Abstract:
A full-color light emitting device includes four leads and three light emitting diode chips which have different light emission wavelengths and can be individually controlled to realize emission of light beams of more diverse colors. The device has a simplified connection structure, so that it can be implemented even when a limited bonding area is provided.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a full-color light emitting device capable of emitting light beams of three or more colors in which three light emitting diode chips thereof having different light emission wavelengths are individually adjusted in light emission density, so that it can emit light beams of three or more colors, and more particularly to a full-color light emitting device with four leads in which three light emitting diode chips thereof having different light emission wavelengths can be individually controlled to realize emission of light beams of more diverse colors, while having a simplified connection structure, so that the light emitting device can be implemented even in the case in which a limited bonding area is provided. 
     2. Description of the Related Art 
     Light emitting devices, which use semiconductor light emitting elements, are configured by arranging, for example, a plurality of light emitting diodes (LEDs) as light emitting semiconductor elements on a panel. In such a case, each LED emits red, green or blue light in accordance with the kind of its compound semiconductor. 
     In the case of a light emitting device adapted to emit monochrome light, using LEDs as semiconductor light emitting elements, each LED constitutes one pixel. In the case of a light emitting device adapted to emit full-color light composed of the three primary colors, that is, red (R), green (G), and blue (B), using LEDs, each light emitting element thereof consists of three LEDs of red, green, and blue, that is, the three primary colors. In the latter case, each full-color light emitting element constitutes one pixel. 
       FIG. 1  illustrates a conventional full-color light emitting device for one light emitting element thereof. As shown in  FIG. 1 , first through third LEDs  14  to  16  are mounted on a main lead  11 . The first LED  14  has a first electrode electrically connected to the main lead  11  in accordance with a die-bonding method, and a second electrode electrically connected to a first sub-lead  12  at its second electrode in accordance with a wire-bonding method. The second LED  15  has first and second electrodes respectively electrically connected to the first sub-lead  12  and a second sub-lead  13  in accordance with a wire-bonding method. The third LED  16  has a first electrode electrically connected to the main lead  11  in accordance with a die-bonding method, and a second electrode electrically connected to the second sub-lead  13  in accordance with a wire-bonding method. The first and second electrodes of each LED may be an anode and a cathode, or vice versa, respectively. 
     In each of the first and third LEDs  14  and  16 , one of its anode and cathode is arranged on the upper surface of its chip, whereas the remaining electrode is arranged on the lower surface of the chip. The electrode arranged on the lower chip surface is electrically connected to the main lead  11  in accordance with a die-bonding method, whereas the electrode arranged on the upper chip surface is electrically connected to an associated one of the first and second sub leads  12  and  13  in accordance with a wire-bonding method. On the other hand, in the case of the second LED  15 , both electrodes thereof are arranged on the upper surface of its chip. In this case, the chip of the second LED  15  is mounted on the main lead  11  such that its lower surface is in contact with the main lead  11  via an insulating substrate. In this state, the electrodes on the upper chip surface are electrically connected to the first and second sub-leads  12  and  13  in accordance with a wire-bonding method, respectively. 
     The first LED  14  is an red (R) LED, the second LED  15  is a green (G) LED, and the third LED  16  is a blue (B) LED. 
       FIG. 2  illustrates an equivalent circuit of the full-color light emitting device implemented as shown in  FIG. 1 . 
     Now, operation of the full-color light emitting device will be described with reference to the equivalent circuit of  FIG. 2 . The color of light emitted from the light emitting device can be adjusted by controlling respective voltages applied to the three leads  11  to  13 , thereby controlling respective operations of the first through third LEDs  14  to  16 . 
     For instance, when a “+” voltage is applied to the main lead  11 , and a “−” voltage is applied to the first sub-lead  12 , the first LED  14  is activated. When the “+” voltage is applied to the main lead  11 , and the “−” voltage is applied to the second sub-lead  12 , the third LED  16  is activated. On the other hand, when the “+” voltage is applied to the first sub-lead  12 , and the “−” voltage is applied to the second sub-lead  13 , the second LED  15  is activated. When each LED of the full-color light emitting device is activated, it serves as a red, green or blue light source. 
     However, the above mentioned conventional full-color light emitting device has a high possibility of error generation because its operation condition is determined in accordance with the polarity of the control voltage applied to each of the first and second sub-lead  12  and  13 . 
     Furthermore, the conventional full-color light emitting device has a complex electrical circuit configuration for implementation of full-color light emission, as shown in  FIG. 2 , because it uses a small number of lead frames. 
     In the above mentioned structure, there is also a problem in that it is difficult to configure a desired circuit where each of the LEDs  14  and  16 , to which a die-bonding technique is to be applied, only has two or more wire bonding pads due to the substrate material of its chip. 
       FIG. 3  is a plan view illustrating another conventional full-color light emitting device with four leads.  FIG. 4  illustrates an equivalent circuit of the full-color light emitting device shown in  FIG. 3 . 
     The full-color light emitting device shown in  FIGS. 3 and 4  includes an R LED  35 , a G LED  36 , and a B LED  37  which have different light emission wavelengths, respectively. The R, G, and B LEDs  35  to  37  are bonded to a main lead frame  31  by means of an adhesive. The LEDs  35  to  37  are electrically connected to first through third sub-lead frames  32  to  34  for supply of electric power, respectively, while being electrically connected to the main lead frame  31  as a common electrode. 
     The electrical connection of each LED is achieved in accordance with a die-bonding method and a wire-bonding method using electrical connecting members (for example, conductive wires). 
     In this full-color light emitting device, as shown in  FIG. 4 , each of the three LEDs  35  to  37  is connected to the main lead frame  31  at one electrode thereof (anode) while being connected to an associated one of the first through third sub-lead frames  32  to  34  at the other electrode thereof (cathode). Each of the first through third LEDs  35  to  37  is turned on/off when the control voltage to be applied to an associated one of the first through third sub-lead frames  32  to  34  is switched on/off. Light beams emitted from the LEDs  35  to  37  in their ON state are mixed so that light of full color including red, green and blue, and mixed colors thereof is generated. 
     This conventional 4-lead full-color light emitting device can have a simple circuit configuration, as shown in  FIG. 4 . However, the main lead frame  31  must have a substantial area because the first through third LEDs  31  to  33  are commonly connected to the main lead frame  31  at their one-side ends. For this reason, where the full-color light emitting device has a limited size, an insufficient bonding area may be provided. 
     In other words, the above mentioned conventional 4-lead full-color light emitting device has a problem in that it cannot implement a full-color light emitting device including a main lead frame having a limited area. 
     SUMMARY OF THE INVENTION 
     The present invention has been made in view of the above mentioned problems involved with the related art, and an object of the invention is to provide a full-color light emitting device with four leads in which three light emitting diode chips thereof having different light emission wavelengths can be individually controlled to realize emission of light beams of more diverse colors, while having a simplified connection structure, so that the light emitting device can be implemented even in the case in which a limited bonding area is provided. 
     In accordance with one aspect, the present invention provides a 4-lead full-color light emitting device comprising: first through third sub-lead frames respectively having first through third leads each made of a conductive material, and wire bonding pads each formed at one end of an associated one of the first through third leads; a main lead frame having a fourth lead made of a conductive material, and a reflecting cup formed at one end of the fourth lead while having a side wall and a bottom surface, the reflecting cup being formed with a reflecting surface at an inner surface of the side wall while having, at the bottom surface, an insulating portion, and a non-insulating portion electrically connected to the fourth lead; and first through third light emitting diodes (LEDs) of different light emitting wavelengths mounted on the bottom surface of the reflecting cup in the main lead frame, each of the LEDs having first and second electrodes of different characteristics; wherein the first electrode of the first LED and the first electrode of the second LED are commonly electrically connected to the first lead of the first sub-lead frame; wherein the second electrode of the second LED and the first electrode of the third LED are commonly electrically connected to the second lead of the second sub-lead frame; wherein the second electrode of the first LED is electrically connected to the fourth lead of the main lead frame; and wherein the second electrode of the third LED is electrically connected to the third lead of the third sub-lead frame. 
     The electrical connection of the second electrode of the first LED to the third lead of the main lead frame may be achieved by die-bonding the second electrode of the first LED to the bottom surface of the reflecting cup in the main lead frame, using a conductive bonding agent. 
     The electrode-to-lead electrical connection of the first through third LEDs to the first through third sub-lead frames may be achieved in accordance with a wire-bonding method. 
     The mounting of the second and third LEDs to the main lead frame may be achieved by die-bonding the second and third LEDs to the bottom surface of the reflecting cup in the main lead frame, using a non-conductive bonding agent. 
     Preferably, the bottom surface of the reflecting cup in the main lead frame has a circular or oval shape. 
     In accordance with another aspect, the present invention provides a 4-lead full-color light emitting device comprising: first through light emitting diodes (LEDs) of different light emission wavelengths, each of the LEDs having first and second electrodes; a first lead connected to respective first electrodes of the first and second LEDs, and adapted to apply a first control voltage to the first electrodes of the first and second LEDs; a second lead connected to both the second electrode of the second LED and the first electrode of the third LED, and adapted to apply a second control voltage to the second electrode of the second LED and the first electrode of the third LED; a third lead connected to the second electrode of the third LED, and adapted to apply a third control voltage to the second electrode of the third LED; and a fourth lead connected to the second electrode of the first LED, and adapted to apply a fourth control voltage to the second electrode of the first LED. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above objects, and other features and advantages of the present invention will become more apparent after reading the following detailed description when taken in conjunction with the drawings, in which: 
         FIG. 1  is a plan view schematically illustrating a conventional 3-lead full-color light emitting device; 
         FIG. 2  is an equivalent circuit diagram of the 3-lead full-color light emitting device shown in  FIG. 1 ; 
         FIG. 3  is a plan view schematically illustrating a conventional 4-lead full-color light emitting device; 
         FIG. 4  is an equivalent circuit diagram of the 4-lead full-color light emitting device shown in  FIG. 3 ; 
         FIG. 5  is a plan view schematically illustrating a full-color light emitting device according to the present invention; 
         FIG. 6  is a sectional view schematically illustrating the full-color light emitting device according to the present invention; and 
         FIG. 7  is an equivalent circuit diagram of the full-color light emitting device according to the present invention. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Now, preferred embodiments of the present invention relating to a 4-lead full-color light emitting device will be described. 
       FIG. 5  is a plan view illustrating a 4-lead full-color light emitting device according to the present invention. As shown in  FIG. 5 , the full-color light emitting device designated by the reference numeral includes first through third sub-lead frames  52  to  54  respectively provided with first through third leads, a main lead frame  51  provided with a fourth lead made of a conductive material, and a reflecting cup having a side wall and a bottom surface, and first through third LEDs  55  to  57  of different light emission wavelengths mounted on the bottom surface of the reflecting cup in the main lead frame  51 . The reflecting cup of the main lead frame  51  is formed with a reflecting surface at the inner surface of its side wall while having, at its bottom surface, an insulating portion, and a non-insulating portion electrically connected to the fourth lead. Each of the LEDs  55  to  57  has two electrodes having different characteristics, that is, first and second electrodes. The first electrode of the first LED  55  and the first electrode of the second LED  56  are commonly electrically connected to the first lead of the first sub-lead frame  52 . The second electrode of the second LED  56  and the first electrode of the third LED  57  are commonly electrically connected to the second lead of the second sub-lead frame  53 . The second electrode of the first LED  55  is electrically connected to the fourth lead of the main lead frame  51 . The third LED  57  is electrically connected at its second electrode to the third lead of the third sub-lead frame  54 . 
     The electrode-to-lead electrical connection of the first through third LEDs  55  to  57  to the first through third sub-lead frames  52  to  54  is achieved by bonding an electrical connecting member, for example, a conductive wire, to bonding pads respectively provided at each electrode and each lead, to be connected to each other, in accordance with a wire-bonding technique. 
     The electrical connection of the first LED  55  to the main lead frame  51  at its second electrode is achieved in accordance with a die-bonding technique. Although the second electrode of the first LED  55  has a structure for implementing a die-bonding process for its electrical connection, it may have a structure capable of implementing a wire-bonding process for its electrical connection. 
     In the full-color light emitting device having the above described configuration, its bonding to the main lead frame  51 , on which the LEDs  55  to  57  are mounted, is achieved only at one site. Accordingly, although two or more electrodes for wire-bonding connection are provided by the first through third LEDs  55  to  57 , the main lead frame  51  may have a bonding area for only one bonding site. 
     Where the first LED  55  has a die-bonding structure at one electrode thereof, for example, the second electrode thereof, this second electrode is bonded to the non-insulating portion of the main lead frame  55  by use of a fluxing agent or adhesive. In this case, the main lead frame  55  may have only an area for mounting the three LEDs  55  to  57  thereon. 
     The main lead frame  51  has an oval reflecting cup structure formed with a reflecting surface at its inner surface. As described above, the first through third LEDs  55  to  57  are mounted on the bottom surface of the reflecting cup. The first LED  55  is directly bonded to the lead frame  51  at its second electrode formed on its lower surface, using a conductive material such as Ag. On the other hand, each of the second and third LEDs  56  and  57  is bonded to the lead frame  51  at its surface opposite to its surface formed with its first and second electrodes, that is, at its lower surface, using a non-conductive material such as epoxy. The reflecting cup of the main lead frame  51  may have a round structure. 
     The first LED  55  is bonded to the first lead of the first sub-lead frame  52  at its first electrode formed on its upper surface by bonding opposite ends of an Au wire to bond pads respectively formed on the first electrode of the first LED  55  and the first lead. Similarly, each of the second and third LEDs  56  and  57  is bonded to the lead of an associated one of the second and third sub-lead frames  53  and  54  at each of its first and second electrodes by bonding opposite ends of an Au wire to bond pads respectively formed on the electrode of the LED and the associated lead. 
     The first through third LEDs  55  to  57  have red, green, and blue light emission wavelengths, respectively. 
       FIG. 6  is a sectional view illustrating the full-color light emitting device according to the present invention. Referring to  FIG. 6 , respective cross-sectional shapes of the main lead frame  51  and first through third sub-lead frames  52  to  54  are shown. 
     The sub-lead frames  52  to  54  have lead portions E 1  to E 3  made of a conductive material while extending vertically to have a certain vertical length, respectively. Respective lead portions E 1  to E 3  of the sub-lead frames  52  to  54  correspond to the above described first through third leads. Accordingly, these lead portions E 1  to E 3  will be referred to as the first through third leads, respectively. Each of the first through third leads E 1  to E 3  has a certain area at its upper end to provide a wire-bonding area. Each of the first through third leads E 1  to E 3  is mounted to a printed circuit board at its lower end. 
     The main lead frame  51  has a lead portion E 4  made of a conductive material while extending vertically to have a certain vertical length. The lead portion E 4  of the main lead frame  51  corresponds to the above described fourth lead. Accordingly, this lead portion E 4  will be referred to as the fourth lead. The main lead frame  51  also has a cup-shaped mounting portion  511  formed at the upper end of the fourth lead E 4  and adapted to carry a chip thereon. The cup-shaped mounting portion  511  corresponds to the above described reflecting cup. Accordingly, the cup-shaped mounting portion  511  will be referred to as the reflecting cup. 
     The reflecting cup  511  has an oval or circular shape at its bottom surface. A reflecting material is coated over the inner surface of the side wall of the reflecting cup  511  so as to form a reflecting surface. The first through third LEDs  55  to  57  are mounted on the bottom surface of the reflecting cup  511  within the reflecting cup  511 . Light emitted from each of the first through third LEDs  55  to  57  is reflected by the reflecting surface of the reflecting cup  511 , and then upwardly advanced. 
     Such structures of the main lead frame  51  and first through third sub-lead frames  52  to  54  are well known as the lead frame structures of a general full-color light emitting device (for example, a lamp). The full-color light emitting device of the present invention is not limited to the structure of  FIG. 6 . 
     The electrical circuit configuration of the full-color light emitting device implemented as described above in accordance with the present invention can be represented by an equivalent circuit shown in  FIG. 7 . 
     Referring to  FIG. 7 , the circuit configuration of the full-color light emitting device includes the first through third LEDs  55  to  57  having different light emission wavelengths, and the first through fourth leads E 1  to E 4 . Respective anodes of the first and second LEDs  55  and  56  are commonly connected to the first lead E 1 , whereas respective cathodes of the second and third LEDs  56  and  57  are commonly connected to the second lead E 2 . The anode of the third LED  57  alone is connected to the third lead E 3 , whereas the cathode of the first LED  55  alone is connected to the fourth lead E 4 . 
     In this circuit configuration, emission of light of full color including three primary colors, that is, red, green, blue, and mixtures of at least two of the three primary colors can be achieved by applying a “+” voltage to one or both of the first and third leads E 1  and E 3  while applying a “−” voltage to one or both of the second and fourth leads E 2  and E 4 . 
     The level of the control voltage applied to each of the first through fourth leads E 1  to E 4  is set to an appropriate operating level in accordance with the kind of the first through third LEDs  55  to  57 . 
     The first LED  55  is a red (R) LED, the second LED  56  is a green (G) LED, and the third LED  57  is a blue (B) LED. 
     The operation of the above described circuit configuration will now be described in more detail. When 0V is applied to the first lead E 1 , and −1.9V is applied to the fourth lead E 4 , the first LED  55  is activated, thereby emitting light of a first wavelength (red). On the other hand, when 0V is applied to the first lead E 1 , and −3.0V is applied to the second lead E 2 , the second LED  56  is activated, thereby emitting light of a second wavelength (green). Also, when 0V is applied to the first lead E 1 , and −3V and −1.9V are applied to the second and fourth leads E 2  and E 4 , respectively, the first and second LEDs  55  and  56  are activated, thereby emitting light of a mixed color of first and second wavelengths (that is, a mixed color of red and green), that is, yellow. 
     When 0V is applied to the third lead E 3 , and −3.0V is applied to the second lead E 2 , the third LED  57  is activated, thereby emitting light of a third wavelength (blue). 
     On the other hand, when 0V is applied to both the first lead E 1  and the third lead E 3 , and −3.0V is applied to the second lead E 2 , the second and third LEDs  56  and  57  are activated, thereby emitting light of a mixed color of second and third wavelengths (that is, a mixed color of green and blue), that is, cyan. 
     Also, when 0V is applied to both the first lead E 1  and the third lead E 3 , and −3.0V and −1.9V are applied to the second and fourth leads E 2  and E 4 , respectively, all the first through third LEDs  55  to  57  are activated, thereby emitting light of a second wavelength (green). Also, when 0V is applied to the first lead E 1 , and −3V and −1.9V are applied to the second and fourth leads E 2  and E 4 , respectively, the first and second LEDs  55  and  56  are activated, thereby emitting light of a mixed color of red, green and blue, that is, white. 
     Thus, the full-color light emitting device according to the present invention can emit light of full color by adjusting respective control voltages of the first through fourth leads E 1  to E 4 . 
     As apparent from the above description, the present invention provides a full-color light emitting device which includes four leads in order to independently control three LEDs of different light emission wavelengths, while reducing the number of bonding sites on a main lead frame, on which the three LEDs are mounted, to one, thereby being capable of implementing a 4-lead full-color light emitting device even where the main lead frame has a limited bonding space. 
     Although the preferred embodiments of the invention have been disclosed for illustrative purposes, those skilled in the art will appreciate that various modifications, additions and substitutions are possible, without departing from the scope and spirit of the invention as disclosed in the accompanying claims.