Abstract:
An LED drive circuit includes: a rectifier circuit that outputs a full-wave rectification waveform voltage; a primary LED row that includes a first partial LED row and a second partial LED row; an auxiliary LED row-; a bypass circuit that is connected to part connecting the first partial LED row and the second partial LED row and that returns current from the first partial LED row to the rectifier circuit; and a current limiting circuit that limits the current flowing through the auxiliary LED row wherein the primary LED row and the auxiliary LED row are connected in parallel to the rectifier circuit; and the bypass circuit blocks the current passing through the bypass circuit in accordance with the current flowing through the primary LED row, and controls and blocks the current flowing through the auxiliary LED row.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to an LED drive circuit, and more particularly to an LED drive circuit comprising as a light source an LED array constructed by connecting a plurality of LEDs (light-emitting diodes) in series. 
       BACKGROUND 
       [0002]    An LED drive circuit is known which drives LEDs to emit light by applying a full-wave rectified waveform obtained from an AC commercial power supply directly to an LED array. The LED array here is a series connection of a plurality of LED arrays and is constructed to be able to withstand a high voltage. Compared with other types of LED drive circuits that drive LEDs to emit light by generating a constant voltage from an AC commercial power supply, the above LED drive circuit has the advantage that the circuit configuration is simple and compact. 
         [0003]    However, if a full-wave rectified waveform is simply applied to an LED array, a problem occurs in which the LEDs light only during the periods when the full-wave rectified waveform exceeds the threshold value of the LED array. For example, when the forward voltage Vf of each LED is 3 V, and the LED array is constructed by connecting 40 such LEDs in series, the threshold value of the LED array is 120 V. Suppose that the rms value of the AC commercial power supply is 100 V; then, with the above LED drive circuit, the LEDs light only during short periods when the full-wave rectified waveform exceeds 120 V. As a result, with the above LED drive circuit, not only do the LEDs become dim or produce perceivable flicker, but the power factor and distortion factor also decrease. 
         [0004]    To address this, it is known to provide methods for extending the LED ON period; in one known method, the LED array is divided into a number of LED sub-arrays, and provisions are made to turn on only some of the LED sub-arrays during the low voltage period of the full-wave rectified waveform and to increase the number of LED sub-arrays to be turned on as the voltage increases (refer to patent document 1). 
         [0005]      FIG. 4  is a circuit diagram showing one example of an LED drive circuit; the diagram is redrawn from  FIG. 26  given in patent document 1 without departing from the purpose thereof. In the diagram, the current-limiting resistor is replaced by a current-limiting circuit. Therefore, it should be noted that the LED drive circuit shown in  FIG. 4  is given only for illustrative purposes and does not directly represent the LED drive circuit known in the art. 
         [0006]    In the example of  FIG. 4 , a bypass circuit which turns on (conducts) during the low voltage period of the full-wave rectified waveform and turns off (does not conduct) during the high voltage period is connected to a connection node between each LED sub-array. The bypass circuit is controlled on and off according to the voltage value of the full-wave rectified waveform or the value of the current that flows through the LED array. 
         [0007]    The LED drive circuit  400  shown in  FIG. 4  comprises a diode bridge circuit  405 , LED sub-arrays  410  and  430 , a bypass circuit  420 , and a current-limiting resistor  440 . A commercial power supply  406  is connected to input terminals of the diode bridge circuit  405 . 
         [0008]    The bridge rectifier circuit  405  is constructed from four diodes  401 ,  402 ,  403 , and  404 , and has a terminal A as an output terminal for outputting a full-wave rectified waveform and a terminal B as a terminal for providing a reference voltage. The LED sub-array  410  is constructed from a series connection of a large number of LEDs including LEDs  411  and  412 . Similarly, the LED sub-array  430  is constructed from a series connection of a large number of LEDs including LEDs  431  and  432 . The bypass circuit  420  includes a pull-up resistor  421 , a current-sensing resistor  424 , a field-effect transistor (FET)  422 , a bipolar transistor (hereinafter simply “transistor”)  423 , a first current input terminal  427 , a second current input terminal  428 , and a current output terminal  429 . Similarly to the bypass circuit  420 , the current-limiting circuit  440  includes a pull-up resistor  441 , a current-sensing resistor  444 , an FET  442 , a transistor  443 , a current input terminal  447 , and a current output terminal  449 . The FETs  422  and  442  are enhancement-mode n-type MOS-FETs. 
         [0009]      FIG. 5(   a ) is a waveform diagram depicting a full-wave rectified waveform, and  FIG. 5(   b ) is a waveform diagram depicting a circuit current I in the LED drive circuit  400 . The same time axis is used for both 
         [0010]      FIGS. 5  ( a ) and  5 ( b ). 
         [0011]    In  FIG. 5 , no circuit current I flows during the period t1 because the voltage value of the full-wave rectified waveform is smaller than the threshold value of the LED sub-array  410 . 
         [0012]    In the period t2, the voltage value of the full-wave rectified waveform exceeds the threshold value of the LED sub-array  410  but is smaller than the sum of the threshold value of the LED sub-array  410  and the threshold value of the LED sub-array  430 . In this case, the circuit current I passes through the bypass circuit  420  and returns to the bridge rectifier circuit  405 . During the period t2, feedback is applied so as to maintain the base-emitter voltage of the transistor  423  at 0.6 V, and the bypass circuit  420  thus operates in a constant current mode. Actually, in the last short portion of the period t2, the voltage value of the full-wave rectified waveform becomes slightly larger than the sum of the threshold value of the LED sub-array  410  and the threshold value of the LED sub-array  430 , and a current flows in from the LED sub-array  430  via the current input terminal  428 . 
         [0013]    In the period t3, the voltage value of the full-wave rectified waveform exceeds the sum of the threshold value of the LED sub-array  410  and the threshold value of the LED sub-array  430 , and the current flows to the current input terminal  428  by passing through the LED sub-array  430 . At this time, the transistor  423  saturates, and the gate voltage of the FET  422  becomes equal to the reference voltage (the voltage at the terminal B), so that the FET  422  is cut off. As a result, the current flowing into the bypass circuit  420  via the current input terminal  427  rapidly drops. On the other hand, in the current-limiting circuit  440 , feedback is applied so as to maintain the base-emitter voltage of the transistor  443  at 0.6 V, and the current-limiting circuit  440  thus operates in a constant current mode. The process that takes place during the period that the voltage of the full-wave rectified waveform falls is the reverse of the process that takes place during the period that the voltage of the full-wave rectified waveform rises. 
       PRIOR ART LITERATURE 
     Patent Literature 
       [0014]    Patent document 1: WO2011020007 
       SUMMARY 
       [0015]    However, in the LED drive circuit  400  shown in 
         [0016]      FIG. 4 , compared with the current that flows during the high voltage period t3 (see  FIG. 5 ) of the full-wave rectified waveform, the current that flows during the low voltage period t2 (see  FIG. 5 ) is small, and the number of LEDs (LEDs  111 ,  112 , etc.) turned on is also small; as a result, the brightness greatly decreases. Furthermore, since the proportions of the LED OFF period t1 (see  FIG. 5 ) and the low voltage period t2 are large, motion breaks (a phenomenon in which a high-speed moving object appears to be moving discontinuously) become noticeable. 
         [0017]    It is an object of the present invention to provide an LED drive circuit in which a simple circuit is added to increase emission brightness during the low voltage period of a full-wave rectified waveform. 
         [0018]    There is provided an LED drive circuit includes, a rectifier circuit which outputs a full-wave rectified voltage waveform, a main light-emitting LED array which includes a first LED sub-array constructed by connecting a plurality of LEDs in series and a second LED sub-array constructed by connecting a plurality of LEDs in series, an auxiliary light-emitting LED array constructed by connecting a plurality of LEDs in series, a bypass circuit, connected to a connection node between the first LED sub-array and the second LED sub-array, for allowing a current passing through the first LED sub-array to return to the rectifier circuit, and a current-limiting circuit for limiting a current flowing through the auxiliary light-emitting LED array, and wherein the main light-emitting LED array and the auxiliary light-emitting LED array are connected in parallel with respect to the rectifier circuit, and the bypass circuit shuts off the current passing through the bypass circuit in accordance with the current flowing through the main light-emitting LED array, while also shutting off the current flowing to the auxiliary light-emitting LED array. 
         [0019]    Preferably, in the LED drive circuit, the bypass circuit performs control so that the auxiliary light-emitting LED array turns on during a low voltage period of the full-wave rectified voltage waveform and turns off during a high voltage period of the full-wave rectified voltage waveform. 
         [0020]    There is provided an LED drive circuit includes a main light-emitting LED array which is driven by a full-wave rectified voltage waveform, and which includes a first LED sub-array constructed by connecting a plurality of LEDs in series and a second LED sub-array constructed by connecting a plurality of LEDs in series, an auxiliary light-emitting LED array constructed by connecting a plurality of LEDs in series, a bypass circuit which includes a first current input terminal, a second current input terminal, and a first field-effect transistor, the first current input terminal being connected to a connection node between the first LED sub-array and the second LED sub-array, wherein the first field-effect transistor acts to limit the current flowing in via the first current input terminal in accordance with the current flowing in via the second current input terminal; and an auxiliary light-emitting circuit which includes a second field-effect transistor, and which causes the auxiliary light-emitting LED array to emit light, wherein the first field-effect transistor and the second field-effect transistor are chosen to have the same gate voltage. 
         [0021]    The first field-effect transistor in the bypass circuit is used to control the current flowing in via the first current input terminal of the bypass circuit in accordance with the current flowing in via the second current input terminal. During the low voltage period of the full-wave rectified waveform, since no or little current flows in via the second current input terminal, the first field-effect transistor turns on, and the current flows in via the first current input terminal. On the other hand, during the high voltage period of the full-wave rectified waveform, since the current flowing in via the second current input terminal increases, the first field-effect transistor turns off, and the current no longer flows in via the first current input terminal. In the auxiliary light-emitting circuit, the second field-effect transistor whose gate voltage is equal to the gate voltage of the first field-effect transistor turns on during the low voltage period of the full-wave rectified waveform and turns off during the high voltage period. As a result, the auxiliary light-emitting LED array contained in the auxiliary light-emitting circuit emits light during the low voltage period of the full-wave rectified waveform. 
         [0022]    Preferably, in the LED drive circuit, an anode of the auxiliary light-emitting LED array is connected to an anode of the first LED sub-array, and a cathode of the auxiliary light-emitting LED array is connected to a drain of the second field-effect transistor. 
         [0023]    Preferably, in the LED drive circuit, the first field-effect transistor and the second field-effect transistor are enhancement-mode devices. 
         [0024]    Preferably, in the LED drive circuit, the bypass circuit further includes a current output terminal, a bipolar transistor, a pull-up resistor, and a current-sensing resistor, one end of the pull-up resistor and a drain of the first field-effect transistor are connected to the first current input terminal, a gate of the first field-effect transistor and a collector of the bipolar transistor are connected to the other end of the pull-up resistor, a source of the first field-effect transistor, a base of the bipolar transistor, and one end of the current-sensing resistor are connected to the second current input terminal, and an emitter of the bipolar transistor and the other end of the current-sensing resistor are connected to the current output terminal. 
         [0025]    Preferably, in the LED drive circuit, the first field-effect transistor and the second field-effect transistor are depletion-mode devices. 
         [0026]    Preferably, in the LED drive circuit, the bypass circuit further includes a current output terminal and a current sensing resistor, a drain of the first field-effect transistor is connected to the first current input terminal, a source of the first field-effect transistor and one end of the current-sensing resistor are connected to the second current input terminal, and a gate of the first field-effect transistor and the other end of the current-sensing resistor are connected to the current output terminal. 
         [0027]    In the LED drive circuit, since the gate voltage of the second field-effect transistor contained in the auxiliary light-emitting circuit is equal to the gate voltage of the first field-effect transistor contained in the bypass circuit, the gate voltage of the second field-effect transistor need not be provided separately, and thus the circuitry to be added can be simplified. Furthermore, in the LED drive circuit, the light emission of the auxiliary light-emitting circuit serves to increase the amount of light to be produced during the low voltage period of the full-wave rectified waveform. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0028]      FIG. 1  is a circuit configuration diagram of an LED drive circuit  100 . 
           [0029]      FIG. 2(   a ) is a waveform diagram depicting a full-wave rectified waveform, and 
           [0030]      FIG. 2(   b ) is a waveform diagram depicting a circuit current J in the LED drive circuit  100 . 
           [0031]      FIG. 3  is a circuit configuration diagram of an alternative LED drive circuit  300 . 
           [0032]      FIG. 4  is a circuit diagram showing one example of an LED drive circuit; the diagram is redrawn from  FIG. 26  given in patent document 1 without departing from the purpose thereof. 
           [0033]      FIG. 5(   a ) is a waveform diagram depicting a full-wave rectified waveform, and 
           [0034]      FIG. 5(   b ) is a waveform diagram depicting a circuit current I in an LED drive circuit  400 . 
       
    
    
     DESCRIPTION 
       [0035]    An LED drive circuit will be described below with reference to the drawings. It will, however, be noted that the technical scope of the present invention is not limited by any particular embodiment described herein, but extends to the inventions described in the appended claims and their equivalents. Further, in the description of the drawings, the same or corresponding component elements are designated by the same reference numerals, and the description of such component elements, once given, will not be repeated thereafter. 
         [0036]      FIG. 1  is a circuit configuration diagram of an LED drive circuit  100 . 
         [0037]    The LED drive circuit  100  comprises a bridge rectifier circuit  105 , LED sub-arrays  110  and  130 , a bypass circuit  120 , a current-limiting circuit  140 , and an auxiliary light-emitting circuit  160 . An AC commercial power supply  106  is connected to input terminals of the bridge rectifier circuit  105 . 
         [0038]    The bridge rectifier circuit  105  is constructed from four diodes  101 ,  102 ,  103 , and  104 , and has a terminal A as an output terminal for outputting a full-wave rectified waveform and a terminal B as a terminal for providing a reference voltage. The LED array contained in the LED drive circuit  100  comprises two LED arrays, a main light-emitting LED array and an auxiliary light-emitting LED array. The LED sub-array  110  (first LED sub-array) and the LED sub-array  130  (second LED sub-array) constitute the main light-emitting LED array. The auxiliary light-emitting LED array is implemented as an auxiliary light-emitting LED array  150  in the auxiliary light-emitting circuit  160  to be described later. In the main light-emitting LED array, the LED sub-arrays  110  and  130  are connected in series. The LED sub-array  110  is constructed from a series connection of a large number of LEDs including LEDs  111  and  112 . Similarly, the LED sub-array  130  is constructed from a series connection of a large number of LEDs including LEDs  131  and  132 . The anode of the LED sub-array  110  is connected to the terminal A of the bridge rectifier circuit  105 . The connection node between the LED sub-arrays  110  and  130  is connected to a current input terminal  127  (first current input terminal) of the bypass circuit  120 . The cathode of the LED sub-array  130  is connected to a current input terminal  147  of the current-limiting circuit  140 . 
         [0039]    The bypass circuit  120  includes, in addition to the current input terminal  127  (first current input terminal), a current input terminal  128  (second current input terminal) and a current output terminal  129 . The current input terminal  128  is connected to a current output terminal  149  of the current-limiting circuit  140 . The current output terminal  129  is connected to the terminal B of the bridge rectifier circuit  105 . The bypass circuit  120  is configured so that the current flowing in via the current input terminal  127  is limited in accordance with the current flowing in via the current input terminal  128 . 
         [0040]    The bypass circuit  120  comprises a pull-up resistor  121 , a current-sensing resistor  124 , a field-effect transistor (first field-effect transistor)  122 , and a bipolar transistor (hereinafter simply “transistor”)  123 . One end of the pull-up resistor  121  and the drain of the FET  122  are connected to the current input terminal  127 . The other end of the pull-up resistor  121  is connected to the gate of the FET  122  and the collector of the transistor  123 . The source of the FET  122 , the base of the transistor  123 , and one end of the current-sensing resistor  124  are connected to the current input terminal  128 . The emitter of the transistor  123  and the other end of the current-sensing resistor  124  are connected to the current output terminal  129 . 
         [0041]    The current-limiting circuit  140  differs from the bypass circuit  120  only by the absence of the second current input terminal (the terminal that corresponds to the current input terminal  128  of the bypass circuit  120 ). In the current-limiting circuit  140 , the connections of the pull-up resistor  141 , the current-sensing resistor  144 , the FET  142 , and the transistor  143  are the same as those of the corresponding components in the bypass circuit  120 . The resistance value of the current-sensing resistor  144  is set smaller than that of the current-sensing resistor  124 . 
         [0042]    The auxiliary light-emitting circuit  160  comprises an FET  162  and a resistor  164  in addition of the auxiliary light-emitting LED array  150 . The auxiliary light-emitting LED array  150  is constructed from a series connection of a large number of LEDs including LEDs  151  and  152 , and its anode is connected to the anode of the LED sub-array  110 , while its cathode is connected to the drain of the FET  162 . The number of series-connected stages in the auxiliary light-emitting LED array  150  is set equal to the number of series-connected stages in the LED sub-array  110  so that the threshold value becomes the same for both. The gate of the FET  162  is connected to the gate of the FET  122  in the bypass circuit  120 , while the source of the FET  162  is connected to one end of the resistor  164 . The other end of the resistor  164  is connected to the terminal B of the bridge rectifier circuit  105 . 
         [0043]    The FETs  122 ,  142 , and  162  are enhancement-mode n-type MOS-FETs. 
         [0044]      FIG. 2(   a ) is a waveform diagram depicting a full-wave rectified waveform, and  FIG. 2(   b ) is a waveform diagram depicting a circuit current J in the LED drive circuit  100 . The circuit current J indicates the current that flows out from the terminal A of the bridge rectifier circuit  105  toward the LED sub-array  110 . The same time axis is used for both  FIGS. 2(   a ) and  2 ( b ). 
         [0045]    In  FIG. 2 , no circuit current J flows during the period t1 because the voltage value of the full-wave rectified waveform is smaller than the threshold value of the LED sub-array  110  and the auxiliary light-emitting LED array  150 . 
         [0046]    In the period t2, the voltage value of the full-wave rectified waveform exceeds the threshold value of the LED sub-array  110  and the auxiliary light-emitting 
         [0047]    LED array  150  but is smaller than the sum of the threshold value of the LED sub-array  110  and the threshold value of the LED sub-array  130 . In this case, a portion of the circuit current J passes through the bypass circuit  120  and returns to the bridge rectifier circuit  105 . During the period t2, feedback is applied so as to maintain the base-emitter voltage of the transistor  123  at 0.6 V, and the bypass circuit  120  thus operates in a constant current mode. The remaining portion of the circuit current J passes through the auxiliary light-emitting LED array  150  and flows through the FET  162 , and returns to the bridge rectifier circuit  105 . Since the FET  122  is operating in a constant current mode, the FET  162  also operates in a constant current mode. The amount of current to be diverted to the auxiliary light-emitting LED array  150  is determined by adjusting the value of the resistor  164  and the die size of the FET  162 . 
         [0048]    In this way, the circuit current J that flows during the period t2 is the sum of the current that flows through the LED sub-array  110  and the current that flows through the auxiliary light-emitting LED array  150 , and thus becomes large as depicted in  FIG. 2(   b ). For example, if it is assumed that the number of LEDs in the LED sub-array  110  is the same as that in the auxiliary light-emitting LED array  150 , and that the die sizes of the FETs  122  and  162  and the values of the resistors  124  and  164  are respectively the same, the current that flows through the auxiliary light-emitting LED array  150  is approximately the same as the current that flows through the LED sub-array  110 . That is, the amount of current approximately twice the amount of current that flows through the LED sub-array  110  flows during the period t2. 
         [0049]    In the period t3, the voltage value of the full-wave rectified waveform exceeds the sum of the threshold value of the LED sub-array  110  and the threshold value of the LED sub-array  130 , and the current flows to the current input terminal  128  by passing through the LED sub-array  130 . At this time, the transistor  123  saturates, and the gate voltage of the FET  122  becomes equal to the reference voltage (the voltage at the terminal B), so that the FET  122  is cut off. At the same time, the FET  162  is also cut off. As a result, the circuit current J flows only through the LED sub-arrays  110  and  130 . The circuit current J is held constant by the current-limiting circuit  140 . 
         [0050]    The process that takes place during the period that the voltage of the full-wave rectified waveform falls is the reverse of the process that takes place during the period that the voltage of the full-wave rectified waveform rises. Further, the LED drive circuit  100  is set up so that the circuit current J that flows during the low voltage period (the period t2) of the full-wave rectified waveform is larger than the circuit current J that flows during the high voltage period (the period t3) of the full-wave rectified waveform. 
         [0051]    As described above, in the LED drive circuit  100 , the main light-emitting LED array and the auxiliary light-emitting LED array  150  are connected in parallel with respect to the rectifier circuit  105  and, during the period t2, only the LED sub-array  110  and auxiliary light-emitting LED array  150  connected in parallel with respect to the rectifier circuit  105  are caused to emit light. In the period t3, the bypass circuit  120  shuts off the current returning to the rectifier circuit  105  via the bypass circuit  120 , while also shutting off the current flowing to the auxiliary light-emitting LED array  150  by controlling the auxiliary light-emitting circuit  160 . As a result, during the period t3, only the LED sub-array  110  and  130  connected in series to the rectifier circuit  105  are caused to emit light. In this way, the LED drive circuit  100  performs control so that, during the low voltage period of the full-wave rectified waveform, the two LED arrays connected in parallel with respect to the rectifier circuit  105  are caused to emit light and, during the high voltage period of the full-wave rectified waveform, the two LED arrays connected in series to the rectifier circuit  105  are caused to emit light. The above configuration is the same for the alternative LED drive circuit  300  described hereinafter. 
         [0052]      FIG. 3  is a circuit configuration diagram of the alternative LED drive circuit  300 . 
         [0053]    The FETs  122  and  142  used in the bypass circuit  120  and current-limiting circuit  140  shown in  FIG. 1  were enhancement-mode FETs; however, if depletion-mode FETs were used instead, the bypass circuit and the current-limiting circuit could be simplified while accomplishing equivalent functions.  FIG. 3  shows the LED drive circuit  300  using depletion-mode FETs. 
         [0054]    The LED drive circuit  300  comprises a bridge rectifier circuit  105 , LED sub-arrays  110  and  130 , a bypass circuit  320 , a current-limiting circuit  340 , and an auxiliary light-emitting circuit  360 . The bridge rectifier circuit  105  and the LED sub-arrays  110  and  130  are the same as those in the LED drive circuit  100  of  FIG. 1 , and the AC commercial power supply  106  is connected to the input terminals of the bridge rectifier circuit  105 . 
         [0055]    The LED array contained in the LED drive circuit  300  comprises two LED arrays, a main light-emitting LED array and an auxiliary light-emitting LED array. The LED sub-array  110  (first LED sub-array) and the LED sub-array  130  (second LED sub-array) constitute the main light-emitting LED array. The auxiliary light-emitting LED array is implemented as an auxiliary light-emitting LED array  350  in the auxiliary light-emitting circuit  360  to be described later. The anode of the LED sub-array  110  is connected to the terminal A of the bridge rectifier circuit  105 , the connection node between the LED sub-arrays  110  and  130  is connected to a current input terminal  327  (first current input terminal) of the bypass circuit  320 , and the cathode of the LED sub-array  130  is connected to a current input terminal  347  of the current-limiting circuit  340 . 
         [0056]    The bypass circuit  320  includes, in addition to the current input terminal  327  (first current input terminal), a current input terminal  328  (second current input terminal) and a current output terminal  329 . The current input terminal  328  is connected to a current output terminal  349  of the current-limiting circuit  340 . The current output terminal  329  is connected to the terminal B of the bridge rectifier circuit  105 . The bypass circuit  320  is configured so that the current flowing in via the current input terminal  327  is limited in accordance with the current flowing in via the current input terminal  328 . 
         [0057]    The bypass circuit  320  comprises a field-effect transistor (first field-effect transistor)  322  and a current-sensing resistor  324 . The drain of the FET  322  is connected to the current input terminal  327 . The source of the FET  322  and one end of the current-sensing resistor  324  are connected to the current input terminal  328 . The gate of the FET  322  and the other end of the current-sensing resistor  324  are connected to the current output terminal  329 . 
         [0058]    The circuit configuration of the current-limiting circuit  340  is substantially the same as that of the bypass circuit  320 , the only difference being the absence of the second current input terminal (the terminal that corresponds to the current input terminal  328  of the bypass circuit  320 ). The connections of the FET  342  and the current-sensing resistor  344  are the same as those of the corresponding components in the bypass circuit  320 . The resistance value of the current-sensing resistor  344  is smaller than that of the current-sensing resistor  324 . 
         [0059]    The FETs  322  and  342  are depletion-mode n-type MOS-FETs. In the depletion-mode FET, the drain current 
         [0060]    Id has a negative threshold value and the gate voltage Vg must be made sufficiently lower than the source voltage Vs in order to turn off the FET. However, since the drain current Id has a negative threshold value, the bypass circuit  320  whose configuration is simplified by omitting the resistor  121  and transistor  123  from the bypass circuit  120  shown in  FIG. 1  can be made to operate in a constant current mode in the same manner as the bypass circuit  120 . For example, in the bypass circuit  320 , when the drain current Id of the FET  322  increases, the voltage drop across the resistor  324  increases, and as a result, the current flowing between the source and drain of the FET  322  decreases. Conversely, in the bypass circuit  320 , when the drain current Id of the FET  322  decreases, the voltage drop across the resistor  324  decreases, and as a result, the current flowing between the source and drain of the FET  322  increases. That is, the drain current Id and the current flowing between the source and drain form a negative feedback loop. On the other hand, in the case of the enhancement-mode FET, since the drain current Id has a positive threshold value, a transistor has to be added in order to form such a negative feedback loop. 
         [0061]    The auxiliary light-emitting circuit  360  comprises an FET  362 , an PNP transistor  363 , and an operational amplifier  364 , in addition of the auxiliary light-emitting LED array  350 . The FET  362  is a depletion-mode n-type MOS-FET. The auxiliary light-emitting LED array  350  is constructed from a series connection of a large number of LEDs including LEDs  351  and  352 . The anode of the auxiliary light-emitting LED array  350  is connected to the anode of the LED sub-array  110 , and the cathode of the auxiliary light-emitting LED array  350  is connected to the drain of the FET  362 . The number of series-connected stages in the auxiliary light-emitting LED array  350  is set equal to the number of series-connected stages in the LED sub-array  110  so that the threshold value becomes the same for both. The gate of the FET  362  is connected to the gate of the FET  322  in the bypass circuit  320 , while the source of the FET  362  is connected to the emitter of the transistor  363  as well as to the negative input terminal of the operational amplifier  364 . The output terminal of the operational amplifier  364  is connected to the base of the transistor  363 , and the collector of the transistor  363  is connected to the terminal B of the bridge rectifier circuit  105 . 
         [0062]    Next, the operation of the LED drive circuit  300  will be described with reference to  FIG. 2 . The waveform diagrams of  FIGS. 2(   a ) and  2 ( b ) also apply to the LED drive circuit  300 . The circuit current J in the LED drive circuit  300  also indicates the current that flows out from the terminal A of the bridge rectifier circuit  105  toward the LED sub-array  110 . 
         [0063]    No circuit current J flows during the period t1 because the voltage value of the full-wave rectified waveform is smaller than the threshold value of the LED sub-array  110  and the auxiliary light-emitting LED array  350   
         [0064]    In the period t2, the voltage value of the full-wave rectified waveform exceeds the threshold value of the LED sub-array  110  and the auxiliary light-emitting LED array  350  but is smaller than the sum of the threshold value of the LED sub-array  110  and the threshold value of the LED sub-array  130 . In this case, a portion of the circuit current J passes through the bypass circuit  320  and returns to the bridge rectifier circuit  105 . During the period t2, feedback is applied from the current-sensing resistor  324  to the source of the FET  322 , and the bypass circuit  320  thus operates in a constant current mode. The remaining portion of the circuit current J passes through the auxiliary light-emitting LED array  350  and flows through the FET  362 , and returns to the bridge rectifier circuit  105 . Since the FET  322  is operating in a constant current mode, the FET  362  also operates in a constant current mode. 
         [0065]    The operational amplifier  364  and the transistor  363  are inserted so that the source voltage of the FET  322  becomes equal to the source voltage of the FET  362 . If the current supply capability of the operational amplifier  364  is large, the transistor  363  may be omitted. The amount of current to be diverted to the auxiliary light-emitting LED array  350  is determined by adjusting the die size of the FET  362 . In this way, the circuit current J that flows during the period t2 is the sum of the current that flows through the LED sub-array  110  and the current that flows through the auxiliary light-emitting LED array  350 , and thus becomes large as depicted in the figure. 
         [0066]    In the period t3, the voltage value of the full-wave rectified waveform exceeds the sum of the threshold value of the LED sub-array  110  and the threshold value of the LED sub-array  130 , and the current flows to the current input terminal  328  by passing through the LED sub-array  130 . At this time, the source voltage of the FET  322  increases, thus increasing the source-to-gate voltage of the FET  322 , so that the FET  322  is cut off. At the same time, the FET  362  is also cut off. As a result, the circuit current J flows only through the LED sub-arrays  110  and  130 . The circuit current J is held constant by the current-limiting circuit  340 . 
         [0067]    The process that takes place during the period that the voltage of the full-wave rectified waveform falls is the reverse of the process that takes place during the period that the voltage of the full-wave rectified waveform rises. Further, the LED drive circuit  300  is set up so that the circuit current that flows during the low voltage period (the period t2) of the full-wave rectified waveform is larger than the circuit current that flows during the high voltage period (the period t3) of the full-wave rectified waveform. 
         [0068]    While the LED drive circuits  100  and  300  have each been described as being constructed using the current-limiting circuit  140  or  340 , a current-limiting resistor or a constant-current diode may be used instead of the current-limiting circuit. However, compared with the current-limiting resistor, using the constant-current circuit offers the further advantage of being able to ensure relatively stable operation even when the amplitude of the AC commercial power supply is unstable. 
         [0069]    In the LED drive circuits  100  and  300 , the main light-emitting LED array is constructed from two LED sub-arrays (the LED sub-arrays  110  and  130 ), and the bypass circuit  120 ,  320  and the current-limiting circuit  140 ,  340  are identical in configuration, except for the current input terminal (second current input terminal)  128 ,  328 . In view of this, in the case of the LED drive circuit  100 , a block may be constructed from a combination of the LED sub-array  110  and the bypass circuit  120 , and a number of such blocks may be cascaded in a multistage configuration. In this case, the bypass circuit  120  is connected to the connection node between each LED sub-array and, between any two adjacent bypass circuits  120 , the current input terminal (second current input terminal)  128  is connected to the current output terminal  129 . By thus cascading the blocks in a multistage configuration, it becomes easier to increase brightness and improve distortion factor. Further, in this case, the resistance values of the current-limiting resistors contained in the respective blocks must be set in such a manner that the current-limiting resistor contained in a block farther away from the bridge rectifier circuit  105  has a smaller resistance value. In the LED drive circuit  300  also, such blocks can be cascaded in a multistage configuration. 
       DESCRIPTION OF REFERENCE NUMERALS 
       [0000]    
       
           100 ,  300  . . . LED DRIVE CIRCUIT 
           101 ,  102 ,  103 ,  104  . . . DIODE 
           105  . . . BRIDGE RECTIFIER CIRCUIT 
           106  . . . AC COMMERCIAL POWER SUPPLY 
           110  . . . LED SUB-ARRAY (FIRST LED SUB-ARRAY) 
           111 ,  112 ,  131 ,  132 ,  151 ,  152 ,  351 ,  352  . . . LED 
           102 ,  320  . . . BYPASS CIRCUIT 
           121 ,  141  . . . PULL-UP RESISTOR 
           122  . . . FET (FIRST ENHANCEMENT-MODE FIELD-EFFECT TRANSISTOR) 
           123 ,  143  . . . TRANSISTOR (BIPOLAR TRANSISTOR) 
           124 ,  144 ,  324 ,  344  . . . CURRENT-SENSING RESISTOR 
           127 ,  327  . . . CURRENT INPUT TERMINAL (FIRST CURRENT INPUT TERMINAL) 
           128 ,  328  . . . CURRENT INPUT TERMINAL (SECOND CURRENT INPUT TERMINAL) 
           129 ,  329  . . . CURRENT OUTPUT TERMINAL 
           130  . . . LED SUB-ARRAY (SECOND LED SUB-ARRAY) 
           142  . . . FET (ENHANCEMENT-MODE FIELD-EFFECT TRANSISTOR) 
           150 ,  350  . . . AUXILIARY LIGHT-EMITTING LED ARRAY 
           160 ,  360  . . . AUXILIARY LIGHT-EMITTING CIRCUIT 
           162  . . . FET (SECOND ENHANCEMENT-MODE FIELD-EFFECT TRANSISTOR) 
           164  . . . RESISTOR 
           322  . . . FET (FIRST DEPLETION-MODE FIELD-EFFECT TRANSISTOR) 
           342  . . . FET (DEPLETION-MODE FIELD-EFFECT TRANSISTOR) 
           362  . . . FET (SECOND DEPLETION-MODE FIELD-EFFECT TRANSISTOR) 
           363  . . . TRANSISTOR (PNP BIPOLAR TRANSISTOR) 
           364  . . . OPERATIONAL AMPLIFIER