Abstract:
A lighting controller for a lighting device for a vehicle includes a semiconductor light source; a power source for supplying electric power; and control circuitry for receiving the electric power from the power source and controlling a current supplied to the semiconductor light source. The control circuitry determines an amount of time the semiconductor light source is in a turned on state and an amount of time the semiconductor light source is in a turned off state. The control circuitry controls a value of the current supplied to the semiconductor light source based on both the determined amount of time the semiconductor light source is in a turned on state and the determined amount of time the semiconductor light source is in a turned off state. A method of controlling a lighting device for a vehicle includes receiving electric power from a power source; supplying a current to a semiconductor light source, determining an amount of time the semiconductor light source is in a turned on state and an amount of time the semiconductor light source is in a turned off state, and controlling a value of the current supplied to the semiconductor light source based on both the determined amount of time the semiconductor light source is in a turned on state and the determined amount of time the semiconductor light source is in a turned off state.

Description:
BACKGROUND OF INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention relates to a lighting controller for a lighting device for a vehicle, and more particularly to a lighting controller for a lighting device for a vehicle constructed so as to control the lighting of a semiconductor light source composed of semiconductor light emitting element.  
         [0003]     2. Background Art  
         [0004]     As a lighting device for a vehicle, a lighting device using a semiconductor light emitting element such as an LED (light Emitting Diode) as a light source has been hitherto known. On such kind of lighting device for a vehicle, a lighting control circuit for controlling the lighting of the LED is mounted.  
         [0005]     When the LED is controlled to be turned on using the lighting control circuit, a control for always supplying a constant current to the LED can be employed. However, the LED has characteristics that when the temperature of the LED rises, even if the same forward current is supplied to the LED, the light flux of emitted light (a quantity of light) is lowered. Therefore, when a control for always supplying a constant current to the LED is carried out, the quantity of light is sequentially lowered due to the self-heat generation of the LED itself. Especially, when a heat radiating structure is used for the purpose of raising the temperature to a temperature as high as the maximum junction temperature of the LED in view of cost or size, the amount of decrease of the quantity of light is more drastic. When the quantity of light is lowered, a visibility is lowered for a driver, so that there is a fear that the driver cannot perform driving safely. Further, when the LED is used as a headlight or a signal light of a vehicle, the LED may possibly not satisfy required product standards.  
         [0006]     Further, because the LED has an unevenness of the forward voltage Vf, when a prescribed current is supplied to the LED, an electric power applied to the LED having a higher forward voltage Vf is high, so that a heat generation is greater. Accordingly, as the lighting control circuit, a lighting control circuit needs to be used that has a capability and a size that allows supply of the electric power anticipating the unevenness of the forward voltage Vf of the LED. Further, the heat radiating structure of the LED needs to have a size, a form, and a thermal resistance anticipating the heat generation of the LED.  
         [0007]     Thus, to ensure a necessary quantity of light even when the temperature of the LED rises, a lighting control circuit has been proposed in which a time during which the LED continuously emits light is measured and a current supplied to the LED is increased in accordance with the measured time (see Patent Document 1).  
         [0008]     [Patent Document 1] JP-A-2004-330819.  
         [0009]     As described in Patent Document 1, the lighting time during which the LED continuously emits light is measured and the current supplied to the LED is controlled to increase in accordance with the measured time, so that the quantity of emitted light of the LED can be prevented from falling in accordance with the rise of temperature.  
       SUMMARY OF INVENTION  
       [0010]     When the quantity of emitted light of the LED is always controlled to be constant, the current of the LED needs to be controlled by considering not only the time that the LED is turned on, but also, the time that the LED is turned off. That is, the temperature of an LED at the time of initial turning on of the LED is different depending on the amount of time that the LED was turned off before the initial turning on. For instance, when the LED is in a turned on state for a long time, then, is turned off and turned on again in a short time, because the LED is already in a state of a high temperature, a larger current than that required at a low temperature needs to be supplied to the LED. In contrast, when the LED is turned off for a long time and then turned on under a sufficiently cooled state, because the LED is in a state of the low temperature, a smaller current than that required at the high temperature needs to be supplied to the LED.  
         [0011]     One or more embodiments of the present invention maintain a quantity of emitted light of a semiconductor light source to be constant irrespective of the temperature of the semiconductor light source.  
         [0012]     In one or more embodiments, a lighting controller for a lighting device for a vehicle comprises: a current supply control unit for receiving the supply of an electric power from a power source to control the supply of a current to a semiconductor light source; and a time measuring unit for measuring a turned on time and a turned off time of the semiconductor light source. The current supply control unit sequentially further increases the value of the current supplied to the semiconductor light source as the turned on time measured by the time measuring unit is longer and further increases the value of the current supplied to the semiconductor light source at the time of initial turning on of the semiconductor light source when the turned off time is shorter.  
         [0013]     When the supply of the current to the semiconductor light source is controlled, because the temperature of the semiconductor light source is indirectly measured, the turned on time and the turned off time of the semiconductor light source are measured. Then, as the turned on time of the semiconductor light source is longer, the temperature of the semiconductor light source is determined to be sequentially more elevated and the value of the current supplied to the semiconductor light source is sequentially further increased. Accordingly, a quantity of the emitted light of the semiconductor light source can be prevented from being lowered in accordance with the rise of the temperature of the semiconductor light source and the quantity of the emitted light of the semiconductor can be maintained to be constant. Further, when the semiconductor light source is turned on, as the turned off time is shorter, it is determined that the heat of the semiconductor light source is not adequately radiated, and accordingly, the semiconductor light source is in a state of a high temperature. Thus, the value of the current supplied to the semiconductor light source is increased, so that the quantity of the emitted light of the semiconductor light source can be prevented from being lowered during turning on the semiconductor light source and the quantity of the emitted light of the semiconductor light source can be maintained to be constant. That is, the current of the semiconductor light source is controlled to meet the change of the temperature of the semiconductor light source, and accordingly, the quantity of the emitted light of the semiconductor light source can be maintained to be constant irrespective of the temperature of the semiconductor light source.  
         [0014]     In one or more embodiments, a lighting controller for a lighting device for a vehicle further comprises: a voltage detecting unit for detecting the forward voltage of the semiconductor light source. The current supply control unit sequentially further increases the value of the current supplied to the semiconductor light source as the forward voltage detected by the voltage detecting unit is lower.  
         [0015]     When the current is supplied to the semiconductor light source, the forward voltage of the semiconductor light source is detected. As the forward voltage is lower, namely, as the temperature of the semiconductor light source is higher, the value of the current supplied to the semiconductor light source is sequentially further increased, so that the quantity of the emitted light of the semiconductor light source can be maintained to be constant. In this case, the detected result of the voltage detecting unit is used as a back up. Thus, even when the time measuring unit is failed, the quantity of the emitted light of the semiconductor light source can be maintained to be constant irrespective of the temperature of the semiconductor light source.  
         [0016]     In one or more embodiments, in a lighting controller for a lighting device for a vehicle, the current supply control unit limits the current supplied to the semiconductor light source to a limit value or lower when the value of the current supplied to the semiconductor light source reaches the limit value.  
         [0017]     When the value of the current supplied to the semiconductor light source reaches the limit value, the current supplied to the semiconductor light source is limited to a value not higher than the limit value, so that the thermo-runaway of the semiconductor light source can be prevented and a heat radiating structure for radiating the heat of the semiconductor light source can be miniaturized.  
         [0018]     As apparent from the above-description, according to the lighting controller for a lighting device for a vehicle in accordance with one or more embodiments, the quantity of the emitted light of the semiconductor light source can be maintained to be constant irrespective of the temperature of the semiconductor light source.  
         [0019]     According to one or more embodiments, even when the time measuring unit is failed, the quantity of the emitted light of the semiconductor light source can be maintained to be constant irrespective of the temperature of the semiconductor light source.  
         [0020]     According to one or more embodiments, the thermo-runaway of the semiconductor light source can be prevented and the heat radiating structure for radiating the heat of the semiconductor light source can be miniaturized.  
         [0021]     Other aspects and advantages of the invention will be apparent from the following description and the appended claims. 
     
    
     BRIEF DESCRIPTION OF DRAWINGS  
       [0022]      FIG. 1  is a circuit block diagram of a lighting controller for a lighting device for a vehicle showing a first embodiment of the present invention.  
         [0023]      FIG. 2  is a circuit block diagram of a switching regulator.  
         [0024]      FIG. 3  is a circuit block diagram of a control circuit.  
         [0025]      FIG. 4  is a wave form diagram for explaining the operation of the control circuit.  
         [0026]      FIG. 5  is a circuit block diagram of a controlling power source.  
         [0027]      FIG. 6  is a wave form diagram for explaining the relation between a turned on time and a turned off time and a supplied current.  
         [0028]      FIG. 7  is a circuit block diagram of a lighting controller for a lighting device for a vehicle showing a second embodiment of the present invention.  
         [0029]      FIG. 8  is a circuit block diagram of a lighting controller for a lighting device for a vehicle showing a third embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION  
       [0030]     Now, embodiments of the present invention will be described below.  FIG. 1  is a circuit block diagram of a lighting controller for a lighting device for a vehicle showing a first embodiment of the present invention.  FIG. 2  is a circuit block diagram of a switching regulator.  FIG. 3  is a circuit block diagram of a control circuit.  FIG. 4  is a wave form diagram for explaining an operation of the control circuit.  FIG. 5  is a circuit block diagram of a controlling power source.  FIG. 6  is a wave form diagram for explaining the relation between a turned on time and a turned off time and a supplied current.  FIG. 7  is a circuit block diagram of a lighting controller for a lighting device for a vehicle showing a second embodiment of the present invention.  FIG. 8  is a circuit block diagram of a lighting controller for a lighting device for a vehicle showing a third embodiment of the present invention.  
         [0031]     In these drawings, the lighting controller  10  for a lighting device for a vehicle includes, as shown in  FIG. 1 , a switching regulator  12 , a controlling power source  14 , a control circuit  16 , a time measuring circuit  18  and shunt resistances R 1  and R 2 . To the switching regulator  12 , an LED  20  as a load is connected. The LED  20  is connected in parallel with the output side of the switching regulator  12  as a semiconductor light source composed of semiconductor light emitting elements.  
         [0032]     As the LED  20 , a plurality of LEDs mutually connected in series may be used, or the plurality of LEDs mutually connected in series may be used as a power source block, or a plurality of power source blocks respectively connected in parallel may be used. Further, a plurality of LED chips mutually accommodated in series in a package may be used in place of the LED  20 . Further, the LED  20  may be formed as light sources of various kinds of lighting devices for vehicles such as a head lamp, a stop and tail lamp, a fog lamp and a turn signal lamp.  
         [0033]     As shown in  FIG. 2 , the switching regulator  12  includes a transformer T 1 , a capacitor C 1 , an NMOS transistor  22 , a diode D 1  and a capacitor C 2 . The capacitor C 1  is connected in parallel with a primary side of the transformer T 1  and the NMOS transistor  22  is connected in series to the primary side of the transformer T 1 . One end side of the capacitor C 1  is connected to a positive terminal of a battery  26  to be mounted on a vehicle (a dc power source) through a power supply input terminal  24  and the other end side is connected to a negative terminal of the battery  26  to be mounted on a vehicle through a power supply input terminal  28  and grounded. The NMOS transistor  22  has a drain connected to the primary side of the transformer T 1 , a source grounded, and a gate connected to the control circuit  16 . With the secondary side of the transformer T 1 , the capacitor C 2  is connected in parallel through the diode D 1 . A node of the diode D 1  and the capacitor C 2  is connected to an anode side of the LED  20  through an output terminal  30 . One end side of the secondary side of the transformer T 1  is grounded together with one end side of the capacitor C 2  and connected to a cathode side of the LED  20  through the shunt resistance R 1  and an output terminal  32 . The output terminal  32  is connected to the control circuit  16  through the shunt resistance R 2  and a current detecting terminal  34 . The shunt resistance R 1  is formed as a current detecting unit for detecting a current supplied to the LED  20 . Voltage generated at both the ends of the shunt resistance R 1  is fed back to the control circuit  16  as the current of the LED  20 .  
         [0034]     The NMOS transistor  22  is formed as a switching element turned on and off in response to an on/off signal (a switching signal) outputted from the control circuit  16 . When the NMOS transistor  22  is turned on, an input voltage from the battery  26  to be mounted on a vehicle is accumulated in the transformer T 1  as electromagnetic energy. When the NMOS transistor  22  is turned off, the electromagnetic energy accumulated in the transformer T 1  is discharged to the LED  20  as light emitting energy from the secondary side of the transformer T 1  through the diode D 1 .  
         [0035]     That is, the switching regulator  12  is constructed as a current supply control unit for receiving the supply of an electric power from the battery  26  to be mounted on a vehicle and controlling the supply of the current to the LED  20  together with the control circuit  16 . In this case, the switching regulator  12  compares the voltage of the current detecting terminal  34  with a prescribed voltage to control an output current in accordance with the result of the comparison.  
         [0036]     Specifically, the control circuit  16  for controlling the switching regulator  12  includes, as shown in  FIG. 3 , a comparator  36 , an error amplifier  38 , a saw tooth wave generator  40 , a resistance voltage  42 , resistances R 3 , R 4  and R 5  and a capacitor C 3 . An output terminal  44  of the comparator  36  is directly connected to the gate of the NMOS transistor  22  or through a current amplifying preamplifier (not shown in the drawing). An input terminal  46  connected to one end of the resistance R 3  is connected to the current detecting terminal  34 . To the input terminal  46 , voltage fed back from the current detecting terminal  34  is applied. The resistances R 3  and R 4  divide the voltage applied to the input terminal  46  to apply the voltage obtained by dividing the voltage to a negative input terminal of the error amplifier  38 . The error amplifier  38  outputs voltage corresponding to the difference between the voltage applied to the negative input terminal and the reference voltage  42  to a positive input terminal of the comparator  36  as a threshold value Vth. The comparator  36  takes in a saw tooth wave Vs to a negative input terminal from the saw tooth wave generator  40  to compare the saw tooth wave Vs with the threshold value Vth and outputs an on/off signal corresponding to the compared result to the gate of the NMOS transistor  22 .  
         [0037]     As shown in FIGS.  4 ( a ) and  4 ( b ), when the level of the threshold value Vth is located at a substantially intermediate part of the saw tooth wave Vs, the on/off signal of on duty as high as about 50% is outputted. On the other hand, when the level of the voltage fed back from the current detecting terminal  34  is lower than the reference voltage  42  as the output current of the switching regulator  12  is decreased, the level of the threshold value Vth by the output of the error amplifier  38  is high. Thus, as shown in FIGS.  4 ( c ) and  4 ( d ), the on/off signal of on duty higher than 50% is outputted from the comparator  36 . As a result, the output current of the switching regulator  12  is increased.  
         [0038]     On the contrary, when the level of the voltage fed back from the current detecting terminal  34  is higher than the reference voltage  42  as the output current of the switching regulator  12  is increased and the level of the threshold value Vth by the output of the error amplifier  38  is lowered, the on/off signal of on duty lower than 50% is outputted from the comparator  36 , as shown in FIGS.  4 ( e ) and  4 ( f ). As a result, the output current of the switching regulator  12  is decreased. A chopping wave generator for generating a chopping wave (a chopping wave signal) can be used in place of the saw tooth wave generator  40 .  
         [0039]     Further, to the control circuit  16 , the electric power is supplied from the controlling power source  14 . The controlling power source  14  includes, as shown in  FIG. 5 , an NPN transistor  48  as a series regulator, a resistance R 6 , a Zener diode ZD 1  and a capacitor C 4 . A collector of the NPN transistor  48  is connected to the power supply input terminal  24  and an emitter is connected to the control circuit  16  through an output terminal. When a supply voltage is applied to the NPN transistor  48  from the power supply input terminal  24 , the NPN transistor  48  outputs voltage corresponding to Zener voltage generated at both the ends of the Zener diode ZD 1  to the control circuit  16  from the emitter through the output terminal.  
         [0040]     As shown in  FIG. 1 , the time measuring circuit  18  includes PNP transistors  50  and  52 , an NPN transistor  54 , operation amplifiers  56  and  58 , resistances, R 7 , R 8 , R 9 , R 10 , R 11 , and R 12 , and capacitor C 5 .  
         [0041]     The PNP transistors  50  and  52  form a current mirror circuit. The PNP transistor  50  has a collector connected to the current detecting terminal  34  and connected to the output terminal  32  through the resistance R 2 . The PNP transistor  52  has a collector connected to the collector of the NPN transistor  54  together with a base. The NPN transistor  54  has an emitter connected to a negative input terminal of the operation amplifier  56  and connected to the output side of the operation amplifier  58  through the resistance R 7 . To the negative input terminal of the operation amplifier  56 , the output voltage of the operation amplifier  58  is applied through the resistance R 7 . To a positive input terminal of the operation amplifier  56 , a voltage V 1  obtained by dividing a reference voltage Vref by the resistance R 9  and the resistance R 10  is applied. The voltage V 1  obtained by dividing the reference voltage by the resistance R 9  and the resistance R 10  is set so as to meet voltage at the time of full charge, of the voltage V 2  generated at both the ends of the capacitor C 5  and a current I 1  corresponding to a potential difference between the output voltage V 3  of the operation amplifier  58  and the voltage V 1  is supplied through the resistance R 7 . When the current I 1  is supplied to the PNP transistor  52  of the current mirror circuit, a current  12  equal to the current I 1  is allowed to flow through the PNP transistor  50  and the resistance R 2 . Each of the currents I 1  and I 2  is set to be “0” when the voltage V 1 =V 3 . To the positive input terminal of the operation amplifier  58 , the voltage generated at both the ends of the capacitor C 5  or the voltage V 2  obtained by dividing the reference voltage Vref by the resistance R 11  and the resistance R 12  is applied. The voltage V 2  generated at both the ends of the capacitor C 5  is gradually boosted in accordance with a time constant determined from the resistances R 11  and R 12  and the capacitor C 5  when the LED  20  is turned on by turning on a power source. That is, as the turned on time is longer, the voltage V 2  is sequentially more elevated. Then, when the capacitor C 5  is fully charged, the voltage V 2  is maintained to a prescribed value. The voltage V 2  is amplified by the operation amplifier  58  and outputted as the voltage V 3 . As the turned on time is longer, the voltage V 3  is also more elevated like the voltage V 2 . When the capacitor C 5  is fully charged, the potential difference between the voltage V 3  and the voltage V 1  becomes 0 so that the currents I 1  and I 2  are not supplied to the current mirror circuit.  
         [0042]     On the other hand, when a power switch is turned off so that the LED  20  is turned off, an electric charge accumulated in the capacitor C 5  is discharged through the resistances R 11  and R 12  and the voltage V 2  is sequentially lowered in accordance with the time constant. As the turned off time is longer, the voltage V 2  is further lowered. When the electric charge of the capacitor C 5  is exhausted, the voltage V 2  becomes 0V. However, as the turned off time is shorter like a case that the LED  20  is turned on again in a short time after the LED  20  is turned off, the electric charge is accumulated in the capacitor C 5 , so that the voltage V 2  is higher than 0V. Therefore, when the turned off time is long and the LED  20  is turned on after the electric charge of the capacitor C 5  is exhausted, the potential difference between the voltage V 1  and the voltage V 3  is large. Thus, the value of the currents I 1  and I 2  at the beginning to turn on the LED  20  is large. On the contrary, when the turned off time is short and a large quantity of electric charge is accumulated in the capacitor C 5 , if the LED  20  is turned on, the potential difference between the voltage V 1  and the voltage V 3  is small. Thus, the value of the currents I 1  and I 2  at the beginning to turn on the LED  20  is small.  
         [0043]     Here, the control circuit  16  performs a control in such a way that, as the current I 2  acting on the resistance R 2  is smaller (as the turned on time is longer) so as to make the voltage of the current detecting terminal  34  constant, the supply current (output current) of the switching regulator  12  is gradually increased as shown in  FIG. 6 . Therefore, when the LED  20  is turned on, as the electric charge is accumulated in the capacitor C 5 , the voltage V 2  is elevated, so that the current I 2  acting on the resistance R 2  is sequentially decreased in accordance with the rise of the voltage V 2 . Accordingly, the current supplied to the LED  20  is sequentially increased.  
         [0044]     In such a way, when the LED  20  is turned on, the current supplied to the LED  20  is increased at the time of initial turning on of the LED  20  in accordance with the rise of the temperature of the LED  20 . Thus, the light flux of the LED  20  can be prevented from being decreased and the quantity of light of the LED  20  can be controlled to be constant. As a result, The LED  20  can be prevented from being dark.  
         [0045]     When the capacitor C 5  is fully charged and the voltage V 1  is equal to the voltage V 3  under a state that the LED  20  is turned on, the current I 2  acting on the resistance R 2  becomes 0 and the control circuit  16  shifts to a constant current control for maintaining the output current of the switching regulator  12  to a prescribed current (a limit value). In this case, the current supplied to the LED  20  is limited to a value not higher than the limit value (the prescribed current) so that the thermo-runaway of the LED  20  can be prevented.  
         [0046]     On the other hand, when the LED  20  is turned on again after the LED  20  is turned off, as the turned off time is shorter, the value of the current  12  acting on the resistance R 2  is smaller as shown in  FIG. 6 , so that the value of the current of the LED  20  at the time of initial turning on of the LED  20  is high. Thus, the quantity of light of the LED  20  can be maintained to be constant even at the time of initial turning on of the LED  20 . Accordingly, the LED  20  can be prevented from being dark.  
         [0047]     According to this embodiment, because the temperature of the LED  20  is indirectly measured, the turned on time and the turned off time of the LED  20  are measured. Then, as the turned on time of the LED  20  is longer, the value of the current supplied to the LED  20  is sequentially further increased. Accordingly, a quantity of the emitted light of the LED  20  can be prevented from being lowered in accordance with the rise of the temperature of the LED  20  and the quantity of the emitted light of the LED  20  can be maintained to be constant. Further, when the LED  20  is initially turned on, as the turned off time is shorter, the value of the current supplied to the LED  20  is further increased, so that the quantity of the emitted light of the LED  20  can be prevented from being lowered during turning on the LED  20  and the quantity of the emitted light of the LED  20  can be maintained to be constant. That is, according to this embodiment, the current of the LED  20  is controlled to meet the change of the temperature of the LED  20 , and accordingly, the quantity of the emitted light of the LED  20  can be maintained to be constant irrespective of the temperature of the LED  20  and the LED  20  can be prevented from being dark.  
         [0048]     Now, a second embodiment of the present invention will be described below with reference to  FIG. 7 . In this embodiment, a voltage detecting circuit  60  for detecting the forward voltage of an LED  20  is provided in place of the time measuring circuit  18  and other structures are the same as those shown in  FIG. 1 . The voltage detecting circuit  60  includes a resistance R 13 , a Zener diode ZD 2  and a capacitor C 6  as a voltage detecting unit for detecting the forward voltage of the LED  20 . The resistance R 13  is connected in series to the Zener diode ZD 2 . One end side of the resistance R 13  is connected to an output terminal  30  and an anode side of the Zener diode ZD 2  is connected to a current detecting terminal  34 . To the anode side of the Zener diode ZD 2 , the capacitor C 6  is connected and one end side of the capacitor C 6  is grounded.  
         [0049]     The Zener voltage of the Zener diode ZD 2  is set so as to meet a forward voltage Vf at a low temperature of the forward voltage Vf generated at both the ends of the LED  20 . As the voltage applied to the LED  20  is higher, a larger current as a Zener current Iz is supplied to the Zener diode ZD 2 . On the contrary, as the forward voltage Vf of the LED  20  is lower with the rise of the temperature of the LED  20 , a smaller current as the Zener current Iz is allowed to flow to the Zener diode.  
         [0050]     Accordingly, at the time of initial turning on of the LED  20 , when the voltage applied to the LED  20  is higher than the Zener voltage of the Zener diode ZD 2 , the Zener current Iz is supplied to a resistance R 2  through the Zener diode ZD 2 . After that, the LED  20  is continuously turned on and as the turned on time of the LED  20  is longer, the forward voltage Vf of the LED  20  is sequentially lowered. Accordingly, the value of the Zener current Iz is also sequentially decreased. At this time, a control circuit  16  performs a control in such a way that as the turned on time of the LED  20  is longer, namely, the forward voltage Vf is lower, the value of the current supplied to the LED  20  is sequentially further increased to maintain the voltage of the current detecting terminal  34  to be constant. As a result, even when the forward voltage Vf is sequentially lowered with the rise of the temperature of the LED  20 , because the value of the current supplied to the LED  20  is sequentially increased, the quantity of light of the LED  20  can be maintained to be constant and the LED  20  can be prevented from being dark.  
         [0051]     During a process that the current supplied to the LED  20  is increased, when the forward voltage Vf of the LED  20  is equal to the Zener voltage of the Zener diode ZD 2 , the Zener current Iz is 0 and the current acting on the resistance R 2  also becomes 0. When the current Iz acting on the resistance R 2  is 0, the control circuit  16  shifts to a constant current control for maintaining the output current of a switching regulator  12  to be a prescribed current (a limit value). In this case, the current supplied to the LED  20  is limited to the limit value (the prescribed current) or lower so that the thermo-runaway of the LED  20  can be prevented.  
         [0052]     In this embodiment, as the turned on time of the LED  20  is longer, the control is performed that the value of the current supplied to the LED  20  is sequentially increased. Accordingly, even when the forward voltage Vf is sequentially lowered in accordance with the rise of the temperature of the LED  20 , since the value of the current supplied to the LED  20  is sequentially increased, the quantity of light of the LED  20  can be maintained to be constant and the LED  20  can be prevented from being dark.  
         [0053]     Now, a third embodiment of the present invention will be described with reference to  FIG. 8 . In this embodiment, the first embodiment is combined with the second embodiment and a limiter circuit  62  is provided.  
         [0054]     The limiter circuit  62  includes an operation amplifier  64 , a resistance R 14 , a diode D 2  and a reference voltage  66 . To the negative input terminal of the operation amplifier  64 , the reference voltage  66  is applied. A positive input terminal of the operation amplifier  64  is connected to an output terminal  32  and to one end side of a resistance R 2 . An output side of the operation amplifier  64  is connected to a current detecting terminal  34  through the diode D 2  and the resistance R 14 .  
         [0055]     The reference voltage  66  is set to the same voltage as a voltage drop when the value of a current desired to be limited is supplied to a resistance R 1 . The operation amplifier  64  does not operate until the voltage of the positive input terminal of the operation amplifier  64  is equal to the reference voltage  66  of the negative input terminal during a process that as the forward voltage Vf of an LED  20  is lowered in accordance with the rise of the temperature of the LED  20 , and accordingly, a Zener current Iz is sequentially decreased. Along therewith, a control circuit  16  performs a control for sequentially increasing the output current of a switching regulator  12 . Then, when the forward voltage Vf is sequentially lowered in accordance with the rise of the temperature of the LED  20 , the current supplied to the LED  20  is increased and the voltage drop of the resistance R 1  reaches the reference voltage  66 , the operation amplifier  64  supplies the current as a source.  
         [0056]     Namely, the output of the operation amplifier  64  is maintained to be a low level until the positive input terminal of the operation amplifier  64  corresponds to the reference voltage  66 . As the forward voltage of the LED  20  is lowered, the current value of the Zener current Iz supplied to the resistance R 2  is also sequentially decreased. Then, when the voltage of the positive input terminal of the operation amplifier  64  corresponds to the reference voltage  66 , the output of the operation amplifier  64  becomes a high level, so that a current through the diode D 2  and the resistance R 14  is supplied to the resistance R 2  in addition to the Zener current Iz. At this time, the current supplied to the shunt resistance R 1  serves as a limit value (a prescribed current). The current limited to a value not higher than the limit value is supplied to the LED  20  and the switching regulator  12  shifts to a constant current control.  
         [0057]     In this case, before the forward voltage Vf of the LED  20  is equal to the Zener voltage of a Zener diode ZD 2 , the value of the current to be supplied to the LED  20  is controller to a value not higher than the limit value by the limiter circuit  62 .  
         [0058]     In this embodiment, as the turned on time of the LED  20  is longer, the control is performed that the value of the current supplied to the LED  20  is sequentially increased. Accordingly, even when the forward voltage Vf is sequentially lowered in accordance with the rise of the temperature of the LED  20 , because the value of the current supplied to the LED  20  is sequentially increased, the quantity of light of the LED  20  can be maintained to be constant and the LED  20  can be prevented from being dark. Further, because the current supplied to the LED  20  can be limited to the value not higher than the limit value (the prescribed current), the thermo-runaway of the LED  20  can be prevented.  
         [0059]     Further, in this embodiment, because the temperature of the LED  20  is indirectly measured, the turned on time and the turned off time of the LED  20  are measured. Then, as the turned on time of the LED  20  is longer, the value of the current supplied to the LED  20  is sequentially further increased. Accordingly, a quantity of the emitted light of the LED  20  can be prevented from being lowered in accordance with the rise of the temperature of the LED  20  and the quantity of the emitted light of the LED  20  can be maintained to be constant. Further, when the LED  20  is initially turned on, as the turned off time is shorter, the value of the current supplied to the LED  20  is further increased, so that the quantity of the emitted light of the LED  20  can be prevented from being lowered during turning on the LED  20  and the quantity of the emitted light of the LED  20  can be maintained to be constant  
         [0060]     Further, in this embodiment, as the turned on time of the LED  20  is longer, the control is performed that the value of the current supplied to the LED  20  is sequentially increased. Accordingly, even when the forward voltage Vf is sequentially lowered in accordance with the rise of the temperature of the LED  20 , because the value of the current supplied to the LED  20  is sequentially increased, the quantity of light of the LED  20  can be maintained to be constant and the LED  20  can be prevented from being dark.  
         [0061]     Further, because a voltage detecting circuit  60  is used as a back up of a time measuring circuit  18 . Thus, even when the time measuring circuit  18  is failed, because, as the turned on time of the LED  20  is longer, the control is performed that the value of the current supplied to the LED  20  is sequentially increased. Accordingly, the quantity of light of the LED  20  can be maintained to be constant and the LED  20  can be prevented from being dark.  
         [0062]     The limiter circuit  62  in this embodiment may be provided in the first embodiment or the second embodiment.  
         [0063]     [Description of Reference Numerals and Signs] 
         [0064]      10  . . . lighting controller for lighting device for vehicle  12  . . . switching regulator  14  . . . controlling power source  16  . . . control circuit  18  . . . time measuring circuit  20  . . . LED  62  . . . limiter circuit  
         [0065]     [ FIG. 1 ] 
         [0066]      12  . . . switching regulator  14  . . . controlling power source  16  . . . control circuit  
         [0067]     [ FIG. 6 ] 
         [0068]     a . . . supplied current b . . . turned on c . . . turned off d . . . time  
         [0069]     While the invention has been described with respect to a limited number of embodiments, those skilled in the art, having benefit of this disclosure, will appreciate that other embodiments can be devised which do not depart from the scope of the invention as disclosed herein. Accordingly, the scope of the invention should be limited only by the attached claims.