FIELD OF THE INVENTION

The invention relates to the field of LED assemblies, and more specifically to the field of LED assemblies comprising multiple LEDs integrated in a single monolithic chip.

BACKGROUND OF THE INVENTION

Lighting systems comprising LEDs are becoming more popular due to advantages of LEDs over incandescent light sources, e.g. the efficiency and life span.

LEDs are usually manufactured as a stack of semiconductor layers formed on or in a wafer substrate. In this layer stack, the necessary pn-junctions are formed for instance by doping. After forming of the layers, the wafer is cut into pieces which are referred to as chips (or dice). These chips are monolithic blocks of material which are then further assembled and connected into higher-level assemblies that allow the LEDs to be used in an application.

It is to be understood that in the context of this application, the reference to LED means a reference to a single-junction LED. In prior art documents, reference may be made to multi-junction LEDs. This reference will not be used in this application. A multi- junction LED is in fact a collection of single-junction LEDs and thus will be referred to as multiple LEDs integrated into the same monolithic chip.

It has been a recent development to use multiple LEDs which are integrated into a monolithic chip instead of a single LED in a chip. Advantages are the improved current spreading across the different LEDs and the fact that the multiple LEDs can be connected in series thereby increasing the required output of a corresponding LED driver that has an efficiency advantage, because electronic drivers with high output voltage (e.g. above 50V) and low current are generally smaller and more efficient than drivers with low voltage and high current.

It is to be understood that throughout the application the use of terms such as 'connect(ed)' and 'interconnect(ed)' refers to electrical connections and that mutual spatial locations will be described by terms such as 'position(ed)' or 'arrange(d)'. Mechanical connections will be referred to by terms such as 'mount(ed)' and 'attach(ed)'.

An example of an assembly comprising multiple LEDs integrated in a single monolithic chip can be found in US application 2008/0246040. The LEDs are serially connected to produce a high drive voltage and a small drive current. Two LED arrays are connected in inverse parallel, so that an AC power supply can be used as the power supply.

A disadvantage of this type of lighting system is that a high voltage at the output side of the driver may not be desirable because of the associated electrical safety issues and approbations. This is especially the case when there is a certain spatial separation between a driver unit and the light generating unit, i.e. the chip. It is possible to provide the driver unit with basic electrical insulation, so that more relaxed safety requirements may apply, but that also involves lowering the drive voltages (depending on country or region) and thus adapting the series connection of the LEDs on the chip.

SUMMARY OF THE INVENTION

It would be desirable to provide an improved LED assembly, in particular a more versatile LED assembly.

To address one or more of these concerns, a LED assembly is provided comprising multiple LED groups consisting of at least two serially connected LEDs, wherein all LEDs are integrated in a single chip, and wherein each LED group has two electrical connection pads configured to connect at least two LED groups in a serial or parallel manner to an external LED driver.

An advantage of the LED assembly according to the invention is that the LED assembly can be used with high- voltage drivers, which is ideal for electrical efficiency, and with low- voltage drivers, which is preferable when electrical safety is critical, without having to adapt the structure of the LED assembly. This reduces the component diversity leading to lower supply chain complexity in the manufacturing process and shorter development times as the LED assembly can be used as an off-the-shelf component.

In an embodiment, the sum of forward voltages of the LEDs of each group does not exceed 60V, preferably does not exceed 50V. This makes the LED assembly in particular suitable for low-voltage applications.

In an embodiment, the sum of forward voltages of all LEDs on the monolithic substrate is substantially equal to the mains voltage. This makes the LED assembly in particular suitable for high-voltage applications. In an embodiment, the groups of LEDs are arranged parallel to each other. This makes the connection to a LED driver in case of a parallel arrangement easy.

In an embodiment, about half of the groups of LEDs has a forward direction opposite to a forward direction of the other groups of LEDs. This makes the possible interconnection of LED groups easier.

In an embodiment, each group of LEDs has a forward direction opposite to the forward direction of adjacent groups of LEDs. This makes the serial connection between adjacent groups easier.

In an embodiment, the serially connected LEDs of each group are connected via anode-cathode through-connections formed during manufacturing of the wafer. In this way, the connections are also integrated in the chip and are reliable permanent connections.

There is further provided a luminaire comprising a LED assembly according to the invention and a LED driver electrically connected to the LED assembly to drive the LED assembly.

In an embodiment, the LED driver is an integrated high- voltage driver or a non- integrated low- voltage driver.

In an embodiment, the LED driver is electrically connected to the LED assembly via bond wires.

It is possible that the LED driver and/or the LED assembly are mounted on a respective submount, wherein the electrical connections between the LED driver and the LED assembly run via the submount of the LED driver and/or the submount of the LED assembly. The electrical connections from the LED assembly to the submount are then preferably formed by bond wires.

There is also provided separately a lighting system, illumination system, projection system, or a display system, wherein said system comprises a LED assembly according to the invention.

There is further provided a method for assembly of a luminaire according to the invention, comprising the following steps:

providing a LED assembly according to the invention;

providing a LED driver;

mounting the LED assembly and the LED driver together;

connecting the LED assembly to the LED driver.

There is also provided the use of a LED assembly according to the invention as a light source. These and other aspects of the invention will be more readily appreciated as the same becomes better understood by reference to the following detailed description and considered in connection with the accompanying drawings in which like reference symbols designate like parts.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 depicts schematically a LED assembly according to an embodiment of the invention;

Figure 2 depicts schematically a luminaire with the LED assembly of Figure 1 according to an embodiment of the invention; and

Figure 3 depicts schematically a luminaire with the LED assembly of Figure 1 according to another embodiment of the invention.

DETAILED DESCRIPTION OF THE DRAWINGS

Figure 1 depicts schematically a LED assembly LA according to an embodiment of the invention, comprising a monolithic chip MS and integrated into the monolithic chip MS multiple LED groups A,B,C,D,E consisting, in this particular embodiment, of five serially connected LEDs. For LED group E, the individual LEDs are designated respectively El, E2,E3,E4,E5. Each LED is schematically indicated by a corresponding square.

Each LED group has two electrical connection pads shown in Figure 1 by small circles and indicated only for LED group E by reference symbols CP1,CP2. The connection pads are configured to connect at least two LED groups in a serial or parallel manner to an external LED driver as will be explained below.

The LEDs of a group are serially interconnected via anode-cathode through connections C. Such connections are embedded, i.e. integrated, into the layer stack formed during wafer manufacturing and thus form a reliable permanent connection between the different LEDs of a group.

An advantage of this LED assembly is that the LED assembly can be used with high- voltage drivers, which is ideal for electrical efficiency, and with low- voltage drivers, which is preferable when electrical safety is critical, without having to adapt the structure of the LED assembly. This reduces the component diversity leading to lower supply chain complexity in the manufacturing process and shorter development times as the LED assembly can be used as an off-the-shelf component. In the embodiment of Figure 1, a LED group consists of five LEDs, and the LED assembly comprises five groups. However, as will be understood by a skilled person, the amount of LEDs in a group and the amount of groups can be optimized for an application depending on the specific requirements of the application. It is further not essential that each group has the same amount of LEDs.

Figure 2 depicts schematically a luminaire LU comprising a submount SM with a LED driver LD provided on the submount SM, and the LED assembly LA of Figure 1 mounted on the submount SM.

The LED groups are connected to the LED driver LD in a parallel manner. The submount therefore comprises conductive traces CT extending from the LED driver to the LED assembly so that a bond wire BW can be provided between a connection pad and the corresponding conductive trace as indicated by reference BW for one connection pad only.

As the LED groups are connected in parallel, the required drive voltage is equal to the forward voltages of five serially interconnected LEDs of a group, so in case the forward voltage of a LED is 3 V, the required drive voltage is approximately 15 V.

Figure 3 depicts schematically the same LED assembly LA of Fig. 1 and 2 mounted in a luminaire LU according to another embodiment of the invention. The structural outline of the luminaire is similar to the embodiment of Fig. 2, but in the embodiment of Fig. 3, the LED groups of LED assembly LA are connected in series to a LED driver LD. The LED driver LD and LED assembly LA are arranged on respective submounts SMI and SM2.

The submount of the LED assembly LA is provided with conductive traces CT. One of the contacting pads is connected using a bond wire BW to one conductive trace CT, and another contacting pad (which belongs to another LED group) is connected using a bond wire BW to the other conductive trace CT. The groups of LEDs are interconnected by bond wires BW to form a series connection of LEDs. For the series connection of the LEDs, an anti-parallel configuration of the groups is advantageous as connecting one group to the other now only requires a bond wire BW between neighboring contacting pads as shown in Fig. 3.

The conductive traces TC come together into a connector CO to which a cable

CA can be connected that connects the LED assembly to the submount SM2 of the LED driver using connector CO' on the submount SM2. The connector CO' in turn is connected to the LED driver LD. The required drive voltage of the LED driver is in this case equal to the forward voltages of twenty-five serially connected LEDs, so in case the forward voltage of a LED is 3V, the required drive voltage amounts to approximately 75V.

As required, detailed embodiments of the present invention are disclosed herein; however, it is to be understood that the disclosed embodiments are merely exemplary of the invention, which can be embodied in various forms. Therefore, specific structural and functional details disclosed herein are not to be interpreted as limiting, but merely as a basis for the claims and as a representative basis for teaching one skilled in the art to variously employ the present invention in virtually any appropriately detailed structure. Further, the terms and phrases used herein are not intended to be limiting, but rather, to provide an understandable description of the invention.

The terms "a" or "an", as used herein, are defined as one or more than one. The term plurality, as used herein, is defined as two or more than two. The term another, as used herein, is defined as at least a second or more. The terms including and/or having, as used herein, are defined as comprising (i.e., open language, not excluding other elements or steps). Any reference signs in the claims should not be construed as limiting the scope of the claims or the invention.

The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage.

The term coupled, as used herein, is defined as connected, although not necessarily directly, and not necessarily mechanically.

A single processor or other unit may fulfill the functions of several items recited in the claims.

The terms program, software application, and the like as used herein, are defined as a sequence of instructions designed for execution on a computer system. A program, computer program, or software application may include a subroutine, a function, a procedure, an object method, an object implementation, an executable application, an applet, a servlet, a source code, an object code, a shared library/dynamic load library and/or other sequence of instructions designed for execution on a computer system.

A computer program may be stored and/or distributed on a suitable medium, such as an optical storage medium or a solid-state medium supplied together with or as part of other hardware, but also be distributed in other forms, such as via the Internet or other wired or wireless telecommunication systems.