Patent Publication Number: US-9854631-B2

Title: LED illumination device

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This is the U.S. National Phase application of PCT/JP2015/057918, filed Mar. 17, 2015, which claims priority to Japanese Patent Application No. 2014-053284, filed Mar. 17, 2014, the disclosures of each of these applications being incorporated herein by reference in their entireties for all purposes. 
     TECHNICAL FIELD 
     The present invention relates to an LED illuminator including an LED drive circuit configured to drive an LED with a full-wave rectified waveform. 
     BACKGROUND ART 
     There is known an LED illuminator including an LED drive circuit having an LED string in which a plurality of LEDs is connected in series and configured to improve luminance and to prevent a flicker by increasing/decreasing the number of serial stages of the LED string in accordance with an increase/decrease in the voltage of the full-wave rectified waveform and by lengthening an on-state period. Among such LED drive circuits, there is an LED drive circuit configured to improve a power factor and a distortion factor by increasing/decreasing a current that flows through the LED string in accordance with an increase/decrease in the full-wave rectified waveform. 
       FIG. 11  is a circuit diagram of a light source circuit  2600  described in Patent Document 1. The light source circuit  2600  includes a bridge rectifier  2605  and an LED string. The LED string includes an LED group  2601 , an LED group  2602 , and an LED group  2603 , in each of which a plurality of LEDs is connected in series. The light source circuit  2600  further includes a bypass circuit  2610  configured to operate so as to decrease an effective forward turn-on voltage. The bypass circuit  2610  includes resistors R 2  and R 3 , an enhancement type field effect transistor Q 1 , and a bipolar transistor Q 2 . 
     With reference to  FIG. 12 , a relationship between a current and a voltage of the light source circuit  2600  is explained.  FIG. 12A  is a waveform diagram illustrating a relationship between a full-wave rectified voltage waveform V 1  corresponding to one period and a time t in the light source circuit  2600  and  FIG. 12B  is a waveform diagram illustrating a relationship between a circuit current I and the time t of the light source circuit  2600 . The scales of the time axis are the same in  FIG. 12A  and  FIG. 12B . 
     During a period of time t 30  during which the voltage of the full-wave rectified voltage waveform V 1 , which is an output of the bridge rectifier  2605 , is less than a threshold voltage (effective forward turn-on voltage) determined by the LED groups  2601  and  2602  in the light source circuit  2600 , the current I does not flow through the LED groups  2601  and  2602 . During a period of time t 31  during which the voltage of the full-wave rectified voltage waveform V 1  is greater than or equal to the threshold voltage determined by the LED groups  2601  and  2602  and less than a threshold voltage of the LED string, the current I flows through the bypass circuit  2610  from the LED groups  2601  and  2602 . At this time, the bypass circuit  2610  performs a constant-current operation with a current value I 31 . During a period of time t 32  during which the voltage value of the full-wave rectified voltage waveform V 1  is greater than or equal to the threshold voltage of the LED string, a current flows through an LED group  3  from LED groups  1  and  2 . At this time, if a current with a predetermined value or more flows into the bypass circuit  2610  from the right terminal of the resistor R 1 , the field effect transistor Q 1  cuts off and all the current I comes to flow through the LED group  2603 . In this case, the current that flows through the resistor R 2  is ignored. When the voltage of the full-wave rectified voltage waveform V 1  decreases, the processes take place in the opposite order. 
     As described above, the light source circuit  2600  has an LED string in which a plurality of LEDs is connected in series and increases/decreases the current I that flows through the LED string in accordance with an increase/decrease in the full-wave rectified voltage waveform V 1  as well as increasing/decreasing the number of serial stages of the LED string in accordance with an increase/decrease in the full-wave rectified voltage waveform V 1 . As a result of this, an attempt to improve the luminance, the flicker, the power factor, and the distortion factor is made to a certain extent. 
     PRIOR ART DOCUMENT 
     Patent Document 
     
         
         Patent Document 1: Japanese Unexamined Patent Application Publication No. 2013-502081 
       
    
     SUMMARY OF THE INVENTION 
     The waveform of the current I illustrated in  FIG. 12B  is made to resemble a sinusoidal wave, but the current I has large modified portions in the form of a ladder, and therefore, the current I considerably differs from a sinusoidal wave. Consequently, in the light source circuit  2600 , harmonic noise occurs and the total harmonic distortion (THD) is not reduced sufficiently. That is, there is a possibility that the light source circuit  2600  affects the outside by the harmonic noise when driving with a large current although the influence on the outside is small when driving with a small current. 
     The objective of the invention of the application is to provide an LED illuminator capable of further reducing the total harmonic distortion. 
     An LED illuminator has a rectifier, a first LED string connected to the rectifier and including a first partial LED string and a second partial LED string connected in series with the first partial LED string, a second LED string connected to the rectifier in parallel to the first LED string and including a third partial LED string and a fourth partial LED string connected in series with the third partial LED string, a first switching circuit configured to switch between a state where only the first partial LED string is connected to the rectifier and a state where the first partial LED string and the second partial LED string connected in series are connected to the rectifier as a full-wave rectified voltage waveform that is output from the rectifier increases/decreases, and a second switching circuit configured to switch between a state where only the third partial LED string is connected to the rectifier and a state where the third partial LED string and the fourth partial LED string connected in series are connected to the rectifier as the full-wave rectified voltage waveform that is output from the rectifier increases/decreases, and the switching timing by the first switching circuit and the switching timing by the second switching circuit are set so as to differ from each other. 
     In the above-described LED illuminator, it is preferable for the first switching circuit to detect a current that flows through at least part of the first LED string and to switch between a state where only the first partial LED string is connected to the rectifier and a state where the first partial LED string and the second partial LED string connected in series are connected to the rectifier in accordance with the detected current. 
     In the above-described LED illuminator, it is preferable for the first switching circuit to have current detection resistors for detecting a current for each of the first partial LED string and the second partial LED string. 
     In the above-described LED illuminator, it is preferable for the first switching circuit to have one current detection resistor for detecting a current for the first partial LED string and the second partial LED string. 
     In the above-described LED illuminator, it is preferable for the first switching circuit to detect a voltage of a full-wave rectified voltage waveform that is output from the rectifier and to switch between a state where only the first partial LED string is connected to the rectifier and a state where the first partial LED string and the second partial LED string connected in series are connected to the rectifier in accordance with the detected voltage. 
     In the above-described LED illuminator, it is preferable for a combination of the number of LEDs included in the first partial LED string and the number of LEDs included in the second partial LED string to be set so as to differ from a combination of the number of LEDs included in the third partial LED string and the number of LEDs included in the fourth partial LED string. 
     In the above-described LED illuminator, it is preferable for the number of serial stages of LEDs included in the partial LED string that lights up during the period of time during which the voltage of the full-wave rectified voltage waveform is the lowest between the first partial LED string and the second partial LED string to be set so as to differ from the number of serial stages of LEDs included in the partial LED string that lights up during the period of time during which the voltage of the full-wave rectified voltage waveform is the lowest between the third partial LED string and the fourth partial LED string. 
     In the above-described LED illuminator, it is preferable for the first LED string to further include another partial LED string and for the second LED string to further include another partial LED string. 
     In the above-described LED illuminator, it is preferable for the number of partial LED strings included in the first LED string to be set so as to differ from the number of partial LED strings included in the second LED string. 
     In the above-described LED illuminator, it is preferable for the first LED string and the first switching circuit to be configured as one LED module and for the second LED string and the second switching circuit to be configured as another LED module. 
     In the above-described LED illuminator, the switching timing of the connection state of the first LED string by the first switching circuit and the switching timing of the connection state of the second LED string by the second switching circuit are set so as to differ from each other, and therefore, it is made possible to further reduce the total harmonic distortion. 
     In the LED illuminator including an LED drive circuit configured to increase/decrease the number of serial stages within an LED string and a current that flows through the LED string as a voltage of a full-wave rectified waveform increases/decreases, the LED illuminator includes: a first LED drive circuit including a first LED string in which a plurality of LEDs is connected in series and configured to increase/decrease the number of serial stages of LEDs included in the first LED string in accordance with the voltage of the full-wave rectified waveform; and a second LED drive circuit including a second LED string in which a plurality of LEDs is connected in series and configured to increase/decrease the number of serial stages of LEDs included in the second LED string in accordance with the voltage of the full-wave rectified waveform, and the first LED drive circuit and the second LED drive circuit are connected in parallel, and the timing at which the number of serial stages of the first LED string switches and the timing at which the number of serial stages of the second LED string switches are different. 
     The above-described LED illuminator has the first and second LED drive circuits configured to increase/decrease the number of serial stages within the LED string and the current that flows through the LED string as the voltage of the full-wave rectified waveform increases/decreases. The first and second LED drive circuits have the first and second LED strings, respectively and the timing at which the number of serial stages of the first LED string switches in accordance with the change in the voltage of the full-wave rectified waveform and the timing at which the number of serial stages of the second LED string switches are made to differ from each other. In the LED illuminator, a current that is the sum of the current flowing through the first LED string and the current flowing through the second LED string flows and this current changes at small steps in accordance with the change in the voltage of the full-wave rectified waveform. That is, as a result of the current waveform becoming closer to a sinusoidal wave, the total harmonic distortion is reduced. 
     In the LED illuminator, it is preferable for the combination relating to the number of serial stages of a partial LED string obtained by dividing the first LED string and the combination relating to the number of serial stages of a partial LED string obtained by dividing the second LED string to differ from each other. 
     In the LED illuminator, the number of serial stages of the partial LED string that is included in the first LED string and which lights up during the period of time during which the voltage of the full-wave rectified waveform is the lowest and the number of serial stages of the partial LED string that is included in the second LED string and which lights up during the period of time during which the voltage of the full-wave rectified waveform is the lowest may be different from each other. 
     In the LED illuminator, the first and second LED drive circuits may each include only one current detection resistor and the numbers of serial stages of the first and second LED drive circuits may be switched based on the voltage between both ends of the current detection resistor or the divided voltage thereof. 
     In the LED illuminator, it may also be possible for the first and second LED drive circuits to switch the numbers of serial stages of the first and second LED strings by measuring the voltage of the full-wave rectified waveform. 
     The purpose and the effect of the present invention will be recognized and obtained by using components that are pointed out particularly in the claims and combinations thereof. Both the foregoing general explanation and the following detailed explanation are merely illustrative and explanatory and do not limit the present invention described particularly in the claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram of an LED illuminator  10 . 
         FIG. 2  is a circuit diagram of the LED illuminator  10  illustrated in  FIG. 1 . 
         FIG. 3A  is a waveform diagram illustrating a relationship between a full-wave rectified voltage waveform V 1  corresponding to one period and a time t in the LED illuminator  10 . 
         FIG. 3B  is a waveform diagram illustrating a relationship between a current I 1  that flows into a first LED drive circuit  13  and the time t. 
         FIG. 3C  is a waveform diagram illustrating a relationship between a current I 2  that flows into a second LED drive circuit  14  and the time t. 
         FIG. 3D  is a waveform diagram illustrating a relationship between a total current I 0  and the time t. 
         FIG. 4A  is a plan view of the first LED drive circuit  13 . 
         FIG. 4B  is a front view of the first LED drive circuit  13 . 
         FIG. 5  is a diagram illustrating a connection situation of a first module  13 P and a second module  14 P. 
         FIG. 6  is a circuit diagram of another LED illuminator  50 . 
         FIG. 7A  is a waveform diagram illustrating a relationship between the full-wave rectified voltage waveform V 1  corresponding to one period and the time t in the LED illuminator  50 . 
         FIG. 7B  is a waveform diagram illustrating a relationship between a current I 51  that flows into a first LED drive circuit  53  and the time t. 
         FIG. 7C  is a waveform diagram illustrating a relationship between the current I 2  that flows into the second LED drive circuit  14  and the time t. 
         FIG. 7D  is a waveform diagram illustrating a relationship between a total current I 50  and the time t. 
         FIG. 8  is a circuit diagram of still another LED illuminator  60 . 
         FIG. 9  is a circuit diagram of still another LED illuminator  70 . 
         FIG. 10A  is a waveform diagram illustrating a relationship between the full-wave rectified voltage waveform V 1  corresponding to one period and the time t in the LED illuminator  70 . 
         FIG. 10B  is a waveform diagram illustrating a relationship between a current I 71  that flows into a first LED drive circuit  73  and the time t. 
         FIG. 10C  is a waveform diagram illustrating a relationship between a current I 72  that flows into a second LED drive circuit  74  and the time t. 
         FIG. 10D  is a waveform diagram illustrating a relationship between a total current I 70  and the time t. 
         FIG. 11  is a circuit diagram of a light source circuit  2600  described in Patent Document 1. 
         FIG. 12A  is a waveform diagram illustrating a full-wave rectified voltage waveform corresponding to one period in the light source circuit  2600  illustrated in  FIG. 11 . 
         FIG. 12B  is a waveform diagram illustrating a circuit current of the light source circuit  2600  illustrated in  FIG. 11 . 
     
    
    
     EMBODIMENTS OF THE INVENTION 
     Hereinafter, with reference to the drawings, embodiments of an LED illuminator according to the present invention are described in detail. However, it should be noted that the technical scope of the present invention is not limited to those embodiments but encompasses the inventions described in the claims and the equivalents thereof. The dimension in each drawing does not reflect the exact dimension and sometimes the size of parts is drawn in an exaggerated manner or some parts are omitted for explanation. The same numerals are attached to the same elements and duplicated explanation is omitted. 
       FIG. 1  is a block diagram of an LED illuminator  10 . 
     As illustrated in  FIG. 1 , the LED illuminator  10  includes a bridge rectifier circuit  11 , a first LED drive circuit  13 , and a second LED drive circuit  14 . For convenience, in  FIG. 1 , a commercial AC power source  12  connected to the bridge rectifier circuit  11  is illustrated. 
     The commercial AC power source  12  connects to the input terminal of the bridge rectifier circuit  11 . The bridge rectifier circuit  11  applies a full-wave rectified waveform to the first and second LED drive circuits  13  and  14  via a wire  15 . As a result of this, a current I 0  is output from the bridge rectifier circuit  11  and currents I 1  and I 2  flow into the first and second LED drive circuits  13  and  14 , respectively. From the first and second LED drive circuits  13  and  14 , the currents return to the bridge rectifier circuit  11  via a wire  16 . That is, the wire  16  is a ground wire. 
     The first LED drive circuit  13  includes a first LED string in which a plurality of LEDs is connected in series and the number of serial stages of LEDs included in the first LED string increases/decreases in accordance with the voltage of the full-wave rectified waveform. Similarly, the second LED drive circuit  14  also includes a second LED string in which a plurality of LEDs is connected in series and the number of serial stages of LEDs increases/decreases in accordance with the voltage of the full-wave rectified waveform. 
     The currents I 1  and I 2  that flow through the first and second LED drive circuits  13  and  14  also increase/decrease in accordance with the full-wave rectified waveform, but the timing at which the number of serial stages of the first LED string switches and the timing at which the number of serial stages of the second LED string switches are set so as to differ from each other. As a result of this, the timing at which the current value of the current I 1  changes and the timing at which the current value of the current I 2  changes differ therebetween. Consequently, the LED illuminator  10  is configured so that the state where the total harmonic distortion is lower is brought about by increasing/decreasing the total current I 0  at small steps, which is the sum of the current I 1 , the current I 2 , etc. 
       FIG. 2  is a circuit diagram of the LED illuminator  10  illustrated in  FIG. 1 . 
     As illustrated in  FIG. 2 , the bridge rectifier circuit  11  consists of four diodes and includes an input terminal and an output terminal. To the input terminal of the bridge rectifier circuit  11 , the commercial AC power source  12  is connected, and to the output terminal, the wire  15  for applying a full-wave rectified waveform and the wire  16 , which is the ground wire, are connected. 
     In the first LED drive circuit  13 , five partial LED strings  31   a ,  31   b ,  31   c ,  31   d , and  31   e  are connected in series. In each of the partial LED strings  31   a ,  31   b ,  31   c ,  31   d , and  31   e , a plurality of LEDs  33   a , a plurality of LEDs  33   b , a plurality of LEDs  33   c , a plurality of LEDs  33   d , and a plurality LEDs  33   e  are connected in series, respectively. The LED string in which the partial LED strings  31   a ,  31   b ,  31   c ,  31   d , and  31   e  are connected in series corresponds to the first LED string included in the first LED drive circuit  13 . 
     In the first LED drive circuit  13 , to the connection portion of the partial LED strings  31   a  and  32   b , to that of the partial LED strings  31   b  and  31   c , to that of the partial LED strings  31   c  and  31   d , and to that of the partial LED strings of  31   d  and  31   e , bypass circuits  32   a ,  32   b ,  32   c , and  32   d  are connected, respectively, and to the cathode of the partial LED string  31   e , a constant current circuit  32   e  is connected. The bypass circuits  32   a ,  32   b ,  32   c , and  32   d  and the constant current circuit  32   e  include depletion-type FETs  34   a ,  34   b ,  34   c ,  34   d , and  34   e , respectively, and resistors  35   a ,  35   b ,  35   c ,  35   d , and  35   e , respectively. The bypass circuits  32   a ,  32   b ,  32   c , and  32   d  and the constant current circuit  32   e  function as a switching circuit configured to switch the numbers of serial stages of LEDs included in the first LED string in accordance with the voltage of the full-wave rectified waveform. 
     In each of the bypass circuits  32   a ,  32   b ,  32   c , and  32   d  and the constant current circuit  32   e , the drain of each of the FETs  34   a ,  34   b ,  34   c ,  34   d , and  34   e  is the current input terminal, respectively, and the left terminal of each of the resistors  35   a ,  35   b ,  35   c ,  35   d , and  35   e  is the current output terminal, respectively. In each of the bypass circuits  32   a ,  32   b ,  32   c , and  32   d , the right terminal of each of the resistors  35   a ,  35   b ,  35   c , and  35   d  is the other current input terminal, respectively, and to each of the other current input terminals, the current output terminal of each of the bypass circuits  32   b ,  32   c , and  32   d  and the constant current circuit  32   e  is connected, respectively. 
     In the second LED drive circuit  14 , five partial LED strings  41   a ,  41   b ,  41   c ,  41   d , and  41   e  are connected in series. In each of the partial LED strings  41   a ,  41   b ,  41   c ,  41   d , and  41   e , a plurality of LEDs  43   a , a plurality of LEDs  43   b , a plurality of LEDs  43   c , a plurality of LEDs  43   d , and a plurality of LEDs  43   e  are connected in series, respectively. The LED string in which the partial LED strings  41   a ,  41   b ,  41   c ,  41   d , and  41   e  are connected in series corresponds to the second LED string included in the second LED drive circuit  14 . 
     In the second LED drive circuit  14 , to the connection portion of the partial LED strings  41   a  and  41   b , to that of the partial LED strings  41   b  and  41   c , to that of the partial LED strings  41   c  and  41   d , and to that of the partial LED strings of  41   d  and  41   e , bypass circuits  42   a ,  42   b ,  42   c , and  42   d  are connected, respectively, and to the cathode of the partial LED string  41   e , a constant current circuit  42   e  is connected. The bypass circuits  42   a ,  42   b ,  42   c , and  42   d  and the constant current circuit  42   e  include depletion-type FETs  44   a ,  44   b ,  44   c ,  44   d , and  44   e , respectively, and resistors  45   a ,  45   b ,  45   c ,  45   d , and  45   e , respectively. The bypass circuits  42   a ,  42   b ,  42   c , and  42   d  and the constant current circuit  42   e  function as a switching circuit configured to switch the numbers of serial stages of LEDs included in the second LED string in accordance with the voltage of the full-wave rectified waveform. 
     In each of the bypass circuits  42   a ,  42   b ,  42   c , and  42   d  and the constant current circuit  42   e , the drain of each of the FETs  44   a ,  44   b ,  44   c ,  44   d , and  44   e  is the current input terminal, respectively, and the left terminal of each of the resistors  45   a ,  45   b ,  45   c ,  45   d , and  45   e  is the current output terminal, respectively. In each of the bypass circuits  42   a ,  42   b ,  42   c , and  42   d , the right terminal of each of the resistors  45   a ,  45   b ,  45   c , and  45   d  is the other current input terminal, respectively, and to each of the other current input terminals, the current output terminal of each of the bypass circuits  42   b ,  42   c , and  42   d  and the constant current circuit  42   e  is connected, respectively. 
     In the first LED drive circuit  13 , the number of serial stages of LEDs  33   a , that of serial stages of LEDs  33   b , that of serial stages of LEDs  33   c , that of serial stages of LEDs  33   d , and that of serial stages of LEDs  33   e  in each of the partial LED strings  31   a ,  31   b ,  31   c ,  31   d , and  31   e  are set to 20, 20, 20, 17, and 13, respectively. In the second LED drive circuit  14 , the number of serial stages of LEDs  43   a , that of serial stages of LEDs  43   b , that of serial stages of LEDs  43   c , that of serial stages of LEDs  43   d , and that of serial stages of LEDs  43   e  in each of the partial LED strings  41   a ,  41   b ,  41   c ,  41   d , and  41   e  are set to 10, 20, 20, 17, and 23, respectively. The numbers of serial stages are different between the partial LED string  31   a  and the partial LED string  41   a , and the numbers of serial stages are different between the partial LED string  31   a  and the partial LED string  41   e . Both the total number of serial stages of the first LED string and the total number of serial stages of the second LED string are 90 and equal. 
     The forward voltage of the LED is about 3 V and the total numbers of serial stages of the first and second LED strings are 90, and therefore, the voltage at which all the LEDs light up is about 270 V. That is, the first and second LED drive circuits  13  and  14  are designed so as to adapt to the commercial AC power source the effective value of which is 240 V (maximum voltage is about 336 V). 
       FIG. 3A  is a waveform diagram illustrating a relationship between a full-wave rectified voltage waveform V 1  corresponding to one period and a time t in the LED illuminator  10 .  FIG. 3B  is a waveform diagram illustrating a relationship between the current I 11  that flows into the first LED drive circuit  13  and the time t.  FIG. 3C  is a waveform diagram illustrating a relationship between the current I 2  that flows into the second LED drive circuit  14  and the time t.  FIG. 3D  is a waveform diagram illustrating a relationship between the total current I 0  and the time t. The scale of the time axis is the same in  FIG. 3A  to  FIG. 3D . 
     By using  FIG. 3A  and  FIG. 3B , the operation of the first LED drive circuit  13  is explained. A period of time t 0  is a period of time during which the full-wave rectified voltage waveform V 1  does not reach a threshold value (product of the forward voltage and the number of serial stages of the LEDs  33   a , hereinafter, this also applies) of the partial LED string  31   a . During the period of time to, the current I 1  does not flow through the partial LED string  31   a.    
     A period of time t 1  is a period of time during which the full-wave rectified voltage waveform V 1  exceeds the threshold value of the partial LED string  31   a  and is less than or equal to the sum value of the threshold value of the partial LED string  31   a  and a threshold value of the partial LED string  31   b . During the period of time t 1 , the current I 1  flows through the bypass circuit  32   a  from the partial LED string  31   a  and returns to the bridge rectifier circuit  11 . At this time, the voltage drop of the resistor  35   a  is fed back to the FET  34   a , and therefore, a constant current I 11  flows through the bypass circuit  32   a . The transitional situation where the current I 1  changes from 0 (A) to the current I 11  is ignored (hereinafter, this also applies). 
     A period of time t 2  is a period of time during which the full-wave rectified voltage waveform V 1  exceeds the sum value of the threshold value of the partial LED string  31   a  and the threshold value of the partial LED string  31   b  and is less than or equal to the sum value of the threshold value of the partial LED string  31   a , the threshold value of the partial LED  31   b , and a threshold value of the partial LED string  31   c . During the period of time t 2 , a current flows from the partial LED string  31   b  to the bypass circuit  32   b . Due to this current, the FET  34   a  cuts off because the source voltage increases, the current I 1  flows between the source and the drain of the FET  34   b , and the current value becomes that of a current I 12 . 
     When the current begins to flow through the partial LED strings  31   c ,  31   d , and  31   e  as described above, the bypass circuits  32   b ,  32   c , and  32   d  cut off in order, and the value of the current I 1  during each of period of times t 3 , t 4 , and t 5  becomes the value of each of currents I 13 , I 14 , and I 15 , respectively. During the period of time t 5 , the current I 1  is set so as to change considerably from the current I 14  to the current I 15 , and therefore, in  FIG. 3B , the transitional state of the period of time t 5  is also illustrated. During periods of time (period of time t 6  to period of time t 10 ) during which the full-wave rectified voltage waveform V 1  decreases, the first LED drive circuit  13  follows the processes in the order opposite to that when the full-wave rectified voltage waveform V 1  increases. 
     By using  FIG. 3A  and  FIG. 3C , the operation of the second LED drive circuit  14  is explained. As illustrated in  FIG. 3C , the first rise of the current I 2  exists in the middle of the period of time t 0  in  FIG. 3B . In the first LED drive circuit  13 , when the full-wave rectified voltage waveform V 1  is 60 V (3 V*20 stages), the first rise of the current I 1  appears (see  FIG. 3B ). On the other hand, in the second LED drive circuit  14 , when the full-wave rectified voltage waveform V 1  is 30 V (3 V*10 stages), the first rise of the current I 2  appears. Similarly, the second to fourth rises of the current I 2  appear in the middle of the period of times t 1 , t 2 , and t 3 , respectively. Both the fifth rises of the current I 1  and the current I 2  appear when the full-wave rectified voltage waveform V 1  is 270 V (3 V*90 stages) (see FIG.  3 B and  FIG. 3C ). 
     In the first LED drive circuit  13  and the second LED drive circuit  14 , the FETs  34   a  to  34   e  and the FETs  44   a  to  44   e  are all the same. The resistor  35   a  and the resistor  45   a  are set to 54Ω, the resistor  35   b  and the resistor  45  are set to 32.4Ω, the resistor  35   c  and the resistor  45   c  are set to 21.6Ω, the resistor  35   d  and the resistor  45   d  are set to 10.8Ω, and the resistor  35   e  and the resistor  45   e  are set to 5.4Ω. As a result of this, for example, the current value at the first flat part (current I 11 ) of the current I 1  becomes equal to the current value at the first flat part of the current I 2 . 
     The current I 0  illustrated in  FIG. 3D  is the sum of the current I 1  in  FIG. 3B  and the current I 2  in  FIG. 3C , and increases/decrease at small steps except for the period of time t 5 . By increasing/decreasing the current I 0  at small steps as described above, the total harmonic distortion is reduced. During the period of time t 5 , the current I 0 , which is a comparatively large current, is caused to flow through the entire first and second LED strings so as to improve luminance. 
     In the LED illuminator  10  illustrated in  FIG. 2 , it is possible to connect more LED drive circuits to the bridge rectifier circuit  11  in parallel to the first and second LED drive circuits  13  and  14 , in addition to the first and second LED drive circuits  13  and  14 . By making the switching timing of the number of serial stages of the added LED drive circuit differ from the switching timing of the number of serial stages of the first and second LED drive circuits  13  and  14 , it is possible, to cause the current I 0  to increase/decrease at smaller steps. 
     In the LED illuminator  10 , both the numbers of partial LED strings included in the first and second LED drive circuits  13  and  14  are set to five, but the number is not limited to this and it may also be possible to set another number. Further, the number of LEDs included in each partial LED string and the total number of LEDs included in all the LED strings are also not limited to the numbers described above and it is possible to appropriately select the numbers in accordance with the effective value or the like of the commercial AC power source that is made use of. Furthermore, the number of LEDs included in one partial LED string may be one. 
       FIG. 4A  is a plan view of the first LED drive circuit  13  and  FIG. 4B  is a front view of the first LED drive circuit  13 . In  FIG. 4A  and  FIG. 4B , the case is illustrated where the first LED drive circuit  13  is configured as a first module  13 P. 
     As illustrated in  FIG. 4A  and  FIG. 4B , the first module  13 P includes areas demarcated by dam materials  132  and  133  on a packaging substrate  131 . In the circular area surrounded by the dam material  132 , the LEDs  33   a  to  33   e  (see  FIG. 2 ) are packaged and connected in series with one another by wires. In the two areas demarcated by the dam material  132  and the dam material  133 , the FETs  34   a  to  34   e  and the resistors  35   a  to  35   e  are packaged. The LEDs  33   a  to  33   e , the FETs  34   a  to  34   e , and the resistors  35   a  to  35   e  are covered with a resin containing phosphors. On the surface of the packaging substrate, a terminal  135  to which the full-wave rectified waveform is input and a terminal  137  to which the ground wire is connected are provided and wires  136  and  138  that connect to the terminals  135  and  137 , respectively, extend to the inside of the dam materials  132  and  133 . 
       FIG. 5  is a diagram illustrating a connection situation of the first module  13 P and a second module  14 P obtained by configuring the second LED drive circuit  14  as a module. 
     As illustrated in  FIG. 5 , the first module  13 P and the second module  14 P are connected in parallel as a single module, respectively. The wire  15  is a wire through which the full-wave rectified waveform is applied and the wire  16  is a ground wire. In the second module  14 P obtained by configuring the second LED drive circuit  14  as a module, the number of LEDs included in each partial LED string is different, and the way the LEDs  43   a  to  43   e  packaged in the circular area surrounded by the dam material are wire-bonded is different. The other configurations of the second module  14 P are the same as those of the first module  13 P described previously. It may also be possible to configure the first LED drive circuit  13  and the second LED drive circuit  14  as one module. 
     As illustrated in  FIG. 1  and  FIG. 2 , the LED illuminator  10  has the two LED drive circuits (the first LED drive circuit  13  and the second LED drive circuit  14 ) connected in parallel. However, the number of LED drive circuits connected in parallel in the LED illuminator is not limited to two. For example, it may also be possible to connect the two first LED drive circuits  13  and the two second LED drive circuits  14  in parallel. Further, it may also be possible to connect in parallel third LED drive circuits of which the switching timing of the numbers of serial stages of the LED strings is different from that of the first and second LED drive circuits  13  and  14 . 
     The number of partial LED strings included in the first LED drive circuit  13  is not limited to five. For example, it may also be possible to have only two partial LED strings. In this case, it may be possible to configure the first LED drive circuit  13  only by the partial LED strings  31   a  and  31   e , the bypass circuit  32   a , and the constant current circuit  32   e . This is also true with the second LED drive circuit  14 . 
     In the LED illuminator  10 , the combination of the numbers of serial stages of the partial LED strings  31   a ,  31   b ,  31   c ,  31   d , and  31   e  obtained by dividing the first LED string included in the first LED drive circuit  13  is set to 20 stages, 20 stages, 20 stages, 17 stages, and 13 stages. Further, the combination of the numbers of serial stages of the partial LED strings  41   a ,  41   b ,  41   c ,  41   d , and  41   e  obtained by dividing the second LED string included in the second LED drive circuit  14  is set to 10 stages, 20 stages, 20 stages, 17 stages, and 23 stages. In this manner, in the LED illuminator  10 , the combination of the numbers of serial stages of the partial LED string in the first LED drive circuit  13  is set so as to differ from that in the second LED drive circuit  14 . 
     However, as illustrated in the first LED drive circuit  13  and the second LED drive circuit  14 , it is not necessary to considerably change the combination of serial stages of the partial LED string. For example, it may also be possible to set so that only the number of serial stages (20 stages) of the partial LED string  31   a  that lights up during the period of time during which the voltage is the lowest in the first LED drive circuit  13  differs from the number of serial stages (10 stages) of the partial LED string  41   a  that lights up during the period of time during which the voltage is the lowest in the second LED drive circuit  14 . 
     The resistor  35   a  or the like illustrated in  FIG. 2  is a single element, but for example, in the case where a gate protection resistor is inserted additionally between the left end of the resistor  35   a  and the FET  34   a , it may also be possible to integrate the gate protection resistor and the resistor  35   a  into one network resistor. The above-describe change can also be applied to all the other bypass circuits and constant current circuits. 
       FIG. 6  is a circuit diagram of another LED illuminator  50 . 
     The difference between the LED illuminator  50  illustrated in  FIG. 6  and the LED illuminator  10  illustrated in  FIG. 2  lies only in that a first LED drive circuit  53  included in the LED illuminator  50  differs from the first LED drive circuit  13  included in the LED illuminator  10 . The other configurations are the same as those of the LED illuminator  10 , and therefore, explanation thereof is omitted. 
     In the first LED drive circuit  53 , four partial LED strings  51   a ,  51   b ,  51   c , and  51   d  are connected in series. In each of the partial LED strings  51   a ,  51   b ,  51   c , and  51   d , a plurality of LEDs  53   a , a plurality of LEDs  53   b , a plurality of LEDs  53   c , and a plurality of LEDs  53   d  are connected in series, respectively. The LED string in which the partial LED strings  51   a ,  51   b ,  51   c , and  51   d  are connected in series corresponds to the first LED sting included in the first LED drive circuit  53 . 
     In the first LED drive circuit  53 , to the connection portion of the partial LED strings  51   a  and  51   b , to that of the partial LED strings  51   b  and  51   c , and to that of the partial LED strings  51   c  and  51   b , bypass circuits  52   a ,  52   b , and  52   c  are connected, respectively, and to the cathode of the partial LED string  51   d , a constant current circuit  52   d  is connected. The bypass circuits  52   a ,  52   b , and  52   c  and the constant current circuit  52   d  include depletion-type FETs  54   a ,  54   b ,  54   c , and  54   d , respectively, and resistors  55   a ,  55   b ,  55   c , and  55   d , respectively. The bypass circuits  52   a ,  52   b , and  52   c  and the constant current circuit  52   d  function as a switching circuit configured to switch the numbers of serial stages of LEDs included in the first LED string in accordance with the voltage of the full-wave rectified waveform. 
     In each of the bypass circuits  52   a ,  52   b , and  52   c  and the constant current circuit  52   d , the drain of each of the FETs  54   a ,  54   b ,  54   c , and  54   d  is the current input terminal, respectively, and the left terminal of each of the resistors  55   a ,  55   b ,  55   c , and  55   d  is the current output terminal, respectively. In each of the bypass circuits  52   a ,  52   b , and  52   c , the right terminal of each of the resistors  55   a ,  55   b , and  55   c  is the other current input terminal, respectively, and to each of the other current input terminals, the current output terminal of each of the bypass circuits  52   b  and  52   c  and the constant current circuit  52   d  is connected, respectively. 
     In the first LED drive circuit  53 , the number of serial stages of LEDs  53   a , that of serial stages of LEDs  53   b , that of serial stages of LEDs  53   c , and that of serial stages of LEDs  53   d  in each of the partial LED strings  51   a ,  51   b ,  51   c , and  51   d  are set to 20, 20, 20, and 30, respectively. In the second LED drive circuit  14 , the number of serial stages of LEDs  43   a , that of serial stages of LEDs  43   b , that of serial stages of LEDs  43   c , that of serial stages of LEDs  43   d , and that of serial stages of LEDs  43   e  in each of the partial LED strings  41   a ,  41   b ,  41   c ,  41   d , and  41   e  are set to 10, 20, 20, 17, and 23, respectively. Both the total number of serial stages of the first LED string and the total number of serial stages of the second LED string are 90 and equal. 
     The forward voltage of the LED is about 3 V and both the total numbers of the first and second LED strings are 90, and therefore, the voltage at which all the LEDs light up is about 270 V. That is, the first LED drive circuit  53  and the second LED drive circuit  14  are designed so as to adapt to the commercial AC power source the effective value of which is 240 V (maximum voltage is about 336 V). 
       FIG. 7A  is a waveform diagram illustrating a relationship between the full-wave rectified voltage waveform V 1  corresponding to one period and the time t in the LED illuminator  50 .  FIG. 7B  is a waveform diagram illustrating a relationship between a current I 51  that flows into the first LED drive circuit  53  and the time t.  FIG. 7C  is a waveform diagram illustrating a relationship between the current I 2  that flows into the second LED drive circuit  14  and the time t.  FIG. 7D  is a waveform diagram illustrating a relationship between a total current I 50  and the time t. The scale of the time axis is the same in  FIG. 7A  to  FIG. 7D .  FIG. 7A  illustrates the same waveform as that in  FIG. 3A  and  FIG. 7C  illustrates the same waveform as that in  FIG. 3C . 
     As illustrated in  FIG. 7B , for the full-wave rectified voltage waveform V 1  (see  FIG. 7A ), the current I 51  that flows through the first LED drive circuit  53  has five stages (including I 51 =0 (A)). Here, a period of time (t 11 ) during which the current I 51  has the current value I 15  is equal to the period of time, which is the sum of the period of time t 4 , the period of time t 5 , and the period of time t 6  in  FIG. 3B . The resistance of the resistor  55   d  is set to the same resistance of the resistor  35   e  in  FIG. 2  so that the maximum current of the LED illuminator  10  is equal to that of the LED illuminator  50 . The current I 50  that flows through the LED illuminator  50  illustrated in  FIG. 7D  is the sum of the current I 51  illustrated in  FIG. 7B  and the current I 2  illustrated in  FIG. 7C . 
     In the LED illuminator  50  also, the timing at which the current I 51  that flows through the first LED drive circuit  53  rises and the timing at which the current I 2  that flows through the second LED drive circuit  14  rises are set so to differ from each other. As a result of this, the current I 50  illustrated in  FIG. 7D  is the sum of the current I 51  in  FIG. 7B  and the current I 2  in  FIG. 7C , and the current I 50  increases/decreases at small steps except for the period of time t 11 . By increasing/decreasing the current I 50  at small steps in this manner, the total harmonic distortion is reduced. During the period of time t 11 , the current I 50 , which is a comparatively large current, is caused to flow through the entire first and second LED strings so as to improve luminance. 
     In the LED illuminator  10  described previously, the number of partial LED strings included in the first LED drive circuit  13  and the number of partial LED strings included in the second LED drive circuit  14  are set so as to be equal to each other (both, five). Further, in the LED illuminator  10 , the timing at which the numbers of partial LED strings included in the first LED drive circuit  13  are switched and the timing at which the numbers of partial LED strings included in the second LED drive circuit  14  are switched are set so as to differ from each other. As a result of this, it is made possible to suppress the occurrence of noise by changing the total current (I 0 ) flowing through the LED illuminator  10  at small steps. However, it is also possible to suppress the occurrence of noise by making the number of partial LED strings included in the first LED drive circuit  53  differ from the number of partial LED strings included in the second LED drive circuit  14  to change the total current (I 50 ) at small steps as in an LED illuminator  50 . 
       FIG. 8  is circuit diagram of the LED illuminator  60 , which is still another LED illuminator. 
     In the  FIG. 8 , the commercial AC power source  12  (see  FIG. 1 ) and the bridge rectifier circuit  11  (see  FIG. 1 ) included in the LED illuminator  60  are the same as those included in the LED illuminator  10  illustrated in  FIG. 1 , and therefore, they are not illustrated. As illustrated in  FIG. 8 , the LED illuminator  60  includes a first LED drive circuit  63  and a second LED drive circuit  64 . In the LED illuminator  60 , the same numerals are attached to the same configurations as those of the LED illuminator  10  illustrated in  FIG. 2 , and explanation thereof is omitted. 
     The first LED drive circuit  13  included in the LED illuminator  10  illustrated in  FIG. 2  has the configuration in which the circuit blocks including the partial LED string  31   a , the bypass circuit  32   a , etc., are connected in the form of a ladder. Each of the resistors  35   a  to  35   e  included in the first LED drive circuit  13  is a current detection resistor for feedback-controlling (setting the current constant) and cutting off each of the FETs  34   a  to  34   e , respectively (this also applies to the second LED drive circuit  14 ). In contrast to this, in each of the first LED drive circuit  63  and the second LED drive circuit  64  of the LED illuminator  60 , only one current detection resistor is provided and the FETs  34   a  to  34   e  are controlled only by divided voltages thereof. 
     As illustrated in  FIG. 8 , in the first LED drive circuit  63 , the sources of the FETs  34   a ,  34   b ,  34   c ,  34   d , and  34   e  are connected and are connected to the right terminal of an only current detection resistor  62 . In the first LED drive circuit  63 , the FETs  34   a  to  34   e  are controlled by the terminal-to-terminal voltage of the current detection resistor  62  or the divided voltages thereof. First, the resistance of the current detection resistor  62  is set to the same value (54Ω) as that of the resistor  35   a  (see  FIG. 2 ). Next, if the ratio of resistance between resistors  61   a ,  61   b ,  61   c ,  61   d , and  61   e  is set equal to that between the resistors  35   a ,  35   b ,  35   c ,  35   d , and  35   e  (see  FIG. 2 ), the first LED drive circuit  63  and the first LED drive circuit  13  perform substantially the same operation. Here, it is assumed that each of the resistors  61   a  to  61   e  has a sufficiently high resistance value. As indicated by a dot line  67 , the FETs  34   a ,  34   b ,  34   c ,  34   d , and  34   e , the resistors  61   a ,  61   b ,  61   c ,  61   d , and  61   e , and the current detection resistor  62  function as a switching circuit configured to switch the numbers of serial stages of LEDs included in the first LED string in accordance with the voltage of the full-wave rectified waveform. 
     As illustrated in  FIG. 8 , in the second LED drive circuit  64 , the sources of the FETs  44   a ,  44   b ,  44   c ,  44   d , and  44   e  are connected and are connected to the right terminal of an only current detection resistor  66 . In the second LED drive circuit  64 , the FETs  44   a  to  44   e  are controlled by the terminal-to-terminal voltage of the current detection resistor  66  or the divided voltages thereof. In the second LED drive circuit  64  also, first, the resistance of the current detection resistor  66  is set to the same value (54Ω) as that of the resistor  45   a  (see  FIG. 2 ). Next, if the ratio of resistance between resistors  65   a ,  65   b ,  65   c ,  65   e , and  65   e  is set equal to that between the resistors  45   a ,  45   b ,  45   c ,  45   d , and  45   e  (see  FIG. 2 ), the second LED drive circuit  64  and the second LED drive circuit  14  perform substantially the same operation. Here, it is assumed that each of the resistors  65   a  to  65   e  has a sufficiently high resistance value. As indicated by a dot line  68 , the FETs  44   a ,  44   b ,  44   c ,  44   d , and  44   e , the resistors  65   a ,  65   b ,  65   c ,  65   d , and  65   e , and the current detection resistor  66  function as a switching circuit configured to switch the numbers of serial stages of LEDs included in the second LED string in accordance with the voltage of the full-wave rectified waveform. 
     In the LED illuminator  60 , the transitional state where the first LED drive circuit  63  makes a transition from one constant current state into another constant current state is improved, and therefore, the luminance is improved more than in the LED illuminator  10  illustrated in  FIG. 2  (this is also true with the second LED drive circuit  64 ). 
     In the LED illuminator  60 , it is possible to increase the resistances of and downsize the resistors  61   a  to  61   e . Further, the resistors  61   a  to  61   e  are required only to be capable of stably reproducing the mutual ratio, and therefore, there is such an advantage that it is easy to configure as a network resistor by combining the resistors  61   a  to  61   e  with the current detection resistor  62  the resistance of which is comparatively low, and therefore, the permitted power of which needs to be increased (this is also true with the resistors  65   a  to  65   e  of the second LED drive circuit  64 ). Here, in the first LED drive circuit  13  included in the LED illuminator  10  illustrated in  FIG. 2 , a gain G 10  of the FET  34   e  during the transitional period from the period of time t 4  to the period of time t 5  is considered to be drain resistance Rd 10 /source resistance Rs 10  (R 35   a +R 35   b +R 35   c +R 35   d +R 35   e ) (“R 35   a ” represents the resistance value of the resistor  35   a . This also applied to the other resistors). Similarly, in the first LED drive circuit  63  included in the LED illuminator  60  illustrated in  FIG. 8 , a gain G 60  of the FET  34   e  during the transitional period from the period of time t 4  to the period of time t 5  is considered to be drain resistance Rd 60 /source resistance Rs 60  (R 62 ). The value of Rd 10  and that of Rd 60  are substantially the same and Rs 10 &gt;Rs 60 , and therefore, G 60 &gt;G 10  holds. That is, in the LED illuminator  60 , the gain G 60  of the FET  34   e  is larger, and therefore, the transitional response characteristics improve more than those in the LED illuminator  10 . 
       FIG. 9  is a circuit diagram of an LED illuminator  70 , which is still another LED illuminator. 
     In the LED illuminators  10 ,  50 , and  60  described previously, the numbers of serial stages of the first or second LED string are switched by detecting the current that flows through the first or second LED string. However, the switching of the numbers of serial stages of the first or second LED string is not limited to the method of detecting a current, and it is possible to employ a method of detecting a voltage. The LED illuminator  70  illustrated in  FIG. 9  includes first and second LED drive circuits  73  and  74  that switch the numbers of serial stages of the first and second LED strings by detecting a voltage of a full-wave rectified waveform. 
     In  FIG. 9 , the commercial AC power source  12  and the bridge rectifier circuit  11  are common to those in  FIG. 2 , however, a wire  75  is added, which transmits a signal obtained by reducing the voltage of a full-wave rectified waveform by resistors  71  and  72  in order to control the number of serial stages at a low voltage. In the LED illuminator  70 , the same numerals are attached to the same configurations as those of the LED illuminator  10  illustrated in  FIG. 2  and explanation thereof is omitted. 
     As illustrated in  FIG. 9 , in the first LED drive circuit  73 , three partial LED strings  81   a ,  81   b , and  81   c  are connected in series. In each of the partial LED strings  81   a ,  81   b , and  81   c , a plurality of LEDs  83   a , a plurality of LEDs  83   b , and a plurality of LEDs  83   c  are connected in series, respectively. The LED string in which the partial LED strings  81   a ,  81   b , and  81   c  are connected in series corresponds to the first LED string included in the first LED drive circuit  73 . 
     In the first LED drive circuit  73 , to the connection portion of the partial LED strings  81   a  and  81   b , and to that of the partial LED strings  81   b  and  81   c , a bypass circuit is connected, respectively, and to the cathode of the partial LED string  81   c , a constant current circuit is connected. The bypass circuit that is connected to the connection portion of the partial LED strings  81   a  and  81   b  includes a comparator  84   a , an AND element  85   a , an enhancement type FET  86   a , and a current limiting circuit  87   a . The bypass circuit that is connected to the connection portion of the partial LED strings  81   b  and  81   c  includes a comparator  84   b , an AND element  85   b , an enhancement type FET  86   b , and a current limiting circuit  87   b . The constant current circuit includes a comparator  84   c , an enhancement type FET  86   c , and a current limiting circuit  87   c . To each plus input terminal of the comparators  84   a  to  84   c , the wire  75  is connected and to the minus input terminals, reference voltages Vref 1 , Vref 2 , and Vref 3  are input respectively, which are output from a reference voltage generation circuit  88 . As illustrated by a dot line  76 , the comparators  84   a  to  84   c , the AND elements  85   a  and  85   b , the FETs  86   a  to  86   c , the current limiting circuits  87   a  to  87   c , and the reference voltage generation circuit  88  function as a switching circuit configured to switch the numbers of serial stages of LEDs included in the first LED string in accordance with the voltage of the full-wave rectified waveform. 
     As illustrated in  FIG. 9 , in the second LED drive circuit  74 , three partial LED strings  91   a ,  91   b , and  91   c  are connected in series. In each of the partial LED strings  91   a ,  91   b , and  91   c , a plurality of LEDs  93   a , a plurality of LEDs  93   b , and a plurality of LEDs  93   c  are connected in series, respectively. The LED string in which the partial LED strings  91   a ,  91   b , and  91   c  are connected in series corresponds to the second LED string included in the second LED drive circuit  74 . 
     In the second LED drive circuit  74 , to the connection portion of the partial LED strings  91   a  and  91   b , and to that of the partial LED strings  91   b  and  91   c , a bypass circuit is connected, respectively, and to the cathode of the partial LED string  91   c , a constant current circuit is connected. The bypass circuit that is connected to the connection portion of the partial LED strings  91   a  and  91   b  includes a comparator  94   a , an AND element  95   a , an enhancement type FET  96   a , and a current limiting circuit  97   a . The bypass circuit that is connected to the connection portion of the partial LED strings  91   b  and  91   c  includes a comparator  94   b , an AND element  95   b , an enhancement type FET  96   b , and a current limiting circuit  97   b . The constant current circuit includes a comparator  94   c , an enhancement type FET  96   c , and a current limiting circuit  97   c . To each plus input terminal of the comparators  94   a  to  94   c , the wire  75  is connected and to the minus input terminals, reference voltages Vref 4 , Vref 5 , and Vref 6  are input, respectively, which are output from a reference voltage generation circuit  98 . As illustrated by a dot line  77 , the comparators  94   a  to  94   c , the AND elements  95   a  and  95   b , the FETs  96   a  to  96   c , the current limiting circuits  97   a  to  97   c , and the reference voltage generation circuit  98  function as a switching circuit configured to switch the numbers of serial stages of LEDs included in the second LED string in accordance with the voltage of the full-wave rectified waveform. 
     The maximum number of serial stages of the first and second LED strings included in the first and second LED drive circuits  73  and  74  is 90 as in the first and second LED drive circuits  13  and  14  illustrated in  FIG. 2 . The number of serial stages of the partial LED strings  81   a  to  81   c  and the number of serial stages of the partial LED strings  91   a  to  91   c  are determined based on the reference voltages Vref 1  to Vref 3  and the reference voltages Vref 4  to Vref 6 , respectively, as will be described later. For example, it may also be possible to set all the numbers of stages to the same (30 stages). The upper limit current of the current limiting circuit  87   a  and that of the current limiting circuit  97   a  are set equal, the upper limit current of the current limiting circuit  87   b  and that of the current limiting circuit  97   b  are also set equal, and the upper limit current of the current limiting circuit  87   c  and that of the current limiting circuit  97   c  are also set equal. The upper limit current of the current limiting circuits  87   a  and  97   a  is set to the smallest value, the upper limit current of the current limiting circuits  87   b  and  97   b  is set to an intermediate value, and the upper limit current of the current limiting circuits  87   c  and  97   c  is set to the largest value. 
     The reference voltages Vref 1  to Vref 6  are set so as to have a relationship below.
         Vref 1 &lt;Vref 4 &lt;Vref 2 &lt;Vref 5 &lt;Vref 3 &lt;Vref 6         

       FIG. 10A  is a waveform diagram illustrating a relationship between the full-wave rectified voltage waveform V 1  corresponding to one period and the time t in the LED illuminator  70 .  FIG. 10B  is a waveform diagram illustrating a relationship between a current I 71  that flows into the first LED drive circuit  73  and the time t.  FIG. 10C  is a waveform diagram illustrating a relationship between a current I 72  that flows into the second LED drive circuit  74  and the time t.  FIG. 10D  is a waveform diagram illustrating a relationship between a total current I 70  and the time t. The scale of the time axis is the same in  FIG. 10A  to  FIG. 10D . Further, the waveform in  FIG. 10A  is the same as that in  FIG. 3A . 
     By using  FIG. 10A  and  FIG. 10B , the operation of the first LED drive circuit  73  is explained. A period of time t 20  is a period of time during which the full-wave rectified voltage waveform V 1  is smaller than the reference voltage Vref 1 . During the period of time t 20 , the outputs of the comparators  84   a  to  84   c  are at the low level, and therefore, the FETs  86   a  to  86   c  turn off and the current I 71  does not flow. 
     A period of time t 21  is a period of time during which the full-wave rectified voltage waveform V 1  is between the reference voltage Vref 1  and the reference voltage Vref 2 , and the output of the AND element  85   a  turns to the high level, the FET  86   a  turns on, and a current flows through the current limiting circuit  87   a , the magnitude of which is the same as that of the upper limit current thereof. 
     A period of time t 22  is a period of time during which the full-wave rectified voltage waveform V 1  is between the reference voltage Vref 2  and the reference voltage Vref 3 . Through the current limiting circuit  87   b , a current which is the same as the upper limit current thereof flows. 
     A period of time t 23  is a period of time during which the full-wave rectified voltage waveform V 1  is larger than or equal to the reference voltage Vref 3  and a current flows through the current limiting circuit  87   c , the magnitude of which is the same as that of the upper limit current thereof. During periods of time (period of time t 24  to period of time t 26 ) during which the full-wave rectified voltage waveform V 1  decreases, the first LED drive circuit  73  follows the processes in the order opposite to that when the full-wave rectified voltage waveform V 1  increases. 
     Through the second LED drive circuit  74  also, the current I 72  having three levels flows. However, the reference voltages Vref 4  to Vref 6  are different from the reference voltages Vref 1  to Vref 3 , respectively, and therefore, the timing at which the current I 72  rises is set so as to differ from the timing at which the current I 71  rises. 
     In the partial LED string  81   a , the number of LEDs (number of stages) is set so that it is possible to cause the current I 71  to flow sufficiently at the timing determined by the reference voltage Vref 1  and in the partial LED string  91   a  also, the number of LEDs (number of stages) is set so that it is possible to cause the current I 72  to flow sufficiently at the timing determined by the reference voltage Vref 4 . In the partial LED string  81   b , the number of LEDs (number of stages) is set so that it is possible to cause the current I 71  to flow sufficiently at the timing determined by the reference voltage Vref 2  and in the partial LED string  91   b  also, the number of LEDs (number of stages) is set so that it is possible to cause the current I 72  to flow sufficiently at the timing determined by the reference voltage Vref 5 . In the partial LED string  81   c , the number of LEDs (number of stages) is set so that it is possible to cause the current I 71  to flow sufficiently at the timing determined by the reference voltage Vref 3  and in the partial LED string  91   c  also, the number of LEDs (number of stages) is set so that it is possible to cause the current I 72  to flow sufficiently at the timing determined by the reference voltage Vref 6 . 
     The current I 70  illustrated in  FIG. 10D  is the sum of the current I 71  in  FIG. 10B  and the current I 72  in  FIG. 10C  and the current I 70  increases/decreases at small steps in accordance with the increase/decrease in the full-wave rectified voltage waveform V 1 . By causing the current I 70  to increase/decrease at small steps as described above, the total harmonic distortion is reduced. 
     In the LED illuminator  70  illustrated in  FIG. 9 , it is possible to connect more LED drive circuits other than the first and second LED drive circuits  73  and  74  to the bridge rectifier circuit  11  in parallel to the first and second LED drive circuits  73  and  74 . By making the switching timing of the number of serial stages of the added LED drive circuit differ from the switching timing of the number of serial stages of the first and second LED drive circuits  73  and  74 , it is possible to cause the current I 70  to increase/decrease at smaller steps. 
     In the LED illuminator  70 , both the number of partial LED strings included in the first LED drive circuit  73  and the number of partial LED strings included in the second LED drive circuit  74  are set to three, but the number is not limited to this and may be set to another number. Further, the number of LEDs included in each partial LED string and the total number of LEDs included in all the LED strings are not limited to the above-described numbers and it is possible to appropriately select the numbers in accordance with the effective value or the like of the commercial AC power source that is made use of. 
     In the LED illuminators  10 ,  50 ,  60 , and  70  described above, it is important for the timing at which the numbers of partial LED strings that emit light in each LED string switch to differ from one another. It is possible to adjust the timing at which the numbers of partial LED strings that emit light in each LED string switch by changing the number of LEDs (number of stages) included in the partial LED string and the number of partial LED strings. 
     Further, it is also possible to adjust the timing at which the numbers of partial LED strings that emit light in each LED string switch by changing the method of detecting the value of a current that flows through each partial LED string. For example, by making the value of the resistor  35   a  differ from that of the resistor  45   a  in  FIG. 2 , it is possible to adjust the timing at which the partial LED string  31   a  emits light and the timing at which the partial LED string  41   a  emits light. Further, it is also possible to adjust the timing at which the numbers of partial LED strings that emit light in each LED string switch by changing the method of detecting the voltage of the full-wave rectified waveform. 
     In the LED illuminators  10 ,  50 ,  60 , and  70  described above, the first LED string (LEDs  33   a  to  33   e , etc.) and the second LED string (LEDs  43   a  to  43   e , etc.) are connected in parallel to the one bridge rectifier circuit  11 . However, the LED illuminator is not limited to the case where the first LED string and the second LED string are connected in parallel to one bridge rectifier circuit. For example, it may also be possible to connect a first bridge rectifier circuit and a second bridge rectifier circuit in parallel to the commercial AC power source  12  (see  FIG. 2 ), and to connect the first LED string to the first bridge rectifier circuit and to connect the second LED string to the second bridge rectifier circuit. 
     EXPLANATION OF LETTERS OR NUMERALS 
     
         
         
           
               10 ,  50 ,  60 ,  70  LED illuminator 
               11  bridge rectifier circuit 
               12  commercial AC power source 
               13 ,  53 ,  63 ,  73  first LED drive circuit 
               14 ,  64 ,  74  second LED drive circuit 
               31   a  to  31   e ,  41   a  to  41   e ,  51   a  to  51   d ,  81   a  to  81   c ,  91   a  to  91   c  partial LED string 
               32   a  to  32   d ,  42   a  to  42   d ,  52   a  to  52   c  bypass circuit 
               32   e ,  42   e ,  52   d  constant current circuit 
               33   a  to  33   e ,  43   a  to  43   e ,  53   a  to  53   d ,  83   a  to  83   c ,  93   a  to  93   c  LED 
               34   a  to  34   e ,  44   a  to  44   e ,  54   a  to  54   d  FET (depletion type) 
               35   a  to  35   e ,  45   a  to  45   e ,  55   a  to  55   d ,  61   a  to  61   e ,  65   a  to  65   e ,  71 ,  72  resistor 
               62 ,  66  current detection resistor 
               84   a  to  84   c ,  94   a  to  94   c  comparator 
               85   a ,  85   b ,  95   a ,  95   b  AND element 
               86   a  to  86   c ,  96   a  to  96   c  FET (enhancement type) 
               87   a  to  87   c ,  97   a  to  97   c  current limiting circuit 
               88 ,  98  reference voltage generation circuit