Abstract:
A system including a switch configured to supply power to a load. A first comparator is configured to compare a first current through the switch to a first threshold. A second comparator is configured to compare the first current through the switch to a second threshold. The second threshold is greater than the first threshold. A current control module is configured to turn off the switch (i) for a first duration in response to the first current through the switch being greater than or equal to the first threshold and (ii) for a second duration in response to the first current through the switch being greater than or equal to the second threshold. The current control module is configured to adjust the second duration based on a difference between an estimated current through the load and a desired current through the load.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the benefit of U.S. Provisional Application No. 61/451,003, filed on Mar. 9, 2011, and U.S. Provisional Application No. 61/454,442, filed on Mar. 18, 2011. The disclosures of the above applications are incorporated herein by reference in their entirety. 
     
    
     FIELD 
       [0002]    The present disclosure relates generally to power supplies and more particularly to controlling output current of flyback voltage converters supplied to LED-based illumination systems. 
       BACKGROUND 
       [0003]    The background description provided herein is for the purpose of generally presenting the context of the disclosure. Work of the presently named inventors, to the extent the work is described in this background section, as well as aspects of the description that may not otherwise qualify as prior art at the time of filing, are neither expressly nor impliedly admitted as prior art against the present disclosure. 
         [0004]    Flyback voltage converters are used to convert AC line voltage into DC voltage. The DC voltage can be supplied to systems including LED-based illumination systems. The output of the flyback voltage converters can be sensitive to line voltage imbalances and can have large total harmonic distortion (THD). For example, the line voltage can become unbalanced due to unequal system impedances and/or unequal distribution of single-phase loads. Unbalanced line voltage can cause flicker in the light output by LEDs in an LED-based illumination system. Further, the THD can cause transformer heating, secondary voltage distortion, increased power losses, interference with communication systems, and so on. 
       SUMMARY 
       [0005]    A system includes a switch configured to supply power to a load in response to the switch being turned on by pulse width modulation pulses. A first comparator is configured to compare a first current through the switch to a first threshold. A second comparator is configured to compare the first current through the switch to a second threshold. The second threshold is greater than the first threshold. A current control module is configured to turn off the switch (i) for a first duration in response to the first current through the switch being greater than or equal to the first threshold and (ii) for a second duration in response to the first current through the switch being greater than or equal to the second threshold. The current control module is configured to adjust the second duration based on a difference between an estimated current through the load and a desired current through the load. 
         [0006]    In other features, the power is generated based on an AC voltage, and the adjustment of the second duration prevents variation of a second current through the load due to an imbalance in the AC voltage. 
         [0007]    In other features, the power is generated based on an AC voltage having a predetermined period, and the current control module is configured to reduce a total harmonic distortion in a second current through the load by limiting the second duration to (i) greater than or equal to one-fifth of half the predetermined period and (ii) less than or equal to one-third of half the predetermined period. 
         [0008]    In still other features, a method includes supplying power to a load through a switch in response to the switch being turned on by pulse width modulation pulses. The method further includes comparing a first current through the switch to a first threshold and comparing the first current through the switch to a second threshold. The second threshold is greater than the first threshold. The method further includes turning off the switch for a first duration in response to the first current through the switch being greater than or equal to the first threshold. The method further includes turning off the switch for a second duration in response to the first current through the switch being greater than or equal to the second threshold. The method further includes adjusting the second duration based on a difference between an estimated current through the load and a desired current through the load. 
         [0009]    In other features, the method further includes generating the power based on an AC voltage, and preventing variation of a second current through the load due to an imbalance in the AC voltage by adjusting the second duration. 
         [0010]    In other features, the method further includes generating the power based on an AC voltage having a predetermined period, and reducing a total harmonic distortion in a second current through the load by limiting the second duration to (i) greater than or equal to one-fifth of half the predetermined period and (ii) less than or equal to one-third of half the predetermined period. 
         [0011]    Further areas of applicability of the present disclosure will become apparent from the detailed description, the claims and the drawings. The detailed description and specific examples are intended for purposes of illustration only and are not intended to limit the scope of the disclosure. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0012]    The present disclosure will become more fully understood from the detailed description and the accompanying drawings, wherein: 
           [0013]      FIG. 1A  depicts a flyback voltage converter according to the present disclosure; 
           [0014]      FIG. 1B  depicts a half cycle of an AC line voltage and windows for controlling current through a load according to the present disclosure; 
           [0015]      FIG. 2  depicts a circuit for controlling current through LEDs according to the present disclosure; and 
           [0016]      FIG. 3  is a flowchart of a method for controlling current through LEDs according to the present disclosure. 
       
    
    
     DESCRIPTION 
       [0017]    The present disclosure relates to controlling output current of flyback voltage converters. The flyback voltage converters according to the present disclosure output a constant current despite imbalances in line voltage. Specifically, the output current is maintained at a desired value by providing a fixed amount of current and a variable amount of current. The variable amount of current is controlled within a predetermined range to reduce the total harmonic distortion (THD) in the output current to less than a predetermined level. 
         [0018]    Referring now to  FIGS. 1A and 1B , a flyback voltage converter  100  according to the present disclosure is shown. In  FIG. 1A , the flyback voltage converter  100  includes a bridge rectifier  102  that rectifies AC line voltage into a DC voltage. A flyback transformer  104  couples the output of the bridge rectifier  102  to a load  106 . A switch  108  (e.g. a MOSFET) controls the current supplied to the load  106  by the flyback transformer  104 . 
         [0019]    A current control module  110  controls the switch  108 . The current control module  110  includes a pulse width modulation (PWM) controller  112  that generates PWM pulses to control the switch  108 . The current through the switch  108  represents the current supplied to the load  106 . An amplifier  114  amplifies the current through the switch  108 . The output of the amplifier  114  is input to a first comparator  116  and a second comparator  118 . The first comparator  116  compares the output of the amplifier  114  to a first threshold (Th 1 ), which is set to a first peak current value I peak1  shown in  FIG. 1B . The second comparator  118  compares the output of the amplifier  114  to a second threshold (Th 2 ), which is set to a second peak current value I peak2  shown in  FIG. 1B . The second peak current value I peak2  is greater than the first peak current value I peak1  as shown in  FIG. 1B . 
         [0020]    The current control module  110  controls the current supplied to the load  106  by varying the width of the variable window shown in  FIG. 1B  as follows. As the AC line voltage increases, the current through the switch  108 , which represents the current through the load  106 , increases. The first comparator  116  outputs a first control signal when the current through the switch  108  becomes greater than or equal to the first peak current value I peak1 . The second comparator  118  outputs a second control signal when the current through the switch  108  becomes greater than or equal to the second peak current value I peak2 . The first control signal and the second control signal are input to a multiplexer  120 . The multiplexer  120  is controlled by a window control module  126 . The output of the multiplexer  120  is used by the PWM controller  112  to turn off the switch  108 . 
         [0021]    The window control module  126  controls the output of the multiplexer  120  as follows. A current estimating module  122  estimates the current supplied to the load  106 , I avg , based on the output of the amplifier  114 . A difference generator  123  generates a difference, I err , between the estimated current supplied to the load  106 , I avg , and a reference current I ref . The reference current I ref  represents the desired current through the load  106 . The difference is input to a proportional integral (PI) controller  124 . 
         [0022]    Based on the difference I err , the window control module  126  selects the first control signal output by the first comparator  116  or the second control signal output by the second comparator  118 . Additionally, based on the difference, the window control module  126  determines the duration for which the second control signal output by the second comparator  118  is selected (i.e., the duration of the variable window shown in  FIG. 1B ). The PWM controller  112  turns off the switch  108  based on the first control signal or the second control signal selected by the window control module  126 . 
         [0023]    Specifically, when the estimated current supplied to the load  106 , I avg , is less than the reference current I ref , based on the difference I err , the window control module  126  increases the duration of the variable window shown in  FIG. 1B  by increasing the duration for which the output of the second comparator  118  is selected. Accordingly, the switch  108  is turned off when the current through the switch  108  becomes greater than or equal to the second peak current value I peak2  and not when the current through the switch  108  becomes greater than or equal to the first peak current value I peak1 . Since the turn-off threshold for the switch  108  is increased for a longer duration, more power (i.e., more current) is delivered to the load  106 . 
         [0024]    Conversely, when the estimated current supplied to the load  106 , I avg , is greater than the reference current I ref , based on the difference I err , the window control module  126  decreases the duration of the variable window shown in  FIG. 1B  by decreasing the duration for which the output of the second comparator  118  is selected. Accordingly, the switch  108  is turned off when the current through the switch  108  becomes greater than or equal to the second peak current value I peak2  for a shorter duration. Since the turn-off threshold for the switch  108  is increased for a shorter duration, less power (i.e., less current) is delivered to the load  106 . Thus, the current through the load  106  is maintained at the reference current I ref  irrespective of variation in the DC voltage caused by unbalanced AC line voltage. 
         [0025]    Additionally, the total harmonic distortion (THD) in the current through the load  106  can be reduced by controlling the duration of the variable window (i.e., the duration for which the second control signal output by the second comparator  118  is selected). For example, the THD can be reduced to less than 20% by maintaining the duration of the variable window between ⅕ th  and ⅓ rd  of half the period (T half /2) of the AC line voltage. 
         [0026]    Referring now to  FIG. 2 , a circuit for controlling current through LEDs according to the present disclosure is shown. The circuit includes all of the components shown in  FIG. 1A . The circuit includes a TRIAC for dimming the LEDs. The circuit includes an integrated circuit  150  that includes the current control module  110 . The integrated circuit  150  controls the TRIAC. In the circuit, PSVR indicates primary side voltage regulation, OCP/CS indicates over-current protection/current sensing, and N p  and N s  respectively indicate the number of turns of the primary and second windings of the flyback transformer  104 . While the load  106  is shown to include only four LEDs, the load  106  may include any number of LEDs. 
         [0027]    Referring now to  FIG. 3 , a method  200  for controlling LED current according to the present disclosure is shown. At  202 , control compares the current through the switch  108  to a first threshold. At  204 , control compares the current through the switch  108  to a second threshold. At  206 , control determines whether an estimated LED current is greater than a reference current (i.e., a desired current). At  208 , if the estimated LED current is greater than the reference current, control decreases the variable window to reduce power (and current) delivered to the LEDs. At  210 , control determines if the window size is less than or equal to ⅕ th  of half the period (T half /2) of the AC line voltage. Control returns to  202  if the window size is not less than or equal to 115 th  of half the period (T half /2) of the AC line voltage. Control ends if the window size is less than or equal to ⅕ th  of half the period (T half /2) of the AC line voltage. 
         [0028]    At  212 , if the estimated LED current is less than the reference current, control increases the variable window to increase power (and current) delivered to the LEDs. At  214 , control determines if the window size is greater than or equal to ⅓ rd  of half the period (T half /2) of the AC line voltage. Control returns to  202  if the window size is not greater than or equal to ⅓ rd  of half the period (T half /2) of the AC line voltage. Control ends if the window size is greater than or equal to ⅓ rd  of half the period (T half /2) of the AC line voltage. Thus, the current delivered to the LEDs is maintained at the reference current irrespective of variation in the AC line voltage. Additionally, the THD is reduced to less than 20% by maintaining the duration of the variable window between ⅕ th  and ⅓ rd  of half the period (T half /2) of the AC line voltage. 
         [0029]    The foregoing description is merely illustrative in nature and is in no way intended to limit the disclosure, its application, or uses. The broad teachings of the disclosure can be implemented in a variety of forms. Therefore, while this disclosure includes particular examples, the true scope of the disclosure should not be so limited since other modifications will become apparent upon a study of the drawings, the specification, and the following claims. For purposes of clarity, the same reference numbers will be used in the drawings to identify similar elements. As used herein, the phrase at least one of A, B, and C should be construed to mean a logical (A or B or C), using a non-exclusive logical OR. It should be understood that one or more steps within a method may be executed in different order (or concurrently) without altering the principles of the present disclosure. 
         [0030]    As used herein, the term module may refer to, be part of, or include an Application Specific Integrated Circuit (ASIC); an electronic circuit; a combinational logic circuit; a field programmable gate array (FPGA); a processor (shared, dedicated, or group) that executes code; other suitable hardware components that provide the described functionality; or a combination of some or all of the above, such as in a system-on-chip. The term module may include memory (shared, dedicated, or group) that stores code executed by the processor. 
         [0031]    The term code, as used above, may include software, firmware, and/or microcode, and may refer to programs, routines, functions, classes, and/or objects. The term shared, as used above, means that some or all code from multiple modules may be executed using a single (shared) processor. In addition, some or all code from multiple modules may be stored by a single (shared) memory. The term group, as used above, means that some or all code from a single module may be executed using a group of processors. In addition, some or all code from a single module may be stored using a group of memories. 
         [0032]    The apparatuses and methods described herein may be implemented by one or more computer programs executed by one or more processors. The computer programs include processor-executable instructions that are stored on a non-transitory tangible computer readable medium. The computer programs may also include stored data. Non-limiting examples of the non-transitory tangible computer readable medium are nonvolatile memory, magnetic storage, and optical storage.