Patent Publication Number: US-9854636-B2

Title: Lighting device and method for operating a lighting device

Description:
TECHNICAL FIELD 
     The present invention refers to a lighting device and a method for operating the lighting device. 
     BACKGROUND ART 
     The lighting device contains a lighting module comprising a light element series connection of at least two semiconductor light elements. Particularly the light elements series connection contains at least four, five or more semiconductor light elements. A drive circuit provides an output voltage and an output current at its output. The output voltage and the output current are supplied to the lighting module to provide electrical energy for lighting the light elements. In a preferred embodiment the semiconductor light elements are light emitting diodes (LEDs) or organic light emitting diodes (OLEDs). The lighting device according the present invention is particularly usable in outdoor applications, for example for illuminating streets, parks, gardens, boardwalks, cycleways or other public or private locations. 
     Such lighting devices are generally known. DE 10 2009 041 957 A1 discloses a device for operating LEDs. A cooling device is provided to actively cool the LEDs. A temperature sensor can be provided for measuring the temperature of the circuit board, on which the LEDs are mounted. The cooling device can be activated depending on the measured temperature. 
     US 2007/0108843 A1 discloses a series connected power supply for semiconductor-based vehicle lighting systems. The power supply includes a constant current source to supply current to the semiconductor light elements. For each semiconductor light element a bypass switch is provided. If the bypass switch is closed, the current flows through the bypass switch around the respective semiconductor light element. In so doing a failure of one single semiconductor light element does not affect the lighting of the other semiconductor light elements in the series connection. The bypass switches can also be used for modulating the brightness of the lighting device. 
     SUMMARY OF INVENTION 
     Technical Problem 
     Particularly in outdoor applications, the lighting device is exposed to the environmental conditions and respective temperature variations. Because of such varying environment temperatures a large operating temperature range for the lighting device is required. The forward voltage of the non-illuminated semiconductor light elements changes depending from the temperature of the lighting module. If this forward voltage increases due to a low environment temperature it can happen, that the output voltage of the drive circuit is insufficient to start illumination of the lighting module. 
     It is therefore the object of the present invention to make sure that lighting of the semiconductor light elements connected in series is possible also when the ambient temperature is low. 
     Solution to Problem 
     According to an aspect of the present invention, the lighting device contains a control means having a temperature dependent element. The control means can change its condition. It adopts a heating condition if a heating requirement is fulfilled. Fulfilling the heating requirement requires at least that the temperature of the lighting module has decreased below a predetermined minimum temperature value at which the forward voltage of the light element series connection reaches an upper limit value. 
     When the control means adopts the heating condition, a heating means is activated to heat the lighting module for increasing the temperature of the lighting module or at least to avoid that the temperature of the lighting module is further lowered. In so doing the lighting device avoids that the output voltage provided by the drive circuit is insufficient to start the illumination of the light element series connection. The temperature of the lighting module is kept in a temperature range in which the forward voltage is lower or at most as high as the output voltage of the drive circuit. Accordingly, the lighting device can be illuminated independent from the environment temperature. 
     In a preferred embodiment the heating means contains one ore more semiconductor light elements. Preferably at least one of the semiconductor light elements of the light element series connection is used as heating element and thus forms the heating means. This at least one semiconductor light element is lighted if the control means adopts the heating condition. Accordingly, an additional device for heating the lighting module is not necessary. The heat produced by the at least one semiconductor light element is sufficient and used to heat the lighting module. An easy configuration at low costs can be achieved. 
     Preferably, the control means can be configured to short-circuit at least one of the semiconductor light elements of the light element series connection if the control means adopts the heating condition. A current cannot flow through a short-circuited semiconductor light element. In so doing, the forward voltage of the light element series connection is reduced. Preferably the number of semiconductor light elements which are short-circuited is selected so that the semiconductor light elements, which are not short-circuited, provide a forward voltage of the series connection which is definitely not exceeding the output voltage of the drive circuit over the entire possible operating temperature range under consideration of the expected environment temperature range. In so doing independent from the environment temperature the illumination of at least some of the semiconductor light elements of the light element series connection is possible. The illuminated semiconductor light elements can in such an embodiment be used as heating elements of the heating means for heating the lighting module. After the temperature of the lighting module has increased or is sufficiently raised, the short-circuit of the respective semiconductor light elements can be annulated. Annulating the short-circuit can be performed either sequentially for one short-circuited semiconductor light element after another or for all of the short-circuited semiconductor light element at the same time. 
     For creating a short-circuit, the control means can comprise at least one bypass element or bypass circuit connected in parallel with the at least one of the semiconductor light elements to be short-circuited. The bypass element or bypass circuit can contain for example at least one of a temperature dependent element, like a resistor or capacitor, a bimetallic element, a silicon sensor element and/or a controllable switch. The temperature dependent element can have a negative or a positive temperature coefficient. The temperature dependent element of the control means can be any temperature dependent electric and/or electronic element having a positive or a negative temperature coefficient so that the temperature or a temperature change can be detected. The controllable switch is for example a transistor, particularly a field-effect transistor. 
     The control means can also contain a microcontroller for evaluation of a characteristic of the temperature depending element in order to detect the temperature change and/or to determine a temperature value of the lighting module. 
     The lighting device can contain a module carrier, for example a printed circuit board. The lighting module and the control means can be placed together on this module carrier. In so doing, the wiring of the lighting device is simplified. 
     In a preferred embodiment of the lighting device the heating means contains an electric and/or electronic heating arrangement. This heating arrangement is thermally coupled with the lighting module. The heating means can for example contain at least one electric heating component like an electrical resistor and/or an electrical heating coil or the like. The heating means can provide radiation heating and/or convection heating and/or conduction heating. 
     The lighting device can also contain a cooling means. The cooling means can adopt a first condition for cooling the lighting module and a second condition in which the cooling effect is at least reduced. The cooling means can provide cooling via thermal radiation and/or thermal conduction and/or thermal convection. The cooling means can for example contain at least one peltier element for thermoelectric cooling. The cooling means can alternatively or additionally contain a fan. In one embodiment the fan of the cooling means can also be used for heating. Therefore the fan can be part of the heating means and/or the cooling means. 
     In one embodiment the cooling means can contain a heat sink. In the first condition of the cooling means the heat sink and the lighting module and/or the module carrier are in contact with each other for dissipating heat. A drive, particularly an electric drive can be provided to separate the heat sink from the lighting module and/or the module carrier in the second condition of the cooling means. 
     Particularly fulfilling the heating requirement requires additionally that the semiconductor light elements of the light element series connection are all switched off. Accordingly the semiconductor light elements are unlighted. During operation, when the semiconductor light elements are lighted, enough heat is produced to avoid that the light module temperature drops below a predetermined lower value. 
     The heating means can preferably be configured to not only heat the lighting module in the heating condition, but to additionally heat other electric and/or electronic components of the lighting device which are for example arranged on the module carrier, like an electrolytic capacitor and/or a battery and/or an accumulator or the like. 
     In one embodiment the drive circuit and the control means can be switched into a standby-mode. In this standby-mode the semiconductor light elements are turned off. Preferably after the expiration of a predetermined time interval since the beginning of the standby-mode the control means are activated or waked up to check whether the heating requirement is fulfilled. This check can be performed regularly. If the heating requirement is not fulfilled, the control means are switched back into the standby-mode. Otherwise the control means adopts its heating condition and the lighting module is heated. The heating can be continued for a predetermined duration until a predetermined temperature of the lighting module is reached. After this heating period the lighting device is switched back into the standby-mode unless the lighting of the lighting device is requested for illumination. 
     The control means for operating the heating means and/or the cooling means contains a temperature depending element. In one embodiment of the invention the control means can additionally contain at least one of the following devices: a device with calendar function, a timing device, a clock, a brightness sensor, a global position sensor, e.g. a satellite based position sensor. Under use of at least one of these devices the control means can determine under consideration of the global location and/or the time and/or the calendar day whether cooling or heating of the lighting module is necessary. Heating of the lighting module can for example be considered to be necessary if the semiconductor light elements are switched off and winter and/or nighttime is determined. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       Other preferable features of the invention are contained in the dependent claims, the description and the drawing. In the following, preferable embodiments of the invention are explained in detail with reference to the drawing. 
         FIG. 1  is a schematic block diagram of a first embodiment of the lighting device. 
         FIG. 2  is a schematic block diagram of a second embodiment of the lighting device. 
         FIG. 3  is the illustration of a third embodiment of the lighting device in which a cooling means is in a first condition. 
         FIG. 4  is the illustration of a third embodiment according to  FIG. 3  in which the cooling means is in a second condition. 
         FIG. 5  is a block diagram of a fourth embodiment of the lighting device. 
         FIG. 6  is a block diagram of an embodiment of the control means according to the fourth embodiment of the lighting device shown in  FIG. 5 . 
         FIG. 7  is a block diagram of a fifth embodiment of the lighting device. 
         FIG. 8  is a schematic illustration of the dependency of the forward voltage Vf of the series connection of semiconductor light elements in the unlit condition depending from the temperature T of a lighting module. 
         FIG. 9  is a schematic illustration of another embodiment of a heating means. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
       FIG. 1  shows a block diagram of a first embodiment of a lighting device  10 . The lighting device  10  contains a drive circuit  11  having an output  12 . At its output  12  the drive circuit provides an output voltage Vout. This output voltage Vout can be applied to a lighting module  13  which is connected to the output  12 . At the output  12  the drive circuit  11  also supplies an output current for the lighting module  13 . The output current can be controlled for brightness control or dimming. 
     The lighting module  13  contains several semiconductor light elements  14 . At least one light element series connection  15  of at least two and particularly at least four or five semiconductor light elements  14  is contained in the lighting module  13 . The light element series connection  15  is for example illustrated in  FIG. 5 or 7 . Different to the illustrated preferred embodiments the lighting module  13  can contain more than one light element series connection  15 . The lighting module  13  with its semiconductor light elements  14  is arranged for example on a module carrier  16 , which is for example a printed circuit board. 
     A control means  20  is provided which comprises a temperature dependent element  21 . The temperature dependent element  21  is configured to provide a characteristic C that is characteristic for a temperature value and/or the change of a temperature value. In the described preferred embodiments the temperature dependent element  21  is thermally coupled to the lighting module  13  so that the temperature T of the lighting module  13  and/or changes of this temperature T are detected. The temperature dependent element  21  is preferably arranged on the module carrier  16 . 
     The temperature dependent element can have either a negative temperature coefficient or a positive temperature coefficient. Any suitable electric and/or electronic element can be used, such as a temperature dependent resistor, a temperature dependent capacitor, a bimetallic element, any temperature dependent semiconductor element or the like. 
     The characteristic C of the temperature dependent element  21  is evaluated in the control means  20 . The characteristic C can be an electric signal, for example a temperature dependent current or voltage. The characteristic C can also be a mechanic characteristic, for example if the temperature dependent element is a bimetallic element. In this case the characteristic C is the shape and/or length and/or position of the bimetallic element. However the characteristic C depends on the temperature T of the lighting module  13  and can thus be evaluated in an evaluating part  22  of the control means in order to determine the temperature T or temperature changes of the temperature T of the lighting module  13 . The evaluating part  22  can contain a microcontroller for evaluating the characteristic C. The evaluating part  22  can be part of the drive circuit  11  and thus it is possible to use a microcontroller of the drive circuit  11  for receiving and evaluating the characteristic C ( FIG. 1 ). Alternatively the evaluating part  22  can be a separate unit preferably arranged on the module carrier  16  ( FIG. 2 ). 
     The evaluating part  22  of the control means  20  is configured to adopt the control means  20  into a heating condition if a predetermined heating requirement is fulfilled. Adopting the heating condition can be performed by means of a microcontroller and/or a controllable switch  23  and/or the temperature dependent element  21  of the control means  20  ( FIG. 6 ). As controllable switch it is for example possible to use a transistor, such as a bipolar transistor or field-effect transistor. As one illustrative example an enhancement-mode, n-channel MOSFET is shown in  FIG. 6 . 
     Lighting device  10  contains a heating means  26  and in the preferred embodiment additionally a cooling means  27 . The cooling means  27  is optional. The heating means  26  is arranged on the module carrier  16  and is thermally coupled with the lighting module. The heating means  26  is provided for heating the lighting module  13  if the temperature T of the lighting module  13  is low. The cooling means  27  can be used to dissipate heat from the lighting module  13  if the temperature T of the lighting module  13  is high. Accordingly, the heating means  26  and the cooling means  27  are not operated at the same time. 
     According to the first embodiment shown in  FIG. 1 , the heating means and/or the cooling means  27  is controlled by means of the control means  20  which is at least partly integrated in the drive circuit  11 . The energy, which is necessary for heating or cooling can be provided by the drive circuit  11 . Since both means  26 ,  27  do not operate at the same time, one common interface  28  of the drive circuit  11  can be used to provide the necessary electric energy for heating or cooling. 
     Different to the first embodiment shown in  FIG. 1 , the evaluating part  22  of the control means  20  can be arranged on the module carrier  16  ( FIG. 2 ). The evaluating part  22  can contain a microcontroller and/or a controllable switch  23  and can have any configuration as described above. According to the embodiment of  FIG. 2  the heating means  26  is controlled using the control means  20 , whereas the necessary electric energy for creating the heat is provided by means of the drive circuit  11  and particularly the interface  28  as explained with regard to the first embodiment of  FIG. 1 . The lighting device  10  of  FIG. 2  can also contain the cooling means  27  as explained with regard to the first embodiment of  FIG. 1 . 
     The light element series connection  15  contains several semiconductor light elements  14 , for example light emitting diodes (LEDs). The forward voltage Vf depends on the temperature T of the lighting module  13  as schematically illustrated in  FIG. 8 . Particularly if the lighting device  10  is used in outdoor applications, for example for illumination of streets, gardens, boardwalks, cycleways or other public or private locations the ambient temperature can change remarkably. The lighting device  10  must be able to light the semiconductor light elements  14  independent from the ambient temperature, which is particularly relevant in the non-lighted condition, in which no current flows through the light element series connection  15 , so that the semiconductor light elements  14  are not heated. Accordingly the lighting device  10  must be able to light the semiconductor light elements  14  over an entire operating temperature range R, which can for example cover the range from −30 degrees Celsius up to over +120 degrees Celsius. The temperature T of the lighting module  13  can vary over the complete temperature range R. At every temperature value the drive circuit  11  must be able to start lighting. 
     As illustrated in  FIG. 8  the forward voltage Vf of the light element series connection  15  increases with decreasing temperature T. As can be taken from this diagram at very low temperatures T of the lighting module  13  below a minimum temperature value Tmin the forward voltage Vf can exceed the output voltage Vout of the drive circuit  11 . Consequently the output voltage Vout would be insufficient to start lighting of the semiconductor light elements  14  of the light element series connection  15 . It is undesired to use a drive circuit  11  which is able to provide an output voltage Vout having an amount which is sufficient to start lighting over the entire possible operating temperature range R. Such drive circuit  11  would remarkably increase the costs for the lighting device  10 . To solve this problem the heating means  26  are used to heat the lighting module  13  in order to avoid that the forward voltage Vf exceeds the output voltage Vout so that lighting is possible at each temperature. 
     When the heating requirement is fulfilled, the control means  20  adopt the heating condition. For fulfilling the heating requirement it is at least necessary that the temperature T of the lighting module  13  drops down to the minimum temperature value Tmin. At this minimum temperature value Tmin the forward voltage Vf corresponds to an upper limit value Vlim. For fulfilling the heating requirement it is additionally necessary that the semiconductor light elements  14  of the light element series connection  15  are all switched off so that no current flows through the light element series connection  15 . 
     If the heating requirement is fulfilled, the control means  20  changes to the heating condition and the heating means  26  heats a lighting module  13 . In so doing it is possible to avoid that the forward voltage Vf exceeds the output voltage Vout so that lighting of the semiconductor light elements  14  can be guaranteed. 
     As shown in  FIG. 8  it is preferred that the minimum temperature value Tmin is selected so that the amount of the upper limit value Vlim of the forward voltage VF is less than the amount of the output voltage Vout by a predetermined voltage difference DV. If the voltage difference DV has a sufficient amount it can be made sure, that enough time is provided to produce heat for the lighting module  13 . It can happen that immediately after the start of the heating, the temperature T of the lighting module continues to decrease. This is because a certain time delay can exist between starting of the heating operation and the production of enough heating energy to stop the temperature T decreasing and/or to start increasing the temperature T. 
     As shown in the embodiments according to  FIGS. 5 through 7  the heating means can be formed by some of the semiconductor light elements  14  of the light element series connection  15 . When the semiconductor light elements  14  are lighted by means of a current flowing through the semiconductor light elements  14 , not only light but also heat is produced. This heat can be used to heat the lighting module  14 . Accordingly some of the semiconductor light elements  14  form heating elements  30 . Because it is not necessary or even impossible to use all of the semiconductor light elements  14  as heating elements  30  at least one or more semiconductor light elements  14  are short-circuited by the control means  20  when the control means  20  adapts the heating condition. As shown in  FIG. 7 , the control means  20  can comprise at least one bypass element  31  which is connected in parallel with at least one of the semiconductor light elements  14  which is not used as heating element  30 . It is possible that each of the bypass elements  31  is assigned to one of the semiconductor light elements  14  to be short-circuited. 
     The bypass element  31  can be formed in one embodiment from a resistor having a positive temperature coefficient so that the resistance increases as the temperature increases. If the temperature T of the lighting module  13  is low, the resistance of the bypass elements  31  is low enough to short-circuit the respective semiconductor light element  14 . Only the semiconductor light elements  14  used as heating elements  30  are lighted and produce heat (illustrated schematically by the corrugated arrows in  FIG. 7 ) which heats the lighting module  13  and also increases the resistance of the bypass elements  31 . If the temperature T is sufficiently increased the short-circuiting is suspended due to the increase of the resistance. The forward voltage Vf is reduced accordingly and all of the semiconductor light elements  14  of the light element series connection  15  can be lighted. 
     In a further embodiment shown in  FIGS. 5 and 6  the control means  20  is connected via a first node  32  with a tap  33  of the light element series connection  15 . The control means  20  is connected via a second node  24  with the ground GND. The light element series connection  15  is connected to the output  12  at one side and to the ground GND at the other side. The control means  20  is used to bypass and short circuit those semiconductor light elements  14  which are connected in parallel with the control means  20  between the tap  33  and the ground GND if the control means  20  is in the heating condition. The other semiconductor light elements  14  which are arranged between the output  12  of the drive circuit  11  and the tap  33  are used as heating elements  30  and thus form the heating means  26 . A capacitor  35 , for example an electrolytic capacitor, is connected in parallel with the lighting module  13  and/or the light element series connection  15  between the output  12  and the ground GND. 
     An embodiment of the control means  20  of the embodiment of the lighting device  10  shown in  FIG. 5  is illustrated in  FIG. 6 . A voltage divider  39  is provided containing a first resistor  40  connected via a center tap  41  with the temperature dependent element  21 . The temperature dependent element  21  is preferably formed by a temperature dependent resistor  42  having a negative temperature coefficient. The temperature dependent resistor  42  is thermally coupled with the lighting module  13  as illustrated schematically by the corrugated arrows. The voltage divider  39  is connected at the side of the first resistor  40  to a supply voltage Vcc and at the side of the temperature dependent resistor  42  to the ground GND or the second node  34 . 
     The control means  20  further contains the controllable switch  23  which is in this embodiment formed by a field-effect transistor  43 . This controllable switch  23  is used as bypass element to short-circuit some of the semiconductor light elements  14  in the heating condition of the control means  20 . A control input  44  of the controllable switch  23  is formed by the gate of the field-effect transistor  43 . The controllable switch  23  is inserted into the connection between the first node  32  and the second node  34  so that depending on the condition of the controllable switch  23  a conductive connection between the two nodes  32 ,  34  can be provided or interrupted. In the present embodiment the drain of the field-effect transistor  43  is connected with the first node  32  and the source is connected with the second node  34 . 
     The embodiment of the lighting device  10  shown in  FIGS. 5 and 6  works as follows: 
     When the temperature T of the lighting module  13  decreases and reaches the minimum temperature value Tmin, the resistance of the temperature dependent resistor  42  has increased to a value so that the voltage across the temperature dependent resistor  42  has reached a value which switches the controllable switch  23  in its conductive condition. Accordingly some of the semiconductor light elements  14  which are connected between the tap  33  and the ground GND are short-circuited. In order to heat the lighting module  13  the output voltage Vout is applied to the lighting module  13  so that a current flows through those semiconductor light elements  14  that are used as heating elements  30 , via the tap  33  through the controllable switch  23  to the ground GND. The semiconductor light elements  14  used as heating element  30  are lighted and produce heat for heating the lighting module  13 . Accordingly a further drop of the temperature T of the lighting module  13  can be prevented. 
     It is also possible to heat other electric or electronic components, e.g. the electrolytic capacitor  35 , of the lighting device  10  by using the heating means  26 . Those components are thermally coupled with the heating means  26 . 
     Another embodiment of the lighting device  10  is shown in  FIGS. 3 and 4 . The light element series connection  15  and the control means  20  can have a configuration shown in any of the embodiments according to  FIG. 1, 2, 5, 6 or 7  as explained above. The embodiment shown in  FIGS. 3 and 4  has a cooling means  27  containing a heat sink  47 . The cooling means  27  can alternatively or additionally contain a fan  48 . The heat sink  47  and/or the fan  48  is used for dissipating heat from the lighting module  13  if the semiconductor lighting elements  14  are lighted. Accordingly an undesired increase of the temperature T of the lighting module  13  can be avoided. 
     The cooling means  27  can adopt a first condition I for cooling the lighting module  13  and a second condition II in which the cooling effect of cooling the lighting module  13  is at least reduced or suspended. For example a fan  48  can be operated in the first condition I whereas in the second condition II the fan  48  is switched off. 
     In the embodiment according to  FIGS. 3 and 4  the cooling means  27  comprises a drive arrangement  49  for moving the heat sink  47  between a first position P 1  in the first condition I of the cooling means  27  and a second position P 2  in the second condition II of the cooling means  27 . The drive arrangement  49  includes in this embodiment an electric drive  50  connected with the heat sink  47  via a gear  51 . The gear  51  can for example be formed by a rack and pinion gear. 
     A spring arrangement  52  is optionally provided and is used to create a spring force which presses the module carrier  16  and/or the lighting module  13  against the heat sink  47  if the cooling means  27  are in the first condition I. Accordingly a good heat conduction or transfer can be provided between the lighting module  13  or the module carrier  16  respectively and the heat sink  47 . To improve this thermal coupling a graphite layer  53  can be attached either to the module carrier  16  and/or the lighting module or else to the heat sink  47 . The graphite layer  53  is thus arranged between the heat sink  47  and the module carrier  16  and/or the lighting module  13  in the first condition I of the cooling means  27 . 
     The drive arrangement  49  can be operated to move the heat sink  47  against the force of the spring arrangement  52  away from the module carrier  16  or the lighting module  13  and accordingly from the first position P 1  into the second position P 2 . In this second position P 2  a gap  54  exists between the module carrier  16  and/or the lighting module  13  and the heat sink  47  so that the thermal dissipation of heat produced in the lighting module  13  via the heat sink  47  is reduced or even completely blocked. Accordingly in the second condition II of the cooling means  27  no or only a negligible cooling effect is provided. The cooling means  27  are changed into this second condition II if the heating requirement is fulfilled and heating of the lighting module  13  is necessary. Preferably the cooling means  27  are kept in the second condition II unless the lighting module  13  or respectively the semiconductor light elements  14  of the lighting module  13  are lighted for illumination and cooling is necessary. This avoids that the heat produced during a heating operation or after the end of a heating operation is dissipated too quickly through the cooling means  27 . Such cooling is only necessary when the lighting device  10  is used for illumination and the semiconductor light elements  14  are lighted. 
     The cooling means  27  with the drive arrangement  49  and the heat sink  47  can be provided in all of the described embodiments. Alternatively and/or additionally the cooling means  27  can comprise a fan  48  in all of the above described embodiments. The fan  28  is rotating in the first condition I of the cooling means  27  whereas the fan  48  is standing still in the second condition II of the cooling means  27 . At least one Peltier element can alternatively or additionally be used as thermoelectric cooling element in the cooling means  27  and arranged on the module carrier  16  and/or at the lighting module  13 . 
     The above mentioned embodiments can have a modified heating means  26  having alternatively or additionally a heating arrangement  57  containing at least one heating component  58  as schematically illustrated in  FIG. 9 . Each heating component  58  can be formed by an electrical resistor and/or an electrical heating coil. The at least one heating component  58  is arranged directly at the lighting module  13  or else on the module carrier  16  and thermally coupled with the lighting module  13 . The number of heating component  58  depends on the size and the shape of the lighting module  13  and/or on the heating power of the heating component  58 . 
     Further in all of the described embodiments the lighting device  10  and particularly the drive circuit  11  and the control means  20  can be switched into a standby-mode. During this standby mode it is possible to wake up the control means  20  after a predetermined period of time has expired to check the temperature T of the lighting module  13 . If this temperature T has reached or even fallen below the minimum temperature value Tmin the control means  20  adopt its heating condition and the heating of a lighting module  13  is provided as explained above. 
     The control of the heating and/or cooling can not only depend on the temperature T of the lighting module  13 , but can alternatively depend on additional parameters, such as the season, the calendar day, the day time, the global position of the lighting device  10 , etc. Accordingly the control means  20  can contain devices such as timing devices, clocks, positioning sensors (for example global satellite based positioning sensors), etc. to provide such parameters. For example the heating duration and/or the heating energy and/or the heating power or the like can be controlled depending on the temperature T and/or one or more of such additional parameters. 
     The present invention refers to a lighting device  10  and a method of operating this lighting device  10 . A lighting module  13  having a light element series connection  15  of several semiconductor light elements  14  is connected to the output  12  of a drive circuit  11 . The control means  20  is provided which is configured to adapt a heating condition if a heating requirement is fulfilled. In the heating condition a heating means  26  is activated by the control means  20  to heat the lighting module. The heating requirement is fulfilled when the temperature T of the lighting module  13  drops down to a minimum temperature value Tmin. In so doing an undesired increase of the forward voltage Vf of the light element series connection  15  can be avoided. 
     REFERENCE SIGNS LIST 
     
         
         
           
               10  lighting device 
               11  drive circuit 
               12  output 
               13  lighting module 
               14  semiconductor light elements 
               15  light element series connection 
               16  module carrier 
               20  control means 
               21  temperature dependent element 
               22  evaluating part 
               23  controllable switch 
               26  heating means 
               27  cooling means 
               28  common interface 
               30  heating element 
               31  bypass element 
               32  first node 
               33  tap 
               34  second node 
               35  capacitor 
               39  voltage divider 
               40  first resistor 
               41  center tap 
               42  temperature dependent resistor 
               43  field effect transistor 
               47  heat sink 
               48  fan 
               49  drive arrangement 
               52  spring arrangement 
               53  graphite layer 
               54  gap 
               57  heating arrangement 
               58  heating component 
             I first condition 
             II second condition 
             C characteristic of the temperature dependent element 
             DV difference voltage 
             P 1  first position 
             P 2  second position 
             R temperature range 
             T temperature of the lighting module 
             Tmin minimum temperature value 
             Vcc supply voltage 
             Vf forward voltage 
             Vlim upper limit value of the forward voltage 
             Vout output voltage