthermore, a diode  86  is added between the base of the second transistor  58  and the Miller-effect capacitor  56  so that the capacitance of capacitor  56  is only seen during the dwell-on phase  108 ,  110 ,  112 , and  114 , and is isolated from the switching circuit during the dwell-off phase  106 . The resistor  88  and diode  90  provide the discharge path for the capacitor  56  during the dwell-off phase  106 .  
         [0028]     To control the first and second transistor  50 ,  58  thereby charging and discharging of capacitor  56 , a dwell control input circuit is provided. A dwell control signal is provided to an input node  60 . The dwell control signal for the ignition dwell period is generated by a controller that is synchronized with the engine crankshaft and camshaft positions. Typically this is done using the vehicle&#39;s Powertrain Control Module (PCM). For this embodiment, the control signal is defined as 0 Volts outside of the dwell period and a positive voltage during the dwell period. The dwell control voltage is provided for reference and denoted by reference numeral  100 .  
         [0029]     Resistor  62  is connected between the input node  60  and a base of a third transistor  64 . The emitter of the third transistor  64  is connected to an electrical ground through resistor  68 . The collector of the third transistor  64  is connected to the base of a fourth transistor  74 . In addition, the collector of the third transistor  64  is connected to a voltage source  72  through resistor  70 . The emitter of the fourth transistor  74  is connected to an electrical ground through resistor  76 . The collector of the fourth transistor  74  is connected to the base of the second transistor  58 . In addition, the base of the second transistor is connected to the power source  72  through resistor  78 . The collector of the second transistor  58  is connected to the voltage source  72 , while the emitter of the second transistor  58  is connected to an electrical ground through resistor  80 . In addition, the emitter of the second transistor  58  is connected with the gate of the first transistor through a resistor  82 . The first side of capacitor  56  is connected to the collector of the first transistor  50 , the cathode of the Zener diode  84  and the negative of the primary side  44  of the ignition coil  42 . The second side of capacitor  56  is connected to a cathode of diode  86 . The anode of diode  86  is connected to the base of the second transistor  58 .  
         [0030]     In addition, the second side of capacitor  56  is connected to ground through resistor  88  and diode  90 . The cathode of diode  90  is connected to ground. The anode of diode  90  is connected to the first side of resistor  88  and the second side of resistor  88  is connected to the second side of capacitor  56 .  
         [0031]     To control the first transistor  50  as required, a series of transistors and diodes are used between the input node  60  and the gate of the first transistor  50  along with capacitor  56 . A third transistor  64  and fourth transistor  74  are switching transistors which operate in only in the cut-off and saturation regions. The following table lists the states of the third and fourth transistors  64 ,  74  in relation to the state of dwell control.  
                                                       Dwell Control   Transistor 64   Transistor 74                           Off (low)   Cut-Off   Saturated           On (high)   Saturated   Cut-Off                      
 
         [0032]     When dwell control  24  is off, fourth transistor  74  is saturated keeping first and second transistor  58 ,  50  off. When dwell control  24  is turned on, the fourth transistor  74  turns off allowing the base voltage of the second transistor  58  to start increasing as current flows from a voltage source  72  through resistor. When the base Voltage of the second transistor  58  reaches a level where it begins to conduct, the emitter current flows through resistor  80 . Since the zener diode  84  is not conducting at this time, and the gate of the first transistor  50  is sufficiently high so that the current into the gate can be neglected; it can be considered that no current flows through resistor  82  and the gate Voltage of the first transistor  50  is equal to the emitter Voltage of the second transistor  58 . The gate Voltage of the first transistor  50  is designated by reference numeral  104  in  FIG. 4 . The first transistor gate Voltage  104  through four charge rate regions while dwell control is on; these regions are labeled  108 ,  110 ,  112 , and  114  in  FIG. 4 .  
         [0033]     In region  108  both the first transistor  50  and the second transistor  58  start out in cut-off mode; the base voltage of the second transistor  58  increases and starts turning the second transistor  58  on; as the second transistor  58  begins to turn on and the voltage at the gate of the first transistor  50  starts to increase. The time that it takes to move through region  108  is determined by the time constant τ=R*Cm where R is the resistance of resistor  78  and Cm is the capacitance of capacitor  56 .  
         [0034]     When the gate voltage of the first transistor  50  reaches the level where the first transistor  50  begins to turn on region  110  is entered. The first transistor  50  and the second transistor  58  are both operating in the linear mode and become an inverting amplifier. The collector voltage of the first transistor  50  begins to drop and the Miller-effect becomes active. The time that it takes to move through region  110  is determined by the time constant τ=R*Cm* [1+K], where K is the gain of the inverting amplifier.  
         [0035]     When the collector voltage of the first transistor  50  drops to the point where the first transistor  50  becomes saturated the inverting amplifier no longer exists, therefore, region  112  is entered and the time constant becomes τ=R*Cm again. During region  112  the base voltage of the second transistor  58  continues to rise until the second transistor  58  becomes saturated.  
         [0036]     In region  114  both the first transistor  50  and the second transistor  58  are saturated, and capacitor  56  is totally charged. In region  114  the circuit is in a steady-state condition. When K is sufficiently large, the time constant in regions  108  and  112  are much smaller than in region  110 . If the Miller effect is not used, large, often unpredictable delays occur before current begins to flow in the coil&#39;s primary winding. The delay is a result of a large RC time constant in effect while the base and gate voltages of the first transistor  50  and the second transistor  58  transition to linear mode levels.  
         [0037]     Diodes  86  and  90  are used for switching the capacitor  56  in and out when the state of the dwell control pulse changes state. When dwell control switches high, the fourth transistor  74  switches off. Diode  86  is forward biased and current can flow thereby charging capacitor  56  and providing soft turn-on. When dwell control is switched low, the fourth transistor  74  switches on. Diode  86  is now reverse biased and capacitor  56  does not effect the turn-off of the second transistor  58  so secondary energy is not lost during the spark event. Capacitor  56  discharges through resistor  88  and diode  90 . Resistor  88  needs to be selected such that it is sufficiently large to limit the current through diode  90  when diode  86  is forward biased, and sufficiently small to discharge capacitor  86  during the smallest possible dwell control off time.  
         [0038]      FIG. 6  illustrates the reduced oscillation in the secondary voltage  122  when using soft turn-on. Also, when comparing  FIGS. 3 and 6 , note the reduced oscillation in the dwell current  120  and the shallower slope in the primary low-side voltage  102  when soft turn-on is used. Since the ignition coil has very high inductance, the current through the coil primary cannot change instantaneously in time, and since the soft turn-on occurs early in the dwell period when the current is low, the primary current and dwell time are not significantly impacted.  
         [0039]     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 the spirit of this invention, as defined in the following claims.