Patent Abstract:
A circuit capable of receiving, in series with at least one light-emitting diode, a rectified A.C. voltage, comprising: a first gate turn-off thyristor connected to first and second terminals of the circuit; and a control circuit for turning off the first thyristor when the voltage between the first and second terminals exceeds a threshold.

Full Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the priority benefit of French patent application number 09/53922, filed on Jun. 12, 2009, entitled “CIRCUIT FOR CONTROLLING A LIGHTING UNIT WITH LIGHT-EMITTING DIODES,” which is hereby incorporated by reference to the maximum extent allowable by law. 
       BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The present invention relates to lighting units with light-emitting diodes intended to receive an A.C. supply voltage. It more specifically relates to circuits for powering such devices. 
         [0004]    2. Discussion of the Related Art 
         [0005]    For a long time, illumination devices have been formed based on incandescent light bulbs or on fluorescent tubes capable of receiving an A.C. supply voltage, for example, a 220-V mains voltage at 50 Hz. More recently, it has been desired to use light-emitting diodes. Such diodes especially have a long lifetime and a high light output. They however require a power supply circuit capable of receiving the A.C. voltage from the mains. 
         [0006]    Conventional circuits operate in linear mode, that is, they provide a D.C. voltage and a power adapted to the electrical characteristics of the diodes. The diodes are then maintained on for the entire duration of each halfwave of the mains voltage. This power supply mode has the disadvantage of decreasing their lifetime. Further, linear power supply circuits generally comprise high-voltage capacitors having the disadvantage of being expensive and bulky. 
       SUMMARY OF THE INVENTION 
       [0007]    An object of an embodiment of the present invention is to overcome all or part of the disadvantages of circuits for powering light-emitting diodes. 
         [0008]    An object of an embodiment of the present invention is to provide such a circuit improving the lifetime of the diodes. 
         [0009]    An object of an embodiment of the present invention is to provide such a circuit which has low cost and is easy to form. 
         [0010]    Thus, an embodiment of the present invention provides a circuit capable of receiving, in series with at least one light-emitting diode, a rectified A.C. voltage, comprising: a first gate turn-off thyristor connected to first and second terminals of the circuit; and a control circuit for turning off the first thyristor when the voltage between the first and second terminals exceeds a threshold. 
         [0011]    According to an embodiment of the present invention, said circuit capable of receiving, in series with at least one light-emitting diode, a rectified A.C. voltage, comprises a second thyristor connecting the gate of the first thyristor to said second terminal; and a first resistive element connecting the gate of the first thyristor to said first terminal or to a terminal of application of the rectified A.C. voltage. 
         [0012]    According to an embodiment of the present invention, said circuit capable of receiving, in series with at least on light-emitting diode, a rectified A.C. voltage, comprises, in series with the first thyristor, a voltage dividing bridge for setting said threshold, the midpoint of the voltage dividing bridge being connected to a gate of the second thyristor. 
         [0013]    According to an embodiment of the present invention, said circuit capable of receiving, in series with at least one light-emitting diode, a rectified A.C. voltage, further comprises a circuit of temporary power storage between the midpoint of the voltage dividing bridge and said gate of the second thyristor. 
         [0014]    According to an embodiment of the present invention, said storage circuit comprises: a second resistive element in series with a capacitive storage element, connecting said gate of the second thyristor to said second terminal; and a diode connecting the midpoint of the voltage dividing bridge to said gate of the second thyristor. 
         [0015]    According to an embodiment of the present invention, the resistivity of said voltage dividing bridge is low as compared to the resistivity of the first resistive element. 
         [0016]    According to an embodiment of the present invention, a capacitive electromagnetic disturbance attenuation element is connected between said first and second terminals. 
         [0017]    According to an embodiment of the present invention, the first thyristor is maintained on at the beginning of each halfwave of said rectified A.C. voltage, for a period ranging between 5% and 30% of the duration of said halfwave. 
         [0018]    An embodiment of the present invention further provides an illumination device intended to receive an A.C. voltage comprising: a bridge for rectifying the A.C. voltage; at least one light-emitting diode; and a circuit capable of receiving, in series with at least one light-emitting diode, a rectified A.C. voltage, according to any of the above-mentioned embodiments, series-connected with said at least one light-emitting diode, between output terminals of said rectifying bridge. 
         [0019]    According to an embodiment of the present invention, said circuit capable of receiving, in series with at least one light-emitting diode, a rectified A.C. voltage, forms a dipole. 
         [0020]    The foregoing objects, features, and advantages of the present invention will be discussed in detail in the following non-limiting description of specific embodiments in connection with the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0021]      FIG. 1  schematically shows an illumination device with light-emitting diodes; 
           [0022]      FIG. 2  shows the simplified electric diagram of the illumination device of  FIG. 1 ; 
           [0023]      FIG. 3  shows the simplified electric diagram of the illumination device of  FIG. 1 ; and 
           [0024]      FIGS. 4A to 4G  are simplified timing diagrams illustrating the operation of the illumination device of  FIG. 3 . 
       
    
    
     DETAILED DESCRIPTION 
       [0025]    For clarity, the same elements have been designated with the same reference numerals in the different drawings. Further, the timing diagrams of  FIGS. 4A to 4G  are not to scale. 
         [0026]      FIG. 1  is a simplified view of an illumination device  1  with light-emitting diodes  3 . 
         [0027]      FIG. 2  is a simplified electric diagram of device  1 . 
         [0028]    Device  1  comprises an assembly of light-emitting diodes  3  in series. Terminals A and C ( FIG. 2 ) of diode assembly  3  are connected to a power supply circuit  5  (POWER). In the shown example, terminals A and C respectively correspond to the anode and cathode connection terminals of the assembly of diodes  3  in series. 
         [0029]    Circuit  5  is capable of receiving an A.C. voltage V AC  ( FIG. 2 ), for example, the mains voltage, and of providing a power adapted to the electrical characteristics of the assembly of diodes  3 . Input terminals of power supply circuit  5  are connected to terminals E and F of a base  6 . Base  6  may have any shape adapted to a connection on a socket, for example, a screw thread. Other connections may be provided, for example, a direct wiring to a power supply connector. The entire device is assembled in a package  7  only leaving access to base  6  and diodes  3 . Transparent glass, not shown, may protect diodes  3 . 
         [0030]    A switch  9  is generally provided, for example, between a terminal of application of the phase of mains voltage V AC  and terminal E of base  6 , to control the powering-on of device  1 . Switch  9  may correspond to a wall switch that may be actuated by a user. 
         [0031]    To improve the lifetime and the efficiency of the diodes, it is provided to maintain them on for a fraction only of each period of the mains voltage, which is sufficient, in relation with the eye&#39;s persistence of vision, to guarantee a continuous illumination. 
         [0032]    Thus, an aspect of an embodiment of the present invention is to provide a power supply circuit capable of providing a pulse control signal, the electric power received by the diodes depending on the duration of the pulse. 
         [0033]      FIG. 3  is the electric diagram of an embodiment of the illumination device of  FIG. 2  showing power supply circuit  5  in more detailed fashion. 
         [0034]    Terminals E and F of the base are connected to A.C. input terminals of a fullwave rectifying bridge  20  capable of providing, between high and low output terminals H and M, a rectified A.C. voltage V ACR . Terminal M for example corresponds to the reference voltage terminal of the circuit or ground. In the shown example, bridge  20  comprises four diodes  21 ,  23 ,  25 , and  27 . 
         [0035]    In this example, power supply circuit  5  further comprises a dipole or control circuit  31  connected, in series with diode assembly  3 , between output terminals H and M of rectifying bridge  20 . Terminal A of diode assembly  3  is connected to terminal H. Terminals I and J of dipole  31  are respectively connected to terminal C of diode assembly  3  and to terminal M. 
         [0036]    Circuit  31  comprises a gate turn-off thyristor  33  having its anode connected to terminal I. A voltage dividing bridge formed, for example, of two resistors  35  and  37  in series, is connected between the cathode of thyristor  33  and terminal J. A resistor  39 , of strong value with respect to resistors  35  and  37 , is connected between the anode and the gate of thyristor  33 . Resistor  39  is used to turn on thyristor  33  at the beginning of each halfwave of voltage V ACR . A cathode gate thyristor  41  is forward connected between the gate of thyristor  33  and terminal J for controlling the turning off of thyristor  33 . The voltage dividing bridge conditions the turning on of thyristor  41 . 
         [0037]    For the case where thyristor  41  would not be fast enough, a temporary power storage circuit is preferably provided. This circuit, for example, comprises a resistor  43 , series connected with a capacitor  45 , between the gate of thyristor  41  and terminal J, and a diode  47 , forward connected between the midpoint of the voltage dividing bridge and the gate of thyristor  41 . 
         [0038]    Optionally, a capacitor  49  connects terminal I to terminal J, to attenuate electromagnetic disturbances linked to the switching of thyristors  33  and  41 . 
         [0039]    Power supply circuit  5  may further comprise, between its input terminals E and F, a varistor  29  of protection against possible overvoltages. 
         [0040]    As a specific embodiment: 
         [0041]    resistances  37  and  43  are on the order of a few tens of Ω, for example, on the order of 10Ω; 
         [0042]    resistance  35  is on the order of a few hundreds of Ω, for example, on the order of 250Ω; 
         [0043]    resistance  39  is on the order of a few tens of kΩ, for example, on the order of 75Ω; 
         [0044]    capacitance  45  is on the order of a few hundreds of nF, for example, on the order of 100 nF; and 
         [0045]    capacitance  49  is on the order of a few tens of nF, for example, on the order of 10 nF. 
         [0046]      FIGS. 4A to 4G  are simplified timing diagrams showing examples of the variation of the voltages and currents at different points of the illumination device of  FIG. 3 .  FIG. 4A  shows the variation of rectified A.C. voltage V ACR .  FIG. 4B  shows the variation of current I 33  flowing through thyristor  33 .  FIG. 4C  shows the variation of current I G41  flowing between the cathode gate and the cathode of thyristor  41 .  FIG. 4D  shows the variation of voltage V 41  across thyristor  41 .  FIG. 4E  shows the variation of current I 41  flowing through thyristor  41 .  FIG. 4F  shows the variation of current I LED  flowing through diode assembly  3 .  FIG. 4G  shows the variation of voltage V CM  between terminals C and M. 
         [0047]    A steady state is assumed, that is, switch  9  is assumed to be on. 
         [0048]    At a time t 0  of beginning of a halfwave of voltage V ACR , thyristor  41  is off. A current flows through diodes  3 , resistor  39 , the gate of thyristor  33 , and resistors  35  and  37 . Thyristor  33  thus starts conducting. A conduction path is thus established between terminal A and the ground, running through diode assembly  3 , thyristor  33 , and resistors  35  and  37  of low value of the voltage dividing bridge. The diodes turn on. Current I LED  is then equal to current I 33 . 
         [0049]    From time t 0 , gate current I G41  of thyristor  41  increases proportionally to current I 33 , to within a factor especially depending on the value of resistors  35  and  37  of the voltage dividing bridge. A current for charging capacitor  45  further flows between the cathode of diode  47  and terminal M. Voltage V 41  across thyristor  41  is equal to rectified voltage V ACR  minus the voltage drop caused by diode assembly  3  and by resistor  39 . Voltage V CM  is equal to rectified voltage V ACR  minus the voltage drop of diode assembly  3 . 
         [0050]    At a time t 1 , current I G41  reaches a turn-on threshold I TH  of thyristor  41 . The turning on of thyristor  41  brings the gate of thyristor  33  to ground (V 41 =0 V), thus turning it off. Current I 33  flowing through thyristor  33  thus becomes zero. There then is a conduction path between terminal A of diode assembly  3  and terminal M, running through diode assembly  3 , resistor  39 , and thyristor  41 . At time t 1 , current I LED  becomes equal to current I 41 . The value of resistor  39  is selected to be sufficiently high for this current to be very low (it is shown as being zero in  FIG. 4F ). Thus, diodes  3  turn off substantially at time t 1 . The values of resistors  35  and  37  of the voltage dividing bridge define the threshold of voltage V ACR  for which current I G41  reaches turn-on threshold I TH  of thyristor  41 . The values of resistors  35  and  37  are for example selected so that the time for which diodes  3  are on ranges between 5% and 30% of duration T of a halfwave of rectified voltage V ACR . Further, at time t 1 , voltage V CM  is abruptly taken up to the value of voltage V ACR  (neglecting the voltage drop in diodes  3  with respect to the voltage drop in resistor  39 ). 
         [0051]    To ensure the priming of thyristor  41 , capacitor  45  maintains a non-zero current I G41  for some time after time t 1 . 
         [0052]    Thyristor  41  remains conductive until the current that it conducts cancels, that is, until end time t 0 +T of the halfwave. Thus, diodes  3  are maintained off between times t 1  and t 0 +T. 
         [0053]    In a transient state of turning on of switch  9 , this turning on may occur at any time of the halfwave. Thyristor  33  turns on at this time but current I G41  immediately reaches turn-on threshold I TH  of thyristor  41 , thus turning off of thyristor  33  and turning off diodes  3  until the beginning of the next halfwave. This enables the diodes to see across their terminals a voltage close to the maximum mains voltage for a very short time, thus avoiding their destruction. The diodes are thus protected. 
         [0054]    The use of a fullwave rectifying bridge provides a frequency of the diode control pulses equal to twice the frequency of the A.C. power supply voltage. Such a frequency is sufficient to get rid of possible flickering effects with a mains voltage of 50 Hz or 60 Hz. 
         [0055]    An advantage of the provided circuit is that it has a low cost, a small bulk, and is easy to form. 
         [0056]    To form a power supply circuit capable of providing a pulse control signal, it could have been devised to use, instead of gate turn-off thyristor  33 , a gate turn-on thyristor. This thyristor would then have to be turned on at a time close to the end of each halfwave of the rectified voltage, the diodes remaining substantially conductive until the end of each halfwave. However, spurious voltage peaks may appear, in particular at the turning on of switch  9 . Such peaks would be capable of causing the turning-on of the diode turn on thyristor. In this case, the diodes would remain on substantially until the end of the halfwave. If switch  9  had been turned on, for example, at the beginning of a halfwave, the diodes would receive a much greater power than that for which they have been provided, which would cause their destruction. 
         [0057]    Specific embodiments of the present invention have been described. Various alterations and modifications will occur to those skilled in the art. In particular, the present invention applies whatever the available A.C. supply voltage. Further, the number of light-emitting diodes and their connection may vary. 
         [0058]    Further, the illumination device may comprise a dimming function if one of the resistors of the voltage dividing bridge is replaced with a variable resistor. 
         [0059]    Moreover, in the circuit described in relation with  FIG. 3 , terminals A and C of the assembly of light-emitting diodes are respectively connected to terminal H, corresponding to the high terminal of the rectified supply voltage, and to anode terminal I of gate turn-off thyristor  33 . According to an alternative embodiment, resistor  37  is connected to terminal M, no longer directly but via diode assembly  3 . According to another variation, resistor  39  is no longer connected to terminal C but to terminal H. Such variations may be combined. 
         [0060]    Such alterations, modifications, and improvements are intended to be part of this disclosure, and are intended to be within the spirit and the scope of the present invention. Accordingly, the foregoing description is by way of example only and is not intended to be limiting. The present invention is limited only as defined in the following claims and the equivalents thereto.

Technology Classification (CPC): 8