Abstract:
An optoelectronic circuit including a full-wave rectifier circuit including light-emitting diodes and a circuit limiting the current passing through the light-emitting diodes.

Description:
[0001]    The present patent application claims the benefit of French patent application FR14/51230 which will be incorporated herein by reference. 
       BACKGROUND 
       [0002]    The present description relates to an optoelectronic circuit, particularly an optoelectronic circuit comprising light-emitting diodes. 
       DISCUSSION OF THE RELATED ART 
       [0003]    It is desirable to be able to power an optoelectronic circuit comprising light-emitting diodes with an AC voltage, particularly a sinusoidal voltage, for example, the mains voltage. 
         [0004]      FIG. 1  shows an optoelectronic circuit  10  comprising input terminals IN 1  and IN 2  having an AC voltage VIN applied therebetween. Optoelectronic circuit  10  further comprises a rectifying circuit  12  comprising a diode bridge  14 , receiving voltage VIN and supplying a rectified voltage VALIM which powers light-emitting diodes  16 , for example, series-connected. Call IALIM the current flowing through light-emitting diodes  16 . 
         [0005]      FIG. 2  shows a curve CVALIM of variation of power supply voltage VALIM and a curve CIALIM of variation of power supply current IALIM along time for an example where AC voltage VIN corresponds to a sinusoidal voltage. However, voltage VIN may be non-sinusoidal. As an example, voltage VIN may be supplied by a regulation circuit, particularly using triacs. Even if the regulator element is powered with a sinusoidal voltage, voltage VIN generally does not have a sinusoidal shape. When voltage VALIM is greater than the sum of the threshold voltages of light-emitting diodes  16 , light-emitting diodes  16  become conductive and substantially behave as resistors. Power supply current IALIM then follows power supply voltage VALIM. 
         [0006]    A disadvantage is that power supply current IALIM is not constant. This causes variations of the light intensity provided by light-emitting diodes  16 , which may be perceived by an observer. 
         [0007]    A current-limiting circuit may be interposed between rectifying circuit  12  and light-emitting diodes  16  to keep the power supply current at a substantially constant level. The structure of the optoelectronic circuit can then be relatively complex and the bulk of the optoelectronic circuit may be significant. Further, it may be difficult to at least partly form the rectifying circuit and the current-limiting circuit in integrated fashion with the light-emitting diodes to still further decrease the bulk of the optoelectronic circuit. 
       SUMMARY 
       [0008]    An object of an embodiment is to overcome all or part of the disadvantages of the previously-described optoelectronic circuits. 
         [0009]    Another object of an embodiment is to decrease the bulk of the optoelectronic circuit. 
         [0010]    Another object of an embodiment is to decrease the variations of the light intensity provided by the optoelectronic circuit. 
         [0011]    Another object of an embodiment is to be able to form a significant number of components of the optoelectronic circuit in integrated fashion. 
         [0012]    Thus, an embodiment provides an optoelectronic circuit comprising: 
         [0013]    a fullwave rectifying circuit comprising light-emitting diodes; and 
         [0014]    a current-limiting circuit flowing through the light-emitting diodes. 
         [0015]    According to an embodiment, the circuit comprises first and second input terminals intended to receive an AC voltage and the fullwave rectifying circuit comprises: 
         [0016]    first, second, third, and fourth branches, the first and second branches having a first common node connected to the first input terminal, the third and fourth branches having a second common node connected to the second input terminal, the first and third branches having a third common node and the second and fourth branches having a fourth common node; 
         [0017]    a first assembly of light-emitting diodes assembled on the first branch in the forward direction from the third node to the first node; and 
         [0018]    a second assembly of light-emitting diodes assembled on the second branch in the forward direction from the first node to the fourth node or on the third branch in the forward direction from the third node to the second node, 
         [0019]    and the current-limiting circuit flowing through the light-emitting diodes comprises at least one component assembled between the third node and the fourth node. 
         [0020]    According to an embodiment, the fullwave rectifying circuit further comprises a third assembly of light-emitting diodes assembled on the third branch in the forward direction from the third node to the second node. 
         [0021]    According to an embodiment, the second assembly of light-emitting diodes is assembled on the second branch and the fullwave rectifying circuit comprises a fourth assembly of light-emitting diodes assembled on the fourth branch in the forward direction from the second node to the fourth node. 
         [0022]    According to an embodiment, the current-limiting circuit comprises an inductance assembled between the third node and the fourth node. 
         [0023]    According to an embodiment, the current-limiting circuit comprises a sensor of the current flowing through the inductance. 
         [0024]    According to an embodiment, the current-limiting circuit comprises at least one first switch provided on one of the first or third branches, between the third and fourth nodes, between the first input terminal and the first node or between the second input terminal and the second node and a unit for controlling the turning off and the turning on of the first switch, the control unit being connected to the sensor. 
         [0025]    According to an embodiment, the current-limiting circuit is capable of keeping the current between a first threshold and a second threshold when the first assembly of light-emitting diodes or the second assembly of light-emitting diodes is conductive. 
         [0026]    According to an embodiment, the optoelectronic circuit further comprises means for modifying the first and second thresholds. 
         [0027]    According to an embodiment, the control unit is capable of controlling the turning-off of the first switch when the current flowing through the light-emitting diodes of the first assembly or of the second assembly is greater than the first threshold. 
         [0028]    According to an embodiment, the control unit is capable of controlling the turning-on of the first switch when the current flowing through the light-emitting diodes of the first assembly or of the second assembly is smaller than the second threshold. 
         [0029]    According to an embodiment, the first switch is assembled on the first branch or the fourth branch and the current-limiting circuit comprises a second switch assembled on the second branch or the third branch, the control unit being capable of controlling the turning-off of the first switch when the current flowing through the light-emitting diodes of the first assembly is greater than the first threshold and the AC voltage is of a first sign, and being capable of controlling the turning-off of the second switch when the current flowing through the light-emitting diodes of the second assembly is greater than the first threshold and the AC voltage is of a second sign opposite to the first sign. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0030]    The foregoing and other features and advantages will be discussed in detail in the following non-limiting description of specific embodiments in connection with the accompanying drawings, among which: 
           [0031]      FIG. 1 , previously described, is an electric diagram of an example of an optoelectronic circuit comprising light-emitting diodes; 
           [0032]      FIG. 2 , previously described, is a timing diagram of the voltage and of the current for supplying the light-emitting diodes of the optoelectronic circuit of  FIG. 1 ; 
           [0033]      FIG. 3  is an electric diagram of an embodiment of an optoelectronic circuit comprising light-emitting diodes; 
           [0034]      FIGS. 4 and 5  illustrate two layouts of the light-emitting diodes of the optoelectronic circuit of  FIG. 3 ; 
           [0035]      FIG. 6  is a more detailed electric diagram of a portion of the optoelectronic circuit of  FIG. 3 ; 
           [0036]      FIG. 7  shows a curve of the variation of the input voltage of the optoelectronic circuit of  FIG. 3  and a curve of the variation of the power supply current of an inductance of the optoelectronic circuit of  FIG. 3 ; 
           [0037]      FIG. 8  shows a detail of the curve of variation of the power supply current of the inductance of  FIG. 7  and of the curves of variation of currents flowing through overall light-emitting diodes of the optoelectronic circuit of  FIG. 3 ; 
           [0038]      FIG. 9  is a more detailed electric diagram of a portion of the optoelectronic circuit of  FIG. 3 ; 
           [0039]      FIGS. 10, 11, and 12  are electric diagrams of other embodiments of an optoelectronic circuit comprising light-emitting diodes; 
           [0040]      FIG. 13  is a drawing similar to  FIG. 8  obtained with the optoelectronic circuit of  FIG. 12 ; 
           [0041]      FIG. 14  shows an embodiment of an overall light-emitting diode; and 
           [0042]      FIGS. 15 to 18  each show an equivalent electric diagram of the overall light-emitting diode of  FIG. 13  in four operating configurations. 
       
    
    
     DETAILED DESCRIPTION 
       [0043]    For clarity, the same elements have been designated with the same reference numerals in the various drawings and, further, the various drawings are not to scale. In the following description, unless otherwise indicated, terms “substantially”, “approximately”, and “in the order of” mean “to within 10%”. 
         [0044]    According to an embodiment, the light-emitting diodes of the optoelectronic circuit are used to form the diode bridge of the rectifying circuit. This enables to decrease the total bulk of the optoelectronic circuit. Further, a current-limiting circuit is directly integrated to the diode bridge. This enables to decrease the variations of the power supply current of the light-emitting diodes while decreasing the total bulk of the optoelectronic circuit. 
         [0045]      FIG. 3  shows an embodiment of an optoelectronic circuit  20  comprising two input terminals IN 1  and IN 2  receiving input voltage VIN. As an example, input voltage VIN may be a sinusoidal voltage having a frequency, for example, in the range from 10 MHz to 1 MHz. Voltage VIN corresponds, for example, to the mains voltage which may possibly have been modified by a regulation circuit. For example, the mains voltage may be lowered or chopped by the regulation circuit. 
         [0046]    Circuit  20  comprises a fullwave rectifying circuit  21  comprising a diode bridge formed of four assemblies D 1 , D 2 , D 3  and D 4  of light-emitting diodes, called overall light-emitting diodes in the following description. Each overall light-emitting diode is formed of the series and/or parallel assembly of a plurality of elementary light-emitting diodes. Overall light-emitting diode D 1  is assembled on a first branch  22  between a node E and a node F in the forward direction from node E to node F. Overall light-emitting diode D 2  is assembled on a second branch  23  between node F and a node G in the forward direction from node F to node G. Overall light-emitting diode D 3  is assembled on a third branch  24  between node E and a node H in the forward direction from node E to node H. Overall light-emitting diode D 4  is assembled on a fourth branch  25  between node H and node G in the forward direction from node H to node G. 
         [0047]    Preferably, all the light-emitting diodes of optoelectronic circuit  20  belong to one of overall light-emitting diodes D 1 , D 2 , D 3  and D 4 . Overall light-emitting diodes D 1 , D 2 , D 3  and D 4  may comprise the same number of elementary light-emitting diodes or different numbers of elementary light-emitting diodes. 
         [0048]      FIG. 4  shows an embodiment of overall light-emitting diode D 1 . Overall light-emitting diode D 1  comprises R branches  26  assembled in parallel, each branch comprising S series-connected elementary light-emitting diodes  27 , R and S being integers greater than or equal to 2. 
         [0049]      FIG. 5  shows another embodiment of overall light-emitting diode D 1 . Overall light-emitting diode D 1  comprises P series-connected blocks  28 , each block comprising Q elementary light-emitting diodes  27  assembled in parallel, P and Q being integers greater than or equal to 2 and Q being likely to vary from one block to the other. 
         [0050]    Overall light-emitting diodes D 2 , D 3  and D 4  may have a structure similar to that of overall light-emitting diode D 1  shown in  FIG. 4 or 5 . 
         [0051]    Returning to  FIG. 3 , optoelectronic circuit  20  comprises a current-limiting circuit  30  comprising an inductance  32  assembled between nodes E and G. As an example, inductance  32  has a value in the range from 0.1 μH to 10 μH. Call IL, ID 1 , ID 2 , ID 3  and ID 4  the current respectively flowing through inductance  32 , overall light-emitting diode D 1 , overall light-emitting diode D 2 , overall light-emitting diode D 3 , and overall light-emitting diode D 4 . Current-limiting circuit  30  further comprises a current sensor  34  capable of supplying a signal SI representative of current IL to a control unit  36 . Current-limiting circuit  30  further comprises a switch  38  provided between input terminal IN 1  and node E and controlled by a signal SC supplied by control unit  36 . Node H is connected to input terminal IN 2 . Control unit  36  may be formed by a dedicated circuit. 
         [0052]    According to an embodiment, control unit  36  is capable of ordering the turning-off and the turning-on of switch  38  so that current IL remains between a lower threshold IINF and an upper threshold ISUP. Upper threshold ISUP is greater than lower threshold IINF. Lower threshold IINF is greater than 0 A. As an example, current thresholds IINF and ISUP may be from a few milliamperes to several hundreds of milliamperes.  FIG. 6  is an electric diagram of an embodiment of control unit  36 . Control unit  36  may comprise a hysteresis comparator  40  receiving signal SI representative of current IL and supplying a signal OUT capable of taking two values OUT+ and OUT−. As an example, when signal SI increases, signal OUT is at value OUT− when current IL is smaller than threshold ISUP and switches to value OUT+ when current IL becomes greater than threshold ISUP. When current IL decreases, signal OUT is at value OUT+ when current IL is greater than threshold ISUP and switches to value OUT− when current IL becomes smaller than threshold ISUP. Control unit  36  comprises a shaping unit  42  receiving signal OUT and supplying signal SC. 
         [0053]    Switch  38  is for example a bidirectional switch based on transistors, particularly on field-effect metal-oxide gate transistors or enrichment (normally on) or depletion (normally off) MOS transistors. 
         [0054]    Elementary light-emitting diodes  27  are for example planar light-emitting diodes or light-emitting diodes formed from three-dimensional elements, particularly semiconductor microwires or nanowires, comprising a semiconductor material based on a compound mainly comprising a group-III element and a group-V element (for example, gallium nitride GaN), called III-V compound hereafter, or mainly comprising at least one group-II element and one group-VI element (for example zinc oxide ZnO), called II-VI compound hereafter. 
         [0055]    Advantageously, switch  38  may be formed based on a III-V compound, for example, gallium nitride GaN. In this case, switch  38  may be formed in integrated fashion with the light-emitting diodes. 
         [0056]      FIG. 7  is a timing diagram of input voltage VIN and of current IL. As an example, voltage VIN is a sinusoidal voltage.  FIG. 8  is a detail view of the curve of variation of current IL of  FIG. 7  and further shows curves of the variation of currents ID 1 , ID 2 , ID 3  and ID 4 . Times t 0  to t 13  are successive times. 
         [0057]    An embodiment of a method of controlling switch  38  during a positive halfwave and a negative halfwave of input voltage VIN will now be described. 
         [0058]    Input voltage VIN increases from zero at time t 0 . Switch  38  is initially on. Overall light-emitting diodes D 2  and D 3  are forward-biased while overall light-emitting diodes D 1  and D 4  are reverse-biased. When input voltage VIN is sufficiently high at time t 1 , the current starts flowing between terminals IN 1  and IN 2  successively through overall light-emitting diode D 2 , through inductance  32 , from node G to node E, and through light-emitting diode D 3 . 
         [0059]    At time t 2 , current IL exceeds threshold ISUP. Control unit  36  then orders the turning-off of switch  38 , which causes a discharge of inductance  32 . Current IL then keeps on flowing through inductance IL while decreasing and divides into a first portion which successively flows through overall light-emitting diodes D 1  and D 2  and a second portion which successively crosses overall light-emitting diodes D 3  and D 4 . 
         [0060]    At time t 3 , current IL decreases below threshold IINF. Control unit  36  then orders the turning-on of switch  38 . Current IL starts flowing again while rising between terminal IN 1  and terminal IN 2 , successively through overall light-emitting diode D 2 , through inductance  32 , from node G to node E, and through light-emitting diode D 3 . Current IL keeps on increasing until it exceeds threshold ISUP at time t 4 . Switch  38  is then off until current IL decreases below threshold IINF at time t 5 . 
         [0061]    The cycle between times t 2  and t 4  is repeated as long as input voltage VIN is sufficiently high. Currents ID 1 , ID 2 , ID 3  and ID 4  then remain between IINF and ISUP. 
         [0062]    At time t 6 , input voltage VIN decreases so that current IL remains below threshold ISUP. Switch  38  then remains on. 
         [0063]    At time t 7 , input voltage VIN is no longer sufficiently high for a current to flow between input terminals IN 1  and IN 2 . 
         [0064]    At time t 8 , input voltage VIN cancels and starts a negative halfwave. Switch  38  is on. Overall light-emitting diodes D 1  and D 4  are forward-biased while overall light-emitting diodes D 2  and D 3  are reverse-biased. When input voltage VIN is sufficiently high in absolute value at time t 9 , the current starts flowing between terminals IN 1  and IN 2  successively through overall light-emitting diode D 4 , through inductance  32 , from node G to node E, and through light-emitting diode D 1 . 
         [0065]    At time t 10 , current IL exceeds threshold ISUP. The current regulation between IINF and ISUP is performed as previously described between times t 2  and t 6 . 
         [0066]    At time t 11 , input voltage VIN decreases so that current IL remains below threshold ISUP and switch  38  remains on. 
         [0067]    At time t 12 , input voltage VIN is no longer sufficiently high in absolute value for a current to flow between input terminals IN 1  and IN 2 . 
         [0068]    The halfwave stops at time t 13  when input voltage VIN reaches zero. 
         [0069]    When input voltage VIN is sufficiently high for overall light-emitting diode D 1  or D 2  to be conductive, current-limiting circuit  30  enables to keep the current, flowing through the overall light-emitting diode D 1  or D 2  which is conductive, between thresholds IINF and ISUP. Advantageously, optoelectronic circuit  20  comprises means for modifying thresholds IINF and ISUP. Current-limiting circuit  30  then enables to control the current flowing through the overall light-emitting diodes and thus to control the light intensity emitted by optoelectronic circuit  20 . 
         [0070]    When the interval between thresholds IINF and ISUP is small, as is the case in  FIG. 8 , limiting circuit  20  plays the role of a regulation circuit capable of keeping the current flowing through the light-emitting diodes substantially equal to a current set point, for example equal to the average of thresholds IINF and ISUP. The interval between thresholds IINF and ISUP then represents the accuracy of the regulation around the current set point. As an example, the interval between thresholds IINF and ISUP is smaller than 10%, preferably smaller than 5%, of threshold IINF. 
         [0071]    Advantageously, control unit  36  may be powered by a voltage obtained from the voltages across overall light-emitting diodes D 1  to D 4  or any other diode present in the assembly. 
         [0072]      FIG. 9  is an electric diagram of an embodiment of a portion of optoelectronic circuit  20 . Overall light-emitting diode D 2  is shown in the form of two assemblies  52  and  54  of series-connected light-emitting diodes. A capacitor  50  is assembled in parallel across assembly  52  of light-emitting diodes. Control unit  36  is powered with voltage VM across capacitor  50 . Capacitor  50  is charged each time overall light-emitting diode D 2  is conductive. Voltage VM across capacitor  50  is substantially constant and may be used as a voltage for supplying the control unit. The number of elementary light-emitting diodes of assembly  52  is selected according to the desired voltage VM. As an example, voltage VM may be a few volts. 
         [0073]    In the previously-described embodiment, when switch  38  is off, current IL flowing through inductance  32  distributes between branch  22  and branch  24 . It may however be desirable to select in which branch the current will flow when switch  38  is off. 
         [0074]      FIG. 10  shows another embodiment of an optoelectronic circuit  60  enabling to perform such a selection. Optoelectronic circuit  60  comprises all the elements of optoelectronic circuit  20  shown in  FIG. 3  and further comprises a switch  62  located on branch  25 , for example, between overall light-emitting diode D 4  and node G. As a variation, switch  62  may be located on branch  24 . Switch  62  is controlled by a signal S′C provided by control unit  36 . Advantageously, the current always flows in the same direction between nodes H and G so that switch  62  can be a one-way switch. 
         [0075]    Switch  38  may be controlled as previously described for optoelectronic circuit  20 . Preferably, switch  62  is on when switch  38  is on and switch  62  is off when switch  62  is off. As a variation, switch  62  may be kept off all along the positive halfwave of voltage VIN and may be controlled as previously indicated for the negative halfwave of VIN. This advantageously enables to decrease the circuit power consumption and not to have to control switch  62  during positive halfwaves of power supply voltage VIN. 
         [0076]    When switches  38  and  62  are off, inductance  32  discharges and the current flows through overall light-emitting diodes D 1  and D 2 . As a variation, switch  62  may be located on branch  22  or on branch  23  if the current is desired to flow through overall light-emitting diodes D 3  and D 4  when switch  38  is off. As a variation, in addition to switch  62 , another switch may be located on branch  23  or on branch  24 . This enables to select one of branches  22  or  24  where the current will flow when switch  38  is off, such a selection being likely to vary along time. 
         [0077]      FIG. 11  shows another embodiment of an optoelectronic circuit  70 . Optoelectronic circuit  70  comprises all the elements of optoelectronic circuit  20  shown in  FIG. 3 , except that switch  38  is replaced with a switch  72 , located between node G and a node K, inductance  32  and current sensor  34  being series-connected between node E and node K. Switch  72  is controlled by control unit  36 . Optoelectronic circuit  70  further comprises a diode  74  assembled in parallel with inductance  32 . As an example, the anode of diode  74  is connected to node E and the cathode of diode  74  is connected to node K. Diode  74  may be light-emitting. 
         [0078]    Advantageously, the current always flows in the same direction between nodes G and E so that switch  72  can be a one-way switch. 
         [0079]    The method of controlling switch  72  may be the same as that previously described for switch  32  in relation with optoelectronic circuit  20 . Diode  74  enables to prevent the stopping of the current flowing through inductance  32  when switch  72  is off. 
         [0080]      FIG. 12  shows another embodiment of an optoelectronic circuit  80 . Optoelectronic circuit  80  comprises all the elements of optoelectronic circuit  20  shown in  FIG. 3 , with the difference that switch  38  is replaced with a first switch  82 , located on branch  22 , for example, between node E and overall light-emitting diode D 3 , and a second switch  84 , located on branch  24 , for example, between node E and overall light-emitting diode D 2 . As a variation, switch  82  may be located on branch  25  and switch  84  may be located on branch  23 . 
         [0081]    Switches  82  and  84  are controlled by control unit  36 . Advantageously, the current always flows in the same direction between nodes E and F and between nodes E and H so that each switch  82 ,  84  may be a one-ways switch. Control unit  36  is further capable of detecting the sign of power supply voltage VIN. This may be performed by measuring the voltage across one of the elementary light-emitting diodes of one of overall light-emitting diodes D 1  to D 4 . 
         [0082]    An embodiment of the method of controlling switches  82 ,  84  will be described in relation with  FIGS. 7 and 13 . 
         [0083]    Input voltage VIN increases from the zero value at time t 0 . Switches  82  and  84  are initially on. Overall light-emitting diodes D 2  and D 3  are forward-biased while overall light-emitting diodes D 1  and D 4  are reverse-biased. When input voltage VIN is sufficiently high at time t 1 , the current starts flowing between terminal IN 1  and terminal IN 2  successively through overall light-emitting diode D 2 , through inductance  32 , from node G to node E, and through light-emitting diode D 3 . 
         [0084]    At time t 2 , current IL exceeds threshold ISUP. Control unit  36  then controls the turning off of switch  84 , switch  82  remaining on. Current IL then keeps on flowing through inductance IL while decreasing and successively crosses overall light-emitting diodes D 1  and D 2 . 
         [0085]    At time t 3 , current IL decreases below threshold IINF. Control unit  36  then orders the turning on of switch  84 . Current IL starts flowing again while rising between terminals IN 1  and IN 2 , successively through overall light-emitting diode D 2 , through inductance  32 , from node G to node E, and through light-emitting diode D 3 . Current IL keeps on increasing until it exceeds threshold ISUP at time t 4 . 
         [0086]    The cycle between times t 2  and t 4  is repeated several times as long as input voltage VIN is sufficiently high. Currents ID 1 , ID 2 , ID 3  and ID 4  then remain between IINF and ISUP. 
         [0087]    At time t 6 , input voltage VIN decreases so that current IL remains below threshold ISUP. Switch  84  then remains on. 
         [0088]    At time t 7 , input voltage VIN is no longer sufficiently high for a current to flow between input terminals IN 1  and IN 2 . 
         [0089]    At time t 8 , input voltage VIN cancels and starts a negative halfwave. Switches  82  and  84  are on. Overall light-emitting diodes D 1  and D 4  are forward-biased while overall light-emitting diodes D 2  and D 3  are reverse-biased. When input voltage VIN is sufficiently high in absolute value at time t 9 , the current starts flowing between terminals IN 1  and IN 2  successively through overall light-emitting diode D 4 , through inductance  32 , from node G to node E, and through light-emitting diode D 1 . 
         [0090]    At time t 10 , current IL exceeds threshold ISUP. The current regulation between IINF and ISUP is performed as previously described from time t 2 , with the difference that switch  84  remains on and switch  82  is off. 
         [0091]    At time t 11 , input voltage VIN decreases so that current IL remains below threshold ISUP and switch  84  remains on. 
         [0092]    As a variation, current sensor  34  may be replaced with two current sensors, one being arranged on branch  22  or  25  and the other being arranged on branch  23  or  24 . 
         [0093]    In  FIG. 7 , between times t 0  and t 1 , t 7  and t 9 , and t 12  and t 13 , input voltage VIN is not sufficiently high for overall light-emitting diodes D 1  and D 4  or D 2  and D 3  to be conductive. There thus is no light emission. To decrease the duration of phases when no light is emitted, the elementary light-emitting diodes which form each overall light-emitting diode may be connected to one another by a switch network. The switches are then controlled to modify the connection of the elementary light-emitting diodes so as to modify the threshold voltage of the overall light-emitting diode. 
         [0094]      FIG. 14  shows an embodiment of an overall light-emitting diode DG having a variable threshold voltage which may correspond to one of previously-described overall light-emitting diodes D 1 , D 2 , D 3  and D 4 . Overall light-emitting diode DG comprises, as an example, N elementary light-emitting diodes d 1 , d 2 , d 3  and d 4 , N being an integer, preferably even, equal to four in  FIG. 14 . Overall light-emitting diode DG comprises an anode AG and a cathode CG. Each elementary light-emitting diode di, with i being an integer varying from 1 to N, comprises an anode Ai and a cathode Ci. For i varying from 1 to N−1, anode Ai is connected to anode Ai+1 by a switch SW 1   i.  For i varying from 1 to N−1, cathode Ci is connected to cathode Ci+1 by a switch SW 2   i.  For i varying from 1 to N−1, cathode Ci is connected to cathode Ai+1 by a switch SW 3   i.    
         [0095]      FIGS. 15 to 18  are equivalent electric diagrams of overall light-emitting diode DG of  FIG. 14  for different on and off configurations of switches SW 1   i,  SW 2   i  and SW 3   i,  with i varying from 1 to N−1. 
         [0096]    In  FIG. 15 , switches SW 1   i  and SW 2   i  are on and switches SW 3   i  are off for i varying from 1 to N. The N elementary light-emitting diodes di are then assembled in parallel. 
         [0097]    In  FIG. 16 , for i varying from 0 to N/2, switches SW 12   i+ 1 and SW 22   i+ 1 are on, switches SW 32   i+ 1 are off, switches SW 12   i  and SW 22   i  are off, and switches SW 32   i  are on. Elementary light-emitting diodes di are assembled in parallel in pairs, the pairs being series-connected. 
         [0098]    In  FIG. 17 , switches SW 11  and SW 21  are on, switch SW 31  is off, and for i varying from 2 to N, switches SW 1   i  and SW 2   i  are off and switch SW 3   i  is on. Elementary light-emitting diodes d 1  and d 2  are assembled in parallel, this pair being series-connected to the other elementary light-emitting diodes. 
         [0099]    The threshold voltage of overall light-emitting diode DG increases from the configuration shown in  FIG. 15  to the configuration shown in  FIG. 18 . Thereby, switches SW 1   i,  SW 2   i  and SW 3   i  may be controlled according to input voltage VIN or according to the current flowing between input terminals IN 1  and IN 2  to successively pass through the configurations shown in  FIGS. 15, 16, 17, and 18  when input voltage VIN increases. As an example, the passing from a configuration to another may be ordered when input voltage VIN exceeds, in absolute value, a threshold. As an example, the passing from a configuration to another may be ordered when the current flowing between input terminals IN 1 , IN 2  decreases below a threshold. 
         [0100]    Thereby, overall light-emitting diode DG may be conductive for a longer time period and the duration of light emission of the optoelectronic circuit may be increased. 
         [0101]    Specific embodiments have been described. Various alterations, modifications, and improvements will occur to those skilled in the art. In particular, in the previously-described embodiments, the current-limiting circuit comprises an inductance  32  assembled between nodes E and G. However, the current-limiting circuit may be formed differently. It may in particular comprise constant current or current-limiting diodes (CLD). Further, in the previously-described embodiments, overall light-emitting diodes D 1 , D 2 , D 3  and D 4  are provided on each branch  22 ,  23 ,  24 ,  25 . However, as a variation, overall light-emitting diodes D 1 , D 2  may be provided on branches  22  and  23  only, each overall light-emitting diode D 3  and D 4  being replaced with a switch controlled by control unit  36  and which is off when the overall light-emitting diode D 3  or D 4  that it replaces would be forward-biased and which is on when the overall light-emitting diode D 3  or D 4  that it replaces would be reverse-biased during the variation of input voltage VIN.