Electric lamp and discharge devices: systems – Current and/or voltage regulation
Reexamination Certificate
2001-11-16
2003-01-21
Philogene, Haissa (Department: 2821)
Electric lamp and discharge devices: systems
Current and/or voltage regulation
C315S2090PZ, C315SDIG005, C310S318000, C310S319000, C310S316030
Reexamination Certificate
active
06509700
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a device and a method for driving a cathode discharge tube that is used as a light source for a liquid crystal display, display panel and the like.
2. Description of the Related Art
In recent years, for back-lights in liquid crystal displays of notebook computers and the like, there have been used cold cathode fluorescent tubes and hot cathode fluorescent tubes, which consume a comparatively small amount of electric power and have high luminous efficacy.
So far, in a cathode discharge lighting device that lights these cathode discharge tubes, a DC voltage is converted into an AC voltage by a DC/AC inverter circuit, and then using the AC voltage the cold or hot cathode discharge tube is lighted. The discharge starting voltage for a cold cathode discharge tube is higher than that for a hot cathode discharge tube. Also, the discharge starting voltage becomes higher, as the length of a cold cathode discharge tube becomes greater.
FIG. 16
shows prior circuitry of a cold cathode discharge device that lights a cold cathode discharge tube. As shown by
FIG. 16
, the cold cathode discharge device has an inverter circuit
311
. The inverter circuit
311
comprises switching elements
304
a
and
304
b
such as transistors and a step-up transformer
302
that transforms the input voltage into a high voltage. An AC voltage is generated from a DC voltage output from a DC power supply
307
by alternately switching the switching elements
304
a
and
304
b
. This AC voltage is transformed to a higher voltage by the step-up transformer
302
and supplied to a cold cathode discharge tube
210
.
The operation at the start of lighting is described with reference to FIG.
17
.
FIG. 17A
shows the envelope of the voltage (tube voltage) across the cold cathode discharge tube
210
, while
FIG. 17B
shows the envelope of the current (tube current) flowing through the cold cathode discharge tube
210
. In order to light the cold cathode discharge tube
210
, a high AC voltage generated on the secondary side of the step-up transformer
302
is applied to the cold cathode discharge tube
210
. At this time, before the cold cathode discharge tube
210
is lighted, the voltage across the cold cathode discharge tube
210
rises (see FIG.
17
A), but current does not flow because there is almost no load (FIG.
17
B). After that, when the voltage across the cold cathode discharge tube
210
further rises and reaches the lighting start voltage, the current suddenly starts to flow while the voltage starts to fall. Subsequently the cold cathode discharge tube
210
has negative resistance, so that the tube voltage falls, and the tube current rises to become a setting current (a predetermined current necessary to maintain the lighting). In the case of cold cathode discharge tube
210
, in order to limit the pouring current, a current control element
301
such as a capacitor is connected in serial to cold cathode discharge tube
210
. In short, at the start of lighting cold cathode discharge tube
210
, an applied voltage higher than the voltage necessary for maintaining the lighting is required by a large margin. Further, the lighting maintenance voltage and lighting start voltage tend to become higher as cold cathode discharge tube
210
becomes longer.
In general, as a method of lowering this discharge starting voltage in a cathode discharge device, there is a method of lowering the discharge starting voltage by grounding a near-by conductor at the perimeter of the cold cathode discharge tube (or a hot cathode discharge tube).
In the method of grounding a nearby-conductor at the perimeter of a cathode discharge tube to lower the discharge starting voltage, a potential difference occurs between the electrode to which a high voltage is input and the near-by conductor, so that an effect of lowering the discharge starting voltage is obtained by a discharge prompting effect. However, in a cathode discharge lighting device, the other cathode and the near-by conductor are both grounded, so that there occurs no potential difference between them. Therefore, in the discharge device shown as a prior art, glow discharge starting from Townsend discharge reaches whole-tube discharge from the high-voltage electrode toward the GND electrode of the cathode discharge tube. In this way, in the prior discharge device, the discharge prompting effect is obtained only at the high-voltage electrode and not at the other electrode, so that the effect is not sufficient for a method of lowering the discharge starting voltage.
Further, as a method for solving the above problem, there is a method disclosed in the Japanese Laid-open Patent Publication No. 8-31588. In the method proposed by the Publication No. 8-31588, a near-by conductor is connected to the middle potential point of the high AC voltage to make the potential of the nearby conductor the middle potential, and thus Townsend discharge is induced from both electrodes to lower the discharge starting voltage. However, in this method, the sustaining voltage for lighting becomes higher as the cold cathode discharge tube becomes longer, so that a leak current is generated by a floating capacity between the nearby conductor and the cold cathode discharge tube. As a result, there are such problems as the lowering of luminance and the enlarging of the discharge device due to reactive power. Also, there is another problem that it is difficult to detect a current flowing through the discharge tube.
SUMMARY OF THE INVENTION
The present invention is made to solve the above problems. The object of the present invention is thus to provide a device and a method for lighting a cold cathode discharge tube which can lower the discharge starting voltage by a simple method without degrading the characteristics of the lighting device for a cathode discharge tube even if the cathode discharge tube becomes longer.
In a first aspect of the invention, an apparatus for driving a cathode discharge tube by applying an AC voltage to the cathode discharge tube is provided. The apparatus comprises a voltage application section which outputs an AC voltage to be applied to the cathode discharge tube, and a voltage controller which controls the output of the voltage application section. In order to light the cathode discharge tube, the voltage controller controls the output of the voltage application section so that the AC voltage applied to the cathode discharge tube is raised at a speed slower than a rise speed of the cathode discharge tube.
In a second aspect of the invention, a driving apparatus for driving a cathode discharge tube by applying an AC voltage to the cathode discharge tube is provided. The apparatus comprises a voltage application section which outputs an AC voltage to be applied to the cathode discharge tube, and a voltage controller which controls the output of the voltage application section. In order to light the cathode discharge tube, the voltage controller controls the output of the voltage application section so that the cathode discharge tube is half-lighted by the AC voltage, and subsequently the AC voltage is raised at a speed slower than a rise speed of the cathode discharge tube.
In a third aspect of the invention, a driving apparatus for driving a cathode discharge tube by applying an AC voltage to the cathode discharge tube is provided. The apparatus comprises a voltage application section which outputs an AC voltage to be applied to the cathode discharge tube, and a voltage controller which controls the output of the voltage application section. In order to light the cathode discharge tube, the voltage controller controls the output of the voltage application section so that the cathode discharge tube is half-lighted by the AC voltage, the state of half-lighting is maintained for a predetermined period, and then the AC voltage is raised to a voltage level at which the cathode discharge tube starts discharging.
In the above driving apparatus, the voltage controller may vary the AC voltage
Moritoki Katsunori
Nakatsuka Hiroshi
Okuyama Kojiro
Takeda Katsu
Yamaguchi Takeshi
Greenblum & Bernstein P.L.C.
Matsushita Electric - Industrial Co., Ltd.
Philogene Haissa
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