Thyristor Ignition Control Device

Abstract

A thyristor ignition control device for internal combustion engine ignition circuits. A capacitor is charged from a voltage source and discharges through the primary of a step-up transformer when a thyristor connected across the source is fired, producing a high voltage impulse at the transformer secondary to induce sparking across a gap. To limit the current through the thyristor during the initial stages of firing the transformer has a core, for example of ferrite, with a high initial reluctance, thereby reducing wear in the thyristor.

Description

BACKGROUND OF THE INVENTION

This invention relates to ignition control devices, especially for ignition circuits in internal combustion engines, and more particularly the invention concerns the ignition coil or transformer of such ignition control devices.

Electronic ignition control devices are known, in which a capacitor is pre-charged from a voltage source and in which the firing of a thyristor induces the discharge of the capacitor through the primary winding of a step-up transformer, inducing a high voltage pulse across the transformer secondary winding sufficient to induce discharge across a spark-gap, such as, for example, in a sparking plug of an internal combustion engine, a flash light, or some other device.

Ideally, the thyristor should cause an instantaneous short circuit between its anode and cathode, so that the current pulse in the primary winding of the transformer or coil is of the shortest possible duration, with the object of achieving a large voltage excursion in the secondary winding. In practice, however, a thyristor has a finite striking time. The impedance of a thyristor in fact decreases hyperbolically in the initial stage of firing, rising again after an interval of a few microseconds. Since the thyristor is traversed by considerable currents while its impedance is still relatively high, it will be appreciated that considerable power has to be dissipated within the thyristor in the initial discharge stage. This harms the thyristor and causes its premature deterioration, with marked shortening of its useful life.

In order to avoid excessive current through the thyristor during the initial firing phase, the time constant of the transformer or ignition coil should be sufficiently long. In practice, however, this time constant is not sufficiently long for this purpose in conventional coils: the time constant of a transformer is proportional, inter alia, to the input inductance of the primary winding and therefore, to the equivalent loss resistance. The latter, as is known, is proportional to f.sup.-.sup..alpha., where f is the frequency of the applied voltage and .alpha. is an empirical parameter roughly equal to 1.6. Consequently the loss resistance is initially very low during the sharp initial transition of the applied voltage signal upon firing of the thyristor, the frequency of this applied voltage being very high. This has the effect of reducing the initial value of the time constant, allowing the current to build up too quickly, and consequently adversely affecting the working conditions of the thyristor.

Apart from contributing to premature deterioration of the thyristor, this high initial current in the thyristor reduces the magnitude of the voltage excursion in the secondary winding of the transformer or coil.

A main object of this invention, therefore, is to provide an ignition control device for a thyristor firing circuit which avoids rapid deterioration of the thyristor by limiting the initial current flow through the latter.

Another object of the invention is to provide a thyristor ignition control device which for given circuit characteristics causes a greater voltage excursion at the secondary winding of the coil than earlier known thyristor ignition control devices.

SUMMARY OF THE INVENTION

The invention accordingly provides an ignition control device including a capacitor arranged to be charged from a continuous voltage source, and a thyristor arranged when fired to discharge the capacitor through the primary winding of a transformer, in which the transformer includes a core of high initial reluctance, in order to limit the current through the thyristor during the initial phase of its firing.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be further described, by way of example, with reference to the attached drawings, in which:

FIG. 1 is a circuit diagram of an ignition control device of known type;

FIG. 2 is a graph showing the variation of the impedance of a thyristor plotted against time from the moment of application of a firing pulse;

FIG. 3 is a graph showing the variation of the current in the primary winding of a conventional transformer, plotted against time, from the moment of application of a voltage thereacross, and

FIG. 4 is a graph, similar to that of FIG. 3, showing the primary current variation in a transformer forming part of a device according to the present invention.

DETAILED DESCRIPTION WITH REFERENCE TO THE DRAWINGS

Referring to FIG. 1, a capacitor 10 is connected across a direct voltage source 12 in series with the primary winding 14 of a coil or step-up transformer. The secondary winding 16 of the coil or transformer is connected across a spark-gap 18, formed by, for example, a sparking plug in an internal combustion engine.

A thyristor 20 is connected in parallel with the series combination of the capacitor 10 and the primary winding 14. The thyristor 20 is normally, non-conducting and can be rendered conducting, or `fired,` by the application of a firing or trigger pulse to a control electrode 22.

According to the known manner of operation the capacitor 10 is charged from the source 12 while the thyristor 20 is non-conducting and, when the thyristor 20 is fired, the capacitor 10 discharges into the primary winding 14 of the step-up transformer, inducing in the secondary winding of the latter a high voltage step such as to induce a spark discharge across the spark-gap 18.

From the moment at which the trigger pulse is applied to the control electrode 22 of the thyristor 20, a finite time t.sub.o elapses before complete firing of the thyristor, that is, before the latter is fully conductive. The impedance Z of the thyristor 20 in fact changes with time as shown graphically in FIG. 2, in which t = 0 is the instant at which the trigger pulse is applied. The impedance Z decreases rapidly in a short but finite time, reaching virtually zero at time t.sub.o. During the interval 0 - t.sub.o the current I in the primary winding 14 of a conventional step-up transformer increases according to the curve shown in FIG. 3. This curve has a marked bend, the initial steeply sloping part of the curve being due to the high initial permeability of the materials normally used for the core of the step-up transformer, and to losses through parasitic currents.

As already explained, the combination of the two effects illustrated graphically in FIGS. 2 and 3 gives rise to a large initial current through the thyristor 20 while the latter still has a relatively high impedance. Consequently the thyristor 20 heats up, and, especially in control devices which are intended to operate repeatedly with high frequency, as, for example in the ignition circuit of an internal combustion engine, this heating up causes rapid deterioration of the thyristor, with drastic reduction of its useful life.

Moreover, the substantial voltage drop which occurs across the thyristor 20 as a result of the high current flowing through its initially still high impedance, considerably reduces the magnitude of the voltage excursion at the transformer, primary winding 14, therefore reducing also the extent of variation of voltage across its secondary winding 16.

According to this invention, the step-up transformer 14, 16, has a ferrite core of a type having low parasitic current loss and low initial permeability, so that the variation with time of the current I in the primary winding, upon application of a voltage step, has the form shown in FIG. 4. It will be seen that the current I remains low initially and then increases very rapidly.

The coil is so dimensioned that the rapid increase in current occurs when the impedance of the thyristor 20 has decreased to a sufficiently low value as not to give rise to excessive heating up of the thyristor. To this end the windings of the coil are formed, in the known manner, with low parasitic capacity, in order to keep the amount of stored energy low.

Claims

I claim:

1. Ignition control device comprising transformer having a primary and secondary winding, spark electrodes connected across said secondary winding, a capacitor in series with said primary winding, means connecting a continuous voltage source across the series combination of said capacitor and transformer primary winding to charge said capacitor, and a thyristor connected in parallel with said series combination, control electrode means for firing said thyristor thereby causing discharge of said capacitor through said transformer primary winding, said transformer having a core of high initial reluctance effective to limit current through said thyristor in the initial phase of firing of said thyristor.

2. Ignition control device as claimed in claim 1, wherein said transformer core comprises ferrite material.