Patent Publication Number: US-10772168-B2

Title: LED driving circuit and driving method thereof

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of International Patent Application No. PCT/CN2017/084305 with a filing date of May 15, 2017, designating the United States, now pending, and further claims priority to Chinese Patent Application No. 201710048486.X entitled “LED DRIVING CIRCUIT AND DRIVING METHOD THEREOF” filed on 2017 Jan. 20, the entire content of which is incorporated herein by reference. 
    
    
     TECHNICAL FIELD 
     The present disclosure relates to the field of LED driving, and particularly to an LED driving circuit and a driving method thereof. 
     BACKGROUND 
     At present, there are two common problems existing in a high-voltage linear integrated circuit (IC) on the market: 1. a voltage of a Light Emitting Diode (LED) loaded lamp bead is severely restricted by an input voltage, and a total voltage drop of the lamp bead cannot be too much lower than the input voltage. 2. LED illumination stroboflash is severe when the input voltage fluctuates. 
     In the prior art, a current flowing through an LED lamp string under the control of a control circuit be a constant value in the control of an LED power source, and the constant value does not change along with the change of the input voltage of an external circuit. The control circuit is as shown in  FIG. 1 . When the input voltage is greater than a set voltage, redundant voltage drop may be applied to the current control circuit IC, so that the temperature of the IC is increased, thereby increasing power consumption, and burning out the IC, when the input voltage is smaller than the set voltage, a charge voltage on a capacitor may be reduced Under the same condition, the discharge time of the capacitor is shortened and the stroboflash of the LED lamp string is severe. 
     A relationship of a voltage and a current on an output end of a rectifying circuit may be shown in  FIG. 2 . In  FIG. 2 , V 1  refers to a waveform diagram of a voltage that is rectified and filtered, and I 3  refers to a waveform diagram of a current of an LED lamp string. Since the voltage drop on the LED lamp string is a constant value, a voltage waveform V 3  of an input end of a current control circuit  50  as shown in  FIG. 2  is consistent with the waveform V 1 . 
     Generally, a filter circuit  20  may include an electrolytic capacitor C, and thus a magnitude of the voltage V 1  is related to the following factors: a magnitude of an input voltage of a power source, a magnitude of an electrolytic capacitor C, and a magnitude of a circuit charge/discharge current. 
     At a moment t 11 , a positive voltage V 1  of an LED lamp string exceeds a voltage required for conducting the LED lamp string. When the LED lamp string is conducted, the charge current in the filter circuit  20  is the largest, and may be reduced along with the increase of the voltage. A current I 3  flowing through the LED lamp string under the control of the current control circuit  50  may be a constant value, and the voltage drop on both ends of the LED lamp string is equal to V 11 , at a moment t 2 , the input voltage of the power source reaches a maximum value, the voltage on the filter circuit  20  also reaches a maximum value, the charging of the capacitor in the filter circuit  20  is stopped, and the voltage V 3  on the input end of the current control circuit  50  also reaches a maximum value; then, the input voltage is gradually reduced, the capacitor in the filter circuit  20  begins discharging, a sum of a discharge current of the capacitor and a current flowing in from the power source is equal to the current in the LED lamp string to maintain the LED lamp string conducted, and the voltage V 3  on the input end of the current control circuit  50  is also reduced accordingly, at a moment t 3 , the voltage of the power source is lower than the voltage of the filter circuit  20 , the power source stops inputting the current, the filter circuit  20  discharges to maintain the LED lamp string conducted, and the voltage of the filter circuit  20  drops; at a moment t 31 , the voltage V 1  of the filter circuit  20  is equal to the conduction voltage V 11  of the LED lamp string, and the current in the LED lamp string is reduced along with the decrease of the voltage V 1 ; at a moment t 5 , the voltage V 1  is reduced to a minimum value, the current in the LED lamp string is the smallest, and then the voltage V 1  is increased with the increase of the input voltage, and the current in the LED lamp string is also increased; at a moment t 51 , the voltage V 1  reaches the conduction voltage of the LED lamp string again, and the above process is repeated. 
     At the moments t 1  to t 3 , the voltage V 1  changes according to a sine rule, at the moments t 3  to t 5 , the voltage V 1  drops according to an exponential curve, and the constant of the discharge time is related to the magnitude of the capacitor, the magnitude of the discharge current, and the like; at the moments t 3  to t 5 , the capacitor in the filter circuit  20  discharges, the magnitude of the discharge current is controlled by the current control circuit  50 , and the voltage V 1  is reduced. 
     When the input voltage of the power source is smaller than a nominal voltage, energy stored in the filter circuit  20  may be insufficient to discharge to maintain the input voltage reaching the voltage required for conduction again, which may worsen the stroboflash of the LED lamp string. 
     When the input voltage is greater than the nominal voltage, the voltages V 1  of the filter circuit  20  and a positive end of the LED lamp string are increased. During the charge and discharge of the filter circuit  20 , the next charge begins before the previous discharge is finished, so that the voltage on the filter circuit is increased, the voltages V 1  are entirely moved up, and are always greater than the conduction voltage of the LED lamp string during a full period, and the current in the LED lamp string is a constant value. The LED lamp string has no stroboflash at this time, as shown by a waveform V 12  in  FIG. 3 . However, a negative voltage V 3  of the LED lamp string is also increased, as shown by a waveform V 32 . That is, since the voltage drop on the current control circuit  50  is increased, the power consumption is increased, and heat is also increased, thereby resulting in sharp increase of temperature and reduction of reliability. In the drawing, V 11  refers to a waveform diagram of a positive voltage of the LED lamp string when an input voltage is equal to a nominal voltage, and V 31  refers to a waveform diagram of a negative voltage of the LED lamp string. 
     SUMMARY 
     To overcome defects existing in the prior art, the present disclosure provides an LED driving circuit, including a rectifying circuit, a filter circuit, a current-limiting circuit, a detection and control circuit and an LED lamp string; the rectifying circuit is connected with an alternating-current power source, and a negative pole of the rectifying circuit is connected with a second end of the filter circuit and a second end of the detection and control circuit respectively; an input end of the current-limiting circuit is connected with a positive pole of the rectifying circuit, a first output end of the current-limiting circuit is connected with a positive pole of the LED lamp string and an input end of the filter circuit respectively, and a second output end of the current-limiting circuit is connected with a third end of the detection and control circuit; a negative pole of the LED lamp string is connected with an input end of the detection and control circuit. 
     The detection and control circuit may include a control circuit and a voltage-dividing circuit, where the voltage-dividing circuit may include voltage-dividing resistors R 1  and R 2  which are connected in series. 
     The second output end of the current-limiting circuit is connected with the third end of the control circuit, and the negative pole of the LED lamp string is connected with the input end of the control circuit and an input end of the resistor R 1  respectively; the resistors R 1  and R 2  are connected in series, and a fourth end of the control circuit is connected with an output end of the resistor R 1  and an input end of the resistor R 2  respectively, the negative pole of the rectifying circuit is connected with the second end of the control circuit and an output end of the resistor R 2  respectively. 
     A filter capacitor C 1  may be added on the fourth end of the control circuit, where the capacitor C 1  is connected in parallel with the resistor R 2 . 
     The current-limiting circuit may be used to control the charge current of the filter circuit and a current I 3  of the LED lamp string. 
     The detection and control circuit may be used to detect the current in the LED lamp string and control the current-limiting circuit and the filter current according to a detection result. 
     The detection and control circuit may detect a voltage V 3  at the negative pole of the LED lamp string, where the voltage V 3  is referred to as a first detection voltage. 
     When the first detection voltage V 3  is smaller than a third set voltage V 33 , a first output current I 2  flowing out of the first output end of the current-limiting circuit is a first constant current I 21 , the first constant current I 21  supplies a charge current of the filter circuit and the current I 3  of the LED lamp string, the current I 3  flowing through the LED lamp string is controlled by the detection and control circuit, no current is output from the second output end of the current-limiting circuit, and a second output current I 5  is zero. 
     When the first detection voltage V 3  is greater than or equal to the third set voltage V 33  and smaller than a fourth set voltage V 34 , the first output current I 2  of the current-limiting circuit is reduced along with the increase of the first detection voltage V 3 , and the second output current I 5  flowing from the current-limiting circuit to the detection and control circuit is increased along with the increase of the first detection voltage V 3  at the same time. 
     When the first detection voltage V 3  is greater than or equal to the fourth set voltage V 34 , the first output current I 2  of the current-limiting circuit remains as a second constant current I 22 , and the second output current I 5  of the current-limiting circuit remains as a third constant current I 51 , at this time, the second constant current I 22  is a minimum current flowing from the current-limiting circuit to the LED lamp string and the filter circuit, and the third constant current I 51  is a maximum current flowing from the current-limiting circuit to the detection and control circuit. 
     The present disclosure also provides a driving method of an LED driving circuit, by which the detection and control circuit detects a first detection voltage V 3  at the negative pole of the LED lamp string and controls a magnitude of a current I 3  flowing through the LED lamp string according to a magnitude of the first detection voltage V 3 . 
     When the first detection voltage V 3  is smaller than a first set voltage V 31 , no current flows through the LED lamp string, and a first output current I 2  of the current-limiting circuit is a first constant current I 21  used for charging a capacitor in the filter circuit. At this time, a charge current I 4  of the filter circuit is the largest, and a second output current I 5  of the current-limiting circuit is zero. 
     When the first detection voltage V 3  is equal to the first set voltage V 31 , the LED lamp string is conducted and there is a current flowing through the LED lamp string. At this time, the first output current I 2  flowing out of the current-limiting circuit remains as the first constant current I 21 , where the first constant current I 21  may be divided into two parts, one part of which is used for charging the filter circuit, and the other part flows through the LED lamp string for turning on the LED lamp string, and the second output current I 5  of the current limiting circuit continues to be zero. 
     When the first detection voltage V 3  is greater than the first set voltage V 31  and smaller than a second set voltage V 32 , the first output current I 2  flowing out of the current-limiting circuit continues to remain as the first constant current I 21  With the increase of the first detection voltage V 3 , the current I 3  flowing through the LED lamp string is increased and the charge current I 4  of the filter circuit is reduced, and the second output current I 5  of the current-limiting circuit continues to be zero. 
     When the first detection voltage V 3  is greater than or equal to the second set voltage V 32  and smaller than a third set voltage V 33 , the first output current I 2  flowing out of the current-limiting circuit is the first constant current I 21 , the current I 3  flowing through the LED lamp string under the control of the detection and control circuit may be a fourth constant current I 32 . At this time, the fluctuation of the first detection voltage V 3  may not cause the fluctuation of the current I 3 , and the second output current I 5  of the current-limiting circuit continues to be zero. 
     When the first detection voltage V 3  is greater than or equal to the third set voltage V 33  and smaller than a fourth set voltage V 34 , the first output current I 2  flowing out of the current-limiting circuit is reduced along with the increase of the first detection voltage V 3 ; at the same time, the detection and control circuit draws the second output current I 5  from the current-limiting circuit, the second output current I 5  flowing from the current-limiting circuit to the detection and control circuit is increased with the increase of the first detection voltage V 3 , the current I 3  flowing through the LED lamp string under the control of the detection and control circuit remains as the fourth constant current I 32 , and the second output current I 5  is far smaller than the first output current I 2 . 
     When the first detection voltage V 3  is greater than or equal to the fourth set voltage V 34 , the first output current I 2  flowing out of the current-limiting circuit remains as a second constant current I 22 , and the second output current I 5  flowing out of the current-limiting circuit remains as a third constant current I 51 ; at this time, the second constant current I 22  is a minimum current flowing from the current-limiting circuit to the LED lamp string and the filter circuit, the third constant current I 51  is a maximum current flowing from the current-limiting circuit to the detection and control circuit, and the third constant current I 51  is far smaller than the second constant current I 22 . 
     A second detection voltage V 4  may be obtained by performing voltage division for the first detection voltage V 3 , and the second detection voltage V 4  may have corresponding smaller voltage fluctuation, so that the first output current I 2  of the current-limiting circuit has corresponding smaller fluctuation. In this way, the current I 3  on the LED lamp string and the voltage drop on the detection and control circuit are relatively stable. Further, different voltage division ratios may be correspondingly adapted to different input voltages. 
     A constant second detection voltage V 4  may be obtained by filtering the second detection voltage V 4 , so that the first output current I 2  of the current-limiting circuit is also a constant value correspondingly, thereby ensuring that the current I 3  on the LED lamp string is constant and the voltage drop on the detection and control circuit is stable. 
     The current-limiting circuit may include a second power supply circuit, a control and driving circuit, a second driving circuit, a second current-sampling circuit and a power tube Q 2 , where the second driving circuit is connected to the second power supply circuit, the control and driving circuit, the second current-sampling circuit and a control end of the power tube Q 2 , and the second power supply circuit is connected to the control and driving circuit. 
     The control circuit may include a first power supply circuit, a reference circuit, a first driving circuit, a first current-sampling circuit, a voltage-sampling circuit, a pull-down current circuit and a power tube Q 1 , where the reference circuit is connected to the first power supply circuit and the first driving circuit, the first driving circuit is connected to a control end of the power tube Q 1  and the first current-sampling circuit, and the voltage-sampling circuit is connected to the pull-down current circuit and the first current-sampling circuit. 
     The current-limiting circuit may be provided with a second over-temperature protection circuit connected with the second driving circuit. 
     The control circuit may be provided with a first over-temperature protection circuit connected with the first driving circuit. 
     According to the present disclosure, the problems that the power consumption is increased and the burnout of the current control circuit may be caused when the voltage of the power source is increased in the prior art may be solved. Also, the problem of worsening stroboflash of the LED lamp string when the voltage of the power source is reduced may be solved. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a schematic diagram illustrating an LED control circuit in the prior art. 
         FIG. 2  is a schematic diagram illustrating a relationship of a voltage and a current on an output end of a rectifying circuit in an LED control circuit in the prior art. 
         FIG. 3  is a schematic diagram illustrating a relationship of a voltage and a current of a rectifying circuit at different input voltages in an LED control circuit in the prior art. 
         FIG. 4  is a schematic diagram illustrating an LED driving circuit according to an example of the present disclosure. 
         FIG. 5  is a schematic diagram illustrating a working principle of a current-limiting circuit of an LED control circuit according to an example of the present disclosure. 
         FIG. 6  is a schematic diagram illustrating a control situation of a detection and control circuit and a current-limiting circuit of an LED control circuit according to an example of the present disclosure. 
         FIG. 7  is a waveform diagram illustrating an output voltage V 2  and a first output current I 2  of a current-limiting circuit in a low input voltage range according to an example of the present disclosure. 
         FIG. 8( a )  is a waveform diagram illustrating an output voltage V 2  and a first output current I 2  of a current-limiting circuit in a high input voltage range according to an example of the present disclosure. 
         FIG. 8( b )  is another waveform diagram illustrating an output voltage V 2  and a first output current I 2  of a current-limiting circuit in a high input voltage range according to an example of the present disclosure. 
         FIG. 9  is a schematic diagram illustrating drawing waveform diagrams at different input voltages together according to an example of the present disclosure. 
         FIG. 10  illustrates an LED driving circuit according to a preferred example of the present disclosure. 
         FIG. 11  is a waveform diagram illustrating the circuit of  FIG. 10  when an input voltage of a power source is greater than a second threshold and smaller than a fourth threshold according to an example of the present disclosure. 
         FIG. 12  illustrates another LED driving circuit according to a preferred example of the present disclosure. 
         FIG. 13  is a waveform diagram illustrating voltages V 2 /V 3  and currents I 2 /I 3  on both ends of an LED lamp string in the circuit of  FIG. 12  according to an example of the present disclosure. 
         FIG. 14  is a schematic diagram illustrating a structure of a current-limiting circuit according to an example of the present disclosure. 
         FIG. 15  is a schematic diagram illustrating a structure of a current-limiting circuit provided with an over-temperature protection circuit according to an example of the present disclosure. 
         FIG. 16  is a schematic diagram illustrating a structure of a control circuit according to an example of the present disclosure. 
         FIG. 17  is a schematic diagram illustrating a structure of a control circuit provided with an over-temperature protection circuit according to an example of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     The present disclosure will be further described below in detail in combination with  FIGS. 1-17 . 
       FIG. 4  is a schematic diagram illustrating an LED driving circuit according to an example of the present disclosure. The LED driving circuit may include a rectifying circuit  40 , a filter circuit  20 , a current-limiting circuit  10 , a detection and control circuit  30  and an LED lamp string. 
     The rectifying circuit  40  is connected with an alternating-current power source for converting an alternating-current voltage into a direct-current voltage; a negative pole of the rectifying circuit  40  is connected to a second end of the filter circuit  20  and a second end of the detection and control circuit  30 . 
     An input end of the current-limiting circuit  10  is connected with a positive pole of the rectifying circuit  40 , a first output end of the current-limiting circuit  10  is connected with a positive pole of an LED lamp string and an input end of the filter circuit  20 , and a second output end of the current-limiting circuit  10  is connected with a third end of the detection and control circuit  30 . The current limiting circuit  10  is used to control the quantity of energy charged into an electrolytic capacitor in the filter circuit  20 , that is, to control a charge current of the filter circuit  20  and a current I 3  in the LED lamp string; the detection and control circuit  30  is used to detect the magnitude of a voltage in the negative pole of the LED lamp string and feed the voltage back to the current-limiting circuit  10 , and control the magnitude of the current I 3  flowing through the LED lamp string. 
     The input end of the filter circuit  20  is connected with the first output end of the current-limiting circuit  10 , and the filter circuit  20  is used to reduce a ripple in the direct-current voltage. 
     The negative pole of the LED lamp string is connected with the input end of the detection and control circuit  30 . Since the voltage drop on the LED lamp string is fixed, the voltage on the input end of the detection and control circuit  30  may change with the change of the voltages V 2  in the filter circuit  20  and the positive pole of the LED lamp string, and the waveform of the voltage V 3  on the input end of the detection and control circuit  30  is consistent with the waveforms of the voltages V 2  in the filter circuit  20  and the positive pole of the LED lamp string. 
     The third end of the detection and control circuit  30  is connected with the second output end of the current-limiting circuit  10 . The detection and control circuit  30  is used to detect the current in the LED lamp string and control the current-limiting circuit  10  and the filter circuit  20  according to a detection result. 
     As shown in  FIG. 5 , a working principle of the current-limiting circuit  10  of an LED driving circuit according to an example of the present disclosure is described below. 
     The waveform of the voltage V 3  in the negative pole of the LED lamp string is same as the waveform of the voltage V 2  in the positive pole of the LED lamp string, the detection and control circuit  30  detects the voltage V 3  in the negative pole of the LED lamp string, and the voltage V 3  is referred to as a first detection voltage. The current-limiting circuit  10  may work under the following circumstances. 
     1. When the first detection voltage V 3  is smaller than a third set voltage V 33 , a first output current I 2  flowing out of the first output end of the current-limiting circuit  10  is a first constant current I 21 , the first constant current I 21  supplies a charge current of the filter circuit  20  and a current I 3  of the LED lamp string, the current I 3  flowing through the LED lamp string is controlled by the detection and control circuit  30 , no current is output from the second output end of the current-limiting circuit  10 , and a second output current I 5  is zero. 
     2. When the first detection voltage V 3  is greater than or equal to the third set voltage V 33  and smaller than a fourth set voltage V 34 , the first output current I 2  of the current-limiting circuit  10  is reduced with the increase of the first detection voltage V 3 ; at the same time, the second output current I 5  flowing from the current-limiting circuit  10  to the detection and control circuit  30  is increased with the increase of the first detection voltage V 3 . 
     3. When the first detection voltage V 3  is greater than or equal to the fourth set voltage V 34 , the first output current I 2  of the current-limiting circuit  10  remains as a second constant current I 22 , and the second output current I 5  of the current-limiting circuit  10  remains as a third constant current I 51 , at this time, the second constant current I 22  is a minimum current flowing from the current-limiting circuit  10  to the LED lamp string and the filter circuit  20 , and the third constant current I 51  is a maximum current flowing from the current-limiting circuit to the detection and control circuit  30 . 
     The magnitude of the second output current I 5  is far smaller than the magnitude of the first output current I 2 . 
     As shown in  FIG. 6 , a control situation of the detection and control circuit  30  of an LED driving circuit according to an example of the present disclosure is described below. 
     The detection and control circuit  30  detects the first detection voltage V 3  in the negative pole of the LED lamp string and controls the magnitude of the current I 3  flowing through the LED lamp string according to a magnitude of the first detection voltage V 3 . 
     1. When the first detection voltage V 3  is smaller than a first set voltage value V 31 , no current flows through the LED lamp string, and the first output current I 2  of the current-limiting circuit  10  is the first constant current I 21  used for charging a capacitor in the filter circuit  20 ; at this time, the charge current I 4  of the filter circuit  20  is the largest, and the second output current I 5  of the current-limiting circuit  10  is zero. 
     2. When the first detection voltage V 3  is equal to the first set voltage V 31 , the LED lamp string is conducted and there is a current flowing through the LED lamp string; at this time, the first output current I 2  flowing out of the current-limiting circuit  10  remains as the first constant current I 21 , where the first constant current I 21  is divided into two parts, one part of which is used for charging the filter circuit  20 , the other part flows through the LED lamp string for turning on the LED lamp string, and the second output current I 5  of the current-limiting circuit  10  continues to be zero. 
     3. When the first detection voltage V 3  is greater than the first set voltage V 31  and smaller than a second set voltage V 32 , the first output current I 2  flowing out of the current-limiting circuit  10  continues to remain as the first constant current I 21 . With the increase of the first detection voltage V 3 , the current I 3  flowing through the LED lamp string is increased, the charge current I 4  of the filter circuit  20  is reduced, and the second output current I 5  of the current-limiting circuit  10  continues to be zero. 
     4 When the first detection voltage V 3  is greater than or equal to the second set voltage V 32  and smaller than the third set voltage V 33 , the first output current I 2  flowing out of the current-limiting circuit  10  is the first constant current I 21 , and the current I 3  flowing through the LED lamp string under the control of the detection and control circuit  30  may be a fourth constant current I 32 ; at this time, the fluctuation of the first detection voltage V 3  may not cause the fluctuation of the current I 3 , and the second output current I 5  of the current-limiting circuit  10  continues to be zero. 
     5. When the first detection voltage V 3  is greater than or equal to the third set voltage V 33  and smaller than a fourth set voltage V 34 , the first output current I 2  flowing out of the current-limiting circuit  10  is reduced with the increase of the first detection voltage V 3 , at the same time, the detection and control circuit  10  draws the second output current I 5  from the current-limiting circuit  10 , the second output current I 5  flowing from the current-limiting circuit  10  to the detection and control circuit  30  is increased with the increase of the first detection voltage V 3 , the current I 3  flowing through the LED lamp string under the control of the detection and control circuit  30  remains as the fourth constant current I 32 , and the second output current I 5  is far smaller than the first output current I 2 . 
     6. When the first detection voltage V 3  is greater than or equal to the fourth set voltage V 34 , the first output current I 2  flowing out of the current-limiting circuit  10  remains as the second constant current I 22 , and the second output current I 5  flowing out of the current-limiting circuit remains as the third constant current I 51 , at this time, the second constant current I 22  is a minimum current flowing from the current-limiting circuit to the LED lamp string and the filter circuit  20 , the third constant current I 51  is a maximum current flowing from the current-limiting circuit to the detection and control circuit  30 , and the third constant current I 51  is far smaller than the second constant current I 22 . 
     7. When the fluctuation of the first detection voltage V 3  is small, the fluctuation of the output current of the current-limiting circuit  10  is also reduced; when the first detection voltage V 3  is a constant value, the first output current I 2  of the current-limiting circuit  10  is also a constant value correspondingly, thereby ensuring that the current I 3  on the LED lamp string is constant and the voltage drop on the detection and control circuit  30  is stable. 
     8. The first set voltage V 31  to the fourth set voltage V 34  of the first detection voltage V 3  may correspond to a first threshold voltage V 101  to a fourth threshold voltage V 104  of the output voltage of the rectifying circuit  40  respectively. 
     Waveform diagrams of output voltage V 2  and a first output current I 2  of a current-limiting circuit  10  of an LED driving circuit at different input voltages according to an example of the present disclosure are shown in  FIG. 7 ,  FIG. 8( a )  and  FIG. 8( b ) . 
     I. When an input voltage range is between a first threshold V 101  and a third threshold V 103 , as shown in  FIG. 7 , the maximum value of the input voltage is V 11  at this time. After passing through the rectifying circuit  40 , as shown in  FIG. 7 , V 1  refers to a voltage on the input end of the current-limiting circuit  10 , V 2  refers to a voltage on the output end of the current-limiting circuit  10 , I 2  refers to a first output current of the current-limiting circuit  10 , V 3  refers to a first detection voltage on an input end of the detection and control circuit  30 , I 3  refers to a current on the input end of the detection and control circuit  30 , and I 5  refers to a second output current of the current-limiting circuit  10 ; since the second output current I 5  is far smaller than the first output current I 2 , the currents shown in the same drawing, are not in a same scale and used only for convenience of understanding. 
     At a moment t 1 , the voltage on both ends of the LED lamp string is greater than a conduction voltage of the LED lamp string. When the LED lamp string is conducted, the first detection voltage V 3  is a minimum value and smaller than the second set voltage value V 32 , the current I 3  flowing into the LED lamp string is also the minimum value, but the first output current I 2  of the current-limiting circuit  10  is a maximum value. 
     With the increase of the first detection voltage V 3 , the current I 3  is also increased correspondingly, the first output current I 2  remains constant; at a moment t 11 , the first detection voltage V 3  is equal to the second set voltage value V 32 , the current I 3  is equal to the fourth constant current I 32 , and the first output current I 2  is equal to the first constant current I 21 , in this phase, the second output current I 5  does not flow from the current-limiting circuit  10  to the detection and control circuit  30 . 
     At a moment t 12 , the first detection voltage V 3  is equal to the third set voltage V 33 , the first detection voltage V 3  then continues to increase, the first output current I 2  begins to decrease, and the second output current I 5  flowing from the current limiting circuit  10  to the detection and control circuit  30  is increased with the increase of the first detection voltage V 3 . 
     At a moment t 3 , the voltage V 2  on the output end of the current-limiting circuit  10  is equal to a rectified input voltage V 21  of a power source, a capacitor in the filter circuit  20  begins discharging, the first output current I 2  of the current-limiting circuit  10  is zero, the magnitude of the current discharged by the capacitor is controlled by the detection and control circuit  30  and is a constant value, the voltage V 2  is linearly reduced, the first detection voltage V 3  is also linearly reduced, and the second output current I 5  is correspondingly reduced. At a moment t 31 , the first detection voltage V 3  is reduced to be equal to the third set voltage V 33 , at this time, the second output current I 5  is zero. At a moment close to t 5 , the voltage V 2  on the filter circuit  20  is reduced to be close to the minimum value, the first detection voltage V 3  is smaller than the second set voltage value V 32 , and the current I 3  begins to decrease from the constant value. At the moment t 5 , the first detection voltage V 3  is a minimum value, the current I 3  is also a minimum value, but the current I 2  flowing from the first output end of the current-limiting circuit  10  is a maximum value. 
     The above process may be repeated with the increase of time. 
     II, when the range of the input voltage of the power source is greater than the third threshold V 103 , as shown in  FIG. 8( a )  and  FIG. 8( b ) , after passing through the rectifying circuit, V 1  refers to a voltage on the input end of the current-limiting circuit  10 , V 2  refers to a voltage on the output end of the current-limiting circuit  10 , I 2  refers to the first output current of the current-limiting circuit  10 , V 3  refers to the first detection voltage on the input end of the detection and control circuit  30 , I 3  refers to the current on the input end of the detection and control circuit  30 , and I 5  refers to the second output current of the current-limiting circuit  10 . Since the second output current I 5  is far smaller than the first output current I 2 , the currents shown in the same drawing are not in the same scale and used only for convenience of understanding. 
     At a moment t 1 , the voltage on both ends of the LED lamp string is greater than the conduction voltage of the LED lamp string and the LED lamp string is conducted. At this time, the first detection voltage V 3  is a minimum value, which is greater than the third set voltage value V 33  and smaller than the fourth set voltage V 34 , the current I 3  flowing into the LED lamp string is the fourth constant current I 32 , the first output current I 2  of the current-limiting circuit  10  is a maximum value, and the second output current I 5  of the current-limiting circuit  10  is a minimum value. With the increase of the first detection voltage V 3 , the current I 3  remains constant, the current I 2  is gradually reduced, and the second output current I 5  of the current-limiting circuit  10  is correspondingly increased. 
     At a moment t 2 , the first detection voltage V 3  is equal to the fourth set voltage V 34 , the current I 3  remains constant, the first output current I 2  is the second constant current I 22 , and the second output current I 5  is the third constant current I 51 . Then, the voltage V 3  is increased, but the current I 3 , the first output current I 2  and the second output current I 5  all remain constant. 
     At a moment t 3 , the first detection voltage V 3  is same as the voltage passing through the rectifying circuit, the first output current I 2  of the current-limiting circuit  10  is zero, the filter circuit  20  stops charging and turns to supply the current I 3  for the LED lamp string. Since the discharge current is a constant value, the voltage on the filter circuit  20  is linearly reduced, the voltage V 3  is also linearly reduced. At a moment t 31 , the first detection voltage V 3  is smaller than the fourth set voltage V 34 , the current I 3  remains constant, and the second output current I 5  is reduced with the decrease of the first detection voltage V 3 , at the moment T 5 , the voltage V 3  and the second output current I 5  reach the minimum value. 
     Then, the above process may be repeated. 
     III, the waveform diagrams at different input voltages are drawn in a same drawing. As shown in  FIG. 9 , it is assumed that the detection voltage V 3  is between the voltage V 33  and the voltage V 34  at this time. 
     As can be seen from  FIG. 9 , time T 11  when the current-limiting circuit  10  supplies the first output current I 2  in the case that the input voltage of the power source is low (the highest voltage is V 11 ) is smaller than time T 1  when the current-limiting circuit  10  supplies the first output current I 2  in the case that the input voltage of the power source is high (the highest voltage is V 12 ) (the charging process is started again before the previous discharge is finished when the input voltage is high). That is, the charging time of the filter circuit  20  when the input voltage of the power source is low is smaller than the charging time of the filter circuit  20  when the input voltage of the power source is high, but discharging time T 21  of the filter circuit  20  when the input voltage of the power source is low is greater than discharging time T 2  of the filter circuit  20  when the input voltage of the power source is high. To maintain the voltage on the filter circuit basically unchanged when the input voltage of the power source changes and maintain the current on the LED lamp string constant, it is desired to reduce the input current when the input voltage of the power source is increased, so that the energy stored in the electrolytic capacitor in the filter circuit at different input voltages is same in each period, and the current-limiting circuit  10  controls the first output current by phases according to the different input voltages of the power source, thereby increasing an efficiency of the power source. 
     Meanwhile, when the input voltage of the power source is low, the electric energy stored in a filter capacitor in the filter circuit  20  is required to ensure that the current on the LED lamp string is uninterrupted during discharge, that is, the LED lamp string has no stroboflash During the discharge, the detection and control circuit  30  controls the current on the LED lamp string to decrease so as to prolong the discharging time of the filter capacitor. 
     Due to the existence of the current-limiting circuit  10 , the voltage V 22  on the filter circuit  20  when the input voltage of the power source is high does not change much compared with the voltage V 21  on the filter circuit  20  when the input voltage of the power source is low. 
     When the range of the input voltage of the power source is between the third threshold V 103  and the fourth threshold V 104 , the first output current I 2  of the current-limiting circuit  10  is a constant value and does not fluctuate with the fluctuation of the input voltage. However, with the increase of the input voltage of the power source, the first output current I 2  of the current-limiting circuit  10  is reduced with the increase of the input voltage of the power source. When the input voltage of the power source is as shown by  110  in the drawing (the maximum value is V 11 ), the first output current I 2  is as shown by a waveform I 24 . When the input voltage of the power source is as shown by  210  in the drawing (the maximum value is V 12 ), the first output current I 2  is as shown by a waveform I 25 . It can be seen that the first output current I 2  is correspondingly reduced when the input voltage of the power source is increased, but the current I 3  in, the LED lamp string remains as a constant value. 
     IV, specifically,  FIG. 10  illustrates an LED driving circuit according to a preferred example of the present disclosure. As shown in  FIG. 10 , the detection and control circuit  30  may include a control circuit  70  and a voltage-dividing circuit  60 . The voltage-dividing circuit  60  may include voltage-dividing resistors R 1  and R 2  which are connected in series. The second output end of the current-limiting circuit  10  is connected with a third end of the control circuit  70 , and the negative pole of the LED lamp string is connected with an input end of the control circuit  70  and an input end of the resistor R 1  The resistor R 1  is connected in series with the resistor R 2 , and a fourth end of the control circuit  70  is connected with an output end of the resistor R 1  and an input end of the resistor R 2  The negative pole of the rectifying circuit  40  is connected with a second end of the control circuit  70  and an output end of the resistor R 2 . 
     The numerical value of the second detection voltage V 4  may be changed by setting a numerical value of a voltage-dividing resistor in the voltage-dividing circuit, thereby changing a detectable range of the input voltage. In this way, the circuit can be adapted to different ranges of the input voltage of the power source. A voltage value of the voltage-dividing voltage V 4  (the second detection voltage) may be detected by performing voltage division for the first detection voltage V 3 , and the range of the voltage value of the second detection voltage may be changed by adjusting a resistance value of the resistor R 1 /R 2 , thereby adapting the circuit to different changes of the input voltage. 
       FIG. 11  is a waveform diagram illustrating the circuit of  FIG. 10  when an input voltage of a power source is greater than a second threshold and smaller than a fourth threshold according to an example of the present disclosure. 
     In  FIG. 11 , V 1  refers to a rectified voltage of the input voltage, V 2  refers to a voltage on the output end (i.e., the positive pole of the LED lamp string) of the current-limiting circuit  10 , V 3  refers to the first detection voltage having the same waveform as the voltage V 2 . After voltage division is performed by a resistor, the fluctuation of the second detection voltage V 4  is smaller than the fluctuation of the first detection voltage V 3 , the fluctuation of a signal fed back from the detection and control circuit  30  to the current limiting circuit  10  is correspondingly reduced, the fluctuation of the output current I 2  of the current-limiting circuit  10  is also correspondingly reduced, and the voltage drop on the detection and control circuit  30  remains stable. The current I 3  in the LED lamp string remains as a constant value. 
     The second detection voltage V 4  may be obtained by performing voltage division for the first detection voltage V 3 , and the second detection voltage V 4  has corresponding smaller voltage fluctuation, so that the first output current I 2  of the current-limiting circuit  10  has corresponding smaller voltage fluctuation. In this way, the current I 3  on the LED lamp string and the voltage drop on the detection and control circuit  30  are relatively stable. Further, different voltage division ratios may be correspondingly adapted to different input voltages. 
     V. It can be known from the above that the smaller the fluctuation of the first detection voltage V 3  is, the more stable the current output by the current-limiting circuit  10  becomes. To reduce the voltage fluctuation on the second detection voltage V 4 , a filter capacitor C 1  may be added on the fourth end of the control circuit  70 , and the stability of the second detection voltage V 4  may, in turn, affect the stability of the first output current I 2  and the second output current I 5  of the current-limiting circuit  10 . 
     A constant second detection voltage V 4  may be obtained by filtering the second detection voltage V 4 , so that the first output current I 2  of the current-limiting circuit  10  is also a constant value correspondingly, thereby ensuring that the current I 3  on the LED lamp string is constant and the voltage drop on the detection and control circuit is stable. 
       FIG. 12  is a schematic diagram illustrating an LED driving circuit according to a preferred example of the present disclosure. It can be seen from  FIG. 12  that a filter capacitor C 1  is added in a sampling point of the voltage-dividing circuit, and the capacitor C 1  is connected in parallel with the resistor R 2 . When the filter capacitor C 1  is capable of filtering out the voltage fluctuation on the second detection voltage V 4 , a ripple of the voltage V 4  may be neglected. The current waveform of the first output current I 2  at different input voltages may be as shown in  FIG. 13 . 
       FIG. 13  is a waveform diagram illustrating voltages V 2 /V 3  and currents I 2 /I 3  on both ends of an LED lamp string in the circuit of  FIG. 12  according to an example of the present disclosure. 
     In  FIG. 13 , V 1  refers to a voltage obtained by rectifying the input voltage of the power source, V 2  refers to a voltage on the output end of the current-limiting circuit  10 , V 3  refers to a first detection voltage on the negative pole of the LED lamp string, V 4  refers to a second detection voltage filtered again, I 2  refers to a first output current of the current-limiting circuit  10 , and I 3  refers to a current on the LED lamp string. 
     Since the ripple of the second detection voltage V 4  may be neglected, according to the second detection voltage V 4  being a constant value, it is determined that the first output current I 2  and the second output current I 5  of the current-limiting circuit  10  are also constant values, so that the voltage V 2  on the output end of the current-limiting circuit  10  is linearly increased or reduced, and the current I 3  in the LED lamp string is also a constant value. 
     The second detection voltage V 4  is a fifth constant value, and the second output current I 5  flowing out of the current-limiting circuit  10  is also a constant value, and is a sixth constant value. According to the principle of the current-limiting circuit  10 , the first output current I 2  of the current-limiting circuit  10  is also a constant value, that is, a seventh constant value, and the current I 3  in the LED lamp string is an eighth constant value. 
     When the input voltage is increased or reduced, the first detection voltage V 3  changes within all set ranges, the second detection voltage V 4  is a constant value and may correspondingly be moved up or down in parallel with the increase or decrease of the input voltage of the power source, and the constant value of the first output current I 2  of the current-limiting circuit  10  and the constant value of the current I 3  in the LED lamp string may also be correspondingly moved up or down. 
     In the solution of the present disclosure, the current-limiting circuit is provided on the output end of the rectifying circuit, the positive pole of the LED lamp string and the input end of the filter circuit, the detection and control circuit detects the voltage change in the negative pole of the LED lamp string at the time of controlling the current of LED lamp string, and controls the current-limiting circuit according to the voltage change in the negative pole of the LED lamp string. The current-limiting circuit controls the magnitude of the charge current of the filter circuit and the magnitude of the current in the LED lamp string according to the detected voltage change, which is detailed as follows. 
     When the input voltage of the power source is smaller than the third threshold V 103 , the current-limiting circuit is in an open state, the input voltage is applied to the filter circuit, the LED lamp string and the detection and control circuit, and the input current flows into the filter circuit and the LED lamp string; when the input voltage of the power source is greater than the third threshold V 103  and smaller than the fourth threshold V 104 , the output current of the current-limiting circuit is reduced, that is, the charge current I 4  of the filter circuit and the conduction current I 3  of the LED lamp string are reduced, and the voltage drop on the detection and control circuit is reduced; at this time, the input current I 1  of the power source is correspondingly reduced with the increase of the voltage of the power source. Since the output current I 2  of the current-limiting circuit  10  is approximately equal to the input current I 1  of the power source, the output current I 2  of the current-limiting circuit  10  is correspondingly reduced with the increase of the input voltage of the current-limiting circuit  10 , when the voltage of the power source is greater than the fourth threshold V 104 , the current-limiting circuit  10  maintains the charge current I 4  of the filter circuit and the conduction current I 2  of the LED lamp string as constant values. The input current I 1  of the power source is the smallest at this time. 
     It is to be noted that the first threshold, the second threshold, the third threshold and the fourth threshold in the present disclosure are all preset. 
     As shown in  FIG. 14 , the current-limiting circuit  10  may include a second power supply circuit  170 , a control and driving circuit  180 , a second driving circuit  190 , a second current-sampling circuit  300  and a power tube Q 2 , where the second driving circuit  190  is connected to the second power supply circuit  170 , the control and driving circuit  180 , the second current-sampling circuit  300  and a control end of the power tube Q 2 , and the second power supply circuit  170  is connected to the control and driving circuit  180 . 
     As shown in  FIG. 15 , the current-limiting circuit  10  is provided with a second over-temperature protection circuit  320  connected with the second driving circuit  190  for performing over-temperature protection on the power tube Q 2 . 
     As shown in  FIG. 16 , the control circuit  70  may include a first power supply circuit  110 , a reference circuit  120 , a first driving circuit  130 , a first current-sampling circuit  140 , a voltage-sampling circuit  150 , a pull-down current circuit  160  and a power tube Q 1 , where the reference circuit  120  is connected to the first power supply circuit  110  and the first driving circuit  130 , the first driving circuit  130  is connected to a control end of the power tube Q 1  and the first current-sampling circuit  140 , and the voltage-sampling circuit  150  is connected to the pull-down current circuit  160  and the first current-sampling circuit  140 . 
     As shown in  FIG. 17 , the control circuit  70  is provided with a first over-temperature protection circuit  310  connected with the first driving circuit  130  for performing over-temperature protection on the power tube Q 1 . 
     When the second detection voltage V 4  belongs to a first set range, the voltage-sampling circuit  150  sends a V 11  signal to the first current-sampling circuit  140 , and the first driving circuit  130  controls the conduction of the power tube Q 1  according to the V 11  signal and a sampling result of the first current-sampling circuit  140 , thereby controlling the magnitude of the current flowing through the LED lamp string. 
     When the second detection voltage V 4  belongs to a second set range, the voltage-sampling circuit  150  sends a V 12  signal to the pull-down current circuit  160 , the pull-down current circuit  160  outputs an ICS signal to the control and driving circuit  180  according to the V 12  signal, and the second driving circuit  190  controls the conduction of the power tube Q 2  according to the output signal of the control and driving circuit  180 , thereby controlling the input current of the power source. 
     The numerical value of the first set range is smaller than the numerical value of the second set range, or the numerical value of the first set range is partially overlapped with the numerical value of the second set range. 
     Preferably, the first set range is 0 Vdim 1.2V, and the second set range is 1.2V Vdim 2.4V. 
     When Vdim exceeds the second set range, the output of the pull-down current circuit  160  is maintained as a maximum value. 
     Although the present disclosure is described by the above examples, those skilled in the art may make a plurality of modifications and changes to the present disclosure without departing from the spirit of the present disclosure, and these modifications and changes should fall within the scope of the appended claims.