Patent Publication Number: US-2012025731-A1

Title: Ballast for high pressure discharge lamp

Description:
TECHNICAL FIELD 
     The present invention relates to a ballast for a high pressure discharge lamp for driving a high pressure discharge lamp. 
     BACKGROUND ART 
     Conventionally, a startup light source for assisting ignition of a high pressure discharge lamp has been known (for example, Patent document 1). This document describes a high pressure discharge lamp including a bulb serving as a main light source and an auxiliary startup light source. The startup light source includes a pair of electrodes. One of the pair of electrodes of the startup light source is an inner electrode, and the other is an outer electrode. The electrodes are capacitively coupled. Application of a high voltage therebetween causes electrical breakdown, and a glow discharge is initiated, generating ultraviolet radiation. The ultraviolet radiation excites an ignition gas in the bulb. Thereby, the high pressure discharge lamp is ready to ignite. 
       FIG. 7  shows a conventional ballast for a high pressure discharge lamp. In the diagram, a high pressure discharge lamp  50  includes a bulb  51  and a startup light source  52 . The ballast includes a step-down chopper circuit  20 , a full-bridge circuit  30 , and a resonant circuit  80 . 
     During the steady driving of the bulb  51 , the step-down chopper circuit  20  controls a current flowing through the bulb  51 , and the full-bridge circuit  30  inverts the alternating waveform of the current, and the alternating current of a relatively low frequency (for example, approximately 50 to 400 Hz) is supplied to the bulb  51 . 
     At the ignition of the bulb  51 , the full-bridge circuit  30  is driven at a frequency in proximity to a resonance frequency of the resonant circuit  80  or the resonance frequency divided by an odd number, and a high voltage of several kV is generated in the resonant circuit  80 . This high voltage causes the startup light source  52  to emit light, and the ultraviolet irradiation and the high voltage cause the bulb  51  to ignite. 
       FIG. 8  shows a waveform of the voltage applied to the startup light source  52  in the above-described example. The waveform in the chart is of a case where the driving frequency of the full-bridge circuit  30  is ⅓ of the resonance frequency of an inductor  81  and a capacitor  82 . 
     Here, the inductor  81  used in the resonant circuit  80  greatly varies (i.e., the inductance thereof varies) in comparison with the other circuit elements. Accordingly, the resonance frequency of the resonant circuit  80  also greatly varies. As shown in  FIG. 9 , at the ignition, even if the operating frequency of the full-bridge circuit  30  is constant, the voltage generated in the resonant circuit  80  greatly varies as shown from V 0 , V 1  to V 2 . For this reason, in a conventional example (for example, Patent document 2), at the ignition of the bulb  51 , the voltage generated in the resonant circuit  80  is detected while the operating frequency of the full-bridge circuit  30  is being finely changed. The result is fed back to a control circuit (not shown). Then, the operating frequency of the full-bridge circuit  30  is tuned so as to optimize the voltage generated in the resonant circuit  80 . 
     Prior Art Documents 
     Patent Documents 
     Patent Document 1: Japanese Patent Application Publication No. H1-134848 
     Patent Document 2: Published Japanese Translation of PCT International Application No. 2005-520294  
     SUMMARY OF INVENTION 
     Technical Problems 
     In the ballast for high pressure discharge lamp of conventional example ( FIG. 7 ), the resonant circuit  80  including the inductor  81  and the capacitor  82  is operated substantially in a resonating state. Thus, a high voltage is applied to the inductor  81  and the capacitor  82 , and a large current flows therethrough. For this reason, the inductor  81  has to have a large core size so as to prevent saturation. In addition, the capacitor  82  has to withstand the high voltage, and accordingly multiple capacitors are connected in series, for example. 
     Moreover, the high voltage generated by the resonant circuit  80  is large in energy, creepage distance and clearance distance must be designed so long as to prevent electrical breakdown in the ballast for high pressure discharge lamp and also to prevent fire accident. This results in a problem of increased ballast size. 
     Further, a feedback circuit and the like need to be provided to tune the operating frequency of the full-bridge circuit  30  in the conventional example as described above. The conventional example has a problem that the ballast configuration is complicated and costly. 
     Therefore, in a high pressure discharge lamp ballast for driving a high pressure discharge lamp using a startup light source, an object is to provide the ballast having a small and simple configuration. 
     Solution to Problems 
     The present invention provides a ballast for a high pressure discharge lamp including a driving circuit for supplying an alternating power to a high pressure discharge lamp ( 50 ). In the ballast of the present invention, the high pressure discharge lamp includes: a bulb ( 51 ); and a startup light source ( 52 ) disposed in a vicinity of the bulb and assisting ignition of the bulb. The startup light source has a pair of electrodes which are capacitively coupled. The driving circuit includes: a bridge unit ( 30 ,  70 ) for inverting a lamp current; a current regulator unit ( 20 ,  75 ) for regulating the lamp current; and an igniter unit ( 40 ) for generating an ignition voltage. At the ignition, a voltage containing a frequency component higher than a driving frequency during a steady driving of the bulb is applied to the startup light source. 
     Herein, the startup light source is connected in parallel to the bulb. The igniter unit includes a transformer ( 41 ) and a capacitor ( 42 ). The transformer is connected in series to the high pressure discharge lamp. The capacitor is connected in parallel to a series circuit of the transformer and the high pressure discharge lamp. 
     Furthermore, the bridge unit is a full-bridge circuit ( 30 ). An inductor ( 60 ) is connected between the bridge unit and the igniter unit. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a diagram showing a ballast for high pressure discharge lamp according to a first embodiment of the present invention. 
         FIG. 2  is a chart for explaining the first embodiment of the present invention. 
         FIG. 3  is a diagram showing a ballast for high pressure discharge lamp according to a second embodiment of the present invention. 
         FIG. 4  is a chart for explaining the second embodiment of the present invention. 
         FIG. 5  is a diagram showing a ballast for high pressure discharge lamp according to third and fourth embodiments of the present invention. 
         FIG. 6  is a chart for explaining the third embodiment of the present invention. 
         FIG. 7  is a diagram showing a conventional ballast for high pressure discharge lamp. 
         FIG. 8  is a chart for explaining the conventional example. 
         FIG. 9  is a chart for explaining the conventional example. 
     
    
    
     BEST MODES FOR CARRYING OUT THE INVENTION 
     Incidentally, in the conventional technique, to ensure the ignition operation of the high pressure discharge lamp  50  (i.e., the bulb  51  and the startup light source  52 ), a high voltage is applied thereto. However, the amount of ultraviolet radiation generated from the startup light source  52  depends on the current flowing therethrough. Hence, it is important not to increase the voltage to be applied but to increase the current flowing. Specifically, since the startup light source  52  is capacitive, the current can be increased by increasing the frequency of the voltage to be applied (the current flowing). 
     A ballast for high pressure discharge lamp generally includes: a bridge unit for inverting a lamp current; a current regulator unit for regulating the lamp current; and an igniter unit for generating a ignition voltage. At the ignition, these units are configured to apply to a startup light source  52  a voltage containing a frequency component higher than a driving frequency during the steady driving of a bulb  51 . 
     Embodiment 1 
       FIG. 1  shows a circuit configuration diagram of a ballast for high pressure discharge lamp according to a first embodiment. The ballast in the diagram includes a step-down chopper circuit  20  as a current regulator unit, a full-bridge circuit  30  as a bridge unit, and an igniter  40 . 
     A driver (not shown) drives the full-bridge circuit  30  at several ten Hz to several hundred Hz during normal driving phase, while driving at several hundred Hz to several ten kHz at the ignition. To put it differently, the voltage applied to a startup light source  52  has a waveform shown in  FIG. 2 . 
     At the ignition, an output voltage from an igniter  40  causes the startup light source  52  to be electrically broken down. Subsequently, a glow discharge in the startup light source  52  is continued by a voltage supplied from the bridge circuit  30 . Once the startup light source  52  is electrically broken down, a pair of electrodes in the startup light source  52  are brought into a capacitively coupled state. Hence, the higher the frequency component of the voltage supplied from the bridge circuit  30 , the more smoothly a current flows through the startup light source  52 , and the more efficiently ultraviolet radiation is generated. The larger the amount of the ultraviolet radiation generated from the startup light source  52 , the more an ignition gas in a bulb  51  is excited. Thus, even a relatively low ignition voltage can bring the bulb  51  into the ignition state. 
     Since the bulb  51  is made ready to ignite by the efficient ultraviolet irradiation, the output voltage generated by the igniter circuit  40  may be smaller in energy than an ignition voltage generated by a resonant circuit in the conventional example (for example, the resonant circuit  80  in  FIG. 8 ). In other words, advantageously, the size of the igniter circuit can be made smaller. 
     Embodiment 2 
       FIG. 3  shows a circuit configuration diagram of a second embodiment. A ballast for high pressure discharge lamp in the diagram includes a step-down chopper  20  as a current regulator unit, a full-bridge circuit  30  as a bridge unit, an inductor  60 , and an igniter  40 . 
     At the startup, a driver (unillustrated) drives the full-bridge circuit  30  at a frequency in proximity to the resonance frequency of the inductor  60  and a capacitor  42  divided by an even number.  FIG. 4  shows a voltage applied to a startup light source  52  at the ignition. The waveform in the chart is of a case where the driving frequency of the full-bridge circuit  30  is ¼ of the resonance frequency of the inductor  60  and the capacitor  42 . 
     Since the inductor  60  and the capacitor  42  oscillate at the resonance frequency, a voltage containing a frequency component higher than the operating frequency of the full-bridge circuit  30  is applied to the startup light source  52 . Thereby, a large amount of current flows through the startup light source  52  in the capacitively coupled state, and ultraviolet radiation emission is achieved efficiently. 
     In this embodiment, since the voltage generated in and the current through the inductor  60  and the capacitor  42  are small, small and inexpensive parts can be used for these. 
     Moreover, even if the value of the inductor  60  varies, the voltage to be generated varies little because the driving frequency of the full-bridge circuit  30  is far different from the resonance frequency of the inductor  60  and the capacitor  42 . Hence, it is not necessary to finely tune the operating frequency of the full-bridge circuit  30 , and the control configuration can be advantageously simplified. 
     Embodiment 3 
       FIG. 5  shows a circuit configuration diagram of a third embodiment. A ballast for high pressure discharge lamp in the diagram includes a half-bridge circuit  70  as a bridge unit, an inductor  75  as a current regulator unit, and an igniter circuit  40 . 
     During the normal driving of a bulb  51 , when a switching element  71  is turned ON-OFF at a high frequency with a switching element  72  in the OFF state, a current flows from the bulb  51  toward a coil  41   b  of a transformer  41 . Meanwhile, when the switching element  72  is turned ON-OFF at a high frequency with the switching element  71  in the OFF state, a current flows from the coil  41   b  toward the bulb  51 . The bulb  51  is AC-driven at a frequency at which the switching elements  71  and  72  are alternately operated. 
     At the ignition of the bulb  51 , a driver (not shown) drives the switching elements  71  and  72  at a frequency in proximity to the resonance frequency of the inductor  75  and a capacitor  42  divided by an odd number. Immediately after the operation is switched from the ON state of the switching element  71  to the ON state of the switching element  72 , or immediately after the operation is switched from the ON state of the switching element  72  to the ON state of the switching element  71 , the inductor  75  and the capacitor  42  generate an oscillating voltage. 
     As a result, a square wave voltage on which a high frequency component is superimposed as shown in  FIG. 6  is applied to a startup light source  52 . By the high frequency component, a large amount of current flows through the capacitive startup light source, and ultraviolet radiation emission is achieved efficiently. 
     When the operation is switched between the switching elements  71  and  72 , the oscillating voltage generated by the inductor  75  and the capacitor  42  is not a high voltage unlike the case of the resonant circuit in the conventional example. Thus, both the inductor  75  and the capacitor  42  may be small parts. 
     Moreover, if the frequency at which the switching elements  71  and  72  are alternately operated at the ignition of the bulb  51  is higher than that during the steady driving phase, a higher frequency component can be applied to the startup light source  52 , and the ignition operation can be facilitated. 
     Embodiment 4 
     The circuit configuration of this embodiment is the same as that of Embodiment 3 in  FIG. 5 , except for the operations of the switching elements  71  and  72 . 
     At the ignition of the bulb  51 , an unillustrated driver drives the switching elements  71  and  72  at a frequency in proximity to the resonance frequency of the inductor  75  and the capacitor  42  divided by an even number. The switching elements  71  and  72  are turned ON-OFF alternately. The voltage applied to the startup light source  52  is the same as that in  FIG. 4  of Embodiment 2. 
     Although the inductor  75  and the capacitor  42  generate an oscillating voltage of a high frequency similarly to Embodiment 2, the voltage is small, and both the parts may be advantageously small. 
     With the above configuration, a ballast for high pressure discharge lamp can be produced with a simple circuit under simple control using small and inexpensive parts. 
     EXPLANATION OF REFERENCE NUMERALS 
       10 . DC power supply 
       20 . step-down chopper circuit 
       30 . full-bridge circuit 
       40 . igniter circuit 
       50 . high pressure discharge lamp 
       51 . bulb 
       52 . startup light source 
       60 . inductor 
       70 . half-bridge circuit 
       75 . inductor