Patent Publication Number: US-9414452-B1

Title: Light-emitting diode lighting device with synchronized PWM dimming control

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of U.S. provisional application No. 62/104,087 filed on Jan. 16, 2015. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention is related to an LED lighting device, and more particularly, to an LED lighting device capable of providing synchronized PWM dimming control. 
     2. Description of the Prior Art 
     An LED lighting device directly driven by a rectified alternative-current (AC) voltage usually adopts a plurality of LEDs coupled in series in order to provide required luminance. LED lighting has been widely utilized in different application scenarios. To save energy or provide different brightness, dimming technologies have also been developed so that the lighting can be dimmed in different situations. Traditionally, there are different categories of dimming methods, including pulsed width modulation (PWM) dimming and analog dimming. Analog dimming changes LED light output by directly adjusting the DC current in the LED string, while PWM dimming achieves the same effect by varying the duty cycle of a constant current in the LED string to effectively change the average current in the LED string. A user may be provided with a means to control the LED dimming. 
     In the prior art, PWM dimming may be achieved by periodically switching on and off the LED current according to a PWM signal. The duty cycle of the LED current may thus be adjusted, thereby changing the overall luminance of an LED lighting device. However, if the frequency of the PWM signal is not synchronized with the frequency of the rectified AC voltage, the waveform of the LED current may vary during different cycles of the rectified AC voltage, thereby causing flicker or shimmer. LED flicker or shimmer, whether perceptible or not, has been a concern of the lighting community because of its potential human impacts, which range from distraction, mild annoyance to neurological problems. Therefore, there is a need for an LED lighting device capable of providing synchronized PWM dimming control. 
     SUMMARY OF THE INVENTION 
     The present invention provides an LED lighting device which includes a luminescent circuit, an adjustable current source, a detecting circuit, and a dimming control circuit. The luminescent circuit is driven by a rectified AC voltage for providing light. The adjustable current source is configured to vary a duty cycle of the LED current according to a PWM signal. The dimming control circuit is configured to generate the PWM signal and synchronize a frequency of the PWM signal with the frequency of the rectified AC voltage at the detected rising edge or the falling edge of the rectified AC voltage. 
     These and other objectives of the present invention will no doubt become obvious to those of ordinary skill in the art after reading the following detailed description of the preferred embodiment that is illustrated in the various figures and drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagram illustrating an LED lighting device according to an embodiment of the present invention. 
         FIG. 2  is a diagram illustrating an LED lighting device according to another embodiment of the present invention. 
         FIG. 3  is a diagram illustrating an LED lighting device according to another embodiment of the present invention. 
         FIG. 4  is a diagram illustrating the operation of a dimming control circuit according to the present invention. 
         FIG. 5  is a diagram illustrating the operation of a dimming control circuit according to the present invention. 
         FIG. 6  is a diagram illustrating the operation of a dimming control circuit according to the present invention. 
     
    
    
     DETAILED DESCRIPTION 
       FIGS. 1-3  are diagrams illustrating LED lighting devices  101 ˜ 103  according to embodiments of the present invention. Each of the LED lighting devices  101 ˜ 103  includes a power supply circuit  110 , a luminescent circuit  120 , an adjustable current source  130 , a regulating control circuit  140 , a dimming control circuit  150 , and a detecting circuit  160 . Each of the LED lighting devices  102 ˜ 103  further includes a driving circuit  170 . The power supply circuit  110  is configured to receive an AC voltage VS having positive and negative periods and convert the output of the AC voltage VS in the negative period using a bridge rectifier  112 , thereby providing a rectified AC voltage V AC , whose value varies periodically with time, for operating the luminescent circuit  120 . In another embodiment, the power supply circuit  110  may receive any AC voltage VS, perform voltage conversion using an AC-AC converter, and rectify the converted AC voltage VS using the bridge rectifier  112 , thereby providing the rectified AC voltage V AC  whose value varies periodically with time. However, the configuration of the power supply circuit  110  does not limit the scope of the present invention. 
     In the present invention, the luminescent circuit  120  may include multiple luminescent devices A 0 ˜A N  (N is a positive integer), each of which may adopt a single LED or multiple LEDs coupled in series.  FIGS. 1 ˜ 3  depict the embodiment of N=2 using multiple LEDs which may consist of single-junction LEDs, multi-junction high-voltage (HV) LEDs, or any combination of various types of LEDs. However, the types and configurations of the luminescent circuit  120  do not limit the scope of the present invention. 
     In the present invention, the driving circuit  170  is configured to regulate the LED current I LED  flowing through the luminescent circuit  120  in multiple stages. In the LED lighting device  102  depicted in  FIG. 2 , the driving circuit  170  includes M current controller CC 1 ˜CC M  each coupled in parallel to M luminescent devices among the luminescent devices A 1 ˜A N  of the luminescent circuit  120 , respectively. In the LED lighting device  103  depicted in  FIG. 3 , the driving circuit  170  includes M current controller CC 1 ˜CC M  each having a first end coupled to a first end of a corresponding luminescent device among the luminescent devices A 1 ˜A N  of luminescent circuit  120 , a second end coupled to a second end of the corresponding luminescent device, and a third end coupled to a first end of another luminescent device among the luminescent devices A 1 ˜A N  of luminescent circuit  120 . In the present invention, M is a positive integer which does not exceed N.  FIGS. 2 and 3  depict the embodiment when M=N=2. However, the configuration of the driving circuit  170  does not limit the scope of the present invention. 
     Since the LED current I LED  flowing through the LED lighting devices  101 ˜ 103  is associated with the rectified AC voltage V AC  whose value varies periodically with time, the rising edge and the falling edge of the LED current I LED  are related to the frequency of the rectified AC voltage V AC , and the value of the LED current I LED  is related to the level of the rectified AC voltage V AC . In an embodiment of the present invention, the detecting circuit  160  may include a resistor R coupled in series to the adjustable current source  130  for providing a feedback voltage V FB  associated with the rising edge or the falling edge of the LED current I LED , as well as associated with the level of the rectified AC voltage V AC . However, the configuration of the detecting circuit  160  does not limit the scope of the present invention. 
     In the present invention, the adjustable current source  130  is coupled in series to the luminescent circuit  120  and operates based on a regulating signal S REG  and a PWM signal S PWM . In an embodiment, the adjustable current source  130  may be implemented using an N-type metal-oxide-semiconductor (NMOS) transistor and/or one or multiple devices providing similar function. However, the configuration of the adjustable current source  130  does not limit the scope of the present invention. The adjustable current source  130  may be turned on or turned off by the PWM signal S PWM , thereby varying the duty cycle of the LED current I LED . The value of the adjustable current source  130  may be adjusted based on the regulating signal S REG . 
     In the present invention, the regulating control circuit  140  includes a comparator  44 . The comparator  44  is configured to compare the levels of the voltage V FB  with a reference voltage V REF1 , thereby outputting the regulating signal S REG  accordingly. If the regulating signal S REG  indicates that V FB &lt;V REF1 , the adjustable current source  130  may increase its value; if the regulating signal S REG  indicates that V FB &gt;V REF1 , the adjustable current source  130  may decrease its value. Therefore, if the rectified AC voltage V AC  somehow fluctuates, the LED current I LED  may be kept at a constant value. 
     In the present invention, the level of the PWM signal S PWM  is associated with the amount of dimming selected by a user. The user may adjust the brightness of the LED lighting devices  101 ˜ 103  using various types of dimmer switches including, but not limited to, rotary, paddle, slider and wireless switches. However, the means of providing dimming control to the user does not limit the scope of the present invention. 
     In the present invention, the dimming control circuit  150  includes a comparator  54  and a PWM signal generator  56 . The PWM signal generator  56  is configured to provide the PWM signal Sp PWM  according to a dimming signal S DIM  and a synchronization signal SYNC. 
       FIGS. 4 ˜ 6  are diagrams illustrating the operation of the dimming control circuit  150  according to the present invention.  FIG. 4  depicts the relationship between the PWM signal S PWM  and the dimming signal S DIM .  FIGS. 5 and 6  depict the relationship between the PWM signal S PWM  and the synchronization signal SYNC. In  FIG. 5 , the operation of the LED lighting device  102  with multiple driving stages (N=2) is depicted for illustrative purpose. In  FIG. 6 , the operation of the LED lighting device  103  with multiple driving stages (N=2) is depicted for illustrative purpose. The turn-on periods of the LED current I LED  are represented by striped regions. 
     Generally, the dimming signal S DIM  is kept at a nominal level V NOM  when the LED lighting devices  101 ˜ 103  are requested to provide full brightness, and the level of the dimming signal S DIM  is lowered when the user instructs the LED lighting devices  101 ˜ 103  to lower its brightness. The PWM signal generator  56  may compare the levels of the dimming signal S DIM  with an oscillation signal S OSC , thereby outputting the PWM signal S PWM  accordingly. In the embodiment depicted in  FIG. 4 , the PWM signal S PWM  is set to a high level V H  when S DIM &gt;S OSC , and the PWM signal S PWM  is set to a low level V L  when S DIM &lt;S OSC . More specifically, the PWM signal S PWM  is maintained at a 100% duty cycle when the dimming signal S DIM  is at the nominal level V NOM , thereby allowing full LED current I LED  to provide maximum brightness. Similarly, the PWM signal S PWM  is maintained at a 40% duty cycle when the dimming signal S DIM  drops to V 40 , thereby allowing the LED current I LED  to be turned on only during 40% of a period for lowering the brightness. 
     During the rising cycle of the rectified AC voltage V AC  when V AC  becomes higher than the barrier voltage (or cut-in voltage) of the luminescent circuit  120  and the adjustable current source  130 , the LED current I LED  starts to flow and the feedback voltage V FB  established across the detecting circuit  160  ramps up. The comparator  54  is configured to compare the levels of the voltage V FB  with a reference voltage V REF2 , thereby outputting the synchronization signal SYNC accordingly. Upon receiving the synchronization signal SYNC, the PWM signal generator  56  is configured to restart or reset the PWM signal S PWM , thereby synchronizing the frequency of the PWM signal S PWM  with the frequency of the rectified voltage V AC . 
     In the embodiment depicted in  FIG. 5 , the comparator  54  is configured to output the synchronization signal SYNC when the feedback voltage V FB  associated with the frequency of the rectified voltage V AC  exceeds the reference voltage V REF2 . Upon receiving the synchronization signal SYNC, the PWM signal generator  56  is configured to restart the PWM signal S PWM . Therefore, the turn-on periods of the current I LED  during each driving cycle may be synchronized at the rising edge, thereby improving the flicker phenomenon. 
     In the embodiment depicted in  FIG. 6 , the comparator  54  is configured to output the synchronization signal SYNC when the feedback voltage V FB  associated with the rectified voltage V AC  drops below the reference voltage V REF2 . Upon receiving the synchronization signal SYNC, the PWM signal generator  56  is configured to reset the PWM signal S PWM . Therefore, the turn-on periods of the current I LED  during each driving cycle may be synchronized at the falling edge, thereby improving the flicker phenomenon. 
     With the above-mentioned dimming control circuit, the present LED lighting device can synchronize the frequency of the PWM signal with the frequency of the rectified voltage V AC  so that the turn-on periods of the LED current I LED  during each driving cycle may be synchronized. Therefore, the present invention can provide an LED lighting device capable of providing synchronized PWM dimming control without causing flicker or shimmer. 
     Those skilled in the art will readily observe that numerous modifications and alterations of the device and method may be made while retaining the teachings of the invention. Accordingly, the above disclosure should be construed as limited only by the metes and bounds of the appended claims.