Switching power-supply device and luminaire

This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2011-074676, filed on Mar. 30, 2011, and Japanese Patent Application No. 2011-150085, filed on Jul. 6, 2011; the entire contents of which are incorporated herein by reference.

Embodiments described herein relate generally to a switching power-supply device and a luminaire.

In recent years, concerning illumination light sources for luminaires, more and more incandescent lamps and fluorescent tubes are replaced with light sources that consume less power and have longer life such as an LED (Light Emitting Diode). New illumination light sources such as an EL (Electro-Luminescence) and an OLED (Organic light-emitting diode) are developed. Since the luminance of these illumination light sources depends on a current value flowing thereto, when the illumination light sources are lit, a power-supply circuit that supplies a constant current is necessary. In order to adjust a direct-current power supply voltage to a rated voltage of an illumination light source, usually, step-down means is used. As step-down means having high current usage efficiency, a self-excitation DC-DC converter is proposed (see, for example, JP-A-2004-119078).

In an LED lighting device described in JP-A-2004-119078, an FET (Field-Effect Transistor), a resistor for current detection, a first inductor, and an LED circuit are connected to a direct-current power supply in series to form a loop-shape main current path. A voltage generated by resistance division of an output of the direct-current power supply is applied between a source and a gate of the FET. A voltage between both ends of the resistor for current detection is also applied between the source and the gate. A diode is connected between both ends of the first inductor and the LED circuit to form a loop-shape feedback circuit. Further, a second inductor magnetically coupled to the first inductor is provided such that an electromotive force of the second inductor is applied to the gate of the FET.

In such an LED lighting device, when a power-supply is turned on, potential generated by resistance division of a power-supply voltage is applied to the gate of the FET and the FET changes to an ON state. An electric current starts to flow to the main current path. When this electric current increases, an electromotive force is generated in the second inductor and the FET is kept on. Consequently, the LED circuit is lit and magnetic energy is accumulated in the first inductor. Thereafter, when the electric current flowing through the main current path reaches a predetermined amount, a voltage drop amount between both the ends of the resistor for current detection reaches a predetermined amount, gate potential with respect to the source potential of the FET falls to be lower than a threshold, and the FET changes to an OFF state. Consequently, the main current path is shut off. An electric current flows to the feedback circuit with the magnetic energy accumulated in the first inductor and lights the LED circuit. At this point, since this electric current decreases with time, an opposite electromotive force is generated in the second inductor and the FET is kept off. Thereafter, when the electric current decreases to zero, the direction of the electromotive force of the second inductor is reversed again and the FET changes to the ON state. According to the repetition of such operation, self-excitation DC-DC conversion is performed and a stepped-down DC voltage is supplied to the LED circuit.

However, in the LED lighting device in the past, the resistor for current detection is necessary. When the FET is on, an electric current always flows to the resistor for current detection. Therefore, a loss of electric power is large. If the resistor for current detection is not used, a heavy current is likely to flow during the start.

It is an object of the present invention to provide a switching power supply and a luminaire in which a loss of power is small and an overcurrent during the start is suppressed.

FIG. 1 is a circuit diagram of an example of a luminaire according to a first embodiment;

FIG. 2 is a circuit diagram of an example of a constant voltage circuit in the first embodiment;

FIG. 3 is a circuit diagram of an example of a constant voltage circuit in a second embodiment;

FIG. 4 is a circuit diagram of an example of a luminaire according to a third embodiment;

FIG. 5 is a circuit diagram of an example of a luminaire according to a fourth embodiment; and

FIG. 6 is a circuit diagram of an example of a luminaire according to a fifth embodiment.

According to one embodiment, a switching power-supply device includes a switching element, a constant current element, a rectifying element, a first inductor, a second inductor, and a constant voltage circuit. The switching element supplies, when the switching element is on, a power-supply voltage of a direct-current power supply to and feeds an electric current to the first inductor. The constant current element is connected to the switching element in series and turns off the switching element when the electric current of the switching element exceeds a predetermined current value. The rectifying element is connected to any one of the switching element and the constant current element in series and feeds the electric current of the first inductor when the switching element is turned off. The second inductor is magnetically coupled to the first inductor and has induced therein potential for turning on the switching element when the electric current of the first inductor increases and has induced therein potential for turning off the switching element when the electric current of the first inductor decreases and supplies the induced potential to a control terminal of the switching element. The constant voltage circuit applies control potential to a control terminal of the constant current element.

According to another embodiment, a switching power-supply device includes a switching element, a constant current element, a rectifying element, a first inductor, a second inductor, and a constant voltage circuit. A first terminal of the switching element is connected to one terminal of a direct-current power supply. A first terminal of the constant current element is connected to a second terminal of the switching element. A first terminal of the first inductor is connected to a second terminal of the constant current element. The second inductor is magnetically coupled to the first inductor, supplies control potential for turning on the switching element to a control terminal of the switching element when an electric current flowing to the first inductor increases, and supplies control potential for turning off the switching element to the control terminal of the switching element when the electric current flowing to the first inductor decreases. The rectifying element is connected between the other terminal of the direct-current power supply and a first terminal of the first inductor and feeds an electric current in a direction in which an electric current in the same direction as the electric current supplied to the first inductor is supplied to the first inductor via the switching element and the rectifying element. The constant voltage circuit applies a control voltage between a second terminal and a control terminal of the constant current element.

According to still another embodiment, a luminaire includes a switching power-supply device and a lighting load connected between output terminals of the switching power-supply device. The switching power-supply device includes a switching element, a constant current element, a rectifying element, a first inductor, a second inductor, and a constant voltage circuit. The switching element supplies, when the switching element is on, a power-supply voltage of a direct-current power supply to and feeds an electric current to the first inductor. The constant current element is connected to the switching element in series and turns off the switching element when the electric current of the switching element exceeds a predetermined current value. The rectifying element is connected to any one of the switching element and the constant current element in series and feeds the electric current of the first inductor when the switching element is turned off. The second inductor is magnetically coupled to the first inductor and has induced therein potential for turning on the switching element when the electric current of the first inductor increases and has induced therein potential for turning off the switching element when the electric current of the first inductor decreases and supplies the induced potential to a control terminal of the switching element. The constant voltage circuit applies control potential to a control terminal of the constant current element.

Embodiments are explained below with reference to the drawings.

First, a first embodiment is explained.

FIG. 1 is a circuit diagram of an example of a luminaire according to this embodiment.

FIG. 2 is a circuit diagram of an example of a constant voltage circuit in this embodiment.

As shown in FIG. 1 , a luminaire 1 according to this embodiment is connected to a commercial alternating-current power supply AC and used. In the luminaire 1 , a direct-current power supply 11 connected to the alternating-current power supply AC and configured to convert an alternating current supplied to the alternating-current power supply AC into a direct current, a DC-DC converter 12 configured to drop a direct-current voltage supplied from the direct-current power supply 11 , and a lighting load 13 connected between output terminals of the DC-DC converter 12 are provided. In the lighting load 13 , an illumination light source E configured to receive the supply of the direct current from the DC-DC converter 12 and emit light, for example, an LED element is provided. A switching power-supply device according to this embodiment is configured by the direct-current power supply 11 and the DC-DC converter 12 .

In the direct-current power supply 11 , a full-wave rectifier circuit B including a diode bridge is provided. An input terminal of the full-wave rectifier circuit B is connected to the alternating-current power supply AC. Output terminals of the full-wave rectifier circuit B are output terminals T 1 and T 2 of the direct-current power supply 11 . The output terminal T 1 is a terminal on a high-potential side and the output terminal T 2 is a terminal on a low-potential side. The output terminals T 1 and T 2 of the direct-current power supply 11 are also input terminals of the DC-DC converter 12 . “Terminal” is a concept indicating a position on a circuit diagram. A member equivalent to only the “terminal” is not always provided in an actual device.

In the DC-DC converter 12 , a capacitor C 1 is connected between the output terminal T 1 and the output terminal T 2 of the direct-current power supply 11 . A switching element Q 1 , a constant current element Q 2 , and a rectifying element D 1 are provided and connected in series in this order between the output terminal T 1 and the output terminal T 2 .

The switching element Q 1 and the constant current element Q 2 are, for example, field effect transistors, high electron mobility transistors (HEMTs), or so-called GaN HEMTs formed on a substrate of silicon carbide (SiC). Channels of the GaN HEMTs are formed of a gallium nitride (GaN) or indium gallium nitride (InGaN). The switching element Q 1 and the constant current element Q 2 are elements of a normally on type. The rectifying element D 1 is, for example, a Schottky barrier diode and is formed in the same manner as the switching element Q 1 and the constant current element Q 2 .

A drain (a first terminal) of the switching element Q 1 is connected to the output terminal T 1 . A source (a second terminal) of the switching element Q 1 is connected to a drain (a first terminal) of the constant current element Q 2 . A source (a second terminal) of the constant current element Q 2 is connected to a cathode of the rectifying element D 1 via a connection point N 5 . An anode of the rectifying element D 1 is connected to the output terminal T 2 .

In the DC-DC converter 12 , a first inductor L 1 and a smoothing capacitor C 2 are provided. One terminal (a first terminal) of the first inductor L 1 is connected to the connection point N 5 and the other terminal of the first inductor L 1 is connected to an output terminal T 3 on a high-potential side of the DC-DC converter 12 . The smoothing capacitor C 2 is connected between the output terminal T 3 and an output terminal T 4 on a low-potential side of the DC-DC converter 12 . The output terminal T 4 is connected to the output terminal T 2 on a low-potential side of the direct-current power supply 11 . The potential of the output terminals T 2 and T 4 is, for example, ground potential.

Further, in the DC-DC converter 12 , a second inductor L 2 , a coupling capacitor C 3 , and a diode D 2 are provided. The second inductor L 2 is connected between the connection point N 5 and one terminal of the coupling capacitor C 3 and is magnetically coupled to the first inductor L 1 . In the second inductor L 2 , when an electric current flowing from the connection point N 5 to the output terminal T 3 in the first inductor L 1 increases, an electromotive force for setting the coupling capacitor C 3 to potential higher than the potential at the connection point N 5 is generated. When the electric current decreases, an electromotive force for setting the coupling capacitor C 3 to potential lower than the potential at the connection point N 5 is generated. The other terminal of the coupling capacitor C 3 is connected to a gate, which is a control terminal, of the switching element Q 1 . An anode of the diode D 2 is connected to the other terminal of the coupling capacitor C 3 and a gate of the switching element Q 1 . A cathode of the diode D 2 is connected to the connection point N 5 . The diode D 2 clamps a voltage between the gate of the switching element Q 1 and a source of the constant current element Q 2 to a voltage equal to or lower than a forward voltage. The gate potential of the switching element Q 1 (the control potential of the switching element) is level-shifted to a negative potential side. The switching element Q 1 can be surely turned on and off.

Furthermore, in the DC-DC converter 12 , a constant voltage circuit V 1 and bias resistors R 1 and R 2 are provided. A terminal N 1 of the constant voltage circuit V 1 is connected to the output terminal T 1 . A terminal N 2 of the constant voltage circuit V 1 is connected to the connection point N 5 . A terminal N 3 of the constant voltage circuit V 1 is connected to a gate of the constant current element Q 2 (a control terminal of the constant current element). The bias resistor R 1 is connected between the terminal N 3 and the output terminal T 2 . The bias resistor R 2 is connected between the output terminal T 1 and the terminal N 2 . The constant voltage circuit V 1 is a circuit that receives the supply of high potential from the terminal N 1 , receives the supply of low potential from the terminal N 3 , and outputs intermediate potential between the high potential and the low potential from the terminal N 2 . A voltage between the terminal N 2 and the terminal N 3 is fixed. A gate-to-source voltage of the constant current element Q 2 (a control voltage of the constant current element) is a negative fixed value.

An LED element is connected as the illumination light source E between the output terminal T 3 and the output terminal T 4 of the DC-DC converter 12 . An anode of the LED element E is connected to the output terminal T 3 and a cathode of the LED element E is connected to the output terminal T 4 . Consequently, a loop-shape current path of “the full-wave rectifier circuit B→the output terminal T 1 →the switching element Q 1 →the constant current element Q 2 →the connection point N 5 →the first inductor L 1 →the output terminal T 3 →the LED element E→the output terminal T 4 →the output terminal T 2 →the full-wave rectifier circuit B” is formed. A loop-shape regenerative current path of “the first inductor L 1 →the output terminal T 3 →the LED element E→the output terminal T 4 →the rectifying element D 1 →the connection point N 5 →the first inductor L 1 ” is also formed. In this way, the constant current element Q 2 is interposed between an input terminal of the DC-DC converter 12 (the output terminal T 1 of the direct-current power supply 11 ) and the output terminal T 3 . The rectifying element D 1 is connected such that an electric current in the same direction as the electric current supplied to the first inductor L 1 flows via the switching element Q 1 and the constant current element Q 2 .

As shown in FIG. 2 , in the constant voltage circuit V 1 , bipolar transistors Q 11 and Q 12 are provided. Characteristic of the bipolar transistors Q 11 and Q 12 are substantially the same. In the constant voltage circuit V 1 , resistors R 11 , R 12 , and R 13 and a differential amplifier DA are provided. Collectors of the bipolar transistors Q 11 and Q 12 are connected to the terminal N 1 . An emitter of the bipolar transistor Q 11 is connected to the terminal N 3 via the resistor R 12 and the resistor R 13 . An emitter of the bipolar transistor Q 12 is connected to the terminal N 3 via the resistor R 11 . A contact point N 11 of the resistor R 12 and the resistor R 13 is connected to an input terminal on a positive pole side of a differential amplifier DA. A contact point N 12 of the emitter of the bipolar transistor Q 12 and the resistor R 11 is connected to an input terminal on a negative pole side of the differential amplifier DA. An output terminal of the differential amplifier DA is connected to bases of the bipolar transistors Q 11 and Q 12 and connected to the terminal N 2 .

The constant voltage circuit V 1 can output, as a voltage V ref between the terminal N 2 and the terminal N 3 , a voltage based on a base emitter voltage V BE of the bipolar transistors Q 11 and Q 12 . Specifically, when temperature is represented as T, a Boltzmann constant is represented as k, a charge is represented as q, and resistances of the resistors R 11 , R 12 , an R 13 are respectively represented as R 11 , R 12 , and R 13 , the voltage V ref is calculated as indicated by Expression 1 below. A temperature coefficient of the base emitter voltage V BE of the bipolar transistors Q 11 and Q 12 has a negative value. However, if a resistance ratio is properly adjusted using a diffusion layer resistor, polysilicon, or the like, which has a positive temperature coefficient, as the resistors R 11 to R 13 , a temperature coefficient of the voltage V ref can be reduced to substantially zero.

The operation of the luminaire according to this embodiment is explained.

Since the switching elements Q 1 and Q 2 are the elements of the normally on type, in an initial state, both the switching elements Q 1 and Q 2 are in an ON state.

(1) When the alternating-current power supply AC is connected to the direct-current power supply 11 , an alternating current output from the alternating-current power supply AC is input to the direct-current power supply 11 . In the direct-current power supply 11 , the alternating current is converted into a direct current by the full-wave rectifier circuit B. The direct current is output from the output terminals T 1 and T 2 and input to the DC-DC converter 12 . At this point, high potential is output from the output terminal T 1 and low potential is output from the output terminal T 2 .

(2) In the DC-DC converter 12 , after a high-frequency component is removed by the capacitor C 1 , the potential of the output terminal T 1 is input to the terminal N 1 of the constant voltage circuit V 1 , the potential of the output terminal T 1 is input to the terminal N 2 of the constant voltage circuit V 1 via the bias resistor R 2 , and the potential of the output terminal T 2 is input to the terminal N 3 via the bias resister R 1 . Consequently, the constant voltage circuit V 1 operates and sets the voltage V ref between the terminal N 2 and the terminal N 3 to a constant voltage specified by Expression 1 above. As a result, potential lower than the potential of the source of the constant current element Q 2 is applied to the gate of the constant current element Q 2 . An electric current flowing between the drain and the source of the constant current element Q 2 is limited by a source-to-gate voltage of the constant current element Q 2 .

(3) An electric current flows through a path of “the input terminal T 1 →the switching element Q 1 →the constant current element Q 2 →the first inductor L 1 ”. At this point, the electric current does not flow to the LED element E until a voltage applied to the LED element E reaches a forward voltage of the LED element E. Therefore, the smoothing capacitor C 2 is charged. In other words, a voltage is applied between the source and the gate of the constant current element Q 2 such that an absolute value of a negative voltage between the source and the gate of the constant current element Q 2 is smaller than the forward voltage of the LED element E. Therefore, the electric current does not flow to the LED element E and the capacitor C 2 is charged.

(4) When the capacitor C 2 is charged and the voltage applied to the LED element E exceeds the forward voltage of the LED element E, an electric current flows through a path of “the input terminal T 1 →the switching element Q 1 →the constant current element Q 2 →first inductor L 1 →the LED element E→the input terminal T 2 ). Consequently, the LED element E is lit and magnetic energy is accumulated in the first inductor L 1 . Since this current increases, an electromotive force for setting the coupling capacitor C 3 side to high potential is generated in the second inductor L 2 . As a result, the gate potential of the switching element Q 1 increases to be higher than the source potential of the switching element Q 1 and the ON state of the switching element Q 1 is maintained.

(5) When a value of an electric current flowing through the constant current element Q 2 including an HEMT reaches a saturation current, according to the increase of the electric current, a voltage between the source and the drain of the constant current element Q 2 suddenly rises. The saturation current of the constant current element Q 2 is specified by a source-to-gate voltage given by the constant voltage circuit V 1 . According to the sudden rise of the voltage between the source and the drain of the constant current element Q 2 , the source potential of the switching element Q 1 rises to be higher than the gate potential of the switching element Q 1 and the switching element Q 1 changes to an OFF state. As a result, the current path is shut off.

(6) Consequently, the magnetic energy accumulated in the first inductor L 1 is radiated and an electric current flows through a regenerative current path of “the first inductor L 1 →the LED element E→the rectifying element D 1 →the first inductor L 1 ). The lighting of the LED element E is maintained. Since this electric current decreases with time, an electromotive force for setting the coupling capacitor C 3 side to low potential is generated in the second inductor L 2 . As a result, potential lower than the potential of the source of the switching element Q 1 is applied to the gate of the switching element Q 1 and the OFF state of the switching element Q 1 is maintained.

(7) When the magnetic energy accumulated in the first inductor L 1 decreases to zero, the direction of the electromotive force of the second inductor L 2 is reversed again and an electromotive force for setting the coupling capacitor C 3 side to high potential is generated. Consequently, potential higher than the potential of the source of the switching element Q 1 is applied to the gate of the switching element Q 1 and the switching element Q 1 is turned on. Consequently, the switching element Q 1 returns to the state of (4).

Thereafter, (4) to (7) are repeated. Consequently, on and off of the switching element Q 1 are automatically repeated. A direct current subjected to voltage drop is supplied to the LED element E.

Effects of this embodiment are explained.

In this embodiment, when an electric current flowing through the constant current element Q 2 reaches a saturation current, the voltage between the source and the drain of the constant current element Q 2 suddenly rises to change the switching element Q 1 to the OFF state. In other words, the saturation current of the constant current element Q 2 controlled by the constant voltage circuit V 1 is used to detect that the magnitude of the electric current reaches a predetermined value. Therefore, a loss of electric power is small compared with electric power lost when a resistor is used to detect that the magnitude of the electric current reaches the predetermined value. Since a resistor for current detection is unnecessary, it is possible to reduce the size of the LED lighting circuit.

Further, the LED element E can be dimmed and stopped by arbitrarily changing an output of the constant voltage circuit V 1 . Specifically, if the resistor for current detection is used to detect that the magnitude of the electric current reaches the predetermined value, the predetermined value is a fixed value. However, since the constant current element Q 2 is used instead of the resistor for current detection, a predetermined current value to be detected can be arbitrarily changed. Furthermore, the constant voltage circuit V 1 can be caused to operate to correct temperature characteristics of the switching element Q 1 or the constant current element Q 2 . For example, the constant voltage circuit V 1 can add a negative characteristic as a temperature characteristic.

Furthermore, in this embodiment, since the HEMT is used as the switching element Q 1 and the constant current element Q 2 , a high-frequency operation is possible. For example, operation in a megahertz order is possible. In particular, since the GaN HEMT is used, a higher-frequency operation is possible. Since a withstand voltage is high, a chip size can be reduced.

Moreover, when the element of the normally on type is used as the constant current element Q 2 , if the saturation current of the constant current element Q 2 is not controlled, it is likely that an excessive current flows in a period in which an electric current immediately after power-on is unstable or when the LED element E starts lighting. On the other hand, in this embodiment, the saturation current of the constant current element Q 2 is controlled by the constant voltage circuit V 1 , after the power supply is turned on, even during a period until a power-supply voltage is stabilized and when the LED element E starts lighting, it is possible to surely limit an electric current and prevent an excessive current from flowing.

A second embodiment is explained.

FIG. 3 is a circuit diagram of an example of a constant voltage circuit in this embodiment.

As shown in FIG. 3 , this embodiment is different from the first embodiment in the configuration of a constant voltage circuit. Specifically, in this embodiment, a constant voltage circuit V 2 is provided instead of the constant voltage circuit V 1 in the first embodiment. Components other than the constant voltage circuit of a luminaire according to this embodiment are the same as the components shown in FIG. 1 .

As shown in FIG. 3 , in the constant voltage circuit V 2 , p-channel MOS transistors (hereinafter, PMOSs) M 21 and M 23 and n-channel MOS transistors (hereinafter, NMOSs) M 22 and M 24 are provided. The NMOS M 22 is a transistor of a normally on type. The NMOS M 24 is a transistor of a normally off type. Sources of the PMOSs M 21 and M 23 are connected to the terminal N 1 and gates of the PMOSs M 21 and M 23 are connected to a drain of the PMOS M 21 . The drain of the PMOS M 21 is connected to a drain of the NMOS M 22 . A drain of the PMOS M 23 is connected to a drain of the NMOS M 24 . Sources of the NMOSs M 22 and M 24 are connected to the terminal N 3 . A gate of the NMOS M 22 is connected to the terminal N 3 . A gate of the NMOS M 24 is connected to the terminal N 2 . The drain of the PMOS M 23 and the drain of the NMOS M 24 are also connected to the terminal N 2 .

The constant voltage circuit V 2 can output, as the voltage V ref between the terminal N 2 and the terminal N 3 , a voltage based on a difference between a threshold voltage V th22 of the NMOS M 22 of the normally on type and a threshold voltage V th24 of the NMOS M 24 of the normally off type. Specifically, when proportionality constants (gain coefficients) of an electric current to an overdrive voltage of the PMOSs M 21 and M 23 and NMOSs M 22 and M 24 are respectively represented as β 21 , β 23 , β 22 , and β 24 , the voltage V ref between the terminal N 2 and the terminal N 3 is given by Expression 2 below. At this point, temperature coefficients of the threshold voltages V th22 and V th24 cancel each other in first approximation. Therefore, temperature dependency of the voltage V ref is small and can take a substantially fixed value.

In this embodiment, the constant voltage circuit V 2 can apply the constant voltage V ref specified by Expression 2 between the source and the gate of the constant current element Q 2 and control the saturation current of the constant current element Q 2 to a predetermined current value.

Components, operations, and effects in this embodiment other than those explained above are the same as the components, the operations, and the effects explained in the first embodiment.

Third embodiment is explained.

FIG. 4 is a circuit diagram of an example of a luminaire according to this embodiment.

As shown in FIG. 4 , this embodiment is different from the first embodiment in the configurations of a direct-current power supply and the first inductor L 1 and a constant voltage circuit V 3 in a DC-DC converter. Specifically, in this embodiment, a direct-current power supply 21 is provided instead of the direct-current power supply 11 according to the embodiments explained above. The first inductor L 1 connected between the connection point N 5 of the DC-DC converter 12 and the output terminal T 3 on the high-potential side in the first embodiment is connected between the output terminal T 2 on the low-potential side and the output terminal T 4 on the low-potential side. Further, the constant voltage circuit V 3 is provided instead of the constant voltage circuit V 1 of the DC-DC converter 12 in the first embodiment. Components other than the direct-current power supply 21 , the position of the first inductor L 1 of the DC-DC converter 22 , and the constant voltage circuit V 3 of a luminaire 2 according to this embodiment are the same as the components shown in FIG. 1 .

The direct-current power supply 21 is, for example, a battery. The direct-current power supply 21 generates a direct-current voltage VDCin between the output terminal T 1 and the output terminal T 2 and supplies the direct-current voltage VDCin to the DC-DC converter 22 .

In the DC-DC converter 22 , the second inductor L 2 is connected between the output terminal T 4 on the low-potential side and one terminal of the coupling capacitor C 3 and is magnetically coupled to the first inductor L 1 . In the second inductor L 2 , when an electric current flowing from the connection point N 5 to the output terminal T 3 through the first inductor L 1 increases, an electromotive force for setting the coupling capacitor C 3 to potential higher than the potential at the connection point N 5 is generated. When the current decreases, an electromotive force for setting the coupling capacitor C 3 to potential lower than the potential at the connection point N 5 is generated. The other terminal of the coupling capacitor C 3 is connected to the gate, which is the control terminal, of the switching element Q 1 . The diode D 2 in the first embodiment is not provided. However, the diode D 2 does not have to be provided as long as the switching element Q 1 can be turned on or off according to the gate potential of the switching element Q 1 .

In the constant voltage circuit V 3 , a constant voltage diode ZD and an impedance element Z are provided. The constant voltage diode ZD is connected between the connection point N 5 and the gate of the constant current element Q 2 (the control terminal of the constant current element). The impedance element Z is connected between the gate of the constant current element Q 2 and the output terminal T 2 on the low-potential side of the direct-current power supply 21 . Voltages at both ends of the smoothing capacitor C 2 are applied to both ends of the constant voltage diode ZD and the impedance element Z, which are connected in series, via the first inductor L 1 . Therefore, both the ends of the constant voltage diode ZD have a constant voltage. The impedance element Z only has to capable of feeding a reverse current to the constant voltage diode ZD and generating a constant voltage. For example, the impedance element Z only has to feed an electric current of about several microamperes.

In this embodiment, as in the second embodiment, the constant voltage circuit V 3 can apply the constant voltage at the both ends of the constant voltage diode ZD between the source and the gate of the constant current element Q 2 and control the saturation current of the constant current element Q 2 to a predetermined current value.

In this embodiment, the first inductor L 1 is connected between the output terminal T 2 on the low-potential side of the direct-current power supply 21 and the output terminal T 4 on the low-potential side of the DC-DC converter 22 . However, the operation of the DC-DC converter 22 is the same as the operation of the DC-DC converter 12 in the first embodiment. Components, operations, and effects in this embodiment other than those explained above are the same as the components, the operations, and the effects explained in the first embodiment.

A fourth embodiment is explained.

FIG. 5 is a circuit diagram of an example of a luminaire according to this embodiment.

As shown in FIG. 5 , this embodiment is different from the first embodiment in that a direct-current power supply is not provided and in the configuration of a constant voltage circuit V 4 in a DC-DC converter 32 . Specifically, in this embodiment, the direct-current power supplies 11 and 21 in the first and second embodiments are not provided. The direct-current power-supply voltage VDCin is supplied from the outside. The constant voltage circuit V 4 is provided instead of the constant voltage circuit V 1 of the DC-DC converter 12 in the first embodiment. Components other than the constant voltage circuit V 4 of the DC-DC converter 32 of a luminaire 3 according to this embodiment are the same as the components shown in FIG. 1 .

The terminal N 1 of the constant voltage circuit V 4 is connected to the connection point N 5 . The terminal N 2 of the constant voltage circuit V 4 is connected to the gate of the constant current element Q 2 (the control terminal of the constant current element). The terminal N 3 of the constant voltage circuit V 4 is connected to the output terminal T 2 . The constant voltage circuit V 4 is a circuit that receives the supply of high potential VCC+ from the terminal N 1 , receives the supply of low potential VCC− from the terminal N 3 , and outputs intermediate potential, which can be adjusted between the high potential VCC+ and the low potential VCC−, from the terminal N 2 . A voltage between the terminal N 1 and the terminal N 2 can be adjusted. A gate-to-source voltage of the constant current element Q 2 (the control voltage of the constant current element) is an adjustable negative fixed value. The high potential VCC+ and the low potential VCC− supplied to the constant voltage circuit V 4 are voltages at both the ends of the smoothing capacitor C 2 supplied via the first inductor L 1 . The voltages at both the ends of the smoothing capacitor C 2 change to a forward voltage of the LED element E when the LED element E is lit. Therefore, it is possible to cause the constant voltage circuit V 4 to operate. A diode D 3 is connected between the gate and the source of the constant current element Q 2 in order to protect the gate of the constant current element Q 2 .

In this embodiment, when the LED element E is lit, the constant voltage circuit V 4 can apply the adjustable negative constant voltage between the gate and the source of the constant current element Q 2 and control the saturation current of the constant current element Q 2 to a predetermined current value. Therefore, it is possible to adjust an average of electric currents flowing through the LED element E and adjust the luminance of the LED element E.

Components, operations, and effects in this embodiment other than those explained above are the same as the components, the operations, and the effects explained in the first embodiment.

A fifth embodiment is explained.

FIG. 6 is a circuit diagram of an example of a luminaire according to this embodiment.

As shown in FIG. 6 , this embodiment is different from the fourth embodiment in the configuration of a constant voltage circuit V 5 in a DC-DC converter. Specifically, in this embodiment, the constant voltage circuit V 5 is provided instead of the constant voltage circuit V 4 of the DC-DC converter 32 in the fourth embodiment. Components other than the constant voltage circuit V 5 of a DC-DC converter 42 of a luminaire 4 according to this embodiment are the same as the components shown in FIG. 5 .

The terminal N 1 of the constant voltage circuit V 5 is connected to the output terminal T 3 on the high-potential side. The terminal N 2 of the constant voltage circuit V 5 is connected to the gate of the constant current element Q 2 . The terminal N 3 of the constant voltage circuit V 5 is connected to the output terminal T 4 on the low-potential side. The constant voltage circuit V 5 is a circuit that receives the supply of high potential VCC+ from the terminal N 1 , receives the supply of low potential VCC− from the terminal N 3 , and outputs intermediate potential, which can be adjusted between the high potential VCC+ and the low potential VCC−, from the terminal N 2 . A voltage between the terminal N 2 and the terminal N 3 can be adjusted. A gate-to-source voltage of the constant current element Q 2 is an adjustable negative fixed value. The high potential VCC+ and the low potential VCC− supplied to the constant voltage circuit V 5 are voltages at both the ends of the smoothing capacitor C 2 . The voltages at both the ends of the smoothing capacitor C 2 change to a forward voltage of the LED element E when the LED element E is lit. Therefore, it is possible to cause the constant voltage circuit V 5 to operate.

In this embodiment, when the LED element E is lit, the constant voltage circuit V 5 can apply the adjustable negative constant voltage between the gate and the source of the constant current element Q 2 and control the saturation current of the constant current element Q 2 to a predetermined current value. Therefore, it is possible to adjust an average of electric currents flowing through the LED element E and adjust the luminance of the LED element E.

Components, operations, and effects in this embodiment other than those explained above are the same as the components, the operations, and the effects explained in the first embodiment.

The present invention is explained above with reference to the embodiments. However, the scope of the present invention is not limited to the embodiments explained above. Appropriate additions, changes, and omissions of components by those skilled in the art are included in the present invention without departing from the spirit of the present invention.

For example, in the example explained in the first to fifth embodiments, the switching element Q 1 is the element of the normally on type. However, the present invention is not limited to this example. The switching element Q 1 may be an element of the normally off type. In this case, the direction of the diode D 2 is reversed. Specifically, the anode of the diode D 2 is connected to the connection point N 5 and the cathode of the diode D 2 is connected to the coupling capacitor C 3 and the gate of the switching element Q 1 . The diode D 2 clamps a voltage between the gate of the switching element Q 1 and the source of the constant current element Q 2 to a voltage equal to or lower than a forward voltage. The gate potential of the switching element Q 1 is level-shifted to a positive potential side. The switching element Q 1 of the normally off type can be surely turned on and off.

In the example explained in the first and second embodiments, the constant current element Q 2 is the element of the normally on type. However, the present invention is not limited to this example. The constant current element Q 2 may be an element of the normally off type. In this case, the connection of the terminal N 2 and the terminal N 3 in the constant voltage circuit V 1 or V 2 is reversed. Specifically, the relatively high-potential terminal N 2 is connected to the gate of a switching element Q 2 . The relatively low-potential terminal N 3 is connected to the source of the switching element Q 2 , i.e., the connection point N 5 . The gate-to-source voltage of the constant current element Q 2 is a positive fixed value.

The configuration of the DC-DC converter is not limited to the configuration shown in FIGS. 1 and 2 . The DC-DC converter is not limited to a voltage falling type and may be, for example, a rising voltage type or a rising-falling type. The switching power-supply device may be only the DC-DC converter.

The switching element Q 1 and the constant current element Q 2 are not limited to the GaN HEMTs. For example, the switching element Q 1 and the constant current element Q 2 may be semiconductor elements formed using a semiconductor having a wide band gap such as silicon carbide (SiC), gallium nitride (GaN), or diamond (a wide band gap semiconductor) on a semiconductor substrate. The wide band gap semiconductor means a semiconductor, a band gap of which is wider than a band gap of gallium arsenide (GaAs) of about 1.4 eV. Examples of the wide band gap semiconductor include a semiconductor, a band gap of which is equal to or larger than 1.5 eV, gallium phosphide (GaP, a band gap is about 2.3 eV), gallium nitride (GaN, a band gap is about 3.4 eV), diamond (C, a band gap is about 5.27 eV), aluminum nitride (AlN, a band gap is about 5.9 eV), and silicon carbide (SiC). In such a wide band gap semiconductor, parasitic capacitance can be reduced. As a result, since high-speed operation is possible, the switching power-supply device can be further reduced in size.

Further, the configuration of the constant voltage circuit is not limited to the configuration shown in FIGS. 2 and 3 . The constant voltage circuit only has to be a circuit that can supply a constant voltage. Furthermore, the illumination light source E is not limited to the LED and may be an EL or an OLED. Plural illumination light sources E may be connected to the lighting load 13 in series or in parallel.

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 may be made without departing from the spirit of the inventions. These embodiments and modifications thereof are included in the scope and the spirit of the invention and included in the inventions described in the claims and the scope of equivalents of the inventions.