Patent Publication Number: US-9888532-B2

Title: Lighting circuit and illumination system

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application claims the benefit of priority of Japanese Patent Application Number 2015-049894 filed on Mar. 12, 2015, the entire content of which is hereby incorporated by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present disclosure relates to a lighting circuit which supplies current to a solid-state light-emitting element module including a solid-state light-emitting element such as an LED (light-emitting diode), and to an illumination system including the lighting circuit. 
     2. Description of the Related Art 
     A lighting circuit which supplies current to a solid-state light-emitting element module including a solid-state light-emitting element such as an LED is conventionally known (for example, PTL (Patent Literature) 1: Japanese Unexamined Patent Application Publication No. 2011-181295 and PTL 2: Japanese Unexamined Patent Application Publication No. 2013-004370). In the techniques disclosed in PTL 1 and PTL 2, the solid-state light-emitting element module is so configured as to be removably attached to the lighting circuit. In a situation such as where the solid-state light-emitting element module is damaged, this configuration allows only the solid-state light-emitting element module to be replaced. 
     SUMMARY OF THE INVENTION 
     Furthermore, PTL 1 discloses a configuration of a solid-state light-emitting element module that includes a connection terminal for outputting characteristics setting signals in order that a plurality of solid-state light-emitting element modules having different electrical characteristics are available with a single lighting circuit. With this, the lighting circuit disclosed in PTL 1 outputs, based on the characteristics setting signals, current adapted to the electrical characteristics of the solid-state light-emitting element modules. 
     However, the lighting circuit disclosed in PTL 1 requires a terminal to which the characteristics setting signals are input, and therefore has a complicated circuit configuration. 
     In PTL 2, in order to make available a plurality of solid-state light-emitting element modules having different electrical characteristics with a single lighting circuit, a resistor or the like for identifying electrical characteristics of a solid-state light-emitting element included in the solid-state light-emitting element module is connected in parallel with the solid-state light-emitting element. The lighting circuit detects voltage that is applied when the resistor or the like is supplied with current, thereby identifying the electrical characteristics of the solid-state light-emitting element module. Therefore, the lighting circuit disclosed in PTL 2 outputs current adapted to the electrical characteristics of the solid-state light-emitting element module. 
     However, with the lighting circuit disclosed in PTL 2, there is a power loss because the current also flows through the above-stated resistor constantly when the lighting circuit causes the solid-state light-emitting element module to emit light. In order to reduce the power loss, PTL 2 discloses the configuration in which a switching element is connected in series with the above-stated resistor and remains in OFF state except when the electrical characteristics of the solid-state light-emitting element module are identified. However, with this configuration in which the switching element is connected, the solid-state light-emitting element module has a complicated configuration. 
     An object of the present disclosure is to provide a lighting circuit that is capable of supplying current adapted to electrical characteristics of a solid-state light-emitting element module and has a simplified circuit configuration and high efficiency, and to provide an illumination system including the lighting circuit. 
     In order to achieve the aforementioned object, a lighting circuit according to one aspect of the present disclosure is a lighting circuit which supplies current to a solid-state light-emitting element module including: a solid-state light-emitting element; a first input terminal connected to one end of the solid-state light-emitting element; a second input terminal connected to another end of the solid-state light-emitting element; and a rectifying element connected in inverse-parallel connection with the solid-state light-emitting element between the first input terminal and the second input terminal, and the lighting circuit includes: a power supplier that supplies current between the first input terminal and the second input terminal of the solid-state light-emitting element module selectively in one of a forward direction and a reverse direction of the solid-state light-emitting element; a voltage detector circuit that detects voltage that is applied between the first input terminal and the second input terminal, when the power supplier supplies current between the first input terminal and the second input terminal in the reverse direction of the solid-state light-emitting element; and a control circuit that controls, based on a result of the detection by the voltage detector circuit, supply of current from the power supplier to the solid-state light-emitting element module. 
     According to the present disclosure, it is possible to provide a lighting circuit that is capable of supplying current adapted to electrical characteristics of a solid-state light-emitting element module and has a simplified circuit configuration and high efficiency, and to provide an illumination system including the lighting circuit. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       The figures depict one or more implementations in accordance with the present teaching, by way of examples only, not by way of limitations. In the figures, like reference numerals refer to the same or similar elements. 
         FIG. 1  is an external perspective view schematically illustrating illumination system according to Embodiment 1; 
         FIG. 2  is a schematic circuit diagram of an illumination system according to Embodiment 1; 
         FIG. 3A  is a circuit diagram illustrating one example of a light source according to Embodiment 1; 
         FIG. 3B  is a circuit diagram illustrating another example of a light source according to Embodiment 1; 
         FIG. 4  is a circuit diagram illustrating a configuration of a lighting circuit according to Embodiment 1; 
         FIG. 5  is a graph showing time waveforms of signals input to switching elements of a lighting circuit according to Embodiment 1; 
         FIG. 6  is a circuit diagram illustrating a configuration of a lighting circuit according to Embodiment 2; and 
         FIG. 7  is an external view of an illumination system according to Embodiment 3. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Hereinafter, exemplary embodiments are described with reference to the accompanying drawings. Note that each of the embodiments described below shows a preferred specific example of the present disclosure. Therefore, the numerical values, shapes, materials, structural elements, arrangement and connection of the structural elements, steps, the processing order of the steps etc., shown in the following embodiments are mere examples, and are not intended to limit the present disclosure. Consequently, among the structural elements in the following embodiments, structural elements not recited in any one of the independent claims which indicate the broadest concepts of the present disclosure are described as arbitrary structural elements. 
     Note that the respective figures are schematic diagrams and are not necessarily precise illustrations. Additionally, substantially the same structural elements in the figures share the same reference signs, and description that would overlap may be omitted or simplified. 
     Embodiment 1 
     1:1. Configuration of Illumination System 
     First, a configuration of an illumination system according to Embodiment 1 is described with reference to the drawings. 
       FIG. 1  is an external perspective view schematically illustrating illumination system  10  according to this embodiment. 
       FIG. 2  is a schematic circuit diagram of illumination system  10  according to this embodiment. 
     As illustrated in  FIG. 1 , illumination system  10  includes luminaire  4  and LED module  2 . 
     Luminaire  4  is a device for supplying current to LED module  2 , and includes power supply box  5  including lighting circuit  1 , and socket  6 . 
     LED module  2  is a solid-state light-emitting element module that emits light when supplied with current from luminaire  4 . As illustrated in  FIG. 2 , LED module  2  includes LED  21 , first input terminal  221  connected to one end of LED  21 , and second input terminal  222  connected to the other end of LED  21 . In addition, LED module  2  further includes diode  23  connected in inverse-parallel connection with LED  21 , between first input terminal  221  and second input terminal  222 . In this embodiment, LED module  2  includes plug  22  which is connected to socket  6  of luminaire  4 , and light source  20  having LED  21 . 
     Lighting circuit  1  supplies current to LED module  2 . Details of lighting circuit  1  are described later. 
     Socket  6  is a coupling part structured so as to be removably attached to plug  22  of LED module  2 , and includes first socket terminal  61  and second socket terminal  62 . Socket  6  is connected to lighting circuit  1 , and an output current of lighting circuit  1  is supplied to LED module  2  via socket  6 . The shape, structure, etc., of socket  6  are not particularly limited as long as they are adapted to plug  22 . 
     Plug  22  is a coupling part structured so as to be removably attached to socket  6  of luminaire  4 , and includes first input terminal  221  and second input terminal  222 . Furthermore, plug  22  is connected to light source  20 , and current input from luminaire  4  to LED module  2  is supplied to light source  20  via plug  22 . The shape, structure, etc., of plug  22  are not particularly limited as long as they are adapted to socket  6 . 
     First input terminal  221  is one of the terminals of plug  22 , and is connected to an anode-side end of LED  21  and a cathode-side end of diode  23 . 
     Second input terminal  222  is one of the terminals of plug  22 , and is connected to a cathode-side end of LED  21  and an anode-side end of diode  23 . 
     First input terminal  221  and second input terminal  222  are connected to first socket terminal  61  and second socket terminal  62  of socket  6 , respectively. 
     Light source  20  is one example of a light source of LED module  2 , and includes LED  21 , diode  23 , first connection terminal  201 , second connection terminal  202 , and a substrate (not illustrated in the drawings) on which these parts are provided. In this embodiment, the substrate is formed of a planar substrate. 
     LED  21  is a solid-state light-emitting element that is used as a light emitter of LED module  2 . LED  21  is formed of a SMD (surface mount device) LED element, for example. Furthermore, LED  21  includes one or more LED elements. 
     Diode  23  is a rectifying element connected in inverse-parallel connection with LED  21 , between first input terminal  221  and second input terminal  222 . Diode  23  is used for identifying electrical characteristics of LED module  2 . In this embodiment, the electrical characteristics of diode  23  correspond to the electrical characteristics of LED module  2 . An example of the electrical characteristics of diode  23  corresponding to the electrical characteristics of LED module  2  is a forward voltage. The forward voltage of diode  23  is approximately 0.6 V, for example. Note that diode  23  may be formed of a single diode or may include a plurality of diodes connected in series or in parallel. Moreover, diode  23  has a function of conducing when a reverse bias voltage is applied to LED  21 , to reduce the occurrence of an excessively high reverse bias voltage being applied to LED  21 . 
     First connection terminal  201  is connected to the anode-side end of LED  21  and the cathode-side end of diode  23 . Connection terminal  201  is connected to first socket terminal  61  of luminaire  4  via plug  22 . 
     Second connection terminal  202  is connected to the cathode-side end of LED  21  and the anode-side end of diode  23 . Second connection terminal  202  is connected to second Socket terminal  62  of luminaire  4  via plug  22 . 
     Lighting circuit  1  according to this embodiment identifies the electrical characteristics of LED module  2  based on the configuration of an element such as diode  23  connected in inverse-parallel connection with LED  21 . Next, another example of the configuration of light source  20  in LED module  2  illustrated in  FIG. 2  is described with reference to the drawings. 
       FIG. 3A  is a circuit diagram illustrating light source  20 A according to this embodiment. 
       FIG. 3B  is a circuit diagram illustrating source  20 B according to this embodiment. 
     Light source  20 A is one example of the light source according to this embodiment. As illustrated in  FIG. 3A , light source  20 A is different from light source  20  described above, in terms of the configuration of diode  23 A. 
     Diode  23 A is a rectifying element including a plurality of diodes connected in series, and has a higher forward voltage than diode  23  included in light source  20 . For example, diode  23 A has a configuration in which three diodes  23  are connected in series. The forward voltage of diode  23 A is approximately 1.8 V, for example. 
     For example, in order to identify light source  20  illustrated in  FIG. 2  and light source  20 A, it is sufficient that voltage applied between first connection terminal  201  and second connection terminal  202  is detected in a state where current flows as a result of a reverse bias for LED  21  being applied, between first connection terminal  201  and second connection terminal  202 . Since this voltage is equivalent to the forward voltage of diode  23  or diode  23 A, it is possible to identify light source  20  and light source  20 A based on the forward voltage. Note that although diode  23 A includes the plurality of diodes connected in series, the configuration of diode  23 A is not limited, to this example. For example, diode  23 A may be formed of a single diode having a high forward voltage. 
     Light source  20 B is another example of the light source according to this embodiment. As illustrated in  FIG. 3B , light source  20 B is different from light source  20  described above in that resistor  24  is connected in series with diode  23 . 
     For example, in order to identify light source  20  illustrated in  FIG. 2  and light source  20 B, it is sufficient that voltage applied between first connection terminal  201  and second connection terminal  202  is detected, as in the case of identifying light source  20  and light source  20 A. This voltage is equivalent to the forward voltage of diode  23  or a sum of the forward voltage and voltage applied to resistor  24 . Accordingly, it is possible to identify light source  20  and light source  20 B based on the voltage applied between first connection terminal  201  and second connection terminal  202 . 
     1-2. Configuration of Lighting Circuit 
     Next, a configuration of lighting circuit  1  according to this embodiment is described with reference to the drawings. 
       FIG. 4  is a circuit diagram illustrating a configuration of lighting circuit  1  according to this embodiment.  FIG. 4  illustrates lighting circuit  1 , illumination system  10  including lighting circuit  1 , and AC (alternating current) power supply  3  which supplies AC voltage to lighting circuit  1 . 
     AC power supply  3  outputs AC voltage and is a system power supply such as a commercial power supply which outputs AC voltage of 100 V to 242 V, for example. 
     As illustrated in  FIG. 4 , lighting circuit  1  includes power supplier  11 , control circuit  13 , and voltage detector circuit  15 . Furthermore, lighting circuit  1  includes first output terminal  101  and second output terminal  102 . 
     First output terminal.  101  and second output terminal  102  are terminals from which current is output to LED module  2  and that are respectively electrically connected to first input terminal  221  and second input terminal  222  of LED module  2 . 
     Power supplier  11  is a circuit that supplies current between first input terminal  221  and second input terminal.  222  of LED module  2  selectively in one of a forward direction and a reverse direction of LED  21 . In this embodiment, power supplier  11  converts to DC (direct-current) voltage AC voltage input from AC power supply  3 , and additionally performs DC-to-DC conversion, thereby generating constant DC. The direction and value of this constant DC are controlled by control circuit  13 . As illustrated in  FIG. 4 , power supplier  11  includes rectifier  111 , capacitor  112 , resistor  113 , inductor  114 , and switching elements  121 ,  122 ,  123 , and  124 . 
     Rectifier  111  is a circuit that rectifies AC voltage input from AC power supply  3 . Rectifier  111  includes a diode bridge, for example. 
     Capacitor  112  is an element for smoothing pulsing DC voltage output from rectifier  111 . Capacitor  112  has one end connected at node N 1  to a high-voltage output terminal of rectifier  111  and the other end connected at node N 2  to a low-voltage output terminal of rectifier  111 . In this embodiment, capacitor  112  is formed of an electrolytic capacitor. 
     Resistor  113  is a sense resistor for detecting current that flows to LED  21 , that is, an output current of power supplier  11 . Resistor  113  has one end connected at node N 2  to the low-voltage output terminal of rectifier  111  and the other end connected to node N 5 . 
     Inductor  114  is a choke coil, and stores and releases energy according to a switching operation of each of the switching elements. Inductor  114  has one end connected to node N 3  and the other end connected to first output terminal  101 . 
     Switching elements  121 ,  122 ,  123 , and  124  perform switching (repeat turning ON and OFF) under control of control circuit  13 . In this embodiment, each of the switching elements is an N-channel (MOSFET) metal-oxide semiconductor field-effect transistor connected in series with inductor  114 . A series circuit including switching elements  121  and  122  and a series circuit including switching elements  123  and  124  are connected in parallel. This means that switching elements  121 ,  122 ,  123 , and  124  form a full-bridge circuit. Each of drain electrodes of switching elements  121  and  123  is connected to node N 1 , and each of source electrodes of switching elements  122  and  124  is connected to node N 5 . Node N 3  which is a connection point between switching element  121  and switching element  122  is connected to first output terminal  101  via inductor  114 . Node N 4  which is a connection point between switching element  123  and switching element  124  is connected to second output terminal  102 . With the above-described configurations of the switching elements and inductor  114 , it is possible for control circuit  13  to control the direction and value of current which power supplier  11  outputs. An operation performed by power supplier  11  will be described later in detail. 
     Voltage detector circuit  15  detects voltage that is applied between first output terminal  101  and second output terminal  102 , that is, voltage that is applied between first input terminal  221  and second input terminal  222  of LED module  2 . With this, it is possible to detect voltage that is applied across diode  23  of LED module  2  (i.e., the forward voltage of diode  23 ). In this embodiment, voltage detector circuit  15  includes resistors  151 ,  152 ,  153 , and  155 , and operational amplifier  154 , and functions as a differential amplifier circuit. Voltage detector circuit  15  generates at an output terminal of operational amplifier  154  voltage corresponding to the voltage that is applied across diode  23 . Assuming that resistors  152  and  153  have respective resistance values R 152  and R 153 , the differential amplifier circuit generates at the output terminal of operational amplifier  154  voltage that is R 152 /R 153  times as high as the voltage that is applied across diode  23 . In this embodiment, the resistance values of resistors  152  and  153  are set so that R 152 /R 153  is nearly 1. To put it differently, voltage detector circuit  15  outputs almost the same voltage as the voltage that is applied across diode  23 . Furthermore, voltage detector circuit  15  outputs the detected voltage to hold circuit  135  of control circuit  13 . 
     Control circuit  13  controls, based on the result of the detection by voltage detector circuit  15 , supply of current from power supplier  11  to LED module  2 . More specifically, control circuit  13  controls the direction and value of current which power supplier  11  outputs by controlling the switching elements of power supplier  11 . Control circuit  13  includes comparator  130 , driver circuits  131  and  132 , oscillator circuit  133 , timer circuit  134 , hold circuit  135 , NOT circuit  136 , comparator  137 , and IC (integrated circuit)  14 . 
     Timer circuit  134  switches an operation mode of lighting circuit  1  between a detection mode and a lighting mode. The detection mode is a mode of detecting voltage that is applied between first output terminal  101  and second output terminal  102 . The lighting mode is a mode of lighting LED module  2 . Timer circuit  134  outputs a signal for lighting circuit  1  to operate in the detection mode over a predetermined time after lighting circuit  1  starts operating, and outputs a signal for lighting circuit  1  to operate in the lighting mode after the predetermined time elapses. Specifically, timer circuit  134  outputs a HIGH signal over a predetermined time after AC power supply  3  starts applying AC voltage to lighting circuit  1 , and outputs a LOW signal after the predetermined time elapses. Timer circuit  134  outputs the signals to oscillator circuit  133 , hold circuit  135 , and NOT circuit  136  via, node N 13 . Note that the above predetermined time is not particularly limited as long as it is long enough for voltage detector circuit  15  to detect voltage. 
     Oscillator circuit  133  is a signal generator for determining a signal output timing for driver circuit  132 . Oscillator circuit  133  outputs an AC signal to driver circuit  132  when lighting circuit  1  is in the detection mode, that is, when receiving a HIGH signal from timer circuit  134 . 
     Driver circuit  132  performs control of causing switching elements  122  and  123  to repeat turning ON and OFF (i.e., perform a switching operation) when lighting circuit  1  is in the detection mode. With driver circuit  132  causing switching elements  122  and  123  to repeat turning ON and OFF at the same time, power supplier  11  supplies a constant forward current to diode  23  (that is, applies a reverse bias voltage to LED  21 ). Driver circuit  132  receives an AC signal from oscillator circuit  133  and causes switching elements  122  and  123  to perform a switching operation in synchronization with the cycle of the AC signal. 
     Hold circuit  135  maintains voltage output from voltage detector circuit  15  when hold circuit  135  receives a HIGH signal from timer circuit  134 . Hold circuit  135  outputs the output voltage to an inverting input terminal of comparator  137 . 
     Comparator  137  is a circuit that compares reference voltage V0 and voltage output; from voltage detector circuit  15 . When the output voltage is higher than reference voltage V0 which is a predetermined threshold value, comparator  137  outputs a LOW signal. On the other hand, when the output voltage is lower than reference voltage V0, comparator  137  outputs a HIGH signal. Furthermore, comparator  137  outputs an output signal to buffer circuit  141  of IC  14 . For example, when LED module  2  includes either diode  23  having a forward voltage of approximately 0.6 V or diode  23 A having a forward voltage of approximately 1.8 V, it is sufficient that reference voltage V0 is set to about an intermediate value between the above two forward voltages, that is, approximately 1.2 V. 
     IC  14  is a circuit that switches, according to an output signal of comparator  137 , voltage that is output to an inverting input terminal of comparator  130 . The output voltage corresponds to a target value of current that is output from power supplier  11  to LED module  2 . IC  14  includes buffer circuit;  141  and changeover switch  142 . 
     Buffer circuit  141  is for shaping a waveform of an output signal of comparator  137 . 
     Changeover switch  142  is an element that connects an output terminal and terminal  143  or  144  of IC  14 . Changeover switch  142  connects the output terminal and terminal  144  of IC  14  when a HIGH signal is input thereto, and connects the output terminal and terminal  143  of IC  14  when a LOW signal is input thereto. Reference voltages V1 and V2 different from each other are applied to terminals  144  and  143 , respectively. Reference voltages V1 and V2 correspond to a first current value and a second current value, respectively, which are target values of the output current of power supplier  11 . Specifically, when the voltage that is detected by voltage detection circuit  15  is lower than reference voltage V0 which is a predetermined, threshold value, control circuit  13  sets the target value of the output current of power supplier  11  to the first current value. On the other hand, when the voltage that is detected by voltage detection circuit  15  is higher than reference voltage V0 which is a predetermined threshold value, control circuit  13  sets the target value of the output current of power supplier  11  to the second current value. 
     Comparator  130  is a circuit that compares voltage corresponding to current output from power supplier  11  and reference voltage V1 or V2 corresponding to the target value of the current. Output of driver circuit  131  is controlled based on an output signal of comparator  130  to allow feedback control so that the output current of power supplier  11  approaches the target value. Voltage at node N 5 , that is, voltage that is applied to resistor  113 , is input to a non-inverting input terminal of comparator  130 . Reference voltage V1 or V2 which is output of IC  14  is input to an inverting input terminal of comparator  130 . 
     NOT circuit  136  inverts a signal received from timer circuit  134  and outputs the signal to driver circuit  131 . 
     Driver circuit  131  performs control of causing switching elements  121  and  124  to repeat turning ON and OFF (i.e., perform a switching operation) when lighting circuit  1  is in the lighting mode. With driver circuit  131  causing switching elements  121  and  124  to repeat turning ON and OFF at the same time, power supplier  11  supplies a constant forward current to LED  21 . Furthermore, driver circuit  131  receives, from NOT circuit  136 , a signal resulting from inverting the output signal of timer circuit  134 , and determines, based on the signal, whether the current mode is the lighting mode. In this embodiment, the current mode is determined as the lighting mode when the signal is a HIGH signal, and is determined as the detection mode when the signal is a LOW signal. Furthermore, driver circuit  131  receives a signal from comparator  130 . When driver circuit  131  receives a HIGH signal from comparator  130 , that is, when current that is output from power supplier  11  is greater than the target value of the current, driver circuit  131  outputs a HIGH signal having a reduced pulse width to switching elements  121  and  124 . With this, driver circuit  131  reduces the output current of power supplier  11 . On the other hand, when driver circuit  131  receives a LOW signal from comparator  130 , that is, when current that is output from power supplier  11  is less than the target value of the current, driver circuit  131  outputs a HIGH signal having an increased pulse width to switching elements  121  and  124 . With this, driver circuit  131  increases the output current of power supplier  11 . Thus, control circuit  13  performs such feedback control that the output current of power supplier  11  approaches the target value. 
     With the above-described configuration, lighting circuit  1  is capable of identifying electrical characteristics of LED module  2  according to a forward voltage of diode  23  of LED module  2  and supplying an output current adapted to the electrical characteristics. Specifically, control circuit  13  detects a forward voltage of diode  23  by detecting voltage that is applied between first input terminal  221  and second input terminal  222  of LED module  2 . When the forward voltage is lower than a predetermined, threshold voltage, control circuit  13  controls power supplier  11  in such a way that current that is supplied in a forward direction to LED  21  is of a first current value. On the other hand, when the forward voltage is higher than the threshold value, control circuit  13  controls power supplier  11  in such a way that the current is of a second current value different from the first current value. 
     1-3. Operation Performed by Lighting Circuit 
     Next, an operation performed by lighting circuit  1  according to this embodiment is described with reference to the drawings. 
       FIG. 5  is a graph showing time waveforms of signals input to switching elements of lighting circuit  1  according to this embodiment. In  FIG. 5 , time waveforms of voltages V121, V122, V123, and V124 of the signals that are input to respective gate electrodes of switching elements  121 ,  122 ,  123 , and  124  are shown. 
     First, at time point t0 in  FIG. 5 , AC power supply  3  applies AC voltage to lighting circuit  1 , and timer circuit  134  then starts counting time and outputs, to NOT circuit  136 , oscillator circuit  133 , and hold circuit  135 , HIGH signals indicating that the current mode is the detection mode. 
     When oscillator circuit  133  receives the HIGH signal from timer circuit  134 , oscillator circuit  133  outputs an AC signal to driver circuit  132 . 
     When driver circuit  132  receives the AC signal from oscillator circuit  133 , driver circuit  132  repeatedly outputs a HIGH voltage signal pulse to the respective gate electrodes of switching elements  122  and  123  in synchronization with the AC signal as shown in  FIG. 5 . The width of the pulse is a parameter that determines current which lighting circuit  1  outputs in the detection mode. In this embodiment, the current which lighting circuit  1  outputs increases as the width of the pulse increases. The width of the pulse is determined based on characteristics, etc., of diode  23  and LED  21  in such a way that diode  23  and LED  21  are not damaged. 
     Input of the HIGH signal to the gate electrodes of switching elements  122  and  123  causes switching elements  122  and  123  to turn ON. In this case, current flows along the path from AC power supply  3  sequentially to rectifier  111 , switching element  123 , diode  23 , inductor  114 , switching element  122 , resistor  113 , rectifier  111 , and AC power supply  3 , and a forward voltage is applied across diode  23 . Note that in this case, voltage is applied to LED  21  in a reverse direction (that is, a reverse bias voltage is applied thereto), and therefore no current flows to LED  21 . 
     In the state where the forward voltage is applied across diode  23 , voltage detector circuit  15  detects the forward voltage and outputs, to hold circuit  135 , voltage corresponding to the forward voltage. 
     When hold circuit  135  receives the HIGH signal from timer circuit  134 , hold circuit  135  maintains the voltage output from voltage detector circuit  15  that corresponds to the forward voltage of diode  23 , and outputs the voltage to the inverting input terminal of comparator  137 . 
     Comparator  137  compares the voltage received from hold circuit  135  and reference voltage V0 corresponding to the predetermined threshold value. When the voltage received from hold circuit  135  that corresponds to the forward voltage of diode  23  is lower than reference voltage V0, comparator  137  outputs a HIGH signal to IC  14 . When the voltage received from hold circuit  135  that corresponds to the forward voltage of diode  23  is higher than reference voltage V0, comparator  137  outputs a LOW signal to IC  14 . 
     When IC  14  receives the HIGH signal from comparator  137 , that is, when the forward voltage of diode  23  is lower than the predetermined threshold value, IC  14  outputs, to the inverting input terminal of comparator  130 , reference voltage V1 which corresponds to the first current value. When IC  14  receives the LOW signal from comparator  137 , that is, when the forward voltage of diode  23  is higher than the predetermined threshold value, IC  14  outputs, to the inverting input terminal of comparator  130 , reference voltage V2 which corresponds to the second current value. 
     Note that when timer circuit  134  outputs a HIGH signal indicating that the current mode is the detection mode, driver circuit  131  receives a LOW signal from NOT circuit  136 . With this, driver circuit  131  keeps outputting LOW voltage signals to the gate electrodes of switching elements  121  and  124  as shown in  FIG. 5 . 
     As described above, lighting circuit  1  identifies electrical characteristics of LED module  2  by detecting a forward voltage of diode  23  (that is, voltage that is applied between first output terminal  101  and second output terminal  102 ) in the detection mode. Furthermore, lighting circuit  1  determines a target value of the output current to be the first current value or the second current value so that the output current is adapted to the electrical characteristics of LED module  2 . 
     Subsequently, on and after time point t1 in  FIG. 5 , timer circuit  134  outputs, to NOT circuit  136 , oscillator circuit  133 , and hold circuit  135 , a LOW signal indicating that the current mode is the lighting mode. 
     When NOT circuit  136  receives the LOW signal from timer circuit  134 , NOT circuit  136  outputs a HIGH signal to driver circuit  131 . 
     When driver circuit  131  receives the HIGH signal from NOT circuit  136 , driver circuit  131  repeatedly outputs a HIGH voltage signal pulse to the respective gate electrodes of switching elements  121  and  124  as shown in  FIG. 5 . The width of the pulse is a parameter that determines current that is output from lighting circuit  1  in the lighting mode. In this embodiment, the current which lighting circuit  1  outputs increases as the width of the pulse increases. The width of the pulse is controlled according to a signal that is input from comparator  130  to driver circuit  131 . 
     Input of the HIGH signal to the gate electrodes of switching elements  121  and  124  causes switching elements  121  and  124  to turn ON. In this case, current flows along the path from AC power supply  3  sequentially to rectifier  111 , switching element  121 , inductor  114 , LED  21 , switching element  124 , resistor  113 , rectifier  111 , and AC power supply  3 , and LED  21  is turned ON. Note that in this case, voltage is applied to diode  23  in a reverse direction (that is, a reverse bias voltage is applied thereto). Therefore, no current flows to diode  23 . 
     As described above, in order to turn LED  21  ON, voltage that is applied to resistor  113  is input to the non-inverting input terminal of comparator  130 , and compared therein with reference voltage V1 or reference voltage V2 which corresponds to a target value of the output current of power supplier  11 , that is, the first current value or the second current value. When the voltage that is applied to resistor  113  is higher than the reference voltage, that is, when the output current of power supplier  11  is greater than the target value, comparator  130  outputs a HIGH signal to driver circuit  131 . When driver circuit  131  receives the HIGH signal from comparator  130 , driver circuit  131  outputs a HIGH signal having a reduced pulse width to switching elements  121  and  124 , thereby reducing the output current of power supplier  11 . On the other hand, when current that is applied to resistor  113  is lower than the reference voltage, that is, when the output current of power supplier  11  is less than the target value, comparator  130  outputs a LOW signal to driver circuit  131 . When driver circuit  131  receives the LOW signal from comparator  130 , driver circuit  131  outputs a HIGH signal having an increased pulse width to switching elements  121  and  124 , thereby increasing the output current of power supplier  11 . Thus, control circuit  13  performs such feedback control that the output current of power supplier  11  approaches the target value. 
     By operating as described above, lighting circuit  1  is capable of identifying electrical characteristics of LED module  2  according to a forward voltage of diode  23  of LED module  2 , and supplying an output current adapted to the electrical characteristics. 
     1-4. Advantageous Effects, etc. 
     As described above, lighting circuit  1  according to this embodiment supplies current to LED module  2 . LED module  2  includes: LED  21 ; first input terminal  221  connected to one end of LED  21 ; second input terminal  222  connected to the other end of LED  21 ; and diode  23  connected in inverse-parallel connection with LED  21  between first input terminal  221  and second input terminal  222 . Lighting circuit  1  includes power supplier  11  which supplies current between first input terminal  221  and second input terminal  222  of LED module  2  selectively in one of the forward direction and the reverse direction of LED  21 . In addition, lighting circuit  1  further includes voltage detector circuit  15  which detects voltage that is applied between first input terminal  221  and second input terminal  222  when power supplier  11  supplies current in the reverse direction of LED  21 , between first input terminal  221  and second input terminal  222 . Moreover, lighting circuit  1  further includes control circuit  13  which controls, based on the result of the detection by voltage detector circuit  15 , supply of current from power supplier  11  to LED module  2 . 
     With this, lighting circuit  1  is capable of identifying electrical characteristics of LED module  2  by detecting a forward voltage of diode  23  that corresponds to the electrical characteristics. Thus, lighting circuit  1  is capable of supplying current adapted to LED module  2 . Furthermore, since LED module  2  has a configuration in which diode  23  is connected in inverse-parallel connection with LED  21 , no current flows to diode  23  when LED module  2  is ON. Accordingly, the power loss at diode  23  can be reduced. 
     This means that lighting circuit  1  according to this embodiment has high efficiency. Furthermore, in lighting circuit  1 , there is no need to provide a separate terminal for detecting electrical characteristics of LED module  2 , and therefore it is possible to simplify the circuit configuration. 
     Furthermore, in lighting circuit  1 , power supplier  11  includes four switching elements  121 ,  122 ,  123 , and  124  which form a full-bridge circuit. 
     With this, lighting circuit  1  is capable of supplying current in the forward and reverse directions of LED  21  by use of a single circuit. Thus, it is possible to simplify the circuit configuration of lighting circuit  1 . 
     Furthermore, in lighting circuit  1 , control circuit  13  controls power supplier  11  in such a way that current that is supplied in the forward direction to LED  21  is of the first current value when the voltage detected by voltage detector circuit  15  is lower than a predetermined threshold value. On the other hand, when the voltage is higher than the threshold value, control circuit  13  controls power supplier  11  in such a way that the current is of the second current value different from the first current value. 
     With this, lighting circuit  1  is capable of switching current that is supplied to LED module  2  between two different current values according to the electrical characteristics of LED module  2 . 
     Furthermore, illumination system  10  according to this embodiment includes lighting circuit  1  and LED module  2 . 
     This allows illumination system  10  to produce the same or similar effects as those produced by lighting circuit  1 . For a configuration of LED module  2  included in illumination system  10 , it is possible to adopt a relatively simple configuration in which diode  23  is connected in inverse-parallel connection with LED  21 . 
     Embodiment 2 
     Next, a configuration of an illumination system according to Embodiment 2 is described. The illumination system according to this embodiment adopts, as a light source to be included in the LED module, light source  20 B illustrated in  FIG. 3B . The illumination system according to this embodiment includes a lighting circuit adapted to the LED module, and therefore is capable of continuously changing a preset value of current that is supplied to the LED module. 
     The following description will focus on the configuration of the illumination system according to this embodiment that is different from that of illumination system  10  according to Embodiment 1 described above; as such, description of configurations common to these embodiments will be omitted. 
     2-1. Configuration of Lighting Circuit 
     First, a configuration of a lighting circuit and a configuration of an illumination system including the lighting circuit according to this embodiment are described with reference to the drawings. 
       FIG. 6  is a circuit diagram illustrating a configuration of lighting circuit  1 B according to this embodiment.  FIG. 6  illustrates lighting circuit  1 B, illumination system  10 B including lighting circuit  1 B, and AC power supply  3  which supplies AC voltage to lighting circuit  1 B. 
     As illustrated in  FIG. 6 , illumination system  10 B includes lighting circuit  1 B and LED module  2 B. 
     LED module  2 B is a solid-state light-emitting element module including light source  20 B illustrated in  FIG. 3B . In LED module  2 B, diode  23  is connected in inverse-parallel connection with LED  21 , and resistor  24  is connected in series with diode  23 . 
     Lighting circuit  1 B supplies current to LED module  2 B and includes power supplier  11 , control circuit  13 B, and voltage detector circuit  15 . Furthermore, lighting circuit  1 B includes first output terminal  101  and second output terminal  102 . When compared to lighting circuit  1  according to Embodiment 1 described above, lighting circuit  1 B is different in terms of the configuration of control circuit  13 B, but is the same in the other configurations. A configuration of control circuit  13 B which is a difference between lighting circuit  1 B and lighting circuit  1  is described below. 
     As with control circuit  13  according to Embodiment 1 described above, control circuit  13 B controls, based on the result of the detection by voltage detector circuit  15 , supply of current from power supplier  11  to LED module  2 B. Control circuit  13 B includes comparator  130 , driver circuits  131  and  132 , oscillator circuit  133 , timer circuit  134 , hold circuit  135 , and NOT circuit  136 , as does control circuit  13 . Control circuit  13 B is different from control circuit  13  in that comparator  137  and IC  14  are not provided and that the output voltage of hold circuit  135  is directly input to comparator  130 . 
     With a configuration such as that described above, control circuit  13 B inputs to comparator  130  voltage corresponding to voltage that is applied between first output terminal  101  and second output terminal  102 . The voltage corresponding to voltage that is applied between first output terminal  101  and second output terminal  102  is equivalent to a sum of voltage that is applied to diode  23  and voltage that is applied to resistor  24 . Thus, in this embodiment, voltage corresponding to the sum of these voltages is input to the inverting input terminal of comparator  130  as a value corresponding to a target value of the output current of power supplier  11 . With this, lighting circuit  1  according to this embodiment is capable of continuously changing the target value of the output current according to the forward voltage of diode  23  and the resistance value of resistor  24 . For example, it is possible to continuously change the target value of the output current by continuously changing the resistance value of resistor  24  in this embodiment, the target value of the output current of lighting circuit  1 B has a positive correlation with the voltage that is applied between first output terminal  101  and second output terminal  102 . Therefore, it is sufficient, for example, to increase the resistance value of resistor  24  as current to be supplied to LED module  2  increases. In order to set the target value of the output current to a desired value, the resistance values of resistors  152  and  153  of voltage detector circuit  15  may be set as appropriate. 
     2-2. Advantageous Effects, etc. 
     As described above, in lighting circuit  1 B according to this embodiment, control circuit  13 B continuously changes, according to the voltage detected by voltage detector circuit  15 , the value of current that is supplied in the forward direction to LED  21 . 
     With this, lighting circuit  1 B is capable of changing the output current according to LED module  2 B and therefore is capable of supplying current to LED module  2 B that has various electrical characteristics. 
     Illumination system  10 B according to this embodiment includes lighting circuit  1 B and LED module  2 B, and further includes resistor  24  which is connected in series with diode  23 . 
     This allows illumination system  10 B to produce the same or similar effects as those produced by lighting circuit  1 B. 
     Embodiment 3 
     Next, an illumination system according to Embodiment 3 is described.  FIG. 7  is an external view of illumination system  10 C according to this embodiment. Illumination system  10 C illustrated in  FIG. 7  includes luminaire  4 C and LED module  2 C. Luminaire  4 C includes one of lighting circuits  1  and  1 B according to the above embodiments and socket  6  (not illustrated in the drawings) for connecting LED module  2 C. In this embodiment, luminaire  4 C is a downlight, and includes lamp mount  41  which houses the lighting circuit and to which LED module  2 C is fitted. LED module  2 C includes the same or similar circuit as that included in LED module  2  or  2 B, and includes housing  250  having, on an external surface, plug  22  for connecting to socket  6  of luminaire  4 C. 
     Since such illumination system  10 C includes one of lighting circuits  1  and  1 B and one of LED modules  2  and  2 B according to the above embodiments, illumination system  10 C is capable of producing the same or similar advantageous effects as those produced by one of illumination systems  10  and  10 B according to the above embodiments. 
     Variations and Others 
     Although the lighting circuit and the illumination system according to the embodiments have been described above, the present disclosure is not limited to these embodiments. 
     For example, although LED  21  is formed of an SMD LED element in the above embodiments, this is not the only example. For example, an LED chip mounted on a substrate per se may be adopted as LED  21 . 
     Furthermore, although LEI)  21  is used as a solid-state light-emitting element in the above embodiments, other solid-state light-emitting elements such as an organic EL (electroluminescence) element may be used. 
     Furthermore, although diode  23  or  23 A is used as a rectifying element in the above embodiments, the rectifying element is not limited to diode  23  or  23 A. It is sufficient that the rectifying element is an element that exhibits rectifying behavior 
     Furthermore, although power supplier  11  includes four switching elements that form a full-bridge circuit, the configuration of power supplier  11  is not limited to this example. Power supplier  11  can be any power supply that can control the direction and value of current that is to be output. 
     Furthermore, although one threshold value is used in Embodiment 1 described above, a plurality of threshold, values may be provided to set three or more target values of current. 
     Furthermore, although the target value of the output current of lighting circuit  1 B has a positive correlation with voltage that is applied between first output terminal  101  and second output terminal  102  in Embodiment 2 described above, the relationship between the target value and the voltage is not limited to this example. For example, it is possible to provide a lighting circuit in which the target value has a negative correlation with the voltage by inserting an inverse proportion operational circuit between hold circuit  135  and comparator  130  in lighting circuit  1 B. 
     While the foregoing has described what are considered to be the best mode and/or other examples, it is understood that various modifications may be made therein and that the subject matter disclosed herein may be implemented in various forms and examples, and that they may be applied in numerous applications, only some of which have been described herein. It is intended by the following claims to claim any and all modifications and variations that fall within the true scope of the present teachings.