JPH10308289A - Cold cathode tube lighting circuit - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide such a cold cathode tube lighting circuit as being capable of ensuring lighting of a cold cathode tube and preventing the starting malfunction. SOLUTION: A cold cathode tube lighting circuit has an alternating current generating circuit 14 to drive a load 15 as a cold cathode tube with a high frequency, a light control circuit to control the brightness of the cold cathode tube in accordance with a light control signal LC, a control circuit 13 to control and stabilize a current in the load 15, a starting circuit 11 to control the control circuit 13 and the alternating current generating circuit 14 for generating voltage required for discharge start in accordance with a starting signal ST and an operation control circuit 16 to control the operation of the light control circuit 12.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a cold-cathode tube lighting circuit for driving a cold-cathode tube at a high frequency.

[0002]

2. Description of the Related Art Cold cathode fluorescent lamps have a small outer diameter and a long service life, and are widely applied as a backlight source of a liquid crystal display panel or a lighting fixture. Cold cathode tube is several tens kHz
It is driven at a high frequency of several hundred kHz, and its luminance adjustment method includes burst dimming and current dimming.

In the burst dimming, a high-frequency current flowing through the cold-cathode tube is intermittently operated, and the conduction period is adjusted to control the time-average luminance of the cold-cathode tube. The intermittent frequency is usually set to 100 Hz or higher so that flicker is not perceived by human eyes, and the luminance decreases as the duty ratio of the burst waveform decreases.

In the current dimming, the high frequency current flowing through the cold-cathode tube is operated continuously, and the brightness of the cold-cathode tube is controlled by adjusting the magnitude of the high-frequency current. Since the luminance is almost proportional to the power consumption of the cold-cathode tube, the lower the current, the lower the luminance.

The lighting circuit of a cold cathode tube is composed of a high-frequency, high-voltage power supply circuit, generally has an input power supply terminal and a terminal for adjusting brightness, and further has a lighting / turning-on / off state according to an external logic signal.
Some devices have a terminal that can control turning off the light. The luminance adjustment terminal is configured to directly connect an external variable resistor or to input an external dimming signal as a voltage.

FIG. 5 is a block diagram showing an example of a conventional cold-cathode tube lighting circuit. This lighting circuit adjusts the conduction period or the magnitude of the high-frequency current based on the dimming signal LC and the AC generation circuit 4 for driving the load 5 which is a cold-cathode tube at a high frequency, thereby increasing the brightness of the cold-cathode tube. A dimming control circuit 2 for controlling, a control circuit 3 for controlling and stabilizing the current of the load 5, and a control circuit 3 and an AC circuit for generating a voltage necessary for starting discharge based on a start signal ST. Generation circuit 4
And a starter circuit 1 for controlling the power supply.

The operation will be described. When the voltage of the input power supply terminal (not shown) reaches the starting voltage or a lighting signal is input and the starting signal ST of FIG. 5 changes from a low level to a high level, for example, the starting circuit 1 operates and the control circuit 3 By generating a pulse signal, the AC generation circuit 4 generates a voltage equal to or higher than the discharge starting voltage of the load 5 (cold cathode tube). Once the discharge of the load 5 starts, the discharge continues only by the control circuit 3 and the AC control circuit 4 and the role of the start circuit 1 ends, so that the operation time of the start circuit 1 is usually about 1 millisecond. is there.

On the other hand, assuming that the load 5 has failed,
A safety circuit (not shown) for stopping the entire operation when the load current does not flow even after the operation of the start-up circuit 1 is provided. Further, after the operation of the start-up circuit 1 is completed, the dimming control circuit 2 operates to adjust the conduction period or the magnitude of the high-frequency current according to the dimming signal LC from the outside, so that the brightness of the cold-cathode tube is reduced. Controlled.

[0009]

Immediately after the start of discharge of the cold-cathode tube, the number of electrons necessary for sustaining the discharge is not sufficient. Therefore, if brightness is adjusted immediately after the start of the discharge, the discharge becomes unstable, May be interrupted. At this time, the safety circuit determines that the cold cathode fluorescent lamp has failed, and
Stops its own operation.

Such a start-up failure often occurs when the ambient temperature of the cold-cathode tube is cold, for example, about 0 ° C., or when the cold-cathode tube is not lit for a long time and is stored in a cool dark place.
Also, recently, liquid crystal display panels are widely used in portable devices such as electronic notebooks and mobile phones, so that there is a concern that starting failure may occur in a cold environment such as a ski resort, and in particular, lighting of a backlight light source is indispensable. It is fatal in the case of a device configuration.

An object of the present invention is to provide a cold-cathode tube lighting circuit capable of reliably lighting a cold-cathode tube and preventing occurrence of start-up failure.

[0012]

SUMMARY OF THE INVENTION The present invention provides an AC generator for intermittently driving a cold cathode fluorescent lamp at a high frequency and a conduction period of a high frequency current flowing through the cold cathode fluorescent lamp to increase the brightness of the cold cathode fluorescent lamp. A dimming control circuit for controlling, a starting circuit for controlling an AC generating circuit so that a voltage necessary for starting discharge is generated at the time of starting the cold cathode tube, and a predetermined starting from the start of starting by the starting circuit An operation control circuit for switching the intermittent operation by the dimming control circuit to a continuous operation only during a period.

According to the present invention, the maximum luminance within the controllable luminance range of the dimming control circuit is maintained for a predetermined activation period from the start of activation by the activation circuit, regardless of a preset luminance adjustment value. Turn on the cold cathode tube. Thereafter, when the discharge is stabilized, control is performed so that the luminance adjustment value is set in advance.

In the case of burst dimming in which the cold-cathode tube is intermittently driven at a high frequency and the luminance is controlled by adjusting the conduction period of the high-frequency current, the dimming control circuit is switched to a continuous operation during the starting period, and the starting period is changed. After the end of the operation, the operation is switched to the intermittent operation.

By driving in such a manner as to have the maximum possible brightness only during the first start-up period, the number of electrons in the cold-cathode tube increases rapidly, so that a stable discharge state is maintained, and start-up failure occurs. Can be prevented.

The present invention also provides an AC generator for continuously driving a cold cathode fluorescent lamp at a high frequency, and an adjusting circuit for controlling the intensity of the high frequency current flowing through the cold cathode fluorescent lamp to control the brightness of the cold cathode fluorescent lamp. A light control circuit, a start-up circuit for controlling an AC generation circuit so that a voltage necessary for starting discharge is generated when the cold-cathode tube is started, and dimming for a predetermined start-up period from the start-up time by the start-up circuit. An operation control circuit for switching a current control value of the control circuit to a controllable maximum value.

According to the present invention, the maximum luminance within the controllable luminance range of the dimming control circuit is maintained for a predetermined activation period from the start of activation by the activation circuit, regardless of a preset luminance adjustment value. Turn on the cold cathode tube. Thereafter, when the discharge is stabilized, control is performed so that the luminance adjustment value is set in advance.

In the case of current dimming in which the cold-cathode tube is continuously driven at a high frequency and the luminance is controlled by adjusting the magnitude of the high-frequency current, the maximum current control value by the dimming control circuit can be controlled during the startup period. The current value is set to a predetermined value after the start-up period.

By driving in such a manner as to have the maximum possible brightness only during the first start-up period, the number of electrons in the cold-cathode tube rapidly increases, so that a stable discharge state is maintained and a start-up failure occurs. Can be prevented.

Further, the invention is characterized in that the activation period is at least 10 milliseconds and at most 1 second from the start of activation.

According to the present invention, by setting the end point of the start-up period to be at least 10 milliseconds from the start point of the start-up, the occurrence rate of start-up failure (light-out rate) can be suppressed to a level that does not cause a practical problem. In addition, if the start-up period is somewhat long, start-up failures can be prevented.However, the shorter the time from the start of energization of the device to the start of user operation, it is more convenient. Therefore, the upper limit of the activation period is preferably about 1 second.

Further, the present invention is characterized in that the AC generation circuit includes a piezoelectric transformer that boosts voltage by mutual conversion between an electric signal and mechanical vibration.

According to the present invention, a compact and lightweight lighting circuit with high power conversion efficiency can be realized by driving a cold cathode tube with a piezoelectric transformer.

[0024]

FIG. 1 is a block diagram showing an embodiment of the present invention. The cold-cathode tube lighting circuit includes an AC generation circuit 1 for driving a load 15 which is a cold-cathode tube at a high frequency.
4, a dimming control circuit 12 for controlling the brightness of the cold-cathode tube based on the dimming signal LC, a control circuit 13 for controlling and stabilizing the current of the load 15, and a start signal ST. The control circuit 1 generates a voltage necessary for starting discharge.
3 and an activation circuit 1 for controlling the AC generation circuit 14
1 and an operation control circuit 16 for controlling the operation of the dimming control circuit 12. 1 is common to both burst dimming and current dimming.

First, the case of burst dimming will be described. In this case, the AC generation circuit 14 intermittently drives the load 15 at a high frequency, and the dimming control circuit 12 controls the luminance by controlling the conduction period of the high-frequency current. Operation control circuit 16
Switches the intermittent operation of the dimming control circuit 12 to a continuous operation for a predetermined activation period from the start of activation by the activation circuit 11.

Next, the operation will be described. When the voltage of an input power supply terminal (not shown) reaches a starting voltage or a lighting signal is input and the starting signal ST in FIG. 1 changes from a low level to a high level, for example, the starting circuit 11 operates and the control circuit 13 A pulse signal is output and the AC generation circuit 14
Generates a voltage higher than the discharge starting voltage of the load 15 (cold cathode tube). Once the discharge of the load 15 starts, the discharge continues only by the control circuit 13 and the AC control circuit 14, and the role of the starting circuit 11 ends. The operation time of the starting circuit 1 is usually about 1 millisecond.

Further, the operation control circuit 16 operates at the same time as the start-up by the start-up circuit 11, and switches the dimming control circuit 12 to the continuous operation for a predetermined start-up period so that the brightness of the cold-cathode tube is maximized as much as possible. Under control, the load 15 is warmed up. The activation period is 1 from the start of activation.
It is set in the range of 0 ms or more and 1 second or less.

After the end of the start-up period, the operation control circuit 16 switches the dimming control circuit 12 to the intermittent operation, and the dimming signal LC
, The conduction period of the high-frequency current is adjusted in accordance with the control so that a desired luminance is obtained.

Next, the case of current dimming will be described. The AC generation circuit 14 drives the load 15 continuously at high frequency, and the dimming control circuit 12 controls the brightness by adjusting the magnitude of the high frequency current. The operation control circuit 16 includes the activation circuit 1
The current control value of the dimming control circuit 12 is switched to a controllable maximum value only for a predetermined start period from the start of the start by the control unit 1.

Next, the operation will be described. When the voltage of an input power supply terminal (not shown) reaches a starting voltage or a lighting signal is input and the starting signal ST in FIG. 1 changes from a low level to a high level, for example, the starting circuit 11 operates and the control circuit 13 A pulse signal is output and the AC generation circuit 14
Generates a voltage higher than the discharge starting voltage of the load 15 (cold cathode tube). Once the discharge of the load 15 starts, the discharge continues only by the control circuit 13 and the AC control circuit 14, and the role of the starting circuit 11 ends. The operation time of the starting circuit 1 is usually about 1 millisecond.

Further, the operation control circuit 16 operates at the same time as the start-up by the start-up circuit 11, and switches the current control value by the dimming control circuit 12 to the maximum controllable value for a predetermined start-up period, so that the brightness of the cold-cathode tube is reduced. The load 15 is warmed up by controlling so that it becomes maximum as much as possible. The activation period is set in a range from 10 milliseconds to 1 second from the start of activation.

After the end of the start-up period, the operation control circuit 16 switches the current control value of the dimming control circuit 12, and the dimming control circuit 12 adjusts the magnitude of the high-frequency current according to the dimming signal LC. , So as to obtain a desired luminance.

FIG. 2 is a circuit diagram showing an example of a CCFL lighting circuit using burst dimming. First, in the starting circuit 11, the starting signal ST is input to the inverter Q1 via the differentiating circuit of the capacitor C1 and the resistor R1. The output of the inverter Q1 is connected via a resistor R2 to a transistor T1.
Connected to the collector. Resistor R2 and transistor T1
A resistor R3 is connected between the base and the base, and the base is grounded via a capacitor C2, and the emitter is grounded as it is. A backflow preventing diode D1 is connected between the collector of the transistor T1 and the control circuit 13 at the next stage.

Next, in the control circuit 13, a VCO (voltage controlled oscillator) 2 whose oscillation output is controlled by the input voltage
1 is provided, to which the output of the starting circuit 11 is supplied. The oscillation frequency of the VCO 21 is about 100 kHz. On the other hand, an I / V (current-voltage conversion) circuit 22 for monitoring the load current I of the load 15 as a cold-cathode tube is provided, and its output is also supplied to the VCO 21 to constitute a feedback circuit. The I / V circuit 22 includes a monitor resistor and a rectifier circuit, and a smoothing capacitor C5 is connected between its output and ground. Further, the output of the I / V circuit 22 is grounded via the pull-down resistor R6 and the transistor T4, and forcibly drops the control voltage of the VCO 21 to zero for a predetermined period.

The AC generation circuit 14 includes a piezoelectric transformer for boosting the voltage by the mutual conversion between the electric signal and the mechanical vibration, and a driving circuit thereof. The AC generation circuit 14 boosts the high frequency output from the VCO 21 and supplies the high frequency output to the load 15 which is a cold cathode tube. Supply and dimming control circuit 1
From 2, the high frequency is generated intermittently according to the ON period of the pulse signal.

Next, in the dimming control circuit 12, the dimming signal LC is an analog signal with a constant current and is input to the inverter Q2, and the capacitor C4 and the transistor T3 are connected between the input line and the ground. The dimming oscillator 20 is connected to the base of the transistor T3.
Pulses with a duty ratio of 50% at z to 200 Hz are supplied. The output of the inverter Q2 is connected to the AC generation circuit 1 described above.
4 and input to the base of the transistor T4 via the resistor R7.

In the operation control circuit 16, the inverter Q
2, a pull-down transistor T2 is connected between the input line and the ground, resistors R4 and R5 are connected in series between the base and the ground, and a capacitor is connected between the intermediate connection point and the output of the inverter Q1. C3 is connected.

FIG. 3 is a timing chart showing the operation. First, before the time t1, the output S of the inverter Q1 is output.
a is at a high level, the transistor T1 is turned on, and the control voltage to the VCO 21 becomes zero.
5 is not working.

Next, when the start signal ST goes high at time t1, the output Sa of the inverter Q1 goes low once, the capacitor C2 is discharged, and after a period determined by the time constant of the capacitor C1 and the resistor R1, It returns to the high level again at time t2. Then, the capacitor C2
Until is charged, the transistor T1 is turned off, the control voltage Vf is supplied to the VCO 21, and the high frequency driving of the load 15 is started. When a voltage equal to or higher than the discharge start voltage is applied to the load 15 some time after the time t2, the load current I starts to flow, and becomes almost stable at the time t3. Note that the load current I has a high frequency, and is shown as an effective value in FIG.

On the other hand, the signal Sb of the operation control circuit 16 rises once via the capacitor C3, and
2 is turned on, the input of the inverter Q2 is forcibly dropped to near zero, and the output Sc of the inverter Q2 is held at a high level. Then, since the AC generation circuit 14 outputs a continuous high frequency, the load 15 performs a continuous operation.

Further, after time t2, since the capacitor C3 is gradually charged via the resistors R4 and R5, the voltage of the signal Sb slowly decreases, and the time t4 when the voltage of the signal Sb drops below the threshold value Vth determined by the base-emitter voltage of the transistor T2.
, The transistor T2 is turned off. Then, the dimming signal LC is switched according to the output of the dimming oscillator 20, the output Sc of the inverter Q2 becomes a pulse signal, and the duty ratio thereof is adjusted according to the magnitude of the current of the dimming signal LC. Since the AC generation circuit 14 generates a high frequency only during the ON period of the output Sc and does not generate a high frequency during the OFF period, the load 15 is driven intermittently. Transistor T4
Is ON, the resistance of the transistor T4 becomes substantially zero, so that the time constant of the control circuit 13 has a value determined by the capacitor C5 and the resistor R6. When the transistor T4 is off, the resistance of the transistor T4 becomes almost infinite,
The time constant of the control circuit 13 becomes infinite, and the feedback operation does not work.

In this way, after time t4, the brightness adjustment by the dimming signal LC becomes effective, and stable discharge continues. As described above, the lighting control is performed by switching the dimming control circuit 15 to the continuous operation only during the start-up period defined between the time t2 and the time t4. The operation has been switched to the normal intermittent operation.

FIG. 4 is a graph showing the relationship between the starting period and the unlighting rate. The extinction rate on the vertical axis is a ratio (%) of the number of times that the cold cathode tube was not activated to the number of times that the cold cathode tube was activated, and the horizontal axis (logarithmic display) is the activation period in which the cold cathode tube is activated at the maximum luminance ( Seconds). The cold cathode tube used had an outer diameter of 3 mm, a length of 250 mm, and a power consumption of 3 W.
It was measured in a calm environment.

As can be seen from the graph, when the activation period is short, the unlighting rate tends to increase extremely. When the activation period is 0.01 seconds (10 milliseconds) or more, the unlighting rate becomes almost zero. It can be seen that stable startup can be realized.

[0045]

As described above in detail, according to the present invention, the luminance range which can be controlled by the dimming control circuit for a predetermined activation period from the start of activation by the activation circuit regardless of the preset luminance adjustment value. By turning on the cold-cathode tube so that the maximum brightness is obtained, a stable discharge state is maintained, so that occurrence of start-up failure can be prevented.

[Brief description of the drawings]

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

FIG. 2 is a circuit diagram showing an example of a cold-cathode tube lighting circuit using burst dimming.

FIG. 3 is a timing chart showing an operation.

FIG. 4 is a graph showing a relationship between a start-up period and an unlit rate.

FIG. 5 is a block diagram showing an example of a conventional cold-cathode tube lighting circuit.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 11 Start-up circuit 12 Dimming control circuit 13 Control circuit 14 AC generation circuit 15 Load (cold-cathode tube) 16 Operation control circuit

Claims (4)

[Claims] An alternating current generation circuit for intermittently driving a cold-cathode tube at a high frequency, and a dimming control circuit for controlling a conduction period of a high-frequency current flowing through the cold-cathode tube to control the brightness of the cold-cathode tube. And a starting circuit for controlling the AC generating circuit so that a voltage necessary for starting discharge is generated at the time of starting the cold cathode tube, and for a predetermined starting period from the start of starting by the starting circuit.
An operation control circuit for switching intermittent operation by the dimming control circuit to continuous operation. 2. An AC generation circuit for continuously driving a CCFL at a high frequency, and a dimming control circuit for controlling the intensity of the CCFL by adjusting the magnitude of a high-frequency current flowing through the CCFL. And a starting circuit for controlling the AC generating circuit so that a voltage necessary for starting discharge is generated at the time of starting the cold cathode tube, and for a predetermined starting period from the start of starting by the starting circuit.
An operation control circuit for switching a current control value of the dimming control circuit to a controllable maximum value. 3. The startup period is 10 minutes from the start of startup.
3. The cold-cathode tube lighting circuit according to claim 1, wherein the lighting time is from one millisecond to one second. 4. The cold-cathode tube lighting circuit according to claim 1, wherein the AC generation circuit includes a piezoelectric transformer that boosts voltage by mutual conversion between an electric signal and mechanical vibration.