Electronic circuit with an LED module

This Application claims priority to German Application Number 102018122649.7, filed on Sep. 17, 2018, the entire content of which is incorporated herein by reference.

This disclosure in general relates to an electronic circuit with at least one LED (Light Emitting Diode) string.

LED strings, which include a plurality of LEDs connected in series, are widely used for lighting purposes in various kinds of applications such as interior or exterior lighting in automobiles, or lighting in buildings, to name only a view. An LED string may be driven by a drive circuit that receives an input voltage and generates a drive current received by the LED string. The drive circuit may also be configured to monitor the LED string and detect a defect such as a short circuit of one of the LEDs in the LED string. The input voltage received by the drive circuit may be generated by a control circuit based on an input signal. When the input signal indicates that it is desired to switch on the LED string, the control circuit generates a voltage level of the input voltage high enough for the drive circuit to cause the LED string to light up. When the input signal indicates that it is desired to switch off the LED string, the control circuit generates a voltage level of the input voltage low enough to prevent the drive circuit from causing the LED string to light up.

In some applications it is desirable for the drive circuit to report the detection of an failure in the LED string to the control circuit. A dedicated communication channel connected between the control circuit and the drive circuit may be used by the drive circuit to report the failure to the control circuit. Such a dedicated communication channel, however, requires additional ports (pins) at each of the control circuit and the drive circuit and, therefore, increases the costs and the complexity of the overall system.

There is therefore a need for the drive circuit to indicate the detection of failure without requiring additional communication pins.

One example relates to an electronic circuit with an LED circuit. The LED circuit includes an input configured to receive an input voltage, a drive circuit connected to the input, and an LED module connected to the drive circuit and including an LED string with at least one LED. The drive circuit is configured to monitor the LED module for the occurrence of an LED short in the LED string and to change from a normal mode to a defect mode upon detection of the LED short. In the defect mode, the drive circuit is configured, to operate the LED string in at least one defect cycle that includes deactivating the LED string for a deactivation period, activating the LED string for an activation period, and checking for the persistence of the LED short in the activation period.

Another example relates to a method. The method includes monitoring an LED module for the occurrence of an LED short in an LED string and changing from a normal mode to a defect mode upon detection of the LED short by a drive circuit included in an LED circuit. The method further includes, in the defect mode of the drive circuit, operating the LED string in at least one defect cycle that includes deactivating the LED string for a deactivation period, activating the LED string for an activation period, and checking for the persistence of the LED short in the activation period.

Examples are explained below with reference to the drawings. The drawings serve to illustrate certain principles, so that only aspects necessary for understanding these principles are illustrated. The drawings are not to scale. In the drawings the same reference characters denote like features.

FIG. 1 shows one example of an electronic circuit that includes an LED circuit with a drive circuit and an LED module;

FIG. 2 shows one example of an LED module including one LED string;

FIG. 3 shows one example of an LED module including a plurality of LED strings;

FIG. 4 shows a state diagram that illustrates one example of how the LED circuit may operate;

FIG. 5 shows timing diagrams that illustrates one example of how the LED circuit may operate;

FIGS. 6 to 8 show modifications of the state diagram shown in FIG. 4 ;

FIG. 9 shows one example of the drive circuit in greater detail;

FIG. 10 shows one example of a diagnostic circuit implemented in the drive circuit shown in FIG. 9 ;

FIG. 11 shows one example of an error handling circuit implemented in the drive circuit shown in FIG. 9 ;

FIG. 12 shows signal waveforms that illustrate the function of an input filter implemented in the error handling circuit shown in FIG. 11 ;

FIG. 13 shows a further example of the drive circuit;

FIG. 14 shows one example of an LED circuit that includes several drive circuits and several LED modules;

FIG. 15 shows one example of an electronic circuit that includes an LED circuit and a control circuit;

FIG. 16 shows signal waveforms that illustrate the function of the control circuit shown in FIG. 15 ; and

FIG. 17 shows one example of the control circuit in greater detail.

In the following detailed description, reference is made to the accompanying drawings. The drawings form a part of the description and for the purpose of illustration show examples of how the invention may be used and implemented. It is to be understood that the features of the various embodiments described herein may be combined with each other, unless specifically noted otherwise.

FIG. 1 schematically illustrates one example of an electronic circuit that includes an LED (Light Emitting Diode) circuit 1 . The LED circuit 1 includes an input 11 , 12 , at least one drive circuit 2 A connected to the input 11 , 12 , and an LED module 3 A connected to the drive circuit 2 A. The input 11 , 12 is configured to receive an input voltage V IN and an input current I IN . According to one example, the input includes a first input node 11 and a second input node 12 . According to one example, the drive circuit 2 A is connected to the input 11 , 12 of the LED circuit 1 in that an input 21 A, 22 A of the drive circuit 2 A is connected to the input 11 , 12 of the LED circuit 1 . The LED module 3 A includes one or more LED strings with at least one LED. More specifically, the LED module 3 A may include one LED string or may include a plurality of LED strings, wherein each LED string includes at least one LED.

FIG. 2 shows one example of the LED module 3 A. In this example, the LED module 3 A includes one LED string 4 1 . The LED string 4 1 includes one or more LEDs. Referring to FIG. 2 , the LED string 4 1 may include a plurality of LEDs 4 11 , 4 12 , 4 lo connected in series. The LED string 4 1 is connected to the drive circuit 2 A and receives a string current I 4 1 from the drive circuit 2 A. Dependent on a current level of the string current I 4 1 , the LEDs of the string 4 1 light up or do not light up. In the following, “on” and “off” are used interchangeably in place of “light up” and “do/does not light up”, respectively.

FIG. 3 shows one example of an LED module 3 A that includes plurality of LED strings 4 1 , 4 2 , 4 n . Each of these LED strings is connected to the drive circuit 2 A and receives a respective string current I 4 1 , I 4 2 , I 4 n from the drive circuit 2 A, wherein the LEDs of each string 4 1 , 4 2 , 4 n light up dependent on a current level of the respective string current I 4 1 , I 4 2 , I 4 n . According to one example, as illustrated in FIG. 3 , each of these LED strings 4 1 , 4 2 , 4 n includes a plurality of LEDs. In this example, a first LED string 4 1 includes a number of o LEDs 4 11 - 4 1o , a second LED string 4 2 includes a number of p LEDs 4 21 - 4 2p , and an n-th LED string 4 n includes a number of q LEDs 4 n1 - 4 nq . The number of LEDs in the individual LED strings 4 1 - 4 n can be the same, so that o=p=q. However, it is also possible that the LED strings 4 1 - 4 n include different numbers of LEDs, so that o≠p≠q.

In each of the examples illustrated in FIGS. 2 and 3 , the overall number of LEDs in one LED string can range from 1 to several 10, such as from 1 to 50, in particular, from 1 to 30. Further, the number n of LED strings in one LED module 3 A may range from one to several 10 such as from one to 50. Just for the purpose of illustration, the LED module 3 A shown in FIG. 3 includes n=3 LED strings 4 1 - 4 n .

According to one example, the LEDs connected in series in one LED string 4 1 - 4 n are of the same type so that, at given string current I 4 1 -I 4 n , they light with essentially the same intensity. LEDs of different strings can be of the same type or of different types. That is, the different strings 4 1 - 4 n in the LED module 3 A can light up at the same string current or at a different string current. That a string is lit up means that the LEDs of the string are lit up.

According to one example, the drive circuit 2 A is configured to generate the at least one string current I 4 1 -I 4 n such that the LEDs of the at least one LED string 4 1 - 4 n light up whenever the input voltage V IN received by the drive circuit 2 A is high enough for the drive circuit 2 A to generate the at least one string current I 4 1 -I 4 n such that it causes the at least one string 4 1 to light up. During operation of the LED module 3 A, a defect may occur. One type of defect that may occur is a short circuit of one single LED, which is briefly referred to as LED short in the following. In the case of an LED short, the defect (shorted) LED is off, while the remainder of the LEDs in the LED string are still on. An example of an LED short in one 4 12 of the LEDs of string 4 1 is illustrated in bold lines in FIGS. 2 and 3 .

According to one example, the drive circuit 2 A is configured to monitor the LED module 3 A for the occurrence of an LED short in the at least one LED string 4 1 - 4 n and to change from a normal mode to a defect mode upon detection of such LED short. This is explained with reference to FIGS. 2, 3 and 4 in the following.

FIG. 4 illustrates a state diagram of the drive circuit 2 A. Referring to FIG. 4 , the drive circuit 2 A either operates in a normal mode 110 or a defect mode 120 . In the normal mode 110 , the drive circuit 2 A, based on the input voltage V IN received at the input 11 , 12 , generates the string current I 4 1 -I 4 n received by the at least one LED string 4 1 - 4 n such that the LEDs of the at least one LED string 4 1 - 4 n light up. Upon detecting an LED short in one of the one or more LED strings 4 1 - 4 n connected to the drive circuit 2 A, the drive circuit 2 A changes from the normal mode 110 to the defect mode 120 . The string in which the LED short (defect) has been detected is referred to as “defective string” in the following.

Referring to FIG. 4 , in the defect mode, the drive circuit 2 A operates the defective LED string in at least one defect cycle by deactivating the defective LED string for a deactivation period, activating the LED string for an activation period after the deactivation period, and checking for the persistence of the LED short in the activation period. When checking for the persistence of the LED short reveals that the defect persists, the defective LED string is again deactivated for the deactivation period. When, however, checking for the persistence of the LED short during the activation period reveals that the LED short does not persist, operation of the drive circuit 2 A changes from the defect mode 120 back to the normal mode 110 .

FIG. 5 shows timing diagrams that illustrate how the drive circuit 2 A operates in the defect mode. In particular, FIG. 5 shows timing diagrams that illustrate the operating state of the drive circuit 2 A, the input current I IN received by the drive circuit 2 A, and the string current I 4 1 of the defective string 4 1 (just for the purpose of illustration it is assumed that string 4 1 is defective). In the example illustrated in FIG. 5 , an LED short in the defective LED string 4 1 is detected at a first time instance t 0 so that the drive circuit 2 A changes from the normal mode 110 to the defect mode 120 at the first time instance t 0 . In the normal mode, the string current I 4 1 has a current level I 4 1_ON that is high enough for the LEDs of the string 4 1 to light up. Referring to the above, operating the defective string in the defect mode 120 includes at least one defect cycle, wherein several of such defect cycles are illustrated in FIG. 5 . In each of these defect cycles, the defective string 4 1 is deactivated for a deactivation period T D and activated for an activation period T A .

“Deactivated” means that the string current I 4 1 of the defective string is significantly lower than the level I 4 1_ON in the normal mode 110 , such as less than 20% or less than 10% of the current level in the normal mode 110 , so that the LEDs of the defective string 4 1 are off. According to example, the current level of the string current I 4 1 during the deactivation period T D is zero. During the activation period T A the current level of the string current I 4 1 is such that the non-defect LEDs of the defective LED strings are on. According to one example, the current level of the string current I 4 1 during the activation period T A essentially equals the current level I 4 1_ON in the normal mode 110 . According to another example, the current level during the activation period T A is even higher than the current level I 4 1_ON in the normal mode 110 , such as between 100% and 120% of the current level I 4 1_ON in the normal mode.

In the example illustrated in FIG. 5 , the defect disappears at a second time instance t 1 , so that, in the activation period following this time instance t 1 , the defect can no longer be detected and the drive circuit 2 A returns to the normal mode 110 . An LED short may be due to bad soldering process to, conductive dust, etc. In these cases, an LED short may occur and again disappear after a certain time.

According to one example, the activation period T A is long enough to enable the drive circuit 2 A to detect whether there still an LED short in the defective string. According to one example, the duration of the activation period T A is selected from between 50 microseconds (μs) and 100 microseconds, in particular between 70 microseconds and 80 microseconds. According to one example, the duration of the deactivation period T D is significantly longer than the duration of the deactivation period T A . According to one example, the duration of the deactivation period T D is at least 50 times or at least 100 times the duration of the activation period T A . This has the effect that the “average” string current I 4 1 in defect mode 120 is significantly lower than the current level I 4 1_ON of the string current I 4 1 in the normal mode 110 . The defect mode 120 can therefore also be referred to as low current consumption mode. During the defect mode 120 , not only the (average) current consumption of the defective LED string 4 1 is reduced, but also the input current I IN of the LED circuit 1 is reduced. Upon entering the defect mode (lower current consumption mode), the LED circuit 1 , by only using the input nodes 11 , 12 , can communicate that an LED short has been detected in an LED string connected to the drive circuit 2 A. How strongly the (average) input current I IN decreases when the drive circuit 2 A changes from the normal mode 110 to the defect mode 120 is dependent on a ratio between the durations of the activation periods T A and the deactivation periods T D , but may also be dependent on the overall number of LED strings connected to the drive circuit 2 A and the way non-defective LED strings are operated in the defect mode 120 . Examples of how these non-defect LED strings may be operated are explained herein further below.

According to one example, durations of the activation periods T A and the deactivation periods T D are adapted to one another such that a defect cycle period T, which is given by the duration of one activation period T A and the duration of one deactivation period T D , is between five milliseconds (ms) and twenty milliseconds. In this case, a defect cycle frequency, which is the reciprocal 1/T of the defect cycle period is between 200 Hz and 50 Hz. At this defect cycle frequency, the non-defective LEDs (which are the LEDs that do not include an LED short) of the defective string 4 1 switch on and off. At a defect cycle frequency of between 50 Hz and 200 Hz, a visible flickering of the non-defective LEDs of the defective LED string can be widely avoided.

Referring to the above, a plurality of LED strings 4 1 - 4 n can be connected to the drive circuit 2 A, wherein the drive circuit 2 A is configured to monitor each of these LED strings 4 1 - 4 n for the occurrence of an LED short and enters the defect mode 120 when an LED short in one of the plurality of LED strings 4 1 - 4 n is detected. Thus, the drive circuit 2 A enters the defect mode 120 when one of the LED strings, such as LED string 4 1 explained with reference to FIG. 5 , is defective, while the remainder 4 2 - 4 n of the LED strings 4 1 - 4 n are not defective. In the defect mode 120 there are several ways to operate the non-defective LED strings 4 2 - 4 n . Some examples of how the non-defective LED strings 4 2 - 4 n may be operated are explained with reference to FIGS. 6 to 8 in the following. Each of these FIGS. 6 to 8 shows a state diagram of the drive circuit 2 A. Each of these state diagrams is based on the state diagram illustrated in FIG. 4 .

Referring to FIG. 6 , the non-defective strings 4 2 - 4 n are activated in the defect mode 120 so that the non-defective strings 4 2 - 4 n light up in the same way as in the normal mode 110 . In this case, the reduction of the input current I IN in the defect mode as compared to the normal mode 110 is only caused by the reduction of the (average) string current I 4 1 of the defective string 4 1 .

According to another example illustrated in FIG. 7 , the non-defective strings are deactivated in the defect mode 120 . In this case, the input current I IN increases to zero during the deactivation periods T D . Further, during the activation periods T A , the input current I IN is only given by current I 4 1 flowing into the defective string during the activation periods T A . In this case, the input current I IN is significantly reduced when the drive circuit changes from the normal mode 110 to the defect mode 120 .

According to another example (not shown) some of the non-defective strings are activated and others of the non-defective strings are deactivated during the defect mode 120 . By suitably selecting a ratio between the number of activated and the number of deactivated non-defective strings, the reduction of the input current I IN when the drive circuit changes from the normal mode 110 to the defect mode 120 can be adjusted. Referring to the above, a detectable reduction of the input current I IN when the drive circuit 2 A changes from the normal mode 110 to the defect mode 120 . may be desired as this reduction is used to communicate the detection of an LED short from the drive circuit 2 A to an external circuit connected to the input nodes 11 , 12 via the input nodes 11 , 12 . An example of such an external circuit is explained in detail herein further below.

According to another example illustrated in FIG. 8 , in the defect mode 120 , the defective string 4 1 and the non-defective strings 4 2 - 4 n are operated in defect cycles of the type explained with reference to FIG. 5 as long as the defect in the defective string 4 1 persists. This also causes a significant reduction of the input current I IN because the input current I IN is zero during the deactivation periods T D and is different from zero only during the activation periods T A . As, referring to the above, the durations of the activation periods T A can be significantly shorter than the durations of the deactivation periods T D the (average) input current I IN is significantly reduced during the defect mode 120 .

According to one example, the drive circuit 2 A cyclically activates and deactivates the defective string 4 1 as long as the defect persists. According to another example, the drive circuit 2 A counts the number of defect cycles and deactivates the defect string 4 1 when a predefined number of defect cycles has been reached. The drive circuit 2 A may again activate the defective cycle only after a shutdown, that is, after the input voltage V IN has fallen below a threshold that causes the LED circuit 1 to switch off the LED module and after the input voltage V IN has increased to above this threshold.

FIG. 9 shows one example of the drive circuit 2 A that is configured to monitor the LED module 3 A and operates in the defect mode 120 upon detecting an LED short in an LED string. Just for the purpose of explanation, there is only one LED string 4 1 connected to the drive circuit 2 A in the example shown in FIG. 9 . An example of the drive circuit 2 A having more than one LED strings connected thereto is explained herein further below. Referring to FIG. 9 , the drive circuit 2 A includes a current source 23 connected in series to the LED string 4 1 , wherein a series circuit including the LED string 4 1 and the current source 23 is connected between the input nodes 21 A, 22 A of the drive circuit 2 A. The current source 23 is a controllable current source that is controlled by a control signal S 23 received from a control circuit 24 . The current source 23 provides the string current I 4 1 , wherein the current level of the string current I 4 1 is dependent on the control signal S 23 . According to one example, the control signal S 23 can have two signal levels, a first signal level that activates the current source 23 so that the string current I 4 1 provided by the current source 23 has a first current level (I 4 1_ON in FIG. 5 ) that causes the (non-defective) LEDs of the LED string 4 1 to light up, and a second signal level, that switches off the current source 23 so that the current level of the string current I 4 1 is zero. This, however, is only an example. The control signal S 23 may have more than two signal levels may that cause the current source 23 to generate more than two different current levels of the string current I 4 1 .

Referring to FIG. 9 , the drive circuit 2 A further includes a diagnostic circuit 25 . The diagnostic circuit 25 is connected to the LED string 4 1 and is configured to detect a voltage V 4 1 across the LED string 4 1 and generate the diagnostic signal 25 based on monitoring the string voltage V 4 1 . The control circuit 24 receives the diagnostic signal S 25 and drives the current source 25 dependent on the diagnostic signal S 25 . According to one example, the diagnostic signal S 25 can have two different signal levels, a first signal level, which is also referred to as normal level in the following, that indicates that a defect has not been detected by a diagnostic circuit 25 , and a second signal level, which is also referred to as defect level in the following, that indicates that an LED short has been detected by the diagnostic circuit 25 .

One example of the diagnostic circuit 25 is illustrated in FIG. 10 . In this example, the diagnostic circuit 25 compares the string voltage V 4 1 with a threshold voltage V 4 TH and generates the diagnostic signal S 25 based on comparing the string voltage V 4 1 with the threshold voltage V 4 TH . According to one example, a normal level of the diagnostic signal S 25 is generated when the string voltage V 4 1 is higher than the threshold voltage V 4 TH , and a defect level is generated when string voltage V 4 1 is below the threshold voltage V 4 TH . In other words, a string voltage V 4 1 below the threshold voltage V 4 TH indicates that there is an LED short in at least one of the LEDs 4 11 - 4 1o of the LED string 4 1 . The threshold voltage V 4 TH is selected such that the string voltage V 4 1 is higher than the threshold voltage V 4 TH when each of the LEDs 4 11 - 4 1o is non-defect and when the current source 23 , in the normal mode of the drive circuit 2 A, generates the string current I 4 1 with the first current level, which is high enough to cause the LEDs to light up. The string current I 4 1 causes a voltage drop across each non-defective LED, wherein a voltage value of the respective voltage drop is dependent on the current level of the string current I 4 1 and the specific type of the LEDs. The voltage drop across one LED is associated with the string current that causes the voltage drop by the characteristic curve of the respective LED. When the string 4 1 is not defective, the string voltage V 4 1 is given by the sum of the voltage drops caused by the string current I 4 1 across the individual LEDs 4 11 - 4 1o . When a LED short in one of the LEDs occurs, the string current 4 1 is reduced by the voltage across the defective LED before the LED short occurred. The threshold voltage V 4 TH is adapted to the string voltage V 4 1 that occurs when the LED string is not defect in such a way that the string voltage V 4 1 is higher than the threshold voltage V 4 TH when the string is not defect and falls below the threshold voltage V 4 TH when an LED short in one of the LEDs occurs.

Referring to FIG. 10 , the threshold voltage V 4 TH may be provided by a threshold voltage source 252 and the diagnostic signal S 25 may be provided by a comparator 251 that receives the string voltage V 4 1 at one input and the threshold voltage V 4 TH at another input. In the example in FIG. 10 , the comparator receives the string voltage V 4 1 at a non-inverting input. In this example, the defect level is a low signal level and the normal level is a high signal level of the diagnostic signal. This, however, is only an example.

Referring to FIG. 9 , the drive circuit 2 A may further include a power supply 26 . The power supply 26 is connected to the input nodes 21 A, 22 A, receives the input voltage V IN and generates supply voltages (not shown in FIG. 9 ) received by the control circuit 24 and the diagnostic circuit 25 . According to one example, the control circuit 24 is configured to activate the current source 23 in order to light up the LEDs immediately after start up, that is immediately after the power supply 26 supplies the control circuit 24 and the diagnostic circuit 25 .

According to one example, the diagnostic circuit 25 is implemented such that the defect level of the diagnostic signal S 25 persists when the LED string 4 1 is deactivated by switching off the string current I 4 1 . The signal level of the diagnostic signal S 25 only changes from the defect level to the normal level when, during the activation period T A , the string voltage V 4 1 rises to above the threshold voltage V 4 TH and the drive circuit returns to the normal mode 110 .

The control circuit 24 is configured to operate the drive circuit 2 A in the defect mode 120 when the diagnostic circuit S 25 indicates that an LED short has been detected. Referring to the above, operating the drive circuit 2 A in the defect mode includes cyclically activating and deactivating the LED string 4 1 .

One example of the control circuit 24 is illustrated in FIG. 11 . The control circuit shown in FIG. 24 is configured to generate the control signal S 23 received by the current source ( 23 in FIG. 9 ) such that the current source 23 is activated when the diagnostic signal S 25 has the normal level, and such that the current source is cyclically deactivated and activated when the diagnostic signal S 25 has the defect level. Referring to FIG. 11 , the control circuit 24 may include a logic gate 24 that receives the diagnostic signal S 25 and an output signal S 242 from a defect handling circuit 242 and is configured to generate the control signal S 23 based on the diagnostic signal S 25 when the diagnostic signal has the normal level and based on the output signal S 242 from the defect handling circuit 242 when the diagnostic signal S 25 has the defect level. According to one example, the logic gate 241 is an OR gate, the normal level of the diagnostic signal S 25 is a high level and the signal level of the control signal S 23 that activates the current source 23 is a high level. The defect handling circuit 242 receives the diagnostic signal S 25 and is configured to generate its output signal S 242 such that the current source 23 is cyclically deactivated for the deactivation period T D and activated for the activation period T A explained above.

According to one example, the control circuit (as shown in FIG. 11 ) includes a filter 243 that filters the diagnostic signal S 25 to provide a filter output signal S 243 , wherein the filter output signal S 243 is received by the logic gate 241 and the defect handling circuit 242 . In this example, the current source is activated when the filter output signal has a normal level and is cyclically deactivated and activated when the filter output signal S 243 has a defect level. According to one example, the filter 243 has a low-pass characteristic so that the filter output signal S 243 changes from the normal level (which is a high level in the example shown in FIG. 12 ) to the defect level (which is a low level in the example shown in FIG. 12 ) only when the defect level of the diagnostic signal S 25 persists longer than a predefined time period. This ensures that short (erroneous) changes of the diagnostic signal S 25 from the normal level to the defect level do not cause the control circuit 24 to enter the defect mode.

FIG. 13 shows one example of a drive circuit 2 A configured to have an LED module 3 A with several LED strings 4 1 - 4 n connected thereto. In this example, the drive circuit 2 A includes a plurality of current sources 23 1 , 23 2 , 23 n , wherein each of these current sources is connected to a respective one of the plurality of LED strings 4 1 , 4 2 , 4 n . Further, the drive circuit 2 A includes a plurality of diagnostic circuits 25 1 - 25 n , wherein each of these diagnostic circuits is configured to monitor one of these LED strings 4 1 - 4 n for the occurrence of an LED short. The control circuit 24 receives a diagnostic circuit S 25 1 , S 25 2 , S 25 n from each of these diagnostic circuits 25 1 - 25 n and drives the current sources 25 1 - 25 n dependent on these diagnostic circuits S 25 1 -S 25 n in accordance with one of the examples explained with reference to FIGS. 6 to 8 . If, for example, an LED short is detected string 4 1 by diagnostic circuits 25 1 , the control circuit 24 enters the defect mode and cyclically activates and deactivates the defective LED string 4 1 by cyclically activating and deactivating the respective current source 23 1 . The current sources 23 2 - 23 n connected to the non-defective LED strings 4 2 - 4 n can be activated by the control circuit 24 (so that the non-defective LED strings are activated) or can be deactivated (so that the non-defective LED strings are deactivated).

FIG. 14 shows a modification of the LED circuit 1 shown in FIG. 1 . The LED circuit 1 shown in FIG. 14 includes a plurality of drive circuits 2 A- 2 C, wherein each of these drive circuits 2 A- 2 C has a respective LED module 3 A- 3 C connected thereto. Each of the drive circuits 2 B, 2 C can be implemented in the same way as drive circuit 2 A explained herein before. Further each of the LED modules 3 B, 3 C can be implemented in accordance with the LED module 3 A explained herein before. Referring to FIG. 14 , each of the drive circuits 2 A- 2 C is connected to the input 11 , 12 , so each of these drive circuits 2 A- 2 C receives the input voltage V IN .

According to one example, the drive circuits 2 A- 2 C are in signal communication with each other so that each of these drive circuits 2 A- 2 C is configured to communicate the detection of an LED short in the respective LED module connected thereto to the other drive circuits. Communication paths between the drive circuits 2 A- 2 C are only schematically illustrated in FIG. 14 . These communication paths can be implemented as unidirectional signal lines, between each one of the drive circuits 2 A- 2 C and each other one of the drive circuits 2 A- 2 C, as bidirectional signal lines (as illustrated) between each pair of drive circuits 2 A- 2 C, or as a signal bus to which each of the drive circuits is connected to.

Just for the purpose of explanation it is assumed that a first LED module 3 A connected to a first drive circuit 2 A is defect, that is, there is an LED short in an LED in one of the one or more LED strings (see 4 1 in FIG. 2 or 4 1 - 4 n shown in FIG. 3 ) implemented in the first LED module 3 A. In this case, the first drive circuit 2 A may change from the normal mode to the defect mode and operate the LED module 3 A in one of the ways explained with reference to FIGS. 6 to 8 . Upon receiving the defect information from the first drive circuit 2 A, the drive circuits 2 B, 2 C connected to non-defective LED modules 3 B, 3 C may switch off these LED modules in order to reduce the overall input current I IN as long as the first drive circuit 2 A is in the defect mode. “Switching off an LED module” includes decreasing the string current of the at least LED string included in the LED module to zero.

Referring to FIG. 15 , the input voltage V IN may be provided to the LED circuit 1 by a control circuit 5 connected to the input 11 , 12 of the LED circuit 1 . The LED circuit 1 may include one drive circuit, as explained with reference to FIG. 1 , or several drive circuits, as explained with reference to FIG. 14 , wherein each of these drive circuits is connected to an LED module that includes one or more LED strings. The control circuit 5 includes an input 51 , 52 configured to receive a supply voltage V SUP from a supply voltage source 6 and a control input 53 configured to receive a control signal S IN . According to one example, the control circuit 5 is configured to generate the input voltage V IN received by the LED circuit 1 dependent on the input signal S IN . According to one example, the control circuit 5 generates the input voltage V IN such that the input voltage V IN has a first voltage level that causes the at least one drive circuit (not shown in FIG. 15 ) in the normal mode to switch on the LEDs of the at least one LED string (also not shown in FIG. 15 ) connected to the drive circuit 2 A. Further, the control circuit 5 may generate a second signal level of the input voltage V IN , wherein this voltage level is such that it is not high enough for the at least one drive circuit 2 A to switch on the LEDs. The control circuit 5 generates the first and second voltage level of the input voltage V IN based on the input signal S IN , so that in the normal mode, the at least one LED module included in the LED circuit 1 switches on or switches off dependent on the input signal S IN .

This is illustrated in FIG. 16 that shows one example of signal waveforms of the input signal S IN and the input voltage V IN . In the example shown in FIG. 16 , the signal level of the input signal S IN that causes an on-level of the input voltage is a high signal level and a signal level that causes an off-level of the input voltage V IN is a low signal level. This, however, is only an example.

FIG. 17 shows one example of the control circuit 5 . In this example, the control circuit 5 includes an electronic switch 53 connected between a first input node 51 of the control circuit 5 and the first input node 11 of the LED circuit (not shown in FIG. 15 ). Further, a second input node 52 of the control circuit 5 may be connected to the second input node 12 of the LED circuit. The controller 54 receives the input signal S IN and switches on or switches off the electronic switch 53 dependent on the input signal S IN . Just for the purpose of illustration, the electronic switch is a MOSFET, more specifically an n-type MOSFET in the example illustrated in FIG. 17 . However, any other type of electronic switch may be used as well. Referring to FIG. 17 , the control circuit 5 further includes a current measurement circuit 55 configured to measure an input current I IN received by the LED circuit 1 . According to one example, the controller 54 receives a current measurement signal S 55 that represents the (average) input current I IN and monitors the current measurement signal S 55 . According to one example, the controller 54 outputs an error signal S ERR based on the current measurement signal S 55 . According to one example, the controller 54 generates the error signal S ERR such that the error signal S ERR indicates that an error in the LED circuit 1 has occurred when the current measurement signal S 55 indicates that the LED circuit 1 is operating in the defect mode. “The LED circuit 1 operating the in the defect mode” means that at least one drive circuit included in the LED circuit 1 operates in the defect mode.

Although the present disclosure is not so limited, the following numbered examples demonstrate one or more aspects of the disclosure.