JP2006351685A - Light emitting device driving apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a light emitting element driving apparatus capable of restraining a temperature variation in the light emission luminance of a light emitting element with a miniaturized and inexpensive constitution. <P>SOLUTION: The light emitting element driving apparatus 10 comprises a current driving circuit 1, a current detection circuit 5, a control circuit 3, and an oscillator 4. The control circuit 3 controls the current driving circuit 1 in a current discontinuous mode on the basis of a pulse signal from the oscillator 4 and of an output signal from the current detection circuit 5, and the light emitting element 2 is directly coupled between output terminals of the current driving circuit. Moreover, the light emitting element driving apparatus 10 preferably includes a voltage detection circuit 6 for detecting an output voltage Vo of the current driving circuit 1, and the oscillator 4 varies an oscillation frequency in response to the output of the voltage detection circuit 6. Consequently, an output current Io is increased accompanied by the reduction of the output voltage Vo of the current driving circuit 1 to restrain the reduction of light emission luminance caused by a temperature rise. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to an apparatus for driving a light emitting element such as a light emitting diode, and more particularly to a light emitting element driving apparatus capable of suppressing a change in luminance due to a temperature change.

  The light emission luminance of a light emitting element such as a light emitting diode (LED) depends on the magnitude of a current passed through the light emitting element. For this reason, conventionally, in order to light the light emitting element stably, a light emitting element driving device that drives the light emitting element with a constant current is often used. FIG. 7 shows a conventional constant current type light emitting element driving apparatus 100. As shown in FIG. 7A, the light emitting element driving apparatus 100 includes a current driving circuit 101, a control circuit 103, and an oscillator 104. An input for supplying a DC voltage is input to the input side of the current driving circuit 101. A power source Vi is connected, and a light emitting element 2 connected in series to the resistor R2 and the resistor R2 is connected to the output side. The current driving circuit 101 includes a switching element Q1, an inductor L1, a diode D1, and a capacitor C1. The switching circuit Q1 is turned on / off based on an output signal of the control circuit 103, and a DC voltage supplied from the DC power source Vi is supplied. The voltage is boosted and output. At this time, a voltage signal corresponding to the output current Io detected by the resistor R2 is input to the control circuit 103, and the current drive circuit 101 is controlled by the control circuit 103 so that the output current Io becomes a predetermined constant current. Feedback controlled. Further, a voltage signal corresponding to the current flowing through the switching element Q1 detected by the resistor R1 is also input to the control circuit 103, and an excessive current flows through the switching element Q1 to prevent damage.

  For example, the control circuit 103 includes a flip-flop 111, a sawtooth wave generation circuit 112, an amplifier 113, a comparator 114, an error amplifier 115, a driver 116, and an adder 117, as shown in FIG. The switching element Q1 is controlled by the method. The operation of the control circuit 103 will be briefly described as follows. When the S input of the flip-flop 111 becomes High level by the pulse signal from the oscillator 104, the Q output is set, and the switching element Q1 is turned on by the output signal (for example, gate voltage) output through the driver. At this time, the pulse signal from the oscillator 104 is also input to the sawtooth wave generation circuit 112, and the generated sawtooth wave is added to the output of the amplifier 113 and input to the (+) input terminal of the comparator 114. The output voltage of the error amplifier 115 is input to the (−) input terminal of the comparator 114. When the output voltage from the adder 117 rises and exceeds the output voltage of the error amplifier 115, the output voltage of the comparator 114 is output. That is, the R input of the flip-flop is switched to the high level. As a result, the Q output of the flip-flop 111 is reset, and the switching element Q1 is turned off.

  In the operation as described above, a predetermined reference voltage Vref is input to the (+) input terminal of the error amplifier 115, and a voltage corresponding to the output current Io detected by the resistor R2 is input to the (−) input terminal. ing. Therefore, the timing at which the output voltage from the adder 117 rises and exceeds the output voltage from the error amplifier 115 varies according to the increase or decrease in the output voltage from the error amplifier 115. For example, the output current Io increases and the error amplifier 115 increases. When the output voltage of the switching element Q1 decreases, the ON time of the switching element Q1 is shortened. Conversely, when the output current Io decreases and the output voltage from the error amplifier 115 increases, the ON time of the switching element Q1 extends. In this way, the output current Io is maintained at a desired constant current. Further, since a voltage signal corresponding to the current flowing through the switching element Q1 detected by the resistor R1 is input to the (+) input terminal of the amplifier 113, if an excessive current flows through the switching element Q1, an adder As a result of the increase in the output voltage of 117, the output of the comparator 114 becomes a high level regardless of the phase of the sawtooth wave, and the switching element Q1 is turned off, so that the switching element Q1 is prevented from being damaged.

Here, in the light emitting element 2, the light emission luminance in a state where the same current is applied has a temperature dependency that decreases as the temperature increases. Accordingly, when the light emitting element 2 is turned on by controlling the output current Io to be constant by the light emitting element driving apparatus 100, the light emission luminance is lowered as the temperature rises as shown in FIG. When such a light emitting element is used as, for example, a primary light source of a backlight of a liquid crystal display device, there is a problem that the liquid crystal display screen becomes dark as the temperature rises.
In order to cope with such a problem, conventionally, the light emission luminance of the light emitting element is detected by using an optical sensor such as a photodiode, and the light emission luminance of the light emitting element is controlled by feedback control based on the detected light emission luminance. A light emitting element driving device that keeps constant (for example, refer to Patent Document 1), or a temperature sensor such as a thermistor detects the temperature of the light emitting element, and feedback control is performed based on the detected temperature, thereby A light-emitting element driving device that keeps the light emission luminance constant (for example, see Patent Document 2) has been proposed.

JP-A-5-327450 JP 2002-369506 A

  However, since the light emitting element driving apparatus described in Patent Document 1 and Patent Document 2 requires some sensor such as a temperature sensor or an optical sensor, the light emitting element driving apparatus becomes large and expensive. Since it is necessary to arrange a sensor near the light emitting element, there is a problem that wiring and layout of the apparatus become complicated.

  The present invention has been made in view of the above problems, and an object of the present invention is to provide a light emitting element driving device capable of suppressing temperature fluctuations of light emission luminance of a light emitting element with a small and inexpensive configuration.

  In order to achieve the above object, the present invention provides a light-emitting element driving apparatus for lighting a light-emitting element, a current driving circuit including an inductor, a capacitor, a switching element, and a diode, and a current flowing through the switching element. A current detection circuit that controls the on / off operation of the switching element, and an oscillator that supplies a pulse signal to the control circuit, the control circuit including the pulse signal from the oscillator and the current Based on the output signal from the detection circuit, the on / off operation of the switching element is controlled so that the output power of the current driving circuit is substantially constant when the frequency of the oscillator is constant, The current drive circuit is directly connected between output terminals.

  According to the present invention, the control circuit performs switching based on the pulse signal from the oscillator and the output signal from the current detection circuit so that the output power of the current driving circuit is substantially constant when the frequency of the oscillator is constant. The on / off operation of the element is controlled, and the light emitting element is directly connected between the output terminals of the current driving circuit, so that the output current of the current driving circuit decreases as the forward voltage of the light emitting element decreases due to temperature rise. Therefore, the decrease in the light emission luminance of the light emitting element due to the temperature rise can be suppressed without using a sensor for detecting the light emission luminance and temperature. In addition, since the light emitting element is directly connected between the output terminals of the current driving circuit as described above, a highly efficient light emitting element driving apparatus can be realized without power loss due to resistance or the like.

  Preferably, the current drive circuit operates in a current discontinuous mode, whereby a good constant power characteristic can be obtained.

  In one aspect of the present invention, the light-emitting element driving device according to the present invention further includes a voltage detection circuit that detects an output voltage of the current drive circuit, and the oscillator has an oscillation frequency according to the output of the voltage detection circuit. Fluctuate. Accordingly, the frequency of the oscillator can be increased with a decrease in the forward voltage of the light emitting element due to a temperature rise, and a decrease in the light emission luminance of the light emitting element due to the temperature rise can be more effectively suppressed.

  Furthermore, in the oscillator, it is preferable that the amount of change in frequency with respect to the change in output voltage of the current drive circuit is variable, and this makes it possible to deal with various light emitting elements having different temperature characteristics of light emission luminance. .

  In another aspect of the present invention, the light-emitting element driving device according to the present invention further includes a voltage detection circuit that detects an output voltage of the current driving circuit, and the control circuit outputs an output from the voltage detection circuit. Accordingly, the reference voltage is varied. Accordingly, the reference voltage can be increased with a decrease in the forward voltage of the light emitting element due to the temperature rise, and the decrease in the light emission luminance of the light emitting element due to the temperature rise can be more effectively suppressed.

  Furthermore, in the control circuit, it is preferable that the amount of change in the reference voltage with respect to the change in the output voltage of the current drive circuit is variable, and thus it is possible to deal with various light emitting elements having different temperature characteristics of light emission luminance. It becomes.

  In the present invention, the current detection circuit may be a resistor having a low resistance value. As a result, the current detection circuit can be configured easily and inexpensively.

  Since the present invention is configured as described above, a light emitting element driving device capable of suppressing temperature fluctuations of the light emitting luminance of the light emitting element with a small and inexpensive configuration without using a sensor for detecting the light emitting luminance or temperature is provided. It becomes possible to do.

Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings. FIG. 1 is a circuit configuration diagram showing a light emitting element driving apparatus 10 according to an embodiment of the present invention.
In FIG. 1, the light emitting element driving device 10 includes a current driving circuit 1, a control circuit 3, an oscillator 4, a current detection circuit 5, and a voltage detection circuit 6. The current driving circuit 1 includes an inductor L1, a capacitor C1, a switching element Q1 made of, for example, a power MOSFET, and a diode D1, and an input power source Vi is connected between the input terminals, and between the output terminals, The light emitting element 2 which is preferably a light emitting diode is directly connected without a resistor or the like. In the present embodiment, the current drive circuit 1 is a booster circuit that turns on and off the switching element Q1 based on an output signal from the control circuit 3 and boosts and outputs a DC voltage supplied from the input power source Vi. . The current detection circuit 5 includes a resistor R1 having a low resistance value, detects a current flowing through the switching element Q1, and outputs an output signal corresponding to the value to the control circuit 3. The voltage detection circuit 6 detects the output voltage Vo of the current drive circuit 1 and outputs an output signal corresponding to the value to the oscillator 4.

  In the present embodiment, the oscillator 4 varies the frequency of the output pulse signal in accordance with the output signal from the voltage detection circuit 6, and further changes the frequency change amount with respect to the variation of the output voltage Vo to a predetermined value. It can be set to the value of. The present invention is not limited by the specific configuration of the oscillator 4 as described above. For example, the oscillator 4 adjusts a variable resistor or a variable capacitor in a circuit that determines an oscillation frequency according to a control voltage. It can be any suitable voltage controlled oscillator including possible circuit constants.

  As shown in FIG. 1B, the control circuit 3 includes a flip-flop 11, a comparator 12, and a driver 13. The pulse signal output from the oscillator 4 is connected to the S input of the flip-flop 11. The comparator 12 receives an output signal from the current detection circuit 5 at its (+) input terminal, a predetermined reference voltage Vref at its (−) input terminal, and its output is the R of the flip-flop 11. Connected to the input. The Q output of the flip-flop 11 is given to the switching element Q1 via the driver 13 (for example, as the gate voltage of the power MOSFET), and becomes an output signal for controlling the on / off operation of the switching element Q1.

Next, the basic operation of the light emitting element driving apparatus 10 in this embodiment will be described on the assumption that the frequency of the oscillator 4 is fixed. Here, the current drive circuit 1 in the present embodiment is controlled so as to operate in a current discontinuous mode in which the current flowing through the inductor L1 is zero every cycle.
First, when the S input of the flip-flop 11 becomes High level by the pulse signal output from the oscillator 4, the Q output of the flip-flop 11 is set and the switching element Q1 is turned on. As a result, a path including the inductor L1, the switching element Q1, and the resistor R1 is formed, and a current that rises from zero and increases with a slope proportional to the input voltage Vi flows through the inductor L1. After that, when the output signal of the current detection circuit 5 (that is, “current flowing through the switching element Q1” × R1) reaches the reference voltage Vref, the output voltage of the comparator 12 and therefore the R input of the flip-flop 11 goes to the high level. The Q output of the flip-flop 11 is reset, and the switching element Q1 is turned off. At this point, an induced electromotive voltage is generated in the inductor L1 and superimposed on the input voltage Vi, and the capacitor C1 is charged via the diode D1. At this time, the current flowing out of the inductor L1 decreases with a slope proportional to the difference between the output voltage Vo of the current drive circuit 1 and the input voltage Vi, and eventually becomes zero. Thereafter, after a lapse of a certain time according to the frequency of the oscillator 4, the next pulse signal is applied from the oscillator 4 to the S input of the flip-flop 11, and thereafter this operation is repeated.

  In general, in such a current discontinuous mode, the energy stored in the inductor L1 is transmitted to the light emitting element 2 every time the switching element Q1 is on, and the energy flows through the inductor L1 by the reference voltage Vref. By keeping the peak value of the current constant, it is controlled so as to be constant over the entire period. Therefore, when the frequency of the oscillator 4 is constant, the current driving circuit 1 in the present embodiment supplies constant power from the input power source Vi to the light emitting element 2 as a load. As shown in FIG. The output current Io increases as Vo decreases. Here, FIG. 2 is a graph showing the output voltage-output current characteristics of the current driving circuit 1 for various frequencies.

  Here, when the temperature of the light emitting element 2 rises, due to the characteristics of the light emitting element 2, the light emission luminance decreases and the forward voltage Vf also decreases. On the other hand, in the present embodiment, since the light emitting element 2 is directly connected between the output terminals of the current driving circuit 1, the output voltage Vo of the current driving circuit 1 is equal to the forward voltage Vf of the light emitting element 2. Therefore, as the temperature rises, the output voltage Vo decreases, and as a result, as shown in FIG. 3, the output current Io increases to suppress a decrease in light emission luminance. Here, FIG. 3 is a graph showing an example of temperature characteristics of the output current Io and the light emission luminance of the light emitting element 2 when the frequency of the oscillator 4 is fixed.

  Furthermore, since the temperature characteristics of such light emission luminance vary depending on the type of light emitting element, depending on the light emitting element to be used, the output current Io increases sufficiently under the condition that the frequency is fixed. In some cases, it may not be possible to suppress it. In the light emitting element driving apparatus 10 according to the present embodiment, an output signal from the voltage detection circuit 6 is input to the oscillator 4, and the frequency of the oscillator 4 is changed according to the signal, and the amount of change in the frequency is used. By setting in advance according to the temperature characteristic of the emission luminance, it is possible to cope with such a situation. That is, in the light emitting element driving apparatus 10, since the output power of the current driving circuit 1 increases as the operating frequency of the switching element Q1 increases, the frequency of the oscillator 4 increases as shown in FIG. At the same time, the output current Io for a specific output voltage Vo increases. Therefore, by setting the frequency of the oscillator 4 in the present embodiment so as to increase according to the temperature characteristics of the light emitting element to be used when the output voltage Vo decreases, as shown in FIG. When the output voltage Vo decreases as the temperature rises, the output current Io increases to compensate for the decrease in light emission luminance, and the light emission luminance can be kept substantially constant. Here, FIG. 4 is a graph showing an example of the temperature characteristics of the output current Io and the light emission luminance of the light emitting element 2 when the frequency of the oscillator 4 is variable.

  Here, as an example of the case where the temperature characteristic of the light emission luminance varies depending on the type of the light emitting element, FIG. 5 is a graph comparing the temperature characteristics of the light emission luminance for the blue light emitting element, the green light emitting element, and the red light emitting element. Show. As shown in FIG. 5, the degree of decrease in light emission luminance with respect to temperature rise is the smallest for blue light emitting elements, and then increases in the order of green light emitting elements and red light emitting elements. Therefore, for example, when the blue light emitting element shown in FIG. 5 is turned on by applying the light emitting element driving apparatus 10 in the present embodiment, the change in the frequency of the oscillator 4 with respect to the fluctuation of the output voltage Vo according to the temperature characteristics. When the amount is set small and the red light emitting element is turned on, the amount of change in the frequency of the oscillator 4 with respect to the fluctuation of the output voltage Vo is similarly set according to the temperature characteristic.

  FIG. 6 is a circuit configuration diagram showing a light emitting element driving device 20 according to another embodiment of the present invention.

  In FIG. 6, the light emitting element driving device 20 includes a current driving circuit 21, a control circuit 23, an oscillator 24, a current detection circuit 25, and a voltage detection circuit 26. The current driving circuit 21 includes an inductor L1, a capacitor C1, a switching element Q1 made of, for example, a power MOSFET, and a diode D1, and an input power source Vi is connected between the input terminals, and between the output terminals, The light emitting element 2 which is preferably a light emitting diode is directly connected without a resistor or the like. In the present embodiment, the current driving circuit 21 is a boosting circuit that turns on and off the switching element Q1 based on an output signal from the control circuit 23, boosts and outputs a DC voltage supplied from the input power source Vi. . The current detection circuit 25 includes a resistor R1 having a low resistance value, detects a current flowing through the switching element Q1, and outputs an output signal corresponding to the value to the control circuit 23. The voltage detection circuit 26 detects the output voltage Vo of the current drive circuit 21 and outputs an output signal corresponding to the value to the control circuit 23.

  In the present embodiment, the control circuit 23 varies the reference voltage Vref according to the output signal from the voltage detection circuit 26, and further changes the amount of change of the reference voltage Vref with respect to the variation of the output voltage Vo. It can be set to a predetermined value. The present invention is not limited by the specific configuration of the reference voltage Vref.

  The basic operation of the light emitting element driving apparatus 20 in the present embodiment is the same as the basic operation of the light emitting element driving circuit 10 in the embodiment shown in FIG. To do.

  As in the light emitting element driving circuit 10 shown in FIG. 1, when the reference voltage Vref is fixed, an example of the temperature characteristics of the output current Io and the light emitting luminance of the light emitting element 2 is as shown in FIG. Since the characteristics vary depending on the type of the light emitting element, depending on the light emitting element used, there is a possibility that the decrease in the light emission luminance cannot be sufficiently suppressed due to the increase in the output current Io under the condition where the reference voltage Vref is fixed. There is sex. In the light emitting element driving device 20 according to the present embodiment, an output signal from the voltage detection circuit 26 is input to the control circuit 23, the reference voltage Vref of the control circuit 23 is changed in accordance with this signal, and the change amount of the reference voltage Vref. Is set in advance in accordance with the temperature characteristics of the light emission luminance of the light emitting element to be used, to cope with such a situation. That is, in the light emitting element driving device 20, since the output power of the current driving circuit 21 increases as the reference voltage Vref of the switching element Q1 increases, the current driving circuit 21 is specified as the reference voltage Vref of the control circuit 23 increases. The output current Io with respect to the output voltage Vo increases. Therefore, the temperature of the light emitting element 2 is increased by setting the reference voltage Vref of the control circuit 23 in the present embodiment so as to increase according to the temperature characteristic of the light emitting element to be used when the output voltage Vo decreases. When the output voltage Vo decreases, the output current Io increases to compensate for the decrease in light emission luminance, and the light emission luminance can be kept substantially constant.

  As shown in FIG. 5, the degree of decrease in light emission luminance with respect to temperature rise is the smallest for blue light emitting elements, and then increases in the order of green light emitting elements and red light emitting elements. Therefore, for example, when the blue light emitting element shown in FIG. 5 is turned on by applying the light emitting element driving device 20 in the present embodiment, the reference voltage of the control circuit 23 with respect to the fluctuation of the output voltage Vo according to the temperature characteristic. When the change amount of Vref is set small and the red light emitting element is turned on, the change amount of the reference voltage Vref of the control circuit 23 with respect to the change of the output voltage Vo is similarly set according to the temperature characteristic. .

  Through the above description, the current driving circuit 1 has been described as a booster circuit. However, in the light emitting element driving device according to the present invention, the current driving circuit may be a step-down circuit or a step-up / step-down circuit. The current detection circuit 5 may use a current transformer or a thermistor.

(A) is a circuit block diagram which shows the light emitting element drive device in one Embodiment of this invention, (b) is a circuit block diagram which shows the control circuit used with the light emitting element drive device shown to (a). is there. It is a graph which shows the output voltage-output current characteristic of the current drive circuit used with the light emitting element drive device shown in FIG. 2 is a graph showing an example of temperature characteristics of output current and light emission luminance of a light emitting element when the frequency is fixed in the light emitting element driving device shown in FIG. 1. 2 is a graph showing an example of temperature characteristics of output current and light emission luminance of a light emitting element when the frequency is variable in the light emitting element driving device shown in FIG. 1. It is a graph which shows the light-emitting luminance-temperature characteristic of a light emitting element for a blue light emitting element, a green light emitting element, and a red light emitting element as an example. (A) is a circuit block diagram which shows the light emitting element drive device in another one Embodiment of this invention, (b) is a circuit structure which shows the control circuit used with the light emitting element drive device shown to (a). FIG. (A) is a circuit block diagram which shows an example of the conventional light emitting element drive device, (b) is a circuit block diagram which shows the control circuit used with the light emitting element drive device shown to (a). 8 is a graph showing an example of temperature characteristics of output current and light emission luminance of a light emitting element in the light emitting device driving device shown in FIG. 7.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1, 2: 1: Current drive circuit, 2: Light emitting element, 3, 23: Control circuit, 4, 24: Oscillator, 5, 25: Current detection circuit, 6, 26: Voltage detection circuit, 10, 20: Light emitting element drive device , C1: capacitor, D1: diode, L1: inductor, Q1: switching element, R1: resistor, Vi: input power supply

Claims (7)

In the light emitting element driving device for lighting the light emitting element,
A current drive circuit including an inductor, a capacitor, a switching element, and a diode; a current detection circuit that detects a current flowing through the switching element; a control circuit that controls an on / off operation of the switching element; and the control circuit An oscillator for supplying a pulse signal to
Based on the pulse signal from the oscillator and the output signal from the current detection circuit, the control circuit controls the switching so that the output power of the current driving circuit is substantially constant when the frequency of the oscillator is constant. Control the on / off operation of the element,
The light emitting element driving device, wherein the light emitting element is directly connected between output terminals of the current driving circuit.   The light emitting element driving apparatus according to claim 1, wherein the current driving circuit operates in a current discontinuous mode.   The light emitting device according to claim 1, further comprising a voltage detection circuit that detects an output voltage of the current drive circuit, wherein the oscillator varies an oscillation frequency in accordance with an output of the voltage detection circuit. Drive device.   4. The light emitting element driving device according to claim 3, wherein a change amount of a frequency with respect to a change of an output voltage of the current driving circuit of the oscillator is variable.   The light emitting device according to claim 1, further comprising a voltage detection circuit that detects an output voltage of the current drive circuit, wherein the control circuit varies a reference voltage according to an output of the voltage detection circuit. Element driving device.   The light emitting element driving device according to claim 5, wherein a change amount of a reference voltage with respect to a change of an output voltage of the current driving circuit of the control circuit is variable. The light-emitting element driving device according to claim 1, wherein the current detection circuit is a resistor having a low resistance value.

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