LED driving system

Abstract

A method for enabling a light source comprising a set of at least two parallel strings (21) 22) of light-emitting diodes, each string having a forward voltage, wherein a first string (21), has a forward voltage higher than the forward voltage of the other strings (22). A method for manufacturing a driving system and a driving system. A light source comprising the driving system and a vehicle lamp.

Description

[0001] The present invention relates to driving systems for driving light sources, especially light-emitting diodes (LEDs). It also relates to lighting devices, notably for an automotive vehicle, comprising such driving systems, such as vehicle lamp modules. The invention also relates to a method for enabling a light source.

[0002] In general, a lighting device comprises one or more light sources, notably light-emitting diodes (LEDs), which are driven by a driving system for illuminating the lighting device. For example, for a vehicle lamp module, the driving system uses an array of LEDs to illuminate the lamp. An LED array usually comprises two or more LED strings, each string comprising a group of LEDs connected in series, and specific lighting and / or signaling functions are each associated to one string or a combination of strings. The driving system usually further comprises a converter for converting an input voltage into a minimal output voltage for enabling the LEDs and start current flow through the LEDs. Each string may require a different minimum voltage which can vary according to several parameters, namely current and temperature. Furthermore, the current flowing through each string can be varied so that the same string can demonstrate different luminosity and / or color, notably in accordance with standards and regulations for vehicle lamps, for example rear lights can comprise red tail lights and bright red brake lights. In order to generate desired light outputs with uniform brightness, each string is typically controlled by its own driver circuit to regulate the current flowing therethrough.

[0003] Such driving systems require a large pin-count and are usually bulky due to their large number of electronic components which also largely contribute to high heat dissipation leading to damage prone circuits.

[0004] Prior art patent application US 2015 / 0035443 proposes an LED driving system including a bus connected to a main LED string equipped with a current regulator to control the output color in the string. The driving system further includes a voltage regulator circuit which may comprise at least a secondary LED string coupled to the bus. This secondary LED string is coupled between the bus and a ground node that provides a current return path to the controlled current source. The secondary string is able to regulate voltage of the main one as it varies the current passing therethrough before reaching the main LED string. In such a driving circuit, when the main LED string needs to be operated, the secondary LED string is always activated in order to act as a voltage regulator. The system does not allow to activate the first string only without activating the other string.

[0005] The purpose of the present invention is to provide a cost-friendly LED driving system with reduced bulkiness and reduced heat dissipation while at the same time allowing optimal independent control of each string associated to a specific lighting and / or signaling functions in a vehicle.

[0006] To that end, the invention is related to a method for driving light sources comprising a set of at least two parallel strings of light-emitting diodes, each string having a forward voltage, wherein a first string has a forward voltage higher than the forward voltage of the other strings, the method comprising a step of operating the light sources in a low-voltage mode, consisting of inputting a current in the strings at a voltage lower than the forward voltage of the first string, to enlighten at least one of the other strings and not the first string.

[0007] Herein, a forward voltage difference between strings is provided to enable the selective activation of a particular string. Especially in the low-voltage operating mode, at least one string among the other strings can be activated. To do so, a current is supplied at a voltage that is lower than the forward voltage of the first string. Since, the voltage supplied is lower than the required voltage for activating the first string, said first string is automatically blocked or disabled. The supplied current is advantageously suitable to allow activation of the one of the other strings.

[0008] In the description, to "disable a string" should be understood as meaning either "actively disable" if the string was enabled, or "keep disabled" if the string was not enabled, for instance, when the string is the first string, because an earlier idle state already set the output voltage below the forward voltage of the first string.

[0009] Furthermore, contrary to prior art, here no additional control means is required to operate in the low-voltage mode, thus reducing cost of production and heat dissipation. With the proposed method, the first string, having the highest forward voltage, can be used as a back-up string, ready to replace a damaged string of a lower forward voltage. Hence, with the method of the invention, a back-up string can be included in the set of light sources without additional means specific to said back-up string. When a failure of the damaged lower forward voltage string is detected, according to the method, it is sufficient to replace the low-voltage operating mode with a high-voltage operating mode, wherein a current is inputted in the parallel strings with a voltage higher than the forward voltage of the first string.

[0010] In a particular embodiment, the at least one of the other strings is provided with a switch or a current control means.

[0011] The independency of the strings is optimized by the presence of a switch or current control means on at least one of the other strings having a forward voltage lower than that of the first string. Advantageously, such current control means are not required in the first string, thus reducing bulkiness of the assembly.

[0012] Strings can be selectively activated and independently controlled even though all the strings are not provided with current control means.

[0013] This is particularly advantageous since a particular lighting and / or signaling function is associated to each LED string or combination of LED strings. According to a desired function, a single string or a combination of strings can therefore be independently activated upon request and independently controlled, based on their forward voltage.

[0014] In a particular embodiment, the method comprises: a step of selecting one operating mode in a set of at least two operating modes, said set comprising mode A: enlighten at least the first string and mode B: enlighten at least one of the other strings and not the first string, a step of carrying out the selected operating mode according to the following operating instructions: * to carry out mode A, input a current in the strings at a voltage higher than the forward voltage of first string, then control the current in the strings to manage the light-emitting diodes, * to carry out mode B, input a current in the strings at a voltage lower than the forward voltage of the first string, then control the current in the other strings to manage the light-emitting diodes,

[0015] Here, contrary to the prior art, one string can be activated without activating the other one(s).

[0016] Indeed, to enlighten the first string, notably in operating mode A, the switch or the current control means of at least one of the other strings is disabled. By disabling the switch or the current control means of the other strings, their activation is blocked or disabled. Though the first string is not provided with current control means specific to this string, the current flowing through it can still be controlled to control brightness, enable or disable a function and, in addition, to compensate temperature influence on LED light output.

[0017] Here again, to "disable the switch or current control means" should be understood as meaning either "actively disable" if the switch or control means was enabled, or "keep disabled" if said switch or said control means was not enabled.

[0018] In order to activate one of the other strings in operating mode B, there is a voltage or current output which is preferably well below the activation voltage level of the highest forward voltage string. Hence, lower forward voltage strings can be selectively activated without activating the first string. Therefore current control means, which tend to increase bulkiness of a light source circuitry, is not required for the first string which has the highest forward voltage.

[0019] Advantageously, in operating mode A, the first string and at least one of the other strings can be simultaneously activated according to a desired function.

[0020] The invention also relates to a method for manufacturing a driving system as described above.

[0021] Such a method for manufacturing a driving system is able to drive a set of parallel strings of light-emitting diodes, each having a string forward voltage, the method comprising the steps of: selecting the string of light-emitting diodes having the highest forward voltage, thus obtaining a first string, distinct from the other strings, connecting said strings to a DC / DC converter configured to operate either in a current control mode or in a voltage control mode, connecting the DC / DC converter to a microcontroller able to toggle the DC / DC converter from the current control mode to the voltage control mode and vice-versa, said microcontroller comprising a processor and a memory readable by the processor, said memory containing a set of instructions implementing the method for enabling a light source as described above, when executed by said processor.

[0022] The method for manufacturing a driving system may further present one or more of the following features, taken separately or in combination. After selecting the string of light-emitting diodes having the highest forward voltage, at least one of the other strings is provided with a current control mean or a switch, thus obtaining a controlled string.

[0023] The invention further relates to a driving system for driving a set of parallel strings of light-emitting diodes, wherein each string has a forward voltage, wherein a first string has a forward voltage higher than the forward voltage of the other strings, said driving system comprising: a DC / DC converter configured to operate either in a current control mode or in a voltage control mode, a microcontroller, connected to the DC / DC converter and able to toggle the DC / DC converter from the current control mode to the voltage control mode and vice-versa, said microcontroller comprising a processor and a memory readable by the processor, said memory containing a set of instructions implementing the method for enabling a light source as described above, when executed by said processor.

[0024] Hence, the DC / DC converter integrates a "smart function" allowing it to function either in a current control mode for controlling the current supply towards the strings, notably for controlling the luminosity or color of the selected string, or a voltage control mode for controlling the voltage supply towards the strings.

[0025] The driving system may further present one or more of the following features, taken separately or in combination. At least one of the other strings are provided with a current control means or a switch, said microcontroller being connected to the current control means or a switch, said microcontroller being connected to the current control means or switch and being able to to enable and disable the current control means or the switch.

[0026] By providing only the lower forward voltage strings with control means, bulkiness and cost efficiency of the driving system is already optimized. Furthermore, the microcontroller allows the DC / DC converter to operate in a voltage control mode in order to control the voltage towards the strings, or in a current control mode to control the current.

[0027] This is particularly advantageous for the first string, when it needs to be controlled, supplementary current control means are not required, the first string can be accordingly controlled by the DC / DC converter.

[0028] Such a system assembly optimizes the independent control of each string. For activation or deactivation of lower forward voltage strings, the microcontroller only has to enable or disable the current control means. The current control means can be positioned upstream or downstream of the corresponding string.

[0029] Therefore, the current control means are flexible in terms of their positioning, they can be placed in the circuitry of the driving system according to the available space. As each string can be associated to a lighting and / or a signaling function, the temperature of the series of light-emitting diodes (LEDs) is usually different. The temperature of an LED typically refers to its junction temperature. The forward voltage required to enable a string depends on the temperature. The larger the forward voltage gap between the consecutive strings, the easier it is to maintain the independent control of the strings. Temperature difference may work in both directions, either make voltage gap bigger or smaller, depending on which LED string has higher temperature at certain moment of time. The forward voltage gap being larger, the strings can be more easily isolated to be controlled, and a string, especially the highest forward voltage string, can be maintained disabled when required. One of the consecutive strings can be arranged into at least two sub-strings to increase the forward voltage gap between two consecutive strings.

[0030] This arrangement allows an optimal independent control of the LED strings while reducing the number of electronic components in the LED driving system, thus reducing the bulkiness of the system as well as its cost.

[0031] The invention also relates to a light source made of a set of parallel strings of light-emitting diodes, wherein each string has a forward voltage, wherein one string has a forward voltage higher than the forward voltage of the other strings, and wherein the other strings are provided with current control means said light source comprising a driving system as described above.

[0032] The invention further relates a vehicle lamp module comprising a light source as described above.Brief description of figures

[0033] The invention will be better understood, and other characteristics and advantages will appear when reading the following description, given by way of example and not by way of limitation, referring to the annexed figures, in which, [Fig. 1a] is a schematic diagram of an LED driving system according to one embodiment of the invention. [Fig. 1b] is a schematic diagram of an LED driving system according to another embodiment of the invention. [Fig. 1c] is a schematic diagram of an LED driving system according to another embodiment of the invention. [Fig. 2] is a schematic diagram of an embodiment of the LED driving system according to another embodiment of the invention. [Fig. 3] is a schematic diagram of an embodiment of the LED driving system according to another embodiment of the invention. [Fig. 4a] is a schematic diagram of an embodiment of the LED driving system according to another embodiment of the invention. [Fig. 4b] is a schematic diagram of an embodiment of the LED driving system according to another embodiment of the invention. [Fig. 5] is a schematic diagram of a method for enabling a light source according to the invention. Detailed description of the figures

[0034] Figures 1a to 1c are schematic diagrams of an LED driving system 1 comprising the circuitry of a lighting device of a vehicle lamp. The LED driving system is connected to a lamp connector 2 provided on the lighting device.

[0035] LED driving system 1 can be supported on a printed circuit board (PCB) (not represented) preferably when system 1 is adapted for rear lamps, or the system can be mounted on a heat sink (not represented) while being connected to a PCB, for example by means of a gold wire connection, preferably when system 1 is adapted for higher power front lamps.

[0036] LED driving system 1 comprises a DC / DC converter 10 and a set 20 of n LED strings, n being a natural integer number greater than or equal to two. Each string of set 20 comprises multiple LEDs connected in series. In the examples of figures 1a to 1c, 2, 3, n equals two. In the examples of figures 4a, 4b, n equals three. Additionally, it is also possible that a string comprises multiple LEDs connected in parallel.

[0037] DC / DC converter 10 is configured to be fed in electrical energy by a power source (not represented), for example a supply line from the battery of the vehicle. A power protection module 3 can be provided upstream of DC / DC converter 10 for producing a safe current and / or a safe voltage protecting DC / DC converter 10 from high, interrupted or otherwise distorted electrical supply.

[0038] A particular lighting and / or signaling function is associated to each LED string or combination of LED strings.

[0039] Each string has a forward voltage, i.e. the voltage at which current starts to flow in the string, required to activate its LED string, hence activating the desired lighting and / or signaling function of the vehicle lamp.

[0040] DC / DC converter 10 is configured to convert a direct current (DC) from one voltage level range to another. DC / DC converter 10 is configured to operate either in a current control mode or in a voltage control mode and to toggle from one mode to another on request.

[0041] To do so, DC / DC converter 10 integrates a "smart function" allowing toggling between the two modes. The "smart function" can be carried out by a microcontroller 12 (not represented on figures 1a to 1c) configured to drive DC / DC converter 10.

[0042] Microcontroller 12 is configured to receive lighting instructions from a central computer (not represented) of the vehicle, which manages the interface with a user and other parts of the vehicle that may require to enlighten the lamp. Such instructions can take the form of a physical or logical signal, for example a direct control line (physical signal) or a CAN-FD (for "Controller Area Network Flexible Date-Rate") signal or a LIN ("Local Interconnect Network") signal.

[0043] Microcontroller 12 comprises a database holding information on the LEDs of each string, for example on the luminous flux change as a function of temperature, forward voltage change as a function of temperature or / and current. Microcontroller 12 is also configured to receive feedback on the current flow passing through the strings and voltage over all or certain selected LEDs in string in order to detect failures in LED strings and / or self-diagnose internal circuits of DC / DC converter and drive control circuits, for example by detecting damaged LEDs. Microcontroller 12 is configured to receive said feedback through communication lines C1, C2.

[0044] As per design constraints, the database and the feedback allow calculating a setpoint voltage required to power a selected string, as well as current output or current variation output that needs to be supplied by the DC / DC converter 10 towards the selected string.

[0045] Hence, DC / DC converter 10 is able to control the voltage towards the strings in the voltage control mode, or to control the current towards the strings in the current control mode. Also, DC / DC converter 10 is configured to toggle from current control mode to voltage control mode and vice-versa.

[0046] With reference to figures 1a, 1b, 1c, 2 and 3, two LED strings 21, 22 are arranged in parallel and are each connected to the DC / DC converter 10 by a supply line S1, S2.

[0047] The LEDs of the strings 21, 22 are selected so that one of the strings (designated as the "first" string 21) has a forward voltage that is the highest forward voltage among all the strings, i.e., a higher forward voltage that the forward voltage of string 22. The other LED string 22 has a lower forward voltage.

[0048] In the embodiment of figure 1a, the current supply in the first supply line S1 and the second supply line S2 is controlled by the "smart function" of DC / DC converter 10, i.e., microcontroller 12.

[0049] First string 21 is included in the set of string but is not intended for use as long as second string 22 is able to work. Indeed, in the example of Fig. 1a, string 21 is a backup string in case string 22 is damaged.

[0050] Indeed, when the activation of the second string 22 only is requested, in a low-voltage operating mode, the DC / DC converter 10 works in voltage control mode and outputs a setpoint voltage that is higher than the forward voltage required for powering second string 22 and lower than the forward voltage required for powering first string 21. Microcontroller 12 then controls the current required to vary the brightness of the light-emitting diodes of second string 22 only. Hence, second string 22 can be activated without activating first string 21 with the condition that forward voltage of string 22 is lower than the forward voltage of first string 21.

[0051] In case of open circuit fault on string 22, when the activation of first string 21 is requested as a back-up light source, DC / DC converter 10, preferably microcontroller 12, inputs a current at a voltage above the forward voltage of first string 21, then it toggles the DC / DC converter 10 to current control mode manage the light-emitting diodes (LEDs) of the first string, for example for managing the brightness of the LEDs.

[0052] It is also possible to work continuously in current control mode especially when DC / DC converter is not controlled by microcontroller. In this case, DC / DC converter, working in current control mode, enables current flow. When string 22 is working correctly, the current flows through string 22. When string 22 has an open circuit fault condition, voltage set point will automatically adjust for higher forward voltage string and current will flow through string 21.

[0053] In the embodiment of Fig. 1b, two operating modes are possible: mode A: at least first string 21 is lit, no matter if second string 22 will be activated or not, mode B: second string 22 is lit and not first string 21.

[0054] For an independent activation of the two strings 21, 22, a switch 4 is provided on the second supply line S2. Switch 4 is configured to be controlled by DC / DC converter 10, preferably by microcontroller 12, between a closed position (not shown) allowing activation of second string 22, and an open position, as shown allowing deactivation of second string 22.

[0055] Herein, when the activation of first string 21 only is requested, for example in operating mode A, DC / DC converter 10 orders switch 4 towards its open position, notably via communication line C2', so that only first string 21 is controlled by said converter, be it in a voltage control mode or a current control mode. When activation of second string 22 only is requested, notably in operating mode B, DC / DC converter 10 orders switch 4 towards its closed position and supplies a current at a voltage lower than the forward voltage of first string 21 such that only second string 22 is controllable. Hence, such an arrangement allows independent activation of the two strings 21, 22 with the condition that forward voltage of LED string 22 is lower than forward voltage of the second string 22.

[0056] Such a system 1 is advantageously less bulky and more cost-effective as first string 21 is devoid of any additional electronic components, and the control of each string is optimized in case first LED string 21 and second LED string 22 activation is mutually exclusive (both strings 21 and 22 are never activated at the same time, meaning that either string 21 is active or string 22 is active or none of strings 21 and 22 is active). Of course, it is understood that both strings 21, 22 can be activated at the same time, provided that there is some other device (for example a series transistor) controlling current flow in string 22, when it is activated together with string 21. In this case, DC / DC converter 10 orders switch 4 towards its closed position and supplies a current at a voltage higher than the forward voltage of the first string 21 such that both strings can be powered. Here, when the microcontroller 12 toggles to current control mode, it can enable both strings at the same time and control current in string 21, as current in string 22 is controlled by some other device (such as a series transistor).

[0057] For maintaining the independent activation of strings 20, the switch can be alternatively replaced by a driver circuit D2, as shown in figure 1c. In this embodiment, second supply line S2 is provided with a driver circuit D2 which constitutes a current control means. In this embodiment, driver circuit D2 is upstream of second LED string 22, on high side, but it may also be on low side, downstream, or somewhere in between. Driver circuit D2 is configured to control the current flow towards second string 22. Driver circuit D2 can be a transistor or a linear driver, or any other driver circuit known in the field. Driver circuit D2 is controlled by the "smart function" of the DC / DC converter 10, i.e. microcontroller 12 via communication lines C2'. In particular, microcontroller 12 can enable and disable driver circuit D2. Herein, it is understood that for activating second string 22 in operating mode B, DC / DC converter 10 supplies a voltage lower than the forward voltage of the first string so as to prevent activation of the latter as explained the previous paragraphs. DC / DC converter 10, preferably microcontroller 12, is able to enable driver circuit D2 for current control of second string 22 so as to relieve the DC / DC converter from current controlling second string 22.

[0058] A similar example is represented in figure 2, wherein microcontroller 12 is preferably integrated in the DC / DC converter 10. The microcontroller 12 can communicate instructions with a toggle module 14. Alternatively, microcontroller 12 can be external to the DC / DC converter.

[0059] First supply line S1 is devoid of any driver circuit constituting a current control means, upstream, downstream or in between. Accordingly, the current in first supply line S1 is only controlled by DC / DC converter 10.

[0060] Such an arrangement allows to independently control each LED string without providing for a driver circuit on each string.

[0061] Indeed, when the activation of second string 22 only is requested, in operating mode B, DC / DC converter 10 works in voltage control mode and outputs a setpoint voltage that is lower than the forward voltage required for powering first string 21. Microcontroller 12 enables driver circuit D2 to manage the light-emitting diodes of second string 22. Hence, first string 21 remains deactivated while only second string 22 illuminates. Its lighting properties are controlled by driver circuit D2 which controls the current in second supply line S2.

[0062] When only the activation of first string 21 is requested, in operating mode A, microcontroller 12 disables driver circuit D2 and orders DC / DC converter 10 to set a voltage above the forward voltage of first string 21, then it toggles DC / DC converter 10 to current control to control the current required to vary the brightness of the light-emitting diodes of first string 21.

[0063] When the activation of both strings 21, 22 is requested, microcontroller 12 orders DC / DC converter 10 to set a voltage above the forward voltage of first string 21 and enables driver circuit D2 to manage the light-emitting diodes of second string 22. The operation of second string 22 is then controlled by driver circuit D2. Then, microcontroller 12 toggles DC / DC converter 10 to current control mode to manage the light-emitting diodes of first string 21 while taking into account the current consumed by second string 22.

[0064] The absence of a specific driver circuit dedicated to the first string is cost efficient and reduces the need for heat dissipation, assembly and bulk management.

[0065] Advantageously, to guarantee that the higher forward voltage LED string (first string 21) stays disabled when required, the forward voltage of two consecutive LED strings 21, 22 present a voltage gap. The larger the forward voltage gap between the strings 21, 22, the easier to maintain the independent control.

[0066] More advantageously, the lower forward voltage string (second string 22) can be arranged into at least two sub-strings to increase the forward voltage gap between the highest forward voltage string and the lower forward voltage string.

[0067] Furthermore, such an arrangement allows an optimal independent control of the LED strings 20 while reducing the number of electronic components in LED driving system 1, thus reducing the bulkiness of system 1 as well as its cost.

[0068] When power is enabled in LED driver system 1, DC / DC converter 10 sets an idle output voltage at a safety level, ranging from 0V (volt) to slightly lower value than the highest forward voltage.

[0069] When activation of the highest forward voltage LED string (first LED string 21) is requested, a setpoint is calculated by microcontroller 12 and communicated to DC / DC converter 10 which increases the voltage to the calculated setpoint in order to start the current flow towards the highest forward voltage LED string. Microcontroller 12 monitors, by means of communication lines C1, that the required voltage is reached, and then drives DC / DC converter 10 to toggle from voltage control mode to current control mode.

[0070] In the voltage control mode, the setpoint voltage is supplied to highest forward voltage LED string 21 in order to power its activation. The current control mode in turn, allows to compensate temperature influence on LED light output. The current control mode can also change the type of function or signaling requirement. For example, in an autonomous vehicle that can reverse its front and rear directions, or in a train or a tramway that goes back and forth without turning around, the light output of a front lamp can be changed into a rear lamp. Another possible use is the addition of "braking" lights in the front lamp of a car to inform pedestrians that the driver is pressing the brake pedal.

[0071] When deactivation of highest forward voltage LED string 21 is requested, DC / DC converter 10, driven by microcontroller 12, decreases the ongoing output current to zero, then drives DC / DC converter 10 to toggle from current control mode to voltage control mode in order to ensure deactivation of string 21 by setting a voltage lower than the setpoint voltage calculated by microcontroller 12 for activating highest forward voltage LED string 21.

[0072] When activation of a lower forward voltage LED string (second LED string 22) is requested, microcontroller 12 sets the output voltage of the DC / DC converter well below the forward voltage of highest forward voltage LED string 21, but above forward voltage required to activate LED string 22, increased by minimum required voltage drop on driving control circuits, and enables driver circuit D2 of second string 22 to regulate the current flow towards lower forward voltage LED string 22.

[0073] When deactivation of lower forward voltage LED string (or second LED string 22) is requested, microcontroller 12 disables driver circuit D2. If no other lighting in the lamp is needed, DC / DC converter 10 set output voltage to idle.

[0074] It is understood here that highest forward voltage LED string 21 and lower forward voltage LED string 22 can both be activated at the same time. Microcontroller 12 calculates a setpoint voltage appropriate for highest forward voltage string 21 and then, in current control mode, DC / DC converter 10 controls highest forward voltage string 21 while driver circuit D2 is enabled to control current flowing through second string 22. This current flowing through second string 22 is taken into account when driving the DC / DC converter.

[0075] This embodiment provides the advantage that half of driving control circuits are removed and replaced by cheap component, providing highest relative (%) cost improvement.

[0076] Referring to figure 3, LED driver system 1 is similar to the ones illustrated in figures 1 and 2. Here, DC / DC converter 1 is further provided with a current sensor 16. More precisely, the current sensor 16 is connected to microcontroller 12. In the example of figure 3, current sensor 16 is integrated in microcontroller 12. However, it could be located elsewhere. The signal from current sensor 16 is evaluated by microcontroller 12. It can be directly used to control the DC / DC converter or, more likely, to only tune up the algorithm for a better precision. It can be used for diagnostics if DC / DC converter works correctly. Current sensor 16 is configured to detect the current flowing through different LED strings 20 to allow precise calculation of the setpoint voltage expected at each string 20 for current flow. Indeed, it is possible that one string contains a faulty LED, for example a broken LED, which may decrease the current flow. Current sensor 16 determines the current flowing therethrough and on basis of this information, microcontroller 12 can take appropriate actions depending on fault type and predefined fait-safe strategy. Current sensor 16 helps therefore optimizing the microcontroller 12 calculations.

[0077] When current sensor 16 is integrated in the DC / DC converter unit, the pin-count of LED driving system 1 is reduced and thus bulkiness of the system 1 is likely to be reduced.

[0078] The arrangement of current sensor 16 is not limited to that illustrated in figure 3. In an alternate embodiment not represented, the current sensor may be placed along the communication lines C1, C2. If a separate current sensor 16 is used, it should be connected to strings 20 and to DC / DC converter 10 via communication lines.

[0079] Figure 4a represents another embodiment wherein LED driving system 1 comprises three LED strings (n=3) 21, 22, 23. Each string 21, 22, 23 constitutes a different loop with the DC / DC converter 10 via supply lines S1, S2, S3 and communication lines C1, C2, C2', C3, C3'. First supply line S1 for the highest forward voltage string (first string 21) is devoid of any driver circuit, and each of other two supply lines S2, S3 for the two lower forward voltage strings (second 22 and third 23 strings) comprise a driver circuit D2, D3 for current control.

[0080] The two lower forward voltage strings 22, 23 may both require the same setpoint voltage for powering their respective strings, or different setpoint voltages. In the same way, both lower forward voltage strings 22, 23 may be configured to have a same lighting and / or signaling function, or different ones.

[0081] Referring to figure 4b, one of the lower forward voltage strings, here third string 23, comprises a switch 4 configured to be controlled by microcontroller 12 between a closed position (not shown) allowing activation of third string 23 and an open position, as shown, allowing deactivation of third string 23. In such a layout, third string 23 does not require a driver circuit, the current variation is accordingly controlled by the DC / DC converter 10. Such a system 1 is advantageously less bulky and more cost-effective as electronic components is further reduced while maintaining control of each string in case first LED string 21 and third LED string 23 activation is mutually exclusive (both strings 21 and 23 are never activated at the same time, meaning that either string 21 is active or string 23 is active or none of strings 21 and 23 is active). In this case, LED string 22 can be controlled independently from strings 21 and 23 with the condition that forward voltage of LED string 22 is lower than forward voltages of both other strings 21 and 23.

[0082] Once again, this embodiment brings the cost beneficial usage as in case of two functions only (figs. 1, 2 and 3), where half of driving control circuits are removed and replaced by cheap component, providing highest relative (%) cost improvement.

[0083] Figure 5 shows a flow diagram of method for enabling a light source comprising a set 20 of at least two parallel strings 21, 22, 23 of light-emitting diodes, each string 21, 22, 23 having a forward voltage, wherein a first string 21 has a forward voltage higher than the forward voltage of the other strings 22, 23, the method comprising: a step of selecting one operating mode in a set of two operating modes, said set comprising mode "A" 100: enlighten first string 21 and mode "B" 200: enlighten at least one of other strings 22, 23 and not first string 21, a step of carrying out the selected operating mode according to the following operating instructions: * to carry out mode "A" 100, input 120 a current in the strings at a voltage higher than the forward voltage of first string 21, then control 130 the current in the strings to manage the light-emitting diodes, * to carry out mode "B" 200, input 210 a current in the strings at a voltage lower than the forward voltage of first string 21, then control 220 the current in the other strings 22, 23, to manage the light-emitting diodes.

[0084] Understandably, the control of the other strings can be carried out solely by the DC / DC converter 10, notably with the condition that one of the LED strings (the first string 21) presents a forward voltage higher than forward voltages of both other strings 22, 23. Furthermore, the DC / DC converter can manage a switch 4 and / or a driver circuit D2, D3 for controlling the current in the other strings 22, 23.List of references

[0085] C1, C2, C2', C3, C3': communication lines D1, D2, D3: driver circuit S1, S2, S3: supply lines 1: driving system 2: lamp connector 3: power protection module 4: switch 10: DC / DC converter 12: microcontroller 14: toggle module 16: current sensor 20: set of LED strings 21: first string 22: second string 23: third string 100: to enlighten the highest forward voltage string only 120: input a current in the strings 130: control the current in the strings 200: to enlighten at least one of the other strings without the highest forward voltage string 210: input a current in the strings 220: control the other strings

Examples

Embodiment Construction

[0034]Figures 1a to 1c are schematic diagrams of an LED driving system 1 comprising the circuitry of a lighting device of a vehicle lamp. The LED driving system is connected to a lamp connector 2 provided on the lighting device.

[0035]LED driving system 1 can be supported on a printed circuit board (PCB) (not represented) preferably when system 1 is adapted for rear lamps, or the system can be mounted on a heat sink (not represented) while being connected to a PCB, for example by means of a gold wire connection, preferably when system 1 is adapted for higher power front lamps.

[0036]LED driving system 1 comprises a DC / DC converter 10 and a set 20 of n LED strings, n being a natural integer number greater than or equal to two. Each string of set 20 comprises multiple LEDs connected in series. In the examples of figures 1a to 1c, 2, 3, n equals two. In the examples of figures 4a, 4b, n equals three. Additionally, it is also possible that a string comprises multiple LEDs connected in par...

[0001] The present invention relates to driving systems for driving light sources, especially light-emitting diodes (LEDs). It also relates to lighting devices, notably for an automotive vehicle, comprising such driving systems, such as vehicle lamp modules. The invention also relates to a method for enabling a light source.

[0002] In general, a lighting device comprises one or more light sources, notably light-emitting diodes (LEDs), which are driven by a driving system for illuminating the lighting device. For example, for a vehicle lamp module, the driving system uses an array of LEDs to illuminate the lamp. An LED array usually comprises two or more LED strings, each string comprising a group of LEDs connected in series, and specific lighting and / or signaling functions are each associated to one string or a combination of strings. The driving system usually further comprises a converter for converting an input voltage into a minimal output voltage for enabling the LEDs and start current flow through the LEDs. Each string may require a different minimum voltage which can vary according to several parameters, namely current and temperature. Furthermore, the current flowing through each string can be varied so that the same string can demonstrate different luminosity and / or color, notably in accordance with standards and regulations for vehicle lamps, for example rear lights can comprise red tail lights and bright red brake lights. In order to generate desired light outputs with uniform brightness, each string is typically controlled by its own driver circuit to regulate the current flowing therethrough.

[0003] Such driving systems require a large pin-count and are usually bulky due to their large number of electronic components which also largely contribute to high heat dissipation leading to damage prone circuits.

[0004] Prior art patent application US 2015 / 0035443 proposes an LED driving system including a bus connected to a main LED string equipped with a current regulator to control the output color in the string. The driving system further includes a voltage regulator circuit which may comprise at least a secondary LED string coupled to the bus. This secondary LED string is coupled between the bus and a ground node that provides a current return path to the controlled current source. The secondary string is able to regulate voltage of the main one as it varies the current passing therethrough before reaching the main LED string. In such a driving circuit, when the main LED string needs to be operated, the secondary LED string is always activated in order to act as a voltage regulator. The system does not allow to activate the first string only without activating the other string.

[0005] The purpose of the present invention is to provide a cost-friendly LED driving system with reduced bulkiness and reduced heat dissipation while at the same time allowing optimal independent control of each string associated to a specific lighting and / or signaling functions in a vehicle.

[0006] To that end, the invention is related to a method for driving light sources comprising a set of at least two parallel strings of light-emitting diodes, each string having a forward voltage, wherein a first string has a forward voltage higher than the forward voltage of the other strings, the method comprising a step of operating the light sources in a low-voltage mode, consisting of inputting a current in the strings at a voltage lower than the forward voltage of the first string, to enlighten at least one of the other strings and not the first string.

[0007] Herein, a forward voltage difference between strings is provided to enable the selective activation of a particular string. Especially in the low-voltage operating mode, at least one string among the other strings can be activated. To do so, a current is supplied at a voltage that is lower than the forward voltage of the first string. Since, the voltage supplied is lower than the required voltage for activating the first string, said first string is automatically blocked or disabled. The supplied current is advantageously suitable to allow activation of the one of the other strings.

[0008] In the description, to "disable a string" should be understood as meaning either "actively disable" if the string was enabled, or "keep disabled" if the string was not enabled, for instance, when the string is the first string, because an earlier idle state already set the output voltage below the forward voltage of the first string.

[0009] Furthermore, contrary to prior art, here no additional control means is required to operate in the low-voltage mode, thus reducing cost of production and heat dissipation. With the proposed method, the first string, having the highest forward voltage, can be used as a back-up string, ready to replace a damaged string of a lower forward voltage. Hence, with the method of the invention, a back-up string can be included in the set of light sources without additional means specific to said back-up string. When a failure of the damaged lower forward voltage string is detected, according to the method, it is sufficient to replace the low-voltage operating mode with a high-voltage operating mode, wherein a current is inputted in the parallel strings with a voltage higher than the forward voltage of the first string.

[0010] In a particular embodiment, the at least one of the other strings is provided with a switch or a current control means.

[0011] The independency of the strings is optimized by the presence of a switch or current control means on at least one of the other strings having a forward voltage lower than that of the first string. Advantageously, such current control means are not required in the first string, thus reducing bulkiness of the assembly.

[0012] Strings can be selectively activated and independently controlled even though all the strings are not provided with current control means.

[0013] This is particularly advantageous since a particular lighting and / or signaling function is associated to each LED string or combination of LED strings. According to a desired function, a single string or a combination of strings can therefore be independently activated upon request and independently controlled, based on their forward voltage.

[0014] In a particular embodiment, the method comprises: a step of selecting one operating mode in a set of at least two operating modes, said set comprising mode A: enlighten at least the first string and mode B: enlighten at least one of the other strings and not the first string, a step of carrying out the selected operating mode according to the following operating instructions: * to carry out mode A, input a current in the strings at a voltage higher than the forward voltage of first string, then control the current in the strings to manage the light-emitting diodes, * to carry out mode B, input a current in the strings at a voltage lower than the forward voltage of the first string, then control the current in the other strings to manage the light-emitting diodes,

[0015] Here, contrary to the prior art, one string can be activated without activating the other one(s).

[0016] Indeed, to enlighten the first string, notably in operating mode A, the switch or the current control means of at least one of the other strings is disabled. By disabling the switch or the current control means of the other strings, their activation is blocked or disabled. Though the first string is not provided with current control means specific to this string, the current flowing through it can still be controlled to control brightness, enable or disable a function and, in addition, to compensate temperature influence on LED light output.

[0017] Here again, to "disable the switch or current control means" should be understood as meaning either "actively disable" if the switch or control means was enabled, or "keep disabled" if said switch or said control means was not enabled.

[0018] In order to activate one of the other strings in operating mode B, there is a voltage or current output which is preferably well below the activation voltage level of the highest forward voltage string. Hence, lower forward voltage strings can be selectively activated without activating the first string. Therefore current control means, which tend to increase bulkiness of a light source circuitry, is not required for the first string which has the highest forward voltage.

[0019] Advantageously, in operating mode A, the first string and at least one of the other strings can be simultaneously activated according to a desired function.

[0020] The invention also relates to a method for manufacturing a driving system as described above.

[0021] Such a method for manufacturing a driving system is able to drive a set of parallel strings of light-emitting diodes, each having a string forward voltage, the method comprising the steps of: selecting the string of light-emitting diodes having the highest forward voltage, thus obtaining a first string, distinct from the other strings, connecting said strings to a DC / DC converter configured to operate either in a current control mode or in a voltage control mode, connecting the DC / DC converter to a microcontroller able to toggle the DC / DC converter from the current control mode to the voltage control mode and vice-versa, said microcontroller comprising a processor and a memory readable by the processor, said memory containing a set of instructions implementing the method for enabling a light source as described above, when executed by said processor.

[0022] The method for manufacturing a driving system may further present one or more of the following features, taken separately or in combination. After selecting the string of light-emitting diodes having the highest forward voltage, at least one of the other strings is provided with a current control mean or a switch, thus obtaining a controlled string.

[0023] The invention further relates to a driving system for driving a set of parallel strings of light-emitting diodes, wherein each string has a forward voltage, wherein a first string has a forward voltage higher than the forward voltage of the other strings, said driving system comprising: a DC / DC converter configured to operate either in a current control mode or in a voltage control mode, a microcontroller, connected to the DC / DC converter and able to toggle the DC / DC converter from the current control mode to the voltage control mode and vice-versa, said microcontroller comprising a processor and a memory readable by the processor, said memory containing a set of instructions implementing the method for enabling a light source as described above, when executed by said processor.

[0024] Hence, the DC / DC converter integrates a "smart function" allowing it to function either in a current control mode for controlling the current supply towards the strings, notably for controlling the luminosity or color of the selected string, or a voltage control mode for controlling the voltage supply towards the strings.

[0025] The driving system may further present one or more of the following features, taken separately or in combination. At least one of the other strings are provided with a current control means or a switch, said microcontroller being connected to the current control means or a switch, said microcontroller being connected to the current control means or switch and being able to to enable and disable the current control means or the switch.

[0026] By providing only the lower forward voltage strings with control means, bulkiness and cost efficiency of the driving system is already optimized. Furthermore, the microcontroller allows the DC / DC converter to operate in a voltage control mode in order to control the voltage towards the strings, or in a current control mode to control the current.

[0027] This is particularly advantageous for the first string, when it needs to be controlled, supplementary current control means are not required, the first string can be accordingly controlled by the DC / DC converter.

[0028] Such a system assembly optimizes the independent control of each string. For activation or deactivation of lower forward voltage strings, the microcontroller only has to enable or disable the current control means. The current control means can be positioned upstream or downstream of the corresponding string.

[0029] Therefore, the current control means are flexible in terms of their positioning, they can be placed in the circuitry of the driving system according to the available space. As each string can be associated to a lighting and / or a signaling function, the temperature of the series of light-emitting diodes (LEDs) is usually different. The temperature of an LED typically refers to its junction temperature. The forward voltage required to enable a string depends on the temperature. The larger the forward voltage gap between the consecutive strings, the easier it is to maintain the independent control of the strings. Temperature difference may work in both directions, either make voltage gap bigger or smaller, depending on which LED string has higher temperature at certain moment of time. The forward voltage gap being larger, the strings can be more easily isolated to be controlled, and a string, especially the highest forward voltage string, can be maintained disabled when required. One of the consecutive strings can be arranged into at least two sub-strings to increase the forward voltage gap between two consecutive strings.

[0030] This arrangement allows an optimal independent control of the LED strings while reducing the number of electronic components in the LED driving system, thus reducing the bulkiness of the system as well as its cost.

[0031] The invention also relates to a light source made of a set of parallel strings of light-emitting diodes, wherein each string has a forward voltage, wherein one string has a forward voltage higher than the forward voltage of the other strings, and wherein the other strings are provided with current control means said light source comprising a driving system as described above.

[0032] The invention further relates a vehicle lamp module comprising a light source as described above.Brief description of figures

[0033] The invention will be better understood, and other characteristics and advantages will appear when reading the following description, given by way of example and not by way of limitation, referring to the annexed figures, in which, [Fig. 1a] is a schematic diagram of an LED driving system according to one embodiment of the invention. [Fig. 1b] is a schematic diagram of an LED driving system according to another embodiment of the invention. [Fig. 1c] is a schematic diagram of an LED driving system according to another embodiment of the invention. [Fig. 2] is a schematic diagram of an embodiment of the LED driving system according to another embodiment of the invention. [Fig. 3] is a schematic diagram of an embodiment of the LED driving system according to another embodiment of the invention. [Fig. 4a] is a schematic diagram of an embodiment of the LED driving system according to another embodiment of the invention. [Fig. 4b] is a schematic diagram of an embodiment of the LED driving system according to another embodiment of the invention. [Fig. 5] is a schematic diagram of a method for enabling a light source according to the invention. Detailed description of the figures

[0034] Figures 1a to 1c are schematic diagrams of an LED driving system 1 comprising the circuitry of a lighting device of a vehicle lamp. The LED driving system is connected to a lamp connector 2 provided on the lighting device.

[0035] LED driving system 1 can be supported on a printed circuit board (PCB) (not represented) preferably when system 1 is adapted for rear lamps, or the system can be mounted on a heat sink (not represented) while being connected to a PCB, for example by means of a gold wire connection, preferably when system 1 is adapted for higher power front lamps.

[0036] LED driving system 1 comprises a DC / DC converter 10 and a set 20 of n LED strings, n being a natural integer number greater than or equal to two. Each string of set 20 comprises multiple LEDs connected in series. In the examples of figures 1a to 1c, 2, 3, n equals two. In the examples of figures 4a, 4b, n equals three. Additionally, it is also possible that a string comprises multiple LEDs connected in parallel.

[0037] DC / DC converter 10 is configured to be fed in electrical energy by a power source (not represented), for example a supply line from the battery of the vehicle. A power protection module 3 can be provided upstream of DC / DC converter 10 for producing a safe current and / or a safe voltage protecting DC / DC converter 10 from high, interrupted or otherwise distorted electrical supply.

[0038] A particular lighting and / or signaling function is associated to each LED string or combination of LED strings.

[0039] Each string has a forward voltage, i.e. the voltage at which current starts to flow in the string, required to activate its LED string, hence activating the desired lighting and / or signaling function of the vehicle lamp.

[0040] DC / DC converter 10 is configured to convert a direct current (DC) from one voltage level range to another. DC / DC converter 10 is configured to operate either in a current control mode or in a voltage control mode and to toggle from one mode to another on request.

[0041] To do so, DC / DC converter 10 integrates a "smart function" allowing toggling between the two modes. The "smart function" can be carried out by a microcontroller 12 (not represented on figures 1a to 1c) configured to drive DC / DC converter 10.

[0042] Microcontroller 12 is configured to receive lighting instructions from a central computer (not represented) of the vehicle, which manages the interface with a user and other parts of the vehicle that may require to enlighten the lamp. Such instructions can take the form of a physical or logical signal, for example a direct control line (physical signal) or a CAN-FD (for "Controller Area Network Flexible Date-Rate") signal or a LIN ("Local Interconnect Network") signal.

[0043] Microcontroller 12 comprises a database holding information on the LEDs of each string, for example on the luminous flux change as a function of temperature, forward voltage change as a function of temperature or / and current. Microcontroller 12 is also configured to receive feedback on the current flow passing through the strings and voltage over all or certain selected LEDs in string in order to detect failures in LED strings and / or self-diagnose internal circuits of DC / DC converter and drive control circuits, for example by detecting damaged LEDs. Microcontroller 12 is configured to receive said feedback through communication lines C1, C2.

[0044] As per design constraints, the database and the feedback allow calculating a setpoint voltage required to power a selected string, as well as current output or current variation output that needs to be supplied by the DC / DC converter 10 towards the selected string.

[0045] Hence, DC / DC converter 10 is able to control the voltage towards the strings in the voltage control mode, or to control the current towards the strings in the current control mode. Also, DC / DC converter 10 is configured to toggle from current control mode to voltage control mode and vice-versa.

[0046] With reference to figures 1a, 1b, 1c, 2 and 3, two LED strings 21, 22 are arranged in parallel and are each connected to the DC / DC converter 10 by a supply line S1, S2.

[0047] The LEDs of the strings 21, 22 are selected so that one of the strings (designated as the "first" string 21) has a forward voltage that is the highest forward voltage among all the strings, i.e., a higher forward voltage that the forward voltage of string 22. The other LED string 22 has a lower forward voltage.

[0048] In the embodiment of figure 1a, the current supply in the first supply line S1 and the second supply line S2 is controlled by the "smart function" of DC / DC converter 10, i.e., microcontroller 12.

[0049] First string 21 is included in the set of string but is not intended for use as long as second string 22 is able to work. Indeed, in the example of Fig. 1a, string 21 is a backup string in case string 22 is damaged.

[0050] Indeed, when the activation of the second string 22 only is requested, in a low-voltage operating mode, the DC / DC converter 10 works in voltage control mode and outputs a setpoint voltage that is higher than the forward voltage required for powering second string 22 and lower than the forward voltage required for powering first string 21. Microcontroller 12 then controls the current required to vary the brightness of the light-emitting diodes of second string 22 only. Hence, second string 22 can be activated without activating first string 21 with the condition that forward voltage of string 22 is lower than the forward voltage of first string 21.

[0051] In case of open circuit fault on string 22, when the activation of first string 21 is requested as a back-up light source, DC / DC converter 10, preferably microcontroller 12, inputs a current at a voltage above the forward voltage of first string 21, then it toggles the DC / DC converter 10 to current control mode manage the light-emitting diodes (LEDs) of the first string, for example for managing the brightness of the LEDs.

[0052] It is also possible to work continuously in current control mode especially when DC / DC converter is not controlled by microcontroller. In this case, DC / DC converter, working in current control mode, enables current flow. When string 22 is working correctly, the current flows through string 22. When string 22 has an open circuit fault condition, voltage set point will automatically adjust for higher forward voltage string and current will flow through string 21.

[0053] In the embodiment of Fig. 1b, two operating modes are possible: mode A: at least first string 21 is lit, no matter if second string 22 will be activated or not, mode B: second string 22 is lit and not first string 21.

[0054] For an independent activation of the two strings 21, 22, a switch 4 is provided on the second supply line S2. Switch 4 is configured to be controlled by DC / DC converter 10, preferably by microcontroller 12, between a closed position (not shown) allowing activation of second string 22, and an open position, as shown allowing deactivation of second string 22.

[0055] Herein, when the activation of first string 21 only is requested, for example in operating mode A, DC / DC converter 10 orders switch 4 towards its open position, notably via communication line C2', so that only first string 21 is controlled by said converter, be it in a voltage control mode or a current control mode. When activation of second string 22 only is requested, notably in operating mode B, DC / DC converter 10 orders switch 4 towards its closed position and supplies a current at a voltage lower than the forward voltage of first string 21 such that only second string 22 is controllable. Hence, such an arrangement allows independent activation of the two strings 21, 22 with the condition that forward voltage of LED string 22 is lower than forward voltage of the second string 22.

[0056] Such a system 1 is advantageously less bulky and more cost-effective as first string 21 is devoid of any additional electronic components, and the control of each string is optimized in case first LED string 21 and second LED string 22 activation is mutually exclusive (both strings 21 and 22 are never activated at the same time, meaning that either string 21 is active or string 22 is active or none of strings 21 and 22 is active). Of course, it is understood that both strings 21, 22 can be activated at the same time, provided that there is some other device (for example a series transistor) controlling current flow in string 22, when it is activated together with string 21. In this case, DC / DC converter 10 orders switch 4 towards its closed position and supplies a current at a voltage higher than the forward voltage of the first string 21 such that both strings can be powered. Here, when the microcontroller 12 toggles to current control mode, it can enable both strings at the same time and control current in string 21, as current in string 22 is controlled by some other device (such as a series transistor).

[0057] For maintaining the independent activation of strings 20, the switch can be alternatively replaced by a driver circuit D2, as shown in figure 1c. In this embodiment, second supply line S2 is provided with a driver circuit D2 which constitutes a current control means. In this embodiment, driver circuit D2 is upstream of second LED string 22, on high side, but it may also be on low side, downstream, or somewhere in between. Driver circuit D2 is configured to control the current flow towards second string 22. Driver circuit D2 can be a transistor or a linear driver, or any other driver circuit known in the field. Driver circuit D2 is controlled by the "smart function" of the DC / DC converter 10, i.e. microcontroller 12 via communication lines C2'. In particular, microcontroller 12 can enable and disable driver circuit D2. Herein, it is understood that for activating second string 22 in operating mode B, DC / DC converter 10 supplies a voltage lower than the forward voltage of the first string so as to prevent activation of the latter as explained the previous paragraphs. DC / DC converter 10, preferably microcontroller 12, is able to enable driver circuit D2 for current control of second string 22 so as to relieve the DC / DC converter from current controlling second string 22.

[0058] A similar example is represented in figure 2, wherein microcontroller 12 is preferably integrated in the DC / DC converter 10. The microcontroller 12 can communicate instructions with a toggle module 14. Alternatively, microcontroller 12 can be external to the DC / DC converter.

[0059] First supply line S1 is devoid of any driver circuit constituting a current control means, upstream, downstream or in between. Accordingly, the current in first supply line S1 is only controlled by DC / DC converter 10.

[0060] Such an arrangement allows to independently control each LED string without providing for a driver circuit on each string.

[0061] Indeed, when the activation of second string 22 only is requested, in operating mode B, DC / DC converter 10 works in voltage control mode and outputs a setpoint voltage that is lower than the forward voltage required for powering first string 21. Microcontroller 12 enables driver circuit D2 to manage the light-emitting diodes of second string 22. Hence, first string 21 remains deactivated while only second string 22 illuminates. Its lighting properties are controlled by driver circuit D2 which controls the current in second supply line S2.

[0062] When only the activation of first string 21 is requested, in operating mode A, microcontroller 12 disables driver circuit D2 and orders DC / DC converter 10 to set a voltage above the forward voltage of first string 21, then it toggles DC / DC converter 10 to current control to control the current required to vary the brightness of the light-emitting diodes of first string 21.

[0063] When the activation of both strings 21, 22 is requested, microcontroller 12 orders DC / DC converter 10 to set a voltage above the forward voltage of first string 21 and enables driver circuit D2 to manage the light-emitting diodes of second string 22. The operation of second string 22 is then controlled by driver circuit D2. Then, microcontroller 12 toggles DC / DC converter 10 to current control mode to manage the light-emitting diodes of first string 21 while taking into account the current consumed by second string 22.

[0064] The absence of a specific driver circuit dedicated to the first string is cost efficient and reduces the need for heat dissipation, assembly and bulk management.

[0065] Advantageously, to guarantee that the higher forward voltage LED string (first string 21) stays disabled when required, the forward voltage of two consecutive LED strings 21, 22 present a voltage gap. The larger the forward voltage gap between the strings 21, 22, the easier to maintain the independent control.

[0066] More advantageously, the lower forward voltage string (second string 22) can be arranged into at least two sub-strings to increase the forward voltage gap between the highest forward voltage string and the lower forward voltage string.

[0067] Furthermore, such an arrangement allows an optimal independent control of the LED strings 20 while reducing the number of electronic components in LED driving system 1, thus reducing the bulkiness of system 1 as well as its cost.

[0068] When power is enabled in LED driver system 1, DC / DC converter 10 sets an idle output voltage at a safety level, ranging from 0V (volt) to slightly lower value than the highest forward voltage.

[0069] When activation of the highest forward voltage LED string (first LED string 21) is requested, a setpoint is calculated by microcontroller 12 and communicated to DC / DC converter 10 which increases the voltage to the calculated setpoint in order to start the current flow towards the highest forward voltage LED string. Microcontroller 12 monitors, by means of communication lines C1, that the required voltage is reached, and then drives DC / DC converter 10 to toggle from voltage control mode to current control mode.

[0070] In the voltage control mode, the setpoint voltage is supplied to highest forward voltage LED string 21 in order to power its activation. The current control mode in turn, allows to compensate temperature influence on LED light output. The current control mode can also change the type of function or signaling requirement. For example, in an autonomous vehicle that can reverse its front and rear directions, or in a train or a tramway that goes back and forth without turning around, the light output of a front lamp can be changed into a rear lamp. Another possible use is the addition of "braking" lights in the front lamp of a car to inform pedestrians that the driver is pressing the brake pedal.

[0071] When deactivation of highest forward voltage LED string 21 is requested, DC / DC converter 10, driven by microcontroller 12, decreases the ongoing output current to zero, then drives DC / DC converter 10 to toggle from current control mode to voltage control mode in order to ensure deactivation of string 21 by setting a voltage lower than the setpoint voltage calculated by microcontroller 12 for activating highest forward voltage LED string 21.

[0072] When activation of a lower forward voltage LED string (second LED string 22) is requested, microcontroller 12 sets the output voltage of the DC / DC converter well below the forward voltage of highest forward voltage LED string 21, but above forward voltage required to activate LED string 22, increased by minimum required voltage drop on driving control circuits, and enables driver circuit D2 of second string 22 to regulate the current flow towards lower forward voltage LED string 22.

[0073] When deactivation of lower forward voltage LED string (or second LED string 22) is requested, microcontroller 12 disables driver circuit D2. If no other lighting in the lamp is needed, DC / DC converter 10 set output voltage to idle.

[0074] It is understood here that highest forward voltage LED string 21 and lower forward voltage LED string 22 can both be activated at the same time. Microcontroller 12 calculates a setpoint voltage appropriate for highest forward voltage string 21 and then, in current control mode, DC / DC converter 10 controls highest forward voltage string 21 while driver circuit D2 is enabled to control current flowing through second string 22. This current flowing through second string 22 is taken into account when driving the DC / DC converter.

[0075] This embodiment provides the advantage that half of driving control circuits are removed and replaced by cheap component, providing highest relative (%) cost improvement.

[0076] Referring to figure 3, LED driver system 1 is similar to the ones illustrated in figures 1 and 2. Here, DC / DC converter 1 is further provided with a current sensor 16. More precisely, the current sensor 16 is connected to microcontroller 12. In the example of figure 3, current sensor 16 is integrated in microcontroller 12. However, it could be located elsewhere. The signal from current sensor 16 is evaluated by microcontroller 12. It can be directly used to control the DC / DC converter or, more likely, to only tune up the algorithm for a better precision. It can be used for diagnostics if DC / DC converter works correctly. Current sensor 16 is configured to detect the current flowing through different LED strings 20 to allow precise calculation of the setpoint voltage expected at each string 20 for current flow. Indeed, it is possible that one string contains a faulty LED, for example a broken LED, which may decrease the current flow. Current sensor 16 determines the current flowing therethrough and on basis of this information, microcontroller 12 can take appropriate actions depending on fault type and predefined fait-safe strategy. Current sensor 16 helps therefore optimizing the microcontroller 12 calculations.

[0077] When current sensor 16 is integrated in the DC / DC converter unit, the pin-count of LED driving system 1 is reduced and thus bulkiness of the system 1 is likely to be reduced.

[0078] The arrangement of current sensor 16 is not limited to that illustrated in figure 3. In an alternate embodiment not represented, the current sensor may be placed along the communication lines C1, C2. If a separate current sensor 16 is used, it should be connected to strings 20 and to DC / DC converter 10 via communication lines.

[0079] Figure 4a represents another embodiment wherein LED driving system 1 comprises three LED strings (n=3) 21, 22, 23. Each string 21, 22, 23 constitutes a different loop with the DC / DC converter 10 via supply lines S1, S2, S3 and communication lines C1, C2, C2', C3, C3'. First supply line S1 for the highest forward voltage string (first string 21) is devoid of any driver circuit, and each of other two supply lines S2, S3 for the two lower forward voltage strings (second 22 and third 23 strings) comprise a driver circuit D2, D3 for current control.

[0080] The two lower forward voltage strings 22, 23 may both require the same setpoint voltage for powering their respective strings, or different setpoint voltages. In the same way, both lower forward voltage strings 22, 23 may be configured to have a same lighting and / or signaling function, or different ones.

[0081] Referring to figure 4b, one of the lower forward voltage strings, here third string 23, comprises a switch 4 configured to be controlled by microcontroller 12 between a closed position (not shown) allowing activation of third string 23 and an open position, as shown, allowing deactivation of third string 23. In such a layout, third string 23 does not require a driver circuit, the current variation is accordingly controlled by the DC / DC converter 10. Such a system 1 is advantageously less bulky and more cost-effective as electronic components is further reduced while maintaining control of each string in case first LED string 21 and third LED string 23 activation is mutually exclusive (both strings 21 and 23 are never activated at the same time, meaning that either string 21 is active or string 23 is active or none of strings 21 and 23 is active). In this case, LED string 22 can be controlled independently from strings 21 and 23 with the condition that forward voltage of LED string 22 is lower than forward voltages of both other strings 21 and 23.

[0082] Once again, this embodiment brings the cost beneficial usage as in case of two functions only (figs. 1, 2 and 3), where half of driving control circuits are removed and replaced by cheap component, providing highest relative (%) cost improvement.

[0083] Figure 5 shows a flow diagram of method for enabling a light source comprising a set 20 of at least two parallel strings 21, 22, 23 of light-emitting diodes, each string 21, 22, 23 having a forward voltage, wherein a first string 21 has a forward voltage higher than the forward voltage of the other strings 22, 23, the method comprising: a step of selecting one operating mode in a set of two operating modes, said set comprising mode "A" 100: enlighten first string 21 and mode "B" 200: enlighten at least one of other strings 22, 23 and not first string 21, a step of carrying out the selected operating mode according to the following operating instructions: * to carry out mode "A" 100, input 120 a current in the strings at a voltage higher than the forward voltage of first string 21, then control 130 the current in the strings to manage the light-emitting diodes, * to carry out mode "B" 200, input 210 a current in the strings at a voltage lower than the forward voltage of first string 21, then control 220 the current in the other strings 22, 23, to manage the light-emitting diodes.

[0084] Understandably, the control of the other strings can be carried out solely by the DC / DC converter 10, notably with the condition that one of the LED strings (the first string 21) presents a forward voltage higher than forward voltages of both other strings 22, 23. Furthermore, the DC / DC converter can manage a switch 4 and / or a driver circuit D2, D3 for controlling the current in the other strings 22, 23.List of references

[0085] C1, C2, C2', C3, C3': communication lines D1, D2, D3: driver circuit S1, S2, S3: supply lines 1: driving system 2: lamp connector 3: power protection module 4: switch 10: DC / DC converter 12: microcontroller 14: toggle module 16: current sensor 20: set of LED strings 21: first string 22: second string 23: third string 100: to enlighten the highest forward voltage string only 120: input a current in the strings 130: control the current in the strings 200: to enlighten at least one of the other strings without the highest forward voltage string 210: input a current in the strings 220: control the other strings

Examples

Embodiment Construction

[0034]Figures 1a to 1c are schematic diagrams of an LED driving system 1 comprising the circuitry of a lighting device of a vehicle lamp. The LED driving system is connected to a lamp connector 2 provided on the lighting device.

[0035]LED driving system 1 can be supported on a printed circuit board (PCB) (not represented) preferably when system 1 is adapted for rear lamps, or the system can be mounted on a heat sink (not represented) while being connected to a PCB, for example by means of a gold wire connection, preferably when system 1 is adapted for higher power front lamps.

[0036]LED driving system 1 comprises a DC / DC converter 10 and a set 20 of n LED strings, n being a natural integer number greater than or equal to two. Each string of set 20 comprises multiple LEDs connected in series. In the examples of figures 1a to 1c, 2, 3, n equals two. In the examples of figures 4a, 4b, n equals three. Additionally, it is also possible that a string comprises multiple LEDs connected in par...

Claims

1. A method for driving light sources comprising a set of at least two parallel strings (21, 22, 23) of light-emitting diodes, each string (21, 22, 23) having a forward voltage, wherein a first string (21) has a forward voltage higher than the forward voltage of the other strings (22, 23), the method comprising a step of operating the light sources in a low-voltage mode, consisting of inputting a current in the strings (21, 22, 23) at a voltage lower than the forward voltage of the first string (21), to enlighten at least one of the other strings (22, 23) and not the first string (21).

2. A method according to the preceding claim, wherein at least one of the other strings (22) is provided with a switch (4) or a current control means (D2, D3).

3. A method according to any one of the preceding claims, comprising: - a step of selecting one operating mode in a set of at least two operating modes, said set comprising mode A: enlighten at least the first string (21) and mode B: enlighten at least one of the other strings (22, 23) and not the first string (21), - a step of carrying out the selected operating mode according to the following operating instructions: * to carry out mode A (100), input (120) a current in the strings at a voltage higher than the forward voltage of the first string (21), then control (130) the current in the strings to manage the light-emitting diodes, * to carry out mode B (200), input (210) a current in the strings at a voltage lower than the forward voltage of the first string (21), then control (220) the current in the other strings (22, 23) to manage the light-emitting diodes.

4. A method for manufacturing a driving system able to drive a set of parallel strings (20) of light-emitting diodes, each having a string forward voltage, the method comprising the steps of: - selecting a string of light-emitting diodes having the highest forward voltage, thus obtaining a first string, distinct from the other strings, - connecting said strings to a DC / DC converter configured to operate either in a current control mode or in a voltage control mode, - connecting the DC / DC converter to a microcontroller able to toggle the DC / DC converter from the current control mode to the voltage control mode and vice-versa, said microcontroller comprising a processor and a memory readable by the processor, said memory containing a set of instructions implementing the method of claim 1 when executed by said processor.

5. A method for manufacturing a driving system according to the preceding claim, wherein after selecting the string of light-emitting diodes having the highest forward voltage, at least one of the other strings is provided with a current control mean or a switch, thus obtaining controlled strings.

6. A method for manufacturing a driving system according to claim 5, wherein the DC / DC converter, preferably the microcontroller, is connected to the other strings, said DC / DC converter being able to enable and disable the current-control means and the switch.

7. A driving system (1) for driving a set of at least two parallel strings (20) of light-emitting diodes, wherein each string (20, 21, 22, 23) has a forward voltage, wherein a first string (21) has a forward voltage higher than the forward voltage of the other strings (22, 23), said driving system comprising: -a DC / DC converter (10) configured to operate either in a current control mode or in a voltage control mode, -a microcontroller (12), connected to the DC / DC converter (10) and able to toggle the DC / DC converter (10) from the current control mode to the voltage control mode and vice-versa, said microcontroller (12) comprising a processor and a memory readable by the processor, said memory containing a set of instructions implementing the method of claim 1 when executed by said processor.

8. A driving system (1) according to the preceding claim, wherein at least one of other strings (22, 23) are provided with a current control means (D2, D3) or a switch (4), said microcontroller (12) being connected to the current control means (D2, D3) or switch (4) and being able to to enable and disable the current control means (D2, D3) or the switch (4).

9. A driving system (1) according to the preceding claim, wherein the current control means (D1, D2) can be positioned upstream or downstream of the corresponding string (22, 23).

10. A driving system (1) according to any one of preceding claims 8 and 9, wherein one of the consecutive strings (22, 23) can be arranged into at least two sub-strings to increase the forward voltage gap between the two consecutive strings.

11. A light source made of a set (20) of at least two parallel strings (21, 22, 23) of light-emitting diodes, wherein each string (21, 22, 23) has a forward voltage, wherein one string (21) has a forward voltage higher than the forward voltage of the other strings (22, 23), and wherein the other strings (22, 23) are provided with current control means (D2, D3) or a switch (4), said light source comprising a driving system (1) according to claim 7.

12. A vehicle lamp module comprising a light source according to claim 11.

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