WO2006000948A2

WO2006000948A2 - Backlighting unit for a display with improved cooling facilities

Info

Publication number: WO2006000948A2
Application number: PCT/IB2005/051959
Authority: WO
Inventors: Gwendolyn A. Luiten; Genevieve T. Martin; Franciscus J. Stommels; Gerardus H. J. Antonis; Cornelis J. Mies
Original assignee: Koninklijke Philips Electronics N.V.
Priority date: 2004-06-22
Filing date: 2005-06-14
Publication date: 2006-01-05
Also published as: WO2006000948A3

Abstract

The invention relates to a backlighting unit for a display unit, wherein the display unit comprises at least one HCFL-lamp and cooling means for cooling said at least one lamp, wherein the cooling means are adapted to cool only at least a portion of the lamp which is thermally insulated from the hot cathode. Cooling of only those parts of the lamps which are thermally insulated from the source of the heat greatly improves the effectiveness of the cooling, as no heat is withdrawn from an area where it is needed. This leads to substantial savings in energy needed for heating the hot cathodes, while the cooling requirements are substantially reduced. Preferably, the at least one lamp has a longitudinal shape, and the cooling means are adapted to cool a portion of the lamp envelope at one of the ends of the lamp envelope.

Description

Backlighting unit for a display with improved cooling facilities

The invention relates to a backlighting unit for a display unit, wherein the display unit comprises at least one HCFL-lamp and cooling means for cooling said at least one lamp. With the ongoing proliferation of flat displays which do not generate light themselves, such as Liquid Crystal Displays (LCDs), there is a need for backlighting units which are located behind the display units. It is now generally accepted that HCFL-lamps are appropriate for this purpose. A prior art unit is described in US-B-6,417,832. This document states that it is advantageous to maintain a uniform lamp temperature. The efficacy of Hot Cathode Fluorescent Lamps (HCFL-lamps), however, is determined to a major extent by the temperature of the so-called 'cold spot' of the lamp, as this temperature plays an important part in the mercury vapor pressure equilibrium. The temperature of the hot cathodes is much higher. In prior art HCFL lamps this is a minor problem as the cold spot is usually located in the middle of the lamps, so that the distance between the hot cathodes of a lamp and the cold spot is considerable. Several advantages can be gained, however, when the cold spot is located at the ends of the lamps, i.e. adjacent the hot cathodes, but this leads to a thermal short-circuit, and hence to the need for a larger cooling capacity to keep the luminous efficacy of the lamp at its optimum. In this respect reference is made to a patent application filed simultaneously with the present application [attorney's docket PHNL040716]. The object of the invention is to mitigate the problems referred to above. This object is achieved in that the cooling means are adapted to cool only at least a portion of the lamp which is thermally insulated from the hot cathode. Cooling only those parts of the lamps which are thermally insulated from the source of the heat greatly improves the effectiveness of the cooling, as no heat is withdrawn from the area where it is needed. This leads to substantial savings in energy needed for heating the hot cathodes, while the cooling requirements are substantially reduced. A preferred embodiment provides the feature that the at least one lamp has a longitudinal shape and that the cooling means are adapted to cool a portion of the lamp envelope at one of the ends of the lamp envelope. This embodiment is designed for the situation in which the cold spot is located at one of the ends of the lamp envelope. The copending patent application referred to above describes the advantages of the location of the cold spot at one of the lamp ends. Cooling of the hot cathode and the problems related therewith are avoided in that cooling takes place only for both or one of the lamp ends. Despite the advantages of the features mentioned above there is still a thermal short-circuiting path outside the lamp envelope. The thermal leak through this path is minimized by thermal insulation means extending outside the lamp envelope and forming a thermal insulation between the vicinity of the hot cathode and the vicinity of the lamp portion cooled by the cooling means. These thermal insulation means may be implemented by an insulating wall which extends substantially perpendicularly to the axis of the at least one lamp and which intersects the axis between the projection of the cooled part of the envelope on the axis and the nearest hot cathode, while the lamp envelope extends through an aperture in the insulating wall. The insulating wall here serves as thermal insulating means between the vicinity of the heat source, that is the hot cathode, and the vicinity of the lamp portion of which the temperature is to be kept low. The insulating wall also functions as a wall between the space in which the cooling means are located and the space where this is not the case. This offers the possibility to employ cooling means which could have adverse effects on the parts of the lamp envelope where the hot cathodes are located and where the light is emitted. This may be the case for cooling with liquids or gases. As displays usually have a rectangular shape, it is attractive to make use of several longitudinal lamps arranged mutually parallel so as to cover the whole surface area of the display. In such a situation it is attractive to provide an insulating wall common to all lamps, each of the lamps extending through a respective aperture in the insulating wall. As the longitudinal lamps used in these embodiments are usually symmetrical, except for the location of the cold spot, it is attractive to provide a second insulating wall between the distant hot cathode and the distal end of the lamp envelope nearest to the distant hot cathode, such that the lamps extend through apertures provided in the second insulating wall. This offers the possibility to make a substantially symmetrical construction for the backlight, which is usually advantageous. Another advantage of the above feature is the possibility to construct an insulating housing by interconnecting the first and the second insulating wall by means of a rear wall and a diffuser at the side of the display so that the combination of the rear wall, the first and the second insulating wall and the diffuser constitutes the insulating, inner housing. The advantage of this construction is that the inner housing offers the possibility to close off the contents of the box from the adjacent space. As in the preceding embodiment, this has the advantage of separating the cooling facilities from the inner space of the inner housing. This is particularly advantageous when the cooling means comprise an air flow. The use of an air flow brings about a precipitation of dust and hence a soiling of the lamps resulting in a lower light output, which is avoided by the inner housing. Another preferred embodiment provides the feature that an outer housing is provided surrounding the inner housing, which outer housing is at least partly made of a material with thermally conducting properties. This feature improves the cooling capacity substantially at a small cost. This cooling is effective at the location where it is needed most, i.e. at the lamp ends. However, this configuration also offers the possibility to improve the cooling by other means located in the channel. These cooling means may be formed by solid cooling means, such as a metal heat conductor, but they may alternatively be formed by an air flow. In such a configuration the thermal conduction properties of the channel walls themselves are not needed, but the inner space of the channel can be used for conducting an air flow. To make use of these properties, means are provided for generating a gas flow through a channel enclosed between the insulating housing and the second housing. A particularly attractive embodiment is obtained when two channels are formed between the insulating housing and the second housing, into which channels the lamp ends project. In this embodiment it is only the lamp ends that are in intimate contact with the cooling means, so that a good transfer of thermal energy is obtained and indeed at the location where it is needed most. It is noted, however, that the heat transfer mechanism referred to above may be combined with the conducting heat transfer mechanism of the outer housing. The convective heat transfer mechanism is improved by the provision of a heat sink which is coupled to the distant end of the lamp envelope. As the lamp ends extend into the convective channel, it is imperative that the heat sinks also extend into the convective channel. This considerably improves the transfer of heat from the cooled parts of the lamp envelope to the air flowing in the convective channel. It was found that the heat transfer capacity of a wire is well suited to effect a heat transfer such that the temperature of the cold spot of the lamp envelope is reached. An advantage of the feature is the simplicity of its construction. A constructionally attractive embodiment provides the feature that the outer housing is thermally coupled to at least a part of the housing of an apparatus of which the LCD-unit forms a part. The invention is embodied in the backlighting unit, of which the cooling unit forms a part. The invention is also applicable, however, to a combination of a display unit and a backlighting unit. The invention may also be incorporated into a TV-set or a personal computer. In both situations it is attractive to have a cooling air flow passing through the housing of the preceding embodiment. This is in particular but not exclusively the case when the outer housing is made of a thermally conductive material and when this outer housing is in contact with the cooling means. Indeed, such an air flow, for example passing behind the apparatus, greatly enhances the cooling capabilities of the cooling unit. Such an air flow can be effected by having the apparatus in which the invention is incorporated mounted at a distance from the wall against which it is mounted. It is presumed herein that the display is mounted vertically, so that a natural air convection is established between the wall against which the display unit or TV is mounted and the rear wall of the apparatus. This is achieved by the feature of a mounting unit for mounting the display unit in a substantially vertical orientation at a distance from a substantially vertical wall.

The invention will be elucidated below with reference to the accompanying drawings, in which: Fig.l is a cross-sectional view of an end of a longitudinal HCFL- lamp, which is used to explain the function of the invention; Fig. 2 is a schematic front elevation of a backlight unit for an LCD-display, with the LCD-display taken away; Fig. 3 is a horizontal cross-sectional view of the combination of the LCD- display and the backlight unit depicted in Fig. 2; Fig. 4 shows a detail of the view shown in Fig. 3; and Fig. 5 is a vertical cross-sectional view of the combination of the LCD-unit and the back light unit depicted in Fig. 4. As was noted above, the invention is applicable to displays such as LCD- displays which require backlighting. HCFL- lamps of tubular shape are commonly used for backlighting purposes. Fig. 1 shows an end of such an HCFL- lamp generally referenced 1. The lamp 1 comprises a glass envelope 2 on the inside whereof a phosphor is applied. The inner cavity of the lamp contains mercury vapor. At the end of the lamp 1, the envelope 2 is closed off with a flange 3 in which a stem 4 is located. The stem 4 protrudes from the flange 3. The stem or exhaust tube 3 is used during lamp manufacture to evacuate the lamp 1, after which the stem is closed by fusion. A cap 5 commonly made from metal is arranged around each of the ends of the lamp 1. This cap 5 is preferably in good thermal contact with a portion of the glass envelope 2 , such as the rims 6 of the cylindrical portion of the envelope 2. To improve the thermal contact between the rims 6 of the envelope 2 and the lamp cap 5, a thermal paste 7 is applied in between. It is alternatively possible, however, to make use of other parts of the glass envelope, e.g. the stem 4, with or without the application of thermal paste. The lamp 1 further comprise a hot cathode 8, of which the leads 9 are guided through the glass envelope 2 by a pinched connection 10. The leads are connected to connecting pins 11 incorporated into the lamp cap 5 but electrically insulated therefrom. An important factor for the luminous efficacy of the lamp is the temperature of the so-called cold spot, the point of the envelope which has the lowest temperature. To avoid a thermal short-circuit it is advantageous to locate the cold spot, which is determined by the external cooling means, as far away from the hot cathode as possible. Other considerations make it attractive to locate the cold spot not too far away from one of the lamp ends. This is illustrated in Figure 1, which shows that the cold spot is arranged at the rim 6 of the envelope 2, this being the location where the cooling means are most effective. The invention provides means for avoiding a thermal short-circuit outside the lamp envelope in the form of the structure depicted in Figures 2, 3 and 4, showing an elevational view, a plan view, and a detailed view thereof, respectively. This structure comprises an array of lamps 1 in a mutually parallel arrangement. They are mounted in an outer housing 12, 13 which comprises a rear wall 12 extending at the rear of the lamps 1 and wall parts 13 extending to the front of the structure, that is the side of the LCD-display 14. Provisions have been made at the side walls 13 to mount the lamps 1 by their lamp caps 5 . Preferably, the outer housing 12, 13 is made of a thermally well conducting material such as metal. An inner housing, preferably made of a thermally insulating material such as a plastic, is provided inside the outer housing. Again this inner housing comprises a rear wall 16 and two side walls 17 extending from the rear wall to the front. Apertures are provided in these side walls 17 through which the tubular lamps 1 extend. This structure results in channels 20 extending between the inner side walls 17, the outer side walls 13, and the outer rear wall 12. At their front sides these channels may be closed from the environment by any suitable structure, such as a front part of the housing in which the display is used, a side part of the display unit, or a part of a light diffuser 18 . The diffuser is a plate which is transparent light but which has diffusing properties. It is commonly located between the array of lamps and the LCD-display. These channels 20 may be used for ventilation purposes to cool the lamp caps 5 with an air flow. Preferably, the surface area of the lamp caps can be enlarged in that heat sinks or other structures are provided in the channels. It is alternatively possible, however, to make use of other cooling mechanisms, such as a solid thermal conductor which may conveniently be located in the channels. In the embodiment shown, these channels are present at both sides of the lamps. As only one cold spot is required for each of the lamps 1, it makes sense to use only one side channel 20 for cooling purposes. The other side channels may then be dispensed with, resulting in a smaller, simpler construction. An effect of this construction is that any possible air flow in the channel or channels 20 is confined to these channels. The area between the inner side walls is not affected. This has the advantage that the areas of the of the lamps in which the hot cathodes 8 are located are not subjected to cooling, while furthermore the problem of soiling of the lamps, which would lead to a lower efficacy, is avoided. The lamps and the housing are dimensioned such that the hot cathodes are located within the inner housing 16, 17 and the cold spot of each of the lamps is located outside the inner housing but within the outer housing 12, 13. The advantage of this feature is that a possible thermal path between the hot cathodes 8 and the cold spot outside the lamp is broken by the walls of the inner housing 16,17, thus providing a thermal insulation. The result thereof is twofold: the energy required to heat the hot cathodes is less as less heat leaks away from the hot cathode, and the cooling requirements for the cold spot are lower as less heat flows towards the cold spot. It is possible to implement the invention in other forms; for example to make use of other locations for the cold spot, like the stem 4. In this case a sleeve is located around the stem, with a thermally conducting paste between the glass of the stem and the sleeve. The sleeve is then in thermal contact with the lamp cap. It is alternatively possible, however, to make use of a metal part extending through the envelope of the lamp at one of the ends. The metal part is separate from the electrical connections to the electrode, and it serves another purpose. Its aim is to facilitate the establishment of a cold spot. The metal part should be thermally connected to the cooling means at its outer end. This feature provides an efficient method of removing heat from the cold spot, as metal has better heat-conducting properties than glass. The connection through the glass can be made. Another possibility is the provision of a heat sink of a different shape. One of the inventors found that a heat sink 21 having a wire shape as depicted in Figure 4 is very effective. The structures mentioned above make use of the lamp cap 5 as a component in the thermal path from the portion of the glass envelope 2 to the cooling means in the channel 20 . It is very well possible to make use of other constructions, such as a single element which is in close thermal contact with the glass envelope and the air flow within the channel. The structures described above define one or two channels 20 which function as a confinement for an air stream. It is possible to use other media for transporting heat from the heat sink to the environment, such as a different gas or a liquid like water. It is furthermore possible to make use of solid media for heat transport. In this respect attention is drawn to the copending application, which describes such a construction in detail. The present invention distinguishes itself from the invention described in the copending application by the presence of the channels. Any possible thermal conduction means can be located in the channels. It is to be appreciated, however, that the advantages of the present invention are most prevalent when a gas, like air, is used as a cooling medium. It is not only the contents, but also the walls of the channel 20 that are suitable for thermal transport. An embodiment which makes use of this feature is depicted in Figure 5. It will be clear that the wall through which the tubular lamps extend is less appropriate for this purpose as the materials thereof are chosen to have a low thermal conductivity such that they will be able to act as a thermal insulation between the inner parts of the lamps and the ends thereof. The outer walls 12, 13, however, are very well suited for this purpose, provided that an appropriate material with a high thermal conductivity is chosen. This avoids the need for separate cooling members, such as rods extending through the channels, or means for causing a gas flow through the channel, such as a ventilator fan. It is possible, however, to make use of natural ventilation if appropriate conditions are met. As was noted above, the outer walls 12, 13 are preferably made of thermally conducting material, such as metal, so that the outer walls 12, 13 are capable of thermal conduction. This conduction may take place in the longitudinal direction of the channels 20, conforming with a vertical direction when the display 14 is used in its common vertical direction, but the thermal conduction may also take place in other directions, e.g. in a direction perpendicular to the longitudinal direction of the channels 20. Thermal conduction in this direction leads to a distribution of the heat over the rear wall of the outer housing 12 of the unit. If this rear wall 12 of the unit is also the rear wall of the apparatus in which the display is arranged, the rear wall is in contact with the environment. This offers the possibility to dispose of the heat by convection of the air forming the environment of the apparatus. This transfer of heat from the wall to the air of the environment is greatly improved when there is a layer of air behind the apparatus. This can be achieved by mounting the apparatus to a wall by means of brackets which leave a layer of air between the rear wall of the apparatus and the wall against which it is mounted. The situation is slightly different when the apparatus comprises a separate rear wall separate from the rear wall 12 of the unit. It is advantageous in such a situation to arrange an air gap 25 between the rear wall 26 of the apparatus and the rear wall 12 of the backlighting unit. This is the situation shown in Figure 5. An arrow 27 indicates the air flow caused by natural ventilation, receiving the heat from the apparatus and transferring it to the environment.

Claims

CLAIMS:

1. Backlighting unit for a display unit, which display unit comprises at least one HCFL-lamp (1) and cooling means for cooling said at least one lamp (1), characterized in that the cooling means are adapted to cool only at least a portion of the lamp (1) which is thermally insulated from the hot cathode (8) .

2. Unit as claimed in claim 1, characterized in that the at least one lamp (1) has a longitudinal shape, and in that the cooling means are adapted to cool a portion of the lamp envelope (2) located at one of the ends of the lamp envelope (2).

3. Unit as claimed in claim 1 or 2, characterized in that the unit comprises thermal insulation means (16,17) extending outside the lamp envelope (2) and forming a thermal insulation between the vicinity of the lamp portion which is cooled by the cooling means and the vicinity of the hot cathode (8) .

4. Unit as claimed in claim 3, characterized in that the thermal insulation means comprise at least a first insulating wall (17) which extends substantially perpendicularly to the axis of the at least one lamp (1) and which intersects the axis of the at least one lamp (1) between the projection of the cooled part of the envelope (2) on the axis and the nearest hot cathode (8), and in that the lamp envelope (2) extends through an aperture in said first insulating wall (17) .

5. Unit as claimed in claim 4, characterized in that the unit comprises a plurality of lamps (1) in a mutually parallel arrangement, in that the first insulating wall (17) is common to said plurality of lamps (1), and in that each of said plurality of lamps extends through a respective aperture in the first insulating wall (17).

6. Unit as claimed in claim 5, characterized in that the unit comprises a second insulating wall (17) between the distant hot cathode (8) and the distant end of the lamp envelope (2) nearest to the distant hot cathode, wherein the lamps (1) extend through apertures provided in the second insulating wall (17).

7. Unit as claimed in claim 6, characterized in that the first and the second insulating walls (17) are interconnected by a rear wall (16) and by a diffuser (18) at the side of the display (14), and in that the combination of the rear wall (16), the first and the second insulating walls (17), and the diffuser (18) forms an inner housing.

8. Unit as claimed in claim 7, characterized in that an outer housing (12,13) is provided surrounding the inner housing (16,17), which outer housing (12,13) is at least partly made of a material with thermally conducting properties.

9. Unit as claimed in claim 8, characterized in that cooling means are provided for cooling those parts of the lamps (1) that extend into the space between the inner housing and the outer housing.

10. Unit as claimed in claim 9, characterized in that the cooling means comprise generating means for generating a gas flow through a convection channel (20) enclosed between the outer housing (12,13) and the inner housing (16,17).

11. Unit as claimed in claim 9 or 10, characterized in that housing channels (20) are formed between the inner housing (16,17) and the outer housing (12,13), into which channels the lamp ends project.

12. Unit as claimed in one of the claims 6 - 11, characterized in that heat sinks (21) are provided which are thermally coupled to the distal ends of the lamp envelopes (2).

13. Unit as claimed in claim 12, characterized in that lamp caps (5) are provided at the ends of the lamps (1), and in that the heat sinks (21) are connected to the respective lamp caps (5).

14. Unit as claimed in claim 12, characterized in that at least some of the heat sinks (21) are formed by a wire.

15. Unit as claimed in any of the claims 8 to 14, characterized in that the outer housing (12,13) is thermally coupled to at least a part of the housing (26) of an apparatus of which the LCD-unit forms a part.

16. LCD display unit, characterized by a cooling unit as claimed in one of the preceding claims.

17. Apparatus, such as a TV-set or a personal computer, characterized by a display unit as claimed in claim 16.

18. Apparatus as claimed in claim 17, characterized by a mounting unit for mounting the display unit in a substantially vertical orientation at a distance from a substantially vertical wall.