WO2002005351A1

WO2002005351A1 - Led light source

Info

Publication number: WO2002005351A1
Application number: PCT/AT2001/000224
Authority: WO
Inventors: Stefan Tasch; Peter Pachler; Hans Christian Hoschopf
Original assignee: Tridonic Optoelectronics Gmbh
Priority date: 2000-07-12
Filing date: 2001-07-06
Publication date: 2002-01-17
Also published as: AU2002218796A1

Abstract

The invention relates to several LEDs (1) without housings, which are mounted directly onto a printed circuit board (6). Said LEDs (1) are potted using a highly transparent polymer (2) to protect them (1) from mechanical damage. A reflector (3) is placed on the printed circuit board from above around each LED (1). According to the invention, the diameter of the potting compound (2) is at least equal to the internal diameter of the reflectors (3), in such a way that the reflector (3) lies in direct contact with the printed circuit board (6) and the surface of the potting compound (2) is configured as an optically active lens surface. The printed circuit board (6) can consist of a highly thermally conductive material and the reverse of the printed circuit board (6) can be coupled to a heat sink (7), for efficiently transporting the dissipated heat.

Description

"LED light source"

TECHNICAL AREA

The present invention relates to an LED light source in which a plurality of unhoused LEDs are assembled directly on a printed circuit board, the LEDs are cast using a highly transparent polymer to protect the LEDs from mechanical damage, and in which a reflector is parabolic around each LED is formed, is placed on the circuit board from above.

STATE OF THE ART

By processing LEDs in chip-on-board technology (COBT), efficient, bright and small-area lighting units can be manufactured. Due to the high luminous flux values that can be achieved, these are not only interesting as signal and backlighting, but can also be used directly as lamps.

LED arrays in COBT technology, in which the LEDs are assembled directly on the circuit board without imaging optics, have a wide beam angle, which is determined by the beam characteristics of the LED die. By applying a polymer layer, which is applied to protect against mechanical damage to the LED array, this is influenced in accordance with the shape of the protective layer. LED dice in general and those with a transparent substrate in particular (such as GaN on sapphire) have a considerable emission through the side surface of the LED dice. Without imaging optics, this portion of the emitted light is lost, especially as is the case with parallel encapsulation. Therefore, it is also in applications where a broad emission distribution is required, it is advantageous to use imaging optics.

An essential aspect of an imaging optical system is therefore that the light which is emitted from the side surfaces which are arranged perpendicular to the printed circuit board is imaged by the imaging optical system in the half space in front of the printed circuit board. Mirrors can be used for this.

An LED light source of the type mentioned is known from US-4603496-A. It has an imaging optic that fulfills this aspect. Each LED die is first cast with a protective layer. The diameter of the protective layer is slightly larger than the inner diameter of the reflector, which is then put on. The reflector is therefore not on the circuit board, but on the protective layer. Then the reflector is poured out and an approximately spherical lens is pressed into this casting compound. The protective layer, casting compound and lens should have a refractive index that is as similar as possible (see column 3, lines 13-16). Thus only the top of the lens is optically active. A disadvantage of this design is that the reflector is placed on the protective layer. If this is not carried out precisely, this affects the position of the reflector, so that the imaging geometry is changed.

DISCLOSURE OF THE INVENTION

It is an object of the present invention to overcome this disadvantage. This object is achieved according to the invention by an LED light source of the type mentioned at the outset in that the diameter of the casting compound is at most equal to the inside diameter of the reflector, so that the reflector lies directly on the printed circuit board, and in that the surface of the casting compound borders on air, the Interface is convex. According to the present invention, the reflectors sit directly on the circuit board and are therefore always exactly positioned in the vertical direction. The sealing compound borders on air, so that the surface is a breaking surface. In order to keep losses through reflection low, it should be convex.

Another important aspect of the potting compound is the reduction of the internal reflection losses within the LED at the transition from LED semiconductor material / potting compound / air compared to a direct transition from LED semiconductor material / air. This is possible due to the fact that the refractive index of the casting compound (typically. N = 1.3-2.0) is close to that of the semiconductor material (typically 2 <n <4).

The aim should be if the circuit board is made of thermally highly conductive material and if the back of the circuit board is coupled to a heat sink. This is known per se from US-5936353-A. As a result, the density of the LED dies and the current through them can be chosen to be relatively high. So that no short circuits can occur, it is expedient if the reflector consists of a highly reflective, metallic material which is insulated on the underside, or if the reflector consists of a plastic, the inside of which is mirrored. The arrangement of the optical components can either take place in the immediate vicinity of the LED dice (individual optics), or as a common optic around several LEDs (overall optics). From the point of view of mapping effectiveness, both approaches are comparable. Even the shape is identical for the different sized optics. The prerequisite for this is that the geometric relationships between the expansion of the light source and the imaging optics are comparable. In both cases it should be noted that a suitably shaped casting compound must be applied directly above the LED. However, a key difference is the size. While the size in comparison ^¬ is equal to increase only slightly to the LED array with no imaging optics in the single optics, is taken into account in the overall appearance as a rule of thumb into account that the minimum inner diameter of the Ge ^¬ velvet look be at least twice the maximum distance of the LED dice on the board to minimize imaging losses.

In particularly expedient versions, the LEDs are mounted face down and there is a die in each reflector, or else the LEDs are face up mounted and up to 4 dice are arranged in each reflector.

For easier assembly, it is advantageous if a reflector plate is provided which has a large number of parabolically shaped reflectors. This means that not every reflector needs to be positioned individually.

If additional lenses are to be provided, it is expedient if Fresnel lenses or Gaussian lenses in the form of a lens plate are positioned centrally over each reflector and glued to the side. This means that the lenses can also be easily installed.

According to a special embodiment of the invention, it is provided that the heat sink is thermally coupled on the one hand to transmit the heat to a housing or a light source holder thereon and for the electrical connection. tacting is designed as a thread for screwing into a socket analogous to the incandescent lamp. The LED light source can thus be used directly as a replacement for an incandescent lamp. The cooling takes place via the thread. Furthermore, it is possible for the LED light source to be designed as a traffic light module, the LED circuit board being thermally coupled to the traffic light housing and a lens plate being arranged in front of the LED light source. So here the traffic light housing takes over the cooling function. BRIEF DESCRIPTION OF THE DRAWING

The present invention is explained in more detail with reference to the accompanying drawing. 1 shows a section through the first embodiment of the present invention, and FIG. 2 shows a section through a second embodiment of the present invention.

BEST EMBODIMENT OF THE INVENTION

1, LED dice 1 are attached to a conductor track 5. Each LED die 1 is cast in a casting compound 2, which is strongly convex (e.g. hemispherical). A reflector 3 is arranged around each LED die 1 or each potting compound 2, which is seated directly on the base plate body 6 and is therefore precisely positioned. Below the conductor track 5, which consists of thermally highly conductive material, there is the base plate body 6 and underneath the cooling body 7. The reflectors 3 can be attached either individually or in assembled matrices.

The following procedure is proposed for attaching the reflectors 3:

After the LED dice 1 have been applied to the printed circuit board 5 by means of die and wire bonding or by means of flip-chip technology, a reflector 3 is placed over each die 1 with an assembly system. The underside of the reflector 3 uss be non-conductive, otherwise a short circuit between the conductor tracks over which the reflector is placed arises. The inside of the reflector should be highly reflective. For example, a plastic mirrored on the inside can be used. In a preferred variant, instead of many individual optics, an optic array consisting of individual elements, which is positioned exactly above the LED dice 1, is used. The casting compound 2 should have the following properties: highly transparent, softening point> 100 ° C., linear thermal expansion coefficients as low as possible. To achieve good decoupling of the light generated in the LED die, the potting compound must be dome-shaped.

Instead of individual reflectors, a reflector array can be used, which e.g. consists of a thin plastic plate, into which parabolic or funnel-shaped openings with a defined shape and mirrored on the inside are made. Such a mirror array must be specially matched to the respective LED array and is placed on it in one process step.

While it seems sensible for LED dices that are processed using flip-chip technology to assign a single lens to each LED array, this is disadvantageous for LEDs that are processed using die and wire bonding:

The reason for this lies in the bond wires, which have a typical length between 0.5 and 2 mm and lead from the electrical contact on the LED die to a conductor track. When assembling a single optic, the diameter: LED radius + 2 x length of the bonding wire must be provided for the reflector. (The LED-Die should sit in the middle of the reflector if possible.) For this reason, the packing density is significantly loosened compared to the possible density, which is not desirable for many applications. It therefore proves to be advantageous in the case of LED dice, processed by means of die and wire bonding, to use an optical system for several LED dice, since this enables a higher packing density to be achieved (see FIG. 2). A number of between 2 and 5 LED dice is preferably arranged within an optical system. This is also important for traffic light applications.

In a preferred variant, a light pane (eg Fresnel lens) is arranged above the reflector for the final determination of the light emission. The LED light source according to the invention has high luminous intensity, defined radiation characteristics and a low overall height

At least part of the housing in which the LED array is fastened is preferably used as the heat sink, which is to be used in the case of highly loaded LEDs. For this purpose, this is at least partially metallic. In this way, the housing of the reflector can also be used as a heat sink.

Certain colors (e.g. white) cannot be generated by a single LED due to the fact that LEDs generally only have a narrow-band emission spectrum. Options for realizing white light are e.g. in US-5851905-A, in WO-00/02262-A and in US-5836676-A. The generation of white emissions is particularly important for lighting technology. In addition to the white emission color, the color rendering is also of great importance. Since no intrinsically white-emitting LEDs have been able to be produced to date, this color must be produced by a special arrangement or by a special structure, as described as follows:

1) Color conversion: by arranging at least one lumino phosphor directly above the LED dice, which absorbs the emission of the dice and subsequently emits photoluminescent light in a different emission color. With a view to optimum color rendering, it is preferred to use a plurality of luminophores which emit in a different visible spectral range from green to red. According to the prior art, the luminophore is arranged in layers above the LED array or mixed into the lens. This approach has the disadvantage that the emission color is not constant over the radiation angle, since the path through the color conversion layer changes with the radiation angle. In order to obtain a relatively constant emission color, the path of the emitted light through the color conversion medium must therefore be kept constant. This cannot be done by a layered Medium - as a rule - can be achieved through the shape of the lens, but must be realized through a spherical or elliptical shape.

2) White emission can be generated by mixing the emission colors of suitable differently colored LEDs. This approach is particularly attractive for LEDs, processed in COBT (chip-on-board technology), since the local distances between the LED dice can be selected to be very small. With a view to good color rendering, the color mixture must be generated using red, green and blue emitting LED dice (three-band white). These are arranged in a special ratio on a circuit board, and the emission color to be generated is set by defined setting of the operating conditions for the respective die type.

Electrical wiring:

The LEDs, each of which have the same emission color of an array, are electrically connected in a combined parallel and series connection and operated together with control electronics. In this way, the operating voltage of the LED array can be adapted to the available voltage. In such an arrangement, on the one hand, optimal power efficiencies can be achieved since only low voltages drop on the electronics. This also minimizes the thermal load on the arrangement. In order to be able to operate them independently of the temperature dependence of the operating voltage of the LEDs and thus to obtain brightnesses that are as constant as possible over the operating current, these are preferably operated with a current specification. Furthermore, they are provided with reverse polarity protection. In order to ensure optimal adaptation to the available operating voltage and thus to achieve optimal energy utilization, a clocked or linear current regulator is preferably provided for the operation of the LEDs. It is expedient if the control electronics are arranged on a separate circuit board and are electrically connected to the LED circuit board.

Claims

PATENT CLAIMS:

1.LED light source, in which several unhoused LEDs are assembled directly on a printed circuit board, the LEDs are encapsulated using a highly transparent polymer to protect the LEDs from mechanical damage, and in which each LED has a reflector that is parabolic or funnel-shaped is placed on the circuit board from above, characterized in that the diameter of the sealing compound is at most equal to the inside diameter of the reflector, so that the reflector lies directly on the circuit board, and that the surface of the sealing compound is designed as an optically active lens surface.

2. LED light source according to claim 1, characterized in that the circuit board consists of thermally highly conductive material and that the back of the circuit board is coupled to a heat sink.

3. LED light source according to claim 1 or 2, characterized in that the reflector consists of a highly reflective, metallic material which is insulated on the underside.

4. LED light source according to claim 1 or 2, characterized in that the reflector consists of a plastic, the inside of which is mirrored.

5. LED light source according to one of claims 1 to 4, characterized in that the LEDs are mounted face down and one is arranged in each reflector.

6. LED light source according to one of claims 1 to 4, characterized in that the LEDs are mounted face up and up to 4 dice are arranged in each reflector.

7. LED light source according to one of claims 1 to 6, characterized in that a reflector plate is provided which has a plurality of parabolically shaped reflectors.

8. LED light source according to one of claims 1 to 7, characterized in that Fresnel lenses in the form of a lens plate are positioned centrally over each reflector and glued laterally.

9. LED light source according to one of claims 1 to 7, characterized in that Gaussian lenses in the form of a lens plate are positioned centrally above each reflector and glued to the side.

10. LED light source according to one of claims 1 to 9, characterized in that the heat sink is thermally coupled on the one hand for the transfer of heat to a housing or a light source holder thereon and analog for the electrical contacting as a thread for screwing into a socket is designed for the incandescent lamp.

11. LED light source according to one of claims 1-10, characterized in that it is thermally coupled to a housing which at least partially fulfills the cooling function.