JPH04245217A - Back light for liquid crystal display - Google Patents

Abstract

PURPOSE:To eliminate a back-light remaining out of lighting when it is maximally dimmed at a low temp. by furnishing a voltage controlling/frequency changing circuit in the fore-stage of an inverter, and thereby changing the DC voltage level and heightening the frequency at the time of maximum dimming. CONSTITUTION:In the fore-stage of an inverter 1 to convert DC into AC, a voltage controlling/frequency changing circuit 4 is installed, which exerts the function of a voltage control circuit 3 to change the DC voltage level and also heightens the frequency at the time of maximum dimming. The output waveform of this circuit 4 is such that the voltage level at maximum dimming is lower than at min. dimming, but that the frequency is higher than at the min. dimming. Accordingly a high frequency is fed to the inverter 1 from the circuit 4 when the output is minimum, and the drop voltage of the inverter capacitor is sunk.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to backlights for liquid crystal displays.

0002

2. Description of the Related Art As a conventional backlight for a liquid crystal display, for example, one disclosed in Japanese Patent Application Laid-open No. 123428/1983 is known. This structure, as shown in Figure 7,
A cold cathode tube 2 is connected to an inverter 1 that converts direct current to alternating current.
A voltage control circuit 3 for changing the DC voltage is provided before the inverter 1, and the brightness of the cold cathode tube 2 is adjusted by this voltage control circuit 3.

0003

[Problems to be Solved by the Invention] The above-mentioned prior art does not take into consideration the temperature characteristics of the cold cathode tube 2, and has the problem that the cold cathode tube will not light up at the minimum dimming at low temperatures. there were.

SUMMARY OF THE INVENTION An object of the present invention is to provide a backlight for a liquid crystal display that can eliminate non-lighting during minimum dimming at low temperatures.

[0005]

[Means for Solving the Problems] In order to achieve the above object, in a backlight for a liquid crystal display in which a cold cathode tube is lit by an inverter that converts direct current into alternating current, the voltage level of the direct current is changed at a stage before the inverter. At the same time, voltage control /
It is equipped with a variable frequency circuit.

[0006]

[Operation] When dimming is at its minimum, the frequency increases due to the voltage control/frequency variable circuit. This reduces the drop voltage of the output capacitor of the inverter section, effectively increasing the voltage applied to the cold cathode tube, and preventing non-lighting at minimum dimming at low temperatures.

[0007]

Embodiment An embodiment of the present invention will be described below with reference to FIGS. 1 to 6. As shown in Figure 1, in the front stage of the inverter 1 that converts DC to AC, in addition to the function of a voltage control circuit 3 that changes the voltage level of DC as in the conventional example, it also has a function of increasing the frequency when dimming is at its minimum. Voltage control/frequency variable circuit 4
is provided. The output waveform of this voltage control/frequency variable circuit 4 is as shown in FIG. 2, and when the dimming is at its minimum, the voltage level is lower than when the dimming is at its maximum, but the frequency is higher than when it is at its maximum. ing.

[0008] In this way, when the output is at its minimum, the voltage control
Since a high frequency is input to the inverter 1 from the variable frequency circuit 4, the drop voltage of the capacitor in the inverter 1 is reduced. This increases the voltage applied to the cold cathode tube 2 and prevents it from turning off at low temperatures.

FIG. 3 shows the inverter 1 of FIG. 2 in more detail. That is, the input waveform of the inverter 1 from the voltage control/frequency variable circuit 4 becomes a waveform as shown in FIG. The drop voltage of the output capacitor C2 of inverter 1 decreases. Therefore, this prevents the light from turning off at low temperatures.

FIG. 4 shows a detailed embodiment of the voltage control/frequency variable circuit 4 shown in FIG. In FIG. 4, when VR1, that is, variable resistor R1 and variable capacitor C are changed, the output frequency changes. Furthermore, when the switch SW1 is switched, the output voltage changes. Therefore, when switching the switch SW1 for dimming, if VR1 is also made variable at the same time, voltage control and frequency control can be performed at the same time.

FIG. 5 shows an exploded perspective view of a liquid crystal display device using a backlight according to this embodiment, and FIG. 6 is a sectional view of FIG. 5 when assembled. In FIGS. 5 and 6, 1
1 is a frame, 12 is a liquid crystal display element, 13 is a backlight inverter power supply, 14 is a mold, 15 is a cold cathode ray tube (CFL), 16 is a diffusion plate, 17 is a light guide, and 18 is a reflector/back cover. . By employing the backlight shown in this embodiment for such a liquid crystal display element, it is possible to prevent the liquid crystal display device from turning off at low temperatures.

0012

According to the present invention, by increasing the frequency at the minimum dimming time, the voltage applied to the cold cathode tube increases, thereby preventing non-lighting at low temperatures.

[Brief explanation of the drawing]

FIG. 1 is a block diagram showing one embodiment of the present invention.

[Figure 2] shows the output waveform of the voltage control/frequency variable circuit,
(a) is a waveform diagram when dimming is maximum, and (b) is a waveform diagram when dimming is minimum.

FIG. 3 is a circuit block diagram showing details of the inverter in FIG. 2;

FIG. 4 is a detailed diagram of the voltage control/frequency variable circuit in FIG. 3;

FIG. 5 is an exploded perspective view of a liquid crystal display device incorporating a backlight according to the present invention.

FIG. 6 is a cross-sectional view of the liquid crystal display device when FIG. 5 is assembled.

FIG. 7 is a block diagram of a conventional example.

[Explanation of symbols]

1 Inverter 2 Cold cathode tube 4 Voltage control/frequency variable circuit

Claims (1)

[Claims] 1. In a backlight for a liquid crystal display in which a cold cathode tube is lit by an inverter that converts direct current to alternating current, a voltage is provided at a stage before the inverter that changes the voltage level of the direct current and increases the frequency when dimming is at its minimum. A backlight for a liquid crystal display characterized by being equipped with a control/frequency variable circuit.