Patent Publication Number: US-7719207-B2

Title: Apparatus for controlling light emitting devices

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention generally relates to apparatus for controlling light emitting devices, and more particularly to apparatus for driving light emitting diodes with different spectrums by a feedback control system to produce different stable colors. 
   2. Description of the Prior Art 
   For the advantages of less volume, less input power, longer life and lower cost, light-emitting diodes (LEDs) are replacing conventional lighting devices, and novel applications thereof are emerging. For example, various colors could be generated by independently controlling the illuminance (or intensity) of two (or more) LEDs with distinct spectrum (or color) and mixing the color optically. 
   The LED is composed of N-type semiconductor and P-type semiconductor. The resistance of the interface (or node) between the N-type semiconductor and P-type semiconductor is susceptible to ambient temperature, and subsequently, the illuminance of the LED is likely to be affected by the resistance change. Specifically, the varying ambient temperature may result in an over-heated and over-lighted LED with high output, or alternately may result in an under-lighted LED with insufficient output. For example, in the constant-voltage driving mode when the ambient temperature rises, the interface resistance decreases, causing high operation power and heat for the LED and thus disadvantageously shortens the life of the LED; on the other hand, when the ambient temperature falls, the increased interface resistance causes low operating power for the LED, which renders the LED useless for its insufficient illuminance. Alternatively, in the constant-current driving mode, when the ambient temperature rises, the decreased interface resistance causes low operating power of the LED, which renders the LED useless for insufficient illuminance; and when the ambient temperature falls, the increased interface resistance causes high operating power and heat of the LED, which disadvantageously shortens the life of the LED. Further, the LEDs with different spectrums are susceptible to the ambient temperature with different degrees. Accordingly, it is difficult to precisely arrive at a required color by mixing the different spectrums. 
   For the foregoing reasons, a need has arisen to propose apparatus for controlling the LEDs that is capable of reducing the temperature affect on the LEDs, protecting to lengthen the life of the LEDs, stabilizing the output illuminance of the LEDs, and precisely mixing the colors of the LEDs. 
   SUMMARY OF THE INVENTION 
   In view of the foregoing, it is an object of the present invention to provide apparatus for controlling the LEDs, that is capable of reducing the temperature effects on the operating (or input) power of light emitting devices (such as LEDs), and reducing the unstable input voltage/current effects on the operating power of the light emitting devices. Accordingly, the present invention could protect and lengthen the life of the light emitting devices, stabilize the output illuminance of each light emitting device, and precisely mix the colors of the light emitting devices. 
   According to the object, the present invention provides apparatus for driving light emitting devices with different colors. The input powers of the light emitting devices are measured by power measuring devices, returned by feedback controllers to control the power input to the light emitting devices, and then individually configured by controlling the luminance of different spectrums, thus obtaining the desired colors. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1A  shows an electrical connecting flow illustrating apparatus for controlling light emitting devices according to one embodiment of the present invention; 
       FIG. 1B  shows an electrical connecting flow illustrating apparatus for controlling light emitting devices according to another embodiment of the present invention; 
       FIG. 2A  shows an electrical connecting flow illustrating apparatus for controlling light emitting devices according to another embodiment of the present invention; 
       FIG. 2B  shows an electrical connecting flow illustrating apparatus for controlling light emitting devices according to further embodiment of the present invention; 
       FIG. 3A  shows a portion of the apparatus of  FIG. 2A , particularly a pulse width modulation (PWM) switch being practiced as the switch; 
       FIG. 3B  shows an exemplary waveform illustrating the relationship between the DC voltage (or power) and the duty cycle control signal in  FIG. 3A ; and 
       FIG. 4  illustrates mixing two LEDs by a light mixing device to obtain a required color. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1A  shows an electrical connecting flow illustrating apparatus  100  for controlling light emitting devices according to one embodiment of the present invention. In the embodiment, the light emitting devices are light-emitting diodes (LEDs)  12 A and  12 B, which have different spectrums (or colors). More than two LEDs with at least two spectrums (or colors) may also be used. The output illuminance of the LED  12 A and the LED  12 B are independent, and can be controlled to mix optically to arrive at a specific color. For example, light from the LEDs with the three primary colors could be mixed to obtain different colors. 
   The LEDs  12 A and  12 B are influenced by input DC (i.e., direct current), voltage V DC  and ambient temperature T a . The equivalent circuits of the LEDs  12 A and  12 B are shown in the figure, in which gain G vi  represents the function between the current flowing through the LEDs ( 12 A and  12 B) and the input DC voltage, and gain G ai  represents the function between the current flowing through the LEDs ( 12 A and  12 B) and the ambient temperature. 
   The input DC voltages V DC  to the LEDs  12 A and  12 B are provided by AC-to-DC (or AC/DC) converters (or adapters)  14 A and  14 B respectively. The AC/DC converters  14 A and  14 B convert the AC (i.e., alternating current) voltage V ac  (such as the power voltage provided from indoor power outlet) into the DC voltage V DC . 
   The apparatus  100  according to the present embodiment includes two power measuring devices (or detectors)  16 A and  16 B, which are electrically coupled to the LEDs  12 A and  12 B for measuring the input power P of the LEDs  12 A and  12 B respectively. In the embodiment, taking the power measuring device  16 A for example, a current measuring device  160 A is coupled (in series) to one node of the LED  12 A for measuring the current I of the LED  12 A; and a voltage measuring device  162 A is coupled (in parallel) to another node of the LED  12 A for receiving and measuring the DC voltage V DC . The detected current I from the current measuring device  160 A and the detected DC voltage V DC  from the voltage measuring device  162 A are inputted to a multiplier  164 A whose resultant product represents the power P. With respect to another power measuring device  16 B, the operation of its current measuring device  160 B, voltage measuring device  162 B, and multiplier  164 B is the same as the power measuring device  16 A. In the embodiment, the power measuring principle P=V×I is used in constructing the power measuring devices  16 A and  16 B. 
   The measured powers P from the power measuring devices  16 A and  16 B are inputted to the feedback controller  18 A and  18 B respectively, which generate output signals that further control the AC/DC converter  14 A and  14 B. For example, when the rising/falling ambient temperature changes the input power P of the LEDs  12 A and  12 B, the feedback controller  18 A and  18 B change their output signals according to a predetermined reference power P set , and further control a digital variable resistor in the AC/DC converter  14 A and  14 B in order to change the generated DC voltage V DC  and the current flowing through the LEDs ( 12 A and  12 B), thereby maintaining the input power, the output illuminance, and spectrum (or color) of the LEDs  12 A and  12 B. Therefore, the apparatus  100  could maintain the specific mixed color. 
   In the embodiment, taking the feedback controller  18 A for example, a substractor  180 A is coupled to receive the predetermined reference power P set  and the detected power P from the power measuring device  16 A, and the resultant difference is inputted to a controller  182 A, which controls the AC/DC converter  14 A according to the resultant difference, until the power of the LED  12 A is equal to the predetermined reference power P set . For example, when the resultant difference is negative, the AC/DC converter  14 A is controlled (by the controller  182 A) to lower the DC voltage V DC ; alternately, when the resultant difference is positive, the AC/DC converter  14 A is controlled to raise the DC voltage V DC . The controller  182 A may be a circuit, or a program-controlled controller (such as a microprocessor). With respect to another feedback controller  18 B, the operation of its substractor  180 B and controller  182 B is the same as the feedback controller  18 A. In other embodiments, the substractors  180 A and  180 B could be omitted, and the detected power P from the power measuring devices  16 A and  16 B are inputted into an individual or shared controller, which directly generates corresponding output via, for example, a look-up table, to the AC/DC converter  14 A and  14 B according to power P. In the present embodiment, the predetermined reference powers P set  of the feedback controllers  18 A and  18 B may be distinct or the same. The aforementioned predetermined reference powers P set  are fixed; however they could be dynamically adjusted at different time (or interval) by the controller (or other device) to change the illuminance of the LEDs  12 A and  12 B according to different applications, thereafter mixing the light to obtain dynamic color lighting. 
     FIG. 1B  shows an electrical connecting flow illustrating apparatus  102  for controlling light emitting devices according to another embodiment of the present invention. The components such as the LEDs  12 A and  12 B, and the power measuring devices  16 A and  16 B are the same as the components of  FIG. 1A , using same reference numerals or characters, and therefore their discussion is omitted. The primary difference between the present embodiment and the embodiment of  FIG. 1A  is the DC current output I DC  in the present embodiment rather than the DC voltage V DC  in the previous embodiment. Further, in the present embodiment, the equivalent circuits of the LEDs  12 A and  12 B are shown in the figure, in which gain G iv  represents the function between the LED output voltage and the input DC current, and gain G av  represents the function between the LED output voltage and the ambient temperature. The present embodiment functions substantially the same as the embodiment of  FIG. 1A , that is, the measured powers P from the power measuring devices  16 A and  16 B are returned to the feedback controller  18 A and  18 B respectively, which further control the AC/DC converter  14 A and  14 B, thereby maintaining the input power, the output illuminance, and spectrum (or color) of the LEDs  12 A and  12 B. 
     FIG. 2A  shows an electrical connecting flow illustrating apparatus  200  for controlling light emitting devices according to another embodiment of the present invention. The components such as the LEDs  12 A and  12 B, and the power measuring devices  16 A and  16 B are the same as the components of  FIG. 1A , using the same reference numerals or characters, therefore their discussion is omitted. In the embodiment, no AC/DC converter is used, and the DC voltage V DC  is directly provided by a DC voltage power (not shown). However, an AC/DC converter may be used to provide the DC voltage V DC . The value of the DC voltage V DC  may fluctuate (such as in solar power or battery) or be fixed (such as in constant-voltage power supply). 
   The primary difference between the present embodiment and the embodiment of  FIG. 1A  is the switching (or on-off) current driving of the LEDs  12 A and  12 B in the present embodiment compared to the continuous current driving of the LEDs  12 A and  12 B in the previous embodiment. In the present embodiment, taking the LED  12 A for example, one node of the LED  12 A is coupled in series to a switch  191 A of the feedback controller  19 A. The LED  12 A accordingly emits intermittently owing to the intermittent switching of the switch  191 A. The control of the duty cycle of the switch  191 A is utilized to control the proportion of light emitting in time, and therefore control the input power P of the LED  12 A. Human eyes do not perceive the intermittence when the switching frequency of the switch  191 A is high enough. The switch  191 A may be a metal oxide semiconductor field effect transistor (MOSFED), or other electronic devices capable of performing switching. With respect to another feedback controller  19 B, the operation of its switch  191 B is the same as the switch  191 A. 
   In the present embodiment, each of the current measuring devices  160 A and  160 B and the voltage measuring devices  162 A and  162 B includes a signal processor that is capable of converting the detected switching current I and the direct voltage V DC  into a continuous signal representing the average value, which is then respectively inputted to the multiplier  164 A to generate the average input power P of the LEDs  12 A and  12 B. The measured powers P from the power measuring devices  16 A and  16 B are fed back to the feedback controller  19 A and  19 B respectively. T a  king the feedback controller  19 A for example, a substractor  190 A is coupled to it to receive a predetermined reference power P set  and the detected power P from the power measuring device  16 A, and the resultant difference is inputted to a controller  192 A, which generates a duty cycle control signal D to control the switch  191 A and the light emitting of the LED  12 A, thereby maintaining the input power, the output illuminance, and spectrum (or color) of the LED  12 A. The apparatus  200  is then subjected to light mixing to obtain the desired color stably. With respect to another feedback controller  19 B, the operation of its substractor  190 B, switch  191 B, and controller  192 B is the same as the feedback controller  19 A. 
   Similar to the previous embodiment, the controllers  192 A and  192 B may be circuits, or program-controlled controllers (such as microprocessors). The substractors  190 A and  190 B could be omitted, and the detected power P from the power measuring devices  16 A and  16 B are inputted into an individual or shared controller, which directly generates corresponding duty cycle control signals via, for example, a look-up table, to the switches  191 A and  191 B according to power P. 
     FIG. 3A  shows a portion of the apparatus  200  in  FIG. 2A , particularly a pulse width modulation (PWM) switch being practiced as the switch  191 A or  191 B. One end of the PWM switch  191 A/ 191 B is electrically coupled to one node of the LED  12 A/ 12 B, and another end is coupled to the ground.  FIG. 3B  shows an exemplary waveform illustrating the relationship between the DC voltage V DC  (or power P) and the duty cycle control signal D in  FIG. 3A . As shown in the figure, the DC voltage V DC  fluctuates. When the DC voltage V DC  (or power P) is overly high, for example, at time t 1 , the duty cycle control signal has a narrower width, which causes low proportion of light emitting from the LEDs  12 A and  12 B; alternately when the DC voltage V DC  (or power P) is overly low, for example, at time t 2 , the duty cycle control signal has a wider width, which causes high proportion of light emitting of the LEDs  12 A and  12 B. Accordingly, the input power of the LEDs  12 A and  12 B could still be maintained at a fixed value even when the DC voltage fluctuates. Further, when the falling/rising ambient temperature causes the increase/decrease in the P-N interface resistance, the feedback controllers  19 A and  19 B operate the PWM switches  191 A and  191 B according to the principle discussed above to maintain the input power. Therefore, the LEDs  12 A and  12 B could be protected from burned down in an overly high ambient temperature, or be prevented from unsatisfactorily emitting dim light in a cold temperature. 
     FIG. 2B  shows an electrical connecting flow illustrating apparatus  202  for controlling light emitting devices according to further embodiment of the present invention. The present embodiment uses the same components as the embodiment in  FIG. 2A  but is controlled in a different manner. The interconnection of the present embodiment is similar to that in  FIG. 1A . 
   The primary difference between the present embodiment and the embodiment of  FIG. 2A  is the serial connection of the switches  191 A and  191 B (for example, PWM switches) and the inputs (rather than outputs) of the corresponding LEDs  12 A and  12 B in the present embodiment. The outputs of the LEDs  12 A and  12 B are coupled to the power measuring devices  16 A and  16 B. Accordingly, the feedback controllers  19 A and  19 B determine a proper duty cycle under which the DC voltage V DC  controllably provides power to drive the LEDs  12 A and  12 B. 
   The embodiments discussed above are capable of reducing the temperature effects and the unstable input voltage/current effects on the operating (or input) power of the light emitting devices. Accordingly, the present invention could protect and lengthen the life of the light emitting devices, stabilize the output illuminance of the light emitting devices, and precisely mix the colors of the light emitting devices. 
     FIG. 4  illustrates how a light mixing device  40  mixes two or more LEDs (for example, LED 1  and LED 2 ) to obtain a required color. In the embodiment, the LED 1  is characterized with a spectrum L 1 , and the LED 2  is characterized with a different spectrum L 2 . The spectrums L 1  and L 2  together may compose the required spectrum L 1 +L 2  by arranging the relative position of the LEDs (LED 1  and LED 2 ), for example, or by using the accompanied light mixer or reflector. If three LEDs with the three primary colors are used, they could be mixed to obtain various different colors. 
   Although the specific embodiments have been illustrated and described, it will be appreciated by those skilled in the art that various modifications may be made without departing from the scope of the present invention, which is intended to be limited solely by the appended claims.