Patent Publication Number: US-2015084510-A1

Title: Power Supply Device and Luminaire

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is based upon and claims the benefit of priority from Japanese Patent Application No. 2013-198382, filed on Sep. 25, 2013; the entire contents of which are incorporated herein by reference. 
     FIELD 
     Embodiments described herein relate generally to a power supply device and a luminaire. 
     BACKGROUND 
     In a luminaire, as an illumination light source, an incandescent lamp or a fluorescent lamp are replaced with a light source that consumes less energy and has long life, for example, a light-emitting element such as a light-emitting diode (LED). In a power supply device that supplies electric power to such a light source, an element of a normally on type is used as a switching element that converts electric power according to switching. Improvement of safety is desired in the power supply device including the switching element of the normally on type. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a circuit diagram schematically showing a luminaire according to a first embodiment; 
         FIG. 2  is a circuit diagram schematically showing a luminaire according to a second embodiment; 
         FIG. 3  is a circuit diagram schematically showing a luminaire according to a third embodiment; 
         FIG. 4  is a circuit diagram schematically showing a luminaire according to a fourth embodiment; 
         FIG. 5  is a circuit diagram schematically showing another luminaire according to the fourth embodiment; 
         FIG. 6  is a circuit diagram schematically showing a luminaire according to a fifth embodiment; 
         FIG. 7  is a graph showing an example of waveforms of an inductor current; 
         FIG. 8  is a circuit diagram schematically showing another luminaire according to the fifth embodiment; and 
         FIG. 9  is a circuit diagram schematically showing a luminaire according to a sixth embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     In general, according to one embodiment, there is provided a power supply device including: a DC-DC converter; and a protection circuit. The DC-DC converter includes: a switching element of a normally on type; a current control element of the normally on type connected to the switching element in series and configured to control an electric current flowing to the switching element; and a rectifying element connected to the current control element in series. The DC-DC converter converts, according to ON and OFF of the switching element, an input first direct-current voltage into a second-direct current voltage, which has an absolute value different from an absolute value of the first direct-current voltage, and supplies the second direct-current voltage to a load. When an electric current equal to or larger than a predetermined value flows to the switching element and the current control element, the protection circuit cuts off the electric current flowing to the switching element and the current control element. 
     According to another embodiment, there is provided a power supply device including a DC-DC converter. The DC-DC converter includes: a switching element of a normally on type; a current control element of the normally on type connected to the switching element in series and configured to control an electric current flowing to the switching element; and a rectifying element connected to the current control element in series. The DC-DC converter converts, according to ON and OFF of the switching element, an input first direct-current voltage into a second-direct current voltage, which has an absolute value different from an absolute value of the first direct-current voltage, and supplies the second direct-current voltage to a load. A saturation current value of the current control element is equal to or larger than a maximum rated current of the switching element. 
     According to still another embodiment, there is provided a luminaire including: a lighting load; and a power supply device configured to supply electric power to the lighting load. The power supply device includes: a DC-DC converter; and a protection circuit. The DC-DC converter includes: a switching element of a normally on type; a current control element of the normally on type connected to the switching element in series and configured to control an electric current flowing to the switching element; and a rectifying element connected to the current control element in series. The DC-DC converter converts, according to ON and OFF of the switching element, an input first direct-current voltage into a second-direct current voltage, which has an absolute value different from an absolute value of the first direct-current voltage, and supplies the second direct-current voltage to a load. When an electric current equal to or larger than a predetermined value flows to the switching element and the current control element, the protection circuit cuts off the electric current flowing to the switching element and the current control element. 
     According to still another embodiment, there is provided a luminaire including: a lighting load; and a power supply device configured to supply electric power to the lighting load. The power supply device includes a DC-DC converter. The DC-DC converter includes: a switching element of a normally on type; a current control element of the normally on type connected to the switching element in series and configured to control an electric current flowing to the switching element; and a rectifying element connected to the current control element in series. The DC-DC converter converts, according to ON and OFF of the switching element, an input first direct-current voltage into a second-direct current voltage, which has an absolute value different from an absolute value of the first direct-current voltage, and supplies the second direct-current voltage to a load. A saturation current value of the current control element is equal to or larger than a maximum rated current of the switching element. 
     Embodiments are explained below with reference to the drawings. 
     The drawings are schematic or conceptual. Relations between thicknesses and widths of sections, ratios of the sizes among the sections, and the like are not always the same as real ones. Even if the same sections are shown, dimensions and ratios of the sections may be shown different depending on the drawings. 
     In this specification and the drawings, components same as the components already shown in the drawings and explained are denoted by the same reference numerals and signs and detailed explanation of the components is omitted as appropriate. 
     First Embodiment 
       FIG. 1  is a circuit diagram schematically showing a luminaire according to a first embodiment. 
     As shown in  FIG. 1 , a luminaire  10  includes a power supply device  12  and a lighting load  14  (a load). 
     The power supply device  12  includes a pair of input terminals  12   a  and  12   b  and a pair of output terminals  12   c  and  12   d . Potential set at the output terminal  12   c  is higher than potential set at the output terminal  12   d . In the following explanation, the output terminal  12   c  is referred to as high-potential output terminal  12   c  and the output terminal  12   d  is referred to as low-potential output terminal  12   d.    
     The lighting load  14  is connected between the high-potential output terminal  12   c  and a low-potential output terminal  12   d . In this specification, “connection” means electrical connection and includes physical non-connection and connection via other components. 
     The lighting load  14  includes an illumination light source  16 . As the illumination light source  16 , for example, an LED is used or a light-emitting element having a forward drop voltage is used. The lighting load  14  lights the illumination light source  16  according to application of an output voltage VOUT and supply of an output current IOUT from the power supply device  12 . The lighting load  14  can change at least one of the output voltage VOUT and the output current IOUT and perform dimming. Values of the output voltage VOUT and the output current IOUT are specified according to the illumination light source  16 . 
     The power supply device  12  is connected to an alternating-current power supply  2  and a dimmer  3 . The alternating-current power supply  2  is connected between the pair of input terminals  12   a  and  12   b . The dimmer  3  is connected between the alternating-current power supply  2  and the input terminal  12   a.    
     The alternating-current power supply  2  is, for example, a commercial power supply. The dimmer  3  generates an alternating-current voltage VCT subjected to conduction angle control from an alternating-current power supply voltage VIN of the alternating-current power supply  2 . The power supply device  12  converts the alternating-current voltage VCT supplied from the dimmer  3  into the output voltage VOUT and outputs the output voltage VOUT to the lighting load  14  to thereby light the illumination light source  16 . The power supply device  12  performs dimming of the illumination light source  16  in synchronization with the alternating-current voltage VCT subjected to the conduction angle control. The dimmer  3  is provided according to necessity and can be omitted. When the dimmer  3  is not provided, the power supply voltage VIN of the alternating-current power supply  2  is supplied to the power supply device  12 . In an example explained below, the dimmer  3  is connected to the alternating-current power supply  2 . 
     As the conduction angle control by the dimmer  3 , there are a system of phase control (a leading edge type) for cutting off an alternating-current voltage immediately after a zero cross and conducting the alternating-current voltage in a specific phase and a system of antiphase control (a trailing edge type) for conducting the alternating-current voltage immediately after the zero cross and cutting off the alternating-current voltage in a specific phase. 
     The phase-controlling dimmer  3  has a simple circuit configuration and can handle a relatively large power load. However, when a triac is used, a light load operation is difficult. The dimmer  3  tends to fall into an unstable operation when a so-called power supply dip occurs in which a power supply voltage temporarily falls. When a capacitive load is connected to the dimmer  3 , the dimmer  3  is incompatible with the capacitive load because a rush current occurs. 
     On the other hand, the antiphase-controlling dimmer  3  can operate even with a light load. A rush current does not occur even if a capacitive load is connected to the dimmer  3 . The operation of the dimmer  3  is stable even if a power supply dip occurs. However, the dimmer  3  has a complicated circuit configuration. The temperature of the dimmer  3  tends to rise. Therefore, the dimmer  3  is not suitable for a heavy load. When an inductive load is connected to the dimmer  3 , a surge occurs. 
     In this embodiment, as an example, the dimmer  3  is connected in series between the alternating-current power supply  2  and the input terminal  12   a  but another configuration may be applied. 
     The power supply device  12  includes an AC-DC converter  20 , a DC-DC converter  22 , and a protection circuit  24 . The power supply device  12  includes a substrate  13 . The AC-DC converter  20 , the DC-DC converter  22 , and the protection circuit  24  are mounted on the substrate  13 . The substrate  13  includes, for example, an organic material. Heat resistant temperature of the substrate  13  is, for example, equal to or higher than 150° C. 
     The AC-DC converter  20  converts the alternating-current voltage VCT into a direct-current voltage. The DC-DC converter  22  generates the output voltage VOUT from the direct-current voltage supplied from the AC-DC converter  20  and supplies the output voltage VOUT to the lighting load  14 . The protection circuit  24  protects the lighting load  14  from an overcurrent or heat generation involved in the overcurrent. 
     The AC-DC converter  20  includes, for example, a rectifying circuit and a smoothing capacitor. The rectifying circuit rectifies the alternating-current voltage VCT and generates a pulsating voltage from the alternating-current voltage VCT. For example, the rectifying circuit subjects the alternating-current voltage VCT to full-wave rectification. The smoothing capacitor smoothes the pulsating voltage rectified by the rectifying circuit and converts the pulsating voltage into a direct-current voltage. Consequently, the alternating-current voltage VCT is converted into a direct-current voltage. The AC-DC converter  20  includes a high-potential terminal  20   a  and a low-potential terminal  20   b . The AC-DC converter  20  generates a direct-current voltage between the high-potential terminal  20   a  and the low-potential terminal  20   b.    
     The DC-DC converter  22  includes, for example, a switching element  40 , a current control element  41 , a rectifying element  42 , an inductor  43 , a feedback winding  44 , a coupling capacitor  45 , voltage dividing resistors  46  and  47 , and an output capacitor  48 . 
     The switching element  40  and the current control element  41  are, for example, field effect transistors (FETs) or high electron mobility transistors (HEMTs) and are elements of the normally on type. The switching element  40  and the current control element  41  include, for example, a wide band gap semiconductor such as GaN or SiC. As the switching element  40  and the current control element  41 , for example, wide band gap semiconductor elements are used. 
     A drain of the switching element  40  is connected to the high-potential terminal  20   a  of the AC-DC converter  20 . A source of the switching element  40  is connected to a drain of the current control element  41 . That is, the current control element  41  is connected to the switching element  40  in series. A current path between the drain and a source of the current control element  41  is connected to a current path between the drain and the source of the switching element  40  in series. A gate (a control terminal) of the switching element  40  is connected to one end of the feedback winding  44  via the coupling capacitor  45 . A protection diode is connected to the gate of the switching element  40 . 
     The source of the current control element  41  is connected to the lighting load  14  in series. A gate of the current control element  41  is an electrode for controlling an electric current flowing between the drain and the source of the current control element  41 . 
     The source of the current control element  41  is connected to one end of the inductor  43  and the other end of the feedback winding  44 . A voltage obtained by dividing source potential of the current control element  41  with the voltage dividing resistors  46  and  47  is input to the gate of the current control element  41 . The source of the current control element  41  is connected to the lighting load  14  via the inductor  43 . The voltage dividing resistor  46  is connected to the lighting load  14  in parallel. That is, a voltage obtained by dividing a voltage (a forward voltage) of the lighting load  14  with the voltage dividing resistors  46  and  47  is input to the gate of the current control element  41 . Potential lower than the potential of the source is supplied to the gate of the current control element  41 . 
     A capacitor  50  and a Zener diode  52  are connected to the voltage dividing resistor  46  in parallel. The voltage dividing resistor  46 , the capacitor  50 , and the Zener diode  52  function as a low-pass filter. 
     A time constant of the voltage dividing resistor  46  and the capacitor  50  is, for example, equal to or smaller than a half cycle of the power supply voltage VIN (the alternating-current voltage). That is, the time constant of the voltage dividing resistor  46  and the capacitor  50  is, for example, equal to or smaller than 120 Hz or equal to or larger than a cycle of a switching frequency of the switching element  40 . For example, when the switching frequency of the switching element  40  is 1 MHz and the frequency of the power supply voltage VIN is 60 Hz, the time constant of the voltage dividing resistor  46  and the capacitor  50  is equal to or larger than 1 μsec and equal to or smaller than 8.3 msec. For example, the capacity of the capacitor  50  is set to satisfy the time constant. 
     When an increasing current flows from the one end to the other end of the inductor  43 , the inductor  43  and the feedback winding  44  are magnetically coupled in polarity in which a voltage having a positive polarity is supplied to the gate of the switching element  40 . 
     The rectifying element  42  is connected between the source of the current control element  41  and the low-potential terminal  20   b  with a direction from the low-potential terminal  20   b  to the current control element  41  set as a forward direction. As the rectifying element  42 , for example, a Schottky barrier diode is used. 
     The other end of the inductor  43  is connected to the high-potential output terminal  12   c . The low-potential terminal  20   b  is connected to the low-potential output terminal  12   d . The output capacitor  48  is connected between the high-potential output terminal  12   c  and the low-potential output terminal  12   d . The lighting load  14  is connected in parallel to the output capacitor  48  between the high-potential output terminal  12   c  and the low-potential output terminal  12   d.    
     In this example, a semiconductor element  54  is provided between the rectifying element  42  and the source of the current control element  41 . For the semiconductor element  54 , for example, GaN-HEMT is used. The semiconductor element  54  includes, for example, a wide band gap semiconductor. As the semiconductor element  54 , for example, a wide band gap semiconductor element is used. The semiconductor element  54  is, for example, the normally on type. A gate of the semiconductor element  54  is connected to the low-potential terminal  20   b . Consequently, the semiconductor element  54  is retained in an ON state. 
     The protection circuit  24  includes a semiconductor element  60 , a current detecting resistor  61 , a capacitor  62 , and a fuse  63 . One end of the current detecting resistor  61  is connected to the low-potential output terminal  12   d . The other end of the current detecting resistor  61  is connected to the low-potential terminal  20   b . Consequently, the output current IOUT flows to the current detecting resistor  61 . 
     A drain of the semiconductor element  60  is connected to a source of the semiconductor element  54 . A source of the semiconductor element  60  is connected to the low-potential terminal  20   b . A gate of the semiconductor element  60  is connected between the low-potential output terminal  12   d  and the current detecting resistor  61  via the capacitor  62 . Consequently, a voltage corresponding to the current detecting resistor  61  and the output current IOUT is applied to the gate of the semiconductor element  60 . As the semiconductor element  60 , a switching element of a normally off type is used. The semiconductor element  60  is turned on when an output current IOUT equal to or larger than a predetermined value flows to the semiconductor element  60 . 
     The fuse  63  is connected between the input terminal  12   a  and the AC-DC converter  20 . When an electric current equal to or larger than a predetermined value flows to the switching element  40  and the current control element  41 , the fuse  63  fuses an element portion and cuts off the electric current flowing to the switching element  40  and the current control element  41 . More specifically, the fuse  63  cuts off the electric current flowing between the drains and the sources of the switching element  40  and the current control element  41 . 
     As explained above, for example, when an electric current equal to or larger than the predetermined value flows to the switching element  40  and the current control element  41 , the protection circuit  24  fuses the fuse  63  to thereby cut off the electric current flowing to the switching element  40  and the current control element  41 . 
     A rated current of the fuse  63  is larger than a maximum current flowing to the switching element  40  and the current control element  41  in a state in which the semiconductor element  60  is turned off. The rated current of the fuse  63  is smaller than an electric current flowing to the switching element  40  and the current control element  41  in a state in which the semiconductor element  60  is turned on. 
     A position to which the fuse  63  is connected is not limited to between the input terminal  12   a  and the AC-DC converter  20 . The fuse  63  may be connected, for example, between the high-potential terminal  20   a  of the AC-DC converter  20  and the drain of the switching element  40 . The position to which the fuse  63  is connected may be an arbitrary position where the electric current flowing to the switching element  40  and the current control element  41  can be cut off. 
     The operation of the power supply device  12  is explained. 
     First, a dimming degree of the dimmer  3  is set to nearly 100% and the input power supply voltage VIN is substantially directly transmitted, that is, a highest direct-current voltage is input to the DC-DC converter  22 . 
     When the power supply voltage VIN is supplied to the power supply device  12 , since the switching element  40  and the current control element  41  are the elements of the normally on type, both of the switching element  40  and the current control element  41  are on. An electric current flows through a route of the switching element  40 , the current control element  41 , the inductor  43 , and the output capacitor  48 . The output capacitor  48  is charged. A voltage across both ends of the output capacitor  48 , that is, a voltage between the high-potential output terminal  12   c  and the low-potential output terminal  12   d  is supplied to the illumination light source  16  of the lighting load  14  as the output voltage VOUT of the power supply device  12 . Since the switching element  40  and the current control element  41  are on, a reverse voltage is applied to the rectifying element  42 . An electric current substantially does not flow to the rectifying element  42 . 
     When the output voltage VOUT reaches a predetermined voltage, the output current IOUT flows to the illumination light source  16  and the illumination light source  16  is lit. At this point, an electric current flows through a route of the switching element  40 , the current control element  41 , the inductor  43 , the output capacitor  48 , and the illumination light source  16 . For example, when the illumination light source  16  is an LED, the predetermined voltage is a forward voltage of the LED and is set according to the illumination light source  16 . When the illumination light source  16  is extinguished, the output current IOUT does not flow. Therefore, the output capacitor  48  retains a value of the output voltage VOUT. 
     A direct-current voltage input to the DC-DC converter  22  is sufficiently high compared with the output voltage VOUT. That is, a potential difference ΔV between an input and an output is sufficiently large. Therefore, an electric current flowing through the inductor  43  increases. The feedback winding  44  is magnetically couple to the inductor  43 . Therefore, an electromotive force having polarity with the coupling capacitor  45  side set to high potential is induced in the feedback winding  44 . Therefore, potential positive with respect to the source of the switching element  40  is supplied to the gate of the switching element  40  via the coupling capacitor  45 . The switching element  40  maintains an ON state. 
     When an electric current flowing through the current control element  41  exceeds an upper limit value, a drain-to-source voltage of the current control element  41  suddenly rises. Therefore, a gate-to-source voltage of the switching element  40  falls below a threshold voltage and the switching element  40  is turned off. The upper limit value is a saturation current value of the current control element  41  and is specified by potential input to the gate of the current control element  41 . The gate potential of the current control element  41  is set according to a direct-current voltage supplied to the voltage dividing resistors  46  and  47 , a voltage of the illumination light source  16 , and a voltage division ratio of the voltage dividing resistors  46  and  47 . As explained above, the gate potential of the current control element  41  is the negative potential with respect to the source. Therefore, the saturation current value can be limited to a proper value. 
     The inductor  43  continues to feed an electric current through a route of the rectifying element  42 , the output capacitor  48 , and the lighting load  14 . At this point, since the inductor  43  emits energy, the electric current of the inductor  43  decreases. Therefore, an electromotive force having polarity with the coupling capacitor  45  side set to low potential is induced in the feedback winding  44 . Potential negative with respect to the source of the switching element  40  is supplied to the gate of the switching element  40  via the coupling capacitor  45 . The switching element  40  maintains an OFF state. 
     When energy accumulated in the inductor  43  decreases to zero, the electric current flowing through the inductor  43  decreases to zero. The direction of the electromotive force induced in the feedback winding  44  is reversed again and an electromotive force with the coupling capacitor  45  side set to high potential is induced. Consequently, potential higher than the potential of the source is supplied to the gate of the switching element  40  and the switching element  40  is turned on again. Consequently, the power supply device  12  returns to the state in which the output voltage VOUT reaches the predetermined voltage. 
     Thereafter, the operation explained above is repeated. Consequently, the switching from ON to OFF of the switching element  40  is automatically repeated. The output voltage VOUT obtained by dropping the power supply voltage VIN is supplied to the illumination light source  16 . That is, in the power supply device  12 , the switching frequency of the switching element  40  is set by the voltage dividing resistors  46  and  47 . The electric current supplied to the illumination light source  16  is a substantially fixed current, an upper limit value of which is limited by the current control element  41 . Therefore, it is possible to stably light the illumination light source  16 . The current control element  41  is a constant current element in other words. 
     As explained above, the DC-DC converter  22  converts an input first direct-current voltage into a second direct-current voltage, which has an absolute value different from an absolute value of the first direct-current voltage, according to ON and OFF of the switching element  40  and supplies the second direct-current voltage to the lighting load  14 . In this example, the first direct-current voltage is a direct-current voltage supplied from the AC-DC converter  20 . The second direct-current voltage is the output voltage VOUT. The first direct-current voltage input to the DC-DC converter  22  is not limited to a direct-current voltage generated by the AC-DC converter  20 . For example, the first direct-current voltage may be directly supplied to the DC-DC converter  22  from an external power supply or the like. 
     When a dimming degree of the dimmer  3  is set to a value smaller than 100% and the input alternating-current voltage VCT is subjected to the conduction angle control and transmitted, that is, when a high direct-current voltage is input to the DC-DC converter  22 , the above explanation also applies when the switching element  40  can continue oscillation. According to the dimming degree of the dimmer  3 , a value of a direct-current voltage input to the DC-DC converter  22  changes and an average value of the output current IOUT can be controlled. Therefore, it is possible to dim the illumination light source  16  of the lighting load  14  according to the dimming degree. 
     When the dimming degree of the dimmer  3  is set to a smaller value, that is, when the direct-current voltage input to the DC-DC converter  22  is lower, since a potential difference between both ends of the inductor  43  is small even if the switching element  40  is turned on, an electric current flowing through the inductor  43  cannot increase. Therefore, the switching element  40  does not change to the OFF state and outputs a fixed direct current. That is, when the dimming degree of the dimmer  3  is small, that is, when the potential difference ΔV between an input and an output is small, the power supply device  12  operates like a series regulator. 
     As explained above, the power supply device  12  performs the switching operation when the potential difference ΔV is larger than a predetermined value and operates like the series regulator when the potential difference ΔV is smaller than the predetermined value. When the potential difference ΔV is large, a product of the potential difference ΔV and the electric current is large and a loss increases when the operation of the series regulator is performed. Therefore, it is suitable for a reduction in power consumption to perform the switching operation when the potential difference ΔV is large. When the potential difference ΔV is small, since a loss is small, there is no problem in operating as the series regulator. 
     In the power supply device  12 , when the potential difference ΔV is smaller than the predetermined value, the electric current oscillates while the switching element  40  does not change to the OFF state and continues the ON state. The switching element  40  lights the illumination light source  16  of the lighting load  14  with an average value of the electric current. When potential difference ΔV is smaller, the switching element  40  outputs a direct current to the lighting load  14  and lights the illumination light source  16  while continuing the ON state. As a result, in the power supply device  12 , it is possible to continuously change an output current to zero. For example, in the luminaire  10 , it is possible to smoothly extinguish the illumination light source  16  of the lighting load  14 . 
     In the power supply device  12 , it is possible to continuously change, according to the potential difference ΔV, the output current IOUT from a maximum value during the switching operation of the switching element  40  to a minimum value in outputting a direct current while continuing the ON state of the switching element  40 . For example, in the luminaire  10 , it is possible to continuously dim the illumination light source  16  in a range of 0 to 100%. 
     In the power supply device  12 , the switching frequency of the switching element  40  is set by dividing the voltage of the illumination light source  16  with the voltage dividing resistors  46  and  47  and inputting the divided voltage to the gate of the current control element  41 . The voltage of the illumination light source  16  is stable to a certain degree even if an input voltage such as the power supply voltage VIN or the alternating-current voltage VCT is distorted. Therefore, by dividing the voltage of the illumination light source  16  with the voltage dividing resistors  46  and  47  and inputting the divided voltage to the gate of the current control element  41  as explained above, it is possible to suppress a change in the brightness of the illumination light source  16  due to the distortion of the input voltage without preparing a special control reference or the like. For example, it is possible to suppress flickering of the illumination light source  16 . For example, it is possible to suppress fluctuation in the voltage of the illumination light source  16  involved in fluctuation of the input voltage. 
     In the power supply device  12 , the capacitor  50  is connected to the voltage dividing resistor  46  to form the low-pass filter. The time constant of the voltage dividing resistor  46  and the capacitor  50  is set to, for example, equal to or smaller than the half cycle of the power supply voltage VIN. Consequently, for example, it is possible to suppress the gate potential of the current control element  41  from fluctuating according to the switching of the switching element  40 . For example, even if the inductor  43  is interposed, a voltage appears in the voltage dividing resistors  46  and  47 . It is possible to further stabilize the gate potential of the current control element  41 . That is, it is possible to more appropriately suppress a change in the brightness of the illumination light source  16 . For example, the switching frequency is increased and the capacity of the capacitor  50  is reduced. Consequently, for example, responsiveness is improved. It is possible to suppress a current ripple in a relatively wide range. 
     In the power supply device  12 , because of breakage of the gate or the like, the switching element  40  or the current control element  41  sometimes falls into an uncontrolled state. In this case, for example, an excessively large electric current exceeding a rated current of the lighting load  14  flows to the switching element  40  and the current control element  41 . 
     In the power supply device  12 , when the excessively large output current IOUT flows, the gate potential of the semiconductor element  60  of the protection circuit  24  increases and the semiconductor element  60  is turned on. When the semiconductor element  60  is turned on, the rectifying element  42  is short-circuited. When the rectifying element  42  is short-circuited, the gate-to-source voltage of the semiconductor element  54  increases and a larger electric current flows to the switching element  40  and the current control element  41 . As explained above, the rated current of the fuse  63  is smaller than an electric current flowing to the switching element  40  and the current control element  41  in a state in which the semiconductor element  60  is turned on. Consequently, when the semiconductor element  60  is turned on and a larger electric current is fed to the switching element  40  and the current control element  41 , the fuse  63  is fused and the electric current flowing to the switching element  40  and the current control element  41  is cut off. That is, the output current IOUT is cut off. 
     In a power supply device not including the protection circuit  24 , when the switching element  40  or the current control element  41  changes to the uncontrolled state, the output current IOUT equal to or larger than the rated current sometimes continues to flow to the lighting load  14 . In this case, for example, the lighting load  14  is broken. In particular, when the wide band gap semiconductor elements are used as the switching element  40  and the current control element  41 , temperature resistance between the drains and the sources is high compared with a Si semiconductor and the like. Therefore, even if temperature rises, the switching element  40  and the current control element  41  are not broken between the drains and the sources and an over current continues to flow. For example, the temperature of the elements exceeds the heat resistant temperature of the substrate  13  and damages the substrate  13 . 
     On the other hand, in the power supply device  12  according to this embodiment, when an electric current equal to or larger than the predetermined value flows to the switching element  40  and the current control element  41 , the fuse  63  is fused and the electric current flowing to the switching element  40  and the current control element  41  is cut off. Consequently, it is possible to suppress damage to the lighting load  14  and the substrate  13  due to the overcurrent. 
     As explained above, in the luminaire  10  and the power supply device  12  according to this embodiment, it is possible to suppress the damage to the lighting load  14  and the substrate  13  due to the overcurrent and improve safety. 
     In this example, the electric current flowing to the switching element  40  and the current control element  41  is cut off by fusing the fuse  63 . Besides, for example, it is also possible to, by turning on the semiconductor element  60  and feeding a larger electric current to the switching element  40  and the current control element  41 , break at least one of the switching element  40  and the current control element  41  to thereby cut of the electric current flowing to the switching element  40  and the current control element  41 . 
     Second Embodiment 
       FIG. 2  is a circuit diagram schematically showing a luminaire according to a second embodiment. 
     As shown in  FIG. 2 , in a power supply device  102  of a luminaire  100 , the protection circuit  24  includes the fuse  63 , a thermistor  64  (a temperature sensitive resistance element), and a rectifying element  65 . 
     The thermistor  64  is connected between the feedback winding  44  and the gate of the switching element  40 . More specifically, the thermistor  64  is connected between the feedback winding  44  and the coupling capacitor  45 . The thermistor  64  has a temperature characteristic for increasing a resistance value according to a temperature rise. The thermistor  64  is a so-called PCT (Positive Temperature Coefficient) thermistor. The temperature sensitive resistance element is not limited to the thermistor and may be an arbitrary element that changes a resistance value according to a change in temperature. 
     The rectifying element  65  is connected to the thermistor  64  in parallel. The rectifying element  65  is connected with a direction from the coupling capacitor  45  to the feedback winding  44  set as a forward direction. 
     In the power supply device  102 , when an overcurrent flows to the switching element  40  and the current control element  41 , the thermistor  64  detects a temperature rise involved in the overcurrent and the resistance of the thermistor  64  rises. Even if the resistance of the thermistor  64  rises, during discharging of the coupling capacitor  45 , there is no change because charges flow via the rectifying element  65 . On the other hand, during charging of the coupling capacitor  45 , a voltage drop occurs because of to the increased resistance value of the thermistor  64  and the gate voltage of the switching element  40  falls. Therefore, the switching element  40  is always in the ON state. 
     As explained above, when an electric current equal to or larger than the predetermined value flows to the switching element  40  and the current control element  41 , the protection circuit  24  changes the resistance value of the thermistor  64  and retains the switching element  40  in the ON state. As the thermistor  64 , an element having a temperature characteristic for reducing the resistance value according to a temperature rise may be used. The circuit configuration of the protection circuit  24  including the thermistor  64  may be an arbitrary circuit that can retain the switching element  40  in the ON state. 
     In the power supply device  102 , for example, when the current control element  41  changes to the uncontrolled state and an overcurrent flows, the switching element  40  is always set in the ON state, whereby a larger current flows to the switching element  40  and the current control element  41 . Consequently, the fuse  63  is fused and the electric current flowing to the switching element  40  and the current control element  41  is cut off. 
     As explained above, in the luminaire  100  and the power supply device  102  according to this embodiment, as in the first embodiment, it is possible to suppress damage to the lighting load  14  and the substrate  13  due to an overcurrent and improve safety. 
     Third Embodiment 
       FIG. 3  is a circuit diagram schematically showing a luminaire according to the third embodiment. 
     As shown in  FIG. 3 , in a power supply device  112  of a luminaire  110 , the protection circuit  24  is omitted. In the power supply device  112 , a saturation current value of the current control element  41  is equal to or larger than a maximum rated current of the switching element  40 . 
     In the power supply device  112 , for example, when the current control element  41  changes to an uncontrolled state and an overcurrent flows, an electric current exceeding the maximum rated current of the switching element  40  flows to the switching element  40 . Consequently, the switching element  40  is broken and the electric current flowing to the switching element  40  and the current control element  41  is cut off. 
     As explained above, in the luminaire  110  and the power supply device  112  according to this embodiment, as in the first and second embodiments, it is possible to suppress damage to the lighting load  14  and the substrate  13  due to an overcurrent and improve safety. 
     Fourth Embodiment 
       FIG. 4  is a circuit diagram schematically showing a luminaire according to a fourth embodiment. 
     As shown in  FIG. 4 , in a power supply device  122  of a luminaire  120 , the protection circuit  24  includes the fuse  63 , a resistor  66 , and a thermistor  67  (a temperature sensitive resistance element). 
     The resistor  66  is connected between the high-potential terminal  20   a  and the gate of the semiconductor element  54 . The thermistor  67  is connected between the gate of the semiconductor element  54  and the low-potential terminal  20   b . Consequently, in the DC-DC converter  22  of the power supply device  122 , a voltage obtained by dividing a direct-current voltage output from the AC-DC converter  20  with the resistor  66  and the thermistor  67  is applied to the gate of the semiconductor element  54 . 
     The thermistor  67  has a temperature characteristic for increasing a resistance value according to a temperature rise. The thermistor  67  is a so-called PTC thermistor. The gate voltage of the semiconductor element  54  increases according to an increase in the resistance of the thermistor  67 . An electric current flowing between the drain and the source of the semiconductor element  54  increases according to the increase in the resistance of the thermistor  67 . 
     In the power supply device  122 , the rectifying element  42  includes silicon. The rectifying element  42  is, for example, a silicon diode. More specifically, the rectifying element  42  is a silicon Schottky barrier diode. The withstand voltage of the rectifying element  42  is larger than an absolute value of a threshold voltage of the gate of the semiconductor element  54  and smaller than an input voltage of the DC-DC converter  22 . The input voltage of the DC-DC converter  22  is a direct-current voltage supplied from the AC-DC converter  20 . The breakdown voltage in the opposite direction of the rectifying element  42  is larger than the absolute value of the threshold voltage of the gate of the semiconductor element  54  and smaller than the input voltage of the DC-DC converter  22 . 
     In the power supply device  122 , for example, when at least one of the switching element  40  and the current control element  41  changes to an uncontrolled state and an overcurrent flows, the thermistor  67  detects a temperature rise involved in the overcurrent and the resistance of the thermistor  67  increases. When the resistance of the thermistor  67  increases, an electric current flowing to the semiconductor element  54  increases and an electric current equal to or larger than the withstand voltage flows to the rectifying element  42 . Consequently, the rectifying element  42  is short-circuited and broken. When the rectifying element  42  is short-circuited and broken, an electric current flowing to the switching element  40  and the current control element  41  increases according to a rise of a gate-to-source voltage of the semiconductor element  54  and the short circuit of the rectifying element  42 . Consequently, the fuse  63  is fused and the electric current flowing to the switching element  40  and the current control element  41  is cut off. 
     As explained above, in the luminaire  120  and the power supply device  122  according to this embodiment, as in the first to third embodiments, it is possible to suppress damage to the lighting load  14  and the substrate  13  due to an overcurrent and improve safety. 
     When the rectifying element  42  is short-circuited and broken, an electric current equal to or larger than a maximum rated current of the switching element  40  may be allowed to flow to the switching element  40  and the current control element  41 . In this case, the switching element  40  is broken by the electric current equal to or larger than the maximum rated current. The electric current flowing to the switching element  40  and the current control element  41  is cut off by the breakage of the switching element  40 . 
       FIG. 5  is a circuit diagram schematically showing another luminaire according to the fourth embodiment. 
     As shown in  FIG. 5 , in a luminaire  124  and a power supply device  126 , the protection circuit  24  includes the semiconductor element  60 , the fuse  63 , the resistor  66 , and the thermistor  67 . 
     The drain of the semiconductor element  60  is connected between the source of the semiconductor element  54  and the rectifying element  42 . The source of the semiconductor element  60  is connected to the low-potential terminal  20   b . The resistor  66  is connected between the high-potential terminal  20   a  and the gate of the semiconductor element  60 . The thermistor  67  is connected between the gate of the semiconductor element  60  and the low-potential terminal  20   b . As explained above, the thermistor  67  is a PTC thermistor. 
     A voltage obtained by dividing a direct-current voltage output from the AC-DC converter  20  with the resistor  66  and the thermistor  67  is applied to the gate of the semiconductor element  60 . The gate voltage of the semiconductor element  60  increases according to an increase in the resistance of the thermistor  67 . When the resistance of the thermistor  67  increases to be equal to or larger than a predetermined value, the semiconductor element  60  transitions from an OFF state to an ON state. When the semiconductor element  60  changes to the ON state, the rectifying element  42  is short-circuited. Consequently, as in the embodiments explained above, a large electric current flows to the switching element  40  and the current control element  41  and the fuse  63  is fused. The electric current flowing to the switching element  40  and the current control element  41  is cut off. 
     In the luminaire  124  and the power supply device  126  according to this embodiment, as in the embodiments explained above, it is possible to suppress damage to the lighting load  14  and the substrate  13  due to an overcurrent and improve safety. As explained above, the protection circuit  24  may be a circuit that applies a voltage equal to or larger than a withstand voltage to both ends of a silicon diode (the rectifying element  42 ) or short-circuit the silicon diode using a switch (the semiconductor element  54  or  60 ) driven using the temperature sensitive resistance element (the thermistor  67 ), a resistance value of which changes according to temperature. As the temperature sensitive resistance element, an element having a temperature characteristic for reducing a resistance value according to a temperature rise may be used. 
     Fifth Embodiment 
       FIG. 6  is a circuit diagram schematically showing a luminaire according to a fifth embodiment. 
     As shown in  FIG. 6 , in a power supply device  132  of a luminaire  130 , the DC-DC converter  22  further includes a rectifying element  56 . The rectifying element  56  is connected between the rectifying element  42  and the low-potential terminal  20   b . The rectifying element  56  is connected with a direction from the low-potential terminal  20   b  to the rectifying element  42  set as a forward direction. That is, in this example, the rectifying element  42  and the rectifying element  56  are connected in series between the semiconductor element  54  and the low-potential terminal  20   b . In the power supply device  132 , the rectifying elements  42  and  56  include silicon. The rectifying elements  42  and  56  are silicon diodes. 
     The power supply devices explained in the embodiments are a so-called falling voltage chopper circuit. A switching loss involved in ON and OFF of the switching element  40  can be reduced by causing the falling voltage chopper circuit to operate in an inductor current critical mode. When the junction capacitance of the rectifying element  42  is large, at a point when an electric current of the inductor  43  decreases to zero, the switching element  40  is turned on. Then, an electric current flows from the cathode to the anode in order to charge the junction capacitance of the rectifying element  42 . At a point when the junction capacitance is charged to a direct-current voltage supplied from the AC-DC converter  20 , the switching element  40  is turned on. Consequently, it is possible to maintain zero current switching. 
     A load operates with a direct current. A direct-current equivalent to an average of an inductor current flows to the load. For example, when an LED is assumed as the load, in order to fix brightness, that is, in order to fix a direct-current current flowing to the LED, it is possible to perform control for determining timing for turning off the LED such that a peak value of a switching current is fixed. 
       FIG. 7  is a graph showing an example of waveforms of an inductor current. 
     For example, as shown in  FIG. 7 , in order to fix a load current, as indicated by a first waveform WP 1  represented by a thin line, it is possible to determine, to fixe peaks of a switch current (peaks of the inductor current), timing for turning off a switch element and turn on the switch element at timing when the inductor current is zero. On the other hand, when the junction capacitance of a diode is large, as indicated by a second waveform WP 2  represented by a thick line in the figure, an electric current for charging the junction capacitance flows in the opposite direction of an inductor. In this case, in order to equalize the load current, it is necessary to increase the peaks of the switch current. Consequently, a reactive current flowing through a circuit occurs and increases a circuit current. Since the electric current does not contribute to the load, the electric current increases a loss of components through which the electric current flows. This phenomenon appears more conspicuously as a switching frequency is higher. 
     On the other hand, in the power supply device  132  according to this embodiment, two diodes are connected in series. Consequently, junction capacitances of the two diodes are connected in series. It is possible to equivalently reduce capacitance components present in parallel to the diodes. Therefore, it is possible to reduce an electric current for charging the junction capacitances and complete the charging in a short time. Accordingly, the reactive current flowing through the circuit decreases and the circuit loss can be reduced. It is possible to more effectively reduce the reactive current by controlling ON and OFF of the switching element  40  at a frequency equal to or higher than 100 kHz. 
     In the power supply device  132 , the protection circuit  24  is provided to short-circuit or short-circuit and break the rectifying elements  42  and  56  when an overcurrent flows. Consequently, as explained in the embodiments, for example, the fuse  63  is fused and the electric current flowing to the switching element  40  and the current control element  41  is cut off. Consequently, it is also possible to improve safety. In  FIG. 6 , as an example, the protection circuit  24  that short-circuits and breaks the rectifying elements  42  and  56  is shown. However, the configuration of the protection circuit  24  may be any one of the configurations in the embodiments. In this example, two rectifying elements  42  and  56  are connected in series. However, the number of rectifying elements connected in series may be three or more. 
       FIG. 8  is a circuit diagram schematically showing another luminaire according to the fifth embodiment. 
     As shown in  FIG. 8 , in a power supply device  136  of a luminaire  134 , the rectifying element  42  is a high-speed rectifying diode and the rectifying element  56  is a Schottky barrier diode. In order to cause the power supply device  136  to operate at a frequency equal to or higher than 100 kHz, it is necessary to turn on and off a diode at high speed. When an electric current is flowing to the diode in a forward direction, in order to reduce a loss of the diode, a forward voltage is preferably small. 
     Therefore, in the power supply device  136 , the two kinds of diodes are connected in series. The high-speed rectifying diode has small junction capacitance, can be switched at high speed, and has high voltage resistance performance. The Schottky barrier diode can be switched at high speed and has a low forward voltage. Therefore, by connecting the diodes in this way, it is possible to suppress the forward voltage and reduce equivalent junction capacitance without spoiling high-speed switching performance. Accordingly, it is possible to further reduce a reactive current flowing through the circuit and further reduce the circuit loss. 
     Sixth Embodiment 
       FIG. 9  is a circuit diagram schematically showing a luminaire according to a sixth embodiment. 
     As shown in  FIG. 9 , in a power supply device  142  of a luminaire  140 , the DC-DC converter  22  further includes an integrated circuit  70 . The integrated circuit  70  includes the switching element  40 , the current control element  41 , the rectifying element  42 , the semiconductor element  54 , and the rectifying element  56 . The integrated circuit  70  is one component obtained by integrating the components as one chip. 
     The integrated circuit  70  includes first to sixth terminals  70   a  to  70   f . The first terminal  70   a  is connected to the drain of the switching element  40 . The second terminal  70   b  is connected to the source of the current control element  41 . The third terminal  70   c  is connected to the gate of the switching element  40 . The fourth terminal  70   d  is connected to the gate of the current control element  41 . The fifth terminal  70   e  is connected to the anode of the rectifying element  56 . The sixth terminal  70   f  is connected to the gate of the semiconductor element  54 . 
     In the integrated circuit  70 , the semiconductor element  54  and the rectifying element  56  are provided according to necessity and can be omitted. When the rectifying element  56  is omitted, the fifth terminal  70   e  is connected to the anode of the rectifying element  42 . 
     For example, when the rectifying elements  42  and  56  are configured as external separate components, a terminal provided in a package including a diode chip is necessary. A diode and parasitic capacitance are connected by the terminal. The parasitic capacitance is also connected to the junction capacitance of the diode in parallel. Therefore, a reactive current involved in the connection increases. 
     On the other hand, in the power supply device  142  according to this embodiment, the integrated circuit  70  is provided and integrated to include the rectifying elements  42  and  56 . Consequently, in the power supply device  142 , it is possible to reduce the parasitic capacitance. Therefore, it is possible to further reduce the reactive current and further reduce the circuit loss. 
     The embodiments are explained above with reference to the specific examples. However, various modifications of the embodiments are possible. 
     For example, the switching element  40  and the current control element  41  are not limited to the GaN-HEMT. For example, the switching element  40  and the current control element  41  may be a semiconductor element formed using, in a semiconductor substrate, a semiconductor having a wide band gap (a wide band gap semiconductor) such as silicon carbide (SiC), gallium nitride (GaN), or diamond. The wide band gap semiconductor refers to a semiconductor having a band gap wider than a band gap of about 1.4 eV of gallium arsenide (GaAs). The wide band gap semiconductor includes semiconductors having band gaps equal to or larger than 1.5 eV such as gallium phosphide (GaP; having a band gap of about 2.3 eV), gallium nitride (GaN; having a band gap of about 3.4 eV), diamond (C; having a band gap of about 5.27 eV), aluminum nitride (AlN; having a band gap of about 5.9 eV), and silicon carbide (SiC). When a withstand voltage is set equal, since the wide band gap semiconductor element can be formed smaller than a silicon semiconductor element, parasitic capacitance is small and a high-speed operation is possible. Therefore, it is possible to reduce a switching cycle and reduce the sizes of a winding component, a capacitor, and the like. 
     The illumination light source  16  is not limited to the LED and may be, for example, an organic EL (Electro-Luminescence) and an OLED (Organic light-emitting diode). A plurality of the illumination light sources  16  may be connected to the lighting load  14  in series or in parallel. 
     In the embodiments, the lighting load  14  is described as the direct-current load. However, the direct-current load is not limited to this and may be other direct-current loads such as a heater. In the embodiment, the power supply device used in the luminaire is described as the power supply device. However, the power supply device is not limited to this and may be an arbitrary power supply device corresponding to the direct-current load. 
     While certain embodiments have been described, these embodiments have been presented by way of example only, and are not intended to limit the scope of the inventions. Indeed, the novel embodiments described herein may be embodied in a variety of other forms; furthermore, various omissions, substitutions and changes in the form of the embodiments described herein may be made without departing from the spirit of the inventions. The accompanying claims and their equivalents are intended to cover such forms or modifications as would fall within the scope and spirit of the inventions.