Abstract:
A high utilization AC-line input light emitting diode driver that can automatically transition to the most favorable configuration of the LEDs based on the instantaneous line voltage input.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This patent application claims priority to U.S. Provisional Patent Application No. 61/979,147 filed on Apr. 14, 2014 and entitled “High Utilization LED Driver”, the contents of which are incorporated herein by reference. 
     
    
     FIELD OF THE DISCLOSURE 
       [0002]    The present disclosure relates to providing a high utilization alternating current (AC)-line input light emitting diode (LED) driver, and more particularly, to providing a driver that automatically transitions to the most (or more) favorable configuration of the LEDs based on the instantaneous line voltage input. 
       BACKGROUND 
       [0003]    It is frequently desirable to power LEDs from the AC line. In North America, this is nominally 120VAC or 277VAC; in other parts of the world, this is nominally 240VAC. The actual line voltage may deviate from this nominal by +10% or more on a regular basis. 
         [0004]    LEDs typically have a forward voltage while conducting current of approximately 3V. This voltage varies somewhat as a function of the drive current and temperature, typically ±20%. However, LEDs, being diodes, need to be driven with a current rather than a voltage. For this reason, LEDs are frequently driven by switch-mode power supplies (SMPS), which convert the high-voltage AC line voltage to a low-voltage current. 
         [0005]    However, SMPS tend to be expensive, and may have relatively low lifetime compared with that of the LEDs they are driving. For this reason, some designs use a string of LEDs, with a sufficient number of LEDs in series in the string to present a forward voltage of approximately the line voltage. Some designs place the LED string directly across the AC line; however, since LEDs are unidirectional, the LEDs in this arrangement conduct only during half of each line cycle. Other designs first rectify the AC line and then apply the rectified voltage to the string of LEDs; in this arrangement, the LEDs conduct during both halves of the line cycle, thus providing double the light output of the first configuration. 
         [0006]    However, such designs suffer from a number of problems. One of these problems is the low utilization of the LEDs, which is to say, the amount of light produced per LED is relatively low. Since the string of LEDs has a forward voltage roughly comparable with the line voltage, the LEDs don&#39;t turn on at all until a substantial fraction of the peak line voltage is reached by the AC line. They are thus off for a significant fraction of the line cycle, resulting in less light output per LED than if they were on longer. Furthermore, since the LEDs are off for a significant fraction of the line cycle, line frequency flicker may be more noticeable with this system than if they were on longer. 
         [0007]    It would be desirable to have an AC drive circuit that conducts current through the LEDs for a larger fraction of the line cycle, to improve LED utilization and reduce flicker. It would also be desirable that it would be inexpensive and have a long lifetime. 
       SUMMARY 
       [0008]    Some embodiments described herein relate to an AC-line driver for LEDs, such that the above-described primary problem is effectively solved. An AC-line driver for LEDs can produce a certain current throughout a specified range of the instantaneous line voltage, and then re-configure to produce another certain current throughout another specified range of the instantaneous line voltage. It provides for high LED utilization and low flicker, and also provides for high efficiency, low cost and long lifetime. 
         [0009]    In some embodiments, a rectifier bridge and two sets of strings of LEDs can be included. The first set of strings can be connected from the output of the bridge, through a controllable element such as a transistor or a current sink, to ground. The second set of strings of LEDs can be connected through a transistor to the output of the bridge, and is then connected, either directly or through a controllable element such as a transistor or a current sink, to ground. The output of the first set of strings of LEDs is, in addition to being connected to a controllable element, also connected to a diode, and potentially also to additional components as described below, which in turn connects to the input of the second set of strings of LEDs. 
         [0010]    In such embodiments, while the instantaneous line voltage is in a first, lower, input range of the voltage, the controllable element for the first set of strings of LEDs is on, as is also the controllable element from the output of the bridge to the input to the second set of strings of LEDs. The controllable element for the second set of strings of LEDs, if present, is on in this configuration. In this configuration, both sets of strings of LEDs are connected in parallel to the output of the bridge, and are both powered on. In one embodiment, the strings of LEDs and the controllable elements are configured such that a specific current is produced in the first range of the instantaneous line voltage. 
         [0011]    While the instantaneous line voltage is in a second, higher, input range of the line voltage, the controllable element for the first set of strings of LEDs is off, as is also the controllable element from the output of the bridge to the input to the second set of strings of LEDs. The controllable element for the second set of strings of LEDs, if present, remains on in this configuration. In this configuration, the current from the bridge goes through the first set of strings of LEDs, then through the diode and additional components if present, and then through the second set of strings of LEDs, and thence through the controllable element, if present, to ground. In one embodiment for a 120VAC line input, the first input voltage range is 0-120V and the second input voltage range is 120V-168V. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]    The accompanying drawings are included to provide a further understanding, and are incorporated in and constitute a part of this specification. 
           [0013]      FIG. 1   a  is a system block diagram of an AC-line input LED circuit  10 , according to an embodiment. 
           [0014]      FIG. 1   b  is an example graph showing voltage as a function of time for the AC line shown in  FIG. 1   a.    
           [0015]      FIG. 2  is a diagram of an AC-line input LED circuit, in which two sets of strings of LEDs are configured to re-configure as the instantaneous line voltage from a AC source rises from one range to another, according to an embodiment. 
           [0016]      FIG. 3  is a diagram of the AC-line input LED circuit of  FIG. 2 , operating in a low range of the instantaneous line voltage. 
           [0017]      FIG. 4  is a diagram of the AC-line input LED circuit of  FIG. 2 , operating in a high range of the instantaneous line voltage. 
       
    
    
     DETAILED DESCRIPTION 
       [0018]    Reference will now be made in detail to various embodiments, examples of which are illustrated in the accompanying drawings. Wherever possible, the same reference numbers are used in the drawing and the description to refer to the same or like parts. 
         [0019]    According to the design characteristics, a detailed description of the embodiments is given below. 
         [0020]      FIG. 1   a  is a system block diagram of an AC-line input LED circuit  10 , according to an embodiment.  FIG. 1   b  is an example graph showing voltage as a function of time for the AC line shown in  FIG. 1   a . The LED circuit  10  includes an AC source  100 , controllable elements  35  and LED set  120 , which includes for example a first string of LEDs  121  and a second string of LEDs  122 . The LED set  120  can be re-configurable as a function of the input, and including configuration  40  (where the first string of LEDs  121  and second string of LEDs  122  are connected in parallel) and configuration  50  (where the first string of LEDs  121  and second string of LEDs  122  are connected in series). Although not shown, the LED circuit  10  can include a control system that is operatively coupled to the controllable elements  35 , and can select the state of each controllable element  35 . 
         [0021]    As shown in  FIG. 1   a , two sets of strings of LEDs  40  are configured to be in parallel when the instantaneous line voltage of AC source  100  is in a low range  20  (shown in  FIG. 1   b ), and the two sets of strings of LEDs  50  are configured to be in series when the instantaneous line voltage of AC source  100  is in a high range  30  (shown in  FIG. 1   b ). The instantaneous line voltage from AC source  100  can be rectified, for example, by a diode bridge (not shown in  FIG. 1 ). More specifically, when the instantaneous line voltage of AC source  100  is in a low range  20 , the first  121  and the second  122  of two sets of strings of LEDs are configured in parallel as configuration  40  and powered from the instantaneous line voltage of AC source  100 . When the instantaneous-line voltage of AC source  100  is in a high range  30 , the first  121  and the second  122  of two sets of strings of LEDs are configured in series as configuration  50  and powered from the instantaneous line voltage of AC source  100 . 
         [0022]      FIG. 2  is a diagram of an AC-line input LED circuit  110 , in which two sets of strings of LEDs  120  are configured to re-configure as the instantaneous -line voltage from AC source  100  rises from one range to another. As shown in  FIG. 2 , the instantaneous line voltage from AC source  100  is rectified by a diode bridge  130 . The output voltage of the diode bridge  130  is fed to the first  121  of the two sets of strings of LEDs  120 . This first  121  of the two sets of strings of LEDs  120  is connected through a transistor  140  to ground. In other embodiments, the transistor  140  may be replaced by a current sink. Such a current sink can be either a controllable current sink or a non-controllable current sink. The output voltage of the diode bridge  130  is also fed to the transistor  150 , and thence to the second  122  of the two sets of strings of LEDs  120 . The second  122  of the two sets of strings of LEDs  120  is connected through a transistor  160  to ground, although the transistor  160  need not be present in all cases. In other embodiments, the transistor  160  may be replaced by a current sink. Again, such a current sink can be either a controllable current sink or a non-controllable current sink. 
         [0023]    The connection of the first  121  of the two sets of strings of LEDs  120  to the transistor  140  is also connected to a diode  170 . The diode  170  is connected to a third set of strings of LEDs  180 , although this third set of strings of LEDs  180  may not be present in all cases. The third set of strings of LEDs  180  may instead be replaced or supplemented by one or more resistors and/or one or more zener diodes. The third set of strings of LEDs  180 , if present, or the one or more resistors and/or one or more zener diodes, if present, is then connected to the connection between the transistor  150  and the second  122  of the two sets of strings of LEDs  120 . If the third set of strings of LEDs  180  is not present, nor the one or more resistors and/or one or more zener diodes, then the diode  170  is instead connected directly to the connection between the transistor  150  and the second  122  of the two sets of strings of LEDs  120 . 
         [0024]      FIG. 3  is a diagram of the AC-line input LED circuit  110  of  FIG. 2 , operating from a low range of instantaneous line voltage. A control system (e.g., comparator  230 ) determines the range of the instantaneous line voltage and then controls the controllable elements based on the range of the instantaneous line voltage to re-configured the LED circuit  110 , as discussed below. 
         [0025]    As shown in  FIG. 3 , the output voltage of the diode bridge  130  is divided down by a resistor divider  210 . The divided down voltage is compared by a comparator  230  with a reference voltage  240 . Since the instantaneous line voltage is in the low range, the divided down voltage is lower than the reference voltage  240 , and thus the comparator  230  has an output  250  which is high. 
         [0026]    When the output  250  of the comparator  230  is high, all three transistors  140 ,  150 , and  160  if present, are in their ‘on’ state, shown as a closed switch. Transistor  140  connects the first  121  of the two sets of strings of LEDs  120  to ground, causing them to experience voltage equal to the line voltage and conduct current. Transistor  150  connects the output voltage of the bridge  130  to the input of the second  122  of the two sets of strings of LEDs  120 . Transistor  160  or a current sink, if present, connects the second  122  of the two sets of strings of LEDs  120  to ground. If transistor  160  or a current sink is not present, the second  122  of the two sets of strings of LEDs  120  may be connected directly to ground. As the second  122  of the two sets of strings of LEDs  120  is connected to the output of the bridge  130  and ground, through the transistor  150  which is on, they also experience voltage equal to the line voltage, and so they also conduct current. Since the diode  170  and the third set of strings of LEDs  180  and/or resistors and/or zener diodes has the output of the bridge  130  and ground applied across them, the diode  170  is reverse-biased, and is non-conducting in this situation. In this configuration, the two sets of strings of LEDs  120  are in parallel, thus producing the correct current in each string while the line voltage is in this lower voltage range. 
         [0027]      FIG. 4  is a diagram of the AC-line input LED circuit  110  of  FIG. 2 , operating from a high range of the instantaneous line voltage. As shown in  FIG. 4 , since the instantaneous line voltage is in the high range, the divided down voltage is higher than the reference voltage  240 , and thus the comparator  230  has an output  250  that is low. When the output  250  of the comparator  230  is low, the transistors  140  and  150  are in their ‘off’ state, shown as open switches, while transistor  160  or a current sink, if present, remains in its ‘on’ state, shown as a closed switch. In this condition, current flows through the first  121  of the two sets of strings of LEDs  120 , through the diode  170  and the third set of strings of LEDs  180  and/or resistors and/or zener diodes, and thence through the second  122  of the two sets of strings of LEDs  120 , and then through the transistor  160  or current sink, if present, which is connected to ground. In this configuration, the two sets of strings of LEDs  120  are in series, thus producing the correct current in each string while the line voltage is in this higher voltage range. 
         [0028]    Although the above-discussed embodiment is shown with two possible ranges of the instantaneous line voltage, any number of ranges is possible in other embodiments with an appropriately alternative control system(s). Similarly, although the above-discussed embodiment is shown with two possible LED set configurations, series and parallel, additional configurations are possible in other embodiments. For example, in such alternative embodiments, the additional configurations can include various combinations of LEDs connected in series and LEDs connected in parallel, effectively forming various possible hybrid configurations. Such additional possible hybrid configurations can be implemented, for example, with the alternative control system(s) having more than two ranges of instantaneous line voltage. 
         [0029]    It will be apparent that various modifications and variation can be made to the disclosed embodiments. In view of the foregoing, it is intended that the disclosed embodiments cover modifications and variations.