JPH0256593A - Light source device and image display device - Google Patents

Abstract

PURPOSE:To compensate the brightness of a fluorescent discharge tube at low temperature by connecting such a capacitor that the temperature coefficient of the capacity value indicates a negative temperature coefficient for the tube current limitation of the fluorescent discharge tube in series with the fluorescent discharge tube. CONSTITUTION:This is the backlighting device using the fluorescent discharge tube, the capacitor 11 is connected in series with the fluorescent discharge tube for the current limitation of the fluorescent discharge tube, and the temperature coefficient of the capacity value of the capacity indicates the negative temperature coefficient in an optional temperature range below the room temperature. For example, a capacitor whose low-temperature electrostatic capacity value increases by tens of % at the room temperature is used as the current limiting capacitor 11. Consequently, the brightness at low temperature can be compensated.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a backlight device and an image display device used in a transmission type image display board.

[Conventional technology]

In an image display device using an image display board such as a transmissive liquid crystal panel for image display, a backlight device such as a firefly or a photodischarge tube is generally used as a light source. Fluorescent discharge tubes are generally inexpensive, have a high color temperature, high brightness, and are highly efficient, so they are often used in image display devices. However, the drawback is that the brightness at low temperatures, especially below freezing, is extremely low. :The temperature had dropped by 11℃.

Figure 2 shows a cold shade (
A characteristic diagram showing an example of temperature-brightness characteristics of a backlight device using fluorescent discharge tubes is shown.

You can understand it with the same diagram! 1t has extremely low brightness at low temperatures,
At around minus 20 degrees Celsius, the temperature is 41, which is about 1/4 of that at room temperature.

Conventionally, as a method to compensate for the drawbacks of backlight devices using such fluorescent discharge tubes, there has generally been a method of varying the power input to the backlight device depending on the ambient temperature. An example of a specific configuration is shown in FIG. In the same figure,
By varying the voltage applied to the lighting circuit 17 in accordance with the ambient temperature, the power input to the fluorescent discharge tube 18 can be varied.

The applied voltage is varied by using a variable resistor 16 to convert the voltage from the power source 14 into a constant voltage power source 15. As another prior art, in JP-A-58-80299, the oscillation frequency of a high-frequency inverter is varied by a variable resistor in accordance with the ambient temperature, thereby compensating for the tube current at low temperatures.

[Problem to be solved by the invention]

The former method of varying the applied voltage using the constant voltage power supply 2 is basically a method of dividing the voltage of the power supply 1 by the constant voltage power supply 2, so the constant voltage power supply 2 has a loss of 11↑. big. Among image display devices, devices used indoors have a relatively large degree of freedom in terms of the power source they use, so even if the power loss of the devices is large, this does not pose a serious problem. However,
For 4M type devices used outdoors, such as pocket televisions and electronic viewfinders built into video cameras, there is often no way to receive power other than using a battery. Therefore, in the former conventional example, the magnitude of power loss was a serious problem. Therefore, as in the latter prior art, a method was devised to increase or decrease the tube current by changing the impedance of an impedance element inserted into the load side using a Q-factor that varies the oscillation frequency depending on the ambient temperature. Although this method requires less work 1f to cause power loss as in the former method, it is necessary to provide a temperature detector for detecting the ambient temperature and to manually vary the variable resistor 7 in response to changes in the temperature detector. Furthermore, in order to adequately compensate for the tube current at low temperatures, it is necessary to have a wide variable range of oscillation frequency, which requires an independent oscillation circuit, and it is difficult to achieve a wide variable range with a self-excited type using a transistor.

Therefore, the present invention attempts to solve the problems of image display devices using conventional transmission type image display plates, and its purpose is to improve The purpose is to compensate for the brightness of the fluorescent discharge tube at least at low temperatures by using the temperature characteristics of the capacitor used to limit the tube current of the tube.

[Means to solve the problem]

In order to solve the above problems, the backlight device of the present invention has the following features:
For all backlight devices using fluorescent discharge tubes, a capacitor is connected in series with the fluorescent discharge tube for the fluorescent discharge tube 'KL 9M IIJ, and the temperature coefficient of the capacitance value of the capacitor is However, at least in any temperature range below room temperature, η1, which exhibits a negative temperature coefficient, is expressed as f,
It is assumed to be a y sign.

Further, an image display device incorporating such a backlight device is constructed.

〔Example〕

ffj Figure 1 shows the lighting circuit of the backlight device of the present invention. In the figure, a cold cathode fluorescent discharge tube 12 is used for the backlight, and the circuit type is a self-excited bush-pull circuit. are doing. The circuit operation will be explained below.

The description will start from a state in which the power supply 1 is applied to the circuit via the choke coil 2. First, assume that transistor 8 is energized when the power is turned on. The current flows into the collector winding 4, and at the same time a voltage is induced in the base winding 5, causing the transistor 8
is further driven in the energizing direction. The transistor 8 rapidly becomes saturated due to the positive feedback action of the base winding, and since voltage is applied from the power supply 1 to both ends of the collector winding 4, the core magnetic flux increases linearly and the core reaches saturation. When the transformer 3 is saturated, the current increases rapidly and reaches a value limited by the drive voltage provided by the base winding 5, after which the voltage across the collector winding 4 drops,
Transistor 8 begins to switch off. Therefore, an induced voltage is generated in the transformer 3 in the opposite direction to the previous one, and a voltage is similarly induced in the collector winding 4 of the Ranjistaf (to which it is coupled), and as a result, the transistor 7 starts energizing.

Transistor 7 is then quickly switched on and transistor 8 is switched off.

Thereafter, the operation is similar to that when the transistor 7 is switched on, and this cycle is repeated, resulting in continuous oscillation. The voltage generated by the continuous oscillation is generated in the load winding 6, which is a secondary winding, according to the winding ratio with respect to the collector winding 4, and is passed through the cold cathode through the current limiting capacitor 11 that limits the tube current. The fluorescent discharge tube 12 is turned on. The relationship between the brightness of the cold cathode fluorescent discharge tube 12 and the tube current is proportional, and as the tube current increases, the brightness also increases. Therefore, it can be seen that in order to increase the brightness, it is sufficient to increase the capacitance of the current limiting capacitor 11 that limits the tube current and lower the impedance. In the present invention, the low temperature capacitance value of the current limiting capacitor 11 is several 10% of that at room temperature.
I used a capacitor that was larger than that. Note that, herein, normal temperature is defined as any temperature between +10 degrees Celsius and 30 degrees Celsius.

FIG. 4 shows the capacitance change rate versus ambient temperature of the capacitor used in the present invention. The capacitor is a ceramic capacitor whose main material is strontium titanate, and the Curie point of the capacitance is selected to be around minus several tens of degrees Celsius, so that the capacitance change at low temperatures, especially around minus 20 degrees Celsius, is less than normal temperature ( For example, it is more than 30% larger than 25 degrees Celsius). - Capacitors with similar characteristics between the shells are called YN type, Y5S type, Y5T type, or D type. FIG. 5 shows the results of improving brightness by increasing the tube current at low temperatures using the capacitor described above.

The solid line graph shows the conventional characteristics shown in FIG. 2 for comparison. According to FIG. 5, the brightness at low temperatures, especially around minus 20 degrees Celsius, is about twice that of the conventional technology, and even around zero degrees Celsius, the brightness is significantly improved by about 30%.

Therefore, it can be seen that the present invention greatly compensates for the brightness at low temperatures.

In the present invention, in addition to the example shown in FIG. 4, a material showing various temperature characteristics above room temperature as shown in FIG. 6 may be used. Graph (a) shows the temperature characteristics of the capacitor explained in FIG. 4, and is shown for comparison. Graph (b) has the same characteristics as graph (a) below room temperature, but exhibits approximately flat temperature characteristics above room temperature. Similarly, graph (C) shows the same characteristics as graph (a) below normal temperature, but shows a characteristic that changes to a positive temperature coefficient above normal temperature. Graph (b)
The effect of using a capacitor with these characteristics is to reduce the reduction in brightness at high temperatures. In other words, as shown in Figure 5, with conventional characteristics, the brightness decreases at high temperatures (above 40 degrees Celsius), but with the capacitor having the temperature characteristics shown in graph (a) of Figure 6, the brightness can be further reduced. This will cause it to decline. Therefore, if a capacitor having the temperature characteristic shown in graph (b) is used, the influence of the decrease in brightness at high temperatures can be somewhat reduced, but the effect is small. Therefore, by using a capacitor with temperature characteristics as shown in graph (C), it is possible to significantly improve the decrease in brightness at high temperatures. FIG. 7 shows temperature-luminance characteristics when several types of capacitors having different temperature characteristics are used. In the figure, the brightness compensation effect at low temperatures is the same as that in FIG. 2, but a large 1 degree compensation effect appears at high temperatures, especially at 40 degrees Celsius or higher.

Graphs (1), (2), and (3) are the graphs in Figure 6 (
a), (b), and (c), and the graph (3
), the brightness is completely compensated at high temperatures.

Therefore, it can be understood that brightness compensation is performed not only at low temperatures but also at high temperatures, depending on the temperature characteristics of the capacitor. Although it is desirable for a single capacitor to exhibit the above-mentioned temperature characteristics, similar effects can be expected by using a plurality of capacitors with different temperature characteristics. In the embodiment of the present invention, further improvement of the compensation effect at low temperatures can be achieved by using a capacitor with a larger temperature coefficient at temperatures below room temperature. Furthermore, in order to maximize the effects of the present invention,
The temperature coefficient of the tube current limiting capacitor exhibits a temperature-brightness characteristic completely opposite to that of a cold cathode fluorescent discharge tube.

Further, FIGS. 8 and 9 show an example in which the connection method of the tube current limiting capacitor is slightly improved in the lighting circuit of the backlight device of the present invention shown in FIG. 1. In both figures, only the secondary side of the oscillation transformer 19 is shown, and the primary side circuit is omitted.

In Fig. 8, a ceramic capacitor 20 such as a YN type having the temperature characteristics shown in Fig. 4 and a ceramic capacitor 21 such as an SL type having a small capacitance change rate with respect to temperature changes are connected in series as a tube current limiting capacitor. Here is an example. The effect shown in the figure is that by limiting the upper limit of the change in the capacitance of the composite capacitor 20.21, which is a tube current limiting capacitor, at low temperatures, the tube current can be unlimitedly reduced by cold cathode fluorescent discharge at extremely low temperatures. The purpose is to prevent it from flowing into the pipe 22.

In FIG. 9, contrary to FIG. 8, ceramic capacitors 20 such as YN type and ceramic capacitors 21 such as S L W are connected in six rows. The effect shown in the figure limits the lower limit of the change in the capacitance value of the composite capacitor at high temperatures,
The purpose is to limit the lower limit of the tube current flowing through the cold cathode fluorescent discharge tube 22 at high temperatures.

In the present invention, a self-excited bush-pull circuit is shown as the backlight lighting circuit, but the same effect can be obtained even if it is a separately excited type or a single circuit. Further, although a cold cathode fluorescent discharge tube is shown as an example of the fluorescent discharge tube, the same effect can be obtained even if a hot cathode fluorescent discharge tube is used.

Furthermore, in the embodiment of the backlight device of the present invention, the temperature characteristics of the capacitor for limiting the tube current are utilized;
By replacing or combining any temperature-impedance conversion elements such as resistors or thermistors, a brightness compensation effect at low temperatures or high temperatures can be expected.

FIG. 10 shows a block diagram of an image display device using the backlight device of the present invention. This figure shows an example of a color liquid crystal electronic viewfinder that uses a color liquid crystal panel as an electronic viewfinder used in a video camera among image display devices.

A video composite signal is input to an input 23, and a portion thereof is input to a Y-C separation circuit 24 that separates a luminance signal and a color signal, and passes through a color demodulation circuit 25 to produce three primary color signals of RlG and B. The primary color signals are input to the liquid crystal panel 27 through an RGB rotation circuit 26 that rotates the three primary colors in accordance with the arrangement of color filters on the liquid crystal panel 27. Further, a part of the video composite signal inputted to the input 23 passes through a synchronization division circuit 28 and a synchronization circuit 29 to produce a synchronization signal. The signal is then input to the liquid crystal panel 27 via a clock generation circuit 30 and a level shift circuit 31 in order to create a lock necessary for driving the liquid crystal panel 27. Here, the liquid crystal panel 27 used is an active matrix type having a shift 1/register and a driver built therein.

Further, power is supplied to the input 32 from the video camera body. The supplied power is used to generate necessary voltages for each part by the switching power supply circuit 33. Furthermore, the input 32 is also directly connected to the lighting circuit 34 of the backlight device of the present invention, and supplies power to the backlight lighting circuit 34 to light the cold cathode fluorescent discharge tube 35. Further, the switching power supply circuit 33 and the lighting port i/334 may be configured as a ml'B7 path to improve the efficiency of the power supplied from the human power 32. As explained above, the backlight lighting circuit 34 compensates for the brightness at low and high temperatures based on the temperature characteristics of the current limiting capacitor.

As mentioned above, we have shown an example of an electronic viewfinder as an image display device, but other applications include video tape recorders with a built-in liquid crystal display, which are often taken outdoors like video cameras, or liquid crystal televisions. Naturally, the present invention can be used for equipment, etc., and can also be applied to light sources such as projection type liquid crystal display devices.

Further, although products using liquid crystal display elements as transmissive display elements have been mainly described, the present invention can also be applied to light sources for other 3n transmissive display elements.

〔Effect of the invention〕

As described above, the backlight device of the present invention reduces the reduction in brightness of the fluorescent discharge tube at low temperatures by providing a tube current limiting capacitor with a temperature of 1-7, and the reduction in brightness reduces the brightness of the image display device 1. Solved the problem that made it difficult to see the displayed contents of the screen. In this case, the present invention performs temperature compensation for brightness without adding any negative parts or requiring any adjustment, which greatly contributes to product miniaturization and high reliability. Furthermore, since the method increases the tube current only in a temperature environment where brightness decreases, there is no power loss at room temperature.
"f rj:" is the optimal method for power saving, and extends the battery usage time, which is always a problem with equipment used outdoors. Since the battery can be made smaller, it will greatly contribute to the miniaturization of products, which is the most important issue for equipment used outdoors.Also, changing compensation 1 for brightness reduction is possible by using capacitors with different temperature characteristics. Since it is only necessary to change to , it becomes possible to correspond to fluorescent tubes with various different characteristics.

Temperature-brightness characteristic diagram when using the sensor.

FIG. 8 is a backlight lighting circuit diagram showing a part of another embodiment of the present invention.

FIG. 9 is a backlight lighting circuit diagram showing a part of another embodiment of the present invention.

[Brief explanation of the drawing]

FIG. 1 is a lighting circuit diagram of a backlight device for explaining one embodiment of the present invention. FIG. 2 is a temperature-luminance characteristic diagram of the backlight device from FNOC. FIG. 3 is a block diagram of a conventional backlight device. Figure 4 shows the temperature-capacitance change 1 of the capacitor used in the present invention. De sex diagram. FIG. 5 is a temperature-luminance characteristic diagram when the capacitor shown in the characteristic diagram of FIG. 4 is used. FIG. 6 is a graph showing an example of improved temperature-brightness characteristics of a capacitor used in the backlight device of the present invention. FIG. 7 is a block diagram of the compact image display device shown in the characteristic diagram of FIG. 6. 13...Capacitor 9.10...Resistance or more Applicant Seiko Epson Co., Ltd. Agent Patent attorney Masayoshi Ueyanagi 1 person Figure 1 Figure 2 Broom 4 Figure 6 Figure 7 Figure 8 Figure

Claims (2)

[Claims] (1) In a backlight device using fluorescent discharge tubes,
A capacitor is connected in series with the fluorescent discharge tube for limiting the tube current of the fluorescent discharge tube, and the temperature coefficient of the capacitance value of the capacitor is at least in any temperature range below room temperature,
A backlight device characterized by exhibiting a negative temperature coefficient. (2) An image display device characterized by incorporating the backlight device according to claim 1.