Patent Publication Number: US-9854633-B1

Title: Light emitting device array and light source device using the same

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims benefit of priority under 35 U.S.C. §119 to Korean Patent Application No. 10-2016-0105737 filed on Aug. 19, 2016, with the Korean Intellectual Property Office, the disclosure of which is incorporated herein by reference in its entirety. 
     BACKGROUND 
     1. Field 
     The present disclosure relates to a light emitting device array and a light source device. 
     2. Description of Related Art 
     Semiconductor light emitting devices (e.g., including light emitting diodes (LEDs)) may emit light using the principle of recombination of electrons and holes when an electric current is applied thereto. Due to various advantages thereof, such as low power consumption, high luminance, miniaturization, and the like, semiconductor light emitting devices (e.g., including semiconductor LEDs) are widely used as light sources of various electronic products, as well as light sources for lighting devices. For example, since the development of nitride-based light emitting devices, the range of use thereof has been further extended, and thus, nitride-based light emitting devices have been employed in light source modules, domestic lighting devices, automotive lighting devices, and the like. In particular, semiconductor light emitting devices are commonly used as light sources for various display devices such as TVs, mobile phones, PCs, notebook computers, personal digital assistants (PDAs), and the like. 
     In addition, as the range of use of semiconductor light emitting devices has been extended, semiconductor light emitting devices have gradually been applied to light source devices having a high level of electric current and power. As semiconductor light emitting devices have been applied to light source devices having a high level of electric current and power, research into methods of improving reliability of semiconductor light emitting device packages has been undertaken. 
     SUMMARY 
     An aspect of the present disclosure may provide a light emitting device array and a light source device, having improved reliability due to a reduction in a forward voltage deviation (ΔVf). 
     According to an aspect of the present disclosure, a light emitting device array may comprise a plurality of light emitting diode (LED) strings connected in parallel with each other, each LED string including a plurality of light emitting devices connected in series, wherein a sum of forward voltages (Vf) of corresponding plurality of light emitting devices included in at least one LED string among the plurality of LED strings is less than a sum of forward voltages of corresponding plurality of light emitting devices included in a different LED string, and the at least one LED string includes a voltage compensation unit configured to compensate for a difference in forward voltage levels between the at least one LED string and the different LED string. 
     According to an aspect of the present disclosure, a light source device may comprise a plurality of LED strings connected in parallel, each LED string including a plurality of light emitting devices connected in series, wherein each of the plurality of LED strings includes an impedance controlling pattern electrically connected to a corresponding plurality of light emitting devices, and is configured such that a level of a forward voltage applied to the corresponding plurality of light emitting devices included in at least one LED string among the plurality of LED strings is lower than a level of a forward voltage applied to the corresponding plurality of light emitting devices included in a different LED string, and the at least one LED string includes an impedance controlling pattern, different from an impedance controlling pattern of the different LED string and connected to the corresponding plurality of light emitting devices in series, and the impedance controlling pattern included in the at least one LED string is configured to compensate for a difference in forward voltage levels applied to the at least one LED string and to the different LED string. 
     According to an aspect of the present disclosure, a light emitting device array may comprise a first light emitting diode (LED) string connected in parallel with a second LED string, each of the first and second LED strings including a plurality of light emitting devices connected in series, a voltage compensation unit connected in series to one of opposing ends of the plurality of light emitting devices included in the first LED string, wherein a sum of forward voltages (Vf) of the plurality of light emitting devices included in first LED string is less than a sum of forward voltages of the plurality of light emitting devices included in the second LED string, and the voltage compensation unit is configured to compensate for a difference in forward voltage levels between the first LED string and the second LED string. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       The above and other aspects, features and other advantages of the present disclosure will be more clearly understood from the following detailed description when taken in conjunction with the accompanying drawings, in which: 
         FIG. 1  is a circuit diagram of a light source device according to an example embodiment of the present inventive concept; 
         FIG. 2  is a top plan view of a first LED string according to an example embodiment; 
         FIG. 3  is a top plan view of a circuit board of the first LED string according to an example embodiment; 
         FIG. 4  is a perspective view of an LED package employable in the first LED string in the exemplary embodiment of  FIG. 3 ; 
         FIG. 5  is a top plan view of a light emitting device array according to an example embodiment; 
         FIG. 6A  is a top view of a voltage compensation unit according to the exemplary embodiment of  FIG. 5 ; 
         FIG. 6B  is a modified example embodiment of the voltage compensation unit in  FIG. 6A ; 
         FIG. 7  is a comparative example embodiment of the light source device in  FIG. 1 ; and 
         FIGS. 8 and 9  are respective graphs illustrating a current value applied to a first LED string, a second LED string, and a third LED string of the light source device according to the exemplary embodiments of  FIGS. 7 and 1 . 
     
    
    
     DETAILED DESCRIPTION 
     Although the figures described herein may be referred to using language such as “one embodiment,” or “certain embodiments,” these figures, and their corresponding descriptions are not intended to be mutually exclusive from other figures or descriptions, unless the context so indicates. Therefore, certain aspects from certain figures may be the same as certain features in other figures, and/or certain figures may be different representations or different portions of a particular exemplary embodiment. 
     It will be understood that, although the terms first, second, third etc. may be used herein to describe various elements, components, regions, layers and/or sections, these elements, components, regions, layers and/or sections should not be limited by these terms. Unless the context indicates otherwise, these terms are only used to distinguish one element, component, region, layer or section from another element, component, region, layer or section, for example as a naming convention. Thus, a first element, component, region, layer or section discussed below in one section of the specification could be termed a second element, component, region, layer or section in another section of the specification or in the claims without departing from the teachings of the present invention. In addition, in certain cases, even if a term is not described using “first,” “second,” etc., in the specification, it may still be referred to as “first” or “second” in a claim in order to distinguish different claimed elements from each other. 
     It will be understood that when an element is referred to as being “connected” or “coupled” to or “on” another element, it can be directly connected or coupled to or on the other element or intervening elements may be present. In contrast, when an element is referred to as being “directly connected” or “directly coupled” to another element, or as “contacting” or “in contact with” another element, there are no intervening elements present. Other words used to describe the relationship between elements should be interpreted in a like fashion (e.g., “between” versus “directly between,” “adjacent” versus “directly adjacent,” etc.). 
     The term “substantially” may be used herein to emphasize this meaning, unless the context or other statements indicate otherwise. For example, items described as “substantially the same,” “substantially equal,” or “substantially planar,” may be exactly the same, equal, or planar, or may be the same, equal, or planar within acceptable variations that may occur, for example, due to manufacturing processes. 
     As is traditional in the field of the inventive concepts, embodiments are described, and illustrated in the drawings, in terms of functional blocks, units and/or modules. Those skilled in the art will appreciate that these blocks, units and/or modules are physically implemented by electronic (or optical) circuits such as logic circuits, discrete components, microprocessors, hard-wired circuits, memory elements, wiring connections, and the like, which may be formed using semiconductor-based fabrication techniques or other manufacturing technologies. In the case of the blocks, units and/or modules being implemented by microprocessors or similar, they may be programmed using software (e.g., microcode) to perform various functions discussed herein and may optionally be driven by firmware and/or software. Alternatively, each block, unit and/or module may be implemented by dedicated hardware, or as a combination of dedicated hardware to perform some functions and a processor (e.g., one or more programmed microprocessors and associated circuitry) to perform other functions. Also, each block, unit and/or module of the embodiments may be physically separated into two or more interacting and discrete blocks, units and/or modules without departing from the scope of the inventive concepts. Further, the blocks, units and/or modules of the embodiments may be physically combined into more complex blocks, units and/or modules without departing from the scope of the inventive concepts. 
       FIG. 1  is a circuit diagram of a light source device according to an example embodiment of the present inventive concept;  FIG. 2  is a top plan view of a first light emitting diode (LED) string according to an example embodiment; and  FIG. 3  is a top plan view of a circuit board of the first LED string according to an example embodiment. 
     As illustrated in  FIG. 1 , a light source device  10 , according to an example embodiment, may include a light emitting device array  200  in which a plurality of LED strings  210 ,  220 , and  230  are connected in parallel, and may include a power supply unit  100  supplying driving power to the light emitting device array  200 . The light emitting device array  200  may also be referred to as an LED module. 
     The light emitting device array  200  may include the plurality of LED strings  210 ,  220 , and  230 , connected in parallel (e.g., between a potential difference), while the plurality of LED strings  210 ,  220 , and  230  may include a plurality of light emitting devices D 1  to D 6 , D 7  to D 12 , and D 13  to D 18 , connected in series, respectively. A number of light emitting devices included in each of the plurality of LED strings  210 ,  220 , and  230  may be equal. The example embodiment illustrates a case in which the light emitting device array  200  may include a first LED string  210 , a second LED string  220 , and a third LED string  230 , while the first LED string  210 , the second LED string  220 , and the third LED string  230  may include six light emitting devices D 1  to D 6 , D 7  to D 12 , and D 13  to D 18 , respectively. However, the case described above is to facilitate a description thereof. A number of light emitting devices configuring LED strings and a number of LED strings are not limited to a specific number, and may be determined by power that the power supply unit  100  may supply to the light emitting device array  200 . For example, the light emitting device array  200  may include more than three or less than three LED strings and each LED string may include more than six or less than six light emitting devices. 
     A portion among the first LED string  210 , the second LED string  220 , and the third LED string  230  may include a voltage compensation unit  211 . The voltage compensation unit  211  may increase a forward voltage Vf of the first LED string  210 , thus reducing a deviation (ΔVf) of forward voltage levels between the first LED string  210  and the two other strings, the second LED string  220  and the third LED string  230 . Therefore, electric currents I 1 , I 2 , and I 3 , applied to the first LED string  210 , the second LED string  220 , and the third LED string  230 , respectively, may be maintained to be substantially uniform, which will be subsequently described. 
     A first LED string  210 , a second LED string  220 , and a third LED string  230  will be described in detail, with reference to  FIGS. 2 and 3 . Since the second LED string  220  and the third LED string  230  are different from the first LED string  210  only in that a voltage compensation unit  211  is omitted, only the first LED string  210  will be described, in order to avoid an overlapping description. 
       FIG. 2  is an enlarged view of a portion of the first LED string  210 , while  FIG. 3  is an enlarged view of a portion of a circuit board  212  in which light emitting devices D 1  to D 6  and the voltage compensation unit  211  are removed from the first LED string  210 . 
     As illustrated in  FIG. 2 , the first LED string  210  may include the circuit board  212 , the voltage compensation unit  211  and a plurality of light emitting devices D 1  to D 6 , mounted on the circuit board  212  and connected in series, a wiring  213  connecting the voltage compensation unit  211  to the plurality of light emitting devices D 1  to D 6 . 
     The circuit board  212  may provide a region in which the voltage compensation unit  211  and the plurality of light emitting devices D 1  to D 6  are mounted, and may be provided as a printed circuit board. The wiring  213  may be provided as a printed circuit of the printed circuit board. With reference to  FIG. 3 , the circuit board  212  may include voltage compensation unit mounting regions  211   a  and  211   b  in which a voltage compensation unit  211  may be mounted, and may include light emitting device mounting regions D 1   a  to D 6   a  in which a light emitting device may be mounted. In this exemplary embodiment, the voltage compensation unit  211  is connected to the first LED string  210  in series. A pair of electrode pads among electrode pads  214  to  219  may be disposed in each of the regions. The wiring  213  may be connected to each of the electrode pads  214  to  219 , thus electrically connecting the voltage compensation unit to a plurality of light emitting devices mounted on the circuit board  212 . The electrode pads  214  to  219  may be chip pads. The various pads of a device described herein may be conductive terminals connected to internal wiring of the device, and may transmit signals and/or supply voltages between an internal wiring and/or internal circuit of the device and an external source. For example, chip pads of a semiconductor chip may electrically connect to and transmit supply voltages and/or signals between an integrated circuit of the semiconductor chip and a device to which the semiconductor chip is connected. The various pads may be provided on or near an external surface of the device and may generally have a planar surface area (often larger than a corresponding surface area of the internal wiring to which they are connected) to promote connection to a further terminal, such as a bump or solder ball, and/or an external wiring. 
     The voltage compensation unit mounting regions  211   a  and  211   b  may be disposed on respective opposing ends of the light emitting device mounting regions D 1   a  to D 6   a . In this exemplary embodiment, the light emitting devices D 1  to D 6  are provided in series in the light emitting device mounting regions D 1   a  to D 6   a , respectively, such that the light emitting device D 1  is connected to the voltage compensation unit  211  mounted in the voltage compensation unit mounting region  211   a  and the light emitting device D 6  is connected to the voltage compensation unit  211  mounted in the voltage compensation unit mounting region  211   b  via the wiring  213 . 
     The voltage compensation unit may be mounted in one of two voltage compensation unit mounting regions  211   a  and  211   b . In a region in which the voltage compensation unit is not mounted, among the voltage compensation unit mounting regions  211   a  and  211   b , a pair of electrode pads may be short-circuited. The example embodiment illustrates that electrode pads  218  and  219 , in a lower portion of the circuit board  212 , are short-circuited. The electrode pads  218  and  219  are illustrated to be short-circuited by a solder S, but the present disclosure is not limited thereto. A pair of electrode pads  218  and  219  may be connected by an electric wire. 
     Electrode pads  215  and  218 , connected by the light emitting device mounting regions D 1   a  to D 6   a  and the wiring  213 , among an electrode pad  214 , an electrode pad  215 , an electrode pad  218 , and an electrode pad  219  of the voltage compensation unit mounting regions  211   a  and  211   b , may be used as a connection terminal measuring a forward voltage of the plurality of light emitting devices D 1  to D 6  that are mounted, before the voltage compensation unit  211  is mounted. 
     Any device that may emit light when an electrical signal is applied thereto may be used as one of the plurality of light emitting devices D 1  to D 6 . The example embodiment illustrates a case in which an LED package is used as an example.  FIG. 4  illustrates an example of an LED package employable as light emitting devices D 1  to D 6 . The LED package may include a package body  1400  having lead frames  1200  and  1300 , as well as an LED chip  1100 . 
     The package body  1400  may include a first lead frame  1200  and a second lead frame  1300 , while the LED chip  1100  may be mounted in a region of the second lead frame  1300 . The LED chip  1100  may be connected the first lead frame  1200  and the second lead frame  1300  by an electric wire  1180 . The package body  1400  may be formed in such a manner that an insulating resin is molded in a region of the first lead frame  1200  and the second lead frame  1300 . A region in which the LED chip  1100  of the package body  1400  is mounted may have a concave circumferential surface inclined inwardly toward the LED chip  1100 . 
     In some embodiments, the LED chip  1100  may be mounted on a surface of the second lead frame  1300 . In addition, any device that may emit light when an electrical signal is applied thereto may be used as the LED chip  1100 . For example, a semiconductor LED chip in which a semiconductor layer is epitaxially grown on a semiconductor growth substrate may be used. 
     A voltage compensation unit  211  may be connected to one of opposing ends of a plurality of light emitting devices D 1  to D 6 , in series, and compensate for a relatively low forward voltage (Vf) of the plurality of light emitting devices D 1  to D 6 , so that forward voltages of LED strings may be adjusted to be uniform. The voltage compensation unit  211  may be provided as at least one of a resistor and a diode, or the like, in order to compensate for a forward voltage deviation (ΔVf) of the LED strings  210 ,  220 , and  230  having a relatively low forward voltage Vf. 
     Various types of resistors having an additional device may be used according to exemplary embodiments. However, an impedance controlling pattern controlling an entirety of a resistance value in such a manner that a wiring having a specific unit resistance value is provided to have a predetermined length and width, and a length and a width of the wiring are controlled may be used. A detailed description thereof will be subsequently provided. 
     In addition, various types of diodes, to which a specific level of a forward voltage is applied, such as a rectifier diode, a zener diode, or the like, may be used according to exemplary embodiments. 
     A level of a forward voltage applied to the voltage compensation unit  211  may be determined in such a manner that respective forward voltages of a first LED string  210 , a second LED string  220 , and a third LED string  230  are measured, and a forward voltage deviation (ΔVf) among the first LED string  210 , the second LED string  220 , and the third LED string  230  is calculated. The example embodiment illustrates an example in which a level of a forward voltage of the first LED string  210  is lower than a level of a forward voltage of the first LED string  220  and the third LED string  230 . For example, a sum of forward voltages (Vf) of the plurality of light emitting devices D 1  to D 6  included in the first LED string  210  is less than a sum of forward voltages of a plurality of light emitting devices D 7  to D 12  included in the second LED string  220  and a sum of forward voltages (Vf) of the plurality of light emitting devices D 1  to D 6  included in the first LED string  210  is less than a sum of forward voltages of a plurality of light emitting devices D 13  to D 18  included in the third LED string  230 . 
     In a case in which the level of a forward voltage applied to the voltage compensation unit  211  is determined, a resistor or a diode having the level of a forward voltage may be selected and mounted in one of the voltage compensation unit mounting regions  211   a  and  211   b  of the first LED string  210 , thus forming the voltage compensation unit  211 . 
     An example in which a voltage compensation unit includes an impedance controlling pattern will be described, with reference to the exemplary embodiments as illustrated in  FIGS. 5 and 6 .  FIG. 5  is a top plan view of a light emitting device array according to an example embodiment;  FIG. 6A  is a top plan view of a voltage compensation unit in  FIG. 5 ; and  FIG. 6B  is a modified example of the voltage compensation unit in  FIG. 6A . 
     A light emitting device array  300 , in an example embodiment, may have a composition similar to that of an example embodiment described above, but may be different in that the voltage compensation unit is provided as impedance controlling patterns  311 ,  321 , and  331 . 
     The light emitting device array  300 , in the example embodiment, may include a first LED string  310 , a second LED string  320 , and a third LED string  330 . In addition, the first LED string  310 , the second LED string  320 , and the third LED string  330  may include a plurality of light emitting devices D 1  to D 6 , D 7  to D 12 , and D 13  to D 18 , respectively. In an example embodiment described above, a resistor or a diode corresponding to a forward voltage applied to the voltage compensation unit may be selected, and may be mounted in a voltage compensation unit mounting region, thus providing the voltage compensation unit. In the example embodiment, the impedance controlling pattern may be formed in a region of a wiring printed on a circuit board, thus providing the voltage compensation unit. For example, the impedance controlling pattern  311  may be mounted in one of two voltage compensation unit mounting regions  312   a  and  312   b , the impedance controlling pattern  321  may be mounted in one of two voltage compensation unit mounting regions  322   a  and  322   b , and the impedance controlling pattern  331  may be mounted in one of two voltage compensation unit mounting regions  332   a  and  332   b . In a region in which the impedance controlling pattern  311  is not mounted, among the voltage compensation unit mounting regions  312   a  and  312   b , a pair of electrode pads  314  and  315  may be short-circuited. This example embodiment illustrates that electrode pads  314  and  315 , in a lower portion of a circuit board where the first LED string  310  is mounted, are short-circuited. The electrode pads  314  and  315  are illustrated to be short-circuited by a solder S, but the present inventive concept is not limited thereto. A pair of electrode pads  314  and  315  may be connected by an electric wire. In a region in which the impedance controlling pattern  321  is not mounted, among the voltage compensation unit mounting regions  322   a  and  322   b , a pair of electrode pads  324  and  325  may be short-circuited. The example embodiment illustrates that electrode pads  324  and  325 , in a lower portion of a circuit board where the first LED string  320  is mounted, are short-circuited. The electrode pads  324  and  325  are illustrated to be short-circuited by a solder S, but the present inventive concept is not limited thereto. A pair of electrode pads  324  and  325  may be connected by an electric wire. In a region in which the impedance controlling pattern  331  is not mounted, among the voltage compensation unit mounting regions  332   a  and  332   b , a pair of electrode pads  334  and  335  may be short-circuited. The example embodiment illustrates that electrode pads  334  and  335 , in a lower portion of a circuit board where the first LED string  330  is mounted, are short-circuited. The electrode pads  334  and  335  are illustrated to be short-circuited by a solder S, but the present inventive concept is not limited thereto. A pair of electrode pads  334  and  335  may be connected by an electric wire. 
     The first LED string  310 , the second LED string  320 , and the third LED string  330  in  FIG. 5  may include impedance controlling patterns  311 ,  321 , and  331 , respectively. In this exemplary embodiment, the light emitting devices D 1  to D 6  are provided in series in the first LED string  310  in a manner such that the light emitting device D 1  is connected to the impedance controlling pattern  311  and the light emitting device D 6  is connected to the electrode pad  314  among the electrode pads  314  and  315  mounted in the voltage compensation unit mounting region  312   b  via the wiring  313 . In this exemplary embodiment, the light emitting devices D 7  to D 12  are provided in series in the second LED string  320  in a manner such that the light emitting device D 7  is connected to the impedance controlling pattern  321  and the light emitting device D 12  is connected to the electrode pad  324  among the electrode pads  324  and  325  mounted in the voltage compensation unit mounting region  322   b  via the wiring  323 . In this exemplary embodiment, the light emitting devices D 13  to D 18  are provided in series in the third LED string  330  in a manner such that the light emitting device D 13  is connected to the impedance controlling pattern  331  and the light emitting device D 18  is connected to the electrode pad  334  among the electrode pads  334  and  335  mounted in the voltage compensation unit mounting region  332   b  via the wiring  333 . 
     In addition, the impedance controlling patterns  311 ,  321 , and  331  may determine a shape of a pattern, thus controlling an impedance value. The example embodiment illustrates a case in which a level of a forward voltage of a plurality of light emitting devices D 1  to D 6  included in the first LED string  310  is lower than that of a forward voltage of a plurality of light emitting devices D 7  to D 12  and light emitting devices D 13  to D 18 , included in the second LED string  320  and the third LED string  330 . In addition, the example embodiment illustrates a case in which only a shape of the impedance controlling pattern  311  of the first LED string  310  is adjusted, as an example. The impedance controlling patterns  311 ,  321 , and  331  may be formed to be integrated with wirings  313 ,  323 , and  333 . 
     With reference to exemplary embodiments as illustrated in  FIGS. 5 and 6A , it can be determined that the impedance controlling pattern  311  of the first LED string  310  has a pattern path longer than those of the impedance controlling patterns  321  and  331  of the first LED string  310  and the second LED string  320 . The pattern path may be provided in such a manner that a quadrangular wiring, such as wirings in the impedance controlling patterns  321  and  331  of the first LED string  310  and the second LED string  320 , is laser trimmed, and grooves  311   b ,  311   c , and  311   d  are formed. As illustrated in  FIG. 6A , in a case in which the impedance controlling pattern  311  is laser trimmed, a length of a path  311   a  may be increased by three times or more, and a width thereof may be reduced to a seventh or less of the width thereof, than before a laser trimming process is performed thereon. Therefore, a resistance value of the impedance controlling pattern  311  may be increased, so that a level of a forward voltage applied to the impedance controlling pattern  311  may be increased. Therefore, the impedance controlling pattern  311  may be laser trimmed, thus controlling a forward voltage applied thereto. 
       FIG. 6B  is a modified example of an impedance controlling pattern, and illustrates a case in which a width of a path  411   a  of an impedance controlling pattern  411  is reduced. The example embodiment illustrates a case in which the path  411   a  of an impedance controlling pattern  411  may be divided into four sub-paths  411   b ,  411   c ,  411   d , and  411   e , in advance, and a portion of the sub-paths  411   b ,  411   c ,  411   d , and  411   e  may be cut using a laser trimming process, thus reducing the width of the path  411   a  and increasing a resistance value of the impedance controlling pattern  411 . The example embodiment illustrates a region  411   f  in which a sub-path  411   e  at the rightmost end, among the four sub-paths  411   b ,  411   c ,  411   d , and  411   e , is cut and separated. The impedance controlling pattern  411  may be formed to be integrated with a wiring  413 . 
     The impedance controlling pattern may provide the voltage compensation unit by forming a wiring pattern in a region of a printed wiring, using the laser trimming process without a need to mount a separate device. Therefore, the impedance controlling pattern may be used in a case in which the space of a circuit board is narrow. 
     Referring to  FIG. 1 , a power supply unit  100  may rectify alternating current (AC) power applied by a separate power supply unit, and may supply the rectified AC power to the light emitting device array  200  as driving power supply. An AC/DC converter, used to convert a converted direct current (DC) voltage into an electric current appropriate for driving the light emitting device array  200 , may be used as the power supply unit  100 . For example, in a case in which a level of a voltage of an external power supply is higher than a driving voltage of a light emitting device, a buck converter may be used. In a case in which the level of a voltage of the external power supply is lower than the driving voltage of the light emitting device, a boost converter may be used. In the example embodiment, the boost converter may be used. 
     The power supply unit  100  may supply substantially the same level of electric currents to a first LED string  210 , a second LED string  220 , and a third LED string  230 . 
     The light emitting device array  200  having a composition described above may improve reliability of the light source device  10  in such a manner that an electric current applied to respective LED strings is uniform. Even in the case of light emitting devices in the same rank, a forward voltage deviation (ΔVf) among the light emitting devices may occur by a nominal error in a manufacturing process. Therefore, in the case of an LED string connecting a plurality of light emitting devices in series, a forward voltage deviation may occur. In a case in which a plurality of LED strings are connected in parallel, and power is applied thereto, a phenomenon of non-uniform distribution of an electric current, in which a relatively high level of an electric current is applied to an LED string having a relatively low forward voltage value, may occur. 
     In a case in which the phenomenon of non-uniform distribution of an electric current occurs in a portion of LED strings among light emitting device arrays, an excessive electric current may be applied to an LED string, and power exceeding rated power is consumed. Therefore, a temperature in the LED string may be increased to a high degree. Consequently, physical alteration, such as damage to the light emitting device of the LED string or a gap between the circuit board and the wiring of the circuit board including the light emitting device mounted thereon, may occur. The physical alteration may cause a problem in which a lifespan of the light emitting device array or the light source device may be reduced. 
     Therefore, in order to prevent the phenomenon of non-uniform distribution of an electric current from occurring, it is desirable to reduce the forward voltage deviation applied to respective LED strings. In the example embodiment, a forward voltage of respective LED strings may be measured, the forward voltage deviation among the LED strings may be calculated, and the voltage compensation unit, compensating for the forward voltage deviation, may be disposed in the LED strings having a relatively low forward voltage, thus reducing the forward voltage deviation among the LED strings. 
     An effect of a reduction in a forward voltage deviation among a plurality of LED strings, in an example embodiment, will be described, with reference to  FIGS. 7 to 9 . 
       FIG. 7  is a comparative example of the light source device in  FIG. 1 , while  FIGS. 8 and 9  are graphs illustrating a current value applied to respective LED strings of the light source device in  FIGS. 7 and 1 . A light source device  20  in  FIG. 7  may comprise a light emitting device array  600  including a plurality of LED strings  610 ,  620 , and  630  connected to each other in parallel, as well as a power supply unit  500 . In addition, the plurality of LED strings  610 ,  620 , and  630  may include a plurality of light emitting devices D 1  to D 6 , D 7  to D 12 , and D 13  to D 18 , connected in series, respectively, but a voltage compensation unit may be omitted. Graphs of  FIGS. 8 and 9  illustrate results produced under a condition in which a current input to a power supply unit  100  or the power supply unit  500  is 1.5 A, a level of a forward voltage of an LED string  210  is lower than that of respective different LED strings  220  and  230 , and a level of a forward voltage of an LED string  610  is lower than that of respective different LED strings  620  and  630 , by 0.1 to 0.15V. 
     In the case of the light source device  20  in  FIG. 7 , in which a voltage compensation unit is omitted, it has been determined that an electric current I 4  of 575 mA is applied to the LED string  610  having a relatively low forward voltage, while an electric current I 5  of 462 mA and an electric current I 6  of 462 mA are applied to the different LED strings  620  and  630 , respectively, as illustrated in  FIG. 8 , and thus a current deviation of 113 mA may occur. Therefore, it can be determined that an electric current having a level about 20% higher than that of an electric current of respective different LED strings  620  and  630  is applied to the LED string  610 , which has a relatively low forward voltage. 
     In the case of a light source device  10  in  FIG. 1 , in which a voltage compensation unit is adopted, it has been determined that an electric current I 1  of 501 mA is applied to the LED string  210  having a relatively low forward voltage, while an electric current I 2  of 499 mA and an electric current I 3  of 499 mA are applied to the different LED strings  220  and  230 , respectively, as illustrated in  FIG. 9 , and thus a current deviation of 2 mA may occur. Therefore, compared to a case described above, in which the voltage compensation unit is omitted, a current deviation may be decreased by 111 mA, thus reducing a phenomenon of non-uniform distribution of an electric current. 
     As set forth above, according to example embodiments of the present disclosure, a light emitting device array in which a forward voltage deviation among a plurality of LED strings is reduced, thus reducing a phenomenon of non-uniform distribution of an electric current, and a light source device using the same may be provided. 
     While example embodiments have been shown and described above, it will be apparent to those skilled in the art that modifications and variations could be made without departing from the scope of the present inventive concept as defined by the appended claims.