Light-emitting diode lighting apparatus

This application is the National Stage Entry of International Application PCT/KR2013/006021, filed on Jul. 8, 2013, and claims priority from and the benefit of Korean Patent Application No. 10-2012-0074376, filed on Jul. 9, 2012, each of which is incorporated herein by reference for all purposes as if fully set forth herein.

The present invention relates to a light-emitting diode lighting apparatus, and more particularly, to a light-emitting diode lighting apparatus which rectifies output of an alternating current (AC) power source and supplies uniform power to light emitting diodes.

Discussion of the Background

A light-emitting diode lighting apparatus is used as a backlight unit for portable devices or as a general lighting apparatus.

A lighting apparatus used as the backlight unit for portable devices employs direct current (DC) voltage of a portable power source. Accordingly, studies on improvement of efficiency or power factor of the power source used therefor have been not carried out in practice. This is caused by characteristics of a light emitting diode which consumes power of the lighting apparatus, and insignificant improvement of power consumption through a particular circuit design. In addition, when DC voltage is used by the power source, harmonics components according to Fourier analysis become insignificant. Thus, reduction in power factor resulting from complex frequency components becomes insignificant.

On the other hand, when light emitting diodes are used in a general lighting apparatus, ripple voltage is applied to the light emitting diodes. The ripple voltage causes a problem of improvement in power consumption and power factor. In addition, each of the light emitting diodes using ripple voltage is also required to emit light with uniform brightness as well as improvement in power consumption and power factor. Brightness of the light emitting diode depends upon current flowing through the light emitting diode rather than voltage applied thereto. This is because the light emitting diode has a mechanism of emitting light through recombination of excited electrons and holes. Thus, it is necessary for the lighting apparatus to allow uniform current to flow through the light emitting diodes.

Some lighting apparatuses employ switching elements in order to allow uniform current to flow through the light emitting diodes.

Uniform current can be supplied to the light emitting diodes through on/off control of the switching elements and current path thereby, whereby each of the light emitting diodes can emit light with the same level of brightness.

Further, a current source is used in order to allow the light emitting diodes to emit light with the same level of brightness. In order to realize the same level of brightness between the light emitting diodes, it is necessary to set a variety of current paths. Therefore, there is a need for a technology that can operate a plurality of light emitting diode groups using an active element and can supply uniform current to the light emitting diodes to secure brightness uniformity while improving power factor and efficiency.

Embodiments of the invention provide a lighting apparatus that allows easy control of current supplied to light emitting diodes such that the amount of current flowing through each light-emitting group composed of the light emitting diodes can be controlled through control of the current.

One aspect of the invention provides a light-emitting diode lighting apparatus, which includes: an AC voltage supply supplying AC voltage; a rectification unit rectifying the AC voltage; a light-emitting unit receiving rectified voltage supplied from the rectification unit and performing light emitting operation through at least one light-emitting group; and a constant current unit including current diodes branched from nodes between light-emitting groups of the light-emitting unit and setting a current of each of the light-emitting groups.

Another aspect of the present invention provides a light-emitting diode lighting apparatus, which includes: an AC voltage supply supplying AC voltage; a rectification unit rectifying the AC voltage to supply rectified voltage to a first node; a light-emitting unit is connected to the first node and performing light emitting operation; and a constant current unit branched from each node of the light-emitting unit and setting a current flowing through the light-emitting unit.

According to embodiments of the invention, light emitting diodes of a light-emitting unit can be operated to emit light with uniform brightness through suitable arrangement of the light emitting diodes constituting light-emitting groups.

In addition, a difference in power consumption between distribution resistors can be minimized by a difference in resistance therebetween. As a result, it is possible to prevent damage caused by overcurrent in the constant current unit.

FIG. 1 is a block diagram of a lighting apparatus according to one exemplary embodiment of the invention.

FIG. 2 is a circuit diagram of a lighting apparatus according to another exemplary embodiment of the invention.

FIG. 3 is a directed graph representing a circuit network model of the lighting apparatus shown in FIG. 2 .

FIG. 4 is a circuit diagram of a lighting apparatus according to a further exemplary embodiment of the invention.

FIG. 5 is a directed graph representing a circuit network model of the lighting apparatus shown in FIG. 4 .

FIG. 6 is a voltage-current graph representing operation of the lighting apparatuses shown in FIG. 2 and FIG. 4 .

Exemplary embodiments of the invention will now be described in detail with reference to the accompanying drawings. However, it should be understood that the present invention is not limited thereto and may be embodied in various ways.

As used herein, although the terms “first”, “second”, “third”, and the like may be used to describe various elements, components, regions, layers and/or sections, these elements, components, regions, layers and/or sections are not limited by these terms, and are only used to distinguish one element, component, region, layer or section from another element, component, region, layer or section.

Embodiments

FIG. 1 is a block diagram of a lighting apparatus according to one exemplary embodiment of the invention.

Referring to FIG. 1 , the lighting apparatus according to this exemplary embodiment includes a power voltage unit 10 , a light-emitting unit 300 , and a constant current unit 400 .

The power voltage unit 10 supplies voltage to the light-emitting unit 300 . The voltage may be supplied in the form of ripple voltage through full-wave rectification. Here, any power source may be used as the power voltage unit so long as the power source can supply a suitable level of power for light emitting operation of the light-emitting unit 300 .

In addition, the light-emitting unit 300 receives the voltage supplied from the power voltage unit and performs light emitting operation. The operation of the light-emitting unit 300 is controlled by the constant current unit 400 . Further, the light-emitting unit 300 includes a plurality of light-emitting groups. Each of the light-emitting groups constitutes the light-emitting unit 300 . Each of the light-emitting groups includes at least one light emitting diode.

Corresponding to the light-emitting groups of the light-emitting unit 300 , the constant current unit 400 is provided with current sources. For example, a single light-emitting group may be operated to emit light corresponding to a single power source. That is, the current source determines the amount of current flowing through the light-emitting group.

FIG. 2 is a circuit diagram of a lighting apparatus according to another exemplary embodiment of the invention

Referring to FIG. 2 , the lighting apparatus according to this exemplary embodiment includes a power voltage unit 10 , a light-emitting unit 310 , and a constant current unit 410 .

The power voltage unit 10 includes an AC power supply 100 and a rectification unit 200 .

The AC power supply 100 supplies AC voltage. The AC power supply 100 includes an AC power source 110 , resistors Rin and Rout, and a capacitor C. The AC power source 110 may be a domestic power source having an RMS value of 220V, or another AC power source having a different RMS value. In addition, the resistors Rin and Rout and the capacitor C connected to the AC power source 110 act as low pass filters. For example, when the AC power source 110 supplies AC voltage at a frequency of 60 Hz, harmonics components included in the AC voltage are filtered by the resistors Rin and Rout and the capacitor C. The AC voltage supplied from the AC power supply 100 enters the rectification unit 200 .

The rectification unit 200 rectifies the AC voltage. For example, the rectification unit 200 may have a bridge structure including four diodes D 1 , D 2 , D 3 and D 4 . With the configuration as shown in FIG. 1 , the rectification unit 200 can perform full-wave rectification of the AC voltage supplied from the AC power supply 100 . For example, when the AC voltage is a sine wave having a frequency of 60 Hz, the rectification unit 200 performs full-wave rectification of the AC voltage. As a result, (−) ripple components of the AC voltage are inverted and the rectification unit 200 outputs only (+) ripple components between a first node N 1 and a sixth node N 6 .

The light-emitting unit 310 is electrically connected to the first node N 1 , which is an output terminal of the rectification unit 200 . The light-emitting unit 310 includes a plurality of light-emitting groups LED 1 , LED 2 , LED 3 and LED 4 . Each of the light-emitting groups LED 1 , LED 2 , LED 3 and LED 4 includes at least one light emitting diode or at least one light emitting diode chip. In addition, the light emitting diodes constituting each of the light-emitting groups LED 1 , LED 2 , LED 3 and LED 4 may have different features.

In FIG. 2 , the light-emitting groups LED 1 , LED 2 , LED 3 and LED 4 may be connected to one another in series. In addition, the number of light-emitting groups connected to one another in series may differ according to embodiments.

The constant current unit 410 may be connected between the light-emitting unit 310 and the sixth node N 6 . The constant current unit 410 includes a current source. The current source may be composed of current diodes. The current diodes refer to electronic elements capable of supplying a predetermined amount of current even in the case of variation in voltage applied thereto. Thus, current diodes CRD 1 , CRD 2 , CRD 3 and CRD 4 can stabilize brightness of the light emitting diodes in the light-emitting unit 310 .

Further, the current source may be realized in various forms. For example, any two-terminal elements capable of generating current may be used as the current source.

The constant current unit 410 includes current diodes CRD 1 , CRD 2 , CRD 3 and CRD 4 , which are branched from nodes N 2 , N 3 , N 4 and N 5 between the light-emitting groups LED 1 , LED 2 , LED 3 and LED 4 connected to one another in series. In addition, the constant current unit 410 further includes distribution resistors R 1 , R 2 , R 3 and R 4 connected to the current diodes CRD 1 , CRD 2 , CRD 3 and CRD 4 , respectively. Accordingly, the current diodes CRD 1 , CRD 2 , CRD 3 and CRD 4 and the distribution resistors R 1 , R 2 , R 3 and R 4 are connected to circuit branches extending from nodes between the light-emitting groups LED 1 , LED 2 , LED 3 and LED 4 .

For example, a first current diode CRD 1 and a first distribution resistor R 1 are connected to a second node N 2 , and a second current diode CRD 2 and a second distribution resistor R 2 are connected to a third node N 3 . In addition, a third current diode CRD 3 and a third distribution resistor R 3 are connected to a fourth node N 4 , and a fourth current diode CRD 4 and a fourth distribution resistor R 4 are connected to a fifth node N 5 .

FIG. 3 is a directed graph representing a circuit network model of the lighting apparatus shown in FIG. 2 .

Referring to FIG. 3 , assuming that pull-in current In flows through a branch between the first node N 1 and the second node N 2 . In FIG. 2 , the current Iin flows through a first light-emitting group LED 1 .

In addition, the first current diode CRD 1 is provided to a branch between the second node N 2 and the sixth node N 6 . A current flowing through the first current diode CRD 1 is defined as a first current I 1 . Thus, a current flowing through a branch between the second node N 2 and the third node N 3 becomes Iin-I 1 . That is, the current flowing through a second light-emitting group LED 2 becomes Iin-I 1 .

Further, the second current diode LED 2 is provided to a branch between the third node N 3 and the sixth node N 6 . A current flowing through the second current diode LED 2 is defined as a second current I 2 . Thus, a current flowing through a branch between the third node N 3 and the fourth node N 4 becomes Iin-I 1 -I 2 . Accordingly, the current Iin-I 1 -I 2 flows through a third light-emitting group LED 3 .

The current Iin-I 1 -I 2 flowing through the third light-emitting group LED 3 enters the fourth node N 4 . The third current diode CRD 3 is provided to a branch between the fourth node N 4 and the sixth node N 6 , and a current flowing through the third current diode CRD 3 is defined as a third current I 3 . Accordingly, a current flowing through a fourth light-emitting group LED 4 placed between the fourth node N 4 and the fifth node N 5 becomes Iin-I 1 -I 2 -I 3 . This current is the same as a fourth current I 4 flowing through the fourth current diode CRD 4 provided to a branch between the fifth node N 5 and the sixth node N 6 .

This means that the pull-in current Iin flowing from the first node is I 1 +I 2 +I 3 +I 4 . That is, a current flowing through each of the light-emitting groups connected to one another in series does not exhibit characteristics according to a typical voltage-current curve, and depends on a current value set to a current diode branched from a node between the light-emitting groups connected to each other in series.

That is, the current flowing through each of the light-emitting groups is represented by the following equations.

Current I in flowing through first light-emitting group= I 1+ I 2+ I 3+ I 4  Equation 1

Current I in- I 1 flowing through second light-emitting group= I 2+ I 3+ I 4  Equation 2

Current I in- I 1- I 2 flowing through third light-emitting group= I 3+ I 4  Equation 3

Current I in- I 1- I 2- I 3 flowing through fourth light-emitting group= I 4  Equation 4

According to these equations, when the pull-in current Iin enters the second node N 2 , a current of a current diode provided to the farthest node from the second node N 2 flows through light-emitting groups connected to the farthest node from the second node N 2 . In FIG. 3 , the fourth current I 4 of the fourth current diode CRD 4 commonly flows through all of the light-emitting groups LED 1 , LED 2 , LED 3 and LED 4 . This means that a current of a current diode provided to a branch between light-emitting groups connected to each other in series commonly flows through other light-emitting groups provided to previous branches.

The current values I 1 , I 2 , I 3 and I 4 set to the current diodes to flow through the branches may be selected in various ways.

Preferably, the current value of the current diode branched from the farthest node from the second node N 2 , which the pull-in current Iin enters, is set to the highest value. For example, the current values of the current diodes may be set according to the following equation.

I 4> I 3> I 2> I 1  Equation 5

In this equation, the fourth current I 4 of the fourth current diode CRD 4 has the highest current value. In addition, the first current I 1 has the lowest current value. As a result, a difference in current between the light-emitting groups can be minimized. Further, the current flowing through each of the light-emitting groups LED 1 , LED 2 , LED 3 and LED 4 provided to the branches may be individually set through adjustment of the current of each of the current diodes CRD 1 , CRD 2 , CRD 3 and CRD 4 .

For example, the current flowing through the fourth light-emitting group LED 4 is the fourth current of the fourth current diode CRD 4 , and the current flowing through the third light-emitting group LED 3 is determined by the third current I 3 and the fourth current I 4 . Here, since the fourth current I 4 of the fourth current diode CRD 4 is previously determined, the current flowing through the third light-emitting group LED 3 may be arbitrarily determined by setting the third current I 3 .

This means that a maximum amount of current flowing through each of the light-emitting groups may be controlled by setting the current flowing through the current diodes instead of adjustment of resistance or voltage.

Further, in this embodiment, magnitude of resistance provided to directional branches connected to the sixth node N 6 may be changed in various ways.

For example, distribution resistors provided to respective directional branches may have different resistance values. For example, the resistance values of the distribution resistors may be set to increase with decreasing distance to the first node N 1 . Such a difference in resistance between the distribution resistors may be set after determination of the current values as represented by Equation 5. With such a feature, power consumed by the respective directional branches can be efficiently distributed. The difference in resistance between the distribution resistors may be set according to the following equation.

R 1> R 2> R 3> R 4  Equation 6

In this equation, the current values I 1 , I 2 , I 3 and I 4 flowing through the distribution resistors are determined according to Equation 5. The difference in resistance between the distribution resistors allows efficient distribution of power applied to each of the current diodes. For example, voltage of the second node N 2 is kept higher than that of the third node N 3 . In the case where the distribution resistors are not used, the current diodes are directly connected to the sixth node N 6 . As a result, voltage between both terminals of each of the current diodes CRD 1 and CRD 2 is directly affected by the voltage of each of the second node N 2 and the third node N 3 . Accordingly, the voltage between both terminals of the current diode CRD 1 is higher than the voltage between both terminals of the current diode CDR 2 . This condition is undesirable to drive the current diodes. In addition, power generated by each of the distribution resistors is R*I 2 . That is, power values generated by the distribution resistors are R 1 *I 1 2 , R 2 *I 2 2 , R 3 *I 3 2 and R 4 *I 4 2 , respectively. Considering a relationship of I 1 <I 2 <I 3 <I 4 , it is desirable that the resistance values of the distribution resistors be set such that power generated by each of the distribution resistors is uniformly distributed. To this end, the relationship represented by Equation 6 is used. With this structure, power generated between the nodes through each of the distribution resistors can be efficiently distributed. In addition, this structure minimizes power consumed by the current diodes, thereby preventing damage to the current diodes.

FIG. 4 is a circuit diagram of a lighting apparatus according to a further exemplary embodiment of the invention.

Referring to FIG. 4 , the lighting apparatus according to this embodiment includes a power voltage unit 10 , a light-emitting unit 320 , and a constant current unit 420 .

The power voltage unit 10 includes an AC power supply 100 and a rectification unit 200 .

The AC power supply 100 supplies AC voltage. The AC power supply 100 includes an AC power source 110 , resistors Rin and Rout, and a capacitor C. The structures of the AC power source 110 , the resistors Rin and Rout and the capacitor C are the same as those of the lighting apparatus described with reference to FIG. 2 . Thus, for a description of these components, refer to FIG. 2 . The AC voltage supplied from the AC power supply 100 enters the rectification unit 200 .

The rectification unit 200 rectifies the AC voltage. For example, the rectification unit 200 may have a bridge structure including four diodes D 1 , D 2 , D 3 and D 4 . Operation of the rectification unit 200 is the same as described with reference to FIG. 2 . That is, the rectification unit 200 performs full-wave rectification of the AC voltage supplied from the AC power supply 100 . Output from the rectification unit 200 is delivered to the light-emitting unit 320 through a first node N 1 and an eleventh node N 11 .

The light-emitting unit 320 is electrically connected to the first node N 1 and the eleventh node N 11 , which are output terminals of the rectification unit 200 . The light-emitting unit 320 includes a plurality of light-emitting groups LED 5 , LED 6 , LED 7 and LED 8 . Each of the light-emitting groups LED 5 , LED 6 , LED 7 and LED 8 includes at least one light emitting diode or at least one light emitting diode chip. Accordingly, the light-emitting groups LED 5 , LED 6 , LED 7 and LED 8 are realized through series connection, parallel connection or a combination of series/parallel connection between the light emitting diodes. Further, the light emitting diodes constituting each of the light-emitting groups LED 5 , LED 6 , LED 7 and LED 8 may have different features.

In FIG. 4 , a fifth light-emitting group LED 5 is connected between the first node N 1 and a seventh node N 7 . In addition, a sixth light-emitting group LED 6 is connected between the seventh node N 7 and an eighth node N 8 , a seventh light-emitting group LED 7 is connected between the eighth node N 8 and a ninth node N 9 , and an eighth light-emitting group LED 8 is connected between the ninth node N 9 and a tenth node N 10 . Thus, the light-emitting groups LED 5 , LED 6 , LED 7 and LED 8 are connected to one another in series. In addition, the number of light-emitting groups connected to one another in series may differ according to embodiments.

The constant current unit 420 may be connected to the light-emitting unit 320 . The constant current unit 420 includes a current source. The current source may be composed of current diodes CRD 5 , CRD 6 , CRD 7 and CRD 8 . The current diodes CRD 5 , CRD 6 , CRD 7 and CRD 8 are electronic elements capable of supplying a predetermined amount of current even in the case of variation in voltage applied thereto. Accordingly, current flowing in the light-emitting diode of the light-emitting unit 320 through the current diodes CRD 5 , CRD 6 , CRD 7 and CRD 8 may be determined. This means that brightness of each of the light-emitting group LED 5 , LED 6 , LED 7 and LED 8 may be controlled by a current set to each of the current diodes CRD 5 , CRD 6 , CRD 7 and CRD 8 .

Further, the current source may be realized using two-terminal elements as well as the current diodes. That is, any two-terminal elements capable of generating current may be used as the current source.

The constant current unit 420 includes the current diodes CRD 5 , CRD 6 , CRD 7 and CRD 8 and distribution resistors R 5 , R 6 , R 7 and R 8 .

The current diodes CRD 5 , CRD 6 , CRD 7 and CRD 8 are branched from the nodes N 7 , N 8 , N 9 and N 10 between the light-emitting groups LED 5 , LED 6 , LED 7 and LED 8 , respectively. For example, a fifth current diode CRD 5 is branched from the seventh node N 7 and a sixth current diode CRD 6 is branched from the eighth node N 8 . In addition, a seventh current diode CRD 7 is branched from the ninth node N 9 and an eighth current diode CRD 8 is branched from the tenth node N 10 .

Further, distribution resistors are connected between the current diodes. For example, a fifth distribution resistor R 5 is connected between a twelfth node N 12 and a thirteenth node N 13 , and a sixth distribution resistor R 6 is connected between the thirteenth node N 13 and a fourteenth node N 14 . Further, a seventh distribution resistor R 7 is connected between the fourteenth node N 14 and a fifteenth node N 15 . An eighth distribution resistor R 8 is connected between the fifteenth node N 15 and the eleventh node N 11 .

The aforementioned circuit configuration means that the light-emitting groups, the current diodes and the distribution resistors are arranged to constitute a trapezoidal circuit.

In the trapezoidal circuit, a current flowing through each of the distribution resistors is determined by current values set to the current diodes.

FIG. 5 is a directed graph representing a circuit network model of the lighting apparatus shown in FIG. 4 .

Referring to FIG. 5 , assuming that pull-in current In flows through a branch between the first node N 1 and the seventh node N 7 . In FIG. 4 , the current Iin flows through the fifth light-emitting group LED 5 .

In addition, the fifth current diode CRD 5 is provided to a branch between the seventh node N 7 and the twelfth node N 12 . A current flowing through the fifth current diode CRD 5 is defined as a fifth current I 5 . Thus, a current flowing through a branch between the seventh node N 7 and the eighth node N 8 becomes Iin-I 5 . That is, the current flowing through the sixth light-emitting group LED 6 becomes Iin-I 5 .

In addition, the sixth current diode CRD 6 is provided to a branch between the eighth node N 8 and the thirteenth node N 13 . A current flowing through the sixth current diode CRD 6 is defined as a sixth current I 6 . Thus, a current flowing through a branch between the eighth node N 8 and the ninth node N 9 becomes Iin-I 5 -I 6 . Thus, the current Iin-I 5 -I 6 flows through a seventh light-emitting group LED 7 .

The current Iin-I 5 -I 6 flowing through the seventh light-emitting group LED 7 enters the ninth node N 9 . The seventh current diode CRD 7 is provided to a branch between the ninth node N 9 and the fourteenth node N 14 , and a current flowing therethrough is defined as a seventh current I 7 . Thus, a current flowing through the eighth light-emitting group LED 8 provided to a branch between the ninth node N 9 and the tenth node N 10 is set to Iin-I 5 -I 6 -I 7 .

A current flowing through the eighth light-emitting group LED 8 is the same as an eighth current I 8 that flows through the eighth current diode CRD 8 disposed between the tenth node N 10 and the fifteenth node N 15 .

This means that the pull-in current Iin flowing from the first node N 1 is I 5 +I 6 +I 7 +I 8 . That is, a current flowing through each of the light-emitting groups LED 5 , LED 6 , LED 7 and LED 8 connected to one another in series does not exhibit characteristics according to a typical voltage-current curve, and depends on a current value set to a current diode branched from a node between the light-emitting groups connected in series.

That is, the current flowing through each of the light-emitting groups is represented by the following equations.

Current I in flowing through fifth light-emitting group= I 5+ I 6+ I 7+ I 8  Equation 7

Current I in- I 5 flowing through sixth light-emitting group= I 6+ I 7+ I 8  Equation 8

Current I in- I 5- I 6 flowing through seventh light-emitting group= I 7+ I 8  Equation 9

Current I in- I 5- I 6- I 7 flowing through eighth light-emitting group= I 8  Equation 10

According to these equations, when the pull-in current Iin enters the seventh node N 7 , a current of a current diode provided to the farthest node from the seventh node N 7 flows through light-emitting groups connected to the farthest node from the seventh node N 7 . In FIG. 5 , the eighth current I 8 of the eighth current diode CRD 8 commonly flows through all of the light-emitting groups LED 5 , LED 6 , LED 7 and LED 8 . This means that a current of each of the current diodes CRD 5 , CRD 6 , CRD 7 and CRD 8 provided to each of the branches between the light-emitting group LED 5 , LED 6 , LED 7 and LED 8 connected to one another in series commonly flows through the branches between previous nodes.

The current values I 5 , I 6 , I 7 and I 8 set to the current diodes to flow in the branches may be selected in various ways.

For example, the current value of the current diode branched from the farthest node from the second node N 2 which the pull-in current Iin enters is set to the highest value. Thus, the current values of the current diodes may be set according to the following equation.

I 8> I 7> I 6> I 5  Equation 11

In this equation, the eighth current I 8 set to the eighth current diode CRD 8 has the highest current value. In addition, the fifth current I 5 set to the fifth current diode CRD 5 has the lowest current value.

Further, in this embodiment, magnitude of resistance provided to the directional branches may be changed in various ways.

For example, distribution resistors provided to respective directional branches may have different resistance values. For example, the resistance values of the distribution resistors may be set to increase with decreasing distance to the first node N 1 . Such a feature may be set after determination of the current values as represented by Equation 11. With such a difference in resistance between the distribution resistors, power consumed by the respective directional branches can be efficiently distributed.

In addition, it is desirable that a fifth distribution resistor R 5 connected between the twelfth node N 12 and the thirteenth node N 13 have a higher resistance than a sixth distribution resistor R 6 connected between the thirteenth node N 13 and the fourteenth node N 14 . Further, it is desirable that a seventh distribution resistor R 7 have a lower resistance than the sixth distribution resistor R 6 .

With such a difference in resistance between the distribution resistors, power consumed by the respective current diodes can be efficiently distributed. For example, voltage of the seventh node N 7 is kept higher than that of the eighth node N 8 . In the case where the distribution resistors are not used, the current diodes are directly connected to the eleventh node N 11 . As a result, voltage between both terminals of each of the current diodes CRD 5 and CRD 6 is directly affected by the voltage of each of the seventh node N 7 and the eighth node N 8 . Accordingly, the voltage between both terminals of the current diode CRD 5 is higher than the voltage between both terminals of the current diode CDR 6 . This condition is undesirable to drive the current diodes. Accordingly, it is desirable that the distribution resistors be arranged such that the current diodes have the same or similar voltages between both terminals thereof. To this end, the distribution resistor is set to a higher resistance value with decreasing distance to the seventh node N 7 .

In addition, power generated by the fifth distribution resistor R 5 is R 5 *I 5 2 and power generated by the seventh distribution resistor R 7 is R 7 *(I 5 +I 6 +I 7 ) 2 . This means that power generated by the resistors can be concentrated on the seventh distribution resistor R 7 . Accordingly, the seventh distribution resistor R 7 may be set to a low resistance value to minimize power loss due to an operation of adding the current values. In addition, the fifth distribution resistor R 5 may be set to the highest resistance value to achieve efficient distribution of power generated by each of the resistors. With this structure, power generated between the nodes through each of the distribution resistors can be efficiently distributed. In addition, it is possible to prevent damage to the current diodes due to concentration of power consumption on a certain node caused by concentration of current.

Distribution of the resistance values may be determined according to the following equation.

R 5> R 6> R 7  Equation 12

With the aforementioned operation, a current flowing through each of the light-emitting groups may be determined based only on the current values set to the current diodes. In addition, it is possible to prevent concentration of power on a certain node through suitable distribution of resistance.

FIG. 6 is a voltage-current graph representing operation of the lighting apparatuses shown in FIG. 2 and FIG. 4 .

Referring to FIG. 6 , voltage V N1 is applied to the first node N 1 . Assuming the voltage has a partial sine waveform and is a ripple voltage. In addition, when each of the current diodes is activated, current values set to the current diodes are represented by Equation 11 or 5. Accordingly, FIG. 6 will be described on the assumption of a relationship of I 8 >I 7 >I 6 >I 5 .

In FIG. 4 , as a level of the applied voltage V N1 is increased, the fifth light-emitting group LED 5 is turned on and the current I 5 flows through the fifth current diode CRD 5 . Further, a terminal voltage of the fifth current diode CRD 5 increases with increasing applied voltage V N1 .

Then, as the applied voltage V N1 is further increased, the sixth light-emitting group LED 6 is also turned on. In addition, the sixth current diode CRD 6 also starts to operate. Accordingly, the pull-in current Iin is the sum of the fifth current I 5 and the sixth current I 6 . Further, a terminal voltage V 6 of the sixth current diode CRD 6 also gradually increases. At this time, a constant current flows through the fifth current diode CRD 5 .

Then, as the applied voltage V N1 is further increased, the seventh light-emitting group LED 7 is also turned on. In addition, the seventh current diode CRD 7 also starts to operate. Accordingly, the pull-in current Iin becomes I 5 +I 6 +I 7 , and a terminal voltage V 7 of the seventh current diode CRD 7 also gradually increases with increasing applied voltage.

When the applied voltage V N1 is additionally increased, all of the light-emitting groups are turned on, and the current diode CRD 8 also starts to operate, whereby the eighth current I 8 flows therethrough. Accordingly, the pull-in current Iin becomes I 5 +I 6 +I 7 +I 8 .

As the applied voltage V N1 drops after reaching a peak value, the lighting apparatus operates in a symmetrical way to the case where the applied voltage V N1 is increased. Namely, light emitting operation is sequentially stopped in order of the eighth light-emitting group LED 8 , the seventh light-emitting group LED 7 , the sixth light-emitting group LED 6 and the fifth light-emitting group LED 5 . Likewise, the current values are set corresponding thereto.

The aforementioned operation is equally applied to FIG. 4 . In this embodiment, I 5 corresponds to the first current I 1 , I 6 corresponds to the second current I 2 , I 7 corresponds to the third current I 3 , and I 8 corresponds to the fourth current I 4 . In addition, CRD 5 corresponds to the first current diode CRD 1 , CRD 6 corresponds to the second current diode CRD 2 , CRD 7 corresponds to the third current diode CRD 3 , and CRD 8 corresponds to the fourth current diode CRD 4 . Likewise, the fifth light-emitting group LED 5 corresponds to the first light-emitting group LED 1 , the sixth light-emitting group LED 6 corresponds to the second light-emitting group LED 2 , the seventh light-emitting group LED 7 corresponds to the third light-emitting group LED 3 , and the eighth light-emitting group LED 8 corresponds to the fourth light-emitting group LED 4 .

Through the aforementioned operation, a maximum current flowing through each of the light-emitting groups may be determined by the current values set to the current diodes. In addition, it is possible to prevent concentration of power on a certain node through suitable distribution of resistance.

Although some embodiments have been described herein, it should be understood by those skilled in the art that these embodiments are given by way of illustration only and do not restrict the scope of the present invention, and that various modifications, variations and alterations can be made without departing from the spirit and scope of the present invention.