Patent Publication Number: US-8975830-B2

Title: Light emitting system, optical power control device, and control signal module

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority to Taiwanese Application No. 102101806, filed on Jan. 17, 2013. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to a light emitting system, an optical power control device, and a control signal module. 
     2. Description of the Related Art 
       FIG. 1  shows a conventional optical power control device adapted to receive a direct-current input voltage and generate a working current to drive a light-emitting diode (LED)  1 . When the input voltage is constant, the working current has a constant magnitude. 
     However, the conventional optical power control device has the following drawbacks: 
     1. The working current resulting from the direct-current input voltage will increase temperature of the LED  1 , and characteristics of the LED  1  will vary with temperature. 
     2. Referring to  FIG. 2 , a forward voltage of the LED  1  varies with ambient temperature, and LEDs  1  with different colors (e.g., blue, green and red) follow different forward voltage-temperature curves. When the LED  1  is driven with a constant current (e.g., 20 mA), rise of the ambient temperature may result in drop of the forward voltage, so that the output power of the LED  1  (=forward voltage×working current) drops with rise of the ambient temperature. 
     3. In application, several LEDs  1  with different colors are frequently used together to obtain light with a desired color temperature and a desired color rendering index. When each of the LEDs  1  with different colors is driven by a corresponding conventional optical power control device, the power ratio thereamong may drift due to different drop levels among the LEDs  1 , such that the desired color temperature and the desired color rendering index may not be maintained. 
     SUMMARY OF THE INVENTION 
     Therefore, an object of the present invention is to provide a light emitting system that may have a relatively stable color temperature and a relatively stable color rendering index. 
     According to one aspect of the present invention, a light emitting system comprises: 
     a light emitting device that has a forward voltage with a magnitude dependent on an ambient parameter when driven with current; and 
     an optical power control device including:
         a control signal module including:
           a reference voltage unit coupled to the light emitting device for detecting the forward voltage thereof, and outputting a reference voltage according to the forward voltage of the light emitting device; and   a control signal generator coupled to the reference voltage unit for receiving the reference voltage, and operable to generate, according to the reference voltage, a control signal having a parameter associated with the reference voltage; and   
               

     a current controller coupled to the light emitting device, and coupled to the control signal generator for receiving the control signal, the current controller being operable to permit flow of a driving current through the light emitting device, the driving current being associated with the parameter of the control signal. 
     Another object of the present invention is to provide an optical power control device that may alleviate output power drop of a light emitting device. 
     According to another aspect of the present invention, an optical power control device is adapted to control a light emitting device that has a forward voltage, and comprises: 
     a control signal module including:
         a reference voltage unit to be coupled to the light emitting device for detecting the forward voltage thereof, and outputting a reference voltage according to the forward voltage of the light emitting device; and   a control signal generator coupled to the reference voltage unit for receiving the reference voltage, and operable to generate, according to the reference voltage, a control signal having a parameter associated with the reference voltage; and       

     a current controller to be coupled to the light emitting device, and coupled to the control signal generator for receiving the control signal, the current controller being operable to permit flow of a driving current through the light emitting device, the driving current being associated with the parameter of the control signal. 
     Yet another object of the present invention is to provide a control signal module used in the light emitting system of this invention. 
     According to yet another aspect of the present invention, a control signal module is adapted for use with a current controller to control flow of a driving current through a light emitting device that has a forward voltage, and comprises: 
     a reference voltage unit to be coupled to the light emitting device for detecting the forward voltage thereof, and outputting a reference voltage according to the forward voltage of the light emitting device; and 
     a control signal generator coupled to the reference voltage unit for receiving the reference voltage, and operable to generate, according to the reference voltage, a control signal having a parameter associated with the reference voltage, the control signal to be provided to the current controller. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Other features and advantages of the present invention will become apparent in the following detailed description of the preferred embodiment with reference to the accompanying drawings, of which: 
         FIG. 1  is a schematic circuit diagram of a conventional optical power control device; 
         FIG. 2  is a plot illustrating relationships between ambient temperature and forward voltages of light emitting diodes with different colors; 
         FIG. 3  is a block diagram of a preferred embodiment of a light emitting system according to the present invention; 
         FIG. 4  is a schematic circuit diagram of a reference voltage unit of the preferred embodiment; 
         FIG. 5  is a schematic circuit diagram to illustrate another implementation of a level shifter of the reference voltage unit of the preferred embodiment; 
         FIG. 6  is a plot illustrating generation of a control signal by a control signal generator of the preferred embodiment; and 
         FIG. 7  is a block diagram of an application of the preferred embodiment. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to  FIG. 3 , a preferred embodiment of the light emitting system according to the present invention is shown to include a light emitting device  1  and an optical power control device  2 . 
     In this embodiment, the light emitting device  1  is a light emitting diode (LED) device that has a forward voltage with a magnitude dependent on an ambient parameter when driven with current. For the LED device in this embodiment, the ambient parameter is an ambient temperature. 
     The optical power control device  2  includes a control signal module  21  and a current controller  22 . 
     The control signal module  21  includes a reference voltage unit  211  and a control signal generator  212 . 
     The reference voltage unit  211  is coupled to the light emitting device  1  for detecting the forward voltage thereof, and outputs a reference voltage, which is a direct-current (DC) voltage, having a magnitude associated with an average magnitude of the forward voltage. 
     The reference voltage unit  211  includes a forward voltage detector  2111 , a voltage integrator  2112 , a power amplifier  2113  and a level shifter  2114 . 
     Referring to  FIG. 4 , the forward voltage detector  2111  is coupled to the light emitting device  1  (see  FIG. 3 ) for detecting the forward voltage thereof, and outputs a detection signal that is a pulse voltage, and that has a magnitude varying with the magnitude of the forward voltage of the light emitting device  1 . 
     The forward voltage detector  2111  includes first, second and third operational amplifiers OP 1 , OP 2 , OP 3 , and first to seventh resistors R 11 -R 17 . Each of the first, second and third operation amplifiers OP 1 , OP 2 , OP 3  has a non-inverting input (“+”; first input), an inverting input (“−”; second input) and an output. 
     The first resistor R 11  is coupled between the second input and the output of the first operational amplifier OP 1 . The second resistor R 12  is coupled between the second input and the output of the second operational amplifier OP 2 . The third resistor R 13  is coupled between the second input and the output of the third operational amplifier OP 3 . The fourth resistor R 14  is coupled between the second inputs of the first and second operational amplifiers OP 1 , OP 2 . The fifth resistor R 15  is coupled between the output of the first operational amplifier OP 1  and the second input of the third operational amplifier OP 3 . The sixth resistor R 16  is coupled between the output of the second operational amplifier OP 2  and the first input of the third operational amplifier OP 3 . The seventh resistor R 17  is coupled between the first input of the third operational amplifier OP 3  and a ground node. 
     The first inputs of the first and second operational amplifiers OP 1 , OP 2  are coupled to the light emitting device  1  for receiving the forward voltage thereof, and the output of the third operational amplifier OP 3  outputs the detection signal. 
     The voltage integrator  2112  is coupled to the forward voltage detector  2111  for receiving the detection signal, and integrates the detection signal for generating an integration signal that is a direct-current (DC) voltage signal. 
     The voltage integrator  2112  includes an operational amplifier OP 4 , first and second resistors R 21 , R 22 , and a capacitor C. 
     The operational amplifier OP 4  has a grounded non-inverting input (“+”; first input), an inverting input (“−”; second input) and an output. The first resistor R 21  has a first terminal coupled to the forward voltage detector  2111  for receiving the detection signal, and a second terminal coupled to the second input of the operational amplifier OP 4 . The second resistor R 22  is coupled between the second input and the output of the operational amplifier OP 4 . The capacitor C is coupled across the second resistor R 22 . The output of the operational amplifier OP 4  outputs the integration signal. 
     The power amplifier  2113  is coupled to the voltage integrator  2112  for receiving the integration signal, and amplifies the integration signal for generating an amplified integration signal. Amplification of the power amplifier  2113  is designed with consideration of electro-optic conversion efficiency of the light emitting device  1 . In detail, if the light emitting device  1  has greater reduction of the electro-optic conversion efficiency with rise of the ambient temperature, the amplification of the power amplifier  2113  is accordingly designed to be greater. Moreover, the amplification of the power amplifier  2113  is also determined according to a relationship between variation of the forward voltage of the light emitting device  1  and the ambient temperature. 
     The power amplifier  2113  includes an operational amplifier OP 5 , a first resistor R 31  and a second resistor R 32 . 
     The operational amplifier OP 5  has a grounded non-inverting input (“+”; first input), an inverting input (“−”; second input) and an output. The first resistor R 31  has a first terminal coupled to the voltage integrator  2112  for receiving the integration signal, and a second terminal coupled to the second input of the operational amplifier OP 5 . The second resistor R 32  is coupled between the second input and the output of the operational amplifier OP 5 . The output of the operational amplifier OP 5  outputs the amplified integration signal. 
     The level shifter  2114  is coupled to the power amplifier  2113  for receiving the amplified integration signal, and shifts a voltage level of the amplified integration signal according to a predetermined DC voltage, so as to generate the reference voltage. 
     In one embodiment, the level shifter  2114  may be a voltage adder which adds the predetermined DC voltage to the amplified integration voltage to generate the reference voltage, as shown in  FIG. 4 . In another embodiment, the level shifter  2114  is a voltage subtractor which subtracts the predetermined DC voltage from the amplified integration voltage to generate the reference voltage, as shown in  FIG. 5 . The predetermined DC voltage has a voltage level determined according to a relationship between variation of the forward voltage of the light emitting device  1  and the ambient temperature. 
     Referring to  FIG. 4 , the level shifter  2114 , which is a voltage adder, includes an operational amplifier OP 6 , and first, second and third resistors R 41 , R 42 , R 43 . 
     The operational amplifier OP 6  has a grounded non-inverting input (“+”; first input), an inverting input (“−”; second input) and an output. The first resistor R 41  has a first terminal coupled to the power amplifier  2113  for receiving the amplifier integration signal, and a second terminal coupled to the second input of the operational amplifier OP 6 . The second resistor R 42  has a first terminal disposed to receive the predetermined DC voltage, and a second terminal coupled to the second input of the operational amplifier OP 6 . The third resistor R 43  is coupled between the second input and the output of the operational amplifier OP 6 . The output of the operational amplifier OP 6  outputs the reference voltage. 
     Referring to  FIG. 5 , the level shifter  2114 , which is a voltage subtractor, includes an operational amplifier OP 7 , and first, second, third and fourth resistors R 41 ′, R 42 ′, R 43 ′ and R 44 ′. 
     The operational amplifier OP 7  has a non-inverting input (“+”; first input), an inverting input (“−”; second input) and an output. The first resistor R 41 ′ has a first terminal coupled to the power amplifier  2113  for receiving the amplifier integration signal, and a second terminal coupled to the second input of the operational amplifier OP 7 . The second resistor R 42 ′ has a first terminal disposed to receive the predetermined DC voltage, and a second terminal coupled to the first input of the operational amplifier OP 7 . The third resistor R 43 ′ is coupled between the ground node and the first input of the operational amplifier OP 7 . The fourth resistor R 44 ′ is coupled between the second input and the output of the operation amplifier OP 7 . The output of the operational amplifier OP 7  outputs the reference voltage. 
     Referring to  FIGS. 3 and 6 , the control signal generator  212  is coupled to the reference voltage unit  211  for receiving the reference voltage, and generates, according to the reference voltage, a control signal having a parameter associated with the reference voltage. In this embodiment, the control signal is a pulse signal, and the parameter of the control signal is a duty cycle of the pulse signal, which is associated with the magnitude of the reference voltage. 
     The control signal generator  212  includes a sawtooth wave circuit  2121  and a comparator circuit  2122 . 
     The sawtooth wave circuit  2121  is adapted for generating a sawtooth pulse signal. The comparator circuit  2122  is coupled to the sawtooth wave circuit  2121  for receiving the sawtooth pulse signal, and is coupled to the reference voltage unit  211  for receiving the reference voltage. The comparator circuit  2122  generates the control signal according to comparison of the reference voltage and the sawtooth pulse signal, such that the duty cycle of the control signal has an inverse relation to a magnitude of the reference voltage. 
     The current controller  22  is coupled to the light emitting device  1 , and is coupled to the control signal generator  2  for receiving the control signal that is a pulse signal. The current controller  22  permits flow of a driving current through the light emitting device  1 . The driving current is thus a pulse current that has an average magnitude proportional to the duty cycle of the control signal. 
     In this embodiment, when rise of the ambient temperature results in drop of the forward voltage of the light emitting device  1 , the magnitude of the reference voltage outputted by the optical power control device  2  will become smaller, causing an increase in the duty cycle of the control signal. The increased duty cycle of the control signal makes the average magnitude of the driving current larger, thereby promoting the optical power and brightness of the light emitting device  1 . The brightness of the light emitting device  1  is thus substantially non-varying with the ambient temperature via detection and feedback features of the control signal module  21  of the preferred embodiment. It should be noted that, with rise of the ambient temperature, although reduction of the forward voltage is associated with reduction of the optical power of the light emitting device  1 , there are differences existing therebetween. If the forward voltage is directly used as the reference voltage (i.e., amplification is 1, and the predetermined DC voltage is 0) for adjusting the duty cycle of the control signal, although the electric power (product of the forward voltage and the driving current) of the light emitting device  1  may be non-varying with the ambient temperature, the optical power (measured by instrument) thereof may still vary with the ambient temperature due to lack of consideration of the electro-optic conversion characteristic. In detail, the electrical power P=I×V, and the optical power L=P×N (t), where I is the driving current flowing through the light emitting device  1 , V is the forward voltage of the light emitting device  1 , and N(t) is the electro-optic conversion efficiency of the light emitting device  1 . It is known from the equations that even if the electrical power P is non-varying with the ambient temperature, the optical power L may vary with the ambient temperature since the electro-optic conversion efficiency varies with the ambient temperature t. Generally, N(t) is reduced with rise of the ambient temperature. Accordingly, sensitivity of the duty cycle versus ambient temperature may be set via adjustment of the amplification of the power amplifier  2113 , so as to compensate the temperature effect resulting from the electro-optic conversion efficiency N (t), and to make the optical power L non-varying with the ambient temperature. 
     In practice, if the light emitting device  1  has greater reduction of the electro-optic conversion efficiency with rise of the ambient temperature, the amplification of the power amplifier  2113  is accordingly designed to be greater, so as to make the optical power L non-varying with the ambient temperature. 
     Furthermore, if the dynamic range of the duty cycle is required to be larger versus the same temperature range (i.e., more sensitive), the amplification of the power amplifier  2113  may be designed to be larger. In the following example, it is assumed that temperature variation from 40° C. to 80° C. results in 0.1V variation of the reference voltage when the amplification of the power amplifier  2113  is 1, and the duty cycle of the control signal correspondingly rises by 2%, which is insufficient to effectively promote the optical power of the light emitting device  1 . However, when the amplification is designed to be 5, the same temperature variation will result in 0.5V variation (five times 0.1V) of the reference voltage, resulting in 10% (=2%×5) increment of the duty cycle of the control signal, which is five times the original increment, so as to effectively promote the optical power of the light emitting device  1 . 
     Since human eyes have different sensitivities to lights with different wave lengths (colors), the preferred embodiment uses the lever shifter  2114  to shift the voltage level of the amplified integration signal according to the color of the light emitting device  1 , so as to optimize the optical power of the light emitting device  1 . Furthermore, the level shifter  2114  may be used to inversely offset a dynamic range of the duty cycle of the control signal. For example, when the light emitting device  1  is required to have a greater brightness, the level shifter  2114  maybe used to add a relatively smaller voltage to, or to subtract a voltage from the amplifier integration signal, to thereby result in a relatively higher dynamic range of the duty cycle of the control signal, such as 50%˜80%, having a dynamic range of 30%. When the light emitting device  1  is required to have a smaller brightness, the level shifter  2114  may be used to add a relatively greater voltage to the amplifier integration signal, to thereby result in a relatively lower dynamic range of the duty cycle of the control signal, such as 40%˜70%, having a dynamic range of 30%. 
     Referring to  FIG. 7 , an application of the light emitting system is a light-mixing control system with high color rendering index, which includes three light emitting systems of the preferred embodiment that share one sawtooth wave circuit  2121  and that respectively include the light emitting devices  1  with different colors, such as red, blue and green. 
     The amplification of the power amplifier  2113  of each reference voltage unit  211  is determined as mentioned above, so that the optical power of the corresponding light emitting device  1  is non-varying with the ambient temperature, and the predetermined DC voltage of the level shifter  2114  of each reference voltage unit  211  is determined upon a visual function of the human eyes for the corresponding color, so as to maintain a desired color temperature and color rendering index. 
     To sum up, the aforementioned application using the optical power control device  2  according to this invention has the following advantages: 
     1. Temperature increment of the LED is relatively small. The LED is driven by the driving current, which is a pulse current, such that the light emitting device  1  emits light in an active duration and dissipates heat in an inactive duration, resulting in a relatively small temperature increment. For example, when the duty cycle is 0.1, the LED emits light for one-tenth of a cycle time, and dissipates heat for the other nine-tenths of the cycle time, so as to alleviate the first drawback mentioned hereinbefore. 
     2. The optical power of the light emitting device  1  is maintained to be stable. The optical power control device  2  detects and feeds back the forward voltage variation resulting from the ambient temperature variation, so that the duty cycle of the control signal is adjusted for enabling the light emitting device  1  to operate with stable optical power, and the second drawback mentioned hereinbefore is thus alleviated. 
     3. The color temperature and the color rendering index of the resulting mixed light is relatively stable. Since the corresponding duty cycles of the light emitting devices  1  with different colors are controlled by a respective one of the optical power control devices  2  with consideration of the individual characteristics of the light emitting devices  1 , the optical power of each of the light emitting devices  1  is maintained independently, resulting in the relatively stable color temperature and color rendering index. 
     While the present invention has been described in connection with what is considered the most practical and preferred embodiment, it is understood that this invention is not limited to the disclosed embodiment but is intended to cover various arrangements included within the spirit and scope of the broadest interpretation so as to encompass all such modifications and equivalent arrangements.