LIGHT DEVICE FOR A MOTOR VEHICLE, COMPRISING A MATRIX LIGHT SOURCE

Abstract

The invention relates to a light device for a motor vehicle, comprising a matrix light source having a matrix of elementary light sources with an electroluminescent semiconductor element. Each elementary light source has an emitting surface that is less than or equal to 0.2 mm2. The measures proposed by the invention allow such a matrix light source to be powered without any risk of damage due to overheating of the semiconductive junctions of the elementary light sources that make up the light source.

Claims

1. A lighting device for a motor vehicle, comprising a matrix-array light source having a matrix array of electroluminescent semiconductor element-based elementary light sources, and a circuit for managing the supply of electrical power to at least one group of elementary light sources, the circuit for managing the supply of electrical power comprising a control unit and, for each elementary light source of the group, a switch element for selectively connecting said elementary light source to an electricity source, the control unit being further intended to control the open state of the switch elements by means of a pulse-width-modulation binary control signal, wherein the emitting area of each of the elementary light sources is smaller than or equal to 0.2 mm.sup.2, and in that the control unit is able to transmit a control signal having a frequency higher than or equal to 300 Hz.

2. The lighting device as claimed in claim 1, wherein the frequency of the control signal is higher than or equal to 1 kHz.

3. The lighting device as claimed in claim 1, wherein said group of elementary light sources comprises all of the elementary light sources of the matrix array of elementary light sources.

4. The lighting device as claimed in claim 1, wherein the circuit for managing the supply of electrical power is configured to control the open state of the switch elements of at least two groups of elementary light sources of the matrix array of elementary light sources in alternation.

5. The lighting device as claimed in claim 1, wherein the matrix-array light source comprises an integrated circuit in contact with the matrix array of light sources, and in that the integrated circuit comprises at least a portion of the circuit for managing the supply of electrical power to the elementary light sources.

6. The lighting device as claimed in claim 1, wherein the device comprises means for attenuating acoustic frequencies.

7. The lighting device as claimed in claim 5, wherein the integrated circuit comprises, for each of the elementary light sources of the matrix array of elementary light sources, a delay unit configured to delay, by a predetermined duration, the supply of power to the elementary source following reception of a command from the control signal.

8. The lighting device as claimed in claim 7, wherein the delay unit of each elementary light source is functionally connected to the delay unit of another elementary light source, the arrangement being such that the delay for the second elementary light source starts only to elapse once the delay of the first elementary light source has elapsed.

9. The lighting device as claimed in claim 7, wherein the delay for each elementary light source is identical.

10. The lighting device as claimed in claim 7, wherein the delay unit comprises a memory element for recording a delay value.

11. The lighting device as claimed in claim 2, wherein said group of elementary light sources comprises all of the elementary light sources of the matrix array of elementary light sources.

12. The lighting device as claimed in claim 2, wherein the circuit for managing the supply of electrical power is configured to control the open state of the switch elements of at least two groups of elementary light sources of the matrix array of elementary light sources in alternation.

13. The lighting device as claimed in claim 2, wherein the matrix-array light source comprises an integrated circuit in contact with the matrix array of light sources, and in that the integrated circuit comprises at least a portion of the circuit for managing the supply of electrical power to the elementary light sources.

14. The lighting device as claimed in claim 2, wherein the device comprises means for attenuating acoustic frequencies.

15. The lighting device as claimed in claim 6, wherein the integrated circuit comprises, for each of the elementary light sources of the matrix array of elementary light sources, a delay unit configured to delay, by a predetermined duration, the supply of power to the elementary source following reception of a command from the control signal.

16. The lighting device as claimed in claim 8, wherein the delay for each elementary light source is identical.

17. The lighting device as claimed in claim 8, wherein the delay unit comprises a memory element for recording a delay value.

18. The lighting device as claimed in claim 3, wherein the matrix-array light source comprises an integrated circuit in contact with the matrix array of light sources, and in that the integrated circuit comprises at least a portion of the circuit for managing the supply of electrical power to the elementary light sources.

19. The lighting device as claimed in claim 3, wherein the device comprises means for attenuating acoustic frequencies.

20. The lighting device as claimed in claim 9, wherein the delay unit comprises a memory element for recording a delay value.

Description

[0036] Other features and advantages of the present invention will be better understood with the aid of the description of the examples and of the drawings, in which:

[0037] FIG. 1 schematically shows a lighting device according to one preferred embodiment of the invention;

[0038] FIG. 2 schematically shows a section through a lighting device according to one preferred embodiment of the invention.

[0039] Unless specified otherwise, technical features that are described in detail for one given embodiment may be combined with the technical features that are described in the context of other embodiments described by way of example and without limitation. Similar reference numerals will be used to describe similar concepts across various embodiments of the invention. For example, the references 100 and 200 denote two embodiments of a lighting device according to the invention.

[0040] The description focuses on the elements of a lighting device for a motor vehicle which are necessary for an understanding of the invention. Other elements, which in a known manner form part of such devices, will not be mentioned or described in detail. It goes without saying, for example, that a lighting device comprises structural elements for the installation of the described components, or, for example, a heat dissipation element such as a radiator.

[0041] The illustration in FIG. 1 shows a lighting device 100 comprising a pixelated or matrix-array light source 105 according to one preferred embodiment of the invention. The matrix-array light source 105 comprises a plurality of electroluminescent semiconductor element-based elementary light sources 110 and a common substrate, not illustrated. Each elementary light source comprises an emitting area extending over 0.14×0.14 mm, or less. When an elementary light source of this size is traversed by an electric current with an intensity higher than or equal to the intensity of the forward current, it experiences a temperature gradient of the order of 20° C. after a few milliseconds, even one millisecond.

[0042] The matrix-array light source 105 preferably comprises a monolithic matrix-array component, in which the semiconductor layers of the elementary light sources 110 are, for example, arranged on the common substrate. The matrix array of elementary light sources 110 preferably comprises a parallel assembly of a plurality of branches, each branch comprising electroluminescent semiconductor light sources 110.

[0043] The device 100 comprises a circuit for managing the supply of electrical power 130 to a group of elementary light sources. In the example of FIG. 1, this group comprises all of the light sources of the matrix array 105, without, however, the invention being limited to this embodiment. The managing circuit comprises, for each elementary light source, a switch device which is controlled by means of a control signal 136 so as to take an open state or a closed state. The arrangement is such that, when the switch is in the closed state, the corresponding elementary light source is connected to an electricity source, while in the open state, the elementary light source is not connected thereto. Other arrangements are conceivable for controlling the light-emitting/off state of an elementary light source, without, however, departing from the scope of the present invention.

[0044] The electricity source may for example comprise a converter circuit configured to transform an input electric current/voltage, delivered for example by a source internal to the motor vehicle, such as a battery, into a load current/voltage, with an intensity/value suitable for supplying the matrix array 105 with electrical power.

[0045] A control unit 132 generates the control signal 136. The signal has a binary appearance, corresponding to the two distinct states that the switch elements 134 may take. It is a pulse-width-modulation (PWM) signal, the frequency of which is higher than or equal to 300 Hz, and preferably higher than or equal to 1 kHz. At this frequency, the phase during which a switch 134 is closed and during which the corresponding elementary light source 110 emits light and heats up is short enough that its semiconductor junction does not heat up by more than 20° C. This prevents damage to the junction. The heating phase, which is equivalent to the duration for which an elementary light source is supplied with power continuously, does not exceed the thermal time constant of said elementary light source. Electronic circuits able to generate PWM signals as just described are known per se and their arrangement and operation will not be described in greater detail in the context of this invention.

[0046] When the matrix-array light source is voltage-driven, the driving of each elementary source, or equivalently, of each pixel, merely entails controlling a switch device 134 as shown schematically in FIG. 1. By controlling the state of the device 134, the elementary light source 110 may be selectively connected to the voltage source Vin. The switch device is for example formed by a field-effect transistor of MOSFET type, preferably characterized by a low voltage drop between its drain and source terminals, and controlled by a control signal 136 from the circuit for managing the supply of power 132. The control signal 136 is a pulse-width-modulation (PWM) signal. This is a cyclic binary signal. The choice of the duty cycle, that is to say the respective duration of the non-zero phase and of the zero phase of the cycle, directly influences the average value of the signal, which is between the extreme values of the signal. The cyclic signal 136 forms a sequence of binary commands for opening/closing the switch device 134. The average intensity of the electric current flowing through the elementary light source 110, and therefore the average light intensity emitted by this elementary light source, reflects the average value of the PWM control signal 136.

[0047] By way of example and without limitation, the matrix array of elementary light sources 105 comprises, along the thickness of the substrate and starting at the end opposite the location of the elementary sources 110, a first electrically conductive layer deposited on an electrically insulating substrate. This is followed by an n-doped semiconductor layer whose thickness is between 0.1 and 2 μm. This thickness is much smaller than that of known light-emitting diodes, for which the corresponding layer has a thickness of the order of 1 to 2 μm. The following layer is the active quantum well layer having a thickness of around 30 nm, followed by an electron-blocking layer, and finally a p-doped semiconductor layer, the latter having a thickness of around 300 nm. Preferably, the first layer is an (Al)GaN:Si layer, the second layer is an n-GaN:Si layer, and the active layer comprises quantum wells made of InGaN alternating with barriers made of GaN. The blocking layer is preferably made of AlGaN:Mg and the p-doped layer is preferably made of p-GaN:Mg. n-Doped gallium nitride has a resistivity of 0.0005 ohm/cm, whereas p-doped gallium nitride has a resistivity of 1 ohm/cm. The thicknesses of the proposed layers make it possible in particular to increase the internal series resistance of the elementary source, while at the same time significantly reducing its manufacturing time, as the n-doped layer is not as thick in comparison with known LEDs and requires a shorter deposition time. By way of example, a time of 5 hours is typically required for MOCVD depositions for a standard-configuration LED with 2μ of n layer, and this time may be reduced by 50% if the thickness of the n layer is reduced to 0.2μ.

[0048] In order to achieve elementary light sources 110 having semiconductor layers having homogeneous thicknesses, the monolithic component 105 is preferably manufactured by depositing the layers homogeneously and uniformly over at least part of the surface of the substrate so as to cover it. The layers are deposited for example using a metal oxide chemical vapor deposition (MOCVD) method.

[0049] Such methods and reactors for implementing them are known for depositing semiconductor layers on a substrate, for example from patent documents WO 2010/072380 A1 or WO 01/46498 A1. Details on their implementation will therefore not be described in the context of the present invention. The layers thus formed arc then pixelated. By way of example and without limitation, the layers arc removed using known lithographic methods and by etching at the sites that subsequently correspond to the spaces between the elementary light sources 110 on the substrate. A plurality of several tens or hundreds or thousands of pixels 110 having an area smaller than one square millimeter for each individual pixel, and having a total area greater than 2 square millimeters, having semiconductor layers with homogeneous thicknesses, and therefore having homogeneous and high internal series resistances, are thus able to be produced on the substrate of a matrix-array light source 105. Generally speaking, the more the size of each LED pixel decreases, the more its series resistance increases, and the more this pixel is able to be driven by a voltage source. As an alternative, the substrate comprising the epitaxial layers covering at least part of the surface of the substrate is sawn or divided into elementary light sources, each of the elementary light sources having similar characteristics in terms of their internal series resistance.

[0050] The invention also relates to types of semiconductor element-based elementary light sources involving other configurations of semiconductor layers. In particular the substrates, the semiconductor materials of the layers, the arrangement of the layers, their thicknesses and any vias between the layers may be different from the example that has just been described.

[0051] The illustration in FIG. 2 shows a lighting device 200 comprising a pixelated light source or matrix-array light source 205 according to another preferred embodiment of the invention. The matrix-array light source 205 comprises a plurality of electroluminescent semiconductor element-based elementary light sources 210, of LED type, and a common substrate, not illustrated. Each elementary light source comprises an emitting area extending over 0.14×0.14 mm, or less. When an elementary light source of this size is traversed by an electric current greater than or equal to the intensity of the forward current, it experiences a temperature gradient of the order of 20° C. after a few milliseconds, even one millisecond or less.

[0052] The device 200 comprises a circuit for managing the supply of electrical power 230 to two groups of elementary light sources. In the example of FIG. 2, a first the elementary light sources of the first group 210 alternate with the elementary light sources 210′ of the second group. The managing circuit comprises, for each elementary light source, a switch device 234, 234′ which is controlled by means of a control signal 236, 236′, respectively, so as to take an open state or a closed state. The arrangement is such that, when the switch is in the closed state, the corresponding elementary light source is connected to an electricity source, while in the open state, the elementary light source is not connected thereto.

[0053] A control unit 232 generates the control signal 236. The signal has a binary appearance, corresponding to the two distinct states that the switch elements 234 may take. It is a pulse-width-modulation (PWM) signal, the frequency of which is higher than or equal to 300 Hz, and preferably higher than or equal to 1 kHz. At this frequency, the phase during which a switch 234 is closed and during which the corresponding elementary light source 210 emits light and heats up is short enough that its semiconductor junction does not heat up by more than 20° C. This prevents damage to the junction.

[0054] The control unit 232 also generates the PWM-type control signal 236 intended for the second group of light sources 210′. The signals 236 and 236 are such that the two groups of elementary light sources 210, 210′ are not always connected together to the electricity source. For example, the signal 236 may have a duty cycle of 50% and be formed of one half-cycle intended to control the switch devices so that they take their closed state, followed by a second half-cycle intended to control the switch devices so that they take their open state. The signal 236′ may then be a copy shifted by a half-cycle of the signal 236, so that the groups of elementary light sources 210, 210′ arc supplied with electricity in alternation. The signal 236′ may in this case be generated on the basis of the signal 236 by means of a delay circuit, known in the prior art. Other embodiments are conceivable without, however, departing from the scope of the invention. The advantage of this arrangement is that the elementary light sources of one group 210 do not always contribute to the heating of the elementary light sources of another group 210′, the elementary light sources of which are spatially very close to those of the first group. This arrangement also applies to a larger plurality of groups of elementary sources within the matrix array 205.

[0055] In the embodiment of FIG. 2, the circuit for managing the supply of electrical power 230 is housed by an integrated circuit 220, soldered to the lower face of the common substrate, which houses the elementary light sources 210, 210′ on its upper face, so as to establish mechanical and electrical contact with the substrate and the elementary light sources.

[0056] Using an integrated circuit 120 in mechanical and electrical contact with the substrate on which the elementary light sources reside makes it possible to dispense with wired connections, the number of which would be at least equal to the number of pixels of the matrix-array light source. Since the managing circuit 230 is spatially close to the matrix array 205 of elementary light sources 210, 210′, the control times are negligible. Specifically, the components of the circuit 230 are located a few micrometers below the elementary light sources.

[0057] Preferably, a power supply circuit may be integrated into the substrate when the monolithic component 205 is manufactured.

[0058] According to another embodiment which is not illustrated, the shift between the respective on times of the groups of elementary light sources (210, 210′ in the case of FIG. 2) is produced by a delay unit integrated beneath each light source. A PWM-type control signal such as just described for other embodiments is used to control the elementary light sources. However, the delay units of each elementary light source apply the control only after their respective delay has elapsed. The arrangement may preferably be such that the delay units are functionally connected in a chain, and such that the delay of one elementary light source starts to elapse only once the delay of the preceding elementary light source of the chain has elapsed. In this way, the simultaneous heating of all of the elementary light sources is also reduced.

[0059] It goes without saying that the integrated circuit may comprise other electronic circuits and/or memory elements used for other functions in connection with the matrix-array light source and/or with the elementary light sources. This includes but is not limited to circuits for detecting a short circuit or an open circuit fault with an elementary light source.

[0060] The scope of protection is defined by the claims.