Light-emitting diode driver circuit and LED light source

A light-emitting diode driver circuit and an LED light source which are capable of reliably turning OFF an LED even when an input voltage having a value smaller than a predetermined value is supplied. A light-emitting diode driver circuit for turning ON an LED includes: an inverter which outputs electric power for driving the LED; and an inverter control circuit which controls an operation of the inverter, and when a DC voltage applied to the inverter control circuit has a value smaller than the predetermined value, the inverter control circuit stops the operation of the inverter.

TECHNICAL FIELD

The present invention relates to a light-emitting diode driver circuit and an LED light source including the same.

BACKGROUND ART

Light-emitting diodes (LEDs) are expected to be used as a next-generation new light source in conventionally known lighting apparatuses including fluorescent lights and incandescent lamps because of their high efficiency and long life. Thus, research and development for LED light sources using the LEDs are proceeding. Along with this, development of light-emitting diode driver circuits for driving the LEDs is also proceeding.

Conventionally, as such a light-emitting diode driver circuit, a light-emitting diode driver apparatus as shown inFIG. 4is proposed (Patent Literature (PTL) 1).FIG. 4is a diagram showing a circuit configuration of a conventional light-emitting diode driver apparatus disclosed in PTL 1.

As shown inFIG. 4, a conventional light-emitting diode driver apparatus100is an LED driver circuit for turning ON an LED200, and includes a rectifier circuit110, a light-emitting diode driver semiconductor circuit120, a smoothing capacitor111, a choke coil112, and a diode113.

The rectifier circuit110is a bridge type full-wave rectifier circuit including four diodes, and two opposite ends are connected to an AC power source and the other two opposite ends are connected to the smoothing capacitor. An AC supply voltage from the AC power source is full-wave rectified by the rectifier circuit110and smoothed by the smoothing capacitor111, generating a DC input voltage Vin.

An end of the choke coil112is connected to the high electrical potential side of the smoothing capacitor111and the other end of the choke coil112is connected to the anode side of the LED200. Moreover, the cathode terminal of the diode113is connected to the high electrical potential side of the smoothing capacitor111. The diode113is connected to the choke coil112and the LED200in parallel, and supplies back electromotive force generated in the choke coil112to the LED200.

The light-emitting diode driver semiconductor circuit120is connected to the cathode terminal of the LED200, and controls an LED block circuit including the choke coil112, the diode113, and the LED200.

The light-emitting diode driver semiconductor circuit120includes a drain terminal120D for receiving an output voltage from the LED200, a ground/source terminal120GS connected to a ground potential, and a VCC terminal (reference voltage terminal)120Vcc for outputting a reference voltage Vcc. The light-emitting diode driver semiconductor circuit120further includes, in the circuit configuration, a switching device block121for controlling a current flowing to the LED200, a control circuit122for controlling the switching device block121based on a voltage VJ of the switching device block121, a drain current detection circuit123for detecting a current flowing in the switching device block121, and a start/stop circuit124for controlling a start and a stop of operations of the switching device block121. It is to be noted that a capacitor114is connected between the VCC terminal120Vcc and the ground/source terminal120GS of the light-emitting diode driver semiconductor circuit120.

The switching device block121includes a junction FET121aand a switching device121bthat is an N-type MOSFET connected in series to each other.

The control circuit122includes a regulator122afor regulating the reference voltage Vcc at a constant value, an oscillator122bfor outputting a MAXDUTY signal and CLOCK, and an ON-time blanking pulse generator122cfor providing, to an AND circuit, pulses for setting time period in which the detection of the current is not performed. The control circuit122intermittently turns ON/OFF the switching device121bat a predetermined oscillation frequency based on an output signal from the start/stop circuit124and an output signal from the drain current detection circuit123. An end of the regulator122ain the control circuit122is connected between the junction FET121aand the switching device121b, and the other end is connected to the VCC terminal120Vcc. The regulator122areceives the voltage VJ, regulates the reference voltage Vcc at a constant level, and provides the reference voltage Vcc to the VCC terminal120Vcc.

The drain current detection circuit123is a comparator, which outputs a signal indicating High when the voltage VJ is higher than a detected reference voltage Vsn and outputs a signal indicating Low when the voltage VJ is lower than the detected reference voltage Vsn. The current flowing in the switching device121bis detected by comparing an ON-voltage of the switching device121band the detected reference voltage Vsn of the drain current detection circuit123.

The start/stop circuit124receives the reference voltage Vcc, and outputs a start signal (output signal indicating High) when the reference voltage Vcc is higher than or equal to a predetermined value, and outputs a stop signal (output signal indicating Low) when the reference voltage Vcc is lower than the predetermined value.

In the conventional light-emitting diode driver apparatus100configured as above, an ON/OFF control of the switching device121bis performed as described above by the control circuit122in the light-emitting diode driver semiconductor circuit120.

When the switching device121bis ON, the input voltage Vin causes a current to flow in a direction from the choke coil112to the LED200, and then to the light-emitting diode driver semiconductor circuit120to turn ON the LED200. At this time, magnetic energy is accumulated in the choke coil112due to the current flowing in the choke coil112.

Moreover, when the switching device121bis OFF, the back electromotive force generated by the magnetic energy accumulated in the choke coil112causes a current to flow in a closed loop of the LED block circuit including the choke coil112, the LED200, and the diode113in a direction from the choke coil112to the LED200, and then to the diode113. As a result, the LED200is turned ON.

As described above, the conventional light-emitting diode driver apparatus100is capable of controlling the current flowing to the LED200at a constant current even when the input voltage varies.

CITATION LIST

Patent Literature

SUMMARY OF INVENTION

Technical Problem

However, the conventional light-emitting diode driver apparatus100requires the light-emitting diode driver semiconductor circuit120, which is an integrated circuit (IC), to operate even when the supply of the input voltage Vin is stopped, and thus needs to provide a current to the regulator122a(an IC power source) and the like.

Therefore, the conventional light-emitting diode driver apparatus100has a problem in that, even when the supply of the input voltage Vin is stopped for turning the LED200OFF, very small current flows in the LED200via the junction FET121a, and thus the LED is not completely turned OFF. That is, there is a problem that the LED200keeps emitting light without being completely turned OFF even when the LED200is intended to be turned OFF. This problem is more significantly caused especially when the number of LEDs200is small.

More specifically, there is a need to supply a current to the power circuit inside the IC to cause the circuit inside the IC to normally operate and maintain the state of the signal which indicates the switch is OFF. The IC driver current has to flow into the switching device121ainside the IC via the LED200. As a result, the very small current for driving the IC causes the LED200to emit weak light.

Other than in the light-emitting diode driver apparatus100shown inFIG. 4, the problem as above may arise also in the case of a light-emitting diode driver apparatus without the light-emitting diode driver semiconductor circuit120.

For example, it is the case where an LED lighting apparatus such as a ceiling light has a remote controller switch for remotely turning ON the LED or controlling lighting of the LED, or has a night-light switch (a firefly switch) which is a wall switch having a small green LED lamp that emits light when the lighting apparatus is OFF so that the position of the wall switch can be specified in a dark room when the lighting apparatus is OFF.

When the apparatus has an electronic switch including the above remote control switch and the night-light switch, a circuit is used which includes a triac (thyristor) connected to an AC power source and a controller (IC) for controlling the triac. The triac requires a current flowing therein for maintaining an operation state. Therefore, a problem arises that even when the LED is intended to be turned OFF, an input voltage lower than an input voltage Vd, which is a voltage for turning ON the LED, is supplied. As a result, the LED keeps emitting light without being completely turned OFF as described above, or the electronic switch erroneously works.

The present invention was conceived in view of the above problem and has an object to provide a light-emitting diode driver circuit and an LED light source which is capable of reliably turning OFF the LED even when an input voltage lower than an expected voltage is supplied.

Solution to Problem

In order to solve the above problem, a light-emitting diode driver circuit according to the present invention is a light-emitting diode driver circuit for turning ON a light-emitting diode, the circuit including: an electric power output unit configured to output electric power for driving the light-emitting diode; and a control circuit which controls an operation of the electric power output unit, in which when a DC voltage applied to the control circuit has a value smaller than a predetermined value, the control circuit stops the operation of the electric power output unit.

In this case, it is preferable that the control circuit includes: a start trigger circuit which starts and maintains the operation of the electric power output unit; and a stop circuit which stops an operation of the start trigger circuit, in which the DC voltage is applied to the stop circuit, and when the DC voltage applied to the stop circuit has a value smaller than the predetermined value, the stop circuit stops the operation of the start trigger circuit and the operation of the electric power output unit.

Moreover, it is preferable that the electric power output unit includes a switching device for converting electric power provided to the electric power output unit, the electric power being provided for driving the light-emitting diode.

Moreover, it is preferable that the start trigger circuit (a) includes: a first resistor; a capacitor connected to the first resistor in series; and a trigger diode connected to a connection point between the first resistor and the capacitor, and (b) starts the operation of the electric power output unit as a result of a predetermined voltage held in the capacitor causing the trigger diode to conduct electric power.

Moreover, it is preferable that the stop circuit (a) includes: a second resistor; and another switching device which is different from the switching device and is connected to the second resistor and connected in parallel to the capacitor in the start trigger circuit, and (b) stops the operation of the start trigger circuit as a result of said another switching device being turned ON.

Moreover, another light-emitting diode driver circuit according to the present invention is a light-emitting diode driver circuit for turning ON a light-emitting diode, the circuit including: an inverter which outputs electric power for driving the light-emitting diode; and an inverter control circuit which controls an operation of the inverter, in which when a DC voltage applied to the inverter control circuit has a value smaller than a predetermined value, the inverter control circuit stops the operation of the inverter. The inverter includes: a first switching device; a second switching device connected to the first switching device in series; and a driving transformer, in which the first switching device and the second switching device are alternately turned ON and OFF due to inducing oscillation generated by the driving transformer.

In this case, it is preferable that the first switching device and the second switching device are bipolar transistors.

Moreover, an LED light source according to the present invention includes: any of the above light-emitting diode driver circuits; and a light-emitting diode which is turned ON by the light-emitting diode driver circuit.

Advantageous Effects of Invention

According to the present invention, it is possible to reliably turn OFF an LED even when an input voltage having a value smaller than a predetermined value is supplied.

Moreover, it is possible to prevent an error in a semiconductor switch such as a firefly night-light switch.

DESCRIPTION OF EMBODIMENTS

Hereinafter, a light-emitting diode driver circuit and an LED light source according to embodiments of the present invention will be described with reference to the drawings. It is to be noted that each of the embodiments described below shows a preferred illustrative embodiment of the present invention. Therefore, the numerical values, shapes, materials, structural elements, the arrangement and connection of the structural elements, and so on are mere examples, and therefore do not limit the scope of the present invention. The present invention is limited only by the scope of Claims. Thus, among the structural elements in the following embodiments, structural elements not recited in any one of the independent claims defining the most generic part of the present invention are not necessarily required to achieve the object of the present invention, but will be described as structural elements for preferable embodiments.

The following describes a circuit configuration of a light-emitting diode driver circuit according to Embodiment 1 of the present invention with reference toFIG. 1.FIG. 1is a diagram showing the circuit configuration of the light-emitting diode driver circuit according to Embodiment 1 of the present invention.

As shown inFIG. 1, a light-emitting diode driver circuit1according to Embodiment 1 of the present invention is an LED driver circuit for turning ON an LED2(LED lighting circuit), and includes a first rectifier circuit10, an inverter20, an inverter control circuit30, and a second rectifier circuit40.

The light-emitting diode driver circuit1has input terminals P1and P2for receiving an input of an AC voltage. The input terminals P1and P2are connected to an AC power source and to input terminals of the first rectifier circuit10. For example, the input terminals P1and P2of the light-emitting diode driver circuit1are connected to a commercial AC power source through a wall switch. It is to be noted that the commercial AC power source is an AC power source of 100V, that is, a household AC power source. Moreover, each of the input terminals P1and P2is, for example, a cap of an LED bulb lamp to be attached to a socket to which AC power is supplied.

Moreover, the light-emitting diode driver circuit1has output terminals P3and P4for outputting a DC voltage. The output terminals P3and P4are connected to the LED2and to output terminals of the second rectifier circuit40. The high electrical potential side of the output terminal P3is connected to the anode side of the LED2, and the low electrical potential side of the output terminal P4is connected to the cathode side of the LED2. The LED2is turned ON by the DC voltage supplied from the light-emitting diode driver circuit1. It is to be noted that, in this embodiment, a zener diode ZD is connected in parallel to the LED2for electrostatic protection of the LED2.

The following describes details of structural elements of the light-emitting diode driver circuit1according to this embodiment.

First, the first rectifier circuit10is described. The first rectifier circuit10(DB1) is a bridge type full-wave rectifier circuit including four diodes. In the first rectifier circuit10, two input terminals are connected to the AC power source via the input terminals P1and P2, and two output terminals are connected to a smoothing capacitor C1and others. It is to be noted that the smoothing capacitor C1is provided for the purpose of stabilizing an output voltage of the first rectifier circuit10, and is an electrolytic capacitor, for example.

The first rectifier circuit10receives an AC voltage from the commercial AC power source through the wall switch, for example, and full-wave rectifies the received AC voltage to output a DC voltage. The DC voltage outputted from the first rectifier circuit10is smoothed by the smoothing capacitor C1to be an input voltage Vin which is a DC voltage. The input voltage Vin is supplied to the inverter20and the inverter control circuit30.

Next, the inverter20is described. The inverter20(INV) is an example of an electric power output unit which outputs electric power for driving the LED2. In this embodiment, an inverter for converting a DC voltage into an AC voltage is employed. The inverter20includes a first switching device Q1, a second switching device Q2connected to the first switching device Q1in series, a driving transformer CT, an inductor L1, and a resonance capacitor C5.

In this embodiment, the inverter20is a half-bridge type self-excited inverter including a series circuit connected to a DC power source. The series circuit includes the first switching device Q1and the second switching device Q2connected in series to each other which alternately perform switching operations. Moreover, in this embodiment, the first switching device Q1and the second switching device Q2are bipolar transistors. It is to be noted that, in this embodiment, the self-excited inverter is an inverter which receives a feedback using a driving transformer and plural switching devices.

The collector of the first switching device Q1is connected to the positive electrode of the DC voltage output terminals in the first rectifier circuit10and to the resonance capacitor C5. The emitter of the first switching device Q1is connected to the collector of the second switching device Q2and a coil of the driving transformer CT. Moreover, the base of the first switching device Q1is connected to the coil of the driving transformer CT via a resistor R7.

The collector of the second switching device Q2is connected to the emitter of the first switching device Q1and the coil of the driving transformer CT. The emitter of the second switching device Q2is connected to the negative electrode of the DC voltage output terminals in the first rectifier circuit10and to the coil of the driving transformer CT. Moreover, the base of the second switching device Q2is connected to the coil of the driving transformer CT.

The driving transformer CT includes winding coils which are a primary winding (input winding) and a secondary winding (output winding).

It is to be noted that the inductor L1is a choke inductor, an end of which is connected to the output side of the driving transformer CT and the other end of which is connected to the input side of the second rectifier circuit40. Moreover, an end of the resonance capacitor C5is connected to the positive electrode of the DC voltage output terminals in the first rectifier circuit10, and the other end of the resonance capacitor C5is connected to the input side of the second rectifier circuit40.

The inverter20configured as above operates with a predetermined input voltage Vin applied to the both terminals of the series circuit including the first switching device Q1and the second switching device Q2(to the input terminals of the inverter20), and a start control signal (trigger signal) supplied from the inverter control circuit30. In the inverter20, alternate ON/OFF operations performed by the first switching device Q1and the second switching device Q2which are caused by a self-oscillation induced by the driving transformer CT causes series resonance between the inductor L1and the resonance capacitor C5to induce an AC secondary voltage, which is supplied to the second rectifier circuit40.

Next, the following describes the inverter control circuit30for controlling the inverter20which is the electric power output unit. The inverter control circuit30controls operations of the inverter20. More specifically, the inverter control circuit30stops an operation of the inverter20when the DC input voltage Vin provided to the inverter control circuit30has a value smaller than a predetermined value (Vth), for example, when the DC input voltage is lower than a reference input voltage (Vd) and is a very low voltage (Vmin) within a range in which the LED2can be turned ON. In this regard, the reference input voltage (Vd) is a voltage for turning ON/OFF the LED2.

In this embodiment, the inverter control circuit30includes a start trigger circuit31(TRG) for starting an operation of the inverter20and maintaining the operation, and a stop circuit32(STP) for stopping the operation of the start trigger circuit31. When the DC input voltage Vin has a value smaller than the above predetermined value, the stop circuit32operates to stop the operation of the start trigger circuit31. As a result, the inverter20stops operating and the power supply to the LED2completely stops. Here, the following describes details of specific configurations of the start trigger circuit31and the stop circuit32in the inverter control circuit30.

The start trigger circuit31includes a resistor R1which is a first resistor, a capacitor C3connected to the resistor R1in series, and a trigger diode TD connected to a connection point between the resistor R1and the capacitor C3.

The resistor R1is connected to the positive electrode of the DC voltage output terminals in the first rectifier circuit10via the resistor R2, and to the negative electrode of the DC voltage output terminals in the first rectifier circuit10via the capacitor C3. The capacitor C3is a capacitor for controlling conduction of the trigger diode TD, and the high electrical potential side of the capacitor C3is connected to the resistor R1and the low electrical potential side of the capacitor C3is connected to the negative electrode of the DC voltage output terminals in the first rectifier circuit10. It is to be noted that, in the start trigger circuit31, the resistor R1and the capacitor C3constitute a time constant circuit.

Moreover, the trigger diode TD is a trigger device including a diode and enters a conducting state when a voltage exceeding a specified voltage (breakover voltage) is applied. In this embodiment, the trigger diode TD enters the conducting state when a voltage held in the capacitor C3exceeds the breakover voltage. Moreover, the trigger diode TD is connected to the base of the second switching device Q2which is a terminal for controlling the inverter20, and the inverter20starts operating as a result of the trigger diode TD entering the conducting state.

That is, the second switching device Q2is turned ON by the start trigger circuit31and thus a current starts flowing in the inverter circuit20. The current flows from the inductor L1to the resonance capacitor C5, the second rectifier circuit40, the driving transformer CT, the LED2, and the second switching device Q2in this order. As a result, resonance among the inductor L1, the resonance capacitor C5, and the driving transformer CT inverts voltages generated in the bases of the first switching device Q1and the second switching device Q2at a resonance frequency, so that the first switching device Q1and the second switching device Q2start a regular operation of switching alternately ON/OFF.

It is to be noted that, for example, a diode AC switch (DIAC) having a voltage breakover of from 30 to 34V can be used as the trigger diode TD.

As described above, the start trigger circuit31is a circuit for starting the inverter20and includes the time constant circuit having the resistor R1and the capacitor C3, and the trigger diode TD which breaks over according to the voltage of the capacitor C3. Then, an input of a trigger signal from the start trigger circuit31to the inverter20starts the self-oscillation of the inverter20.

Moreover, in this embodiment, the start trigger circuit31includes the resistor R2connected to the resistor R1in series and the diode D1connected to the resistor R1in parallel. The diode D1is a diode for rectification and the anode side of the diode D1is connected to a connection point between the resistor R1and the capacitor C3and to the trigger diode TD. Moreover, the cathode side of the diode D1is connected to the connection point between the resistor R1and the resistor R2, to the connection point between the first switching device Q1(emitter) and the second switching device Q2(collector), and to a capacitor C4. It is to be noted that the high potential side of the capacitor C4is connected to the positive electrode of the DC voltage output terminals in the first rectifier circuit10and the collector of the first switching device Q1, and the low electrical potential side of the capacitor C4is connected to the cathode of the diode D1. The capacitor C4is a capacitor for soft switching, and is optionally used for suppressing generation of noise caused by switching.

The stop circuit32is a circuit for completely turning OFF the LED2by stopping the operation of the inverter circuit20, for example, when the LED2is turned OFF through the semiconductor switch such as the firefly night-light switch. The stop circuit32according to this embodiment includes the resistor R1as a second resistor, a third switching device Q3connected to the resistor R1, and the fourth switching device Q4connected to the third switching device Q3. The resistor R1is shared by the start trigger circuit31and the stop circuit32. Moreover, the third switching device Q3and the fourth switching device Q4constitute a switching circuit SW, and switching ON the switching circuit SW stops the operation of the start trigger circuit. In this embodiment, the stop circuit32further includes the resistor R2, the resistor R3, the resistor R4, the resistor R5, and the resistor R6.

In the switching circuit SW, the third switching device Q3and the fourth switching device Q4are bipolar transistors.

The third switching device Q3is connected in parallel to the capacitor C3in the start trigger circuit31, and the collector of the third switching device Q3is connected to the high electrical potential side of the capacitor C3and to the positive electrode of the DC voltage output terminals in the first rectifier circuit10via the resistor R1and the resistor R2. The emitter of the third switching device Q3is connected to the low electrical potential side of the capacitor C3and to the negative electrode of the DC voltage output terminals in the first rectifier circuit10. It is to be noted that the base of the third switching device Q3is connected to the collector of the fourth switching device Q4.

The collector of the fourth switching device Q4is connected to the resistor R6and the base of the third switching device Q3. The emitter of the fourth switching device Q4is connected to the resistor R5and to the negative electrode of the DC voltage output terminals in the first rectifier circuit10. Moreover, the base of the fourth switching device Q4is connected to the connection point between the resistor R4and the resistor R5.

An end of the resistor R3is connected to the positive electrode of the DC voltage output terminals in the first rectifier circuit10, and the other end is connected to the connection point between the resistor R4and the resistor R6. An end of the resistor R4is connected to the connection point between the resistor R3and the resistor R6, and the other end is connected to the connection point between the resistor R5and the base of the fourth switching device Q4. An end of the resistor R5is connected to the connection point between the resistor R4and the base of the fourth switching device Q4, and the other end is connected to the negative electrode of the DC voltage output terminals in the first rectifier circuit10. An end of the resistor R6is connected to the connection point between the resistor R3and the resistor R4, and the other end is connected to the connection point between the base of the third switching device Q3and the collector of the fourth switching device Q4. It is to be noted that details of the resistor R1and the resistor R2are as described in the description of the start trigger circuit31.

In the stop circuit32configured as above, resistance values of the resistors R1to R6are set as follows: the forth switching device Q4is turned OFF and the third switching device Q3is turned ON when the input voltage Vin has a value smaller than the above predetermined value (Vth), and the forth switching device Q4is turned ON and the third switching device Q3is turned OFF when the input voltage Vin has a value greater than or equal to the above predetermined value (Vth). That is, the predetermined value (Vth) can be set appropriately as a threshold voltage of the switching circuit SW, for example, to a voltage value which is smaller than the value of the reference input voltage (Vd) and is the value of the very small voltage (Vmin) within a range in which the LED2can be turned ON. In this regard, the reference input voltage (Vd) is a voltage for turning ON/OFF the LED2.

As described above, when the input voltage Vin has a value smaller than the above predetermined value (Vth), the switching circuit SW in the stop circuit32enters an ON state. That is, in this case, the ON current does not flow to the base of the fourth switching device Q4and thus the fourth switching device Q4is turned OFF, and a predetermined ON current flows to the base of the third switching device Q3and thus the third switching device Q3is turned ON. As a result, electric charges in the capacitor C3are discharged, so that the trigger diode TD enters a non-conducting state. Accordingly, the stop circuit32stops the operation of the start trigger circuit31when the input voltage Vin has a value smaller than the above predetermined value (Vth).

On the other hand, when the input voltage Vin has a value greater than or equal to the above predetermined value (Vth), the switching circuit SW in the stop circuit32enters an OFF state. That is, in this case, an ON current determined by a voltage dividing ratio among the resistors R3to R5flows in the base of the fourth switching device Q4to turn ON the fourth switching device Q4, and electrical potential of the base and the emitter of the third switching device Q3reaches GND electrical potential to turn OFF the third switching device Q3. As a result, electric charges are charged in the capacitor C3. Accordingly, the capacitor C3is charged, the voltage of the trigger diode TD reaches a conducting voltage, and then the inverter circuit20starts oscillating as described above. Accordingly, the stop circuit32does not stop the start trigger circuit31when the input voltage Vin has a value greater than or equal to the above predetermined value (Vth).

Next, the second rectifier circuit40is described. As the first rectifier circuit10, the second rectifier circuit40(DB2) is a bridge type full-wave rectifier circuit including four diodes. Two input terminals of the second rectifier circuit40are connected to two output terminals of the inverter20. Among the two output terminals of the second rectifier circuit40, an output terminal having higher electrical potential is connected to the anode side of the LED2via the output terminal P3and an output terminal having lower electrical potential is connected to the cathode side of the LED2via the output terminal P4.

The second rectifier circuit40receives an AC voltage from the inverter20, full-wave rectifies the AC voltage, and supplies the full-wave rectified AC voltage to the LED2.

It is to be noted that the second rectifier circuit40can be, for example, a combination of two semiconductor components each of which includes two schottky diodes connected in series to each other.

The light-emitting diode driver circuit1according to this embodiment is configured as described above. It is to be noted that, as examples of the above circuit devices in the light-emitting diode driver circuit1according to this embodiment, chip resistors having the following resistance are used as the resistors R1to R8: R1=120 kΩ, R2=150 kΩ, R3=390 kΩ, R4=82 kΩ, R5=9.3 kΩ, R6=100 kΩ, R7=R8=12Ω, and capacitors having the following capacitance are used as the capacitors C1and C3to C5: C1=2.2 μF, C3=6800 pF, C4=1500 pF, C5=0.047 μF. Moreover, one LED2is provided in this embodiment, but plural LEDs2may be provided. In this case, the plural LEDs2may be connected in series or in parallel, or the series connection and the parallel connection may be combined.

Next, operations of the light-emitting diode driver circuit1according to this embodiment are described.

For example, when a user operates the wall switch for turning the LED2ON, AC power is supplied to the input terminals P1and P2, and thus the first rectifier circuit10generates a smoothed DC input voltage Vin. The input voltage Vin is supplied to the input terminals of the inverter20, to the input terminals of the start trigger circuit31, and to the input terminals of the stop circuit32. At this time, the reference input voltage (Vd), which is a voltage for turning ON the LED2, is supplied as the input voltage Vin.

Accordingly, the start trigger circuit31and the inverter20operate. That is, by supplying the reference input voltage (Vd) to the start trigger circuit31as the input voltage Vin, the capacitor C3in the start trigger circuit31is charged and causes the trigger diode TD to break over. As a result, the trigger diode TD enters a conducting state, a trigger signal (trigger pulse) is supplied to the base of the second switching device Q2in the inverter20, and thus the second switching device Q2is turned ON.

After the second switching device Q2is turned ON in response to the trigger signal, the inverter is started, and the self-oscillation induced by the driving transformer CT causes the first switching device Q1and the second switching device Q2to alternately perform ON/OFF operations, which induces the AC secondary voltage. Accordingly, the secondary voltage is increased by the series resonance between the inductor L1and the resonance capacitor C5, and the resulting AC voltage is supplied to the second rectifier circuit40. Subsequently, the AC voltage is full-wave rectified by the second rectifier circuit40, and a predetermined DC voltage (forward voltage VF) is supplied to the LED2via the output terminals P3and P4. With this, the LED2emits light with desired brightness.

It is to be noted that, at this time, supply of the reference input voltage (Vd) causes the switching circuit SW in the stop circuit32to enter an OFF state. That is, the fourth switching device Q4is turned ON, the third switching device Q3is turned OFF, and the stop circuit32does not act on the start trigger circuit31.

Next, the user's operation on the wall switch for turning OFF the LED2stops the supply of the AC power to the input terminals P1and P2, thereby basically turning OFF the LED2.

However, in the conventional light-emitting diode driver apparatus, other IC circuits (not shown) or electronic switches such as a remote control switch and a night-light switch (not shown) may supply, as an input voltage Vin, a voltage which is lower than the reference input voltage (Vd) and is the very low voltage (Vmin) within a range in which the LED can be turned ON.

Conventionally, there has been a problem that supply of the very low voltage (Vmin) in the circuit turns ON the LED2. However, in this embodiment, supply of the very low voltage (Vmin) does not turns ON the LED2. The details are described below.

In this embodiment, even when the very low voltage (Vmin) is supplied as the input voltage Vin, the stop circuit32is started in which an operation start voltage (Vth) is set to a voltage higher than the very low voltage (Vmin), to stop the operation of the inverter20.

More specifically, supply of the very low voltage (Vmin) as the input voltage Vin turns OFF the third switching device Q3and turns ON the fourth switching device Q4in the stop circuit32. With this, the electric charges stored in the capacitor C3in the start trigger circuit31are discharged, and thus the trigger diode TD enters a non-conducting state. As a result, the start trigger circuit31stops supplying the trigger pulse to the base of the second switching device Q2in the inverter20, so that the inverter20stops. Accordingly, power supply to the second rectifier circuit40is completely stopped, and thus the LED2is turned OFF without fail.

The following describes the result of an experiment that was performed with regard to the stop circuit32in the light-emitting diode driver circuit1according to this embodiment. Tables 1 and 2 below show, with regard to the LED lamp having the light-emitting diode driver circuit1according to this embodiment, the relationship among power consumption (standby electricity) of the stop circuit32, an ON/OFF operation state of the LED lamp, and lighting state of an LED indicator of an experimental switch. It is to be noted that an LED bulb lamp was used as the LED lamp for the experiment.

Moreover, Table 1 shows the case where a remote control switch (“Tottara Rimocon” made by Panasonic corp.: WTC53215K) is used as the experimental switch, and Table 2 shows the case where a night-light switch (“Hotaru Switch” made by Panasonic corp.: WN5052) is used as the experimental switch. Criteria for judgment in the tables is as follows: regarding the ON/OFF operation state of the lamp, “O” is shown when the ON/OFF operation works and “X” is shown when the ON/OFF operation does not work; regarding the lighting state of the LED indicator, “O” is shown when the LED indicator correctly emits light and “X” is shown when the LED indicator does not emit light. It is to be noted that, regarding the lighting state of the LED indicator, “Δ” is shown when the LED indicator emits light slightly weaker although having no problem as a product.

As is seen from Tables 1 and 2, it was found that the light-emitting diode driver circuit1and the electronic switches correctly operate when the power consumption (standby electricity) in the stop circuit32is more than or equal to 0.08 W. It is to be noted that since the stop circuit32consumes electricity also when the LED lamp is ON, too much standby electricity decreases an efficiency of the LED lamp. Therefore, considering rating of the night-light switch at 20% and variations, as a practical range of use, the standby electricity of the stop circuit32is preferably up to 0.11 W.

As described above, with the light-emitting diode driver circuit1according to Embodiment 1 of the present invention, ON/OFF of the LED2can be controlled by a start and a stop of the inverter20, and the inverter control circuit30completely stops the operation of the inverter20when the input voltage Vin has a value smaller than the predetermined value. With this, it is possible to completely interrupt a path of current flowing to the LED2. Therefore, when the LED2is desired to be turned OFF, it is possible to turn the LED2completely OFF.

Moreover, the light-emitting diode driver circuit1according to this embodiment has a configuration in which a current is bypassed from the resistors through the transistors (the third transistor Q3and the fourth transistor Q4) in the stop circuit32. Therefore, even when electronic switches such as a remote control switch and a night-light switch are equipped, the electronic switches do not erroneously work.

Moreover, according to this embodiment, since electric power is supplied to the LED2using the inverter20, the AC voltage from the commercial low-frequency AC power source is converted to have a higher frequency and thus the flicker in the LED2at the ON state can be suppressed. It is to be noted that a flicker in the LED2can be prevented by high frequencies of more than or equal to 15 kHz. Furthermore, it is also possible to binarize the brightness by supplying the electric power to the LED2using the inverter20.

It is to be noted that, in this embodiment, electric power consumed by the resistors and the switching devices included in the stop circuit32is, as described above, preferably more than or equal to 0.08 W and less than or equal to 0.11 W in the case where the above very low voltage (Vmin) is supplied to the stop circuit32.

Moreover, in this embodiment, the driving transformer CT is not limited to the driving transformer CT disclosed inFIG. 1. For example, although a circuit including the resistor R8, the base of the second switching device Q2, the emitter of the second switching device Q2, and coils of the driving transformer connected to each other is provided in this embodiment, this circuit configuration is not necessarily needed.

Moreover, although the half-bridge type self-excited inverter is used as the electricity output unit in this embodiment, the electricity output unit according to the present invention is not limited to this. For example, as the electricity output unit, a separately-excited inverter may be used, and a full-bridge type inverter may be used instead of the half-bridge type inverter.

The following describes a light-emitting diode driver circuit1A according to Embodiment 2 of the present invention with reference toFIG. 2.FIG. 2is a diagram showing the circuit configuration of the light-emitting diode driver circuit according to Embodiment 2 of the present invention.

A basic configuration of the light-emitting diode driver circuit1A according to Embodiment 2 of the present invention is the same as the light-emitting diode driver circuit1according to Embodiment 1 of the present invention. InFIG. 2, the same reference numerals are assigned to structural elements which are the same as the structural elements shown inFIG. 1, and the detailed descriptions are omitted.

The light-emitting diode driver circuit1A according to Embodiment 2 of the present invention shown inFIG. 2is different in the configuration of the inverter from the light-emitting diode driver circuit1according to Embodiment 1 of the present invention shown inFIG. 1.

That is, in the light-emitting diode driver circuit1A according to this embodiment, an inverter20A includes a fifth switching device Q5the number of which is one, one diode D2, and an inductor L2.

The fifth switching device Q5is a bipolar transistor. The emitter of the fifth switching device Q5is connected to an end of the inductor L2and the anode side of the diode D2. Moreover, the emitter of the fifth switching device Q5is connected to the cathode side of the diode D1in the start trigger circuit31. Furthermore, the collector of the fifth switching device Q5is connected to the negative electrode of the DC voltage output terminals in the first rectifier circuit10. Furthermore, the base of the fifth switching device Q5is connected to the trigger diode TD in the start trigger circuit31.

The cathode side of the diode D2is connected to the positive electrode of the DC voltage output terminals in the first rectifier circuit10. Moreover, the anode side of the diode D2is connected to a connection point between the inductor L2and the fifth switching device Q5.

It is to be noted that an end of the inductor L2is connected to a connection point between the fifth switching device Q5and the diode D2, and the other end of the inductor L2is connected to the input side of the second rectifier circuit40.

When an input voltage Vin provided from the first rectifier circuit10is applied to the inverter20A configured as above, the inverter20A operates according to a start control signal from the inverter control circuit30. As a result, a DC voltage is supplied from the inverter20A to the second rectifier circuit40.

It is to be noted that, in this embodiment, other configurations are the same as those in Embodiment 1.

Next, operations of the light-emitting diode driver circuit1A according to this embodiment is described.

For example, when a user operates the wall switch for turning the LED2ON, as in Embodiment 1, a reference input voltage (Vd), which is a voltage for turning ON the LED, is supplied as the input voltage Vin, and thus the start trigger circuit31and the inverter20A operate.

That is, the capacitor C3in the start trigger circuit31are charged, the trigger diode TD enters a conducting state, a trigger signal (trigger pulse) is supplied to the base of the fifth switching device Q5in the inverter20A, and then the fifth switching device Q5is turned ON.

When the fifth switching device Q5is turned ON in response to the trigger signal, the input voltage Vin causes a current to flow from the second rectifier circuit40to the LED2, the second rectifier circuit40, the inductor L2, and the fifth switching device Q5in this order, to turn ON the LED2.

Subsequently, the electric charges in the capacitor C3are discharged through the diode D1and the base of the switching device Q5, the switching device Q5and the trigger diode TD are turned OFF, and then energy stored in the inductor L2returns to the smoothing capacitor C1as a feedback through the diode D2and immediately starts to be charged in the capacitor C3. When the above feedback current finishes flowing, the trigger diode TD is turned ON again and the switching device Q5is turned ON. Moreover, for the purpose of changing an ON-time of the switching device Q5, a resistor may be added between the capacitor C3and the diode D1.

It is to be noted that, in the stop circuit32, supply of the reference input voltage (Vd) causes the switching circuit SW to enter an OFF state.

Next, when the user operates the wall switch for turning OFF the LED2, the LED2is turned OFF.

In this case, even when a very low voltage (Vmin) is supplied as the input voltage Vin, as in Embodiment 1, the stop circuit32is started in which an operation start voltage (Vth) is set to a voltage higher or equal to the very low voltage (Vmin), to stop the operation of the inverter20A. As a result, power supply to the second rectifier circuit40completely stops, and thus the LED2is turned OFF without fail.

As described above, with the light-emitting diode driver circuit1A according to Embodiment 2 of the present invention, it is also possible to control ON/OFF of the LED2by a start and a stop of the inverter20A, and the inverter control circuit30completely stops an operation of the inverter20A when the input voltage Vin has a value smaller than a predetermined value. With this, it is possible to completely interrupt a path of current flowing to the LED2. Therefore, when the LED2is desired to be turned OFF, it is possible to turn the LED2completely OFF.

Moreover, as in Embodiment 1, the light-emitting diode driver circuit1A according to this embodiment has a configuration in which a current is bypassed from the resistors through the transistors (the third transistor Q3and the fourth transistor Q4) in the stop circuit32. Therefore, even when electronic switches such as a remote control switch and a night-light switch are equipped, the electronic switches do not erroneously work.

The following describes, as Embodiment 3, an exemplary application of the light-emitting diode driver circuits according to Embodiments 1 and 2.FIG. 3is a partial sectional view of a lighting apparatus according to Embodiment 3 of the present invention.

The light-emitting diode driver circuits1and1A according to Embodiments 1 and 2 can be used as an LED light source together with the LED2that is turned ON by the light-emitting diode driver circuits1and1A. It is to be noted that the LED light source in the present invention refers to an apparatus having an LED that is turned ON by any light-emitting diode driver circuit. Besides an apparatus in which an LED and a light-emitting diode driver circuit are simply combined, examples of the LED light source include various lighting apparatuses such as a lighting apparatus which is substituted for a conventional fluorescent bulb and a lighting apparatus which is substituted for a halogen bulb described below.

A lighting apparatus50according to Embodiment 3 of the present invention is an example of the above LED light source and a halogen bulb-substitution lighting apparatus having a light-emitting diode driver circuit1, a case51, a heat sink52, and a light-emitting unit53as shown inFIG. 3.

The light-emitting diode driver circuit1has the same configuration as that of the light-emitting diode driver circuit1according to Embodiment 1.

The case51is made of an insulating material such as ceramics and includes a cylinder part51aand a protruding part51bextending from an end of the cylinder part51a. Inside the cylinder part51a, the light-emitting diode driver circuit1is contained as a lighting circuit for turning ON the LED2. A metal shell54is provided on the outer periphery of the protruding part51band a metal eyelet55is provided at a sharp end of the protruding part51b. Each of the shell54and the eyelet55is connected to the light-emitting diode driver circuit1via a lead wire, for example, and is a feed terminal for receiving electric power supply from an external power source (commercial AC power source). It is to be noted that the shell54and the eyelet55constitutes a cap, and the power is supplied when the cap is inserted into a socket of a lighting apparatus (not shown).

The heat sink52is made of metal material such as aluminum, and is cup-shaped with a bottom part52aand a side face part52bextending from the bottom part52a. It is to be noted that the heat sink52may be made of, other than the metal material, materials such as a non-translucent ceramic material and a non-translucent resin material. On the bottom part52ain the heat sink52, the light-emitting unit53is adhered using an adhesive and the like.

Moreover, to an opening of the heat sink52, a front cover56is attached which is fixed to the heat sink52with a metal fitting57. The front cover56is made of translucent materials such as a resin, glass, and ceramics to transmit light from the light-emitting unit53to the exterior. It is to be noted that the front cover56is preferably made of a transparent material among the above translucent materials. The inner periphery of the side face part52bof the heat sink52is a reflective surface for reflecting light, so that the heat sink52is also used as a reflecting mirror. It is to be noted that the heat sink52has a size smaller than or approximately equal to a conventional halogen bulb having a reflecting mirror. For example, in the case where a halogen bulb having a reflection mirror with an opening diameter of approximately from 50 mm to 70 mm is substituted, the heat sink52may be formed to have an opening diameter of approximately from 50 mm to 70 mm, or smaller.

The light-emitting unit53includes the LED2, a substrate53a, a wavelength converting member53b, and a lens53c. The LED2is a bare chip, and one or more LEDs2are mounted on the substrate53a. As the LED2, for example, a blue LED chip which emits blue light can be used. The substrate53ais a substrate on which the LED is mounted, and examples of the substrate53ainclude a ceramic substrate and a metal substrate coated with insulating resin. As a wavelength converting member53b, a phosphor-containing resin that is a resin containing phosphor, which is a light wavelength converting material, can be used. For example, in the case where the LED2is the blue LED chip, a phosphor-containing resin which is a silicon resin containing yellow phosphor particles of yttrium, aluminum, garnet (YAG) series dispersed therein can be used for obtaining white light. Accordingly, the yellow phosphor particles are excited by the blue light from the blue LED chip and thus emit yellow light. As a result, white light is emitted from the wavelength converting member53bdue to the excited yellow light and the blue light from the blue LED chip. The lens53cis made of a translucent material such as a resin, and formed to include the wavelength converting member53btherein. The LED2in the light-emitting unit53is electrically connected to wiring that is formed on the substrate53athrough patterning. When the wiring is electrically connected to the light-emitting diode driver circuit1, electric power is supplied to the LED2from the light-emitting diode driver circuit1. It is to be noted that the light-emitting unit53is positioned in such a manner that the optical axis of the light-emitting unit53and the central axis of the cup-shaped heat sink52align.

The lighting apparatus3configured as above is attached to a socket of a lighting apparatus and the like and used. When turned ON, the LED2receives the electric power supply from the light-emitting diode driver circuit1to emit light. As a result, the light from the light-emitting unit53is emitted, as spot light, from the opening of the heat sink52through the front cover56. Moreover, when the LED2is turned OFF, the electric power supply from the light-emitting diode driver circuit1completely stops and thus the LED2is completely turned OFF.

Although the light-emitting diode driver circuit and the LED light source according to the embodiments of the present invention are described as above, the present invention is not limited to these embodiments. For example, various modifications to the embodiments that are conceived by the person skilled in the art and other embodiments obtainable by combining the structural elements in the embodiments without materially departing from the scope of the present invention are included within the scope of the present invention.

INDUSTRIAL APPLICABILITY

The light-emitting diode driver circuit according to the present invention can be used in an apparatus, machine, or the like which utilizes LEDs. The LED light source according to the present invention that is the lighting apparatus using the LED is useful, specifically as an LED lighting apparatus including a small number of LEDs, for example, an LED lighting apparatus substituted for a halogen bulb.

REFERENCE SIGNS LIST