Abstract:
A circuit is provided for actuating a lighting device in a motor vehicle. The circuit includes first and second light sources, that may include one or more light-emitting diodes. The light sources are connected in parallel, and each have a respective heat sink. Measuring means is configured to determine the functionality of the first and second light sources.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims priority from and the benefit of PCT Application No. PCT/EP2007/007010, filed on Aug. 8, 2007, entitled “Circuit for a Motor Vehicle, in Particular for Actuating a Lighting Device”, which is herein incorporated by reference. 
     BACKGROUND 
     The present invention relates to a circuit for a motor vehicle, in particular for actuating a lighting device, the circuit having a first light source and a second light source connected in parallel, the first light source being assigned a first current sink and the second light source being assigned a second current sink, and the circuit being assigned a measuring means. 
     Such circuits for motor vehicles are generally known. For example, the German laid-open specification DE 101 15 388 A1 discloses a drive circuit for an LED array, the drive circuit comprising at least one first LED strand and at least one second LED strand, a switch being arranged in series with each LED strand and each LED strand having a supply connection, a control loop being designed in such a manner that it drives the first switch of the first LED strand so as to achieve a constant mean value of the current flowing through the first LED strand, the control loop also being designed to drive the switches of the further LED strands. The drive circuit also comprises a total current detection apparatus which can be used to determine the sum of currents through at least two LED strands. 
     The disadvantage of this is that a distinction is made between a so-called master LED strand and further LED strands, with the result that, in the event of a functional disturbance in the so-called master LED strand, the circuit fails, which has an adverse affect on the functional reliability of the circuit. 
     The object of the present invention is to provide a drive circuit for a motor vehicle, in particular for actuating a lighting device, in which lighting failures or failures of the light source or other fault states are detected and a flexible response is given to such faults. Furthermore, the intention is to minimize the thermal power loss. 
     SUMMARY 
     The object is achieved by means of a circuit for a motor vehicle, in particular for actuating a lighting device, the circuit having a first light source and a second light source connected in parallel, the first light source being assigned a first current sink and the second light source being assigned a second current sink, the circuit being assigned a measuring means, and the measuring means being intended to determine the functionality of the first light source and to determine the functionality of the second light source. According to the invention, this makes it possible to easily check the functionality of each of the light sources periodically in succession or else at the same time in a parallel manner and to bring about an adapted response. Furthermore, the parallel connection of the light sources likewise makes it possible to keep the power loss as small as possible. 
     According to the invention, it is also preferred for the circuit to have a first switch which is assigned to the first light source and a second switch which is assigned to the second light source, the circuit being assigned a control means in such a manner that the first switch can be switched on the basis of the functionality of the first light source and the second switch can be switched on the basis of the functionality of the second light source. According to the invention, this makes it possible to bring about selective disconnection in the event of total failure of one of the light sources and thus to minimize the power loss. Furthermore, this also reduces the costs by virtue of the fact that a higher permitted ambient temperature of the circuit is possible by reducing the thermal power loss, with the result that more cost-effective components with lower requirements can be used according to the invention in the circuit and the durability or service life of the circuit components and also of the components in the light sources (in particular light-emitting diodes), in particular, is increased. 
     According to the invention, it is also preferred for the circuit to have a regulator, in particular a current regulator or a voltage regulator, which is provided for the first light source and the second light source. According to the invention, this makes it possible to adapt the electrical voltages or the electric current provided at the light sources in a specific manner by virtue of the fact that only the required voltage is respectively applied in order to produce a certain light power or to achieve a particular current through the light source or through the light-emitting diodes. This also makes it possible, for instance in the event of partial failure of the light sources (alloying of one or more light-emitting diodes), to regulate the current or voltage in an adapted manner, such that the power loss continues to be reduced or the respective power loss is also reduced in other possible fault situations. If the energy loss caused by the alloying of light-emitting diodes is too high, the respective light source can be disconnected. 
     According to the invention, it is also preferred for the regulator to be able to be driven on the basis of the functionality of the first light source and the second light source. This makes it possible to flexibly adapt the method of operation of the circuit according to the invention to the respective functional conditions of the light sources, with the result that a particularly low level of power loss can be achieved and, furthermore, the lighting properties of the light sources also change, at most, in a manner which is invisible or at least largely invisible to a user. 
     According to the invention, it is also preferred that, following a change in the functionality of the first light source or the second light source, the regulator can be driven with a time delay of less than or equal to one second, preferably less than or equal to 500 ms, particularly preferably less than or equal to 300 ms, very particularly preferably less than or equal to 100 ms. This enables a rapid response or makes it possible to rapidly adapt the method of operation of the circuit to a fault situation, with the result that an impermissibly high power loss is present, at most, over a very short period of time and therefore does not damage any parts or components of the circuit. This further reduces the production costs of the circuit because more cost-effective components can be used. 
     According to the invention, it is also preferred for the first light source to have a first light-emitting diode, preferably a plurality of first light-emitting diodes, and/or for the second light source to have a second light-emitting diode, preferably a plurality of second light-emitting diodes. This advantageously makes it possible to use a so-called light-emitting diode chain as a light source, which has the advantage that, on the one hand, a high level of flexibility is achieved because the number of light-emitting diodes can be varied for adaptation to different lighting intensities or voltage levels or the like. Furthermore, this is also associated with a greater degree of reliability because a larger number of individual light sources are used in the lighting apparatus, with the result that the failure of a particular individual light source (in particular a light-emitting diode) has a far smaller effect than when comparatively few incandescent lamps or the like are used, for example. 
     According to the invention, it is also preferred that, with an adequate repetition rate, the circuit can be operated in clocked fashion, in particular with a duty ratio of the switch-on time interval to the switch-off time interval of less than or equal to 1:100, preferably less than or equal to 1:200, particularly preferably less than or equal to 1:500, very particularly preferably less than or equal to 1:1000. This makes it possible to vary the dynamic response of the lighting device in very wide limits, thus enabling a larger field of use of the circuit according to the invention as a drive circuit for lighting devices. 
    
    
     
       DRAWINGS 
       Exemplary embodiments of the invention are illustrated in the drawing and are explained in more detail in the following description. 
         FIG. 1  shows a schematic illustration of a first embodiment of the circuit according to the invention. 
         FIG. 2  schematically shows the first embodiment of the circuit according to the invention with more details. 
         FIG. 3  schematically shows a second embodiment of the circuit according to the invention. 
         FIG. 4  schematically shows a third embodiment of the circuit according to the invention. 
         FIG. 5  schematically shows a timing diagram for illustrating different switching states at important points of the third embodiment of the circuit according to the invention. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  schematically illustrates a circuit  10  according to the invention, the circuit  10  being intended, in particular, to actuate a lighting device. For this purpose, the circuit  10  has, for example, a first light source  11  and a second light source  12 . The light sources  11 ,  12  are provided, in particular, in the form of so-called chains of light-emitting diodes, the first light source having a first light-emitting diode  11 ′ and preferably a plurality of first light-emitting diodes  11 ′ and the second light source  12  having a second light-emitting diode  12 ′ and preferably a plurality of second light-emitting diodes  12 ′. According to the invention, the light sources  11 ,  12  are each connected to a current sink (alternatively a current source) or are assigned to the latter, the first light source  11  being assigned to a first current sink  21  and the second light source  12  being assigned to a second current sink  22 . In this case, an arrangement comprising a transistor, in particular a so-called MOSFET transistor (metal oxide semiconductor field effect transistor) or alternatively a bipolar transistor, and an operational amplifier is provided, in particular, as the current sinks  21 ,  22 . The circuit  10  also has a regulator  60  which regulates the voltage level (or current level) which is applied to the light sources  11 ,  12  which are connected in parallel. The invention provides a connection to a measuring means  40  both from the arrangement comprising the first light source  11  and the first current sink  21  and from the arrangement comprising the second light source  12  and the second current sink  22 , said measuring means being able to check the functionality of the first light source  11  and the second light source  12 . According to the invention, this checking operation takes place either continuously (that is to say at the same time for the first light source  11  and for the second light source  12  in a parallel manner) or else sequentially (that is to say the light sources  11 ,  12  are switched on in succession and the respective result is supplied to the measuring means  40 ). The measuring means  40  is also connected to a control means  50  or is combined or else integrated with the latter, the control means  50  being used to influence the behavior of the circuit  10  in such a manner that, in the event of the first light source  11 , for example, failing, the corresponding first current sink  21  and thus the entire circuit branch of the first light source  11  (also referred to below as the light-emitting diode strand or LED strand) are also disconnected. It goes without saying that the same also applies to the second light source  12 . The invention also provides for the measuring means  40  to also make it possible to determine whether, for example, one of the first light-emitting diodes  11 ′ and one of the second light-emitting diodes  12 ′ are alloyed (that is to say have a low impedance) and thus no longer produce a voltage drop which, with the feed voltage applied to the corresponding LED strand remaining the same, results in a larger power loss which should be kept as small as possible according to the invention. If the power loss in the current sink becomes too large on account of the alloying of light-emitting diodes, the latter may be selectively disconnected. According to the invention, the regulator  60  comprises, in particular, a current regulator or a voltage regulator, a first regulating component  61 , for example in the form of an integrated circuit, being able to be used, for instance, as an example of a current regulator, and the regulator  60  also being able to have a second regulating component  62 , for example in the form of a so-called DC/DC converter part. In this case, on the basis of the battery voltage U batt  applied to the circuit  10 , the regulator  60  generates or regulates the respectively required voltage at the input  65  of the LED strands or light sources  11 ,  12 , which are connected in parallel, with the result that the current required by the light sources  11 ,  12  can flow. 
     The circuit according to the first embodiment of the invention functions as follows: 
     The second regulating component  62  is connected to an output  64  of the first regulating component  61  (actual current regulator) and the light sources  11 ,  12  are connected to said second regulating component. Each light source  11 ,  12  comprises one of the light-emitting diode branches which are regulated to a current intensity which is exactly the same as far as possible by means of the current sinks  21 ,  22 . In this case, the current sinks  21 ,  22  may also be in the form of current sources. The currents flowing through the light sources  11 ,  12  are respectively added by an adder  50 ′ via a so-called sense resistor  41  (for example of a size of 470 mΩ). In parallel with this operation, the information relating to the functionality of the first and second light sources  11 ,  12  is supplied to the measuring means  40  by means of corresponding measuring lines  42 . The measuring means  40  and the control means  50  are now able to drive the adder  50 ′ (with a downstream voltage amplification regulation means  50 ″) in such a manner that a voltage level, which is supplied to a control connection (feedback connection, likewise denoted using the reference symbol  51  in the text below) of the first regulating component  61 , is applied to an output  51  of the adder  50 ′ and the voltage amplification regulation means  50 ″, with the result that the voltage regulation at the input  65  of the light sources  11 ,  12  can respond to the changed situation (for example failure of one of the LED strands) in an adequate manner, that is to say by applying a sufficient voltage level. 
     This method of operation shall be described below using an example. If, for example, two LED branches (light sources  11 ,  12 ) are each operated with 150 mA, the regulator  60  is intended to regulate the current flowing on the line  65  to a total of 300 mA. If only one of the LED branches is operated, the regulator  60  is intended to supply only 150 mA. The voltage amplification regulation means  50 ″ which can be switched over is required for this purpose in the first embodiment of the circuit  10  according to the invention. If a total of two LED branches are operated (each with 150 mA) and one of them fails, the voltage at the output of the arrangement comprising the adder  50 ′ and the voltage amplification regulation means  50 ″ must be amplified by a factor of two so that the required voltage is doubled at the feedback input  51  of the first regulating component  61  (which, on account of the method of operation of the first regulating component  61 , results in the current regulator  60  regulating only to 150 mA). If, in the event of such a failure of one of the LED strands, the flow of current through this failed LED strand stops, the regulation means of the circuit  10  first of all responds in such a manner that the voltage level at the input  65  of the light sources  11 ,  12  is stepped up to the maximum set limit voltage of the first regulating component  61  (because an excessively low voltage is applied to the feedback input  51  of the first regulating component  61 ). However, in such a situation, the current in the LED branch or light source  11 ,  12  which is still functioning is still kept constant at 150 mA, which can be attributed to the function of the constant current sinks  21 ,  22 . Therefore, the remaining voltage drop must finally be brought under control using the functioning current sink  21 ,  22 , with the result that a very large power loss momentarily occurs. This state must be prevented with a delay time which is as short as possible. For this purpose, the measuring means  40  uses the measuring lines  42  to detect that one of the LED strands or one of the light sources  11 ,  12  has failed, and the corresponding LED branch is disconnected. The voltage at the output of the voltage amplification regulation means  50 ″ is then raised again to the level in the steady state, with the result that the regulator  60  or the first regulating component  61  can regulate back to the normal voltage level at the input  65  of the light sources  11 ,  12  again. The circuit  10  according to the invention thus ensures that there are no unstable states. There is thus no flashing either or some other impairment in the function of the light source which disturbs a user. Precisely because the defective light source is selectively disconnected, the situation is precluded in which the light source functions at times and then does not function again at times. When one of the light sources  11 ,  12  fails, only the brightness of the lighting device (which, according to the invention, is in the form of a background lighting device for a display device, in particular) decreases only by a defined value and then remains stable again, which is precisely the desired behavior of such an arrangement and is possibly not perceived at all by a user, for instance when the light sources  11 ,  12  fail in daylight and the decrease in the background lighting is not noticed at all on account of the ambient illumination. The brightness can be adjusted to the desired value of two or more light sources in regions by means of longer switch-on times of the light sources which are still functioning. 
       FIG. 2  illustrates a schematic illustration of the first embodiment of the circuit according to the invention in greater detail for the case of two light sources  11 ,  12 . The first light source  11 , the second light source  12 , the first light-emitting diode  11 ′, the second light-emitting diode  12 ′, the first current sink  21 , the second current sink  22 , the regulator  60  with the first regulating component  61  and the second regulating component  62 , the input  65  of the light sources  11 ,  12  as well as the regulating input  51  and the adder  50 ′ with the voltage amplification regulation means  50 ″ are again illustrated. Furthermore, the measuring means  40  and the control means  50  are likewise illustrated in the left-hand part of  FIG. 2 . In addition to the elements in  FIG. 1 ,  FIG. 2  furthermore also illustrates a first switch  31  and a second switch  32 , the switches  31 ,  32  being provided for the controlled disconnection of individual light-emitting diode branches or individual light sources  11 ,  12 . This is carried out via the control means  50  by means of corresponding control lines (not individually provided with a reference symbol). 
     In addition to the first and second light sources  11 ,  12 , it goes without saying that it is possible to use further light sources in the form of further light-emitting diode branches, which are connected in parallel, in the circuit according to the first embodiment (and also the further embodiments). In this case, corresponding current sinks (not illustrated), corresponding switches (likewise not illustrated) and the corresponding control lines are required. 
       FIG. 3  illustrates a schematic illustration of a second embodiment of the circuit  10  according to the invention. In addition to the first light source and the second light source  11 ,  12 , three further light sources  13 ,  14 ,  15  are schematically illustrated in this case in the form of light-emitting diode chains. According to the second exemplary embodiment of the circuit  10  according to the invention, each of the light sources  11  to  15  again has a current sink, only the first current sink  21  being illustrated in  FIG. 3 , however. The same applies to the switch, that is to say only the first switch  31  for the first light source  11  is illustrated in  FIG. 3 . In the second embodiment of the invention, the light sources  11 ,  12  are driven in a similar manner to that in the first embodiment of the invention by means of the regulator  60  and corresponding current sinks  21 . Therefore, the regulator  60  with the first regulating component  61  and the second regulating component  62 , the input  65  of the light sources  11  to  15 , the regulating input  51  and the adder  50 ′ are again illustrated. Furthermore, the measuring means  40  and the control means  50  are likewise illustrated in the left-hand part of  FIG. 3 . 
     There are differences with regard to the measuring means  40  and the evaluation of the individual light-emitting diode strands. In the second embodiment of the invention (in contrast to the first embodiment of the invention), the functions of the voltage amplification regulation means  50 ″ (according to the first embodiment of the invention) are also undertaken by the control means  50 . For this purpose, the control means  50  has an output line  52  which acts on the adder  50 ′, with the result that the voltage needed to stabilize the control loop is applied to the output  51  of the adder  50 ′ even when individual light-emitting diode chains or light sources fail. For each light-emitting diode branch which fails, the measuring means  40  first of all uses measuring lines  42  (only one measuring line  42  of which is illustrated for the sake of simplicity) to detect that the light-emitting diode branch has failed. The control means  50  then steps up (in particular by means of a digital/analog converter or by means of a so-called PPG (programmable pulse generator)) the input  52  of the adder  50 ′ in such a manner that the voltage at the output  51  of the adder remains at the level needed for a regulated voltage at the input  65  of the light sources. In the example of the second embodiment shown in  FIG. 3 , it is the case, for example, that the current through each of the light sources is 100 mA but this value should be understood only as an example. Therefore, the voltage level on the control line  52  must be increased by (depending on the resistor configuration provided) 100 mV, for example, for each light-emitting diode chain which fails. 
       FIG. 4  schematically illustrates a third embodiment of the circuit  10 . The first light source  11 , the second light source  12 , the first current sink  21 , the second current sink  22 , the first regulating component  61 , the second regulating component  62 , the first switch  31  and the second switch  32  as well as the measuring means  40  and the control means  50  are illustrated again. In contrast to the first and second embodiments of the circuit  10 , there is no adder  50 ′ for providing the voltage level required at the feedback input  51  of the first regulating component  61 , but rather a first diode D 1 , in particular a Schottky diode, between the first current sink  21  and the feedback input  51  is used to ensure that, when the voltage level on the cathode side of the first diode D 1  falls, to a very great extent, toward the ground potential on account of the first light source  11  failing, a corresponding response (on account of the diode D 1  turning on) at the feedback input  51  (drop in the voltage applied there) is given in such a manner that the input voltage at the input  65  of the light sources  11 ,  12  is stepped up again. After the corresponding LED branch (of the first light source  11 , for example, in the present case) has been switched off, the turning-on of the first diode D 1  is reversed again. In a manner corresponding to the function with the first LED branch, a second diode D 2 , in particular a Schottky diode, between the second current sink  22  and the feedback input  51  is used to ensure that, when the voltage level on the cathode side of the second diode D 2  falls, to a very great extent, toward the ground potential on account of the second light source  12  failing, a corresponding response (on account of the second diode D 2  turning on) at the feedback input  51  (drop in the voltage applied there) is given in such a manner that the input voltage at the input  65  of the light sources  11 ,  12  is stepped up again. 
     The information relating to whether or not one of the light sources  11 ,  12  has failed is passed to the measuring means  40  via a measuring line  43  (or measuring connection  43 ′) which is common to all light sources  11 ,  12 . It is therefore necessary to sequentially sample the different light-emitting diode branches. The LED branch which has failed is disconnected using the switches  31 ,  32 . A third diode D′ (likewise a Schottky diode for example) and a fourth diode D″ (likewise a Schottky diode for example) are used for this purpose (for the example of two light sources  11 ,  12 ). According to the third embodiment of the circuit  10  according to the invention, the input of the current sinks  21 ,  22  (that is to say in each case between the light sources  11 ,  12  and the current sinks  21 ,  22 , thus implementing a measuring tap at this point) is connected to the anode side of either the third diode D′ (first current sink) or the fourth diode D″ (second current sink) for this purpose. A query regarding which voltage drop is present across the light sources  11 ,  12  is effected in a temporally clocked manner (that is to say sequentially) by means of control using the control means  50 . If, for example, one of the light-emitting diodes has failed in both light sources  11 ,  12 , this can be detected in the control means  50  on the basis of the measured voltage drop, and the voltage level at the input  65  of the light sources  11 ,  12  can be reduced in order to reduce the power loss. A further measuring connection  42 ′ is used to measure the voltage drop across the entire light-emitting diode chain for a particular light source  11 ,  12 . In order to illustrate the method of operation,  FIG. 4  also depicts the control lines S 1 , S 2 , S 3 , S 4  and S 5  of the control means  50  or of the measuring and control means  50 . In this case, the control line S 1  is used to periodically switch the circuit  10  on and off when the lighting device (for instance background lighting for a display apparatus) is activated and with a duty ratio of the switch-on time interval to the switch-off time interval that is dependent on the required brightness of the light sources. The control line S 2  is used to switch the first light source  11  on or off. The control line S 3  is used to switch the second light source  12  on or off. The control line S 4  is used to disconnect the feedback input  51  from ground potential. The control line S 5  is used to sample the voltage regulation (or current regulation) at the input  65  of the light sources and on the measuring line  42  or at the measuring connection  42 ′. 
       FIG. 5  schematically illustrates a timing diagram relating to different states of the control lines S 1 , S 2 , S 3 , S 4  and S 5  of the control means  50 . According to the invention, the circuit  10  is operated in clocked fashion. This means that switching-on and switching-off operations are carried out at regular intervals at a clock generator output (control line S 1 ) which switches the entire circuit on and off. When both light sources  11  and  12  are functioning, a switching-on operation is likewise carried out synchronously on the individual switching lines (control lines S 2  and S 3 ) for the individual light sources. The result here is only a minimum time delay, between the time at which the circuit is switched on and the actual functioning of the circuit (control line S 5 ), of, for example, approximately 1 to 40 microseconds, preferably approximately 5 microseconds (the delay time is indicated in the drawing using an arrow; in this case, the delay time can be adjusted). This can be attributed, in particular, to the fact that the adding stage is dispensed with in  FIG. 4 , the response times of the first and second diodes D 1 /D 2  and of the switches  31 ,  32  are in the nanosecond range and the response time of the operational amplifiers of the current sinks  21 ,  22  could additionally be accelerated to a considerable extent because a) said operational amplifier is supplied with a high voltage (for example the battery voltage, and the voltage is limited to a defined limit value in this case) and because b) the output of the operational amplifier is biased (for example by means of a resistance to its voltage supply). As a result, when switching on the circuit, it is possible for the edge gradient to be so large that the circuit is ready for operation in 5 microseconds, for example. In the extreme case, it is thus possible for the circuit to be operated with a dynamic response of 1:1000 or even higher within a repetition time of, for example, 5 milliseconds (with a repetition rate of, for example, approximately 200 per second or else from approximately 50 per second to approximately 600 per second).