Abstract:
An LED lighting device can compensate a brightness change and a color change caused by a temperature change and exhibit an in-plane uniform luminance and color. A lighting device includes: at least one line connected in parallel, a constant voltage source for applying a constant voltage to the line, an ON/OFF controller, and a current detector for measuring a value of current flowing in the line. The line is formed by one or more light-emitting diodes and a switch for turning ON/OFF the current flowing in the light emitting diodes connected in series. According to the current value of each of the lines measured by the current detector, the ON/OFF controller controls the ON/OFF period of the switch of each of the lines.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to an LED lighting device drive circuit and an image display device using the same. 
     Recently, with increase of the light emission efficiency, the light emitting diode (LED) is employed for a lighting device more and more. Especially the LED is tried to be employed for a backlight arranged at the rear surface of a liquid crystal display (LCD) device. As a display method of the liquid crystal display device, in addition to the TN (twisted nematic) as a mainstream, an IPS (in-plane switching) and an MVA (multi-domain vertical alignment) characterized by a wide visual angle are used. These devices form an image by introducing the light of the lighting device arranged on the rear surface of the display unit into a liquid crystal panel capable of controlling the transmittance of the light. As a light source of the lighting device, in addition to the LED, it is possible to use a cold cathode fluorescent light (CCFL), a hot cathode fluorescent light (HCFL), an organic light emitting diode (OLED), and the like. The LED has a high color purity and can increase the color reproduction range of the liquid crystal display device. Moreover, since no lead is used, it is appropriate for the environment. Furthermore, since the LED has a high-speed response, it is possible to easily perform modulation with a light emission time width, which easily reduces power consumption. 
     The lighting device as a liquid crystal display device should have a characteristic that the brightness and color will not change. However, the LED has a characteristic that the light emission efficiency changes depending on the temperature and its brightness is changed by the affect of self-heating during ON state. Accordingly, the LED requires a compensation technique so that the brightness and the color will not be changed by the temperature change. 
     JP-A-2005-310997 discloses a technique for compensating the fluctuation of the brightness of the LED as the time elapses. That is, a photo sensor is divided to detect an emitted light quantity of each LED and feedback is performed on the LED drive condition so as to prevent the brightness change of the LED. 
     Moreover, JP-A-2004-199896 discloses a technique for arranging a temperature sensor in the vicinity of the LED so as to detect the LED temperature state and performing feedback on the LED drive condition. 
     Recently, however, a large-screen liquid crystal television exceeding 32 inches is spread. In order to realize this, it is necessary to arrange a plenty of LEDs substantially uniformly in the wide area. For this, it is difficult to compensate the brightness fluctuations of the plenty of LEDs by the techniques disclosed in the aforementioned JP-A-2005-310997 and the JP-A-2004-199896. 
     For example, in the compensation technique using a photo sensor, when a plenty of light sources are used, the positional relationship between the light source and the photo sensor differs among the light sources and the light quantity received by the photo sensor differs depending on the respective light sources. Accordingly, it is necessary to prepare a compensation table for compensating the positional relationship between the photo sensors and the light sources and estimate the light source emitted from each of the light sources. Such a complicated process in turn increases the circuit cost. 
     Alternatively, a plenty of photo sensors are required and increase of the number of photo sensors increases the production cost. Moreover, when using light sources of multi-primary colors such as LED for each of the RGB, it is necessary to arrange a color filter for the photo sensor and detect the light of each color, which also increases the production cost. 
     Moreover, in the compensation technique using the temperature sensor, when a plenty of LEDs are arranged on a large-area surface such as a large-size television, a temperature distribution is caused in the plane due to thermal convection and heat discharge structure. Accordingly, it is actually difficult to detect the temperatures of all the LEDs in the plane by a single temperature sensor and a plenty of temperature sensors are required, which increases the production cost. 
     SUMMARY OF THE INVENTION 
     It is therefore an object of the present invention to provide a technique for not causing brightness fluctuations by a temperature change and not causing brightness in-plane irregularities even if a temperature distribution is generated in the plane. 
     In order to achieve the aforementioned object, the present invention provides a lighting device comprising: at least one line connected in parallel, a constant voltage source for applying a constant voltage to the line, an ON/OFF controller, and a current detector for measuring a value of current flowing in the line, wherein the line is formed by one or more light-emitting diodes and a switch for turning ON/OFF the current flowing in the light emitting diodes connected in series, and according to the current value of each of the lines measured by the current detector, the ON/OFF controller controls the ON/OFF period of the switch of each of the lines. 
     Moreover, the present invention provides a lighting device in which a constant current source is used instead of the constant voltage source and a voltage detector is used instead of the current detector. 
     Furthermore, a liquid crystal display device using these lighting devices is provided. 
     The present invention can provide a lighting device whose brightness is not changed by a temperature change so as to output stable luminance. Furthermore, the present invention can realize an LED lighting device in which no in-plane brightness irregularities are caused by a temperature distribution even if the temperature distribution is generated in the plane. 
     Other objects, features and advantages of the invention will become apparent from the following description of the embodiments of the invention taken in conjunction with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a configuration of the present invention. 
         FIG. 2  explains an example of a circuit configuration of the present invention. 
         FIG. 3  explains a drive sequence of the present invention. 
         FIG. 4  explains a light emission duty. 
         FIG. 5  shows a configuration of the present invention. 
         FIG. 6  shows a relationship between the LED junction temperature and current when a constant voltage is applied. 
         FIG. 7  shows a relationship between the LED current and the emitted light quantity when a constant voltage is applied. 
         FIG. 8  shows the relationship of the measured current and light emission duty. 
         FIG. 9  shows a circuit configuration of the present invention. 
         FIG. 10  explains an example of the circuit configuration of the present invention. 
         FIG. 11  shows a drive sequence of the present invention. 
         FIG. 12  shows an example of circuit configuration of the present invention. 
         FIG. 13  shows the relationship between the LED junction temperature and voltage when a constant current flows. 
         FIG. 14  shows the relationship between the LED voltage and the emitted light quantity when a constant current flows. 
         FIG. 15  shows the relationship between the measured voltage and light emission duty. 
         FIG. 16  explains a configuration of the present invention. 
         FIG. 17  explains a configuration of the present invention. 
         FIG. 18  shows the relationship between the LED efficiency and the junction temperature. 
         FIG. 19  shows voltage and current characteristics for the junction temperature. 
         FIG. 20  shows a liquid crystal display device. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
     As has been described above, the LED has a characteristic that its light emission efficiency is changed by the temperature.  FIG. 18  is a graph showing the relationship between the light emission efficiency and the temperature (Tj) of the light emission portion of the LED. When the temperature of the light emission portion increases, the light emission efficiency is lowered. Accordingly, when a constant power is supplied to the LED, the emitted light quantity is lowered by the temperature increase by the self-heating. In order to maintain a certain emitted light quantity against a temperature fluctuation, it is necessary to change the power supplied to the LED at each moment. For this, it is necessary to know the light emission efficiency of the LED at each moment. 
       FIG. 19  is a graph showing an example of Tj dependence of current characteristic with respect to the LED forward direction voltage. In  FIG. 19 , when a constant voltage is applied, the Tj increases together with the current. That is, by applying a constant voltage to the LED and detecting the current flow, it is possible to know the Tj from the graph of  FIG. 19 . When the Tj is known, it is possible to know the light emission efficiency from  FIG. 18 . Accordingly, it is possible to supply a power corresponding to the light emission efficiency at each moment. The supplied power can be changed by adjusting the application time under a constant voltage applied. 
     Moreover, as is clear from  FIG. 19 , when a constant current is applied, as the Tj increases, the voltage is lowered. Accordingly, when a constant current is applied, it is possible to know the Tj similarly by measuring the voltage applied. 
     The present invention was made by noting that the Tj can be indirectly measured by measuring the electric characteristic of the LED as has been described above. By using this method, it is possible to estimate the light emission efficiency of the LED not depending on the difference in the physical position of the detector with respect to the LED. Accordingly, it is possible to provide a lighting device which can be appropriately employed for a large-screen liquid crystal display device. 
     Hereinafter, explanation will be given on specific embodiments of the present invention. 
     Embodiment 1 
     Detailed explanation will be given on the first embodiment of the present invention with reference to  FIGS. 1 to 8 . 
       FIG. 1  shows a concept of a circuit configuration according to the present embodiment. The present embodiment uses five lines, each including five LEDs  101  connected in series and a switch  304  connected in series to the LEDs. The five lines are connected in parallel and each line is driven by a constant voltage source  201 . 
     A current detector  302  and a bypass switch  303  are arranged at a portion where currents of all the lines are concentrated. With this configuration, it is possible to measure the current values of all the lines by the single current detector  302 . That is, only the switch  304  of the line whose current value is to be detected is turned ON and the current flow is measured by the current detector  302 . Thus, it is possible to measure a current value of each line. 
     Moreover, when no current is to be measured, the bypass switch  303  is turned ON so that no current is fed to the current detector  302 . An ON/OFF controller  301  adjusts the ON/OFF period of the switch  304  of each line according to the current value of the line detected, thereby compensating the brightness fluctuation caused by the temperature change. 
       FIG. 2  shows a configuration of the present embodiment through a specific circuit diagram. The switch  304  of each line may be a semiconductor switch such as a MOS transistor or a bipolar transistor. In this embodiment, the MOS transistor is used as the switch  304  of each line. 
     The current detector  302  is formed by a highly accurate resistor  306  and an A/D converter  305 . That is, by measuring a voltage drop in the resistor  306  caused by current flow, it is possible to calculate the current value by the Ohm&#39;s law. It should be noted that in this embodiment, the voltage drop of the resistor  306  is converted into a digital signal by the A/D converter  305  and transmitted to the ON/OFF controller  301 . Moreover, the bypass switch  303  may be formed similarly by a semiconductor switch. 
     Hereinafter, explanation will be given on the operation of the circuit shown in  FIG. 2  by using  FIG. 3 .  FIG. 3  shows a drive sequence of the circuit of  FIG. 2 . In this embodiment, the drive sequence is divided into a current detection period and a lamp lighting period. The current detection period is a period for detecting a current value of each line. The lamp lighting period is a period for causing the lamp to turn ON to emit light. The current detection period and the lamp lighting period constitute one cycle, which is repeated at 60 Hz (about 16.6-millisecond cycle). This disables human eyes to know that the light sources of each line are turned ON intermittently. When the frequency is lower than 60 Hz, the human eyes feel that the light sources are blinking. 
     Moreover, the current detection period is set to 100 micro-seconds. As will be detailed later, during the current detection period, current is supplied to each line in time division way and the current value is measured. If this period is long, the human eyes can catch that only one line is lit. Accordingly, current is supplied for only an extremely short time to measure the current value so that human eyes cannot feel that only one line is lit. 
     The time required for measuring the current value of each line may be a time for stabilizing the circuit time constant, i.e., current plus a time required for converting the analog signal into a digital signal by the A/D converter. That is, several tens of microseconds are sufficient. In this embodiment, current of each line is measured during 20 microseconds. Accordingly, the current measurement period for measuring the current of the five lines was 100 microseconds. 
     As shown in  FIG. 3 , during the current measurement period, Vg 1  to Vg 5  are successively set to High voltage to turn ON the transistors. That is, during the current measurement period, each switch of at least one line is turned ON in time division way and current of each line is separately measured. The ON/OFF controller  301  has a built-in condition table  307  and calculates the efficiency of the LED  101  from the current value of each line so as to control the ON/OFF period ratio (light emission duty) of each line during the lamp lighting period, thereby compensating the fluctuation of the brightness. That is, the time average power is adjusted so that the product of the efficiency and the power supply is always constant to obtain a constant brightness all the time. 
     It should be noted that as shown in  FIG. 4 , the light emission duty is the ON time ratio during the lamp lighting period. 
     During the lamp lighting period, Vb is set to high voltage to turn ON the bypass switch  303  and bypass the current. Thus, it is possible to prevent heating of the resistor  306  during the lamp lighting period. 
       FIG. 5  shows the entire configuration of the present embodiment. The ON/OFF controller  301 , the current detector  302 , and the switches  304  of the respective lines are arranged on a control substrate  309 . Moreover, the gate of the MOS switch  304  of each line is driven by a shift register  308 . The ON/OFF controller  301  serially transmits the ON/OFF information on each line to the shift register  308 . The shift register  308  parallel-converts the ON/OFF information so as to control ON/OFF of the switch  304  of each line. By using the shift register  308 , it is possible to reduce the number of pins of the ON/OFF controller. This is advantageous when the number of lines is very large. 
       FIG. 6  shows the relationship between the temperature Tj of the light emission portion of the LED when a constant voltage is applied to the LED  101  used in this embodiment and the current flow. Moreover,  FIG. 7  shows the emitted light quantity of the LED  101  at each current value similarly when a constant voltage is applied. As is clear from  FIG. 6  and  FIG. 7 , the current value becomes maximum and the emitted light quantity becomes minimum when the Tj has reached the saturation temperature. The condition table can be created from  FIG. 7 . That is, since the brightness is proportional to the product of the emitted light quantity and the light emission duty, the light emission duty at each current value is set so that the light emission duty is maximum at the current value when the emitted light quantity becomes minimum and the light emission duty is minimum at the current value when the emitted light quantity becomes maximum, thereby preventing fluctuation of brightness.  FIG. 8  shows a condition table when the maximum light emission duty is set to 100%. 
     Here, explanation will be given on the method how to set the condition table  307  with reference to  FIG. 6 ,  FIG. 7 , and  FIG. 8 . The junction temperature of the LED  101  gradually increases after lighting and is saturated at a certain temperature. 
       FIG. 6  shows the relationship between the temperature Tj of the light emitting portion of the LED when a constant voltage is applied to the LED  101  and the current which has flown. Moreover,  FIG. 7  shows the emitted light quantity of the LED  101  at each current value when a constant voltage is applied. As is clear from  FIG. 6  and  FIG. 7 , when the Tj has reached the saturation temperature, the current value is maximum and the emitted light quantity is minimum. The condition table  307  can be created from  FIG. 7 . That is, the brightness is proportional to a product of the emitted light quantity and the light emission duty. Accordingly, the light emission duty at each current value is set so that the light emission duty is maximum at the current value when the emitted light quantity is minimum and the light emission duty is minimum at the current value when the emitted light quantity is maximum, thereby preventing brightness fluctuation.  FIG. 8  shows a condition table when the maximum light emission duty is set to 100%. 
     On the contrary, there is an LED whose efficiency is lowered as the junction temperature increases but the emitted light quantity increases because the current increases. When using such an LED, control is performed to lower the light emission duty when the current is large and increase the light emission duty when the current is small. 
     Embodiment 2 
     Explanation will be given on the second embodiment of the present invention. 
     The explanation will be given by referring to  FIG. 9 . 
     The present embodiment shows 12 lines connected in parallel. Each of the lines is connected to an LED  101  and a switch  304  in series. These lines are driven by a constant voltage source  201 . The switch  304  of each line is controlled to be ON/OFF by an ON/OFF controller  301 . Moreover, the current detector  302  and the bypass switch  303  are arranged in parallel at a position where currents of all the lines are concentrated. 
     Furthermore, temperature detection means  310  is provided. A condition table  307  built in the ON/OFF controller  301  decides the ON/OFF period of each line according to the current value of each line and the detection result of the temperature detection means  310 . 
     By detecting the current of each line, it is possible to adjust the ON/OF period ratio of each line to compensate the brightness fluctuation in the same way as has been described in the first embodiment. 
     The LED junction temperature gradually increases immediately after lighting and the temperature is saturated when the heating amount of the LED itself is balanced with the heat release of the substrate  401  on which the LED is mounted. However, the saturation is also affected by the ambient temperature where the light device is placed. That is, the saturation temperature differs depending on the temperature of the environment in which the lighting device is placed. 
     In this invention, the duty at each junction temperature is set according to the efficiency of the LED  101  in the junction saturation temperature. Accordingly, by measuring the environment temperature, it is possible to predict the saturation temperature. Consequently, it is possible to set an optimal light emission duty according to various environment temperatures. 
     The temperature detection means  310  may be arranged in an environment where the lighting device is placed such as a place for measuring the indoor temperature or at the back side of a radiation plate where the LED  101  is mounted. By checking the correlation between the temperature of the place where the temperature detection means  310  is arranged and the saturation temperature of the Tj in advance, it is possible to predict the Tj saturation temperature. 
     Embodiment 3 
     Explanation will be given on the third embodiment of the present invention with reference to  FIG. 10  and  FIG. 11 . 
       FIG. 10  shows a circuit configuration of the present embodiment. The AC power input is amplified by a transformer  311  and subjected to a rectifier circuit  312  and a smoothing circuit  313  so as to generate a constant voltage. The voltage is adjusted by a switching regulator  314  and used as a constant voltage source  201  of green (G) and blue (B). The G and B voltages are reduced by a step down chopper and the voltage is used as a constant voltage source  201  of red (R). Thus, a plurality of primary colors share the constant voltage source  201 , thereby reducing the power cost. 
     In this embodiment, three LEDs are connected in series for each of the primary colors and the switch  304  is connected in series in each line. Moreover, two lines are connected in parallel for each of the primary colors. 
     The low potential side of each line is commonly connected and a current detection resistor  306  is connected between the potential point and the circuit reference potential (GND). The current detection resistor  306  is connected to the bypass switch  303  in parallel. 
     With this configuration, it is possible to measure current of each line by one current measuring means. Even when LEDs  101  of a plurality of primary colors are used, it is possible to perform measurement by one current detector  302 . 
       FIG. 11  shows a drive sequence in the present embodiment. Here, SW 1  to SW 6  show the ON/OFF state of the switch  304  of each line. The switch  304  is turned ON when the voltage becomes high and turned OFF when the voltage becomes low. Moreover, Idet represents a current detected by the current detector  302 . Itotal represents a current flowing in the bypass switch  303 . 
     In this embodiment, during the current detection period, the switches  304  of line  1  to line  3  are turned ON to measure the current value of each line and the light emission duty during the lamp light emission period is decided according to the measured current value. Moreover, during the next current detection period, the switches  304  of line  4  to line  6  are successively turned ON to measure the current value of each line. Thus, the lines measured during the current detection period may be freely selected. For example, it is possible to measure one line during one cycle. 
     As shown by Itotal in  FIG. 11 , during the lamp light emission period, a large current flows in the bypass switch  303 . Accordingly, it is necessary to lower the voltage drop generated by the current flow in the bypass switch  303  to a degree which can be ignored as compared to the drive voltage of the LED  101 . For this, the ON resistance of the bypass switch  303  should be very low. In this embodiment, the bypass switch  303  is realized by MOS transistors. In order to reduce the ON resistance, two MOS transistors are connected in parallel. This suppresses the heating by the detection resistor  306  during lamp light emission period and reduces the voltage drop of the bypass switch  303 . 
     Embodiment 4 
     Explanation will be given on the fourth embodiment with reference to  FIG. 12  to  FIG. 15 . 
       FIG. 12  shows a circuit configuration of the present embodiment. 
     In this embodiment, each line which has two LEDs  101  of RGB connected in series and also a resistor are driven by a constant current source  202 . The constant current source  202  feeds back voltage applied to the resistor of each line and changes the output voltage so that the current is always constant. That is, a constant current flows in each line. 
     Moreover, the constant current source  202  has a built-in switch for performing current ON/OFF control. In this embodiment, the junction temperature is recognized by measuring the voltage applied to the LED  101  when a constant current is fed to each line. By making the light emission duty based on each temperature, it is possible to compensate brightness fluctuation caused by a temperature change. 
       FIG. 13  shows the relationships between voltages applied to the respective LEDs  101  of R, G, B and Tj. Moreover,  FIG. 14  shows the relationships between the voltages and the emitted light quantity (normalized value). For this, a condition table  307  changing the light emission duty for the measured voltages is created to suppress the brightness fluctuation. 
     Moreover, a voltage selector  317  is provided to switch the voltage applied to the LED  101  of each line so that a single voltage detector  316  can measure voltage applied to the LEDs  101  of all the lines. Thus, it is possible to measure LEDs of a plurality of lines and of a plurality of primary colors by using the single voltage detector  316 . 
     Embodiment 5 
     Explanation will be given on the fifth embodiment of the present invention with reference to  FIG. 16  and  FIG. 17 . 
       FIG. 16  is a plan view of a configuration of the lighting device of the present embodiment. The LEDs  101  are mounted on a metal substrate  401 . The metal substrate  401  is mounted on a radiation plate  402 . Moreover, on the rear side of the surface where the substrate  401  having the LED  101 s  101  is mounted, a plurality of radiation fins  403  and air cooling fans  404 .  FIG. 17  is a cross sectional view of the configuration of  FIG. 16 . 
     The radiation fins  403  are arranged in parallel to the substrate  401  and the longitudinal axis direction of the protruding portion is arranged to be parallel to the substrate  401 . The air cooling fans are arranged so as to send an air flow in that direction. 
     The LEDs  101  are driven by the constant voltage source  201 . The control unit  309  includes a current detector  302  and an ON/OFF controller  301  similar to those described in the first embodiment are provided. 
     In this embodiment, according to the current value of each line detected by the current detector  302 , i.e., the junction temperature information on the LEDs, the air cooling fans  404  are separately driven. That is, when the junction temperature exceeds a set temperature, the air cooling fan  404  immediately below the line is driven. Thus, it is possible to selectively cool the line which has exceeded the set temperature. That is, the air cooling fan  404  is driven only when required and it is possible to reduce the power consumption of the air cooling fan  404 . 
     Embodiment 6 
     Explanation will be given on the sixth embodiment with reference to  FIG. 20 . 
       FIG. 20  shows a configuration using the lighting device explained through the aforementioned embodiments 1 to 5 as a light source of the liquid crystal display device. The lighting device is used as a light source  501  which supplies light to a liquid crystal panel having a pair of polarization panels  503 , a pair of substrates  504 , and a liquid crystal layer  505  sandwiched by them. 
     By using the present invention as a light source, it is possible to realize a liquid crystal display device improving the brightness stability and eliminating fluctuation caused by the temperature distribution. 
     It should be further understood by those skilled in the art that although the foregoing description has been made on embodiments of the invention, the invention is not limited thereto and various changes and modifications may be made without departing from the spirit of the invention and the scope of the appended claims.