Power switching devices are routinely used in a number of applications; commonly, a power switching device is implemented by means of a transistor—for example, an Insulated Gate Bipolar Transistor (IGBT)—capable of sustaining high voltages (such as up to 100-1,000V) and of driving large currents (such as up to 0.1-10 A). A typical example of an application of the IGBT is in the automotive field, wherein the IGBT can be used to control the ignition sparks of plugs in an internal-combustion engine.
Particularly, in such application the IGBT is coupled with a primary winding of a transformer; the transformer has a plurality of secondary windings, each one coupled with a respective spark plug. The IGBT is firstly turned on by applying a suitable voltage to its gate terminal. As a result, the IGBT passes from an off (blocking) state—wherein a collector-emitter voltage thereof is about equal to a voltage provided by an automotive battery (typically 12V, with respect to a reference or ground voltage)—to an on state—wherein the same collector-emitter voltage reaches a saturation voltage (such as lower than 1V). In this way, a voltage across the primary winding passes from the ground voltage (i.e., 0V) to approximately the battery voltage (i.e., 12V). This causes the charging of the primary winding with a current having a linear-like pattern. The peak value of this charging current is determined by the length of a time interval, during which the IGBT is kept on. At this point, when the shooting of an ignition spark is required the IGBT is turned off so as to cause an abrupt cut of the charging current. Consequently, an extra-voltage appears across the primary winding; this generates a very high voltage at each secondary winding (of the order of some thousands of volts), which high voltage causes the generation of the ignition spark.
When the IGBT turns on in order to charge the primary winding, the voltage across the primary winding undergoes a sharp variation, having a duration corresponding to a turn-on transient period of the IGBT (while switching from the off state to the on state). Typically, by applying a step voltage to the gate terminal of the IGBT, the duration of the turn-on transient period is of the order of hundreds of nanoseconds. However, this results in a very high incremental ratio ΔV/Δt of the voltage across the primary winding, which generates an overshoot that may cause an undesired ignition spark.
In order to solve this problem, the IGBT is generally controlled to obtain a so-called soft turn-on thereof, wherein the collector-emitter voltage of the IGBT is gradually decreased (from 12V to 1V). For this purpose, it is possible to apply a direct turn-on current to the gate terminal of the IGBT; the turn-on current charges corresponding stray capacitors, so as to increase the gate voltage relatively slowly until the IGBT turns on. In this way, the incremental ratio ΔV/Δt of the voltage across the primary winding of the transformer is greatly reduced (thereby avoiding any undesired ignition sparks).
Unfortunately, the above described soft turn-on procedure may increase a turn-on delay between the application of the signal required to turn-on the IGBT and its actual switching. The turn-on delay may cause a corresponding reduction of the maximum charging current that is reached when the IBGT is turned off to generate the ignition spark, and consequently a reduction of the energy stored in the transformer (for the same time available); this may cause a poor ignition spark when the turn-on delay is too long.
A solution known in the art for reducing the length of the turn-on delay (without causing any overshoots) consists of pre-charging the gate terminal of the IGBT—to a pre-charging voltage lower than its threshold voltage—before the application of the above-described turn-on current. However, the threshold voltage of the IGBT is strictly related to manufacturing process spreads and to temperature variations. Therefore, the pre-charging voltage takes a worst-case value sufficiently low to ensure that the IGBT is kept off (before applying the turn-on current) in any condition. Accordingly, such solution may not be completely satisfactory, since in most practical situations the length of the turn-on delay remains significantly high.
In any case, the length of the turn-on delay varies according to the actual operative conditions. Therefore, it may be impossible to control the generation of the ignition sparks accurately.