JPH06302387A - Dimming device for cold cathode discharge tube - Google Patents

Abstract

PURPOSE:To ensure freedom from an influence due to the change of ambient temperature by detecting current flowing through a cold cathode discharge tube, converting a detected current value to a voltage value for supply to a voltage control circuit, and then controlling a dimming circuit on the basis of an output signal from the voltage control circuit. CONSTITUTION:DC voltage is supplied from a power circuit 1 to a lighting circuit 2, and converted to AC voltage in the circuit 2. Then, the AC voltage outputted from the circuit 2 is fed to a cold cathode discharge tube 3, and this discharge tube 3 generates luminous energy corresponding to the supplied AC voltage. Current flowing in the tube 3 is fed to a tube current detecting circuit 4 and the amount thereof is detected. This current is supplied to a controller 5. Then, this controller 5 performs a control process so that voltage supplied from the circuit 4 becomes equal to the predetermined value. The controller 5 sends a PWM signal to the power circuit 1, thereby keeping the output voltage of the circuit 4 at the predetermined value.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a cold cathode discharge tube light control device suitable for use as a backlight of a display panel of a personal computer having a liquid crystal display, a liquid crystal television or the like.

[0002]

2. Description of the Related Art Televisions and personal computers having a liquid crystal display panel are known. A backlight is attached to the back surface of the liquid crystal display panel, which makes it easy to visually see images and characters displayed on the display panel. For example, the brightness of the backlight (cold-cathode discharge tube) is adjusted when sunlight or light from a fluorescent lamp enters the liquid crystal display being used from the outside, and images and characters on the display panel are displayed. You can see it with optimum contrast.

[0003]

By the way, in the brightness adjustment of the above-mentioned backlight, the duty ratio of the lighting time is changed simply by changing the power supply voltage of the lighting circuit or by PWM (pulse width modulation). It is done by

When the brightness of the backlight is adjusted in this way, the brightness level of the backlight fluctuates due to variations in the characteristics of the parts used in the lighting circuit. In addition, if the ambient temperature changes, especially at low temperature, the lighting of the backlight becomes unstable. Therefore, the brightness level cannot be set too low. Further, the brightness level also changes due to fluctuations in the power supply voltage. Due to such a cause, the image quality of the image signal displayed on the display panel is deteriorated. Further, once the brightness level is set, the brightness is maintained. Therefore, it is not possible to change the brightness level or change the brightness level changing speed depending on the operating state of the device.

Therefore, an object of the present invention is to suppress the change in the brightness level due to the characteristics of the components used in the lighting circuit and the change in the power supply voltage, and to stably light the cold cathode discharge tube without being affected by the change in the ambient temperature. It is an object of the present invention to provide a cold cathode discharge tube light control device capable of controlling the temperature.

[0006]

According to the present invention, there is provided a cold cathode discharge tube, a tube current detecting circuit for detecting a current flowing through the cold cathode discharge tube, and an integrating circuit for integrating a detection output obtained from the tube current detecting circuit. A cold cathode discharge tube light control device comprising a voltage control circuit which is controlled according to an output signal of an integrating circuit and a lighting circuit which controls an AC voltage based on an output signal of the voltage control circuit.

[0007]

The tube current detection circuit detects the current flowing through the cold cathode discharge tube. The detected current value is converted to a voltage value and then supplied to the controller. In the controller, a PWM signal based on this voltage value is formed and supplied to the dimming circuit.

[0008]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described below with reference to the drawings. FIG. 1 shows a schematic diagram of a cold cathode discharge tube light control device according to the present invention. In FIG. 1, for example, a DC voltage in the range of 0 to 12 V is supplied from the power supply circuit 1 to the lighting circuit 2.
Is output to. The lighting circuit 2 includes a DC-AC inverter circuit, in which a DC voltage is converted into an AC voltage. The AC voltage output from the lighting circuit 2 is supplied to the cold cathode discharge tube 3. In the cold cathode discharge tube 3, a light quantity corresponding to the supplied AC voltage is generated. The current flowing through the cold cathode discharge tube 3 is supplied to the tube current detection circuit 4,
The amount of current is detected. This current is the controller 5
It is output to the A / D port of. The controller 5 generates a voltage corresponding to the supplied current value.

The controller 5 controls the voltage supplied from the tube current detection circuit 4 to a predetermined value. A PWM signal is supplied from the controller 5 to the power supply circuit 1, and the above operation is repeated based on this signal. Finally, the output voltage of the tube current detection circuit 4 is set to a predetermined value.

FIG. 2 shows a circuit diagram for realizing the cold cathode discharge tube light control device according to the present invention. The configuration of this circuit will be described below. The cold cathode discharge tube light control device shown in FIG. 2 includes a light control circuit, a lighting circuit, a cold cathode discharge tube,
It consists of a tube current detection circuit and a controller. The input terminal 11 to which a predetermined DC voltage is input is connected to one end of the capacitor 12, the emitter of the transistor 13 and one end of the resistor 14. The other end of the capacitor 12 is grounded.

An input terminal 15 to which a PWM signal output from a controller (not shown) is supplied is connected to one end of a diode 17 and one end of a coil 18 via a resistor 16. The other end of the diode 17 is grounded. Coil 1
The other end of 8 is connected to one end of the capacitor 19 and the transistor 2
It is connected to the base of 0. The other end of the capacitor 19 is grounded. The emitter of the transistor 20 is grounded via the resistor 21. The collector of the transistor 20 is connected to the other end of the resistor 14 and the base of the transistor 13.

The collector of the transistor 13 is connected to one end of the capacitor 22 and one end of the coil 23. The other end of the capacitor 22 is grounded. The capacitor 22 prevents fluctuations in the output voltage of the lighting circuit. Coil 2
The other end of 3 is one end of the resistor 24 and the winding 2 of the transformer 25.
5a. The coil 23 prevents fluctuations in the power supply voltage and makes the output waveform of the transformer 25 a sine wave. The other end of the resistor 24 is connected to the winding 25b of the transformer 25. The resistor 24 causes a base current to flow in a transistor described later. One end of the winding wire 25b is connected to the base of the transistor 26, and the other end of the winding wire 25b is connected to the base of the transistor 27.
The collector of the transistor 26 is connected to one end of the capacitor 28, and the collector of the transistor 27 is connected to the other end of the capacitor 28. The oscillation frequency is determined by the capacitor 28. Transistors 26 and 2
The respective emitters of 7 are connected to each other at a connection point 29 and are grounded.

One end of the winding 25c of the transformer 25 is grounded, and the other end is connected to one end of a cold cathode discharge tube 31 via a capacitor 30. The capacitor 30 limits the current flowing through the cold cathode discharge tube. The other end of the cold cathode discharge tube 31 is connected to one end of the resistor 32 and the anode side of the diode 33. The other end of the resistor 32 is grounded. The cathode side of the diode 33 is connected to the output terminal 37 via one ends of the capacitor 34, the resistor 35, and the zener diode 36. The other ends of the capacitor 34, the resistor 35, and the zener diode 36 are grounded.
The Zener diode 36 limits the maximum voltage supplied to the output terminal 37.

The operation of this circuit will be described below with reference to FIGS. It is assumed that a predetermined voltage is supplied to the input terminal 11. Further, as shown in FIG. 3A, the input terminal 15 has a P signal from a controller (not shown).
The WM output voltage is provided. The PWM voltage is converted into a DC voltage according to the duty ratio of the pulse signal by the integrating circuit including the coil 18 and the capacitor 19 (see FIG. 3B). This DC voltage is supplied to the base of the transistor 20. As a result, the signal input via the input terminal 11 is supplied to the transistor 13. Note that the transistor 13 and the transistor 20 form a voltage control circuit.

At the collector of the transistor 13, the voltage supplied to the circuit in the subsequent stage is formed (see FIG. 3C). This voltage is supplied to the connection point between the capacitor 22 and the coil 23. The DC voltage passed through the coil 23 is DC-A
It is supplied to the C inverter circuit (composed of 24-28).

The current output from the coil 23 is the resistance 2
4 to the transistor 26. As a result, the transistor 26 is turned on. Therefore, the current from the power source is applied to the coil 23, the winding 25a, the transistor 26.
Then, an electromotive force is generated in the winding 25b by electromagnetic induction, which flows between the base-emitter and the connection point 29. This electromotive force makes it difficult for the current supplied to the transistor 26 to flow. On the contrary, the current output from the coil 23 is the resistance 2
4, the transformer 25, the base-emitter of the transistor 27, and the connection point 29. As a result, the transistor 27 is turned on, and the current from the power source
The winding 25a, the collector-emitter of the transistor 27, and the connection point 29 flow in this order. The direction of electromagnetic induction at this time is opposite to that described above.

The AC voltage that has passed through the winding wire 25c is supplied to the capacitor 30. The input voltage to the capacitor 30 is shown in Figure 3D. The output voltage of the capacitor 30 is
It is supplied to the cold cathode discharge tube 31. The cold cathode discharge tube 3
When a high voltage is applied to No. 1, it illuminates brightly, while when a low voltage is applied, it illuminates with low illuminance.

The value of the current flowing through the cold cathode discharge tube 31 is detected by the tube current detection circuit in the subsequent stage. The current is supplied to the A / D port of the controller via the output terminal 37. A voltage corresponding to the supplied current is generated at the A / D port. In the controller, the duty ratio of the PWM signal supplied to the input terminal 15 is determined so that the voltage of the A / D port becomes a preprogrammed voltage. This PWM signal is supplied to the input terminal 15 from the controller.

By the way, when the cold cathode discharge tube 31 is turned on,
When the output voltage of the lighting circuit and the ambient temperature are constant, the cold cathode discharge tube 31 lights up at a substantially constant brightness level. However, if the output voltage from the lighting circuit changes or the ambient temperature changes due to variations in the characteristics of the components forming the lighting circuit (in particular, the inductance of the winding 25c of the transformer), a constant voltage is applied from the input terminal 11. Although applied, the current flowing through the cold cathode discharge tube 31 changes. Therefore, the brightness level of the cold cathode discharge tube 31 changes and the lighting becomes unstable.

Therefore, the cold cathode discharge tube 3 is used in the tube current detection circuit.
The actual current flowing through 1 is detected, and the current value is converted into a voltage value in the controller. Cold cathode discharge tube 3
Even if the characteristics of each element in the lighting circuit vary, the voltage value corresponding to the output current of the tube current detection circuit with respect to the brightness level during lighting of No. 1 is set in the controller in advance, even if the characteristics of each element in the lighting circuit vary. The tube 31 can be turned on at a predetermined brightness level.

When the cold cathode discharge tube 31 is lit at the lowest brightness level, the cold cathode discharge tube 31 is lit based on the setting condition by setting the lowest brightness level for lighting at the lowest operating temperature in the controller. To be done. Further, the brightness of the cold cathode discharge tube 31 can be freely controlled by setting the controller to change the brightness level according to the operating state of the device, or to gradually change the brightness level at the time of turning on and off. It will be possible.

[0022]

According to the present invention, it is possible to prevent a change in the brightness level due to a variation in each of the parts used in the lighting circuit and a change in the brightness level due to a change in the power supply voltage. Further, even when the ambient temperature changes, the current flowing in the discharge tube is kept constant, so that the lamp can be lit with stable brightness. Further, by inputting into the software a program for gradually converting the brightness level at the time of turning on or off, various controls can be performed on the cold cathode discharge tube.

[Brief description of drawings]

FIG. 1 is a schematic view of a cold cathode discharge tube light control device according to the present invention.

FIG. 2 is a circuit diagram of a cold cathode discharge tube light control device.

FIG. 3 is a signal waveform of each circuit constituting the cold cathode discharge tube light control device.

[Explanation of symbols]

 2 Lighting circuit 3, 31 Cold cathode discharge tube 4 Tube current detection circuit 5 Controller

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁵ Identification code Office reference number FI technical display location H05B 41/29 Z 9249-3K

Claims (3)

[Claims] 1. A cold cathode discharge tube, a tube current detecting circuit for detecting a current flowing through the cold cathode discharge tube, an integrating circuit for integrating a detection output obtained from the tube current detecting circuit, and an output of the integrating circuit. A cold cathode discharge tube light control device comprising a voltage control circuit which is controlled according to a signal, and a lighting circuit which controls an AC voltage based on an output signal of the voltage control circuit. 2. The cold cathode discharge tube light control device according to claim 1, wherein a PWM signal is supplied to the integrating circuit. 3. The cold cathode discharge tube light control device according to claim 1, wherein the cold cathode discharge tube is used for a backlight.