EP4489524A1

EP4489524A1 - Protected lighting circuit

Info

Publication number: EP4489524A1
Application number: EP23184326.9A
Authority: EP
Inventors: Pierrick GEORGE; Eric BUSSON; Mehdi ABDELMOULA
Original assignee: Valeo Vision SAS
Current assignee: Valeo Vision SAS
Priority date: 2023-07-07
Filing date: 2023-07-07
Publication date: 2025-01-08
Also published as: WO2025012054A1; CN121488591A

Abstract

The invention relates to a lighting circuit comprising a first branch of LEDs 20 serially mounted with a second branch of LEDs 30 having at least two parallel LED branches 30A and 30B. Each parallel LED branch has two extremities, one of the extremities being connected to a reference voltage. An unbalance detection circuit 40 is connected to the two parallel LED branches 30A and 30B for measuring a difference of voltage and/or current between said parallel LED branches 30A and 30B, and for activating a control signal when a difference is detected. A switch 50 is placed for short-circuiting the parallel LED branches 30A and 30B when the control signal is activated. The invention also relates to an automotive lighting device and to an automotive vehicle incorporating said lighting circuit.

Description

Technical Field

The invention relates to a protected lighting circuit for avoiding cascading problems in case of failure of a lighting element. More Particularly, the invention relates to lighting circuits using LEDs and having a branch of LEDs serially mounted with at least two parallel LED branches.

Background Art

Light Emitting Diodes (hereinafter named LEDs) are commonly used for lighting. They can be mounted serially or in parallel and a LED driver supplies a regulated current or a regulated voltage for powering the LEDs. In case of failure of a LED creating an open or a short circuit, the LED driver can monitor the voltage and/or the current for adapting the output current or voltage for avoiding a more serious problem. Depending on the LED circuit type and of the LED driver type, the LED driver may reduce current or voltage, or simply switch off.
For ornamental reasons, a LED circuit may comprise parallel LED branches connected serially to another branch of LED as shown in figure 1. In such a configuration, it becomes difficult for the LED driver to adapt the current or the voltage for having a degraded mode of lighting which could be acceptable for lighting. More particularly, when a LED is in open or short circuit in a parallel branch, all the current will flow either in this or in the other parallel branch with an increase or a decrease of the total voltage that can be detected by the LED driver for reducing the current but, in that case, the current will be strongly reduced in the serial branch. If the serial branch constitutes a main light source and the parallel branches are only for ornamentation, it is not acceptable to make such a light reduction.

Summary of the Invention

The invention provides a lighting circuit comprising a first branch of Light Emitting Diodes (hereinafter LEDs) serially mounted with a second branch of LEDs having at least two parallel LED branches, each parallel LED branch having two extremities, one of the extremities being connected to a reference voltage. The lighting circuit further comprises an unbalance detection circuit and a switch. The unbalance detection circuit is connected to the at least two parallel LED branches for measuring a difference of voltage and/or current between said parallel LED branches and for activating a control signal when a difference is detected. The switch short-circuits the parallel LED branches when the control signal is activated.
With such a configuration, the unbalance detection circuit can be configured to detect a LED failure in the second branch of LEDs.
As an advantage, the lighting circuit can be configured to maintain an on-state in the first branch of LEDs, and to turn-off the second branch of LEDs based on the detected LED failure.
According to a specific embodiment, the unbalance detection circuit can comprise at least two comparators and a control circuit. Each parallel LED branch can comprise a pick-up point which is located at the same place for all parallel LED branches. Each comparator can compare a voltage at the pick-up point of one of the at least two parallel LED branches with a voltage at the pick-up point of another of the at least two parallel LED branches.
An output of the comparator can be active when the voltage of one parallel LED branch is higher than the voltage of another parallel LED branch, the comparators being respectively connected to each of the parallel LED branches. The control circuit can be connected to the output of all comparators for providing, at an output, the control signal which is activated if one of the outputs of the comparators is active.
For making the comparison on a current, each parallel LED branch can comprise a sense resistor serially mounted with LEDs, the sense resistor having one terminal corresponding to an extremity of the parallel LED branch which is connected to the reference voltage and the other terminal of the sense resistor connected to the pick-up point.
For supplying the unbalance detection circuit and the switch on the current flowing through the lighting circuit, each comparator can comprise a transistor having a base connected to the pick-up point of one of the at least two parallel LED branches, an emitter connected to the pick-up point of another of the at least two parallel LED branches, and a collector forming the output of the comparator and connected to the control circuit. The sense resistors can have a resistance determined for providing a voltage higher than a threshold voltage of the transistor of each comparator when said sense resistor is crossed by a current flowing through its parallel LED branch when one parallel LED branch is off.
The control circuit can comprise a first transistor and a second transistor, a base of the first transistor being connected to the outputs of all comparators, an emitter of the first transistor being connected to one extremity of the at least two parallel LED branches, a collector of the first transistor being connected to the base of the second transistor, an emitter of the second transistor being connected to another extremity of the at least two parallel LED branches which is connected to the reference voltage, the collector of the second transistor constituting the output of the control circuit.
For reducing the consumption, the control circuit can comprise an entry resistor serially connected at its input and having a resistance sufficiently high in such a way the resistance of the sense resistor is negligible compared to the resistance of the entry resistor.
In case of a pulsed power supply, the control circuit can comprise a memorization capacitor connected between the collector of the first transistor and the extremity of the at least two parallel LED branches which is connected to the reference voltage to be charged through the collector of the first transistor and discharged through the base of the second transistor.
When the lighting circuit is powered by a LED driver using pulse wide modulation (PWM) of the current, then the memorization capacitor can be sized to maintain a sufficient voltage on the base of the second transistor to maintain the control signal activated for at least one PWM cycle.
For reducing the size of the charging capacitor, the control circuit can comprise a discharging resistor serially connected between the collector of the first transistor and the base of the second transistor, the memorization capacitor being connected to the base of the second transistor through the discharging resistor.
Preferentially, the switch can comprise a switching transistor having a base connected to the output of the control circuit, an emitter connected to one extremity of the parallel LED branches, and a collector connected to the pick-up point of one of the at least two parallel LED branches.
According to a specific embodiment, the reference voltage can be the ground. The transistor of each comparator and the second transistor can be NPN type transistor. The first transistors and the switch transistor can be PNP type transistors.
In a particular application to an automotive vehicle, the first branch of LEDs corresponds to a light source of a position light and the second branch of LEDs corresponds to a light source backlighting a logo in an automotive vehicle.
Another object of the invention is an automotive lighting device comprising a position light and a light source for backlighting a logo affixed to an automotive vehicle. The automotive lighting device comprises a LED driver and a lighting circuit according to the invention, the first branch of LEDs corresponding to the position light and the second branch of LEDs corresponding to the light source for backlighting the logo.
The invention is also an automotive vehicle comprising at least one position light and at least one logo affixed near the position light. The automotive vehicle comprises at least one lighting circuit according to the invention, in which the first branch of LEDs is a light source for one position light and the second branch of LEDs is a light source for backlighting the logo.

Brief description of the Drawings

The invention will be detailed with reference to the annexed drawings in which:

Figure 1 shows a LED circuit comprising serial and parallel connections of the LED according to prior art,
Figure 2 shows an example of application of the invention to an automotive vehicle,
Figure 3 shows the LED circuit of figure 1 modified according to the invention,
Figure 4 details a first embodiment of the invention,
Figure 5 details an improvement of the embodiment of figure 4,
Figure 6 shows an improvement of the embodiment of figure 5,
Figure 7 shows another improvement of the embodiment of figure 6,
Figure 8 is a variant of the embodiment of figure 5.

Detailed Description of the invention

For simplifying the specification, a same reference is used in several drawings for designating a same element or a similar element. In addition, the element already described in one figure will not necessarily be described in a subsequent figure.
As indicated before, the prior art corresponds to figure 1 which is improved by the invention. The figure 1 corresponds to a LED arrangement used for lighting and for ornamental reason. The LED arrangement comprises a LED driver 10 connected to a first branch of LED 20 serially mounted with a second branch of LED 30 comprising two parallel LED branches 30A and 30B. Each LED branch 20, 30A and 30B comprises a plurality of LEDs serially mounted as well known in the art. For regulating the power of light, the LED driver 10 is preferably a regulated current source. In an embodiment, the LED driver 10 is preferably a regulated voltage source.
In this figure 1, the first branch of LED 20 constitutes the main source of light while the second branch of LEDs 30 constitutes an ornamental source of light. The main source of light provides a light more important than the ornamental source. So, the LED driver 10 regulates the current to correspond to an optimal current for the first LED branch 20. In case of failure of a LED, the LED driver may adjust the current in such a way the current will continue to be optimal for the main source of light. In case of a failure of one LED which transforms a LED in a short circuit or in an open circuit, the LED driver 10 can compensate the failure or it can switch off as commonly known in the art.
A failure of one LED in the first LED branch 20 is automatically compensated by the LED driver. If one LED fails and is transformed in a short circuit, the LED driver 10 maintains the nominal current and reduces the voltage. If one LED fails and is transformed in an open circuit, the LED driver 20 switches off and the LED circuit must be changed or repaired.
For a failure of one LED in a parallel LED branch 30A or 30B, the adjustment by the LED driver 10 could be used but it cannot be an optimum solution. As an example, if one LED of the parallel LED branch 30A fails and is transformed in a short circuit, the voltage in the parallel LED branch 30A will instantaneously fall from one LED voltage causing the switch-off of the parallel LED branch 30B and the doubling of the current flowing in the parallel LED branch 30A. As another example, if one LED of the parallel LED branch 30A fails and is transformed in an open circuit, the current in the parallel LED branch 30A will instantaneously stop causing the doubling of the current flowing in the parallel LED branch 30B. The doubling of the current in one parallel LED branch 30A or 30B doubles the ornamental light intensity (which is not desirable) but the doubled current may also be out of the current range of the LEDs of the parallel LED branches 30A and 30B causing destruction of remaining LEDs and a risk of firing of the LEDs.
At the driver side, the failure of a LEDs in a parallel LED branch 30A or 30B could be detected because the voltage/current changes are not the same than a failure of a LED from the first LED. Nevertheless, the only adjustment to do is to reduce the current provided by the LED driver 10. Reducing the current automatically reduces the current in the first LED branch 20 and decreases the lighting power which is not acceptable in the example of application of the invention shown in figure 2.
Figure 2 shows an automotive vehicle 100 comprising a position light 110 using LED technology and wherein the light source of said position light is the first branch of LEDs 20. The automotive vehicle 10 comprises a logo 120, which can be a trademark corresponding to the name of the vehicle or the manufacturer brand. For ornamental reason, the logo 120 is illuminated when position lights 110 are switched on. To this aim the logo 120 is made in a translucent material for illuminating the logo by backlighting.
The logo 120 being light-on simultaneously with the position light 110, a same LED driver 10 is used for economical reason and the logo backlighting is performed by the second branch of LEDs 30 which is located behind the logo 120. The use of parallel LED branches 30A and 30B is preferred because there is no need to have the same light power for illuminating the logo and for position light. But in this example, a failure of a LED in the second branch of LEDs 30 must not cut nor reduce the light produced by the first branch of LEDs 20.The invention provides a solution for maintaining an on-state in the first branch of LEDs 20 corresponding to the position light, and while turning-off the second branch of LEDs 30 in case of a LED failure in said second branch 30. The solution is presented in relation with figure 3. Figure 3 shows a LEDs arrangement which comprises the LED driver 10 connected to the first branch of LED 20 serially mounted with the two parallel LED branches 30A and 30B connected as indicated in figure 1 and which further comprises an unbalance detection circuit 40 and a switch 50. The unbalance detection circuit 40 is connected to the two parallel LED branches 30A and 30B for measuring a difference of voltage and/or a difference current between the parallel LED branches 30A and 30B. The switch 50 has two terminals respectively connected to the two extremities of the parallel LED branches 30A and 30B for bypassing the parallel LED branches 30A and 30B and a control terminal connected to an output of the detection circuit.
Typically, when all the LEDs are functional, the current is approximately the same in the two parallel LED branches and the voltage at the terminals of each LED or the parallel LED branches 30A and 30B is approximately the same. In case of failure of a LED, only one of the two branches 30A and 30B is traversed by the current while the other of the two branches 30A and 30B is blocked. In an embodiment, the unbalance detection circuit 40 is configured to detect whether the current is approximately the same at two common middle points in the two parallel LED branches 30A and 30B. In another embodiment, the unbalance detection circuit 40 is configured to detect whether the voltage is approximately the same at the two common mid-points in the two parallel LED branches 30A and 30B. In another embodiment, the unbalance detection circuit 40 is configured to detect whether both the current and the voltage is approximately the same at the two common middle points in the two parallel LED branches 30A and 30B. If the current or if the voltage is approximately the same, the output of the unbalance detection circuit 40 is inactive and the switch 50 is open. If the current or if the voltage is not the same, the output of the unbalance detection circuit 40 becomes active and the switch 50 is closed to bypass the parallel LED branches 30A and 30B.
In case of failure of one LED of the parallel LED branches 30A and 30B, the unbalance detection circuit 40 activates the switch 50 for bypassing the parallel LED branches 30A and 30B. Such a bypassing is detected by the LED driver 10 and an adjustment of voltage and current will be made for maintaining the nominal current into the first LED branch 20. In that case, the main light source is maintained optimal while the ornamental light source is switched off.
Now a first detailed embodiment will be disclosed in relation with figure 4. In this first embodiment, each parallel LED branch 30A or 30B comprises a pick-up point located between the LEDs. Preferably, the pick-up point is located at the middle of each parallel LED branch 30A or 30B, but it could be located at any place having a voltage which is dependent on the fact that a current flows through the parallel LED branch 30A or 30B. For simplifying a comparison, the pick-up point is preferably located at the same place for the two parallel LED branches 30A and 30B.
On this figure 4, the unbalance detection circuit 40 comprises two comparators 60A and 60B and a control circuit 70. Each comparator 60A or 60B comprises a first input, a second input and an output. The control circuit 70 comprises a first input, a second input and output. The first input of the comparator 60A is connected to the pick-up point of parallel LED branch 30A and the second input of the comparator 60A is connected to the pick-up point of parallel LED branch 30B. The first input of the comparator 60B is connected to the pick-up point of parallel LED branch 30B and the second input of the comparator 60B is connected to the pick-up point of parallel LED branch 30A. The outputs of the comparators 60A and 60B are respectively connected to the first and second inputs of the control circuit 70. The output of the control circuit 70 is connected to the control terminal of the switch 50.
The comparators 60A and 60B can be operational amplifiers used in comparator mode, hysteresis comparators, Schmitt triggers or any other type of circuit capable to compare a first voltage with another voltage and to have an output that becomes active when the voltage on a first input is superior to the voltage at the second input. Thus, each comparator 60A or 60B compares the voltage at the pick-up point of one of the two parallel LED branches 30A and 30B with a voltage at the pick-up point of the other of the two parallel LED branches 30B or 30A and provides an active output signal if the voltage at the pick-up point of one of the two parallel LED branches 30A and 30B is higher than the voltage at the pick-up point of the other of the two parallel LED branches 30B or 30A.
The control circuit 70 is a logic circuit with memorization capability. After power on the control circuit, the output of the control circuit 70 is in an inactive state in such a way the switch 50 remains open. If one of the first and second inputs of the control circuit 70 receive an active signal, then the output of the control circuit 70 becomes active and remains active for closing and for maintaining closed the switch 50.
The circuit of figure 4, using comparators circuits and logic control circuit needs to be powered for functioning and necessitate to have a power supply near the parallel LED branches, which is not practical. The circuit of figure 5 shows a second embodiment having the same functionalities than figures 2 and 3 but which is powered directly by the current supplied for the LEDs.
The circuit of figure 5 makes a comparison of the current between the two parallel LED branches 30A and 30B. In view to measure the current, each of the parallel LED branches 30A and 30B comprises a sense resistor 31A and 31B serially mounted with LEDs. The sense resistors 31A and 31B are identical and located at a same extremity of the parallel LED branches 30A and 30B, one terminal of the sense resistors 31A and 31B being connected to a ground voltage, and the other terminal of the sense resistors 31A and 31B being connected to the pick-up point. Thus, the sense resistors transform the current into voltage that can be compared by any kind of comparator.
The example of figure 5 provides an improvement of the circuit of figure 4 which is capable to function under the LED supply only. Each of the comparators 60A and 60B comprises a transistor 61A or 61B, for example a bipolar transistor of NPN type. The base of each transistor 61A or 61B corresponds to the first input of each comparator 60A or 60B. The emitter of each transistor 61A or 61B corresponds to the second input of each comparator 60A or 60B. The collector of each transistor 61A or 61B corresponds to the output of each comparator 60A or 60B. For triggering the transistors 61A and 61B, it is necessary to have a difference between the pick-up points of the two parallel LED branches that is higher than the triggering voltage between base and emitter of each transistor 61A or 61B. The sense resistors 31A and 31B have a resistance which is sufficiently high for providing a voltage higher than the threshold voltage of the transistors 61A and 61B when said sense resistors 31A and 31B are crossed by the maximum current flowing through the parallel LED branches 30A and 30B in case of failure, this maximum current corresponding to the nominal current of the first LED branch 20.
In figure 5, the control circuit 70 comprises a first transistor 71 and a second transistor 72. The first transistor 71 is for example a bipolar transistor of PNP type while the second transistor is for example a bipolar transistor of NPN type. The base of the first transistor 71 is connected to the collectors of the transistors 61A and 61B. The emitter of the first transistor 71 being connected to the extremity of the two parallel LED branches 30A and 30B, which is opposed to the ground voltage. The collector of the first transistor 71 is connected to the base of the second transistor 72. The emitter of the second transistor 72 is connected to the ground voltage. The collector of the second transistor 72 corresponds to the output of the control circuit 70.
The switch 50 comprises a switching transistor 51, for example a bipolar of PNP type. The base of the switching transistor 51 is connected to the collector of the second transistor 72. The emitter of the switching transistor 51 is connected to the extremity of the two parallel LED branches 30A and 30B, which is opposed to the ground voltage. The collector of the switching transistor 51 is connected to the pick-up point of one of the two parallel LED branches 30B.
In normal use (i.e., without LED failure), the current in the two parallel LED branches 30 and 30B is the same, causing a same voltage at the pick-up points of each parallel LED branches 30A and 30B. Transistors 61A and 61B are blocked, no current is drawn on the base on the first transistor 71, blocking the transistor 71. The collector of the first transistor 71 being in high impedance, no voltage is applied on the base of the second transistor 72, which is also blocked. The collector of the second transistor 72 being in high impedance, the switching transistor 51 is blocked.
As previously explained in case of a LED failure, the current stops to flow for example in the parallel LED branch 30B and double in the parallel LED branch 30A for attaining the nominal current of the first LED branch 20. The transistor 61A turns on and a current is drawn on the base of the first transistor 71, which turns on too. First transistor 71 being switched on, the second transistor 72 turns on, making the switching transistor 51 turning on. The switching transistor 51 being on, the voltage at the extremities of parallel LED branches 30A and 30B is not sufficient for maintaining the on state of the LEDs. Thus, the entire nominal current flows through the transistor 51 and the sense resistor 31B. The transistor 61A is blocked and the transistor 61B is turned on holding first transistor 71, second transistor 72 and switch transistor 51 in the on state.
In case of failure of one LED, all the nominal current is diverted through all the transistors and through the sense resistor 31B with an equilibrium of currents between the transistors made automatically. The dissipated power is equal to the nominal current multiplied by the voltage made at the equilibrium of current in the transistors. But a part of the power is dissipated in the sense resistor 31A and 31B. In particular, the sizing of the sense resistor 31A and 31B is made to provide a voltage higher than the threshold voltage of transistors 61A and 61B when crossed by the nominal current. But this voltage should be higher than the threshold voltage of transistor 61A or 61B added to the voltage of a sense resistor 31A or 31B crossed by the emitter current of the transistor 61A or 61B when said transistor 61A or 61B is on. For reducing this voltage, one solution consists in reducing the emitter current of transistors 61A and 61B when they are on. For that reason, the control circuit 70 may have an entry resistor 73 serially connected at its input between the base of the first transistor 71 and the collectors of transistors 61A and 61B. Such an entry resistor 73 reduces the current flowing into sense resistor 31B, preferentially the resistance of entry resistor 73 is sufficiently high in such a way the resistance of the sense resistors 31A and 31B is negligible compared to the resistance of the entry resistor 73.
The use of an entry resistor 73 has also the advantage to reduce the base current of the first transistor 71 and thus to reduce the power dissipated by first and second transistors 71 and 72, enabling to choose smaller transistors than the switch transistor 51.
As it can be understood by a person skilled in the art, it exists a switching time for bypassing the parallel LED branches 30A and 30B. This switching time is not critical if the LED driver provides a pure DC current. But for having an easier adjustment of the current and/or voltage by the LED driver, it is well known to use pulsed signal. In case of pulsed signal, the switching time will be repeated for each pulse, such a repetition will cause a repetition of overcurrent in the LEDs of the parallel LED branch 30A or 30B where the current flows. For preventing such a repetition, figure 6 proposes a variant of the circuit of figure 5 in which a memorization of the failure of a LED is made between two pulses of power supply.
The circuit of figure 6 reproduces all elements of figure 5. In addition, the control circuit 70 comprises a memorization capacitor 74 connected between the collector of the first transistor 71 and the ground voltage to be charged through the collector of the first transistor 71 and discharged through the base of the second transistor 72. Such a memorization capacitor 74 can maintain a voltage on the base of the second transistor 72 between two pulses of the current supply for holding the switching transistor 51 on.
For using a smaller memorization capacitor 74, the control circuit 70 may comprise a discharging resistor 75 serially connected between the collector of the first transistor 71 and the base of the second transistor 72, the memorization capacitor 74 being connected to the base of the second transistor 72 through this discharging resistor 75. The memorization capacitor 74 being discharged through the discharging resistor 75, it is easier to adjust the capacity value of the memorization capacitor 74 for fitting with the time period of the power supply pulses.
As explained before, if a LED failure causes the switching-off of parallel LED branch 30B, the transistors 61A causes the commutation of the switching transistor 51 but the transistor 61B is in charge to keep the on-state of the switching transistor 51. This commutation of transistor 61A to transistor 61B may cause some trouble, commonly known as "glitches", which are depending on the commutation time of the transistors. To avoid any problem, an improvement is provided in relation with figure 7, which is based on the circuit of figure 6.
Each comparator 60A or 60B can comprise stabilization resistors R1 and R2 and a stabilization capacitor C1. The stabilization resistor R1 and the stabilization capacitor C1 are mounted in parallel on the base and the emitter of transistors 61A and 61B. The stabilization resistor R2 is connected between the base of transistor 61A, or respectively 61B, and the pick-up point of the parallel LED branch 30B, or respectively 30A. The control circuit 70 can comprise stabilization resistors R3 and R4 and stabilization capacitor C3 and C4. The stabilization resistor R3 and the stabilization capacitor C3 are mounted in parallel on the base and the emitter of the first transistor 71. The stabilization resistor R4 and the stabilization capacitor C4 are mounted in parallel on the base and the emitter of the second transistor 72. The switch 50 can comprise stabilization resistors R5 and R6 and a stabilization capacitor C5. The stabilization resistors R1, R2, R3, R4, R5 and R6 and the stabilization capacitors C1, C3, C4 and C5 are used for adding commutation delay of to the transistors 61A, 61B, 71, 72 and 51 for preventing eventual glitches during the propagation of the communication states of the transistors and, in particular, when the switching transistor 51 turns on and changes the polarization voltage of all other transistors 61A, 61B, 71 and 72.
The control circuit 70 may comprise a charging resistor 76 serially connected between the collector of the first transistor 71 and the discharging resistor 75, the memorization capacitor 74 being connected to the node between the charging resistor 76 and the discharging resistor 75. The memorization capacitor 74 is charged through the charging resistor 76 in such a way to prevent sudden abnormal increase in voltage between the commutation of the first transistor 71 and the commutation of the switching transistor 51.
The invention was describing a LED circuit having two parallel LED branches 30A and 30B but it can be used with more parallel LED branches, even if the increase of parallel LED branches reduces the overcurrent in the other parallel LED branches when a LED fails. A variant of a LED arrangement according to the invention is shown on figure 8 to explain how to adapt the invention to more than two parallel LED branches. The figure 8 differs from the figure in the addition of a third parallel LED branch 30C and a third comparator 60C. The comparators 60A, 60B and 60C are circularly connected to the parallel LED branches 30A, 30B and 30C. In other words, the first inputs of comparators 60A, 60B and 60C are respectively connected to the pick-points of parallel LED branches 30A, 30B and 30C, while the second inputs of comparators 60A, 60B and 60C are respectively connected to the pick-points of parallel LED branches 30C, 30A and 30B. The person skilled in the art can observe that only one additional comparator 60X is necessary for any additional parallel LED branch 30X.
Other variants of circuit can be made by someone skilled in the art without departing from the scope of the invention. The improvement shown on figures 6 and 7 can be used with the arrangement of figure 8. The circuits of figures 4 to 8 show embodiments using preferred circuits made with bipolar transistors and the person skilled in the art can combine the comparators 60A, 60B and 60C with another control circuit 70. Also, all NPN transistors can be replaced by PNP transistors and PNP transistors can be replaced by NPN transistors, in that cases the resistors should be located at the extremity corresponding to the highest voltage of the parallel LED branches 30A, 30B and 30C. The switch transistor 51 could be also replaced by a MOSFET. In addition, the example of use in relation with figure 2 is related to position light of car headlights but the person skilled in the art could understand that the invention is not limited to front light or rear light or turn indicator or position light or daytime running lamps nor limited to automotive.

Claims

A lighting circuit comprising a first branch of Light Emitting Diodes (hereinafter LEDs) (20) serially mounted with a second branch of LEDs (30) having at least two parallel LED branches (30A, 30B, 30C), each parallel LED branch having two extremities, one of the extremities being connected to a reference voltage, wherein the lighting circuit further comprises:
- an unbalance detection circuit (40) connected to the at least two parallel LED branches (30A, 30B, 30C) for measuring a difference of voltage and/or current between said parallel LED branches (30A, 30B, 30C) and for activating a control signal when a difference is detected,

- a switch (50) for short-circuiting the parallel LED branches (30A, 30B, 30C) when the control signal is activated.
The lighting circuit according to claim 1, wherein the unbalance detection circuit (40) is configured to detect a LED failure in the second branch of LEDs (30).
The lighting circuit according to claim 2, wherein the lighting circuit is configured to maintain an on-state in the first branch of LEDs (20), and to turn-off the second branch of LEDs (30) based on the detected LED failure.
The lighting circuit according to one of the claims 1 to 3, wherein the unbalance detection circuit (40) comprises at least two comparators (60A, 60B, 60C), and a control circuit (70), and wherein each parallel LED branch (30A, 30B, 30C) comprises a pick-up point which is located at the same place for all parallel LED branches (30A, 30B, 30C), each comparator (60A, 60B, 60C) comparing a voltage at the pick-up point of one of the at least two parallel LED branches (30A, 30B, 30C) with a voltage at the pick-up point of another of the at least two parallel LED branches (30A, 30B, 30C), an output of the comparator (60A, 60B, 60C) being active when the voltage of one parallel LED branch (30A, 30B, 30C) is higher than the voltage of another parallel LED branch (30C, 30A, 30B), the comparators being respectively connected to each of the parallel LED branches (30A, 30B, 30C), the control circuit (70) being connected to the output of all comparators (60A, 60B, 60C) and providing at an output the control signal which is activated if one of the outputs of the comparators (60A, 60B, 60C) is active.
The lighting circuit according to claim 4, wherein each parallel LED branch (30A, 30B, 30C) comprises a sense resistor (31A, 31B, 31C) serially mounted with LEDs, the sense resistor (31A, 31B, 31C) having one terminal corresponding to an extremity of the parallel LED branch (30A, 30B, 30C) which is connected to the reference voltage and the other terminal of the sense resistor (31A, 31B, 31C) connected to the pick-up point.
The lighting circuit according to claim 5, wherein each comparator (60A, 60B, 60C) comprises a transistor (61A, 61B, 61C) having a base connected to the pick-up point of one of the at least two parallel LED branches (30A, 30B, 30C), an emitter connected to the pick-up point of another of the at least two parallel LED branches (30A, 30B, 30C), and a collector forming the output of the comparator (60A, 60B, 60C) and connected to the control circuit,
and wherein the sense resistors (31A, 31B, 31C) have a resistance determined for providing a voltage higher than a threshold voltage of the transistor (61A, 61B, 61C) of each comparator (60A, 60B, 60C) when said sense resistor (31A, 31B, 31C) is crossed by a current flowing through its parallel LED branch (30A, 30B, 30C) when one parallel LED branch is off.
The lighting circuit according to one of the claims 4 to 6, wherein the control circuit (70) comprises a first transistor (71) and a second transistor (72), a base of the first transistor (71) being connected to the outputs of all comparators (60A, 60B, 60C), an emitter of the first transistor (71) being connected to one extremity of the at least two parallel LED branches (30A, 30B, 30C), a collector of the first transistor (71) being connected to the base of the second transistor (72), an emitter of the second transistor (72) being connected to another extremity of the at least two parallel LED branches (30A, 30B, 30C) which is connected to the reference voltage, the collector of the second transistor (72) constituting the output of the control circuit (70).
The lighting circuit according to claim 6 or to claims 5 and 7, wherein the control circuit (70) comprises an entry resistor (73) serially connected at its input and having a resistance sufficiently high in such a way the resistance of the sense resistor (31A, 31B, 31C) is negligible compared to the resistance of the entry resistor (73).
The lighting circuit according to one of the claims 7 to 8, wherein the control circuit (70) comprises a memorization capacitor (74) connected between the collector of the first transistor (71) and the extremity of the at least two parallel LED branches (30A, 30B, 30C) which is connected to the reference voltage to be charged through the collector of the first transistor (71) and discharged through the base of the second transistor (72).
The lighting circuit according to claim 9, wherein the lighting circuit is powered by a LED driver (10) using pulse wide modulation (PWM) of the current and wherein the memorization capacitor (74) is sized to maintain a sufficient voltage on the base of the second transistor (72) to maintain the control signal activated for at least one PWM cycle.
The lighting circuit according to of the claims 9 to 10, wherein the control circuit (70) comprises a discharging resistor (75) serially connected between the collector of the first transistor (71) and the base of the second transistor (72), the memorization capacitor (74) being connected to the base of the second transistor (72) through the discharging resistor (75).
The lighting circuit according to one of the claims 5 to 11, wherein the switch (50) comprises a switching transistor (51) having a base connected to the output of the control circuit (70), an emitter connected to one extremity of the parallel LED branches (30A, 30B, 30C), and a collector connected to the pick-up point of one of the at least two parallel LED branches (30A, 30B, 30C).
The lighting circuit according to one of the claims 5 to 12 wherein the reference voltage is the ground, wherein the transistor (61A, 61B, 61C) of each comparator (60A, 60B, 60C) and the second transistor (72) are NPN type transistor, and wherein the first transistors (71) and the switch transistor (51) are PNP type transistors.
The lighting circuit according to one of the claims 1 to 13 wherein the first branch of LEDs (20) corresponds to a light source of a position light and the second branch of LEDs (30) corresponds to a light source backlighting a logo in an automotive vehicle.
An automotive lighting device comprising a position light and a light source for backlighting a logo affixed to an automotive vehicle, wherein the automotive lighting device comprises a LED driver and a lighting circuit according to one of the claims 1 to 13, and wherein the first branch of LEDs corresponds to the position light and the second branch of LEDs corresponds to the light source for backlighting the logo.