Abstract:
The invention is a method and apparatus to provide a temperature compensation for LED brightness utilizing temperature dependence of voltage-current characteristic of the LED. Methods ensure the precise compensation of brightness of LED using two or more temperatures to tune an LED driver circuit without the use of thermistors or other temperature sensor. 
     This method also allows for circuits that compensate brightness not only of a single LED but of an assembly of LEDs. The method allows to manufacture LED modules with built-in temperature brightness compensation circuit. These modules can be used in a variety of illuminating equipment including but not limited to home lights, vehicle turn or brake signals, traffic lights, and a backlight for LCD display.

Description:
TECHNICAL FIELD OF THE INVENTION 
       [0001]    The present invention relates to lighting control systems for Light Emitting Diode (LED) equipment, notably temperature brightness compensation of LED for lighting systems. 
       BACKGROUND OF THE INVENTION 
       [0002]    Light-emitting diodes (LED) lighting has become increasingly more popular in recent years. As a source of light, LED has many advantages over incandescent lighting, including high efficiency, reliability, and long life. However, the brightness of LED depends on temperature P-N junction and ambient temperature fluctuations. This imperfection limits the use of LED in some areas of applications. 
         [0003]    There are many patents which describe methods to address LED brightness, attempting to correct operating conditions to create stable lighting brightness. Most of these methods use external temperature sensors, such as PTC and NTC thermo-resistors, semiconductor diodes, and transistors of various sorts. 
         [0004]    All circuits that use external temperature sensors have one shortcoming: they assume that the temperature of an LED is the same as the temperature of the sensor. This assumption is not valid; the temperatures of the LED and the sensor are not the same. Because of this, the temperature brightness compensation of LED with external temperature sensors is far from inexact. Moreover, if ambient temperature varies quickly, compensation of brightness occurs after the circuitry reaches thermal equilibrium. Additionally, the added external sensors and components reduce reliability and increases the costs of illuminators. 
         [0005]    Currently, the following LED drivers integrated circuits (ics) include temperature brightness compensation: 
         [0006]    A) Driver CL25 is a two-terminal element that functions as a current stabilizer. A typical application for the CL25 is to drive LEDs with constant current of 25 ma. It can also be used in parallel to provide higher current such as 50 ma, 75 ma or 100 ma. Typical temperature coefficient about +0.01%/° C. is insufficient for brightness compensation for most LEDs. 
         [0007]    B) Driver MT7910 accepts a wide ranging dc voltage input to a switching regulator and used to power high brightness LEDs, and can be tuned to supply a wide range of current from just a few milliamps to more than an amp. It also includes a 0-245 mv linear dimming and temperature compensation of the LED current. For proper operation, this IC needs an external temperature sensor and a few ancillary components, such as a power transistor, damping diode, inductor, and capacitor. 
         [0008]    The necessity of an external temperature sensor is a significant limitation in use of LED drivers. 
         [0009]    Because an LED is a semiconductor diode, its forward voltage decreases when temperature rises and increases when temperature drops; this realization allows an LED to be used as a temperature sensor. U.S. Pat. No. 7,683,864 “LED driving apparatus with temperature compensation function” describes the following method: the electric circuit measures the forward voltage of an LED, compares it with a reference voltage and then manages current flow—increasing current when the voltage difference indicates a temperature rise and decreasing current when the temperature drops. 
         [0010]      FIG. 1A  is a block diagram of a prior art LED driving unit. Reference to  FIG. 1A , the prior art&#39;s LED driving unit includes: reference voltage generator  100 , the amplification unit  200 , driving unit  300 , LED  400 , repeating amplifier  500 , and current limiter  600 . The forward voltage across LED  400  is fed into the pass repeating amplifier  500  on the amplification unit  200  where is compared with reference voltage generator  100 . Difference signal pass current limiter  600  and come to driving unit  300 . A driving unit  300  adjusts a supply voltage of LED  400  in response to the voltage of the differential amplification unit  200 . An increased temperature causes a decrease of the forward voltage across LED  400 . A difference voltage between the reference voltage generator  100  and a forward voltage to LED  400  is increased and force driving unit  300  increases LED supply current. 
         [0011]    The prior-described prior art has difficulties. The forward voltage across the LED depends not only on temperature but also on forward current. An increase of forward current with increasing temperature causes an additional increase in forward voltage. The control system perceives this fact as decreasing of temperature and stops correcting the brightness. Therefore the compensation of brightness is partial, as confirmed by the experimental evidence shown on  FIG. 1B  from the aforementioned patent. 
         [0012]    The temperature dependence of brightness is determined by the type of semi-conductive material. For example, ultra-bright LEDs TLCX510 of Company Vishay are made of material aluminium indium gallium phosphide on gallium arsenide to radiate light of different color. Each color LED has different operating characteristics, including temperature dependence on brightness. 
         [0013]    The brightness of an LED can be reported as a comparison relative to its reference brightness level, typically at 25° C. By definition, the relative brightness of an LED at +25° C. is one; in the range of temperatures −40° C. to +85° C., the change of relative brightness is 2.2 to 0.6 for red LEDs, 2.3 to 0.4 for yellow LEDs, and 2.6 to 0.48 for green. 
         [0014]    Other materials show similar changes in brightness. For example, ultra bright LEDs type TLHB580 made of gallium nitride on silicon carbide can radiate blue or white (with phosphor) light. As temperature ranges from −10° C. to +100° C., brightness changes 1.15 to 0.3. 
         [0015]    The LED industry is continuously in need of better ways of providing consistent brightness in LEDs over a range of temperature and operating conditions that does not depend on external sensors whose operational characteristics are susceptible to temperature change. 
       SUMMARY OF THE INVENTION 
       [0016]    The disclosed invention teaches a method and circuit which calculates a constant voltage power source and series resistance to power an LED or LED assembly so that the brightness of the LED will be the same throughout its expected operating temperature range. Throughout this application, references to an LED can also refer to an LED assembly of multiple LEDs. 
         [0017]    These aforesaid parameters of supply, values of voltage and resistance, will be denoted as optimal voltage and optimal resistance. The method is designed to obtain the optimal voltage of power supply and the optimal resistance for temperature brightness compensation of LED. It is also possible to select a power supply voltage and series resistor such that the brightness of the LED will be increase or decrease with a rise in temperature. 
         [0018]    The present invention solves the problems of temperature brightness compensation of a LED using an inexpensive power supply and simple design. The present invention proposes a method for choosing the parameters for the power supply. The method ensures the exact compensation of brightness for two, three and more pre-specified temperatures. This patent gives examples of circuits implementing the method and provides experimental confirmation of its efficiency. The patent also describes examples of actual circuits that limit the working current or engage an emergency shut-down in case when maximum allowed current is exceeded. 
         [0019]    Electrical circuits for supplying LED utilizing the method comprise a voltage stabilizer with an optimal output voltage and an internal resistance. If the internal resistance of the voltage stabilizer is much less than the optimal resistance then the external resistor must be equal to the optimal resistance. In general, the sum of the internal resistance of the voltage stabilizer and the external resistor must be equal the optimal resistance. The method proves to be useful for illuminating equipment containing series circuit of LEDs. In this case, the output voltage stabilizer must be equal to the product of the optimal voltage for a single LED and the number of LEDs in a series circuit. The optimal resistance must be equal to the product of the optimal resistance of a single LED and the number of LEDs in a series circuit. The same series circuit of LEDs can be connected in parallel. In this case the optimal voltage is the same as in the case of a single series circuit of LEDs. The optimal resistance is equal to the optimal resistance of one series circuit divided by the number of parallel branches. But the best design uses a resistor in every series of LEDs so current is equalized in all parallel circuits. A voltage drop across the external resistor may be used to protect the circuit of LEDs or to turn off LEDs in case of alert condition. The aforesaid voltage stabilizer may be designed as an analogous circuit as well as a pulse circuit. The aforesaid method can execute precise brightness compensation of LED at two, three or more temperatures. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0020]      FIG. 1A  is a block diagram of a prior art LED driving apparatus. 
           [0021]      FIG. 1B  is a graph showing results from a temperature-compensating circuit as described in  FIG. 1A . 
           [0022]      FIG. 2A  is a schematic diagram of a set of devices that implements the method of precise temperature brightness compensation. 
           [0023]      FIG. 2B  is a circuit for connecting an LED to an optimal power supply. 
           [0024]      FIG. 2C  is a graph showing a method of precise temperature brightness compensation for two temperatures. 
           [0025]      FIG. 3  is a block diagram of the method of precise temperature brightness compensation to precisely compensate brightness of LED for two temperatures. 
           [0026]      FIG. 4A  is a graph showing experimental result of the precise temperature brightness compensation for TLCY5100 LEDs at two temperatures. 
           [0027]      FIG. 4B  is a graph showing experimental results of the precise temperature brightness compensation for HB5-439AWD LEDs at two temperatures. 
           [0028]      FIG. 5A  is a power supply electrical circuit to drive an LED or series of LEDs using the method disclosed in claim  1 . 
           [0029]      FIG. 5B  is an electrical circuit of a power supply for several connected in-parallel series of LEDs utilizing the method of claim  1  to produce precise brightness compensation of LED for two temperatures. 
           [0030]      FIG. 6  is a graph explaining a method of precise temperature brightness compensation using three temperatures points. 
           [0031]      FIG. 7  is a schematic block diagram disclosing the method compensating for a temperature range of LED brightness using three temperatures. 
           [0032]      FIG. 8  is an electrical circuit for a power supply to drive a series of LEDs using the method disclosed in claim  7 . 
       
    
    
       [0033]    Manufacturers recommend powering LEDs by a current source. However, using a constant current source to power LEDs has the natural result that LED brightness changes with temperature. 
         [0034]    Manufacturers recommend against use of constant voltages to supply LEDs, as the LED can be ruined by excessive current and resulting heat. However, if the LED is supplied from a voltage source with a voltage value equal to the forward voltage of the LED with a current set at a value less than the LED&#39;s maximum current, then the LED can safely operate. Moreover, under such conditions the LED demonstrates surprising behavior; the brightness of an LED increases with increasing temperature and decrease with decreasing temperature. This occurs because an increase in temperature decreases the forward voltage of the LED and increases the current through LED, leading to a higher brightness. 
         [0035]    Conversely, a decrease in temperature increases the forward voltage of LED and current through the LED decreases, leading to a decreasing brightness. Therefore, it is possible to select a value of voltage of power supply and resistance of series resistor such that the brightness of the LED will be the same both at low and at high temperatures. These aforesaid parameters of supply, values of voltage and resistance, will be denoted as optimal voltage and optimal resistance. The method is designed to obtain the optimal voltage of power supply and the optimal resistance for temperature brightness compensation of LED. It is also possible to select a power supply voltage and series resistor such that the brightness of the LED will be increase or decrease with a rise in temperature. 
         [0036]    The method presented in the invention, develops an optimal voltage power supply output and optimal resistance, so that LED brightness both at low and at high temperatures will be the same. To achieve excellent temperature-compensation of LED brightness, an LED must receive power from an optimal voltage though optimal resistor. 
         [0037]    To get optimal parameters of power supply it is necessary to use a measuring system that includes the following widely available equipment: an adjustable power supply unit, a voltmeter, an ammeter, a temperature-controlled chamber with a window for light to exit, and a light meter. 
         [0038]    To get optimal parameters of power supply of LED it is necessary to execute the following steps: place the LED into the temperature-controlled chamber and connect the LED to an adjustable power supply unit, the voltmeter measuring forward voltage across the LED, and the ammeter set in series with the LED. 
         [0039]    A user installs the LED so it emits the light onto the light meter through the window of the temperature-controlled chamber. Set high working temperature of the LED in the temperature-controlled chamber and after the temperature stabilizes, adjust the constant voltage power supply unit so that the light meter reads a required brightness of the LED. 
         [0040]    Because the heat caused by the current of the LED can disturb the thermal equilibrium, it is necessary to wait until the light meter is steady. Thereafter keep adjusting the brightness of the LED until the required brightness is achieved. 
         [0041]    A user measures and records the values of the current through the LED, and forward voltage across the LED at a high working temperature, and then plot these values as a first point on a two-dimensional graph using current and voltage as the two axes. 
         [0042]    A user then repeats the process by setting a low working temperature of the LED in the temperature-controlled chamber, and after a thermal equilibrium is reached adjust power supply unit until the light meter reads the same the brightness of the LED as at the brightness at the high working temperature. 
         [0043]    The user then records the values of the current and the forward voltage of the LED and plots these values as the second point on a two-dimensional graph using current and voltage as the two axes. 
         [0044]    To find the optimal voltage, a user connects the first and the second points with a straight line and continues this line until it crosses the current and the voltage axes. 
         [0045]    The resulting intercept with voltage axis gives optimal voltage power supply of the LED. The ratio of the voltage to current intercepts gives the optimal resistance of resistor for supply of LED. The order in which the temperature of the temperature-controlled container is set is not essential. It can be first set to low working temperature and then to high working temperature. Users can obtain axis intercepts of the current and the voltage axes analytically using the first and second coordinate points. 
         [0046]    The method can be applied to both single LED and an assembly of LEDs connected in series and in-parallel series circuits. The assembly of LEDs must be constructed with heatsinks. 
         [0047]    The described method is excellent when the temperature dependence of brightness is linear. The majority of devices for indoor use operate in a temperature range from +5° C. to +35° C., so the typical temperature range of 0° C. +40° C. easily covers a temperature range for all indoor light equipment. These devices include, but not limited to LCD screen, TV, and computers with LED backlight, home light, etc. A precise compensation of brightness is very important for reserve color temperature (white balance) in color LCD displays and TV. 
         [0048]    At the temperature range of −40° C. +85° C., the LED&#39;s temperature dependence of brightness is nonlinear, and the brightness compensation may be less efficient. The method can be easily modified to achieve perfect brightness compensation of LEDs in a wide range of temperatures. 
         [0049]    The idea is to choose several temperature values, in which constant brightness level must be maintained. Without loss generality, the method is expanded for three temperature values. 
         [0050]    First, it is necessary to choose three temperature points for high, middle and low temperatures, surrounding the range where precise temperature compensation is needed. The temperature range is divided into two temperature intervals: from high to middle temperature and from middle to low temperature. 
         [0051]    Using aforesaid method, the user finds the optimal voltage of the first power supply and the first optimal resistor value for the temperature intervals from high to middle temperature. 
         [0052]    The user then repeats the previous step, finding the optimal voltage of the second power supply and the second optimal resistor value for the temperature intervals from middle to low temperature and recording the value of the forward voltage at middle temperature. If the actual forward voltage of the LED or the assembly of LEDs is greater than the recorded forward voltage at middle temperature, then the user connects the LED or the assembly of LEDs to the second power supply through the second optimal resistor. 
         [0053]    If the actual forward voltage of the LED or the assembly of LEDs is less than or equal to the recorded forward voltage at middle temperature, then the user connects the LED to first power supply through the first optimal resistor. 
         [0054]    The middle temperature can be chosen in the middle temperature range. The best performance of the method is achieved by selection the middle temperature where the function of brightness versus temperature is curved the most. The middle temperature value is defined by using the following steps. First, using the described method of temperature brightness compensation for two temperatures the optimal voltage and the optimal resistance are found for low and high temperature points. Then the graph of brightness versus temperature is plotted. Finally, the middle temperature is found by choosing a temperature value that maximizes the deviation of brightness from the desired brightness level. 
         [0055]    The method can adopted to achieve precise brightness compensation of LED at any number of temperature values. The method works as follows with n temperatures: T 1 , T 2 , T i  . . . T n-1 , T n  such that T 1 &gt;T 2 &gt;T i &gt; . . . &gt;T n , which divides the temperature range into n−1 temperature intervals: first—from T 1  to T 2 , second—from T 2  to T 3 , etc., and at last from T n-1  to T n  temperatures. 
         [0056]    The user follows the previously described process to gather optimal parameters of supply for every temperature interval, recording the forward voltage values at each temperature. Comparing the forward voltage of the LED to the recorded forward voltage, the LED or the assembly of LEDs should be connected to the previous temperature interval power supply with the corresponding optimal voltage and optimal resistance that is less than the recorded forward voltage for Ti. 
         [0057]    If the forward voltage of the LED becomes greater or equal to the recorded forward voltage for Ti, the LED should be connected to the following temperature interval power supply with the corresponding optimal voltage and optimal resistance. 
       DESCRIPTION OF EMBODIMENTS 
       [0058]    Experiments have confirmed the performance of the precise temperature brightness compensation method for two temperatures by testing super bright LEDs manufactured by company Vishay. Two types of LEDs, one with yellow color light (TLCY5100) and the other one with white color light (HB5-439AWD) were chosen to demonstrate the performance of the method. 
       Example 1 
       [0059]    The experiment was conducted by the instrumentality measuring the system shown on  FIG. 1A  for temperature range from 0° C. to +40° C. with intervals of 5° C. This experimental data is presented in the Table 1. First, the brightness of LED type TLCY5100 was measured at temperature 20° C. and current 17 ma. This brightness value was taken as a reference and was used to compute relative brightness by dividing the brightness of the LED by the reference brightness. Next, relative brightness was measured in the specified temperature range using two power sources: a current power supply, and the aforesaid optimal supply. The data of the relative brightness of the LED with supply by current source 17 ma is presented in the second row of Table 1. This data show that in the temperature range from 0° C. to +40° C., the relative brightness of the LED varies in the range of ±20%. The data of relative brightness of the LED with supply by the optimal voltage source 2.16 V and through the optimal resistor 8.49 Ohm is presented in the third row of Table 1. This data show that in the temperature range from 0° C. to +40° C., the relative brightness of the LED varies in the range of ±1.85%. Thus the temperature compensation of brightness reduces the variation of brightness of LED more than 10 times. 
         [0000]    
       
         
               
               
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                 Temperature ° C. 
                 0 
                 5 
                 10 
                 15 
                 20 
                 25 
                 30 
                 35 
                 40 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 Relative brightness 
                 1.2 
                 1.151 
                 1.1 
                 1.052 
                 1.0 
                 0.948 
                 0.892 
                 0.847 
                 0.791 
               
               
                 without compensation 
               
               
                 Relative brightness 
                 1.0 
                 1.008 
                 1.019 
                 1.028 
                 1.037 
                 1.031 
                 1.021 
                 1.01 
                 1.0 
               
               
                 with compensation 
               
               
                   
               
             
          
         
       
     
         [0060]      FIG. 3A  presents experimental data for LED yellow color type TLCY5100 before and after the precise temperature brightness compensation. 
         [0061]    Annex 1—The optimal voltage and the optimal resistance were calculated by the instrumentality the aforesaid measuring system. First, the brightness of LED type TLCY5100 was measured at temperature 20° C. and current 17 ma. This brightness was taken as the reference brightness and all the following brightness values were normalized by this reference level. 
         [0062]    However, at temperature of 40° C., the reference brightness level was achieved by adjusting the current value (19.7 ma). The forward voltage value of the LED is 1.99 V. This pair of current-voltage (19.7 ma:1.99V) was plotted in the current-voltage coordinates. Next, at temperature 0° C., the reference brightness level was achieved by adjusting the current value (13.8 ma). The forward voltage value of the LED is 2.04 V. This pair of current-voltage (13.8 ma:2.04V) was also plotted in the current-voltage coordinates. The mentioned points with coordinates (19.7 ma:1.99V) and (13.8 ma:2.04V) were connected by a straight line until the crossed the voltage (V) and the current (I) axes. The intercept of the voltage axis (2.16 V) was defined as the optimal voltage of the supply. The intercept of the current axis was I=254.5 ma. The ratio of voltage intercept (2.16 V) to the current intercept (254.5 ma) of the straight line was defined as the optimal resistance (8.49 Ohm) value for the supply of LED, ensuring equal brightness values for both low and high ambient temperature. 
       Example 2 
       [0063]    The Experiment was Conducted by the instrumentality measuring the system shown on  FIG. 1A  for temperature range from 0° C. to +40° C. with intervals of 5° C. This experimental data is presented in the Table 2. First, the brightness of LED type HB5-439AWD was measured at temperature 20° C. and current 17 ma. This brightness value was taken as a reference and was used to compute relative brightness by dividing the brightness of the LED by the reference brightness. Next, relative brightness was measured in the specified temperature range using two power sources: a current power supply, and the aforesaid optimal supply. The data of the relative brightness of the LED with supply by current source 17 ma is presented in the second row of Table 2. This data show that in the temperature range from 0° C. to +40° C., the relative brightness of the LED varies in the range of ±8%. The data of relative brightness of the LED with supply by the optimal voltage source 3.51 V and through the optimal resistor 24.0 Ohm is presented in the third row of Table 2. This data show that in the temperature range from 0° C. to +40° C., the relative brightness of the LED varies in the range of ±0.2%. Thus the temperature compensation of brightness reduces the variation of brightness of LED in 40 times. 
         [0000]    
       
         
               
               
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
               
               
             
           
               
                 TABLE 2 
               
               
                   
               
               
                 Temperature ° C. 
                 0 
                 5 
                 10 
                 15 
                 20 
                 25 
                 30 
                 35 
                 40 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 Relative brightness 
                 1.08 
                 1.06 
                 1.04 
                 1.02 
                 1.0 
                 0.98 
                 0.96 
                 0.94 
                 0.92 
               
               
                 without compensation 
               
               
                 Relative brightness 
                 1.0 
                 1.001 
                 1.002 
                 1.004 
                 1.004 
                 1.003 
                 1.002 
                 1.001 
                 1.0 
               
               
                 with compensation 
               
               
                   
               
             
          
         
       
     
         [0064]      FIG. 3B  presents experimental data for LED white color type HB5-439AWD before and after the precise temperature brightness compensation. 
         [0065]    Annex 2. The optimal voltage and the optimal resistance were calculated by the instrumentality the aforesaid measuring system. First, the brightness of LED type HB5-439AWD was measured at 20° C. and current 17 ma. This brightness was taken as the reference brightness and all the following brightness values were normalized by this reference level. Then is at +40° C., the reference brightness level was achieved by adjusting the current value (18.2 ma). The forward voltage value of the LED is 3.07 V. This pair of current-voltage (18.2 ma:3.07V) was plotted in the current-voltage coordinates. Next, at temperature 0° C., the reference brightness level was achieved by adjusting the current value (15.7 ma). The forward voltage value of the LED is 3.13 V. This pair of current-voltage (15.7 ma:3.13V) was also plotted in the current-voltage coordinates. 
         [0066]    The mentioned points with coordinates (18.2 ma:3.07V) and (15.7 ma:3.13V) were connected by a straight line until the crossed the voltage (V) and the current (I) axes. The intercept of the voltage axis (3.51 V) was defined as the optimal voltage of the supply. The intercept of the current axis was I=146.1 ma. The ratio of voltage intercept (3.51 V) to the current intercept (146.1 ma) of the straight line was defined as the optimal resistance (24.0 Ohm) value for the supply of LED, ensuring is equal brightness values for both low and high ambient temperature. 
       DETAILED DESCRIPTION 
       [0067]    For the purposes of understanding the principles of the invention, reference will now be made to the embodiments illustrated in the drawings and the block diagrams, and specific language will be used to describe the them. It will nevertheless be understood that no limitation of the scope of the invention is thereby intended. Any alterations and further modifications in the described embodiments, and any further applications of the principles of the invention as described herein are contemplated as would normally occur to one skilled in the art to which the invention relates. The present invention can be implemented proposal with various mixtures of analog and digital circuitry. 
         [0068]      FIG. 2A  illustrates a schematic diagram of a system for method implementation of precise temperature brightness compensation for the LED or the assembly of LEDs. The system includes regular equipment: adjustable power supply unit  010 , voltmeter  020 , ammeter  030 , temperature-controlled chamber  040  with window  050  for exiting light and light meter  060 . An LED or is an assembly of LEDs  070  are placed in the temperature-controlled chamber  040  and are connected to adjustable power supply unit  010  through ammeter  030 . Voltmeter  020  is connected in parallel with the LED or the assembly of LEDs  070 . The LED or the assembly of LEDs  070  placed into temperature-controlled chamber  040  so that the LED or assembly of LEDs  070  emit the light through the temperature-controlled chamber&#39;s window  050  onto the light meter  060 . The light meter  060  is facing the temperature-controlled chamber&#39;s window  050 . 
         [0069]      FIG. 2B  illustrates a circuit for connecting the LED to the optimal power supply. The circuit  200  includes a series-connected voltage source  201  with the optimal output voltage, resistor  202  with the optimal resistance and the LED  203  or the assembly of LEDs. 
         [0070]      FIG. 2C  illustrates the dependence of the voltage-current characteristics of the LED or the assembly of LEDs for high and low temperatures. It explains the method of the precise temperature brightness compensation of the LED or the assembly of LEDs for two temperatures. 
         [0071]      FIG. 2C  show the work of circuit  200  in current (I)-voltage (V) coordinates axes for high (curve  210 ) and low (curve  220 ) temperatures. Point  211  denotes a state of the LED or the assembly of LEDs that given current  212  and voltage  213  it has a reference brightness value for high temperature. Point  221  denotes a state of the LED or the assembly of LEDs that given current  222  and voltage  223  it has a reference brightness value for low temperature. Points  211  and  221  define a straight line  230  with two intercepts: the current axis intercept, point  231 , and the voltage axis intercept, point  232 . Points  231  and  232  (V 232  and I 231 ) can be obtained analytically using coordinates points  211  and  221  (V 213 ,I 212  and V 223 ,I 222 ) and the following formulas: 
         [0000]    
       
         
           
             
               V 
               232 
             
             = 
             
               
                 
                   
                     V 
                     223 
                   
                    
                   
                     I 
                     212 
                   
                 
                 - 
                 
                   
                     V 
                     213 
                   
                    
                   
                     I 
                     222 
                   
                 
               
               
                 
                   I 
                   212 
                 
                 - 
                 
                   I 
                   222 
                 
               
             
           
         
       
       
         
           
             
               I 
               231 
             
             = 
             
               
                 
                   
                     V 
                     223 
                   
                    
                   
                     I 
                     212 
                   
                 
                 - 
                 
                   
                     V 
                     213 
                   
                    
                   
                     I 
                     222 
                   
                 
               
               
                 
                   V 
                   223 
                 
                 - 
                 
                   V 
                   213 
                 
               
             
           
         
       
     
         [0072]    With changing temperature, the working point of the LED or the assembly of LEDs moved along this straight line. With increasing temperature, the working point moves from point  221  to point  211 . In points  221  and  211  the brightness of the LED or the assembly of LEDs the same. Thus the precise compensation of brightness is ensured at two temperatures points. In the intermediate temperatures the brightness is slightly higher. 
         [0073]      FIG. 3  illustrates the block diagram of the precise method of the temperature brightness compensation and process:
       1. Placing the LED or the assembly of LEDs  070  ( FIG. 2A ) into temperature-controlled chamber  040  with output window  050 .   2. Connecting the LED or the assembly of LEDs  070  to the adjustable power supply unit  010 , voltmeter  020  and ammeter  030 .   3. Directing the light of the LED or the assembly of LED  070  to the light meter  060  through output window  050 .   4. Setting a high working temperature in the temperature-controlled chamber  040 .   5. Adjusting the adjustable power supply unit  010  so that the light meter  060  reads the required brightness of the LED or the assembly of LEDs.   6. Recording the current  212  and the forward voltage  213  of the LED or the assembly of LEDs and plot first point  211  into coordinate space current—voltage.   7. Setting a low working temperature in the temperature-controlled chamber  040 .   8. Adjusting the adjustable power supply unit  010  so that the light meter  060  reads the same brightness.   9. Recording the current  222  and the forward voltage  223  of the LED or the assembly of LEDs and plot second point  221  into coordinate space current—voltage.   10. Connecting points  211  and  221  with straight line  230  until it crosses the current and the voltage axes.   11. Using the voltage intercept point  232  of line  230  to calculate the optimal voltage value of source for supply of the LED or the assembly of LEDs  070 .   12. Using the ratio of the intercept voltage value to the intercept current value, points  232  and  231  of line  230  gives the value of the optimal resistance of the resistor for supply the LED or the assembly of LEDs.       
 
         [0086]      FIG. 4A  illustrates experimental data for single LED yellow color type TLCY5100 without any temperature brightness compensation and the same LED using the method of precise temperature brightness compensation described above. The brightness of LED at temperature 20° C. was taken as a reference level. 
         [0087]    The temperature ranges from 0° C. to +40° C. and is presented by the X-axis. The relative brightness (brightness divided by the reference brightness) is presented by the Y-axis. Without compensation (curve “x”) with supply of direct current ma in the temperature range from 0° C. to +40° C. relative is brightness of LED varies in the range of ±20%. 
         [0088]    With the method of precise temperature brightness compensation (curve “∘”) and that implies the optimal power supply with voltage 2.16 V and series of the optimal resistor 8.47 Ohm, in the temperature from 0° C. to +40° C., the relative brightness of LED varies in the range of only ±1.85%. 
         [0089]      FIG. 4B  illustrates experimental data for single LED white color type HB5-439AWD without any temperature brightness compensation and the same LED using the method of precise temperature brightness compensation described above. 
         [0090]    The brightness of LED at temperature 20° C. was taken as a reference level. The temperature ranges from 0° C. to +40° C. and is presented by the X-axis. The relative brightness (brightness divided by the reference brightness) is presented by the Y-axis. 
         [0091]    Without compensation (curve “x”) with supply of direct current 17 ma in the temperature range from 0° C. to +40° C. relative brightness of LED varies in the range of ±8%. With the method of precise temperature brightness compensation (curve “∘”) and that implies the optimal power supply with voltage 3.51 V and series of the optimal resistor 24.0 Ohm, in the is temperature from 0° C. to +40° C., the relative brightness of LED varies in the range of only ±0.2%. 
         [0092]      FIG. 5A  illustrates a schematic diagram of a circuit according to the first embodiment of the present invention. In this drawing, the assembly of LEDs is represented as a series circuit of the LEDs. The circuit dynamically resists changes in brightness caused by ambient temperature and adjusts the current through the assembly of LEDs to maintain the prespecified brightness of the LEDs. 
         [0093]    As shown in  FIG. 5A , compensation circuit  500  includes the assembly of LEDs  530 . The circuit connects to power source  510 . Compensation circuit  500  includes voltage stabilizer  520  with optimal output voltage, the assembly of LEDs  530  (that functions as temperature sensors), optimal resistor  540  and output terminal  550  for a safety system. 
         [0094]    Input Vin of voltage stabilizer  520  is connected to power supply  510 . Terminal GND of voltage stabilizer  520  is connected to ground  560 . Positive terminal of the assembly of LED  530  connects output Vout of voltage stabilizer  520 , negative terminal of the assembly of LED  530  connects to the resistor is  540 . Other terminal of resistor  540  is connected to ground  560 . 
         [0095]    Common point of the assembly of LED  530  and resistor  540  is connected to output terminal  550 . Voltage of output terminal  550  may be used for a safety system. 
         [0096]    A temperature rise causes a decline in forward voltage of the assembly of LEDs to produce more current, which compensates a decrease in brightness. A temperature reduction causes an increase in forward voltage of the assembly of LEDs to produce less current which compensate the increase in brightness. The optimal voltage of the voltage stabilizer and the optimal resistance maintain brightness at the same level at the boundaries of the selected temperature range. Experimental data show, that in the middle of the selected temperature range, the brightness of the LED is slightly higher. 
         [0097]    The optimal voltage and the optimal resistance for the assembly of LEDs can be calculated from the optimal voltage and the optimal resistance for a single LED. The optimal voltage for the assembly of LEDs equals optimal voltage for a single LED multiplied by the number of LEDs in the series circuit. The optimal resistance for the assembly of LEDs equals optimal is resistance for a single LED multiplied by the number of LEDs in the series circuit. 
         [0098]    It is important to choose the correct internal resistance of voltage stabilizer. As a rule, a resistance of external resistor equals to the optimal resistance because the internal resistance of the voltage stabilizer is usually much less than the optimal resistance. 
         [0099]    In a non-typical cases, the internal resistance of voltage stabilizer may be comparable with the optimal resistance, may be equal, or even higher than the optimal resistance. When internal resistance of voltage stabilizer is comparable with the optimal resistance, the value of the external resistor equals the optimal resistance minus the internal resistance of the voltage stabilizer. If internal resistance of the voltage stabilizer equals the optimal resistance, the external resistor is excluded from the circuit. 
         [0100]    If internal resistance of voltage stabilizer is higher than the optimal resistance, the circuit of the precise brightness compensation does not work. Brightness at high temperature will be less than at low temperature. Thus, the internal resistance of voltage stabilizer must be always smaller is than the optimal resistance. 
         [0101]    If the internal resistance of the voltage stabilizer equals the optimal resistance and the external resistor is absent, the signal for safety system can be obtained from the current mirror. In this case the output current of the current mirror can be used for safety system. 
         [0102]    Terminal  550  can be used for safety system. Some voltage stabilizers have a special input for turning off output voltage. In such cases, terminal  550  may be used for protection of the assembly of LEDs if maximum allowed current of the LED in the assembly is exceeded. If the voltage stabilizer has a built-in current limiter or current protection, then the output terminal  550  is not used. 
         [0103]      FIG. 5B  illustrates a schematic diagram of a circuit according to the first embodiment of the present invention. This assembly of LEDs is represented as several branch in-parallel of series circuit of LEDs. The circuit dynamically resists changes in brightness due to ambient temperature and adjusts the current applied to the assembly of LEDs to maintain the pre-specified brightness of the LEDs. 
         [0104]    As shown in  FIG. 5B , a compensation circuit  500  includes the assemblies of LEDs  530 , (which function as a temperature sensor). The circuit connects to a power source  510 . The circuit  500  includes voltage stabilizer  520  with optimal output voltage, the assembly of LEDs  530 , optimal resistors  541  and  542 , output terminals  551  and  552  for safety system. 
         [0105]    Input Vin of voltage stabilizer  520  is connected to power supply  510 . GRD terminal of voltage stabilizer  520  is connected to ground  560 . This circuit assembly  530  includes several series of LEDs. Each series of LED has the same number of LEDs and the same optimal resistor  541  and  542  that are connected between the series of LEDs  531  and  532  to the ground  560 . Positive terminals each series of LEDs  531  and  532  is connected to output Vout of voltage stabilizer  520  and every negative terminal is connected to optimal resistor  541  and  542 . Other terminals of resistors  541  and  542  are connected to the ground  560 . Common points of series of LED  531  and  532  and resistors  541  and  542  are connected to output terminals  551  and  552  accordingly. 
         [0106]    As described above, a temperature increase causes a is decline of forward voltage of series circuit of LEDs to produce more current that compensates the drop in brightness. A temperature decrease causes an increase of forward voltage of series circuit of the LEDs to produce less current that compensates the increase in brightness. The optimal voltage of the voltage stabilizer and the optimal resistance maintain brightness on the same level at the boundaries of the selected temperature range. 
         [0107]    Connection in-parallel of the series of LEDs may be done in two ways: a parallel connection of the series of the LEDs that utilizes balancing resistors as showed in  FIG. 5B , and a simple connection in-parallel without balancing resistors. The simple connection in-parallel of the series of LEDs without balancing resistors is incorrect, because a small change in the forward voltages of any series of LEDs can cause a very big difference of currents in the series circuits, leading to the thermal failure of one of the series circuits of LED. Therefore, the use of balancing resistors is preferred. 
         [0108]    The optimal voltage of the voltage stabilizer and the optimal resistance of the resistors included in an assembly of is LEDs maintain the brightness at the same level as the boundaries of the selected temperature range. The optimal voltage for the in-parallel assembly of LEDs equals the optimal voltage for a single LED multiplied by the number of LEDs in the series circuits. 
         [0109]    The resistance of the external resistors depends on the internal resistance of the voltage stabilizer. When the internal resistance of the voltage stabilizer is substantially smaller then the optimal resistance divided by the number of parallel branches, the external resistors equal to the optimal resistance of a single LED multiplied by the number of LEDs in series circuit. Generally, the resistance of the external resistors equals to the optimal resistance of the series circuits minus the internal resistance of the voltage stabilizer multiplied by the number of parallel branches. If the resistance of voltage stabilizer is greater than the optimal resistance series circuits divided by the number of parallel branches, then this voltage stabilizer does not ensure temperature compensation brightness. 
         [0110]    If the primary voltage supply has a stable output voltage and negligible internal resistance, then the voltage is stabilizer can be replaced with a voltage divider. The values of the resistors for the voltage divider are chosen so that the open circuit voltage equals the optimal voltage of an assembly of LEDs, and the resistors in-parallel of the voltage divider should have optimal resistance. This makes it possible to manufacture LEDs modules with embedded precise temperature brightness compensation. Such modules can be used in automobile industry. 
         [0111]      FIG. 6  illustrates the dependence of the voltage-current characteristics of the LED for high, middle and low temperatures and explains the method of precise temperature brightness compensation of the LED for three temperatures values.  FIG. 6  shows voltage-current characteristics of the LED in current (I)-voltage (V) coordinates for high  610 , middle  620  and low  630  temperatures. For high temperature (curve  610 ) point  611  defines values of current  612  and voltage  613  of the LED corresponding to a specified brightness level. For middle temperature (curve  620 ), point  621  defines values of current  622  and voltage  623  of the LED corresponding to the same brightness level as in point  611 . Points  611  and  621  define a straight line  640  with voltage (V) and current (I) intercepts. This part of the drawing explains the work of LED in the range from high to middle temperatures. Thus for a temperature interval from high to middle temperatures, the optimal voltage of supply is voltage (V) intercept point  642 . 
         [0112]    The optimal resistance of the resistor equals to the ratio of voltage (point  642 ) to current axis intercept (point  641 ) of line  640 . 
         [0113]    For low temperature (curve  630 ), point  631  defines values of current  632  and voltage  633  of LED that correspond to the same brightness level as in points  611  and  621 . Points  621  and  631  define a straight line  650  with voltage (V) and current (I) axes intercepts. This part of the drawing explains the work of LED in the range from middle to low temperatures. For a temperature interval from middle to low temperatures, the optimal voltage (point  652 ) is the voltage (V) axis intercept. The optimal resistance equals a ratio of voltage intercept (point  652 ) to current intercept (point  651 ) of line  650 . 
         [0114]    In order to compensate brightness of the LED at three is temperatures values, it is necessary to switch values the optimal voltage and the optimal resistors when the LED is operated in a different temperature interval. If the actual forward voltage of the LED is less than the voltage in middle temperature (point  623 ), then the circuit should be powered by the high-middle interval settings. If the actual forward voltage of the LED is more than the voltage in middle temperature (point  623 ), then the circuit should be powered by the middle-low interval settings. The working point of the LED is located the other the linear segments ( 631 ,  621 ) or ( 621 ,  611 ) of the straight lines  640  or  650 . 
         [0115]    For three temperature points there are two temperature intervals. For each temperature interval we define two independent parameters: optimal voltage and an optimal resistance. We call these parameters an “interval setting”. The power supply of the circuit can be switched from one “interval setting” to another. When the power supply switches onto the high-middle “interval setting” it means that the voltage of the power supply becomes equal to the optimal voltage of the high-middle temperature interval and the resistance of the circuit becomes equal to the optimal resistance value of the high-middle is temperature interval. 
         [0116]    Thus, with an increase of temperature from low to high the working point moves along the straight line  650  from point  631  to point  621 , and after a power supply switch, it moves along the straight line  640  from point  621  to point  611 . In the points  631 ,  621  and  611 , the brightness of the LED is the same. Thus, precise compensation of brightness at three temperatures is ensured. 
         [0117]      FIG. 7  illustrates a schematic block diagram of the method of the precise temperature compensation for the LED or the assembly of LEDs at three temperatures which includes the steps:
       1. Dividing the temperature operating range of the LED or the assembly of LEDs at two temperature intervals: from high to middle temperature and from middle to low temperature.   2. For temperature interval from high to middle temperatures, using the aforesaid method to find first optimal voltage and first optimal resistance.   3. For temperature interval from middle to low temperatures, using aforesaid method to find the second optimal voltage is and second optimal resistance.   4. Recording the value of the forward voltage of the LED or assembly of LEDs at middle temperature.   5. With an increasing temperature and a decreasing forward voltage of the LED or the assembly of LEDs is less than recorded the forward voltage at the middle temperature connect the LED to the power supply with first optimal voltage through resistor with first optimal resistance.   6. With a decreasing temperature and increasing the forward voltage of the LED or the assembly of LEDs equal or more then recorded the forward voltage at the middle temperature connect the LED or the assembly of LEDs to the power supply with second optimal voltage through resistor with second optimal resistance.       
 
         [0124]    There are several ways to choose the middle temperature to minimize the variation of brightness. The easiest way is to choose the middle temperature in the center of the temperature range. Experiments show that at temperature range 0° C. to +40° C. maximal deviation of brightness is in the center of the temperature range +20° C. For a very broad temperature range, −40° is to +85° C., for example, choosing the center point is inappropriate. A better method to find a middle temperature point includes the following steps.
       1. Using the method of the precise temperature brightness compensation for two temperatures, high and low, and finding the optimal voltage and the optimal resistance.   2. Connecting the LED to the power source with optimal voltage and optimal resistor and recording the data of the brightness of the LED for each temperature point.   3. Plotting this data, and finding a temperature point on this graph such that the deviation of brightness from the specified brightness is the largest.   4. Choosing this temperature as the middle temperature point.       
 
         [0129]      FIG. 8  illustrates a schematic diagram of a circuit according to the second embodiment of the present invention. The circuit dynamically adjusts the current applied to the LED assembly to maintain the brightness of the LED and ensures precise compensation of brightness of the LED or the assembly of LED at three temperatures. 
         [0130]    As follows from the explanation, to implement the circuit for precise temperature brightness compensation at three is temperatures points, it is necessary to have two voltage stabilizers with different values of the optimal voltage, two optimal resistors and four switches—two for switching the optimal voltage and two for switching the optimal resistors. The optimal voltage and the optimal resistors must be switched in the corresponding temperature intervals. But it is also possible to use a single adjustable voltage stabilizer and only two switches. 
         [0131]    As shown in  FIG. 8  the compensation circuit includes: adjustable voltage stabilizer  820 , resistors  821 ,  822  and  823  for choosing optimal voltage of stabilizer  820  on different of temperature intervals, the assembly of LEDs  830  as a light source and as the temperature sensor, resistors  831  and  832  for choosing optimal resistance on different of temperature intervals, reference voltage source  850  and the control system  840  for switching the optimal voltage and the optimal resistors at two temperature intervals. 
         [0132]    The circuit is supplied from a power source  810 . Input V in  of adjustable voltage stabilizer  820  is connected to power supply  810 . Resistor  821  connects output V out  with adjustable is input ADJ of adjustable voltage stabilizer  820 . Resistor  822  connects adjustable input ADJ of adjustable voltage stabilizer  820  with ground  860 . Resistors  821  and  822  predetermine the optimal voltage of adjustable voltage stabilizer  820  at temperature interval from middle to low temperatures. 
         [0133]    The common point of resistors  821  and  822  is connected to resistor  823 . The other terminal of resistor  823  is connected to switch  845  of control system  840  for switching the optimal voltage at temperature interval from high to middle temperatures. Resistors  823 ,  822  and resistor  821  defines the optimal voltage of voltage stabilizer  820  at temperature interval from high to middle temperatures. The assembly of LEDs  830  connects between output V out  of the adjustable voltage stabilizer  820  and optimal resistor  831 . 
         [0134]    The other terminal of resistor  831  is connected to ground  860 . Resistance of resistor  831  equals the optimal resistance at temperature interval from middle to low temperatures. The common point of the assembly of LEDs  830  and resistor  831  is connected to resistor  832 . 
         [0135]    The other terminal of resistor  832  is connected to switch  846  of the control system  840  for switching the optimal is resistor at temperature intervals from middle to high temperature. The resistance of in-parallel resistors  831  and  832  equals to optimal resistance at temperature interval from high to middle temperatures. 
         [0136]    Control system  840  switches the optimal voltage and resistors at different temperature intervals and includes: op-amp  841 , resistors  842  and  843 , resistor of negative feedback  844 , comparator  849  and two MOSFET switches  845  and  846 . 
         [0137]    Resistor  842  connects between inverting input op-amp  841  and common point of LED or assembly of LED  830  and resistor  831 . Resistor  843  connects non-inverting input op-amp  841  with common point of the assembly of LEDs  830  and with output V out  of adjustable voltage stabilizer  820 . Resistor of negative feedback  844  connects the output with the inverting input op-amp  841 . Output of op-amp  841  is connected to the inverting input of comparator  849 . Non-inverting input of comparator  849  is connected to the output of the reference voltage source  850 . Output of comparator  849  is connected to gates of MOSFET switches  845  and  846 . Sources of MOSFET switches  845  and  846  are connected to ground  860 . The drain of MOSFET switch  845  is is connected to resistor  823  and drain of MOSFET switch  846  is connected to resistor  832 . 
         [0138]      FIG. 8  shows that resistor  843  connects to the common point of the assembly of LEDs and the output V out  of adjustable voltage stabilizer  820 . Resistor  843  may be connects to one, two or any number of LEDs of series at the assembly of LEDs. 
         [0139]    Reference voltage source  850  controls the switching threshold of optimal voltage for power source and optimal resistors at different temperature intervals. The value voltage of reference voltage source  850  depends on the gain factor of op-amp  841  and the forward voltage of the assembly of LEDs  830  at middle temperature. If the gain factor of op-amp  841  equals to one, then the value reference voltage source  850  equals to the forward voltage of assembly of LEDs  830  at middle temperature. If not, then the value voltage of reference voltage source  850  equals to the forward voltage of the assembly of LEDs multiplied by the gain factor of op-amp  841 . 
         [0140]    When the ambient temperature is between low and middle, the value of the forward voltage on the assembly of LED  830  is greater than the forward voltage on the assembly of LEDs  830  at middle temperature. 
         [0141]    In this case the output voltage op-amp  841  is greater than the voltage of reference source  850 , and the voltage on output comparator  849  has low potential. Now MOSFET switches  845  and  846  are closed. The assembly of LEDs supply from the source with optimal voltage assignable the divider consist of resistors  821  and  822  through optimal resistor  831 . 
         [0142]    When ambient temperature is between high and middle, the value of the forward voltage on the assembly of LED  830  is less than the forward voltage on the assembly of LEDs  830  at middle temperature. In this case the output voltage op-amp  841  less than voltage of reference source  850 , and voltage on output comparator  849  has high potential. Now MOSFET switches  845  and  846  are opened. The assembly of LEDs supply from the source with optimal voltage assignable the divider consist of resistors  821 ,  822 ,  823  arranged in parallel through optimal resistor  831  in parallel with resistor  832 . 
         [0143]    MOSFET switches  845  and  846  of the control system  840  can be replaced by FET, bipolar transistors, optoelectronic switch, optoelectronic relay, reed relay or other electronic switches. 
         [0144]    While the invention has been illustrated and described in details in the drawings and foregoing description, the same is to be considered as illustrative and not restrictive in character, it being understood that only certain embodiments have been shown and described and that all equivalents, changes, and modifications that come within the spirit of the inventions as described herein and/or by the following claims are desired to be protected. 
         [0145]    Hence, the proper scope of the present invention should be determined only by the broadest interpretation of the appended claims so as to encompass all such modifications as well as all relationships equivalent to those illustrated in the drawings and described in the specification. 
         [0000]    
       
         
               
             
               
               
               
               
               
             
           
               
                   
               
               
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