Lamp ballast circuit

Abstract: Disclosed is an electronic ballast circuit for a high pressure gas discharge lamp with a reduced parts-count starting circuit. The circuit comprises an arrangement for supplying d.c. voltage from a d.c. source, and a main inductor for receiving energy from the d.c. source and supplying the energy to the lamp. The main inductor has a plurality of windings. A first winding of the inductor is coupled to receive energy from the d.c. source. Further included is a current-switching arrangement conductive during periodic first part cycles for transferring energy from the d.c. source to the main inductor, and non-conductive during periodic second part cycles. A first part cycle is followed by a second part cycle in successive periods of switching operation of the current-switching arrangement. The first winding of the main inductor has impressed across it substantially the d.c. source voltage during the first part cycles, and reflects substantially the lamp voltage during the second part cycles. The circuit further includes a starting circuit for providing a pulse in excess of 15,000 volts across the lamp to facilitate starting the lamp. The starting circuit comprises a spark gap device, a pulse transformer, and a voltage-generating circuit. The voltage-generating circuit comprises a starting capacitor for storing energy and impressing across a spark gap an increasing voltage, which reaches the breakdown voltage of the spark gap during a lamp start-up period, and also a circuit for charging the starting capacitor with energy from the main inductor during the second part cycles.

Description: FIELD OF THE INVENTION The present invention relates to an electronic ballast, or power supply, circuit for a high pressure gas discharge lamp. BACKGROUND OF THE INVENTION High pressure discharge lamps, such as xenon metal halide lamps, employ an electronic ballast circuit for starting the lamp, and for powering the lamp during steady state lamp operation. High pressure discharge lamps, especially those operating at pressures greater than about 0.5 atmospheres, require a very high voltage impressed across the lamp to start. Such starting voltage typically exceeds more than about 15,000 volts, and may even exceed more than about 20,000 volts. A prior art approach to designing an appropriate electronic ballast circuit has been to utilize two separate sets of electrical components: one set of components for starting the lamp and another set of components for running the lamp. Electronic ballast circuits for discharge lamps of this kind are known from EP-A-0567408 and FR-A-2686762. In particular, electronic ballast circuits generally in accordance with the preamble of Claim 1 hereof are described in EP-A-0567408. As a general matter, however, it is desirable to reduce the number of electrical components that make up an electronic ballast circuit. This reduces the cost of the circuit, and also its size, a factor becoming increasingly important in size-limited environments, such as in automobiles. It would, accordingly, be desirable to provide an electronic ballast circuit for a high pressure discharge lamp that requires fewer parts to implement a lamp-starting function. An illustrative embodiment of the invention seeks to provide: an electronic ballast circuit for a high pressure gas discharge lamp, which ballast circuit employs a reduced-parts count starting circuit; the foregoing electronic ballast circuit without sacrificing reliability over prior art circuits having a separate starting circuit; an electronic ballast circuit of the foregoing type that places the start circuit in an automatic idling mode after the lamp starts; and/or an electroinc ballast circuit of the foregoing type that provides an automatic restart function in the event of accidental lamp turn-off. In accordance with an illustrative embodiment of the invention, there is provided an electronic ballast circuit for a high pressure gas discharge lamp with a reduced parts-count starting circuit. The circuit comprises means for supplying d.c. voltage from a d.c. source, and a main inductor for receiving energy from the d.c. source and supplying the energy to the lamp. The main inductor has a plurality of windings. A first winding of the inductor is coupled to receive energy from the d.c. source. Further included is current-switching means conductive during periodic first part cycles for transferring energy from the d.c. source to the main inductor, and non-conductive during periodic second part cycles. A first part cycle is followed by a second part cycle in successive periods of switching operation of the current-switching means. The first winding of the main inductor has impressed across it substantially the d.c. source voltage during the first part cycles, and reflects substantially the lamp voltage during the second part cycles. The circuit further includes a starting circuit for providing a pulse in excess of 15,000 volts across the lamp to facilitate starting the lamp. The starting circuit comprises a spark gap device, a pulse transformer, and a voltage-generating circuit. The spark gap device includes a spark gap that has a breakdown voltage at which the device conducts to provide a pulse of current. The pulse transformer has a primary winding coupled to the pulse-generating device for receiving the pulse of current, and has a secondary winding coupled to the lamp for applying to the lamp a voltage in excess of 15,000 volts while the pulse is present, so as to facilitate starting of the lamp. The voltage-generating circuit is effective for impressing across the pulse-generating device a voltage that reaches the breakdown voltage of the spark gap during a lamp start-up period, resulting in a pulse of current through the spark gap device. The voltage-generating circuit comprises a starting capacitor for storing energy and impressing across the spark gap an increasing voltage, which reaches the breakdown voltage of the spark gap during a lamp start-up period, and also a circuit for charging the starting capacitor. The latter circuit comprises a one-way current valve and a starting winding of the main inductor coupled to the starting capacitor via the one-way current valve, so as to be effective to charge the capacitor with energy from the main inductor during the second part cycles. The invention is characterised in that said main inductor is a boost inductor, and comprising a boost capacitor coupled to said first winding of said boost inductor to receive energy from said boost inductor at a rate determined by switching of said current-switching means between its first and second part cycles; and said circuit for charging said starting capacitor further comprises said starting winding being serially coupled to said first winding of said boost inductor, so as to minimize the number of windings turns needed in said starting winding. A better understanding of the invention will become apparent from the following illustrative description when read in conjunction with the drawing, in which: Fig. 1 is a schematic diagram, partially in block form, of an electronic ballast circuit for a high pressure discharge lamp employing a boost converter circuit topology and having a reduced parts-count starting circuit. Fig. 2 is a diagram illustrating the conducting state of a current switch used in the circuit of Fig. 1. Fig. 1 shows an exemplary ballast circuit for a high pressure discharge lamp, generally designated 10. Lamp 10 may comprise a high pressure xenon metal halide discharge lamp of the type disclosed, for instance, in U.S. Patent No. 4,868,458, which is assigned to the same assignee as the instant application. Starting of lamp 10 requires that a high voltage be applied across the lamp. For instance, where envelope 1 OA of lamp 1 0 contains a gaseous mixture under a pressure higher than about 0.5 atmospheres, starting of the lamp typically requires that a voltage in excess of about 15,000 volts be applied across the lamp. Some lamps may require even higher starting voltages, for instance, more than 20,000, or even, 25,000 volts. Such high voltage is generated by a starting circuit 12. Starting circuit 12 cooperates with a main ballast circuit 14 that provides steady state power to operate the lamp. In the topology shown in Fig. 1, main ballast circuit 14 incorporates a so-called boost converter topology. In this topology, a main, or boost, inductor 16 operates in alternating first and second part cycles as determined by the switching operation of a MOSFET or other current switch 18 ' whose conductive state is controlled by circuitry 20. Fig. 2 illustrates the conduction states of current switch 18 during its first and second part cycles. In a first part cycle 30, as shown in Fig. 2, switch 18 is conducting, or "on" During first part cycle 30, source 22 of d.c. power supplies power to main, or boost, inductor 16 through its left-shown node; during this part cycle, the right-hand shown node of main inductor 16 is shorted to ground via switch 18, which is then conducing, or "on." Source 22 may, of course, be rectified a.c. power. For the purposes of the invention, it is important to note that during first part cycle 30, the voltage across first winding 16A of main inductor 16 is substantially the voltage of d.c. source 22, a relatively low voltage of typically 12 volts. In fact, during first part cycle 30, first winding 16A of main inductor 16 reflects virtually the voltage of d.c. source 22. During a second part cycle 31 of the conducting state of switch 18, as shown in Fig. 2, the switch is non-conducting, or "off." For the purposes of the invention, it is important to note that during this time, main inductor 16 reflects a voltage that approximately equals the voltage across the lamp, V L , as reduced by the voltage of d.c. source 22. (The voltage drop across an exemplary p-n diode 24 and a pulse transformer winding 26B are negligible, except during lamp starting.) The lamp voltage V L is set by a boost capacitor 28 that is charged from main, or boost, inductor 16, as will be further described below. Typically, the lamp voltage V L is set to about 250 volts before lamp turn-on. After lamp turn-on, the lamp voltage typically falls to about 60 volts. Both the 250-volt and 60-volt values are considerably greater than a typical d.c. source voltage of, for instance, 12 volts. It is important to note that, accordingly, first winding 16A of boost inductor 16 reflects substantially the lamp voltage V L during second part cycle 31 of operation of switch 18. As shown at in Fig. 2, first and second part cycles 30 and 31 collectively define a period P. Period P is cyclically repeated, as shown by the start of subsequent first part cycle 30' when switch 18 again conducts. First part cycle 30' starts a new period P. In normal operation of main ballast circuit 14, which employs boost converter topology, boost inductor 16 becomes charged with energy during the mentioned first part cycles of the switching state of switch 18, in which the d.c. source voltage is applied across first winding 16A. During the mentioned second part cycles of switch 18, the energy stored in boost inductor 16 is provided, via p-n diode 24, to boost capacitor 28, which stores energy for powering the lamp. Control circuitry 20 (Fig. 1) for switch 18 controls the duration of period P (Fig. 2) of the first and second part cycles, and also the respective durations of the first and second part cycles within a period P. By so controlling the first and second part cycles of operation of switch 18, circuitry 20 regulates the transfer of energy from boost inductor 16 to boost capacitor 28. In this manner, circuitry 20 maintains a desired voltage on boost capacitor 28, and hence on the lamp. Circuitry 20 is per se conventional. As mentioned previously, high pressure discharge lamp 10 requires in excess of 15,000 volts to cause it to turn on. Such high voltage is provided in pulse form by novel starting circuit 12. Circuit 12 includes a spark gap device 32, including a spark gap 33, and is non-conductive until a relatively high breakdown voltage of, for instance, 1,000 volts, is impressed across the device. The dielectric isolation between a pair of spaced terminals that define spark gap 33 breaks down, and renders the device conductive. Starting circuit 12 employs a capacitor 34, which is charged from an extra winding 16B on main inductor 16. Winding 16B is poled in such manner as to supply capacitor 34 with current via a one-way current valve, such as a p-n diode 36, during the mentioned second part cycles of operation of switch 18, when first winding 16A reflects substantially the lamp voltage. With the lamp voltage being at a relatively high level before starting, e.g. 250 volts, capacitor 34 repetitively receives energy from main inductor 16 during the second part cycles. The voltage on capacitor 34 sequentially increases in a staircase fashion as a result of the repeated second part cycles. When capacitor 34 reaches the breakdown voltage of spark gap device 32, such device is rendered conductive. This completes a circuit in which capacitor 34 abruptly discharges through spark gap device 32 and a primary winding 26A of a pulse transformer 26. With the secondary-to-primary winding turns ratio of transformer 26 being high, e.g. 50-to-1, a very high voltage, in excess of 15,000 volts, is induced across secondary winding 26B. With boost capacitor 28 acting as a transient short-circuit at this time, most of the high voltage induced on secondary winding 16B is placed across lamp 10, to facilitate turn-on of the lamp. If the lamp does not turn on upon receiving a first high-voltage pulse, starting circuit 12 automatically restarts the above-described procedure of sequentially increasing the voltage on capacitor 34, during the mentioned second part cycles of switch 18, until the breakdown voltage of spark gap device 32 is again reached. A high-voltage pulse is again impressed across the lamp to facilitate its starting. This process beneficially continues automatically until lamp turn-on is achieved. Once the lamp turns on, its voltage drops considerably, e.g., from 250 to 60 volts. Accordingly, during the mentioned second half cycles, starting winding 16B of main inductor 16 is at a proportionately decreased voltage. Such reduced voltage is insufficient to charge capacitor 34 up to the breakdown voltage of spark gap device 32. Beneficially, then, starting circuit 12 remains in an idle condition during normal lamp operation. If, however, lamp 10 should lose its arc discharge state, i.e., were it to turn off, the lamp voltage V L would again rise to e.g., 250 volts, and the correspondingly increased voltage across winding 16B will sequentially charge capacitor 30 up to the breakdown voltage of spark gap device 32. This initiates an automatic re-start procedure in the same manner as the original start procedure described above. Starting circuit 12 beneficially employs considerably fewer parts than the prior art starting circuit mentioned in the "Background of the Invention," above. Secondary winding 16B is merely an extra winding placed on already existing main inductor 16. Only four additional parts are needed to complete a typical starting circuit 12: i.e., pulse transformer 26, spark gap device 32, capacitor 34, and p-n diode 36. In addition, the stress levels imposed on the components of main ballast circuit 14 during the lamp-starting process are, beneficially, no greater than the stress levels during steady state operation of the lamp, and only occur for a very brief duration of lamp operation. As such, circuit reliability is maintained. In the circuit of Fig. 1, the number of winding turns of starting winding 16B is reduced in the following manner. The right end of winding 16B is connected by conductor 50 to the right end of winding 16A, and the bottom of capacitor 34 is connected to the top of d.c. source 22 by conductor 52. From the foregoing, it will be appreciated that the present invention provides an electronic ballast circuit for a high pressure discharge lamp with a reduced parts-count starting circuit. Greater economy, without loss of reliability, is attained. Additionally, automatic idling of the start circuitry during lamp operation, and automatic restarting of the lamp, are provided.

Claims: An electronic ballast circuit for a high pressure gas discharge lamp (10) with a reduced parts-count starting circuit, comprising: (a) means for supplying dc voltage from a dc source (22); (b) a main inductor (16) for receiving energy from the dc source and supplying said energy to the lamp; said inductor having a plurality of windings (16A, 16B); a first winding (16A) of said inductor being coupled to receive energy from the dc source; (c) current-switching means (18) being conductive during periodic first part cycles (30) for transferring energy from the dc source to said main inductor, and being non-conductive during periodic second part cycles (31); a first part cycle being followed by a second part cycle in successive periods of switching operation of said current-switching means; (d) said first winding (16A) of said main inductor having impressed across it substantially the dc source voltage during the first part cycles, and reflecting substantially the lamp voltage during the second part cycles; and (e) a starting circuit (12) for providing a pulse in excess of 15,000 volts across the lamp to facilitate starting the lamp; said starting circuit comprising a spark gap device (32), a pulse transformer (26), and a voltage-generating circuit; (f) said spark gap device including a spark gap (33) that has a breakdown voltage at which said device conducts to provide a pulse of current; (g) said pulse transformer (26) having a primary winding (26A) coupled to said starting circuit (12) to be for receiving the pulse of current, and having a secondary winding (26B) coupled to the lamp (10) for applying to the lamp a voltage in excess of 15,000 volts while the pulse is present, so as to facilitate starting of the lamp; (h) said voltage-generating circuit being effective for impressing across said starting circuit (12) a voltage that reaches the breakdown voltage of said spark gap during a lamp start-up period, resulting in a pulse of current through said spark gap device; said voltage-generating circuit comprising: (I) a starting capacitor (34) for storing energy and impressing across said spark gap an increasing voltage, which reaches the breakdown voltage of said spark gap during a lamp start-up period; and (II) a circuit for charging said starting capacitor, comprising a one-way current valve (36) and a starting winding (16B) of said main inductor coupled to said starting capacitor via said one-way current valve, so as to be effective to charge said capacitor with energy from said main inductor during the second part cycles; characterised in that; (i) said main inductor (16) is a boost inductor, and comprising a boost capacitor (28) coupled to said first winding of said boost inductor to receive energy from said boost inductor at a rate determined by switching of said current-switching means between its first and second part cycles; and (j) said circuit for charging said starting capacitor (34) further comprises said starting winding (16B) being serially coupled to said first winding (16A) of said boost inductor, so as to minimize the number of windings turns needed in said starting winding. The ballast circuit of Claim 1 for a high pressure metal halide lamp. The ballast circuit of Claim 1 for a high pressure xenon metal halide lamp.

Inventors: ALLISON JOSEPH MICHAEL, ALLISON, JOSEPH MICHAEL
Applicants: GEN ELECTRIC, GENERAL ELECTRIC COMPANY
Application year: 1995
Main classification: H05B41
IPC codes: H05B41