EP0406116A1 - Process and device for dimming a fluorescent lamp of an LCD backlight - Google Patents

Abstract

Device for dimming light, in a ratio which can vary from 1 to 1000, for a fluorescent lamp (34) used in a liquid-crystal display backlighting facility. The device comprises a generator (1-13, 15-20) of square-wave signals formed of periodic pulses of adjustable width (potentiometer 1); these square waves are synchronised by the image synchronising signal (14) for the liquid crystal display. An a.c. voltage generator powers the lamp only during the pulses of the square-wave signals. The drop in output from the lamp (34) for square waves of very short duration enables the desired dimming ratio to be achieved without any image flicker effect. …<IMAGE>…

Description

The present invention relates to a method and a device for dimming light for fluorescent lamps used in a rear lighting installation of a liquid crystal display device.

Liquid crystal displays, in particular those used in the color display systems of aircraft and helicopter dashboards, are backlit to provide high luminance so that they can be seen with sufficient comfort in high light bright atmosphere. This luminance must be able to be made variable according to the different conditions of light environment, and it is generally necessary to control this luminance according to the day-night variations of this environment. These variations impose for the light source a dynamic greater than 1000/1, that is, for fluorescent lamps, a brightness of the order of a few Cd / m2 for the minimum luminance and of approximately 15,000 Cd / m2 for the maximum luminance.

It should be noted that the light source uses fluorescent lamps because of their excellent energy efficiency and their colorimetry which is well suited to liquid crystal screens.

In order to obtain an optimal light output for these fluorescent lamps, the supply voltage applied between their two electrodes is an alternating high-voltage, the amplitude of which is generally of the order of 300 to 500 volts, and the frequency of which is of the order of a few tens of kilohertz.

It is known to vary the luminance of a fluorescent lamp by varying the amplitude of its supply voltage, and therefore the intensity of the current flowing through this lamp. This process does not provide a variation in luminance only in a ratio less than 10/1, which is clearly insufficient for the application cited above. In addition, since the starting voltage of fluorescent lamps is a function of temperature and, in particular, increases when the temperature decreases, this method of controlling luminance does not allow operation in a wide temperature range, in particular for temperatures below 0 degrees Celsius.

The range of variation in luminance can, in known manner, be improved by frequency-modulating the alternating supply voltage, and more precisely by using slots of variable frequency, for example from a few tens of hertz to a few tens of kilohertz. But, in this case, to fulfill the abovementioned operating conditions, it is necessary, for the lowest luminances, to work at frequencies below 15 kilohertz, but these frequencies can cause sound vibrations. Finally, at a very low level of luminance, there appears a beat by stroboscopic effect between the periodic lighting of the lamps and the image refreshment whose frequency is of the order of 50 to 60 hertz. This results in a scrolling on the screen of a horizontal bar in overcurrent, which is an unacceptable defect for viewing piloting.

It is also known to vary the luminance of a fluorescent lamp by cutting the supply voltage by means of signals formed by slots whose width is adjustable. But here again, problems of stroboscopic effect appear.

The invention aims to remedy these drawbacks. This is obtained by cutting the alternating supply voltage of a fluorescent lamp for rear lighting of a liquid crystal screen by a signal in slots of adjustable duration as a function of the desired light intensity, the start of the slots of which is synchronized on "image synchronization" signal from the liquid crystal display.

According to the invention there is provided a method of dimming light for a fluorescent lamp for rear lighting of a liquid crystal screen, consisting in applying to the lamp an alternating supply voltage cut by cutting signals formed by time slots. adjustable according to the desired light intensity, characterized in that it consists in synchronizing the cutting signals on a signal corresponding to the image synchronization signal of the liquid crystal screen, divided in frequency by a whole number n greater at 0.

According to the invention, there is also proposed a light dimming device for a fluorescent lamp for rear lighting of a liquid crystal screen, comprising a generator of cutting signals formed by periodic signals in slots, of fixed frequency and adjustable width, an alternating voltage generator for supplying the fluorescent lamp and blocking means controlled by the switching signals so as to authorize the operation of the alternating voltage generator only during the duration of the slots, characterized in that, to implement the method according to any one of the preceding claims, it comprises means for synchronizing the cutting signals to a signal corresponding to the image-synchronization signal of the liquid crystal screen, divided in frequency by an integer n greater than 0.

• - la figure 1, un schéma électrique d'un dispositif de gradation de lumière, selon l'invention, pour lampe fluorescente d'éclairage arrière d'un écran à cristaux liquides ;
• - la figure 2, un diagramme des temps explicatif du fonctionne­ment du dispositif de la figure 1 ; et
• - la figure 3, un schéma électrique partiel d'une va­riante de réalisation du dispositif de la figure 1. La fi­gure 1 montre un potentiomètre de commande de luminosité 1, qui reçoit sa tension continue négative d'alimentation sur une borne 2. Une partie de cette tension continue est préle­vée par un curseur 3 du potentiomètre 1, pour constituer un niveau de tension continue, d'amplitude réglable par déplace­ment du curseur 3, qui, après mise à niveau par un amplifica­teur opérationnel 4 (associé à une résistance série 5 et à une résistance de contre-réaction 6), est appliqué via une résistance 7 à l'entrée inverseuse 8 d'un comparateur de tension 9, lui-même alimenté par une tension continue +Vo, -Vo.

The invention will be better understood, and other characteristics will emerge from the description below and the figures relating thereto which represent:

• - Figure 1, an electrical diagram of a light dimming device, according to the invention, for a fluorescent lamp for rear lighting of a liquid crystal screen;
• - Figure 2, a time diagram explaining the operation of the device of Figure 1; and
• - Figure 3, a partial electrical diagram of an alternative embodiment of the device of Figure 1. Figure 1 shows a brightness control potentiometer 1, which receives its negative DC supply voltage on a terminal 2. Part of this DC voltage is taken by a slider 3 of the potentiometer 1, to constitute a level of DC voltage, of amplitude adjustable by displacement of the slider 3, which, after leveling by an operational amplifier 4 (associated with a series resistor 5 and to a feedback resistance 6), is applied via a resistance 7 to the inverting input 8 of a voltage comparator 9, itself supplied by a DC voltage + Vo, -Vo.

The non-inverting input 10 of the comparator is connected, via a resistor 11, to the output 12 of a sawtooth signal generator 13, the signals of which are synchronized with the pulse image synchronization signal of a liquid crystal display; this impulse signal is applied at 14, to the generator 13.

This generator 13 includes an operational amplifier 15 which is mounted as an integrator using a capacitor 17 which connects its input and its output and a resistor 16 which connects its input to a terminal 18 to which a voltage of reference V2.

The rapid return of the saw teeth is achieved by a rapid analog switch 19, of the CMOS type, which is connected in parallel to the capacitor 17 and which is controlled by image synchronization pulses, shaped by a monostable rocker 20 .

In FIG. 2 which is a diagram of Amplitude (A) / time (t) curves, the (negative) image synchronization pulses 21 are represented on the upper curve A, while the saw teeth at the output 12 of the generator 13 are shown in B. The adjustable continuous level applied at 8 is drawn in dashed lines at 22.

As long as the curves B and 22 are intersecting, negative and periodic voltage slots 23, of width L adjustable by adjustment of the cursor 3, are generated at the output 24 of the comparator 9, the amplitude of these slots being equal to Vo.

The elements referenced 1 to 20 constitute a generator of periodic slots, of fixed frequency but of adjustable width, these slots being synchronized by the image synchronization pulses 21 of the liquid crystal screen whose rear lighting is to be produced.

The output 24 of the comparator 9 (slots 23) and the output 25 of the monostable flip-flop 20 (pulses 21) are respectively applied to the two diodes 27, 22 of an OR circuit 26; the output of circuit 26 is coupled, by a resistor 29 followed by a fitness amplifier 30, to the control input 31 of another analog switch 32. This switch 32 is open when a negative slot 23 or a negative pulse 21 is applied at 31, and it is closed otherwise. It constitutes a blocking switch for an oscillator 33 for supplying alternating high-voltage to the fluorescent lamp 34.

This oscillator 33 comprises: a transformer with a primary primary winding 35 at midpoint 36, a reaction winding 40 at midpoint 41 and a secondary winding 44, two NPN transistors 37, 38, a capacitor 39, three resistors 42, 43, 60 and a shock coil, 48. The transistors 37, 38 have their emitters connected to ground, their collectors respectively connected to the two ends of the winding 35 and their bases connected respectively to the resistors 42, 43; these resistors are respectively connected to the two ends of the reaction winding 40. The capacitor 39 is disposed between the ends of the winding 35. The secondary high-voltage winding 44 of the transformer has a terminal connected directly to ground and another terminal connected, through a ballast capacitor 45, to an electrode 46 of the fluorescent lamp 34; the other electrode, 47, of the lamp is connected to ground.

The positive supply voltage + V1 of the oscillator 33 is applied, via the shock coil 48, to the midpoint 36 and, from there, through the resistor 60, to the midpoint 41, while a DC voltage negative blocking -V3 is applied, when the switch 32 is closed, on point 41, and through the resistor 60, on point 36.

The operation of the circuit in FIG. 1 is as follows:

When the cursor 3 of the potentiometer 3 is in the high stop position (in FIG. 1), the positive voltage applied to the terminal 8 is of maximum amplitude, greater than that of the saw teeth 8, so that a level of DC voltage, of amplitude equal to -Vo, is applied at 24.

The voltage applied to the control input 31 of the switch 32 is then a continuous level, so that the switch 32 remains permanently open and, that the oscillator 33 operates without interruption, supplying the fluorescent lamp 34 at its maximum luminance.

When, from this high stop position, the cursor is gradually moved downwards (towards the mass), the tension level 22 (FIG. 2) decreases in amplitude and comes to cut the curve 8 of the saw teeth, which generates the slots 23, of width L progressively decreasing as the cursor 3 approaches the mass, and whose front edge is synchronized with that of the pulses 21. The oscillator 33 then operates only during the duration of these slots 23 (curve D of FIG. 2) since, apart from these, the switch 32 is closed and, as a result, the voltage -V3 blocks the oscillator 33.

The light intensity provided by the lamp 34 is then proportional to the width L of the slots 23, which depends on the position of the cursor 3.

When the cursor 3 comes to the bottom stop (earth side), no more signal appears at 24, but however, thanks to the OR circuit 28, the pulses 21 are nevertheless applied to the control terminal 31, which makes the oscillator 33 during the duration of the image synchronization pulses 21: it is thus advantageously obtained a non-zero minimum of brightness for the lamp 34.

The circuit according to FIG. 3 is a representation of another device according to the invention, in its part where it differs from that according to FIG. 1; this circuit includes a series resistor 49, or "foot resistor", which is inserted between the electrode 47 of the lamp 34 and the ground. The voltage across this foot resistor 49 is applied, via a rectifier 50 and a series resistor 51, to a first input 52 of a differential amplifier 53. The other input 55 of this differential amplifier 53 receives, by the through a reference voltage V4 and an adjustable resistor 54, a DC voltage of adjustable value.

The output of the differential amplifier 53 is connected to the control input 56 of a voltage regulator 57, which is inserted between the supply terminal + V1 and the shock inductor 48 and which is capable of varying the DC voltage on its output 58 as a function of the control voltage it receives at 56.

The part of the device in FIG. 3, where the references 49 to 57 appear, constitutes a control loop whose role is to control the current in the resistor 49, and therefore in the lamp 34, at the value given by the reference voltage applied to input 55, value which depends on that of the adjustment resistor 54; thus it is possible to optimize the value of the supply voltage tion of the lamp 34 as a function of its operating point, thereby minimizing the dissipated power and avoiding temperature variations.

In addition, the circuit of FIG. 3 allows the lamp 34 to be started at low luminosity or at very low ambient temperature.

It should be remembered that the starting voltage of fluorescent lamps is a function of the temperature of the electrodes and of the enclosure which contains mercury vapor. At low light, the average current flowing through the lamp is very low and does not heat up the lamp. The ignition voltage is then higher than in the case where the lamp is very bright. This ignition voltage then also increases when the ambient temperature decreases.

In the event of non-priming, due to the too low brightness or the too low ambient temperature, no voltage is applied to the terminal 52 of the differential amplifier 53, so that the maximum control voltage of the regulator 57 is applied at 53, then increasing the effective supply voltage of oscillator 33 above its ignition voltage under these unfavorable conditions, which of course assumes that the voltage + V1 is of sufficient amplitude.

The circuit of FIG. 3 makes it possible to pair the lamps with low light.

In the case of lighting with two or more fluorescent lamps, it is necessary to pair these lamps for low levels of brightness in order to obtain for them identical starting voltages, otherwise one of the lamps would risk d 'enlighten and the other not; for this, each lamp has its own circuit according to FIG. 3. This pairing is carried out by adjusting the resistors 54 of each circuit so that all of the lamps start for the same operating conditions. For this purpose, it is also possible to adjust the foot resistors 49, but this solution is not as good as it may increase losses.

It was mentioned above that the minimum luminance was obtained by cutting, or modulating, the alternating voltage of the oscillator 33 by slots whose duration corresponds to the width of the image-synchronization pulses 21. Now, these pulses 21 have a width of the order of microseconds. In theory, to obtain, as required, a variation in luminance of the fluorescent tube 34 in a ratio of 1 to 1000, it would be necessary to vary the width L of the pulses 23 ranging from 50 microseconds to 1000 times more, that is to say 50 milliseconds. However, a cut at 50 milliseconds corresponds to a frequency of 20 hertz, which would introduce a visible flicker ("flicker" in Anglo-Saxon literature) of the image provided by the liquid crystal screen so that in fact, in purely and simply according to this theory, the device according to the invention could not operate under the required conditions (brightness ratio of 1000/1).

In reality, this is not so because, when the lamp 34 is only allowed to operate for 50 microseconds, it does not have time to heat up, the ignition operation itself is not sufficient not to raise the lamp temperature. Its cold light output is then three times lower than that corresponding to continuous or quasi-continuous operation, that is to say hot, so that the ratio of 1 to 1000 in luminance is finally obtained by the way, for the width L of the chopping pulses of the sinusoidal voltage of the oscillator 33, from 50 microseconds to fifteen milliseconds, which corresponds to a chopping frequency markedly higher than those which show a phenomenon of flicker.

The invention is not limited to the exemplary embodiments which have just been described. This is for example that, in the context of an automatic servo-control on the lighting environment, the luminosity control potentiometer, 1, can be replaced by a photodetector providing a voltage proportional to the desired luminance. In the example above, the start of each slot 23 for cutting the sinusoidal voltage of the oscillator 33 is synchronized with the image synchronization signal of the liquid crystal screen. In order to extend the operating dynamics of the device, it is also possible to synchronize this time slot by the image synchronization signal divided in frequency by an integer greater than 1. This is obviously only possible if the frequency of this signal divided by this number is not low enough to introduce flickering. It is also possible, when several fluorescent lamps are necessary, to use only one switch 32, provided that a resistance is inserted in the connection between this switch and point 41 of each oscillator specific to each of these lamps.

Claims (4)

1 - Method for dimming a fluorescent lamp (34) for back lighting a liquid crystal screen, consisting in applying to the lamp (34) an alternating supply voltage cut by cutting signals formed by slots (23 ) of duration (L) adjustable as a function of the desired light intensity, characterized in that it consists in synchronizing the cutting signals (23) with a signal corresponding to the image synchronization signal (21) of the screen to liquid crystal, divided in frequency by an integer n greater than 0. 2 - A method of dimming light according to claim 1, characterized in that a minimum luminance of the lamp is obtained by using, as cutting signals, the pulses of the image synchronization signal (21) of the crystal screen liquids. 3 - Device for dimming a fluorescent lamp (34) for back lighting a liquid crystal screen, comprising a generator (1 to 20) of cutting signals formed by periodic signals (23) in slots, of fixed frequency and of adjustable width (L), an alternating voltage generator (33) for supplying the fluorescent lamp (34) and blocking means (32) controlled by the switching signals (23) so as not to authorize the operation of the voltage generator alternative (33) that during the duration of the slots (23), characterized in that, to implement the method according to any one of the preceding claims, it comprises means (13) for synchronizing the switching signals to a signal corresponding to the synchronization-image signal (21) of the liquid crystal screen, divided in frequency by an integer n greater than 0. 4 - Light dimming device according to claim 3, characterized in that it uses, as cutting signals, the pulses of the synchronization-image signal (21) of the liquid crystal screen, thereby obtaining a minimum luminance value for the fluorescent lamp (34).

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