Patent Publication Number: US-9420652-B2

Title: LED driver

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims the benefit of Chinese Patent Application No. 201310460841.6, filed on Sep. 29, 2013, which is incorporated herein by reference in its entirety. 
     TECHNICAL FIELD 
     The present disclosure relates to a driving circuit, and more specifically to a LED driver. 
     BACKGROUND 
     The power factor (PF) of an AC-DC LED driver should be limited within a certain range to meet harmonic standard. A conventional AC-DC LED driver may be of a single-stage type or of a two-stage type. The single-stage type of AC-DC LED driver may have a large PF value by providing suitable parameters. However, control variables are selected in a limited scope, and may not be suitable for optimizing efficiency of the LED driver. An electrolytic capacitor should be included as an essential part, which, however, reduces lifetime of the LED driver and causes flicker which can be observed by human eyes. The two-stage type of AC-DC LED driver may be controlled with control variables in a full range, has a high PF value, and is suitable for optimizing efficiency of the LED driver, without flicker which can be observed by human eyes. The LED driver has an input for receiving a pulsed power supply and an output for providing a flat power supply, and has a storage capacitor in an intermediate bus for balancing the power supplies. If the storage capacitor is an electrolytic capacitor, it will adversely influence lifetime of the LED driver. If the storage capacitor is a ceramic capacitor or a thin-film capacitor, its operation voltage range will limit an intermediate bus voltage, or its footprint will be large for providing a large voltage range. The capacitor will increase cost of the LED driver if having a large footprint. 
     An AC-DC LED driver using multiple bus voltages has been proposed, which uses no electrolytic capacitor by means of power discretion control, and hence increases lifetime of the LED driver and has no problem of flicker. A control scheme for multiple-stage sequential LED strings configuration has been proposed by Seoul Semiconductor Co. Ltd., in which the LED strings are basically configured as shown in  FIG. 1 . The control scheme has a beneficial effect of avoiding flicker, but has an adverse effect of non-uniform usage of LEDs. For example, LED La maintains an on state when switch Sa is closed; LEDs La and Lb maintain an on state when switch Sb is closed; LEDs La, Lb and Lc maintain an on state when switches Sa and Sb are both opened. LED La has usage higher than that of LEDs Lb and Lc, which adversely influences lifetime of the whole system. Thus, the usage of LEDs is low or non-uniform in an inappropriate design of LED configuration scheme, which results in reduced lifetime of the whole system. 
     RELATED APPLICATIONS 
     One object of the present disclosure is to provide an LED driver which balances usage of LEDs to increase lifetime of the LED driver while reducing power dissipation or increasing PF value. 
     According to one aspect of the present disclosure, there is provided a LED driver comprising a rectifier circuit, a driving current generating circuit, a bus voltage detection circuit, a LED configuration control circuit, and a LED array, 
     wherein the rectifier circuit has an input for receiving an AC voltage and an output for providing a DC output voltage which is obtained by rectifying the AC voltage; 
     the driving current generating circuit has an input for receiving the DC voltage and an output for providing a driving current to the LED array for its operation; 
     the bus voltage detection circuit detects a bus voltage in the driving current generating circuit and has an output for providing a value of the bus voltage; 
     the LED configuration control circuit controls on and off states of LEDs in the LED array in accordance with the value of the bus voltage; and 
     the LED array comprises a plurality of LEDs connected in series with each other and one or more switches each connected in parallel with at least one of the plurality of LEDs, and the LED configuration control circuit controls on and off states of the switches. 
     According to another aspect of the present disclosure, there is provided a driving method using the above LED driver, comprising: 
     step 1, a rectifier circuit receives an AC voltage at an input, rectifies the AC voltage, and provides a DC voltage at an output to a driving current generating circuit; 
     step 2, the driving current generating circuit converts the DC voltage into a driving current for driving the LED array; 
     step 3, a bus voltage detection circuit detects a bus voltage in the driving current generating circuit and provides the bus voltage to a LED configuration control circuit; 
     step 4, the LED configuration control circuit controls on and off states of LEDs in a LED array in accordance with the value of the bus voltage; and 
     step 5, the driving current generating circuit adjusts the driving current output therefrom in accordance with a number of the LEDs in an on state so as to reduce power dissipation or increase power factor of the LED driver. 
     Preferably, the driving current generating circuit divides the bus voltage into a plurality of levels, and divides the driving current into a plurality of corresponding levels. 
     Preferably, the driving current generating circuit outputs the driving current which is adjusted in accordance with an actual operation state of each LED in the LED array, 
     when the bus voltage detection circuit detects that the level of the bus voltage decreases gradually, a number of the LEDs connected in series in an on state decreases gradually and the driving current increases so that the LED driver has a constant output power, 
     or the driving current decreases so that the LED driver has a high power factor; 
     when the bus voltage detection circuit detects that the level of the bus voltage increases gradually, the number of the LEDs connected in series in an on state increases gradually and the driving current decreases so that the LED driver has a constant output power, 
     or the driving current increases so that the LED driver has a high power factor. 
     Preferably, the driving current output from the driving current generating circuit is adjusted in accordance with an actual operation state of each LED in the LED array, in a case that the output power of the LED driver is constant, 
     when the bus voltage detection circuit detects that the bus voltage has a first voltage value, the number of the LEDs connected in series in an on state has a minimum value, and the driving current, which has a maximum value below a rated current of the LEDs, is adjusted to be a first current value; 
     when the bus voltage detection circuit detects that the bus voltage has a second voltage value, the number of the LEDs connected in series in an on state has a value between the minimum value and a maximum value, and the driving current is adjusted to be a second current value; 
     when the bus voltage detection circuit detects that the bus voltage has a third voltage value, the number of the LEDs connected in series in an on state has the maximum value, and the driving current is adjusted to be a third current value. 
     Preferably, the driving current generating circuit outputs the driving current which is adjusted in accordance with an actual operation state of each LED in the LED array, in a case that the LED driver has a high power factor, 
     when the bus voltage detection circuit detects that the bus voltage has a first voltage value, the number of the LEDs connected in series in an on state has a minimum value, and the driving current is adjusted to be a third current value; 
     when the bus voltage detection circuit detects that the bus voltage has a second voltage value, the number of the LEDs connected in series in an on state has a value between the minimum value and a maximum value, and the driving current is adjusted to be a second current value; 
     when the bus voltage detection circuit detects that the bus voltage has a third voltage value, the number of the LEDs connected in series in an on state has the maximum value, and the driving current, which has a maximum value below a rated current of the LEDs, is adjusted to be a first current value. 
     Preferably, the LED array comprises a plurality of switches and a plurality of LEDs which are connected to each other, and wherein the plurality of LEDs are connected in series between a high output voltage terminal and a low output voltage terminal of the driving current generating circuit, and the plurality of switches are each connected in parallel with one or more of the plurality of LEDs. 
     Preferably, the LED array comprises two LEDs and two switches, 
     the two LEDs comprises a first LED and a second LED, a cathode of the first LED is connected to an anode of the second LED, and an anode of the first LED and a cathode of the second LED are connected to the high voltage output terminal and the low voltage output terminal, respectively; 
     the two switches comprise a first switch and a second switch, the first switch is connected in parallel with the first LED, and the second switch is connected in parallel with the second LED. 
     Preferably, the first voltage value is the minimum voltage level, and the third voltage value is the maximum voltage level, and the first current value is the maximum current value, and the third current value is the minimum current value which is one half of the first current value, 
     in a case that the output power of the LED driver is constant, 
     when the bus voltage reaches the first voltage value, the LED configuration control circuit turns off the first switch or the second switch, and the driving current generating circuit adjusts the driving current to be the first current value; 
     when the bus voltage reaches the third voltage value, the LED configuration control circuit turns off the first switch and the second switch, and the driving current generating circuit adjusts the driving current to be the third current value; 
     in a case that the LED driver has a high power factor, 
     when the bus voltage reaches the first voltage value, the LED configuration control circuit turns off the first switch or the second switch, and the driving current generating circuit adjusts the driving current to be the third current value; and 
     when the bus voltage reaches the third voltage value, the LED configuration control circuit turns off the first switch and the second switch, and the driving current generating circuit adjusts the driving current to be the first current value. 
     Preferably, the LED array comprises three LEDs and two switches, 
     the three LEDs comprises a third LED, a fourth LED and a fifth LED, a cathode of the third LED is connected to an anode of the fourth LED, a cathode of the fourth LED is connected to an anode of the fifth LED, and an anode of the third LED and a cathode of the fifth LED are connected to the high voltage output terminal and the low voltage output terminal, respectively; 
     the two switches comprise a third switch and a fourth switch, the third switch is connected in parallel with the third LED, and the fourth switch is connected in parallel with the fourth LED and the fifth LED which are connected in series with each other; 
     or, the third switch is connected in parallel with the fifth LED, and the fourth switch is connected in parallel with the third LED and the fourth LED which are connected in series with each other. 
     Preferably, the first voltage value is the minimum voltage level, the third voltage value is the maximum voltage level, and the second voltage value is a value between the minimum voltage level and the maximum voltage level, and the first current value is the maximum current value, the second current value is a value between the minimum current value and the maximum current value and is one half of the first current value, and the third current value is the minimum value and is one third of the first current value, 
     in a case that the output power of the LED driver is constant, 
     when the bus voltage reaches the first voltage value, the LED configuration control circuit turns off the third switch and turns on the fourth switch, and the driving current generating circuit adjusts the driving current to be the first current value; 
     when the bus voltage reaches the second voltage value, the LED configuration control circuit turns on the third switch and turns off the fourth switch, and the driving current generating circuit adjusts the driving current to be the second current value; 
     when the bus voltage reaches the third voltage value, the LED configuration control circuit turns off the third switch and the fourth switch, and the driving current generating circuit adjusts the driving current to be the third current value; 
     in a case that the LED driver has a high power factor, 
     when the bus voltage reaches the first voltage value, the LED configuration control circuit turns off the third switch and turns on the fourth switch, and the driving current generating circuit adjusts the driving current to be the third current value; 
     when the bus voltage reaches the second voltage value, the LED configuration control circuit turns on the third switch and turns off the fourth switch, and the driving current generating circuit adjusts the driving current to be the second current value; 
     when the bus voltage reaches the third voltage value, the LED configuration control circuit turns off the third switch and the fourth switch, and the driving current generating circuit adjusts the driving current to be the first current value; 
     Preferably, the LED array comprises three LEDs and four switches, 
     the three LEDs comprises a sixth LED, a seventh LED and an eighth LED, a cathode of the sixth LED is connected to an anode of the seventh LED, a cathode of the seventh LED is connected to an anode of the eighth LED, and an anode of the sixth LED and a cathode of the eighth LED are connected to the high voltage output terminal and the low voltage output terminal, respectively; 
     the four switches comprise a fifth switch, a sixth switch, a seventh switch and an eighth switch, the fifth switch is connected in parallel with the sixth LED, the seventh switch is connected in parallel with the eighth LED, the sixth switch is connected in parallel with the seventh LED and the eighth LED which are connected in series with each other, and the eighth switch is connected in parallel with the sixth LED and the seventh LED which are connected in series with each other. 
     Preferably, the LED array has a first operation mode and a second operation mode. In the first operation mode, the seventh switch and the eighth switch both maintain an off state. The first voltage value is the minimum voltage level, the third voltage value is the maximum voltage level, and the second voltage value is a value between the minimum voltage level and the maximum voltage level, and the first current value is the maximum current value, the second current value is a value between the minimum current value and the maximum current value and is one half of the first current value, and the third current value is the minimum value and is one third of the first current value, 
     in a case that the output power of the LED driver is constant, 
     when the bus voltage reaches the first voltage value, the LED configuration control circuit turns off the fifth switch and turns on the sixth switch, and the driving current generating circuit adjusts the driving current to be the first current value; 
     when the bus voltage reaches the second voltage value, the LED configuration control circuit turns on the fifth switch and turns off the sixth switch, and the driving current generating circuit adjusts the driving current to be the second current value; 
     when the bus voltage reaches the third voltage value, the LED configuration control circuit turns off the fifth switch and the sixth switch, and the driving current generating circuit adjusts the driving current to be the third current value; 
     in a case that the LED driver has a high power factor, 
     when the bus voltage reaches the first voltage value, the LED configuration control circuit turns off the fifth switch and turns on the sixth switch, and the driving current generating circuit adjusts the driving current to be the third current value; 
     when the bus voltage reaches the second voltage value, the LED configuration control circuit turns on the fifth switch and turns off the sixth switch, and the driving current generating circuit adjusts the driving current to be the second current value; 
     when the bus voltage reaches the third voltage value, the LED configuration control circuit turns off the fifth switch and the sixth switch, and the driving current generating circuit adjusts the driving current to be the first current value; 
     In the second operation mode, the fifth switch and the sixth switch both maintain an off state. The first voltage value is the minimum voltage level, the third voltage value is the maximum voltage level, and the second voltage value is a value between the minimum voltage level and the maximum voltage level, and the first current value is the maximum current value, the second current value is a value between the minimum current value and the maximum current value and is one half of the first current value, and the third current value is the minimum value and is one third of the first current value, 
     in a case that the output power of the LED driver is constant, 
     when the bus voltage reaches the first voltage value, the LED configuration control circuit turns off the seventh switch and turns on the eighth switch, and the driving current generating circuit adjusts the driving current to be the third current value; 
     when the bus voltage reaches the second voltage value, the LED configuration control circuit turns on the seventh switch and turns off the eighth switch, and the driving current generating circuit adjusts the driving current to be the second current value; 
     when the bus voltage reaches the third voltage value, the LED configuration control circuit turns off the seventh switch and the eighth switch, and the driving current generating circuit adjusts the driving current to be the first current value; 
     in a case that the LED driver has a high power factor, 
     when the bus voltage reaches the first voltage value, the LED configuration control circuit turns off the seventh switch and turns on the eighth switch, and the driving current generating circuit adjusts the driving current to be the first current value; 
     when the bus voltage reaches the second voltage value, the LED configuration control circuit turns on the seventh switch and turns off the eighth switch, and the driving current generating circuit adjusts the driving current to be the second current value; 
     when the bus voltage reaches the third voltage value, the LED configuration control circuit turns off the seventh switch and the eighth switch, and the driving current generating circuit adjusts the driving current to be the third current value. 
     The present disclosure can advantageously provide the following beneficial effects over the prior art: 
     The LED driver according to the present disclosure reconfigures the prior LED array, balances usage of each LED by switching operation of each switch, which results in long lifetime of the LED driver while reducing power dissipation or increasing a PF value. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The disclosure will be further illustrated in conjunction with the accompanying drawings. 
         FIG. 1  is a schematic diagram showing a basic configuration of a LED string according to the prior art; 
         FIG. 2  is a schematic diagram showing an example LED driver according to one embodiment of the present disclosure; 
         FIG. 3  is a schematic diagram showing an example LED array according to the first embodiment of the present disclosure; 
         FIG. 4  is a schematic diagram showing an example LED array according to the second embodiment of the present disclosure; 
         FIG. 5  is a schematic diagram showing another example LED array according to the second embodiment of the present disclosure; 
         FIG. 6  is a schematic diagram showing an example LED array according to the third embodiment of the present disclosure; and 
         FIG. 7  is a schematic diagram showing an example LED array according to the fourth embodiment of the present disclosure. 
     
    
    
     In  FIGS. 1 to 7 ,
         La, Lb, Lc, Lm 1 , Lm 2 , . . . , Lmn: LED;   Sa, Sb, Sm 1 , Sm 2 , . . . , Sm(n−1), Sm 1 ′, Sm 2 °, . . . , Sm(n−1)′: switches;   L 1 : the first LED; L 2 : the second LED; L 3 : the third LED; L 4 : the fourth LED;   L 5 : the fifth LED; L 6 : the sixth LED; L 7 : the seventh LED; L 8 : the eighth LED;   S 1 : the first switch; S 2 : the second switch; S 3 : the third switch; S 4 : the fourth switch;   S 5 : the fifth switch; S 6 : the sixth switch; S 7 : the seventh switch; S 8 : the eighth switch;   V in : AC voltage; V g : DC voltage; V o : output voltage; i L : driving current.       

     DESCRIPTION OF EMBODIMENTS 
     Reference may now be made in detail to particular embodiments of the LED driver according to the present disclosure, examples of which are illustrated in the accompanying drawings. Other advantages and features of the present disclosure will become readily apparent from claims and the detailed description of preferred embodiments below. The accompanying drawings are illustrative and not intended to be limiting, but are examples of embodiments of the disclosure, are simplified for explanatory purposes, and are not drawn to scale. 
     The inventive idea of the present disclosure is to provide a LED driver comprising a rectifier circuit, a driving current generating circuit, a bus voltage detection circuit, a LED configuration control circuit, and a LED array, wherein the rectifier circuit has an input for receiving an AC voltage and an output for providing a DC output voltage which is obtained by rectifying the AC voltage; the driving current generating circuit has an input for receiving the DC voltage and an output for providing a driving current to the LED array for its operation; the bus voltage detection circuit detects a bus voltage in the driving current generating circuit and has an output for providing a value of the bus voltage; the LED configuration control circuit controls on and off states of LEDs in the LED array in accordance with the value of the bus voltage; and the LED array comprises a plurality of LEDs connected in series with each other and one or more switches each connected in parallel with at least one of the plurality of LEDs, and the LED configuration control circuit controls on and off states of the switches. The LED driver according to the present disclosure reconfigures the prior LED array, balances usage of each LED by switching operation of each switch, which results in long lifetime of the LED driver while reducing power dissipation or increasing a PF value. 
     Embodiments of the present disclosure will be described with reference to  FIGS. 2 to 6 , in which  FIG. 2  is a schematic diagram showing an example LED driver according to one embodiment of the present disclosure;  FIG. 3  is a schematic diagram showing an example LED array according to the first embodiment of the present disclosure;  FIG. 4  is a schematic diagram showing an example LED array according to the second embodiment of the present disclosure;  FIG. 5  is a schematic diagram showing another example LED array according to the second embodiment of the present disclosure;  FIG. 6  is a schematic diagram showing an example LED array according to the third embodiment of the present disclosure; and  FIG. 7  is a schematic diagram showing an example LED array according to the fourth embodiment of the present disclosure. 
     Referring to  FIG. 2 , one embodiment of the present disclosure is a LED driver comprising a rectifier circuit, a driving current generating circuit, a bus voltage detection circuit, a LED configuration control circuit, and a LED array, 
     wherein the rectifier circuit has an input for receiving an AC voltage and an output for providing a DC output voltage which is obtained by rectifying the AC voltage; 
     the driving current generating circuit has an input for receiving the DC voltage and an output for providing a driving current to the LED array for its operation; 
     the bus voltage detection circuit detects a bus voltage in the driving current generating circuit and has an output for providing a value of the bus voltage; 
     the LED configuration control circuit controls on and off states of LEDs in the LED array in accordance with the value of the bus voltage; and 
     the LED array comprises a plurality of LEDs connected in series with each other and one or more switches each connected in parallel with at least one of the plurality of LEDs, and the LED configuration control circuit controls on and off states of the switches. 
     Another one embodiment of the present disclosure is a driving method using the above LED driver comprising: 
     step 1, a rectifier circuit receives an AC voltage at an input, rectifies the AC voltage, and provides a DC voltage at an output to a driving current generating circuit 
     step 2, the driving current generating circuit converts the DC voltage into a driving current for driving the LED array; 
     step 3, a bus voltage detection circuit detects a bus voltage in the driving current generating circuit and provides the bus voltage to a LED configuration control circuit; 
     step 4, the LED configuration control circuit controls on and off states of LEDs in a LED array in accordance with the value of the bus voltage; and 
     step 5, the driving current generating circuit adjusts the driving current output therefrom in accordance with a number of the LEDs in an on state so as to reduce power dissipation or increase power factor of the LED driver. 
     Preferably, the driving current generating circuit divides the bus voltage into a plurality of levels, and divides the driving current into a plurality of corresponding levels. 
     Preferably, the driving current generating circuit outputs the driving current which is adjusted in accordance with an actual operation state of each LED in the LED array, 
     when the bus voltage detection circuit detects that the level of the bus voltage decreases gradually, a number of the LEDs connected in series in an on state decreases gradually and the driving current increases so that the LED driver has a constant output power, 
     or the driving current decreases so that the LED driver has a high power factor; 
     when the bus voltage detection circuit detects that the level of the bus voltage increases gradually, the number of the LEDs connected in series in an on state increases gradually and the driving current decreases so that the LED driver has a constant output power, 
     or the driving current increases so that the LED driver has a high power factor. 
     Preferably, the driving current outputted from the driving current generating circuit is adjusted in accordance with an actual operation state of each LED in the LED array, in a case that the output power of the LED driver is constant, 
     when the bus voltage detection circuit detects that the bus voltage has a first voltage value, the number of the LEDs connected in series in an on state has a minimum value, and the driving current, which has a maximum value below a rated current of the LEDs, is adjusted to be a first current value; 
     when the bus voltage detection circuit detects that the bus voltage has a second voltage value, the number of the LEDs connected in series in an on state has a value between the minimum value and a maximum value, and the driving current is adjusted to be a second current value; 
     when the bus voltage detection circuit detects that the bus voltage has a third voltage value, the number of the LEDs connected in series in an on state has the maximum value, and the driving current is adjusted to be a third current value. 
     Preferably, the driving current generating circuit outputs the driving current which is adjusted in accordance with an actual operation state of each LED in the LED array, in a case that the LED driver has a high power factor, 
     when the bus voltage detection circuit detects that the bus voltage has a first voltage value, the number of the LEDs connected in series in an on state has a minimum value, and the driving current is adjusted to be a third current value; 
     when the bus voltage detection circuit detects that the bus voltage has a second voltage value, the number of the LEDs connected in series in an on state has a value between the minimum value and a maximum value, and the driving current is adjusted to be a second current value; 
     when the bus voltage detection circuit detects that the bus voltage has a third voltage value, the number of the LEDs connected in series in an on state has the maximum value, and the driving current is adjusted to be a first current value. 
     Preferably, the LED array comprises a plurality of switches and a plurality of LEDs which are connected to each other, and wherein the plurality of LEDs are connected in series between a high output voltage terminal and a low output voltage terminal of the driving current generating circuit, and the plurality of switches are each connected in parallel with one or more of the plurality of LEDs. 
     Embodiment One 
     In the present embodiment, the LED array comprises two LEDs L 1  and L 2 , and two switches S 1  and S 2 , as shown in  FIG. 3 . The two LEDs comprise a first LED L 1  and a second LED L 2 . A cathode of the first LED L 1  is connected to an anode of the second LED L 2 . Moreover, an anode of the first LED L 1  and a cathode of the second LED L 2  are connected to the high voltage output terminal and the low voltage output terminal of an output voltage V o , respectively. The two switches comprise a first switch S 1  and a second switch S 2 . The first switch S 1  and the second switch S 2  are connected in parallel with the first LED L 1  and the second LED L 2  respectively. The LED configuration control circuit is used for controlling on and off states of the two switches S 1  and S 2 . 
     In the present embodiment, the first voltage value is the minimum voltage level, and the third voltage value is the maximum voltage level, and the first current value is the maximum current value, and the third current value is the minimum current value which is one half of the first current value. 
     Moreover, in step 4, the LED configuration control circuit turns off the first switch S 1  or the second switch S 2  when the bus voltage reaches the first voltage value, and turns off both of the first switch S 1  and the second switch S 2  when the bus voltage reaches the third voltage value. 
     In the present embodiment, the first switch S 1  and the second switch S 2  may be turned off alternatively each time when the bus voltage reaches the first voltage value, so that the usage of the LEDs is balanced. 
     Moreover, in step 5, it is assumed in one case that the output power of the LED driver is constant. 
     When the bus voltage reaches the first voltage value, only one of the first LED L 1  and the second LED L 2  is turned on, and V o  is equal to V L , which is a voltage drop across the first LED L 1  or the second LED L 2 . The driving current generating circuit adjusts the driving current i L  to be I L  which is the first current value. Hence, the output power P o  is P L . When the bus voltage reaches the third voltage value, both of the first LED L 1  and the second LED L 2  are turned on, and V o  is equal to 2V L . The driving current generating circuit adjusts the driving current i L  to be I L /2, which is the second current value. Hence, the output power P o  is still P L . The output voltage of the present circuit is constant and the power dissipation is minimized. 
     It is assuming in another case that the LED driver has a high power factor. 
     When the bus voltage reaches the first voltage value, only one of the first LED L 1  and the second LED L 2  is turned on, and V o  is equal to V L , which is a voltage drop across the first LED L 1  or the second LED L 2 . The driving current generating circuit adjusts the driving current i L  to be I L /2 which is the second current value. Hence, the output power P o  is P L /2. When the bus voltage reaches the second voltage value, both of the first LED L 1  and the second LED L 2  are turned on, and V o  is equal to 2V L . The driving current generating circuit adjusts the driving current i L  to be I L , which is the first current value. Hence, the output power P o  is 2PL. The present circuit has an increased voltage level and thus a high PF value. 
     Embodiment Two 
     As compared with the first embodiment, the LED array and steps 4 and 5 of the driving method according to the second embodiment are different. Other technical features are the same as or substantially similar to those of the first embodiment and will not be described in detail. 
     In the present embodiment, the LED array comprises three LEDs L 3 , L 4  and L 5 , and two switches S 3  and S 4 , as shown in  FIGS. 4 and 5 . The three LEDs comprise a third LED L 3 , a fourth LED L 4  and a fifth LED L 5 . A cathode of the third LED L 3  is connected to an anode of the fourth LED L 4 . A cathode of the fourth LED L 4  is connected to an anode of the fifth LED L 5 . Moreover, an anode of the third LED L 3  and a cathode of the fifth LED L 5  are connected to the high voltage output terminal and the low voltage output terminal of an output voltage V o , respectively. The two switches comprise a third switch S 3  and a fourth switch S 4 . The third switch S 3  is connected in parallel with the third LED L 3  or the fifth LED L 5 . The fourth switch S 4  is connected in parallel with other two of the LEDs L 3  to L 5 . The LED configuration control circuit is used for controlling on and off states of the two switches S 3  and S 4 . 
     In the present embodiment, the first voltage value is the minimum voltage level, the third voltage value is the maximum voltage level, and the second voltage value is a value between the minimum voltage level and the maximum voltage level, and the first current value is the maximum current value, the second current value is a value between the minimum current value and the maximum current value which is one half of the first current value, and the third current value is the minimum value which is one third of the first current value. 
     Moreover, in step 4, the LED configuration control circuit turns off the third switch S 3  when the bus voltage reaches the first voltage value, turns on the third switch S 3  and turns off the fourth switch S 4  when the bus voltage reaches the second voltage value, and turns off both of the third switch S 3  and the fourth switch S 4  when the bus voltage reaches the third voltage value. 
     Moreover, in step 5, it is assumed in one case that the output power of the LED driver is constant. 
     When the bus voltage reaches the first voltage value, only one of the third LED L 3  and the fifth LED L 5  is turned on, and V o  is equal to V L , which is a voltage drop across the third LED L 3  or the fifth LED L 5 . The driving current generating circuit adjusts the driving current i L  to be I L  which is the first current value. Hence, the output power P o  is P L . When the bus voltage reaches the second voltage value, the fourth LED L 4  and the fifth LED L 5  are turned on, or the third LED L 3  and the fourth LED L 4  are turned on, and V o  is equal to 2V L . The driving current generating circuit adjusts the driving current i L  to be I L /2, which is the second current value. Hence, the output power P o  is still P L . When the bus voltage reaches the third voltage value, all of the third LED L 3 , the fourth LED L 4  and the fifth LED L 5  are turned on, and V o  is equal to 3V L . The driving current generating circuit adjusts the driving current i L  to be I L /3, which is the third current value. Hence, the output power P o  is still P L . The output voltage of the present circuit is constant and the power dissipation is minimized. 
     It is assuming in another case that the LED driver has a high power factor. 
     When the bus voltage reaches the first voltage value, only one of the third LED L 3  and the fifth LED L 5  is turned on, and V o  is equal to V L , which is a voltage drop across the third LED L 3  or the fifth LED L 5 . The driving current generating circuit adjusts the driving current i L  to be I L /3 which is the third current value. Hence, the output power P o  is P L /3. When the bus voltage reaches the second voltage value, the fourth LED L 4  and the fifth LED L 5  are turned on, or the third LED L 3  and the fourth LED L 4  are turned on, and V o  is equal to 2V L . The driving current generating circuit adjusts the driving current i L  to be I L /2, which is the second current value. Hence, the output power P o  is P L . When the bus voltage reaches the third voltage value, all of the third LED L 3 , the fourth LED L 4  and the fifth LED L 5  are turned on, and V o  is equal to 3V L . The driving current generating circuit adjusts the driving current i L  to be I L , which is the first current value. Hence, the output power P o  is 3PL. The present circuit has an increased voltage level and thus a high PF value. 
     Embodiment Three 
     As compared with the above two embodiments, the LED array and steps 4 and 5 of the driving method according to the third embodiment are different. Other technical features are the same as or substantially similar to those of the above two embodiments and will not be described in detail. 
     In the present embodiment, the LED array comprises three LEDs L 6 , L 7  and L 8 , and four switches S 5 , S 6 , S 7  and S 8 , as shown in  FIG. 6 . The three LEDs comprise the sixth LED L 6 , the seventh LED L 7  and the eighth LED L 8 . A cathode of the sixth LED L 6  is connected to an anode of the seventh LED L 7 . A cathode of the seventh LED L 7  is connected to an anode of the eighth LED L 8 . Moreover, an anode of the sixth LED L 6  and a cathode of the eighth LED L 8  are connected to the high voltage output terminal and the low voltage output terminal of an output voltage V o , respectively. The four switches comprise the fifth switch S 5 , the sixth switch S 6 , the seventh switch S 7  and the eighth switch S 8 . The fifth switch S 5  is connected in parallel with the sixth LED L 6 . The seventh switch S 7  is connected in parallel with the eighth LED L 8 . The sixth switch S 6  is connected in parallel with the seventh LED L 7  and the eighth LED L 8 . The eighth switch S 8  is connected in parallel with the sixth LED L 6  and the seventh LED L 7 . The LED configuration control circuit controls on and off states of the four switches S 5 , S 6 , S 7  and S 8 . 
     In the present embodiment, the first voltage value is the minimum voltage level, the third voltage value is the maximum voltage level, and the second voltage value is a value between the minimum voltage level and the maximum voltage level, and the first current value is the maximum current value, the second current value is a value between the minimum current value and the maximum current value which is one half of the first current value, and the third current value is the minimum value which is one third of the first current value. 
     Moreover, the LED configuration control circuit has two switching control modes in step 4. The two switching control modes can be changed from one to the other, instead of being performed simultaneously, so as to avoid flicker. 
     In the first switching control mode, the seventh switch S 7  and the eighth switch S 8  maintain an off state. The LED configuration control circuit turns off the fifth switch S 5  when the bus voltage reaches the first voltage value, turns on the fifth switch S 5  and turns off the sixth switch S 6  when the bus voltage reaches the second voltage value, and turns off both of the fifth switch S 5  and the sixth switch S 6  when the bus voltage reaches the third voltage value. 
     In the second switching control mode, the fifth switch S 5  and the sixth switch S 6  maintain an off state. The LED configuration control circuit turns off the seventh switch S 7  when the bus voltage reaches the first voltage value, turns on the seventh switch S 7  and turns off the eighth switch S 8  when the bus voltage reaches the second voltage value, and turns off both of the seventh switch S 7  and the eighth switch S 8  when the bus voltage reaches the third voltage value. 
     In the present embodiment, the two switching control modes may be performed alternatively so as to balance the usage of the LEDs. 
     Moreover, in step 5, it is assumed in one case that the output power of the LED driver is constant. 
     When the bus voltage reaches the first voltage value, only one of the sixth LED L 6  and the eighth LED L 8  is turned on, and V o  is equal to V L , which is a voltage drop across the sixth LED L 6  or the eighth LED L 8 . The driving current generating circuit adjusts the driving current i L  to be I L  which is the first current value. Hence, the output power P o  is P L . When the bus voltage reaches the second voltage value, the seventh LED L 7  and the eighth LED L 8  are turned on, or the sixth LED L 6  and the seventh LED L 7  are turned on, and V o  is equal to 2V L . The driving current generating circuit adjusts the driving current i L  to be I L /2, which is the second current value. Hence, the output power P o  is still P L . When the bus voltage reaches the third voltage value, all of the sixth LED L 6 , the seventh LED L 7  and the eighth LED L 8  are turned on, and V o  is equal to 3V L . The driving current generating circuit adjusts the driving current i L  to be I L /3, which is the third current value. Hence, the output power P o  is still P L . The output voltage of the present circuit is constant and the power dissipation is minimized. 
     It is assuming in another case that the LED driver has a high power factor. 
     When the bus voltage reaches the first voltage value, only one of the sixth LED L 6  and the eighth LED L 8  is turned on, and V o  is equal to V L , which is a voltage drop across the sixth LED L 6  or the eighth LED L 8 . The driving current generating circuit adjusts the driving current i L  to be I L /3 which is the third current value. Hence, the output power P o  is P L /3. When the bus voltage reaches the second voltage value, the seventh LED L 7  and the eighth LED L 8  are turned on, or the sixth LED L 6  and the seventh LED L 7  are turned on, and V o  is equal to 2V L . The driving current generating circuit adjusts the driving current i L  to be I L /2, which is the second current value. Hence, the output power P o  is P L . When the bus voltage reaches the third voltage value, all of the sixth LED L 6 , the seventh LED L 7  and the eighth LED L 8  are turned on, and V o  is equal to 3V L . The driving current generating circuit adjusts the driving current i L  to be I L , which is the first current value. Hence, the output power P o  is 3PL. The present circuit has an increased voltage level and thus a high PF value. 
     Embodiment Four 
     The present embodiment is an extension of the LED array according the third embodiment. As shown in  FIG. 7 , LEDs Lm 1 , Lm 2 , . . . , Lmn are connected in series between a high output voltage terminal and a low output voltage terminal of an output voltage V o . Switches Sm 1 , Sm 2 , . . . , Sm(n−1) each have one end connected to the high output voltage terminal of the output voltage V o , and the other end connected to a cathode of the respective one of the LEDs Lm 1 , Lm 2 , . . . , Lm(n−1). Switches Sm 1 ′, Sm 2 ′, . . . , Sm(n−1)′ each have one end connected to the low output voltage terminal of the output voltage V o , and the other end connected to an anode of the respective one of the LEDs Lm 2 , . . . , Lmn. The LED configuration control circuit has a plurality of switching control modes. The plurality of switching control modes, including the states m 1 , m 2 , . . . , and state m(n−1), can be changed from one to another, instead of being performed simultaneously. 
     The above configuration extension can control on and off states and usage of the LEDs as required. Each time when the bus voltage increased to a higher level, one more LED is turned on and the driving current is adjusted, so that the LED driver has a constant output power or a high power factor. The LED driver has an increased lifetime by balancing usage of the LEDs, while reducing power dissipation or increase PF value. 
     Apparently, one skilled person may introduce various modifications and alternatives into the present disclosure, without departing from the spirit and scope of the disclosure. The disclosure is intended to cover all of alternatives, modifications and equivalents that may be included within the spirit and scope of the disclosure as defined by the appended claims.