METHOD FOR OPERATING A LIGHT EMITTING DIODE ARRANGEMENT, METHOD FOR CHARACTERIZING A LIGHT EMITTING DIODE, AND LIGHT EMITTING DIODE ARRANGEMENT

Abstract

A method for operating a light emitting diode arrangement with at least one light emitting diode includes the steps of: a) determining at least one instantaneous current-voltage value pair; b) matching the instantaneous current-voltage value pair with an original current-voltage value pair; and c) determining an updated current feed based on the matching and driving the light emitting diode with the updated current feed.

Claims

1. A method for operating a light emitting diode arrangement with at least one light emitting diode comprising the steps: a) determining at least one instantaneous current-voltage value pair; b) matching the instantaneous current-voltage value pair with an original current-voltage value pair; and c) determining an updated current feed based on the matching and driving the light emitting diode with the updated current feed.

2. The method of claim 1, wherein aging of the light emitting diode is at least partially compensated for by determining the updated current feed.

3. The method according to claim 1, wherein the light emitting diode arrangement comprises at least one further light emitting diode and wherein an updated current feed is determined for the further light emitting diode on the basis of an individually determined instantaneous current-voltage value pair.

4. The method according to claim 1, wherein a current is in the low current regime of the light emitting diode when the instantaneous current-voltage value pair is determined.

5. The method according to claim 1, wherein the light emitting diode arrangement is configured to operate the light emitting diode with a plurality of dimming levels, and an associated updated current feed is determined for each dimming level.

6. The method according to claim 1, wherein an updated efficiency of the light emitting diode is determined for different dimming levels based on the deviation between the instantaneous current-voltage value pair and the original current-voltage value pair.

7. The method according to claim 6, wherein the determination of the updated efficiency of the light-emitting diode is based on an equivalent circuit in which an ideal radiating diode is bridged by a parasitic non-radiating diode with a series resistor.

8. The method according to claim 7, wherein an updated series resistor is determined based on the instantaneous current-voltage value pair, and the updated current feed is determined based on the updated series resistor.

9. The method according to claim 1, wherein a respective instantaneous current-voltage-value pair is determined for a plurality of dimming levels, and a correspondingly updated current feed for the dimming level is determined for the dimming levels on the basis of the respective associated instantaneous current-voltage value pair.

10. A method for characterizing a light-emitting diode with regard to its internal efficiency of light generation with the steps: a) determining current-voltage value pairs for the light-emitting diode; b) approximating the current-voltage value pairs based on an equivalent circuit in which an ideal radiating diode is bridged by a parasitic non-radiating diode with a series resistor; and c) matching the series resistor with a characteristic series resistor for the light emitting diode.

11. The method according to claim 10, in which the light-emitting diode is selected out if a deviation of the series resistor from the characteristic series resistor exceeds a specified tolerance value.

12. A light-emitting diode arrangement with at least one light-emitting diode and a drive circuit, wherein the light-emitting diode arrangement is configured to determine an instantaneous current-voltage value pair of the light-emitting diode and to determine an updated current feed for the light-emitting diode on the basis of the instantaneous current-voltage value pair.

13. The light emitting diode arrangement according to claim 12, wherein the light emitting diode arrangement is a display device with a plurality of light emitting diodes, wherein the light emitting diode arrangement is configured to determine an instantaneous current-voltage value pair for each of the light emitting diodes and to determine an updated current feed for the light emitting diodes based on the respective instantaneous current-voltage value pairs.

14. The light emitting diode arrangement according to claim 12, wherein the light-emitting diode arrangement comprises a memory in which characteristic values for the light-emitting diode are stored for its original current-voltage characteristic.

15. The light-emitting diode arrangement according to claim 12, wherein the light-emitting diode arrangement comprises a memory in which a plurality of original current-voltage value pairs are stored for the light-emitting diode.

16. The light emitting diode arrangement according to claim 12, wherein the light emitting diode arrangement can be calibrated on the basis of an original current-voltage characteristic.

17. The light emitting diode arrangement according to claim 12, wherein the light emitting diode arrangement is configured for an operation according to a method according to claim 1.

18. A method for operating a light emitting diode arrangement with at least one light emitting diode comprising the steps: a) determining at least one instantaneous current-voltage value pair; b) matching the instantaneous current-voltage value pair with an original current-voltage value pair; and c) determining an updated current feed based on the matching and driving the light emitting diode with the updated current feed; wherein an updated efficiency of the light emitting diode is determined for different dimming levels based on the deviation between the instantaneous current-voltage value pair and the original current-voltage value pair, and the determination of the updated efficiency of the light-emitting diode is based on an equivalent circuit in which an ideal radiating diode is bridged by a parasitic non-radiating diode with a series resistor.

Description

[0034] In the Figures:

[0035] FIG. 1 shows an exemplary embodiment for a method for operating a light emitting diode arrangement in schematic representation;

[0036] FIG. 2 shows an exemplary embodiment for a light emitting diode arrangement in schematic representation;

[0037] FIG. 3A shows measurement results of a luminous flux in arbitrary units (left scale, logarithmic) and the internal quantum efficiency in arbitrary units (right scale) as a function of the current density I/(A/cm.sup.2) for a light emitting diode, wherein the different measurement curves were recorded at operating times between 0 hours and 600 hours;

[0038] FIG. 3B shows measurements of the change of the luminous flux as a function of the operating time nominated to the luminous flux at time t=0 for different light emitting diodes;

[0039] FIG. 4A shows an equivalent circuit diagram for a light emitting diode;

[0040] FIG. 4B shows a measurement of the voltage as a function of the current density I/(A/cm.sup.2) of a light emitting diode (measured values in diamonds) with an associated fitting function (solid line) as well as curves of a current-voltage characteristic of the non-radiating diode according to the equivalent circuit of FIG. 4A for different series resistances as dotted lines;

[0041] FIG. 4C shows an enlarged view of a portion of FIG. 4B;

[0042] FIG. 4D shows a relative change of the luminous flux as a function of a relative change of the series resistance for different light emitting diodes, each normalized to the luminous flux as well as the series resistance at time t=0;

[0043] FIG. 4E shows measurements of luminous flux in arbitrary units for different light emitting diodes as a function of the ratio e*U/EPh, wherein e is the elementary charge, U is the operating voltage and EPh is the respective photon energy of the generated radiation;

[0044] FIG. 5 shows an exemplary embodiment of a method for operating a light emitting diode arrangement; and

[0045] FIG. 6 shows an exemplary embodiment of a method for characterizing a light emitting diode.

[0046] Elements that are the same, similar, or have the same effect are indicated in the figures with the same reference signs.

[0047] The figures are each schematic representations and therefore not necessarily to scale. Rather, comparatively small elements and, in particular, layer thicknesses may be shown exaggeratedly large for clarification or better understanding.

[0048] In the exemplary embodiment shown in FIG. 1, a light emitting diode of a light emitting diode arrangement is controlled with an instantaneous current feed through the light emitting diode (illustrated by step S11).

[0049] For the exemplary embodiment of the method, an instantaneous current-voltage value pair is determined. For this purpose, the associated voltage is measured for a specified current. Alternatively, the associated current can be measured for a specified voltage. The current present when determining the current-voltage value pair can, but does not necessarily have to, be at an intensity of current typical for operation of the light emitting diode arrangement (step S12). Depending on the sensitivity of the measurement, it may be more appropriate to perform the measurement in a region that is not typical for the actual operation of the light emitting diode arrangement.

[0050] The instantaneous current-voltage value pair is matched with an original current-voltage value pair (step S13). An updated current feed is determined based on the matching so that the light emitting diode can be further driven with the updated current feed (step S14). For example, the updated current feed differs from the instantaneous current feed by a changed intensity of current and/or a changed pulse width modulation.

[0051] The method is particularly suitable for at least partially compensating for aging of the light emitting diode by determining the updated current feed.

[0052] FIG. 3A illustrates how the luminous flux (and the internal quantum efficiency IQE for a light emitting diode change over an operating period of 600 hours. In particular, the curves show that at comparatively small currents, such as at current densities between and including

[0053] 1×10.sup.−4 A/mm.sup.2 and including 0.01 A/mm.sup.2, aging effects occur. At higher current densities, however, the luminous flux and the internal quantum efficiency change only slightly.

[0054] FIG. 3B illustrates the change in luminous flux for various light-emitting diodes as a function of the operating time t in hours. From this figure, it can be seen that even the qualitative aging behavior differs for different light emitting diodes. For example, some light emitting diodes show a continuous decrease in luminous power over time, while for other light emitting diodes the luminous flux even increases in the meantime.

[0055] It is therefore not readily possible to store a typical aging curve for a light-emitting diode arrangement and, on the basis of the operating time already completed, to adjust the operating current so that the luminous power remains constant.

[0056] With the method described, the compensation of the aging can be done on the basis of a concrete measurement of current and associated voltage.

[0057] The method described is generally suitable for driving light-emitting diode arrangements in which a change in the efficiency of the light-emitting diodes due to aging leads to a significant change in the characteristic properties of the radiation. This is especially the case for light emitting diode arrangements which comprise light emitting diodes emitting in different spectral ranges. For example, the light emitting diode arrangements are part of a color mixing system or a display.

[0058] To determine the updated current feed, an equivalent circuit 5 for the light emitting diode can be used, as shown in FIG. 4A. Here, an ideal light-emitting diode DR is brigded by a parasitic non-radiating diode DNR with a series resistor RP. The equivalent circuit diagram 5 further shows a series resistor RS. RS represents the cumulative series resistances of the light-emitting diode. However, RS is only of minor importance for the aging of the light emitting diode. Thus, the ideal light-emitting diode DR represents an idealized light-emitting diode without non-radiative defect-assisted recombination, i.e., with an internal efficiency of light generation in the low-current regime of 100%. The ideality factor of this light-emitting diode is 1.

[0059] In contrast, the non-radiative diode DNR represents, in particular, the tunneling current without light emission at very small currents. The ideality factor of the parasitic nonradiative diode DNR is greater than 1 and is, for example, between 2 and 7 inclusive.

[0060] According to this model, the low-current efficiency of a light-emitting diode is directly proportional to the relative current flow through the radiating diode DR.

[0061] Based on this equivalent circuit, an adjustment to a real current-voltage characteristic can be made. This is shown in FIG. 4B. FIG. 4B also shows curves for the branch of the non-radiating diode DNR with series resistances RP of 5000 Ω, 220Ω and 0Ω. From this it is clear that the series resistor RP determines the transition from a predominant current flow through the non-radiating diode DNR to a predominant current flow through the radiating diode DR. The higher the series resistor RP, the earlier this transition occurs. The ideality factor of the parasitic non-radiating diode DNR is 2.7.

[0062] It can be seen from FIG. 4D that the relative change in luminous flux ΔΦ, nominated at time t=0 is immediately accompanied by a relative change in resistance ΔRP, also nominated at time t=0.

[0063] Thus, when describing a light emitting diode with the equivalent circuit 5 shown in FIG. 4A, the aging can be described directly by a change in the series resistance RP, wherein the other parameters, in particular the ideality factors of the diodes, do not change.

[0064] By comparing a measured voltage value at a certain operating current in the low current regime with the original voltage at this operating current, it is therefore possible to deduce the series resistor RP. The series resistor RP thus provides a measure of the efficiency of the light-emitting diode and, in particular, of the change in efficiency due to aging. Based on the instantaneous current-voltage value pair, an updated efficiency of the light emitting diode can thus be determined.

[0065] From FIG. 4E, in which the luminous flux (is shown in terms of the ratio of elementary charge e multiplied by the operating voltage U to the photon energy EPh of the generated light, it is further clear that aging can be read directly from the voltage change and compensated for with a corresponding adjustment of the current feed. The photon energy EPh can be used to take into account that the measured light-emitting diodes emit radiation with different peak wavelengths.

[0066] In particular, it can be deduced over the entire operating range of the light emitting diode how the current feed must be adjusted for different dimming levels in order to compensate for aging effects.

[0067] More conveniently, the measurement of the instantaneous current-voltage value pair is performed at an operating current in the low current regime of the light emitting diode. When operating a light emitting diode arrangement with several light emitting diodes, the described method can be carried out for several light emitting diodes, in particular for all light emitting diodes individually, so that an updated current feed can be determined for each light emitting diode.

[0068] FIG. 5 shows an exemplary embodiment of a method for operating a light emitting diode arrangement. Here, steps S21 and S22 correspond to steps S11 and S12 described in connection with FIG. 1. In contrast to the method described in connection with FIG. 1, several instantaneous current-voltage value pairs are determined. The different operating currents correspond to different dimming levels of the light emitting diode arrangement.

[0069] In step S23, the instantaneous current-voltage value pairs are matched with the stored corresponding original current-voltage value pairs. From this, a correspondingly updated current feed is determined for each of the dimming levels on the basis of the respective instantaneous current-voltage value pair (step S24).

[0070] In contrast to the exemplary embodiment described in connection with FIG. 1, the measurement of several current-voltage value pairs is thus required. In this case, it is not necessary to determine an updated efficiency of the light emitting diode based on the equivalent circuit shown in FIG. 4A.

[0071] FIG. 2 illustrates an exemplary embodiment of a light emitting diode arrangement which is particularly suitable for the operating methods described. The light emitting diode arrangement 1 comprises a plurality of light emitting diodes 2 and a drive circuit 3. The light emitting diode arrangement is configured to determine at least one instantaneous current-voltage value pair of the light emitting diode and to determine an updated current feed for the light emitting diodes based on the instantaneous current-voltage value pair.

[0072] This can be done as explained in connection with the described operating method. For example, the light emitting diodes 2 are arranged in a matrix. For example, the light emitting diode arrangement comprises different types of light emitting diodes which are provided for generating radiation in mutually different spectral ranges, such as for generating radiation in the red, green and blue spectral ranges. For simplified illustration, only a section with eight light emitting diodes is shown, wherein one of the light emitting diodes 2 is identified as another light emitting diode 2B for simplified reference.

[0073] In particular, the light emitting diode arrangement 1 is configured to determine at least one instantaneous current-voltage value pair operating voltage for the light emitting diodes 2 in each case and to determine an updated current feed for the light emitting diodes on the basis of the respective instantaneous current-voltage value pair.

[0074] The change in efficiency can thus be derived from the change in the instantaneous current-voltage value pair compared to the corresponding original current-voltage value pair, and the current feed can be readjusted accordingly.

[0075] For example, the light emitting diode arrangement 1 comprises a memory 4 in which characteristic values for the light emitting diode 2 are stored for its original current-voltage characteristic. For example, the characteristic values are based on an equivalent circuit as shown in FIG. 4A. For example, the ideality factor of the parasitic non-radiating diode DNR, the series resistor RP and, if applicable, the series resistor RS are stored.

[0076] Alternatively or additionally, a plurality of original current-voltage value pairs are stored in the memory. By matching the instantaneous value pairs with respect to the original value pairs, aging can be deduced and a corresponding adjustment of the current feed can be made.

[0077] In such a light emitting diode arrangement 1 with a plurality of light emitting diodes, for example a light emitting diode arrangement in the form of a display device 10, it would not be readily possible to monitor the radiation emitted by the individual light emitting diodes 2 during operation. As a result, there is a risk that a change in the efficiency of the light-emitting diodes due to aging would lead to incorrect color control and/or color reproduction. This can be compensated for as described above.

[0078] In particular, light emitting diodes 2 intended for generating radiation in different spectral ranges may comprise, for example, different aging characteristics, so that by individually adjusting the aging behavior based on concrete measurements on the respective light emitting diodes 2, a reliable control of the current feed can be performed.

[0079] In such a display device 10, the color control and dimming of the individual pixels can be implemented by changing the current feed. The change of the current feed can be realized by a pulse width modulation and/or a change of the operating current of the individual pixels. For the second case, the operating current intensity can change over several orders of magnitude depending on the luminosity required in each case. Over this entire operating range, the aging effects can be compensated or at least largely compensated.

[0080] FIG. 6 schematically illustrates a method for characterizing a light emitting diode. The method is particularly suitable for checking the internal efficiency of the light generation of the light emitting diode after its manufacture. For this purpose, current-voltage value pairs for the light-emitting diode are determined in a step S31. The value pairs are approximated based on an equivalent circuit in which an ideal radiating diode DR is bridged by a parasitic non-radiating diode DNR with a series resistor (step S32).

[0081] The series resistor determined in this way is matched with a characteristic series resistor for the light-emitting diode (step S33). The internal efficiency of the light-emitting diode can be deduced from this matching. In this way, the light-emitting diode can be checked with respect to an optical parameter, namely the internal efficiency of light generation, without the need for an optical measurement of the emitted light power. Rather, the characterization of the light emitting diode can be based on a purely electrical measurement of the current-voltage characteristic.

[0082] For example, the light emitting diode can be selected out if a deviation of the series resistance from the characteristic series resistance exceeds a specified tolerance value. In other words, an excessively low series resistance in the underlying equivalent circuit can be used to conclude that the internal efficiency of the light generation is too low.

[0083] This patent application claims priority to German patent application 10 2019 115 817.6, the disclosure content of which is hereby incorporated by reference.

[0084] The invention is not limited by the description based on the exemplary embodiments. Rather, the invention encompasses any new feature as well as any combination of features, which in particular includes any combination of features in the patent claims, even if that feature or combination itself is not explicitly specified in the patent claims or the exemplary embodiments.

LIST OF REFERENCE SIGNS

[0085] 1 light emitting diode arrangement [0086] 2 light emitting diode [0087] 2B further light emitting diode [0088] 3 drive circuit [0089] 4 memory [0090] 5 equivalent circuit [0091] DR ideal radiating diode [0092] DNR parasitic non-radiating diode [0093] RP series resistor [0094] RS serial resistor [0095] S11, S12, S13, S14 step [0096] S21, S22, S23, S24 step [0097] S31, S32, S33 step