Abstract
Methods and apparatus for driving an emitter array are described. A method includes determining a thermal environment profile for a plurality of emitters of the emitter array, computing a current pulse profile for at least one of the plurality of emitters based on the thermal environment profile, and applying a current pulse with the computed current pulse profile to the at least one of plurality of emitters.
Claims
1. A display comprising: an emitter comprising a matrix of light emitting diodes; a thermal environment module configured to determine a thermal environment profile of the light emitting diodes, the thermal environment profile including thermal responses of each light emitting diode affected by others of the light emitting diodes; a current profile computation module configured to compute, for each light emitting diode based on the thermal environment profile, a current pulse profile tailored to a respective one of the light emitting diodes; and a driver configured to apply, to each light emitting diode, a current pulse with the computed current pulse profile for the respective light emitting diode to produce substantially uniform light output from the respective light emitting diode over a time interval of the current pulse even though a temperature of the respective light emitting diode changes during the current pulse.
2. The display according to claim 1, wherein a shape of the current pulse is substantially rectangular in which the current pulse monotonically increases from a start of the time interval to a later point of the time interval.
3. The display according to claim 2, wherein the current pulse monotonically increases from the start of the time interval to an end of the time interval.
4. The display according to claim 2, wherein the current pulse monotonically increases from the start of the time interval to a point at which the current pulse remains constant until an end of the time interval.
5. The display according to claim 1, wherein an increase of the current pulse varies with position of the respective light emitting diode within the matrix.
6. The display according to claim 1, wherein the current profile computation module is configured to use at least one previous pulse event to compute the current pulse profile.
7. The display according to claim 1, wherein the current pulse applied to at least two different light emitting diodes in the matrix have different pulse shapes.
8. The display according to claim 1, wherein a pulse shape of the current pulse reflects an increase in current over a time period of the current pulse.
9. The display according to claim 1, wherein the thermal environment module is configured to consider a thermal time constant of a sub mount or printed circuit board (PCB) on which the light emitting diodes are disposed to determine the thermal environment profile.
10. The display according to claim 1, further comprising a forward voltage measuring module coupled to the thermal environment module, the forward voltage measuring module configured to measure forward voltages of the light emitting diodes and provide the forward voltages to the thermal environment module, the thermal environment module configured to estimate actual temperatures of the light emitting diodes based on the forward voltages to predict a temperature behavior of the light emitting diodes.
11. The display according to claim 1, further comprising a split current module configured to deploy a split current algorithm that combines information from a light emitting diode color table with information from a specification source to divide available current into current pulse sets for two emitters, the light emitting diode color table to be compiled using information provided by the thermal environment module to determine a ratio of a first emitter color to a second emitter color during the current pulse to achieve a uniform color temperature distribution.
12. The display according to claim 1, wherein the light emitting diodes are micro-light emitting diodes.
13. A method of driving an emitter array in a display, the method comprising: determining a thermal environment profile of light emitting diodes in the emitter array, the thermal environment profile including thermal responses of each light emitting diode affected by others of the light emitting diodes; computing, for each light emitting diode based on the thermal environment profile, a current pulse profile tailored to a respective one of the light emitting diodes; and applying, to each light emitting diode, a current pulse with the computed current pulse profile for a respective one of the light emitting diodes to produce uniform light output from the respective light emitting diode over a time interval of the current pulse even in response to a change in a temperature of the respective light emitting diode changes during the current pulse.
14. The method according to claim 13, wherein a shape of the current pulse is substantially rectangular in which the current pulse monotonically increases from a start of the time interval to a later point of the time interval.
15. The method according to claim 13, wherein an increase of the current pulse varies with position of the respective light emitting diode within the array.
16. The method according to claim 13, further comprising using at least one previous pulse event to compute the current pulse profile.
17. The method according to claim 13, further comprising considering a thermal time constant of a sub mount or printed circuit board (PCB) on which the light emitting diodes are disposed to determine the thermal environment profile.
18. The method according to claim 13, further comprising: measuring forward voltages of the light emitting diodes; and estimating actual temperatures of the light emitting diodes based the forward voltages to predict a temperature behavior of the light emitting diodes.
19. A display comprising: a printed circuit board (PCB), the PCB comprising: an emitter comprising a matrix of light emitting diodes disposed on the PCB; a thermal environment module configured to determine a thermal environment profile of the light emitting diodes, thermal responses of each light emitting diode affected by a thermal time constant of the PCB and others of the light emitting diodes determined to form the thermal environment profile; a current profile computation module configured to compute, for each light emitting diode based on the thermal environment profile, a current pulse profile tailored to a respective one of the light emitting diodes; and a driver configured to apply, to each light emitting diode, a current pulse with the computed current pulse profile for the respective light emitting diode to produce uniform light output from the respective light emitting diode over an entire time interval of the current pulse even in response to a change in a temperature of the respective light emitting diode changes during the current pulse.
20. The display according to claim 19, wherein the PCB further comprises a forward voltage measuring module coupled to the thermal environment module, the forward voltage measuring module configured to measure forward voltages of the light emitting diodes and provide the forward voltages to the thermal environment module, the thermal environment module configured to estimate actual temperatures of the light emitting diodes based the forward voltages to predict a temperature behavior of the light emitting diodes.
Description
BRIEF DESCRIPTION OF THE DRAWINGS
[0029] FIG. 1 shows an array of direct-emitting LEDs;
[0030] FIG. 2 shows rectangular current pulses to drive three emitters of the array of FIG. 1 using a prior art approach;
[0031] FIG. 3 shows thermal response of the nine emitters of the array of FIG. 1 using a prior art approach;
[0032] FIG. 4 shows light output of the three driven emitters of the array of FIG. 1 using a prior art approach;
[0033] FIG. 5 shows tailored current pulses to drive three emitters of the array of FIG. 1 using the inventive method;
[0034] FIG. 6 shows thermal response of the nine emitters of the array of FIG. 1 using the inventive method;
[0035] FIG. 7 shows light output of the three driven emitters of the array of FIG. 1 using the inventive method;
[0036] FIG. 8 shows rectangular current pulses to drive nine emitters of the array of FIG. 1 using the prior art approach;
[0037] FIG. 9 shows thermal response of the nine emitters of the array of FIG. 1 using the prior art approach;
[0038] FIG. 10 shows light output of the nine driven emitters of the array of FIG. 1 using the prior art approach;
[0039] FIG. 11 shows tailored current pulses to drive nine emitters of the array of FIG. 1 using the inventive method;
[0040] FIG. 12 shows thermal response of the nine emitters of the array of FIG. 1 using the inventive method;
[0041] FIG. 13 shows light output of the nine driven emitters of the array of FIG. 1 using the inventive method;
[0042] FIG. 14 shows tailored subsequent current pulses to drive nine emitters of the array after a preceding switching event of the array of FIG. 1 using the inventive method;
[0043] FIG. 15 shows light output as a function of emitter position in the matrix of FIG. 1 when driven using a prior art approach;
[0044] FIG. 16 shows an embodiment of the inventive LED arrangement;
[0045] FIG. 17 shows a simplified block diagram of an embodiment of the inventive LED arrangement for driving two different colour arrays;
[0046] FIG. 18 shows a further simplified block diagram of an embodiment of the inventive LED arrangement.
[0047] In the drawings, like numbers refer to like objects throughout. Objects in the diagrams are not necessarily drawn to scale.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT(S)
[0048] FIG. 1 shows an emitter array 1 with a square arrangement of nine emitters E.sub.11, E.sub.12, . . . , E.sub.33 for use in a camera flash application, for example a flash module of a mobile phone. In this exemplary embodiment, the emitters E.sub.11, E.sub.12, . . . , E.sub.33 are sub-500 micron dies, i.e. the surface area of each emitter is at most 0.25 mm.sup.2. The dies are closely packed and mounted on a common carrier 10 or PCB. The gap between any two adjacent emitters may comprise about 100 μm or less. Alternatively, the emitter array 1 could be realised as a monolithic die. The emitters E.sub.11, E.sub.12, . . . , E.sub.38 are driven individually by a driver module (not shown in the diagram) comprising a driver that issues current pulses for the emitters according to a control algorithm. In the following, graphs of current, temperature and relative light output are shown in a similar 3×3 array layout so that information content of a graph can easily be related to the corresponding emitter.
[0049] In the prior art, the driver is realised to drive selected emitters E.sub.11, E.sub.12, . . . , E.sub.33 with identical current pulses. FIG. 2 shows three rectangular current pulses I.sub.21_pa, In.sub.22_pa, I.sub.23_pa in a time interval t1 extending from time 0 to time t.sub.1 to drive the three emitters E.sub.21, E.sub.22, E.sub.23 of the middle row using the prior art approach. The graphs indicate relative current (Y-axis) against time (X-axis). The emitters E.sub.11, E.sub.12, E.sub.13, E.sub.31, E.sub.32, E.sub.33 of the top and bottom rows are to remain off. The three emitters E.sub.21, E.sub.22, E.sub.23 of the middle row will heat up as a result of their respective current pulses I.sub.21_pa, I.sub.22_pa, I.sub.23_pa, but each of these emitters will also heat its neighbours. The temperature development during and after the current pulses is shown in FIG. 3, which indicates relative temperature (Y-axis, in percent) against time (X-axis). Because a hot emitter heats its neighbours, the temperature T.sub.22_pa of the central emitter E.sub.22 will rise to the greatest level. The temperatures T.sub.21_pa, T.sub.23_pa of the two outer emitters E.sub.21, E.sub.23 of the middle row will also be influenced by the central emitter E.sub.22, and the emitters E.sub.11, E.sub.12, E.sub.13, E.sub.31, E.sub.32, E.sub.33 of the top and bottom rows—even though they were not turned on—will become heated by the middle row of emitters E.sub.21, E.sub.22, E.sub.23. The central emitter E.sub.22 is heated by both adjacent emitters E.sub.21, E.sub.23, and because the central emitter E.sub.22 is hottest, the middle emitters E.sub.12, E.sub.32 of the top and bottom rows are also heated more than the outermost four emitters E.sub.11, E.sub.13, E.sub.31, E.sub.33.
[0050] The temperature of an emitter affects the light output by that emitter. FIG. 4 illustrates this effect, and shows the relative light output L.sub.21_pa, L.sub.22_pa, L.sub.23_pa (in percent) by the three emitters E.sub.21, E.sub.22, E.sub.23 of the middle row. The diagram shows that the initially high light output decreases rapidly as the emitters E.sub.21, E.sub.22, E.sub.23 heat up, and that the relative light output L.sub.22_pa of the central emitter E.sub.22 decreases to the greatest extent. Regarding this light output level as 100%, the light output of the outer two emitters E.sub.21, E.sub.23 decreases to a lesser extent, to a level above the 100% mark on the Y-axis. This is because the central emitter E.sub.22 is also heated the most, being flanked by two hot emitters E.sub.21, E.sub.23. Instead of delivering a uniform light output pattern, the three emitters driven using the prior art approach will deliver an uneven light output pattern.
[0051] The temperature of an emitter decreases after termination of the current pulses, but may be still significantly higher than an ambient temperature by the end of a second time interval after which a subsequent pulse is to be applied to the emitter array.
[0052] In the case of an emitter array that implements a combination of warm-white and cold-white emitters, the different emitter behaviours over temperature result in deviations from the desired on-scene colour temperature.
[0053] The inventive method provides a solution to these problems and is explained with the aid of FIGS. 5-7, using the same 3×3 emitter array of FIG. 1 as a basis. Assuming that the middle row of emitters E.sub.21, E.sub.22, E.sub.23 is to be driven, the approach taken by the invention is to predict the temperature environment that will affect the relevant emitters, and to compute the current pulse shapes that will be needed to counteract the negative effects of emitter temperatures. An example of three tailored current pulses I.sub.21, I.sub.22, I.sub.23 is shown in FIG. 5. Instead of the simple rectangular shapes shown in FIG. 2, the three current pulses I.sub.21, I.sub.22, I.sub.23 applied to the middle row of emitters E.sub.21, E.sub.22, E.sub.23 have shapes or profiles that will ensure a uniform light output even though the emitter temperatures will change during the current pulse event over time interval t1 (again, the pulse event starts at time 0 and ends at time t.sub.1).
[0054] FIG. 6 shows the temperature development in the emitter array following the current pulses I.sub.21, I.sub.22, I.sub.23 shown in FIG. 5. The relative temperatures T.sub.11, . . . , T.sub.33 of the emitters E.sub.21, E.sub.22, E.sub.23 are different from those shown in FIG. 3 owing to the different shapes of the current pulses I.sub.21, I.sub.22, I.sub.23 applied to the middle emitter row.
[0055] FIG. 7 shows the relative light output L.sub.21, L.sub.22, L.sub.23 (in percent) of the three emitters E.sub.21, E.sub.22, E.sub.23 of the middle row after receiving the current pulses I.sub.21, I.sub.22, I.sub.23 shown in FIG. 5. The diagram shows that the initially high light output (close to 100%) is essentially maintained, even as the emitters E.sub.21, E.sub.22, E.sub.23 heat up, and that the light output L.sub.22 of the central emitter E.sub.22 is essentially no different from the light output L.sub.21, L.sub.23 by the two outer emitters E.sub.21, E.sub.23. Therefore, even thought the central emitter E.sub.22 is heated by its neighbours, the adjacent emitters E.sub.21, E.sub.23, these three emitters deliver a favourably uniform light output pattern.
[0056] FIGS. 8-10 respectively show current pulses I.sub.11_pa, . . . I.sub.33_pa, relative temperature development T.sub.11_pa, . . . T.sub.33_pa and relative light output L.sub.11_pa, . . . L.sub.33_pa when all nine emitters E.sub.11, . . . E.sub.33 of the array in FIG. 1 are simultaneously switched using the prior art approach. The drawings show that, in response to nine identical current pulses I.sub.11_pa, . . . I.sub.33_pa, the temperature of the central emitter E.sub.22 is highest, and lowest at the four outer corner emitters E.sub.11, E.sub.13, E.sub.31, E.sub.33. As a result, the light output of the array is uneven, with the hotter emitters E.sub.12, E.sub.21, E.sub.22, E.sub.23, E.sub.32 delivering the lowest light output L.sub.12_pa, L.sub.21_pa, L.sub.22_pa, L.sub.23_pa, L.sub.32_pa.
[0057] FIGS. 11-13 respectively show current pulses I.sub.11, . . . I.sub.33, temperature development T.sub.11, . . . T.sub.33 and light output L.sub.11, . . . L.sub.33 when all nine emitters E.sub.11, . . . E.sub.33 of the array in FIG. 1 are simultaneously switched using the inventive method. The drawings show that, in response to nine tailored current pulses I.sub.11, . . . I.sub.33, the light output of the array is even or homogenous, with all emitters E.sub.11, . . . E.sub.33 delivering essentially the same light output levels L.sub.11, . . . L.sub.33, even though the “inner” emitters are heated by their neighbours. By taking these heating effects into account when computing the current shapes, a favourable homogenous light output is obtained.
[0058] As explained above, the pulse history of an emitter will determine its behaviour during a subsequent pulse, i.e. any preceding pulse may affect the behaviour of an emitter if the preceding pulse was applied to that emitter or to a neighbouring emitter, and if the temperature of any of those emitters is still greater than its steady state or ambient value.
[0059] FIG. 14 shows how the inventive method can take pulse history into consideration when all nine emitters E.sub.11, E.sub.12, . . . , E.sub.33 are to be driven in a pulse event following the pulse event of FIG. 11. The current profile computation module takes into consideration the temperature of each emitter E.sub.11, E.sub.12, . . . , E.sub.33 prior to the intended pulse event interval, as shown in FIG. 12, and computes the necessary current profile shapes accordingly. A set of nine such tailored current pulses I.sub.11, I.sub.12, . . . , I.sub.33 is shown in FIG. 14. The relative light output L.sub.11, L.sub.12, . . . L.sub.33 of the entire emitter array is shown will be the same as in FIG. 13, i.e. all nine emitters deliver the same light output so that the emitter array delivers a favourably uniform light output even though the emitters had different temperatures prior to the second pulse event of FIG. 14.
[0060] FIG. 15 shows light output as a function of emitter position in an array, for example in the 3×3 matrix of FIG. 1, when current pulses do not take thermal crosstalk into consideration. The diagram shows light output curves, each decreasing to different levels. Curve 150 corresponds to an emitter heated by four surrounding emitters, for example the central emitter E.sub.22 in the matrix of FIG. 1. Curve 151 corresponds to an emitter heated by three surrounding emitters, for example emitters E.sub.12, E.sub.21, E.sub.23, E.sub.32 in the middle of each side of the array in the matrix of FIG. 1. Curve 152 corresponds to an emitter heated by two surrounding emitters, for example emitter E.sub.11, E.sub.13, E.sub.31, E.sub.33 at an outer corner of the array in the matrix of FIG. 1. Curve 153 corresponds to an emitter that is not heated by any surrounding emitter.
[0061] FIG. 16 shows an embodiment of the inventive LED arrangement 10, showing an emitter array 1 and a driver 2. The driver 2 is configured to apply individual tailored current pulses 20 to the emitters as explained in FIGS. 5, 11 and 14. The driver 2 can be connected via a suitable bus to deliver the current pulses 20 to the emitters of the array 1. For a 2×2 array, the driver 2 will be realised to generate four current pulses 20; for a 3×3 array, the driver 2 will be realised to generate nine current pulses 20, etc. In this exemplary embodiment, a thermal environment module 3 applies a suitable model that is given the thermal time constants τ of the emitters, and which predicts emitter temperature behaviour following a current pulse. In this embodiment, the thermal environment module 3 uses one thermal time constant for each emitter, for example. The thermal time constants τ may be stored in a memory. The thermal environment module 3 provides a current profile computation module 4 with information 30 necessary to determine the current pulse profiles 40 of the tailored current pulses 20 that will be required to achieve a desired light output pattern. Such information can comprise input temperature data, output current shape data, etc. Here also, the current profile computation module will be realised to generate four current pulse profiles 40 for a 2×2 array, nine current pulse profiles 40 for a 3×3 array, etc.
[0062] A further circuit may be used to obtain information about the actual temperature of the emitters. For example, an initial condition for the thermal environment profile can be established by measuring the forward voltages of the emitters and estimating the actual temperatures. To this end, this embodiment of the inventive LED arrangement 10 also comprises a forward voltage measuring module 5, and the measured forward voltages 50 can be passed to the thermal environment module 3 which uses them to predict the temperature behaviour of the emitters.
[0063] FIG. 17 shows a an embodiment of the inventive LED arrangement 10 for driving two different colour arrays 1A, 1B, for example for a dual colour flash. The diagram shows a specification source 160 that specifies the desired light on scene in terms of lumen and colour temperature. Information from an LED colour table 161 and an LED temperature table 162 is fed to a behaviour model 163, which computes a set of basic current pulse shapes IA, IB for the emitter arrays 1A, 1B. In temperature crosstalk correction modules 164A, 164B, the effect of a hot emitter neighbour is factored into each current pulse, and corrected current pulse shapes IA′, IB′ are forwarded to the matrix drivers 2A, 2B which apply the corrected current pulses (e.g. as shown in FIGS. 5, 11, 14) to the emitters. The result is a homogenous light output L on a scene with a desired colour temperature, even though the emitters heat up in response to current pulses. A product based on the embodiment shown in FIG. 17 will be characterized by very advanced control on account of the feedback mechanism, and may be relatively expensive.
[0064] FIG. 18 shows a more economical realisation. Here, a simpler feed-forward system is used, in which the emitters are divided into two groups 1A, 1B. In this embodiment, a split current module 165 deploys a split current algorithm that combines information from an LED colour table 161 with information from a specification source 160 to divide the available current into current pulse sets IA, IB for the two emitter groups 1A, 1B. The result is a relatively homogenous light output L on a scene. The LED colour table 161 can be compiled using information provided by a thermal environment module such as that described in FIG. 16. With a suitable LED colour table 161, the ratio of a first emitter colour to a second emitter colour during a current pulse can be determined, for example to adjust the ratio of warm white to cool white during a pulse sequence in order to achieve a uniform colour temperature distribution in the scene.
[0065] 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 as described by the appended claims.
[0066] 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.
