JP4971254B2 - LED lighting device - Google Patents

Description

  The present invention relates to an LED lighting device for lighting an LED (light emitting diode) used in, for example, a lighting fixture or a liquid crystal backlight.

  In recent years, with the advent of blue LEDs, white light can be generated, and LEDs are attracting attention as a third light source in the lighting industry. When this LED is used for a lighting fixture, a large LED display, or the like, as in Patent Document 1, a large number of LEDs are generally connected in series. In Patent Document 1, a plurality of series circuits composed of a constant current circuit and a plurality of LEDs are connected in parallel to the drive power supply.

  FIG. 3 schematically shows a circuit configuration of a conventional LED lighting device. The circuit is configured by connecting an LED 2 and a constant current element 3 in series to a DC power source 1. In the figure, the LED 2 is formed by connecting a plurality of n (n is an arbitrary number) LEDs in series from the LEDs 2a to 2n. On the other hand, the constant current element 3 is provided for making the current flowing to the LED 2 constant, that is, for controlling the constant current, and is composed of an element capable of limiting current, such as a constant current diode or a transistor.

The current flowing out from the DC power source 1 is limited by the constant current element 3 and the constant current is passed through the LED 2 so that the LED 2 is well lit.
JP 2005-109025 A

  However, in the above conventional LED lighting device, the LED current is made constant by the constant current element 3, so that equivalently, the resistance value is changed according to the number of LEDs 2, and the maximum number of LEDs 2 are lit. The power consumption (determined by the product of the voltage V of the DC power supply 1 and the LED current) becomes the same amount when one is turned on. That is, when the number of LEDs 2 is small, the constant current element 3 occupies most of the entire power consumption, which causes a problem of heat generation.

  Even if there is no problem of heat generation, the power consumption when several LEDs 2 are lit and when one LED is lit is the same as consuming unnecessary power when one LED is lit. It becomes economy.

  In view of the above problems, an object of the present invention is to provide an LED lighting device that adjusts an output to an LED so as to achieve power consumption corresponding to the number of LEDs to be lit.

The present invention is an LED lighting device comprising: an LED series circuit including a plurality of LEDs connected in series; and a lighting control unit that supplies power to the LED series circuit to light it, and the lighting control unit Is a step-down circuit that steps down a predetermined input voltage and supplies power to the LED series circuit, the step-down circuit including a switching element for controlling power supply to the LED series circuit, and the LED series circuit The output current supplied to the LED is fed back, the output current value fed back is compared with a constant current value preset as the LED drive current in the LED series circuit, and the output current value exceeds the constant current value. A constant current control circuit for generating a current setting feedback signal in order to prevent the output, and the output voltage applied to the LED series circuit is fed back, The output voltage value fed back is compared with a constant voltage value set in advance corresponding to the maximum number of connectable LEDs in the LED series circuit so that the output voltage value does not exceed the constant voltage value. A constant voltage control circuit for generating a voltage setting feedback signal, and a switching control circuit for driving the switching element by the current setting feedback signal and the voltage setting feedback signal, wherein the maximum connectable number When the following arbitrary number of LEDs are connected to the LED series circuit, the power supplied to the LED series circuit by the constant current control circuit is constant current controlled, and there are more LEDs than the maximum connectable number. When connected to the LED series circuit or when a break occurs in the LED series circuit, the constant voltage control circuit More the LED series circuit to the constant voltage control of power supply, characterized in that the connectable to LED any number of the following maximum connectable number.

In this way, the applied voltage to the LED is adjusted to an optimum value according to the number of LEDs connected to the lighting control unit, so that a constant current can be obtained without providing a constant current element that causes heat generation or loss. Control is possible, and power consumption can be set to a minimum value corresponding to the number of LEDs. In addition, when a constant current control circuit and a constant voltage control circuit are provided in combination, and the number of LEDs less than the maximum connectable number is connected to the LED series circuit, the constant current control circuit supplies the LED series circuit. While the power is controlled at a constant current, on the other hand, when more LEDs than the maximum connectable number are connected to the LED series circuit or when the LED series circuit is disconnected, the constant voltage control circuit supplies the LED series circuit. Since power is controlled at a constant voltage, the voltage applied to the LED series circuit when more LEDs than the maximum connectable number are connected to the LED series circuit or when the LED series circuit is disconnected. It is possible to suppress the occurrence of an abnormal voltage that becomes higher than a predetermined constant voltage. Furthermore, due to the use of step-down circuit can be connected to any number of LED below the maximum connectable number, For example, it can cope with the case of connecting the LED of only one.

According to claim 1 of the present invention, the output to the LEDs is adjusted so that the power consumption is in accordance with the number of LEDs to be lit, while the voltage applied to the LED series circuit is higher than a predetermined constant voltage. such can provide a LED lighting device capable of suppressing the occurrence of abnormal voltage and can be connected up connectable number following any number of LED, for example, connecting the LED of only one It can also cope with the case.

  Hereinafter, preferred embodiments of an LED lighting device according to the present invention will be described with reference to the accompanying drawings. In addition, the same code | symbol is attached | subjected to the same location as a prior art example, and it abbreviate | omits as much as possible since description of a common part overlaps.

  FIG. 1 schematically shows the circuit configuration of the LED lighting device of this embodiment. The circuit is configured by connecting the DC power source 1 to the input side of the lighting unit 5 as a lighting control unit, and connecting the LED 2 to the output side of the lighting unit 5. The LED 2 is obtained by connecting a plurality of n (n is an arbitrary number) LEDs in series to the LEDs 2a to 2n as in the conventional example. Vi is an input voltage input from the DC power source 1 to the lighting unit 5, Vo is an output voltage output from the lighting unit 5 and applied to the LED 2, and Io is output from the lighting unit 5 and flows through the LED 2. It is a current (LED current), and Vf is an LED voltage that is a forward voltage of the LED 2a and the like.

  An internal circuit of the lighting unit 5 which is a characteristic part of the present invention will be described with reference to FIG. On the input side, a pair of input terminals 10 and 10 to which the DC power source 1 is connected are provided, and on the output side, a pair of output terminals 17 and 17 to which the LED 2 is connected are provided. A series circuit of a diode 11 and a capacitor 12 is connected between the input terminals 10 and 10. At this time, the diode 11 has an anode on the input terminal 10 side and a cathode on the capacitor 12 side. Here, a line connecting one input terminal 10 connected to the capacitor 12 and the output terminal 17 serving as an LED-output is hereinafter referred to as a common line. The connection point between the cathode of the diode 11 and the capacitor 12 is connected to the emitter of a PNP-type transistor 13 as a switching element. The collector of the transistor 13 is connected to an output terminal 17 serving as an LED + output via a choke coil 15. It is connected to the. A commutation diode 14 is connected between the collector of the transistor 13 and the common line. At this time, the commutation diode 14 has a cathode on the collector side and an anode on the common line side. A smoothing capacitor 16 is connected between the output terminals 17 and 17. Thus, a so-called step-down chopper circuit 19 is configured.

  A switching control circuit 18 for controlling the switching operation (on / off operation) is connected to the base of the transistor 13. Hereinafter, a specific configuration of the switching control circuit 18 will be described. A resistor 20 is connected between the emitter and base of the transistor 13, and the base of the transistor 13 is grounded (connected to a common line) via a resistor 21 and a collector-emitter of an NPN transistor 22. The output terminal of the comparator 23 is connected to the base of the transistor 22, and the drive signal output from the comparator 23 causes the transistor 22 and thus the transistor 13 to perform a switching operation. The comparator 23 compares the triangular wave input from the OSC 24 serving as the triangular wave oscillator to the inverting input terminal and the feedback signal input to the non-inverting input terminal, and the voltage of the triangular wave is equal to or higher than the voltage level of the feedback signal as the threshold value. A pulse-shaped drive signal having a pulse width as a period of time is output.

  In this embodiment, two signals are input in a wired OR connection as feedback signals input to the non-inverting input terminal of the comparator 23.

  The first signal is a voltage setting feedback signal related to the output voltage Vo, and is input from the output terminal of the operational amplifier 32 to the comparator 23 through the backflow prevention diode 34. The diode 34 is connected so that the operational amplifier 32 side is an anode and the comparator 23 side is a cathode. On the input side of the operational amplifier 32, the connection point between the resistor 30 and the resistor 31 connected to each of the output terminals 17 and 17 for outputting the output voltage Vo to the LED 2 is connected to the non-inverting input terminal of the operational amplifier 32, while the reference A reference voltage source 33 having a voltage Vref is connected to the inverting input terminal of the operational amplifier 32. That is, the operational amplifier 32 receives the voltage obtained by dividing the output voltage Vo by the resistors 30 and 31 and the reference voltage Vref.

  The second signal is a current setting feedback signal related to the output current Io, that is, the LED current flowing to the LED 2, and is input from the output terminal of the operational amplifier 42 to the comparator 23 through the backflow prevention diode 44. The diode 44 is connected so that the operational amplifier 42 side is an anode and the comparator 23 side is a cathode. In this embodiment, a current detection resistor 40 is inserted in a common line between the anode of the commutation diode 14 and the smoothing capacitor 16 in order to detect the output current Io. When the output current Io flows through the current detection resistor 40, a detection voltage Vd proportional to the current value is generated. On the input side of the operational amplifier 42, the output terminal 17 on the LED-output side is connected to the non-inverting input terminal of the operational amplifier 42, while the reference voltage source 33 is connected to the inverting input terminal of the operational amplifier 42 via the resistor 43. The inverting input terminal of the operational amplifier 42 is connected to the common line via the resistor 41. In other words, the detection voltage Vd and the voltage obtained by dividing the reference voltage Vref by the resistors 41 and 43 are input to the operational amplifier 42.

  Next, the operation of the lighting unit 5 configured as described above will be described.

  The lighting unit 5 takes out electric power from the DC power source 1 connected to the input terminals 10 and 10 by the switching operation of the transistor 13 and outputs it to the LED 2 connected to the output terminals 17 and 17. When the transistor 13 is on, the power of the DC power source 1 is averaged by the choke coil 15 and the smoothing capacitor 16 through the transistor 13 and output to the output terminals 17 and 17. At this time, excitation current is stored by the current flowing through the choke coil 15. When the transistor 13 is off, the commutation diode 14 is turned on, and the excitation energy stored in the choke coil 15 is released and output to the output terminals 17 and 17 as discharge power. The output voltage Vo is lower than the input voltage Vi.

  The switching control circuit 18 intermittently applies a voltage between the emitter and base of the transistor 13 by switching the transistor 22 in accordance with the drive signal. Specifically, when the drive signal is at the H level, the transistor 22 is turned on, and a current flows through the resistors 20 and 21, whereby a voltage corresponding to the voltage drop of the resistor 20 is generated between the emitter and base of the transistor 13. While the transistor 13 is turned on, the transistor 22 is turned off when the drive signal is at the L level, the emitter and base of the transistor 13 have substantially the same potential, and the transistor 13 is turned off. The drive signal is PWM-controlled based on the two feedback signals described above, and constant voltage control for the output voltage Vo and constant current control for the output current Io are selectively performed.

  Here, for example, the reference voltage Vref is designed so that the set value of the output voltage Vo is 18 V and the set value of the output current Io is 20 mA, and the LED voltage Vf when the LED 2 is lit (the LED current at this time is 20 mA). Is 3.6V.

  When only one LED 2 (for example, LED 2a) is lit by the lighting unit 5, the LED voltage of the entire LED 2 is 3.6V, which is less than the set value 18V. A negative voltage is output to widen. However, if the output current Io exceeds 20 mA at which the LED voltage becomes 3.6 V as the output voltage Vo increases, the operational amplifier 42 outputs a positive voltage so as to narrow the pulse width of the drive signal. The positive voltage is obtained by amplifying the difference between the detection voltage Vd and the divided voltage value of the reference voltage Vref. Therefore, since the positive voltage output from the operational amplifier 42 is input to the comparator 23 by the wired OR, the constant current control for the output current Io becomes dominant. Similarly, constant current control for the output current Io is performed until the LED voltage of the entire LED 2 reaches 18 V, that is, up to five LEDs 2 to be lit.

  On the other hand, when the number of LEDs 2 is 6 or more, the LED voltage as a whole LED2 exceeds 18V before the output current Io reaches 20mA, so that the output voltage Vo does not exceed the set value 18V. In addition, the operational amplifier 32 outputs a positive voltage so as to narrow the pulse width of the drive signal. The positive voltage is obtained by amplifying the difference between the divided value of the output voltage Vo and the reference voltage Vref. At this time, since the output current Io is lower than 20 mA, the operational amplifier 42 outputs a negative voltage so as to widen the pulse width of the drive signal. Therefore, since the positive voltage output from the operational amplifier 32 is input to the comparator 23 by the wired OR, the constant voltage control with respect to the output voltage Vo becomes dominant. The constant voltage control also has an effect of suppressing an abnormal voltage of the output voltage Vo generated when the output current Io stops flowing, for example, when the LED 2 fails in the open mode or disconnection occurs.

In this way, by adjusting the output voltage Vo, which is the voltage applied to the LED 2, to an optimum voltage according to the number of the LEDs 2 connected to the lighting unit 5, the LED 2 can be driven and lit with a constant current. . During the constant current control, the output voltage Vo is a value obtained by (LED voltage Vf) × (number of LEDs 2), and the power consumption can be set to a minimum value corresponding to the number. That is, the lighting unit 5 enables constant current control without providing the constant current element 3 that causes problems of heat generation and loss. Regarding the LED 2, one power consumption is one, and several power consumptions are several. The structure which becomes is realized. The maximum number of LEDs 2 to be lit by the lighting unit 5 is determined by the input voltage Vi, and can be expressed by the equation (maximum number of LEDs to be lit) = (input voltage Vi−circuit element voltage) / (LED voltage Vf). In the circuit of this embodiment, the circuit element voltage is about 3V.

  As described above, in this embodiment, the LED lighting device includes the LED 2 and the lighting unit 5 as a lighting control unit that supplies the LED 2 with power and lights it. The lighting unit 5 has an output that flows through the LED 2. The output voltage Vo as an applied voltage to the LED 2 is adjusted according to the current Io.

  In this way, the output voltage Vo to the LED 2 is adjusted to an optimum value according to the number of the LEDs 2 connected to the lighting unit 5, so that there is no need to provide the constant current element 3 that causes problems of heat generation and loss. Constant current control is possible, and power consumption can be set to a minimum value according to the number of LEDs 2. Accordingly, it is possible to provide an LED lighting device that adjusts the output to the LED 2 so as to achieve power consumption corresponding to the number of the LEDs 2 to be lit.

  In addition, this invention is not limited to the said Example, It can change in the range which does not deviate from the meaning of this invention. Instead of the step-down chopper circuit 19 used in the lighting unit 5, for example, a switching power source such as a step-up chopper circuit or a forward converter can be used.

It is a block diagram which shows the circuit structure of the LED lighting device in the Example of this invention. It is a circuit diagram which shows an internal structure of the lighting unit which comprises an LED lighting device same as the above. It is a block diagram which shows the circuit structure of the LED lighting device in a prior art example.

Explanation of symbols

2 LED
5 Lighting unit (lighting control unit)

Claims (1)

An LED lighting device comprising: an LED series circuit comprising a plurality of LEDs connected in series; and a lighting control unit for supplying power to the LED series circuit to light it,
The lighting control unit
A step-down circuit for stepping down a predetermined input voltage and supplying power to the LED series circuit, the step-down circuit including a switching element for controlling power supply to the LED series circuit;
The output current supplied to the LED series circuit is fed back, the output current value fed back is compared with a constant current value preset as an LED driving current in the LED series circuit, and the output current value is determined. A constant current control circuit for generating a current setting feedback signal so as not to exceed the current value;
The output voltage applied to the LED series circuit is fed back, and the output voltage value fed back is compared with a constant voltage value set in advance corresponding to the maximum connectable number of the LEDs in the LED series circuit. A constant voltage control circuit for generating a voltage setting feedback signal so that the output voltage value does not exceed the constant voltage value;
A switching control circuit for driving the switching element by the current setting feedback signal and the voltage setting feedback signal;
When an arbitrary number of LEDs equal to or less than the maximum connectable number are connected to the LED series circuit, the constant current control circuit controls the power supplied to the LED series circuit by constant current control, and the maximum when disconnection or the LED series circuit when more than connectable number LED is connected to the LED series circuit occurs, the power supplied to the LED series circuit constant voltage controlled by the constant voltage control circuit An LED lighting device characterized in that an arbitrary number of LEDs less than the maximum connectable number can be connected .

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