Abstract:
A current ripple canceling light-emitting diode (LED) driver is disclosed. The input current source contains a current ripple. The LED load is connected to the drain of a power switch. The source of the power switch is connected to a current sensing resistor. The gate of the power switch is connected to the output of an operational amplifier. The operational amplifier compares the voltage signal across the current sensing resistor with a dynamic reference voltage. The dynamic reference voltage is adjusted according to the gate or drain voltage of the power switch. The LED load current is controlled to be a nearly no ripple DC current.

Description:
CROSS-REFERENCE TO RELATED DOCUMENT 
       [0001]    The present application claims priority from CN 201310218482.3 filed on Jun. 4, 2013 the disclosure of which is hereby incorporated herein by reference. 
       TECHNICAL FIELD 
       [0002]    The present disclosure relates generally to Light-Emitting Diode (LED) supply and control circuits, and more specifically to current ripple canceling LED supply and control circuits. The first embodiment is designed for the second stage of an Active Power Factor Correction (APFC) LED driver. 
       BACKGROUND INFORMATION 
       [0003]      FIG. 1  (Prior Art) is a diagram of one traditional LED driver circuit. An Active Power Factor Correction (APFC) LED driver  10  provides a constant current output for the LED load. The output current of the APFC LED driver  10  contains a ripple current, while its average current is regulated and kept constant. 
         [0004]    The ripple current is usually twice the input AC line frequency, for example, if the line frequency is 60 Hz, the constant current output of APFC LED driver  10  contains a 120 Hz current ripple. 
         [0005]    A filtering capacitor C 1  filters the output current of the APFC LED driver  10  and reduces the ripple current in the LED load. However, since the ripple current frequency is low (120 Hz), even with a large size capacitor, the LED load current still contains a ripple current of 120 Hz frequency. 
         [0006]    Since the LED load current contains a line frequency ripple, the luminance output of the LED lamp also contains a line frequency flicker. The line frequency flicker may interfere with video equipment such as cameras and video recorders. 
       SUMMARY OF THE INVENTION 
       [0007]    A current ripple canceling light-emitting diode (LED) driver is disclosed. The input current source contains a current ripple. The LED load is connected to the drain of a power switch. The source of the power switch is connected a current sensing resistor. The gate of the power switch is connected to the output of an operational amplifier. The operational amplifier compares the voltage signal across the current sensing resistor with a dynamic reference voltage. The dynamic reference voltage is adjusted according to the gate or drain voltage of the power switch. The LED load current is controlled to be a nearly no ripple DC current. 
         [0008]    Other structures and methods are described in the detailed description below. This summary does not purport to define the invention. The invention is defined by the claims. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]    The accompanying drawings, where like numerals indicate like components, illustrate embodiments of the invention. 
           [0010]      FIG. 1  (Prior Art) is a diagram of one traditional constant current LED driver circuit. 
           [0011]      FIG. 2  is a diagram of a first embodiment of a current ripple canceling LED driver in accordance with the novel aspect. 
           [0012]      FIG. 3  is a waveform diagram that illustrates the operation of the current ripple canceling LED driver of  FIG. 2   
           [0013]      FIG. 4  is a diagram of a second embodiment of the current ripple canceling LED driver in accordance with the novel aspect. 
           [0014]      FIG. 5  is a diagram of a flyback structure embodiment of the current ripple canceling LED driver 
           [0015]      FIG. 6  is a diagram of a high side buck structure embodiment of the current ripple canceling LED driver 
           [0016]      FIG. 7  is a diagram of a low side buck structure embodiment of the current ripple canceling LED driver 
           [0017]      FIG. 8  is a diagram of a buck-boost structure embodiment of the current ripple canceling LED driver 
       
    
    
     DETAILED DESCRIPTION 
       [0018]      FIG. 2  is a circuit diagram of a current ripple canceling LED driver in accordance with a first embodiment. In the embodiment, the input current source comes from a first stage LED driver, such as an Active Power Factor Correction (APFC) converter. The first stage LED driver delivers a constant input current to the current ripple canceling LED driver. The input current source contains a current ripple that need to be eliminated in the LED load by the disclosed circuits. 
         [0019]    A filtering capacitor C 1  is implemented. The filtering capacitor C 1  is connected between the input current source and the system ground. The positive terminal of the LED load is connected to the positive node of the filtering capacitor C 1 . The LED load could be a number of series-connected or parallel-connected LEDs. 
         [0020]    A power switch M 1  is implemented. The drain D of the power switch M 1  is connected to the negative terminal of the LED load. The power switch M 1  could be a Metal-Oxide-Semiconductor Field-Effect Transistor (MOSFET) or a Bipolar Junction Transistor (BJT). Although the MOSFET device is illustrated here, it is appreciated that other types of transistors may be used as well. 
         [0021]    A current sensing resistor R 1  is implemented. The source S of the power switch M 1  is connected to the current sensing resistor R 1 . The current sensing resistor R 1  senses the information of the LED load current. The voltage across the current sensing resistor is proportional to the LED load current. 
         [0022]    An operational amplifier  20  is implemented. The negative input of the operational amplifier  20  is connected to the current sensing resistor R 1 . The positive input of the operational amplifier  20  is connected to a dynamic reference voltage REF. The output of the operational amplifier  20  is connected to the gate G of the power switch M 1 . 
         [0023]    A comparator  30  is implemented. One input terminal of the comparator  30  is connected to the gate G of the power switch M 1 . Another input terminal of the comparator  30  is connected to a threshold voltage V 1 . The output of the comparator is connected to a discharge switch S 1 . 
         [0024]    A dynamic reference generating circuit is implemented. The discharge switch S 1  is in series with a discharge circuit R 2 . An integrating capacitor C 2  is connected to the discharge switch S 1  and the discharge circuit R 2 . A charge circuit  40  is connected to the integrating capacitor C 2 . The charge circuit  40  is usually a current source or a resistor. The voltage on the integrating capacitor C 2  is scaled by the proportional convertor  50 . The output of the proportional convertor  50  is the dynamic reference voltage REF and is fed into the positive input of the operational amplifier  20 . 
         [0025]      FIG. 3  is a waveform diagram that illustrates the operation of the current ripple canceling LED driver. The input current source contains a current ripple. For example, the first stage APFC LED driver delivers a current source containing a current ripple with twice of the AC line frequency. The filtering capacitor C 1  stores the ripple current of the input current source and a ripple voltage is established on the filtering capacitor C 1 . 
         [0026]    The current in the LED load flows into the drain D of the power switch M 1  and flows out from the source S of the power switch M 1 . The current in the current sensing resistor R 1  is equal to the current in the LED. The voltage CS on the current sensing resistor is proportional to the LED load current. The operational amplifier  20  compares the voltage CS and the dynamic reference voltage REF. The output of the operational amplifier  20  controls the gate G voltage of the power switch M 1 . 
         [0027]    Since the LED load voltage drop is almost constant, the drain D voltage of the power switch M 1  also contains a voltage ripple. When the drain D voltage of the power switch M 1  is higher than the dynamic reference voltage REF, the LED load current is closed loop regulated. The LED load current is a flat shape current without ripple. The LED load current is 
         [0000]    
       
         
           
             
               
                 I 
                 LED 
               
               = 
               
                 
                   V 
                   ref 
                 
                 
                   R 
                   CS 
                 
               
             
             , 
           
         
       
     
         [0000]    where V ref  is the dynamic reference voltage and R Cs  is the value of the current sensing resistor R 1 . When the drain D voltage is lower than the dynamic reference voltage REF, the above loop cannot be closed, the output of the operation amplifier  20  will saturate, the gate G voltage of the power switch M 1  will increase, and the LED load current will be less than 
         [0000]    
       
         
           
             
               
                 V 
                 ref 
               
               
                 R 
                 CS 
               
             
             . 
           
         
       
     
         [0028]    The gate G voltage of the power switch M 1  is fed into the comparator  30 . The comparator  30  compares the gate G voltage of the power switch M 1  with the threshold voltage V 1 . When the gate G voltage is higher than the threshold voltage V 1 , it means the LED load current is less than 
         [0000]    
       
         
           
             
               
                 V 
                 ref 
               
               
                 R 
                 CS 
               
             
             , 
           
         
       
     
         [0000]    and it indicates that the LED load current is no longer flat and current ripple occurs. At this time, the comparator  30  turns on the discharge switch S 1 , the integrating capacitor C 2  voltage and dynamic reference voltage REF decrease, and the LED load current is reduced. Since the average value of the input current of the LED driver is a constant, when the LED load current is reduced, the average input current is higher than the LED load current, and the average voltage of the filtering capacitor and the average voltage of drain D of the power switch M 1  increases. 
         [0029]    When the average voltage of drain D of the power switch M 1  is higher, the on-time of the discharge switch S 1  is reduced, and the average discharge current of the integrating capacitor C 2  is reduced. Thereafter, the integrating capacitor C 2  voltage, dynamic reference voltage REF, and LED load current increases. When the average LED load current is higher than the average input current, the average voltage of filtering capacitor C 1  decreases, and the average voltage of drain D is reduced. A slow voltage loop is closed resulting in the average LED load current to be equal to the average input current, and resulting in the drain D voltage of the power switch M 1  to be higher than the dynamic reference voltage REF for a majority of the time. The LED load current is an almost flat waveform, and the drain D voltage of the power switch M 1  is as low as possible, minimizing the power loss on the power switch M 1 . 
         [0030]    In other words, there is a fast current loop and a slow voltage loop. The fast current loop is formed by the operational amplifier  20 , current sensing resistor R 1  and power switch M 1  resulting in a flat LED load currentwhen drain D voltage of the power switch is sufficient. The slow voltage loop is formed by the comparator  30 , discharge switch S 1 , discharge circuit R 2 , charge circuit  40 , integrating capacitor C 2  and proportional convertor  50 , allowing the drain D voltage of the power switch to be sufficient for a majority of the time. 
         [0031]    In the embodiments, the charge circuit  40  could be a current sourceor a resistor. Discharge switch S 1  could be a transistor or a controlled current source. Discharge circuit R 2  could be a resistor or a current source. The place of the discharge switch S 1  and the discharge circuit R 2  can be interchanged. The proportional convertor  50  can also be omitted. 
         [0032]      FIG. 4  is a diagram of a second embodiment of the disclosed current ripple canceling LED driver. The difference between the second embodiment and first embodiment is that the input signal of the comparator  30  is changed to the drain D voltage of the power switch M 1 . When the drain D voltage is lower than the threshold V 1 , the dynamic reference voltage REF and LED load current is reduced, thereby increasing the average voltage of the filtering capacitor C 1  and increasing the average voltage of the drain D of the power switch M 1 . The voltage loop can be closed to make the LED load current almost flat and to minimize the power loss of the power switch M 1 . 
         [0033]      FIG. 5  is the flyback structure embodiment of the disclosed current ripple canceling LED driver together with the first stage flyback convertor  15 . 
         [0034]      FIG. 6  is the high side buck structure embodiment of the disclosed current ripple canceling LED driver together with the first stage high side buck convertor  16 . 
         [0035]      FIG. 7  is the low side buck structure embodiment of the disclosed current ripple canceling LED driver together with the first stage low side buck convertor  17 . 
         [0036]      FIG. 8  is the buck-boost structure embodiment of the disclosed current ripple canceling LED driver together with the first stage buck-boost convertor  18 . 
         [0037]    Although specific embodiments have been illustrated and described, it will be appreciated by those skilled in the art that various modifications may be made without departing from the scope of the present invention.