METHOD FOR DETECTING A DEFECT IN AN LED STRIP AND ELECTRONIC CIRCUIT WITH AT LEAST ONE LED STRIP

Abstract

A process that exhibits: Detecting at least one operating parameter in an electronic circuit that includes a monitored LED chain (3); Setting a voltage threshold (S3) TH ) based on at least one detected operating parameter; Detecting a chain voltage (S3) across the monitored LED chain (3); Comparing the chain tension (S3) with the tension threshold (S3 TH ), and Detecting a defect in the LED chain (3) based on comparison, where at least one operating parameter is a voltage (V) F3m ) above a monitored LED (3 m ) in the LED chain, where the setting of the voltage threshold (S3) TH ) increasing the voltage threshold (S3 TH ), if the voltage (V F3m ) increases, shows.

Description

[0001] This description generally relates to a method for detecting a defect in an LED (Light Emitting Diode) chain and an electronic circuit with at least one LED chain.

[0002] LED strings, which consist of several LEDs connected in series, are widely used for lighting purposes in various applications, such as interior or exterior vehicle lighting or building lighting, to name just a few. An LED string can be controlled by a driver circuit that generates a drive current supplied by the LED string. The driver circuit can also be configured to monitor the LED string and detect a fault in any of the LEDs. This fault could include a short circuit in one of the LEDs.

[0003] Detecting a fault in a system with at least one LED string can involve measuring the voltage across the LED string and comparing the measured voltage to a threshold value specified by the system manufacturer and stored in the system's memory. This type of fault detection is based on the assumption that the voltage across the LED string (string voltage) is higher when each LED in the string is illuminated, and that the string voltage drops below the threshold value when a short circuit occurs in one of the LEDs. The string voltage is essentially proportional to the number of LEDs in the string and the forward voltage of the LEDs. A decrease in the string voltage when a short circuit occurs in an LED is essentially proportional to an inverse of the number n of LEDs in the string.If the LED chain comprises, for example, n = 2 LEDs and a short circuit occurs in one of the LEDs, the chain voltage drops by approximately 50% (= 1 / n). If the chain comprises, for example, n = 10 LEDs and a short circuit occurs in one of the LEDs, the chain voltage drops by approximately 10% (= 1 / n). The threshold must be set so that the voltage change caused by a short circuit in an LED is detected and that fluctuations in the chain voltage resulting from temperature changes do not erroneously lead to the detection of a defect. Selecting the appropriate threshold is particularly difficult when the number of LEDs increases and the change in chain voltage that needs to be reliably detected decreases.

[0004] German patent application DE 10 2009 017 989 A1 describes a method for detecting a defect in an LED string comprising several LEDs connected in series. This method involves measuring the voltage across the LED string and comparing it to a predefined threshold value. This threshold value is adjusted based on the measured temperature of the LED string to account for temperature-related changes in the voltage across the LED string during fault detection.

[0005] German patent DE 10 2008 058 524 A1 describes a circuit arrangement with several LEDs and a controller designed to regulate the current through the LEDs such that the actual current value corresponds to a setpoint value. The actual value depends on the current flowing through the LEDs and their temperature.

[0006] The object of the invention is to provide a method suitable for reliably detecting a defect in an LED string, in particular an LED string with more than five LEDs, and to provide an electronic circuit suitable for carrying out such a method. This object is achieved by a method according to claim 1 and by an electronic circuit according to claim 6.

[0007] Examples are explained below with reference to the drawings. The drawings serve to illustrate certain principles, so only aspects necessary for understanding these principles are shown. The drawings are not to scale. In the drawings, the same reference symbols denote the same features. Fig. Figure 1 illustrates an example of an electronic circuit that includes an LED string; Fig. Figure 2 illustrates an example of an LED chain in more detail; Fig. Figure 3 schematically illustrates the voltage across the LED string (string voltage) in a fault-free state and after an LED short circuit has occurred; Fig. Figure 4 schematically illustrates the dependence of the chain voltage on the temperature of the LED chain; Fig. Figure 5 schematically illustrates the dependence of the chain voltage on a current through the LED chain; Fig. Figure 6 schematically illustrates the dependence of the chain voltage on the forward voltage of the LEDs in the LED chain; Fig. Figure 7 shows a flowchart of an example of a method for detecting a defect in an LED chain; Fig. Figure 8 shows a circuit diagram of an electronic circuit with an LED chain and an adaptive fault detection circuit; Fig. 9 shows a modification of the in Fig. 8 circuit arrangement shown; Fig. Figure 10 illustrates an example of a fault detection circuit designed to set a threshold based on a current through the LED string; Fig. Figure 11 illustrates an example of a defect detection circuit designed to set a threshold based on the temperature of the LED string; Fig. Figure 12 illustrates an example of a defect detection circuit designed to set a threshold based on both a current through the LED string and a temperature of the LED string; Fig. Figure 13 illustrates another example of a fault detection circuit designed to set a threshold based on a current through the LED string; Fig. Figure 14 illustrates another example of a fault detection circuit designed to set a threshold based on a current through the LED string; Fig. Figure 15 illustrates an example of a defect detection circuit designed to set a threshold based on a forward voltage of an LED in the LED chain; Fig. Figure 16 illustrates an example of a fault detection circuit configured to set a threshold based on the chain voltage of another LED chain in the electronic circuit; and Fig. 17 shows a modification of the in Fig. 16 electronic circuits shown.

[0008] The following detailed description refers to the accompanying drawings. The drawings form part of the description and illustrate examples of how the invention can be used and implemented. Naturally, the features of the various embodiments described herein can be combined unless explicitly stated otherwise.

[0009] Fig. Figure 1 illustrates an example of an electronic circuit comprising an LED string 3 and a fault detection circuit 4 configured to detect a fault in the LED string 3. Referring to Fig. The LED chain 3 comprises a first circuit node 31 and a second circuit node 32, which are hereinafter also referred to as the first chain node 31 and the second chain node 32. A voltage V3 between the first chain node 31 and the second chain node 32 is hereinafter referred to as the chain voltage. The defect detection circuit 4 is configured to detect the chain voltage V3 in order to detect a defect in the LED chain. To detect the chain voltage V3, the defect detection circuit 4 can be connected to the first and second chain nodes 31 and 32, as shown in Fig. 1 is shown.

[0010] The LED chain 3 comprises several (two or more) LEDs. An example of the LED chain 3 is shown in Fig. 2 illustrated. Referring to Fig. 2. The LED chain can have 3 multiple LEDs. 31-3 m include those connected in series between the first chain node 31 and the second chain node 32.

[0011] When the electronic circuit is operating, the LED chain 3 can receive a chain current I3, which is a current that flows between the first chain node 31 and the second chain node 32. Depending on the current level of the chain current I3, the LEDs 31-3 light up. m The chain 3 or do not light up. In the following, "on" and "off" are used synonymously for "light up" and "not light up," respectively.

[0012] The total number of LEDs in the LED string can range from 2 to 50, but especially from 2 to 30. The example shown is for illustrative purposes only and includes... Fig. 2 LED chain shown m = 6 LEDs 31-3 m , which are connected in series.

[0013] According to an example, the LEDs connected in series are 31-3 m The LED chain has 3 LEDs of the same type, so that for a given chain current I3, the LEDs 31-3 messentially glow with the same intensity when the chain current I3 is above a threshold value that causes the LEDs 31-3 to m shine.

[0014] A defect can occur during the operation of LED string 3. One type of defect that can occur is a short circuit in a single LED. This type of defect will be referred to below as an LED short circuit. In the case of an LED short circuit, the defective (short-circuited) LED is off, while the remaining LEDs in LED string 3 remain on. An example of an LED short circuit in 33 of the LEDs 31-3 m The LED chain 3 is in Fig. 2 shown in bold dashed lines. In this example, LED 33 is off and the remaining LEDs 31-32-34-3 are on. m The LEDs are on when a chain current higher than a certain current threshold flows through LED chain 3.

[0015] Referring to Fig. 2. The chain tension V3 is essentially given by a sum of tensions V31-V3. m above the individual LEDs 31-3 m in chain 3. When a chain current I3 is driven into chain 3, it causes the LEDs 31-3 to light up. m light up, and when the LEDs 31-3 m If they are of the same type, the LED voltages are V31-V3. m essentially the same. Therefore, if there is no defect in chain 3, the chain voltage V3 is essentially proportional to the number m of LEDs 31-3 connected in series. m If an LED short circuit occurs and the chain current I3 does not change, the chain voltage V3 drops because the voltage (V33) at the Fig. 2 example shown) above the short-circuited LED (33 in the example shown) Fig. (as shown in example 2) essentially drops to zero.

[0016] Referring to Fig. 1. The chain current I3 can be generated by a current source connected in series with the LED chain 3, wherein a series circuit with the LED chain 3 and the current source 2 is connected between input nodes 11, 12, at which an input voltage V is applied. IN The power source 2 can be a switched current source that switches on or off depending on an input signal S2. According to an example, the power source 2 is configured to generate a chain current I3 that causes the LEDs in the LED chain 3 to light up when the input signal S2 is on, and to generate a chain current level I3 such that the LEDs 31-3 m They are off when the output signal S2 has an off level.

[0017] Basically, detecting an LED short circuit can involve detecting the chain voltage V3 and comparing the chain voltage V3 with a voltage threshold V3. TH include. This is in Fig. Figure 3 shows a schematic time diagram of the chain voltage V3. In this example, an LED short circuit occurs in one of the LEDs 31-3. m at a first time t1, such that at the first time t1 the chain voltage V3 is from a first voltage level V3 N , which is subsequently referred to as the intact level, to a second voltage level V3 D The level , hereinafter referred to as the defect level, decreases. The intact level V3 N is essentially given by V3 N = m · V3 F , where V3 F the voltage across each of the LEDs 31-3 m This is when the LEDs are switched on (lit) and when LEDs 31-3 m are of the same type. This voltage V F This is also referred to below as the forward voltage. The defect level is essentially determined by V3. D = (m-1)·V3 F , so that the difference V3 N -V3 DThe difference between the intact level and the defective level is essentially given by V3. F To detect this type of fault, the voltage threshold V3 is used. TH chosen so that they are between the intact level V3 N and the defect level (V3) D lies, that is, V3 N > V3 TH > V3 D However, for several reasons the chain tension V3 can vary, so situations may arise where comparing the chain tension V3 with a fixed tension threshold V3 TH This does not lead to reliable detection of an LED short circuit. Some examples are explained below.

[0018] Referring to Fig. 4 The chain voltage V3 depends on the temperature of the LED chain 3, with the chain voltage V3 decreasing as the temperature T decreases. Fig. Figure 4 schematically illustrates the intact level V3. N and the defect level V3 Ddepending on the temperature. In this example, the intact level is V3. N and the defect level V3 D each above the V3 threshold TH , if the temperature T is below a temperature threshold T1, then detecting an LED short circuit would not be possible at temperatures below the voltage threshold T1. Furthermore, at high temperatures, the intact level V3 N below the V3 threshold TH drop, so that there is only a certain temperature range in which the detection of an LED short circuit by comparing the chain voltage V3 with a fixed threshold value V3 is possible. TH works reliably.

[0019] Referring to Fig. 5. The chain voltage V3 also depends on the chain current I3, whereby the chain voltage V3 increases when the chain current I3 increases. Fig. 5 shows the intact level V3 N and the defect level V3 Ddepending on the chain current I3. Referring to Fig. 5 becomes the defect level V3 D higher than the fixed threshold V3 TH , if the chain current I3 is higher than a current threshold I31. Therefore, reliable detection of an LED short circuit may no longer be possible if, due to current variations, the chain current I3 rises above the current threshold I31.

[0020] Furthermore, with reference to Fig. 6. The chain tension V3 also depends on the flux voltage V. F the LEDs 31-3 m in the LED chain 3. The forward voltage V F The forward voltage (Vf) is the voltage across a single LED at a specific chain current that causes the LED to illuminate. Due to variations in the LED manufacturing process, different LEDs can have different forward voltages (Vf). FIn some cases, when assembling an LED string, LEDs with essentially the same forward voltage are selected, so that different LED strings with the same number of LEDs and receiving a string current at the same level can have significantly different string voltages V3. Thus, in some LED strings, the defect level at a given string current I3 can be higher than the threshold voltage V3. TH , so that reliable short-circuit detection is not possible in these LED chains.

[0021] Fig. Figure 7 illustrates a flowchart of a process which, despite variations in the chain voltage V3, is based on the Fig. The types described in sections 4 to 6 are capable of reliably detecting an LED short circuit in an LED string 3. Referring to Fig. 7. The method comprises detecting at least one operating parameter in the electronic circuit comprising the LED string 3 (101). This LED string 3 is hereinafter also referred to as the monitored LED string. The method further comprises setting a voltage threshold based on the at least one detected operating parameter (102), measuring the string voltage across the monitored LED string (103), comparing the string voltage with the voltage threshold (104), and detecting a defect in the LED string based on the comparison (105). By detecting the at least one operating parameter and setting the voltage threshold based on the at least one detected operating parameter, the method is robust against changes in the string voltage that are not caused by an LED short circuit, such as changes in the voltage measured by the LED string. Fig. 4 to 6 of the types explained.

[0022] Fig. Figure 8 shows an example of a defect detection circuit 4, which is configured to perform according to the [reference to the relevant section]. Fig. to work with the 7 described procedures. Referring to Fig. 8 The defect detection circuit 4 comprises a comparator 41 and a voltage threshold generation circuit 42, wherein the comparator 41 provides a signal S3 representing the chain voltage V3 and a threshold signal S3 TH , that the voltage threshold V3 TH represents, receives, and is trained to detect a defect signal S DEF depending on comparing the chain voltage signal S3 with the voltage threshold signal S3 TH to generate. According to an example, comparator 41 is configured to generate the defect signal S. DEF to generate such that the defect signal S DEFhas a normal level or a defect level, where the defect level indicates that a defect has been detected. According to one example, the comparator 41 is configured to generate the defect level each time the chain voltage V3, represented by the chain voltage signal S3, falls below the voltage represented by the voltage threshold signal S3. TH represented voltage threshold V3 TH sinks.

[0023] According to an example, the comparator 41 receives the chain voltage signal S3 at an inverting input and the voltage threshold signal S3 TH at a non-inverting input, so that the defect signal S DEF It has a low signal level whenever the chain voltage signal S3 is below the voltage threshold signal S3 TH decreases. However, this is only one example. The inverting and non-inverting inputs of comparator 41 can be reversed, resulting in a high signal level of the defect signal S. DEFrepresents a defect.

[0024] This is merely an example, in the case of the one in Fig. In example 8, the chain voltage signal S3 is identical to the chain voltage V3. Accordingly, the voltage threshold signal S3 TH identical to the voltage threshold V3 TH This is, however, only one example. According to another example, the chain voltage signal S3 is not identical to the chain voltage V3, but represents the chain voltage V3 in an arbitrary way. Accordingly, the voltage threshold signal S3 must be TH not identical to the threshold voltage V3 TH be, but represents the voltage threshold V3 TH in any way. In any case, however, comparing the chain voltage signal S3 with the voltage threshold signal S3 is necessary. TH equivalent to comparing the chain tension V3 with the voltage threshold V3 TH .

[0025] At the in Fig. In the example shown, the chain voltage V3 is referenced to a second input node 12 of the electronic circuit, where the second input node 12 can be a ground node of the electronic circuit. In this example, the voltage threshold generation circuit 42 is also connected to the second input node 12, and the voltage threshold signal S3 is... TH a signal related to the second input node 12. However, this is only one example. According to another example, which is in Fig. As shown in Figure 9, the chain voltage V3 is a voltage between the first input node 11 and the second chain node 32. In this example, the voltage threshold generation circuit 42 generates the voltage threshold signal S3. TH such that this signal is referenced to the first input node 11.

[0026] Referring to the Fig. 8 and Fig. 9 The voltage threshold generation circuit 42 receives an operating parameter signal Sor, where the operating parameter signal Sor represents at least one operating parameter of the electronic circuit.

[0027] According to one example, the detected operating parameter is the chain current I3. In this example, the operating parameter signal S represents OP the chain current I3. A fault detection circuit 4, which is designed to detect the chain current I3 and the voltage threshold signal S3 TH to generate based on the detected chain current I3 is in Fig. Figure 10 shows the voltage threshold generation circuit 42. In this example, the circuit includes a current sensor 45 configured to measure the chain current I3 and a measuring current I. SENSE, which represents the chain current I3. The current sensor 45 can be any type of current sensor designed to measure the chain current I3 and to measure a current I SENSE to provide a current that represents the chain current I3. According to an example, the measuring current I is... SENSE proportional to the chain current I3. In this example, the voltage threshold generation circuit 42 is configured to generate the voltage threshold signal S3. TH (which is identical to the voltage threshold V3) TH is) to be generated in such a way that the voltage threshold signal S3 TH increases when the measuring current I SENSE increases, and therefore, when the chain current I3 increases. In this way, changes in the chain voltage V3 due to changes in the chain current I3 can be compensated for when a defect, such as an LED short circuit, is detected.

[0028] At the in Fig. The voltage threshold V3 shown in example 10 is TH a voltage across a resistor 44, wherein a constant current I43, provided by a current source 43, and the measuring current I SENSE driven by this resistor 44. Thus, the voltage threshold V3 TH proportional to a total current given by the constant current I43 and the measuring current I SENSE , and the voltage threshold V3 increases TH on, when the measuring current I SENSE , which represents the chain current I3, increases.

[0029] According to another example, the operating parameter is a temperature of the LED chain 3, so that the operating parameter signal S OP The temperature of the LED chain 3 is represented. A defect detection circuit 4 is designed to detect the voltage threshold signal S3. TH depending on the temperature of the LED chain 3, is in Fig. Figure 11 shows the voltage threshold generation circuit 42. In this example, the circuit includes a temperature sensor 51 configured to measure the temperature of the LED chain 3 and generate a temperature signal S. TEMP to output a signal representing the chain temperature. This temperature signal S TEMP In this example, the operating parameter signal S OP A control circuit 52 receives the temperature signal S TEMP and controls a variable current source 53 depending on the temperature signal S TEMP According to an example, the control circuit 52 is configured to control the current source 53 such that a current I supplied by the current source 53 TEMP increases when the temperature signal S TEMP The represented chain temperature increases. The defect detection circuit includes the constant current source 43 and the previously determined circuit based on Fig. 10. The resistor 44 is explained, with the variable current source 53 connected in parallel to the resistor 44. In this way, the current I44 through the resistor is proportional to a difference between the constant current I44 and the temperature-dependent current I. TEMP and decreases when the temperature-dependent current I TEMP increases. In this way, the voltage threshold signal S3 decreases. TH from when the temperature-dependent current I TEMP increases, that is, when the chain temperature increases. In this way, the voltage threshold signal S3 can be used to determine the voltage threshold signal. TH represented voltage threshold V3 TH so generated that they are between the intact level V3 N and the defect level V3 D lies, even if these voltage levels decrease with increasing temperature, as in Fig. Figure 3 shows that variations in the chain voltage V3 due to variations in the chain temperature can be compensated for when a defect, such as an LED short circuit, is detected.

[0030] Fig. Figure 12 shows an example of a defect detection circuit 4 designed to detect the voltage threshold signal S3 TH dependent on both the chain current I3 and the chain temperature. This defect detection circuit 4 includes the one in Fig. 10 shown current sensor 54 and temperature sensor 51, control circuit 52 and variable current source 43, which are in Fig. Figure 11 is shown. In this example, the current I44 through the resistor 44 is given by the constant current I43 provided by the current source 43 plus the measuring current I. SENSE minus the temperature-dependent current I TEMP In this way, the voltage threshold signal S3 TH to, when the sensor current ISENSE increases, and decreases when the temperature-dependent current I TEMP increases. In this way, the voltage threshold signal S3 can be TH so that - despite fluctuations in chain temperature and chain current I3 - there is a difference between the intact level V3 N and the defect level V3 D This allows fluctuations in the chain voltage V3, which are present due to fluctuations in the chain current I3, and fluctuations in the chain temperature to be compensated when a defect, such as an LED short circuit, is detected.

[0031] At the in Fig. In example 10, the voltage threshold signal S3 changes. TH continuously, when the chain current I3 changes and thus the measuring current I SENSE changes. Another example of the defect detection circuit 4, which is designed to detect the voltage threshold signal S3 THto generate such that the voltage threshold signal S3 TH increases gradually as the chain current I3 increases, is in Fig. Figure 13 shows that in this example, a switch controller 47 receives a measuring voltage V. SENSE , where the measuring voltage V SENSE The chain current I3 is represented. According to an example, the measuring voltage V SENSE proportional to the chain current I3. The measuring voltage V SENSE can be generated by a current sensor 45 based on Fig. 10 explained type and a resistor 46, which is connected in series to the current sensor 45 and measures the measuring current I SENSE is obtained, so that a voltage across the resistor 46 is proportional to the measuring current I SENSE is and the measuring voltage V SENSE educates.

[0032] The switch controller 47, which may contain an analog-to-digital converter (ADC), receives the measuring voltage V SENSEand controls one of the several switches 472-475 depending on the measuring voltage V SENSE The switches 472-475 are each connected to a respective tap of a resistive voltage divider 441-445, wherein this voltage divider 441-445 is connected in series with the current source 43 and wherein the voltage threshold signal S3 TH The resistive voltage divider 441-445 is available above the resistive voltage divider. The resistive voltage divider 441-445 comprises several resistors 441-445 connected in series between the current source 43 and a reference node, which in this example is the second input node 12. A circuit node between the current source 43 and the resistive voltage divider 441-445 is connected to the input of the comparator 41, which receives the voltage threshold signal S3. THThe comparator input receives (this comparator input is the non-inverting input in this example). Two of the multiple resistors 441-445 are connected to each tap, with each of the controller-controlled switches 472-475 connected between a respective tap and the reference node. The "reference node" is the circuit node to which the voltage threshold signal S3 is applied. TH is related. In the on state, each of these switches 472-475 bridges at least one of the resistors 441-445. For example, switch 472 bridges resistors 442-445, switch 473 bridges resistors 444-445, and so on. This allows a resistance value of the resistive voltage divider between the input of comparator 41 and the reference node, and thus the voltage threshold signal S3, to be determined. TH , by switching on one of the respective switches 472-475. In the case of the Fig. In the example shown in Figure 13, the resistive voltage divider 441-445 has the highest resistance value when each of the switches 472-475 is off, and the lowest resistance value when switch 472 is on. Essentially, the controller 47 is configured to control the switches 471-475 such that the number of resistances bridged by each switch 472-475 being turned on decreases as the current sensing signal V SENSE increases. In this way, the voltage threshold signal S3 decreases. TH in discrete steps when the current measurement signal V SENSE increases.

[0033] At the in Fig. In example 13, the switch controller 47 receives a signal V SENSE , which represents the chain current I3 to determine the voltage threshold signal S3 THThis can be adjusted depending on the chain current. However, this is just one example. According to another (not shown) example, controller 47 receives the temperature signal S. TEMP and controls switches 472-475 such that the number of resistors bridged by switching on each of the switches 472-475 increases when the temperature signal S TEMP increases. In this way, the voltage threshold signal S3 decreases. TH in discrete steps when the temperature signal S TEMP increases.

[0034] At the in Fig. In example 13, the chain current I3 is measured and the voltage threshold signal S3 is measured. TH adjusted based on the measured chain current I3. Fig. 14 shows a modification of the in Fig. Circuit shown in 13. In the circuit shown in Fig. In the example shown in Figure 14, current source 2 is a controlled current source that provides the chain current I3 depending on a control signal. More precisely, the one shown in Figure 14 is a controlled current source that provides the chain current I3 depending on a control signal. Fig. Figure 14 shows current source 2 comprising three sub-sources 2A, 2B, 2C connected in parallel. The control signal comprises three sub-signals S2A, S2B, S2C, each controlling one of the sub-sources 2A, 2B, 2C. Current sources 2A, 2B, 2C are each configured to provide a sub-current I3A, I3B, I3C, respectively, where the series current I3 is given by the sum of the sub-currents I3A, I3B, I3C provided by current sources 2A, 2B, 2C. By way of example, the current provided by each sub-source 2A, 2B, 2C is either zero or has a predetermined current level, which may be the same or different for each sub-source 2A, 2B, 2C. Whether the current provided by a sub-source 2A, 2B, 2C is zero, or has a predetermined current level other than zero, depends on the respective control signal S2A, S2B, S2C.Using a power source 2 of the in . Fig. 14 of the types shown can be 2 N -1 different current levels are generated, each different from zero. N is the number of sub-sources, where in Fig. Example 14 shown N = 3.

[0035] Referring to Fig. 14 The defect detection circuit 4 receives the control signals S2A, S2B, S2C and sets the voltage threshold signal S3 TH depending on these control signals S2A-S2C. More precisely, a logic circuit 61 receives the control signals S2A, S2B, S2C and sets the voltage threshold signal S3. TH by bridging two or more resistors of a resistive voltage divider 441-444 depending on the control signal S2A-S2C. The in Fig. The arrangement shown in 14 comprises 2 N -1 resistors connected in series, and 2 N -2 switches 472-474, so that 2 N -1 different resistance values, and therefore 2N -1 different levels of the voltage threshold signal S3 TH These levels of the voltage threshold signal S3 can be adjusted. TH are each assigned to one of the different current levels of the chain current I3.

[0036] Fig. Figure 15 illustrates a defect detection circuit 4 according to another example. In this example, the defect detection circuit 4 is configured to detect a forward voltage V. Fm one of the several LEDs 31-3 m to detect the LED chain, and sets the voltage threshold signal S3 TH depending on the detected flux voltage V F3m one. According to an example, the voltage threshold signal S3 TH generated such that the voltage threshold signal S3 TH increases when the detected flux voltage V F3m to increase. In the Fig. In example 15, an amplifier 71 receives a forward voltage signal S. F3m, that the flux voltage V F3m represents, and generates the voltage threshold signal S3 TH at an output. According to an example (not shown), the forward voltage signal S F3m the flux voltage V F3m , that means the amplifier can be connected to LED 3 m be connected. According to another example, which is in Fig. As shown in 15, a buffer 72 receives the forward voltage V F3m and represents the forward voltage signal S F3m either directly or, as shown, via an optional voltage divider 731, 732.

[0037] As in the previously explained examples, a comparator 41 receives the defect signal S DEF generated, the chain voltage signal S3, which in this example is equal to the chain voltage V3, and the voltage threshold signal S3 TH Optionally, another comparator 74 compares the forward voltage V. F3m with a minimum voltage V MIN, wherein an output signal of the comparator 41 and the further comparator 74 is obtained by a logic gate, such as an OR gate 75, and the defect signal S DEF provided by logic gate 75. In this example, a defect level of the defect signal S is used. DEF generated each time the forward voltage V F3m drops below a voltage determined by the voltage threshold signal S3 TH is represented, or the flux voltage V F3m below the minimum voltage V MIN decreases. The latter can occur if there is an LED short circuit in LED 3. m occurs.

[0038] According to another example, which is in Fig. As shown in Figure 16, the electronic circuit comprises several LED strings, the monitored LED string and at least one further LED string 31, 32, wherein in the Fig. In the example shown in Figure 16, two further LED strings 31 and 32 are present. These further LED strings 31 and 32 are each connected in series to a further power source 21 and 22, respectively. Furthermore, the string voltage V3 of the monitored LED string 3 is essentially equal to the string voltages V31 and V32 of these further LED strings 31 and 32 – provided there is no defect in the monitored LED string 3 – or a predetermined ratio exists between the string voltage V3 of the monitored LED string 3 and the string voltages V31 and V32 of the further LED strings 31 and 32.

[0039] It can be assumed that, depending on the temperature, the chain voltage V3 of the monitored LED chain 3 and the chain voltages V31, V32 of the other LED chains 31, 32 vary in the same way. Therefore, in the Fig. Example 16 shows the voltage threshold signal S3 THbased on the chain voltages V31, V32 of the other LED chains 31, 32. According to an example, a maximum selector 81 receives the chain voltages V31, V32 of the other LED chains 31, 32 and outputs a signal that represents the maximum of the other chain voltages V31, V32. In this way, generating a suitable voltage threshold signal S3 is TH This is ensured even if an LED short circuit occurs in one of the further LED chains 31, 32. A resistive voltage divider 821, 822 generates the voltage threshold signal S3. TH based on the output signal generated by the maximum selector 81. In this example, the voltage threshold signal S3 is taken into account. TH the temperature, although the temperature is not directly measured.

[0040] According to another example, which is in Fig. As shown in figure 17, each of the LED chains 3, 31, 32, which are in Fig.Figure 16 shows the monitoring of the occurrence of a defect. In this example, the maximum selector 81 receives the chain voltages V3, V31, and V32, and the minimum selector 83 receives the chain voltages V3, V31, and V32. An output signal of the minimum selector 83 is obtained through the second input of the comparator 41, which in this example is the inverting input.

[0041] Although the invention has been described with the aid of illustrative examples, this description should not be considered limiting. Various modifications and combinations of the illustrative examples, as well as other examples of the invention, are obvious to those skilled in the art from the description. The attached claims are therefore intended to encompass any such modifications or examples.

Claims

[1] Method which features: Detecting at least one operating parameter in an electronic circuit that includes a monitored LED chain (3); Setting a voltage threshold (S3) TH ) based on at least one detected operating parameter; Detecting a chain voltage (S3) across the monitored LED chain (3); Comparing the chain tension (S3) with the tension threshold (S3 TH ), and Detecting a defect in the LED chain (3) based on comparison, where at least one operating parameter is a voltage (V) F3m ) above a monitored LED (3 m ) in the LED chain, where the setting of the voltage threshold (S3) TH ) increasing the voltage threshold (S3 TH ), if the voltage (V F3m ) increases, shows. [2] Method according to claim 1, where at least one operating parameter continues to exhibit a chain current (I3) through the LED chain (3); and where the voltage threshold (S3) is set TH ) increasing the voltage threshold (S3 TH ), when the chain current (I3) increases. [3] Method according to claim 1 or 2, where at least one operating parameter still exhibits an estimated temperature of the LED chain (3); and where the voltage threshold (S3) is set TH ) reducing the voltage threshold (S3 TH ), when the estimated temperature rises. [4] The method of claim 1, further comprising: Detecting a fault when the voltage across the monitored LED (3 m ) below a given minimum value (V MIN ) decreases. [5] Method according to any one of the preceding claims, where the electronic circuit still has a variable current source (2A, 2B, 2C), in which the variable current source (2A, 2B, 2C) is connected in series with the monitored LED chain (3) and is designed to provide a current with a variable amplitude depending on a control signal, and where at least one operating parameter has the control signal (S2A, S2B, S2C). [6] Electronic circuit which features: a monitored LED chain (3); and a fault detection circuit (4), the defect detection circuit is designed to: to detect at least one operating parameter in the electronic circuit; a voltage threshold (S3) TH ) based on at least one detected operating parameter; to detect a chain voltage (V3) across the monitored LED chain (3); the chain tension (V3) with the voltage threshold (S3) TH to compare; and to detect a defect in the LED chain (3) based on comparison, where at least one operating parameter is a voltage (V) F3m ) above a monitored LED (3 m ) in the LED chain and wherein the defect detection circuit (4) is configured to detect the voltage threshold (S3) TH ) to increase when the voltage (V F3m ) increases. [7] Electronic circuit according to claim 6, where at least one operating parameter continues to exhibit a chain current (I3) through the LED chain (3); and in which the defect detection circuit (4) is designed to detect the voltage threshold (S3) TH ) to increase when the chain tension increases. [8] Electronic circuit according to claim 6 or 7, where at least one operating parameter still exhibits an estimated temperature of the LED chain (3); and in which the defect detection circuit (4) is designed to detect the voltage threshold (S3) TH ) to decrease as the estimated temperature increases. [9] Electronic circuit according to claim 6, wherein the defect detection circuit (4) is further configured to detect a defect when the voltage across the monitored LED 3 m below a predetermined minimum value (V MIN ) decreases. [10] Electronic circuit according to any one of claims 6 to 9, further comprising: a variable current source (2A, 2B, 2C) connected in series with the monitored LED chain (3) and designed to provide a current with a variable amplitude depending on a control signal (S2A, S2B, S2C), wherein at least one operating parameter has the control signal (S2A, S2B, S2C) and the defect detection circuit (4) is configured to determine the voltage threshold (S3 TH ) based on the control signal (S2A, S2B, S2C).

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