Abstract:
An LED illuminant driving circuit and an automatic brightness compensation method thereof are provided herein. The automatic brightness compensation method includes: providing a target value; detecting an operation period of a pulse of an output of the LED illuminant driving circuit, the pulse is adapted to an LED illuminant for making the light emitting; deciding a peak value according to the target value and the operation period; and setting a peak level of the pulse according to the peak value. The LED illuminant driving circuit and the automatic brightness compensation method thereof provides a stable average current/voltage to the LED illuminant and avoids brightness variations of the light emitting.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims the priority benefit of Taiwan application serial no. 97149535, filed on Dec. 18, 2008. The entirety of the above-mentioned patent application is hereby incorporated by reference herein and made a part of specification. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an illuminant adjustment technique, and more particularly, to an LED illuminant driving circuit and an automatic brightness compensation method thereof. 
     2. Description of Related Art 
       FIG. 1  shows a block diagram of a conventional LED illuminant device. Referring to  FIG. 1 , an LED illuminant device  10  includes an electronic transformer  100 , a rectifier circuit  110 , an input capacitor Cin, a driving circuit  120 , and an LED illuminant  130 . First, an output of an electronic transformer  100  with a conventional alternating current (AC) to AC transformation is a high frequency AC voltage VAC 2 . For example, when an AC voltage VAC 1  with low frequency of 60 Hz is applied, the AC voltage VAC 1  is transformed by the electronic transformer  100  to an AC voltage VAC 2  with high frequency of 25 KHz to 100 KHz, wherein root-mean-squared voltage (Vrms) of the AC voltage VAC 1  and the AC voltage VAC 2  is 110V and 12V respectively. Then, the AC voltage VAC 2  output from the electronic transformer  100  is connected with the rectifier circuit  110  and the input capacitor Cin. The rectifier circuit  110  and the input capacitor Cin produces a direct current (DC) voltage VDC with ripple components on the AC voltage VAC 2 . Furthermore, a power input terminal VIN of the driving circuit  120  receives a DC voltage VDC to make the LED illuminant  130  emit light. 
     Referring to  FIG. 2 ,  FIG. 2  shows the DC voltage VDC with ripple components of  FIG. 1 . When the driving circuit  120  utilizes the DC voltage VDC with ripple components as an input power source, the following conditions may occur: 
     (1) The ripples of the DC voltage VDC causes a periodic operation of a starting time and a stopping time so as to impact on a working stability of the driving circuit  120 . 
     (2) The ripples of the DC voltage VDC influences the LED illuminant  130  at an output terminal of the driving circuit  120 , and the ripple which causes variations of average current or average voltage of the LED illuminant  130  also causes the variations of brightness of the LED illuminant  130   
     In the driving circuit  120  of the related art, due to the impact from the DC voltage VDC with ripples, the average current or the average voltage conducted through the LED illuminant  130  will not be controlled stably. It also means the brightness of the LED emitting light may be different so it causes inconvenience of applications. Therefore, there is a need for an LED illuminant driving circuit and an automatic brightness compensation method thereof accordingly. 
     SUMMARY OF THE INVENTION 
     Accordingly, the present invention provides an automatic brightness compensation method of an LED illuminant driving circuit. The method decides a peak value according to a target value and an operation period of a pulse output from the LED illuminant driving circuit, and then sets a peak level of the pulse according to the peak value so as to achieve stability of average current or average voltage for driving the LED illuminant. 
     An LED illuminant driving circuit is also provided in the present invention. The LED illuminant driving circuit includes a driver unit and an automatic brightness compensation circuit. The LED illuminant driving circuit utilizes the automatic brightness compensation circuit to detect output from the driver unit, decide a peak value according to a target value and an operation period of a pulse output from the driver unit, and then control the driver unit according to the peak value to set a peak level of the pulse so as to achieve stability of driving the LED illuminant. 
     The automatic brightness compensation method of an LED illuminant driving circuit is provided in the present invention. The automatic brightness compensation method includes: providing a target value; detecting an operation period of a pulse output from the LED illuminant driving circuit; deciding a peak value according to the target value and the operation period; and setting a peak level of the pulse according to the peak value. 
     In one embodiment of the present invention, the step of detecting the operation period of the pulse output from the LED illuminant driving circuit in the above-described automatic brightness compensation method of an LED illuminant driving circuit includes: counting a turn-on period and a cut-off period of the pulse; and calculating the operation period according to the turn-on period and the cut-off period. 
     In one embodiment of the present invention, the target value in the above-described automatic brightness compensation method of an LED illuminant driving circuit is a setting value of average current or average voltage for driving the LED illuminant. 
     In one embodiment of the present invention, the step of setting the peak level of the pulse according to the peak value in the above-described automatic brightness compensation method of an LED illuminant driving circuit includes: generating a control signal according to the peak value to set the peak level of the pulse. 
     The LED illuminant driving circuit is provided in the present invention. The LED illuminant driving circuit includes a driver unit and an automatic brightness compensation circuit. The driver unit outputs a pulse to drive an LED illuminant. The automatic brightness compensation circuit is coupled to the driver unit for detecting an operation period of the pulse, deciding a peak value according to a target value and the operation period, controlling the driver unit according to the peak value to set a peak level of the pulse according to the peak value. 
     In one embodiment of the present invention, the automatic brightness compensation circuit of the above-described LED illuminant driving circuit includes a turn-on counter, a cut-off counter, and an operation period calculation circuit. The turn-on counter counts a turn-on period. The cut-off counter counts a cut-off period. The operation period calculation circuit calculates the operation period according to the turn-on period and the cut-off period. 
     In one embodiment of the present invention, the automatic brightness compensation circuit of the above-described LED illuminant driving circuit includes an average load setting circuit and a feedback compensation setting circuit. The average load setting circuit sets the target value as a setting value of the average current or the average voltage of the LED illuminant. The feedback compensation setting circuit generates a control signal to set the peak value according to the target value and the operation period. 
     In one embodiment of the present invention, the LED illuminant of the above-described LED illuminant driving circuit is an MR16 lamp. 
     The present invention detects the operation period of the pulse output from the LED illuminant to acquire the control signal for feedback compensation of the driver unit so as to control the brightness of the LED illuminant. Therefore, an automatic compensation mechanism is produced to provide stable average current or average voltage to the LED illuminant so as to avoid variations of the brightness of the LED illuminant. 
     In order to make the features and advantages of the present invention comprehensible, preferred embodiments accompanied with figures are described in detail below. 
     It is to be understood that both the foregoing general description and the following detailed description are exemplary, and are intended to provide further explanation of the invention as claimed. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings are included to provide a further understanding of the invention, and are incorporated in and constitute a part of this specification. The drawings illustrate embodiments of the invention and, together with the description, serve to explain the principles of the invention. 
         FIG. 1  is a block diagram of a conventional LED illuminant device. 
         FIG. 2  shows a DC voltage VDC with voltage ripple components of  FIG. 1 . 
         FIG. 3  and  FIG. 4  shows waveform diagrams of a (load) current or voltage before compensation. 
         FIG. 5  shows a flowchart of an automatic brightness compensation in an LED illuminant driving circuit of one embodiment of the present invention. 
         FIG. 6  shows a flowchart of an automatic brightness compensation in an LED illuminant driving circuit of another embodiment of the present invention. 
         FIG. 7  shows a block diagram of an LED illuminant device of one embodiment of the present invention. 
         FIG. 8  shows a waveform of a (load) current or voltage after compensation according to one embodiment of the present invention. 
         FIG. 9  is a block diagram of an LED illuminant device of another embodiment of the present invention. 
         FIG. 10  shows a waveform of the pulse output from the driver unit  720  of  FIG. 9 . 
         FIG. 11  shows another embodiment of the automatic brightness compensation circuit. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
       FIG. 3  and  FIG. 4  show waveform diagrams of a (load) current or voltage before compensation. Referring to  FIG. 3  and  FIG. 4 , an input capacitor Cin impacts the DC voltage VDC such that the DC voltage VDC has ripples. Furthermore, the DC voltage VDC having ripple components varies an average current or average voltage on a load. As shown in  FIG. 3 , an operation period of the load current or voltage of a pulse is D 1 , the average load current of voltage is AVG 1 , a maximum peak value of the pulse is P 1 , and a waveform of the varied pulse is illustrated in  FIG. 4 . The operation period of the (load) current or voltage of the pulse as shown in  FIG. 4  is changed to D 2  or D 3 . The possible conditions may be one of the following: D 2 =D 1 −D 3  or D 2 =D 1 +D 3 . However, the maximum peak value at this time is not changed, and the values of average (load) current/voltage is changed to AVG 2 , therefore, it is possible to get a situation of AVG 2 &lt;AVG 1  or a situation of AVG 2 &gt;AVG 1 . As a result, the value of average current or voltage will be varied. 
       FIG. 5  shows a flowchart of an automatic brightness compensation in an LED illuminant driving circuit according to an embodiment of the present invention. Referring to  FIG. 5 , the automatic brightness compensation method may be applied to the LED illuminant driving circuit of an illuminant device in order to solve an unstableness of the illuminant device caused by voltage ripples and variations of illuminant brightness. The automatic brightness compensation method includes the following steps. First, in step S 510 , a target value is provided, and the target value may be utilized as a setting value of the average current or the average voltage to drive the LED illuminant. Next, in step S 520 , an operation period of a pulse output from the LED illuminant driving circuit is detected. Then, in step S 530 , a peak value according to the target value and the operation period is decided. Furthermore, in step S 540 , a peak level of the pulse is set according to the peak value. 
       FIG. 6  shows a flowchart of an automatic brightness compensation in an LED illuminant driving circuit according to another embodiment of  FIG. 5 . Referring to  FIG. 6 , when the above-described automatic brightness compensation method of  FIG. 5  is proceeded in step S 520 , the step may further includes the following steps: in step S 522 , a turn-on period and a cut-off period of the pulse output from the LED illuminant driving circuit are counted; next, in step S 524 , the operation period is calculated according to the turn-on period and the cut-off period. Then, when the above-described automatic brightness compensation method of  FIG. 5  proceeds in step S 530 , step S 532  may be processed to generate a control signal for setting the peak level of the pulse output from the LED illuminant driving circuit. 
       FIG. 7  shows a block diagram of an LED illuminant device according to one embodiment of the present invention. Referring to  FIG. 7 , the LED illuminant device  700  may include an electronic transformer  100 , a rectifier circuit  100 , an input capacitor Cin, an LED illuminant driving circuit  710 , and an LED illuminant  730 . In this embodiment, the LED illuminant driving circuit  710  includes a driver unit  720  and an automatic brightness compensation unit  740 . A power input terminal VIN of the driver unit  720  receives the DC voltage VDC having ripples components, in order to output a pulse to drive the LED illuminant  730 . The automatic brightness compensation circuit  740  is coupled to the driver unit  720  to detect an operation period of the pulse, and then decides a peak value according to a target value and the operation period, and controls the driver unit  720  according to the peak value such that the driver unit  720  then sets a peak level of the pulse according to the peak value. 
     In order to stabilize the average current or the average voltage of the LED illuminant  730 , the automatic brightness compensation circuit  740  is used to generate a feedback control signal Ref to the driver unit  720 . The driver unit  720  is able to stably maintain the average current or the average voltage of the LED illuminant  730  according to the feedback control signal Ref.  FIG. 8  shows a waveform diagram of a (load) current or voltage after compensation according to one embodiment of the present invention. Please refer to  FIG. 8  in accordance with  FIG. 4  and  FIG. 3 . The working principle of the automatic brightness compensation circuit  740  is as the following: before compensation, the output pulse of the driver unit  720  as shown in  FIG. 4  has the operation period of the pulse as D 2 ; after compensation, the operation period of the output pulse of the driver unit  720  is not changed but the peak value of the output pulse of the driver unit  720  is set as P 2  for a result of compensation. Therefore, a situation of P 2 &gt;P 1  or a situation of P 2 &lt;P 1  is obtained. And setting of P 2  may be embodied as follows: for example, if the target value of the average current is AVG 1 , then P 2  is set as the value of AVG 1  divided by D 2 . It means at last the average current or the average voltage values AVG 3  of the output pulse of the driver unit  720  is equal to the target value AVG 1 . This means that the average load current or the average load voltage values of the LED illuminant  730  is not changed, and hence, a functionality of the automatic brightness compensation of the LED illuminant may be obtained to make a stable and uniform emitting light brightness of the LED illuminant. 
       FIG. 9  shows a block diagram of an LED illuminant device of another embodiment of the present invention. Referring to  FIG. 9 , the power input terminal VIN of the driver unit  720  receives a DC voltage VDC as shown in  FIG. 7 , and the automatic brightness compensation circuit  900  is coupled to the driver unit  720  and the LED illuminant  730 . The automatic brightness compensation circuit  900  may include a turn-on counter  910 , a cut-off counter  920 , an operation period calculation circuit  930 , an average load setting circuit  940 , and a feedback compensation circuit  960 . 
       FIG. 10  shows a waveform of the pulse output from the driver unit  720 . Referring to  FIG. 10 , the pulse output from the driver unit  720  is similar to a square wave, and the turn-on period and the cut-off of each operation period is T 1  and T 2  respectively. Referring to  FIG. 9 , the main purpose of the turn-on counter  910  is to count the turn-on period T 1  of the pulse output from the driver unit  720 , and the cut-off counter  920  is used for counting the cut-off period T 2  of the pulse output from the driver unit  720 . Surely, the turn-on counter  910  can be used to count the turn-on period T 1  of current or voltage on the LED illuminant  730  instead of the pulse, and the cut-off counter  920  to count the cut-off period T 2  of current or voltage on the LED illuminant  730  instead of the pulse. These changes belong to the scope of the present invention. Next, the operation period calculation circuit  930  is utilized to generate current operation period D 2  according to counting results from the turn-on counter  910  and the cut-off counter  920  to generate. In this embodiment, D 2 =T 1 /(T 1 +T 2 ). The average load setting circuit  940  is connected to an output terminal of the operation period calculation circuit  930 , and together with a variable resistor  950  to form a setting unit for setting the target value AVG 1  of the average current or the average voltage output from the driver unit  720  or for setting the target value AVG 1  of the average current or the average voltage conducting through the LED illuminant  730 . The feedback compensation circuit  960  is connected to the output end of the average load setting circuit  940  for obtaining the target value AVG 1  and the operation period D 2  of the average current or the average voltage of the LED illuminant  730 . The first compensation is mentioned previously as illustrated in the  FIG. 3  and  FIG. 4 . Then, after processed by the feedback compensation circuit  960 , where the way of process does not change the operation period D 2  of the average current or the average voltage and sets the peak value of the output pulse to P 2 , or alternatively, the way of processing may also set the maximum peak value of the load current or the load voltage to P 2  as a result of compensation so as to achieve a situation of P 2 &gt;P 1  or a situation of P 2 &lt;P 1 . The above-described feedback compensation circuit  960  may operate as the followings. For example, the target value of the average current is AVG 1 , and then P 2  is equal to the value of AVG 1  divided by D 2 . The feedback compensation circuit  960  generates a feedback control signal Ref after processing. The control signal Ref is transferred to the driver unit  720 . The driver unit  720  compensates the average current or the average voltage of the LED illuminant  730  according to the feedback control signal Ref. The embodiment detects the variations of the operation period to compensate the average current or the average voltage so as to maintain them such that the automatic brightness compensation of the LED illuminant is achieved. 
       FIG. 11  shows another embodiment of the automatic brightness compensation circuit  900 . In this embodiment, the average load setting circuit  940  may be implemented with an Analog-to-Digital Converter (ADC), and the feedback compensation circuit  960  may be implemented with an Arithmetic Logic Unit (ALU), where the ALU receives the target value AVG 1  and the operation period D 2  and generates the feedback control signal Ref after computation of P 2  (equal to the value of AVG 1  divided by D 2 ). 
     It is noted that in the above-described embodiments, the illuminant device  700  may be an MR16 lamp, but surely may be an E26 lamp or an E27 lamp. 
     It is understood by people skilled in the field that embodiments of the present invention are not limited to the embodiments disclosed above, embodiments may be varied according to design requirements, so long as realizations, which detect the operation period of the pulse output from the LED illuminant driving circuit and use the operation period and the target value to set the peak level of the pulse, fall within domains of the present invention. 
     In summary, the embodiments of the present invention have at least the following advantages: 
     (1) having an automatic brightness compensation mechanism to provide stable average current/voltage to the LED illuminant (load) so as to avoid variations of illuminant brightness; 
     (2) may applies to the LED illuminant (load) requiring stable average current or average voltage to make brightness of emitting light uniform and in turn enhances convenience of applications, for example, the LED illuminant may be applied to an MR16 lamp, an E26 lamp or an E27 lamp. 
     Though the present invention has been disclosed above by the preferred embodiments, they are not intended to limit the present invention. Anybody skilled in the art can make some modifications and variations without departing from the spirit and scope of the present invention. Therefore, the protecting range of the present invention falls in the appended claims.