JPH1187078A - Cold cathode tube drive circuit - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a cold cathode tube drive circuit whereby a cold cathode tube is made usable to brightness lower compared with conventional situation in a drive circuit for the cold cathode tube used in a back light or the like in a liquid crystal display device. SOLUTION: A switch circuit 13 is provided, between a first/second terminal 15, 16 supplying an output of a high frequency pulse generating circuit 12 and a terminal 17, 18 connecting a cold cathode tube 10, which switching connects by a frequency of about 1 KHZ between a third terminal 17 and the first/second terminal 15, 16 and between a fourth terminal 18 and the second terminal 16, 15.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a driving circuit for a cold cathode tube used for a backlight or the like of a liquid crystal display device.
In particular, the present invention relates to a cold-cathode tube driving circuit suitable for a display of an in-vehicle device that needs to change luminance over a wide range.

[0002]

2. Description of the Related Art In recent years, liquid crystal display devices have been used for displays of in-vehicle navigation devices and in-vehicle televisions. In a liquid crystal display device, a cold cathode tube is usually used as a backlight. This type of cold cathode tube includes a straight tube type and a U-tube type. Further, as shown in FIG. 5, the cold cathode tube 30 is
One or more (two in the figure) are arranged in one. As shown in FIG. 6, the backlight case 31 is disposed on the back of the liquid crystal display panel 32, and a light diffusion sheet 33 for preventing luminance unevenness is provided between the cold cathode tube 30 and the liquid crystal display panel 32. Is arranged.

FIG. 7 is a block diagram showing a cold-cathode tube driving circuit used for a display of a conventional vehicle-mounted navigation device. This type of cold-cathode tube driving circuit includes a DC-DC converter 31 that converts a voltage supplied from a vehicle-mounted battery into a constant voltage, and a voltage supplied from the DC-DC converter 31 as a power supply voltage. A high-frequency pulse generating circuit 32 for generating a high-frequency pulse voltage for driving. The high-frequency pulse generation circuit 32 includes an input terminal 34 to which the control signal PWM is supplied, and an output terminal 3 connected to the cold cathode tube 30.
7, 38. The output terminal 38 is connected to the ground.

[0004] The high-frequency pulse generating circuit 32 outputs a control signal P
For example, a pulse voltage having a frequency of about 40 kHz is output between the output terminals 37 and 38 during a period when WM is “H”. The cold cathode tube 30 emits light by the high frequency pulse voltage. By the way, in the case of displays for in-vehicle televisions and navigation devices, the illuminance inside the vehicle varies extremely from about 200,000 lux to several lux, so in order to improve the visibility of the liquid crystal display in any environment, the backlight It is required that the luminance can be varied over a wide range.

In the cold-cathode tube driving circuit shown in FIG. 7, by changing the duty ratio of the control signal PWM, the period W for supplying the pulse voltage is changed as shown in FIG. 8 to increase the brightness of the cold-cathode tube. Can be changed.

[0006]

There is a demand for a display that can be used to a lower luminance as a display of a vehicle-mounted navigation device and a backlight of a vehicle-mounted television. However, in the conventional cold-cathode tube driving circuit, when the duty ratio of the control signal PWM becomes about 1% or less, FIG.
As shown in (1), the ground side of the cold-cathode tube becomes dark, and display unevenness occurs on the liquid crystal display device.

From the above, it is an object of the present invention to provide a driving circuit for a cold cathode tube used for a backlight of a liquid crystal display device and the like, and a driving circuit for a cold cathode tube capable of using the cold cathode tube to a lower luminance than before. It is to provide.

[0008]

SUMMARY OF THE INVENTION An object of the present invention is to provide a cold-cathode tube driving circuit for supplying a high-frequency pulse voltage to a cold-cathode tube and lighting the cold-cathode tube, thereby generating a high-frequency pulse voltage of a first frequency. A pulse generation circuit, first and second terminals to which the output of the high-frequency pulse generation circuit is supplied, and third and fourth terminals to which the cold cathode tubes are connected,
A switch circuit that switches and connects between the third terminal and the first and second terminals and between the fourth terminal and the second and first terminals at a second frequency; The problem is solved by a cold-cathode tube driving circuit characterized in that:

The operation of the present invention will be described below. In the present invention, the first and second terminals to which the output of the high-frequency pulse generating circuit is supplied and the third terminal to which the cold cathode tube is connected.
And a switch circuit is provided between the switch and the fourth terminal. The switch circuit causes a constant frequency (second frequency) between the third terminal and the first and second terminals and between the fourth terminal and the second and first terminals. Switch connection. That is, in the present invention, since the switching side switches the ground side and the high voltage side of the cold-cathode tube at high speed, even if the ground side of the cold-cathode tube becomes dark, the apparent brightness of the entire cold-cathode tube becomes uniform. . Therefore, it is possible to use the cold-cathode tube in a lower luminance range than before.

It is preferable that the switch circuit switches at the constant frequency only when the duty ratio of the control signal falls below a certain range. As a result, a reduction in power consumption by the switch circuit can be suppressed.

[0011]

Embodiments of the present invention will be described below with reference to the accompanying drawings. FIG. 1 is a block diagram showing a cold cathode tube driving circuit according to an embodiment of the present invention.
The DC-DC converter 11 is connected to a vehicle-mounted battery (not shown), and outputs a constant voltage even when the battery voltage fluctuates. The high frequency pulse generation circuit 12 has an input terminal 1
The control signal PWM is input via the control signal 4. The high-frequency pulse generation circuit 12 uses the voltage supplied from the DC-DC converter 11 as a drive voltage, and has a frequency of about 40 kHz and a voltage of 500 during the period when the control signal PWM is “H”.
A pulse voltage of ~ 600V is output between terminals 15 and 16.

The terminal 16 is connected to the ground.
In the present embodiment, the control signal PWM is generated by a microcomputer (not shown) constituting a liquid crystal display panel driving circuit. For example, when the user operates the brightness adjustment unit of the liquid crystal display device, the microcomputer changes the duty ratio of the control signal PWM.

The switch circuit 13 is connected between terminals 15 and 16 to which the output of the high-frequency pulse generation circuit 12 is supplied and terminals 17 and 18 connected to the cold cathode tube 10. The oscillator 19 generates a pulse signal having a frequency of about 1 kHz, and the signal generated by the oscillator 19 is input to the switch circuit 13 via the switch 20. Switch circuit 1
The reference numeral 3 switches the polarity of the pulse voltage generated by the high-frequency pulse generation circuit 12 in synchronization with the pulse signal input from the oscillator 19, and outputs the voltage between terminals 17 and 18. Switch 2
0 is turned on / off by a signal from the microcomputer that generates the control signal PWM. The microcomputer turns on the switch 20 when the duty ratio of the control signal PWM becomes 1% or less.

FIG. 2 is a circuit diagram showing an example of the high-frequency pulse generation circuit 12. A power supply voltage is supplied to the power supply line 21 from the DC-DC converter 11. Also, this power line 2
Reference numeral 1 is connected to an intermediate contact of a coil L2 of the transformer T via a coil L1. Further, the base of the transistor Q1 is connected to the control signal input terminal 14 via the resistor R1, and the collector of the transistor Q1 and the power supply line 2 are connected.
1, a resistor R2 is connected, and the emitter is connected to ground.

A resistor R3 is connected to the collector of the transistor Q1.
Is connected, and a capacitor C1 is connected between the other end of the resistor R3 and the ground. The interconnection point of the resistor R3 and the capacitor C1 is connected to the transistor Q2,
Connected to each base of Q3. These transistors Q2 and Q3 are connected in series between the power supply line 21 and the ground to form an inverter. That is, the collector of the transistor Q2 is connected to the power supply line 21, the collector of the transistor Q3 is connected to the ground,
2, The emitters of Q3 are connected to each other. The output terminal of the inverter, that is, the interconnection point between the emitters of the transistors Q2 and Q3 is connected to the base of the transistor Q4 via a resistor R4 and to the base of the transistor Q5 via a resistor R5. . The emitters of these transistors Q4 and Q5 are both connected to ground, the collector of transistor Q4 is connected to one end of coil L2 of transformer T, and the collector of transistor Q5 is connected to the other end of coil L2.
A capacitor C2 is connected between the collectors of these transistors Q4 and Q5. Further, the base of the transistor Q4 is connected to one end of the coil L3 of the transformer T, and the base of the transistor Q5 is connected to the other end of the coil L3. These transistors Q4, Q5
The transistor T forms an oscillator.

A boosted pulse voltage is output from the coil L4 of the transformer T. A capacitor C3 is connected between one end of the coil L4 and the terminal 15, and the other end of the coil L4 is connected to the terminal 16 and the ground.
FIG. 3 is a circuit diagram illustrating an example of the switch circuit 13.
The terminal 15 is connected to the emitters of the transistors Q6 and Q9. The terminal 16 is connected to the ground.
The collector of the transistor Q6 is connected to the terminal 17, and a resistor R7 is connected between the base and the emitter.
Further, the base of the transistor Q6 is connected to the collector of the transistor Q7 via the resistor R8. The emitter of the transistor Q7 is connected to ground, and the base is connected to an inverter 27 described later via a resistor R9. The terminal 17 is connected to the collector of the transistor Q8. The emitter of the transistor Q8 is connected to ground, and the base is connected to an inverter 26 described later via a resistor R10.

On the other hand, the collector of the transistor Q9 is connected to the terminal 18, and a resistor R11 is connected between the base and the emitter.
Is connected. Further, the base of the transistor Q9 is connected to the collector of the transistor Q10 via the resistor R12. The emitter of the transistor Q10 is connected to ground, and the base is connected to the inverter 2 via a resistor R13.
6 is connected. The terminal 18 is connected to the transistor Q
Connected to 11 collectors. This transistor Q11
Is connected to the ground, and the base is connected to the inverter 27 via the resistor R14.

The input terminal of the inverter 26 is connected to the switch 20. The output terminal of the inverter 26 is connected to the input terminal of the inverter 27, as described above, to the base of the transistor Q8 via the resistor R10, and to the base of the transistor Q10 via the resistor R13. ing. As described above, the output terminal of the inverter 27 is connected to the base of the transistor Q7 via the resistor R9, and is connected to the base of the transistor Q11 via the resistor R14.

Hereinafter, the operation of the cold cathode tube driving circuit of the present embodiment configured as described above will be described. However,
In the initial state, it is assumed that the duty ratio of the control signal PWM is sufficiently large, the switch 20 is off, and the input terminal of the inverter 26 is kept at "H". While the control signal PWM input to the input terminal 14 is "H", the output of the transistor Q1 is "L". Therefore, transistor Q2
, Q3 becomes "H", an oscillator composed of transistors Q4 and Q5 and a transformer T oscillates, and a high-frequency pulse voltage is output between terminals 15 and 16. Thereby, the switch circuit 1
A high-frequency pulse voltage is supplied to the cold-cathode tube 10 via 3 and the whole cold-cathode tube 10 is turned on.

On the other hand, while the control signal PWM is at "L",
The output of the transistor Q1 becomes "H", and the output of the inverter constituted by the transistors Q2 and Q3 becomes "L".
become. As a result, the oscillation of the oscillator constituted by the transistors Q4 and Q5 and the transformer T is stopped, and the cold-cathode tube 10 is turned off. When the duty ratio of the control signal PWM is changed, the period during which the pulse voltage is output from the high frequency pulse generation circuit 12 (see FIG. 8) changes, and the cold cathode fluorescent lamp 10
, The brightness of the cold-cathode tube 10 changes. That is, the luminance of the cold cathode tube 10 can be changed by changing the duty ratio of the control signal PWM.

When the duty ratio of the control signal PWM becomes 1% or less (that is, the period of "H" becomes 1% or less for one cycle), the microcomputer turns on the switch 20. As a result, the frequency of the switch circuit 13 is about 1 kHz.
A pulse signal of z is supplied, and the transistors Q6 and Q9, the transistors Q7 and Q10, and the transistors Q8 and Q11 are alternately turned on.
It turns off, and the polarity of the pulse voltage output from the terminals 17 and 18 changes at a cycle of about 1 kHz.

When the duty ratio of the control signal PWM becomes about 1% or less, the ground side of the cold cathode fluorescent lamp 10 becomes dark. However, in the present embodiment, the pulse voltage of the pulse voltage supplied to the cold cathode fluorescent lamp 10 at a cycle of about 1 kHz. Since the polarity changes, as shown in FIG. 4, the period in which the upper half of the cold cathode tube 10 is dark and the period in which the lower half is dark alternate alternately, and the entire cold cathode tube 10 appears to be dimly glowing. Therefore, the cold-cathode tube driving circuit according to the present embodiment has an advantage that the cold-cathode tube 10 can be used to a lower luminance than in the related art.

In the above embodiment, the case where the switch 20 is turned on and the switch circuit 13 is switched and driven when the duty ratio of the control signal PWM is 1% or less has been described. When using at normal brightness, place a brightness sensor near the ground side,
The switch circuit 13 may be driven at a fixed frequency when the luminance on the ground side falls below a certain value.

[0024]

As described above, according to the present invention,
The switch circuit switches between the first and second terminals to which the output of the high-frequency pulse generating circuit is applied and the third and fourth terminals to which the cold cathode tubes are connected at the second frequency, so that the cold cathode tubes Even if a dark portion occurs on one side, the position of the dark portion changes alternately, and the brightness of the entire cold-cathode tube can be made uniform. As a result, there is an effect that the cold-cathode tube can be used to a lower luminance than in the related art.

[Brief description of the drawings]

FIG. 1 is a block diagram showing a CCFL drive circuit according to an embodiment of the present invention.

FIG. 2 is a circuit diagram showing an example of a high-frequency pulse generation circuit of the CCFL drive circuit.

FIG. 3 is a circuit diagram showing an example of a switch circuit of the CCFL drive circuit.

FIG. 4 is a schematic diagram showing a position change between a bright part and a dark part of the cold cathode tube according to the embodiment of the present invention.

FIG. 5 is a plan view showing a backlight case of the liquid crystal display device.

FIG. 6 is a perspective view illustrating a structure of a liquid crystal display device.

FIG. 7 is a block diagram showing a driving circuit of a conventional cold cathode tube.

FIG. 8 is a diagram showing waveforms of a control signal supplied to the CCFL drive circuit and an output signal of the CCFL drive circuit.

FIG. 9 is a diagram showing a problem of a conventional cold cathode tube driving circuit.

[Explanation of symbols]

 10, 30 Cold-cathode tube 11, 31 DC-DC converter 12, 32 High-frequency pulse generation circuit 13 Switch circuit 14, 34 Control signal input terminal 15, 16, 17, 18, 37, 38 terminal 19 Oscillator 20 Switch 21 Power line 26 , 27 inverter 31 backlight case 32 liquid crystal display panel 33 light diffusion sheet

Claims (3)

[Claims] A cold-cathode tube driving circuit for supplying a high-frequency pulse voltage to the cold-cathode tube to light the cold-cathode tube; a high-frequency pulse generating circuit for generating a pulse voltage of a first frequency; First and second terminals to which the output of the circuit is supplied; third and fourth terminals to which the cold cathode tubes are connected; and between the third terminal and the first and second terminals. And a switch circuit for switching connection between the fourth terminal and the second and first terminals at a second frequency. 2. The cold-cathode tube according to claim 1, wherein a control signal is supplied to the high-frequency pulse generation circuit, and the pulse voltage is output only during a period when the control signal is at a high level or a low level. Drive circuit. 3. The cold-cathode tube driving circuit according to claim 2, wherein the switch circuit performs switching based on the second frequency only when a duty ratio of the control signal is equal to or less than a predetermined value.