Abstract:
A head lamp device for a vehicle includes a DC power source, a diode array having a plurality of series-connected light emitting diodes and a current control circuit that supplies driving current to the diode array. The current control circuit is constituted of a current supply circuit, a voltage measuring circuit, a comparing circuit that compares the voltage drop with a reference value, a judging circuit for judging that there is a short-circuiting at any of the light emitting diodes based on the comparison by the comparing circuit, and a display unit for displaying an alarm when the judging circuit judges that there is a short-circuiting.

Description:
CROSS REFERENCE TO RELATED APPLICATION  
       [0001]     The present application is based on and claims priority from Japanese Patent Application 2006-4796, filed Jan. 12, 2006, the contents of which are incorporated herein by reference.  
       BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     The present invention relates to a head lamp device for a vehicle that includes light emitting diodes.  
         [0004]     2. Description of the Related Art  
         [0005]     JP-A2004-51014 or its counterpart U.S. Pat. No. 6,870,328 B2, discloses a head lamp device that includes a diode array circuit comprised of plural light emitting diodes. Because the voltage drop of the light emitting diode (hereinafter referred to as LED) is as small as the voltage drop of a common diode in the forward direction, it is easy to adjust its resistance to be connectable to a battery of a certain voltage by connecting a suitable number of LEDs in series. JP-A-10-217851 proposes to detect breakdown of an LED by measuring the impedance thereof. JP-A-2000-98941 discloses an array of series-connected diodes that has a bypass circuit at each junction of the diodes to secure operation of other normal diodes even when one diode breaks down. JP-A-2004-314808 proposes to detect the breakdown of the LED based on a voltage drop of a series-circuit of a current detecting resistor and a series-connected diode.  
         [0006]     In such a head lamp device that includes an array of many LEDs, a short circuiting may take place. Because the voltage drop of the LED is very small, decrease in the light intensity of the array as a whole is not so significant even when one or two LEDs are short-circuited. However, the light intensity at a specific area may become lower than a suitable intensity.  
       SUMMARY OF THE INVENTION  
       [0007]     Therefore, an object of the invention is to solve the above stated problem.  
         [0008]     According to a feature of the invention, a head lamp device for a vehicle includes a DC power source, a diode array of a plurality of series-connected light emitting diodes and a current control circuit, which includes current supply means for supplying controlled current to the diode array, voltage measuring means for measuring forward voltage drop of the light emitting diodes, a comparing means that compares the voltage drop with a reference value, judging means for judging whether there is a short-circuiting at any of the light emitting diodes based on the comparison by the comparing means, and display means for displaying an alarm when the judging means judges that there is short-circuiting.  
         [0009]     If a short-circuiting occurs, the driving current increases due to decrease in the resistance thereof, so that significant decrease in the light intensity of the diode array can be prevented, while the short-circuiting of the LED can be easily detected.  
         [0010]     In the above head lamp device, the current supply means may include a current measuring circuit that measures current supplied to the diode array, a DC-DC converter for converting voltage of the DC power source to a voltage suitable to the diode array, and a feedback control circuit for controlling current supplied to the diode array according to current measured by the current measuring circuit. Therefore, increase in the driving current due to a short-circuiting can be limited to a maximum allowable amount.  
         [0011]     Further, the current control circuit may include a memory for storing data of the voltage drop of the light emitting diodes. In this embodiment, the judging means judges that there is a short-circuiting if the voltage drop of one of the light emitting diode is a predetermined value smaller than the voltage drop of the one of the light emitting diodes that is stored in the memory. In addition, the voltage measuring means may include a plurality of voltage sensors for respectively detecting voltage drops of plural sections. In this case, the judging means judges whether there is a short-circuiting or not based on the comparison of the voltage drop of each of the sections with the reference value. Therefore, the short-circuiting can be accurately detected.  
         [0012]     Furthermore, the current supply means may include a current detecting resistor connected between the diode array and the DC power source and a comparator for comparing a voltage drop of the current detecting resistor with a threshold value. The current supply means may further include an over-current-prevention circuit for stopping current supplied to the diode array if the amount of the current becomes a threshold value or larger. The over-current-prevention circuit may include a current limiting transistor connected between the diode array and the current detecting resistor, a comparator for turning off the current limiting transistor if the current detected by the current detecting resistor exceeds a maximum allowable amount. The comparator may change the its threshold value to increase the driving current if the judging means judges that there is a short-circuiting. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0013]     Other objects, features and characteristics of the present invention as well as the functions of related parts of the present invention will become clear from a study of the following detailed description, the appended claims and the drawings. In the drawings:  
         [0014]      FIG. 1  is a circuit diagram of a vehicle head lamp device according to the first embodiment of the invention;  
         [0015]      FIG. 2  is a circuit diagram of a vehicle head lamp device according to the second embodiment of the invention;  
         [0016]      FIG. 3  is a circuit diagram of a vehicle head lamp device according to the third embodiment of the invention;  
         [0017]      FIG. 4  is a flow diagram of an operation of a vehicle head lamp device according to the fourth embodiment of the invention;  
         [0018]      FIG. 5  is a flow diagram of an operation of a vehicle head lamp device according to the fifth embodiment of the invention; and  
         [0019]      FIG. 6  is a flow diagram of an operation of a vehicle head lamp device according to the sixth embodiment of the invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0020]     Some preferred embodiments according to the present invention will be described with reference to the appended drawings.  
         [0021]     A head lamp device for a vehicle according to the first embodiment of the invention will be described with reference to  FIG. 1 .  
         [0022]     The head lamp device includes an array of LEDs (hereinafter referred to the diode array)  1 , a current control circuit  2  and an electronic control unit (ECU)  13 .  
         [0023]     The diode array  1  is comprised of  20  series-connected LEDs, each of which is applied a DC voltage between 3.3 and 4.5 volts so as to pass current of 0.7 A.  
         [0024]     The current control circuit  2  includes a boosting DC-DC converter  3 , voltage dividing circuits  4 - 9 , a current detecting resistor  10 , a comparator  11  and a microcomputer  12 .  
         [0025]     The boosting DC-DC converter  3  boosts the voltage of a DC power source to drive to the diode array  1  and the current control circuit  2 . The DC-DC converter  3  is a common chopper type converter that includes a switching transistor, a rectifier and a smoothing capacitor. The switching transistor switches on and off current flowing therethrough at a certain frequency to generate AC electric power of a higher voltage, which is rectified by the rectifier and smoothed by the smoothing capacitor to provide the DC power to drive the diode array  1 . The boosting ratio of the boosting DC-DC converter  3  can be changed by changing the duty ratio of the switching transistor.  
         [0026]     The diode array  1  is divided into  5  sections, each of which has  4  series-connected LEDs. The voltage dividing circuit  4  provides a voltage signal of the forward voltage of the diode array  1  to input the voltage signal to the microcomputer  12 . Each of the other voltage dividing circuits  5 - 9  also provides a voltage signal of the forward voltage of the corresponding section of the diode array  1  to input the voltage signal to the microcomputer  12 .  
         [0027]     The comparator  11  compares the voltage drop of the current detecting resistor  10  with a threshold voltage that corresponds to the driving current of 0.7 A and provides a signal when the amount of the driving current becomes larger than 0.7 A. The output signal of the comparator  11  is sent to the DC-DC converter to stop its operation. That is, the current supplied to the diode array  1  is feedback-controlled and limited to be less than 0.7 A.  
         [0028]     The microcomputer  12  examines if there is a short-circuiting at any one of the sections of the LEDs or not based on the voltage signals provided by the voltage dividing circuits  4 - 9 . If there is a short-circuiting at any of the sections of the diode array  1 , the microcomputer sends a short-circuiting signal to the ECU  13 , which gives an alarm or signal to request for replacement of the diode array  1 . Before the head lamp device is shipped, the microcomputer  12  detects forward voltage of each section of the diode array  1  and writes the voltage into a nonvolatile memory (e.g. EEPROM) that is mounted in the ECU  13 .  
         [0029]     Thereafter, the microcomputer periodically detects the voltage of each section of the diode array  1  while the head lamp device operates, and the voltage data are sent to the ECU  13 . Incidentally, the voltage of each section is detected at the same timing in the operation cycle of the switching transistor of the boosting DC-DC converter in order to eliminate an influence of a ripple voltage component included in the output voltage of the DC-DC converter  3 . The ECU  13  compares the voltage data with voltage data that are previously stored the memory such as an EEPROM to examine if there is a short-circuiting or not. If the newly detected voltage of any section of the diode array is a predetermined value lower than the previously stored voltage of the corresponding section of the diode array, it is determined that there is a short-circuiting. If all of the sections of the diode array found normal, the previously stored data of the voltage may be replaced with the new data to compensate variations of the diode array with age.  
         [0030]     An ambient temperature may be detected and stored when the data are stored so that the examination of the short-circuiting can be more accurately achieved by taking a difference between the present ambient temperature and the previously stored ambient temperature into account.  
         [0031]     A head lamp device according to the second embodiment will be described with reference to  FIG. 2 . Incidentally, the same reference numeral corresponds to the same or substantially the same part, portion or composition of the head lamp device according to the first embodiment, hereafter.  
         [0032]     The head lamp device according to the second embodiment has a current control circuit  2  that includes a over-current prevention circuit  14  in addition to the boosting DC-DC converter  3 , voltage dividing circuits  4 - 9 , a current detecting resistor  10 , a comparator  11  and a microcomputer  12 .  
         [0033]     The over-current-prevention circuit  14  includes a power transistor  15 , a low-pass filter  16 , a comparator  17  and a transistor  18 .  
         [0034]     The power transistor  15  is connected in series with the diode array  1  and the current detecting resistor  10 . The voltage drop of the current detecting resistor  10  is inputted to the comparator  17 , which turns off the transistor  15  if the amount of the current flowing through the current detecting resistor  10  becomes as much as 0.75 A. If the microcomputer  12  judges short-circuiting of one or more sections of the diode array or detects failure of the feedback control of the DC-DC converter  3  due to the short-circuiting, the microcomputer  12  controls the transistor  18  to turn on, thereby to turn off the transistor  15 . The low-pass filter  16  removes high frequency ripple components included in the output voltage of the DC-DC-converter  3 . The transistor  18  may be connected to the transistor included in the DC-DC converter  3  to turn off this transistor in stead of transistor  15 . It is also possible to connect this transistor  18  to the low-pass filter  16  so that the low-pass filter  16  can control this transistor  18  without the comparator  17 .  
         [0035]     A head lamp device according to the third embodiment will be described with reference to  FIG. 3 .  
         [0036]     In this head lamp device, the threshold voltage of the comparator is controlled by the microcomputer  12 . The maximum allowable current of the diode array and the minimum allowable current are respectively set to 0.75 and 0.65 A.  
         [0037]     If the microcomputer  12  judges short-circuiting of one of the LEDs in the sections  4 - 9  of the diode array  1 , the microcomputer  12  changes the threshold voltage from a voltage that corresponds to the normal current of 0.7 A to a voltage that corresponds to the maximum allowable current of 0.75 A. Accordingly, the current supplied to the diode array  1  increases to thereby increase the light intensity of the diode array  1 .  
         [0038]     A head lamp device according to the fourth embodiment will be described with reference to a flow diagram shown in  FIG. 4 .  
         [0039]     This head lamp device may be the same in structure as any one of the previous embodiments.  
         [0040]     At first, the forward voltage data (Vfs) of the sections of the diode array  1  are read from the EEPROM at S 100 . Then, whether or not there is a short-circuiting in the diode array  1  is examined based on the data at S 110 . If the result of the examination is No, whether initial forward voltage data of Vfs of the sections of the diode array  1  are stored or not is examined at S 120 . If the result of the step S 120  is No, the forward voltages of the sections of the diode array  1  are measured at S 130 . Then, the data of the forward voltage Vfm are stored as Vfs in the EEPROM at S 140 .  
         [0041]     After a certain time period, the forward voltage Vftn of the sections of the diode array  1  is measured at S 150 , and whether the diode array  1  is normal or not is examined by comparing the measured data of Vfm with the stored data of Vfs at S 160 . At this step  160 , whether the difference in the forward voltage of each corresponding section of the diode array  1  between the measured data of Vfm and the stored data of Vfs is not smaller than a certain value a, is examined after taking the temperature characteristic Vtc of the LED into account, No is given to go to S 170 , where the forward voltage of each section of the diode array  1  is repeatedly (Q times) measured to have an average value Vav of the measured forward voltages. Subsequently, whether or not the difference in the forward voltage of each corresponding section of the diode array  1  between the mean value Vav and the stored data of Vfs is larger than the certain value a after taking the temperature characteristic Vtc of the LED into account is examined at S 180 . If the result of S 180  is Yes, it is determined that there is a short-circuiting at S 190 .  
         [0042]     Thereafter, whether the short-circuiting occurs first time or not is examined at S 200 , this short-circuiting record is stored into the EEPROM at S 210  if the result of S 200  is Yes, an alarm is displayed on a display panel at S 220 , a signal of requesting for replacement of the diode array is given at S 230 , and the data previously stored in the EEPROM is reset at S  240 . Thereafter, the step returns to the start of the program.  
         [0043]     If the result of the examination at S 110  is Yes (there is a short-circuiting), the step goes to S 220 , which is followed by steps S 230  and S 240 . If the result of the examination at S 120  is Yes (the data are stored), the step goes to S 150 . If the result of the examination at S 180  is Yes (the difference in the forward voltage of each corresponding section of the diode array  1  between the mean values and the stored data is not larger than the certain value α after taking the temperature characteristic of the LED into account), the step goes to S 150 . If the result of the examination at S 200  is No (the short-circuiting does not occur first time), the step also goes to S 150 .  
         [0044]     A head lamp device according to the fifth embodiment will be described with reference to a flow diagram shown in  FIG. 5 .  
         [0045]     This head lamp device may be the same in structure as any one of the previous embodiments. Incidentally, because the same reference numeral corresponds to the same or substantially the same step of the flow diagram of the third embodiment shown in  FIG. 4 , only steps different from the third embodiment will be further described.  
         [0046]     After measuring the forward voltage Vfm of the sections of the diode array  1  at step S 150 , the step goes to S 152 , where the average data Vav of the data that have been measured are stored. After a certain time interval, the forward voltages Vfm of the sections of the diode array  1  are measured again at step S 154 , which is followed by S 160 , where whether the difference in the forward voltage of each corresponding section of the diode array  1  between the measured data of Vfm and the stored data of Vfs is not smaller than a certain value α is examined after taking the temperature characteristic Vtc of the LED into account. If the result of S 160  is Yes, whether the difference in the forward voltage of each corresponding section of the diode array  1  between the measured data of Vfm and the stored data of Vfs is not smaller than a first value β is examined at S 182 . If the result of S 182  is No, the forward voltage of each section of the diode array  1  is repeatedly (e.g. M times) measured to have average value data Vav of the measured forward voltages at S 184 , and whether the difference in the forward voltage of each corresponding section of the diode array  1  between the stored data of Vfs and the newly measured data of Vfin is larger than a second value γ is examined at S 186  which is followed by the step S 190 , which is followed by S 200 , S 210 , S 230  and S 240 .  
         [0047]     A head lamp device according to the sixth embodiment will be described with reference to a flow diagram shown in  FIG. 6 .  
         [0048]     This head lamp device may be the same in structure as any one of the previous embodiments. Because the same reference numeral corresponds to the same or substantially the same step of the flow diagram of the third embodiment shown in  FIG. 4 , only steps different from the fourth embodiment will be further described.  
         [0049]     After S 140 , the step goes to S 150 ′, where the forward voltage Vfm of the sections of the diode array  1  and ambient temperature T thereof are measured. Thereafter, the step goes to S 152 ′, where the data of Vfm (which becomes Vfs) and the temperature T are stored. After a certain time interval, the forward voltages Vfm of the sections of the diode array  1  and the ambient temperature T thereof are measured again at step S 154 ′, which is followed by S 160 .  
         [0050]     In the foregoing description of the present invention, the invention has been disclosed with reference to specific embodiments thereof. It will, however, be evident that various modifications and changes may be made to the specific embodiments of the present invention without departing from the scope of the invention as set forth in the appended claims. Accordingly, the description of the present invention is to be regarded in an illustrative, rather than a restrictive, sense.