Abstract:
A dimming method for light emitting diodes (LEDs), which is applied in a pulse width modulation boost circuit system, includes the following steps: (a) equally dividing a cycle into a plurality of intervals; (b) providing a plurality of control signals having the cycle and a pulse duration, wherein the pulse duration of each of the control signals is sequentially generated in the cycle; and (c) using the plurality of control signals to control a plurality of corresponding switches for dimming the LEDs connected to the switches.

Description:
CROSS-REFERENCE TO RELATED U.S. APPLICATIONS 
       [0001]    Not applicable. 
       STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
       [0002]    Not applicable. 
       NAMES OF PARTIES TO A JOINT RESEARCH AGREEMENT 
       [0003]    Not applicable. 
       REFERENCE TO AN APPENDIX SUBMITTED ON COMPACT DISC 
       [0004]    Not applicable. 
       BACKGROUND OF THE INVENTION 
       [0005]    1. Field of the Invention 
         [0006]    The present invention relates to a dimming method for light emitting diodes (LEDs), and more particularly, to a dimming method for the LEDs applied to a pulse width modulation boost circuit system. 
         [0007]    2. Description of Related Art Including Information Disclosed Under 37 CFR 1.97 and 37 CFR 1.98 
         [0008]      FIG. 1  is a schematic view of a circuit for a conventional LED dimming system  10 . The LED dimming system  10  includes a boost circuit  11 , a pulse width modulation controller  12 , a comparator  13 , a feedback signal selector  14 , a DPWM signal generator  15 , a LED group  17  including six LEDs of LED 1 -LED 6 , a switch group  18  including six switches of T 1 -T 6 , and a current source group  19  including six current sources of I S1 -I S6 . The boost circuit  11  includes a voltage source V IN , an input capacitor C 1 , an output capacitor C 2 , a diode D, a boost inductor L, a resistor R, and a switch. The boost circuit  11  receives a pulse width modulation signal SG (with a changeable duty cycle) from the pulse width modulation controller  12  to control the output voltage V OUT . According to a current feedback signal I SEN  and an output signal of the comparator  13 , the pulse width modulation controller  12  timely adjusts the duty cycle of the pulse width modulation signal S G . A control signal DS G  generated by the DPWM signal generator  15  is transferred to each switch T 1 -T 6  in the switch group  18 , so as to simultaneously control the ON or OFF state of the switches T 1 -T 6 . Generally, the forward voltage and the operating current of the white light LED are respectively 3.6 V and 20 mA. 
         [0009]    In order to avoid violating the maximum current specification of the white light LED and thereby sacrificing the reliability (thereby speeding up the aging of the white light LED) and to obtain the predictable and mutually matching luminance and chrominance, the white light LED is generally driven by a fixed current source. Therefore, the current sources I S1 -I S6  in the current source group  19  are respectively connected to the LEDs of LED 1 -LED 6  in the LED group  17  through the switches T 1 -T 6 , so as to control the current flowing through the LEDs of LED 1 -LED 6 . The feedback signal selector  14  receives the feedback signals FB 1 -FB 6  relevant to the LEDs of LED 1 -LED 6 , selects one of the feedback signals FB 1 -FB 6  (for example, the one with the minimum voltage) as the output signal FB 0 , and transfers it to the comparator  13  to be compared with a reference voltage V REF , in order to control the duty cycle of the pulse width modulation signal S G . The brightness of the LEDs of LED 1 -LED 6  is controlled by the control signal DS G . 
         [0010]      FIG. 2  is a timing chart of the control signal DS G , the output current I OUT  and the output voltage V OUT  of the boost circuit  11 . The control signal DS G  has a cycle T and a pulse duration P (i.e., time duration when it is at the high logic level). Such pulse duration P is used to simultaneously close the switches T 1 -T 6  to make an operating current (e.g., 20 mA) flow through each LED of LED 1 -LED 6 , and thereafter, the LEDs emit lights. At this time, the output current I OUT  of the boost circuit  11  is 120 mA (i.e., 20 mA×6). The brightness of the LEDs of LED 1 -LED 6  can be modified by adjusting the duty cycle of the control signal DS G  (i.e., the length of the pulse duration P is adjusted). 
         [0011]    However, the dimming method used in the conventional LED dimming system  10  has the following disadvantages: (1) the output capacitor C 2  of the boost circuit  11  is charged and discharged, so as to generate an excessively large ripple voltage and thereby reducing the service efficiency of the power source; and (2) the dimming method of the boost circuit  11  switches between the totally opened state (with the current value of 120 mA) or the totally closed state (with the current value of zero) of the output current I OUT , and adjusts the ON or OFF time of all the switches T 1 -T 6  according to the duty cycle of the control signal DS G  (the ratio of P/T), but not operating in a continuous conduction mode (CCM). The operating method also reduces the service efficiency of the power source. 
       BRIEF SUMMARY OF THE INVENTION 
       [0012]    An aspect of the present invention is to provide a dimming method for the LEDs, applied in a pulse width modulation boost circuit, wherein a plurality of sequentially generated control signals is used to independently control the corresponding LEDs, so as to reduce the ripple voltage of the output voltage for the pulse width modulation boost circuit, and thereby enhancing the service efficiency of the power source of the pulse width modulation boost circuit. 
         [0013]    Another aspect of the present invention is to provide a dimming method for the LEDs, applied in a pulse width modulation boost circuit, wherein a plurality of sequentially generated control signals are used in such pulse width modulation boost circuit to independently control the corresponding LEDs, such that the pulse width modulation boost circuit is operated in a CCM, and thereby enhancing the service efficiency of the power source of the pulse width modulation boost circuit. 
         [0014]    The present invention discloses a dimming method for the LEDs, applied in a pulse width modulation boost circuit. The dimming method comprises the following steps of: (a) equally dividing a cycle into a plurality of intervals; (b) providing a plurality of control signals having the cycle and a pulse duration, wherein the pulse duration of each of the control signals is sequentially generated in the cycle; and (c) using the plurality of control signals to control a plurality of corresponding switches for dimming the LEDs connected to the switches. 
     
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         [0015]    The invention will be described according to the appended drawings. 
           [0016]      FIG. 1  is a schematic view of a circuit of a conventional LED dimming system. 
           [0017]      FIG. 2  is a timing chart of the signal relevant to  FIG. 1 . 
           [0018]      FIG. 3  is a schematic view of a circuit of a LED dimming system according to an embodiment of the present invention. 
           [0019]      FIG. 4  is a timing chart of a control signal relevant to  FIG. 3 . 
           [0020]      FIGS. 5(   a )- 5 ( c ) are timing charts of the signal relevant to  FIG. 3  under duty cycles of different control signals. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0021]      FIG. 3  is a schematic view of a circuit of the LED dimming system  20  in the dimming method for the LED according to an embodiment of the present invention. The dimming system  20  includes a boost circuit  21 , a pulse width modulation controller  22 , a comparator  23 , a feedback signal selector  24 , a DPWM signal generator  25 , a LED group  27  including six LEDs (not shown), a switch group  28  including six switches T 1 -T 6  (MOS transistor is used as the switch in this embodiment), and a current source group  29  including six current sources (not shown). 
         [0022]    The operating principle of the dimming system  20  is described herein. The boost circuit  21  receives a pulse width modulation signal S′ G  (with a changeable duty cycle) from the pulse width modulation controller  22  to control the output voltage V′ OUT . According to the current feedback signal I′ SEN  and the output signal of the comparator  23 , the pulse width modulation controller  22  timely adjusts the duty cycle of the pulse width modulation signal S′ G . The six control signals DP 1 -DP 6  generated by the DPWM signal generator  25  are respectively transferred to the gates g 1 -g 6  of each switch T 1 -T 6  in the switch group  28 , so as to sequentially control the ON or OFF state of the switches T 1 -T 6 . The six current sources in the current source group  29  are respectively connected to the six LEDs in the LED group  27  through the switches T 1 -T 6  (the connecting method is same as that of  FIG. 1 ), in order to control the current flowing through the six LEDs. The feedback signal selector  24  receives the feedback signals FB 1 ′-FB 6 ′ relevant to the six LEDs, selects one of the feedback signals FB 1 ′-FB 6 ′ (for example, the one with the minimum voltage) as the output signal FB 0 ′, and then transfers it to the comparator  23  to be compared with a reference voltage V′ REF , and thereby controls the duty cycle of the pulse width modulation signal S′ G . The brightness of the six LEDs is controlled by the control signals DP 1 -DP 6 . 
         [0023]      FIG. 4  is a timing chart of the control signals DP 1 -DP 6  of the dimming method for the LED according to an embodiment of the present invention. First, a clock signal having a cycle T′ is provided. Next, the cycle T′ is equally divided into six intervals Ph 1 -Ph 6 . Then, the DPWM signal generator  25  provides six control signals DP 1 -DP 6 , each having the same cycle T′ and having a pulse duration P 1 -P 6  respectively (in this embodiment, the six pulse durations P 1 -P 6  are the same). The six pulse durations P 1 -P 6  are sequentially generated in the cycle T′ and respectively transferred to the gates g 1 -g 6  of the six switches T 1 -T 6 . Therefore, the six pulse durations P 1 -P 6  are corresponding to the operating currents I 1 -I 6  flowing through the six LEDs. In addition, the six control signals DP 1 -DP 6  are generated by the clock signal (with the cycle of T′) and a time-delay circuit. It should be noted in this embodiment that, the six control signals DP 1 -DP 6  have the same duty cycle, i.e., P 1 /T′, and it is referred to as the control signal duty cycle below. 
         [0024]      FIGS. 5(   a )- 5 ( c ) are timing charts of the operating current I 1 -I 6 , and the output current I′ OUT  and the output voltage V′ OUT  of the boost circuit  21  under different control signal duty cycles. Referring to  FIG. 5(   a ), the control signal duty cycle is smaller than ⅙ (about 1/12). Each operating current I 1 -I 6  is sequentially generated in the cycle T′, and is equal to the individual operating current (i.e., 20 mA) in magnitude. Although the boost circuit  21  of  FIG. 5(   a ) is operated in a discontinuous conduction mode (DCM), the ripple voltage of the output voltage V′ OUT  can be reduced by increasing the frequency of the output current I′ OUT , thereby reducing the power loss due to charging and discharging the output capacitor C 2  (in  FIG. 1) . Upon comparing  FIG. 5(   a ) with  FIG. 2 , it is known that, the ripple voltage of the output voltage V′ OUT  in  FIG. 5(   a ) is significantly smaller than that of the V′ OUT  in  FIG. 2 , and the frequency of the former is six times of that of the latter. Under the same duty cycle (e.g., 10%), the average output current shown in  FIG. 5(   a ) (6*10%*20 mA) is equal to that shown in  FIG. 2  (10%*120 mA). However, as mentioned above, since the output voltage of  FIG. 5(   a ) has smaller ripple voltage, the service efficiency of the power supply of the boost circuit  21  is enhanced. 
         [0025]    Referring to  FIG. 5(   b ), the control signal duty cycle is ⅙. Each operating current I 1 -I 6  is sequentially generated in the cycle T′, and is equal to the individual operating current (i.e., 20 mA) in magnitude. The boost circuit  21  of  FIG. 5(   b ) is operated in the CCM, and the output current I′ OUT  is continuously output, with the magnitude of 20 mA. 
         [0026]      FIG. 5(   c ) shows the situation when the control signal duty cycle is larger than ⅙ (about ⅓). Each operating current I 1 -I 6  is sequentially generated in the cycle T′, and is equal to the operating current (i.e., 20 mA) in magnitude. The boost circuit  21  of  FIG. 5(   c ) is operated in the CCM, and the output current I′ OUT  is continuously output, with the magnitude of 40 mA. In other words, when the control signal duty cycle is larger than ⅙, the output current I′ OUT  is larger than the operating current of the LED (i.e., 20 mA). Under the situation of  FIGS. 5(   b ) and  5 ( c ), the output voltage V′ OUT  obviously has no ripple voltages and is substantially equal to the forward voltage of the LED (e.g., 3.6 V). 
         [0027]    In the above embodiment, six LEDs are taken as an example for illustration, but the number of the LEDs is not limited in the dimming method for the LED of the present invention. Compared with the conventional dimming method of  FIG. 2 , the cycle of a clock signal in the present invention is equally divided into a plurality of intervals, and a plurality of control signals corresponding to the plurality of intervals is provided, such that the pulse duration of each control signal is generated in at least one of the plurality of intervals, and an output current is generated in each interval, thereby reducing the ripple voltage of the output voltage or making the boost circuit be operated in the CCM. Therefore, the dimming method for the LED of the present invention surely improves the service efficiency of the power supply for the pulse width modulation boost circuit. 
         [0028]    The above-described embodiments of the present invention are intended to be illustrative only. Numerous alternative embodiments may be devised by persons skilled in the art without departing from the scope of the following claims.