METHOD FOR DETECTING A SHORT CIRCUIT IN AT LEAST ONE LED OF AN LED STRING OF A LIGHT MODULE

Description

[0001] The invention relates to a method for detecting a short circuit of at least one LED of an LED string of a light module, wherein the LED string comprises a number of at least two LEDs connected in series with one another.

[0002] Many technical applications of light modules require high technical reliability. To this end, it is common practice in the industry to guarantee the highest possible reliability by selecting high-quality components and implementing a suitable light module design. Nevertheless, the occurrence of errors cannot generally be completely ruled out.

[0003] Another technical task therefore concerns the question of how to optimally handle errors. In order to react to the presence of an error, it must first be detected. For lighting modules with an LED string, a monitoring method has become known from the state of the art in which each individual LED forward voltage of each LED in the LED string is measured and compared with a target value. If a short circuit occurs in an LED, the forward voltage at that LED collapses, allowing the short-circuit error to be measured. However, such monitoring is associated with considerable complexity.

[0004] Another fault detection method involves measuring the LED forward voltages of the entire LED string and using an additional temperature sensor to adjust the expected value of the LED forward voltage depending on the temperature. The measured LED forward voltage of the entire string can then be compared with the reference value. However, this method complicates the replacement of spare parts because it is very sensitive to the individual LED forward voltages, which can vary significantly depending on the production batch, for example. Furthermore, the influence of temperature can only be predicted to a limited extent, which is why this method is only suitable for monitoring a small number of LEDs in an LED string. Document EP2717653A1 shows a method for detecting LED failure.

[0005] One object of the invention is therefore to provide a method that detects short-circuit faults of an LED in an LED string in a cost-effective and reliable manner, whereby the number of LEDs in the LED string can be freely selected without the method failing. This object is achieved with a method of the type mentioned above, which according to the invention comprises the following steps: a) Providing a reference value that correlates with a differential operating AC target resistance of the LED string and storing the reference value in a data storage device, b) Starting up the LED string with an operating DC voltage so that the LED string emits light, c) Impressing an alternating current superimposed on the operating DC voltage into the LED string, wherein the frequency of the alternating current is at least 60 Hz, d) Measuring the AC voltage drop across the LED string caused by the alternating current according to step c), e) Deriving a comparison actual value of the LED string that correlates with a differential operating AC current actual resistance value with the aid of the AC voltage drop measured according to step d), f) Retrieving the reference value provided in the data storage device according to step a) and comparing it with the comparison actual value,Depending on the result of the comparison, the presence of an LED short circuit is inferred.

[0006] The method according to the invention creates a cost-effective yet reliable solution for detecting short circuits in LEDs in an LED string. If this method is used, for example, in commercially available vehicle headlights, no additional hardware is required for implementation, as modern LED control units already have suitable hardware for executing the method. If a voltage measurement of the entire LED chain is already available in a light module, it can be adapted to implement the method according to the invention by applying an LED alternating current to the LED operating direct current through a software adaptation, thus determining the number of functioning LEDs according to the method according to the invention.The number of LEDs in a string is generally freely selectable and can be, for example, at least four, at least ten, between four and forty, or any other number. The advantages of the invention are particularly evident when a large number of LEDs are present in a string. By using a frequency of over 60 Hz in conjunction with the alternating current used, any slight fluctuations in light intensity are prevented from becoming visible as unpleasant flickering. In particular, the frequency can be at least 100 Hz.

[0007] In particular, it can be provided that the alternating current impressed according to step c) is free of a DC component. This largely minimizes temporal changes in the radiated light output of the LEDs and keeps additional energy consumption low.

[0008] Furthermore, it can be provided that the impressed alternating current has the temporal characteristic of a square-wave signal, a triangular signal or a sine-wave signal.

[0009] According to the invention, the alternating current impressed according to step c) is selected such that its amplitude is between 20% and 60% of the absolute value of the nominal value of an operating direct current of the LED string caused by the operating direct voltage. This suitably large current amplitude simplifies the measurement, since the voltage amplitude to be measured is thus also correspondingly large. Any measurement inaccuracies then have a less pronounced impact on the comparison result according to step f), since the actual comparison value to be calculated, which correlates with the differential operating alternating current actual resistance value, fluctuates less.

[0010] Furthermore, it can be provided that the reference value provided according to step a) is the differential operating AC current target resistance, and wherein the comparison actual value derived according to step e) is a differential operating AC current resistance actual value obtained by forming a quotient of the measured AC voltage drop according to step d) and the impressed AC current according to step c). This differential operating AC current resistance actual value therefore corresponds to the slope of a current-voltage curve of the LED string at the respective operating point.

[0011] In particular, it can be provided that the comparison according to step f) is a limit value comparison, wherein if a predeterminable maximum difference, formed by the difference between the reference value and the actual comparison value, is exceeded, it is concluded that there is an LED short circuit, wherein the maximum difference is a maximum of 10% of the reference value.

[0012] Furthermore, it can be provided that an error routine is triggered when an LED short circuit is detected. In particular, it can be provided that the error routine includes the output of an error signal and / or the change of the operating state of the LED string. The change of the operating state can be achieved, for example, by reducing the operating voltage of the LED string, switching off the LED string, increasing the transmission power of the remaining functioning LEDs of the LED string (e.g., by increasing the switch-on time or the duty cycle in the case of clocked operation), and / or switching on a replacement string.

[0013] Furthermore, it can be provided that if there is no LED short circuit, a signal is issued to confirm that there is no error.

[0014] In particular, it can be provided that if the absence of short-circuit faults is determined in step e), the recorded actual comparison value replaces the existing reference value and is stored in the data memory to serve as an updated reference value in a subsequent iteration of steps a) to f). This allows aging effects to be compensated for. The method thus becomes self-calibrating and self-learning.

[0015] Furthermore, the invention relates to a short-circuit fault-detecting lighting system for carrying out a method according to the invention, wherein the short-circuit fault-detecting lighting system comprises the following: a light module with at least one LED string, wherein the at least one LED string has a number of at least two LEDs connected in series with one another, and a short-circuit detection system, wherein the short-circuit detection system is configured at least for the electrical supply of the at least one LED string, wherein the short-circuit detection system is configured to carry out steps a) to f) of the method according to the invention.

[0016] In particular, the short-circuit detection system can be configured so that the amplitude of the superimposed alternating current is independent of the absolute value of the nominal value of the operating direct current of the LED string. If the superimposed alternating current is independent of the operating point, then the evaluation of the voltage measurement should be continuously adjusted (e.g., due to the nonlinear current-voltage characteristic of the LEDs or temperature influences) in order to achieve correct short-circuit detection at different operating direct currents. The advantage of this variant is that the effort required for correct short-circuit detection can be shifted from the driver side to the measurement side.

[0017] Furthermore, it can be provided that the short-circuit detection system is configured such that the amplitude of the superimposed alternating current is dependent on the absolute value of the nominal value of the operating direct current of the LED string. In contrast to the previously mentioned variant, a dependent alternating current results in the adaptation to the above-mentioned influences being shifted to the driver side. This configuration is considered particularly advantageous, for example, in the case of temperature-dependent derating of the LEDs or in the case of transition effects from one lighting function to another. It can be provided that the amplitude of the impressed alternating current amounts to a percentage, for example between 20% and 50%, in particular exactly 30% of the absolute value of the nominal value of the operating direct current of the LED string, depending on the respective operating state.

[0018] Furthermore, the invention relates to a motor vehicle headlight comprising a short-circuit fault detecting lighting system according to the invention.

[0019] Furthermore, the invention relates to a motor vehicle comprising a short-circuit fault detecting lighting system according to the invention and / or a motor vehicle headlight according to the invention.

[0020] In other words, the considerations of the invention can be described as follows: The dynamic resistance of an LED (semiconductor) changes only very slightly with temperature and can therefore be used to detect short circuits in a chain of LEDs. By superimposing an alternating current on the LED (direct) current (i.e. the operating current), the alternating voltage across the LED chain can be determined using the dynamic resistance, or conversely, the dynamic resistance can be determined by recording the current and voltage. When the LED driver or LED control unit is put into operation for the first time, the number of LEDs and the dynamic resistance in a defined operating state are known (e.g. by providing a reference value R ref according to step a)) in order to compare the data generated with the alternating current during regular operation.

[0021] This makes it possible to detect a short circuit in one or more LEDs in an LED chain with an indeterminate number greater than 1. The alternating current component can be selected so that it has no noticeable effect on the intensity or the thermal load.

[0022] The invention is explained in more detail below with reference to an exemplary and non-limiting embodiment, which is illustrated in the figures. Figure 1 a schematic representation of a short-circuit fault detecting lighting system for carrying out a method according to the invention, Figure 2 an example LED string of a lighting system, Figure 3 three exemplary operating DC voltages for different temperatures and a corresponding current flow through the LED string, Figure 4 an exemplary characteristic curve of an LED 31 of an LED string 3 at a temperature of 25°C, and Figure 5an example design of a driver circuit.

[0023] In the following figures, unless otherwise stated, the same reference symbols denote the same features.

[0024] Fig. 1 shows a schematic representation of a short-circuit fault detecting lighting system 7 for carrying out a method according to the invention. This method for detecting a short circuit of at least one LED 31 of an LED string 3 (see Fig. 2 ) of a light module 8, wherein the LED string 3 comprises a number of at least two LEDs 31 connected in series with one another, the method comprising the following steps: a) Providing a reference value R ref , which correlates with a differential operating AC target resistance of the LED string and storing the reference value R ref on a data memory 6, b) Commissioning the LED string 3 with an operating DC voltage (this can, for example, vary depending on the temperature of the string or be adjusted to achieve a certain operating current; Fig. 3shows three exemplary operating DC voltages for different temperatures, namely U BT1 for a temperature of -40°C, U BT2 for a temperature of -25°C and U BT3 for a temperature of -105°C), so that the LED string 3 emits light, c) impressing an alternating current I W1 or I W2 superimposed on the operating DC voltage U BT1 , U BT2 or U BT3 into the LED string 3, wherein the frequency of the alternating current I W1 or I W2 is at least 60 Hz (I W1 is a rectangular temporal profile of the alternating current and I W2 is a sinusoidal profile of an alternating current - the alternating current component is superimposed on the direct current component; in principle, any desired (advantageously periodic) alternating signal profiles can be used), d) measuring the voltage generated by the alternating current I W1 orI W2 according to step c) caused AC voltage drop U W1 , U W2 at the LED string 3, e) deriving a comparison actual value R ist of the LED string 3 correlating with a differential operating AC current actual resistance value with the aid of the AC voltage drop U W1 or U W2 measured according to step d) (depending on whether, for example, the current I W1 or I W2 was present), f) retrieving the reference value R ref made available on the data memory 6 according to step a) and comparing it with the comparison actual value R ist , wherein the presence of an LED short circuit is inferred depending on the result of the comparison.

[0025] The short-circuit fault detecting lighting system 7 for carrying out the method according to the invention comprises a light module 8 with at least one LED string 3, wherein the at least one LED string 3 has a number of at least two LEDs 31 connected in series with one another, and a short-circuit detection system 9, wherein the short-circuit detection system 9 is configured at least to electrically supply the at least one LED string 3, wherein the short-circuit detection system 9 is configured to carry out steps a) to f) of the method.

[0026] As shown by Fig. 3 which shows a temporal progression of the phase current I(t) and the phase voltage U(t), it can be provided that the alternating current I W1 or I W2 impressed according to step c) is free of a DC component. As in Fig. 3As can be clearly seen, the LED forward voltage or the DC operating voltage of the entire chain changes much more with temperature than the AC voltage resulting from the superimposed AC current in conjunction with the almost constant dynamic resistance or the differential AC operating resistance. The resulting AC voltage is almost temperature-independent.

[0027] In particular, it can be provided that the reference value R ref provided according to step a) is the differential operating AC current target resistance, and wherein the comparison actual value R ist derived according to step e) is a differential operating AC current resistance actual value obtained by forming a quotient from the measured AC voltage drop U W1 , U W2 according to step d) and the impressed AC current I W1 , I W2 according to step c). This value correlates with the slope of the characteristic curve of the respective LED string 3 at the operating point. Figure 4shows an example current-voltage characteristic curve of an LED 31 of an LED string 3 at a temperature of 25°C. The differential AC resistance in an operating range is also shown as an example, with this resistance corresponding to the inverse of the slope k. The impressed AC current I W2 causes the correspondingly correlating voltage drop U w2 in this operating range. For an LED string comprising, for example, four LEDs, this would result in an operating forward voltage of between approximately 12V and 13V for these LEDs.

[0028] In particular, it can be provided that the comparison according to step f) is a limit value comparison, wherein if a predeterminable maximum difference amount formed by the difference between the reference value R ref and the comparison actual value R ist is exceeded, it is concluded that an LED short circuit has occurred, wherein the maximum difference amount is a maximum of 10% of the reference value R ref.

[0029] Furthermore, it can be provided, for example, that an error routine is triggered when an LED short circuit is detected. The error routine can, for example, output an error signal s1 (see Fig. 1 ) and / or the change in the operating state of the LED string 3. If there is no LED short circuit, however, a signal s2 can be output to confirm that there is no error.

[0030] If the absence of short-circuit faults is determined, the recorded actual comparison value R ist can replace the existing reference value R ref and be stored in the data memory 6 in order to serve as an updated reference value in a subsequent iteration of steps a to f.

[0031] With a view to Fig. 1It should be mentioned that the short-circuit detection system 9 can be designed so that the amplitude of the superimposed alternating current I W1 , I W2 is independent of the absolute value of the nominal value of the operating direct current IB of the LED string 3.

[0032] Alternatively, the short-circuit detection system can be designed so that the amplitude of the superimposed alternating current I W1 , I W2 is dependent on the absolute value of the nominal value of the operating direct current IB of the LED string 3.

[0033] Furthermore, the invention relates to a motor vehicle headlight comprising a short-circuit fault-detecting lighting system 7 according to the invention. Furthermore, the invention relates to a motor vehicle comprising a short-circuit fault-detecting lighting system 7 according to the invention and / or a motor vehicle headlight according to the invention.

[0034] Fig. 1shows some further features worth mentioning: The supply of the LED string 3 or the lighting system 7 can be provided by a battery 1 or an energy storage device. The operating direct current IB can be supplied by a driver 2.

[0035] This driver 2 can, for example, be designed as a switching converter or as a linear regulator. If the driver 2 is designed as a switching converter, it has, as is known, an electronic switch (not shown) to control a required output variable. Depending on requirements, the driver 2 can be designed as a combination of several switching converters. For example, a combination of boost converters 21 and buck converters 22, 22a can be used (see Fig. 5). If several LED strings are to be supplied, a step-down converter 22, 22a can be provided for each LED string. The output variable of the switching converter is preferably regulated. The output variable can correspond to the required operating direct current IB. The operating direct current IB can be regulated by modulating the duty cycle of the electronic switch. Alternatively, switching converters have also become known which regulate the output variable, in this case the operating direct current IB, by modulating a switching frequency of the electronic switch. With both types of control, switching frequencies in the upper kHz range up to a few MHz are common.

[0036] It can be provided that the operating direct current IB of the LED string 3 is provided by a current-controlled buck converter 22, 22a. This buck converter has an electronic switch (not shown) whose switching cycles depend on the required operating direct current IB. The switching cycles thus correspond to a manipulated variable which can be changed by the duty cycle and / or the switching frequency. If the LED string 3 is now put into operation by means of an operating direct voltage, for example via the connection to a current or voltage source, for example a battery 1, an alternating current I W1 or I W2 can be superimposed by changing the switching cycles of the electronic switch.The frequency fw of the superimposed alternating current is preferably less than one thousandth of the switching frequency of the electronic switch in the current-controlled buck converter, wherein the frequency fw of the superimposed alternating current is at least 60 Hz.

[0037] Alternatively or additionally, the operating direct current IB can be linearly regulated or limited. For this purpose, an electronic switch (not shown) can be connected in series with the LEDs 31 of the LED string 3. This electronic switch acts like a controlled resistor and thus allows the operating direct current IB of the LED string 3 to be changed. The manipulated variable that determines the operating current IB in this variant is therefore the variable that influences the value of the controlled resistor. As a result, as with a switching converter, the LED string 3 can be supplied with an alternating current I W1 or I W2 superimposed on the operating direct current IB.

[0038] Both in the variant in which the operating direct current IB is provided via a switching converter and in the variant in which the operating direct current IB is linearly controlled, the manipulated variable for the driver 2 and thus the frequency fw of the superimposed alternating current I W1 or I W2 can be specified by a microcontroller 4.

[0039] The superimposed alternating current I W1 or I W2 leads to a corresponding change in the LED string voltage U(t) of LED string 3 around the operating DC voltage point. If one now considers only the small-signal behavior of the LEDs 31 in LED string 3, one can speak of an alternating voltage drop U W1 or U W2 at LED string 31 resulting from the superimposed alternating current I W1 or I W2. This can now be used to infer the presence of a short circuit.

[0040] For this purpose, the alternating voltage drop U W1 or U W2 caused by the alternating current I W1 or I W2 can be measured across LED string 3. By filtering the LED string voltage U(t), for example using an electronic high-pass filter, the resulting alternating voltage drop U W1 or U W2 can be measured separately from the operating direct voltage UB(T1, T2, T3). The measurement of the alternating voltage drop U W1 or U W2 can be achieved by measuring the amplitude value or a peak-to-peak value measurement of the resulting alternating voltage U W1 or U W2 across LED string 31. Furthermore, an effective value of the resulting alternating voltage across LED string 31 can be calculated from the measured alternating voltage drop U W1 or U W2.

[0041] In particular, a continuous measurement of the alternating voltage drop U W1 or U W2 at the LED string 31 can be provided, which continuously measures the LED string voltage U(t) or the alternating voltage drop U W1 or U W2. In practice, such a continuous measurement is preferably carried out at discrete time intervals of at least 1 / (2*fw ), particularly preferably of at least 1 / (5*fw ).

[0042] If an AC voltage measurement is planned in the form of a peak-to-peak value measurement, the individual measurements can be filtered according to maximum and minimum values, which makes it possible to calculate the peak-to-peak value by forming the difference between the maximum and minimum values. This peak-to-peak value corresponds to a comparison actual value R ist that correlates with the differential operating AC current actual resistance value. The peak-to-peak value increases according to the flattening curve of the representative differential resistance with an increasing number of LEDs 31 in LED string 3. If an LED 31 is short-circuited, the peak-to-peak value of LED string 3 decreases. Analogously, the comparison actual value R ist can also correspond to the amplitude value or the effective value of the AC voltage drop U W1 or U W2 of LED string 3. It is advantageous if the maximum and minimum values ​​are each averaged over at least three measurements before the peak-to-peak value is calculated.

[0043] In a preferred embodiment, the comparison actual value R corresponds to a certain rate of change over time. This rate of change over time can correspond to the change over time of a peak-to-peak value or an amplitude value or an effective value within a defined time period. This makes the short-circuit detection independent of slow changes due to environmental influences. Advantageously, in this case the reference value is -x * 0.1 of the comparison actual value per millisecond, where x corresponds to the number of LEDs 31 in the LED string 3.

[0044] In particular, a microcontroller 4 can be provided which is configured to compare the reference value R ref with the actual comparison value R ist. This microcontroller 4 can have a voltage measuring unit 5 with which the LED string voltage U(t) or the AC voltage drop U W1 or U W2 is measured. Particularly preferably, the microcontroller 4 is configured both to determine the actual comparison value R ist and to compare it with the reference value R ref, and to provide the above-mentioned manipulated variable for the driver 2. In a further embodiment, the impressed alternating current I W1 or I W2 can be selected such that its amplitude is independent of the absolute value of the nominal value of the operating direct current IB of the LED string 3.

[0045] The invention is not limited to the embodiments shown, but is defined by the entire scope of the claims.

[0046] Any reference signs in the claims are exemplary and serve only to facilitate the readability of the claims without limiting them.

Claims

1. Method for detecting a short circuit of at least one LED (31) of an LED string (3) of a light module (8), the LED string (3) comprising a number of at least two LEDs (31) connected in series with one another, the method comprising the following steps: a) providing a reference value (Rref) which correlates with a differential operating AC setpoint resistance of the LED string (3) and storing the reference value (Rref) on a data memory (6), b) Commissioning the LED string (3) with a DC operating voltage (UBT1, UBT2, UBT3) so that the LED string (3) emits light, c) impressing an alternating current (IW1, IW2) superimposed on the DC operating voltage (UBT1, UBT2, UBT3) into the LED string (3), the frequency of the alternating current (IW1, IW2) being at least 60 Hz, the impressed alternating current (IW1, IW2) is selected such that its amplitude is between 20% and 60% of the absolute value of the nominal value of an operating direct current (IB) of the LED string (3) caused by the operating direct voltage (UBT1, UBT2, UBT3), d) measuring the AC voltage drop (UW1, UW2) at the LED string caused by the AC current (IW1, IW2) according to step c), e) deriving a comparison actual value (Rist) of the LED string correlating with a differential operating AC actual resistance value with the aid of the AC voltage drop (UW1, UW2) measured in accordance with step d), f) retrieving the reference value (Rref) made available on the data memory (6) in accordance with step a) and comparing it with the comparative actual value (Rist), the presence of an LED short circuit being inferred from the comparison depending on the result.

2. Method according to claim 1, wherein the alternating current (IW1, IW2) impressed according to step c) is free of a DC component.

3. Method according to claim 2, wherein the impressed alternating current (IW1, IW2) has the time characteristic of a rectangular signal, a triangular signal or a sinusoidal signal.

4. Method according to one of the preceding claims, wherein the reference value (Rref) provided according to step a) is the differential operating AC nominal resistance, and wherein the comparison actual value (Rist) derived according to step e) is a differential operating AC resistance actual value which is obtained by forming a quotient from the measured AC voltage drop (UW1, UW2) according to step d) and the impressed AC current (IW1, IW2) according to step c).

5. Method according to one of the preceding claims, wherein the comparison according to step f) is a limit value comparison, wherein if a predeterminable maximum difference amount formed by the difference between the reference value (Rref) and the comparison actual value (Rist) is exceeded, the presence of an LED short circuit is inferred, wherein the maximum difference amount is at most 10% of the reference value (Rref).

6. Method according to one of the preceding claims, wherein an error routine is triggered when an LED short circuit is detected.

7. Method according to claim 6, wherein the error routine comprises the output of an error signal (s1) and / or the change of the operating state of the LED string (3).

8. Method according to any one of the preceding claims, wherein if there is no LED short circuit, a signal (s2) is output to confirm that there is no fault.

9. Method according to one of the preceding claims, wherein, in the event that freedom from short-circuit faults is detected in step e), the detected comparative actual value (Rist) replaces the present reference value (Rref) and is stored in the data memory (6) in order to serve as an updated reference value in a subsequent iteration of steps a) to f).

10. Short-circuit fault-detecting lighting system (7) for carrying out a method according to one of the preceding claims, comprising - a light module (8) with at least one LED string (3), wherein the at least one LED string (3) has a number of at least two LEDs (31) connected in series with each other, - and a short-circuit detection system (9), the short-circuit detection system (9) being set up at least for the electrical supply of the at least one LED string (3), the short-circuit detection system (9) being set up to carry out steps a) to f) of the method according to one of the preceding claims.

11. Short-circuit fault-detecting lighting system (7) according to claim 10, wherein the short-circuit detecting system (9) is set up so that the amplitude of the superimposed alternating current (IW1, IW2) is independent of the absolute value of the nominal value of the operating direct current (IB) of the LED string (3).

12. Short-circuit fault-detecting lighting system (7) according to claim 10, wherein the short-circuit detection system is set up so that the amplitude of the superimposed alternating current (IW1, IW2) is dependent on the absolute value of the nominal value of the operating direct current (IB) of the LED string (3).

13. Motor vehicle headlamp, comprising a short-circuit fault-detecting light system (7) according to one of claims 10 to 12.

14. Motor vehicle, comprising a short-circuit fault detecting light system (7) according to any one of claims 10 to 12 and / or a motor vehicle headlamp according to claim 13.