FIELD OF THE INVENTION

The invention describes a modular LED string, an LED lighting assembly, and a method of manufacturing an LED lighting assembly.

BACKGROUND OF THE INVENTION

With advances in the manufacture of light-emitting diodes (LEDs), their use in lighting applications such as automotive lighting has become widespread. Such lighting applications are subject to many constraints, including the need to suit design goals. For this reason, the concept of a modular LED string - comprising multiple LEDs soldered to a flexible carrier - was developed. Examples of a modular LED string are given in US2015/0092413A1 and in FR3048056A1. In such a modular LED string, complete groups of series-connected LEDs can be cut from the modular LED string. To facilitate this, the first LED of each group is arranged for connection to a power supply, and the final LED of each group is arranged for connection to ground. Suitable conductive tracks for the power supply and for ground can run the length of the carrier. There can be any number of inner LEDs between the first and final LED of a series- connected group, but such groups are usually quite short and comprise only a few LEDs in series. The advantage of such an approach is that, regardless of how many such groups are present on a string (after cutting a section of complete groups from the string), the anodes of all first LEDs of the groups can be connected to a common power supply, and the cathodes of all final LEDs of the groups can be connected to common ground.

Each LED of a group therefore has specific connection requirements, e.g. the cathode of the first LED must be connected to the anode of the second LED; the cathode of the final LED must be connected to ground, etc. For ease of manufacturing, an LED may be provided on a small printed circuit board (PCB) with contact pads on its underside. The contact pads of such an LED module are formed according to the intended position of the LED module in a series-connected string. A driver (for supplying power to the modular LED string) therefore also only needs one voltage output for connection to the corresponding supply voltage track on the carrier, and a return or ground terminal for connection to the corresponding return track on the carrier.

While the modular LED string described above has many advantages, it also suffers from a limitation. This is because of the specific connection requirements of the LEDs of a series-connected group as described above. These connection requirements mean that a prior art modular LED string can only be cut between complete groups. Therefore, if a series-connected LED group has a certain module length (e.g. 30 mm for three LEDs in a group), the length of a string section cut from that modular LED string will always be a multiple of that module length (e.g. 12 cm in the case of four 30 mm groups).

However, certain design requirements may specify a different LED string length (e.g. 10 cm). To achieve "non-standard" lengths, it is known from the prior art to add corrective circuitry or to add a supplemental LED circuit portion to obtain the desired number of LEDs and/or the desired string length. However, these modifications are time-consuming and costly.

Therefore, it is an object of the invention to provide an alternative modular LED string that overcomes the problems outlined above.

SUMMARY OF THE INVENTION

The object of the invention is achieved by the modular LED string of claim 1; by the LED lighting assembly of claim 11; and by the method of claim 15 of manufacturing an LED lighting assembly.

According to the invention, the modular LED string comprises a plurality of LED module groups mounted on a carrier, wherein an LED module group comprises a series-connected row of three or more LED modules commencing with a first LED module followed by a number of inner LED modules and terminated by a final LED module. The inventive modular LED string features a plurality of main contact pads, each of which can be used to make an electrical connection to a driver, wherein each main contact pad is electrically connected to a main supply track formed on the carrier and is arranged between neighbouring LED modules; and also a plurality of secondary contact pads, each of which can be used to make an electrical connection to a driver, and wherein each secondary contact pad is electrically connected to a secondary supply track formed on the carrier and is arranged on either side of an inner LED module. In the inventive modular LED string, an anode contact of each first LED module is electrically connected to the main supply track, and an anode contact of each remaining LED module is electrically connected to the secondary supply track. The inventive modular LED string is manufactured to permit cutting through an LED module group to remove one or more LED modules from a complete group, so that the cut section can commence with an incomplete LED module group, and so that the cut section can be connected to a driver directly, i.e. without any need for modification.

An advantage of the inventive modular LED string is that its length - i.e. the number of LED modules it contains - can easily be adapted to the requirements of the intended lighting application and that the length of a cut string section is not limited to multiples of the LED module length. This is a distinct advantage over the prior art, in which such modular LED strings can only be cut between complete groups as explained in the introduction.

According to the invention, the LED lighting assembly comprises a driver realised to provide a first positive supply voltage at a first voltage output and to provide a second positive supply voltage at a second voltage output; an LED string section cut from an embodiment of the inventive modular LED string; and at least an electrical connection between the first voltage output and the main contact pad at the cut edge of the cut string section.

An advantage of the inventive LED lighting assembly is that it can be used in a greater variety of lighting applications, since the driver can be used to drive a cut LED string section that commences with a complete LED module group, or a cut LED string section that commences with an inner LED module or a final LED module. The cost of the added functionality of the driver (to provide the second positive supply voltage) can be outweighed by the savings in manufacturing, since it is no longer necessary to perform expensive modifications to a cut LED string section in order to achieve a non-modular length. According to the invention, the method of manufacturing such an LED lighting assembly comprises the steps of shortening an embodiment of the inventive modular LED string to remove at least one LED module (e.g. just the first LED module, or the first LED module and also one or more inner LED modules); providing a driver with a first voltage output and a second voltage output; forming an electrical connection between the first voltage output and the main contact pad at the cut edge of the LED string section; and forming an electrical connection between the second voltage output and the secondary contact pad at the cut edge of the LED string section.

The dependent claims and the following description disclose particularly advantageous embodiments and features of the invention. Features of the embodiments may be combined as appropriate. Features described in the context of one claim category can apply equally to another claim category.

In the context of the invention, the carrier may be assumed to be a narrow flexible band or strip onto which the LED modules are mounted. A design feature of such a modular LED string is the ability to create luminaries with different shapes by bending the modular LED string as appropriate. The flexible carrier can be realised as a "flex PCB", i.e. as a flexible printed circuit board primarily comprising a polymer band (e.g. polyimide) or laminated polymer layers upon which copper tracks can be patterned. The LED module components may be soldered directly onto the copper tracks on the flex PCB. Alternatively, the flexible carrier may be a thicker body that can be bent into a desired shape, and which holds its shape after bending. In such an embodiment, the LED modules may be provided as preassembled PCB elements that can be attached to the carrier and connected using thin wires. The LED modules and wiring can be enclosed using a suitable transparent or translucent potting material such as a polysiloxane to protect the circuit elements from damage, to hermetically seal the circuit, to diffuse the light, etc. This protective material coating can be formed in an over-moulding step, as will be known to the skilled person. The inventive modular LED string may comprise any number of LED modules mounted to such a flexible carrier. The terms "modular LED string" and "LED string" may be used interchangeably in the following.

In the following, but without restricting the invention in any way, it may be assumed that the flexible carrier is a "flex PCB" and that each LED module comprises a single LED die such as a surface-mount die (SMD) mounted on a small printed circuit board or interposer. An LED module may also be assumed to have contact pads on its lower surface, arranged to form interconnects with conductive tracks on a flexPCB, for example. In the following, for the sake of simplicity, it may be assumed that the flexible carrier is provided with conductive tracks to form the series-connections between the LED modules. The three different types of LED module (first, inner and final) have appropriate contact pad arrangements, since a first LED module will be connected between the main supply track and the secondary supply track, an inner LED module will be connected between consecutive sections of the secondary supply track, and a final LED module be connected between the secondary supply track and a ground or return track. An LED module can be mounted to the carrier by forming solder interconnects between its contact pads and the conductive tracks of the carrier, for example in a reflow solder process.

The expression "complete LED group" or "complete group" is used to refer to a series-connected group of LED modules, beginning with a first LED module and ending with a final LED module. The modular LED string comprises any number of such complete groups, and the groups are connected in parallel. The anodes of all first LED modules are connected to the main supply track, and the cathodes of all final LED modules are connected to the return track. A complete group may comprise more than one inner LED module. However, in a preferred embodiment of the invention, a complete group comprises a single inner LED module, so that the complete group comprises a total of three series-connected LED modules. In the following, but without restricting the invention in any way, it may be assumed that a complete group comprises three series-connected LED modules as described above. The inventive LED string can be manufactured to comprise any number of such complete groups.

In a particularly preferred embodiment of the invention, the modular LED string is manufactured to permit cutting through the carrier and the supply tracks, for example along marked cutting lines. A "group cutting line" can indicate where to cut the modular LED string to obtain a cut string section that commences with a complete group, i.e. the cut string section commences with a first LED module. Another cutting line can indicate where to cut the modular LED string in order to obtain a cut string section that commences with an inner LED module. Another cutting line can indicate where to cut the modular LED string in order to obtain a cut string section that commences with a final LED module. After cutting along such an "inner" cutting line, the resulting cut LED string section will start with an inner LED module or a final LED module, followed by any number of complete groups. In a flexPCB embodiment that has conductive tracks on its upper surface, the cutting lines preferably extend midway through a set of contact pads, for example through all three contact pads (main contact pad, secondary contact pad and return contact pad) in front of the final LED module. Preferably, the contact pads are large enough to be solderable after being cut in half, and may have any suitable shape such as an oval shape, a figure-of-eight shape, etc. Such a contact pad is preferably arranged symmetrically about the cutting line. The length of a contact pad is preferably significantly longer than the width of the contact pad, so that after cutting through the contact pad, the remaining area (or "half contact pad") is sufficiently large to form a solder connection. In a particularly preferred embodiment of the invention, the aspect ratio of a contact pad is 2: 1, i.e. the contact pad is twice as long as it is wide. With these measures, it is always possible to easily connect a cut string section to the driver, since it is straightforward to solder leads to the favourably large half contact pads. Alternatively, the cutting lines may be positioned to one side of the contact pads so that cutting does not affect the contact pads, which remain on the carrier. In an embodiment in which the LED modules are connected by wires, the cut ends of the wires may simply be used to form the electrical connections to the driver. The driver of a modular LED string is generally realised to provide a voltage difference across the parallel-connected LED module groups, which voltage difference is at least the sum of the forward voltages of the series-connected LEDs of a group. To connect the main positive supply voltage to each complete LED module group, the main supply track preferably extends along the length of the carrier and can be connected via any main contact pad to the first voltage output of the driver. To connect each LED module group to ground, the carrier is patterned with a return track that can be connected via any return contact pad to the negative supply voltage. The return track or ground track can extend along the length of the carrier. The cathode of each final LED module is electrically connected to the return track.

To achieve a series-connection between the first LED module and the inner LED module of a complete group, and to achieve a series-connection between an inner LED module and the final LED module, the secondary supply track is patterned as a number of successive sections, which are preferably arranged in line with each other and separated by gaps. In the case of a modular LED string in which complete groups each comprise a total of three LED modules, the secondary supply track comprises two sections for each group. For a complete group, the first section of the secondary supply track is electrically connected at one end to the cathode contact of the first LED module, and at its other end to the anode contact of the inner LED module; the second section of the secondary supply track is electrically connected at one end to the cathode contact of the inner LED module, and at its other end to the anode of the final LED module.

In the inventive approach, any section of the secondary supply track can be connected via a secondary contact pad to the secondary voltage output of the driver. In this way, when the cut string section commences with an inner LED module or a final LED module, the LED lighting assembly comprises an electrical connection between the second voltage output and the secondary contact pad at the cut edge of the shortened modular LED string. In this way, the anode of the inner LED module (or final LED module) can easily be connected to a suitable supply voltage that is lower than the primary supply voltage. The secondary supply voltage is lower than the primary supply voltage since it does not need to drive a complete string of LED modules.

As explained above, the inventive modular LED string can be cut to obtain a string section of the desired length. Assuming M LED modules in each complete group, the length of a cut string section can be expressed as

L = mL _M + nL _G for 0 < m < M and n > 1 (1) where L _M is the length of an LED module, m is the number of LED modules in the incomplete group, LG is the length of a complete group, and n is the number of complete groups attached to the incomplete group. With an LED module length of 10 mm, a complete group will be 30 mm in length. A cut string section can then comprise an integer multiple of 30 mm, with an additional 10 mm or 20 mm. In this way it is possible to cut an LED strip to a length of 40 mm, 70 mm, 110 mm, etc. This is in contrast to the prior art modular LED string which can only be cut between complete groups, and for which the length of a cut string section can be expressed as

L = nL _G for n > 1 (2) where L _G is the length of a complete group, and n is the number of complete modules in the strip after cutting. In the prior art, it is only possible to cut an LED strip to a length of 30 mm, 60 mm, 90 mm etc., when a complete group has a length of 30 mm.

When a cut string section commences with a complete LED group, the second voltage output of the LED lighting assembly is not connected to the cut string section, i.e. this voltage output of the driver is left open. To drive the complete LED groups of the cut string section, the driver provides the primary supply voltage at the first voltage output. To be able to drive a cut string section commencing with an incomplete or shortened group, the driver is realised to further provide a secondary supply voltage at the second voltage output. To be able to adjust to the LED count of a cut string section, the driver is preferably a programmable driver or a self-adjusting driver. The number of parallel-connected LEDs in complete groups is relevant to the primary voltage output, while the number of series-connected LEDs in the first string section is relevant to the secondary voltage output. An adjustable current-regulated power-source may be used to provide the primary voltage output. Preferably, the driver comprises a current-regulated source to provide the secondary voltage, so that the driver does not need to be re-configured if the first string section has been cut. Other objects and features of the present invention will become apparent from the following detailed descriptions considered in conjunction with the accompanying drawings. It is to be understood, however, that the drawings are designed solely for the purposes of illustration and not as a definition of the limits of the invention. BRIEF DESCRIPTION OF THE DRAWINGS

Fig 1 shows an embodiment of the inventive modular LED string;

Fig 2 shows a pattern of conductive tracks on a carrier of the modular

LED string of Fig 1;

Fig 3 shows the undersides of LED modules of the modular LED string of Fig 1;

Fig 4 shows LED modules of the modular LED string of Fig 1 ;

Figs 5 - 7 show various embodiments of the inventive LED lighting

assembly.

In the drawings, like numbers refer to like objects throughout. Objects in the diagrams are not necessarily drawn to scale.

DETAILED DESCRIPTION OF THE EMBODIMENTS

Fig 1 shows an embodiment of the inventive modular LED string 1. The drawing shows a flexible carrier 10, which can for example be made of a material such as polyimide. Several conductive tracks 11, 12, 13 have been patterned onto the carrier 10, for example by etching from a copper layer, or by printing. Fig 2 shows a carrier 10 with such a pattern of conductive tracks 11, 12, 13. These tracks comprise a main supply track 11 for connection to a primary power supply; secondary supply track sections 12 which are used to serially connect the LEDs in sequence and which can - if necessary - be connected to a secondary power supply; and a return track 13 for connection to a negative power terminal or ground.

Fig 1 also shows groups G of LED modules Ml, M2, M3 attached to the carrier by interconnects (e.g. solder bonds) between contact pads of the LED modules Ml, M2, M3 and the conductive tracks. Each LED module Ml, M2, M3 comprises an SMD LED soldered to a small PCB or interposer which has been prepared with vias and conductive tracks for connecting the anode of the LED die to an anode contact pad, and for connecting the cathode of the LED die to a cathode contact pad. In this exemplary embodiment, each group G comprises three LED modules Ml, M2, M3, namely a first LED module Ml, an inner LED module M2, and a final LED module M3

Fig 3 shows the undersides of the interposers PI, P2, P3 of the LED modules Ml, M2, M3 to indicate the different contact pads required for the different LED modules Ml, M2, M3. Fig 4 shows the circuit components of each LED module Ml, M2, M3. To connect the first LED module Ml to a primary power supply, it has an anode contact pad Pl_a arranged for a solder interconnect to the main supply track 11. A resistor 21 is arranged in series between the anode contact pad Pl_a and the LED 20 of the first LED module ML The first LED module Ml also has a cathode contact pad Pi c arranged for a solder interconnect to a secondary supply track section 12.

To connect the first LED module Ml in series with the inner LED module M2, or to connect the inner LED module M2 directly to the secondary power supply, the inner LED module M2 has an anode contact pad P2_a arranged for a solder interconnect to a secondary supply track section 12. In this exemplary embodiment, a resistor 22 is arranged in series between the anode contact pad P2_a and the LED 20 of the inner LED module M2. The inner LED module M2 also has a cathode contact pad P2_c arranged for a solder interconnect to the next secondary supply track section 12.

To connect the inner LED module M2 in series with the third LED module M3, this has an anode contact pad P3_a arranged for a solder interconnect to the secondary supply track section 12. The cathode contact pad P3_c of the third LED module M3 is arranged for a solder interconnect to the return track 13.

As known from similar prior art products, the inventive modular LED string 1 can be cut between groups G along cutting lines X _cut3 , so that a shorter piece comprising an integer number of groups G can be cut from the long strip 1. In contrast to the prior art, the inventive modular LED string 1 (as shown in Fig 1 and Fig 2) can also be cut between LED modules Ml, M2, M3 along cutting lines X _cut2 , X _cuti , so that a section of carrier with any number of LED modules can be cut from a long strip.

However, instead of then having to carry out modifications in order to be able to drive the shortened section correctly, the inventive modular LED string 1 is provided with strategically placed contact pads. As shown in Fig 1 and Fig 2, main contact pads 11C are provided between adjacent LED modules Ml, M2, M3, so that the primary power supply can always be connected to the main supply track 11, regardless of where the LED string 1 is cut. Secondary contact pads 12C are provided on both sides of every inner LED module M2, so that the secondary power supply can always be connected to a secondary supply track section 12, regardless of where the LED string 1 is cut. Return contact pads 13C are provided between adjacent LED modules Ml, M2, M3, so that a negative or return voltage can always be connected to the return track 13, regardless of where the LED string 1 is cut. Fig 2 indicates a preferred shape for the contact pads l lC, 12C, 13C, in this case a figure-of-eight shape.

Fig 5 shows a first embodiment of the inventive LED lighting assembly 3. The diagram shows a constant current driver 4 connected to a cut section l _cut 2 of the inventive modular LED string 1 described in Figs 1 - 4. In this exemplary embodiment, the driver 4 is an adjustable or self-adjusting (current-limiting) driver.

In this embodiment, the modular LED string section l _cut 2 has been cut along cutting line X _cut 2 so that section l _cut 2 commences with an incomplete group, i.e. with the inner LED module M2 of a two-module shortened group. The cut section l _cut 2 can comprise any number of further complete groups G, and only one such group is shown here as an example. The total length of the cut section l _cut 2 in this example is LG + 2L _M , and is not constrained to a length of n x LG, which would be the case using a prior art LED string which can only be cut between complete groups.

The driver 4 is realised to provide a primary voltage at a first voltage output 41, and - if necessary - to provide a secondary voltage at a second voltage output 42. The driver 4 also provides a negative voltage or ground at a return terminal 43. The primary voltage is at a level that is suitable to drive complete groups G and will be connected via the main contact pad 11C at the cut edge of the shortened group to the anodes of the first LED modules Ml of any complete group G of the cut section l _cut 2. In this exemplary embodiment, the cut section l _cut 2 comprises a two-module shorted group (LED modules M2, M3) and a complete group, i.e. five LED modules in total. The secondary voltage is at a level that is suitable to drive a shortened group and will be connected, via the secondary contact pad 12C at the cut edge of the shortened group, to the anode of an inner LED module M2. The driver 4 is realised to provide essentially the same current level ILED at each voltage output 41, 42 when connected to a load. The return voltage is at ground level or a suitable negative voltage level, and will be connected via the return contact pad 13C at the cut edge of the shortened group to the cathodes of the final LED modules M3 of the cut section l _cut 2. The anode of the inner LED module M2 is connected via the secondary contact pad 12C to the secondary voltage output 42 of the driver 4 by means of an electrical connection 420. The primary voltage output 41 is connected via the main contact pad 11C to the main supply track 11 of the cut section l _cut 2 by means of an electrical connection 410, and the return terminal 43 is connected via the return contact pad 13C to the return track 13 by means of an electrical connection 430.

Fig 6 shows another embodiment of the inventive LED lighting assembly 3, with the same driver 4 as described in Fig 5. In this embodiment, the cut section l _cuti has been cut along cutting line X _cuti so that the cut section l _cuti consists of only one LED module, i.e. the final LED module M3. The cut section l _cuti can continue with any number of further complete groups G, and only one complete group is shown here as an example. The total length of the cut section l _cuti is LG + LM.

The primary voltage output 41 is connected, via the main contact pad 11C at the cut edge of the shortened group, to the main supply track 11 of the cut section lcuti . As explained in Fig 4, the anode of the final LED module M3 is connected via the secondary contact pad 12C at the cut edge of the shortened group, to the secondary voltage output 42 of the driver 4 by means of an electrical connection 420. The cathodes of all final LED modules M3 are connected to the return track 13, which is connected, via the return contact pad 13C at the cut edge of the shortened group, to the return terminal 43 by means of an electrical connection 430.

Fig 7 shows another embodiment of the inventive LED lighting assembly 3. In this embodiment, the cut section l _cut 3 has been cut along cutting line X _cut 3 so that the cut section l _cut 3 commences with a complete group G. The cut section l _cut 3 can continue with any number of further complete groups G. Any suitable driver that is realised to provide a suitable voltage difference can be connected across the main supply track 11 and the return track 13. Alternatively, it is possible to use the same driver 4 as described in Fig 5 and Fig 6 above, in which case the secondary voltage output 42 is not required and can be left unconnected or open. This embodiment would correspond to a prior art LED lighting assembly that is required to use complete LED module groups.

Although the present invention has been disclosed in the form of preferred embodiments and variations thereon, it will be understood that numerous additional modifications and variations could be made thereto without departing from the scope of the invention.

For the sake of clarity, it is to be understood that the use of "a" or "an" throughout this application does not exclude a plurality, and "comprising" does not exclude other steps or elements. The mention of a "unit" or a "module" does not preclude the use of more than one unit or module.

REFERENCE SIGNS:

LED string assembly 1

cut LED string section lcut2, lcutl, lcut3 carrier 10

main supply track 11

main contact pad l ie

secondary supply track 12

secondary contact pad 12C

return supply track 13

return contact pad 13C

LED 20

resistor 21, 22, 23 driver 4

supply voltage terminal 41

supply voltage terminal 42

return terminal 43

electrical connection 410, 420, 430 complete groups G

LED modules Ml, M2, M3 cutting line Xcut3, Xcut2, Xcutl contact pad PI a, Pi c contact pad P2_a, P2_c contact pad P3_a, P3_c

LED module length LM

group length LG