JP2000068088A - Drive method for discharge device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve luminance and light-emitting efficiency, and to lower a discharge maintaining voltage, in a light source for outdoor/indoor lighting, display or a liquid crystal display backlight. SOLUTION: A first voltage V1 having a first frequency is impressed on an electrode 21, in order to let a discharge device discharge itself. In addition, a second voltage V2 having a second frequency higher than the first frequency is superposed on the first voltage V1. The waveform of the voltage having the second frequency may be a damped waveform synchronized with the first frequency. Electrons, ions, or plasma existing in the discharge formed previously are vibrated accompanying electric field changes due to this voltage superposition. If the second frequency is selected appropriately, further resonance of electrons, ions and plasma occur, due to which the temperature of the particles rises. Thereby, the electron temperature becomes close to a value preferred for irradiation of visible rays or ultraviolet rays, which improves light-emitting efficiency.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a discharge device which emits visible light, ultraviolet light, etc., and more particularly to a discharge device used for indoor / outdoor illumination, display, or a light source for a liquid crystal display backlight. It relates to a driving method.

[0002]

2. Description of the Related Art There are various methods for driving a light source using discharge, for example, a fluorescent lamp for illumination or display, a flat discharge device, or the like. A conventional method of driving an inverter for turning on a liquid crystal backlight is disclosed in, for example, Japanese Patent Application Laid-Open No. 63-11096.
As shown in FIG. 2, the fluorescent lamp is turned on using a sine wave of several tens of kHz. For the discharge tube, a cold cathode or hot cathode fluorescent lamp having a tube diameter of about 3 to 6 mm is used, for example, mercury and argon are sealed inside the discharge tube, and ultraviolet rays generated by discharge excite the phosphor, It emits light and illuminates the liquid crystal panel to display characters and images.

[0003] Further, general lighting lamps are generally lit at a commercial frequency of 50 or 60 Hz, but there are also lamps lit at several tens of kHz.

[0004]

However, the discharge device using the above driving method has a problem that the light emission efficiency is reduced and the power consumption is increased particularly when the diameter of the discharge tube is reduced. For example, the luminous efficiency of a fluorescent lamp having a diameter of 2 to 3 mm used as a backlight for a liquid crystal display device is only about half that of a fluorescent lamp for indoor lighting having a diameter of approximately 25 mm, and the power consumption of the liquid crystal display device is reduced. The biggest cause that could not be done. In particular, for a battery-driven liquid crystal display device such as a notebook personal computer, reducing the power of the backlight has been a very important issue.

An object of the present invention is to provide a driving method for improving the luminous efficiency of such a discharge device and at the same time reducing power consumption.

[0006]

In order to achieve the above object, a method of driving a discharge device according to the present invention uses a voltage having a drive waveform different from the conventional one. That is, to discharge the discharge device, a first voltage having a first frequency is applied to the electrode. Further, a second voltage having a second frequency higher than the first frequency is superimposed on the first voltage. The voltage waveform having the second frequency may be an attenuation waveform synchronized with the first frequency.

According to the present invention, a first voltage having a first frequency is applied to a discharge device, and a discharge is formed by this voltage. Next, when a second voltage having the second frequency is superimposed, the voltage superimposition causes the electrons, ions, or plasma existing in the already formed discharge to oscillate with a change in the electric field. If the second frequency is appropriately selected, resonance phenomena of electrons, ions, and plasma will also occur. This increases the temperature of these particles. As a result, the electron temperature approaches a preferable value for emitting visible light or ultraviolet light, and as a result, luminous efficiency is improved.

In a conventional discharge tube, such as a fluorescent lamp,
Most of the space charge required for sustaining the discharge is generated in the negative glow. Relatively large electric power is required to form this negative glow. However, according to this driving method, the electron density and the ion density in the discharge space, particularly in the positive column, increase, so that the negative glow may be small. Therefore, less electric power is injected into the discharge tube, and as a result, the luminous efficiency is improved. Further, the discharge sustaining voltage is reduced, and the power consumption can be significantly reduced.

[0009]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Next, embodiments of the present invention will be described with reference to the drawings. FIG. 1 is a schematic view showing an embodiment of a method for driving an external electrode type discharge tube according to the present invention. In the figure, 20 is a discharge tube, 21 and 22 are electrodes provided on the outer surface of the discharge tube, 2
Reference numerals 3 and 24 denote driving voltage generating circuits having a first frequency, and reference numerals 25 and 26 denote second driving voltage generating circuits having a second frequency connected to the driving voltage generating circuits 23 and 24, respectively. V1 and V2 are electrodes 21 and 22
And a second voltage having a second frequency higher than the first frequency is superimposed on the first voltage in addition to the first voltage having the first frequency. A discharge type so-called electrodeless discharge is performed.

FIG. 2 and FIG. 3 are diagrams of voltage waveforms showing one embodiment of the driving waveforms. The driving voltage generating circuit having the first frequency generates a rectangular wave voltage V shown in FIG.
Twelve rectangular wave voltages V2 whose phases are shifted by a half cycle from 1 and 11 are further generated. Further, a voltage having a second frequency, for example, an attenuated sine wave voltage is applied to these waveforms from a driving voltage generating circuit having a second frequency. 15 and 16 are superimposed on 11 and 12. The voltage waveforms 13 and 14 shown in FIG. 3 are discharge voltage waveforms. By applying this voltage to the discharge tube, high efficiency can be achieved.

A result of driving a discharge tube using the driving waveform according to the present invention and measuring discharge light emission characteristics thereof will be described. FIG. 4 shows the configuration of the apparatus used for the measurement. The discharge tube 20 is made of, for example, soda glass having an inner diameter of 3.1 mm, and has a pair of electrodes 21.
Each of the reference numerals 22 has a length of 10 mm and a width of 5 mm, and is provided on the outer surface of the discharge tube. The distance between the two electrodes was 3 mm, and the inside of the discharge tube was coated with a three-wavelength white phosphor and the one without the three-wavelength white phosphor was measured. For example, mercury, neon 54 Torr, and argon 6
Torr enclosed.

The basic shape of the driving voltage waveform is the same as the waveform shown in FIG. 3, but the coils 25 'and 26' are used as a second driving waveform voltage generating circuit, and the driving having the second frequency is used. The voltage used was a voltage generated by ringing between the coils 25 'and 26' and the stray capacitance of the circuit system. In this embodiment, this driving condition is referred to as driving with a coil.

In the present embodiment, a coil having a simple structure is used as the second drive voltage generating circuit, but any coil having an inductance component may be used.

FIG. 5 is a schematic diagram of a conventional driving method, for example, a discharge state generated by the driving voltage waveform shown in FIG. This is a result of observing a discharge tube to which no phosphor is applied. In the vicinity of the electrodes 21 and 22, a negative glow 3
1 and 32 are formed. A positive column 33 is formed above the negative glow and between the electrodes.

FIG. 6 shows the pulse width (width of the driving voltage pulse shown in FIG. 2) in the conventional driving method while keeping the applied voltage constant.
FIG. 4 is a schematic diagram of a change in a discharge state when is gradually narrowed. FIG. 6A shows a pulse width of about 1.7 μs, and FIG.
In (c), the pulse width is gradually reduced.
It shows a discharge state of 00 ns.

As the pulse width is reduced, the emission intensity of each of the negative glows 31 and 32 and the positive column 33 is weakened. In particular, the negative glow region decreases as the pulse width is reduced. This is because the amount of current decreases as the pulse width decreases, and the wall voltage due to the wall charges formed by the discharge increases, so that the effective voltage applied between the electrodes decreases and the discharge weakens. However, when the pulse width was extremely narrow, ie, 100 ns or less, a phenomenon was observed in which the negative glow region was reduced but the positive column region was increased.

FIG. 7 shows a change in the discharge state when driving with a coil at the same set voltage value. As in FIG. 6, (a) has a pulse width of about 1.7 μs, and (b) and (c) gradually narrow the pulse width, and (d) shows a discharge state of about 100 ns.

Compared with the conventional driving shown in FIG. 6, the driving method according to the present invention is characterized in that the area occupied by the positive column 33 increases and the area occupied by the negative glows 31 and 32 decreases. In the drive with the coil, the voltage applied to the discharge tube becomes larger than that in the conventional drive due to ringing generated in the voltage pulse. Then, when a voltage obtained by adding this increase is used in the conventional driving method, the light emission of the positive column 33 is increased, but the neon red emission (negative glow) seen on the electrode is also increased. It was found that the discharge state was different from that in the drive with a coil.

In the drive with a coil, as the pulse width is gradually reduced, the negative glow region is reduced as in the conventional drive method. However, the length of the positive column is almost constant although the emission intensity is slightly reduced. Even in the case of driving with a coil, when the pulse width is 200 ns or less, an increase in the positive column area is observed as in the conventional driving method. If the pulse width is sufficiently narrow, the discharge will end up with almost no negative glow.

FIG. 8 shows the change in the minimum discharge sustaining voltage with respect to the pulse width measured by changing the number of turns of the coil. By changing the number of turns of the coil, the minimum discharge maintaining voltage changes.
This is because changing the number of turns of the coil changes the inductance and changes the ringing frequency, voltage, damping constant, and the like. When the conventional driving method is used, the minimum discharge sustaining voltage increases as the pulse width decreases. This is presumably because as the pulse width becomes shorter, the wall voltage formed during the pulse application period decreases, and the effective voltage decreases. When the pulse width becomes 100 ns or less, the minimum sustaining voltage decreases, which is exactly the same as the pulse width where the positive column region is increased. As shown in FIG. 8, the minimum discharge sustaining voltage in driving a coil having 30 turns is less than half that of the conventional driving method, and the discharge voltage can be reduced by using the driving method according to the present invention.

In the case of the drive with a coil according to the present invention, the negative glow region can be reduced as compared with the conventional drive. From this, it is considered that the cause of the decrease in the minimum discharge sustaining voltage is a decrease in the cathode drop voltage. In addition, when a coil having 20 or more turns is used, vibration may be observed in a region where the pulse width is small. This is due to the balance between the set pulse width and the ringing period.

FIG. 9 shows a change in luminance with respect to an applied voltage in the driving method with the coil according to the present invention and the conventional driving method at a pulse width of 1.7 μs. This brightness is measured in a circle with a diameter of 1 mm at the center between the electrodes of the discharge tube on which the phosphor is applied. The drive with coil, which has an increased positive column area, has a much higher brightness than the normal drive. I do.

FIG. 10 shows a visible light emission waveform observed by the photomultiplier tube. The measurement location was on the center axis of the discharge tube,
It is a central part between 1 and 22. The emission waveform is shown by a solid line for driving with a 30-turn coil according to the present invention and by a broken line for conventional driving. In the drive with the coil, the light emission intensity is significantly increased as compared with the conventional drive. In particular, the secondary light emission after the end of the pulse application is very strong. This secondary emission waveform and the attenuation waveform of the ringing at the end of the pulse substantially match.

FIG. 11 shows a time-integrated waveform of the discharge current measured by the current transformer. The period of the rectangular wave voltage having the first frequency is 8 μs, the pulse width is 1.7 μs, and the applied voltage is 300 V. The driving with the coil according to the present invention is indicated by a solid line, and the conventional driving is indicated by a broken line. Although this current waveform includes a displacement current flowing when charging / discharging the stray capacitance of the discharge tube, it can be seen from the figure that the current slightly increases when the coil is driven, but there is not much difference. The drive with a coil according to the present invention has little difference between the conventional drive and the discharge current, and has high luminance, that is, good luminous efficiency.

FIG. 12 shows the relationship between the pulse width of the rectangular wave voltage having the first frequency and the luminance when the power supply voltage is maintained at 300 V. In the case of the conventional drive, the brightness decreases as the pulse width becomes shorter, but the drive with the coil according to the present invention maintains a substantially constant brightness. This is because, as described above, in the driving method according to the present invention, even if the pulse width is narrowed, the luminance does not decrease because the positive column region increases irrespective of the decrease in the negative glow region.

Next, the result of comparison between the luminance and the luminous efficiency while keeping the time integral of the current constant will be described. FIG. 13 shows the relationship between the luminance and the pulse width of the rectangular wave voltage having the first frequency, and FIG. 14 shows the relationship between the luminous efficiency and the pulse width of the rectangular wave voltage having the first frequency. The applied voltage was set at each pulse width so as to be equal to the current value at a voltage of 220 V at which the maximum efficiency was obtained by the conventional driving, and the measurement was performed.

In the driving method according to the present invention, the luminance is twice or more as shown in FIG. As shown in FIG. 11, at the same applied voltage, the time integrated value of the current is larger in the drive with the coil than in the drive with the coil, so that the applied voltage for each pulse width is lower in the drive with the coil according to the present invention. As described above, since the luminance is twice or more and the applied voltage is low, the luminous efficiency in the driving with the coil increases to nearly three times as compared with the conventional driving method as shown in FIG.

If the driving method according to the present invention is used for a liquid crystal display device such as a notebook personal computer, the power of the backlight can be reduced, and a display device with high luminance and low power consumption can be realized.

FIG. 15, FIG. 16, or FIG. 17 shows drive voltage waveforms of another embodiment according to the present invention. In the figure, 1
7 is a first voltage waveform having a first frequency, and 18 is a second voltage waveform.
A second voltage waveform 19 having a frequency of ## EQU1 ## is a waveform obtained by superimposing the first and second voltages, and the same effect can be obtained by using the waveforms of these embodiments. Further, the waveforms of the first and second voltages may have a sine wave, a drive wave, a staircase wave, or another waveform. These waveforms can also be created by the circuit of FIG. 18, for example. In the figure, 23 is a first voltage generating circuit having a first frequency, and 27 is a second voltage generating circuit.
, A circuit for superimposing the first and second voltages, and 20 a discharge device.

In the above embodiment, the first voltage has a period of 8 μs and a pulse width of 1.7 μs. However, the frequency of the first voltage is 30 kHz or more and the width of the rectangular wave pulse is 10 μs.
By setting the condition to s or less, the same effect as described above can be obtained. When the peak-to-peak value of the voltage having the second frequency is smaller than the peak-to-peak value of the voltage having the first frequency, the effect of improving the luminous efficiency is large.

In the above embodiment, the external electrode type discharge tube is described. However, the electrode may be provided inside the discharge tube like a fluorescent lamp, or may be applied to a flat type discharge device. Of course.

[0032]

As described above, according to the driving method of the present invention, in addition to the first voltage having the first frequency, the voltage applied to the electrode further includes the first voltage having a frequency higher than the first frequency. With a simple driving method of superimposing a second voltage having a frequency of 2 on the first voltage, the luminance and luminous efficiency of the discharge device can be increased, and further,
The discharge sustaining voltage can also be reduced. As a result,
Since the power consumption of the discharge device can be significantly reduced, a low power consumption liquid crystal display device or the like suitable for driving a battery or the like can be obtained.

[0033]

[Brief description of the drawings]

FIG. 1 is a schematic view showing one embodiment of a driving method of a discharge device according to the present invention.

FIG. 2 is a voltage waveform of one embodiment according to the driving method of the present invention.

FIG. 3 is a voltage waveform of one embodiment according to the driving method of the present invention.

FIG. 4 is a schematic diagram of an apparatus used for measurement.

FIG. 5 is a schematic diagram of a discharge state in a discharge tube.

FIG. 6 is a schematic diagram of a discharge state in a discharge tube by a conventional driving method.

FIG. 7 is a schematic diagram of a discharge state in a discharge tube when the driving method according to the present invention is used.

FIG. 8 shows the relationship between the number of coil turns and the minimum discharge sustaining voltage.

FIG. 9 shows a relationship between a power supply voltage and luminance.

FIG. 10 is a graph showing the change over time in the emission intensity in the discharge tube when the driving method according to the present invention is used.

FIG. 11 is a waveform of a time integration value of a current.

FIG. 12 shows a relationship between a pulse width and luminance.

FIG. 13 shows the relationship between pulse width and luminance when the current-time integral value is constant.

FIG. 14 shows the relationship between pulse width and luminous efficiency when the current-time integration value is constant.

FIG. 15 is a driving voltage waveform showing another embodiment according to the present invention.

FIG. 16 is a driving voltage waveform showing another embodiment according to the present invention.

FIG. 17 is a driving voltage waveform showing another embodiment of the present invention.

FIG. 18 is a system diagram of a circuit according to the present invention.

[Explanation of symbols]

11, 12, 17 ... a drive voltage waveform having a first frequency, 13, 14, 19 ... a discharge tube drive waveform according to the present invention, 15, 16, 18 ... a drive voltage waveform having a second frequency, 20 ..., Discharge tube, 21, 22,..., Electrode, 23, 24,..., Drive voltage generating circuit having first frequency, 25, 26,. , A driving voltage generating circuit having a second frequency, 25 ′, 26 ′... A coil, a driving voltage generating circuit having a second frequency,. And a second voltage superimposing circuit, 31, 32,..., A negative glow, 33,..., A positive column.

Continued on the front page (72) Inventor Tomokazu Shiga 568 Kitanocho, Hachioji-shi, Tokyo

Claims (11)

[Claims] 1. A discharge device provided with at least one pair of electrodes, wherein a first voltage having a first frequency is applied to at least one of the electrodes, and a second frequency higher than the first frequency is applied. A method for driving a discharge device, comprising superimposing a second voltage having the following on the first voltage. 2. A discharge device of an electrodeless type wherein at least one pair of electrodes is provided outside a discharge space, a first voltage having a first frequency is applied to at least one of said electrodes. A method for driving an external electrode type discharge device, wherein a second voltage having a second frequency higher than the first frequency is superimposed on the first voltage. 3. At least one pair of electrodes in which either rare gas such as xenon, krypton, argon, helium, neon, or mercury is sealed alone, or two or more of these gases are mixed and sealed. Is provided, a first voltage having a first frequency is applied to at least one of the electrodes, and a second voltage having a second frequency higher than the first frequency is applied to the first voltage. A driving method of the discharge device, wherein the driving method is superimposed on the voltage of the discharge device. 4. The driving method according to claim 1, wherein the voltage having the second frequency is a voltage whose amplitude is periodically attenuated or increased, and the period of the attenuation or increase is the first. A method for driving a discharge device, wherein the method is synchronized with a cycle of a frequency of the discharge device. 5. A driving method according to claim 1, wherein the amplitude of the voltage having the second frequency is smaller than the amplitude of the voltage having the first frequency. . 6. A driving method according to claim 1, wherein a voltage having a second frequency is generated by an inductance component. 7. The driving method according to claim 1, wherein the second frequency is selected to be the same as or close to the resonance frequency of electrons, ions, or plasma present in the discharge device. Method for driving a discharge device. 8. The driving method according to claim 1, wherein the first frequency is selected to be 30 kHz or more. 9. The driving method according to claim 1, wherein the first voltage is a rectangular pulse, and the pulse width is selected to be 10 μs or less. 10. A backlight configured to illuminate an object to be illuminated by using a discharge device lit by the driving method according to claim 1. 11. A liquid crystal display device using the driving method according to claim 1 and a backlight.