EP4247667A1

EP4247667A1 - System and method for detecting abnormalities in a vehicle lighting device

Info

Publication number: EP4247667A1
Application number: EP21816418.4A
Authority: EP
Inventors: FRANCO Miguel-Angel CANTUDO
Original assignee: Valeo Vision SAS
Current assignee: Valeo Vision SAS
Priority date: 2020-11-20
Filing date: 2021-11-19
Publication date: 2023-09-27
Also published as: FR3116485A1; WO2022106659A1

Abstract

The present invention relates to an abnormality detection system (120) of a vehicle lighting device (100) to detect a short circuit in the lighting unit (110). The abnormality detection system (120) comprises: a voltage detector (125) configured to detect a voltage across a LED in each string of plurality of LED strings (145) of a lighting unit of the vehicle lighting device; a comparison unit (135) configured to compare the detected voltage with a predetermined voltage value and generates a status signal; and a switching unit (130) configured to switch the operation of the plurality of LED strings (145) from a first mode to a second mode based on the information in the status signal.

Description

Titre: SYSTEM AND METHOD FOR DETECTING ABNORMALITIES IN A VEHICLE LIGHTING DEVICE

[0001 ] TECHNICAL FIELD

[0002] The present invention relates to an abnormality detection system, and particularly relates to a system for detecting a short-circuit abnormality or a disconnection abnormality in a LED lamp installed in a vehicle head lamps and/or tail lamps.

[0003] STATE OF THE ART

[0004] A light emitting diode (LED) is being extensively used in vehicle headlamps and tail lamps. As compared with conventional light sources, LEDs have distinctive advantages such as high efficiency, good directivity, high reliability, long service life, and small size. Generally, a plurality of LEDs are arranged into an LED string, for example, a plurality of LED strings are used in the vehicle head lamps and tail lamps. In use of the LEDs, the corresponding LEDs in each LED string might be short circuited or open circuited. Hence, it is desirable to timely, and correctly detect abnormalities in the pluralities of LED strings in the vehicle lamps.

[0005] In one of conventional techniques for abnormality detection in LED strings, an integrated chip embedded with the abnormality detection functionality is employed to detect the abnormalities in the LED strings. In this technique, the LEDs strings are integrated into an integrated chip. Due to manufacture differences among LEDs, for same current, the voltage drops are rather different among the LED strings. In other words, for the same current output, currents might be rather different among the LED strings. This might cause wrong results in detecting short circuit of the LEDs in the LED string. Therefore, by using this technique, it may be optimal to perform match detection of the LED strings before the LED strings are applied to a specific environment. Besides, it might not be possible to adapt this abnormality detection function for different current ratings of LEDs.

[0006] SUMMARY OF THE INVENTION

[0007] One objective of the present invention is to provide an abnormality detection system, which can detect abnormalities in a lighting unit of a vehicle lighting device having the plurality of LED strings. In particular, the present invention provides the abnormality detection system which can detect the abnormalities for various lighting units having different current ratings of LEDs, without using an integrated chip. For example, without limitation, the different current ranges of LEDs include between 20mA to 300mA.

[0008] Other objective of the present invention is to provide an abnormality detection system which can detected both short-circuit abnormalities and opencircuit abnormalities in the lighting unit of the vehicle lighting device.

[0009] Another objective of the present invention is to provide an abnormality detection system, which is adaptive, modular, and cost effective compared to the exiting abnormality detection systems of the vehicle lighting device.

[0010] Yet, another objective of the present invention is to provide an abnormality detection system which uses only discrete passive and analog components.

[0011 ] Further, another objective of the present invention is to timely and correctly detect abnormalities in the pluralities of LED strings in the vehicle lamps.

[0012] According to the present invention, there is provided an abnormality detection system of a vehicle lighting device to detect a short circuit in a lighting unit of the vehicle lighting device. The abnormality detection system comprises: a voltage detector configured to detect a voltage across a LED in each string of plurality of the LED strings of the lighting unit; a comparison unit configured to compare the detected voltage with a predetermined voltage value and generates a status signal based on the comparison; and a switching unit configured to switch the operation of the plurality of LED strings from a first mode to a second mode based on information in the status signal received from the comparison unit. Thus, the abnormality detection system of the present invention detects the short-circuits in the LED strings without employing any integrated chips. Moreover, the abnormality detection system employs only discrete passive and analog components. Hence, the abnormality detection system of the present invention is adaptive, modular, and cost effective compared to the existing abnormality detection systems, and can be used for different current ratings of LEDs.

[0013] In the invention, the detected voltage is connected to the comparison unit through an OR-gate. Such a gate allows for a reduced footprint and cost of the circuit, as it allows using fewer comparison units for detecting a short circuit on a number of LED strings, preferably only one comparison unit for detecting a short circuit on all LED strings.

[0014] According to an embodiment of the present invention, one end of the plurality of LED strings is electrically connected to a power supply unit through the switching unit, and other end of the plurality of LED strings is electrically connected to a drive controller, which is configured to control the current through the plurality of LED strings.

[0015] According to an embodiment of the present invention, the switching unit comprises at least one power transistor, and the switching unit is configured to power ON the LEDs of the plurality of LED strings in the first mode, and power OFF the LEDs of the plurality of LED strings in the second mode. By powering OFF all the LEDs in the LED strings in the second mode, the vehicle lighting regulations can be met, because, If any LED becomes shorted such that its anode is shorted to its cathode, the shorted LED will stop emitting light, but can still provide a current path for the remaining series-connected LEDs to emit light, thus violating some vehicle regulations.

[0016] In an embodiment, the comparison unit generates a short-circuit detection signal indicating that there is at least one short circuiting LED in the LED strings when the detected voltage is greater than the predetermined voltage value. According to the present invention, the voltage detector is configured to consider unevenness of a forward drop voltage across LEDs and a temperature characteristic of the LED, such that the present abnormality detect system is able to accurately detect the voltage across the LEDs and thereby the occurrence of short-circuits in LED strings.

[0017] In an embodiment, the switching unit switches the operation of the LEDs of the plurality of LED strings to the second mode from the first mode when it receives a feedback signal as the short-circuit detection signal from the comparison unit. Thus, the abnormality detection system of the present invention can timely and correctly switch the operation of LEDs based on the feedback signal, and thereby the damage to the LEDs can be avoided. [0018] According to another embodiment, the abnormality detection system is further configured to detect a disconnection abnormality in the plurality of LED strings. The abnormality detection system of the present invention can detect both the short-circuit abnormality and the open-circuit abnormality in the LED strings of the lighting unit, without using an integrated chip and with minimal discrete passive and analog components.

[0019] According to another embodiment, on each branch, the detection of the voltage across a LED is done through a measurement of a voltage at a measurement point on each LED branch. For a given LED branch, the measurement point is located before the last LED of said branch in the direction of the current, preferably between the penultimate LED and the last LED. If so, in a preferred embodiment, the measured voltage is applied at the comparison unit. If so, in a preferred embodiment, the comparison unit includes an operational amplifier, the measured voltage is preferably applied at the non-inverting end of the amplifier through the OR-gate.

[0020] In a preferred, non-limiting embodiment, the OR-gate comprises several diodes arranged to let a current flow when a short circuit abnormality is detected; such an OR-gate is particularly cheap to produce and implement. In particular, such an OR-gate may use diodes with their cathode connected to the voltage measure points, so that they will only let current flow when the measured voltage exceeds their breakdown voltage. Said breakdown voltage value is chosen so as to be slightly lower than the nominal voltage value at the measurement point. In an example, non-shorted LEDs have a 2V nominal voltage across them, the nominal voltage at measurement point is 9V, and the breakdown voltage is 1 1 V.

[0021] According to another aspect, the present invention provides a vehicle lamp comprising: a headlamp housing, a reflector, a lens, a heat sink, a power supply unit and the abnormality detection.

[0022] According to another aspect the present invention, there is provided a method for detecting an abnormality in a plurality of Light Emitting Diode (LED) strings of a vehicle lighting device, the method comprises: obtaining, by a voltage detector, a voltage across a LED in each string of the plurality of the LED strings to detect a short circuit abnormality in the plurality of LED strings; comparing, by a comparison unit, the detected voltage with a predetermined voltage value; and switching, by a switching unit, the operation of the plurality of LED strings from a first mode to a second mode based on a feedback from the comparison unit. In the first mode, the LEDs of the plurality of LED strings are in ON state, and in the second mode, the LEDs of the plurality of LED strings are in OFF state.

[0023] According to an embodiment, the method further comprises generating a short-circuit detection signal when the detected voltage is greater than the predetermined voltage value.

[0024] According to an embodiment, the method further comprises switching, by the switching unit, the operation of the LEDs in the plurality of LED strings to the second mode from the first mode upon receiving the feedback as the short-circuit signal from the comparison unit.

[0025] Thus, the abnormality detection system of the present invention is adaptive, modular, and cost effective compared to the exiting abnormality detection systems, and can be used for different range of LEDs, and detection can be performed without using an integrated chip like the existing solutions.

[0026] BRIEF DESCRIPTION OF THE INVENTION

[0027] To complete the description and to provide a better understanding of the invention, a set of drawings is provided. Said drawings form an integral part of the description and illustrate an embodiment of the invention, which should not be construed as restricting the scope of the invention, but only as an example of how the invention can be carried out.

[0028] [Fig.1 ] is a block diagram of a vehicle lighting device having an abnormality detection system, according to an embodiment of the present invention.

[0029] [Fig.2] shows a schematic diagram of a vehicle lighting device having the abnormality detection system for short-circuit detection, according to an embodiment of the present invention.

[0030] [Fig.3] shows a schematic diagram of a vehicle lighting device having the abnormality detection system for short-circuit detection, according to another embodiment of the present invention. [0031 ] [Fig.4] shows a schematic diagram of a vehicle lighting device having a driver controller and an abnormality detection system of the Fig.2, according to an embodiment of the present invention.

[0032] [Fig.5] illustrates a flow chart of a method for detecting a short-circuit abnormality in LED strings during the operation of the vehicle lighting device, according to an embodiment of the present invention.

[0033] DETAILED DESCRIPTION OF THE INVENTION

[0034] Embodiments of the present application will below be explained in details by ways of examples with reference to the accompanied drawings. Throughout the description, same or similar reference numerals represent same or similar parts. The following description of the embodiments with reference to the drawings is intended to explain the general inventive concept of the present application, instead of limiting to the present invention.

[0035] In the following detailed description, for purposes of explanation, numerous specific details are set forth in order to provide a thorough understanding of the disclosed embodiments. It will be apparent, however, that one or more embodiments may be practiced without these specific details.

[0036] [Fig.1 ] is a block diagram of a vehicle lighting device having an abnormality detection system for detecting a short-circuit abnormality in LED strings during the operation of the vehicle lighting device, according to an embodiment of the present invention. It will be described in detail hereunder. The vehicle lighting device of the present embodiments can be either a head lamp or a tail lamp of an automotive vehicle.

[0037] Embodiments described herein provide lighting units of the vehicle lighting devices having one or more solid state light sources. As used herein, a solid state light source refers to a type of light source using an electroluminescence phenomenon in which a material emits light in response to passage of an electric current or in response to a strong electric field. Examples of light sources include, but are not limited to semiconductor light-emitting diodes (LEDs), organic lightemitting diodes (OLEDs), polymer light-emitting diodes (PLEDs), and monolithic light-emitting diodes (MLEDs). Lighting modules described herein can also include one or more bulb sources, such as a halogen light source or a high intensity discharge (HID) light source.

[0038] As shown in the Fig.1 , the vehicle lighting device 100 comprises a power supply unit 105, a lighting unit 1 10, a drive controller 115 and an abnormality detection system 120. The power supply unit 105 includes a battery of a vehicle. Besides, the power supply unit 105 may also include a boost converter (not shown in the Figures). Certainly, those skilled in the art may understand that the power supply unit 105 may also include a buck converter or a buck-boost converter (not shown in the Figures). The power supply unit 105 is electrically connected to the lighting unit 110, which includes a plurality of LED strings (shown in Fig.2) to provide lighting or signal functionalities for the automotive vehicle. In use of the LEDs, the corresponding LEDs in each LED string might be short circuited or open circuited. The abnormality detection system 120 is configured to detect a short circuit abnormality or a disconnection abnormality, i.e., open circuit abnormality in the LED strings during the operation of the vehicle lighting device. Further, the driver controller 1 15 is configured to control the amount of current through the LED strings. The abnormality detection system 120 comprises a voltage detector 125, a switching unit 130, and a comparison unit 135, as shown in the Fig.1. Details of the abnormality detection system 120 are explained in detail with respect to Fig.2.

[0039] [Fig.2] shows a schematic diagram of a vehicle lighting device having an abnormality detection system for detecting a short-circuit abnormality in LED strings during the operation of the vehicle lighting device, according to an embodiment of the present invention. As shown in the Fig.2, the lighting unit 1 10 comprises a plurality of LEDs 140. Generally, the plurality of LEDs 140 are arranged into a plurality of LED strings 145. In the following description, for the purpose of brevity, each of the LED strings 145 is assumed to have the same number of LEDs. Noticeably, the number of LED strings 145 according to the present invention is not limited to three. The present invention can also be adapted for situations in which the number of LED strings 145 is greater than three or less than three, e.g.., two, four, five or six.

[0040] The power supply unit 105 is arranged to provide an input voltage to the lighting unit 1 10. As can be seen from the Fig.2, the power supply unit 105 is electrically connected to one end of LED strings 145, that is to say, each LED string in the plurality of LED strings 145 is electrically coupled between the power supply unit 105 and a ground. Each LED string 145 in the plurality of LED strings is electrically coupled in series with an LED driver or the driver controller 1 15.

[0041 ] The vehicle lighting device 100 of the Fig.2 further comprises the voltage detector 125 configured to measure the voltages V1 , V2 and V3 across a LED in each LED string. For example, the voltage detector 125 is a voltage sensor. In an aspect, the voltage detector 125 is configured to detect the voltage at the middle of the LED strings 145. In another aspect, the voltage detector 125 is configured to detect the voltage at the end of the LED strings 145 with respect to the ground. According to the present invention, the voltage detector is configured to consider unevenness of a forward drop voltage across LEDs and a temperature characteristic of the LED, such that the present abnormality detect system is able to accurately detect the voltage across the LEDs and thereby the occurrence of short-circuits in LED strings.

[0042] Further, the vehicle lighting device 100 includes the comparison unit 135 as shown in the Fig.2. The comparison unit 135 includes an operational amplifier 150, which is configured to operate as a comparator. An inventing end of the operational amplifier is connected to a reference voltage generator 155. The reference voltage generator 155 is configured to generate a predetermined voltage value for the short circuit abnormality. That is to say, the reference voltage generator 155 generates a voltage which is equal to a voltage across the LED when the circuit operates without any faults. As can be seen from the Fig.2, the reference voltage generator 155 includes a voltage source 160 and a resistance divider 165. Output of the reference voltage generator 155 is applied at the inverting end of the operational amplifier 150. Whereas, the non-inverting end of the operational amplifier 135 is provided with a voltage detected across LEDs in each string of the LED strings 145. For instance, as can be seen from the Fig.2, an OR-Gate 170 is implemented using diodes is arranged at the non-inverting end of the operational amplifier 150. The diodes of the OR gate 170 receives measured voltage across LEDs. The Fig.2 shows three diodes which are configured to receive detected voltages from three LED strings 145 so that if there is a shot-circuit in any one of the LED strings 145, the abnormality detection system 120 is configured to detect the short-circuit. [0043] Although in the present embodiment, the inverting end of the operational amplifier 150 is provided with the reference voltage and the non-inverting end of the operation amplifier 150 with the detected voltage. However, it will be understood to a person skilled in the art that reference voltage and the detected voltage can also be applied at non-inverting end and the inverting end of the operational amplifier 150, respectively. Accordingly, the type of MOSFETs to be used in the switching unit 135 may vary in order to electrically disconnect the LED strings 145 from the power supply unit 105 when there is an abnormality in the LED strings 145.

[0044] In an embodiment, the detected voltage is applied at the non-inverting end of the amplifier through an OR gate 170, and the predetermined voltage value for the short circuit abnormality is applied at the inverting end of the amplifier 150. The operational amplifier 150 which acts as the comparator compares both the voltages at its inputs and generates a status signal or an abnormality signal when the detected voltage applied at the non-inverting end of the operational amplifier 150 is greater than the predetermined voltage value for the short circuit abnormality. That is to say, the comparison unit 135 generate a short-circuit detection signal or the abnormality signal when there is a short-circuit in any one of the LED strings 145.

[0045] The vehicle lighting device 100 of the Fig.2 further comprises the switching unit 130, which includes at least one Metal Oxide Semi-Conductor Field Effect T ransistor (MOSFET) 175 or the at least one bipolar transistor (BJT). Here, without loss of generality, assume the switches to be MOSFETs 175. The number of transistors 175 (S1 , S2 and S3) in the switching unit 130 can include three, according to one embodiment of the present invention. According to another embodiment of the present invention, the switching unit 130 may include a single transistor 175 (S), as shown in the [Fig.3]. The switching unit 130 is configured to receive the abnormality signal as a feedback signal from the comparison unit 135 whenever the abnormality is detected in the LED strings 145. For instance, whenever the detected voltage is higher than the reference voltage, i.e., the predetermined voltage value, the comparison unit 135 generated and sends the abnormality signal, i.e., active LOW signal to the MOSFETs 175 (for example, p- type MOSFETs) of the switching unit 130. This causes the switching unit 135 to switch the operation of the LEDs in the LED strings 145 from a first mode to a second mode. In the first mode, the LEDs 140 are in ON state, and in the second mode the LEDs 140 are in OFF state. Whenever the active LOW signal is received at gates of MOSFET 175 (n-type enhanced MOSFET), the MOSFETs (n-type enhanced MOSFET) 175 of the switching unit 135 switches to OFF state, this in turn causes to switch the mode of the LEDs in LED strings 175 from the first mode to the second mode, i.e., the LED strings 175 are electrically disconnected from the power supply unit 105 in the second mode.

[0046] According to another embodiment of the present invention, if the reference voltage and the detected voltage are applied at non-inverting end and the inverting end of the operational amplifier 150, the comparison unit 135 generates an active HIGH signal to the MOSFETs 175 (for example, p-type enhanced MOSFET) of the switching unit 130 during the abnormality detection such that the LED strings 145 are electrically disconnected from the power supply unit 105.

[0047] Further, the architecture of the drive controller 1 15 shown as the block in the Fig.2 and the Fig.3 is shown in the [Fig.4], As previously mentioned, each LED string in the plurality of LED strings145 includes at least one LED 140 electrically coupled in series with an LED driver or the drive controller 1 15. The drive controller 1 15 is configured to drive the LEDs 140 with constant current and variable voltage. In an embodiment, the drive controller 1 15 is a linear driver and implemented with one or more transistors 180 and a current source 185. More specifically, each transistor 180 may be further defined as a bipolar junction transistor having a collector coupled directly to a cathode terminal of the corresponding LED and an emitter connected via a resistor (not shown in the Figures) to ground. While reference is made to a bipolar junction transistor, other types of transistors (e.g., FETs) as well as other types of electronic switches fall within the broader aspects of this disclosure. It is also envisioned that other types of LED drivers 1 15 may be used in place of the transistor. The operation of the drive controller 1 15 to control the current through the LED strings 145 is very well known to a person skilled in the art and hence not discussed here.

[0048] [Fig.5] illustrates a flow chart of a method for detecting a short-circuit abnormality in a plurality of LED strings 145 during the lighting of the vehicle lighting device 100, according to an embodiment of the present invention, wherein one end of each of the plurality of LED strings 145 is connected to a power supply unit 105, and other end of the LED strings 145 is respectively connected to the ground through the drive controller 1 15.

[0049] As shown in the Fig.5, the method 300 comprises a step 305 for obtaining voltages across at least a LED 140 in each of the LED strings 145. For instance, a voltage sensor is employed to measure voltage across LEDs 140. In an aspect, the voltage at the middle of the LED string is sensed by the voltage sensor. In another aspect, the voltage at the end of the LED string is sensed by the voltage sensor. At step 310, the method includes comparing the detected voltage, by a comparison unit, with a reference voltage or a predetermined voltage value. For instance, the predetermined voltage is equal to a voltage across the LED 140 when the system operates without any faults. In an aspect, the output of the comparison unit 135 is electrically coupled to the switching unit 135, as previously discussed. During normal operation, the reference voltage is greater than the detected voltage, and the output of the comparator is active HIGH signal. Therefore, the transistors (for example n-type enhanced MOSFETS) in the switching unit 135 operates in the first mode, i.e., all the LEDs are in ON state. In case the reference voltage is lesser than the detected voltage, then the comparator outputs active LOW signal, indication of the short-circuit in the LED strings 145. The output of the comparison unit 135, i.e., the active LOW signal is fed to gates of the MOSFETS (for example n-type enhanced MOSFETS) in the switching unit 135, as stated at step 315 of the flow chart. In response to the feedback signal or the abnormality signal, the switching unit 135 changes the mode of the operation of the LEDs from the first mode to the second mode, i.e., all the LEDs are in OFF state, as stated at step 320 of the flow chart.

[0050] Thus, the abnormality detection system 120 of the present invention is adaptive, modular, and cost effective compared to the exiting abnormality detection systems, and can be used for different current ratings of LEDs, as the abnormality detection system of the present invention only employs discrete passive and analog components.

[0051 ] The person skilled in the art realizes that the present invention by no means is limited to the preferred embodiments described above. On the contrary, many modifications and variations are possible within the scope of the appended claims. Also, the skilled person readily realizes that the different embodiments described herein may be combined freely to obtain new combinations.

[0052] Additionally, variations to the disclosed embodiments can be understood and effected by the skilled person in practicing the claimed invention, from a study of the drawings, the disclosure, and the appended claims. In the claims, the word “comprising” does not exclude other elements or steps, and the indefinite article “a” or “an” does not exclude a plurality. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measured cannot be used to advantages.

Claims

Revendications

[Claim 1 ] An abnormality detection system (120) of a vehicle lighting device (100) to detect a short circuit in the lighting unit (110), the abnormality detection system (120) comprises: a voltage detector (125) configured to detect a voltage across a LED in each string of plurality of LED strings (145) of the lighting unit (1 10); a comparison unit (135) configured to compare the detected voltage with a predetermined voltage value and generates a status signal based on the comparison; and a switching unit (130) configured to switch the operation of the plurality of LED strings (145) from a first mode to a second mode based on the information in the status signal received from the comparison unit (130), wherein the detected voltage is connected to the comparison unit through an OR-gate.

[Claim 2] The abnormality detection system (120) as claimed in claim 1 , wherein one end of the plurality of LED strings (145) is electrically connected to a power supply unit (105) through the switching unit (130), and other end of the plurality of LED strings (145) is electrically connected to a drive controller (1 15), which is configured to control the current through the plurality of LED strings.

[Claim 3] The abnormality detection system (120) according to claim 1 or 2, wherein in the switching unit (130) comprises at least one power transistor (175), and the switching unit is configured to power ON the LEDs of the plurality of LED strings (145) in the first mode, and power OFF the LEDs of the plurality of LED strings (145) in the second mode.

[Claim 4] The abnormality detection system (120) according to one of the preceding claims, wherein the status signal generated by the comparison unit (135) is a short-circuit detection signal indicating that there is at least one short circuiting LED in the LED strings when the detected voltage is greater than the predetermined voltage value.

[Claim 5] The abnormality detection system (120) according to any one of the previous claims, wherein the switching unit (130) switches the operation of the LEDs of the plurality of LED strings (145) to the second mode from the first mode when it receives a feedback signal as the short-circuit detection signal from the comparison unit (135).

[Claim 6] The abnormality detection system (120) according to any one of the previous claims, wherein the abnormality detection system (120) is further configured to detect a disconnection abnormality in the plurality of LED strings (145).

[Claim 7] A vehicle lamp comprising: a headlamp housing, a reflector, a lens, a heat sink, a power supply unit and the abnormality detection system (120) according to claims 1 to 6.

[Claim 8] A method (300) for detecting an abnormality in a plurality of Light Emitting Diode (LED) strings (145) of a vehicle lighting device (100), the method comprising: obtaining, by a voltage detector (125), a voltage across a LED in each string of the plurality of the LED strings (145) to detect a short circuit abnormality in the plurality of LED strings (145); comparing, by a comparison unit (135), the detected voltage with a predetermined voltage value; and switching, by a switching unit (130), the operation of the plurality of LED strings (145) from a first mode to a second mode based on a feedback from the comparison unit (135).

[Claim 9] The method (300) according to the preceding claim, wherein the method comprises operating the LEDs of the plurality of LED strings (145) in ON state in the first mode, and operating the LEDs of the plurality of LED strings (145) in OFF state in the second mode.

[Claim 10] The method (300) according to claim preceding, wherein the method further comprises generating a short-circuit detection signal when the detected voltage is greater than the predetermined voltage value.

[Claim 11 ] The method (300) according to preceding claims 8 to 10, wherein the method further comprises switching, by the switching unit (135), the operation of the LEDs in the plurality of LED strings (120) to the second mode from the first mode upon receiving the feedback as the short-circuit detection signal from the comparison unit (135).