Circuit & method for regulating the current flow in a distributorless ignition system coil

Abstract: A control circuit (18) and method for regulating the current flow through a series connected inductor (20) and transistor (16). The circuit comprises an operational amplifier (34) for receiving a first voltage proportional to the current flow, for receiving a variable second voltage, and for providing a control current to the transistor (16) which keeps the transistor (16) out of its saturation region.

Description: The present invention relates to a circuit for controlling the charging and discharging of an inductor. More particularly, it relates to a circuit for regulating the operation of the primary coil winding in a distributorless ignition system. For many years the ignition systems in automobiles employed an electro-mechanical contact breaker, known as a distributor, to sequentially send current pulses to ignite spark plugs. More recently, these systems have been replaced with electronic ignition systems which eliminate the distributor. These so called "distributorless ignition systems" (DIS) rely on electronic switching and control of current pulses. A typical DIS system places the primary winding of an ignition coil (an inductor) in series with a high gain transistor, such as a Darlington transistor. A control circuit is connected to the control electrode of the transistor to turn it on and off as required. During a dwell period while the primary winding is storing energy, the transistor is turned on to allow current to flow through the transistor and primary winding. When the current flow reaches its desired value, the control circuit maintains the current flow by regulating the control current supplied to the control electrode of the transistor. At the end of dwell period, when the spark plug requires a current pulse, the control circuit is disconnected from the control electrode of the transistor shutting the transistor off. This sudden stop in current flow through the transistor causes an inductive high voltage surge in the secondary winding of the ignition coil which provides the energy for the spark. A current control circuit for an ignition device is known from Patent Abstracts of Japan; vol. 8263 (M342); 4-12-1984; JP-A-59.136.571 which is primarily employed for detecting the unsaturated time of a power transistor associated with the ignition device and which is disadvantageously restricted in its mode of controlling the unsaturated operation of the power transistor. Two problems associated with prior DIS system are current overshoot and frequency instability in the primary winding of the ignition coil. Current overshoot is caused by turning the transistor on hard (driving it into saturation) during the dwell period. Although control circuits typically reduce the current to the transistor's control electrode as the current flow in the primary winding approaches its desired value, the transistor develops parasitic capacitances during saturation. These capacitances can build up a relatively large quantity of charge during this time which must be dissipated. Such dissipation can keep the transistor at a higher conductivity than preferred for regulation. This results in the current flow overshooting its desired value. In reducing the overcurrent by choking its flow through the Darlington transistor, a reverse voltage can appear on the primary winding of the coil causing a surge in the secondary winding and resulting in a premature firing of the spark. Overshoot can also contribute to frequency instability wherein oscillations are created in the primary winding as the control circuit attempts to regulate the current flow. Instability can be improved somewhat by using a lower gain controller. However, a low gain controller can produce an undesirable high offset in the coil current. One way of solving overshoot is to use a fixed gain trainsistor. As long as the current source is tightly controlled, overshoot will not occur since the transistor controller maintains a constant current as the inductor current approaches its desired value. However, achieving a fixed gain trainsistor that is stable over time, temperature, power supply variations, and variations in the inductor is impractical for large production quantities. Another way to avoid overshoot is to customize an individual DIS system by adjusting the transistor controller drive current to the transistor gain. As with a fixed gain transistor, this solution can be expensive. It is therefore an object of the present invention to provide a new and improved circuit for regelating the current flow through a series connected inductor and transistor. It is another object of the present invention to provide a new and improved method for controlling the charging and discharging of such an inductor. According to one aspect of the present invention there is provided a control circuit for regulating the current flow through a series connected inductor and transistor characterized by an operational amplifier, means for sensing a first voltage proportional to said current flow and for delivering said first voltage to said operational amplifier, means for producing a variable second voltage and for delivering said second voltage to said operational amplifier and in which said second voltage increases as a function of time and is maintained above a level to maintain a control current from said operational amplifier, wherein said operational amplifier is arranged to provide said control current to said transistor in response to said first and second voltages so as to keep said transistor out of its saturation region. According to another aspect of the present invention there is provided a method for controlling the charging and discharging of an inductor, said inductor being series connected with a transistor between a voltage source and reference potential terminal, characterized by the steps of: sensing a first voltage proportional to the current flow through said inductor; generating a variable second voltage which increases as a function of time; comparing said first and second voltages; and maintaining said second voltage above a level to maintain a control current proportional to the difference of said first and second voltages to a control electrode of said transistor to keep said transistor out of its saturation region. One form of the present invention is a control circuit for regulating the current flow through a series connected inductor and transistor. The circuit comprises an operational amplifier for receiving a first voltage proportional to the current flow, for receiving a variable second voltage, and for providing a control current to the transistor which keeps the transistor out of its saturation region. Yet another form of the invention is a method for controlling the charging and discharging of an inductor series connected with a transistor between a voltage source and reference potential terminal. A first voltage proportional to the current flow through the inductor is sensed, and a variable second voltage is generated. The first and second voltages are compared and a control current proportional to the difference of the first and second voltages is provided to a control electrode of the transistor. The control current is small enough to keep the transistor out of its saturation region. An advantage of the present invention is the provision of an inexpensive control circuit which can reliably prevent the transistor from going into saturation. One example of the present invention will now be described by way of example with reference to the accompanying drawings in which:- Figure 1 is a circuit diagram showing one embodiment of the present invention. Figure 2 is a circuit diagram showing an alternative embodiment of the present invention. Figure 1 shows an electronic ignition system 10. System 10 includes a voltage source 12, an ignition coil 14, a switching transistor 16, resistor R1, and a control circuit 18. Voltage source 12 (V BAT ) is ideally a battery which in a preferred embodiment will supply about 12 volts. Ignition coil 14 includes a primary winding or inductor 20 coupled to a secondary winding 22. Secondary winding 22 is connected to a reference potential terminal 24 through a potential barrier 26, potential barrier 26 being a spark plug gap. Switching transistor 16 is a power Darlington transistor in a preferred embodiment. A Darlington transistor is a very high gain device formed by two transistors with a common collector and the emitter of the first connected to the base of the second. Transistor 16 has a collector c, emitter e, and a control electrode or base b. Transistor 16 is series connected with inductor 20 between voltage source 12 and reference potential terminal 28 (ground), with collector c connected to inductor 20 and emitter e connected to terminal 28 through resistor R1. Base b of transistor 16 is also connected to control circuit 18 as will be described more fully hereinafter. Control circuit 18 regulates the current flow through inductor 20 and transistor 16 by varying the current provided to base b. Circuit 18 includes a sensor in the form of an input line 30 connected to resistor R2 for sensing the voltage on resistor R2. This voltage is proportional to the current flowing through inductor 20 and transistor 16 since resistors R1 and R2 are connected in parallel. Resistor R2 is much greater than resistor R1 with typical values of 1000 and .05 ohms, respectively. Control circuit 18 also includes a voltage controlled current source 32, operational amplifier 34, comparator 36, capacitor C, and switches SW1, SW2 and SW3. Comparator 36 is connected to input line 30 and receives the voltage sensed by line 30 at its (-) input. Comparator 36 receives a reference voltage V REF on its (+) input. Comparator 36 generates a high or low signal on its output line 38 which is connected to switch SW2. Switch SW2 is responsive to the output signal from comparator 36 for connecting voltage controlled current source 32 to capacitor C, as will be discussed more fully hereinafter. Voltage controlled current source 32 has an input line 40 connected to voltage source 12 for receiving V BAT , and an output line 42 for providing a current which is a function of the value of V BAT . Current source 32 is connected to the (+) input of operational amplifier 34 through switch SW2. Capacitor C and switch SW3 are connected in parallel between the (+) input of operational amplifier 34 and reference potential terminal 28, which in a preferred embodiment is ground. When closed, switch SW3 will discharge capacitor C. Operational amplifier 34 is also connected to input line 30 and receives the voltage on line 30 at its (-) input. Operational amplifier 34 compares the voltages appearing on its (+) and (-) inputs and provides an output current proportional to the difference of these voltages on its output line 44. This output current is provided to base b of transistor 16 when connected by switch SW1. Switching logic 46 provides digital on/off signals to switches SW3 and SW1 on lines a and b, respectively. The signals provided are related to the operation of the distributorless ignition system and are generated in a conventional manner. The timing of these signals will be discussed below. The operation of electronic ignition system 10 may be divided into four states or operating regions. The first region is the "at rest" condition. Switch SW3 is closed and switch SW1 is open. Since switch SW1 is open, there is no current provided to the base b of transistor 16 and ideally no current will flow through inductor 20. No voltage will be developed on resistor R2 and the output of comparator 36 will be high thereby closing switch SW2. However, since switch SW3 is closed, no charge will be developed on capacitor C. The second region is the "charge up" condition. Switch SW1 receives a signal to close and switch SW3 receives a signal to open. When switch SW3 opens, charge starts to build up on capacitor C. Operational amplifier 34 starts providing a small output current to the base of transistor 16 which allows transistor 16 to start conducting. The current flow through inductor 20 and transistor 16 develops a voltage on resistor R2. This voltage is provided to comparator 36 and operational amplifier 34. Resistor R2 is sized so that during the charge up condition the voltage on line 30 is less than V REF . Thus, comparator 36 will continue to provide a high output signal thereby keeping switch SW2 closed. An important feature of the present invention is that capacitor C is initially discharged going into the charge up condition. When switch SW3 is opened, the voltage on capacitor C does not change instantaneously but gradually builds up in response to the current from current source 32. Thus, the output current of operational amplifier 34 is initially small thereby preventing transistor 16 from being driven into saturation. As the voltage builds on capacitor C, the output current of operational amplifier 34 will tend to increase which will increase the current flow through inductor 20. However, the output current of amplifier 34 will not increase excessively since the (-) input voltage developed from resistor R2 is also increasing with increased current flow. In addition, a characteristic of capacitor C is that it will charge gradually thereby preventing the output current of operational amplifier 34 from exceeding a value which would otherwise drive transistor 16 into its saturation region. By keeping transistor 16 out of its saturation region and operating solely within its ohmic region, better regulation of the current flow through inductor 20 is achieved. More particularly, problems such as current overshoot and oscillation leading to premature firing of the spark plug are avoided. Devices other than capacitor C, voltage controlled current source 32, and switches SW2 and SW3 may be able to achieve the same result. For example, a clocked digital network and an analog to digital converter having the following operating characteristics may be employed: (1) If the signal on switching logic 46 line "a" is active, then the output of the A/D converter provided to line 60 is zero. (2) If the signal on switching logic 46 line "a" is inactive and the signal on line 38 is active, then the voltage provided to line 60 will be a function of time and V BAT . (3) If the signal on switching logic 46 line "a" is inactive and the signal on line 38 is inactive, then the voltage provided to line 60 is a constant value based on previous conditions. The third region is the "regulation" condition. Switch SW1 is still closed and switch SW3 remains open. Resistor R2 is sized so that when the desired current flow is achieved in inductor 20 the voltage developed on resistor R2 will be slightly greater that V REF . Thus, comparator 36 will generate a low output signal which will open switch SW2 thereby inhibiting capacitor C from further charging. Operational amplifier 34 will hold the base bias on transistor 16 to maintain the current in inductor 20. If the charge on capacitor C should leak off reducing the output current from operational amplifier 34 and decreasing the current flow in inductor 20, the resulting drop in resistor R2 voltage will flip the output of comparator 36 to again close SW2. This will return the electronic ignition system 10 to the charge up condition. The fourth region is the "spark plug fire" condition. Switch SW1 is quickly opened, in response to a signal from switching logic 46, thereby shutting off transistor 16. This sudden stop in current flow through transistor 16 causes an inductive high voltage surge in secondary winding 22 of ignition coil 14. This provides the energy for a spark across gap 26 to reference potential terminal 24. When switch SW1 is opened, switch SW3 is closed to discharge capacitor C. The halt in current through resistor R2 will create a high output signal from comparator 36 thereby closing switch SW2. This returns electronic ignition system 10 to the first region. Figure 2 shows an alternative embodiment of the present invention. Electronic ignition system 10 is similar to that shown in Figure 1 with the exception that the voltage controlled current source 32 is replaced with a resistor 50. The operation of system 10 is similar to that described above for the Figure 1 embodiment. The voltage on line 60 is kept at a value between 0 and 0.5 volts as the value of V BAT varies from about 5 to about 30 volts. Resistor 50 will approximate voltage controlled current source 32 since the current flowing through a resistor is proportional to the voltage across the resistor. In a preferred embodiment of Figures 1 and 2, the elements shown in control circuit 18 absent capacitor C and resistor R2 will be formed as an integrated circuit. Capacitor C and resistor R2 will be connected externally thereto in order to allow sizing changes for different applications. It will be clear to those skilled in the art that the present invention is not limited to the specific embodiment disclosed and illustrated herein. Nor is the invention limited to electronic ignition systems. Rather, the invention may be applied equally to any inductor which must be charged to a defined current and maintained at such current for an indefinite period.

Claims: A control circuit (18) for regulating the current flow through a series connected inductor (20) and transistor (16) characterized by an operational amplifier (34), means (30) for sensing a first voltage proportional to said current flow and for delivering said first voltage to said operational amplifier (34), means (SW2,C) for producing a variable second voltage and for delivering said second voltage to said operational amplifier (34), and in which said second voltage increases as a function of time and is maintained above a level to maintain a control current from said operational amplifier (34), wherein said operational amplifier (34) is arranged to provide said control current to said transistor (16) in response to said first and second voltages so as to keep said transistor (16) out of its saturation region. A control circuit according to claim 1, characterized in that said operational amplifier (34) has first and second inputs for receiving said first and second voltages, respectively, said means for producing a variable second voltage comprising a capacitor (C) connected between said second input (+) and a reference potential terminal (28), and said circuit further comprising a current source (32) connected by a switch (SW2) to said second input (+). A control circuit as claimed in claim 2, characterized in that said means (30) for sensing said first voltage includes a resistor (R2) connected between said transistor (16) and a reference potential terminal (28) and wherein said first input (-) is connected to said resistor (R2). A control circuit as claimed in claim 2, characterized in that said circuit further comprises a comparator (36) arranged to receive a reference voltage (V REF ) on a second input (+) thereof, and arranged to receive the first voltage on a first input (-) thereof, and generate at its output (38), a signal for controlling said switch (SW2). A control circuit as claimed in claim 2, characterized in that the current source (32) comprises a resistor (50) in series with a voltage device and said switch (SW2). A method for controlling the charging and discharging of an inductor (20), said inductor (20) being series connected with a transistor (16) between a voltage source (12) and reference potential terminal (28), characterized by the steps of: sensing a first voltage proportional to the current flow through said inductor (20); generating a variable second voltage which increases as a function of time; comparing said first and second voltages; and maintaining said second voltage above a level to maintain a control current proportional to the difference of said first and second voltages to a control electrode (b) of said transistor (16) to keep said transistor out of its saturation region. A method as claimed in claim 6, characterized in that said generating step includes charging a capacitor (C) from a current source (32), wherein said second voltage is developed on said capacitor (C). A method as claimed in claim 7, characterized in that the method further comprises the steps of: comparing said first voltage to a reference voltage and providing a signal in response thereto; and regulating said current flow by selectively connecting and disconnecting said current source (32) to said capacitor (C) in response to said signal. A method as claimed in claim 7, characterized in that the method further comprises the steps of: coupling a secondary coil (22) to said inductor (20) and connecting said secondary coil (22) to a reference potential terminal (24) through a potential barrier (26); and discharging said inductor (20) by removing said control current from said control electrode (b) thereby inducting a spark across said potential barrier (26); and, discharging said capacitor (C).

Inventors: PERKINS LUKE A, PERKINS, LUKE A.
Applicants: AT & T GLOBAL INF SOLUTION, HYUNDAI ELECTRONICS AMERICA, SYMBIOS LOGIC INC, AT&T GLOBAL INFORMATION SOLUTIONS INTERNATIONAL INC.
Application year: 1990
Main classification: F02P3
IPC codes: F02P3, G05F1, F02P7