JPH0522912Y2 - - Google Patents

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to a backlight device for a liquid crystal display.

[Prior art and its problems]

Recently, there has been a lot of development work on LCD displays such as pocket TVs that use liquid crystal elements in their screens. Images are formed by ON-OFF control. As a way to obtain this back light, if you view it outdoors during the day,
Although it is possible to use sunlight directly or by reflecting sunlight to illuminate the back of the liquid crystal element, generally the screen is dark, and a dedicated device is installed behind the liquid crystal element for viewing at night or indoors with little brightness. A backlight device is required. This backlight device is required to be small and consume little power, and the use of an electroluminescent board has been considered, but at present, this electroluminescent board does not have sufficient brightness, so it consumes little power. A backlight device incorporating a small fluorescent lamp with low power consumption is used.

By the way, when a hot cathode type compact fluorescent lamp is lit at about 70V in a steady state, for example, when used for a long period of time, the amount of emitters attached to the electrodes decreases. Since the emission of thermionic electrons decreases at normal cathode temperatures, to compensate for this, ions are accelerated in front of the cathode, and the cathode temperature is increased by ion collisions, thereby emitting thermionic electrons. Increase emission and secondary electron emission to near normal values. Therefore, for this ion acceleration, the lamp voltage is
It rises to around 130V and the overvoltage light turns on. For this reason,
The amount of heat generated not only by the small fluorescent lamp itself but also by peripheral elements such as the lighting circuit increases, and the backlight device becomes hot. If this state continues, it will have an adverse effect on peripheral elements, small fluorescent lamps, etc., and in severe cases, it will cause failure. Furthermore, the liquid crystal element of the liquid crystal display device, which is in close contact with the light irradiation surface of the backlight device, also becomes hot. Moreover, liquid crystal elements are sensitive to heat, and when they reach high temperatures, they no longer operate correctly, causing the liquid crystal element screen to become distorted.

For this reason, it is preferable to provide an overvoltage lighting detection circuit in the lighting circuit to turn off the small fluorescent lamp when overvoltage lighting occurs. This overvoltage lighting detection circuit has a detection resistor, and uses a rectifier diode to half-wave rectify the alternating current voltage applied to the detection resistor. Here, the emitters attached to the electrodes of a small fluorescent lamp are not necessarily reduced equally on both electrodes, and in reality there is an imbalance in the amount of reduction. And in that case,
As shown by A ₁ and B ₁ in FIG. 5, one of the electrodes may have a substantially normal voltage, but only the other electrode may have an overvoltage. When this one-way overvoltage AC voltage is half-wave rectified with a rectifier diode, if the positive voltage side becomes overvoltage as shown in A ₁ in Figure 5, then
After half-wave rectification, overvoltage can be detected as shown in _A2 of FIG. However, if the negative voltage side becomes overvoltage as shown in B ₁ in FIG. 5, the half-wave rectified voltage becomes below the overvoltage detection limit as shown in B ₂ in FIG. 5, and overvoltage lighting is not detected. Therefore, the overvoltage lighting detection circuit must be able to reliably detect even such a one-way overvoltage.

[Purpose of invention]

Therefore, the object of the present invention is to provide a highly reliable backlight device for a liquid crystal display that can reliably prevent overvoltage lighting of a small hot cathode type fluorescent lamp even in the case of one-way overvoltage. be.

[Structure of the device and its function]

The configuration of the present invention is that in a backlight device that includes a built-in compact fluorescent lamp and is placed behind the liquid crystal element screen of a liquid crystal display device to illuminate the liquid crystal element screen, the lighting circuit of the small fluorescent lamp is configured such that the small fluorescent lamp is exposed to an overvoltage voltage. It has an overvoltage lighting detection circuit that releases the drive of the transformer that supplies power to the small fluorescent lamp and turns off the small fluorescent lamp when the lamp is turned on.This overvoltage lighting detection circuit is connected in series with an overvoltage detection resistor. The device is characterized in that it includes a leveling capacitor.

In other words, when the overvoltage lighting detection circuit detects overvoltage in the compact fluorescent lamp, the transformer that supplies power to the compact fluorescent lamp is deactivated and the compact fluorescent lamp is turned off, thus preventing overvoltage lighting. be done. In addition, the overvoltage lighting detection circuit includes a leveling capacitor connected in series with an overvoltage detection resistor, so in the case of a one-way overvoltage, the potential difference between the positive voltage side and the negative voltage side is charged to the leveling capacitor and becomes zero. The potential deviates midway between both peak voltages. Therefore, even in the case of one-way overvoltage, it is possible to reliably detect and turn off the light. Therefore, the backlight device does not reach a high temperature, and the reliability of the device is ensured. Further, the liquid crystal element of the liquid crystal display device operates accurately and the liquid crystal element screen is not disturbed.

〔Example〕

The present invention will be specifically described below based on embodiments shown in the drawings.

FIG. 1 is a side view showing the backlight device set in the liquid crystal display device.
The backlight device includes a power supply section 13 containing a transformer and the like, and an illumination section 14 containing a light source lamp and the like, which are connected in a triangular shape on the side. When using the backlight device, from the liquid crystal display device main body 11 to the liquid crystal element screen section 12,
is raised at a predetermined angle, and the backlight device is set in the triangular space thus formed with the illumination section 14 in contact with the back surface of the liquid crystal element screen section 12. Inside the illumination section 14, two hot cathode compact fluorescent lamps 16 with a power consumption of approximately 1W are placed together with a reflector 15 behind them, so as to illuminate the liquid crystal element screen from behind. There is. Once the backlight device is in place,
The lock button 17 provided on the power supply section 13 is pressed to lock the liquid crystal display device main body 11.

This lock button 17 is provided with three power supply terminals, and when the lock button 17 is locked to the liquid crystal display device main body 11, the backlight device is electrically connected to the liquid crystal display device as shown in FIG. . Figure 2 shows a circuit diagram that uses commercial power to drive a liquid crystal element and light a small fluorescent lamp.
The DC power converted in step 1 is supplied to the liquid crystal element drive circuit 22 via the power switch 1 of the liquid crystal display device.
When the power switch 1 is closed, the liquid crystal element screen lights up and is driven. The power input circuit of the compact fluorescent lamp lighting circuit 20 of the backlight device is provided with a pair of input terminals 2, 2 and a signal terminal 3. When the lock button 17 is locked, the input terminals 2, 2 are connected to the front stage of the power switch 1. The signal terminal 3 is connected to the subsequent stage of the power switch 1.

Next, FIG. 3 shows a lighting circuit 20 for a small fluorescent lamp.
The lighting circuit 20 is roughly divided into a power input circuit 23 and a self-excited push-pull inverter circuit 24, and the transistors Tr ₄ and Tr ₅ of the inverter circuit 24 are turned on alternately to turn on the transformer T. A current with a predetermined frequency flows from the secondary side to light the small fluorescent lamp 16.

Overvoltage lighting detection circuit 25 for small fluorescent lamp 16
is the capacitor C ₂ and the resistor R ₉ for overvoltage detection,
R ₁₀ , R ₁₄ , R ₁₅ , rectifier diode D ₁ , D ₂ , D ₃ ,
D ₄ , Zener diode ZD, transistor
It consists of Tr ₁ , programmable union transistor PUT, etc., and leveling capacitors C ₇ and C ₈ are connected in series with resistors R ₉ , R ₁₀ , R ₁₄ , and R ₁₅ for overvoltage detection, respectively. There is.
The overvoltage lighting detection circuit 25 is interposed between the secondary side of the transformer T and the switching transistor _Tr2 that drives the transformer T. Note that the capacitors C ₅ and C ₆ are current limiting elements for the small fluorescent lamp 16, and the capacitor L ₂ is a current limiting element for the inverter circuit 24.

When the power switch 1 is closed, a DC voltage is applied to the signal terminal 3, which creates a potential difference across the resistor _R5 , and a bias voltage is applied to the transistor _Tr2 , turning it on.
Tr ₃ is also turned on. As a result, the inverter circuit 24 starts self-oscillation, and the high frequency voltage generated across the primary winding _N1 is boosted according to the turns ratio.
The lighting voltage for the compact fluorescent lamp 16 is taken from the secondary winding _N2 . At this time, most of the voltage generated in the secondary winding N ₂ is applied to the capacitors C ₅ and C ₆ ,
The discharge current is determined by the capacitance of these capacitors C ₅ and C ₆ .

When the use of the liquid crystal display device is finished and the power switch 1 is turned off, there is no potential difference between both ends of the resistor _R5 , the transistor _Tr2 is turned off, and the transistor _Tr4 is also turned off. Therefore, both transistors Tr ₄ and Tr ₅
OFF, and the self-excited push-pull inverter circuit 24 does not operate, so the small fluorescent lamp 16 goes out.

Next, when the emitters attached to the electrodes of one of the small fluorescent lamps 16 decrease after long-term use, the lamp voltage increases to about 130V, causing overvoltage lighting. Therefore, the potential at point b also increases.
This power is then stored in capacitor _C2 and applied to Zener diode ZD. In addition,
The breakdown voltage at the start of lighting is about 600V, and a large potential is momentarily generated at point b.
That power is stored in the capacitor _C2 , but by increasing the charging time constant, it is not applied to the Zener diode ZD, and the overvoltage lighting detection circuit 2
5 does not work. The Zener voltage of this Zener diode ZD is selected to be lower than the potential at point b and higher than the potential difference during steady lighting. Therefore, when the overvoltage turns on, Zener diode ZD detects this and transistor Tr ₁ turns on. This makes the resistance R ₁ and
The potential at point a between _R2 drops and the programmable union transistor
The anode and cathode of PUT become conductive, and there is no potential difference across resistor _R5 . Therefore, the transistor
Tr ₂ is turned off, resulting in the same result as turning off the power switch 1, the drive of the transformer T is released, and the small fluorescent lamp 16 is turned off.

Next, the amount of reduction of the emitters attached to the electrodes of the small fluorescent lamp 16 is not equal on both electrodes, and as shown in FIG. 4B, the positive voltage side is V ₁ , which is an almost normal voltage. , the negative voltage side is overvoltage V ₂
In the case of a one-way overvoltage as shown in the diagram, the amount of electricity in the shaded area is equal, so the potential difference |V ₂ |−
|V ₁ |=E is charged to the leveling capacitor _C7 . Therefore, the zero potential deviates by (1/2)・E, and both the positive and negative voltages are (1/2)・E.
(V ₂ +V ₁ ), and the one-way overvoltage is eliminated.
Therefore, even if this is rectified by the rectifier diode D ₁ , a potential of (1/2)·(V ₂ +V ₁ ) is always obtained, making it possible to reliably detect overvoltage.

In this way, even if there is a one-way overvoltage, if the overvoltage turns on, the overvoltage lighting detection circuit 25 will reliably detect this and the small fluorescent lamp 16 will automatically turn off, ensuring the reliability of the device. , it is possible to prevent the disturbance of the liquid crystal element screen caused by overvoltage lighting.

[Effect of idea]

As explained above, the backlight device of the present invention can
Since the overvoltage lighting detection circuit releases the drive of the transformer that supplies power to the small fluorescent lamp and automatically turns off the small fluorescent lamp, overvoltage lighting of the small fluorescent lamp can be prevented. Since a leveling capacitor is connected in series with the overvoltage detection resistor, the small fluorescent lamp 16 is reliably turned off even if there is a one-way overvoltage. Therefore, it is possible to provide a backlight device for a liquid crystal display which has high device reliability, and in which the liquid crystal element of the liquid crystal display device operates accurately and the liquid crystal element screen is not disturbed.

[Brief explanation of drawings]

FIG. 1 is an explanatory diagram of the use of the backlight device, FIG. 2 is an explanatory diagram of the connection between the backlight device and the liquid crystal display device, FIG. 3 is a small fluorescent lamp lighting circuit diagram, and FIG. 4 and FIG. 5 are explanatory diagrams of one-way overvoltage. 1... Power switch, 2... Input terminal, 3...
Signal terminal, 16: compact fluorescent lamp, 20: lighting circuit, 23: power input circuit, 24: self-excited push-pull inverter circuit, 25: overvoltage lighting detection circuit, C ₇ , C ₈ : equalizing capacitors.

Claims (1)

[Scope of Claim for Utility Model Registration] In a backlight device that includes a built-in compact fluorescent lamp and is placed behind a liquid crystal element screen of a liquid crystal display device to illuminate the liquid crystal element screen, the lighting circuit for the compact fluorescent lamp is It has an overvoltage lighting detection circuit that releases the drive of the transformer that supplies power to the compact fluorescent lamp and turns off the small fluorescent lamp when the lamp lights up due to overvoltage.This overvoltage lighting detection circuit is connected in series with an overvoltage detection resistor. A backlight device for a liquid crystal display, comprising a leveling capacitor connected to a backlight device.