GB2501758A - LED strip lighting with heat sink - Google Patents

Abstract

A lighting component 30 comprises an array of LEDs 34 supported on a substrate 36. A plurality of conducting tracks or strips 52 provide current carrying connections between the LEDs 34. This connects the LEDs 34 to form a plurality of strings of LEDs connected in series along each string and also provides at least some coupling connections between different strings at intermediate positions along the lengths of the strings. The coupling connections provided by the strips 52 are sufficiently large to serve as heat sinks.

Description

Improvements in or relating to LED lighting The present invention concerns improvements in or relating to LED fighting.

Significant work has been undertaken in recent years to replace fluorescent, incandescent and other traditional lamp technologies with LED light sources, with a view to providing light fittings, such as luminaires, which are more energy efficient, more resistant to failure and longer lasting. An LED represents an efficient light source which io can have a very long life and can be reliable, but only if the environment in which the LED is used is adequate to protect it from adverse thermal conditions, mechanical failure of solder joints or other connections, etc. Examples of the present invention provide a lighting component comprising an array of LEDs supported on a substrate, and a plurality of conducting tracks providing current-carrying connections between the LEDs, during use, and wherein the connections connect the LEDs to form a plurality of strings of LEDs connected in series along each string; and wherein the connections further provide at least some coupling connections between different strings at an intermediate position along the length of the strings; and wherein the coupling connections serve, in use, as heatsinks.

The coupling connections may be areas of conducting track. At least one coupling connection may be provided by an area of conducting track which also provides a series connection in at least two strings. At least one coupling connection may be provided by an area of conducting track which provides a series connection in each string. At least one coupling connection may be provided by a conducting track which extends across the substrate to extend between a corresponding pair of adjacent LEDs in each string, to provide series connections between each of those pairs, and coupling connections between the strings.

The component may comprise a plurality of conducting strips extending across the substrate and separated by gaps, each string of LEDs being provided by an LED connected across each gap. The plurality of conducting strips may be straight and may be parallel. The width of each conducting strip may be at least 10 times the width of s each gap.

The component may comprise a first strip to which the first LED of each string is connected, and a last strip to which the last LED of each string is connected, and in which the first and last strips provide terminals for connection to a power supply. The io first and last strips may comprise terminals for connection to the first and last strips of another like component, to allow the LED strings of both components to be connected in parallel to a power supply.

The component may be connected to a current supply which, in use, supplies sufficient is current for the LEDs to be running at less than 75% of their maximum rated value, when all LEDs are functioning normally. Alternatively, the LEDs may be running at less than 50% of their maximum rated values. The LEDs which populate the array may be chosen to have matched dynamic resistances when operating. The component may comprise at least eight strings of LEDs.

The component may further comprise an emergency LED provided with a power supply which is independent of the supply to the array of LEDs, for illumination when there is a failure in the supply to the array. The emergency LED may be in thermal contact with at least one of the coupling connections. The substrate may carry the conducting tracks on a first face, and a layer of thermally conductive material on another face to serve as a heatsink, there being at least one thermal via between the thermally conducting material and the coupling connection with which the emergency LED is in thermal contact.

In another example, examples of the present invention provide a lighting component comprising a substrate which! in use, supports an array of LEDs, and comprises a plurality of conducting tracks for providing, in use, current-carrying connections between the LEDs, and wherein the connections serve to connect the LEDs to form a plurality of strings of LEDs connected in series along each string; s and wherein the connections further provide at least some coupling connections between different strings at an intermediate position along the length of the strings; and wherein the coupling connections serve, in use, as heatsinks.

Features of the first aspect of the invention may also be incorporated in this aspect.

Examples of the present invention will now be described in more detail, by way of example only, and with reference to the accompanying drawings, in which: Fig. 1 is a circuit diagram of an array of LEDs organised in parallel strings to form a is lighting component; Fig. 2 is a circuit diagram of an improved array of LEDs organised in parallel strings, * with additional connections provided to improve the performance of the array; Fig. 3 is a plan view of a lighting component embodying the circuit diagram of Fig. 2; and Fig. 4 and Fig. 5 are sectional views through the lighting component of Fig. 3, taken at the lines 4-4 and 5-5, respectively.

Fig. 1 illustrates a lighting component 10 comprising an array 12 of LEDs 14 supported on a substrate 16, and a plurality of conducting tracks providing current-carrying connections 18 between the LEDs, during use. The connections 18 connect the LEDs 14 to form a plurality of strings 20 of LEDs 14 connected in series along each string 20.

The strings 20 are connected in parallel by end connectors 22 which are in turn connected to respective terminals of a power supply 24.

It can readily be understood from Fig. 1 that if one of the LEDs 14 fails for any reason (such as the LED 14 indicated at 26 in Fig. 1), the whole of the string 20 containing the failed LED 26 will cease to function. Thus, the failure of a single LED can disable a much larger number of LEDs, because a luminaire may contain strings of 10, 20 or more LEDs. Furthermore, the failure of one string 20 may have the effect of directing additional current through other strings 20, possibly overloading components in those other strings and creating additional failures, disabling additional strings 20. Thus, the geometry with which the LEDs 14 are connected in Fig. 1 has the inherent risk of io failures creating a series of failures which cascade through the array 12.

Failures may arise from the failure of the LED itself, perhaps as a result of overheating, failures of solder joints or other mechanical connections, perhaps due to ageing, or other causes.

Protection by coupling connections Fig. 2 illustrates an alternative circuit geometry. A lighting component 30 comprises an array 32 of LEDs 34 supported an a substrate 36, which is non-conducting, and carries a plurality of conducting tracks providing current-carrying connections 38 between the LEDs 34, during use. As before (Fig. 1), the connections 38 connect the LEDs 34 to form a plurality of strings 40 of LEDs 34 connected in series along each string 40.

Again, end connections 42 provide for connection of the strings 40 to a power supply 44, thus connecting the strings 40 in parallel across the power supply 44.

In addition, the connections 38 further provide at least same coupling connections 46 between different strings 40 at intermediate positions 48 along the length of the strings 40. In the example illustrated in Fig. 2, a coupling connection 46 is provided between each consecutive pair of LEDs 34 in each of the strings 40, and the corresponding position in the two neighbouring strings 40. This creates a circuit geometry which is more robust against failure than the geometry of Fig. 1, as follows. In the event that there is failure of a single LED, such as that illustrated at 50, current may cease to flow through the failed LED 50. However, the other LEDs 34 in the string 40 containing the failed LED 50 are not disabled in this example. Current flowing through the string 40 which contains the failed LED 50 can bypass the failed LED 50, by passing along coupling connections 46, through LEDs 34 in other strings 40, and back through coupling connections 46 to continue flowing through the string 40 of the failed LED 50, downstream of the failed LED 50.

Consequently, the failure of a single LED 50 does not immediately disable other LEDs 34. Moreover, although the failure of the LED 50 may result in additional current flowing through the other LEDs 34 used to bypass the failed LED 50, the coupling connections 46 allow this additional current to be shared among several other LEDs 34, thus reducing the risk that other LED failures are triggered to fail by the failure of the LED 50.

This reduces the risk of cascade failures through the array. Clearly, these risks are is reduced most if every coupling connection 46 is present, as illustrated in Fig. 2, but some could be removed, without losing all of the risk reduction described.

Thermal protection One cause of failure of LEDs in practice relates to their thermal environment. Modern LEDs are desirably kept cool, for maximum working life and greatest efficiency. The mechanical arrangement of Fig. 3 allows the circuit geometry of Fig. 2 to be implemented in a manner which also provides enhanced thermal treatment of the LEDs 34.

In the example of Fig. 3, the component 30 has a plurality of conducting strips 52 extending across the substrate 36 and separated by gaps 54. The strips may be copper strips, allowing them to be formed by conventional PCB production techniques. Each LED 34 is contained in a conventional package, which may be square in shape and which is mounted on the substrate 36 to span one of the gaps 54, between adjacent strips 52, thereby electrically connecting the LED 34 to the two strips 52. This can be seen in Fig. 4 which shows the package of an LED 34 connected across the gap 54 between two strips 52, by solder joints 55. Thus, each string 40 of LEDs 34 is provided by an LED 34 connected across each gap 54. In this example, each of the conducting strips 52 is straight and the strips 52 are parallel with each other. Other geometries could be used. In this example, the width of each conducting strip 52 is much greater than the width of each gap 54. For example, the width of each conducting strip 52 may be at least 10 times the width of each gap 54.

A comparison of Fig. 3 and Fig. 2 reveals that each string 40 is formed by consecutive LEDs which are connected across consecutive gaps 54 along a string 40, so that the strips 52 each provide a series connection in at least two strings. In the example illustrated, each strip 52 provides series connections in each of the strings 40. In addition, each of the strips 52 provides coupling connections 46 between adjacent strings 40. In this example, each coupling connection 46 is provided by a strip 52 which is is a conducting track extending across the substrate 36 to extend between a corresponding pair of adjacent LEDs 34 in each string 40, to provide series connections between each of those pairs, and coupling connections 46 between each of the strings 40. The large width of the strips 52, compared with the width of the gaps 54, results in the coupling connections 46 being provided by areas of conducting track which are sufficiently large to serve, in use, as heatsinks. Thus, the LEDs 34 are thermally coupled with the strips 52, as well as being electrically connected with the strips 52.

The arrangement of Fig. 3 also includes a first strip 56 and a last strip 58. The first LED 34 of each string 40 is connected to the first strip 56. The last LED 34 of each string 40 is connected to the last strip 58. Accordingly, the first strip 56 and the last strip 58 provide the end connections 42 of the circuit geometry of Fig. 2. The first strip 56 and the last strip 58 comprise terminals for connection to the power supply 44.

In addition, the terminals 60 of the first strip 56 and the last strip 58 allow for connection to the first and last strips 56, 58 of another like component 30. In this example, the terminals 60 are provided as notches at the edge of the substrate 36, allowing appropriate electrical connectors to be used to connect the first strip 56 to the first strip of another substrate 36 placed alongside the illustrated substrate 36, as indicated by the broken lines 62, Similarly, the last strip 58 can be connected to the last strip of a neighbouring substrate 62. This results in the LED strings 40 of the two substrates 36, 62 being connected in parallel across the power supply 44.

The arrangement of Fig. 3 provides the advantage of allowing a failed LED to be bypassed, as described above in relation to Fig. 2. In addition, the provision of thermal contact between the packages of the LEDs 34, and the strips 52 allows the whole area of the strips 52 to provide a heatsink function for the LEDs 34. The strips 52 cover almost the whole area of the substrate 36, with the exception of the gaps 54, which are relatively narrow compared with the strips 52, so that almost the whole area of the substrate 36 is available for use as a heatsink in this manner. This is expected to allow adequate control of the thermal operating conditions of the LEDs, without each LED being provided with a dedicated conventional heatsink arrangement. Thus, the heatsink function of the strips 52 is expected to reduce the risk of failure through thermal effects.

In addition, the provision of coupling connections 46 is expected to reduce the risk of faults cascading through the array 32, as noted above. The risk of cascading faults can be further reduced by arranging for the supply current of the power supply 44 to be only sufficient for the LEDs to be running at less than 75%, or less than 50% of maximum rated value, when all LEDs are functioning normally. This will provide significant additional current-carrying capacity in each LED, before maximum rated values are reached, allowing each LED to accommodate some additional current in the event of failure of a neighbouring LED. For example, if the array 32 includes ten parallel strings of LEDs, each running at 50% of the maximum rated value, failure of one of those LEDs will create additional current through the other nine LEDs used as by-pass LEDs. If this additional current is evenly distributed among those other nine LEDs, they will then each be running at around 55% of their maximum rated value, well within their rated ability. Accordingly, the risk of one failure resulting in the failure of one of the bypass LEDs is considered to be acceptably small.

A further possibility for reducing the risk of cascade failures is available by making careful choices of the LEDs 34 used in the array 32. These may be chosen so that the array is populated with LEDs which have closely matched performance, particularly their dynamic resistances when operating. This is expected to improve the degree to which s currents are evenly distributed among the various LEDs, both in normal and in fault conditions.

Emergency provision The arrangement of Fig. 3 also has provision for an emergency LED 64. It is a common requirement for luminaires to be able to provide emergency illumination, in the event that the primary systems fail. In the example of Fig. 3, a large area of one of the strips 52, indicated at 66, is left uncovered by the material of the strip 52. Within the area 66, adthtional conducting tracks 68 are provided for connecting the emergency LED 64 to an emergency power supply 70. In the event that there is failure of the primary system (failure of all of the LED5 34, arising from failure of the power supply 44, or otherwise), the emergency power supply 70 will cause the emergency LED 64 to illuminate.

The emergency LED 64 is advantageously a high-power LED. Accordingly, an issue again arises of removal of heat in order to protect the LED 64. Arrangements for thermal protection of the LED 64 can be understood by considering Fig. 5. Fig. 5 illustrates the package of the LED 64 connected by solder joints 72 to the tracks 68 and also connected to another area 74 of thermally conductive material, such as copper, by a thermally conducting compound 76, of the type which is well known in itself in the field of PCB construction. The area 74 may be provided by the adjacent strip 52, or may be provided separately, on the same face of the substrate 36 as the strips 52. In this example, the reverse face of the substrate 36 is substantially entirely covered with a metal layer 78 providing good thermal conductivity and large surface area over the reverse face of the substrate 36. The area 74 is thermally coupled with the metal layer 78 by one or more thermal vias 80. Thermal vias will be understood by the skilled reader to consist of a solid or hollow column of thermally conducting material, such as a metal layer or pin, in good thermal contact with the area 74 and the layer 78.

Accordingly, a heatsink function is provided for the emergency LED 64 either by the whole of the area 74, or by the whole area of the metal layer 78, or by both, thus greatiy improving the expected life of the LED 64, even when that is a high-power LED S operating under emergency conditions.

Concluding comments In the examples described, the substrates are populated with LEDs but it is to be understood that the substrates can be manufactured without LEDs, for subsequent population. Many variations can be made to the arrangements described above, particularly in relation to the number of LEDs in each string, the number of strings, the geometry of the arrangement, the nature of provision for emergency operation, and other features.

Whilst endeavouring in the foregoing specification to draw attention to those features of the invention believed to be of particular importance it should be understood that the Applicant claims protection in respect of any patentable feature or combination of features hereinbefore referred to and/or shown in the drawings whether or not particular emphasis has been placed thereon.

Claims (4)

1. CLAIMS1. A lighting component comprising an array of LEDs supported on a substrate, and a plurality of conducting tracks providing current-carrying connections between the s LED5, during use, and wherein the connections donnect the LEDs to form a plurality of strings of LEDs connected in series along each string; and wherein the connections further provide at least some coupling connections between different strings at an intermediate position along the length of the strings; io and wherein the coupling connections serve, in use, as heatsinks.
2. 2. A component according to claim 1, wherein the coupling connections are areas of conducting track.is
3. 3. A component according to claim 1 or 2, wherein at least one coupling connection is provided by an area of conducting track which also provides a series connection in at least two strings.
4. 4. A component according to claim 3, wherein at least one coupling connection is * 20 provided by an area of conducting track which provides a series connection in each string. * *5. A component according to any preceding claim, wherein at least one coupling ". connection is provided by a conducting track which extends across the substrate to * 2 extend between a corresponding pair of adjacent LEDs in each string, to provide series " connections between each of those pairs, and coupling connections between the *es.*:. strings.6. A component according to any preceding claim, wherein the component comprises a plurality of conducting strips extending across the substrate and separated by gaps, each string of LED5 being provided by an LED connected across each gap.7. A component according to claim 6, wherein the plurality of conducting strips are straight.s 8. A component according to claim 7, wherein the plurality of conducting strips are parallel.9. A component according to claim 6, 7 or 8, wherein the width of each conducting strip is at least 10 times the width of each gap.10. A component according to any preceding claim, comprising a first strip to which the first LED of each string is connected, and a last strip to which the last LED of each string is connected, and in which the first and last strips provide terminals for connection to a power supply.11. A component according to claim 10, wherein the first and last strips comprise terminals for connection to the first and last strips of another like component, to allow the LED strings of both components to be connected in parallel to a power supply.12. A component according to any preceding claim, connected to a current supply 4 which, in use, supplies sufficient current for the LEDs to be running at less than 75% of r" their maximum rated value, when all LEDs are functioning normally."° 13. A component according to claim 12, wherein, in use, the LED5 run at less than 2 50% of their maximum rated values. e*.a$ 14. A component according to any preceding claim, wherein the LEDs which populate the array are chosen to have matched dynamic resistances when operatihg.15. A component according to any preceding claim, comprising at least eight strings of LEDs.16. A component according to any preceding claim, further comprising an emergency LED provided with a power supply which is independent of the supply to the array of LEDs, for illumination when there is a failure in the supply to the array.17. A component according to claim 16, wherein the emergency LED is in thermal contact with at least one of the coupling connections.18. A component according to any preceding claim, wherein the substrate carries the io conducting tracks on a first face, and a layer of thermally conductive material on another face to serve as a heatsink, there being at least one thermal via between the thermally conducting material and the coupling connection with which the emergency LED is in thermal contact.is 19. A lighting component comprising a substrate which, in use, supports an array of LEDs, and comprises a plurality of conducting tracks for providing, in use, current-carrying connections between the LEDs, and wherein the connections serve to connect the LEDs to form a plurality of strings of LED5 connected in series along each string; and wherein the connections further provide at least some coupling connections between different strings atan intermediate position along the length of the strings; :": and wherein the coupling connections serve, in use, as heatsinks."". 20. A component according to claim 19, the substrate having the features of any of * 25 claims ito 18. *..*. 21. A lighting component substantially as described above, with reference to the accompanying drawings.