Abstract:
An LED lighting drive circuit supplies power for N groups of LED loads. The LED lighting drive circuit comprises N current paths, N groups of LEDs, and a constant current section. The LED lighting drive circuit enables the current of LED lamps to change in a sinusoidal half-wave form with the utility AC input, and keeps the current a constant effective value.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to the technical field of digital-analogue hybrid integrated circuit, and more particularly, to an LED lighting drive circuit. 
       BACKGROUND 
       [0002]    High-power LED, by virtue of its advantages of high efficiency, energy conservation, environmental protection, long service life and high reliability, has become a new generation of lighting source and are gradually replacing conventional incandescent lamps and fluorescent lamps etc. With improvement of performances and reduction of production cost, application areas of high-power lighting LEDs have expanded from screen backlight sources and brightness insensitive landscape lightings to general white lighting. On the other hand, drive integrated circuits matched with high-power LED have been considerably developed due to the increasing popularity of the high-power LED applications. Therefore, it is particularly critical to design an efficient and reliable LED lighting drive circuit. 
         [0003]    In order to improve the efficiency of the power grid, power factors of lighting devices have been paid more and more attention. Power factor of a lighting device is optimal when an input current of the lighting device varies in a sinusoidal form with voltage of the commercial electric supply. In addition, since a luminous intensity of an LED is proportional to a current passing through the LED within a certain range, in order to ensure the service life and the lighting effect of LED lights, the current passing through the LED is required to be within a certain range and be kept at a constant effective value. 
         [0004]    A conventional LED lighting drive circuit generally employs a bulky transformer to transfer power, which takes much space. Moreover, it employs high-voltage electrolytic capacitors with polarities, and the lifetime of the elements will be dramatically shortened after a period of operation, resulting in poor reliability of the product. 
         [0005]    With respect to the above defects of the conventional LED lighting drive circuit, there is a demand to develop an LED drive circuit without a transformer for converting power and high-voltage electrolytic capacitors, while having a high power factor. 
       SUMMARY OF THE INVENTION 
       [0006]    In order to overcome the defects in the conventional circuit, the present invention provides a novel LED lighting drive circuit, which is a novel circuit structure without a transformer for power conversion, without a high-voltage electrolytic capacitor while having a high power factor, and can drive a high-voltage LED with a current of a constant effective value. 
         [0007]    According to one aspect of the present invention, there is provided an LED lighting drive circuit, configured to supply power to N groups of LED loads, the LED lighting drive circuit including N current paths, N groups of LEDs, and a constant current section, N being an integer no less than 2. The N current paths include: a voltage dividing section, connected to a half sine wave power supply, and configured to reduce a supply voltage of each of the N current paths to a voltage available for a corresponding current path; a comparator section, including N comparators for the N current paths, each of the comparators having an input terminal connected to an output terminal of the voltage dividing section, and another terminal connected to a reference voltage, and configured to output a control signal when a voltage varies; a logic control section, having N input terminals connected to N output terminals of the comparator section and configured to output N enabling signals according to the control signal outputted by the comparator section; a power drive section, having N input terminals connected to N output terminals of the logic control section and configured to provide N voltages according to the N enabling signals; and a switch section, including N switch elements, the N switch elements having N input terminals connected to N output terminals of the power drive section and configured to be switched off or on and thereby to control the N groups of LEDs to be turned on or off according to the N voltages provided by the power drive section, wherein the logic control section controls the N switch elements of the N current paths to be successively switched off and successively switched on through logic control, a first group of LEDs of the N groups of LEDs has a terminal connected to the power supply and another terminal connected to a first current path and a terminal of a second group of LEDs; an i th  group of LEDs has a terminal connected to an (i−1) th  group of LEDs and an (i−1) th  current path, and another terminal connected to an (i+l) th  group of LEDs and an i th  current path; an N th  group of LEDs has a terminal connected to an (N−1) th  group of LEDs and an (N−1) th  current path, and another terminal connected to an N th  current path, where 2         i         (N−1), and the constant current section is connected between the power supply and the switch section such that a current passing through the N groups of LEDs varies in a half sine wave form with input power supply and has an effective value being kept constant. 
         [0008]    According to another aspect of the present invention, wherein the voltage dividing section includes N voltage dividing resistors connected in series between the power supply and a ground, except that a first voltage dividing resistor R1 is only connected to a comparator of the first current path at its negative terminal close to a ground terminal, an i th  voltage dividing resistor Ri is connected to a comparator of the (i−1) th  current path at its positive terminal close to a power supply terminal and connected to a comparator of the i th  current path at its negative terminal, where i         N. 
         [0009]    According to another aspect of the present invention, wherein each of the comparators in the comparator section has a positive terminal connected to a negative terminal of a voltage dividing resistor of a corresponding current path, and a negative terminal connected to a constant reference voltage, when the supply voltage increases, voltages at the positive terminals of the N comparators successively increase to be higher than the reference voltage, such that the N comparators successively output a high voltage level; and when the supply voltage decreases, voltages at the positive terminals of the N comparators successively decrease to be lower than the reference voltage, such that the N comparators successively output a low voltage level. 
         [0010]    According to another aspect of the present invention, wherein the logic control section sequentially includes one NOR gate in the first current path, one NOR gate and two inverters in each of the i th  current path, where 2         i         (N−1), and three inverters in the N th  current path, each NOR gate has an input terminal connected to the output terminal of a comparator in a corresponding current path and another input terminal connected between two inverters close to the output terminal of the logic control section. 
         [0011]    According to another aspect of the present invention, wherein the switch elements are power MOS tubes, each of the power MOS tubes has a gate electrode connected to an output terminal of the power drive section to control the power MOS tube to be switched on and off according to an enabling signal, a drain electrode connected to the N groups of LEDs, and a source electrode connected to an output terminal of the constant current section. 
         [0012]    According to another aspect of the present invention, wherein the constant current section includes: voltage dividing resistors Ra and Rb, an error amplifier, a power MOS tube and a sampling resistor connected in series between the power supply and a ground, the error amplifier has a positive terminal connected between the resistors Ra and Rb, a negative terminal connected to a source electrode of the power MOS tube and a terminal of the sampling resistor, and an output terminal connected to a gate electrode of the power MOS tube; another terminal of the sampling resistor is connected to the ground; and a drain electrode of the power MOS tube serves as an output terminal connected to the switch section. 
         [0013]    According to another aspect of the present invention, wherein resistance of the resistor R1 is much larger than that of the resistor Ri, where i         N and resistance of the resistor Ra is much larger than that of the resistor Rb. 
         [0014]    According to another aspect of the present invention, wherein the Power MOS tubes are all high-voltage power tubes which can endure a voltage larger than a maximum threshold of each group of LEDs. 
         [0015]    According to another aspect of the present invention, wherein the reference voltage at the negative terminals of the comparators is supplied by other power supply module with a constant voltage. 
         [0016]    According to another aspect of the present invention, wherein the constant current section outputs an instantaneous current: 
         [0000]    
       
         
           
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         [0017]    where V AMP  is an instantaneous voltage at a positive terminal of the error amplifier, and R set  is a resistance of the sampling resistor. 
         [0018]    The main operation process of the LED lighting drive circuit of the present invention is that: when the supply voltage (a half sine wave from a commercial electric supply through a bridge rectifier) is zero, all of the power MOS tubes M1˜M5 are switched on. As the supply voltage gradually increases to reach the threshold voltage for turning on the first group of high-voltage LEDs, LED 1  is turned on and the current flows to the ground through the tube M 1  and the tube M 6 . The voltage continues to increase, and when the voltage reaches the threshold voltage for turning on the second group of high-voltage LEDs, due to the voltage dividing of the resistors, the voltage at the terminal VP 1  of the comparator COMP 1  firstly increases to be higher than the constant reference voltage at its terminal VREF. The output voltage level of the comparator COMP 1  jumps, through the logic control, to switch off the power MOS tube M 1 . At this time, the power tubes M2, M3, M4 and M5 are remained as being switched on, and the first and second groups of high-voltage LEDs LED1 and LED2 are turned on. As the voltage continues to increase, and when the voltage reaches the threshold voltage for turning on the third group of high-voltage LEDs, due to the voltage dividing of the resistors, the voltage at the terminal VP 2  of the comparator COMP 2  increases to be higher than the constant reference voltage at its terminal VREF. The output voltage of the comparator COMP 2  jumps, through the logic control, to switch off the power MOS tube M 2 . At this time, the power tubes M 3 , M 4  and M 5  are remained as being switched on, and the first, second and third groups of high-voltage LEDs LED 1 , LED 2  and LED 3  are turned on. And so on, the voltage continues to increase, and the comparators COMP 3  and COMP 4  successively give a response to, through the logic control, successively switch off the MOS tubes M 3  and M 4 , and then the third, fourth and fifth groups of high-voltage LEDs are successively turned on. Afterwards, the voltage starts to decrease, the power tubes M 4 , M 3 , M 2  and M 1  are successively switched on again, and the high-voltage LED groups are successively turned off, and such cycle repeats. Since the variation frequency of voltage is faster than the recognition frequency of human eyes, flickering cannot be observed in the whole process. Moreover, with the increase and decrease of the voltage, the power tube M 5  is always in a switch-on state. 
         [0019]    It can be seen from the above operation of the circuit that, no voltage transformer is employed in the circuit. With the variation of the voltage, the high voltage LEDs are successively turned on, then successively turned off, and such cycle repeats. Since the variation frequency of the voltage is sufficiently faster than the recognition frequency of human eyes, and the effective value of the current is kept constant during the process, the whole LED matrix emits uniform and stable lights. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0020]      FIG. 1  is a circuit diagram of an LED lighting drive circuit according to the present invention; 
           [0021]      FIG. 2  is a preferred circuit diagram of a comparator in the LED lighting drive circuit according to the present invention; 
           [0022]      FIG. 3  is a preferred circuit diagram of a logic control section in the LED lighting drive circuit according to the present invention; and 
           [0023]      FIG. 4  is a graph of simulation current and voltage profiles of the LED lighting drive circuit according to the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0024]    In order to make objectives, technical solutions and advantages of the present invention more apparent, hereinafter, the present invention will be further described in detail with reference to specific embodiments and accompanying drawings. It should be understood that, the description are merely illustrative, and by no means limiting the scope of the present invention. In addition, in the following, description of well-known structures and techniques will be omitted to avoid obscuration of the idea of the present invention. 
         [0025]      FIG. 1  is a circuit diagram of an LED lighting drive circuit according to the present invention. 
         [0026]    The LED lighting drive circuit of the present invention is configured to supply power to N groups of LED loads, and therefore includes N current paths (N is an integer no less than 2). The power supply to the entire circuit is of a half sine wave from a commercial electric supply through a bridge rectifier. In a preferred embodiment of the present invention, there are 5 groups of loads for example. However, the present invention is not limited thereto, and any N of groups of LED loads can be applied to the LED lighting drive circuit of the present invention, as long as the number of current paths is increased according to a similar principle. 
         [0027]    As shown in  FIG. 1 , the LED lighting drive circuit includes N current paths and a constant current section  600 . The N current paths respectively drive N groups of LED loads to emit lights. Each current path includes successively: a voltage dividing section  100 , a comparator section  200 , a logic control section  300 , a power drive section  400  and a switch section  500 . 
         [0028]    The voltage dividing section  100  is connected to a power supply, and configured to reduce the voltage of the power supply to a voltage available for a corresponding current path. As shown in  FIG. 1 , the voltage dividing section  100  includes N voltage dividing resistors, for example R 1 ˜R 5 . These resistors are successively connected in series. Wherein the voltage dividing resistor R 1  for a first path has a terminal connected to the power supply, and has a resistance much larger than the voltage dividing resistors R 2 ˜R 5  for other paths. The voltage dividing resistor R 5  for an N th  path has a terminal connected to the ground. Except for the terminal of R 1  which is connected to the power supply, a voltage dividing resistor Ri for an i th  current path is connected to both of a positive terminal of a comparator COMPi of the corresponding path and a positive terminal of a comparator COMPi- 1  of the previous path, where i=2, 3, 4, . . . , N. In the embodiment of  FIG. 1 , terminals of the resistors R 2 ˜R 5  are respectively and successively connected to the positive terminals of the comparators COMP 1 , COMP 2 , COMP 3 , COMP 4  and COMP 5 . 
         [0029]    The comparator section  200  has an input terminal connected to an output terminal of the voltage dividing section  100 , and is configured to output a corresponding control signal to the logic control section  300  when the voltage varies, to control the switch section  500  to be switched on and off, and in turn, to control LEDs in the paths to be turned on and off. As shown in  FIG. 1 , the comparator section  200  includes N comparators, for example COMP 1 ˜COMP 5 . These comparators respectively belong to a current path of one of the LED loads. Each of the comparators has a positive terminal connected to a negative terminal of a voltage dividing resistor of the corresponding current path. Each of the comparators has a negative terminal connected to a constant reference voltage VREF. Optionally, the reference voltage VREF can be supplied by other power supply module with a constant voltage which satisfies the input scope of the comparator, to ensure normal operation of the comparator. 
         [0030]    Operation principle of the comparator section  200  is as follows. The voltage of the power supply varies. Due to the presence of the voltage dividing resistors, when the voltage increases, voltages (for example, VP 1 , VP 2 , VP 3  and VP 4 ) at the positive terminals of the comparators successively increase, until they are higher than the reference voltage VREF at the negative terminals of the comparators. The comparators COMP 1 , COMP 2 , COMP 3  and COMP 4  successively output high voltage levels to the logic control section LOGIC  300 , in turn, to control power MOS tubes M 1 , M 2 , M 3  and M 4  in the switch section  500  to be successively switched off, such that LED strings of a successively increased number are turned on. When the voltage decreases, voltages at positive terminals of comparators successively decrease, until they are lower than the reference voltage VREF at the negative terminals of the comparators. The comparators COMP 1 , COMP 2 , COMP 3  and COMP 4  successively output low voltage levels to the logic control section LOGIC  300 , in turn, to control the power MOS tubes M 1 , M 2 , M 3  and M 4  to be successively switched on, such that LED strings of a successively decreased number are turned on. In addition, a last comparator COMPN (for example the comparator COMPS) of the N th  current path has the positive terminal VP 5  connected to the ground, and its output is maintained at a low voltage level, such that the power MOS tube M 5  of this path can be maintained as switched on. When each of the MOS tubes MN (N&lt;5) is switched off, LED in the path corresponding to M 5  is turned on; and when any one of the MOS tubes MN (N&lt;5) is switched on, the LED in the path corresponding to M 5  is turned off. 
         [0031]      FIG. 2  is a preferred circuit diagram of a comparator in the LED lighting drive circuit according to the present invention. 
         [0032]    As shown in  FIG. 2 , a preferred comparator in the present invention can consist of two stages of operational amplifiers and inverters, wherein a first stage of the two stages of operational amplifiers is of a differential input single-terminal output structure, signals outputted from the operational amplifiers are outputted as logic signals after rectified by the inverters. The two stages of amplifiers amplify a difference between an input signal IN+ and an input signal IN−, and the amplified signal is rectified by the inverters to be output as a logic signal. 
         [0033]    The logic control section  300  is connected to an output terminal of the comparator section  200  and includes a plurality of NOR gates and a plurality of inverters for controlling the successive turning on and off of the switch section  500  during the variation of the voltage. 
         [0034]      FIG. 3  is a preferred circuit diagram of a logic control section in the LED lighting drive circuit according to the present invention. 
         [0035]    As shown in  FIG. 3 , the logic control section  300  includes NOR gates  201 ˜ 204  and inverters  101 ˜ 109 . In addition, the logic control section  300  includes enable terminals E 1 , E 2 , E 3 , E 4  and E 5 . Each of the enable terminals outputs a high voltage level or a low voltage level depending on the variation of the voltage. After being driven by power tube drivers  1 ˜ 5 , the outputs respectively control the power MOS tubes to be switched on or off, and in turn, to control the LED loads in the current paths to be turned on or off. 
         [0036]    The NOR gate  201  in the first current path has an input terminal connected to the output terminal of the comparator COPM 1  and another input terminal connected to an input terminal of the inverter  102  and an output terminal of the inverter  101  in the next current path (i.e. the second current path). The NOR gate  201  has an output terminal connected to the power tube driver  1  in the corresponding current path (i.e. the first current path). 
         [0037]    The NOR gate  202  in the second current path has an input terminal connected to the output terminal of the comparator COPM 2  and another input terminal connected to an input terminal of the inverter  104  and an output terminal of the inverter  103  in the next current path (i.e. the third current path). The NOR gate  202  has an output terminal connected to the first inverter  101  in the corresponding current path (i.e. the second current path). The second inverter  102  is connected to the power tube driver  2  in the corresponding current path. 
         [0038]    The NOR gate  203  in the third current path has an input terminal connected to the output terminal of the comparator COPM 3  and another input terminal connected to an input terminal of the inverter  106  and an output terminal of the inverter  105  in the next current path (i.e. the fourth current path). The NOR gate  203  has an output terminal connected to the first inverter  103  in the corresponding current path (i.e. the third current path). The second inverter  104  is connected to the power tube driver  3  in the corresponding current path. 
         [0039]    The NOR gate  204  in the fourth current path has an input terminal connected to the output terminal of the comparator COPM 4  and another input terminal connected to an input terminal of the inverter  109  and an output terminal of the inverter  108  in the next current path (i.e. the fifth current path). The NOR gate  204  has an output terminal connected to the power tube driver  4  in the corresponding path (i.e. the fourth current path) via the first inverter  105  and the second inverter  106  in the fourth current path. 
         [0040]    The logic control section of the N th  current path (i.e. the fifth current path) includes three inverters  107 ,  108  and  109 , wherein the first inverter  107  has an input terminal connected to the output terminal of the comparator COMP 5 , and an output terminal connected to the power tube driver  5  in the corresponding path (i.e. the fifth current path) via the second inverter  108  and the third inverter  109 . 
         [0041]    The power drive section  400  is connected to the output terminal of the logic control section  300 , and configured to provide a gate voltage required by turning on and off a power MOS tube. As shown in  FIG. 1 , the power drive section  400  includes a plurality of power tube drivers, for example, drivers  1 ˜ 5 , respectively disposed in the first to fifth current paths. Each of the drivers  1 ˜ 5  has an output terminal connected to a gate electrode of a power MOS tube (M 1 ˜M 5 ) in the corresponding current path, to control the power MOS tube to be turned on and off according to the output signal of the enable terminal of the logic control section  300 . 
         [0042]    The switch section  500  has an input control terminal connected to an output terminal of the power drive section  400 , and is configured to control the LED loads to be turned on and off. As shown in  FIG. 1 , the switch section  500  includes a plurality of power MOS tubes, for example M 1 ˜M 5 , respectively disposed in the first to fifth current paths. Each of the power MOS tubes has a gate electrode connected to the output terminal of the power tube driver in the corresponding current path, a drain electrode connected to the LED load in the corresponding current path and a source electrode connected to a drain of the MOS tube M 6  in the constant current section  600 . When each of the power MOS tubes is switched on, the LED load in the corresponding current path is turned off. When the power MOS tubes are successively switched off, the LED loads in the corresponding current paths are successively turned on. In the present invention, the power MOS tubes M 1 ˜M 5  are preferably high-voltage power tubes which can endure a voltage higher than the maximum threshold of all of the LED loads. 
         [0043]    The LED lighting drive circuit of the present invention also includes the constant current section  600 , configured to provide a constant current for the drive circuit. As shown in  FIG. 1 , the constant current section  600  includes voltage dividing resistors Ra and Rb, an error amplifier AMP, a MOS M 6  and a sampling resistor R set , wherein the resistor Ra has a terminal connected to the power supply (a half sine wave from a commercial electric supply through a bridge rectifier) and another terminal connected to a terminal of Rb and a positive input terminal of the error amplifier AMP. Another terminal of Rb is connected to the reference ground. The error amplifier AMP has a negative input terminal connected to a source electrode of the MOS tube M 6  and a terminal of the sampling resistor R set , and an output terminal connected to a gate electrode input terminal of the MOS tube M 6 . Another terminal of the sampling resistor R set  is connected to the reference ground. A drain electrode of the MOS tube M 6  as an output terminal is connected to the switch section  500 . The voltage dividing resistors Ra and Rb are configured to divide the sine-waved supply voltage to a voltage appropriate for the operation of the error amplifier AMP, and the voltage resulted from the dividing varies in a half sine wave form with the supply voltage. When the current is in a normal operation, the error amplifier has its positive input voltage equal to its negative input voltage, then a current passing through the sampling resistor R set  is V AMP /R set . As the supply voltage varies, a current passing through the LEDs varies accordingly. Through the constant current section  600 , the current passing through the LED strings varies in a half sine wave form with the input AC power supply and is maintained at a constant effective value. The effective value of the current can be adjusted through the resistor R set . 
         [0044]    Hereinafter, the operation principle of the LED lighting drive circuit of the present invention will be described with reference to  FIGS. 1 to 3 . 
         [0045]    The supply voltage is a half sine wave from a commercial electric supply through a bridge rectifier. When the supply voltage is 0, each of the comparators COMP 1 ˜COMP 5  outputs “0”. Through the NOR gates and the inverters in the logic control section  300 , each of the enable terminals E 1 , E 2 , E 3 , E 4  and E 5  of the logic control section  300  outputs a high voltage level “1”. Through the driving of the power tube drivers  1 ˜ 5 , each of the power MOS tubes M 1 ˜M 5  is controlled to be switched on. However, since the supply voltage is 0, less than the threshold voltage for turning on the LED lights, so the LED strings are turned off. With the increase of the supply voltage, the light string LED 1  is firstly turned on. 
         [0046]    With the further increase of the supply voltage, due to the characteristic of LED devices, the light string LED 1  will maintain its voltage. When the supply voltage reaches the threshold voltage for turning on both of the LED lights LED 1  and LED 2 , with set resistances of the resistors in the voltage dividing section  100 , a voltage VP 1  at the positive terminal of the comparator COMP 1  can firstly increase to be higher than the reference voltage VREF at its negative terminal. The comparator COMP 1  outputs a high voltage level “1”, through the NOR gate  201 , then the enable terminal E 1  in the corresponding current path outputs a low voltage level “0”. After the driving of the power tube driver  1 , the power MOS tube M 1  is controlled to be switched off. At this time, the power MOS tube M 2 , M 3 , M 4  and M 5  are maintained as being switched on and the high-voltage groups of LEDs LED 1  and LED 2  are turned on to emit light. The current through the LEDs flows to the ground through M 2 , M 6  and R set . The instantaneous current is: 
         [0000]    
       
         
           
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         [0047]    Where V AMP  is the instantaneous voltage at the positive terminal of the error amplifier AMP. 
         [0048]    Afterwards, the supply voltage continues to increase to the threshold voltage for turning on LED 1 , LED 2  and LED 3 . Due to the voltage dividing of the resistors, the voltage at the terminal VP 2  of the comparator COMP 2  increases to be higher than the reference voltage VREF at its negative terminal. The output of the comparator COMP 2  jumps to “1”, through the NOR gate  202  and the inverters  101  and  102 , the enable terminal E 2  outputs “0”, to switch off the power MOS tube M 2 . The power tubes M 3 , M 4  and M 5  are maintained as being switched on. At this time, the high-voltage groups of LEDs LED 1 , LED 2  and LED 3  are turned on. The current through the LEDs flows to the ground through M 3 , M 6  and R set . The instantaneous current is: 
         [0000]    
       
         
           
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         [0049]    Where V AMP  is the instantaneous voltage at the positive terminal of the error amplifier AMP. 
         [0050]    Afterwards, the supply voltage continues to increase, the enable terminals E 3  and E 4  in the other current paths successively output a low voltage level, to successively switch off the MOS tubes M 3  and M 4  and successively turn on LED 4  and LED 5 . 
         [0051]    When the supply voltage reaches a maximum value (a peak value of a half sine wave) and then starts to decrease, the enable terminals E 4 , E 3 , E 2  and E 1  successively outputs a high voltage level to successively switch on the power MOS tubes M 4 , M 3 , M 2  and M 1 , such that the number of LED strings being turned on is successively decreased, and such cycle repeats. 
         [0052]    In addition, during the whole process of the variation of the supply voltage, the enable terminal E 5  in the last path (i.e. the fifth current path) is maintained at a high voltage level such that the MOS tube M 5  is maintained as being switched on. When each of the MOS tubes MN (N&lt;5) is switched off, LED in the path corresponding to M 5  is turned on; and when any one of the MOS tubes MN (N&lt;5) is switched on, the LED in the path corresponding to M 5  is turned off. 
         [0053]    Since the variation frequency of the voltage is faster than the recognition frequency of human eyes, flickering cannot be observed in the whole process. Moreover, with the increase and decrease of the voltage, the power tube M 5  is always in a switch-on state. 
         [0054]    It can be seen from the above operation principle that, no voltage transformer is employed in the circuit. With the variation of the voltage, the high voltage LEDs are successively turned on, then successively turned off, and such cycle repeats. Since the variation frequency of the voltage is sufficiently faster than the recognition frequency of human eyes, and the effective value of the current is kept constant during the process, the whole LED matrix emits uniform and stable lights. 
         [0055]      FIG. 4  is a graph of simulation current and voltage profiles of the LED lighting drive circuit according to the present invention. As shown in  FIG. 4 , the voltage is a 220 V AC wave which is full-wave rectified with a peak value of 310 V. When the voltage reaches the threshold of the first group of LEDs, a current will occur on the LEDs. Afterwards, the wave of the current varies in a half sine wave form with the supply voltage with a frequency of 100 Hz. Since the effective value of the current is kept constant during the process, it can ensure that the LED matrix emits uniform and stable lights. Moreover, since the input current and voltage of the circuit are in a half sine wave form and vary simultaneously, the circuit can achieve a desirable power factor. 
         [0056]    It should be understood that, the above specific embodiments of the present invention are merely for illustration or explanation of the present invention, and not constitute restriction of the present invention. Therefore, any modification, equivalent replacement, improvement and the like without departing from the spirit and scope of the present invention falls within the protective scope of the present invention. In addition, the appended claims of the present invention are intended to cover all the alterations and modifications within the scope and mete of the appended claims and their equivalents.