BALLAST

Abstract: A ballast for powering a high intensity discharge lamp including an ignitor and a resonant circuit. Ignition pulses produced by the ignitor added to a boosted low frequency voltage produced by the circuit operating near its resonant frequency during start up of the lamp. The circuit is disabled after a predetermined period of time has elapsed following ignition of the lamp. Application of the signal precedes application of the ignitor pulses to the lamp. Disablement of the circuit can render the ignitor inoperable.

Description: Ballast. This invention relates generally to a ballast for a high intensity discharge (HID) lamp, including a circuit for aiding ignition of the HID lamp. In order to ignite a conventional HID lamp, the nominal U.S. utility line voltage of 120 volts AC (VAC) is increased and in combination with very high amplitude voltage pulses of short duration is applied to the lamp. A ballast can step up the nominal U.S. utility line voltage by magnetic transformation. Such transformation results in a ballast which is undesirably bulkier and/or having unacceptably higher losses. When the nominal line voltage is sufficiently high (e.g. at about 200 volts RMS or more), transformation is not required. A conventional ballast, without such transformation, is disclosed in U.S. Patent No. 4,461,982. Such ballasts in providing high amplitude voltage pulses of short duration include both an oscillator and an ignitor. The amount of energy supplied by the oscillator to the lamp, however, is limited. Transfer of energy from the oscillator to the lamp occurs only during operation of the ignitor, that is, for the duration of each ignition pulse. The amount of energy stored by the oscillator and therefore available to the lamp is also limited by a storage scheme which imposes sharing of energy to be stored between the oscillator and an ignitor. Accordingly, it is desirable to provide an improved HID ballast which is less bulky and more energy efficient. The improved ballast in providing energy from the boosted nominal line voltage to the HID lamp during starting should not be limited to the duration of the one or more ignition pulses or be limited in the amount of energy supplied to the lamp due to sharing of energy storage with an ignitor. Generally speaking, in accordance with a first aspect of the invention, a ballast for powering a high intensity discharge lamp includes a first input terminal and a second input terminal for receiving an input voltage, a first output terminal and a second output terminal for supplying power to the lamp and a first inductor and a first capacitor serially coupled between the first input terminal and the first output terminal. The ballast also includes a reactive component and a switch serially coupled between the second output terminal and a junction joining the first inductor and the first capacitor together. The reactive component in combination with an element selected from the group consisting of the first inductor and first capacitor is characterized by a first resonant condition at a first frequency. Energy generated by the first resonant condition is supplied to the lamp for aiding ignition of the lamp . Advantageously, even though the ballast is supplied by a 120 VAC nominal line voltage, the energy from the resonant condition is sufficient in aiding ignition to start the lamp without the need for a transformer in boosting the 120 VAC nominal line voltage to at least about 200 volts RMS. The invention provides a far more energy efficient and less bulky ballast than a conventional ballast requiring transformation in boosting the nominal line voltage to an acceptable level for starting the lamp. In accordance with a first feature of the invention, the first inductor and first capacitor are characterized by a second resonant condition at a second frequency for sustaining lamp current during steady state operation of the lamp. In one preferred embodiment of the invention the first frequency is greater than the second frequency. In accordance with another feature of the invention, the ballast further comprises an ignitor which includes the first inductor for generating ignition pulses. During starting of the lamp, the reactive component in combination with the selected element produce a signal associated with the first resonant condition whose ringing is extended by a second inductor when the second inductor is connected between the switch and the second output terminal . The conductive state of the switch is based on the operating state of the lamp such that the switch changes from a conductive state to a non-conductive state in response to current flowing through the lamp for a predetermined period of time. In accordance with the invention, the ballast includes means for applying to the lamp a signal at a low frequency and of a first magnitude and ignition pulses at a high frequency and of a second magnitude prior to its ignition. The signal is produced by a circuit, forming part of the ballast, operating near its resonant frequency. The ignition pulses are produced by an ignitor, which can forma part of the ballast. Application of the signal precedes application of the ignition pulses to the lamp. The ignition pulses are preferably applied to the lamp near at least one peak of the signal. It is a feature of this invention to disable the circuit after a predetermined period of time has elapsed following ignition of the lamp. The second magnitude is preferably at least ten times greater than the first magnitude. Disabling of the ignitor occurs upon or before disablement of the circuit. In yet another feature of this invention, disablement of the circuit can render the ignitor inoperable. Accordingly, it is an object of the invention to provide an improved HID ballast which is less bulky and more energy efficient than a conventional ballast. It is another object of the invention to provide an improved HID ballast which in supplying energy to the lamp from other than an igniter does not limit such supply to the duration of the one or more ignition pulses. It is a further object of the invention to provide an improved HID ballast which in providing energy to the lamp from both an ignitor and a resonant circuit does not limit the amount of energy generated by the resonant circuit due to sharing of energy storage with an ignitor. Still other objects and advantages of the invention, will, in part, be obvious and will, in part, be apparent from the specification. The invention accordingly comprises several steps in a relation of one or more of such steps with respect to each of the others, and the device embodying features of construction, a combination of elements and arrangement of parts which are adapted to effect such steps, all is exemplified in the following detailed disclosure and the scope of the invention will be indicated in the claims. For a fuller understanding of the invention, reference is had to the following description taken in connection with the accompanying drawings, in which: FIG. 1 is a schematic diagram of the invention in accordance with a first embodiment of the invention; and FIG. 2 is a schematic diagram of the invention in accordance with an alternative embodiment of the invention. As shown in FIG. 1, a voltage source VS at 120 VAC, 60 hertz (Hz) is connected to a first input terminal IT1 and a second input terminal IT2 of a ballast 10 and supplying input voltage to the ballast. Ballast 10 is connected at a first output terminal OT1 and a second output terminal OT2 to an HID lamp load LL for supplying powerto the lamp. A series resonant LC circuit formed by a first inductor (choke) LI and a first capacitor Cl are serially coupled between the first input terminal IT1 and the first output terminal OT1. The resonant frequency of choke LI and capacitor Cl is near and above the frequency of the driving voltage, that is, near and above the 60 Hz frequency of voltage source VS (e.g. 75 Hz). During steady state operation of lamp load LL, that is, after lamp load LL has been successfully ignited, choke LI and capacitor Cl serve to ballast lamp load LL. The LC combination of choke LI and capacitor Cl operates on the capacitive side of its resonant frequency (i.e. lag type design) allowing lamps with nominal voltages above 100 VAC to be reliably ballasted from a 120 VAC source. Ballast 10 also includes a bilateral switching device SI, such as a SIDAC, which is connected to a tap T of choke LI. A capacitor C2 is connected between a junction joining together switching device SI and a resistor Rl and a junction joining together choke LI and capacitor Cl. A normally closed switch SWl of a relay is connected between resistor Rl and one end of a choke (inductor) L2. The other end of choke L2 is connected to a junction joining input terminal IT2 to output terminal OT2. SWl need not be part of a relay and can include other suitable switching devices such as, but not limited to, a transistor, bilateral switching device or a positive temperature coefficient resistor (PTC) provided that switch SWl following a predetermined period of time after the detection of current flowing through lamp load LL is turned off (i.e. opened). In other words, once a predetermined period of time has elapsed following lamp ignition, switch SWl in response to the flow of current through lamp load LL should change from a conductive to a nonconductive switching state. A capacitor C3 as a reactive component and switch SWl are serially coupled between the second output terminal OT2 and a junction joining the first inductor LI and the first capacitor Cl together. The reactive component, capacitor C3, forms in combination with the first inductor LI a circuit characterized by a first resonant condition at a first frequency wherein energy is generated that is supplied to the lamp LL for aiding ignition of the lamp. The combination of choke LI, switching device SI, capacitor C2 and resistor Rl serve as an ignitor. During start up of lamp LL, current flows through choke LI, capacitor C2, resistor Rl, switch SWl and choke L2. Capacitor C2 charges based essentially on the RC time constant of resistor Rl and capacitor C2. The inductance of choke L2 is negligible compared to the inductance of choke LI at the 60 Hz frequency of voltage source VS. Once the voltage across capacitor C2 reaches the breakover voltage of switching device SI, capacitor C2 discharges through switching device SI and that portion of choke LI connecting switching device SI to capacitor C2. Choke LI serves as an autotransformer increasing the voltage to several thousand volts. Capacitor C2 discharges very quickly so as to apply this very high voltage (ignition pulse) for a very short duration to lamp LL. The ignition pulse is at a very high frequency (e.g. 100kHz). Choke L2 presents a high impedance to the ignition pulse so that loading placed on the ignition pulse by other than lamp load LL is minimized. As capacitor C2 discharges, the voltage across switching device SI drops below the breakdown voltage of the latter. Switching device SI opens. Capacitor C2 begins charging once again until reaching the breakdown voltage of switching device S 1. Another ignition pulse is generated as described above. Generation of these ignition pulses continues until near or at full arc discharge of lamp load LL (i.e. until stable lamp current is established). During steady state operation of lamp load LL, the voltage across capacitor C2 never reaches the breakdown voltage of switching device SI. No further generation of ignition pulses by the ignitor occurs. The ignitor is disabled. Generation of ignition pulses cannot occur once switch SWl opens. That is, when switch SWl is in its non-conductive state, capacitor C3 can no longer charge. Switch SWl changes from its conductive state to its non-conductive state in response to the flow of current through lamp load LL for a predetermined period of time. The opening of switch SWl therefore renders the ignitor inoperable. Switch SWl is part of a DC actuated relay which also includes resistors R2 and R3, capacitor Cl, an electrolytic capacitor C4, a capacitor C5 and a diode bridge formed by four diodes D1-D4. Once lamp load LL has ignited, current begins to flow through capacitors Cl and C5 and lamp load LL. The current flowing through capacitor C5 is rectified by the diode bridge and charges capacitor C4 to a DC voltage which energizes the relay so as to pull the normally closed contacts of switch SWl apart and into a nonconductive state. The elapsed time for sufficiently charging capacitor C4 resulting in the opening of switch SWl can be in, but is not limited to, the range of tens of milliseconds. The rate at which capacitor C4 charges depends primarily on the capacitance ratio between capacitors Cl and C5 and the capacitance of capacitor C4. Resistor R3, which is connected in parallel with capacitor Cl, serves to bleed off the voltage across capacitor Cl, which can be several hundred volts, within a certain period of time (e.g. 30 seconds to 1 minute) for safety purposes. Resistor R2, which is in series with capacitor C5, serves to prevent an inrush of current through capacitor C5. An HID lamp is particularly difficult to start unless the open circuit voltage is at least about 200 volts RMS. Conventional ballasts boost the voltage applied to the lamp during starting through magnetic transformation of the 120 VAC nominal line voltage. Such transformation results in bulkier ballasts and/or ballasts having much higher losses (i.e. far less efficient). In accordance with the invention, the nominal utility line voltage is increased/boosted to about at least 200 volts RMS through a series resonant LC circuit of choke LI and capacitor C3. The resonant frequency of choke LI and capacitor C3, forming the first frequency, is just above the driving frequency of voltage source VS, that is, just above 60 Hz (e.g. about 85 Hz). The voltage applied to lamp load LL during starting includes the high frequency voltage pulses of several thousand volts (e.g. 3000 to 3500 volts) supplied by the ignitor added to a voltage of at least about 200 volts RMS at about 60 Hz supplied by the series resonant LC circuit of choke LI and capacitor C3. The energy generated by the resonant circuit of choke LI and capacitor C3 supplied to lamp load LL therefore aids in the ignition of the latter. During ignition, choke L2 dampens the ringing produced by the voltage across capacitor C3 which is applied to lamp load LL so as to last in the range of milliseconds. More particularly, during ignition choke L2 limits the amplitude and prolongs the duration of the current delivered to lamp load LL from charged capacitor C3. The period over which the energy in C3 is delivered to lamp load LL is thereby extended. Without choke L2, capacitor C3 could discharge in a few hundred microseconds thereby providing an insufficient boost to successfully ignite lamp load LL. Choke L2 helps to sustain the flow of current through lamp load LL. A resistor R4 serves to bleed off the voltage across capacitor C3 within a certain predetermined period of time (e.g. 30 seconds to 1 minute). The resonant circuit formed by choke LI and capacitor C3 is disabled once switch SWl opens. Choke L2 should be designed so as not to saturate during the discharge of capacitor C3 into lamp load LL. Relative to choke LI, the inductance of choke L2 is relatively small. Prior to lamp ignition, the current flowing through choke L2 can be hundreds of milliamps. The voltage across capacitor C3 is applied to lamp load LL once ballast 10 is energized, that is, prior to the ignitor beginning its generation of ignition pulses. In other words, application of the voltage across capacitor C3 precedes application of the ignition pulses to lamp load LL. Disablement of the ignitor occurs upon or before disablement of the resonant circuit formed by choke LI and capacitor C3. For ballasting a 70 watt, CDM metal halide lamp typical component values for resistors Rl and R2 are about 8.2 k ohms and 100 ohms, respectively; for resistors R3 and R4 are about 2.2 M ohms; for capacitors Cl, C2 , C3, C4 and C5 are about 8.1 Tf, 300 VAC, 0.15 Tf; 630 VDC; 6.3 Tf, 300 VAC; 220 Tf, 25 VDC and 0.33 Tf, IK VDC, respectively; for switching device SI is a 260v SIDAC; for diodes D1-D4 are about 1 amp, lOOOv; for chokes LI and L2 are about 530 mH at 900 ma and 35 mH at 600ma, respectively, and the relay is about 10 A, 250 VAC, 12 VDC. FIG. 2 illustrates an alternative embodiment of the invention in which like reference numerals/letters represent components of similar construction and operation. A ballast 10' includes a series resonant LC circuit of choke LI and a capacitor C5 as first capacitor connected between first input terminal IT1 and first output terminal OT1 for ballasting lamp load LL during steady state operation. The resonant frequency of choke LI and capacitor C5 is near and above the frequency of the driving voltage, that is, near and above the 60 Hz frequency of voltage source VS (e.g. 75 Hz). Capacitor C5 and a choke (inductor) L3, as a reactive component, form a resonant circuit having in a first resonant condition a first frequency just below the driving frequency of voltage source VS (i.e. just below 60 Hz). Switch SI is normally closed. The conductive state of switch SI is controlled by a conventional relay RLY which causes SI to open in response to the flow of current through lamp LL for a predetermined period of time. During ignition, the ignitor circuit formed by choke LI, switching device SI, capacitor C2 and resistor Rl generates ignition pulses which are added to the voltage of at least about 200 volts rms, 60Hz developed across choke L3. These boosted ignition pulses are applied to lamp load LL. A choke L2, which is connected between resistor Rl and the junction joining input terminal IT2 to output terminal OT2, is negligible compared to choke LI at the 60 Hz frequency of voltage source VS. Choke L2 presents a high impedance to the ignition pulse so that loading placed on the ignition pulse by other than lamp load LL is minimized. In both ballasts 10 and 10', the ignition pulses occur near the peaks of the 60 Hz oscillator output. Consequently, when a momentary breakdown of lamp load LL takes place as a result of the ignition pulse, the energy stored by capacitor C3 of ballast 10 or by choke L3 of ballast 10' is delivered to the lamp to help sustain the discharge. As can now be readily appreciated, the resonant circuits formed by choke LI and capacitor L3 and by choke L3 and capacitor C5 of ballasts 10 and 10', respectively, as compared to a transformer within a conventional ballast are far more efficient in increasing the 120 VAC nominal line voltage. The ballasts 10 and 10' as compared to a conventional ballast are also far less bulky by not having to incorporate a transformer for stepping up the 120 VAC nominal line voltage.

Claims: CLAIMS: 1. A ballast for powering a high intensity discharge lamp, comprising: a first input terminal and a second input terminal for receiving an input voltage; a first output terminal and a second output terminal for supplying power to the lamp; a first inductor and a first capacitor serially coupled between the first input terminal and the first output terminal; and a reactive component and a switch serially coupled between the second output terminal and a junction joining the first inductor and the first capacitor together, the reactive component in combination with an element selected from the group consisting of the first inductor and first capacitor characterized by a first resonant condition at a first frequency; wherein energy generated by the first resonant condition is supplied to the lamp for aiding ignition of the lamp. 2. The ballast of claim 1 , wherein the first inductor and first capacitor are characterized by a second resonant condition at a second frequency for sustaining lamp current during steady state operation of the lamp. 3. The ballast of claim 2, wherein the first frequency is greater than the second frequency. 4. The ballast of claim 1, 2 or 3 further including an ignitor which includes the first inductor for generating ignition pulses. 5. The ballast of claim 1, 2, 3 or 4 further including a second inductor connected between the switch and the second output terminal. 6. The ballast of any preceeding claim, wherein during starting of the lamp the reactive component in combination with the selected element produce a signal associated with the first resonant condition whose ringing is extended by the second inductor. 7. The ballast of any preceeding claim, wherein the conductive state of the switch is based on the operating state of the lamp. 8. The ballast of claim 7, wherein the switch changes from a conductive state to a non-conductive state in response to current flowing through the lamp for a predetermined period of time. 9. The ballast of any preceeding claim, wherein the reactive component is a capacitor. 10. The ballast of any preceeding claim, wherein the reactive component is an inductor.

Inventors: MATTAS CHARLES B, MATTAS, CHARLES, B.
Applicants: KONINKL PHILIPS ELECTRONICS NV, PHILIPS SVENSKA AB, KONINKLIJKE PHILIPS ELECTRONICS N.V., PHILIPS AB
Application year: 1998
Main classification: H05B41
IPC codes: H05B41