Patent Publication Number: US-2015084531-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. 2013-199053, 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 recent years, in a luminaire, as an illumination light source, an incandescent lamp and a fluorescent lamp are replaced with energy-saving and long-life light sources such as a light-emitting diode (LED). For example, new illumination light sources such as an EL (Electro-Luminescence) and an OLED (organic light-emitting diode) are also developed. The brightness of the illumination light sources depends on a value of a flowing electric current. In order to stably light the luminaire, a power supply device that supplies a constant current is necessary. It is necessary to convert a voltage in order to adjust an input power supply voltage to a rated voltage of an illumination light source such as an LED. As a highly efficient power supply suitable for power saving and a reduction in size, there are known switching power supplies such as a DC-DC converter of a chopper system. 
     As low-loss switching elements used in the switching power supplies, switching elements formed by a compound semiconductor such as GaN and SiC are put to practical use. These elements are a normally on type. It is necessary to perform surer current limitation for the elements. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a circuit diagram illustrating a luminaire including a power supply device according to a first embodiment; 
         FIGS. 2A to 2D  are waveform charts for describing the operation of the power supply device; 
         FIG. 3  is a characteristic chart showing dependency of an electric current of a switching element on the potential of a control terminal; 
         FIG. 4  is a circuit diagram illustrating a power supply device according to a second embodiment; and 
         FIG. 5  is a circuit diagram illustrating a luminaire including a power supply device according to a third embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     In general, according to one embodiment, a power supply device includes a first inductor, a current control section, a rectifying element, and a second inductor. The current control section is configured to limit a current value of an electric current flowing through the first inductor to a predetermined current value. The current control section includes a first switching element of a normally on type and a resistor connected to a main terminal of the first switching element. The rectifying element is connected to the current control section in series. An electric current flows to the rectifying element when the current control section is off. The second inductor is magnetically coupled to the first inductor and is configured to induce a voltage for turning on the current control section, when the electric current of the first inductor increases, to induce a voltage for turning off the current control section, when the electric current of the first inductor decreases, and to supply the induced voltage to a control terminal of the current control section. 
     According to another embodiment, there is provided a luminaire including a power supply device and a lighting load. The power supply device includes a first inductor, a current control section, a rectifying element, and a second inductor. The current control section is a current control section configured to limit a current value of an electric current flowing through the first inductor to a predetermined current value. The current control section includes a first switching element of a normally on type and a resistor connected to a main terminal of the first switching element. The rectifying element is connected to the current control section in series. An electric current flows to the rectifying element when the current control section is off. The second inductor is magnetically coupled to the first inductor. When the electric current of the first inductor increases, the second inductor induces a voltage for turning on the current control section. When the electric current of the first inductor decreases, the second inductor induces a voltage for turning off the current control section. The second inductor supplies the induced voltage to a control terminal of the current control section. The lighting load functions as a load circuit of the power supply device. 
     Various embodiments will be described with reference to the accompanying drawings. In the following explanation, members same as members already described with reference to drawings are denoted by the same reference numerals and signs. Explanation of the members once described is omitted as appropriate. 
     First Embodiment 
       FIG. 1  is a circuit diagram illustrating a luminaire including a power supply device according to a first embodiment. A luminaire  1  includes a power supply device  3  that converts an output voltage VIN of a direct-current voltage source  2  into a voltage VOUT and a lighting load  4  functioning as a load circuit of the power supply device  3 . The direct-current voltage source  2  includes, for example, a commercial alternating-current power supply and a bridge-type rectifier circuit. The direct-current voltage source  2  full-wave rectifies an alternating-current voltage of the commercial alternating-current power supply in the bridge-type rectifier circuit and outputs a direct-current voltage. The lighting load  4  includes an illumination light source  5 . The illumination light source  5  includes, for example, an LED and is supplied with the voltage VOUT from the power supply device  3  to light. 
     In the power supply device  3 , a current control section  6  and a rectifying element  16  are connected in series between a high-potential input terminal  9  and a low-potential input terminal  10 . The current control section  6  includes a first switching element  14  and a resistor  15  connected in series. A drain functioning as a main terminal of the first switching element  14  is connected to the high-potential input terminal  9 . A source functioning as a main terminal of the first switching element  14  is connected to one end of the resistor  15 . The other end of the resistor  15  is connected to a cathode of the rectifying element  16 . An anode of the rectifying element  16  is connected to the low-potential input terminal  10 . The first switching element  14  is, for example, a field effect transistor (FET) including a compound semiconductor, is, for example, a high electron mobility transistor (HEMT), and is a normally on type element. The rectifying element  16  is, for example, a silicon diode. 
     In the power supply device  3 , a first inductor  18  is connected between the cathode of the rectifying element  16  and a high-potential output terminal  11 . A second inductor  19  is magnetically coupled to the first inductor  18 . One end of the second inductor  19  is connected to the cathode of the rectifying element  16 . The other end of the second inductor  19  is connected to a gate functioning as a control terminal of the first switching element  14  via a capacitor  20 . 
     An input filter capacitor  13  is connected between the high-potential input terminal  9  and the low-potential input terminal  10 . A smoothing capacitor  21  is connected between the high-potential output terminal  11  and a low-potential output terminal  12 . The low-potential input terminal  10  and the low-potential output terminal  12  are connected on the inside of the power supply device  3 . 
     In  FIG. 1 , as the direct-current voltage source  2 , a direct-current voltage source is illustrated that rectifies an alternating-current voltage of an alternating-current power supply  7  such as a commercial alternating-current supply with a rectifier  8  such as a bridge-type rectifier circuit and outputs a direct-current voltage. 
     The operation of the power supply device  3  is described with reference to  FIGS. 1 and 2A  to  2 D. 
       FIGS. 2A to 2D  are waveform charts for describing the operation of the power supply device  3 . 
       FIG. 2A  is a waveform chart showing an electric current IL 1  of the first inductor  18 .  FIG. 2B  is a waveform chart showing an electric current IQ 1  of the first switching element  14 .  FIG. 2C  is a waveform chart of an electric current ID 1  of the rectifying element  16 .  FIG. 2D  is a waveform chart showing a gate-to-source voltage VGS of the first switching element  14 . 
     First, the operation of the current control section  6  is described. The current control section  6  includes the first switching element  14  and the resistor  15 . 
       FIG. 3  is a characteristic chart showing dependency of a drain current of the first switching element  14  to the potential of the control terminal. The abscissa of  FIG. 3  represents a drain-to-source voltage and the ordinate of  FIG. 3  represents a drain current. 
     As it is evident from  FIG. 3 , when a drain current Id reaches a predetermined current value, that is, a threshold, ON resistance of the first switching element  14  rises. That is, the first switching element  14  shows a constant current characteristic. The drain current Id in a state in which the first switching element  14  shows the constant current characteristic depends on a gate-to-source voltage Vgs. As an absolute value of the gate-to-source voltage Vgs is larger, a value of the drain current Id in the constant current characteristic is smaller. 
     The operation of the power supply device  3  is described below with reference to such characteristics of the first switching element  14 . 
     (1) When the power supply voltage VIN is applied to between the high-potential input terminal  9  and the low-potential input terminal  10 , since the first switching element  14  is the normally on type element, the first switching element  14  is in an ON state. Then, an electric current flows through a path of the high-potential input terminal  9 , the first switching element  14 , the resistor  15 , the first inductor  18 , the smoothing capacitor  21 , and the low-potential input terminal  10 . The smoothing capacitor  21  is charged. Electromagnetic energy is accumulated in the first inductor  18 . Since the first switching element  14  is on, the power supply voltage VIN is substantially applied to both ends of the rectifying element  16 . Since a voltage is applied in the opposite direction, the rectifying element  16  changes to a non-conduction state. 
     (2) As time elapses, an electric current flowing through the first inductor  18  increases. Since the second inductor  19  is magnetically coupled to the first inductor  18 , the second inductor  19  induces an electromotive force having polarity for setting the capacitor  20  side to high potential. The capacitor  20  functions as a coupling capacitor. Potential, which is positive with respect to the source, is supplied to a gate of the first switching element  14  via the capacitor  20 . The first switching element  14  maintains the ON state. In this embodiment, the potential of the gate of the first switching element  14  based on an A point of  FIG. 1  is limited to, for example, 0.6 V according to the action of a diode  17 . 
     (3) According to the increase in the electric current flowing through the first inductor  18 , the voltage at both the ends of the resistor  15  also increases. The potential of the gate of the first switching element  14  is limited to, for example, 0.6 V as described above. Therefore, according to the increase in the voltage at both the ends of the resistor  15 , the gate-to-source voltage of the first switching element  14  changes to relatively negative potential. 
     (4) When the electric current flowing through the first inductor  18  exceeds the threshold, the drain-to-source voltage of the first switching element  14  suddenly increases according to the rise in the ON resistance. The increase in the electric current flowing through the first inductor  18  is limited. A counter electromotive force occurs in the first inductor  18 . The voltage of the second inductor  19  is reversed. An electromotive force having polarity for setting the capacitor  20  side to low potential is induced. Potential, which is negative with respect to the source, is supplied to the gate of the first switching element  14  via the capacitor  20 . The first switching element  14  changes to an OFF state. The threshold of the electric current flowing through the first inductor  18  is represented by Ip in  FIGS. 2A to 2C . 
     (5) A forward voltage is applied by the counter electromotive force of the first inductor  18 . Therefore, the rectifying element  16  changes to an ON state. An electric current flows through a path of the rectifying element  16 , the first inductor  18 , and the smoothing capacitor  21 . This state is shown in  FIGS. 2B and 2C . Simultaneously with the electric current IQ 1  decreasing to zero, the electric current ID 1  changes from Ip to 0. Since electromagnetic energy is emitted, the electric current of the first inductor  18  decreases. The negative voltage induced by the second inductor  19  is maintained. The first switching element  14  maintains the OFF state. 
     (6) When the electromagnetic energy accumulated in the first inductor  18  decreases to zero, the electric current flowing through the first inductor  18  decreases to zero. The direction of the electromotive force induced by the second inductor  19  is reversed again. An electromotive force for setting the capacitor  20  side to high potential is induced. A voltage higher than the voltage of the source is supplied to the gate of the first switching element  14 . The first switching element  14  is turned on. Consequently, the power supply device  3  returns to the state of (1). 
     Thereafter, the power supply device  3  repeats (1) to (6). 
     Switching from ON to OFF of the first switching element  14  is automatically repeated. The voltage of the smoothing capacitor  21  changes to the voltage VOUT stepped down from the power supply voltage VIN. The voltage VOUT is supplied to the illumination light source  5  of the lighting load  4  as an output voltage of the power supply device  3 . For example, when the illumination light source  5  is an LED, the voltage is equal to a forward voltage of the LED. 
     As shown in  FIG. 2A , an increasing electric current and a decreasing electric current alternately flow to the first inductor  18 . An average Io of the electric currents is an electric current supplied to the illumination light source  5 . In the electric current flowing through the first inductor  18 , a high-frequency component is bypassed by the smoothing capacitor  21 . The average Io is represented as Io=Ip/2. The average Io is a fixed current irrespective of the power supply voltage VIN and a load. That is, a constant current is supplied to the illumination light source  5 . The illumination light source  5  can be stably lit. 
     The resistor  15  configuring the current control unit  6  is selected taken into account the electric current Ip. Among the constant current characteristics shown in  FIG. 3 , a condition in which a threshold is a current value same as the current Ip is selected. A gate-to-source voltage corresponding to the current value is calculated. When the gate-to-source voltage is represented by Vth, a resistance value R15 of the resistor  15  is calculated as follows: 
         R 15= V th/ Ip    
     The number of turns n2 of the second inductor  19  is determined as described below. 
     When the first switching element  14  is on, a voltage applied to the first inductor  18  is VIN-VOUT. However, a voltage drop due to the current control section  6  is neglected. When the number of turns of the first inductor  18  is represented as n1 and a voltage induced by the second inductor  19  is represented as Vn2, the voltage Vn2 is calculated as follows: 
         Vn 2= n 2×( V IN− V OUT)/ n 1
 
     The capacitor  20  is charged through the diode  17  in a direction in which a terminal connected to the second inductor  19  is set on a positive side. 
     When the first switching element  14  is off, the voltage of the gate of the first switching element  14  is a sum of the voltages of the inductor  19  and the capacitor  20 . When the voltage of the gate of the first switching element  14  is represented as Vg, the voltage Vg is calculated as follows: 
         Vg=− 2× Vg=− 2× n 2×( V IN− V OUT)/ n 1
 
     The voltage n2 is determined such that the voltage Vg is larger than the gate-to-source voltage Vth and smaller than a withstand voltage of the gate. 
     Effects of the first embodiment are described. 
     In applying the normally on type element to the current control section, a negative power supply for applying a negative voltage to between the gate and source is necessary. In this embodiment, a function equivalent to the negative power supply can be obtained by adding a resistor to a source terminal in series and using a voltage at both ends of the resistor. An effect is obtained that it is possible to simplify a circuit and configure the power supply device with a small number of components. 
     In this embodiment, two functions, i.e., a function for limiting an electric current and a function for turning on and off an electric current are imparted to the first switching element  14 . In this regard, the effect is obtained that it is possible to simplify the circuit. 
     Second Embodiment 
       FIG. 4  is a circuit diagram illustrating a power supply device according to a second embodiment. 
     In a power supply device  22  in this embodiment, a constant voltage diode  23  and resistors  24  to  26  are added to the power supply device  3  in the first embodiment. One end of the capacitor  20  is connected to the second inductor  19 . The other end of the capacitor  20  is connected to the gate of the first switching element  14  through the resistor  24 . The constant voltage diode  23  is connected to the capacitor  20  in parallel. A cathode terminal of the constant voltage diode  23  is connected to the second inductor  19 . An anode terminal of the constant voltage diode  23  is connected to the resistor  24 . The resistor  25  is connected to the diode  17  in parallel. The resistor  26  is connected to the smoothing capacitor  21  in parallel. Otherwise, the power supply device  22  can be the same as the power supply device  3  shown in  FIG. 1 . 
     The constant voltage diode  23  limits a charging voltage of the capacitor  20 . This is for the purpose of reducing a voltage applied to the gate of the first switching element  14  to be equal to or lower than the withstand voltage of the switching element  14 . The resistors  24  and  25  divide the voltage of the second inductor  19  and supply the divided voltage to the gate of the first switching element  14 . The resistor  26  feeds a fixed load current and stabilizes the operation of the first switching element  14  during a light load. Otherwise, the operation of the power supply device  22  is the same as the operation of the power supply device  3  shown in  FIG. 1 . 
     Third Embodiment 
       FIG. 5  is a circuit diagram illustrating a luminaire including a power supply device according to a third embodiment. 
     A luminaire  28  includes a power supply device  29  that converts the output voltage VIN of the direct-current voltage source  2  into the voltage VOUT and the lighting load  4  functioning as a load circuit of the power supply device  29 . The lighting load  4  includes the illumination light source  5 . 
     In the power supply device  29 , a second switching element  27 , the current control section  6 , and the rectifying element  16  are connected in series between the high-potential input terminal  9  and the low-potential input terminal  10 . The current control section  6  includes the first switching element  14  and the resistor  15  connected in series. A drain of the second switching element  27  is connected to the high-potential input terminal  9 . A source of the second switching element  27  is connected to a drain of the first switching element  14 . A source of the first switching element  14  is connected to one end of the resistor  15 . The other end of the resistor  15  is connected to the cathode of the rectifying element  16 . The anode of the rectifying element  16  is connected to the low-potential input terminal  10 . Like the first switching element  14 , the second switching element  27  is, for example, a field effect transistor (FET), is, for example, a high electron mobility transistor (HEMT), and is a normally on type element. 
     The first inductor  18  is connected between the cathode of the rectifying element  16  and the high-potential output terminal  11 . One end of the second inductor  19  magnetically coupled to the first inductor  18  is connected to the cathode of the rectifying element  16 . The other end of the second inductor  19  is connected to a gate of the second switching element  27  via the capacitor  20 . An anode of the diode  17  is connected to the gate of the second switching element  27 . The gate of the first switching element  14  is connected to the cathode of the rectifying element  16  through a resistor  30 . A diode  31  is connected to the resistor  30  in parallel. An anode of the diode  31  is connected to the gate of the first switching element  14 . A cathode of the diode  31  is connected to the cathode of the rectifying element  16 . 
     The input filter capacitor  13  is connected between the high-potential input terminal  9  and the low-potential input terminal  10 . The smoothing capacitor  21  is connected between the high-potential output terminal  11  and the low-potential output terminal  12 . The low-potential input terminal  10  and the low-potential output terminal  12  are connected on the inside of the power supply device  3 . The resistor  26  is connected to the smoothing capacitor  21  in parallel. 
     The second switching element  27  turns on and off an electric current of the first inductor  18 . The resistor  30  and the diode  31  stabilize the gate potential of the first switching element  14 . 
     The operation of the power supply device  29  is described. 
     (1a) When the power supply voltage VIN is applied to between the high-potential input terminal  9  and the low-potential input terminal  10 , since the first switching element  14  and the second switching element  27  are the normally on type elements, the first switching element  14  and the second switching element  27  are in an ON state. Then, an electric current flows through a path of the high-potential input terminal  9 , the second switching element  27 , the first switching element  14 , the resistor  15 , the first inductor  18 , the smoothing capacitor  21 , and the low-potential input terminal  10 . The smoothing capacitor  21  is charged. Electromagnetic energy is accumulated in the first inductor  18 . The power supply voltage VIN is substantially applied to both ends of the rectifying element  16 . Since a voltage is applied in the opposite direction, the rectifying element  16  changes to a non-conduction state. 
     (2a) As time elapses, an electric current flowing through the first inductor  18  increases. Since the second inductor  19  is magnetically coupled to the first inductor  18 , the second inductor  19  induces an electromotive force having polarity for setting the capacitor  20  side to high potential. The capacitor  20  functions as a coupling capacitor. Potential, which is positive with respect to the source, is supplied to a gate of the second switching element  27  via the capacitor  20 . The second switching element  27  maintains the ON state. In this embodiment, a voltage between an A point in  FIG. 5  and the gate of the second switching element  27  is limited to, for example, 0.6 V according to the action of the diode  17 . 
     (3a) According to the increase in the electric current flowing through the first inductor  18 , the voltage at both the ends of the resistor  15  also increases. The voltage between the A point in  FIG. 5  and the gate of the second switching element  27  is limited to, for example, 0.6 V as described above. Therefore, according to the increase in the voltage at both the ends of the resistor  15 , the gate-to-source voltage of the second switching element  27  changes to relatively negative potential. 
     (4a) When the electric current flowing through the first inductor  18  exceeds the threshold described above with reference to  FIG. 3 , the drain-to-source voltage of the first switching element  14  suddenly increases according to the rise in the ON resistance. The gate-to-source voltage of the second switching element  27  changes to a negative large value. The second switching element  27  changes to an OFF state. An electric current at this point is represented by Ip as in the circuit shown in  FIG. 1 . 
     (5a) A forward voltage is applied by the counter electromotive force of the first inductor  18 . Therefore, the rectifying element  16  changes to an ON state. An electric current flows through a path of the rectifying element  16 , the first inductor  18 , and the smoothing capacitor  21 . Since electromagnetic energy is emitted, the electric current of the first inductor  18  decreases. The negative voltage induced by the second inductor  19  is maintained. The second switching element  27  maintains the OFF state. 
     (6a) When the electromagnetic energy accumulated in the first inductor  18  decreases to zero, the electric current flowing through the first inductor  18  decreases to zero. The direction of the electromotive force induced by the second inductor  19  is reversed again. An electromotive force for setting the capacitor  20  side to high potential is induced. A voltage higher than the voltage of the source is supplied to the gate of the second switching element  27 . The second switching element  27  is turned on. Consequently, the power supply device  29  returns to the state of (1a). 
     Thereafter, the power supply device  29  repeats (1a) to (6a). Switching from ON to OFF of the second switching element  27  is automatically repeated. The voltage VOUT stepped down from the power supply voltage VIN is supplied to the illumination light source  5 . As in the embodiment shown in  FIG. 1 , a limited current is supplied to the illumination light source  5 . It is possible to stably light the illumination light source  5 . 
     Effects of the second embodiment are described. 
     In this embodiment, as in the first embodiment, the resistor is added to the source terminal of the normally on type element in series to configure the current control section. Therefore, an effect is obtained that it is possible to simplify a circuit and configure the power supply device with a small number of components. 
     In this embodiment, the two normally on type elements, that is, the first switching element  14  and the second switching element  27  are used. A withstand voltage equal to or higher than the power supply voltage VIN is necessary for the second switching element  27 . As a withstand voltage of the first switching element  14 , a value exceeding the gate-to-source voltage of the second switching element  27  is sufficient. That is, a low-withstand voltage element can be used as the first switching element  14 . In general, since the low-withstand voltage element operates at high speed, an increase in the ON resistance at the time when a flowing electric current reaches the threshold is steep. An OFF operation of the second switching element  27  is performed at high speed. Therefore, an effect is also obtained that it is possible to reduce a loss of the second switching element  27  and save power consumption. 
     The embodiments are described above with reference to the specific examples. However, the embodiments are not limited to the specific examples and various modifications of the embodiments are possible. 
     For example, the first switching element  14  and the second switching element  27  are not limited to the GaN-based HEMT. For example, the first switching element  14  and the second switching element  27  may be a semiconductor element formed by using a semiconductor having a wide band gap (a wide band gap semiconductor) such as silicon carbide (SiC), gallium nitride (GaN), or diamond as a semiconductor substrate. The wide band gap semiconductor means a semiconductor having a band gap wide than a band gap of gallium arsenide (GaAs) having the band gap of about 1.4 eV. The wide band gap semiconductor includes, for example, a semiconductor having a band gap equal to or wider than 1.5 eV, gallium phosphate (GaP having 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). 
     The lighting load  4  is not limited to LED and may be, for example, an organic EL (Electro-Luminescence) or an OLED (Organic light-emitting diode). A plurality of the illumination light sources  5  may be connected to the lighting load  4  in series or in parallel. 
     The embodiments are described above with reference to the specific examples. However, the embodiments are not limited to the specific examples. That is, examples obtained by those skilled in the art applying design changes to the specific examples are also included in the scope of the embodiments as long as the examples include the characteristics of the embodiments. The components and the arrangement, the materials, the conditions, the shapes, the sizes, and the like of the components included in the specific examples are not limited to those illustrated in the figures and can be changed as appropriate. 
     The components included in the embodiments can be combined as long as the combination is technically possible. Components obtained by combining the components are also included in the scope of the embodiments as long as the components include the characteristics of the embodiments. Besides, in the category of the idea of the embodiments, those skilled in the art can conceive various modifications and alterations. It is understood that the modifications and the alternations also belong to the scope of the embodiments. 
     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.