Abstract:
The invention relates to a backlight device with light emissive diodes not requiring the supply of a large current. According to the invention, each light emissive diode of the backlight device is combined with a voltage converter capable of storing energy during part of the operating cycle and then discharging this energy into the light emissive diode during another part of the cycle.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention is situated in the field of backlight devices for all types of backlight displays or projectors, such as liquid crystal displays or any type of backlight panel such as advertising panels. 
       Technological Background 
       [0002]    Such displays or projectors equipped with discharge lamps are known for being relatively difficult to control in intensity and colorimetry. The notable advances made in light emissive diodes enable new types of backlight devices to be realized. The backlight device thus comprises a plurality of light emissive diodes or LEDs organised in an array form. These diodes are possibly grouped into basic blocks and are thus controlled per block instead of being individually controlled. These diodes are preferably controlled dynamically to improve the contrast and appearance of the display movements. 
         [0003]    Each diode operates at a low voltage in the order of 2 to 5 volts. This low control voltage constitutes a disadvantage as, when for example the power consumed by the diodes must be 100 watts and this power operates at 2 volts, the necessary current is then 50 Amperes. 
       SUMMARY OF THE INVENTION 
       [0004]    One purpose of the invention is to propose a new type of backlight device not requiring the supply of such a high current. 
         [0005]    According to the invention, it is proposed to combine with each light emissive diode a voltage converter capable of storing energy during part of the operating cycle and then discharging this energy into the light emissive diode during another part of the cycle. 
         [0006]    The present invention relates to a set of light emissive diode elements for backlighting, wherein each element comprises a light emissive diode and wherein at least one element is equipped with a step-down voltage converter circuit to supply the light emissive diode. 
         [0007]    According to one particular embodiment, the voltage converter circuit comprises an inductive element and a switch, the switch being controlled such that the inductive element stores energy during a first operating phase and discharges this energy into the light emissive diode during a second operating phase consecutive to the first operating phase. 
         [0008]    The invention also relates to a backlight device for displays, such as for instance a liquid crystal display, comprising:
       a set of light emissive diode elements organised in rows and columns, each element comprising a light emissive diode and at least one element equipped with a step-down voltage converter to supply the said light emissive diode, and   a control circuit to control the voltage converter circuit of the said one light emissive diode.       
 
         [0011]    According to one particular embodiment, the voltage converter circuit comprises an inductive element and a switch, the switch being controlled such that the inductive element stores energy during a first operating phase and discharges this energy into the light emissive diode during a second operating phase consecutive to the first operating phase. 
         [0012]    In practice, the light emissive diode elements of the set are organised into rows and columns and the control circuit comprises a selection circuit to sequentially select the rows of elements of the set, and a control circuit to trigger and control the time of the first operating phase of the elements of the row selected by the said selection circuit. 
         [0013]    Preferably, the light emissive diodes are arranged into basic blocks, each basic block comprising at least one light emissive diode element, and the elements of a same block thus being controlled in the same manner. In this case, the selection circuit of the device selects sequentially rows of basic blocks and the control circuit triggers and controls the time of the first operating phase of the blocks selected by the selection circuit. 
         [0014]    From a functional viewpoint, the first operating phases of the elements of two basic blocks belonging respectively to one row of current blocks and one row of next blocks are not covering. For each element, the time of the first operating phase is variable. It is comprised between a minimum time and a maximum time. 
         [0015]    Advantageously, the first operating phase of the blocks of the next row begins at the end of a period equal to the maximum time after the start of the first operating phase of the blocks of the current row. 
         [0016]    The invention also relates to a display comprising a backlight device to produce light and an imaging device lit by the light produced by the backlight device to display an image during a video frame. The backlight device of this display is in accordance with the backlight device defined previously. 
         [0017]    Advantageously, the operating period of the backlight device is synchronised with the video frame period of the display. The video frame period is for example a multiple of the time of the operating period of the backlight device. The time of the operating period of the backlight device is preferably less than 50 μs (frequency greater than or equal to 20 kHz) so that the operation is inaudible to the human ear. 
     
    
     
       DESCRIPTION OF THE FIGURES 
         [0018]    The invention will be better understood upon reading the following description, provided as a non-restrictive example and referring to the annexed drawings wherein: 
           [0019]      FIG. 1  shows a backlight device in accordance with the invention, 
           [0020]      FIG. 2  is a detailed diagram of a basic block of light emissive diode elements of the basic block array of the device of  FIG. 1 , 
           [0021]      FIG. 3  is a first example of a circuit diagram of a light emissive diode element of the block of  FIG. 2 , 
           [0022]      FIG. 4  is a first example of a circuit diagram of an AND type logic gate of the element of  FIG. 3 , 
           [0023]      FIG. 5  is a second example of a circuit diagram of an AND type logic gate of the element of  FIG. 3 , 
           [0024]      FIG. 6  shows timing diagrams illustrating a first operating mode of the light emissive diode element of  FIG. 3 , 
           [0025]      FIG. 7  shows timing diagrams illustrating the sequential lighting of the basic block array of the backlight device of  FIG. 1 , 
           [0026]      FIG. 8  shows the timing diagrams illustrating the input and output signals of the logic gate of  FIG. 4  or  5 , 
           [0027]      FIG. 9  shows timing diagrams illustrating a second operating mode of the light emissive diode element of  FIG. 3 , 
           [0028]      FIG. 10  is a second example of a circuit diagram of a light emissive diode element of the block of  FIG. 2 , and 
           [0029]      FIG. 11  shows timing diagrams illustrating an operating mode of the light emissive diode element of  FIG. 10 , 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0030]    According to the invention, each diode element of the element array comprises, besides a light emissive diode, a step-down voltage converter to supply the diode and more particularly to store the energy during a first operating phase and discharge it into the diode during a second operating phase. 
         [0031]      FIG. 1  shows a backlight device for displays that is in accordance with the invention. It comprises:
       a set of 10 light emissive diode elements grouped into basic blocks  20 , said basic blocks being organised into an array of n rows and m columns; each basic block comprising at least one light emissive diode element; the elements themselves are organised into arrays within the basic blocks; in this embodiment, the blocks are identical in size although this is not mandatory; the light emitted by each of these elements is time modulated; the elements are therefore controlled in PWM mode (Pulse Width Modulation)   a row selection circuit  11  to sequentially select the rows of basic blocks of the set  10 ; for a set  10  comprising n rows of basic blocks, the selection circuit  11  comprises n outputs, each output being designed to select a row of blocks of the set  10 ; in the rest of the description, Si designates the output intended to select the row i of blocks of the array  10 ,   a control circuit  12 , to supply each column of blocks of the set  10 , with a control signal for lighting the elements of the basic block located at the point crossed by the column and a row selected by the selection circuit  11 ; the control circuit  12  comprises m outputs, each output being connected to the basic blocks of the set  10 ; in the rest of the description, Cj designates the output connected to the column j of the set  10 .         
         [0035]      FIG. 2  shows the detailed diagram of a basic block  20  comprising an array of n′×m′ light emissive diode elements  30 . Said basic block belongs to the row i and the column j of the set  10 . Each of the elements  30  of the block  20  is therefore connected to the output Si of the selection circuit  11  and the output Cj of the control circuit  12 . 
         [0036]      FIG. 3  shows a circuit diagram of a first example of a light emissive diode element  30 . It comprises a light emissive diode or LED  31  and a step-down voltage converter to supply the LED. This converter comprises a diode  32  fitted in series with a switch  33  of the transistor type between a supply terminal receiving a supply voltage Va (known as the supply terminal) and a terminal receiving a voltage Vss. The latter terminal is for example connected to ground. Transistor  33  is controlled by a control voltage supplied by the output S of an AND logic gate  35 , the said gate receives on a first input E 1  a voltage signal coming from the output Si of the selection circuit  11  and on a second input E 2  a voltage signal coming from the output Cj of the control circuit  12 . The diode  32  is arranged to pass the current moving toward the supply terminal. The point located between the diode  32  and the transistor  33  is connected to the cathode of the LED  31  via an inductive element  34 . The anode of LED  31  is connected to the supply terminal. In the rest of the description, L designates the inductance of the inductive element  34 , V LED  designates the voltage at the terminals of the LED  31 , V L  and I L  respectively designate the voltage at the terminals of the inductive element  34  and the current moving toward the inductive element  34 , V D  designates the voltage at the terminals of the diode  32 , V T  and I T  respectively designate the voltage at the terminals of the switch  33  and the current circulating in the switch  33  and V CTRL  designates the control voltage of the switch  33  present at the output of the gate  35 . 
         [0037]    A first circuit diagram example of the logic gate  35  is provided in  FIG. 4 . This gate comprises a diode  36  and a resistive element  37 . The diode  36  is connected between the input E 2  and the output S of the gate. The diode  36  is arranged to pass the current moving toward the input E 2 . The resistive element  37  is connected between the output S and the input E 1 . 
         [0038]    A second circuit diagram example of the logic gate  35  provided in  FIG. 5 . This gate is a “push-pull” assembly comprising a transistor  38  of the NMOS type in series with a transistor  39  of the PMOS type. The common output (sources) of both transistors constitutes the output S of the logic gate  35 . The input E 2  is connected to the two gates of the transistors and the input E 1  is connected to the drain of the transistor  38 . Finally, the drain of the transistor  39  receives the Vss voltage. 
         [0039]    A first operating mode of this light emissive diode element is explained in the timing diagrams of  FIG. 6 . In this operating mode, the inductive element is completely discharged at each operating period.  FIGS. 6(   a ),  6 ( b ),  6 ( c ),  6 ( d ) and  6 ( e ) respectively show the variation in the current I L , the voltage V L , the current I T , the voltage V T  and the voltage V CTRL  during a period of time T of the control voltage V CTRL . 
         [0040]    In the timing diagrams, it is considered that the control voltage V CTRL  is at a high level between the times 0 and t a  and at a low level (zero) between the times t a  and T. The switch  33  is therefore closed (voltage V T  zero) between the times 0 and t a  and open (voltage V T =V a+ V D ) between the times t a  and T. The voltage V L  at the terminal of the inductive element  34  is therefore equal to V a −V LED  between the times 0 and t a . A current I L  flows through the element  34 , said current increasing in a linear manner until a maximum value I max  equal to 
         [0000]    
       
         
           
             
               
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         [0000]    at the time t a  is reached. The current I T  flowing through the switch  33  is therefore equal to the current I L  flowing through the inductive element  34 . When the switch  33  is open (between the times t a  and T), the voltage V L  applied at the terminals of the inductive element  34  is equal to −V LED −V D  until said inductive element is completely discharged. The discharge operation is complete at time t b . The discharge current of the element  34  thus decreases until a zero value is reached at time t b . When the inductive element  34  is completely discharged, the voltage V L  at its terminals becomes zero after a few oscillations due to a resonance between the inductive element  34  and the interference capacitors of the elements  32 ,  33  and  34 . The same operating cycle starts again at the end of the time T. 
         [0041]    If the overall operation of the set  10  of  FIG. 1  is now considered, the rows of basic blocks are selected sequentially as shown by the timing diagrams of  FIG. 7 . In this figure, it is considered that the set  10  comprises 10 rows of basic blocks. The selection circuit  11  thus comprises 10 outputs S 1  to S 10 . The operating period T of the device is therefore divided into 10 sub-periods of equal time (T/10), a sub-period being assigned to each of the outputs S 1  to S 10 . 
         [0042]      FIG. 8  shows the signal V CTRL  applied at the block of the set  10  connected to the outputs S 1  and C 1  and the signals applied to these outputs. A pulse of time T 1 =T/10 is supplied on the output S 1  as already shown in  FIG. 7 . This time is fixed and corresponds to the maximum time that can be applied to the block. A pulse of variable time T 2  lower than or equal to T 1  is applied to the output C 1 . The result is that the voltage V CTRL  applied at the block is a pulse of time T 2  (identical to the one applied on the output C 1 ). The time T 2  defines the level of lighting required for the block considered and is between a minimum non-null time and the maximum time T 1 . The operating frequency 1/T is preferable greater than 20 KHz (namely T=50 μs) so that the sequential addressing is inaudible to the human ear. According to the number n of rows of blocks in the array  10  (the number is generally predefined), the maximum time T 1  is linked with T through T 1 =T/n. Finally, so that the lighting is synchronised with the display of the images, the period of the video frame is preferably a multiple of the operating period (of time T) of the backlight device. 
         [0043]    The inductance value L of the inductive element  34  is defined for the borderline case t a =T 1  and is equal to: 
         [0000]    
       
         
           
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         [0044]    It is possible to provide a particular embodiment wherein the complete discharge of the inductive element  34  finishes at the end of the period T, namely that t b =T. It should be recalled that in a stable state, the average value at the terminals of the inductive element is equal to zero for a first approximation. It will be noted that in this case, the power transmitted to the LED is equal to 
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         [0045]    A second embodiment of the light emissive diode element shown in  FIG. 3  is explained by the timing diagrams of  FIG. 9 . In this operating mode, the inductive element is not fully discharged at the end of the operating cycle.  FIGS. 9(   a ),  9 ( b ),  9 ( c ),  9 ( d ) and  9 ( e ) respectively show the variation in the current I L , the voltage V L , the current I T , the voltage V T  and the voltage V CTRL  during a period of time T of the control voltage V CTRL . 
         [0046]    These timing diagrams must be compared with those of  FIG. 6 . As shown in  FIG. 9(   a ), the inductive element  34  charges when the switch  33  is closed (V T =0) and discharges into LED  31  when it is open (voltage V T =V a +V D ). At the end of the cycle (time T), the current in the inductive element  34  is not zero. This embodiment enables the power transmitted to the LED  31  to be increased without increasing the conduction time of the switch  33 . 
         [0047]      FIG. 10  shows a circuit diagram of a second example of a light emissive diode element  30 . The first example illustrated by  FIG. 3  is an electrical assembly wherein the anodes of the LEDs of the set  10  are connected in common (to the row receiving the voltage V a ). This second embodiment is a variant in which the cathodes of the LEDs of the set  10  are connected in common. This assembly comprises the same components as those of  FIG. 3  but some of them are placed differently. The components whose position remains unchanged with respect to  FIG. 3  keep the same numerical reference as in  FIG. 3 . 
         [0048]    This assembly comprises a light emissive diode or LED  31 ′ and a step-down voltage converter to supply the LED. This converter comprises an inductive element  34 ′ fitted in series with a switch  33  of the transistor type between a supply terminal receiving the supply voltage Va (known as the supply terminal) and a terminal receiving the voltage Vss. The latter terminal is for example connected to ground. The transistor  33  is controlled by a control voltage supplied by an AND logic gate  35 , the said gate receives on an input a voltage signal coming from the output Si of the selection circuit  11  and a voltage signal coming from the output Cj of the control circuit  12 . The point located between the inductive element  34 ′ and transistor  33  is connected to the anode of the LED′  31  via a diode  32 ′. The diode  32 ′ is arranged to pass the current moving toward the LED  31 ′. The cathode of the LED  31 ′ is connected to the supply terminal. 
         [0049]    This assembly operates globally in the same manner as the assembly of  FIG. 3 , namely that the inductive element  34 ′ charges when the switch  33  is closed and discharges when it is open. However, the currents and voltages at the terminals of the components are a little different. An operating mode with complete discharge of the inductive element  34 ′ is shown in  FIG. 11 .  FIGS. 11(   a ),  11 ( b ),  11 ( c ),  11 ( d ) and  11 ( e ) respectively show the variation in the current I L , the voltage V L , the current I T , the voltage V T  and the voltage V CTRL  during a period of time T of the control voltage V CTRL . 
         [0050]    In the timing diagrams, it is considered that the control voltage V CTRL  is at a high level between the times 0 and t a  and at a low level (zero) between the times t a  and T. The switch  33  is therefore closed (voltage V T  zero) between the times 0 and t a  and open (voltage V T =V a +V D +V LED ) between the times t a  and T. The voltage V L  at the terminal of the inductive element  34  is therefore equal to V a  between the times 0 and t a . A current I L  flows through the element  34 ′, said current increasing in a linear manner until a maximum value I max  equal to 
         [0000]    
       
         
           
             
               
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         [0000]    at time t a  is reached. The current I T  flowing through the switch  33  is therefore equal to the current I L  flowing through the inductive element  34 ′. When the switch  33  is open (between the times t a  and T), the voltage V L  applied to the terminals of the inductive element  34 ′ is equal to −V LED −V D  until this latter is fully discharged. The discharge operation is complete at time t b . The discharge current of the element  34 ′ thus decreases until a zero value is reached at time t b . The same operating cycle starts again at the end of the time T. 
         [0051]    Naturally, the invention is not limited to the embodiments previously described. 
         [0052]    In particular, those skilled in the art will be able to implement a set  10  wherein the blocks are selected by columns (and not rows). A pulse of maximum time is transmitted to the blocks of the column to select and a pulse of variable time is transmitted on the rows of blocks to modulate this time. Moreover, the blocks can have different sizes.