JP2011049197A - Driving circuit of light-emitting diode, and light-emitting device and lighting device using the same, - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To drive an LED by stable luminance relative to the fluctuation of a power supply voltage. <P>SOLUTION: The driving circuit 100 drives an LED array 2 including a plurality of LEDs 4 connected in series. A power source 30 supplies a driving voltage Vdrv to the LED array 2. A first transistor 12 and a first resistor R1 are provided in series between the second terminal P2 of the LED array 2 and a reference voltage terminal P3 on the driving route of the LED array 2. A contact current source 16 is provided between the second terminal P2 of the LED array 2 and the control terminal of the first transistor 12. A second transistor 14 is provided between the control terminal of the first transistor 12 and the reference voltage terminal, and a voltage corresponding to the voltage drop VR1 of the first resistor R1 is applied to the control terminal. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to a driving technique for a light emitting diode.

  In recent years, instead of fluorescent lamps, a technique has been proposed in which a light emitting diode (hereinafter abbreviated as LED), which is expected to have high efficiency and long life, is used as a lighting fixture. For example, Patent Document 1 discloses an LED lighting device that rectifies a commercial AC voltage, smoothes it with a smoothing circuit including a smoothing capacitor, and supplies the smoothed circuit to an array of LEDs that are loads.

JP 2004-273267 A

  The commercial AC voltage is supplied at a predetermined rated voltage of about 100 V or 200 V, but it is known that it actually fluctuates. In order to drive the LED array with stable brightness, it is necessary to provide a constant current circuit on the drive path of the LED array and stabilize the brightness with a constant drive current. When driving the LED array by smoothing the commercial AC voltage, if the commercial AC voltage falls below the operating voltage of the constant current circuit, it becomes impossible to generate a drive current necessary to obtain a desired luminance.

  In order to solve this problem, it is conceivable that the voltage (voltage drop) between both ends of the constant current circuit at the rated time is designed to be high by adding a margin on the assumption that the commercial AC voltage is lowered. However, in this case, another problem that power consumption in the constant current circuit becomes large occurs. From this point of view, a constant current circuit (drive circuit) that can operate at a low voltage is desired.

  Further, when a constant current diode connected in series to the LED array is used as the constant current circuit, there arises a problem that the drive current that can be supplied to the LED array is limited to the rated current of the constant current diode.

  The present invention has been made in view of such circumstances, and one of its purposes is to provide a technique for driving an LED with stable luminance.

  One embodiment of the present invention relates to a drive circuit for driving a light-emitting diode array including a plurality of light-emitting diodes connected in series. The driving circuit includes a power source for supplying a driving voltage to the light emitting diode array, a first transistor and a first transistor provided in series between one end of the light emitting diode array and a reference voltage terminal on a driving path of the light emitting diode array. A resistor, a constant current source provided between the other end of the light-emitting diode array and the control terminal of the first transistor, and a resistor provided between the control terminal of the first transistor and the reference voltage terminal. And a second transistor to which a voltage corresponding to the voltage drop is applied.

  According to this aspect, a stable drive current that does not depend on the power supply voltage can be supplied to the light emitting diode array. The main power loss in the drive circuit is generated by the first transistor and the second resistor, and the power is given by the product of the drive current and the combined resistance of the first transistor and the second resistor. The voltage drop of one transistor and the second resistor can be reduced, and the power consumption can be reduced.

The driving circuit according to an aspect may further include a second resistor provided in series with the constant current source between the other end of the light emitting diode array and the control terminal of the first transistor.
According to this aspect, the constant current source can be protected.

  The driving circuit according to an aspect may further include a third resistor provided between a connection point between the first transistor and the first resistor and a control terminal of the first transistor. According to this aspect, the gate-source voltage (base-emitter voltage) of the first transistor can be clamped, and the drive circuit can be suitably protected.

  The first transistor may be a field effect transistor and the second transistor may be a bipolar transistor.

  Another aspect of the present invention also relates to a drive circuit for a light emitting diode array. The driving circuit includes a power source that supplies a driving voltage to the light emitting diode array, a first transistor and a first transistor that are provided in series between one end of the light emitting diode array and a reference voltage terminal on the driving path of the light emitting diode array. 1 resistor, a second resistor provided instead of the constant current source, between the other end of the light emitting diode array and the control terminal of the first transistor, and between the control terminal of the first transistor and the reference voltage terminal. And a second transistor having a voltage corresponding to the voltage drop of the first resistor applied to the control terminal thereof.

  The drive circuit may further include a constant current source provided in series with the second resistor between the other end of the light emitting diode array and the control terminal of the first transistor.

  Yet another embodiment of the present invention is a light emitting device. This device includes a light-emitting diode array including a plurality of light-emitting diodes connected in series, and the drive circuit according to any one of the above-described modes for driving the light-emitting diode array.

  Yet another embodiment of the present invention is a lighting device. This device includes the light-emitting device described above.

  It should be noted that any combination of the above-described constituent elements and the expression of the present invention converted between methods, apparatuses, etc. are also effective as an aspect of the present invention.

  According to the present invention, it is possible to drive an LED with stable luminance against fluctuations in power supply voltage.

It is a circuit diagram which shows the structure of the light-emitting device which concerns on embodiment. It is a figure which shows the voltage-current characteristic of the drive circuit of FIG. It is a circuit diagram which shows the structure of the drive current generation part which concerns on a modification. It is a figure which shows the operating characteristic of the 1st transistor of FIG. It is a circuit diagram which shows the structure of the drive current generation part which concerns on another modification. It is a circuit diagram which shows the structure of the drive current generation part which concerns on another modification.

  The present invention will be described below based on preferred embodiments with reference to the drawings. The same or equivalent components, members, and processes shown in the drawings are denoted by the same reference numerals, and repeated descriptions are omitted as appropriate. The embodiments do not limit the invention but are exemplifications, and all features and combinations thereof described in the embodiments are not necessarily essential to the invention.

  In this specification, “the state in which the member A is connected to the member B” means that the member A and the member B are physically directly connected, or the member A and the member B are electrically connected. The case where it is indirectly connected through another member that does not affect the state is also included. Similarly, “the state in which the member C is provided between the member A and the member B” refers to the case where the member A and the member C or the member B and the member C are directly connected, as well as an electrical condition. It includes the case of being indirectly connected through another member that does not affect the connection state.

  FIG. 1 is a circuit diagram illustrating a configuration of a light emitting device 200 according to an embodiment. The light emitting device 200 includes an LED array 2 and a drive circuit 100. The light emitting device 200 can be used as, for example, indoor lighting, a vehicle headlight, an electric bulletin board, and the like, but its application is not particularly limited.

  The LED array 2 includes n (n is an integer of 2 or more) LED modules 4_1 to 4_n (hereinafter simply referred to as LEDs 4) connected in series. In FIG. 1, each LED 4 is shown as a single LED, but each may include a plurality of LEDs connected in series and / or in parallel. Each LED 4 has a forward voltage Vf corresponding to the number of LEDs connected in series therein. A high power LED 4 with a high luminance is commercially available with Vf = 10.4 V (350 mA), and a low power LED 4 with Vf = 3 V is commercially available. Below, the case where the LED 4 for high powers of about Vf = 10V is used is demonstrated.

  The drive circuit 100 drives the LED array 2. Specifically, the drive voltage Vdrv is supplied, and the drive current Idrv flowing through the LED array 2 is set and controlled according to the luminance.

  The drive circuit 100 includes a drive current generator 10 and a power supply 30 as basic components.

  The power supply 30 supplies the drive voltage Vdrv to the LED array 2. For example, the power supply 30 includes an AC power supply 34, a rectifier circuit 36, and a smoothing capacitor Cs. The AC power supply 34 generates an AC voltage Vac, and the rectifier circuit 36 rectifies the AC voltage Vac. For example, a diode bridge may be used as the rectifier circuit 36. In this case, the AC voltage Vac is full-wave rectified by the rectifier circuit 36. When a commercial AC voltage is used as the AC voltage Vac, the AC power supply 34 is provided outside the drive circuit 100.

  A smoothing capacitor Cs is connected to the output terminal of the rectifier circuit 36. The smoothing capacitor Cs smoothes the full-wave rectified AC voltage Vac to generate a DC drive voltage Vdrv. The drive voltage Vdrv obtained by rectifying the rated AC voltage Vac of 100V is about 141V.

  The number n of LEDs 4 included in the LED array 2 is designed according to the relationship between the forward voltage Vf and the drive voltage Vdrv. When Vdrv≈141V and Vf≈9.5V, n = 14.

  The drive current generator 10 generates a drive current Idrv to be supplied to the LED array 2. The drive current generator 10 includes a first transistor 12, a second transistor 14, a first resistor R1, and a constant current source 16.

  The first transistor 12 and the first resistor R1 are provided in series on the drive path of the LED array 2 and between one end (second terminal) P2 of the LED array 2 and a reference voltage terminal (ground terminal) P3. The first resistor R1 is preferably a variable resistance element. In this case, the drive current Idrv can be adjusted by adjusting the resistance value of the first resistor R1.

  Specifically, the first transistor 12 is an N-channel MOSFET (Metal Oxide Semiconductor Field Effect Transistor). The drain of the first transistor 12 is connected to the second terminal P2 of the LED array 2, and the source thereof is connected to the first resistor R1.

  The constant current source 16 is provided between the other end (first terminal P1) of the LED array 2 and the control terminal (gate) of the first transistor 12. The constant current source 16 generates a predetermined constant current Ic1. The constant current source 16 may be, for example, a CRD (CRD: Current Regulative Diode). In this case, the circuit can be simplified. Alternatively, a constant current source having another configuration may be used.

  The second transistor 14 is, for example, an NPN-type bipolar transistor, and is provided between the control terminal (gate) of the first transistor 12 and the reference voltage terminal P3. The voltage drop VR1 of the first resistor R1 at the control terminal (base). A voltage corresponding to is applied. That is, the voltage drop VR1 of the first resistor R1 is applied between the base and emitter of the second transistor.

  The above is the basic configuration of the light emitting device 200.

  FIG. 2 is a diagram illustrating voltage-current characteristics of the drive circuit 100 of FIG. FIG. 2 can be divided into three regions.

(I) First Region In the region where the drive voltage Vdrv is lower than the threshold voltage of the LED array, the LED array 2 is not turned on, so the drive current Idrv is substantially zero.

(II) Second Region When the drive voltage Vdrv exceeds the threshold voltage of the LED array, the drive current Idrv begins to flow through the LED array 2. A voltage drop VR1 = Idrv × R1 proportional to the drive current Idrv is generated in the first resistor R1. As the drive voltage Vdrv increases, the degree of turning on of the second transistor 14 increases. In the second region, the drive current Idrv increases according to the voltage-current characteristics of the LED array 2.

(III) Third Region When the drive current reaches a desired current value, the voltage drop VR1 of the first resistor R1 is stabilized around 0.7V. As a result, the bias voltage of the first transistor 12 becomes constant, and accordingly, the drive current Idrv is kept constant over a wide range.

The above is the operation of the drive circuit 100.
The drive current generator 10 can supply a stable drive current Idrv to the LED array 2 when the voltage drop VR1 of the first resistor R1 reaches 0.7V. That is, the voltage drop of the drive current generation unit 10 can be made smaller than before, and the power consumption can be reduced.

  Focusing on the drive current Idrv, the value is not affected by the constant current Ic generated by the constant current source 16, and the drive current Idrv in a wide range can be stably generated.

  Furthermore, since the bias state of the first transistor 12 is stabilized by the first resistor R1 and the second transistor 14, there is an advantage that thermal runaway hardly occurs.

  FIG. 3 is a circuit diagram showing a configuration of the drive current generator 10a according to the modification. The drive current generator 10a further includes a second resistor R2 and a third resistor R3 in addition to the drive current generator 10 of FIG.

  The second resistor R2 is provided in series with the constant current source 16 between the first terminal P1 of the LED array 2 and the control terminal (gate) of the first transistor 12. In FIG. 1, the voltage across the LED array 2 is almost applied to the constant current source 16. Therefore, in an application where the drive voltage Vdrv exceeds 100 V, the constant current source 16 is required to have a very high breakdown voltage. On the other hand, in the modified example of FIG. 3, since about half of the voltage across the LED array 2 is applied to the second resistor R2, the voltage applied to the constant current source 16 can be reduced. The withstand voltage of the constant current source 16 can be lowered. From another viewpoint, the constant current source 16 can be protected from overvoltage by providing the second resistor R2.

  The third resistor R3 is provided between the connection point of the first transistor 12 and the first resistor R1 (that is, the source of the first transistor 12) and the control terminal (gate) of the first transistor 12. The constant current Ic generated by the constant current source 16 branches to the second transistor 14 and the third resistor R3. The constant current Ic flows to the third resistor R3 when the second transistor 14 is turned off, and flows to the second transistor 14 when the second transistor 14 is turned on.

  FIG. 4 is a diagram illustrating operating characteristics of the first transistor 12 of FIG. The vertical axis represents the gate-source voltage Vgs of the first transistor 12, and the horizontal axis represents the drive voltage Vdrv. The solid line is the characteristic of the circuit of FIG. 3, and the broken line is the characteristic of the circuit of FIG. The lower figure is an enlarged view of the vertical axis of the upper figure.

  First, the operation of the circuit of FIG. 1 in which the third resistor R3 is not provided will be described with reference to a broken line. When the third resistor R3 is not provided, the gate-source voltage Vgs of the first transistor 12 increases as the drive voltage Vdrv increases. Therefore, it is necessary to select a transistor having a higher absolute maximum rating between the gate and the source than the gate-source voltage Vgs, and there is a problem that the degree of freedom in design is low. In applications where the drive voltage Vdrv exceeds 100V, the reliability of the first transistor 12 may be impaired.

  Next, the operation of the circuit of FIG. 3 provided with the third resistor R3 will be described with reference to a solid line. When the second transistor 14 is turned off, the constant current Ic generated by the constant current source 16 flows through the third resistor R3. That is, the gate-source voltage Vgs of the first transistor 12 is given by the product (R3 × Ic) of the resistance value R3 and the current value Ic. In FIG. 3, R3 × Ic≈7V, and the gate-source voltage Vgs can be suppressed.

  When the drive voltage Vdrv exceeds a certain threshold value, the LED array 2 is turned on, a current flows through the first transistor 12, the voltage drop VR1 of the first resistor R1 is stabilized to about 0.7V, and the second transistor 14 Turns on. When the second transistor 14 is turned on, the constant current Ic generated by the constant current source 16 flows to the second transistor 14 instead of the third resistor R3, and is substantially equivalent to the circuit of FIG.

  That is, by providing the third resistor R3, the gate-source voltage Vgs of the first transistor 12 can be clamped and the first transistor 12 can be protected when the second transistor 14 is off.

  The present invention has been described based on the embodiments. This embodiment is an exemplification, and it will be understood by those skilled in the art that various modifications can be made to combinations of the respective constituent elements and processing processes, and such modifications are within the scope of the present invention. is there. Hereinafter, such modifications will be described.

  Those skilled in the art can understand that the N-channel MOSFET and the P-channel MOSFET can be replaced, and the NPN-type bipolar transistor and the PNP-type bipolar transistor can be replaced in the circuit of FIG.

  FIG. 5 is a circuit diagram showing a configuration of a drive current generator 10b according to another modification. The drive current generation unit 10b in FIG. 5 has a configuration in which the transistor of the drive current generation unit 10a in FIG. Using the drive current generation unit 10b of FIG. 5, the same functions, operations, and effects as those of the drive current generation unit 10a of FIG. 3 can be obtained.

  FIG. 6 is a circuit diagram showing a configuration of a drive current generator 10c according to still another modification. The drive current generator 10c further includes a fourth resistor R4 and a fifth resistor R5 in addition to the drive current generator 10a of FIG. A voltage obtained by dividing the voltage drop VR1 by the resistors R4 and R5 is applied to the control terminal (base) of the second transistor 14 as a voltage corresponding to the voltage drop VR1 of the first resistor R1. Instead of using the first resistor R1 as a variable resistor, the fourth resistor R4 is a variable resistor. The fifth resistor R5 may be a variable resistance element.

  Also with the drive current generator 10c of FIG. 6, the same functions, operations, and effects as those of the drive current generator 10a of FIG. 3 can be obtained.

  A configuration in which the constant current source 16 is omitted in the circuits of FIGS. 3, 5, and 6 is also effective as an aspect of the present invention. Alternatively, a configuration in which the second resistor R2 is omitted in the circuits of FIGS. 3, 5, and 6 is also effective as an aspect of the present invention.

  Although the present invention has been described using specific terms based on the embodiments, the embodiments only illustrate the principles and applications of the present invention, and the embodiments are defined in the claims. Many modifications and arrangements can be made without departing from the spirit of the present invention.

DESCRIPTION OF SYMBOLS 100 ... Drive circuit, 2 ... LED array, 4 ... LED, P1 ... 1st terminal, P2 ... 2nd terminal, P3 ... Reference voltage terminal, 10 ... Drive current generation part, 12 ... 1st transistor, 14 ... 2nd transistor , 16 ... constant current source, R1 ... first resistor, R2 ... second resistor, R3 ... third resistor, 30 ... power source, Cs ... smoothing capacitor, 36 ... rectifier circuit, 200 ... light emitting device.

Claims (8)

A drive circuit for driving a light emitting diode array including a plurality of light emitting diodes connected in series,
A power supply for supplying a driving voltage to the light emitting diode array;
A first transistor and a first resistor provided in series on a driving path of the light emitting diode array and between one end of the light emitting diode array and a reference voltage terminal;
A constant current source provided between the other end of the light emitting diode array and a control terminal of the first transistor;
A second transistor provided between a control terminal of the first transistor and the reference voltage terminal, and having a voltage corresponding to a voltage drop of the first resistor applied to the control terminal;
A drive circuit comprising:   The drive circuit according to claim 1, further comprising a second resistor provided in series with the constant current source between the other end of the light emitting diode array and a control terminal of the first transistor. A drive circuit for driving a light emitting diode array including a plurality of light emitting diodes connected in series,
A power supply for supplying a driving voltage to the light emitting diode array;
A first transistor and a first resistor provided in series on a driving path of the light emitting diode array and between one end of the light emitting diode array and a reference voltage terminal;
A second resistor provided between the other end of the light emitting diode array and a control terminal of the first transistor;
A second transistor provided between a control terminal of the first transistor and the reference voltage terminal, and having a voltage corresponding to a voltage drop of the first resistor applied to the control terminal;
A drive circuit comprising:   The drive circuit according to claim 3, further comprising a constant current source provided in series with the second resistor between the other end of the light emitting diode array and a control terminal of the first transistor.   5. The drive according to claim 1, further comprising a third resistor provided between a connection point of the first transistor and the first resistor and a control terminal of the first transistor. circuit.   6. The drive circuit according to claim 1, wherein the first transistor is a field effect transistor, and the second transistor is a bipolar transistor. A light emitting diode array including a plurality of light emitting diodes connected in series;
The drive circuit according to any one of claims 1 to 6, which drives the light emitting diode array;
A light emitting device comprising:   An illumination device comprising the light-emitting device according to claim 7.

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