EP2270387A1 - LED flat light - Google Patents

Abstract

The flat light has a lighting unit (30) and housing for retaining the lighting unit. The lighting unit has light emitting diodes (31), which are arranged on a printed circuit board (32). The light emitting diodes are formed as surface mount light emitting diode without outstanding lens part.

Description

The invention relates to a flat lamp comprising a luminous means with LEDs arranged on a printed circuit board and a housing for accommodating the luminous means.

At lights for industrial application, for example in mechanical engineering, increasingly high demands are placed on the height and the performance. In order to take account of the increasing trend in mechanical engineering, to provide more production capacity and functionality on a smaller area, it is desirable to provide a luminaire with as low a height as possible, a so-called flat luminaire, in order to restrict the working space in the machine room as little as possible. Such application scenarios continue to place high demands on the performance of the flat lamps used. For example, a strong, high-contrast, glare- and flicker-free, uniform light with low shadowing is desirable to ensure optimal illumination of the workplaces. Furthermore, such lights must often be resistant to water, dust, oils and emulsions.

Flat lamps based on fluorescent tubes are known from the prior art. Fluorescent tubes have the disadvantage of a comparatively low color temperature, as well as a limitation of the minimum height due to the size of the fluorescent tubes. Furthermore, LED-based machine lights are known, which simulate the principle of a fluorescent lamp, for example, by 4-8 high-power LEDs are arranged in a row. LED-equipped lighting fixtures have the advantage that LEDs are usually shock- and vibration resistant, circuit compatible and easily modulated in their emitted radiation. However, due to the use of punctiform light sources, such a luminaire does not lead to a uniform illumination of the room and thus to disturbing glare effects and shadowing. Another disadvantage is the low light intensity and the specification of the minimum height of the luminaire through the thickness of the high-power LEDs. Out DE 197 47 980 A1 is an LED-based lighting fixture for wet rooms known. The arrangement of the LEDs in this fixture also leads to glare and shadowing and not to a sufficiently uniform illumination as is required for many industrial applications.

It is therefore an object of the present invention to provide a cost-effective flat lamp that emits high-contrast, glare and low-shadow light with high light intensity and color temperature. This object is achieved by a flat lamp according to claim 1. The dependent claims relate to advantageous embodiments of the invention.

According to the invention, the flat lamp comprises a lighting means and a housing for receiving the lighting means. The overall height of the flat lamp in the direction of radiation can be less than 30 mm. Furthermore, the luminous means comprises LEDs arranged on a printed circuit board with an arrangement density of at least 1 LED per 2.5 cm ² over an area of at least 250 cm ² . Due to the high arrangement density according to the invention over a minimum area of 250 cm ² , the LEDs form a luminous surface, ie a surface luminous body based on punctiform LED light sources.

The inventors have observed in the development of the flat lamp according to the invention the surprising effect that at an arrangement density of min. 1 LED per 2.5 cm ² irregularities in the perceptible light distribution can be reliably avoided despite the "point-shaped" LED light sources. An arrangement density of the LEDs of at least 1 LED per 1.75 cm ² is particularly advantageous. It is therefore a particular aspect of the present invention to produce planar light despite the use of point LED-based light sources.

According to the invention can be generated by the high LED array density at the predetermined minimum surface area light, planar light in the context of this invention corresponds to a light or with respect to the light distribution very close to a light that is generated by a conventional surface light body.

The term "surface light body" is understood in the relevant art to mean a body having a uniformly light-emitting luminous area, the thickness of which is small in comparison to the length and width dimensions. With the same light intensity measured in a for a particular application (eg light for a factory machine) typical distance from the luminaire, the emitted light of a surface luminaire compared to the light of a filament with point light sources produces less glare and shadowing, as the light distribution distributed by the flat luminous body spatially evenly is. The LED luminous element according to the invention represents a surface luminous element in this sense, since it has the same advantageous illumination properties with regard to glare effects and shadowing.

The luminous element according to the invention thus combines the advantageous light distribution of a surface luminous body with the advantageous properties one on LED-based filament. The inventive high total number of LEDs used allows a sufficiently high light intensity.

According to the invention, the bulbs of the flats consist of individual LEDs whose forward voltage does not exceed a deviation of +/- 5% by a predetermined value. It is particularly advantageous if the variation width of the forward voltage does not exceed a value of 3.5% by a predetermined value. This is due to the surprising finding in the course of the development that brightness differences and the resulting shadowing in the luminous area can be avoided by observing previously determined tolerances in the LED forward voltage. In addition, a significantly increased lifetime of the LEDs was observed. Investigations by the inventors after realization of the invention have found that customary tolerances of the LEDs used far exceed these fluctuation ranges. Advantageously, the predetermined value of the forward voltage is in the range 2.7-3.7 volts. A higher fluctuation range of the forward voltage, in combination with the planar arrangement density of at least 1 LED per 2.5 cm ² according to the invention, leads to significant temperature and brightness differences, which increase further with increasing operating time of the luminaire.

Preferably, the flat lamp has a light source with a passive LED drive. The passive LED drive can be realized by resistors connected in series to the LED, thereby defining the current flow through the LEDs. This allows a very cost-effective implementation of the bulb.

The flat lamp can also include an active control of the individual LEDs. This has the advantage that in order to comply with small tolerances of the LED forward voltage no preselection of the LEDs used is needed, but this can be realized by an active control circuit.

Preferably, the LEDs of the surface illuminant according to the invention are selected such that the fluctuation of the color temperature of the LEDs does not exceed a value of +/- 15% of a predetermined value. This is based on the statement in the context of the invention that the customary manufacturing tolerances with respect to color temperature and light intensity in combination with the dense arrangement of many individual LEDs according to the invention to shade, especially color shadows, can lead in the surface light. Particularly advantageous is a fluctuation range of the color temperature of less than +/- 10% by a predetermined value. Advantageously, the predetermined value of the color temperature is in the range 6000-7500 ° Kelvin.

Furthermore, the light source can comprise LEDs whose fluctuation of the intensity of the LEDs does not exceed a value of +/- 20% by a predetermined value. Particularly advantageous is a fluctuation range of less than +/- 15% by a predetermined value. The fluctuation ranges of the luminous intensity of commercially available LEDs is +/- 45% by a predetermined value. Advantageously, the predetermined value of the light intensity is in the range of 1000 - 1300 mcd.

The aforementioned magnitudes of the preferred values of light intensity, color temperature, as well as the forward voltage represent preferred embodiments in which currently commercially available LEDs are used. In view of the rapidly advancing development of LED technology are therefore also others predetermined values of the luminous intensity, color temperature and forward voltage for a flat lamp according to the present invention are possible as long as the relative fluctuation width by these predetermined values does not exceed the limits given by the invention.

The LEDs mounted on the surface illuminant are preferably designed as surface-mount LEDs (SMD LEDs) without an outstanding lens part. In the case of the combination according to the invention of a high LED arrangement density over a minimum area of 250 cm ² , disturbing light distortions occur when conventional radial LEDs are used. Radial LEDs have a protruding lens portion where the emergent light is refracted. For lights with few LEDs, these effects are usually imperceptible. The high number of densely arranged LEDs according to the invention, however, leads to dark perceptible "light spots" on the surface illuminant in the case of radial LEDs. The refraction of light on the protruding lens part also produces a warmer light with a higher proportion of yellow and red frequencies. By using SMD LEDs without an excellent lens part these disturbing effects can be reliably avoided.

Moreover, the SMD LED emitted light, which is colder compared to the use of radial LEDs, is particularly advantageous for applications in dark, oily machining areas, as these spaces better reflect cold light compared to warmer light.

Another advantage of SMD LEDs is the better thermal management of the lighting fixture. SMD LEDs generally have two planar bearing surfaces, by means of which the LED is mounted directly on a thermally conductive surface of the circuit board, preferably provided with thermal pads, whereby the resulting heat of the LED can be derived. Radial LEDs are typically mounted on a printed circuit board, with the legs of the LEDs plugged in through the printed circuit board print and soldered from the back. As a result, they can be cooled only with difficulty. In the high number of densely arranged LEDs according to the invention, there is a particularly strong heat generation, the low height of <30 mm and for industrial applications often high ambient temperature (up to 45 ° C) and necessary tightness of the flat light against dust and water and the From this reduced air circulation make the thermal management even more difficult. In order to further improve the thermal management of the flat lamp, it is advantageous to use a metal-based circuit board, as it has an improved thermal conductivity compared to conventional FR2 or FR4 circuit boards.

Preferably, individual LEDs with a power less than 120 mW per LED are used for the luminous means according to the invention. Due to the large number of individual LEDs used produces a sufficiently high light intensity, at the same time too high heat radiation is prevented in the emission direction. LEDs with a power <120 mW are very cost-effective and due to their compact design compared to expensive high-performance LEDs for a low-profile flat luminaire.

In summary, the present invention enables a low-cost flat luminaire with an LED-based surface illuminant, which produces shadow-rich, blinding and stain-poor, high-contrast light with high color temperature and luminous intensity. In contrast to the usual market approaches to improve the lighting quality of a luminaire by means of sophisticated and high-performance LEDs, the LED-based flat light according to the invention allows instead a variety of low-cost and relatively low-power LEDs use. Furthermore, due to the high arrangement density in conjunction with the small fluctuation ranges of the LED performance parameters, a surface luminous element is realized by means of individual LEDs, which avoids the otherwise usual shadowing and irregularities in the luminous area when using "point-shaped" light sources (LEDs) and thus a Flat LED-based light enables the high requirements in industrial applications.

• Fig. 1 zeigt eine perspektivische Ansicht eines Ausführungsbeispiels der erfindungsgemäßen Flachleuchte;
• Fig. 2 zeigt schematisch ein Rahmenelement mit darin montiertem Flächenleuchtmittel und eine Außenabdeckung der Flachleuchte gemäß einem Ausführungsbeispiel der vorliegenden Erfindung;
• Fig. 3 zeigt eine vergrößerte Draufsicht auf das Flächenleuchtmittel gemäß einem Ausführungsbeispiel der vorliegenden Erfindung;
• Fig. 4 zeigt eine Steuerschaltung der Leuchtdiodenstränge des Flächenleuchtmittels gemäß einem Ausführungsbeispiel der vorliegenden Erfindung;
• Fig. 5a-c zeigen verschiedene Ansichten der eingesetzten LEDs.
• Fig. 1 zeigt eine perspektivische Ansicht einer Flachleuchte 1 in 29 Zoll-Ausführung. Die Flachleuchte 1 umfasst ein Rahmenelement 10, das aus Aluminium-Druckguss gefertigt ist und eine dunkle Farbe zur Absorption von Strahlung eines LED-basierten Leuchtmittels 20 aufweist. An einer Stirnfläche des Rahmenelements 10 ist ein Halteelement 13 zur Montage der Flachleuchte 1 in einer entsprechenden Haltung vorgesehen. Die Flachleuchte 1 weist ferner eine sich über einen äußeren Bereich der Oberseite, sowie einen Teil des Umfangrands der Flachleuchte 1 erstreckende Außenabdeckung 12 auf, die über Schrauben 14 mit dem Rahmenelement 10 verschraubt ist. Eine zumindest teilweise lichtdurchlässige Deckscheibe 11 ist in dem Rahmenelement 10 stoßfest festgelegt und besteht beispielsweise aus Polycarbonat oder Polyacrylat oder auch Glas (ESG oder VSG).

Preferred embodiments of the present invention will be described in detail below with reference to the accompanying drawings.

• Fig. 1 shows a perspective view of an embodiment of the flat lamp according to the invention;
• Fig. 2 schematically shows a frame member with surface illuminant mounted therein and an outer cover of the flat lamp according to an embodiment of the present invention;
• Fig. 3 shows an enlarged plan view of the surface lighting means according to an embodiment of the present invention;
• Fig. 4 shows a control circuit of the light-emitting diode strands of the surface lighting means according to an embodiment of the present invention;
• Fig. 5a-c show different views of the LEDs used.
• Fig. 1 shows a perspective view of a flat lamp 1 in 29 inch design. The flat lamp 1 comprises a frame element 10 which is made of die-cast aluminum and has a dark color for absorbing radiation of an LED-based luminous means 20. On an end face of the frame member 10, a holding member 13 for mounting the flat lamp 1 is provided in a corresponding attitude. The flat lamp 1 further has an outer cover 12 which extends over an outer region of the upper side and a part of the peripheral edge of the flat lamp 1 and which is screwed to the frame element 10 via screws 14. An at least partially translucent cover plate 11 is fixed shock-resistant in the frame member 10 and consists for example of polycarbonate or polyacrylate or glass (tempered glass or laminated safety glass).

The outer dimensions of in Fig. 1 shown flat lamp amount to 771 x 171 x 28mm with a thickness of the illuminant of 3.5 mm. The flat lamp thus has a height of 28 mm in the emission direction.

The internal structure of in Fig. 1 shown flat lamp 1 is off Fig. 2 better visible. Fig. 2 schematically shows a frame member with lamp mounted therein and an outer cover of the flat lamp according to an embodiment of the present invention. The frame element has a lateral angle section 41. The LED illuminant 30 rests on a lower surface of the angle section 41 and is fixed there with fastening means. The frame member 10 has a circular recess for receiving the cables from the cable connection 34.

Fig. 3 shows a plan view of the surface illuminant 30. The surface illuminant 30 includes a rectangular FR4 circuit board 32 on the 952 LEDs 31 are attached. The outer dimensions of the Fig. 3 shown PCB are 700 x 138 mm. The LEDs 31 are alternately arranged in rows of 8 and 9 perpendicular to the longitudinal direction of the circuit board with the row of LEDs 8 row parallel to the row of LEDs 9, so that one row 8 LED is equidistant from the next two LEDs of the following row 9 Series has. The LEDs are mounted at a distance of 1.3 cm on the circuit board, which corresponds to an arrangement density of 1 LED per 1.68 cm ² . The LEDs 31 are mounted via two contact surfaces of the LED underside by Printmontage directly to a thermally conductive surface of the circuit board, preferably provided with thermal pads attached. Despite the high arrangement density and the large number of LEDs used, the flat lamp according to the invention knows a max. Surface temperature in the range of 55 ° C at 25 ° C ambient temperature. The LED flat light according to the invention produces a light output in the range of 27,000 Lx per dm ² at a distance of 15 cm and has a luminous area of approximately 7.6 dm ² .

On the two outer long edge strips of the printed circuit board 32 each 136 resistors 33 are soldered in a line perpendicular to the LED 8 and 9 rows. About the tracks of the circuit board 32 7 LEDs and two resistors 33 are connected in series. At two opposite corners of the circuit boards in each case a cable connection 34 is soldered to the power supply. At two other opposite corners of the circuit boards two holes 37 are provided for fastening means to secure the circuit board to the frame member 10.

The inventors have determined in experimental series that the inventive high arrangement density of the 952 LEDs of 1 LED per 1.68 cm ² at market Tolerances of the LEDs with respect to the forward voltage, the color temperature and the light intensity leads to disturbing shadows and light spots. Therefore, only specially preselected LEDs are installed for the surface illuminant according to the invention in the embodiment with a passive LED control, which has a very narrow fluctuation range with respect to the forward voltage (+/- 0.1 V at a mean forward voltage of 3.1 V), the color temperature (+ /. 500 ° K by a predetermined value of 6500 ° K) and the luminous intensity (+/- 150 mcd at a predetermined value of 1150 mcd).

Further, at the outer edges of the circuit board fastener 35 for mounting the circuit board on the frame member 10. Some of these fasteners are pronounced as bolts, which both secure the circuit board to the frame member and serve as a spacer for resting the outer cover 12.

Fig. 4 shows a control circuit according to an embodiment of the invention. Fig. 4 shows 2 LED strands 41 of the surface illuminant according to the invention (the remaining identically switched LED strands are not shown). The arranged on the circuit board 32 LEDs 31 are summarized according to this embodiment to LED strands with 6 series-connected LEDs, wherein at the beginning and end of the strand in each case a resistor 33 is connected in series. All LED strings 41 consisting of 6 LEDs and 2 resistors are connected in parallel. On the circuit board is located on the cable connection 34, a voltage of 24 V, which is applied to each of the parallel-connected LED strands 14 At each of the identical LEDs falls a forward voltage of 3.1 V with a tolerance of +/- 0.1 V and at each Resistor a voltage of 0.8V. The resistors connected in series in the LEDs determine the current flow through the diodes through the resistance variable and the applied voltage and thus form the passive control of the luminous means.

Fig. 5a shows a view from the direction of emission of the LEDs used. Surface Mounted (SMD) LEDs are used in a PLCC 2 package. The light is emitted via a circular opening 52 which has no outstanding lens part with a beam angle of 120 degrees. As a result, disruptive refraction effects are avoided. The recess 53 indicates the polarization in the forward direction. The LEDs used emit white light at a power of 62 mW per LED. The LEDs used have a length of 3.5 mm and a width of 2.8 mm. The LEDs 31 mounted on the circuit board have a forward voltage of 3.1 V, and a tolerance range (forward voltage fluctuation range) of +/- 0.1 V. Furthermore, the LEDs used have a luminous intensity in the range of 1000 □ 1300 mcd, and thus a tolerance range of altogether 300 mcd. Furthermore, the LEDs used have a color tolerance of about +/- 500 ° K around the range of 6500 ° K. Fig. 5b shows a side view of the LEDs used which have a height of 1.9 mm. Fig. 5c shows a bottom view of the LED in the direction of radiation. The foot surfaces 54 are mounted directly on a thermally conductive surface of the circuit board.

Of course, the individual features of the invention are not limited to the described combinations of features in the context of the presented embodiments and can also be used in other combinations depending on predetermined device parameters.

Claims (10)

Flat lamp (1) comprising a lighting means (30) and a housing (10) for receiving the illuminant with a total height of the flat lamp (1) in the emission direction of <30 mm, wherein
the luminous means (30) comprises on a printed circuit board (32) areally arranged LEDs (31) with an arrangement density of at least 1 LED per 2.5 cm ² over an area of at least 250 cm ² , and a fluctuation of the forward voltage of the LEDs has a value of +/- 5% does not exceed a predetermined value. The flat lamp (1) according to claim 1, wherein a fluctuation of the color temperature of the LEDs does not exceed a value of +/- 15% by a predetermined value. A flat lamp (1) according to claim 1 or 2, wherein a fluctuation of the intensity of the LEDs does not exceed a value of +/- 20% by a predetermined value. A flat lamp (1) according to any one of the preceding claims, wherein the predetermined value of the forward voltage is in the range 2.7 to 3.5V. A flat lamp (1) according to any one of the preceding claims, wherein the predetermined value of the luminous intensity is in the range 1000-1300 mcd. A flat lamp (1) according to any one of the preceding claims, wherein the predetermined value of the color temperature is in the range 6000 to 7500 ° Kelvin. Flat lamp according to one of the preceding claims, characterized in that the LEDs (31) are designed as surface mount LEDs (SMD LEDs) without outstanding lens part. Flat lamp according to one of the preceding claims, characterized in that LEDs are used with a power <120 mW per LED. Flat lamp according to one of the preceding claims, characterized in that the circuit board is a metal-based circuit board. Flat light according to one of the preceding claims, characterized in that the surface lighting means comprises a passive LED control.

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