Patent Publication Number: US-9414451-B2

Title: Lighting device and luminaire

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims the benefit of priorities of Japanese Patent Application Nos. 2013-261624, filed on Dec. 18, 2013 and 2013-262717, filed on Dec. 19, 2013, the entire contents of which are hereby incorporated by reference. 
     TECHNICAL FIELD 
     The present invention relates to a lighting device of a solid-state light-emitting element such as an LED (light-emitting diode), and a luminaire having the lighting device. 
     BACKGROUND ART 
     A solid-state light-emitting element such as an LED is attracting attention as a light source for a variety of products since it is smaller, more efficient, and lasts longer. 
     Examples of products using LEDs as a light source include a luminaire. The number of LEDs used in a luminaire is determined based on a desired brightness. Typically, a number of LEDs are used for a single luminaire. When a number of LEDs are used in a luminaire, the LEDs may be connected in series to one another. In this arrangement, the same current is supplied to the LEDs, and accordingly unevenness in brightness of the LEDs can be suppressed. 
     For the arrangement in which LEDs are connected in series to one another, if one of the LEDs has an open-circuit failure, current supply is stopped for all of the LEDs, so that the other normal LEDs are not lit as well. In order to address this problem, a technique is known, in which a bypass circuit is connected in parallel to each of the LEDs, and the bypass circuit is turned on when an open-circuit failure occurs in the corresponding LED to thereby supply current to the other normal solid-state light-emitting elements (see, e.g., Japanese Unexamined Patent Application Publication Nos. 2005-310999, 2008-204866, 2003-208993, and 2009-038247). 
     For such a luminaire, however, excessive current may flow in the other normal LEDs when the bypass circuit is operated. As a result, the normal LEDs may deteriorate or fail. 
     For example, in the disclosure of Japanese Unexamined Patent Application Publication No. 2009-038247, a bypass circuit is connected in parallel to each of LEDs connected in series, and if an increase in the voltage across an LED having an open-circuit failure is detected, a bypass switch in a corresponding bypass circuit is turned on. In this instance, however, immediately after the bypass switch is turned on, excessive current flows in the other LEDs having no open-circuit failure and in the corresponding bypass circuit. Therefore, in the above disclosure, normal LEDs may deteriorate or fail. In order to prevent the LEDs from deteriorating or failing, the LEDs or the like need to be robust to stress due to such excessive current, causing the cost and size to be increased. 
     Hereinafter, such a problem will be described in more detail with reference to  FIGS. 1A and 1B  and  FIG. 2 . 
       FIG. 1A  is a circuit diagram of a luminaire having bypass circuits. The luminaire shown in  FIG. 1A  includes: light-emitting elements  103   a  and  103   b  connected in series; a bypass circuit  104   a  connected in parallel to the light-emitting element  103   a ; a bypass circuit  104   b  connected in parallel to the light-emitting element  103   b ; a constant-current circuit  101  for supplying constant current to the light-emitting elements  103   a  and  103   b ; and a smoothing capacitor  102  connected between output terminals of the constant-current circuit  101 . The light-emitting elements  103   a  and  103   b  are, e.g., LEDs. 
     In this luminaire, if the light-emitting element  103   b  has an open-circuit failure, the bypass circuit  104   b  is turned on as shown in  FIG. 1B . By doing so, current is supplied to the light-emitting element  103   a . As such, the luminaire can prevent that all of the light-emitting elements are lit out when one of them has an open-circuit failure. 
     Further, in this luminaire, the output voltage VC from the constant-current circuit  101  is monitored, for example, and it is detected that the light-emitting element  103  or  103   b  has an open-circuit failure if the voltage VC rises above a predetermined voltage. 
     In this regard, the present inventors have found out that such a luminaire has the following problem. 
       FIG. 2  shows graphs of the voltage VC versus time and a current I flowing in the normal light-emitting element  103   a  versus time, in the case where an open-circuit failure occurs. 
     Before time t 1  at which an open-circuit failure occurs, the voltage VC is equal to the sum of forward voltages of the two light-emitting elements  103   a  and  103   b  (2×Vf). When an open-circuit failure occurs at time t 1 , no current flows in the normal light-emitting element  103   a  and the voltage VC rises. At time t 2 , the voltage VC rises above a predetermined voltage (i.e., VC&gt;2×Vf). Accordingly, the bypass circuit  104   b  is turned on. 
     As the bypass circuit  104   b  is turned on, the voltage VC decreases up to a voltage equal to the forward voltage Vf of the normal light-emitting element  103   a . However, at the moment when the bypass circuit  104   b  is turned on, the voltage VC is higher than the voltage 2×Vf, and electric charges corresponding to this voltage have been accumulated in the smoothing capacitor  102 . Therefore, at the moment when the bypass circuit  104   b  is turned on, electric charges accumulated in the smoothing capacitor  102 , which correspond to a difference voltage (&gt;Vf) between the voltage (&gt;2×Vf) and the forward voltage Vf (i.e., electric charges which correspond to the forward voltage Vf of the light-emitting element  103   b  having the open-circuit failure) flow in the normal light-emitting element  103   a  at a burst (from time t 2  to time t 3 ). 
     As such, excessive current may flow in the normal light-emitting element  103   a  so that the normal light-emitting element  103   a  may deteriorate or break down. In addition, when excessive current flows in the light-emitting element  103   a , the bypass circuit  104   a  may be erroneously turned on. 
     In order to suppress excessive current from flowing in the normal light-emitting element  103   a , the bypass circuit  104   b  having a forward voltage equal to the forward voltage of the light-emitting element  103   b  may be provided. However, this approach may cause another problem in that the bypass circuit  104   b  has more power loss. 
     As a technology to suppress such excessive current, there is known a technique in which a voltage drop unit is provided in a bypass circuit (see, e.g., International Publication No. WO 2012/005239). According to this reference, a resistor is provided in a bypass circuit as a voltage drop unit, so that it reduces current flowing immediately after a bypass switch in the bypass circuit is turned on, thereby suppressing stress exerted on LEDs or the like. 
     In this approach, however, the power loss is continuously generated by the voltage drop unit after connecting two ends of the LED having the open-circuit failure. 
     SUMMARY OF THE INVENTION 
     In view of the above, the present invention provides a lighting device, with solid-state light-emitting elements connected in series and bypass circuits, capable of suppressing excessive current from flowing in normal light-emitting elements at the moment when a bypass circuit is turned on. 
     In accordance with an aspect of the present invention, there is provided a lighting device including: a constant-current circuit configured to supply a constant current to a plurality of solid-state light-emitting elements connected in series; a smoothing capacitor connected between output terminals of the constant-current circuit; a bypass circuit connected in parallel to one or more of the plurality of solid-state light-emitting elements, the bypass circuit configured to bypass the one or more solid-state light-emitting elements; a detection unit configured to detect whether the one or more solid-state light-emitting elements are open-circuited; and a bypass control unit configured to, when the detection unit detects that at least one of the one or more solid-state light-emitting elements is open-circuited, discharge the smoothing capacitor during a discharge period to then bypass the one or more solid-state light-emitting elements through the bypass circuit. 
     Further, during the discharge period, the smoothing capacitor may be discharged until a voltage across the smoothing capacitor becomes smaller than a sum of forward voltages of the plurality of solid-state light-emitting elements. 
     Further, during the discharge period, the smoothing capacitor may be discharged until the voltage across the smoothing capacitor becomes smaller than a sum of forward voltages of other solid-state light-emitting elements than the one or more solid-state light-emitting elements among the plurality of solid-state light-emitting elements. 
     Further, during the discharge period, the bypass control unit may stop the constant-current circuit or may reduce a value of the constant current supplied from the constant-current circuit. 
     Further, the lighting device may further include a discharge circuit connected in parallel to the smoothing capacitor, wherein, during the discharge period, the bypass control unit may turn on the discharge circuit to discharge the smoothing capacitor. 
     Further, the bypass control unit may include a comparator to compare a voltage across the smoothing capacitor with a predetermined reference voltage, and the bypass control unit may terminate the discharge period when the voltage across the smoothing capacitor becomes lower than the reference voltage, and may bypass the one or more solid-state light-emitting elements through the bypass circuit. 
     Further, after the detection unit detects that said at least one of the one or more solid-state light-emitting elements is open-circuited, the bypass control unit may terminate the discharge period after a predetermined time period has elapsed and may bypass the one or more solid-state light-emitting elements through the bypass circuit. 
     Further, the discharge period may be longer than a time constant of a discharge path through which the smoothing capacitor is discharged. 
     Further, the constant-current circuit may be a DC-to-DC converter that is supplied with a current from a DC power source, and the constant-current circuit may include: a switching element; an inductor through which the current from the DC power source flows when the switching element is turned on; a diode through which a current discharged from the inductor is supplied to the plurality of solid-state light-emitting elements; and a control unit for controlling on and off of the switching element. 
     In accordance with another aspect of the present invention, there is provided a lighting device including: a constant-current circuit configured to supply a constant current to a plurality of solid-state light-emitting elements connected in series; a capacitor circuit connected in parallel to one or more of the plurality of solid-state light-emitting elements, the capacitor circuit including a capacitor; a bypass switch circuit connected in parallel to the one or more solid-state light-emitting elements and to the capacitor circuit, the bypass switch circuit including a bypass switch; and a current detection unit configured to measure a current flowing through the capacitor, wherein the current detection unit turns on the bypass switch when the measured current exceeds a predetermined threshold. 
     Further, the capacitor circuit may further include a resistor connected in series to the capacitor, and the current detection unit may measure the current based on a voltage across the resistor. 
     Further, the current detection unit may include a resistor-capacitor filter to attenuate high-frequency components in the current. 
     Further, the bypass switch circuit may further include an impedance element connected in series to the bypass switch. 
     Further, the constant-current circuit may be a DC-to-DC converter that is supplied with current from a DC power source, and the constant-current circuit may include: a switching element; a control circuit that outputs a signal to control on and off of the switching element; an inductive element through which the current from the DC power source flows when the switching element is turned on; and a diode through which a current discharged from the inductive element is supplied to the plurality of solid-state light-emitting elements. 
     Further, the current detection unit may detect a DC component in the current flowing through the capacitor. 
     Further, the constant-current circuit may be driven in a boundary current mode, and the predetermine threshold may be larger than a value of the constant current supplied from the constant-current circuit and may be equal to or less than two times the value. 
     In accordance with yet another aspect of the present invention, there is provided a luminaire including: the lighting device described above; and the plurality of solid-state light-emitting elements that receive the constant current from the lighting device. 
     In accordance with the aspects of the present invention, in a lighting device with solid-state light-emitting elements connected in series and bypass circuits, the lighting device can suppress excessive current from flowing in normal light-emitting elements at the moment when a bypass circuit is turned on. 
     Accordingly, it is possible to prevent the normal light-emitting elements from deteriorating or failing. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The objects and features of the present invention will become apparent from the following description of embodiments, given in conjunction with the accompanying drawings, in which: 
         FIG. 1A  is a circuit diagram of a luminaire having bypass circuits; 
         FIG. 1B  is a circuit diagram showing an operation example of a luminaire having bypass circuits; 
         FIG. 2  is a timing chart showing a voltage and a current when a bypass circuit operates; 
         FIG. 3  is a schematic circuit diagram of a lighting device according to a first embodiment; 
         FIG. 4  is a circuit diagram showing a detailed configuration example of the lighting device according to the first embodiment; 
         FIG. 5  is a circuit diagram showing a configuration example of a bypass control unit according to the first embodiment; 
         FIG. 6  is a timing chart of the lighting device according to the first embodiment; 
         FIG. 7  is a circuit diagram showing a configuration example of a lighting device according to a second embodiment; 
         FIG. 8  is a circuit diagram showing a configuration example of a bypass control unit according to the second embodiment; 
         FIG. 9  is a timing chart of the lighting device according to the second embodiment; 
         FIG. 10  is a circuit diagram showing a configuration example of a lighting device according to a modification of the second embodiment; 
         FIG. 11  is a circuit diagram showing a configuration example of a bypass control unit according to the modification of the second embodiment; 
         FIG. 12  is a circuit diagram showing a configuration example of a lighting device according to a third embodiment; 
         FIG. 13  is a circuit diagram showing a configuration example of a bypass control unit according to the third embodiment; 
         FIG. 14  is a timing chart of the lighting device according to the third embodiment; 
         FIG. 15A  is a circuit diagram showing a configuration example of a timer according to the third embodiment; 
         FIG. 15B  is a timing chart of the timer according to the third embodiment; 
         FIG. 16  is a circuit diagram showing a configuration example of a lighting device according to a fourth embodiment; 
         FIG. 17A  is a flowchart for illustrating processes by in an MCU according to the fourth embodiment; 
         FIG. 17B  is a flowchart for illustrating processes by in an MCU according to a modification of the fourth embodiment; 
         FIG. 18  is a circuit diagram showing a configuration example of light-emitting elements according to a modification of the embodiments; 
         FIG. 19  is a circuit diagram showing a configuration example of a constant-current circuit according to the exemplary embodiments; 
         FIG. 20  is a circuit diagram showing a configuration example of a control unit according to the embodiments; 
         FIG. 21  is a circuit diagram showing another configuration example of a constant-current circuit according to the embodiments; 
         FIG. 22  is a circuit diagram showing another configuration example of a constant-current circuit according to the embodiments; 
         FIG. 23  is a circuit diagram showing another configuration example of a constant-current circuit according to the embodiments; 
         FIG. 24  is a circuit diagram of a lighting device  1   a  according to a fifth embodiment; 
         FIG. 25  shows waveforms of current and voltage of elements in the lighting device  1   a  according to the fifth embodiment; 
         FIG. 26  shows enlarged waveforms of current and voltage of elements in the lighting device  1   a  according to the fifth embodiment; 
         FIG. 27  shows enlarged waveforms of current and voltage of elements in the lighting device  1   a  according to the fifth embodiment; 
         FIG. 28  is a circuit diagram of a lighting device  1   b  according to a sixth embodiment; 
         FIG. 29  shows voltage waveforms of elements in the lighting device  1   b  according to the sixth embodiment; 
         FIG. 30  is a circuit diagram of a lighting device  1   c  according to a seventh embodiment; 
         FIG. 31  shows current waveforms of elements in the lighting device  1   a  according to the fifth embodiment and the lighting device  1   c  according to the seventh embodiment; 
         FIG. 32  is a circuit diagram of a lighting device  1   d  according to an eighth embodiment; 
         FIG. 33  is a circuit diagram of a lighting device  1   e  according to a ninth embodiment; 
         FIG. 34  is an external view of a luminaire according to a tenth embodiment. 
         FIG. 35  is an external view of a luminaire according to the tenth embodiment; and 
         FIG. 36  is an external view of a luminaire according to the tenth embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings. In the following descriptions, embodiments to be described below are all to provide preferable examples of the present invention. Therefore, the numerical values, shapes, materials, elements, arrangement of elements, connection manner and the like are merely illustrative but are not limited to those to be suggested in the following embodiments. Accordingly, among the elements described in the embodiments, those not recited in the broadest independent claims are meant to be selective elements. In addition, the drawings are schematic views and are not strictly depicted. 
     First Embodiment 
     According to the first embodiment, when an open-circuit failure has occurred, a luminaire releases electric charges accumulated in a smoothing capacitor and then turns on a bypass circuit. Specifically, the luminaire releases electric charges accumulated in the smoothing capacitor by interrupting a constant-current circuit for a predetermined time period after the open-circuit failure has occurred. By doing so, it is possible to suppress excessive current flowing in normal light-emitting elements when the bypass circuit is turned on. 
       FIG. 3  is a circuit diagram of a lighting device  210   a  according to the first embodiment of the present invention. 
     The lighting device  210   a  lights solid-state light-emitting elements connected in series to each other, e.g., LEDs  202   a  and  202   b , by using power from a commercial power source  201 . The lighting device  210   a  includes a DC power source  211 , a constant-current circuit  212 , a smoothing capacitor  213 , a detection circuits  214   a  and  214   b , bypass circuits  215   a  and  215   b , and a bypass control unit  216   a.    
     The DC power source  211  is a circuit to convert AC power supplied from the commercial power source  201  into DC power, e.g., an AC-to-DC converter. 
     The constant-current circuit  212  is a circuit to generate a constant current by using DC power supplied from the DC power source  211 , e.g., a DC-to-DC converter. The constant current generated in the constant-current circuit  212  is supplied to the LEDs  202   a  and  202   b.    
     The smoothing capacitor  213  is connected between output terminals of the constant-current circuit  212 . The smoothing capacitor  213  is a capacitive element to smoothen the constant current generated by the constant-current circuit  212 . Although the smoothing capacitor  213  is disposed outside the constant-current circuit  212  in  FIG. 3 , it may be incorporated in the constant-current circuit  212 . 
     The detection circuit  214   a  detects whether the LED  202   a  is open-circuited. In other words, the detection circuit  214   a  detects whether the LED  202   a  has an open-circuit failure. Likewise, the detection circuit  214   b  detects whether the LED  202   b  is open-circuited, i.e., whether the LED  202   b  has an open-circuit failure. 
     The bypass circuit  215   a  is connected in parallel to the LED  202   a  and is for bypassing the LED  202   a . For example, the bypass circuit  215   a  includes a switching element connected in parallel to the LED  202   a . When the bypass circuit  215   a  is turned on, two ends of the LED  202   a  are short-circuited. 
     Likewise, the bypass circuit  215   b  is connected in parallel to the LED  202   b  and is for bypassing the LED  202   b . For example, the bypass circuit  215   b  includes a switching element connected in parallel to the LED  202   b . When the bypass circuit  215   b  is turned on, two ends of the LED  202   b  are short-circuited. 
     The bypass control unit  216   a  controls the bypass circuits  215   a  and  215   b  and the constant-current circuit  212  based on the results detected by the detection circuits  214   a  and  214   b . Specifically, the bypass control unit  216   a  turns on the bypass circuit  215   a  if the detection circuit  214   a  detects an open-circuit failure in the LED  202   a . Further, the bypass control unit  216   a  turns on the bypass circuit  215   b  if the detection circuit  214   b  detects an open-circuit failure in the LED  202   b . Furthermore, if an open-circuit failure has detected, the bypass control unit  216   a  interrupts the constant-current circuit  212  for a predetermined discharge period, and then turns on the bypass circuit  215   a  or  215   b . By doing so, electric charges accumulated in the smoothing capacitor  213  are released during the discharge period. 
       FIG. 4  is a diagram of example circuits of the detection circuits  214   a  and  214   b  and the bypass circuits  215   a  and  215   b.    
     The detection circuit  214   a  detects whether a voltage difference V 1  across the LED  202   a  rise above a predetermined voltage Vf_max, and outputs a failure detection signal LED 1  indicating a result of the detection. The voltage Vf_max is equal to the maximum of the forward voltage of the LEDs  202   a  and  202   b , for example. 
     The detection circuit  214   a  includes voltage-dividing resistors R 1   a  and R 1   b , a zener diode D 1 , and a photo-coupler PC 1 . The voltage-dividing resistors R 1   a  and R 1   b  generate a voltage V 1   a  by dividing the voltage V 1 . If the voltage V 1   a  rises above a voltage Vf_max_a corresponding to the voltage Vf_max, the zener diode D 1  is turned on. Accordingly, current flows in the photo-coupler PC 1  so that the level of the failure detection signal LED 1  is changed to be low. 
     Likewise, the detection circuit  214   b  detects whether a voltage difference V 2  across the LED  202   b  rises above the predetermined voltage Vf_max, and outputs a failure detection signal LED 2  indicating a result of the detection. The detection circuit  214   b  includes voltage-dividing resistors R 2   a  and R 2   b , a zener diode D 2 , and a photo-coupler PC 2 . The voltage-dividing resistors R 2   a  and R 2   b  generate a voltage V 2   a  by dividing the voltage V 2 . If the voltage V 2   a  rises above the voltage Vf_max_a corresponding to the voltage Vf_max, the zener diode D 2  is turned on. Accordingly, current flows in the photo-coupler PC 2  so that the level of the failure detection signal LED 2  is changed to be low. 
     The bypass circuit  215   a  includes a photo MOS relay PMR 1 . The photo MOS relay PMR 1  is turned on if the level of a bypass control signal B 1  is high. Likewise, the bypass circuit  215   b  includes a photo MOS relay PMR 2 . The photo MOS relay PMR 2  is turned on if the level of a bypass control signal B 2  is high. 
       FIG. 5  shows an example of a circuit diagram of the bypass control unit  216   a . As shown in  FIG. 5 , the bypass control unit  216   a  includes flip-flops FF 0 , FF 1 A, FF 1 B, FF 2 A and FF 2 B, and a comparator COM 0 . 
     The comparator COM 0  compares a voltage VCa, obtained by dividing the voltage VC, with a reference voltage Vf_min_a corresponding to a reference voltage Vf_min. 
     The flip-flop FF 0  outputs a stop control signal DC/DC_enable of low level when the level of the failure detection signal LED 1  or LED 2  becomes low. In addition, the flip-flop FF 0  outputs a stop control signal DC/DC_enable of high level in response to an output signal from the comparator COM 0  when the voltage VCa becomes lower than the reference voltage Vf_min_a. 
     After the level of the failure detection signal LED 1  has become low, the flip-flop FF 1 B outputs a bypass control signal B 1  of high level in response to an output signal from the comparator COM 0  when the voltage VCa becomes lower than the reference voltage Vf_min_a. After the level of the failure detection signal LED 2  has become low, the flip-flop FF 2 B outputs a bypass control signal B 2  of high level in response to an output signal from the comparator COM 0  when the voltage VCa becomes lower than the reference voltage Vf_min_a. 
       FIG. 6  is a timing chart when the LED  202   a  has an open-circuit failure. Hereinafter, operations when the LED  202   a  has an open-circuit failure will be described. 
     Before time t 1  at which the open-circuit failure occurs, the voltage V 1  across the LED  202   a  is equal to the forward voltage Vf of the LED  202   a . In addition, the voltage VC (=V 1 +V 2 ) is equal to the sum (2×Vf) of the forward voltages Vf of the LEDs  202   a  and  202   b.    
     At time t 1 , the open-circuit failure occurs in the LED  202   a . At this time, the constant-current circuit  212  keeps supplying current, and thus the voltage VC increases. In addition, the voltage V 2  across the normal LED  202   b  does not increase any further once it has reached the forward voltage Vf, and thus the voltage V 2  stays at the forward voltage Vf. Accordingly, the voltage V 1  increases as the voltage VC increases. As the voltage V 1  increases, so does the voltage V 1   a  that is obtained by dividing the voltage V 1 . 
     At time t 2 , when the voltage V 1   a  reaches the voltage Vf_max_a (when the voltage V 1  reaches the voltage Vf_max), the zener diode D 1  is turned on. Accordingly, current flows in the photo-coupler PC 1  so that the photo-coupler PC 1  is turned on. As a result, the level of the failure detection signal LED 1  becomes low, so that the open-circuit failure in the LED  202   a  is detected. 
     When the open-circuit failure is detected, a high-level signal is inputted to the set terminal of the flip-flop FF 0 . Accordingly, the level of the stop control signal DC/DC_enable becomes low. As the stop control signal DC/DC_enable becomes low, the constant-current circuit  212  stops its operation. 
     As the constant-current circuit  212  stops its operation, electric charges accumulated in the smoothing capacitor  213  are released through, e.g., the resistors R 2   a , R 2   b , R 1   a  and R 1   b . Accordingly, the voltage VC decreases. 
     At time t 3 , if the voltage VC becomes lower than the voltage Vf_min, the level of the stop control signal DC/DC_enable becomes high. Specifically, if the voltage VC decreases, so does the voltage VCa that is inputted to the comparator COM 0 . Then, if the voltage VCa becomes lower than the voltage Vf_min_a corresponding to the voltage Vf_min, the level of the output signal from the comparator COM 0  becomes high. Accordingly, the level of the stop control signal DC/DC_enable becomes high. 
     As the level of the stop control signal DC/DC_enable becomes high, the constant-current circuit  212  starts its operation. 
     In addition, as the level of the bypass control signal B 1  becomes high, the bypass circuit  215   a  is turned on. Specifically, a high-level signal is inputted to the set terminal of the flip-flop FF 1 B. Accordingly, the level of the bypass control signal B 1  becomes high, and thus the photo MOS relay PMR 1  is turned on. 
     If the constant-current circuit  212  starts its operation, the voltage VC increases. At time t 4 , the voltage VC reaches a voltage equal to the forward voltage Vf of the normal LED  202   b , so that current flows in the normal LED  202   b . In other words, the LED  202   b  is lit. 
     As described above, if an open-circuit failure occurs in the LED  202   a , the bypass circuit  215   a  is turned on, and accordingly the current supplied from the constant-current circuit  212  flows in the normal LED  202   b , passing through the bypass circuit  215   a . In this manner, even if one of the LEDs has an open-circuit failure, the other normal LEDs can be supplied with current. 
     Further, according to the first embodiment, when the bypass circuit  215   a  is turned on, electric charges in the smoothing capacitor  213  are released. By doing so, it is possible to suppress excessive current from flowing in the bypass circuit  215   a  and the LED  202   b . Therefore, it is possible to suppress deterioration or failure of the LED  202   b  and malfunction of the bypass circuit  215   b.    
     Although the operations when the LED  202   a  has an open-circuit failure have been described in the foregoing description, the operations can be equally applied to the case where the LED  202   b  has an open-circuit failure. 
     Further, although the two LEDs connected in series have been used in the foregoing description, three or more LEDs connected in series may be used. In the latter instance, the above-described detection circuit and the bypass circuit are provided for each of the LEDs. 
     Furthermore, although each of the LEDs includes the detection circuit and the bypass circuit in the foregoing description, at least one of the LEDs may include the detection circuit and the bypass circuit. 
     As described above, in the lighting device  210   a  according to the first embodiment, the constant-current circuit  212  resumes its operation when the voltage VC becomes lower than the voltage Vf_min. As shown in  FIG. 6 , the voltage Vf_min is, e.g., lower than the sum of the forward voltages of the normal LEDs (the forward voltage Vf of the LED  202   b  in the example of  FIG. 6 ). However, the voltage Vf_min may be higher than the sum of the forward voltages of the normal LEDs. By way of providing a predetermined discharge period, the voltage VC of when the bypass circuit is turned on can be more lowered, compared to the case where no discharge period is provided. Accordingly, currents flowing in the normal LEDs at the time when the bypass circuit is turned on can be reduced, so that deterioration or failure of the normal LEDs can be suppressed. 
     Moreover, by providing a longer discharge period (by setting the voltage Vf_min to be lower), this effect can be enhanced. Therefore, it is preferable that the voltage Vf_min is lower than the voltage VC in a normal operation state with no open-circuit failure, for example. Herein, the voltage VC in a normal operation state refers to the sum of the forward voltages of LEDs (2×Vf in the example of  FIG. 6 ) in a state with no open-circuit failure. Further, as shown in  FIG. 6 , it is desirable that the voltage Vf_min is the sum of the forward voltages of the normal LEDs other than the LED having an open-circuit failure. 
     In the foregoing description, the constant-current circuit  212  stops during the discharge period until the bypass circuit is turned on. However, the output from the constant-current circuit may be lowered than usual, e.g., up to a level at which the smoothing capacitor  213  is discharged. Also in this manner, the voltage VC can be reduced during the discharge period. 
     As described above, the lighting device  210   a  according to the first embodiment includes: the constant-current circuit  212  that supplies a constant current to the plurality of LEDs  202   a  and  202   b  connected in series, the smoothing capacitor  213  connected between output terminals of the constant-current circuit  212 ; the bypass circuits  215   a  or  215   b  connected in parallel to one of the LEDs  202   a  and  202   b  so as to bypass the one LED  202   a  (or  202   b ); the detection unit (detection circuit  214   a  or  214   b ) configured to detect whether the one LED  202   a  (or  202   b ) is open-circuited; the bypass control unit  216   a  configured to, when the detection circuit  214   a  (or  214   b ) detects that the one LED  202   a  (or  202   b ) is open-circuited, discharge the smoothing capacitor  213  during the discharge period to then bypass the one LED  202   a  (or  202   b ) through the bypass circuit  215   a  (or  215   b ). 
     With this configuration, when an open-circuit failure occurs in the LED  202   a , the lighting device  210   a  releases electric charges accumulated in the smoothing capacitor  213  and then turns on the bypass circuit  215   a . By doing so, it is possible to suppress excessive current flowing in normal LEDs when the bypass circuit  215   a  is turned on. 
     Specifically, during the discharge period, the bypass control unit  216   a  may stop the constant-current circuit  212  or may reduce a value of the constant current supplied from the constant-current circuit  212 . 
     By doing so, the lighting device  210   a  can discharge the smoothing capacitor  213  during the discharge period. 
     In addition, during the discharge period, the smoothing capacitor  213  may be discharged until the voltage at the smoothing capacitor  213  becomes smaller than the sum of the forward voltages of the LEDs  202   a  and  202   b . In addition, during the discharge period, the smoothing capacitor  213  may be discharged until the voltage at the smoothing capacitor  213  becomes smaller than the forward voltage of the LED  202   b  other than the LED  202   a  among the LEDs  202   a  and  202   b.    
     In this manner, the lighting device  210   a  can further discharge the smoothing capacitor  213 , so that it is possible to further suppress current flowing in the normal LED  202   b  when the bypass circuit  215   a  is turned on. 
     Additionally, the bypass control unit  216   a  may include the comparator COM 0  to compare the voltage VC at the smoothing capacitor  213  with the reference voltage Vf_min, and may terminate the discharge period when the voltage VC at the smoothing capacitor  213  becomes smaller than the reference voltage Vf_min and may bypass the LED  202   a  through the bypass circuit  215   a.    
     By doing so, the lighting device  210   a  may turn on the bypass circuit  215   a  after the voltage VC has decreased up to a predetermined voltage. 
     Second Embodiment 
     The second embodiment to be described below is a modification of the first embodiment. In addition to the elements of the first embodiment, the lighting device  210   b  according to the second embodiment further includes a discharge circuit for discharging electric charges in the smoothing capacitor  213  during the discharge period. 
     In the following description, descriptions will be made focusing on differences between the first and second embodiments, and redundant descriptions on the same elements will be omitted. 
       FIG. 7  is a circuit diagram of a lighting device  210   b  according to the second embodiment of the present invention. In addition to the elements shown in  FIG. 3 , the lighting device  210   b  shown in  FIG. 7  further includes a discharge circuit  220 . The bypass control unit  216   b  includes the functionality of the bypass control unit  216   a.    
     The discharge circuit  220  is connected in parallel to the smoothing capacitor  213  and includes a switching element connected in parallel to the smoothing capacitor  213 . For example, the discharge circuit  220  includes a photo MOS relay PMR 0  and a resistor R 0 . As the photo MOS relay PMR 0  is turned on, electric charges accumulated in the smoothing capacitor  213  are released through the resistor R 0  and the photo MOS relay PMR 0 . 
     In addition to the functionality of the bypass control unit  216   a , the bypass control unit  216   b  has the functionality of turning on the discharge circuit  220  during a discharge period.  FIG. 8  shows an example of a circuit diagram of the bypass control unit  216   b . As shown in  FIG. 8 , the bypass control unit  216   b  outputs a discharge control signal DISCHARGE that is an inverted signal of the stop control signal DC/DC_enable, in addition to the functionality of the bypass control unit  216   a.    
       FIG. 9  is a timing chart when the LED  202   a  has an open-circuit failure in the lighting device  210   b  according to the second embodiment. 
     As shown in  FIG. 9 , at time t 2 , if the voltage V 1  reaches the voltage Vf_max, the level of the discharge control signal DISCHARGE becomes high. In response to this, the photo MOS relay PMR 0  is turned on, and accordingly electric charges accumulated in the smoothing capacitor  213  are released through the resistor R 0  and the photo MOS relay PMR 0 . 
     By employing the discharge circuit  220  in this manner, the discharge period (from time t 2  to time t 3 ) can be more shortened than that of the first embodiment. 
     Herein, the constant-current circuit  212  stops and the discharge circuit  220  is turned on during the discharge period. However, the constant-current circuit  212  may not stop.  FIG. 10  shows a circuit diagram of a lighting device  210   c  according to this instance. The configuration shown in  FIG. 10  is identical to that of  FIG. 7  except that the bypass control unit  216   c  does not output the stop control signal DC/DC_enable.  FIG. 11  shows an example of a circuit diagram of the bypass control unit  216   c.    
     As such, even if the constant-current circuit  212  does not stop, the smoothing capacitor  213  is discharged through the discharge circuit  220 , and therefore the same effect as the above can be achieved. 
     As described above, the lighting devices  210   b  and  210   c  may further include the discharge circuit  220  connected in parallel to the smoothing capacitor  213 , and the bypass control unit  216   b  or  216   c  may turn on the discharge circuit  220  during the discharge period to discharge the smoothing capacitor  213 . 
     By doing so, the smoothing capacitor  213  can be discharged during the discharge period. 
     Third Embodiment 
     In the above embodiments, the discharge period terminates when the voltage VC becomes lower than the predetermined voltage Vf_min. According to the third embodiment, the discharge period terminates after a predetermined time period has elapsed from the start of the discharge period. 
       FIG. 12  is a circuit diagram of a lighting device  210   d  according to the third embodiment of the present invention. The configuration of the lighting device  210   d  shown in  FIG. 12  is identical to that of  FIG. 7  except that the configuration of a bypass control unit  216   d  is different from that of the bypass control unit  216   b . As in the configuration shown in  FIG. 7 , the configuration in which the discharge circuit  220  is employed and the constant-current circuit  212  stops during the discharge period will be described as an example in this embodiment. However, the discharge circuit  220  may not be employed or the constant-current circuit  212  may not stop during the discharge period. 
     The bypass control unit  216   d  terminates the discharge period after a predetermined time period has elapsed from the start of the discharge period.  FIG. 13  shows an example of a circuit diagram of the bypass control unit  216   d . As shown in  FIG. 13 , the bypass control unit  216   d  includes a timer  230 , and flip-flops FF 3 A and FF 3 B. 
     The timer  230  outputs a discharge control signal DISCHARGE of high level and a stop control signal DC/DC_enable of low level for a predetermined time period after the level of a failure detection signal LED 1  or LED 2  has become low. Further, the timer  230  outputs the discharge control signal DISCHARGE of low level and the stop control signal DC/DC_enable of high level after the predetermined time period has elapsed. 
     After the level of the failure detection signal LED 1  becomes low, the flip-flop FF 3 A outputs a bypass control signal B 1  of high level if the level of the stop control signal DC/DC_enable is high. After the level of the failure detection signal LED 2  becomes low, the flip-flop FF 3 B outputs a bypass control signal B 2  of high level if the level of the stop control signal DC/DC_enable is high. 
       FIG. 14  is a timing chart when the LED  202   a  has an open-circuit failure in the lighting device  210   d  according to the third embodiment. As shown in  FIG. 14 , at time t 2 , when the voltage V 1  reaches the voltage Vf_max, the level of an input signal Tin of the timer  230  becomes high. Then, the timer  230  outputs an output signal Tout of high level for a predetermined time period. Accordingly, for the predetermined time period, the level of the discharge control signal DISCHARGE is high and the level of the stop control signal DC/DC_enable is low. As a result, during the discharge period, the constant-current circuit  212  stops and the discharge circuit  220  is tuned on. 
       FIG. 15A  shows an example of a circuit diagram of the timer  230 .  FIG. 15B  is a timing chart showing relationship between the input signal Tin and the output signal Tout of the timer  230 . As can be seen from  FIGS. 15A and 15B , when the level of the input signal Tin becomes high, the level of the output signal Tout also becomes high and then becomes low after a predetermined time period elapses. 
     Herein, the discharge period from when the level of the output signal Tout becomes high until it becomes low corresponds to the above-described discharge period. Therefore, it is desirable that the discharge period is set to be long enough so that the voltage VC becomes lower than the voltage Vf_min (e.g., the sum of the forward voltages of normal LEDs) when the discharge period terminates. For example, the discharge period is set to be longer than a time constant of a discharge path (the discharge circuit  220 , in this example) through which electric charges in the smoothing capacitor  213  are released during the discharge period. Further, as described above, the voltage VC may not be lowered than the sum of the forward voltages of normal LEDs when the discharge period terminates. Even though the voltage VC is not lowered enough, the voltage VC can be decreased when the bypass circuit is turned on. Therefore, it is possible to suppress excessive current from flowing in normal LEDs, compared to the case where no discharge period is provided. 
     As described above, after the detection circuit  214   a  detects that the LED  202   a  is open-circuited, the bypass control unit  216   d  may terminate the discharge period after a predetermined time period has elapsed and may bypass the LED  202   a  through the bypass circuit  215   a.    
     Accordingly, the discharge period can be set as required. 
     Further, the discharge period may be longer than the time constant of the discharge path through which the smoothing capacitor  213  is discharged. 
     By doing so, electric charges in the smoothing capacitor  213  can be released sufficiently until the bypass circuit  215   a  is turned on. 
     Fourth Embodiment 
     According to the fourth embodiment, the same functionalities of the above embodiments are implemented by using an MCU (microcontroller). 
       FIG. 16  is a circuit diagram of a lighting device  210   e  according to the fourth embodiment of the present invention. The configuration of the lighting device  210   e  shown in  FIG. 16  is identical to that of  FIG. 7  except that the lighting device  210   e  includes an MCU  240  and a group of voltage-dividing resistors  241 , in place of the bypass control unit  216   b  and the detection circuits  214   a  and  214   b . As in the configuration shown in  FIG. 7 , the discharge circuit  220  is employed and the constant-current circuit  212  stops during the discharge period in this embodiment. However, the discharge circuit  220  may not be employed or the constant-current circuit  212  may not stop during the discharge period. 
     By the MCU  240  and the group of voltage-dividing resistors  241 , the same functionality as the above-described bypass control unit  216   b  and the detection circuits  214   a  and  214   b  is achieved. 
     As shown in  FIG. 16 , the group of voltage-dividing resistors  241  generates voltages V 0   a , V 1   a  and V 2   a  by dividing the voltages V 0 , V 1  and V 2 , respectively. 
     The MCU  240  is a microcontroller and detects whether any of the LEDs  202   a  and  202   b  has an open-circuit failure by using the voltages V 0   a , V 1   a  and V 2   a , in addition to the functionality of the bypass control unit  216   b.    
     Hereinafter, the operation of the microcontroller will be described in detail.  FIG. 17A  is a flowchart for illustrating the operation of the MCU  240 . 
     The MCU  240  includes an A/D converter that converts the voltages V 0   a , V 1   a  and V 2   a  into digital signals. The MCU  240  calculates differences in voltages, i.e., V 2   a −V 1   a  and V 1   a −V 0   a , and determines whether each of the differences is greater than Vf_max_a (in step S 101  and S 102 ). By doing so, the MCU  240  determines whether each of the LEDs  202   a  and  202   b  has an open-circuit failure. The voltage Vf_max_a is a value corresponding to the voltage Vf_max (e.g., the maximum of the forward voltages of LEDs). 
     If the difference V 2   a −V 1   a  is greater than the voltage Vf_max_a (Yes in step S 101 ), the MCU  240  determines that the LED  202   b  has an open-circuit failure and sets a variable “n” to be “2” (in step S 103 ). Further, if the difference V 1   a −V 0   a  is greater than the voltage Vf_max_a (Yes in step S 102 ), the MCU  240  determines that the LED  202   a  has an open-circuit failure and sets the variable “n” to be “1” (in step S 104 ). 
     Subsequent to step S 103  or S 104 , the MCU  240  sets the level of the stop control signal DC/DC_enable to be low (in step S 105 ), and sets the level of the discharge control signal DISCHARGE to be high (in step S 106 ). As a result, the constant-current circuit  212  stops and the discharge circuit  220  is tuned on. 
     Then, the voltage V 2 −V 0  across the smoothing capacitor  213  decreases. The MCU  240  calculates the voltage V 2   a −V 0   a , and determines whether a result of the calculation is less than Vf_min_a (in step S 107 ). The voltage Vf_min_a is a value corresponding to the voltage Vf_min (e.g., a value smaller than the sum of the forward voltages of normal LEDs). 
     If the voltage V 2   a −V 0   a  is less than the voltage Vf_min_a (Yes in step S 107 ), the MCU  240  sets the level of the discharge control signal DISCHARGE to be high to thereby turn off the discharge circuit  220 . 
     Subsequently, the MCU  240  sets the level of a bypass control signal Bn (where n is a value (1 or 2) set in step S 103  or S 104 ) to be high to thereby turn on the bypass circuit  215   a  or  215   b  (in step S 109 ). Namely, if the LED  202   a  has an open-circuit failure (n=1), the MCU  240  sets the level of the bypass control signal B 1  to be high to thereby turn on the bypass circuit  215   a . If the LED  202   b  has an open-circuit failure (n=2), the MCU  240  sets the level of the bypass control signal B 2  to be high to thereby turn on the bypass circuit  215   b.    
     Thereafter, the MCU  240  sets the level of the stop control signal DC/DC_enable to be high to thereby operate the constant-current circuit  212  (in step S 110 ). 
     In the above-described manner, the same operations as those of the second embodiment are implemented. 
     As in the third embodiment, the MCU  240  may end the discharge period after a predetermined time period has elapsed from the start of the discharge period.  FIG. 17B  is a flowchart for illustrating the operation of the MCU  240  in this instance. The processes illustrated in  FIG. 17B  are identical to those of  FIG. 17A  except that step S 107  is replaced with step S 107 A. 
     Subsequent to step S 106 , the MCU  240  waits for a predetermined time period (discharge period) (in step S 107 A). Thereafter, the MCU  240  performs the processes of step S 108  and subsequent steps. 
     Thus far, the lighting devices according to the embodiments have been described. However, the present invention is not limited to the above embodiments. 
     For example, although one bypass circuit has been provided for one light-emitting element in the above embodiments, one bypass circuit may be provided for a plurality of light emitting elements. The light-emitting elements may be connected to one another either in parallel or in series. Further, as shown in  FIG. 18 , groups of light-emitting elements, each group having light-emitting elements connected in series, may be connected to one another in parallel. In other words, the light-emitting element may be a single LED or may include LEDs connected in series and/or in parallel. Further, the light-emitting element may be an LED module including a plurality of LED chips or may include a plurality of LED modules. 
     Although an LED has been used as the solid-state light-emitting element in the above embodiments, an organic EL (Electro-Luminescence) element may be used as the solid-state light-emitting element. 
     Further, in the above description, a photo MOS relay has been used as the switching element employed in the bypass circuit and the discharge circuit. However, an MOSFET (Metal Oxide Semiconductor Field Effect Transistor), a thyristor, a triac, a photo-coupler, a power transistor, an IGBT (Insulated Gate Bipolar Transistor), a relay, a bimetal or the like may be used as the switching element. 
     Further, different control may be conducted in a normal operation state (where no open-circuit failure occurs in light-emitting elements) and a bypass state in which the bypass circuit is turned on (after an open-circuit failure has occurred in a light-emitting element). 
     For example, when an open-circuit failure has occurred, a light-emitting element having the open-circuit failure is not lit, and thus a less number of light-emitting elements are lit in a bypass state. Therefore, the brightness degrades in the case where constant current is supplied. To cope with this, the constant-current circuit  212  may supply to the light-emitting element a larger current in the bypass state than in the normal operation state. By doing so, difference in optical power between the bypass state and the normal operation state can be reduced. 
     Further, the constant-current circuit  212  may intermittently supply current to the light-emitting elements in the bypass state. In this case, the light-emitting elements blink on and off in the bypass state, so that a user can notice that a light-emitting element has been open-circuited due to a failure or a bad connection of the light-emitting element. 
     The constant-current circuit ( 212 ) is, e.g., a DC-to-DC converter. Hereinafter, a specific example of the constant-current circuit  212  will be described. 
       FIG. 19  is a circuit diagram showing a specific example of the constant-current circuit  212 . The constant-current circuit  212  shown in  FIG. 19  is of a step-down DC-to-DC converter, and includes a switching element SW 1 , an inductor L 1 , a diode DI 1 , a resistor Rs 1 , and a control unit  250 . The smoothing capacitor  213  is disposed outside the constant-current circuit  212 , but may be included in the constant-current circuit  212 . 
     The switching element SW 1  is connected in series to the DC power source  211  and is turned on and off by the control unit  250 . 
     The inductor L 1  is connected in series to the switching element SW 1 . When the switching element SW 1  is turned on, current from the DC power source  211  flows in the inductor L 1 . 
     The diode DI 1  is an element through which current discharged from the inductor L 1  is supplied to the LEDs  202   a  and  202   b.    
     The resistor Rs 1  is to generate a voltage Rs·i that corresponds to a current flowing in the switching element SW 1  (LEDs  202   a  and  202   b ). 
     The control unit  250  generates a signal GD to control on/off of the switching element SW 1  based on a signal ZCD from a secondary winding of the inductor L 1  and the voltage Rs·i. The signal ZCD is proportional to a time differential of a current flowing in the inductor L 1  and is used to detect whether the current flowing in the inductor becomes zero. 
       FIG. 20  is a circuit diagram of an example of the control unit  250 . In order to start the constant-current circuit  212 , a starter S 1  generates a start pulse signal so that the level of the Q output (signal GD) of a flip-flop FF 4  becomes high. As a result, the switching element SW 1  is turned on. 
     As the switching element SW 1  is turned on, current from the DC power source  211  flows in the switching element SW 1 , the inductor L 1 , the LED  202   a  and the LED  202   b . This current increases over time. When this current reaches a peak current, the level of an output signal from a comparator COM 1  becomes high, so that the level of the Q output (signal GD) of the flip-flop FF 4  becomes low. As a result, the switching element SW 1  is turned off. 
     When the switching element SW 1  is turned off, the diode DI 1  becomes conductive, so that current flows in the inductor L 1  and the diode DI 1 . This current decreases from the peak current over time. When the current flowing in the inductor L 1  becomes zero, the level of the signal ZCD becomes low. In response to this, the level of the Q output (signal GD) of the flop-flop FF 4  becomes high, and accordingly the switching element SW 1  is turned on again. 
     By repeating the above operations, the constant-current circuit  212  supplies constant current to the LEDs  202   a  and  202   b.    
     A step-down DC-to-DC converter shown in  FIG. 21 , a flyback DC-to-DC converter shown in  FIG. 22 , or a step-up/step-down DC-to-DC converter shown in  FIG. 23  may be used as the constant-current circuit  212 . 
     As described above, the constant-current circuit  212  is a DC-to-DC converter, and may include the switching element SW 1  (or SW 2  or SW 3  or SW 4 ), the inductor L 1  (or L 2  or L 3  or L 4 ) in which current from the DC power source  211  flows while the switching element SW 1  (or SW 2  or SW 3  or SW 4 ) is turned on, the diode DI 1  (or DI 2  or DI 3  or DI 4 ) through which current discharged from the inductor L 1  (or L 2  or L 3  or L 4 ) is supplied to the LEDs  202   a  and  202   b , and the control unit  250  that controls on/off of the switching element SW 1  (or SW 2  or SW 3  or SW 4 ). 
     Fifth Embodiment 
     At first, elements of a lighting device according to the fifth embodiment will be described with reference to  FIG. 24 . 
       FIG. 24  is a circuit diagram of a lighting device according to the fifth embodiment of the present invention. 
     As shown in  FIG. 24 , the lighting device  1   a  according to the fifth embodiment receives DC power from a DC power source  10  to light LEDs  40   a  and  40   b  connected in series. The lighting device  1   a  includes a constant-current circuit  20  and bypass circuits  30   a  and  30   b.    
     The LEDs  40   a  and  40   b  shown in  FIG. 24  are solid-state light-emitting elements that are connected in series and are lit upon receiving current from the constant-current circuit  20 . Each of the LEDs  40   a  and  40   b  may be formed of a single LED chip or may be formed of LED chips connected in series or in parallel. 
     The constant-current circuit  20  shown in  FIG. 24  converts current supplied from the DC power source  10  to a predetermined current and supplies the predetermined current to the LEDs  40   a  and  40   b  connected in series. The constant-current circuit  20  includes a control circuit  21 , a diode  22 , an inductor  23 , a FET (field effect transistor)  24 , and a detection resistor  25 . 
     The control circuit  21  of the constant-current circuit  20  outputs a signal to control on/off of the FET  24 . 
     The FET  24  of the constant-current circuit  20  is a switching element that is controlled by the signal outputted from the control circuit  21 . 
     The inductor  23  of the constant-current circuit  20  is an inductive element through which current from the DC power source  10  flows while the FET  24  is tuned on. 
     The diode  22  of the constant-current circuit  20  is an element through which current discharged from the inductor  23  is supplied to the LEDs  40   a  and  40   b.    
     The detection resistor  25  of the constant-current circuit  20  is for detecting current flowing in the FET  24 . 
     In this embodiment, the constant-current circuit  20  is a DC-to-DC converter that performs BCM (boundary current mode) control. Specifically, while the FET  24  is conductive, the control circuit  21  of the constant-current circuit  20  detects whether a current flowing in the detection resistor  25  reaches a peak current and, if so, turns the FET  24  to be non-conductive. Additionally, while the FET  24  is non-conductive, the control circuit  21  detects whether the current flowing in the inductor  23  becomes zero and, if so, turns the FET  24  to be conductive. 
     The bypass circuits  30   a  and  30   b  shown in  FIG. 24  are connected in parallel to the LED  40   a  and  40   b , respectively. The bypass circuits  30   a  and  30   b  provide bypass paths for bypassing the LEDs  40   a  and  40   b , respectively, when open-circuit failures occur in the LED  40   a  and  40   b . The bypass circuit  30   a  includes a capacitor  31   a , a resistor  32   a , a zener diode  33   a  and a thyristor  34   a . The bypass circuit  30   b  includes a capacitor  31   b , a resistor  32   b , a zener diode  33   b  and a thyristor  34   b.    
     The capacitor  31   a  and the resistor  32   a  are connected in series to each other and form a capacitor circuit  37   a . The capacitor circuit  37   a  is connected in parallel to the LED  40   a . Likewise, the capacitor  31   b  and the resistor  32   b  are connected in series to each other and form a capacitor circuit  37   b . The capacitor circuit  37   b  is connected in parallel to the LED  40   b . Herein, the resistors  32   a  and  32   b  are also included in current detection units  300   a  and  300   b , respectively. 
     If open-circuit failures occur in the LEDs  40   a  and  40   b , currents flowing in the capacitors  31   a  and  31   b  increase, respectively. Therefore, the open-circuit failures can be detected by measuring the currents. The capacitors  31   a  and  31   b  also work as smoothing capacitors for the output from the constant-current circuit  20 . Namely, pulsating components in the output current from the constant-current circuit  20  caused by the switching of the FET  24  are smoothened by the capacitors  31   a  and  31   b , so that smooth DC current flows in the LEDs  40   a  and  40   b.    
     The thyristor  34   a  of the bypass circuit  30   a  and the thyristor  34   b  of the bypass circuit  30   b  are bypass switches that are connected in parallel to the capacitor circuits  37   a  and  37   b , respectively. 
     The resistor  32   a  and the zener diode  33   a  of the bypass circuit  30   a  constitute a current detection unit  300   a  that detects whether a current flowing in the capacitor  31   a  exceeds a predetermined threshold Ith. Specifically, a current flowing in the capacitor  31   a  is measured by the zener diode  33   a  based on a voltage across the resistor  32   a  connected in series to the capacitor  31   a . When the current I 31   a  flowing in the capacitor  31   a  exceeds the threshold Ith, a zener voltage Vza is determined so that the voltage across the resistor  32   a  exceeds the zener voltage Vza of the zener diode  33   a . Accordingly, the zener voltage Vza is determined by the following equation:
 
 Vza=Ra×Ith   (Equation 1)
 
     where Ra denotes the resistance of the resistor  32   a.    
     In addition, when the measured current exceeds the threshold Ith, the current detection unit  300   a  allows current to flow from the zener diode  33   a  to the thyristor  34   a  to thereby turn the thyristor  34   a  to be conductive. 
     Likewise, the resistor  32   b  and the zener diode  33   b  of the bypass circuit  30   b  constitute a current detection unit  300   b  that detects whether a current flowing in the capacitor  31   b  exceeds a predetermined threshold Ith. The zener voltage Vzb of the zener diode  33   b  is determined by the following equation:
 
 Vzb=Rb×Ith   (Equation 2)
 
     where Rb denotes the resistance of the resistor  32   b.    
     When the measured current exceeds the threshold Ith, the current detection unit  300   b  allows current to flow from the zener diode  33   b  to the thyristor  34   b  to thereby turn the thyristor  34   b  to be conductive. 
     The threshold Ith is larger than the output current from the constant-current circuit  20  and equal to or less than two times the output current. Herein, the output current from the constant-current circuit  20  corresponds to a peak current flowing in the capacitors  31   a  and  31   b  in the normal operation state (where no open-circuit failure has occured in the LEDs  40   a  and  40   b ). The two times the output current from the constant-current circuit  20  corresponds to a peak current flowing in the capacitors  31   a  or  31   b  when an open-circuit failure has occurred in the LED  40   a  or  40   b.    
     Next, the operations of the lighting device  1   a  and the bypass circuits  30   a  and  30   b  according to the fifth embodiment will be described. As an example of the operations, a scenario where an open-circuit failure occurs in the LED  40   b  will be described with reference to  FIGS. 25 to 27 . 
       FIG. 25  shows graphs of waveforms of voltages V 31   a  and V 31   b  across the capacitors  31   a  and  31   b  of the lighting device  1   a , respectively, versus time.  FIG. 25  also shows graphs of waveforms of currents I 31   a , I 31   b , I 40   a  and I 40   b  flowing in the capacitor  31   a  and  31   b  and the LEDs  40   a  and  40   b , respectively, versus time. 
       FIG. 26  is an enlarged view of a part of the waveforms of voltages and currents shown in  FIG. 25 .  FIG. 26  shows the waveforms of the currents I 40   b  and I 31   b  flowing in the LED  40   b  and the capacitor  31   b , respectively, versus time, and the waveform of the voltage V 31   b  across the capacitor  31   b  versus time. 
       FIG. 27  is an enlarged view of a part of the waveforms of voltages and currents shown in  FIG. 25 , and there is also depicted a waveform of the current I 34   b  flowing in the thyristor  34   b  versus time.  FIG. 27  shows the waveforms of the currents I 31   b , I 34   b  and I 40   b  flowing in the capacitor  31   b , the thyristor  34   b  and the LED  40   b , respectively, versus time.  FIG. 27  further shows the waveform of the voltage V 31   b  across the capacitor  31   b  versus time. 
     For the lighting device  1   a  according to the fifth embodiment, if an open-circuit failure occurs in the LED  40   b , the current I 40   b  flowing in the LED  40   b  becomes zero, as shown in  FIGS. 25 to 27 . When no more current flows in the LED  40   b , the current having flowed in the LED  40   b  before the open-circuit failure occurs flows to the capacitor  31   b  connected in parallel to the LED  40   b . Therefore, as shown in  FIGS. 25 and 26 , a DC component is added to the current I 31   b  flowing in the capacitor  31   b . Herein, the DC component refers to a frequency component lower than the switching frequency of the FET  24 . Then, as described above, the current I 31   b  flowing in the capacitor  31   b  increases up to about two times the peak current of a normal operation state. Further, the voltage V 31   b  across the capacitor  31   b  increases slowly. 
     As the current I 31   b  flowing in the capacitor  31   b  increases, the current flowing through the resistor  32   b  connected in series to the capacitor  31   b  and the voltage across the resistor  32   b  also increase. Further, when the current I 31   b  flowing in the capacitor  31   b  exceeds the threshold Ith and the voltage across the resistor  32   b  exceeds the zener voltage Vzb of the zener diode  33   b , current abruptly flows in the zener diode  33   b . The current flows from the anode of the zener diode  33   b  to the gate of the thyristor  34   b , so that the thyristor  34   b  becomes conductive. Consequently, a bypass path for bypassing the LED  40   b  is turned on. 
     When the bypass path for bypassing the LED  40   b  is turned on, electric charges accumulated in the capacitor  31   b  are released. The current generated by these electric charges flows in a closed circuit that is formed of the capacitor  31   b , the thyristor  34   b  and the resistor  32   b  (see the waveforms of the currents I 13   b  and I 34   b  in  FIG. 27 ) but does not flow in the normal LED  40   a  (see the waveform of the current I 40   a  in  FIG. 25 ). 
     Now, the operation of the LED  40   a  when the thyristor  34   b  is conductive will be described. Immediately after an open-circuit failure has occurred in the LED  40   b , current flows through the capacitor  31   b  (see the waveform of the current I 31   b  in  FIG. 26 ). Therefore, the normal LED  40   a  is kept at a lighted state even during a time period after the open-circuit failure has occurred in the LED  40   b  until the thyristor  34   b  is conductive (see the waveform of the current I 40   a  in  FIG. 25 ). 
     Next, a time period required until the current detection unit  300   b  turns the thyristor  34   b  to be conductive after the open-circuit failure has occurred in the LED  40   b  will be discussed below. The period of the pulsation of the current I 31   b  flowing in the capacitor  31   b  shown in  FIGS. 25 and 26  corresponds to the switching period of the FET  24  of the constant-current circuit  20 . Further, as shown in  FIG. 26 , the current I 31   b  exceeds the threshold Ith until the current I 31   b  reaches the peak of its pulsation after the open-circuit failure has occurred in the LED  40   b  and then the DC component is added to the current I 31   b . Accordingly, the detection time can be reduced below the period of the pulsation of the current I 31   b , i.e., below the switching period of the FET  24 . By doing so, the thyristor  34   b  can become conductive with a less amount of electric charges accumulated in the capacitor  31   b . Accordingly, excessive current to be generated at the instant when the thyristor  34   b  becomes conductive can be suppressed, so that stress to be exerted on the bypass circuits  30   a  and  30   b  can be suppressed. 
     As described above, the lighting device  1   a  according to the fifth embodiment includes: the constant-current circuit  20  that supplies a constant current to the plurality of LEDs  40   a  and  40   b  connected in series; the capacitor circuits  37   a  and  37   b  connected in parallel to the LEDs  40   a  and  40   b , respectively; the thyristors  34   a  and  34   b  connected in parallel to the capacitor circuits  37   a  and  37   b , respectively; and the current detection units  300   a  and  300   b  configured to measure currents flowing through the capacitors  31   a  and  31   b , respectively. The current detection units  300   a  and  300   b  turn on the thyristors  34   a  and  34   b , respectively, when the measured currents exceed the predetermined threshold Ith. 
     In this manner, immediately after the thyristors  34   a  or  34   b  serving as bypass switches become conductive, the current from the capacitor  31   a  or  31   b  does not flow in the normal LED, and thus stress exerted on the normal LED is mitigated. In addition, according to the fifth embodiment, even during the time period after an open-circuit failure has occurred in one of the LEDs  40   a  and  40   b  until the bypass switch is turned on, current flows in the other one of the LEDs  40   a  and  40   b  so that the other one of the LEDs  40   a  and  40   b  is kept at a lighted state. 
     Further, the lighting device  1   a  according to the fifth embodiment may include the resistors  32   a  and  32   b  connected in series to the capacitors  31   a  and  31   b , respectively. The current detection units  300   a  and  300   b  may measure the currents flowing through the capacitors  31   a  and  31   b  based on the voltages across the resistors  32   a  and  32   b , respectively. 
     By doing so, the current detection units  300   a  and  300   b  of the lighting device  1   a  can accurately measure the currents flowing through the capacitors  31   a  and  31   b , respectively. 
     Furthermore, in the lighting device  1   a  according to the fifth embodiment, the constant-current circuit  20  is a DC-to-DC converter that is controlled in a BCM manner. The predetermined threshold Ith is larger than the output current of the constant-current circuit  20  and is equal to or less than two times the output current. 
     By doing so, the threshold Ith can be set so that an open-circuit failure in the LED  40   a  or  40   b  can be detected. 
     Sixth Embodiment 
     Next, a lighting device according to the sixth embodiment will be described. 
     The basic elements and operations of the lighting device according to the sixth embodiment are identical to those according to the fifth embodiment except for the configuration of the current detection unit. Therefore, descriptions will be made focusing on the differences between the fifth and sixth embodiments. 
     According to the above fifth embodiment, when the lighting device  1   a  undergoes a transitional behavior such as start-up, large currents flow in the capacitors  31   a  and  31   b , and thus the current detection units  300   a  and  300   b  may malfunction. 
     In this regard, according to the sixth embodiment, there is provided a lighting device capable of suppressing such malfunction of the current detection units. 
     At first, elements of a lighting device according to the sixth embodiment will be described with reference to  FIG. 28 . 
       FIG. 28  is a circuit diagram of a lighting device according to the sixth embodiment of the present invention. 
     As can be seen from  FIG. 28 , the lighting device  1   b  according to the sixth embodiment is different in the configurations of the current detection unit  300   c  of the bypass circuit  30   c  and the current detection unit  300   d  of the bypass circuit  30   d , compared to the lighting device  1   a  according to the fifth embodiment. In the lighting device  1   b , the current detection unit  300   c  has therein a RC (resistor-capacitor) filter  50   a  and a resistor  35   a , and the current detection unit  300   d  has therein a RC filter  50   b  and a resistor  35   b.    
     The RC filters  50   a  and  50   b  are high-cut filters that attenuate high-frequency components in voltage applied to cathodes of zener diodes  33   a  and  33   b , respectively. The RC filter  50   a  includes a resistor  51   a  and a capacitor  52   a . The RC filter  50   b  includes a resistor  51   b  and a capacitor  52   b . The resistors  35   a  and  35   b  are resistors for preventing malfunction of the current detection units  300   c  and  300   d  by limiting current flowing in the thyristors  34   a  and  34   b , respectively. 
     Next, the operation of the lighting device  1   b  according to the sixth embodiment will be described with reference to  FIG. 29 . 
       FIG. 29  shows graphs of waveforms of a voltage V 32   b  across the resistor  32   b  and a voltage V 52   b  across the capacitor  52   b  versus time, when an open-circuit failure occurs in the LED  40   b.    
     As shown in  FIG. 29 , the pulsation, which is high-frequency component, in the voltage across the resistor  32   b  is suppressed by the RC filter  50   b . Therefore, the current detection units  300   c  and  300   d  can detect the DC component in the current flowing in the capacitors  31   a  and  31   b , respectively, other than the high-frequency component. According to the sixth embodiment, the zener diodes  33   a  and  33   b  are chosen so that the voltages applied to the zener diodes  33   a  and  33   b  exceeds their zener voltages, respectively, when the DC component in the current flowing in the capacitors  31   a  and  31   b  exceeds the threshold Ith. 
     As described above, in the lighting device  1   b  according to the sixth embodiment, the current detection units  300   c  and  300   d  include RC filters  50   a  and  50   b  that attenuate high-frequency components in the current. Further, the current detection units  300   c  and  300   d  detect the DC component in the current flowing in the capacitors  31   a  and  31   b , respectively. 
     In this manner, the lighting device  1   b  according to the sixth embodiment can suppress the malfunction of the current detection units  300   c  and  300   d  due to a transitional behavior such as start-up and the like. 
     In addition, the lighting device  1   b  according to the sixth embodiment includes resistors  35   a  and  35   b  for preventing malfunction. 
     With the resistors  35   a  and  35   b , in the lighting device  1   b  according to the sixth embodiment, currents flowing in the thyristors  34   a  and  34   b  are suppressed, so that malfunction of the thyristors  34   a  and  34   b  can be suppressed. 
     Seventh Embodiment 
     Next, a lighting device according to the seventh embodiment will be described. 
     The basic elements and operations of the lighting device according to the seventh embodiment are identical to those according to the fifth embodiment except for the configuration of the bypass circuit. Therefore, descriptions will be made focusing on the differences between the fifth and seventh embodiments. 
     In the lighting device  1   a  according to the above fifth embodiment, excessive currents flows in the bypass circuits  30   a  and  30   b  immediately after the bypass circuits  30   a  and  30   b  operate, respectively (see the waveforms of the currents I 31   b  and I 34   b  shown in  FIG. 27 ). Consequently, stress may be exerted on the thyristors  34   a  and  34   b  of the bypass circuits  30   a  and  30   b , or the like. 
     In this regard, according to the seventh embodiment, there is provided a lighting device capable of suppressing excessive current flowing immediately after the bypass circuits operate. 
     At first, elements of a lighting device according to the seventh embodiment will be described with reference to  FIG. 30 . 
       FIG. 30  is a circuit diagram of a lighting device according to the seventh embodiment of the present invention. 
     As can be seen from  FIG. 30 , the lighting device  1   c  according to the seventh embodiment is different from the lighting device  1   a  according to the fifth embodiment in the configurations of the bypass circuits  30   e  and  30   f.    
     According to the seventh embodiment, the bypass circuit  30   e  has therein an impedance element  60   a  and a diode  36   a , and the bypass circuit  30   f  has therein an impedance element  60   b  and a diode  36   b.    
     The impedance elements  60   a  and  60   b  are connected in series to the thyristors  34   a  and  34   b , respectively. The impedance element  60   a  and the thyristor  34   a  form a bypass switch circuit  38   a  and the bypass switch circuit  38   a  is connected in parallel to the LED  40   a . Likewise, the impedance element  60   b  and the thyristor  34   b  form a bypass switch circuit  38   b  and the bypass switch circuit  38   b  is connected in parallel to the LED  40   b.    
     The impedance elements  60   a  and  60   b  suppress currents flowing in the bypass circuits  30   e  and  30   f  immediately after the bypass circuits  30   e  and  30   f  operate. The impedance element  60   a  includes a thermistor  61   a  and an inductor  62   a . The impedance element  60   b  includes a thermistor  61   b  and an inductor  62   b.    
     The thermistors  61   a  and  61   b  are NTC (negative temperature coefficient) thermistors whose resistance decreases with increase of temperature. The thermistors  61   a  and  61   b  have high resistance at a low temperature. Therefore, when the current is zero and the temperature is low, the thermistors  61   a  and  61   b  can suppress the current from increasing abruptly. 
     The inductors  62   a  and  62   b  are elements that resist change in current, and thus they can suppress the current from increasing abruptly. Further, the resistance of the inductors  62   a  and  62   b  is almost zero, if there is no change in current. Therefore, in the operation of the bypass circuits  30   e  and  30   f , when currents flowing in the thyristors  34   a  and  34   b  become constant, currents flow in the inductors  62   a  and  62   b  and thus loss can be reduced. 
     The diodes  36   a  and  36   b  are connected in parallel to the LEDs  40   a  and  40   b , respectively, and suppress oscillation of current caused by the inductors  62   a  and  62   b.    
     Next, the operation of the lighting device  1   c  according to the seventh embodiment will be described with reference to  FIG. 31 . 
       FIG. 31  shows graphs of waveforms of the currents I 31   b  and I 34   b  flowing in the capacitor  31   b  and the thyristor  34   b , respectively, versus time in the case where an open-circuit failure occurs in the LED  40   b , according to the fifth and seventh embodiment. 
     As shown in  FIG. 31 , according to the fifth embodiment, when an open-circuit failure occurs in the LED  40   b , the bypass circuit  30   b  operates, and immediately thereafter, the current increases abruptly. On the other hand, according to the seventh embodiment, the current also increases immediately after the bypass circuit  30   f  operates, but the peak value of the current is significantly reduced. 
     As described above, the lighting device  1   c  according to the seventh embodiment includes the impedance elements  60   a  and  60   b  which are connected in series to the thyristors  34   a  and  34   b  serving as bypass switches, respectivelys. 
     With the impedance elements  60   a  and  60   b , it is possible to suppress abrupt increase in current immediately after the bypass circuits  30   e  and  30   f  operate. In addition, in a normal operation state, the bypass circuits  30   e  and  30   f  allow current to flow in the inductors  62   a  and  62   b , so that the loss can be reduced. 
     The lighting device  1   c  according to the seventh embodiment further includes the diodes  36   a  and  36   b  which are connected in parallel to the LEDs  40   a  and  40   b , respectively. 
     With the diodes  36   a  and  36   b , it is possible to suppress oscillation of current caused by the inductors  62   a  and  62   b.    
     Eighth Embodiment 
     Next, a lighting device according to the eighth embodiment will be described. 
     The basic elements and operations of the lighting device according to the eighth embodiment are identical to those according to the fifth embodiment except for the configuration of the bypass circuit. Therefore, descriptions will be made focusing on the differences between the fifth and eighth embodiments. 
     According to the eighth embodiment, there is provided a lighting device capable of more accurately detecting current than the lighting device  1   a  of the fifth embodiment. 
     At first, elements of a lighting device according to the eighth embodiment will be described with reference to  FIG. 32 . 
       FIG. 32  is a circuit diagram of a lighting device according to the eighth embodiment of the present invention. 
     As can be seen from  FIG. 32 , the lighting device  1   d  according to the eighth embodiment is different in the configurations of a bypass circuit  30   g  from the lighting device  1   a  of the fifth embodiment. The bypass circuit  30   g  includes an MCU (micro-control unit)  71   a , photo-couplers  74   a  and  74   b , MOSFETs (metal oxide semiconductor field effect transistors)  73   a  and  73   b , and gate resistors  72   a  and  72   b.    
     The MCU  71   a  of the bypass circuit  30   g  is a processing unit that measures currents flowing in the capacitors  31   a  and  31   b  to output signals corresponding to the measured currents to the photo-couplers  74   a  and  74   b . The MCU  71   a  measures currents flowing in the capacitors  31   a  and  31   b  based on the voltages across the resistors  32   a  and  32   b , respectively. 
     The MOSFETs  73   a  and  73   b  of the bypass circuit  30   g  are bypass switches. When a high voltage is applied between gate and source of the MOSFETs  73   a  and  73   b , source-drain channel becomes conductive. 
     The photo-couplers  74   a  and  74   b  of the bypass circuit  30   g  are elements that transfer electrical signals by using light. The photo-couplers  74   a  and  74   b  transfer signals from the MCU  71   a  to the MOSFETs  73   a  and  73   b , respectively. Output signals from the MCU  71   a  are inputted to the input circuit sides of the photo-couplers  74   a  and  74   b . If the output signals from the MCU  71   a  are at high level, the output circuit sides of the photo-couplers  74   a  and  74   b  become conductive. If the output signals from the MCU  71   a  are at low level, the output circuit sides of the photo-couplers  74   a  and  74   b  is not conductive. Since the MCU  71   a  and the MOSFETs  73   a  and  73   b  are electrically isolated by the photo-couplers  74   a  and  74   b , noise cannot be transmitted. 
     According to the eighth embodiment, the current detection unit that detects currents flowing in the capacitors  31   a  and  31   b  includes the MCU  71   a , the resistors  32   a  and  32   b , and the photo-couplers  74   a  and  74   b.    
     Next, the operation of the bypass circuit  30   g  according to the eighth embodiment will be described. As an example of the operations, a scenario where an open-circuit failure occurs in the LED  40   b  will be described. 
     Similar to the above-described fifth to seventh embodiments, if an open-circuit failure occurs in the LED  40   b , the DC component is added to the current flowing in the capacitor  31   b , and accordingly the current flowing in the capacitor  31   b  rises. If the current flowing in the capacitor  31   b  rises, the MCU  71   a  measures the voltage across the resistor  32   b . Further, the MCU  71  compares the measured value with a reference voltage value, by using a comparator provided therein, to determine whether the current flowing in the capacitor  31   b  exceeds the threshold Ith. The MCU  71   a  outputs a signal of high level to the photo-coupler  74   b  if the current I 31   b  flowing in the capacitor  31   b  does not exceed the threshold Ith, whereas the MCU  71   a  outputs a signal of low level to the photo-coupler  74   b  if the current I 31   b  exceeds the threshold Ith. The output circuit side of the photo-coupler  74   b  becomes conductive when a signal of high level is received from the MCU  71   a . The output circuit side of the photo-coupler  74   b  is not conductive when a signal of low level is received from the MCU  71   a . Accordingly, when the current I 31   b  exceeds the threshold Ith, the level of the gate-source voltage of the MOSFET  73   b  becomes high, so that the source-drain channel becomes conductive. Consequently, a bypass path for bypassing the LED  40   b  is turned on. On the other hand, when the current I 31   b  does not exceed the threshold Ith, the level of the gate-source voltage of the MOSFET  73   b  becomes low, so that the source-drain channel does not become conductive. 
     As described above, similar to the fifth embodiment, the lighting device  1   d  according to the eighth embodiment can turn on the bypass path when an open-circuit failure has occurred in one of the LEDs  40   a  and  40   b , without causing excessive current to flow in the other one of the LEDs  40   a  and  40   b . Further, according to the eighth embodiment, currents are measured by the MCU  71   a , so that detection accuracy of the current can be improved. Furthermore, in order to prevent malfunction in a transitional state such as start-up of the lighting device  1   d , software processing can be performed in the MCU. For example, a mask time period can be set so that the MOSFETs  73   a  and  73   b  of the bypass circuit  30   g  do not become conductive for a certain period of time after the start-up of the lighting device  1   d . In addition, filtering process on a signal inputted to the MCU  71   a  can be performed by software, thereby preventing malfunction. 
     Ninth Embodiment 
     Next, a lighting device according to the ninth embodiment will be described. 
     The basic elements and operations of the lighting device according to the ninth embodiment are identical to those according to the eighth embodiment except for the configuration of the bypass circuit. Therefore, descriptions will be made focusing on the differences between the fifth and ninth embodiments. 
     According to the above eighth embodiment, the currents flowing in the capacitors  31   a  and  31   b  of the bypass circuit  30   g  are measured based on the voltages across the resistors  32   a  and  32   b , respectively. In contrast, according to the ninth embodiment, the currents are measured based on the voltages across the capacitors  31   a  and  31   b.    
     At first, elements of a lighting device according to the ninth embodiment will be described with reference to  FIG. 33 . 
       FIG. 33  is a circuit diagram of a lighting device according to the ninth embodiment of the present invention. 
     As can be seen from  FIG. 33 , the lighting device  1   e  according to the ninth embodiment is different from the lighting device  1   d  of the eighth embodiment in that the voltages across the capacitors  31   a  and  31   b  are measured by an MCU  71   b  of a bypass circuit  30   h . Therefore, according to the ninth embodiment, the resistors  32   a  and  32   b  used for detecting current in the eighth embodiment are not required. In the ninth embodiment, the current detection unit that measures currents flowing in the capacitors  31   a  and  31   b  includes the MCU  71   b  and the photo-couplers  74   a  and  74   b.    
     Next, the operation of the bypass circuit  30   h  in the lighting device  1   e  according to the ninth embodiment will be described. As an example of the operation, a scenario where an open-circuit failure occurs in the LED  40   b  will be described. 
     Similar to the fifth to eighth embodiments, if an open-circuit failure occurs in the LED  40   b , the DC component is added to the current flowing in the capacitor  31   b , and accordingly the current flowing in the capacitor  31   b  rises. As the current flowing in the capacitor  31   b  increases, the voltage across the capacitor  31   b  also increases. The MCU  71   b  measures the voltage across the capacitor  31   b . Further, the MCU  71   b  compares the measured value with a reference voltage value by using a comparator provided therein to determine whether the current flowing in the capacitor  31   b  exceeds the threshold Ith. The subsequent operations by the MCU  71   b , the photo-couplers  74   a  and  74   b  and the MOSFETs  73   a  and  73   b  are identical to those of the eighth embodiment. 
     As described above, the lighting device  1   e  according to the ninth embodiment can also achieve the same effect as that of the eighth embodiment. 
     Tenth Embodiment 
     As the tenth embodiment, a luminaire having any one of the lighting devices  210   a  to  210   e  and  1   a  to  1   e  according to the first to the ninth embodiment will be described with reference to  FIGS. 34 to 36 . The luminaire includes light-emitting elements in addition to the lighting device. 
       FIGS. 34 to 36  are external views of the luminaire having any one of the lighting devices  210   a  to  210   e  and  1   a  to  1   e  according to the first to the ninth embodiments. As examples of the luminaire, a downlight  100   a  (shown in  FIG. 34 ) and spotlights  100   b  and  100   c  (shown in  FIG. 35  and  FIG. 36 , respectively) are illustrated. In  FIGS. 34 to 36 , circuit boxes  110   a  to  110   c  accommodate a circuit of any one of the lighting devices  210   a  to  210   e  and  1   a  to  1   e . The LEDs  40   a  and  40   b  or the LED  202   a  and  202   b  are installed in lamp bodies  120   a  to  120   c . A wire  130   a  in  FIG. 34  and a wire  130   b  in  FIG. 35  electrically connect the circuit boxes  110   a  and  110   b  with the lamp bodies  120   a  and  120   b , respectively. 
     The tenth embodiment can also achieve the same effects as those of the above-described first to ninth embodiments. 
     (Modification) 
     Thus far, the lighting devices and the luminaire of the present invention have been described based on the embodiments. However, the present invention is not limited to the embodiments. 
     For example, in the fifth to ninth embodiments, the two LEDs  40   a  and  40   b  are used as solid-state elements. However, three or more LEDs may be used, each with a capacitor and a bypass switch connected in parallel thereto. 
     Further, in the fifth to ninth exemplary embodiments, every solid-state light-emitting element is provided with a bypass circuit. However, at least one of the solid-state light-emitting elements may be provided with a bypass circuit. In this instance, an additional smoothing capacitor may be provided between output terminals of the constant-current circuit  20 . 
     Further, in the lighting devices  1   a  to  1   c  according to the fifth to seventh embodiments, the zener diodes  33   a  and  33   b  are used in the current detection units  300   a  to  300   d . However, the zener diodes  33   a  and  33   b  may not be included the current detection units  300   a  to  300   d . In other words, two ends of each of the zener diodes  33   a  and  33   b  may be short-circuited. In the case where the zener diodes  33   a  and  33   b  are not employed, however, it is necessary to set characteristics of elements so that the thyristors  34   a  and  34   b  become conductive by the currents flowing to the gate electrodes of the thyristors  34   a  and  34   b  when the currents flowing in the capacitors  31   a  and  31   b  exceeds the threshold Ith. 
     In the fifth to ninth embodiments, the LEDs  40   a  and  40   b  are used as the solid-state light-emitting elements. However, organic EL (Electro-Luminescence) elements may be used. 
     In the fifth to ninth embodiments, the thyristors  34   a  and  34   b  or the MOSFETs  73   a  and  73   b  are used as the bypass switches. However, other switching elements may be used as well. For example, switching transistors other than MOSFETs may be used. 
     The constant-current circuit  20  according to the fifth to ninth embodiments may be replaced with another constant-current circuit, e.g., the constant-current circuit  212  shown in  FIG. 19 ,  FIG. 22  or  FIG. 23 . 
     Further, in the fifth to ninth embodiments, the DC-to-DC converter that performs BCM control is used as the constant-current circuit  20 . However, a DC-to-DC converter that performs CCM (continuous current mode) control may be used. 
     Thus far, the lighting devices of the present invention have been described based on the first to ninth embodiments. However, the present invention is not limited to those embodiments. Aspects implemented by adding a variety of modifications conceived by those skilled in the art to the embodiments or aspects implemented by combining elements in different embodiments also fall within the scope of one or more aspects of the present invention, as long as they do not depart from the gist of the present invention. 
     In addition, at least a part of the processing units included in the lighting devices according to the first to ninth embodiments may be implemented as an LSI (large-scale integration), which is an integrated circuit. Each of them may be implemented as one chip or some or the whole of them may be implemented as one chip. 
     The integrated circuit is not limited to an LSI, but may be implemented by a dedicated circuit or a general-purpose processor. A FPGA (field programmable gate array) that can be programmed after an LSI manufacturing, or a reconfigurable processor capable of reconstructing the setting and connections of circuit cells in the LSI may be used. 
     A part or the whole of the elements in the first to ninth embodiments may be implemented with dedicated hardware or may be implemented by executing software programs appropriate for the elements. The elements may be implemented in a such manner that a program executing unit such as a CPU and a processor reads out a software program stored in a storage medium such as a hard disk and a semiconductor memory to execute it. 
     In the block diagrams, the division of the functional blocks is merely illustrative. Several functional blocks may be implemented as a single functional block or a single functional block may be divided into several functional blocks. Further, some of functionalities in a functional block may be performed by another functional block. Additionally, similar functionalities of several functional bocks may be performed by single hardware or software in parallel manner or in a time-divisional manner. 
     The orders in which the steps of the processes are carried out are merely illustrative, and therefore the steps may be carried out in other orders. In addition, some of the steps may be carried out simultaneously (in parallel) with other steps. 
     The circuit configurations shown in the circuit diagrams are merely illustrative and the present invention is not limited to the circuit configurations. In other words, any circuit that can implement the features of the present disclosure like the above-described circuit configurations is also within the scope of the present disclosure. For example, as long as the same functionality as the above-described circuit configurations is implemented, connecting, in series or in parallel, a switching element (transistor), a resistor or a capacitive element to a particular element is also within the scope of the present invention. In other words, in the above embodiments, a term “connected” refers to not only that two terminals (nodes) are directly connected to each other but also that the two terminals (nodes) are connected to each other through another element, as long as the same functionality is implemented. 
     The numerical values given above are merely illustrative and the present disclosure is not limited to those values. Further, the logic levels represented as High and Low, and the switching states represented as On and Off are merely illustrative. It is also possible to achieve the same result by using combinations of logic levels or switching states different from those described above. Further, the configurations of the logic circuits described above are merely illustrative. It is also possible to achieve the equal input/output relationship by using different configurations of logic circuits. 
     While the invention has been shown and described with respect to the embodiments, it will be understood by those skilled in the art that various changes and modifications may be made without departing from the scope of the invention as defined in the following claims.