JP3574333B2 - Backlight driving method for liquid crystal display device and backlight unit - Google Patents

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a backlight driving method and a backlight unit for a liquid crystal display device mainly used outdoors, such as a liquid crystal display device of a vehicle-mounted AV (audio visual) system or a vehicle-mounted navigation system.
[0002]
[Prior art]
FIG. 5 is a schematic diagram showing a configuration of a liquid crystal display device used for a vehicle-mounted AV system or a navigation system.
The liquid crystal display panel 31 includes a pair of transparent substrates, liquid crystal sealed between the transparent substrates, and a pair of polarizing plates disposed with the pair of transparent substrates interposed therebetween. One of the pair of transparent substrates is provided with a transparent common electrode and a color filter, and the other is formed with transparent pixel electrodes arranged in a matrix. When a voltage is applied between the common electrode and the pixel electrode of the liquid crystal display panel 31, the liquid crystal molecules between the common electrode and the pixel electrode are arranged along the electric field, and the light transmittance of that portion of the liquid crystal display panel 31 is changed. Changes. By controlling the applied voltage for each pixel electrode, a desired image can be displayed on the liquid crystal display panel 31.
[0003]
The liquid crystal display panel 31 is attached to a box-shaped lamp house 30. In the lamp house 30, a cold cathode tube 32 is arranged as a backlight. A heater 33 is wound around the cold cathode tube 32, and the heater 33 heats the cold cathode tube 32. Further, between the liquid crystal display panel 31 and the cold cathode tube 32, a milky white plate 35 for scattering light output from the cold cathode tube 32 is arranged. Further, a temperature sensor 34 such as a thermistor is provided near the cold cathode tube 32.
[0004]
The cold-cathode tube 32 has the property that the luminance greatly changes depending on the temperature. For example, when the temperature is 0 ° C., the luminance of the cold-cathode tube 32 is about 10 to 20% of the maximum luminance. Therefore, in the liquid crystal display device, when the temperature of the cold cathode tube 32 is low, the contrast is significantly deteriorated.
A liquid crystal display device used outdoors such as a liquid crystal display device of an in-vehicle AV system or a navigation system is exposed to severe temperature changes and may be used in an environment of 0 ° C. or lower. For this reason, in the liquid crystal display device used for the in-vehicle AV system or the navigation system, as described above, the heater 33 is wound around the cold cathode tube 32, and the temperature inside the lamp house 30 is detected by the temperature sensor 34. When the detected temperature is lower than a preset temperature, the heater 33 is energized to heat the cold-cathode tube 32 to secure the brightness.
[0005]
However, in the liquid crystal display device having the above-described structure, a step of winding the heater 33 around the cold cathode tube 32 is required, so that there is a disadvantage that the product cost is increased. Further, since it is necessary to energize the heater 33, there is a disadvantage that power consumption is large.
In order to eliminate such disadvantages, a so-called self-heating type cold cathode tube has been developed in recent years, in which the temperature rises sharply due to the heat generated by the lamp itself when energization is started. The use of this type of cold cathode tube eliminates the need for a step of winding a heater around the lamp, thereby simplifying the manufacturing process, reducing product cost, and reducing power consumption.
[0006]
When using a self-heating type cold-cathode tube, it is necessary to supply a large current to the lamp in order to raise the temperature, but even if the temperature increases, the brightness of the lamp will decrease if a large current is continued to be supplied. In addition, the liquid crystal display panel may be damaged due to the high temperature. Therefore, in a self-heating type cold cathode tube, it is necessary to reduce the supply current as the temperature rises. Normally, in a liquid crystal display device using a self-heating type cold cathode tube, a temperature sensor such as a thermistor is arranged near the lamp, and a current corresponding to the detected temperature is supplied to the lamp.
[0007]
[Problems to be solved by the invention]
However, since the on-vehicle AV system and the navigation system are exposed to severe temperature changes, the temperature sensor may fail or the wiring of the temperature sensor may be short-circuited due to dew condensation. In such a case, in the conventional backlight unit, since a correct temperature cannot be detected by the temperature sensor, a large current is continuously supplied to the lamp, and the liquid crystal display panel may be damaged.
[0008]
Accordingly, it is an object of the present invention to provide a backlight driving method and a backlight unit for a liquid crystal display device which can prevent an excessive rise in temperature even if an abnormality occurs in a temperature sensor and can prevent damage to a liquid crystal display panel. It is.
[0009]
[Means for Solving the Problems]
Problems described above, in the backlight driving method of a liquid crystal display panel and liquid crystal display device constituted by a backlight disposed on the back, after starting the energization of the lamp, and detects the temperature at predetermined time intervals The temperature rise due to the heat generation of the lamp is monitored by comparing with the temperature at the time of the previous measurement, and when the temperature rise cannot be detected, the power supplied to the lamp is limited, and the power that is not likely to damage the liquid crystal display panel. The problem is solved by a backlight driving method for a liquid crystal display device.
[0010]
The above-mentioned problem is solved by detecting a temperature by a temperature sensor at predetermined time intervals after starting the energization of the lamp and the lamp disposed on the back surface of the liquid crystal display panel, and comparing the temperature at the previous measurement. detects the temperature rise due to heat generation of the lamp, the power corresponding to the output of the temperature sensor is supplied to the lamp, when the increase in temperature is not detected by said temperature detecting means to limit the power supplied to the lamp the And a power supply unit that supplies power that does not cause damage to the liquid crystal display panel .
[0011]
Hereinafter, the operation of the present invention will be described.
In the method of the present invention, when energization of the lamp is started, a temperature rise due to heat generation of the lamp is monitored. If the temperature rise is not detected even when the lamp is energized, the temperature sensor may be faulty, so the power supplied to the lamp is limited to a preset value. As a result, an excessive rise in temperature can be avoided, and damage to the liquid crystal display panel can be prevented.
[0012]
Further, the backlight unit of the present invention has a temperature sensor and power supply means, and the power supply means supplies power to the lamp according to the output of the temperature sensor. When the temperature sensor does not detect a rise in temperature, the power supply unit limits the power supplied to the lamp to a preset value. Thus, even if the temperature sensor fails, it is possible to prevent the lamp from being supplied with excessive electric power, thereby preventing the liquid crystal display panel from being damaged.
[0013]
As the lamp, a self-heating type cold cathode tube can be used. Further, since the brightness of the cold cathode tube is affected by the temperature of the tube surface, it is preferable that the temperature sensor is disposed on the tube surface.
Further, the power supply means includes, for example, a pulse signal generation unit that generates a pulse signal having a duty ratio corresponding to a temperature detected by a temperature sensor, and a pulse signal generation unit that generates a pulse signal only during a period in which the pulse signal is “1”. And a high-voltage oscillation circuit that supplies lamp driving power to the lamp.
[0014]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings.
FIG. 1 is a schematic diagram showing a liquid crystal display device using a backlight unit according to an embodiment of the present invention.
The liquid crystal display panel 11 includes a pair of transparent substrates, a liquid crystal sealed between the transparent substrates, and a pair of polarizing plates disposed with the pair of transparent substrates interposed therebetween. One of the pair of transparent substrates is provided with a transparent common electrode and a color filter, and the other is formed with transparent pixel electrodes arranged in a matrix. When a voltage is applied between the common electrode and the pixel electrode of the liquid crystal display panel 11, the liquid crystal molecules between the common electrode and the pixel electrode are arranged along the electric field, and the light transmittance of that portion of the liquid crystal display panel 11 is changed. Changes. By controlling the applied voltage for each pixel electrode, a desired image can be displayed on the liquid crystal display panel 11.
[0015]
The liquid crystal display panel 11 is attached to a box-shaped lamp house 10. A self-heating type cold cathode tube 12 is mounted as a backlight in the lamp house 10, that is, on the back side of the liquid crystal display panel 11. For example, a rod-shaped or U-shaped cold-cathode tube 12 is used.
A milky white plate 15 for scattering light output from the cold cathode tube 12 is disposed between the liquid crystal display panel 11 and the cold cathode tube 12. A temperature sensor 14 such as a thermistor is mounted on the surface of the cold cathode tube 12.
[0016]
FIG. 2 is a block diagram illustrating a circuit configuration of the backlight unit according to the present embodiment. The backlight unit 20 according to the present embodiment includes a control unit (pulse signal generation unit) 21 including a microcomputer, a switching element 22, a high-voltage oscillation circuit, in addition to the cold cathode tube 12 and the temperature sensor 14 described above. 23, a sensor amplifier 24, and an A / D (analog / digital) converter 25.
[0017]
The sensor amplifier 24 converts a change in the resistance value of the sensor 14 due to the temperature into an electric signal. The output of the sensor amplifier 24 is input to the A / D converter 25, converted into a digital signal, and input to the control unit 21. The control unit 21 generates a pulse signal having a duty ratio of 100% (DC) to 70% according to the temperature detected by the temperature sensor 14 and outputs the pulse signal to the switching element 22.
[0018]
The switching element 22 is connected to a vehicle-mounted battery (power supply) 29, is turned on during a period in which the pulse signal given from the control unit 21 is “1”, transmits the power supply voltage (about 12 V) to the high-voltage oscillation circuit 23, Are off.
The high-voltage oscillation circuit 23 oscillates at a predetermined frequency to generate a high-frequency high-voltage pulse for driving the lamp while the power supply voltage is being supplied via the switching element 22, and supplies the high-frequency high-voltage pulse to the cold cathode tube 12. I do.
[0019]
FIG. 3 is a diagram showing the relationship between the temperature measured by the temperature sensor 14 and the duty ratio of the pulse signal output from the control unit 21. When the temperature is 15 ° C. or lower, the duty ratio of the pulse signal is set to 100% (the switch element 22 is always on), and when the temperature is 50 ° C. or higher, the duty ratio is set to 70%. When the temperature is between 15 ° C. and 50 ° C., the duty ratio of the pulse signal is controlled to decrease as the temperature increases, as shown in FIG.
[0020]
In the present embodiment, the minimum value of the duty ratio of the pulse signal is set to 70%, and as described later, when an abnormality occurs in the temperature sensor 14, the control unit 21 sets the duty ratio of the pulse signal to 70%. In this case, the minimum value of the duty ratio can secure the brightness of the cold cathode tube 22 necessary for obtaining a good contrast and increase the temperature of the cold cathode tube 12 to a temperature at which the liquid crystal display panel 11 may be damaged. It is necessary to set it on the condition that it is not done. Since this depends on the structure of the cold cathode tube 12 and the lamp house 10 to be used, it is necessary to determine the optimum value by conducting experiments and the like in advance.
[0021]
FIG. 4 is a flowchart illustrating the operation of the backlight unit driving circuit according to the present embodiment.
When the power of the liquid crystal display device is turned on, first, in step S11, the control unit 21 initializes the value of the variable i (i = 0). Thereafter, the process proceeds to step S12, and the control unit 21 detects the current temperature T0 from the output of the temperature sensor 14. Then, the process proceeds to step S13, in which a pulse signal (see FIG. 3) having a duty ratio corresponding to the temperature T0 is output to turn the switching element 22 on and off.
[0022]
Next, the process proceeds to step S14, and the control unit 21 waits for a predetermined time (for example, 2 seconds) to elapse. Then, after a predetermined time has elapsed, the process proceeds to step S15, and the current temperature T1 is detected from the output of the temperature sensor 14. Thereafter, the process proceeds to step S16, and the control unit 21 calculates a temperature difference W between the previous detected temperature T0 and the current detected temperature T1. Then, in step S17, it is determined whether or not the temperature difference W is within a predetermined range (for example, ± 1 ° C.). If the temperature difference W is larger than the predetermined range (YES), the process proceeds to step S18, and the current detection is performed. After setting the temperature T1 to the detected temperature T0, the process returns to step S13.
[0023]
On the other hand, when the temperature difference W is within the predetermined range in step S17, the process proceeds to step S19, and the control unit 21 increments the variable i. Thereafter, the process proceeds to step S20, and it is determined whether the variable i is equal to or more than a predetermined value n (for example, n = 10). If i is smaller than n (NO), the process proceeds to step S18, where the current detected temperature T1 is set to T0, and then returns to step S13.
[0024]
If the variable i is equal to or greater than n in step S20 (YES), it means that the temperature does not rise despite the current being supplied to the cold cathode tube 12, and the temperature sensor 14 has an abnormality. Probably occurred. Therefore, the process proceeds to step S21, and the control unit 21 maintains the duty ratio of the pulse signal supplied to the switching element 22 to a preset value (70% in the present embodiment). As a result, the current supplied to the cold cathode tube 12 is limited to 70% of the maximum current, and a rise in the temperature of the cold cathode tube 12 is avoided. Also, in this case, 70% of the power at the maximum is supplied to the cold-cathode tube 12, so that although the temperature rise speed of the cold-cathode tube 12 becomes slow, the temperature gradually rises to obtain good contrast. Required brightness can be secured.
[0025]
As described above, in the present embodiment, if the temperature rise is not detected by the temperature sensor 14 even when the power supply to the cold cathode tubes 12 is started, the power supplied to the cold cathode tubes 12 is set to a predetermined value. Restrict to Thus, even if the temperature sensor 14 breaks down or a short circuit occurs due to dew condensation, an excessive current is prevented from being supplied to the cold-cathode tube 12, and the liquid crystal display panel 11 can be prevented from being damaged due to a rise in temperature. Further, in the present embodiment, even if an abnormality occurs in the temperature sensor 14, a preset power is supplied to the cold-cathode tube 12, so that it is possible to secure the luminance necessary for obtaining good contrast. it can.
[0026]
In the above-described embodiment, a case has been described in which the present invention is applied to a direct-type backlight. However, the present invention can also be applied to an edge-lit backlight (sidelight). Further, in the above-described embodiment, the case where the present invention is applied to the display device of the in-vehicle AV system or the navigation system has been described. However, the scope of the present invention is not limited to the in-vehicle electronic device. Absent.
[0027]
【The invention's effect】
As described above, according to the present invention, when the temperature sensor cannot detect a rise in temperature after the lamp is energized, the power supplied to the lamp is limited to a constant value. Even if it occurs, the temperature of the lamp can be prevented from becoming high, and damage to the liquid crystal display panel can be prevented. Also in this case, since a certain amount of power is supplied to the lamp, the temperature of the lamp rises slowly, but the temperature of the lamp gradually rises, and the brightness required to obtain good contrast can be secured. . Therefore, according to the present invention, there is an effect that the reliability of a liquid crystal display device used particularly in an environment where temperature change is severe is improved.
[Brief description of the drawings]
FIG. 1 is a schematic diagram showing a liquid crystal display device using a backlight unit according to an embodiment of the present invention.
FIG. 2 is a block diagram illustrating a circuit configuration of a backlight unit according to the embodiment of the present invention.
FIG. 3 is a diagram illustrating a relationship between a temperature measured by a thermistor and a duty ratio of a pulse signal output from a control unit.
FIG. 4 is a flowchart illustrating the operation of the backlight unit driving circuit according to the embodiment of the present invention.
FIG. 5 is a schematic diagram showing a configuration of a liquid crystal display device used for a vehicle-mounted AV system or a navigation system.
[Explanation of symbols]
10,30 lamp house,
11,31 liquid crystal display panel,
12, 32 cold cathode tubes,
14,34 temperature sensor,
20 backlight unit,
21 control part,
22 switching elements,
23 High voltage oscillation circuit,
24 sensor amplifiers,
25 A / D converter,
33 heater.

Claims (5)

In a backlight driving method of a liquid crystal display device including a liquid crystal display panel and a backlight disposed on the back thereof ,
After energization of the lamp is started , the temperature is detected at predetermined intervals and compared with the temperature at the time of the previous measurement to monitor the temperature rise due to the heat generated by the lamp. If the temperature rise cannot be detected, supply the lamp to the lamp A method of driving a backlight of a liquid crystal display device , wherein the power is limited so as not to damage the liquid crystal display panel . A lamp arranged on the back of the liquid crystal display panel,
After starting energizing the lamp, and detects the temperature by the temperature sensor at predetermined time intervals, and detects the temperature rise due to heat generation of the lamp by comparing the temperature at the time of previous measurement, the output of the temperature sensor Power supply means for supplying the corresponding power to the lamp, and limiting the power to be supplied to the lamp when the temperature rise is not detected by the temperature detection means so that the power does not cause damage to the liquid crystal display panel. A backlight unit comprising: The backlight unit according to claim 2, wherein the lamp is a self-heating type cold cathode tube. The backlight unit according to claim 2, wherein the temperature sensor is disposed on a tube surface of the lamp. The power supply unit includes a pulse signal generation unit that generates a pulse signal having a duty ratio corresponding to a temperature detected by the temperature sensor, and a high-voltage supply unit that supplies lamp driving power to the lamp only during a period when the pulse signal is “1”. The backlight unit according to claim 2, further comprising an oscillation circuit.

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