JPH02282220A - Fluorescent lamp driving circuit for back light of liquid crystal display device - Google Patents

Abstract

PURPOSE:To prevent the deterioration in the display of a screen by an electric discharge noise without applying an electrical shield by forming the driving circuit in such a manner that the discharge of a fluorescent lamp is executed within the horizontal or vertical blanking period of the video signal displayed on the screen. CONSTITUTION:The input terminal (i) of the video signal in the circuit for driving, by AC, the lighting of the fluorescent lamp L for back light of the liquid crystal display device to display the screen of the video signal is connected to a separating circuit 1 of a synchronizing pulse generating section IC and an AFC circuit 2 and the output terminal O for vertical oscillation are connected to the separating circuit 1. The oscillation of the oscillation circuit 3 is controlled by the output signal of the AFC circuit 2 and the negative horizontal synchronizing pulse synchronized with the horizontal synchronizing signal of the video signal is outputted from a driving circuit 4. The pulse width of this synchronizing pulse is adjusted to the pulse width within the horizontal blanking period of the video signal and the high voltage for discharge of the pulse width within the blanking period is repeatedly supplied via a switching element to the fluorescent lamp L.

Description

DETAILED DESCRIPTION OF THE INVENTION (a) Industrial Application Field The present invention relates to a fluorescent lamp drive circuit for backlighting a liquid crystal display device, which drives a fluorescent lamp for backlighting a liquid crystal display device such as a liquid crystal television with alternating current. .

(b) Conventional technology Conventionally, color or monochrome liquid crystal display devices for liquid crystal televisions, word processors, and personal computers have been
Since liquid crystals do not emit light by themselves, they usually require a flat light source, that is, a backlight.
6], pages 21 to 26) describes the use of fluorescent lamps driven by high-frequency alternating current lighting as backlights for liquid crystal panels.

In addition, in many cases, this type of liquid crystal display device, except for large ones, is desired to be miniaturized and portable, so a primary or secondary battery is used as the driving power source, and the driving power source is boosted. A high discharge voltage of several hundred volts is created.

By the way, fluorescent lamps for backlights are required to have uniform brightness characteristics, and for example, Japanese Patent Application Laid-Open No. 189154/1989 (HOIJ 61/30) describes a flat fluorescent lamp suitable for backlights when downsizing. Are listed.

In principle, flat fluorescent lamps have the characteristic that uniform brightness characteristics can be obtained with a very short discharge time of several microseconds.

The conventional backlight fluorescent lamp drive circuit that drives a backlight fluorescent lamp such as the above-mentioned flat fluorescent lamp at a low frequency or a high frequency is a third type.
It is configured as shown in the figure, in which (O5C) consists of three inverters (G1), (G2), (G3), two diodes (Dl) (D2), and two resistors (R1). ,
(R2), a capacitor (C1), and outputs a low frequency or high frequency negative pulse.

(Ql) and (C2) are PNP type and NPN type transistors that invert the output pulse of the oscillator (O8C) and amplify the power, and the emitter of the transistor (Ql) is connected to the 12 volt power supply terminal ( 10B), the collector and emitter of the transistor (C2) are connected to the collector and ground of the transistor (Ql), and a resistor for base input is connected to the bases of both transistors (Ql) and (C2). The output pulse of the oscillator (O8C) is applied through each of (R3) and (R4).

(C3) is an N-channel depletion type FET that forms a switching element for the inverter, and its gate is connected to the connection point between the collectors of the transistor (Ql) and (C2), and its source is grounded.
It is turned on and off depending on the low level and low level of the output pulse of O3C).

(T) is a step-up transformer with a FET (C3) installed in the primary circuit, one end of the primary winding (!1) with N1 turns, the other end is the power supply terminal (+B), and the drain of the FET (C3). A secondary winding (22
) are connected to one end and the other end of a backlight fluorescent lamp (L) consisting of a flat fluorescent lamp.

(C2) and (R5) are surge absorbing capacitors and resistors installed in series between the drain of FET (C3) and the ground, and (C3) is installed between the power supply terminal (10B) and the ground. This is a bypass capacitor.

Note that the inverters (G1) to (G3) are formed using integrated circuits, and the other end of the secondary winding (I12) is grounded.

Then, the oscillator (O3C) outputs a pulse with a frequency set by the capacitor (C1) and resistors (R1) and (R2), and at this time, the diodes (Dl) and (D2
), the charging and discharging time constants of the capacitor (C1) are determined by the resistors (R1) and (R2), respectively, and the duty factor of the pulse is set.

By the way, the discharge time of the fluorescent lamp (L) is determined by the output pulse width of the oscillator (O5C).
) at a high frequency, so when the oscillation period of the oscillation part (O5C) is set to approximately 63 μsec, the standard You can get a backlight with a certain brightness.

Therefore, the resistances (R1) and (R2) of the oscillation part (O3C) are usually set to have greatly different resistance values, and the oscillation part (O3C) receives a negative signal with a period of about 64 μsec and a pulse width of 2 to 3 μSec. A thin pulse is output.

Then, the output pulse of the oscillator (O3C) is inverted and amplified by the transistors (Ql) and (C2), and is passed from the connection point of the collectors of the transistors (Ql) and (C2) to the FET (
A polarity inversion pulse of the output pulse of the oscillator (OS C) is applied to the gate of the oscillator (OS C) as a drive pulse. Switching is performed based on the output pulse of O3C).

Furthermore, when FET (Q3) is reversed from off to on,
A FET (Q3) is connected to the primary winding (11) of the step-up transformer (T).
) flows into the secondary winding (12), and the winding Nl
, a positive high voltage of several hundred volts set at a ratio of 2 to 3
μsec occurs and the FET (Q3) is reversed from on to off, the current in the primary winding (!1) decreases, and the current in the secondary winding (!1) decreases.
I! , 2), a negative high voltage of several hundred volts, that is, a flyback voltage, is generated for, for example, 3 to 4 μsec.

And the positive of the secondary winding (22) of the step-up transformer (T),
Based on the negative high voltage, the fluorescent lamp (L) generates an oscillating part (O
A positive discharge occurs for 2 to 3 μsec synchronized with the leading edge of the output pulse of the oscillator (O5C), and a reverse discharge occurs for 3 to 4 μsec synchronized with the trailing edge of the output pulse of the oscillator (O5C).
For each period of the output pulse of 5C), the AC lighting drive is performed by sequentially controlling forward and reverse discharge for 5 to 7 μsec.

Note that the lighting drive frequency of the fluorescent lamp (L) is determined by the oscillation unit (O
5C), and by changing the discharge time, it is also possible to drive the fluorescent lamp (L) at a low frequency of about 60 Hz.

Further, when the fluorescent lamp (L) is other than a flat type fluorescent lamp, the frequency, pulse width, etc. of the output pulse of the oscillation unit (O8C) are changed depending on the fluorescent lamp.

(C) Problems to be Solved by the Invention By the way, as is clear from FIG. Fluorescent lamps are driven asynchronously with the signal.

Therefore, when the discharge noise of the fluorescent lamp mixes into the signal path of the video signal, the discharge noise appears on the screen as periodic noise, and at this time, changes in the brightness of the screen occur, making it impossible to perform a good display.

Therefore, it is possible to electrically shield the entire fluorescent lamp, but shielding the fluorescent screen of a fluorescent lamp in particular requires a special conductive material that is translucent, making it very expensive. There is a point.

Further, if the phosphor screen is shielded with a special conductive member, the amount of illumination light of the liquid crystal is reduced by the member, resulting in a problem that illumination performance is reduced.

(d) Means for solving the problem This invention has been made with the above points in mind,
In a backlight fluorescent lamp drive circuit of a liquid crystal display device that drives a backlight fluorescent lamp of a liquid crystal display device that displays a video signal screen with alternating current, a synchronization pulse is output in synchronization with a horizontal or vertical synchronization signal of the video signal. a synchronization pulse generator that adjusts the pulse width of the synchronization pulse to a pulse width within the horizontal or vertical blanking period of the video signal and outputs the pulse width, the pulse width adjustment unit being driven by the output pulse of the adjustment unit. a switching element; and a step-up transformer, wherein the switching element is provided in a primary circuit and the fluorescent lamp is connected to a secondary circuit, and which repeatedly supplies a discharge high voltage having a pulse width within the blanking period to the fluorescent lamp. This is a backlight fluorescent lamp drive circuit for a liquid crystal display device.

(e) Effect: Therefore, the discharge of the fluorescent lamp is performed within the vertical or horizontal blanking period of the video signal, and discharge noise of the fluorescent lamp does not appear on the screen, making it unnecessary to electrically shield the fluorescent lamp.

(F) Embodiment Next, the present invention will be explained in detail with reference to FIGS. 1 and 2 showing one embodiment thereof.

In Fig. 1, the same symbols as in Fig. 3 indicate the same or equivalent parts, and (IC) is a synchronization pulse generator consisting of an integrated circuit for color television deflection circuits manufactured by Sanyo Electric Co., Ltd., model number LA 7800. It has a synchronous separation circuit (1), an AFC circuit (2), a horizontal oscillation circuit (3), a horizontal drive circuit (4), and a video signal input terminal (
]) is connected to the separation circuit (1), and the separation circuit (1) is connected to the AFC circuit (2) and the vertical oscillation output terminal (0). The oscillation circuit (3) free-running oscillates based on the variable resistor (R6) and the capacitor (C4), and the oscillation of the oscillation circuit (3) is controlled by the output signal of the AFC circuit (2). From the drive circuit (4) connected to the LCD drive power rank output terminal (0') and the AFC circuit (
2) outputs a negative horizontal synchronization pulse synchronized with the horizontal synchronization signal of the video signal at the input terminal (i).

(MM) is a monostable multi-by-break (hereinafter referred to as mono-multi) consisting of a logic integrated circuit with model number 4538, which forms a pulse width adjustment section, and a rising trigger terminal (a).
is grounded, and the falling trigger terminal (b) is connected to the drive circuit (4), and is triggered by the falling edge of the horizontal synchronizing pulse, and the time constant terminal (t1), (t
Based on the time constant variable resistor (R?) and capacitor (C5) connected to 2), the pulse width of the horizontal synchronizing pulse is set to a predetermined pulse within the horizontal blanking period of the video signal of the input terminal (1). Adjust the pulse width to Q
It is output from the output terminal (ear) to the bases of transistors (Ql) and (C2) via resistors (R3) and (R4).

Then, a video signal displayed on the screen, such as a video signal from a TV tuner or an externally input video signal, is input to the separation circuit (1) via the input terminal (i), and at this time,
The separation circuit (1) separates and extracts the 15.73 KHz horizontal synchronizing signal shown in FIG.
, the extracted horizontal synchronization signal is output to the AFC circuit (2).

Further, the AFC circuit (2) generates a fine adjustment signal (A
The fine adjustment signal controls the oscillation frequency of the oscillation circuit (3) to the frequency of the horizontal synchronization signal of the video signal at the input terminal (1).

Then, the output signal of the oscillation circuit (3) is amplified by the drive circuit (4), and at this time, the output signal from the drive circuit (4) is sent to the output terminal (0') and the monomulti (MM) as shown in FIG. As shown in (b), the periodic 6:34 signal whose falling edge is completely synchronized with the rising edge of the leading edge of the horizontal synchronization signal in (a) of the same figure.
A synchronizing pulse of z 5ec (=1715.73 KHz), that is, a synchronizing pulse with a period of IH (H is a horizontal scanning period) is output.

Note that the synchronization signal of the output terminal (0) is output from the liquid crystal panel drive unit (
The signal is supplied to a vertical oscillation circuit (not shown), and a vertical synchronization signal is extracted and formed by the oscillation circuit.

Further, the synchronizing pulse at the output terminal (0') is supplied as a clock pulse to a liquid crystal driving counter of the liquid crystal panel driving section, and the horizontal driving of the liquid crystal panel is controlled by the counter.

On the other hand, the monomulti (MM) to which the synchronization pulse of the drive circuit (4) is input is triggered by the falling edge of the synchronization pulse, and at this time, the leading edge of the synchronization pulse is 0.2H of the video signal of the input terminal (i). At the leading edge of the horizontal blanking period, the resistor (R7) and capacitor (C5)
Based on the adjustment of
From here, a negative pulse whose pulse width of the synchronization pulse is adjusted to a pulse width of 2 to 3 μsec within the horizontal blanking period,
That is, a pulse is output whose cycle and pulse width are set to be similar to those of the output pulse of the oscillation unit (1) in FIG. 3, and whose generation timing is controlled within the horizontal blanking period.

Then, the output pulse of the Q output terminal (η) of the monomulti (MM) is inverted and amplified by the transistors (Ql) and (C2), and the connection point of the collectors of the transistors (Ql) and (C2) is connected to the switching element for the inverter. FE forming
A positive pulse with a pulse width τ of 2 to 3 μsec within the horizontal blanking period of each IH shown in FIG. 2(c) is output to the gate of FET (C3).
) switches.

Furthermore, based on the switching of the FET (C3), positive and negative high voltages similar to those shown in FIG.
) is 900 volts peak-to-peak (900V
The positive and negative high voltages of p-1) ) are connected to the step-up transformer (T).
This occurs in the secondary winding (21) of the

The positive high voltage generated during a period of pulse width τ of 2 to 3 μsec and the 3 to 4 μs generated following the positive high voltage.
Based on the negative high pressure of ec, the fishing hook 2H ('=
Within the horizontal blanking period of 10 to 13μ5ec), during the period τ' of 5 to 7μSec, the backlight fluorescent lamp (L) is sequentially controlled to forward and reverse discharge, and the fluorescent lamp (L) is switched to alternating current at the IH cycle. Driven to turn on.

Therefore, the fluorescent lamp (L) will discharge during the horizontal blanking period of the video signal displayed on the screen, and at this time, even if the lighting noise of the fluorescent lamp (L) mixes into the signal path of the video signal. , the discharge noise is drawn into the horizontal blanking period and does not appear on the screen.

Therefore, the influence of discharge noise can be eliminated from the screen without electrically shielding the fluorescent lamp, and display deterioration due to discharge noise can be prevented at low cost and without deteriorating illumination performance.

In addition, in the case of Fig. 1, by using a flat fluorescent lamp as the fluorescent lamp (L), uniform brightness characteristics can be obtained with a very short discharge within the horizontal blanking period. You can also easily adjust the brightness of the backlight with variable adjustment.

By the way, in the above embodiment, the fluorescent lamp (L) is driven to turn on at the period of the horizontal synchronization signal, but the fluorescent lamp (L) is turned on at the period of the vertical synchronization signal, and at this time, the fluorescent lamp (I-) is turned on and driven at the period of the vertical synchronization signal. A similar effect can be obtained by retracting the discharge time into vertical blanking.

Furthermore, it goes without saying that various types of fluorescent lamps such as straight tube types other than flat fluorescent lamps can be used as the fluorescent lamp (L).

(G) Effects of the Invention As described above, according to the fluorescent lamp drive circuit for backlight of a liquid crystal display device of the present invention, the discharge of the fluorescent lamp is
This is done during the horizontal or vertical blanking period of the video signal displayed on the screen, and the discharge noise of the fluorescent lamp is drawn into the horizontal or vertical blanking period. This can prevent display deterioration.

[Brief explanation of drawings]

FIG. 1 is a wiring diagram of one embodiment of a fluorescent lamp drive circuit for backlight of a liquid crystal display device of the present invention, and FIG.
d) is a timing chart for explaining the operation in Fig. 1;
The figure is a wiring diagram of a conventional backlight fluorescent lamp drive circuit of a liquid crystal display device. (I C)... Synchronous pulse generator, (MM)... Monostable multi-vibration break, (Q3)... FET, (T
)...Step-up transformer, (L)...Fluorescent lamp for backlight.

Claims (1)

[Claims] (1) In a backlight fluorescent lamp drive circuit of a liquid crystal display device that drives a backlight fluorescent lamp of a liquid crystal display device that displays a video signal screen with alternating current, synchronization that is synchronized with a horizontal or vertical synchronization signal of the video signal. a synchronization pulse generation section that outputs a pulse; a pulse width adjustment section that adjusts the pulse width of the synchronization pulse to a pulse width within a horizontal or vertical blanking period of the video signal and outputs the pulse width; and the output pulse of the adjustment section a switching element to be driven; a booster in which the switching element is provided in a primary circuit, the fluorescent lamp is connected to a secondary circuit, and a high discharge voltage having a pulse width within the blanking period is repeatedly supplied to the fluorescent lamp; A fluorescent lamp drive circuit for the backlight of a liquid crystal display device equipped with a transformer.