Patent Publication Number: US-2005116661-A1

Title: Discharge lamp apparatus

Description:
FIELD OF THE INVENTION  
      The present invention relates to a discharge lamp device suited for back light sources for liquid crystal display units, and especially to a power supply source for driving discharge lamps.  
     BACKGROUND TECHNOLOGY  
      Conventional liquid crystal display units are widely used for electronic devices such as personal computers, car navigation systems. In such liquid display devices, cold cathode fluorescent lamps with less heat generation are used as back light sources.  
      Conventionally, a lighting and driving circuit shown in  FIG. 1  is known as a circuit for driving cold cathode fluorescent lamps. That is, as shown in the  FIG. 1 (A), a drive signal circuit  10  connected to a DC power supply  9  outputs a pair of square wave drive signals  11   a  and  11   b  in the circuit. These square wave drive signals  11   a  and  11   b  turn first and second semiconductor switching elements S 1  and S 2 , which constitute an inverter, ON and OFF alternately. These switching elements S 1  and S 2  of each other are connected between the positive terminal V and the ground G of the DC power supply  9  in series. Each of the first and the second semiconductor switching element is connected with induction elements L 3  and L 4  in series respectively for shaping waveforms. A capacitor C 1  and a capacitor C 2  are connected between the positive terminal V of the DC power supply  9  and the ground G in series. Input terminals of the primary coil L 1  of the pulse transformer  12  are connected with a connecting point P of a capacitor C 1  and a capacitor C 2  and with an end Q of a series connected circuit constituting the first switching element S 1  and an induction element L 3 . An external electrode fluorescent lamp  13  is connected between the output terminals of the secondary coil L 2  of the pulse transformer  12 .  
      Then, operation of the lighting and driving circuit is explained. Under the control of the square wave drive signals  11   a  and  11   b  supplied from the drive signal circuit  10 , a current Ia flows in the circuit of the positive terminal V of the DC power supply  9 —the switching element S 1 —the induction element L 3 —the primary coil L 1  of the pulse transformer  12 —the capacitor C 2 —the ground G, when the switching element S 1  is ON and the switching element S 2  is OFF, as shown in  FIG. 1 (A). Under the control of square wave drive signal  11   a  and  11   b , a current Ib flows in the circuit of the positive terminal V of the DC power supply  9 —the capacitor C 1 —the primary coil L 1  of the pulse transformer  12 —the induction element L 4 —the switching element S 2 —the ground G, when the switching element S 1  is OFF and the switching element S 2  is ON, as shown in  FIG. 1 (B). Since such operation is repeated under the control of the square wave drive signals  11   a  and  11   b  supplied from the drive signal circuit  10 , and current Ia and Ib flow by turns in the primary coil L 1  of the pulse transformer  12 , square wave voltage occurs continuously between the output terminals of the secondary coil L 2  of the pulse transformer  12 . This output voltage is applied to the external electrode fluorescent lamp  13  to light and drive it.  
      However, in the lighting and driving device of the discharge lamps mentioned above, there is a problem that a steep and high voltage surge pulse is generated, at the time of ON OFF of the switching elements S 1  and S 2 , by the action of the induction elements L 3  and L 4  having a resistance component. That is, oscillating current is generated at the time of ON and OFF of the switching elements S 1  and S 2 , due to the so-called ringing phenomenon in a LC resonance circuit, which contains capacitor C 1 , the capacitor C 2 , the primary coil L 1  of the pulse transformer  12 , induction elements L 3  and L 4 , etc., and thus the high voltage surge voltage is generated at both ends of the induction elements L 3  and L 4 . For this reason, there is a possibility of destroying the switching elements S 1  and S 2  which constitute the inverter circuit on the primary side of the pulse transformer  12 .  
      On the other hand, a voltage having a waveform containing a high surge pulse S as shown in  FIG. 2  is generated between the output terminals of the secondary side winding L 2  of the pulse transformer  12 , and it is also supplied to the electric discharge lamp  13 . Here, the abscissa of  FIG. 2  indicates time with 5 μs/div, and the ordinate indicates voltage with 5.0V/div. The generation of such steep and high voltage surge pulse voltage may damage safety, reliability or life of the discharge lamp device of this type.  
      There is measure against such high voltage surge pulse to substitute the semiconductor switching elements S 1  and S 2  with those having a withstand voltage exceeding the above-mentioned surge voltage. However, the semiconductor switching element having the withstand voltage exceeding surge voltage becomes large-sized by itself, which does not meet the need of miniaturization desired as a back light of a liquid crystal display.  
      The present invention was made in view of the above-mentioned situation and has an object to provide a discharge lamp device, in which controls or reduces generation of the high voltage surge pulse at the time of lighting, and which is excellent in safety and stability.  
     DISCLOSURE OF THE INVENTION  
      The discharge lamp device according to one aspect of the present invention has a drive signal circuit connected with a DC power supply, first and second switching elements which are turned ON and OFF alternately under the control of a pair of drive signals supplied from the drive signal circuit and are connected in series between the terminals of the DC power supply, an inductance element connected in series with each of the switching elements in series, a pulse transformer having a primary coil and a secondary coil, in which the primary coil is connected to the DC power supply and a direction of current is changed alternately by the switching elements, a discharge lamp which is connected between terminals of the secondary coil of the pulse transformer, and a Zener diode element which is connected in parallel with the second switching element.  
      The discharge lamp device according to another aspect of the present invention has a drive signal circuit which is connected with a DC power supply, first and second switching elements which are turned ON and OFF alternately under the control of a pair of drive signals supplied from the drive signal circuit and are connected in series between the terminals of the DC power supply, first and second inductance elements connected with each of the switching elements in series respectively, a pulse transformer having a primary coil and a secondary coil, in which the primary coil is connected to the DC power supply and a direction of current is changed alternately by the switching elements, a discharge lamp which is connected between terminals of the secondary coil of the pulse transformer, a first uni-directional current element, which is connected in series with the second switching element, and a second uni-directional current element, which is connected in parallel with the first uni-directional element and the second induction element.  
      The discharge lamp device according to yet other aspect of the present invention has a drive signal circuit which is connected with a DC power supply, first and second switching elements which are turned ON and OFF alternately under the control of a pair of drive signals supplied from the drive signal circuit and are connected in series between the terminals of the DC power supply, first and second inductance elements connected with each of the switching elements in series respectively, a pulse transformer having a primary coil and a secondary coil, in which the primary coil is connected to the DC power supply and a direction of current is changed alternately by the switching elements, a discharge lamp which is connected between terminals of the secondary coil of the pulse transformer, a first uni-directional current element, which is connected in series with the second switching element, and a second uni-directional current element, which is connected in parallel with the first uni-directional element.  
      Further, in the discharge lamp device according to yet other aspect of the present invention, the above-mentioned uni-directional current element is one selected from the group consisting of a diode element, a transistor element, an MOSFET element, and a photo-coupler.  
      Further, in the discharge lamp device according to yet other aspect of the present invention, the pair of drive signals supplied from the drive signal circuit is PWM modulated by a light control signal.  
      Thus, in the discharge lamp device according to one aspect of the present invention stated above, the current flowing in an opposite direction is cut or suppressed by providing a uni-directional current element in the current circuit, in which the current flows in the predetermined direction (as shown by arrows Ia and lb) at the time of ON and OFF of the switching elements. Thus, the ringing phenomenon in the circuit on the side of the primary winding of a pulse transformer can be prevented. Moreover, these uni-directional current elements make the surge voltage generated due to the ringing phenomenon bypass the switching element, thereby preventing destruction of a semiconductor switching elements. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      FIGS.  1 (A) and (B) are the block diagrams for explaining constitution and operation of the conventional discharge lamp device.  
       FIG. 2  is a diagram showing a voltage wave form generated by the discharge lamp device shown in  FIG. 1 .  
       FIG. 3  is a longitudinal section showing an example of the external electrode fluorescent lamp used for the discharge lamp device of the present invention.  
       FIG. 4  is a circuit diagram of the discharge lamp device showing a first embodiment of the present invention.  
       FIG. 5  is a voltage waveform which appears in the secondary coil L 2  of the pulse transformer  12  in the discharge lamp device mentioned above.  
       FIG. 6  is a circuit diagram of the discharge lamp device showing a second embodiment of the present invention.  
       FIG. 7  is a diagram showing a voltage waveform generated on the secondary side of the pulse transformer  12  in the above discharge lamp device.  
      FIGS.  8 (A) and (B) are the circuit diagrams for explaining the constitution and drive operation of a discharge lamp according to a third embodiment of the present invention.  
       FIG. 9  is a diagram showing a voltage waveform A and a current waveform B, which are generated in the secondary coil L 2  of the pulse transformer  12  in the third embodiment.  
       FIG. 10  is a graph comparatively showing the relation between lamp input electric power and relative light flux of the discharge lamp device of the embodiment and the conventional discharge lamp device.  
       FIG. 11  is a diagram showing a relation between a pulse waveform output from the drive signal generating circuit  11  and a light control ratio. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
      The embodiments of the present invention will be explained below referring to the figures.  
       FIG. 3  is a cross section showing an example of an external electrode fluorescent lamp used in the present invention. This external electrode fluorescent lamp includes a glass tube  2 , on an inner wall of which a phosphor film  1  is formed and in which a rare gas (discharge medium) composed mainly of xenon is enclosed airtight therein, an inner electrode  4 , which is arranged on one end of inside the glass tube  2 , a lead terminal  3 , which is lead out of the glass tube  2 , and an external electrode  5 , which is spirally wound around the outer surface of the glass tube  2  along almost entire length of the tube axis at a prescribed pitch.  
      Here, the glass tube  2  has an outer diameter of about 1.2 to 10.0 mm and a length of about 50 to 750 mm, in which a rare gas composed mainly of xenon gas is enclosed as an electric discharge medium. In the figure,  5   a  is a lead terminal of an external electrode  5 , and  6  is a translucent heat shrinkage tube for coating the surface of the external electrode  5  for preventing the displacement of the external electrode  5  and for electric insulation.  
      When the square wave voltage of 1-3 kV is impressed between the internal electrode  4  and the external electrode  5  through the voltage supply lines  7  and  8  from the power supply  9  in the external electrode fluorescent lamp, electric discharge will begin between the electrodes  4  and  5 , and an ultraviolet ray will be emitted within the glass tube  2 . The emitted ultraviolet ray is converted into a visible light by the phosphor film  1  on the inner surface of the glass tube  2 .  
       FIG. 4  is a circuit diagram showing the first embodiment of the discharge lamp device according to the present invention. In the figure,  10  is a drive signal circuit connected to a DC power supply (not illustrated), which outputs the square wave drive signal. S 1  and S 2  are the semiconductor switching elements, which constitute an inverter circuit, and are control to be alternately turned ON and OFF by the square wave drive signals  11   a  and  11   b  supplied from the drive signal circuit  10 . That is, square wave drive signals  11   a  and  11   b  have the same repetition frequency and phases, which are different from each other by 180 degrees. Thus, one is in a low level when the other is in a high level (H). Further,  12  is the pulse transformer having a primary coil L 1  and a secondary coil L 2 , and  13  is an external electrode fluorescent lamp, which is arranged by connecting between the terminals of the secondary coil L 2  of the pulse transformer  12 .  
      The switching elements S 1  and S 2  are connected between the positive terminal V of the DC power supply and the ground G in series. Inductance elements L 3  and L 4  are connected to each of the first and the second semiconductor switching elements in series. A capacitor C 1  and a capacitor C 2  are connected between the terminal V of the DC power supply and the ground G in series. Input terminals of the primary coil L 1  of the pulse transformer  12  are connected with a connecting point P of a capacitor C 1  and a capacitor C 2  and with an end Q of a series connected circuit constituting the first switching element S 1  and an induction element L 3 . A circuit containing a Zener diode element D 1  having a forward voltage Vf, which is below a withstanding voltage of the switching element S 2 , is connected in parallel with the semiconductor switching element S 2 , being one end of the circuit grounded.  
      In the next, the operation of the discharge lamp device is explained. When the switching element S 1  is ON, and the switching element S 2  is OFF under the control of the square wave drive signals  11   a  and  11   b  supplied from the drive signal circuit  10 , current Ia flows in the circuit of DC power supply terminal V—the switching element S 1 —inductance element L 3 —primary coil L 1  of the pulse transformer  12 —capacitor C 2 —ground G. When the switching element S 1  is OFF, and the switching element S 2  is ON under the control of the square wave drive signals  11   a  and  11   b  supplied from the drive signal circuit  10 , current Ib flows in the circuit of the DC power supply terminal V—the capacitor C 1 —the primary coil L 1  of the pulse transformer  12 —the inductance element L 4 —the switching element S 2 —ground G.  
      Such operation is repeated under the control of the square wave drive signals  11   a  and  11   b  supplied from the drive signal circuit  10 , and current Ia and Ib flows alternately to the primary coil L 1  of the pulse transformer  12 . Thus, square wave voltage is generated continuously between the output terminals of the secondary coil L 2  of the pulse transformer  12 . This output voltage is applied to the external electrode fluorescent lamp  13 , to light and drive it.  
      Here, current Ia and Ib flows alternately in the primary coil L 1  of the pulse transformer  12  by ON and OFF operation of the switching elements S 1  and S 2 , as mentioned above, which arises ringing by action of the inductance elements L 3  and L 4  inserted in the current passage, resulting in the generation of a big surge voltage. As mentioned above, Vf of the Zener diode element D 1 , which is connected in parallel with the switching element S 2  and one end of which is grounded, is set as higher than the voltage value that does not contain the surge component, which is induced regularly in the primary terminal L 1  of the pulse transformer  12  during usual lighting operation, and is set below the withstand voltage of the semiconductor switching element S 2  (or semiconductor switching element S 1 ). When the steep surge pulse is generated by the alternate switching drive of the semiconductor switching elements S 1  and S 2  while the necessary drive signals are supplied into the pulse transformer  12 , the pulse current based on the surge voltage is superimposed on the current Ib and flows to the switching element S 2 . However, the current bypasses the switching element S 2  through the Zener diode element D 1  to the grounding circuit, since the Zener diode element D 1  is turned ON. Therefore, even if the big surge pulse, which exceeds withstand voltage of the semiconductor switching element S 2  is generated, the semiconductor switching element S 2  is not damaged. In addition, when the surge pulse is superimposed on the current Ia, which flows when the semiconductor switching element S 1  is ON and the semiconductor switching element S 2  is OFF, the semiconductor switching element S 1  is not destroyed since the current Ia flows to the ground through a capacitor C 2 .  
       FIG. 5  is a voltage waveform of the above-mentioned discharge lamp device, which appears in the secondary coil L 2  of the pulse transformer  12 . As compared with the voltage waveform in the conventional discharge lamp device shown in  FIG. 2 , the surge pulse has been decreased sharply. That is, since the steep high voltage surge pulse generated at the time of ON and OFF of the semiconductor switching elements S 1  and S 2 , is released to the ground easily and certainly, the damage of the semiconductor switching elements S 1  and S 2  by the above-mentioned high voltage surge pulse are avoided, and stable lighting operation is maintained.  
       FIG. 6  is a circuit diagram showing the general concept of the discharge lamp device according to the second embodiment of the present invention. In the figure, the same symbol is are assigned to the same portions of the discharge lamp device shown in  FIG. 4 , with detailed explanation being omitted, and different portions are explained below.  
      The drive signal circuit (inverter drive signal unit)  10 , which outputs square wave signals is connected to the primary coil L 1  of the pulse transformer  12  through the semiconductor switching elements S 1  and S 2 . The inductor element L 3  is connected to the semiconductor switching element S 1  in series. A first uni-directional current element D 2  and an induction element L 5  are connected in series to the semiconductor switching element S 2 . Further, a second uni-directional current element D 3  is connected in parallel with the series connected circuit of the uni-directional current element D 2  and the inductor element L 5 . Here, the second uni-directional current element D 3  is connected to the first uni-directional current element D 2  with a polarity being opposite to each other. In addition, the uni-directional current elements D 2  and D 3  are diode elements and the induction element L 5  is a coil, for example.  
      Basic operation of such discharge lamp device is the same as that of the first embodiment. Namely, when the switching element S 1  is ON, and the switching element S 2  is OFF under the control of the square wave drive signals  11   a  and  11   b  supplied from the drive signal circuit  10 , current Ia flows in the circuit of DC power supply terminal V—the switching element S 1 —inductance element L 3 —primary coil L 1  of the pulse transformer  12 —capacitor C 2 —ground G. When the switching element S 1  is OFF, and the switching element S 2  is ON under the control of the square wave drive signals  11   a  and  11   b  supplied from the drive signal circuit  10 , current Ib flows in the circuit of the DC power supply terminal V—the capacitor C 1 —the primary coil L 1  of the pulse transformer  12 —the inductance element L 4 —the switching element S 2 —ground G.  
      Here, when the steep surge pulse is generated in the primary coil L 1  of the pulse transformer  12  by the alternate switching drive of the semiconductor switching elements S 1  and S 2 , a current Ib superimposed by the pulse due to the surge pulse flows into the semiconductor switching element S 2  through the first uni-directional current element D 2  and induction element L 5 . However, a current Ic flows in the second uni-directional current element D 3  by induced voltage of the opposite direction generated between the both ends of the induction element L 5 . A part of current Ib superimposed by the surge pulse is consumed in its energy in the parallel circuit, which consists of the uni-directional current element D 2 , the induction element L 5 , and the second unidirectional current element D 3 , so that no damage is caused to the semiconductor switching element S 2 .  
      Moreover, with the first uni-directional current element D 2 , which is connected in series to the semiconductor switching element S 2 , the current flow in the opposite direction is prevented, thereby preventing the ringing.  
       FIG. 7  is a diagram showing a voltage waveform generated on the secondary side of the pulse transformer  12  in the above-mentioned discharge lamp device. The diagram shows that the surge voltage S in an output voltage waveform curve of the discharge lamp device is drastically suppressed compared with that in the conventional equipment shown in  FIG. 2 . Moreover, it is clear that the size of surge voltage S is lower than that in the embodiment shown in  FIG. 5 .  
      Here, although the second unidirectional current element D 3  was connected in parallel with the in-series connection circuit of the uni-directional current element D 2  and the induction element L 5  in the case of the embodiment of  FIG. 6 , the same effect is obtained even if it is connected in parallel between the terminals of induction element L 5  itself.  
      FIGS.  8 (A) and (B) are the circuit diagrams for explaining a general concept and drive operation of the discharge lamp drive equipment according the third embodiment of the invention. The same symbols are assigned to the same portions as those in the embodiment shown in  FIG. 4  or  FIG. 6 , omitting detailed explanation and the explanation will be focused on the portions which differ from those below.  
      The drive signal circuit (inverter drive signal unit)  10  supplies the square wave signals  7   a  and  7   b  having different phases with each other to a pair of switching elements S 1  and S 2 , which are controlled by being turned ON and OFF alternately. A pair of switching elements S 1  and S 2 , being connected in series between DC power supply V and the grounding G, alternately change the direction of current which flows to the primary coil L 1  of the pulse transformer  12  from DC power supply V. First and second capacitors C 1  and C 2  are connected in series between the ground G and DC power supply V. One of the terminals of primary coil L 1  of the pulse transformer  12  is connected to a connecting point P of the first and second capacitors C 1  and C 2 . Between the switching elements S 1  and S 2 , first and second uni-directional current elements D 1  and D 2  are connected in series with the same polarity. The other terminal of the primary coil L 1  of the pulse transformer  12  is connected to a connecting point Q of the uni-directional current elements D 1  and D 2 .  
      The basic operation of the discharge lamp device described is the same as that of the first or second embodiment. Namely, when the switching element S 1  is ON and the switching element S 2  is OFF, a current Ia flows in the circuit of DC power supply terminal V—switching element S 1 —first uni-directional current element D 1 —primary coil L 1  of pulse transformer  12 —capacitor C 2 —grounding G, under the control of the square wave drive signals  11   a  and  11   b  supplied from the drive signal circuit  10 . When the switching element S 1  is OFF and the switching element S 2  is ON, a current Ib flows in the circuit—DC power supply terminal V—capacitor C 1 —primary winding L 1  of pulse transformer  12 —second uni-directional current element D 2 —switching element S 2 —grounding G, under the control of the square wave drive signals  11   a  and  11   b  supplied from the drive signal circuit  10 .  
      Since the uni-directional current elements D 1  and D 2 , such as diode elements for example, are inserted in series into the circuit, where the currents Ia and Ib flow, any current flowing in the opposite direction to those of the currents Ia and Ib is cut. That is, the ringing phenomenon is completely oppressed, with which oscillating currents flow in the opposite directions accompanying with the currents Ia and Ib, due to the resonance phenomenon of the circuit.  
       FIG. 9  is a diagram showing a voltage waveform A and a current waveform B which are generated in the secondary coil L 2  of the pulse transformer  12  in the third embodiment mentioned above. The abscissa of the diagram indicate time, with a scale of 5 μsec/div, and ordinates indicate voltage V and current A, with scales of 500V/div and 10 mA/div, respectively.  
      In the discharge lamp device according to the embodiment, not only the ringing is prevented to generate but also the peak value of the lamp current supplied to the discharge lamp becomes high compared with that in the conventional equipment. Moreover, it is confirmed that the luminescence efficiency of the discharge lamp increases.  FIG. 10  is a graph which compares the relation between lamp input power and relative light flux in the discharge lamp device according to the embodiment and the conventional discharge lamp device respectively. In the figure, curvilinear C shows the relative light flux of the embodiment, and curvilinear c shows that of the conventional device, respectively.  
      Furthermore, according to the discharge lamp device of the embodiment described, stable light control can be realized at the time adjusting light control ratio of a discharge lamp, since it enables to prevent generating of the ringing and to supply a stable drive voltage to discharge lamp. The light control is one of the performances required for this kind of discharge lamp. Here, a light control means the luminosity adjustment of a discharge lamp, and a ratio of an arbitrary luminosity to the maximum luminosity is called light control ratio. For example, in the case of the discharge lamp for automobiles, it is required that the light is operated stable until the light control ratio reaches to 2%.  
      Such adjustment of the light control ratio is made by changing the number of output pulse per unit time of the drive signal generating circuit  10  in the discharge lamp device described.  
       FIG. 11  shows a pulse waveform for explaining the relation between the output pulse of the drive signal generating circuit  11  and the light control ratio.  FIG. 11 (A) is a waveform of the drive signal  11   a  (or  11   b ) when the light control ratio is 100%. When repetition frequency of the drive signal  11   a  is selected as 20 kHz, for example, the repetition period is 50 μs. Now, let a unit time to be 0.01 s (equal to a repetition frequency of 100 Hz) for the drive signal  11   a , the number of pulse per unit time of the drive signal generating circuit  11  will become 200. Namely, when the light control ratio is 100%, drive signal  11   a  has 200 pulses per unit time, and is repeated at the frequency of 100 Hz.  
       FIG. 11 (B) is a waveform of drive signal  11   a  (or  11   b ) when the light control ratio is 5%. The number of the output pulses of the drive signal generating circuit  11  is 10 per unit time in this case.  
       FIG. 11 (C) is a waveform of the drive signal  11   a  (or  11   b ) when the light control ratio is 1%. The number of the output pulses of the drive signal generating circuit  11  is one per unit time in this case.  
      As it is clear from the waveforms of  FIG. 11 (B) and (C), the number of pulses per unit time is fewer than that shown in  FIG. 11 (A), where the light control ratio is 100%. However, when the light control ratio is equal, the number of pulses per unit time and their timings are always constant, and are repeated at every unit time. Here, a flicker will be caused if the number or timing of pulse per unit time changes at every unit time.  
      Although the drive signal generating circuit  10  is not illustrated, the number of the drive signals  11   a  and  11   b  per unit time of the output signal of the circuit  10  is controlled to correspond to desired light control ratio by the built in microcomputer, for example.  
      In the embodiment described, the pair of drive signals supplied from the drive signal circuit are so called PWM modulated by a light control signal.  
      As mentioned above, the square wave AC voltage wave is supplied to the secondary coil L 2  of the pulse transformer  12 , where the number of pulses per unit time of which is so controlled and modulated by the light control signal as to respond to desired light control ratio. This is carried out by controlling the semiconductor switching elements S 1  and S 2  with the output drive signals  11   a  and  11   b  of the drive signal circuit  10  by which the number of pulses per unit time was controlled.  
      When the above light control was applied to the conventional device shown in  FIG. 1 , the flickering was observed near the inner electrode while the light control ratio was small. However, in the embodiments described, stable lighting operation was confirmed even though the light control ratio was small.  
      The present invention is not limited to the above-mentioned embodiments, but is possible to make various modifications in the range which does not deviate from the purpose of the invention. For example, although the external electrode fluorescent lamp was used, in which one electric discharge electrode was sealed In the luminescence tube, and the electric discharge electrode of another side has been arranged on the outer surface of the luminescence tube, other type of the lamp may be also used, in which both of the discharge electrodes are arranged on the outer surface of the luminescence tube at opposite portions.  
      Further, in the each embodiment described, one pulse transformer  12  was driven by one drive signal circuit  10 . However, two or more pulse transformers may be driven simultaneously by one drive signal circuit  10 , where electric discharge lamp may be connected to each pulse transformer.  
      Further, a diode element, a transistor element, an MOSFET element, a photo-coupler and the like can be used as a uni-directional current element which passes current only in one direction.