Abstract:
A discharge lamp lighting apparatus includes: an inverter circuit including a switch circuit and a high-voltage transformer; a step-up circuit connected at the input stage of the inverter circuit; a control circuit to output a switch circuit controlling signal and a step-up circuit controlling signal; an on-off switching circuit connected at a input voltage line; and a switch signal buffering circuit connected between the control circuit and the switch circuit and adapted to relay the switch circuit controlling signal. A drive voltage is applied to the control circuit without going through the on-off switching circuit, and a drive voltage is applied to the switch signal buffering circuit via the on-off switching circuit. With such a structure, an excess rush current is inhibited from flowing into the discharge lamp thereby preventing its electrode wear and therefore increasing the product life of a lamp and enhancing reliability while achieving cost reduction and downsizing.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention relates to a discharge lamp lighting apparatus, and particularly to a discharge lamp lighting apparatus used in various information equipments, such as a facsimile, a copier, a scanner, and the like, and adapted to provide illumination for reading documents.  
         [0003]     2. Description of the Related Art  
         [0004]     Recently, a light source for providing illumination for reading documents in various information equipments, such as a facsimile, a copier, a scanner, and the like, or a light source for a backlight device for use in a liquid crystal display (LCD) device is required to provide a high brightness, a long product life, and a high reliability. Consequently, a rare gas discharge lamp utilizing dielectric barrier discharge is increasingly used. The rare gas discharge lamp utilizing dielectric barrier discharge is found to have various advantages: for example, it has a high luminous efficiency, and it has no electrodes in the discharge space thus preventing decrease of its product life due to sputtering discharge. An apparatus to light such a rare gas discharge lamp conventionally incorporates a push-pull inverter circuit (refer, for example, to Japanese Patent Application Laid-Open No. 2001-160497).  
         [0005]      FIG. 4  is a block diagram of a typical example of such a discharge lamp lighting apparatus as mentioned above. Referring to  FIG. 4 , a discharge lamp lighting apparatus  104  includes a control circuit  106 , a step-up circuit  105 , a switch circuit  107 , a step-up transformer  118 , and a smoothing capacitor  115 , wherein an input voltage Vi from an external DC voltage supply is stepped by the step-up circuit  105  up to a voltage Vj (boosted voltage) which, while supplied to the smoothing capacitor  115 , is adapted to drive the primary side of the step-up transformer  118  by means of the switch circuit  107  thereby lighting a discharge lamp  101  connected at the secondary side of the step-up transformer  118 . During this operation, the control circuit  106 , according to a feedback signal FB of the boosted voltage Vj, outputs a control signal GC for the step-up circuit  105  and a control signal GS for the switch circuit  107 , thus both the step-up operation of the step-up circuit  105  and the switching operation of the switch circuit  107  are controlled only by one single circuit, namely the control circuit  106 .  
         [0006]      FIG. 5  is a circuit diagram showing a specific circuitry of the discharge lamp lighting apparatus  104  of  FIG. 4 . The step-up circuit  105  is a step-up chopper circuit which includes a choke coil  112  connected in series to the positive terminal of the DC voltage supply, a diode  114  connected in series to the choke coil  112 , and a switching element  113  constituted by, for example, a power MOSFET and connected between ground and the connection point of the choke coil  112  and the diode  114 , and which has its output terminal connected to the smoothing capacitor  115  to smooth the boosted voltage Vj. The step-up circuit  105  further includes a gate resistor  133  connected to the gate terminal of the switching element  113 , and, in order to rapidly charge and discharge the gate capacitance to thereby achieve a high-speed switching operation, a differentiation circuit  136  composed of a capacitor  131  and a resistor  132  is connected to the gate resistor  133 , and then a buffer circuit  135 , which is structured such that the emitter and base terminals of an NPN transistor  129  are connected respectively to the emitter and base terminals of a PNP transistor  130 , is connected to the differentiation circuit  136 .  
         [0007]     The switch circuit  107  includes two switching elements  116  and  117  which are each constituted by, for example, a power MOSFET, and which are connected respectively to the both terminals of the primary winding of the step-up transformer  118 . The midpoint tap of the primary winding of the step-up transformer  118  is connected to the boosted voltage Vj, and the switching elements  116  and  117  and the step-up transformer  118  make up a push-pull inverter circuit. The switch circuit  107  further includes gate resistors  124  and  125  connected to the respective gate terminals of the switching elements  116  and  117 , and the differentiation circuit  136  and the buffer circuit  135  are provided for each of the switching elements  116  and  117  in the same way as in the switching element  113  of the step-up circuit  105 .  
         [0008]     The control circuit  106  includes a PWM (pulse width modulation) control circuit  119 , a switch circuit gate signal generating circuit composed of transistors  120  and  121  disposed at the output stage of the PWM control circuit  119  and resistors  122  and  123 , and a step-up circuit gate signal generating circuit including a signal adder composed of diodes  126  and  127  and a resistor  128 .  
         [0009]     The PWM control circuit  119  includes an error amplifier  137  which receives a feedback signal FB generated such that the boosted voltage Vj is divided by a voltage dividing circuit  111  so as to be fed back, and a reference signal generated such that a reference voltage Vref is divided. A control pulse generated in the PWM control circuit  119  is pulse width modulated according to the comparison of the feedback signal FB with the reference signal thereby turning into switch circuit gate signals GL and GU to be inputted to the respective gate terminals of the switching elements  116  and  117  of the switch circuit  107 , and into a step-up circuit gate signal GC to be inputted to the gate terminal of the switching element  113  of the step-up circuit  105 .  
         [0010]     The operation of the discharge lamp lighting apparatus  104  described above will be explained with reference to  FIGS. 6A  to  6 F showing sequence charts. Referring first to  FIG. 6A , when the input voltage Vi from the external DC voltage supply is applied to the step-up circuit  105  at time t 0 , the control circuit  106  starts its operation, whereby gate signals B and D are outputted respectively to the switching element  113  of the step-up circuit  105  and to the switching elements  116  and  117  of the switch circuit  107  as shown in  FIGS. 6D and 6F , and the switching elements  113 ,  116  and  117  are caused to start their on-off operations. When the switching elements  113  of the step-up circuit  105  transits from an on-state to an off-state, an induction voltage is generated at the choke coil  112 , whereby the boosted voltage Vj is generated across the both terminals of the smoothing capacitor  115  as shown in  FIG. 6B .  
         [0011]     When the input voltage Vi is applied to the step-up circuit  105 , an input current A starts to flow with a large rush current generated instantaneously as shown in  FIG. 6C . If the DC voltage supply does not have a sufficient supply capacity to cover current excess resulting from superposition of the rush current on the input current A, then the input voltage Vi of the DC voltage supply is lowered below the rated voltage. Generally, for preventing the lowering of the voltage of a DC power supply, the DC power supply is required to have an increased supply capacity, which results in an increased dimension of the DC power supply.  
         [0012]     Further, since the gate signal B for the switching element  113  of the step-up circuit  105 , and the gate signal D and the gate signal respectively for the switching elements  116  and  117  of the switch circuit  107  start their operations simultaneously, the rush current flowing into the smoothing capacitor  115  becomes large. Consequently, an output current C with a rush current superposed thereon flows in the discharge lamp  101  as shown in  FIG. 6E , and therefore a stress on electrodes  103  and  103 ′ of the discharge lamp  101  increases thus shortening the life of the discharge lamp  101 . Also, generally, for reducing the rush current flowing into the discharge lamp  101  to any extent at all, delay operation must be performed in the step-up circuit  5 , and consequently the current in the discharge lamp  101  inevitably has a current waveform with a delayed rise.  
         [0013]     A conventional discharge lamp lighting apparatus generally includes an on-off switching circuit and a constant-voltage circuit both provided between a DC voltage supply and a control circuit, and is structured such that an input voltage from the DC voltage supply is preliminarily applied for putting a step-up circuit in a standby-state so that when the on-off switching circuit is turned on, the step-up circuit and the switch circuit are caused to start their operations so as to start lighting a discharge lamp. In such a discharge lamp lighting apparatus, since an input voltage Vi is preliminarily applied to a smoothing capacitor in a standby-state, the voltage at the smoothing capacitor rises from the input voltage Vi, not from 0 V, up to a prescribed boosted voltage Vj for lighting the discharge lamp. Consequently, the rush current is reduced when compared with the discharge lamp lighting apparatus  104  in which the voltage at the smoothing capacitor  115  rises from 0 V up to the boosted voltage Vj for lighting the discharge lamp  101 .  
         [0014]     However, it is still the case with the above-described conventional discharge lamp lighting apparatus that the step-up circuit and the switch circuit starts their operations simultaneously for lighting the discharge lamp, and the output current composed such that the rush current flowing into the smoothing capacitor is superposed on the input current is caused to flow in the discharge lamp thus raising the same problem as the discharge lamp lighting apparatus  104 .  
       SUMMARY OF THE INVENTION  
       [0015]     The present invention has been made in light of the problem described above, and it is an object of the present invention to provide a discharge lamp lighting apparatus in which a supply voltage from a DC voltage supply is inhibited from lowering due to a rush current superposed on an input current, and in which an excess rush current is inhibited from flowing into a discharge lamp thereby increasing the life of the discharge lamp and enhancing reliability.  
         [0016]     In order to achieve the object of the present invention, according to an aspect of the present invention, there is provided a discharge lamp lighting apparatus which includes: an inverter circuit including a switch circuit and a high-voltage transformer, wherein the primary side of the high-voltage transformer is driven by the inverter circuit thereby lighting a discharge lamp connected at the secondary side of the high-voltage transformer; a step-up circuit connected at the input stage of the inverter circuit; a control circuit to output a switch circuit controlling signal for controlling the operation of the switch circuit and a step-up circuit controlling signal for controlling the operation of the step-up circuit; an on-off switching circuit connected at a input voltage line; and a switch signal buffering circuit connected between the control circuit and the switch circuit and adapted to relay the switch circuit controlling signal. In the discharge lamp lighting apparatus, a drive voltage is applied to the control circuit without going through the on-off switching circuit, and a drive voltage is applied to the switch signal buffering circuit via the on-off switching circuit.  
         [0017]     With the structure described above, when an input voltage is applied to the discharge lamp lighting apparatus, the on-off switching circuit is turned off, and the switch signal buffering circuit is out of operation, whereby the switch circuit controlling signal is prevented from getting to the switch circuit. Consequently, the on-off switching circuit is in a standby-state immediately after an input voltage is supplied, and only the step-up circuit is in a position to operate. Thus, it is enabled that the on-off switching circuit is caused to transit to an on-state after a certain period so as to activate the switch signal buffering circuit, and the switch circuit is caused to start its operation, which suppresses a rush current to be superposed on an input current supplied to the discharge lamp lighting apparatus.  
         [0018]     In the aspect of the present invention, the discharge lamp lighting apparatus may further include a start triggering circuit connected to the control circuit. The start triggering circuit operates such that the control circuit increases an output of the step-up circuit temporarily when the on-off switching circuit transits to an on-state. Consequently, the discharge lamp can be rapidly and stably lighted.  
         [0019]     In the aspect of the present invention, the step-up circuit may be a step-up chopper circuit including a choke coil connected in series to the input voltage line, an diode connected in series to the choke coil, and a switching element connected between ground and a connection point of the choke coil and the diode, the switch circuit may include at least one switching element connected to the primary side of the high-voltage transformer, and the control circuit may include a pulse width modulation control circuit to generate, according to a reference voltage, pulse width modulating signals for controlling respective on-off operations of the switching element of the step-up circuit and the at least one switching element of the switch circuit.  
         [0020]     In the aspect of the present invention, the start triggering circuit may be a reference voltage shifting circuit functioning to shift the reference voltage so as to temporarily increase the on-duty of the switching element of the step-up circuit.  
         [0021]     In the aspect of the present invention, the discharge lamp may be a rare gas discharge lamp utilizing a dielectric barrier discharge, and the switch circuit may be a push-pull switching circuit.  
         [0022]     In the aspect of the present invention, the output signal of the on-off switching circuit may be controlled by an on-off signal externally inputted.  
         [0023]     In the aspect of the present invention, the discharge lamp lighting apparatus may further include a constant-voltage circuit connected between the input voltage line and the on-off switching circuit. The constant-voltage circuit supplies respective drive voltages to the control circuit, the switch buffering circuit, and the switch circuit.  
         [0024]     In the discharge lamp lighting apparatus described above, the supply voltage from the DC voltage supply is inhibited from lowering due to a rush current superposed on an input current, and an excess rush current is inhibited from flowing into the discharge lamp thereby increasing the life of the discharge lamp and enhancing reliability. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0025]      FIG. 1  is a block diagram of a discharge lamp lighting apparatus according to an embodiment of the present invention;  
         [0026]      FIG. 2  is a circuit diagram of the discharge lamp lighting apparatus of  FIG. 1 ;  
         [0027]      FIGS. 3A  to  3 G are sequence charts for explaining an operation of the discharge lamp lighting apparatus of  FIG. 1 ;  
         [0028]      FIG. 4  is a block diagram of a conventional discharge lamp lighting apparatus;  
         [0029]      FIG. 5  is a circuit diagram of the discharge lamp lighting apparatus of  FIG. 4 ; and  
         [0030]      FIGS. 6A  to  6 F are sequence charts for explaining an operation of the discharge lamp of  FIG. 4 . 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0031]     An exemplary embodiment of the present invention will hereinafter be described with reference to the accompanying drawings.  
         [0032]     Referring to  FIG. 1 , a discharge lamp lighting apparatus  1  according to an embodiment of the present invention includes an inverter circuit composed of a switch circuit  4  and a high-voltage transformer  5 , a step-up circuit  3  connected at the input stage of the inverter circuit, and a control circuit  2  to generate a switch circuit controlling signal GS for controlling the operation of the switch circuit  4  and a step-up circuit controlling signal GC for controlling the operation of the step-up circuit  3 , wherein the inverter circuit drives the primary side of the high-voltage transformer  5  thereby lighting a discharge lamp  11  connected at the secondary side of the high-voltage transformer  5 . A smoothing capacitor  7  is connected to the output terminal of the step-up circuit  3 .  
         [0033]     The discharge lamp lighting apparatus  1  further includes an on-off switching circuit  6  connected to an input voltage line (a) via a constant-voltage circuit  10 , and a switch signal buffering circuit  8  connected between the control circuit  2  and the switch circuit  4  and adapted to relay the switch circuit controlling signal GS. A drive voltage (b) from the constant-voltage circuit  10  is applied directly to the control circuit  2  without going through the on-off switching circuit  6 , while a drive voltage (c) from the constant-voltage circuit  10  is applied to the switch signal buffering circuit  8  via the on-off switching circuit  6 . Further, a reference voltage shifting circuit  9  as a start triggering circuit is connected between the control circuit  2  and the on-off switching circuit  6 . Thus, the discharge lamp lighting apparatus  1  receives an input voltage Vi from an external DC voltage supply, and receives also an on-off signal from an external on-off signal generating circuit (not shown) or the like, which switch the on- and off-states of the on-off switching circuit  6 .  
         [0034]     Referring to  FIG. 2 , the step-up circuit  3  is a step-up chopper circuit, and includes a choke coil  12  connected in series to an input voltage line (al), a diode  14  connected in series to the choke coil  12 , and a switching element  13  connected between ground and the connection point of the choke coil  12  and the diode  14 . The switching element  13  is preferably constituted by a power MOSFET and has its gate terminal connected to a gate resistor  24  which, in order to rapidly charge and discharge the gate capacitance thereby achieving a high-speed switching performance, has at its input stage a buffer circuit  34  structured such that the emitter and base terminals of an NPN transistor  22  are connected respectively to the emitter and base terminals of a PNP transistor  23 . And, as described earlier, the smoothing capacitor  7  is connected to the output terminal of the step-up circuit  3 .  
         [0035]     The switch circuit  4  is a push-pull switching circuit (hereinafter referred to as “push-pull switching circuit  4  as appropriate), and includes two switching elements  15  and  16  each preferably constituted by a power MOSFET. The switching elements  15  and  16  are connected respectively to the both terminals of the primary winding of the high-voltage transformer  5 , and a boosted voltage Vj from the step-up circuit  3  is inputted to the midpoint tap of the primary winding of the high-voltage transformer  5 . A push-pull inverter circuit is constituted by the switching elements  15  and  16 , and the high-voltage transformer  5 . Like the switching element  13  of the step-up circuit  3 , the switching elements  15  and  16  have their gate terminals connected respectively to gate resistors  20  and  21  which have respective buffer circuits  35  and  36  connected at their input stages.  
         [0036]     The control circuit  2  is a PWM control circuit (hereinafter referred to as a “PWM control circuit  2  as appropriate”), and includes an error amplifier (not shown) to which a feedback signal FB generated according to the boosted voltage Vj from the step-up circuit  3  and a reference signal generated such that a reference voltage Vref is divided by resistors  41  and  42  are inputted. A control pulse generated in the PWM control circuit  2  is pulse width modulated according to the comparison of the feedback signal FB with the reference signal and turned into switch circuit controlling signals (gate signals) GU and GL to be inputted respectively to the gate terminals of the switching elements  15  and  16  of the push-pull switching circuit  4 , and into the step-up circuit controlling signal (gate signal) GC to be inputted to the gate terminal of the switching element  13  of the step-up circuit  3 . In this connection, the PWM control circuit  2  may include, besides the circuit main body thereof, a switch circuit gate signal generating circuit composed of transistors disposed at the output stage of the circuit main body and resistors, and a step-up circuit gate signal generating circuit including a signal adder composed of diodes and a resistor, like in the conventional discharge lamp lighting apparatus  4  shown in  FIG. 5 .  
         [0037]     The constant-voltage circuit  10  is what is called a serial stabilized power supply, and includes a series circuit which is composed of resistors  28  and  29  and a zener diode  40 , and which is connected between an input voltage line (a 2 ) and ground, and a reference voltage generated at the zener diode  40  is inputted to the base terminal of an NPN transistor  26  connected in series to the input voltage line (a 2 ), whereby the voltage across the base and emitter terminals of the NPN transistor  26  is kept constant thus providing a predetermined output voltage. The constant-voltage circuit  10  further includes a transistor  27  and a resistor  30  which in combination function as a protection circuit against a rapid increase of load current. The present invention is not limited to a specific structure of a constant-voltage circuit, and any appropriate constant-voltage circuit with a desired characteristic may be employed. Further, the constant-voltage circuit  10  may be even omitted depending on the external DC voltage supply used and on the specifications of the other circuits in the discharge lamp lighting apparatus  1 .  
         [0038]     The on-off switching circuit  6  is connected to the constant-voltage circuit  10 . The on-off switching circuit  6  principally includes a PNP transistor  19  as a switch main body, and a resistor  39  and a capacitor  40  which are both connected between the base and emitter terminals of the transistor  19  so as to achieve a predetermined base potential. The base terminal of the transistor  19  is connected to a terminal to which an external on-off signal is inputted. The present invention is not limited to a specific structure of an off-off switching circuit, and any of appropriate on-off switching circuits with a desired characteristic may be employed.  
         [0039]     The input voltage Vi (e.g., 24 V) from the external DC voltage supply is applied to the step-up circuit  3  via the input voltage line (al), and the step-up circuit  3  outputs the boosted voltage Vj (e.g., 70 V). A drive voltage Vcc (e.g., 15 V) from the constant-voltage circuit  10  connected to the input voltage line (a 2 ) is applied directly to the PWM control circuit  2 , and a drive voltage Vcc (substantially identical to the aforementioned drive voltage Vcc) from the constant-voltage circuit  10  is applied, via the on-off switching circuit  6 , to the switch signal buffering circuit  8 , the reference voltage shifting circuit  9 , and to the push-pull switching circuit  4 .  
         [0040]     The switch signal buffering circuit  8  includes comparators  17  and  18  to which the drive voltage Vcc coming from the on-off switching circuit  6  is applied. A reference voltage (e.g., Vcc/2) composed of the drive voltage Vcc divided by resistors  43  and  44  is inputted to the inverting input terminals of the comparators  17  and  18 , the switch circuit gate signals GU and GL from the PWM control circuit  2  are inputted respectively to the non-inverting input terminals of the comparators  17  and  18 , and the output terminals of the comparators  17  and  18  are pulled up to the drive voltage lines. The switch signal buffering circuit  8  thus structured functions as a buffer circuit for the switch circuit gate signals GU and GL having a rectangular wave pulse.  
         [0041]     The reference voltage shifting circuit  9  includes a series circuit which is composed of a capacitor  25  and a resistor  37 , one terminal of which is connected to the line for the drive voltage Vcc outputted from the on-off switching circuit  6 , and the other terminal of which is connected to the connection point of the resistors  41  and  42  to divide the reference voltage Vref.  
         [0042]     The operation of the discharge lamp lighting apparatus  1  will be described with reference to  FIGS. 3A  to  3 G.  
         [0043]     The input voltage Vi (e.g., 24 V) from the external DC power supply, when inputted to the discharge lamp lighting apparatus  1  at time t 0  as shown in  FIG. 3A , is fed to the step-up circuit  3 , and also to the constant-voltage circuit  10  and converted thereby into the drive voltage Vcc (e.g., 15 V) to be applied to the PWM control circuit  2 . Then, the PWM control circuit  2  outputs the step-up circuit gate signal GC, and a gate signal shown in  FIG. 3E  is inputted to the switching element  13  of the step-up circuit  3  thereby starting its on-off operation. As a result, the boosted voltage Vj (e.g., 70 V) is generated across the both terminals of the smoothing capacitor  7  as shown in  FIG. 3B .  
         [0044]     At this stage, an on-off signal A is at a high-state as shown in  FIG. 3C , the on-off switching circuit  6  is at an off-state, and the switch signal buffering circuit  8  is out of operation. Accordingly, though the generation of the switch circuit gate signals GU and GL is started in the PWM circuit  2 , the switch circuit gate signals GU and GL generated do not get to the push-pull switching circuit  4  as shown in  FIG. 3G  as gate signals for the switching elements  15  and  16 , thus the push-pull switching circuit  4  is not brought into operation. Consequently, as shown in  FIG. 3D , an input current B flows instantaneously upon supply of the input voltage Vi at time t 0  but immediately ceases to flow. And, since a gate signal E for the switching element  15  of the push-pull switching circuit  4  does not operate as shown in  FIG. 3G , an output current D does not flow at the secondary side of the high-voltage transformer  5  as shown in  FIG. 3F , thus the discharge lamp  11  is not lighted at this time.  
         [0045]     As described above, after the input voltage Vi is inputted at time t 0 , a standby-state (t 0 -t 1 ) lapses where the on-off switching circuit  6  is at an off-state, then the external on-off signal turns on into a low-state at time t 1  putting the on-off switching circuit  6  into an on-state, and a normal operation condition starts from time t 1 .  
         [0046]     When the on-off switching circuit  6  turns into an on-state at time t 1  as shown in  FIG. 3C , the drive voltage Vcc is applied to the switch signal buffering circuit  8  so as to cause the switching signal buffering circuit  8  to start its operation, and thus the switch circuit gate signals GU and GL from the PWM control circuit  2  get to the push-pull switching circuit  4  as the gate signals for the switching elements  15  and  16  thereby causing the push-pull switching circuit  4  to start its operation. Then, the high-voltage transformer  5  is driven by the push-pull switching circuit  4 , and the input current B starts to flow as shown in  FIG. 3D  thereby causing the output current D to flow at the secondary side of the high-voltage transformer  5  as shown in  FIG. 3F , thus the discharge lamp  11  is lighted.  
         [0047]     In the discharge lamp lighting apparatus  1  having the operation sequence described above, the moment (time t 0 ) the input voltage Vi is applied, the output voltage is stepped up to the boosted voltage Vj prescribed, and the charging of the smoothing capacitor  7  is finished. Therefore, when the on-off switching circuit  6  is turned on (time t 1 ), no rush current is superposed on the input current B and the output current D, thus giving no stress on the electrode of the discharge lamp  11  and consequently preventing the shortened life of the discharge lamp  11 . Also, since no rush current flows in the discharge lamp  11 , there is no need to implement a delayed operation of a step-up circuit as conventionally done for suppressing a rush current. Consequently, the waveform of the output current D for the discharge lamp  11  makes a sharp rise, which results in rapid and stable lighting of the discharge lamp  11 .  
         [0048]     Thus, the switch signal buffering circuit  8  functions as a buffer to operate the push-pull switching circuit  4  in conjunction with the startup of the on-off switching circuit  6  after the boosting operation of the step-up circuit  3  is completed, and the respective operations of the step-up circuit  3  and the push-pull switching circuit  4  can be performed independently from each other by one single circuit (the single PWM control circuit  2 ), rather than two separate control circuits.  
         [0049]     The reference voltage shifting circuit  9  will now be described. Since the discharge lamp  11  is not lighted at the above-described standby-state (period t 0 -t 1  in  FIGS. 3A  to  3 G), the load on the step-up circuit  3  is light. Therefore, the step-up circuit gate signal GC, namely, the gate signal C for the switching element  13  of the step-up circuit  3  has a pulse wave with a very small on-duty as shown in  FIG. 3E . Consequently, when the on-off switching circuit  6  is turned on, the step-up circuit gate signal GC, which has such a very small on-duty, cannot gain a sufficient power to drive the discharge lamp  11  thus possibly failing to successfully light the discharge lamp  11 . This problem is solved by providing the reference voltage shifting circuit  9  which generates the step-up circuit gate signal GC to duly enable lighting the discharge lamp  11 .  
         [0050]     The reference voltage shifting circuit  9  operates as follows. The reference signal formed by dividing the reference voltage Vref, and the feedback signal FB from the step-up circuit  3  are inputted to the PWM control circuit  2 , and the pulse width of the step-up circuit gate signal GC is modulated according to the difference voltage between the reference signal and the feedback signal FB and outputted to the step-up circuit  3 . Then, the reference voltage shifting circuit  9  supplies the power supply voltage Vcc to the PWM control circuit  2  by the function of the capacitor  25  only for the moment the on-off switching circuit  6  is turned on, thereby boosting the reference voltage up to a voltage higher than the prescribed voltage, which results in increasing the difference voltage between the reference signal and the feedback signal FB from the step-up circuit  3 , which are inputted to the PWM control circuit  2 . This causes the PWM control circuit  2  to determine that the boosted voltage Vj of the step-up circuit  3  is lowered, and therefore the on-duty of the step-up circuit gate signal GC, that is the on-duty of the gate signal C for the switching element  13  of the step-up circuit  3  is increased as shown in  FIG. 3E . As a result, the output of the step-up circuit  3  is caused to increase temporarily thereby stably lighting the discharge lamp  11 .  
         [0051]     In the discharge lamp lighting apparatus  1  described above, the discharge lamp  11  is preferably a rare gas discharge lamp utilizing dielectric barrier discharge. Also, the switch circuit  4  is not necessarily a push-pull switching circuit, and may be of, for example, a full-bridge or half-bridge structure, or may alternatively be constituted by one single switching element.