Backlight unit and liquid crystal display device having the same for improving image quality

Provided is a backlight unit with an extending portion that extends from one of a plurality of sheets included in an optical sheet. The optical sheet is on a light guide plate, and a light emitting diode chip is disposed at a side of the light guide plate. A portion of a flexible printed circuit board, to which the light emitting diode chip is mounted, overlaps a portion of the light guide plate. The extending portion overlaps the portion of the flexible printed circuit board. Thus, the occurrence of lines of light due to light progressing between the light guide plate and the flexible printed circuit board can be prevented, thereby obtaining uniform brightness and improving image quality.

This nonprovisional application claims priority under 35 U.S.C. §119(a) on Patent Application No. 064396/2007 filed in Republic of Korea on Jun. 28, 2007, the entire contents of which are hereby incorporated by reference.

BACKGROUND

1. Field of the Invention

The invention relates to a backlight unit, and more particularly, to a backlight unit capable of improving image quality and a liquid crystal display (LCD) device having the backlight unit.

2. Related Art

Cathode ray tubes (CRT) are widely used for televisions and monitors for measurement devices, information terminals, etc. However, due to the innate weight and size of CRTs, it is difficult to aggressively deal with the miniaturization and weight reduction of electronic devices.

In contrast, LCD devices have the advantages of lightweight, low consumption power, full color, high resolution, and large scale compared with CRTs. Thus, LCD devices are used in a wide range of fields.

Since LCD devices are light-receiving type device that adjust the amount of light supplied from an external source to display an image, LCD devices require a backlight unit to emit light. According to the positions from which light is emitted, backlight units are largely divided into edge-type and top-bottom-type units. A backlight unit may include a light source that emits light. Types of the light source includes an electro luminescence (EL), light emitting diode (LED), cold cathode fluorescent lamp (CCFL), external electrode fluorescent lamp (EEFL), and hot cathode fluorescent lamp (HCFL).

An LED backlight unit that has an almost limitless service life, high brightness, and capability of being miniaturized is being developed. This LED backlight unit is applicable in everything from the smallest display devices to large-scale display devices.

FIG. 4is a perspective view of an LCD device according to the related art. Referring toFIG. 4, a reflective plate116and the light guide plate115are held in the molded frame117, the LED chip121is disposed so that its top surface faces the bottom of the holding space117aat one side of the molded frame117, and the LED chip121is mounted to a flexible PCB118. The flexible PCB118is disposed between a protruded portion115aand the molded frame117. That is, a first portion118aof the flexible PCB118is disposed on the molded frame117, and a second portion118bof the flexible PCB118is disposed over the protruded portion115aof the light guide plate115. The protruded portion has a slanted and protruded shape. However, as the flexible PCB118is not attached to the light guide plate115, a gap is generated between the flexible PCB118and the protruded portion115aof the light guide plate115.

An optical sheet114is disposed on the light diffusion plate115. The optical sheet114may include a diffusion sheet114aand first and second prism sheets114band114c.

An LCD panel10including a color filter substrate100aand an array substrate100bmay be disposed on the light guide plate115. An upper polarizing plate130aand a lower polarizing plate130bare attached respectively to the top and bottom surfaces of the LCD panel100.

A backlight unit includes the flexible PCB118on which an LED chip121is mounted, the light guide plate115, a reflective plate116, and the optical sheet114.

A light-shielding tape112maybe interposed between the light guide plate115and the LCD panel110to block light leaking upward from the edges of the light guide plate115. However, in a related art LED backlight unit, there is a gap between the PCB and the light guide plate, so that the red, green, and blue light is progressed parallel to the light guide plate through the gap. In this case, since the emitted directions of each light have high brightness compared to other directions, light is radiated in lines. Since light emitted in lines cannot obtain uniform brightness, image quality deteriorates.

SUMMARY OF THE INVENTION

Accordingly, embodiments of the invention is directed to a backlight unit and a liquid crystal display device having the same that substantially obviates one or more of the problems due to limitations and disadvantages of the related art.

Embodiments provide a backlight unit capable of preventing light emitted in lines by expanding a portion of an optical sheet and overlapping the portion of the optical sheet with a PCB, thereby obtaining brightness uniformity and improving image quality, and LCD devices having the backlight unit.

In a first embodiment of the invention, a backlight unit includes: a light guide plate; a light source at a side of the light guide plate; a flexible printed circuit board (PCB) in which the light source is mounted, and a portion of which overlapping a portion of the light guide plate; an optical sheet on the light guide plate and including a plurality of sheets; and an extending portion extending from the optical sheet, wherein the extending portion overlaps the portion of the flexible printed circuit board.

In a second embodiment of the invention, a liquid crystal display device includes: a liquid crystal display panel; and a backlight unit including a light guide plate emitting surface light on the liquid crystal display panel, a light source at a side of the light guide plate and emitting light toward the light guide plate, a flexible printed circuit board in which the light source is mounted, and a portion of which overlapping a portion of the light guide plate, an optical sheet on the light guide plate and including a plurality of sheets, and an extending portion extending from the optical sheet, wherein the extending portion overlaps the portion of the flexible printed circuit board.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Reference will now be made in detail to the embodiments of the invention, examples of which are illustrated in the accompanying drawings.

FIG. 1is an exploded perspective view of an LCD device according to an of the invention.

Referring toFIG. 1, an LCD device includes an LCD panel10that displays an image, and a backlight unit20disposed on the rear side of the LCD panel10to provide light. The LCD device may also include a molded frame17that holds and secures the LCD panel10and the backlight unit20, and a light-shielding tape12disposed between the LCD panel10and the backlight unit20.

The LCD panel10includes a color filter substrate10a, an array substrate10b, and a liquid crystal layer (not shown) interposed between the color filter substrate10aand the array substrate10b. The color filter substrate10ahas a plurality of red color filters, green color filters, and blue color filters, and the array substrate10bhas a plurality of thin film transistors (TFT) and pixel electrodes.

The pad region of the LCD panel10includes a circuit substrate11that supplies a data signal and a driving signal, and a driver13for driving the LCD panel10.

The circuit substrate11is a flexible insulating film on which various driven devices are attached, and may be a heat resistant plastic film such as a polyester (PET) or a polyimide (PI).

The backlight unit20includes a flexible PCB18, an LED chip21, a light guide plate15, an optical sheet14, and a reflective plate16, the LED chip21functions as a light source, and the light source is not limit to the light emitting diode chip, and other types of lamps, for example an external electrode fluorescent lamp (EEFL) or a cold cathode fluorescent lamp (CCFL) can also be used as the light source.

The flexible PCB18is a flexible insulating film, and may be made of a plastic film that is a heat resistant plastic film such as a polyester (PET) or a polyimide (PI). The flexible PCB18also has conductive patterns.

The LED chip21is disposed on the flexible PCB18, and is electrically connected to the conductive patterns. The LED chip21includes red, green, and blue LEDs, or white LEDs.

The light guide plate15is disposed at the side of the LED chip21, and radiates light incident from the LED chip21to the optical sheet14as surface light.

The light guide plate15includes an incident portion15aadjacent to the LED chip21, and a protruded portion15bprotruding to the same height as the LED chip21from the remainder of the light guide plate15, in order to increase the efficiency of light. Thus, the protruded portion15bprotruded to the same height as the LED chip21allows most of the light emitted from the LED chip21to be incident on the light guide plate15, increasing lighting efficiency. The protruded portion15bmay be formed along the incident portion15aof the light guide plate15. Here, a portion of the flexible PCB18may be disposed to overlap on at least the protruded portion15b.

The molded frame17is provided with a holding space17ain which the LED chip21mounted to the flexible PCB18is inserted. The LED chip21is disposed on the flexible PCB18toward the molded frame17. Thus, a first portion18aof the flexible PCB18is seated in the molded frame17, and a second portion18b of the flexible PCB18is mounted to overlap on the light guide plate15. The first portion18adenotes a portion extending from one end of the flexible PCB18to the LED chip21, and the second portion18b denotes a portion extending from the LED chip21to the other end of the flexible PCB18. The LED chip21disposed between the first and second portions18aand18bis inserted into the holding space17a. The undersurface of the LED chip21may or may not contact the top surface of the molded frame17.

In the related art, as shown inFIG. 4, the flexible PCB is not fixed on the light guide plate as above, but is simply placed thereon, so that a gap forms between the flexible PCB and the light guide plate, through which light emitted from the LED chip radiates, such that light is radiated in line shape due to line of more brighter light and another line of more darker light.

In embodiments of the invention, the gaps between the flexible PCB and the light guide plate block, preventing the occurrence of lines of light.

The optical sheet14diffuses and controls direct light emitted from the light guide plate15. The optical sheet14may sequentially include a diffusion sheet14aand a first and second prism14cand14dproximate to the light guide plate15. To protect the second prism sheet14d,a first protective sheet (not shown) may be disposed on the second prism sheet14d. Also, a second protective sheet (not shown) for protecting the diffusion sheet14amay be disposed beneath the diffusion sheet14a.

An extending portion14bmay be formed in any one of the plurality of sheets14a,14c,and14dincluded in the optical sheet14. For example, the extending portion14bmay be formed in the diffusion sheet14ato overlap with a portion of the flexible PCB18. The extending portion14bextends from the light guide plate toward the light emitting diode chip21. The extending portion14bmay be integrally formed with the diffusion sheet14a. While the extending portion14bis described in the present embodiment as being formed on the diffusion sheet14a, the extending portion14bmay alternately be formed on any one of the sheets other than the diffusion sheet14a—for example, one of the first and second prism sheets14cand14dor the first and second protective sheets.

The extending portion14bmay be disposed to overlap with the second portion18bof the flexible PCB18. Being disposed on the flexible PCB18, the extending portion14bblocks light from the LED chip21from passing through a gap between the flexible PCB18and the light guide plate15and prevents the light from progressing along the light guide plate15, so that the occurrence of lines of light is prevented. Accordingly, uniform brightness can be realized across the light guide plate15, and the uniform brightness is incident on the LCD panel10, to improve the image output from the LCD panel10.

A reflective plate16for reflecting light emitted toward the rear of the light guide plate15may be disposed on the rear surface of the light guide plate15.

The reflective plate16and the light guide plate15are stacked in the molded frame17, and the flexible PCB18is inserted and mounted in the molded frame17, so that the LED chip21is inserted into the holding space17aof the molded frame17. The optical sheet14is seated on the light guide plate15, and the extending portion14bof the diffusion sheet14aoverlaps with a portion of the flexible PCB18.

The light-shielding tape12is adhered between the edges of the optical sheet14and the flexible PCB18.

The LCD panel10is disposed on the optical sheet14with the light-shielding tape12in between. The light-shielding tape12may be disposed along the edge of the LCD panel10.

The molded frame17is fixed and fastened by a lower cover25. While not shown, the lower cover25may be fastened to an upper cover disposed along the edge of the LCD panel10.

While the LED chip21is described in the present embodiment as disposed on only one side of the light guide plate15, a first and second LED chips21may be disposed respectively on both sides of the light guide plate15. In the latter case, a first and second protruding portion15bmay be formed on a first and second incident portion15a, respectively, and a first and second extending portion14bmay be formed respectively on both sides of the diffusion sheet14a. The first and second extending portions14bmay be disposed to overlap with the first and second flexible PCBs18in which the first and second LED chips21are respectively mounted. The second LED chip, the second flexible PCB, the second extending portion and the second protruding portion have an substantially symmetry position relationship with the first LED chip, the first flexible PCB, the first extending portion and the first protruding portion with respect to the light guide plate, respectively. Therefore, the occurrence of lines of light through light emitted from the first and second LED chips21and progressing toward the center of the light guide plate15can be prevented.

FIG. 2is a sectional view of the LCD inFIG. 1in an assembled state, andFIG. 3is an enlarged view of region A inFIG. 2.

Referring toFIGS. 1 to 3, the reflective plate16and the light guide plate15are held in the molded frame17, the LED chip21is disposed so that its top surface faces the bottom of the holding space17aat one side of the molded frame17, and the LED chip21is mounted to the flexible PCB18. The first portion18aof the flexible PCB18is mounted on the molded frame17, and the second portion18bis disposed to overlap on the incident portion15aof the light guide plate15.

The optical sheet14is disposed on the light diffusion plate15. The optical sheet14may include the diffusion sheet14aand the first and second prism sheets14cand14d. First and second protective sheets may be provided on the top surface of the second prism sheet14dand the undersurface of the diffusion sheet14a, respectively.

An LCD panel10including a color filter substrate10aand an array substrate10bmay be disposed on the light guide plate15. An upper polarizing plate30aand a lower polarizing plate30bare attached respectively to the top and bottom surfaces of the LCD panel10.

A backlight unit is formed of a flexible PCB18on which an LED chip21is mounted, a light guide plate15, a reflective plate16, and an optical sheet14.

The LCD panel10and the backlight unit20can be held in the molded frame17.

The light-shielding tape12may be interposed between the light guide plate15and the LCD panel10. That is, the light-shielding tape12may be attached on the edges of the light guide plate15and the flexible PCB18, and the LCD panel10may be mounted on the light-shielding tape12. The light-shielding tape12may be disposed to block light leaking upward from the edges of the light guide plate15. The LCD panel10may be adhered to the light-shielding tape12through a double-sided tape27. If an adhesive is provided on the top surface of the light-shielding tape12, the LCD panel10may be directly attached to the light-shielding tape12, so that double-sided tape27is not needed.

One of the plurality of sheets14a,14c, and14dincluded in the optical sheet14—for example, the diffusion sheet14amay have an extending portion14b. The extending portion14bmay be formed integrally with the diffusion sheet14a. While in the present embodiment, the extending portion14bis described as extending from the diffusion sheet14a, the extending portion14bmay alternately be formed to extend from one of the first and second prism sheets14cand14dor the first and second protective sheets.

The extending portion14bmay be disposed to overlap with the second portion18bof the flexible PCB18. The extending portion14bmay overlap with the entire surface of the second portion18bof the flexible PCB18or part of the second portion18b. When the extending portion14bthus overlaps with the second portion18bof the flexible PCB18, light leaking between the flexible PCB18and the light guide plate15and progressing parallel to the light guide plate15is blocked. Accordingly, the occurrence of the lines of the light due to the leaked light progressing parallel to the light guide plate15is prevented, so that uniform brightness may be obtained from the light guide plate15, and the uniformly bright light is incident on the LCD panel10to improve image quality.

While the extending portion14bhas thus far been described as extending from only one of the sheets14a,14c, and14dincluded in the optical sheet14according to the present embodiments, the extending portion14bmay be formed on each of the sheets14a,14c, and14dincluded in the optical sheet14. In this case, the extending portions14bformed on each of the sheets14a,14c, and14dmay be disposed to overlap with the second portion18bof the flexible PCB18.

In embodiments of the invention, the sheets corresponding to the incident portions of the light guide plate are extended to form an extending portion, and the extending portion is disposed to overlap with a portion of the flexible PCB, so that the occurrence of lines of light due to light progressing parallel to the light guide plate through a gap between the light guide plate and the flexible PCB can be prevented. Since the occurrence of lines of light is prevented, uniform brightness can be obtained and image quality can be improved.

It will be apparent to those skilled in the art that various modifications and variations can be made in the invention. Thus it is intended that the invention covers the modifications and variations of this disclosure provided they come within the scope of the appended claims and their equivalents.