GB2434929A

GB2434929A - Controlling an LED array

Info

Publication number: GB2434929A
Application number: GB0702297A
Authority: GB
Inventors: Klaus-Dieter Grebner; Volker Marr
Original assignee: Lear Corp
Current assignee: Lear Corp
Priority date: 2006-02-07
Filing date: 2007-02-07
Publication date: 2007-08-08
Anticipated expiration: 2027-02-07
Also published as: GB2434929B; DE102006005521B3; GB0702297D0

Abstract

A control circuit 1 for controlling an LED array 2 comprises a voltage source, a control unit 10, and at least two clusters 6 connected in parallel, each having at least one LED 3 and a switch 9 which is controllable by the control unit. The switches for the at least two clusters, and preferably for all the clusters, may be controlled independently of one another by the control unit.

Description

Circuit and Method for Controlling an LED Array The present invention

relates to a control circuit for a light-emitting diode field (hereinafter LED array) and a method for controlling such an LED array. In particular it relates to a control circuit for a light-emitting diode field (hereinafter LED array according to the preamble of Claim 1, and a method for controlling such an LED array according to the preamble of Claim 6.

Such a control circuit and method are generally known from EP 1 246 511 Bi. As disclosed therein, LED arrays are used for vehicle tail lights, for example.

Compared to conventional bulb tail lights, LED arrays offer the advantage of redundancy of the light sources: even if some light sources fail there are still enough other light sources available to ensure sufficient brightness. Other application examples for LED arrays are large-screen displays or modifiable warning and traffic signs.

Specifically for safety-relevant applications such as vehicle tail lights and warning panels or traffic signs, in the development of suitable LED arrays a balance must be found between the simplest possible design for cost reasons and the most efficient monitoring of the LED functions for safety reasons. With regard to the balance of these conflicting requirements, EP 1 246 511 Bi proposes to combine the LEDs into clusters, and to associate each of the clusters with an electrically controllable circuit. One of the LED clusters is selected as the "master cluster." The current intensity flowing through this master cluster is measured separately and is sent to a closed-loop control circuit. The closed-loop control circuit then actuates the circuit for the master cluster in such a way that the average current intensity in the master cluster is adjusted to a predetermined setpoint value. The signal at the single control output of the closed-loop control circuit controls, i.e., actuates, the switch for the master cluster and all "slave clusters" in a synchronous manner. Since all clusters are supplied with voltage via a common power supply, on account of the common control of all clusters the same average or effective voltage should be present at each cluster.

In addition to the current measurement in the master cluster, according to EP 1 246 511 61 the overall current intensity over all slave clusters is collectively measured. If one of the slave clusters fails due to a defect in the light-emitting diode or because of some other interruption, the overall current intensity of the slave clusters drops. The failure of individual slave clusters specifically does not trigger an error message. An error signal is not generated until the measured overall current intensity of the slave clusters drops below a predetermined threshold value after the failure of a number of slave clusters.

The control circuit of EP 1 246 511 BI is unsatisfactory in several respects. For example, according to the cited document an error alarm is triggered too late, after a number of slave clusters have failed. If an alarm is triggered, the only certainty is that a given number of, for example, ten, twelve, or more clusters have failed, without the ability to identify the defective clusters. However, the greatest disadvantage of the known control method is probably that a failure of the master cluster has fatal consequences for the operation of the entire LED array due to the paramount importance of this cluster.

An object of the present invention is to improve the known control circuit and the known control method, using means having the simplest possible design, in such a way that the LED array meets high safety requirements. Another object is to provide a control circuit and method which addresses the above described problems and/or which offers improvements generally.

Therefore according to the present invention there is therefore provided a control circuit having the features of Claim 1, and by a method for controlling an LED array, having the features of Claim 6. Advantageous refinements of the invention are stated in the subclaims. Such a control circuit and method addresses the above described objects.

According to an embodiment of the invention, the switches for at least two LED clusters may be controlled independently of one another by the control unit. The independent control or actuation of the switches in different clusters has the advantage that the unsatisfactory and possibly fatal division of the clusters into a master cluster and various subordinate slave clusters may thus be avoided.

Whereas in EP 1 246 511 61 the failure of the master cluster can have uncontrollable consequences, due to the independent control of the clusters in the embodiment of the present invention the failure of a cluster may be compensated for in any event without adversely affecting other clusters.

Electronically controllable switches such as transistors may likewise be used as switches in the present invention. A common power Supply for furnishing power to all clusters of the LED array may be maintained.

It is possible to design the control circuit in such a way that the switches for some clusters are always actuated as a group in order to reduce the complexity of the circuitry. A better option, however, would be to control the switches for all clusters separately, i.e., independently, by the control unit. This allows, for example, precise localization of a defect in a specific cluster, measurement of the brightness of the light-emitting diodes of a cluster, or disconnection of clusters in which an error has been discovered. A further advantage is that when all clusters are independently controllable, each cluster may be switched on or off individually in order to set an effective or average voltage selected for this cluster. Although all clusters are supplied with the same input voltage, when the clusters are individually actuated the effective voltage may be different from that of the IS adjacent cluster or other clusters. Since the perceived brightness of the light- emitting diodes is proportional to the effective voltage, the brightness of the light-emitting diodes of a cluster may thus be controlled individually, i.e., independently of the brightness control for the other LED clusters.

The design of the control circuit according to an embodiment of the invention may be simplified greatly by providing a single current meter for measuring the total current flowing through all switched-on clusters of the LED array. Instead of a number of current measuring devices in each individual cluster, in principle the single current meter is sufficient for measuring the total current in order to obtain a control variable for controlling the circuit. Thus, the control circuit according to the invention may even have a simpler design than that of EP 1 246 511 Bi, since in the latter case two current measuring devices were always necessary, namely, one for the master cluster and at least one for the slave clusters. In the control circuit according to an embodiment of the present invention, in contrast, by use of a single current meter not only can the total current flowing during operation of the control circuit be measured, but as a result of the ability to independently control the individual clusters it is possible at given time periods to operate only a single cluster whose current is measured by the common current meter. If the value obtained from such an "individual" current measurement is significantly lower than the expected value or is even zero, the cluster has obviously been interrupted and is then classified as defective. Despite measurement using a single current measuring device, in this manner an error or defect may be precisely associated with a given cluster for the control circuit according to the invention. For reasons of operational safety, however, it would of course be possible to provide more than one current meter in the switching system.

To further reduce the complexity of controlling the LED array, a plurality of subclusters connected in parallel may be controlled via a common switch, and in this manner combined into a common cluster. This reduces the number of switches provided in the control circuit and simplifies the design of the control unit.

A controllable voltage regulator is preferably provided in the control circuit for regulating the voltage delivered by the power source. In this manner, for example, a particular voltage may be set which is suitable for operating the LED array.

Similarly as for the control circuit according to the invention, the method according to the invention for controlling an LED array is also characterized in that the control unit controls a plurality of switches independently of one another.

The advantages of such an independent control capability of the switches has been discussed above.

The control unit preferably controls all switches separately and independently. In this manner defects may be identified and precisely assigned to a specific cluster.

In addition, each individual cluster may be controlled so that a suitable effective voltage is present at that location.

It is practical for the control unit to control the switches for the clusters in an initialization mode and/or test mode so that each cluster is switched on at least once as a single cluster during an individual measurement interval, while all the other clusters are switched off. Whereas the initialization mode can be performed upon each start of operation of the LED array, it is possible to perform the test mode according to specified time intervals, such as every five or ten minutes. In this manner the function of all LED clusters can be monitored at the start of operation and at regular intervals. If an error is detected by the current meter during a measurement interval in which only a single cluster is switched on, this error can be precisely assigned to the specific cluster that is switched on. The various measurement intervals may follow one another in direct succession, or may also be interrupted by phases in which several or all of the clusters are switched on.

It is advantageous for an individual measurement interval, in which only a single cluster is switched on, to be so short that the human eye does not perceive the other clusters of the LED array being switched off. For this purpose, the individual measurement interval should have a maximum duration of 1 ms, preferably 500 PS.

In order to perform the initialization mode and/or test mode more quickly, it is possible for the control unit to control the switches for a plurality of clusters in common, i.e., as a group. Thus, multiple clusters would be simultaneously switched on in each measurement interval. If a lower current intensity and thus an error were discovered in one of the groups under investigation, this error could then be precisely assigned to a specific cluster by individually measuring once more all clusters in this group.

In one preferred variant of the method according to an embodiment of the invention the voltage supplied by the power source is controllable, and in initialization mode the voltage is increased for each single switched-on cluster, with simultaneous measurement of the current flowing through the cluster, until a predetermined setpoint current or nominal current flows through the cluster at a threshold voltage. In this manner, for each cluster an individual measurement may be made to determine the effective voltage that must be set to achieve the predetermined setpoint current, and thus to achieve a predetermined brightness of the light-emitting diodes. Such an initialization mode could be performed at each start of operation of the LED array.

In initialization mode the threshold voltage is preferably measured in this manner for each cluster, and the respective value of the threshold voltage for each cluster is stored. Based on the stored values, the LED clusters may then be operated such that in each case the desired brightness is achieved.

After the threshold voltages have been measured for each cluster, during operation of the LED array the power source may then be set to an operating voltage which is at least as great as the highest threshold voltage measured at a single cluster. This ensures that the threshold voltage necessary for each cluster may be present at that cluster, so that in any event a required target brightness of the light-emitting diodes is achieved for this cluster.

Because of the ability to independently control the switches for various clusters, in one advantageous variant of the invention for a cluster having a threshold voltage that is lower than the operating voltage it is possible to switch the cluster on and off by clocked control or actuation of the switch thereof in such a way that the effective or average current flowing through the cluster approximately corresponds to the intensity of the nominal current. The cluster is thus switched on exactly long enough so that the average, effective voltage lies in the region of the threshold voltage of the cluster. In this manner an excessively high constant current through the cluster, which otherwise would result from the excessive operating voltage, is avoided. On the other hand, if the cluster is operated by correspondingly clocked control in the region of its threshold voltage, the service life of the light-emitting diodes for this cluster may be significantly extended. The clocked control of a cluster in the described manner may also be referred to as pulse-width modulation (PWM).

The operating voltage is preferably set higher than the highest threshold voltage measured at an individual cluster, and all clusters of the LED array are switched on and off by pulse-width modulation, i.e., clocked control, of the switch thereof in such a way that the effective current flowing through each cluster approximately corresponds to the respective nominal current. By extending the switched-on cycles, the brightness of any LED cluster can be increased at the higher operating voltage without having to modify the set operating voltage.

It has been shown to be practical to select the setting for the operating voltage to be approximately 1% to 20% higher than the highest threshold voltage measured at an individual cluster. At this value an excessive operating voltage that is potentially destructive for the LEDs is avoided, while at the same time a certain increase in brightness is possible in all clusters.

In the method according to an embodiment of the invention an error signal may be produced when, in initialization mode and/or test mode, a measured current intensity is lower than a predetermined first error threshold for the current intensity, or is higher than a predetermined second error threshold, at a cluster that is switched on and supplied with power. If the measured value of the current intensity lies outside the error thresholds and thus outside a "tolerance band," an interruption or a short circuit of the LED cluster, and thus a defect in this cluster, may be presumed. As soon as such an error is detected, the control unit can output an error signal and, for example, acoustically or optically display the error signal.

After an error is detected in a cluster, it is also possible for an error message identifying the defective cluster to be stored in a suitable memory. If the system is undergoing maintenance, such as for use of the LED array in a vehicle tail light in an automotive repair shop, for example, the maintenance personnel can retrieve the stored error message and thus immediately identify the defective LED cluster.

A special advantage provided by the method according to an embodiment of the invention is that when, after an error has been detected in a cluster, the effective current through at least one adjacent cluster is increased by suitably controlling the switch thereof. Extending the switched-on cycles of the adjacent cluster increases its average voltage and therefore the effective current and the brightness emitted by its light-emitting diodes, thereby compensating, at least partially, for the loss in brightness from the defective cluster. This brightness compensation may occur after the failure of a single LED cluster, whereas in the conventional system an error message is not generated until after a plurality of clusters have failed, and it is not possible to compensate for brightness at all since in that case all clusters are always synchronously operated.

The method according to an embodiment of the invention is particularly simple when an overall current intensity is collectively measured over all switched-on clusters, using a single current meter. Compared to current measurement using a plurality of current measuring devices in each individual cluster, this has the advantage that at any point in time only a single measured value is present, thereby simplifying processing of the measurement signals. Furthermore, on account of the capability to individually control the single clusters this measured value can be precisely associated with a specific LED cluster when it is operated alone.

The invention is explained in greater detail below with reference to a preferred exemplary embodiment illustrated in the accompanying drawing, which shows the following: Figure 1 shows an exemplary embodiment of a control circuit according to an embodiment of the invention; Figures 2A through 2F show six different interchangeable LED modules; and Figure 3 shows a portion of the chronological progression of the signal at the output channels of the control unit, by which the switches for the LED clusters are controlled.

Figure 1 shows the circuit diagram of a control circuit I according to the invention for a field or array 2 of light-emitting diodes (LEDs) 3. The control circuit 1 has a power supply 4 whose output voltage can be regulated by a controllable voltage regulator 5.

The light-emitting diodes 3 of the LED array 2 are configured in a plurality of clusters 6 which are connected in parallel and supplied with the same voltage, which is regulated by the regulator 5. The exemplary embodiment illustrated in Figure 1 of a control circuit 1 according to the invention comprises four LED clusters 6 connected in parallel. However, it is also possible for the LED array 2 to be built from two, three, or four or more clusters 6.

At least one LED 3 and a suitable series resistor 7 are connected in series in each cluster 6. The light-emitting diodes 3 and series resistors 7 may be combined into an LED module 8 which is inserted into the respective cluster 6.

Each cluster 6 also contains an electronically controllable switch 9 such as a transistor. The switch 9 can enable or interrupt the current flow through the respective cluster 6. A control unit 10 is provided in the control circuit 1 for controlling or actuating the switch 9. The control unit 10 may be, for example, a suitably programmed microprocessor or an application-specific integrated circuit (ASIC). In the present embodiment the control unit 10 has (at least) as many output channels as the number of clusters 6 in the LED array 2. Each control channel or output channel 11 is used to transmit a control signal to a switch 9 associated therewith. Since an individual control channel 11 is provided for each switch 9, i.e., for each LED cluster 6, the switches 9 for all clusters 6 may be separately and independently controlled or actuated by the control unit.

A current meter 12 is also provided in the control circuit 1 by means of which the total current flowing through the entire LED array 2, i.e., the sum of currents flowing through all clusters 6 of the array, can be measured. The value measured by the current meter 12 is sent as an input signal 13 to the control unit 10. The control unit 10 also has an additional control input 14 via which the control unit 10 can be externally controlled or programmed. The control unit 10 can control the voltage regulator 5 via an additional control output 15 and thereby influence the voltage applied to the LED array 2.

Figures 2A through 2F show six examples for different variants of the LED module 8. In each case, the LED modules 8 used in the various clusters 6 of the LED array 2 may be the same or different. In addition, the light-emitting diodes 3 of a cluster could each have the same color, which is different from the color of the light-emitting diodes 3 of another cluster 6. In this manner different clusters 6, each having blue, green, red, or white light-emitting diodes, could be used.

The simplest embodiment of an LED module 8 is illustrated in Figure 2A. This module 8 has only a single light-emitting diode 3 and a suitable series resistor 7 which limits the current flow through this light-emitting diode 3. The module 8 shown in Figure 28 comprises two light-emitting diodes 3, and in the LED module 8 of Figure 2C multiple light-emitting diodes 3 are additionally provided for the series resistor 7.

Subclusters 16 connected in parallel are provided in each case in the exemplary embodiments according to Figures 2D, 2E, and 2F. In Figure 20, each subcluster 16 has a single LED 3, whereas in Figure 2E two LEDs are provided in each subcluster, and in Figure 2F a multiple of (n) light-emitting diodes 3 are additionally provided for the series resistor 7. Of course, other, more complex LED modules 8 could also be used. Each LED module 8 may be controlled via an associated switch 9, and together with the switch 9 forms a cluster 6 of the LED array 2.

The mode of operation of the control circuit 1 according to an embodiment of the invention and the associated method for controlling the LED array 2 are explained below.

When the LED array 2 is switched on, the control circuit 1 is ready to undergo an initialization mode. In this initialization mode the control unit 10 first switches on only a single LED 6 via the control channels 11, while the other clusters 6 remain switched off. The control unit 10 controls the voltage regulator 5 via the control output 15 in such a way that the voltage present at the switched-on cluster 6 is gradually increased, starting from a relatively low specified voltage. In the meantime, the current meter 12 continuously determines the current flowing through the switched-on cluster 6 and transmits the measured value as an input signal 13 to the control unit 10. At a given voltage, referred to as the threshold voltage, a predetermined setpoint current or nominal current flows through the cluster 6. The value of the current intensity associated with the threshold voltage has previously been selected so that at this nominal current intensity a predetermined desired brightness of the light-emitting diodes 3 is achieved for the cluster 6 in question. The value of the threshold voltage for the switched-on cluster 6 is stored in the control unit. A corresponding measurement of the threshold voltage is then performed for all the other individual clusters 6 of the LED array, and the values of the threshold voltage for each individual cluster 6 are likewise stored in the control unit 10.

In the subsequent operating mode the control unit 10 can control the voltage regulator 5 in such a way that the starting voltage of the voltage regulator corresponds to the highest measured threshold voltage of an LED cluster 6. This would ensure that the required minimum voltage is present at each cluster 6 so that the light-emitting diodes in this cluster have the necessary minimum brightness. However, the control unit 10 preferably adjusts the voltage regulator 5 such that the operating voltage produced by the voltage regulator 5 is approximately 1% to 20% higher than the highest measured threshold voltage for an LED cluster 6. By suitable actuation of each individual switch 9 it is possible to switch each LED cluster 6 on and off in a clocked manner so that the average effective current through this cluster 6 corresponds to its nominal current intensity. Such a pulse-width modulated actuation of the switches 9 ensures that each cluster 6 emits precisely the desired brightness. Operation at a higher effective voltage or at a higher current, which could adversely affect the service life of the light-emitting diodes 3, is avoided. The higher the setting of the operating voltage above the threshold voltage for an LED cluster 6, the longer the intervals at which the particular cluster 6 is switched off.

If the operating voltage is set to be higher than the highest measured threshold voltage for a cluster 6, this allows the LED clusters 6 to be routinely monitored in a test mode. The chronological progression of such a test mode is illustrated in Figure 3, which shows the signal curves at four control channels 11 of the control unit 10. Each control channel is associated with the switch 9 for an LED cluster 6.

A high signal in a control channel 11 closes the associated switch 9, whereas a low control signal opens the switch 9.

In a first individual measuring interval l only channel 1 is switched on, while the remaining channels are switched off. Therefore, during this interval I only the LED cluster 6 associated with channel I is switched on, and the other LED clusters 6 are switched off. In this interval l the current meter 12 measures only the current which flows through the switched-on first LED cluster 6.

In the subsequent operating interval 12 all LED clusters are initially switched on.

Whereas channel I remains switched on over the operating interval 12, first channel 3, then channel 4, and finally channel 2 are switched off, so that on average the specific required threshold voltage is achieved at the respective LED cluster 6 over the entire operating interval 12. The clusters 6 of the LED array 6 are thus operated in a clocked or pulse-width modulated manner in operating interval 12.

In the subsequent measuring interval 13 all channels except channel 2 are switched off. Thus, during measuring interval 13 the current meter 12 measures only the current flowing through the second LED cluster 6.

In its control characteristics for the control channels 11, the operating interval 14 corresponds once again to operating interval 12. In the subsequent measuring interval 15 only the third channel is switched on, so that the current meter 12 determines the current flowing through the third LED cluster 6.

In this test mode the measuring and operating intervals alternate until each LED cluster 6 has been individually operated once. In this manner the current intensities flowing through each LED cluster 6 can be individually determined.

The individual measuring intervals I, 13, 15 are so brief that the human eye does not perceive the other LED clusters 6 being switched off. In particular, the duration of such a measuring interval may be, for example, I ms, preferably only 500 ps or less. On account of the control by pulse-width modulation, the human eye also does not perceive the LED clusters 6 being switched off in the operating intervals 12, 14. Instead, all LED clusters 6 are perceived at the same brightness.

If during a measuring interval I, 13, l the current meter 12 detects in the sole switched-on LED cluster 6 a current intensity of zero or at least below a specified error threshold, it is presumed that this LED cluster has been interrupted. On the other hand, if the current is higher than a second error threshold, it is presumed that the cluster has been short-circuited. The cluster is then classified in the control unit 10 as defective by means of an error message which identifies the defective cluster 6. The error message is stored in the control unit 10. By appropriate programming of the control unit 10, an error signal may also be externally transmitted in order to optically or acoustically signal to the user that the LED cluster 6 has failed. After a specific LED cluster 6 has been identified as defective, this

cluster 6 may remain switched off permanently by corresponding control of the switch 9 thereof.

A particular advantage may also be derived from the control circuit I according to the invention: By extending the switching-on cycles for one or more adjacent LED clusters 6, the current flowing through these clusters 6 may be increased, thereby increasing the brightness emitted by light-emitting diodes 3 in these adjacent clusters. It is thus possible to at least partially compensate for the loss in brightness from the defective LED cluster 6. The brightness of the LED array 2 may be held essentially constant by means of this brightness compensation, which can be performed after the failure of a single LED cluster 6. In particular for safety-relevant applications of the LED array, such as for vehicle tail lights, ensuring a constant brightness of the LED array 2 may be of great importance. If necessary, the operating voltage applied to the LED array 2 may also be increased by corresponding control of the voltage regulator 5 in order to further contribute to the increase in brightness.

The control circuit 1 according to the invention has a number of significant advantages compared to the conventional LED array. The former allows routine diagnosis of each individual LED cluster 6 of the array 2. Since the measurement can be performed using a single current measuring device 12, the complexity of measurement is very low.

As a result of the separate control of the individual clusters 6, each LED cluster 6 may also be operated in an individually pulse-width modulated manner in order to adjust the brightness of the LED clusters 6 and operate each cluster 6 at its suitable threshold voltage. As discussed above, the service life of the LED array 2 is thus prolonged considerably.

A further advantage of the control circuit I is that the system is very easily scaled.

If the control unit 10 is provided with a corresponding number of control channels 11, any given number of LED clusters 6 may be connected and controlled.

Possibly the most important advantage of the control circuit I according to the invention is that the failure of a single LED cluster 6 can be recognized, and the loss in brightness resulting from a defective LED cluster can be compensated for immediately by a corresponding increase in brightness at the adjacent clusters.

Proceeding from the exemplary embodiment illustrated, the control circuit 1 according to the invention may be modified in a variety of ways. A number of modification options have been described above. In particular, the number of LED clusters could be reduced or greatly increased. Instead of the described constant voltage source a constant current source may also be used, the output current then being set corresponding to the controlled LED clusters 6. The configuration of the individual functional blocks in the control circuit I is interchangeable. In particular, the switch 9 in each LED cluster 6 could also be provided in front of the LED module 8.

Claims

Claims 1. A Control circuit for an LED array, the LED array comprising

a voltage source, a control unit, and at least two clusters connected in parallel having at least one LED, and a switch which is controllable by the control unit also being provided in each of the clusters, characterized in that the switches for at least two clusters may be controlled independently of one another by the control unit.

2. The Control circuit according to claim 1, characterized in that the switches for all clusters may be controlled independently of one another by the control unit.

3. The control circuit according to one of the preceding claims, characterized in that the control circuit has a current meter for measuring the total current flowing through all clusters of the LED array.

4. The control circuit according to one of the preceding claims, characterized in that at least one cluster comprises two or more subclusters connected to one another in parallel.

5. The control circuit according to one of the preceding claims, characterized in that a controllable voltage regulator is provided for regulating the voltage supplied by the voltage source.

6. A method for controlling an LED array, the LED array comprising a voltage source, a control unit, and at least two clusters connected in parallel having at least one LED, and a switch which is controllable by the control unit also being provided in each of the clusters, characterized in that the control unit controls a plurality of switches independently of one another.

7. The method according to claim 6, characterized in that the control unit controls all switches independently of one another.

8. The method according to one of claims 6 or 7, characterized in that the control unit controls the switches for the clusters in an initialization mode and/or test mode so that each cluster is switched on at least once as a single cluster during an individual measurement interval, while all the other clusters are switched off.

9. The method according to claim 8, characterized in that an individual measurement interval has a maximum duration of 1 ms.

10. The method according to claim 8, characterized in that an individual measurement interval has a maximum duration of 500 ps.

11. The method according to one of claims 6 through 10, characterized in that the control unit collectively controls the switches for a plurality of clusters in an initialization mode and/or test mode.

12 The method according to one of claims 6 through 11, characterized in that the voltage supplied by the voltage source may be regulated, and in initialization mode the voltage is increased for each single switched-on cluster, with simultaneous measurement of the current flowing through the cluster, until a predetermined setpoint current flows through the cluster at a threshold voltage.

13. The method according to claim 12, characterized in that in initialization mode the threshold voltage is measured for each cluster, and the respective value of the threshold voltage is stored.

14. The method according to one of Claims 12 or 13, characterized in that during operation of the LED array, in particular after the initialization mode, the voltage source is set to an operating voltage which is at least as great as the highest threshold voltage measured at a cluster.

15. The method according to one of Claims 6 through 14, characterized in that a cluster having a threshold voltage that is lower than the operating voltage is switched on and off by clocked control of the switch thereof in such a way that an effective current flowing through the cluster approximately corresponds to the nominal current of the cluster.

16. The method according to one of Claims 6 through 15, characterized in that the operating voltage is set higher than the highest threshold voltage measured at a cluster, and all clusters of the LED array are switched on and off by clocked control of the switch thereof in such a way that an effective voltage is present at each cluster which approximately corresponds to the threshold current of the cluster.

17 The method according to Claim 16, characterized in that the setting for the operating voltage is 1% to 20% higher than the highest threshold voltage measured at a cluster.

18. The method according to one of Claims 6 through 17, characterized in that an error signal is produced when, in initialization mode and/or test mode, a measured current intensity is lower or higher than a predetermined error threshold at a cluster that is switched on and supplied with power.

19. The method according to claim 18, characterized in that after an error is detected in a cluster an error message identifying the defective cluster is stored.

20. The method according to one of claims 17 or 19, characterized in that after an error is detected in a cluster the effective current is increased in at least one adjacent cluster by suitable control of the switch thereof.

21. Method according to one of claims 6 through 20, characterized in that an overall current intensity is collectively measured over all switched-on clusters, using a current meter.

22. A control circuit for an LED array substantially as hereinbefore described with reference to, and/or as shown in any one or more of figures 1 to 3.

23. A method of controlling an LED array substantially as hereinbefore described with reference to, and/or as shown in any one or more of figures 1 to 3.