JP2001338776A - Light regulation circuit - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve the light regulation rate for a liquid crystal display device, using a cold-cathode tube as a backlight. SOLUTION: A current, flowing through the cold-cathode tube 7, is reduced by lowering the output voltage of a variable power circuit 3, in addition to light regulation by the blinking of the cold-cathode tube 7 by the switching of a switch circuit 4 by a control circuit 2 receiving brightness signal from a light detector 1. This circuit is provided with the control circuit 2 connected to the light detector 1 for generating a voltage control signal and a pulse signal according to the brightness signal; and the variable power circuit 3 for generating a power voltage obtained by controlling its output voltage by the voltage control signal from the control circuit 2 to output it to the switch circuit 4. The switching circuit 4 supplies the controlled power voltage from the variable power circuit 3 to an oscillation circuit 5.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a dimming circuit,
In particular, the present invention relates to a dimming circuit for a liquid crystal display for a vehicle such as a navigation device.

[0002]

2. Description of the Related Art Conventionally, a dimming circuit has been used for the purpose of reducing the brightness of a display screen and reducing the burden on a driver's eyes when the surroundings become dark, such as at night or when traveling in a tunnel.

FIG. 4 shows a conventional PWM (pulse wi).
FIG. 3 is a block diagram illustrating an example of a dimming circuit using dth modulation (pulse width modulation). Photodetector 1
01 outputs a dimming signal according to the surrounding brightness. The pulse circuit 102 receives a dimming signal and outputs a pulse signal whose duty ratio varies according to the dimming signal. Power supply circuit 1
03 supplies power of a constant voltage. Switch circuit 1
Reference numeral 04 is provided between the power supply circuit 103 and the oscillation circuit 105, and is applied to the oscillation circuit 105 by a pulse signal to turn the oscillation circuit 105 on and off. Oscillation circuit 105
Generates a sine wave and is applied to the primary side of the high voltage transformer 106. The high-voltage transformer 106 generates a voltage corresponding to a ratio between a primary winding and a secondary winding necessary for lighting the cold cathode tube 107. The cold cathode tube 107 emits light only during a period in which a voltage is applied.

Next, the operation of the conventional example will be described. When the surroundings become dark, the photodetector 101 generates a dimming signal of a DC voltage corresponding to the brightness. Pulse circuit 102
Outputs a pulse signal whose duty ratio changes, that is, when the surroundings are not so dark due to the dimming signal, the high-level period becomes longer as the darkness becomes longer and the high-level period becomes shorter. Usually, this frequency is between 200 and 500 Hz. The switch circuit 104 is closed only during the high-level period of the pulse signal, and power is supplied to the oscillation circuit 105. Oscillation circuit 105 starts oscillating when power is supplied, and generates a sine wave. When the power supply is cut off, the oscillation stops. The output of the oscillation circuit 105 is applied to the primary side of the high voltage transformer 106. On the secondary side of the high-voltage transformer 106, a high voltage having a ratio of the primary winding to the secondary winding is generated only during the period when the voltage is applied to the primary side, and the cold cathode tube 1
07 and blinking. Therefore, the cold cathode fluorescent lamp 1 is generated by the pulse signal generated by the pulse circuit 102.
The lighting of 07 is controlled, and the lighting time becomes shorter as the surroundings become darker. Flashing cycle is 200-500
Because of the high frequency of Hz, it appears to the human eye that the light has been integrated and dimmed.

[0005]

In the above-mentioned conventional example,
In the case of PWM dimming, power must be continuously applied to the oscillation circuit until the lighting of the cold-cathode tube becomes stable, generally for a period of three or more wavelengths of the lighting frequency. For example, PWM
Frequency of 300 Hz, lighting frequency of cold cathode fluorescent lamp 50 k
Hz, the dimmable value is 300Hz / 50kHz ×
In No. 3, there was a problem that the dimming rate was low and 1.8% was the limit.

Accordingly, an object of the present invention is to provide a dimming circuit in which the dimming rate is improved as compared with the conventional system in order to solve the above-mentioned problem.

[0007]

In order to achieve the above object, a dimming circuit according to the present invention comprises a switch circuit for turning on / off a power supply from a power supply circuit by a pulse signal corresponding to a brightness signal, and a switch. An oscillating circuit that generates a sine wave by supplying power from the circuit, and a high-voltage transformer that applies a voltage from the oscillating circuit to the primary side and generates a voltage corresponding to a ratio between the primary winding and the secondary winding. A light detector that detects ambient brightness and outputs a brightness signal corresponding to the ambient brightness in a dimming circuit including a cold cathode tube that is turned on by a voltage from a high-voltage transformer; and a photodetector. And a control circuit for generating a voltage control signal and a pulse signal according to the brightness signal, and a variable power supply circuit for generating a power supply voltage whose output voltage is controlled by the voltage control signal from the control circuit and outputting the power supply voltage to the switch circuit When Provided, the switch circuit sends a control power supply voltage from the variable power supply circuit to the oscillation circuit, characterized in that reduced the amount of current CCFL flows.

It is preferable that the apparatus further comprises a temperature detector connected to the control circuit for detecting the ambient temperature, and the control circuit sends out a signal corresponding to the ambient temperature.

The variable power supply circuit further includes a battery, a switching transistor, and a coil connected to the battery for storing energy during an ON period of the switching transistor and releasing energy during an OFF period of the switching transistor. As a DC voltage.

[0010] Still further, the variable power supply circuit is connected to a switching transistor to control an output voltage.
C and a plurality of resistors for dividing the output voltage.

Preferably, the switching transistor changes the voltage value by changing the ratio of the on / off time.

Further, the control circuit includes a switch connected to a control IC of the variable power supply circuit, a pulse signal for opening and closing the switch circuit based on a brightness signal of the photodetector, and a voltage for controlling an output of the variable power supply circuit. It is preferable to include a microcomputer that outputs a control signal and an open / close signal for controlling opening / closing of a switch.

To summarize the above means, a first embodiment of the dimming circuit according to the present invention comprises a means (photodetector) for detecting the brightness of the surroundings, and a power supply circuit in which the generated power supply voltage can be varied by a control signal. (Variable power supply circuit), means (control circuit) for outputting a voltage control signal for controlling the output voltage of the power supply circuit according to the brightness, and a PWM signal for changing the duty ratio
And means (switch circuit) for opening and closing the circuit by the PWM signal.

In the second embodiment of the light control circuit of the present invention, the light control operation can be restricted at a low temperature. That is, in addition to the above-described first embodiment, specifically, a means (temperature detector) for detecting a temperature is included.

As described above, the dimming circuit of the present invention reduces the current flowing through the cold-cathode tube more than before.

[0016]

Embodiments of the present invention will be described below in detail with reference to the drawings.

FIG. 1 is a block diagram showing the configuration of a first embodiment of the dimming circuit according to the present invention. The light detector 1 outputs a brightness signal according to the surrounding brightness. Control circuit 2
Is connected to the photodetector 1 and generates a voltage control signal and a pulse signal according to the brightness signal. Variable power supply circuit 3
Generates a power supply voltage whose output voltage is controlled by a voltage control signal. The switch circuit 4 is provided between the variable power supply circuit 3 and the oscillating circuit 5, and interrupts the power supply to the oscillating circuit 5 when the signal is at a low level and supplies the power to the oscillating circuit 5 when the signal is at a high level. The oscillation circuit 5 generates a sine wave only while power is supplied by self-excited oscillation. The primary side of the high voltage transformer 6 is connected to the oscillation circuit 5, and the secondary side generates a high voltage obtained by multiplying the peak value of the sine wave generated by the oscillation circuit 5 by the primary and secondary winding ratios. . By applying this voltage to the cold cathode tube 7, the cold cathode tube 7 is turned on. The brightness of the cold-cathode tube 7 changes according to the amount of current flowing.

FIG. 2 is a block diagram showing the configuration of the variable power supply circuit 3 and the control circuit 2 in the first embodiment of the dimming circuit of the present invention. In FIG. 2, the variable power supply circuit 3
Is a switching regulator, a battery 31 as a power supply, a coil 32, and a switching transistor 3.
3, a control IC 35, output voltage dividing resistors 36 and 37, and a smoothing circuit 34.
And a switch 21.

The coil 32 of the variable power supply circuit 3 is connected to the battery 31, and when the switching transistor 33 is turned on / off, energy is stored in the coil 32 during the ON period of the switching transistor 33.
Energy is released during the off period. The released energy is sent to the smoothing circuit 34 and output as a DC voltage. This output voltage is applied to the switching transistor 3
By changing the time of the on / off ratio of 3, the voltage value can be changed. That is, increasing the ratio of the on-time increases the output voltage value, and increasing the ratio of the off-time decreases the output voltage value. The control IC 35 has a function of controlling the output voltage, and is connected to the switching transistor 33. The voltage at the point A obtained by dividing the output voltage by the resistors 36 and 37 is applied to the control IC 35. The control IC 35 increases the ON ratio when the output voltage increases, and increases the OFF ratio when the output voltage decreases,
A signal for controlling the on / off ratio of the switching transistor 33 is output so that the voltage at the point A becomes constant.

A pulse signal for opening and closing the switch circuit 4, a voltage control signal for controlling the output of the variable power supply circuit 3, and an opening and closing for controlling the opening and closing of the switch 21 are sent from the microcomputer 22 according to the brightness signal of the photodetector 1. A signal is output. The pulse signal from the microcomputer 22 is always a high level when dimming is not performed, and is a PWM signal in which the low level period becomes longer as darker when dimming is performed.

FIG. 3 is a block diagram showing the configuration of a second embodiment of the dimming circuit according to the present invention. In the second embodiment of the present invention, a temperature detector 8 is provided as compared with the first embodiment. The cold-cathode tube 7 has a problem that the discharge starting voltage increases at a low temperature and the discharge is difficult to start when the cold-cathode tube 7 is started in the dimming state. In addition, at low temperatures, the luminous efficiency of the cold cathode fluorescent lamp 7 decreases and the luminance decreases, so that it is not necessary to lower the dimming rate as compared with normal temperature. In this embodiment, the temperature detection means 8 such as a thermistor identifies low temperature. When the temperature is low, the control circuit 2 does not output a signal for lowering the output of the variable power supply circuit 3. As a result, the voltage applied to the cold-cathode tube 7 does not decrease, so that the discharge can be started stably.

The second embodiment of the present invention has an effect that the startability at a low temperature is improved in addition to the effect of the first embodiment.

[0023]

Next, an embodiment of the present invention will be described in detail with reference to FIGS.

First, when the surroundings are sufficiently bright and no dimming is required, the open / close signal from the microcomputer 22 is transmitted to the switch 21.
, And a pulse signal always outputs a high level signal. Since the switch 21 is open, the output of the variable power supply circuit 3 is determined only by the divided voltages of the resistors 36 and 37. In this embodiment, it is assumed that +18 V is output. On the other hand, since the pulse signal is always at the high level, the switch circuit 4 remains closed. Therefore, oscillation in the oscillation circuit 5 (in this embodiment, the oscillation frequency is 50 k
Hz. ) Is performed continuously, and the cold-cathode tube 7 is always lit.

Next, the case where the light control is actually performed will be described. In the initial stage of dimming where the surroundings are not too dark, a brightness signal from the photodetector 1 is received, a signal for opening the switch 21 from the microcomputer 22, and a pulse signal (high-level period relatively longer than the low level) ( The frequency in this embodiment is 300 Hz.). Since the switch 21 is open, the output voltage of the variable power supply circuit 3 is +18 V in the same manner as in the non-dimming state. The power supply to the oscillation circuit 5 is interrupted only during the low level by the pulse signal, the oscillation is intermittent, and the cold cathode fluorescent lamp 7 blinks. Further, the normal PWM dimming operation in which the ratio of the low level of the pulse signal increases when the surroundings become darker is performed.

When the high-level period of the pulse signal reaches three wavelengths of the oscillating frequency (the duty ratio in this embodiment is 1.8%), the microcomputer 22 outputs the pulse signal without changing its duty ratio. A voltage control signal and a signal for closing the switch 21 are output. The voltage control signal is a DC voltage, and the voltage increases as the surroundings become darker. The voltage control signal is applied to the point A via the switch 21. As a result, the output voltage of the smoothing circuit 34 is applied to the point A at the resistors 36 and 37.
In addition to the divided voltage, a voltage for voltage control from the microcomputer 22 is applied. The voltage at point A is switch 2.
1 tries to rise from when it is open.
5, the on / off ratio of the transistor is controlled such that the voltage at point A becomes a constant voltage.
The output voltage of No. 4 is +18 V for the voltage control signal added.
Would be lower. As a result, the power supply voltage supplied from the variable power supply circuit 3 to the oscillation circuit 5 decreases, and the peak value of the oscillation of the oscillation circuit 5 decreases. Therefore, high-voltage transformer 6
As the input voltage on the primary side becomes smaller, the secondary side voltage also decreases, and the voltage applied to the cold cathode fluorescent lamp 7 decreases. As a result, the current flowing through the cold-cathode tube decreases, and the luminance decreases. As the voltage of the voltage control signal from the microcomputer 22 increases, the output voltage of the variable power supply circuit 3 decreases, and the luminance further decreases.

The above-described embodiment will be described below with a specific example. Resistance 36 is 16.5 kΩ, resistance 3
7 is 1.5 kΩ, and the impedance of the switch 21 is 1
The output voltage is controlled so that 6.5 kΩ, the input impedance of the control IC 35 is infinite, and the voltage at the point A becomes 1.5 V. When dimming is not performed, the switch 21 is open, so the output voltage value is 1.5 V / 1.5 KΩ ×
(16.5 kΩ + 1.5 kΩ) = 18V.

Next, the switch 21 is closed and the microcomputer 22 is closed.
A case in which a voltage of 3 V is output from is described. A voltage is applied to the point A via the switch 21. But,
Since the output voltage is controlled so that the voltage at the point A is kept at 1.5 V, the output voltage decreases. The output voltage is determined by the superposition theorem and drops to about 13V. That is, it becomes 30% darker than the limit value of the PWM method. Therefore, in the dimming circuit of the present invention, the current flowing through the cold-cathode tube can be reduced while maintaining the period until the lighting of the cold-cathode tube is stabilized, so that the dimming rate can be improved compared to the PWM method.

[0029]

As described above, according to the present invention, the cold cathode fluorescent lamp can be turned on and off, and the voltage applied to the cold cathode fluorescent lamp can be reduced to reduce the current flowing through the cold cathode fluorescent lamp. It has the effect of being able to.

Further, a means for detecting the temperature (for example, a thermistor) can be provided, and there is an effect that stable lighting can be performed even at a low temperature.

[Brief description of the drawings]

FIG. 1 is a block diagram showing a configuration of a first exemplary embodiment of the present invention.

FIG. 2 is a block diagram showing a detailed configuration of a control circuit and a variable power supply circuit of FIG.

FIG. 3 is a block diagram showing a configuration of a second exemplary embodiment of the present invention.

FIG. 4 is a block diagram showing a configuration of a conventional PWM type dimming circuit.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Photodetector 2 Control circuit 3 Variable power supply circuit 4 Switch circuit 5 Oscillation circuit 6 High voltage transformer 7 Cold cathode tube 8 Temperature detector 21 Switch 22 Microcomputer 31 Battery 32 Coil 33 Switching transistor 34 Smoothing circuit 35 Control IC 36, 37 Resistance

Claims (6)

[Claims] A switch circuit for turning on / off a power supply from a power supply circuit by a pulse signal corresponding to a brightness signal;
An oscillation circuit that generates a sine wave by power supply from the switch circuit; and a voltage from the oscillation circuit is applied to a primary side to generate a voltage corresponding to a ratio between a primary winding and a secondary winding. In a dimming circuit including a high-voltage transformer and a cold-cathode tube that is turned on by a voltage from the high-voltage transformer, a photodetector that detects ambient brightness and outputs a brightness signal corresponding to the ambient brightness A control circuit that is connected to the photodetector and generates a voltage control signal and a pulse signal according to the brightness signal; and generates a power supply voltage whose output voltage is controlled by the voltage control signal from the control circuit. A variable power supply circuit for outputting to the switch circuit, wherein the switch circuit sends a controlled power supply voltage from the variable power supply circuit to the oscillation circuit to reduce an amount of current flowing through the cold-cathode tube. A dimming circuit, characterized in that: 2. The apparatus according to claim 1, further comprising a temperature detector connected to said control circuit for detecting an ambient temperature, wherein said control circuit sends a signal corresponding to said ambient temperature. Dimming circuit. 3. A variable power supply circuit, comprising: a battery; a switching transistor; and a coil connected to the battery for storing energy during an ON period of the switching transistor and releasing energy during an OFF period of the switching transistor.
The dimming circuit according to claim 1, further comprising: a smoothing circuit that outputs the released energy as a DC voltage. 4. A control IC connected to the switching transistor and controlling an output voltage.
The dimming circuit according to claim 3, further comprising: a plurality of resistors for dividing the output voltage. 5. The switching transistor according to claim 1, wherein
The dimming circuit according to claim 4, wherein the voltage value is changed by changing a ratio of an off time. 6. A variable power supply circuit comprising: a switch connected to the control IC of the variable power supply circuit; a pulse signal for opening and closing the switch circuit based on a brightness signal of the light detector; 6. The dimming circuit according to claim 4, further comprising a microcomputer that outputs a voltage control signal for controlling the output of the switch and an open / close signal for controlling the opening and closing of the switch.