FIELD OF THE DISCLOSURE

The present disclosure relates to ignition systems and more specifically to a circuit and method for limiting a kickback voltage to prevent a spark when discharging a charged coil according to a soft shutdown (SSD) profile.

BACKGROUND

An ignition system includes circuitry to create a spark to ignite combustion in a combustion engine (e.g., of a vehicle). The circuitry includes a primary coil in series between a voltage source (e.g., a battery) and a switching device coupled to a ground. The primary coil is inductively coupled to a secondary coil that is in series with a spark gap. When the switching device is configured to conduct (i.e., is turned ON), the primary coil is charged by a coil current flowing from the voltage source and through the primary coil and the switching device to the ground. After the coil is charged, the switching device may be configured (i.e., turned OFF) to block the coil current. The abrupt change in the coil current creates a large voltage across the primary coil as the primary coil begins to discharge. This large voltage is transformed by a windings-ratio between the primary and secondary coil. The transformed voltage is sufficiently high to generate a spark at the spark gap.

The electronic processes of a vehicle can be monitored and controlled by an engine control unit (ECU). For example, the ECU may generate a first signal (i.e., a rising edge) to begin charging the coil and a second signal (e.g., a falling edge) to trigger the spark. The period between the first signal and the second signal is known as a dwell period. If the dwell period is too long (i.e., over-dwell), overheating of the circuitry (e.g., the coil, the switching device) may occur. Accordingly, an over-dwell may trigger a discharge of the coil to prevent damage (e.g., thermal damage).

Because the timing of the triggered discharge may not be aligned with the timing of the combustion engine (i.e., engine), circuitry can be configured to discharge the coil gradually to prevent a spark. This gradual discharge of the coil is known as a soft shutdown (SSD). During a soft shutdown, the coil current in the primary coil is gradually reduced to minimize a kickback voltage created by the changing current (i.e., V=L·dI/dt). It is in this context that implementations of the disclosure arise.

SUMMARY

In at least one aspect, the present disclosure generally describes an ignition system. The ignition system includes an ignition coil conducting a coil current. The ignition system further includes a switching device (e.g., an IGBT) coupled to the ignition coil at a collector terminal and controllable at a gate terminal to adjust the coil current. The ignition system further includes a current control circuit that is configured to output a signal at the gate terminal to reduce the coil current according to a soft shutdown profile. The current-control circuit includes a kickback voltage limiter that is configured to monitor a collector voltage at the collector terminal and to adjust the soft shutdown profile according to the collector voltage to limit a kickback voltage.

In another aspect, the present disclosure generally describes a method for shutting down a coil current in a coil. The method includes controlling a switching device to reduce the coil current in the coil according to a soft shutdown profile. The method further includes monitoring a voltage at a terminal of the switching device and adjusting the soft shutdown profile according to the voltage in order to limit a kickback voltage generating by the coil.

In another aspect, the present disclosure generally describes current-control circuit. The current-control circuit includes a coil-current sensor that is configured to sense a coil current flowing through a switching device that is in series with a coil. The current-control circuit further includes a kickback voltage limiter that is configured to sense a collector voltage at a terminal of the switching device, which is directly coupled to the coil. The kickback voltage limiter is further configured to compare the collector voltage to a voltage-limit threshold. The current-control circuit further includes a soft shutdown circuit that is configured to receive a ramp signal and to reduce the coil current based on the ramp signal and the sensed coil current. The soft shutdown circuit is further configured to decrease a rate at which the coil current is reduced when the collector voltage exceeds the voltage-limit threshold.

The foregoing illustrative summary, as well as other exemplary objectives and/or advantages of the disclosure, and the manner in which the same are accomplished, are further explained within the following detailed description and its accompanying drawings.

