Resonance igniter for discharge lamps

Abstract: An igniter device (10) for discharge lamps (16) with an associated supply circuit (30) able to provide a supply signal (V AL ) to the lamp (16) comprises: a primary circuit (12) with at least one switch element (20) controlled to switch to induce in the primary circuit (12) a controlled oscillation phenomenon at a frequency greater than a megahertz; a secondary circuit (14) capable of resonating and producing a signal (V d ) for striking the discharge of the lamp (16), controlled through the said controlled oscillation phenomenon produced in the primary circuit (12); and a control unit (22) acting on the switch element (20) for controlling the oscillation phenomenon as a function of the characteristics of the lamp (16), this unit (22) being capable of inducing the said controlled oscillation phenomenon a programmable number of times in a predetermined time interval (T 1 ), each time for a predetermined time duration (T 2 ).

Description: The present invention relates to an igniter for discharge lamps (or arc lamps, the two terms being here considered equivalent to one another) with an associated supply circuit able to provide a supply signal to the lamp. Such lamps are used in many fields of application where characteristics of high luminance and quality of the radiation are required, for example in cinematographic and television filming, in the photographic field, in scientific and medical applications, and as sources of light in a large range of projectors. During operation such lamps are supplied by a voltage of the order of several tens or hundreds of volts (supply voltage) which can maintain an electric arc between the electrodes of the lamp. Depending on the model of the lamp the supply voltage required is dc, ac (in the majority of actual cases at mains frequency) or a square wave. Upon starting it is necessary to strike the arc between the electrodes of the lamp in such a way that it can then be maintained over time by the supply voltage. In the cold state discharge lamps are perfect insulators since the gas filling the bulb has an insulating effect between the two electrodes, and the application of only the supply voltage (from 15 to 225 volts depending on the type of lamp) does not produce sufficient energy to strike the arc. For starting the lamp, therefore, it is necessary to ionise and render conductive the gas filling the bulb through a voltaic arc caused by a high voltage discharge. Discharge lamps are provided with igniter devices (also simply called "igniters") able to provide a high ignition voltage, of the order of kilovolts, to the electrodes. Although only a few kilovolts are needed for starting when the lamps are cold, to obtain ignition in the hot state after the lamp has been extinguished and the dielectric resistance to discharge is greater, voltages up to 10 times greater (10-70 kV) are necessary. To allow lamps to be started in all temperature conditions it is known in the art to use "superimposition" igniters, the mode of operation of which consists in superimposing, during the ignition phase, the discharge ignition voltage over the supply voltage provided for normal operation. The ignition voltage required is provided in the form of a sequence of pulses obtained as a consequence of the electrical discharge across the terminals of a spark gap, amplified by a transformer coupling the igniter with a supply circuit of the lamp. The ignition voltage depends in such cases on the frequency, and voltages of the order of several tens of kilovolts such as those necessary for igniting the lamp in any temperature state are in practice obtained by means of igniters which generate pulses at frequencies in the megahertz range. A typical pulse generated across the terminals of the lamp by a superimposition igniter with a spark gap operating in the megahertz range is represented in attached Figure 5, in which the damped oscillations are reproduced in the drawing at a frequency not representative of the frequency of oscillation in real cases solely for the purpose of clarity of illustration. Although by now established and reliable, the ignition technique using igniters provided with spark gaps has intrinsic disadvantages, above all from the point of view of the control of the operation. The discharge through the spark gap, constitutes an electrical phenomenon which is generally difficult to control, the parameters of which can vary as a function, for example, of the deterioration of the spark gap itself which must periodically be replaced or adjusted. The discharge to the electrodes is a disruptive discharge having many harmonic components, and is therefore a source of electromagnetic and acoustic noise. Since lamps of the type described are commonly used in environments where electronic circuits sensitive to electromagnetic noise are present, it is important to have available igniter devices which disturb the operation of nearby circuits as little as possible. In a spark gap the electrical discharge across the terminals of the electrodes is, moreover, a phenomenon the instant of activation of which is difficult to control with precision, and the duration of which is severely limited; since the spark gap consumes a considerable amount of energy the current between the electrodes falls rapidly and the electric arc is interrupted after a short time (the duration T d shown is, in practical cases, about 5µs). A circuit for starting and operating fluorescent lamps, having an electronic assist oscillator for providing a high frequency starting signal to the lamp is disclosed in US 4,818,917. However, since operation of fluorescent lamps is different from that of discharge lamps, the generated voltages are of the order of a few hundreds volt, and are maintained also after lamp conduction begins. In US Patent No. 4 060 751 entitled "Dual Mode Solid State Inverter Circuit for Starting and Ballasting Gas Discharge Lamps" published 29 November 1977, there is described an inverter circuit comprising a pair of solid state switches operating in phase opposition used for the production of an igniter/power supply for discharge lamps. A resonant circuit is associated with the lamp and controlled by the inverter circuit at the resonant frequency so as to generate peak voltages across the terminals of the discharge lamp and facilitate ignition thereof. The quality factor Q of the resonant circuit is however very low (a Q greater than 2 or 3 is considered to be sufficient) and the ignition voltage which is generated across the electrodes of the lamp cannot reach values of tens of kilovolts. In the particular circuit arrangement, after starting of the lamp the igniter acts as a supply circuit thereof and limits the supply current thereto. However this arrangement does not provide the necessary versatility and, in particular, the igniter described is not easily controllable to manage the number and duration of pulses as a function of the type of lamp to which it is coupled. An igniter device comprising a pulse generator circuit without spark gaps is also disclosed in US 5,004,960. The pulse generator circuit comprises an electronic switch whose operation causes a short pulse to be generated in a resonant circuit comprising a primary winding of a step-up transformer. The trigger pulse is transformed to a high voltage in secondary windings of the transformer and applied to the lamp. Timing means are provided for synchronising the enabling of the pulse generator with the occurrence of a peak in the waveform of the supply voltage. Although pulse triggering is synchronised with the supply signal, only one brief, decaying pulse train is achieved on each positive going cycle of the supply voltage, thus with frequency of 50 Hz not sufficient to produce a fast and reliable ignition. Moreover, the number and duration of the triggering pulses cannot be controlled. DE 195 31 622 further discloses an igniter device without a spark gap. The device comprises a resonant primary circuit with a threshold element which switches to a conductive state when the voltage across it raises over a threshold value, causing in said circuit an oscillation phenomenon. A pulse transformer couples said oscillation to a secondary circuit comprising the lamp, and a programmable number of oscillation packets are generated with a phase angle of 60 to 90 electrical degrees of the supply voltage signal. The operation of the device cannot be accurately adjusted since it always depends on the switching of a threshold element whose behaviour is similar to that of a spark gap and is not precisely controllable. The object of the present invention is to overcome the prior art disadvantages and provide an igniter device in which it is possible to accurately control the number and duration of high voltage pulses for triggering the lamp, and in particular for achieving ignition with the smallest possible voltage whilst maintaining this voltage across the electrodes of the lamp for a longer time than has been possible with prior art devices. The arrangement according to the invention further proposes to generate a high voltage signal for triggering the lamp having lower harmonic components in such a way as to limit electromagnetic noise. A further object of the invention is to provide an igniter device which makes available an electronic command for optimising ignition of different types of discharge lamps so as to make such igniters adaptable to the type of lamp with which they are associated. To this end the subject of the present invention is a igniter device having the characteristics set out specifically in the subsequent claims. Further advantages and characteristics of the present invention will become apparent from the following description, given purely by way of non-limitative example, with reference to the attached drawings, in which: Figures 1a and 1b illustrate a simplified circuit diagram of an igniter device according to the invention in association with a discharge lamp, respectively including one or two switches in the primary circuit; Figure 2 is a diagram which illustrates the time variation of the supply voltage of a discharge lamp in which the time intervals for operation of the igniter device can be seen; Figure 3 is a diagram which illustrates the sequence of ignition voltage pulses generated by the igniter device across the electrodes of the lamp during each operation time interval; Figure 4 shows the variation of the ignition voltage in correspondence with each ignition pulse generated by an igniter device according to the invention; and Figure 5 illustrate the variation of the ignition voltage generated across the electrodes of the lamp by a prior art igniter device with a spark gap. An igniter device according to the invention is indicated in Figure 1a with the reference numeral 10 and comprises a primary circuit 12 and a secondary circuit 14, this latter being directly coupled to a discharge lamp 16. The primary circuit 12 has a dc voltage source V DC which may be derived from the mains AC voltage or may be taken directly from a battery, and a starting capacitor C A in parallel with this source. The capacitor C A is also disposed in parallel with the primary winding L 1 of a transformer 18 via an electronic switch 20. The switch 20 is a FET transistor, and in the currently preferred embodiment is formed as a MOSFET power transistor. The source electrode S of the transistor 20 is connected to a reference voltage such as the ground voltage, whilst the drain electrode D is directly coupled to a terminal of the primary winding L 1 of the transformer. In parallel with the transistor 20 it is moreover possible to arrange a series of zener diodes DZ 1 , DZ 2 , DZ 3 for protection against over voltages which can be generated between the drain and source electrodes. The transistor 20 is controlled at its gate input G by a control unit 22 such as a microcontroller or microprocessor, or more simply a dedicated logic circuit (such as, for example, a wired logic circuit), via a suitable control stage 24. The secondary circuit 14 is coupled to the primary circuit via the secondary winding L 2 of the transformer 18. A decoupling capacitor C 2 is disposed in series between the secondary winding L 2 and the lamp 16 and is employed to separate the ignition voltage (the predominant harmonic component of which is constituted by frequencies of the order of megahertz) from a supply circuit 30 operating at a few tens or hundreds of hertz or at dc, so that this latter is not subject to the ignition discharges with possible risks of damage. In practice, the capacitor C 2 behaves as a short circuit with respect to the ignition pulse, and the secondary winding L 2 can be seen as connected in parallel to the lamp 16. In parallel to this winding there is further disposed a capacitor C 3 so as to constitute a resonant circuit. The presence of the capacitor C 3 makes the effects of the parasitic capacitance normally present in the circuit, and indicated C p in the circuit diagram illustrated, negligible in the variation of the resonant frequency of the secondary circuit 14. A further capacitor C y is present to connect a terminal (cold pole) of the secondary winding L 2 to the reference voltage (the ground voltage) permitting the localisation of the high voltage solely at the opposite terminal (hot pole) and to protect the power supply and the supply network from high frequency noise. Finally, the igniter is provided with an operating device (not shown) associated with the input to the control unit 22 and through which it can be activated. This device can advantageously be formed utilising remote-controlled receiver apparatus to allow remote activation of the igniter. The operation of the igniter device according the invention is based on the idea of producing and controlling the evolution of an oscillation phenomenon in the primary circuit 12 of the igniter as a function of the characteristics of the secondary circuit and the lamp to which it is coupled. The secondary circuit 14 is formed as a resonant LC circuit having a high quality factor Q, for example with Q equal to at least 100 and preferably equal to about 200. The high Q of the secondary circuit is achieved by forming a secondary winding L 2 with a high quality factor. This winding is formed as a coil wound on a dielectric core, for example simply in air. In the currently preferred embodiment this winding is formed by a coil wound on the ceramic support of the capacitor C 3 so as to limit the dimensions of the LC circuit and further improve its Q by limiting the length of the high frequency connections. The capacitor C 3 itself is chosen from among capacitors which have temperature and frequency stability characteristics in such a way as to limit the drift of the resonant frequency of the secondary circuit with variations in the operating conditions of the igniter. The control unit 22 controls the opening and closing commutations of the switch 20 in such a way as to control the application of the voltage present across the terminals of the starting capacitor C A to the terminals of the primary winding L 1 of the transformer 18. The voltage present across the terminals of the capacitor C A is generally obtained by rectifying the mains voltage and is for this reason of the order of hundreds of volts. An alternative embodiment is possible in which the oscillation phenomenon across the terminals of the primary of the transformer 18 is obtained by utilising two switches connected in a phase opposition arrangement as illustrated in Figure 1b. The capacitor C A is interposed between a common connection line of the switches 20 and an intermediate terminal of the primary winding L 1 which is therefore itself subdivided into two generally symmetrical parts each of which is selectively connected to the voltage present across the terminals of the capacitor C A . The commutations of the switch 20 are controlled in a very precise manner by the control unit 22 in such a way as to induce in the primary circuit 12 an oscillation phenomenon at a frequency of the order of megahertz (typically 4 MHz) corresponding to the resonant frequency of the secondary circuit 14. The frequency of oscillation in the primary circuit is determined by the same control unit 22 equipped to detect the resonant frequency of the secondary circuit 14 according to known criteria. With reference to the diagrams of Figures 2, 3 and 4 the evolution of the ignition voltage across the electrodes of the lamp 16 will be better described hereinafter. The igniter device is activated when the no-load voltage provided by the supply circuit 30 to the electrodes of the lamp reaches a predetermined threshold value. If the lamp operates on AC the control unit 22 controls activation of the igniter for a time interval T 1 coinciding with precisely determined phase angles of the supply voltage V AL (generally 60°-90° and/or 240°-270°, that is to say when the supply voltage is reaching its peaks). In this time interval T 1 an oscillation phenomenon is induced a programmable number of times, this number being determinable as a function of the characteristics of the lamp with which the igniter is associated and typically lying between several units and several tens. The oscillation phenomenon is induced each time for a predetermined duration, of the order of several tens of microseconds. It has been experimentally determined that the greater is the duration (up to times of 30-40 µs and beyond) the smaller is the peak voltage required to strike the discharge between the electrodes of the lamp. The control unit 22 can be programmed in dependence on physical lamp operating parameters (including capacitive load and nominal ignition voltage) in such a way as to repeat the ignition attempts for a predetermined minimum time (typically correlated to the nominal ignition voltage) as a function of different possible cooling stages of the lamp. In Figure 3 there is shown an envelope curve of the ignition voltage V d which is set up across the electrodes of the lamp following amplification produced by the resonant secondary circuit 14 of the oscillations induced in the primary circuit 12. The envelope curve of the ignition voltage V d is represented as a sequence or packet of "pulses" on a time scale corresponding to a time interval T 1 . Each "pulse" is the envelope of a carrier wave the variation of which is qualitatively illustrated in Figure 4 (in this figure the frequency of the carrier wave illustrated with reference to the base time plotted along the abscissa is not representative of the frequency really used, but rather very much less than this, solely for the purpose of making the drawing comprehensible). The voltage signal V d induced across the electrodes of the lamp 16 thanks to the excellent characteristics of stability, and to the low losses of the secondary resonant circuit 14, is maintained by controlling the switch (or switches) 20 to switch for the desired time period T 2 . The train of pulses activated for a time interval T 1 makes it possible to maintain and widen the conductive channel which is created between the electrodes. The possibility of controlling at will the duration of the oscillation phenomenon makes it possible to increase the time interval for which a high ignition voltage is set up across the electrodes of the lamp and facilitates ionisation of the dielectric between the electrodes. It thus becomes possible to ignite the lamp with ignition voltages equal to about 40-70% of the ignition voltages required with igniters with a spark gap. A prior art igniter with a spark gap is not able to achieve such performance because the oscillation phenomenon induced by the discharge of the spark gap is not controllable in duration. The arrangement according to the invention makes it possible on the contrary to have available a programmable electronic control for optimising the starting of different types of discharge lamps present on the market making a few basic types of igniter adaptable to very many models of lamp, different from one another in light emission characteristics and application.

Claims: An igniter device for discharge lamps having an associated supply circuit (30) for providing an alternate supply signal (V AL ) to the lamp (16), comprising: a primary circuit (12) without a spark gap or similar threshold element, having associated oscillator means (20) for inducing in the primary circuit (12) a controlled oscillation phenomenon at a frequency greater than one megahertz, the said oscillator means (20) including at least one controllable switch element; a resonant secondary circuit (14) with a high quality factor for applying an ignition pulse signal (V d ) to said lamp (16); electromagnetic coupling means (18) between the said primary circuit (12) and said secondary circuit (14); and control means (22) acting on the said oscillator means (20) for controlling said oscillation phenomenon, wherein said secondary circuit (14) is arranged for resonating said oscillation phenomenon transferred by said electromagnetic means, so as to produce a pulse sequence ignition signal (V d ) for the said lamp (16) for a consequent attempt to strike the discharge in the lamp (16); said control means (22) are arranged for inducing said phenomenon a programmable number of times in a predetermined time interval (T 1 ) of the full-wave period of said supply signal (V AL ), each time for a predetermined time duration (T 2 ), so as to cause said secondary circuit (14) to generate, in said time interval (T 1 ), a programmed number of pulses, each pulse having said time duration (T 2 ); and said control means (22) are further arranged for adjusting the frequency of oscillation in the said oscillator means (20) at a frequency corresponding to the resonant frequency of said secondary circuit (14). An igniter device according to Claim 1, characterised in that the said control means (22) are arranged to activate the said controlled oscillation phenomenon by inducing the resonance in the said secondary circuit (14) for a time interval (T 1 ) in correspondence with a phase angle of 60° to 90° of the said supply signal (V AL ). An igniter device according to Claim 1, characterised in that the said control means (22) are arranged to activate the said controlled oscillation phenomenon by inducing the resonance in the said secondary circuit (14) for a time interval (T 1 ) in correspondence with a phase angle of 240° to 270° of the said supply signal (V AL ). An igniter device according to any preceding claim, wherein said control means (22) are arranged for controlling the said oscillation phenomenon as a function of the characteristics of the lamp (16), characterised in that the said control means (22) are arranged to induce the said oscillation phenomenon for a predetermined minimum time as a function of physical parameters and/or a particular cooling stage of the said lamp. An igniter device according to Claim 1, characterised in that the said secondary circuit (14) includes at least one inductive component (L 2 ) and a capacitive component (C 3 ) connected together to form said resonant circuit having a high quality factor Q. An igniter device according to Claim 5, characterised in that the said secondary circuit (14) has a quality factor Q at least equal to 100. An igniter device according to Claim 1, characterised in that the said predetermined time duration (T 2 ) is of several tens of microseconds. An igniter device according to any preceding claim, characterised in that the said control means (22) acting on the oscillator means (20) comprise a dedicated logic circuit. An igniter device according to any of Claim 1 to 7, characterised in that the said control means (22) acting on the oscillator means (20) comprise a microprocessor control unit. An igniter device according to any preceding claim, characterised in that the said at least one switch element (20) is a solid state device of FET type. An igniter device according to any preceding claim, characterised in that the said oscillator means (20) comprise two switch elements connected in a phase opposition configuration. An igniter device according to any preceding claim, characterised in that it includes operating means associated with the said control means (22), the said operating means being of the type comprising remote-controlled receiver devices for activating the said igniter device from a distance.

Inventors: CELSO FRANCESCO, CELSO, FRANCESCO
Applicants: QUALITY LIGHT ELECTRONICS S A, QUALITY LIGHT ELECTRONICS S.A.S. DI FRANCESCO CELSO E C.
Application year: 1998
Main classification: H05B41
IPC codes: H05B41