Patent Publication Number: US-2013249420-A1

Title: Led driving system and method

Description:
BACKGROUND 
     1. Technical Field 
     The disclosure relates to backlight driving systems, and particularly to a light emitting diode (LED) driving system and method of a display device. 
     2. Description of Related Art 
     Light emitting diodes (LEDs) with intrinsic power saving performance are increasingly utilized as display backlights. As a good display requires smooth LED backlighting, switches are correspondingly connected to LED strings respectively in series, to balance current flowing through each LED string. Usually, drivers of the LED strings provide sufficient voltage that satisfies voltage drop requirements of the LED strings to make the LED strings have sufficient current. However, because each LED may have different characteristic, different LED strings may have different voltage drops. A switch connected to one of the LED strings with the minimum voltage drop accordingly has the maximum voltage drop, which induce great power loss and thermal stress. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Many aspects of the present embodiments can be better understood with reference to the following drawings. The components in the drawings are not necessarily drawn to scale, the emphasis instead being placed upon clearly illustrating the principles of the present embodiments. Moreover, in the drawings, all the views are schematic, and like reference numerals designate corresponding parts throughout the several views. 
         FIG. 1  is a schematic diagram of one embodiment of a light emitting diode driving system as disclosed. 
         FIG. 2  is a schematic diagram of another embodiment of a light emitting diode driving system as disclosed. 
         FIG. 3  is a flowchart of one embodiment of a light emitting diode driving method. 
     
    
    
     DETAILED DESCRIPTION 
     The disclosure is illustrated by way of example and not by way of limitation in the figures of the accompanying drawings in which like references indicate similar elements. It should be noted that references to “an” or “one” embodiment in this disclosure are not necessarily to the same embodiment, and such references mean at least one. 
       FIG. 1  is a schematic diagram of one embodiment of a light emitting diode (LED) driving system  10   a  as disclosed. In the embodiment, the LED driving system  10   a  comprises a sampling circuit  100 , a control circuit  101 , a pulse width modulation (PWM) controller  102 , a DC/DC converter  103 , and a current balance circuit  105 , to drive a LED array  20 . In the embodiment, the LED array  20  comprises a plurality of LED strings which comprise LED strings  20   a ,  20   b ,  20   c  connected in parallel, and each of the LED strings  20   a ,  20   b ,  20   c  comprises a plurality of LEDs connected in series forwardly. An anode of each LED string  20   a ,  20   b ,  20   c  is an anode of the first LED of each LED string  20   a ,  20   b ,  20   c , and a cathode of each LED string  20   a ,  20   b ,  20   c  is a cathode of the last LED of each LED string  20   a ,  20   b ,  20   c . Accordingly, an anode of the LED array  20  is a common node of the anodes of the LED strings  20   a ,  20   b ,  20   c . The DC/DC converter  103  is connected to an external power supply Vin, the PWM controller  102  and the LED array  20 , to convert external power supplied by the external power supply Vin into suitable direct current voltage according to PWM signals generated by the PWM controller  102 , to drive the LED array  20 . 
     In the embodiment, the current balance circuit  105  is connected to cathodes of the LED strings  20   a ,  20   b ,  20   c  of the LED array  20 , and balances current flowing through the LED strings  20   a ,  20   b ,  20   c . In the embodiment, the current balance circuit  105  comprises a plurality of switches  105   a ,  105   b ,  105   c  correspondingly connected to the cathodes of the LED strings  20   a ,  20   b ,  20   c . In the embodiment, the number of the switches  105   a ,  105   b ,  105   c  is the same to that of the LED strings  20   a ,  20   b ,  20   c . In one example, the number of the switches  105   a ,  105   b ,  105   c  may be three, and the number of LED strings  20   a ,  20   b ,  20   c  may also be three. In the embodiment, the switches  20   a ,  20   b ,  20   c  are bipolar junction transistors or field effect transistors. 
     The sampling circuit  100  is connected to the cathodes of the LED strings  20   a ,  20   b ,  20   c , and detects voltage of the cathodes of the LED strings  20   a ,  20   b ,  20   c , and feeds back the voltages of the cathodes of the LED strings  20   a ,  20   b ,  20   c  to the control circuit  101 . In the embodiment, the sampling circuit  100  detects the voltages of the cathodes of the LED strings  20   a ,  20   b ,  20   c  continuously. 
     The control circuit  101  is connected between the sampling circuit  100  and the PWM controller  102 , to generate and output a control signal to the PWM controller  102 , to control a duty cycle of the PWM signals. In the embodiment, the control circuit  101  comprises a storage circuit  1011 , a subtraction circuit  1012 , a comparing circuit  1013  and a signal generating circuit  1014 . The storage circuit  1011  stores an expected voltage range of the voltages of the cathodes of the LED strings  20   a ,  20   b ,  20   c  and a threshold voltage difference. In the embodiment, the expected voltage range is defined as a stabilization range of the LED driving system  10   a , and can be enacted according to experimental data by users, such as [0.1, 0.25] volt (V). The threshold voltage difference is the maximum voltage difference between the switches  105   a ,  105   b ,  105   c  that can be supported, such as 0.3 volts (V). 
     In the embodiment, the comparing circuit  1013  compares the voltages of the cathodes of the LED strings  20   a ,  20   b ,  20   c , to retrieve a maximum and a minimum voltage of the cathodes of the LED strings  20   a ,  20   b ,  20   c , and determines whether the minimum voltage of the cathodes of the LED strings  20   a ,  20   b ,  20   c  fall within the expected voltage range. The subtraction circuit  1012  subtracts the minimum voltage of the cathodes of the LED strings  20   a ,  20   b ,  20   c  from the maximum voltage of the cathodes of the LED strings  20   a ,  20   b ,  20   c  to retrieve a difference between the maximum voltage and the minimum voltage of the cathodes of LED strings  20   a ,  20   b ,  20   c  when the minimum voltage of the cathodes of the LED strings  20   a ,  20   b ,  20   c  is within expected voltage range, and calculates a difference between the minimum voltage of the cathodes of the LED strings  20   a ,  20   b ,  20   c  and the expected voltage range when the minimum voltage of the cathode of the LED strings  20   a ,  20   b ,  20   c  is not within the expected voltage range. The comparing circuit  1013  also determines whether the difference between the maximum voltage and the minimum voltage of the cathodes of the LED strings  20   a ,  20   b ,  20   c  is greater than the threshold, and elects a minimum voltage of the cathodes of rest of LED strings  20   a ,  20   b ,  20   c  except the LED string with the minimum cathode voltage, and continuously determines whether the minimum voltage of the cathodes of the rest of the LED strings  20   a ,  20   b ,  20   c  fall within the expected voltage range. The signal generating circuit  1014  generates and outputs the control signal according to the difference between the minimum voltage of the cathodes of the LED strings  20   a ,  20   b ,  20   c  and the expected voltage range when the minimum voltage of the cathodes of the LED strings  20   a ,  20   b ,  20   c  is not within the expected voltage range, and generates and outputs the control signal according to the minimum voltage when the difference between the maximum voltage of the cathodes of the LED strings  20   a ,  20   b ,  20   c  and the minimum voltage of the cathodes of the LED strings  20   a ,  20   b ,  20   c  is not greater than the threshold. 
     In the embodiment, the comparing circuit  1013  compares the voltage of cathodes of the LED strings  20   a ,  20   b ,  20   c  to retrieve the maximum and the minimum voltage of the cathodes of the LED strings  20   a ,  20   b ,  20   c , and determines whether the minimum voltage of the cathodes of the LED strings  20   a ,  20   b ,  20   c  fall within the expected voltage range to determine whether the LED driving system  10  is stable. If the minimum voltage of the cathodes of the LED strings  20   a ,  20   b ,  20   c  is within the expected voltage range, the LED driving system  10  is stable. If the minimum voltage of the cathodes of the LED strings  20   a ,  20   b ,  20   c  is not within the expected voltage range, the LED driving system  10  is unstable, and the direct current voltage output by the DC/DC converter  103  is required to adjust. In the embodiment, the subtraction circuit  1012  calculates the difference between the minimum voltage of the cathodes of the LED strings  20   a ,  20   b ,  20   c  and the expected voltage range, which is the difference between the minimum value of the expected voltage range and the minimum voltage of the cathodes of the LED strings  20   a ,  20   b ,  20   c  when the minimum voltage of the cathodes of the LED strings  20   a ,  20   b ,  20   c  is less than the minimum value of the expected voltage range. The subtraction circuit  1012  calculates the difference between the minimum voltage of the cathodes of the LED strings  20   a ,  20   b ,  20   c  and the expected voltage range, which is the difference between the minimum voltage of the cathodes of the LED strings  20   a ,  20   b ,  20   c  and the maximum value of the expected voltage range when the minimum voltage of the cathodes of the LED strings  20   a ,  20   b ,  20   c  is greater than the maximum value of the expected voltage range. 
     In the embodiment, the signal generating circuit  1014  generates and outputs the control signal to the PWM controller  102  according to the difference between the minimum voltage of the cathodes of the LED strings  20   a ,  20   b ,  20   c  and the expected voltage range when the minimum voltage of the cathodes of the LED strings  20   a ,  20   b ,  20   c  is not within the expected voltage range. The signal generating circuit  1014  generates and outputs the control signal with a first duty cycle to control the PWM controller  102  to generate and output the PWM signals with a first duty cycle, and to control the DC/DC converter  103  to generate and output a first direct current voltage to make current and light of the LED array  20  decrease, when the minimum voltage of the cathodes of the LED strings  20   a ,  20   b ,  20   c  is less than the minimum value of the expected voltage range. The signal generating circuit  1014  generates and outputs the control signal with a second duty cycle, to control the PWM controller  102  to generate and output the PWM signals with a second duty cycle, thus the DC/DC converter  103  generates and outputs a second direct current voltage, to make current and light of the LED array  20  increase when the minimum voltage is greater than the maximum value of the expected voltage range. In the embodiment, the first duty cycle is less than the second duty cycle, correspondingly the first direct current voltage is less than the second direct current voltage. 
     In the embodiment, when the minimum voltage of the cathodes of the LED strings  20   a ,  20   b ,  20   c  is within the expected voltage range, the subtraction circuit  1012  retrieves the difference between the maximum voltage of the cathodes of the LED strings  20   a ,  20   b ,  20   c  and the minimum voltage of the cathodes of the LED strings  20   a ,  20   b ,  20   c , and then the difference between the maximum voltage of the cathodes of the LED strings  20   a ,  20   b ,  20   c  and the minimum voltage of the cathodes of the LED strings  20   a ,  20   b ,  20   c  is compared with the threshold by the comparing circuit  1013 . When the difference between the maximum voltage of the cathodes of the LED strings  20   a ,  20   b ,  20   c  and the minimum voltage of the cathodes of the LED strings  20   a ,  20   b ,  20   c  is greater than the threshold, the comparing circuit  1013  elects the minimum voltage of the cathodes of the rest of the LED strings except the LED string with the minimum cathode voltage, and continuously determines whether the minimum voltage of the cathodes of the rest of the LED strings fall within the expected voltage range. The signal generating circuit  1014  generates and outputs the control signal according to the minimum voltage of the cathodes of the LED strings  20   a ,  20   b ,  20   c  when the difference between the maximum voltage of the cathodes of the LED strings  20   a ,  20   b ,  20   c  and the minimum voltage of the cathodes of the LED strings  20   a ,  20   b ,  20   c  is not greater than the threshold. 
     The signal generating circuit  1014  generates and outputs the control signal with a constant duty cycle, to control the PWM controller  102  to generate and output the PWM signals with a constant duty cycle too, thus the DC/DC converter  103  generates and outputs constant direct current voltage to drive the LED array  20  when the difference between the maximum voltage of the cathodes of the LED strings  20   a ,  20   b ,  20   c  and the minimum voltage of the cathodes of the LED strings  20   a ,  20   b ,  20   c  is not greater than threshold. 
     In the embodiment, the control signal and the PWM signals are both square-wave signals. 
     In the embodiment, the control circuit  101  elects the minimum voltage of the cathodes of the rest of the LED strings except the LED string with the minimum cathode voltage, and continuously determines whether the minimum voltage of the cathodes of the rest of the LED strings fall within the expected voltage range, when the minimum voltage of the cathodes of the LED strings  20   a ,  20   b ,  20   c  of the LED array  20  is within the expected voltage range and the difference between the maximum voltage of the cathodes of the LED strings  20   a ,  20   b ,  20   c  and the minimum voltage of the cathodes of the LED strings  20   a ,  20   b ,  20   c  is greater than the threshold. When the minimum voltage of the cathodes of the rest of the LED strings is within the expected voltage range, and the difference between the maximum voltage of the cathodes of the rest of the LED strings and the minimum voltage of the cathodes of the rest of the LED stings is greater than the threshold, the control circuit  101  not elects the minimum voltage of the cathodes of rest of the rest of the LED strings except the LED string with the minimum cathode voltage until the minimum voltage of the cathodes of the rest of the LED strings  20   a ,  20   b ,  20   c  is not within expected voltage range or the difference between the maximum voltage of the cathodes of the rest of the LED strings  20   a , 20   b ,  20   c  and the minimum voltage of the cathodes of the rest of the LED strings  20   a , 20   b ,  20   c  is not greater than the threshold. When the minimum voltage of the cathodes of the rest of the LED strings  20   a ,  20   b ,  20   c  is not within the expected voltage range, the control circuit  101  adjusts the duty cycle of the control signal according to the difference between the minimum voltage of the cathodes of the rest of the LED strings  20   a ,  2   b ,  20   c  and the expected voltage range, thus to adjust the duty cycle of the PWM signals, and to adjust the direct current voltage outputted to the LED array  20 . 
     When the difference between the maximum voltage of the cathodes of the LED strings  20   a ,  20   b ,  20   c  and the minimum voltage of the cathodes of the LED strings  20   a ,  20   b ,  20   c  is not greater than the threshold, the control circuit  101  generates and outputs the control signal with the first duty to control the PWM controller  102  to generate and output the PWM signals with the first duty cycle, and to control the DC/DC converter  103  generate and output the first direct current voltage to make current and light of the LED array  20  decrease. 
     The LED string that has the minimum cathode voltage means the LED string has the maximum voltage drop, so the control circuit  101  elects the minimum voltage of the cathodes of the rest LED strings except the LED string with the minimum cathode voltage, which avoids adjusting the duty cycle of the PWM signals according to the LED string with the maximum voltage drop, and reduces the direct current voltage outputted by the DC/DC converter  103 , and then reduces voltage drop of the switches  105   a ,  105   b ,  105   c  of the current balance circuit  105 , to improve the thermal stress problem caused by the switches  105   a ,  105   b ,  105   c  with a great power loss. 
       FIG. 2  is a schematic diagram of another embodiment of the LED driving system  10   a  as disclosed. The difference between the LED driving system  10   a  and the LED driving system  10  is that the LED driving system  10   a  further comprises a feedback circuit  104 . 
     In the embodiment, the feedback circuit  104  is connected to an output of the DC/DC converter  103 , to receive the direct current voltage outputted by the DC/DC converter  103 , and to output a feedback signal to the PWM controller  102  according to the direct current voltage, to adjust the duty cycle of the PWM signals. In the embodiment, the feedback signal and the control signal adjust the duty cycle of the PWM signals together, and to adjust the direct current voltage outputted by the DC/DC converter  103 . In the embodiment, the feedback plays a chief role, and the control signal plays a secondary role in adjusting the duty cycle of the PWM signals. 
     In the embodiment, the feedback circuit  104  comprises two divider resistor  104   a ,  104   b  connected between the output of the DC/DC converter  103  and ground. The two divider resistor  104   a ,  104   b  connects in serials each other, and the PWM control circuit  102  is connected to a common point of the two divider resistor  104   a ,  104   b . In alternative embodiment, the feedback circuit  104  comprises a coil too, to output the feedback signal to the PWM controller  102  according to the direct current voltage, to adjust the duty cycle of the PWM signals. 
       FIG. 3  is a flowchart of one embodiment of a LED driving method. Firstly, in step S 1000 , the DC/DC converter circuit  103  converts the external power supplied by the external power supply Vin into the suitable direct current voltage, to drive the LED array  20 . In step S 1001 , the sampling circuit  100  detects the voltages of the cathodes of the LED strings  20   a ,  20   b ,  20   c , and feedbacks the voltages of the cathodes of the LED strings  20   a ,  20   b ,  20   c  to the control circuit  101 . In step S 1003 , the control circuit  101  compares the voltages of the cathodes of the LED strings  20   a ,  20   b ,  20   c , and retrieves the maximum voltage of the cathodes of the LED strings  20   a ,  20   b ,  20   c  and the minimum voltage of the cathodes of the LED strings  20   a ,  20   b ,  20   c . The control circuit  101  determines whether the minimum voltage of the cathodes of the LED strings  20   a ,  20   b ,  20   c  fall within the expected voltage range, to determine whether the LED driving system is stable. If the minimum voltage is within the expected voltage range, the LED driving system is stable. If the minimum voltage is not within the expected voltage range, the LED driving system is unstable and the direct current voltage output by the DC/DC converter  103  is required to adjust. So if the minimum voltage of the cathodes of the LED strings  20   a ,  20   b ,  20   c  is not within the expected voltage range, in step S 1005 , the control circuit  101  adjusts the duty cycle of the control signal according to the difference between the minimum voltage of the cathodes of the LED strings  20   a ,  20   b ,  20   c  and the expected voltage range to adjust the duty cycle of the PWM signals generated by the PWM controller  102 , and to adjust the direct current voltage outputting to the LED array  20 . 
     If the minimum voltage of the cathode of the LED strings  20   a ,  20   b ,  20   c  is within the expected voltage range, in step S 1004 , the control circuit  101  counts the difference between the maximum voltage and the minimum voltage. In step S 1009 , the control circuit  101  determines whether the difference between the maximum voltage and the minimum voltage is greater than the threshold, to determine whether the voltage drop of the switches  105   a ,  105   b ,  105   c  causes the thermal stress problem. If the difference between the maximum voltage of the cathodes of the LED strings  20   a ,  20   b ,  20   c  and the minimum voltage of the cathodes of the LED strings  20   a ,  20   b ,  20   c  is greater than the threshold, into step S 1011  to avoid the voltage drop of the switches  105   a ,  105   b ,  105   c  causes the thermal stress problem. In step S 1011 , the control circuit  1011  elects the minimum voltage of the cathodes of the rest LED strings except the LED string with the minimum cathode voltage, and repeatedly in step S 1003 . If the difference between the maximum voltage of the cathodes of the LED strings  20   a ,  20   b ,  20   c  and minimum voltage of the cathodes of the LED strings  20   a ,  20   b ,  20   c  is not greater than the threshold, which means the voltage drop of the switches  105   a ,  105   b ,  105   c  not cause the thermal stress problem, in step S 1013 , the control circuit  101  generates and outputs the control signal to the PWM controller  102  correspondingly. The feedback circuit  104  generates and outputs the feedback signal according to the direct current voltage outputted by the DC/DC converter  103 , to adjust the duty cycle of the PWM signals. 
     The LED driving system  10  and the LED driving method elects the minimum voltage of the cathodes of the rest LED strings except the LED string witch the minimum cathode voltage, when the minimum voltage of the cathode is within the expected voltage range and the difference between the maximum voltage of the cathodes of the LED strings  20   a ,  20   b ,  20   c  and the minimum voltage of the cathodes of the LED strings  20   a ,  20   b ,  20   c  is greater than the threshold. Thus, the LED driving system  10  and the LED driving method adjusts the duty cycle of the control signal according to the minimum voltage of the cathodes of the LED strings  20   a ,  20   b ,  20   c , to control the duty cycle of the PWM signals outputted by the PWM controller  102 , and to control the direct current voltage outputting to the LED array  20 , to improve the thermal stress problem caused by the power loss of the switches  105   a ,  105   b ,  105   c.    
     The foregoing disclosure of the various embodiments has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise forms disclosed. Many variations and modifications of the embodiments described herein will be apparent to one of ordinary skill in the art in the light of the above disclosure. The scope of the invention is to be defined only by the claims appended hereto and their equivalents.