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 the 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.
Two problems associated with prior DIS systems 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 transistor. 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 transistor 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.