Patent Publication Number: US-8120273-B2

Title: Light control system with PWM duty cycle control using current signal feedback

Description:
FIELD OF THE INVENTION 
     The present invention relates to lighting. More particularly, the invention is directed to a lighting system and method for controlling a duty cycle of the lighting system. 
     BACKGROUND OF THE INVENTION 
     Creating a brake light and a tail light function for Light Emitting Diode (LED) automotive tail lamps that use the same LEDs for a tail mode and a brake mode requires that the LEDs be driven “ON” to a lesser degree for the tail mode than for the brake mode. The partially “ON” tail mode can be achieved by principles of Pulse Width Modulation (PWM), wherein the LEDs are “ON” for less than 10% of the time. The PWM timing can be implemented by pulsing the LED driver ON/OFF in an open loop fashion from a timing source. Unfortunately, when the LED driver is a switchmode power supply (SMPS), there can be variability in the time it takes to start-up and shut-down the power supply, especially if the SMPS has soft start or is commanded from its soft start control circuits. The variation in the start-up/shut-down time of the power supply impacts the duty cycle performance of a resultant PWM waveform. Specifically, SMPS circuits take time to start-up and shut-down. The start-up and shut-down time changes in response to external conditions such as ambient temperature, input voltage, and load. 
     A constant PWM duty cycle of an output of the power supply is desirable for combination tail mode and brake mode LEDs. Open loop PWM of the power supply circuits from a soft-start control input allows the duty cycle to drift due to varying start-up and shut-down delay times in the power circuits. A drifting duty cycle causes a drifting perception and measurement of the brightness of the tail mode LEDs. 
     It would be desirable to have a light control system and a method for controlling a duty cycle of the light control system wherein the duty cycle provides a substantially continuous and stable power “ON” (duty cycle) and thereby a light output having a stable and continuous perceived brightness. 
     SUMMARY OF THE INVENTION 
     Concordant and consistent with the present invention, a light control system and a method for controlling a duty cycle of the light control system, wherein the duty cycle provides a substantially continuous and stable power “ON” (duty cycle) and thereby a light output having a stable and continuous perceived brightness, has surprisingly been discovered. 
     In one embodiment, a light control system comprises: a light emitting device for producing a predetermined light output; a power circuit in electrical communication with the light emitting device for selectively transmitting an electrical current to the light emitting device for controlling the light output; a timing circuit in electrical communication with the power circuit, wherein the timing circuit generates a pre-determined timing sequence and regulates a duty cycle of the power circuit in response to the timing sequence; and a feedback circuit in communication with the light emitting devices and the timing circuit, wherein the feedback circuit is adapted to monitor an electrical characteristic of the light emitting devices and control the timing sequence of the timing circuit in response to the electrical characteristics of the light emitting devices. 
     In another embodiment, a light control system comprises: a light emitting device for producing a first light output and a second light output; a power circuit in electrical communication with the light emitting device for selectively transmitting an electrical current to the light emitting device for controlling the first light output and the second light output; a timing circuit in electrical communication with the power circuit, wherein the timing circuit generates a pre-determined timing sequence and regulates a duty cycle of the power circuit in response to the timing sequence; and a feedback circuit in communication with the light emitting devices and the timing circuit, wherein the feedback circuit is adapted to monitor an electrical characteristic of the light emitting devices and control the timing sequence of the timing circuit in response to the electrical characteristics of the light emitting devices. 
     The invention also provides methods for controlling a duty cycle of a light control system. 
     One method comprises the steps of: providing a light emitting device for generating a light output; generating a timing sequence; transmitting an electrical current to the light emitting device in response to the timing sequence, wherein the light emitting device generates the light output in response to the transmitted electrical current; monitoring the electrical current flowing through the light emitting device; generating a feedback signal in response to a pre-determined monitoring threshold based upon the electrical current flowing through the light emitting device; and modifying the timing sequence in response to the feedback signal. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above, as well as other advantages of the present invention, will become readily apparent to those skilled in the art from the following detailed description of the preferred embodiment when considered in the light of the accompanying drawings in which: 
         FIG. 1  is a schematic block diagram of a light control system according to an embodiment of the present invention; 
         FIG. 2  is a graphical plot of voltage v. time for various points in the light control system of  FIG. 1 ; and 
         FIG. 3  is a flow diagram of a method for controlling a duty cycle of the light control system of  FIG. 1 . 
     
    
    
     DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS OF THE INVENTION 
     The following detailed description and appended drawings describe and illustrate various embodiments of the invention. The description and drawings serve to enable one skilled in the art to make and use the invention, and are not intended to limit the scope of the invention in any manner. In respect of the methods disclosed, the steps presented are exemplary in nature, and thus, the order of the steps is not necessary or critical. 
       FIG. 1  illustrates a light control system  10  including a timing circuit  12 , a power circuit  14 , a plurality of light emitting devices  16 , a current sense component  18 , and a feedback circuit  20 . It is understood that the light control system  10  may include additional components and circuits. 
     The timing circuit  12  or integration circuit is in electrical communication with the power circuit  14  and is adapted to generate and transmit a power toggle signal  22  to the power circuit  14  for controlling the operation of the power circuit  14 . The timing circuit  12  may be any circuit adapted to generate and set a sequential timing, analog or digital, for associated electronic circuits. As a non-limiting example, the timing circuit  12  is a 555 timer circuit, known in the art. As a further example, the timing circuit  12  may be a programmable timer chip adapted to function as a single or a multi function unit. However, other circuits, arrangements, and devices may be used, as desired. In the embodiment shown, the timing circuit  12  includes an up counter component  24  and a down counter component  26 . It is understood that the timing circuit  12  may include any number of sequencing components and may generate any timing sequence including a bidirectional sequencing, for example. It is further understood that the timing circuit  12  may be adapted to operate as an astable, monostable, or bistable circuit. 
     The power circuit  14  is in communication with the light emitting devices  16  and is adapted to transmit and control an electrical power  28  transmitted to the light emitting devices  16 . As shown, the power circuit  14  includes a soft-start component  30  to limit inrush current or input surge current to a pre-determined value. As a non-limiting example, the power circuit  14  is an LED driver with a built-in soft-start implementation. As a further example, the power circuit  14  may be a switch mode power supply (SMPS) having a duty cycle for producing a pre-determined output current. However, other drivers, power supplies, and circuits may be used, as desired. 
     The light emitting devices  16  illustrated are light emitting diodes adapted to generate at least one of a pre-determined first light output  32  and a pre-determined second light output  34 . It is understood that other devices and elements may be used to generate the at least one of the light outputs  32 ,  34 . It is further understood that any number of light emitting devices  16  may be used. In one embodiment, the light emitting devices  16  are arranged in a pre-determined array to operate as a brake light/tail light combination for a vehicle. As such, the first light output  32  is suitable for use as a tail light function for a vehicle and the second light output  34  is suitable for use as a brake light function for the vehicle. In certain embodiments, the first light output  32  has a perceived brightness that is less than the second light output  34 . It is understood that any number of the light emitting devices  16  may be adapted to generate any number of light outputs and patterns, as desired. 
     The current sense component  18  is in electrical communication with the light emitting devices  16  and the feedback circuit  20 . As shown, the current sense component  18  includes a plurality of resistors R 1 , R 2 . It is understood that the current sense component  18  may include any number of resistors, as desired. It is further understood that the current sense component  18  may include additional elements and devices for cooperating with the feedback circuit  20  to monitor an electrical characteristic  24  of the light emitting devices  16  such as voltage and current, for example. 
     The feedback circuit  20  is in electrical communication with the current sense component  18  and the timing circuit  12 . As such, the feedback circuit  20  is adapted to monitor the electrical characteristics  24  of the light emitting devices  16  and current sense component  18 , and transmit a feedback signal  36  to the timing circuit  12  in response to the monitored electrical characteristics  24 . It is understood that the feedback circuit  20  may include any components or devices for measuring and monitoring the electrical characteristics  24  of the light emitting devices  16  and current sense component  18 , as desired. It is further understood that the feedback signal  36  may be generated and transmitted in response to a pre-determined current monitoring threshold such as a threshold voltage drop, for example. Other monitoring threshold variables may be used. 
     Referring to  FIGS. 2 and 3 , there is illustrated a method  100  for controlling a duty cycle of the light control system  10 , and thereby at least one of the light outputs  32 ,  34  of the light emitting devices  16 . Specifically,  FIG. 2  illustrates a graphical plot of voltage v. time for various points A, B, C, D in the light control system  10  relative to steps of the method  100 . 
     In step  102 , the timing circuit  12  or integration circuit begins an up count timing sequence. In step  104 , the up count timing sequence continues until an up count value exceeds a pre-determined up count threshold. It is understood that the up count threshold may have any value, as desired. Once the up count threshold value is exceeded, the timing circuit  12  transmits the power toggle signal  22  to the power circuit  14  wherein the power toggle signal  22  represents an active “HIGH” signal. In step  106 , the soft-start component  30  of the power circuit  14  is initiated to an active state in response to the power toggle signal  22 . After a pre-determined delay associated with the settings and characteristics of the soft-start component  30 , the power circuit  14  starts-up and transmits the electrical power  28  to the light emitting devices  16 , as illustrated in step  108 . In step  110 , an electrical current flows through the light emitting devices  16  and the current sense component  18 , wherein the resistors R 1 , R 2  of the current sense component  18  drop a voltage according to Ohm&#39;s law. In step  112 , the feedback circuit  20  detects the voltage drop and where the pre-determined monitoring threshold is exceeded, the feedback circuit  20  transmits the feedback signal  36 , representing an active “HIGH” signal, to the timing circuit  12 . In step  114 , the timing circuit  12  stops the up count timing sequence and initiates a down count timing sequence, in response to the feedback signal  20 . In step  116 , the down count timing sequence continues until a down count threshold value is exceeded. It is understood that the down count threshold may have any value, as desired. Once the down count threshold value is exceeded, the timing circuit  12  transmits the power toggle signal  22  to the power circuits  14 , wherein the power toggle signal  22  goes LOW and represents a “shut-down” command. In step  118 , the power circuit  14  is toggled to an inactive state in response to the “LOW” power toggle signal  22 . In step  120 , the power circuit  14  shuts down, and thereby the transmission of the electrical power  28  to the light emitting devices  16  is stopped. With substantially no electrical current flowing through the light emitting devices  16  and the current sense component  18 , the feedback circuit  20  detects the electrical characteristics  24  representing a low signal, as illustrated in step  120 . In step  122 , the feedback circuit  20  transmits the feedback signal  36 , representing an “LOW” signal, to the timing circuit  12  in response to the substantially no current in the light emitting devices  16  and the current sense component  18 . In step  124 , the timing circuit  12  ends the down count timing sequence in response to step  122  and begins the up count timing sequence, returning the method  100  to step  102  and continuing the cycle. 
     As the method  100  continues to cycle, the first light output  32  is generated in accordance with pulse width modulation (PWM) principles known to someone skilled in the art of light control. In one embodiment, the second light output  34  is generated by a full transmission of electrical current, such that the light emitting devices  16  are continuously “ON”, while the first light output  34  is generated in response to the constant duty cycle of the power circuit  14  such that the light emitting devices  16  are “ON” for less than 10% of the time. It is understood that the duty cycle may be varied to generate any “ON” timing for the first light output  32 . The duty cycle is held substantially constant at the expense of frequency shift and since the human eye perceives brightness based on duty cycle, the constant duty cycle is perceived as constant brightness. Therefore, the first light output  32  is perceived as having a constant brightness that is less than fully “ON”. 
     Additionally, the feedback circuit  20  monitors the electrical characteristics  24  of the light emitting devices  16  and controls the timing sequences of the timing circuit  12  to improve performance of the PWM waveform and to generate the substantially constant duty cycle. It is understood that the duty cycle is based on a slope of the up count timing sequence relative to a slope of the down count timing sequence. Since the slope of the up count timing sequence is independent of any delay in the power circuit  14  and the down count timing sequence begins once the monitoring threshold is detected in the light emitting devices  16 , and since the slope of the down count timing sequence is also independent of any delay in the power circuit  14  the light control system  10  effectively adjusts for changes in the time it takes the power circuit  14  to start-up. 
     Accordingly, the light control system  10  uses feedback or “handshaking” to improve performance of the PWM waveform to generate a constant duty cycle. The improved P M reduces drift in the duty cycle caused by temperature, input voltage, and load. As such, the light control system  10  and method  100  provide a combination brake mode and tail mode functionality for rear LED tail lamps using the same LEDs for tail and brake mode. 
     From the foregoing description, one ordinarily skilled in the art can easily ascertain the essential characteristics of this invention and, without departing from the spirit and scope thereof, make various changes and modifications to the invention to adapt it to various usages and conditions.