JP2009300725A - Liquid crystal backlight device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a liquid crystal backlight device in which heat generation in a power supply stabilization circuit is suppressed by appropriately controlling an output voltage of a direct current power supply circuit even during dimming a backlight. <P>SOLUTION: Upon dimming a CCFL (Cold Cathode Fluorescent Lamp) 2, a second transistor Q2 is turned on to reduce a base potential of a first transistor Q1. Thereby, the potential difference between the base and the emitter of the first transistor Q1 is increased to turn on the first transistor Q1, a current flowing from the direct current power supply circuit 4 to a switching IC 31 is fed back through a current feedback circuit 33 to the direct current power supply circuit 4, and the output voltage of the direct current power supply circuit is appropriately controlled. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a liquid crystal backlight device used in a liquid crystal display device.

  Conventionally, cold cathode fluorescent lamps (hereinafter referred to as CCFLs) have been used for backlights of liquid crystal display devices. CCFLs have high efficiency and long life, but their operating characteristics depend on temperature, and the lamp brightness tends to decrease significantly at low temperatures. Therefore, as a technique for obtaining sufficient luminance immediately after lighting, for example, in Patent Document 1 and Patent Document 2, the voltage applied to the CCFL at startup is set high, and then the current value is fed back to lower the voltage. A circuit is shown.

  Further, in recent liquid crystal display devices, a technology for dimming a backlight by suppressing a current flowing through the CCFL based on a command input by a user under a request for reducing power consumption has also been implemented. (For example, see Patent Document 3 and Patent Document 4).

  FIG. 3 shows an example of an inverter circuit that has a function of driving the CCFL and performing the lighting control of the backlight shown in Patent Literature 1 and Patent Literature 2 and the dimming control shown in Patent Literature 3 and Patent Literature 4. Show. When the backlight of the liquid crystal display device is powered on, a high signal is output from the control unit 6, and accordingly, the transistor Q5 is turned on, the transistor Q6 is turned on, and the transistor Q4 is turned on. The transistor Q1 detects the tube current of the CCFL 2 by monitoring the voltage across the current monitoring resistor R1. For example, the transistor Q1 is turned on when the tube current reaches 1A, and the tube current is fed back to the control of the DC power supply circuit 4. .

During dimming of the backlight, a pulse signal for PWM control is input from the control unit 6 to the transistor Q3, the transistor Q3 is alternately turned on / off, and the transistor Q4 is alternately turned off / on alternately. And the backlight is dimmed.
JP-A-5-343190 Japanese Utility Model Publication No. 4-81500 JP 2005-285476 A Japanese Patent Laid-Open No. 7-110461

  However, when dimming is performed by the switching operation of the transistor Q4, the current flowing through the CCFL2 decreases and the potential difference between both ends of the current monitoring resistor R1 decreases. Therefore, the potential difference between the emitter and base of the transistor Q1 decreases, and the transistor Since Q1 does not turn on, current feedback is stopped. As a result, the current feedback is not applied to the DC power supply circuit 4, and the output voltage of the DC power supply circuit 4 rises excessively. The DC power supply circuit 4 also inputs a DC voltage to a power supply stabilization circuit that is separately provided to stabilize the output of the DC power supply circuit 4, and the DC power supply circuit increases as the output voltage of the DC power supply circuit 4 increases. Since the voltage input to the power stabilization circuit from 4 also rises, the PC (power loss) of the power stabilization circuit increases and the heat generation becomes excessive. In order to suppress the heat generation of the power supply stabilization circuit, it is necessary to take measures against heat such as strengthening the input resistance inside the power supply stabilization circuit and adding a heat sink to cool the heat generation part such as a transistor. This causes an increase in manufacturing costs and is a factor that hinders downsizing.

  The present invention has been made to solve the above problems, and a liquid crystal backlight capable of optimizing the output voltage of the DC power supply circuit and suppressing the heat generation of the power supply stabilization circuit even when the backlight is dimmed. An object is to provide an apparatus.

In order to achieve the above object, the invention of claim 1
A cold cathode fluorescent lamp used for a backlight of a liquid crystal display device, an inverter circuit for applying an alternating voltage to the cold cathode fluorescent lamp, and a direct current power supply circuit for obtaining direct current from a commercial alternating current power source and supplying the direct current to the inverter circuit,
The inverter circuit includes a switching element that switches a DC voltage applied from a DC power supply circuit, an inverter transformer that oscillates when a DC voltage switched by the switching element is applied, and outputs an AC voltage, and the DC power supply circuit A current feedback circuit for detecting a current flowing from the DC power supply circuit to the DC power supply circuit, and a dimming circuit for dimming the cold cathode fluorescent lamp by limiting the current flowing from the DC power supply circuit to the switching element; In a liquid crystal backlight device having
When the dimmer circuit limits the current flowing from the DC power supply circuit to the switching element, the inverter circuit forces the current flowing from the DC power supply circuit to the switching element via a current feedback circuit. It further has a feedback forcing circuit for feeding back to the DC power supply circuit.

The invention of claim 2 is the liquid crystal backlight device according to claim 1,
The current feedback circuit operates according to a current monitoring resistor inserted in a line between the DC power supply circuit and the switching element, and a potential difference between both ends of the current monitoring resistor. A first transistor that feeds back a current flowing from the current to the switching element to the DC power supply circuit,
The feedback forcing circuit includes a second transistor that is turned on when the cold cathode fluorescent lamp is dimmed to change the base potential of the first transistor.

The invention of claim 3 is the liquid crystal backlight device according to claim 2,
The dimming circuit includes a third transistor that performs a switching operation by inputting a pulse signal having a predetermined frequency during dimming of the cold cathode fluorescent lamp, a current monitoring resistor between the current feedback circuit and the switching element. And a fourth transistor that is inserted in a line and performs a switching operation according to the operation of the third transistor.

According to the invention of claim 1, when the cold cathode fluorescent lamp is dimmed, that is, when the dimming circuit limits the current flowing from the DC power supply circuit to the switching element, the feedback forcing circuit includes:
The current flowing from the DC power supply circuit to the switching element is forcibly fed back to the DC power supply circuit. Thereby, since current feedback is always applied to the DC power supply circuit, the output voltage of the DC power supply circuit is not excessively increased, and the heat generation of the power supply stabilization circuit can be suppressed. Therefore, heat measures such as strengthening the input resistance of the power stabilization circuit and adding a heat sink are not necessary, and the manufacturing cost of the liquid crystal display device can be reduced.

  According to the invention of claim 2, since the second transistor of the feedback forcing circuit fluctuates the base potential of the first transistor during dimming of the cold cathode fluorescent lamp, even if the potential difference between both ends of the current monitoring resistor decreases, the second transistor When one transistor operates, the current flowing from the DC power supply circuit to the switching element can be fed back to the DC power supply circuit. Thereby, the configuration of the liquid crystal backlight device can be made simple and inexpensive.

  According to invention of Claim 3, the structure of a light control circuit can be made simple and cheap, and the further cost reduction of a liquid crystal backlight apparatus can be aimed at.

  A liquid crystal backlight device according to an embodiment of the present invention will be described with reference to the drawings. FIG. 1 shows a schematic configuration of a liquid crystal backlight device. The liquid crystal backlight device 1 is disposed on the back surface of the liquid crystal panel, and is used for the backlight, a CCFL 2 used for the backlight, a self-excited inverter circuit 3 that applies an AC voltage to the CCFL 2, and a direct current from a commercial AC power source. A DC power supply circuit 4 to be supplied, a power supply stabilization circuit 5 that stabilizes a DC voltage output from the DC power supply circuit 4, a control unit 6 that controls the entire liquid crystal backlight device 1, and a user that performs dimming or the like An operation unit 7 or the like operated by a user to input a command to the control unit 6 is configured.

  FIG. 2 shows the inverter circuit 3. The inverter circuit 3 includes a switching IC (switching element) 31, an inverter transformer 32, a current feedback circuit 33, a dimming circuit 34, a feedback forcing circuit 35, and the like.

  The switching IC 31 has, for example, a plurality of FETs and switches the DC voltage applied from the DC power supply circuit 4. The inverter transformer 32 has a primary side coil and a secondary side coil, and a DC voltage switched by the switching IC 31 is applied to the primary side coil to oscillate, and an AC is output from the secondary side coil.

  The current feedback circuit 33 includes a current monitoring resistor R1 and a first transistor Q1, and detects a current flowing from the DC power supply circuit 4 to the switching IC 31 and feeds it back to the DC power supply circuit 4. The current monitoring resistor R1 is inserted in a line between the DC power supply circuit 4 and the switching IC 31. The first transistor Q1 performs a switching operation according to the potential difference between both ends of the current monitoring resistor R1, and feeds back the current flowing from the DC power supply circuit 4 to the switching IC 31 to the DC power supply circuit 4. For example, when the potential difference between both ends of the current monitoring resistor R1 becomes 0.6 [V] or more, the first transistor Q1 is turned on and current feedback is applied.

  When dimming is not performed, a power supply voltage of, for example, +22 [V] is input to the current monitor resistor R1 from the DC power supply circuit 4, and the inverter transformer 32 resonates at a resonance frequency of 38 kHz, and the current monitor resistor R1 A current of 1 [A] flows, and the maximum luminance of CCFL2 is obtained. Immediately after the CCFL2 is turned on, the resonance frequency of the inverter transformer 32 is 40 kHz, so that the current flowing through the current monitoring resistor R1 decreases and the brightness of the CCFL2 decreases. Therefore, by raising the output voltage of the DC power supply circuit 4 immediately after the lighting of the CCFL 2 to about +23 [V], for example, the resonance frequency of the inverter transformer 32 can be shifted to 38 kHz, and the luminance of the CCFL 2 is prevented from being lowered. Thereafter, if the output voltage of the DC power supply circuit 4 is kept at +23 [V], the tube current of the CCFL 2 increases, so that a current of, for example, 1 [A] or more flows through the current monitoring resistor R1, and the current monitoring resistor When the potential difference between both ends of R1 becomes 0.6 [V] or more, the first transistor Q1 is turned on, current feedback is applied, and the output voltage of the DC power supply circuit 4 is reduced to about +22 [V].

  The dimming circuit 34 includes a third transistor Q3, a fourth transistor Q4, and transistors Q5 and Q6, and dims CCFL2 by limiting the current flowing from the DC power supply circuit 4 to the switching IC 31. The third transistor Q3 performs a switching operation when a pulse signal having a frequency of, for example, 500 Hz is input from the control unit 6 during dimming of the CCFL2. The fourth transistor Q4 is inserted in a line between the current monitoring resistor R1 of the current feedback circuit 33 and the switching IC 31, and performs a switching operation according to the operation of the third transistor Q3.

  When the power source of the liquid crystal display device is on, a high signal is input from the control unit 6 to the base of the transistor Q5, and the transistors Q5 and Q6 are sequentially turned on. Therefore, the fourth transistor is switched along with the switching operation of the third transistor Q3. A pulse signal having a frequency of 500 Hz is input to the base of Q4. Here, the on-duty of the fourth transistor Q4 is PWM-controlled by PWM controlling the off-duty of the third transistor Q3 in accordance with the output during dimming, and the current flowing from the DC power supply circuit 4 to the switching IC 31 is limited. Will be.

  The feedback forcing circuit 35 has a second transistor Q2, and forcibly feeds back the current flowing from the DC power supply circuit 4 to the switching IC 31 during dimming of the CCFL2. The emitter of the second transistor Q2 is connected to the ground, and when the CCFL2 is dimmed, that is, when the dimming circuit 34 limits the current flowing from the DC power supply circuit 4 to the switching IC 31 by the PWM control of the fourth transistor Q4, The two transistors Q2 are turned on to lower the base potential of the first transistor Q1. As a result, the potential difference between the base and the emitter of the first transistor Q1 is increased, the first transistor Q1 is turned on, and the current flowing from the DC power supply circuit 4 to the switching IC 31 is fed back to the DC power supply circuit 4 via the current feedback circuit 33. The

  As described above, according to the liquid crystal backlight device 1 of the present embodiment, when the CCFL 2 is dimmed, that is, when the dimming circuit 34 limits the current flowing from the DC power supply circuit 4 to the switching IC 31, the feedback forcing circuit 35 forcibly feeds back the current flowing from the DC power supply circuit 4 to the switching element 31 to the DC power supply circuit 4. Thereby, since current feedback is always applied to the DC power supply circuit 4, the output voltage of the DC power supply circuit 4 is not excessively increased, and heat generation of the power supply stabilization circuit 5 can be suppressed. Therefore, heat measures such as strengthening the input resistance of the power supply stabilization circuit 5 and adding a heat sink are not necessary, and the manufacturing cost of the liquid crystal display device can be reduced.

  Also, since the second transistor Q2 of the feedback forcing circuit 35 varies the base potential of the first transistor Q1 during dimming of the CCFL2, the first transistor Q1 operates even if the potential difference between both ends of the current monitoring resistor R1 decreases. As a result, the current flowing from the DC power supply circuit 4 to the switching element 31 can be fed back to the DC power supply circuit 4. Thereby, the structure of the liquid crystal backlight device 1 can be made simple and inexpensive. Further, the configuration of the dimming circuit 34 can be made simple and inexpensive by the third transistor Q3 and the fourth transistor Q4, and the cost of the liquid crystal backlight device 1 can be further reduced.

  Note that the present invention is not limited to the configuration of the above embodiment, and at least during dimming of the CCFL 2, that is, when the dimming circuit 34 limits the current flowing from the DC power supply circuit 4 to the switching IC 31. A configuration corresponding to the feedback forcing circuit 35 that forcibly feeds back the current flowing from the current to the switching element 31 to the DC power supply circuit 4 may be provided.

1 is a block diagram showing a schematic configuration of a liquid crystal backlight device according to an embodiment of the present invention. The circuit diagram of the inverter circuit of the liquid crystal backlight device. The block diagram which shows schematic structure of the conventional liquid crystal backlight apparatus.

Explanation of symbols

1 LCD backlight device 2 CCFL (cold cathode fluorescent lamp)
3 Inverter circuit 4 DC power supply circuit 31 Switching IC (switching element)
32 Inverter transformer 33 Current feedback circuit 34 Dimming circuit 35 Feedback forcing circuit Q1 1st transistor Q2 2nd transistor Q3 3rd transistor Q4 4th transistor R1 Resistance for current monitoring

Claims (3)

A cold cathode fluorescent lamp used for a backlight of a liquid crystal display device, an inverter circuit for applying an alternating voltage to the cold cathode fluorescent lamp, and a direct current power supply circuit for obtaining direct current from a commercial alternating current power source and supplying the direct current to the inverter circuit,
The inverter circuit includes a switching element that switches a DC voltage applied from a DC power supply circuit, an inverter transformer that oscillates when a DC voltage switched by the switching element is applied, and outputs an AC voltage, and the DC power supply circuit A current feedback circuit for detecting a current flowing from the DC power supply circuit to the DC power supply circuit, and a dimming circuit for dimming the cold cathode fluorescent lamp by limiting the current flowing from the DC power supply circuit to the switching element; In a liquid crystal backlight device having
When the dimmer circuit limits the current flowing from the DC power supply circuit to the switching element, the inverter circuit forces the current flowing from the DC power supply circuit to the switching element via a current feedback circuit. A liquid crystal backlight device further comprising a feedback forcing circuit for feeding back to the DC power supply circuit. The current feedback circuit operates according to a current monitoring resistor inserted in a line between the DC power supply circuit and the switching element, and a potential difference between both ends of the current monitoring resistor. A first transistor that feeds back a current flowing from the current to the switching element to the DC power supply circuit,
2. The liquid crystal backlight according to claim 1, wherein the feedback forcing circuit includes a second transistor that is turned on when dimming the cold cathode fluorescent lamp and changes a base potential of the first transistor. apparatus.   The dimming circuit includes a third transistor that performs a switching operation by inputting a pulse signal having a predetermined frequency during dimming of the cold cathode fluorescent lamp, a current monitoring resistor between the current feedback circuit and the switching element. The liquid crystal backlight device according to claim 2, further comprising a fourth transistor that is inserted in a line and performs a switching operation according to an operation of the third transistor.

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