Abstract:
The present invention relates to a lamp lighting apparatus including a voltage control section for supplying a pulse voltage whose peak value rises stepwise, and a power conversion section for converting an output of the voltage control section into a high voltage and applying the high voltage to a lamp to light the lamp and for supplying, after the lamp is lit once, self-sustaining a discharge current to the lamp.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    This invention relates to a lamp lighting apparatus and a projector which uses the lamp lighting apparatus, and more particularly to a lamp lighting apparatus which uses a metal halide lamp or the like and a projector which uses the lamp lighting apparatus.  
           [0002]    A lamp lighting apparatus generates a high voltage for lighting and applies the high voltage to both of electrodes of a lamp to cause glow discharge to be produced by a glow switch or the like provided in the lamp. Then, the glow discharge changes into arc discharge in an arc tube to light the lamp.  
           [0003]    A typical lamp lighting apparatus shown in FIG. 8 includes an active filter, for example. Referring to FIG. 8, the lamp lighting apparatus comprises a down converter  52  which receives a DC voltage E 1  normally of approximately 300 to 400 Vdc from a DC power supply and converts the DC voltage E 1  once into a DC voltage V 1  of approximately 50 to 100 Vdc, a controller  53  which inputs a control signal C 1  obtained by comparing a detection power W 1  from a power detection section  59  hereinafter described and a reference power W 2  with each other to the down converter  52  to control the DC voltage V 1  to keep a fixed power, a full bridge  54  which converts the DC voltage V 1  of approximately 50 to 100 Vdc into an AC current having a frequency of approximately 90 to 200 Hz which is necessary to keep discharge of a lamp  62  and supplies the resulting AC current to the lamp  62 , a controller  55  which sets a reference frequency fs of a drive signal for driving the full bridge  54  and controls on/off of the drive signal, an igniter  56  including an ignition outputting transformer T 2  which generates a high voltage of 5 to 20 kV for lighting of the lamp  62  and transmits an AC voltage V 2  of approximately 50 to 100 V for keeping of arc discharge, a voltage control section  60  for controlling the igniter  56 , a voltage detection section  57  which detects a voltage value of the AC voltage V 2  to be supplied from the full bridge  54  to the igniter  56 , a current detection section  59  which detects a current value of AC current I 2  to be supplied from the full bridge  54  to the igniter  56 , a power detection section  58  which detects a detection power W 1  from the voltage detected by the voltage detection section  57  and the current detected by the current detection section  58 , and a connector  61  for establishing a connection to the lamp  62 . It is to be noted that, when the present lamp lighting apparatus is applied to a projector, the controller  53  can be used also as a system controller, and if a power switch not shown is depressed and the controller  53  discriminates a power supply off state, then the controller  53 , for example, opens a contact of a relay to switch off supply of the DC voltage E 1  and enters a standby state. Then, if the power switch is depressed again and the controller  53  discriminates a power supply on state, then the present lamp lighting apparatus enters a normal operation mode and closes the contact of the relay to supply the DC voltage E 1 .  
           [0004]    The voltage control section  60  performs control for producing a high voltage of 5 to 20 kV upon lighting of the lamp  62 . An exemplary configuration of the voltage control section  60  is shown in FIG. 9. Referring to FIG. 9, the voltage control section  60  shown includes a resister R 31  and a capacitor C 31  which form a series circuit to which a DC power supply DC of 300 Vdc is supplied, a boosting transformer T 31  connected to a node between the resister R 31  and the capacitor C 31  through a switching element H 31  such as a SIDAC, a diode D 31  and a discharging gap element H 32  which are connected to a secondary winding N 2  of the boosting transformer T 31  and form a series circuit, and a capacitor C 32  which is connected to a node between the diode D 31  and the discharging gap element H 32  and which forms a parallel circuit together with the secondary winding N 2  of the boosting transformer T 31 . The output side of the discharging gap element H 32  is connected to a terminal T 5 , and the output side of the secondary winding N 2  of the boosting transformer T 31  and the capacitor C 32  which form the parallel circuit is connected to a terminal T 6 .  
           [0005]    Referring back to FIG. 8, the full bridge  54  is formed from a full bridge including, for example, field effect transistors (FETs) or the like. The gates of the FETs are controlled between on and off based on the reference frequency fs set by the controller  55 .  
           [0006]    Consequently, the full bridge  54  can convert the DC voltage V 1  into an AC current having a frequency of approximately 90 to 200 Hz and can supply an AC current I 2  necessary to keep lighting of the lamp  62  to the lamp  62  through the igniter  56  and the connector  61  which has a Lo terminal and a Hi terminal.  
           [0007]    Referring to FIGS. 8 and 9, in the voltage control section  60  having the connection scheme described above, a voltage of 300 Vdc of the DC power supply first charges the capacitor C 31  through the resister R 31 . Then, if the charging voltage of the capacitor C 31  reaches, for example, 200 V, then the switching device H 31  enters into a conducting state, and as a result, an excitation current flows to the primary side of the boosting transformer T 31  while the capacitor C 31  discharges. The charging voltage of the capacitor C 31  drops as a result of the discharge just described, and finally, the switching device H 31  enters into a non-conducting state and the excitation current does not flow to the boosting transformer T 31  any more. Then, the voltage from the DC power supply charges the capacitor C 31  again through the resister R 31 .  
           [0008]    By the repeating cycle described above, a pulse voltage raised to 2 to 3 kV can be repetitively obtained on the secondary side of the boosting transformer T 31 .  
           [0009]    The pulse voltage repetitively charges the capacitor C 32  through the diode D 31  on the secondary side of the boosting transformer T 31 , and as a result, a charging voltage of the capacitor C 32  gradually rises. If this charging voltage reaches, for example, 1 kV, then the discharging gap element H 32  starts discharge and an excitation current flows to the primary side of the outputting transformer T 2  of the igniter  56  while the capacitor C 32  discharges. The charging voltage drops as the capacitor C 32  discharges, and finally, the discharging gap element H 32  stops the discharge and the excitation current does not flow to the outputting transformer T 2 . Then, the raised pulse voltage charges the capacitor C 32  again through the diode D 31 .  
           [0010]    By the repeating cycle described above, a pulse voltage raised to, for example, 5 kV can be obtained on the secondary side of the outputting transformer T 2  and the lamp  62  is lit.  
           [0011]    The lamp  62  may be, for example, a discharge lamp  11  such as a metal halide lamp shown in FIG. 10. Referring to FIG. 10, the discharge lamp  11  has a pair of electrodes  26  and  27  of the same structure arranged in a spaced relationship from each other in a translucent airtight vessel  25  formed from, for example, a heat resisting glass material so that they may have a predetermined electrode space distance L therebetween. The electrodes  26  and  27  are driven with an AC current.  
           [0012]    The electrodes  26  and  27  are connected to metal conductors  30  and  31  sealed in seal sections  28  and  29  formed at both ends of the airtight vessel  25 . The metal conductor  30  is connected to a terminal Hi of a connector  17  and the other metal conductor  31  is connected to another terminal Lo. Further, the seal section  28  of the airtight vessel  25  is fixed to a central portion of a reflector  32  having a hemispherical shape.  
           [0013]    Then, a discharging medium which includes, for example, rare gas, cesium, rare earth metal, and halogen in addition to mercury and wherein the encapsulated amount of cesium is within a predetermined range is encapsulated in the airtight vessel  25 . Thus, since steam of the several kinds of metals is included in a discharging arc of the mercury steam, peculiar optical spectra to the metals are emitted.  
           [0014]    Consequently, the light emission efficiency is high, and a desired light emission characteristic can be obtained depending upon a combination of metals. Further, also the color temperature is high, and also the color is near to that of the natural light and a point-source light and parallel light can be extracted readily using a reflector or the like. Due to the advantages just described, the lamp  62  is utilized for a liquid crystal projector for which a high picture quality and a high luminance are required.  
           [0015]    However, in the high voltage discharging lamp having such a configuration as described above, the discharge starting voltage is as low as, for example, 5 kV in a state wherein the temperature is low because of lapse of time after it is extinguished. However, in another state wherein the temperature of the lamp is high as at an instance such as immediately after it is extinguished, since the discharge starting voltage is high because the steam pressure is high, the lamp cannot be lit again with a voltage equal to that in the state wherein the temperature of the lamp is low. Only after the temperature drops as the time elapses and the steam pressure drops, discharging becomes possible and the lamp is lit again.  
           [0016]    In this manner, in order to make it possible for the lamp to be lit even in a state wherein the temperature thereof is high immediately after it is extinguished, an unnecessarily high voltage is applied to the lamp.  
           [0017]    Further, in the voltage control section  60  which performs ignition as described above with reference to FIG. 9, since discharge is started when the charging voltage of the capacitor C 32  reaches the discharge starting voltage of the discharging gap element H 32 , the lighting voltage of the lamp depends upon the characteristic of the discharging gap element H 32 . Therefore, the voltage control section  60  of the lamp lighting apparatus has a drawback in that the discharging gap element H 32  must be selected suitably for the lamp.  
           [0018]    Accordingly, it is demanded to provide a lamp lighting apparatus which can light a lamp any time without the necessity for selection of a discharging gap element and without application of an unnecessarily high voltage.  
         SUMMARY OF THE INVENTION  
         [0019]    In order to attain the object described above, according to an aspect of the present invention, there is provided a lamp lighting apparatus comprising voltage controlling means for supplying a voltage whose peak value rises stepwise, and power conversion means for converting an output of the voltage controlling means into a high voltage and applying the high voltage to a lamp to light the lamp and for supplying, after the lamp is lit, an AC current necessary to keep discharge of the lamp to the lamp.  
           [0020]    In the lamp lighting apparatus, a high pulse voltage whose peak value is controlled to rise stepwise by the voltage controlling means is supplied to the lamp, whatever characteristic the lamp has, the lamp can be lit with a necessary but minimum high pulse voltage conforming with the characteristic of the lamp. Consequently, the burden on the lamp can be moderated and the life of the lamp can be increased.  
           [0021]    Further, a high pulse voltage whose peak value is controlled to rise stepwise by the voltage controlling means is supplied to the lamp, only a minimum voltage is applied to a winding of the power conversion means, and consequently, otherwise possible deterioration by a high voltage can be prevented.  
           [0022]    Furthermore, a high pulse voltage whose peak value is controlled to rise stepwise by the voltage controlling means is supplied to the lamp, the lamp can be replaced by another lamp which may have any different discharge starting voltage. This augments the convenience in use and makes the process of manufacture flexible. Also augmentation in service can be anticipated.  
           [0023]    In addition, a high pulse voltage whose peak value is controlled to rise stepwise by the voltage controlling means is supplied to the lamp, when the lamp has a low temperature because time has passed after it was extinguished last, it can be lit with a low lighting voltage, but when the same lamp has a high temperature because much time has not passed after it was extinguished last, it can be lit with a corresponding high voltage. Consequently, the convenience in use can be augmented.  
           [0024]    Where the voltage controlling means includes a charging circuit for supplying the charging voltage, a voltage dividing circuit for dividing the output of the charging circuit and a circuit for lowering the dividing ratio of the voltage dividing circuit stepwise, since the output of the voltage dividing circuit increases the time required until the predetermined voltage is reached, the peak value of the charging voltage can be increased stepwise.  
           [0025]    According to another aspect of the present invention, there is provided a projector comprising voltage controlling means for supplying a voltage whose peak value rises stepwise, power conversion means for converting an output of the voltage controlling means into a high voltage and applying the high voltage to a lamp to light the lamp and for supplying, after the lamp is lit, an AC current necessary to keep discharge of the lamp to the lamp to keep the lamp in the lit state, a display panel for transmitting light therethrough or reflecting light emitted from the lamp lit by the power conversion means, and an optical system for projecting the output light of the display panel.  
           [0026]    In the projector, a high pulse voltage whose peak value is controlled to rise stepwise by the voltage controlling means is supplied to the lamp, even when the lamp is not sufficiently cooled immediately after the power supply to the projector is stopped, the power supply can be made available to light the lamp. Consequently, the projector is augmented in convenience in use.  
           [0027]    The above and other objects, features and advantages of the present invention will become apparent from the following description and the appended claims, taken in conjunction with the accompanying drawings in which like parts or elements denoted by like reference symbols. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0028]    [0028]FIG. 1 is a schematic view showing a configuration of a voltage control section which is a component of a lamp lighting apparatus according to a first embodiment of the present invention;  
         [0029]    [0029]FIGS. 2A through 2E and  3 A through  3 E are waveform diagrams illustrating operation of the voltage control section shown in FIG. 1;  
         [0030]    [0030]FIG. 4 is a schematic view showing a configuration of a voltage control section which is a component of a lamp lighting apparatus according to a second embodiment of the present invention;  
         [0031]    [0031]FIG. 5 is a waveform diagram illustrating operation of the voltage control section shown in FIG. 4;  
         [0032]    [0032]FIG. 6 is a schematic view showing a configuration of a projector according to a third embodiment of the present invention;  
         [0033]    [0033]FIG. 7 is a schematic view showing a configuration of a projector according to a fourth embodiment of the present invention;  
         [0034]    [0034]FIG. 8 is a block diagram showing a configuration of a related-art lamp lighting apparatus;  
         [0035]    [0035]FIG. 9 is a schematic diagram showing a configuration of a voltage control section of the lamp lighting apparatus of FIG. 8; and  
         [0036]    [0036]FIG. 10 is a sectional view schematically showing a configuration of a lamp shown in FIG. 8. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0037]    In the following, a lamp light apparatus and a projector having the lamp lighting apparatus incorporated therein according to the present invention are described with reference to the accompanying drawings. The lamp lighting apparatus according to a first embodiment of the present invention has a general configuration similar to that described hereinabove with reference to FIGS.  8  to  10 , and therefore, it is not shown in the drawings to avoid redundancy, and for description of the lamp lighting apparatus, reference is had also to FIGS.  8  to  10 . The lamp lighting apparatus according to the first embodiment of the present invention is different from the lamp lighting apparatus of FIGS.  8  to  10  in that it includes a voltage control section  19  for controlling lighting of a lamp in place of the voltage control section  60 .  
         [0038]    The voltage control section  19  is particularly shown in FIG. 1. Referring to FIG. 1, the voltage control section  19  includes a power supply EB for supplying a DC voltage Vcc of, for example, 15 Vdc, a switch Sw, a multivibrator circuit  35  for generating a rectangular wave voltage V 3  of a predetermined oscillation frequency, a bootstrap circuit  36  for generating a triangular wave voltage V 4 , a comparator circuit  37  for comparing the triangular wave voltage V 4  and a comparison voltage Vc with each other, an inversion circuit  38  for inverting a pulse signal voltage V 5  from the comparator circuit  37  to produce an inverted voltage V 6 , and an outputting circuit  39 .  
         [0039]    The multivibrator circuit  35  may be a multivibrator circuit which is used commonly, and the DC voltage Vcc from the power supply E B  is applied to power supply terminals T 7  and T 8  of the multivibrator circuit  35  through the switch Sw. The multivibrator circuit  35  outputs the rectangular wave voltage V 3  of an oscillation frequency f (≈0.7·C 0 ·R 0 ) which depends upon a capacitor C 0  and a resistor R 0  built therein.  
         [0040]    The bootstrap circuit  36  may be a bootstrap circuit which is used commonly, and converts a rectangular wave signal of the rectangular wave voltage V 3  inputted thereto from the multivibrator circuit  35  into a triangular wave signal with a time constant which depends upon a capacitor C 1  and a resistor R 1  built therein and outputs the triangular wave signal as the triangular wave voltage V 4 .  
         [0041]    The triangular wave voltage V 4  of the triangular wave signal is inputted to a noninverting input terminal (+) of an operational amplifier Q 1  which composes the comparator circuit  37  while a comparison voltage Vc obtained by division of the DC voltage Vcc by a resistor R 2  and a combined resistance of another resistor R 3  and a capacitor C 2  is applied to an inverting input terminal (−) of the operational amplifier Q 1 . When the triangular wave voltage V 4  exceeds the comparison voltage Vc, a pulse signal voltage V 5  of a rectangular waveform is outputted from an output terminal of the operational amplifier Q 1 .  
         [0042]    The inversion circuit  38  may be composed of a transistor Q 2  and so forth. The pulse signal voltage V 5  of a rectangular wave signal is inputted to the base of the transistor Q 2  while an inverted voltage V 6  of the rectangular wave signal having a polarity inverted from that of the pulse signal voltage V 5  is obtained at the collector of the transistor Q 2 .  
         [0043]    The outputting circuit  39  is composed of an insulated gate bipolar transistor (IGBT) Q 3 . The inverted voltage V 6  from the inversion circuit  38  is applied to the gate of the insulated gate bipolar transistor Q 3 , and the positive side of a DC power supply E P  is connected to the collector of the insulated gate bipolar transistor Q 3  through the primary winding N 1  of the ignition outputting transformer T 2 .  
         [0044]    Now, operation of the voltage control section  19  having the configuration described above is described with reference FIGS. 1, 2 and  3 .  
         [0045]    First, if the power button not shown is depressed and the system controller discriminates a power supply on state, then the switch Sw is switched on so that the DC voltage Vcc is supplied from the power supply E B  to the voltage control section  19 . Consequently, the multivibrator circuit  35  is rendered operative, and a rectangular wave voltage V 3  of a rectangular wave signal having an amplitude which increases as the time passes and having a predetermined frequency f as seen from FIG. 2A is outputted from the output terminal of the multivibrator circuit  35 .  
         [0046]    The rectangular wave voltage V 3  of the rectangular wave signal is inputted to the bootstrap circuit  36  and converted into a triangular wave voltage V 4  as shown in FIG. 2B, which is outputted from the output terminal of the bootstrap circuit  36 .  
         [0047]    The triangular wave voltage V 4  is inputted to the comparator circuit  37 , by which it is compared with comparison voltages Vc 1  and Vc 2  produced from the comparison voltage Vc as seen from FIG. 2C. Consequently, a pulse signal voltage V 5  of a rectangular wave signal (FIGS. 2D and 2E) is outputted from the output terminal of the comparator circuit  37 .  
         [0048]    Since the comparison voltage Vc appearing across the capacitor C 2  increases, for example, from the comparison voltage Vc 2  to the comparison voltage Vc 1  with respect to the triangular wave voltage V 4  as seen from FIG. 2C, the pulse width of the pulse signal voltage V 5  of the output of the comparator circuit  37  exhibits different values depending upon with which one of the comparison voltages Vc 2  and Vc 1  the triangular wave voltage V 4  is compared. For example, if the triangular wave voltage V 4  is compared with the comparison voltage Vc 1 , then the pulse width of the pulse signal voltage V 5  is small as seen from FIG. 2D, but if the triangular wave voltage V 4  is compared with the comparison voltage Vc 2 , then the pulse width of the pulse signal voltage V 5  is great as seen from FIG. 2E.  
         [0049]    In this instance, if the switch Sw is switched on, then since the capacitor C 2  of the comparator circuit  37  shown in FIG. 1 is charged from the DC voltage Vcc, as the time t passes, the comparison voltage Vc rises from 0 V finally to a voltage VM which depends upon Vcc*[(R 3 )/(R 2 +R 3 )].  
         [0050]    Since the comparison voltage Vc rises as the time t passes in this manner, the pulse width of the pulse signal voltage V 5  decreases as the time t passes in a corresponding relationship as seen from FIG. 3B.  
         [0051]    Then, the pulse signal voltage V 5  is inverted by the inversion circuit  38  and inputted as such an inverted voltage V 6  as seen from FIG. 3C to the gate of the insulated gate bipolar transistor Q 3  of the outputting circuit  39 .  
         [0052]    The insulated gate bipolar transistor Q 3  of the outputting circuit  39  exhibits a collector current I L  (FIG. 3D) whose peak current value increases in proportion to the pulse width of the inverted voltage V 6 . When the insulated gate bipolar transistor Q 3  is turned off, avalanche breakdown occurs with the insulated gate bipolar transistor Q 3  due to energy accumulated in the inductance L 1  of the primary winding N 1  of the outputting transformer T 2 , and thereupon, an induced voltage V L  (FIG. 3E) corresponding to the winding number ratio (N 2 /N 1 ) of the ignition outputting transformer T 2  is generated in the secondary winding N 2 .  
         [0053]    The voltage to be generated when the insulated gate bipolar transistor Q 3  is turned off is represented generally by L(di/dt) where the inductance is represented by L and the collector current is represented by i, and the energy value then is represented by 1/2*L*i 2 . Both of the voltage and the energy value can be controlled with the inductance L and the collector current i.  
         [0054]    When the pulse width of the inverted voltage V 6  applied to the insulated gate bipolar transistor Q 3  is small as shown in FIG. 3D, the collector current i of the insulated gate bipolar transistor Q 3  is low and also the energy accumulated in the inductance L is low. Therefore, also the induced voltage V L  generated in the secondary winding N 2  of the ignition outputting transformer T 2  when the insulated gate bipolar transistor Q 3  is turned off is low as seen from FIG. 3E.  
         [0055]    On the contrary, when the pulse width of the inverted voltage V 6  applied to the insulated gate bipolar transistor Q 3  is great, the collector current I L  of the insulated gate bipolar transistor Q 3  is high and also the energy accumulated in the inductance L 1  is high. Therefore, the induced voltage V L  generated in the secondary winding N 2  of the ignition outputting transformer T 2  when the insulated gate bipolar transistor Q 3  is turned off is high.  
         [0056]    Since the pulse width of the inverted voltage V 6  to be applied to the insulated gate bipolar transistor Q 3  gradually increases as seen from FIG. 3D, also the induced voltage V L  generated in the secondary winding N 2  gradually rises.  
         [0057]    In particular, since the induced voltage V L  gradually rises, also where a lamp having a different discharge voltage characteristic is adopted for the lamp  11 , discharge can be started at some voltage point of the induced voltage V L , and the lamp  11  can be lit flexibly in accordance with the characteristic thereof. If is to be noted that, since the role of the voltage control section  19  is completed when discharge is started, if the system controller discriminates that the lamp is lit, then it switches off the switch Sw to stop the supply of the DC voltage Vcc from the power supply E B  thereby to stop the operation of the voltage control section  19 .  
         [0058]    Usually, since various lamps start discharge thereof within a range of approximately 5 to 20 kV, as seen in FIG. 3E, the induced voltage V L  at (1) is 5 kV and the induced voltage V L  at (4) is 20 kV, and at (2) and (3) between (1) and (4), the induced voltage is 10 kV and 15 kV, respectively.  
         [0059]    It is to be noted that the pulse width and the repeating interval of the inverted voltage V 6  inputted to the insulated gate bipolar transistor Q 3  can be changed or selected freely and also the inductance values, the winding number ratio and so forth of the primary winding N 1  and the secondary winding N 2  of the ignition outputting transformer T 2  of the igniter  16  can be selected arbitrarily for various lamps.  
         [0060]    Now, another voltage control section  19  which is incorporated in the lamp lighting apparatus according to a second embodiment of the present invention is described with reference to FIGS. 4 and 5.  
         [0061]    Referring to FIG. 4, the voltage control section  19  includes a charging circuit for supplying a voltage charged in a capacitor C 12 , a voltage dividing circuit for dividing the charging voltage, and a control circuit for controlling so that the charging voltage is applied to the primary winding N 1  of the ignition outputting transformer T 2  when the divided voltage obtained by the voltage dividing circuit becomes equal to a predetermined voltage, and successively inserts resistors R 19 , R 20  and R 21  in parallel to a resistor R 13  after each predetermined interval of time to increase the time required to charge the capacitor C 12  every time thereby to raise the voltage at a point Q stepwise.  
         [0062]    The charging circuit includes a capacitor C 11  to which a DC power supply DC of 300 Vdc is supplied through a resistor R 11 , a boosting transformer T 11  connected to a node between the resistor R 11  and the capacitor C 11  through a switching element H 11  such as a SIDAC, and a diode D 11  and a capacitor C 12  connected to the secondary winding N 2  of the boosting transformer T 11  and forming a series circuit. A node between the diode D 11  and the capacitor C 12  is connected to the terminal T 5  of the igniter  16 .  
         [0063]    The voltage control section  19  includes a capacitor C 13  to which the DC voltage Vcc of, for example, 15 Vdc is supplied through a resistor R 15 , a resistor R 16  connected to a node between the resistor R 15  and the capacitor C 13  and forming a parallel circuit together with the capacitor C 13 , a switching element Q 13  connected at the base thereof to the node between the resistor R 15  and the capacitor C 13  and connected at the collector thereof to a resistor R 19 , a capacitor C 14  which forms a series circuit together with the resistor R 15  through which the DC voltage Vcc is supplied, a resistor R 17  which forms a parallel circuit together with the capacitor C 14 , a switching element Q 14  connected at the base thereof to the node between the resistor R 15  and the capacitor C 14  and connected at the collector thereof to a resistor R 20 , a capacitor C 15  which forms a series circuit together with the resistor R 15  through which the DC voltage Vcc is supplied, a resistor R 18  which forms a parallel circuit together with the capacitor C 15 , a switching element Q 15  connected at the base thereof to the node between the resistor R 15  and the capacitor C 15  and connected at the collector thereof to a resistor R 21 , a switching element Q 12  for discharging charge accumulated in the capacitors C 13 , C 14  and C 15  when a reset voltage is applied thereto at a predetermined timing, a voltage dividing circuit formed from a resistor R 12  and the resistor R 13  connected between a point Q and the grounding point of the charging circuit, a discharging element H 12  and a resistor R 14  which form a series circuit connected to the node between the resistor R 12  and the resistor R 13 , and a thyristor Q 11  connected at the gate thereof to a node between the discharging element H 12  and the resistor R 14 . The anode side of the thyristor Q 11  is connected to the terminal T 6  connected to the primary winding N 1  of the ignition outputting transformer T 2  of the igniter  16 .  
         [0064]    The resistor R 15  for supplying the DC voltage Vcc cooperates with the capacitors C 13 , C 14  and C 15  to form three charging circuits, and the charging voltages charged in the charging circuits are supplied to the bases of the switching elements Q 13 , Q 14  and Q 15 , respectively. When any of the supplied voltages reaches the base driving voltage of the corresponding one of the switching elements Q 13 , Q 14  and Q 15 , then it is turned on and connects the parallel resistors R 19 , R 20  and R 21  to the resistor R 13 . Consequently, the potential at the point P drops.  
         [0065]    Here, if it is assumed that the capacitance values of the capacitors C 13 , C 14  and C 15  have a relationship of C 13 &lt;C 14 &lt;C 15 , then the switching elements Q 13 , Q 14  and Q 15  are successively turned on in order of the switching elements Q 13 , Q 14  and Q 15 .  
         [0066]    By successively turning on the switching elements Q 13 , Q 14  and Q 15  in this order, the potential at the point P can be lowered stepwise, and as a result, the timing at which the thyristor Q 11  is to be turned on can be delayed successively. Since the delaying of the timing at which the thyristor Q 11  is turned on increases the time for which the capacitor C 12  is charged, the voltage charged in the capacitor C 12  becomes higher, and the voltage to be supplied to the primary winding N 1  of the ignition outputting transformer T 2  when the thyristor Q 11  is turned on becomes higher. As a result, the voltage obtained at the secondary winding N 2  rises.  
         [0067]    In short, as charging of the capacitors C 13 , C 14  and C 15  proceeds through the resistor R 15  and the switching elements Q 13 , Q 14  and Q 15  are successively turned on, the output voltage of the ignition outputting transformer T 2 , that is, the voltage to be supplied to the lamp  11 , can be raised gradually. It is to be noted that the charging operation through the resistor R 15  can be reset by causing the switching element Q 12  to be turned on.  
         [0068]    Since the switching elements Q 13 , Q 14  and Q 15  are successively switched on in this order as the time passes, the voltage to be supplied to the lamp  11  rises stepwise in this manner. Since the lamp  11  is lit when the voltage supplied to the lamp  11  reaches the discharge starting voltage necessary for starting of lighting of the lamp  11 , even if the lamp has a high discharge starting voltage, it can be lit as the time passes. Therefore, even if various lamps having different characteristics are applied as the lamp  11 , there is no necessity to take the discharge starting voltage of the lamp  11  into consideration every time.  
         [0069]    Operation of the voltage control section  19  shown in FIG. 4 is described with reference also to a waveform diagram of FIG. 5.  
         [0070]    First, when the system controller is in its standby state, a reset voltage is continuously applied from the system controller to the switching element Q 12 , and consequently, the switching element Q 12  is in an on state. Therefore, the capacitors C 13 , C 14  and C 15  remain in a discharged state and have no charge therein. If the power button not shown is depressed and the system controller discriminates a power supply on state, then the system controller enters into a normal operation mode and ends application of the reset voltage. Consequently, the switching element Q 12  is turned off, and charging of the capacitors C 13 , C 14  and C 15  through the resistor R 15  is started. When the capacitors C 13 , C 14  and C 15  are successively charged up until predetermined voltages are reached as seen in FIG. 5, the switching elements Q 13 , Q 14  and Q 15  are successively switched on in this order.  
         [0071]    Here, the potential at the point P when the switching element Q 13  is not on is equal to a potential obtained by dividing the potential at the point Q by means of the resistor R 12  and the resistor R 13 .  
         [0072]    If the voltage of the capacitor C 11  charged reaches the predetermined value, then the switching element H 11  is rendered conducting, and the primary winding of the boosting transformer T 11  is excited. Consequently, an induced voltage of 2 to 3 kV is generated in the secondary winding of the boosting transformer T 11 , and charging (1) for the first time of the capacitor C 12  is started through the diode D 11 . Consequently, the potential at the point Q rises, and also the potential at the point P rises.  
         [0073]    If this state continues until the charging voltage of the capacitor C 13  reaches the voltage at which the switching element Q 13  is to be turned on, then the switching element Q 13  is turned on. Consequently, the resistor R 19  is connected in parallel to the resistor R 13 , and the voltage at the point P drops by a voltage corresponding to the resistor R 19 . When the resistor R 19  is connected in parallel to the resistor R 13 , since the load becomes heavier, also the voltage at the point Q drops.  
         [0074]    If the charging voltage of the capacitor C 11  rises in this state until the switching element H 11  becomes conducting, then the primary winding of the boosting transformer T 11  is excited and an induced voltage generated in the secondary winding charges the capacitor C 12  through the diode D 11  again. Consequently, the potential at the point Q rises, and also the voltage at the point P rises. When the voltage at the point P reaches the discharge starting voltage of the discharging element H 12 , the discharging element H 12  starts its discharge. Consequently, a voltage is applied to the gate of the thyristor Q 11 , and the thyristor Q 11  is turned on. When the thyristor Q 11  is turned on, an excitation current flows through the primary winding N 1  of the ignition outputting transformer T 2  due to the voltage V 1  charged in the capacitor C 12 , and thereupon, an induced voltage is generated in the secondary winding N 2  of the ignition outputting transformer T 2  and applied to the lamp  11 . The voltage charged in the capacitor C 12  is discharged when the thyristor Q 11  is turned on, and thereupon, the voltages at the points Q and P drop. Consequently, the discharging by the discharging element H 12  stops and the thyristor Q 11  is switched off, and charging (2) for the second time of the capacitor C 12  is started.  
         [0075]    Since the resistor R 19  is already connected in parallel to the resistor R 13  when charging of the capacitor C 12  is started in the charging (2) for the second time, the voltage at the point P is lower than that when the charging for the first time is started. Therefore, the time t 2  after which the thyristor Q 11  is turned on is longer than the time t 1  required for the charging for the first time.  
         [0076]    While the charging (2) for the second time is started in this manner, charging of the capacitor C 14  proceeds. Then, when the charging voltage with which the switching element Q 14  is turned on is reached, then the switching element Q 14  is turned on. Consequently, the resistor R 20  is connected further in parallel to the parallel circuit of the resistor R 13  and the resistor R 19 , and the voltage at the point P drops by an amount corresponding to the resistor R 20 . When the resistor R 20  is connected in parallel in this manner, also the voltage at the point Q drops because the load becomes heavier. If the capacitor C 12  is charged in this manner, then the voltage at the point P reaches the voltage with which the discharging element H 12  discharges after an excess interval of time provided by the resistor R 20  is required.  
         [0077]    Then, discharging of the discharging element H 12  is started and an on-voltage is applied to the gate of the thyristor Q 11  to turn on the thyristor Q 11 . After the thyristor Q 11  is turned on, an excitation current flows through the primary winding N 1  of the ignition outputting transformer T 2  due to the voltage V 2  charged in the capacitor C 12 , and an induced voltage is generated in the secondary winding N 2  of the ignition outputting transformer T 2 . Consequently, the voltage V 2  higher than the voltage for the first time can be supplied to the lamp  11 .  
         [0078]    Then, the voltage charged in the capacitor C 12  is discharged as a result of the turning on of the thyristor Q 11 . Consequently, since the voltages at the points Q and P drop, the discharging by the discharging element H 12  stops, and the thyristor Q 11  is turned off and charging (3) for the third time of the capacitor C 12  is started.  
         [0079]    While the charging (3) for the third time of the capacitor C 12  is started in this manner, the charging of the capacitor C 15  proceeds. Then, when the voltage of the capacitor C 15  reaches the charging voltage with which the switching element Q 15  is turned on, the switching element Q 15  is turned on. Consequently, the resistor R 21  is further connected in parallel to the parallel circuit of the resistors R 13 , R 19  and R 20 , and the voltage at the point P drops by an amount corresponding to the resistor R 21 . As the capacitor C 12  is charged in this state, the voltage at the point P reaches the voltage with which the discharging element H 12  is discharged after lapse of additional time corresponding to the drop of the voltage by the resistor R 21 .  
         [0080]    Thereupon, discharging of the discharging element H 12  is started, and an on-voltage is applied to the gate of the thyristor Q 11  to turn on the thyristor Q 11 . After the thyristor Q 11  is turned on, an excitation current flows through the primary winding N 1  of the ignition outputting transformer T 2  due to the voltage V 3  charged in the capacitor C 12 , whereupon an induced voltage is generated in the secondary winding N 2  of the ignition outputting transformer T 2 . Consequently, an ignition voltage higher than the ignition voltage for the second time can be supplied to the lamp  11 .  
         [0081]    The voltage charged in the capacitor C 12  is discharged when the thyristor Q 11  is turned on. Consequently, since the voltages at the points Q and P drop, the discharging of the discharging element H 12  stops and the capacitor C 12  starts charging again.  
         [0082]    In this manner, when the capacitors C 13 , C 14  and C 15  are charged up to predetermined voltages and the switching elements Q 13 , Q 14  and Q 15  are successively turned on so that the resistors R 19 , R 20  and R 21  are successively connected in parallel to the resistor R 13 , the voltage at the point P drops every time. Therefore, the time required until the thyristor Q 11  is turned on increases every time, and accordingly, the voltage charged in the capacitor C 12  rises stepwise. Accordingly, since the induced voltage generated in the secondary winding N 2  of the ignition outputting transformer T 2  when the thyristor Q 11  is turned on and an excitation current flows through the primary winding N 1  of the ignition outputting transformer T 2  due to the voltage charged in the capacitor C 12  rises stepwise, even where a lamp having a different discharge voltage characteristic is used as the lamp  11 , the lamp  11  can be lit even in a state wherein the temperature thereof is high such as immediately after it is extinguished. It is to be noted that, after the lamp  11  is lit, since application of the reset voltage from the system controller continues and the switching element Q 12  keeps an on state, the capacitors C 13 , C 14  and C 15  are not charged. Further, after the lamp  11  is lit, since the voltage of the DC power supply DC drops, the voltage charged in the capacitor C 11  is so low that the discharging element H 12  cannot be turned on. Therefore, after the lamp  11  is lit, operation of the voltage control section  19  is disabled.  
         [0083]    It is to be noted that, while, in the foregoing description, the DC voltage is raised from the DC voltage of 300 Vdc, first by the boosting transformer T 11  and further by the outputting transformer T 2  in order to obtain an output of the peak voltage of 20 kV if there is no restriction to the size of a transformer, the two transformers can be replaced by a single transformer, for example, if the winding number ratio of the secondary winding to the primary winding is set to 1:100.  
         [0084]    Now, a projector according to a third embodiment of the present invention is described with reference to FIG. 6.  
         [0085]    A projector  40  shown is formed as a projector of the transmission type and includes the lamp lighting apparatus  10  in which the voltage control section  19  according to the first and second embodiments with reference to FIG. 1 or  4  is incorporated.  
         [0086]    The projector  40  of the transmission type includes, in addition to the lamp lighting apparatus  10 , a lamp  11  having a reflector  32  for reflecting light emitted from a light source thereof, a liquid crystal panel  41  of the light transmission type such as, for example, a color liquid crystal panel on which light irradiated from the reflector  32  is projected, a liquid crystal driver  42  for driving the liquid crystal panel  41 , and a housing  43  for accommodating the components therein. The housing  43  has a floodlighting section  44  at a portion thereof opposing to the liquid crystal panel  41 , and a lens  45  serving as an optical system is disposed in the floodlighting section  44 . A power is supplied from a power supply  46  to the lamp lighting apparatus  10  and the liquid crystal driver  42 .  
         [0087]    When a power is supplied to the lamp lighting apparatus  10  in which the voltage control section  19  is incorporated to light the lamp  11 , light is emitted from the lamp  11  and reflected by the reflector  32  so that it is irradiated upon the liquid crystal panel  41 .  
         [0088]    The liquid crystal panel  41  includes R, G and B color filters not shown corresponding to pixels, and the color filters are controlled by the liquid crystal driver  42  which operates in response to a color image signal.  
         [0089]    When the light passes through the liquid crystal panel  41 , it is colored to R, G or B by the color filters. The colored light is condensed by the lens  45  and projected on a screen  47 . Consequently, a color image controlled by the liquid crystal panel  41  is projected on the screen  47 .  
         [0090]    Also in this instance, when the lamp  11  is to be lit, the induced voltage V L  which gradually rises is applied to the lamp  11  to light the lamp  11  in accordance with the characteristic of the lamp  11 . Therefore, also the projector  40  has such a characteristic of the voltage control section  19  that a lamp is lit without application of an unnecessarily high voltage thereto.  
         [0091]    Now, a projector according to a fourth embodiment of the present invention is described with reference to FIG. 7.  
         [0092]    A projector  40 A according to the fourth embodiment of the present invention is formed as a projector of the reflection type and uses the lamp lighting apparatus  10  according to the first and second embodiments with reference to FIG. 1 or  4 .  
         [0093]    The projector  40 A of the reflection type includes, in addition to the lamp lighting apparatus  10  in which the voltage control section  19  is incorporated, a lamp  11  having a reflector  32  for reflecting light emitted from a light source thereof under the control of the lamp lighting apparatus  10 , a first lens section  48   a  for condensing rays of light from the lamp  11 , a reflection type image panel  49  for reflecting the rays of light condensed by the first lens section  48   a , a second lens section  48   b  for projecting the rays of image light reflected by the reflection type image panel  49  onto a screen  47 , and a driving section  50  for receiving RGB signals, a video signal or the like to drive the reflection type image panel  49 .  
         [0094]    The reflection type image panel  49  is formed from a liquid crystal panel of the reflection type or a panel of the DLP (Digital Light Processing) type.  
         [0095]    A panel of the DLP type is a panel which uses a DMD (Digital Micromirror Device). The DMD is formed from a semiconductor optical switch composed of very small movable mirrors. More particularly, the semiconductor optical switch is structured such that a very small mirror of 16 μm square of an aluminum alloy formed on each one of cells of an SRAM (Static Random Access Memory) has an inclination of plus or minus 10 degrees in an on or off state and is supported on a support post and operated by an action of an electrostatic field produced by a memory element disposed immediately therebelow. A number of such mirrors not less than 848×600 are integrated on one chip, and the chip has a degree of integration of not less than 2,300,000 mirrors in the maximum. The switching speed of each mirror is not less than 500,000 times per second, and incoming light to the chip is reflected as light having a digital gray scale. In other words, light can be controlled fully digitally, and there is no necessity for a process of converting a digital signal into a luminance signal by means of a D/A converter as in the related art.  
         [0096]    In the projector  40 A having such a configuration as described above, when the lamp  11  is to be lit, the lamp lighting apparatus  10  in which the voltage control section  19  is incorporated applies a gradually rising induced voltage V L  to the lamp  11  to light the lamp  11  in accordance with the characteristic of the lamp  11 . Therefore, the projector  40 A has such a characteristic of the voltage control section  19  that a lamp is lit without application of an unnecessarily high voltage thereto.  
         [0097]    While preferred embodiments of the present invention have been described using specific terms, such description is for illustrative purposes only, and it is to be understood that changes and variations may be made without departing from the spirit or scope of the following claims.