JP2004335362A - Inverter and liquid crystal display using the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To satisfy both the suppression of a temperature at high temperatures and an improvement in the deterioration of a service life of a fluorescent lamp at low temperatures by reducing a tube current at high temperatures and making a constant tube current flow at room temperature or lower in an inverter to regulate the tube current in accordance with surrounding temperatures. <P>SOLUTION: A midpoint of a thermistor 7 and a voltage dividing resister 8 connected between constant voltages is connected via a diode 9 to a voltage inverted from a tube current flowing in a fluorescent lamp 2 by a resister for detecting a current 3, and rectified and smoothed by a diode 4 and a smoothing capacitor 5. The midpoint voltage of the thermistor 7 and the dividing resister 8 makes the tube current constant by setting the midpoint voltage lower than the voltage obtained by smoothing the tube current at room temperature. The midpoint voltage only at high temperatures becomes higher than the voltage obtained by smoothing the tube current, then, makes the tube current low roughly in inverse proportion to temperatures. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a temperature control circuit for a tube current in an inverter for lighting a fluorescent tube, and more particularly to an inverter having a function of reducing the tube current at a certain temperature or higher.
[0002]
[Prior art]
2. Description of the Related Art Conventionally, an inverter is used to turn on a fluorescent tube used for a backlight unit of a liquid crystal panel connected to a driving circuit such as a liquid crystal display device. In such a conventional inverter for lighting a fluorescent tube, a method of stabilizing luminance by stabilizing a tube current is generally used.
[0003]
FIG. 8 is a circuit diagram showing an example of this type of inverter according to the related art. Referring to FIG. 8, the tube current flowing through the cold-cathode tube (fluorescent tube) 2 is converted into a voltage by the current detecting resistor 3, rectified and smoothed by the rectifying diode 4 and the smoothing capacitor 5, and input to the control circuit 6. The tube current is stabilized. Reference numeral 1 denotes a transformer, and 12 denotes a DC voltage source.
[0004]
On the other hand, in order to prevent an excessive temperature rise at a high temperature, there is a case where a means for reducing the tube current is used which is substantially inversely proportional to the inverter ambient temperature. FIG. 9 is a circuit diagram showing this example. In the example shown in FIG. 9, the thermistor 7 is inserted in series between the rectifier diode 4 and the control circuit 6 in FIG. Is changed by the ambient temperature of the inverter.
[0005]
Patent Document 1 discloses a cold-cathode tube lighting device using the above-described inverter. However, Patent Document 1 discloses a configuration in which a plurality of fluorescent tubes are provided, and all the fluorescent tubes are turned on reliably.
[0006]
[Patent Document 1]
JP-A-6-267675
[Problems to be solved by the invention]
FIG. 10 is a diagram provided for explaining the disadvantages of the prior art. Referring to FIG. 10, the inverter having the temperature control shown in FIG. 9 has a function of suppressing a rise in temperature at a high temperature, but has a disadvantage that a tube current increases at a low temperature and shortens the life of the fluorescent tube.
[0008]
Therefore, the present invention has been made to solve the above-mentioned drawbacks, and one technical problem thereof is to provide an inverter that suppresses a rise in tube current at low temperatures while suppressing a rise in temperature at high temperatures. .
[0009]
Another technical object of the present invention is to provide a liquid crystal display device using the inverter for lighting a fluorescent tube of a backlight unit of a liquid crystal panel.
[0010]
[Means for Solving the Problems]
According to the present invention, in an inverter that stabilizes a tube current by detecting and controlling a current flowing through a fluorescent tube, when the ambient temperature of the inverter exceeds a set temperature, the tube current increases the ambient temperature. The inverter has a function to detect and stabilize the tube current with a resistor connected in series to the fluorescent tube, and to determine the midpoint between the thermistor and the resistor connected between a certain voltage. And an inverter connected to the tube current detection point via a diode.
[0011]
Further, according to the present invention, the inverter includes a plurality of the fluorescent tubes, and has a function of detecting and stabilizing the tube current by a resistor connected so that a total current of the tube currents of the plurality of fluorescent tubes flows. An inverter characterized by having the above.
[0012]
According to the invention, in any one of the inverters, the diode is a Zener diode.
[0013]
Further, according to the present invention, a control circuit for controlling the tube current flowing through the fluorescent tube according to the input voltage so as to be kept constant, and detecting the tube current to generate a first voltage value according to the value. A tube current detection circuit that outputs a voltage, and a temperature detection circuit that outputs a second voltage value according to the ambient temperature, wherein the control circuit is configured to output the first voltage value and the second voltage value. An inverter characterized in that the tube current is controlled using a larger voltage value as the input voltage.
[0014]
According to the invention, there is provided a liquid crystal display device using any one of the inverters, wherein the inverter is used for lighting a fluorescent tube for a backlight unit of a liquid crystal panel. A liquid crystal display device is obtained.
[0015]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, embodiments of the present invention will be described with reference to the drawings. In the following description of the embodiment of the present invention, the same components as those of the conventional inverter shown in FIGS. 8 and 9 are denoted by the same reference numerals.
[0016]
One example of the liquid crystal display device of the present invention includes a liquid crystal panel incorporated in a housing having an opening or a window on one surface, a driving circuit for driving the liquid crystal panel, and an inside of another surface opposite to the one surface of the housing. , And an inverter for lighting the backlight unit. The backlight unit consists of multiple fluorescent tubes, which are cold-cathode tubes, a reflector that concentrates the light from the fluorescent tubes, and a plate-like guide that guides the light from the concentrated reflector to illuminate the entire liquid crystal panel with uniform brightness. Light guide plate.
[0017]
The fluorescent tube is turned on by the energy supplied by the inverter, the light is concentrated in a certain direction of the light guide plate by the reflection plate, and the entire liquid crystal can be illuminated with a constant brightness by the light guide plate. On the other hand, in the liquid crystal panel, it is determined whether or not light is transmitted by a driving circuit, thereby determining the display content of the screen.
[0018]
The liquid crystal display device according to an example of the present invention has a configuration different from that of a conventional liquid crystal display device, and therefore, in the following description, only the inverter will be described.
[0019]
(First Embodiment)
FIG. 1 is a diagram showing an inverter according to a first embodiment of the present invention. FIG. 2 is a diagram provided for describing the operation of the inverter of FIG.
[0020]
Referring to FIG. 1, an inverter 10 according to a first embodiment of the present invention includes a DC voltage source 12, a control circuit 6 connected to both ends of the DC voltage source 12, and both ends connected to the control circuit 6. And a fluorescent tube 2 connected to one end of the transformer 1.
[0021]
The inverter 10 includes a rectifier diode 4 having an anode connected to the other end of the fluorescent tube 2 as a tube current detecting circuit, and a current detecting circuit having one end connected to the other end of the fluorescent tube 2 and the other end grounded. It comprises a resistor 3 and a smoothing capacitor 5 having one end connected to the cathode side of the rectifier diode 4 and the other end grounded.
[0022]
Further, the inverter includes, as a temperature detecting circuit, a thermistor 7, a voltage dividing resistor 8, and a rectifying diode 9 having an anode connected to the midpoint of the thermistor 7 and the voltage dividing resistor 8. The cathode side of the rectifier diode 9 is connected, and this connected portion is connected to the control circuit 6.
[0023]
In the inverter 10 having such a configuration according to the first embodiment of the present invention, the tube current flowing through the fluorescent tube 2 connected to one end of the transformer 1 is converted into a voltage by the current detecting resistor 3, and the rectifier diode 4 The current is rectified and smoothed by the smoothing capacitor 5 and input to the control circuit 6 as a first voltage value. On the other hand, the midpoint of the thermistor 7 and the voltage dividing resistor 8 connected between the constant voltages is input to the control circuit 6 via the rectifier diode 9 as the second voltage value.
[0024]
At room temperature, the voltage at the midpoint of the thermistor 7 and the voltage dividing resistor 8 (second voltage value) is set lower than the voltage (first voltage value) obtained by rectifying and smoothing the tube current. Keep the tube current constant without any effect.
[0025]
At a high temperature, the resistance value of the thermistor 7 decreases, so that the voltage at the midpoint of the thermistor 7 and the voltage dividing resistor 8 (second voltage value) becomes a voltage (first voltage value) obtained by rectifying and smoothing the tube current. ). Therefore, the input voltage to the control circuit 6 is in a state similar to the apparent increase in the tube current, and as a result, the operation of reducing the tube current is performed.
[0026]
That is, as shown in FIG. 2, only when the temperature exceeds the inverter ambient temperature set by the thermistor 7 and the voltage dividing resistor 8, the tube current decreases substantially in inverse proportion to the ambient temperature.
[0027]
(Second embodiment)
FIG. 3 is a circuit diagram showing an inverter according to a second embodiment of the present invention. The inverter 20 according to the second embodiment of the present invention shown in FIG. 3 has a zener diode 11 connected in the opposite direction instead of the rectifier diode 9 of the inverter 10 according to the first embodiment of the present invention of FIG. Things.
[0028]
In the rectifier diode 9 according to the first embodiment, it is necessary to change the resistance value of the thermistor in order to change the set temperature at which the tube current starts decreasing and the inclination of the decrease, but the thermistor generally has no fine constant. However, in the second embodiment, since the Zener diode 11 is connected in the opposite direction instead of the rectifier diode 9, the Zener voltage can be finely set, so that the set temperature and the inclination can be easily changed, and the design can be improved. This has the effect of increasing the degree of freedom. Except for such effects, the inverter 20 according to the second embodiment of the present invention has the same effects as those of the inverter 10 according to the first embodiment of the present invention shown in FIG.
[0029]
(Third embodiment)
FIG. 4 is a circuit diagram showing an inverter according to a third embodiment of the present invention. Although the inverters 10 and 20 according to the first and second embodiments shown in FIGS. 1 and 3 are for controlling the current of one fluorescent tube 2, the third embodiment of the present invention shown in FIG. In the inverter 30 of the embodiment, another fluorescent tube 2 ′ is used in parallel with the fluorescent tube 2. When a plurality of fluorescent tubes 2 and 2 'are used in this case, the screen of the liquid crystal panel of the liquid crystal display device can be displayed brighter.
[0030]
It is apparent that the inverter 30 according to the third embodiment of the present invention has the same effects as the inverters 10 and 20 according to the first and second embodiments of the present invention.
[0031]
(Fourth embodiment)
FIG. 5 is a circuit diagram showing an inverter according to a fourth embodiment of the present invention. In the inverter 40 according to the fourth embodiment shown in FIG. 5, the transformer 1 for input to the fluorescent tubes 2 and 2 'of the inverter 30 in the third embodiment of the present invention shown in FIG. One transformer 1 ′ is provided in parallel with the input transformer 1.
[0032]
When lighting a plurality of fluorescent tubes, there is a case where it is necessary to use a plurality of transformers due to a problem of power of the transformer or a problem of balance accuracy of the tube current. In such a case, the tube current is further stabilized. be able to. It is apparent that the fourth embodiment of the present invention can provide the same effect as the inverter 30 according to the third embodiment using a plurality of fluorescent tubes 2, 2 '.
[0033]
(Fifth embodiment)
FIG. 6 is a circuit diagram showing an inverter according to a fifth embodiment of the present invention. The inverter 50 according to the fifth embodiment of the present invention shown in FIG. 5 is similar to the inverter 30 according to the third embodiment of the present invention shown in FIG. However, like the inverter 20 according to the second embodiment of the present invention shown in FIG. 3, the rectifier diode 9 is used instead of the zener diode 11 in the opposite direction. . As described above, since the zener diode 11 is connected in the opposite direction instead of the rectifier diode 9, the zener voltage can be finely set, so that the set temperature and the inclination can be easily changed, and the degree of freedom in design increases. It has. It is apparent that the same effects as those of the inverter 30 according to the first and third embodiments of the present invention can be obtained.
[0034]
(Sixth embodiment)
FIG. 7 is a circuit diagram showing an inverter according to a sixth embodiment of the present invention. In the inverter 60 according to the sixth embodiment of the present invention shown in FIG. 7, the transformer 1 for input to the fluorescent tubes 2 and 2 'in the fourth embodiment of the present invention shown in FIG. Three transformers 1 'are provided in parallel with the input transformer 1, but similar to the inverter 20 according to the second embodiment of the present invention shown in FIG. The difference is that the Zener diode 11 is used in the direction. Since the zener diode 11 is connected in the opposite direction instead of the rectifier diode 9, the zener voltage can be finely set as in the second and fifth embodiments of the present invention. The inclination and the inclination can be easily changed, which has the effect of increasing the degree of freedom in design. In the case of the inverter 60 according to the sixth embodiment of the present invention, the same effect as that of the inverter 40 according to the fourth embodiment of the present invention using a plurality of transformers 1 and 1 'and fluorescent tubes 2 and 2' is obtained. It is clear that it can be obtained.
[0035]
【The invention's effect】
As described above, according to the present invention, it is possible to realize an inverter in which the tube current is kept constant at a temperature equal to or lower than the set temperature and the tube current is reduced in inverse proportion to the ambient temperature above the set temperature. Thus, it is possible to provide an inverter capable of suppressing a decrease in the service life of a pipe while suppressing a temperature rise at a high temperature.
[Brief description of the drawings]
FIG. 1 is a circuit diagram showing an inverter according to a first embodiment of the present invention.
FIG. 2 is a diagram showing tube current-ambient temperature characteristics of the inverter of FIG.
FIG. 3 is a circuit diagram showing an inverter according to a second embodiment of the present invention.
FIG. 4 is a circuit diagram showing an inverter according to a third embodiment of the present invention.
FIG. 5 is a circuit diagram showing an inverter according to a fourth embodiment of the present invention.
FIG. 6 is a circuit diagram showing an inverter according to a fifth embodiment of the present invention.
FIG. 7 is a circuit diagram showing an inverter according to a sixth embodiment of the present invention.
FIG. 8 is a circuit diagram showing an example of a conventional inverter for controlling a tube current to be constant.
FIG. 9 is a circuit diagram showing another example of a conventional inverter that controls a tube current by temperature.
FIG. 10 is a diagram showing tube current-ambient temperature characteristics of the inverters of FIGS. 8 and 9;
[Explanation of symbols]
1, 1 'Transformer 2, 2' Fluorescent tube 3 Current detection resistor 4 Rectifier diode 5 Smoothing capacitor 6 Control circuit 7 Thermistor 8 Voltage divider 9 Rectifier diode 10, 20, 30, 40, 50, 60, 100, 110 Inverter 11 Zener diode 12 DC voltage source

Claims (5)

An inverter that stabilizes a tube current by detecting and controlling a current flowing through a fluorescent tube has a function of reducing the tube current as the ambient temperature rises when the ambient temperature of the inverter exceeds a set temperature. The inverter has a function of detecting and stabilizing the tube current with a resistor connected in series to the fluorescent tube, and connects the thermistor connected between a certain voltage and the midpoint of the resistor to the tube via a diode. An inverter having a configuration connected to a current detection point. 2. The inverter according to claim 1, wherein a plurality of the fluorescent tubes are provided, and a function of detecting and stabilizing the tube current by a resistor connected so that a total current of the tube currents of the plurality of fluorescent tubes flows is provided. And the inverter. 3. The inverter according to claim 1, wherein the diode is a Zener diode. A control circuit that controls the tube current flowing through the fluorescent tube according to the input voltage to be constant, a tube current detection circuit that detects the tube current and outputs a first voltage value according to the value, A temperature detection circuit that outputs a second voltage value according to an ambient temperature, wherein the control circuit outputs a larger voltage value among the first voltage value and the second voltage value to the input voltage. Controlling the tube current. A liquid crystal display device using the inverter according to claim 1, wherein the inverter is used for lighting a fluorescent tube for a backlight unit of a liquid crystal panel. Liquid crystal display device.

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