Abstract:
A light engine driven directly from the AC power line has multiple series connected arrays of LEDs each with an associated current limiting transistor. The current from each current limiting transistor goes through a corresponding current sensing resistor and all these resistors are connected in series. The voltage across each current limiting transistor is applied across the next LED array so that as the voltage increases during the power line cycle, the next array becomes activated by the increasing voltage across the previous current limiting transistor. When this happens the previous current limiting transistor is turned off. This continues until all the arrays are activated at the peak of the line, at which point the array current is controlled by the last current limiting transistor.

Description:
[0001]    The present invention claims priority from and is a non-provisional of U.S. provisional application No. 62/194,033 entitled AC Led Driver In Constant Current Mode by inventors Thomas O&#39;Neil and Lee Chiang, filed Jul. 17, 2015 the disclosure of which is incorporated herein by reference. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The field of the present invention is direct AC powered LED light engines in which the LEDs are all in series and are driven with a nearly constant current. 
       BACKGROUND TO THE INVENTION 
       [0003]    It has become conventional in LED light engines which are to be driven directly from the rectified AC power line to arrange all the LEDs in series so that their forward voltage is just a little less than the peak of the AC line voltage. The current which passes may be limited by resistors or analog circuit current limiters. Such a circuit will only pass current at the peak of the AC power line, leading to the product having a poor power factor and high current harmonic distortion. This situation is routinely improved by incorporating switches so that when the AC power line voltage is less than its peak value, closing a switch shorts out some of the LEDs so that a lesser forward voltage is presented to the power line, allowing current to flow during a larger fraction of the power line voltage cycle and thus improving the power factor and total harmonic distortion (THD). 
         [0004]    Obviously, this effect can be improved upon by dividing the LED string into multiple subsections so that almost continuous current flow can take place through the LEDs. In practice, three or four subsections of the LED strings are routinely used to piece together an AC line current waveform which is an approximation to a sine wave. This general principle is widely used. A representative example of the prior art is US patent application 2013/0026924 by Jong published Jan. 31, 2013, entitled LED Driving Circuit Package, the disclosure of which is incorporated herein by reference. Jong&#39;s block diagram is shown as prior art  FIG. 1 . The LED string is divided into four subsections, and a switchable current source is provided for each LED subsection. All the switchable current sources bypass the current directly back to the negative terminal of the bridge rectifier. A voltage detection circuit determines which part of the line voltage cycle is current, and then current sensing logic uses this information to determine when to operate the current limiters on and off. The disadvantage of this circuit is that it requires an elaborate digital current comparison circuit which turns the current limiters on and off at the right time. 
         [0005]    Another example of a light engine in which switches are applied across LEDs to match the LED voltage to the varying line voltage is U.S. Pat. No. 8,373,363 by Gradjcar entitled Reduction of Harmonic Distortion For LED Loads, issued Feb. 12, 2013, the disclosure of which is incorporated herein by reference. See for example FIG. 39 of Gradjcar. The Gradjcar circuit senses the current passing through the LED string, and turns the bypass switches off one by one as the current rises until all the LEDs are simply connected across the power line at the peak of the line voltage cycle. The problem with Gradjcar&#39;s approach is that there is almost no current limiting, so that if the power line voltage increases the LED current will increase rapidly and without limit. 
         [0006]    In US patent application 20120081009 by Shteynberg, entitled Apparatus, Method and System for Providing AC Line Power to Lighting Devices issued Apr. 5, 2012, the disclosure of which is incorporated herein by reference. Shteynberg introduces current limiting, which renders the circuit capable of coping with varying voltage levels, however each LED segment has its own independent current sensing resistor and all these resistors are connected in parallel back to the negative terminal of the bridge rectifier. Without any interaction, each of these channels acts independently and so a complex digital control circuit is needed to orchestrate which switch turns on at which current. China invention patent CN 103188848A by Wang, Qin Heng entitled Segmented Linear Constant Current Light Emitting Diode (LED) Driving Circuit published Jul. 3, 2013, the disclosure of which is incorporated herein by reference, describes a multi element LED string light engine with switches and mentions constant current operation, but does not show any connection between the switches and the negative terminal of the bridge rectifier. 
         [0007]    From the foregoing it is apparent that there is a need for an AC LED light engine which has simple analog current limiters which are applied to each LED segment in turn and operate to inherently draw a sinusoidal current from the AC power line without using any complex digital circuitry to switch the current limiters on and off. Such a light engine will generate essentially no emi/rfi, and will be low in cost. 
       SUMMARY OF THE INVENTION 
       [0008]    A light engine is described which uses a bridge rectifier to convert the AC power line voltage into a series of unidirectional half sine wave voltage pulses. Connected to the positive output terminal of the bridge rectifier is a string of LEDs, all connected anode to cathode, which are subdivided into multiple substrings and each substring has an analog current limiter connected between its cathode and the end of a string of current sense resistors. The string of current sense resistors is itself subdivided into substrings, all connected end to end, with the final resistor connected back to the negative terminal of the bridge rectifier. Each sense resistor substring controls the current through a corresponding analog current limiter. The placing of these sense resistors in series is an important feature of the invention because when each subsequent LED substring commences conducting, its current also goes through the sense resistor corresponding to the previous substring. This excessive current thus forces the current limiter for the previous substring to shut off completely. In this way each current limiter in turn is enabled and then automatically shut off as its successor is activated. Finally at the peak of the AC line voltage waveform only the current limiter for the last LED segment is operating and is regulating the current through the whole string. As the line voltage waveform declines from the peak, the situation reverses with each current limiter first operating at constant current and then switching to full on until at the line voltage zero crossing all the current limiters are fully on ready for the next half cycle. This arrangement is advantageous because the current through the LEDs is constant, avoiding extreme currents associated with voltage surges. Also the control circuitry is minimal and simplistic comprising only one quad op amp chip. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]      FIG. 1  is a typical prior art light engine using a plurality of LED strings each with a switch and a constant current regulator. 
           [0010]      FIG. 2  is a diagram showing a complete power line cycle for the operation of a light engine having four LED strings each with a switchable constant current drive associated. 
           [0011]      FIG. 3  is a first embodiment of a light engine according to the present invention. 
           [0012]      FIG. 4  shows the current drawn from the AC power line by the first embodiment of the light engine according to the present invention. 
           [0013]      FIG. 5  shows the current dawn from the AC power line by the preferred embodiment of the light engine according to the present invention. 
       
    
    
       [0014]    The call out list of elements can be a useful guide in referencing the elements of the drawings.
     101  first LED string     102  second LED string     103  third LED string     104  fourth LED string     105  LED 1       106  LED 16       107  MOSFET Q 1       108  LED 17       109  LED 32       110  MOSFET Q 2       111  LED 33       112  LED 48       113  MOSFET Q 3       114  LED 49       115  LED 64       116  MOSFET Q 4       116 A diode D 1       117  diode D 2       118  diode D 3       119  diode D 4       120 A rectified DC voltage V 3       121  VCC     122  low power reference voltage V 2       123  resistor R 1       124  resistor R 2       125  reference voltage Vref     126  operational amplifier (opamp) U 1       127  operational amplifier (opamp) U 2       128  operational amplifier (opamp) U 3       129  operational amplifier (opamp) U 4       130  RS 1       131  RS 2       132  RS 3       133  RS 4       134  feedback resistor R 1 _ 1       136  feedback resistor R 1 _ 2       137  feedback resistor R 2 _ 2       138  feedback resistor R 1 _ 3       139  feedback resistor R 2 _ 3       140  feedback resistor R 1 _ 4       141  feedback resistor R 2 _ 4     
 
       DETAILED DESCRIPTION OF THE INVENTION 
     a) Basic Embodiment 
       [0056]      FIG. 3  shows the arrangement of a constant current ac driven light engine with four strings of LEDs. This AC LED Driver operates the LEDs in constant current mode. In each step of operation through the AC power line cycle the LEDs are operated at a relatively constant current, although that current is increased and decreased through the power line cycle in order to follow the sine wave input voltage waveform as is required to produce good power factor. The power grid input in this case is 230 VAC @60 Hz. However, the same principle applies to other voltages from 90 VAC to 382 VAC, and 47 Hz to 63 Hz applications. 
         [0057]    The 4 steps of operation during a power line voltage half wave of operation involve sequentially turning on the 4 strings of LED&#39;s, namely LED string #1 ( 101 ), LED string #2 ( 102 ), LED string #3 ( 103 ) and LED string #4 ( 104 ). In this embodiment, each LED string has 16 LED&#39;s in series with the cathode of each connected to the anode of the next. LED string #1 ( 101 ): LED 1  ( 105 ) to LED 16  ( 106 ) are connected in series, and driven by MOSFET Q 1  ( 107 ). LED string #2 ( 102 ): LED 17  ( 108 ) to LED 32  ( 109 ) are connected in series, and driven by MOSFET Q 2  ( 110 ). LED string #3 ( 103 ): LED 33  ( 111 ) to LED 48  ( 112 ) are connected in series, and driven by MOSFET Q 3  ( 113 ). LED string #4 ( 104 ): LED 49  ( 114 ) to LED 64  ( 115 ) are connected in series, and driven by MOSFET Q 4  ( 116 ). 
         [0058]    All these LED strings are further connected by connecting cathodes to anodes between LED 16  ( 106 ) and LED 17  ( 108 ), LED 32  ( 109 ) to LED 33  ( 111 ), LED 48  ( 112 ) and LED 49  ( 114 ). The power grid AC voltage is full wave rectified by a bridge rectifier arrangement consisting of diodes D 1  ( 116 A), D 2  ( 117 ), D 3  ( 118 ) and D 4  ( 119 ) which in this embodiment were part number 1N4007. No electrolytic capacitors are used. The rectified DC voltage V 3  ( 120 A) has high ripple (almost 100%) without any “filtering capacitor”. However, small value, extremely long life, ceramic capacitors may be used to construct the low voltage, low power source VCC ( 121 ) for the opamp which is 12 VDC in this illustrative embodiment, and the low voltage low power reference voltage V 2  ( 122 ) which is also 12 VDC. These low power voltages are merely illustrative and many different voltages could be used. The details of the low power VCC ( 121 ) and V 2  ( 122 ) voltage regulator circuits are omitted here, and for clarity just two voltage sources are shown. 
         [0059]    In the this embodiment the 64 LEDs in the 4 strings are Nichia part number NSCW100 white LED with a typical forward voltage Vf of 3.6 VDC and a maximum forward voltage of 4.0 VDC at a forward current If of 20 mA. With suitable adjustments to the circuit, any LED could be used. The AC input voltage V 1  ( 120 ) in this embodiment is 230 VAC, although obviously any line voltage could be used with suitable adjustments to the circuit. The number of LEDs in each LED string is 16. The key advantage of driving the LEDs with constant current is to prolong the usable life of the LED by preventing excess power dissipation. Brand new parts of Nichia NSCW100 usually have a forward voltage of 3.6V and it gradually increases to 4.0V near the end of life. The peak voltage of 230 VAC is 1.414×230V=325V. We need to divide 325V into 5 levels as shown in  FIG. 2  for region 0, 1, 2, 3 and 4. Each level is about 64V. 16 LEDs are used in each string. When the LED are brand new with Vf=3.6 VDC, the voltage steps are 16×3.6V=57.6V. When the LEDs are aged close to the end of life, the voltage steps are increased automatically (due to constant current driving technology) to 16×4.0V=64V. 
         [0060]    The V 2  ( 122 ) voltage is divided by 2 resistors R 1  ( 123 ) and R 2  ( 124 ) to provide the reference voltage Vref ( 125 ). Vref=V 2 ×R 2 /(R 1 +R 2 ). The Vref ( 125 ) voltage is connected to the quad opamp “+ input” pins to make an automatic feedback loop controlling the LED current. The V 2  ( 122 ) voltage and R 1  ( 123 ), R 2  ( 124 ) values can be adjusted in order to set the overall current level provided by the current limiters comprised by MOSFETs Q 1  ( 107 ), Q 2  ( 110 ), Q 3  ( 113 ) and Q 4  ( 116 ). The “− input” pins of the opamps are connected to the four respective sense resistors which enables the control of the current through the LED strings as described below. For this reason the “− input pins” are referred to as the control terminals of the current limiters. 
         [0061]    For properly selected R 1  ( 123 )=14K and R 2  ( 124 )=10K, the Vref ( 125 ) is about 5.0 VDC, which is the minimum gate voltage for MOSFETs Q 1  ( 107 ), Q 2  ( 110 ), Q 3  ( 113 ) and Q 4  ( 116 ) to start conducting in the “linear region” where the LED current is in the constant current mode. When the MOSFETs Q 1  ( 107 ), Q 2  ( 110 ), Q 3  ( 113 ) and Q 4  ( 116 ) are turned ON in the “linear region”, the MOSFET Gate Voltage is about 7 VDC to 8 VDC. If the gate voltage is above 8.5 VDC, the MOSFET will be fully saturated resulting in loss of current control. 
       THE Feedback Mechanism to Produce Constant Current: 
       [0062]    The resulting power line input current of this and other multi segment direct AC driven constant current LED light engines is shown in  FIG. 2 . In  FIG. 2  region 0, the instantaneous voltage from the bridge rectifier is too low for LED strings #1, #2, #3 and #4 ( 101 ,  102 ,  103  and  104 ) to conduct current. There is no current flowing through sense resistors RS 1 , RS 2 , RS 3  and RS 4 . The feedback voltage on the “−input” pin of each of the four operational amplifiers (opamps) U 1 , U 2 , U 3  and U 4  ( 126 ,  127 ,  128  and  129 ) is lower than the “+ input” voltage which is fixed at 5 VDC. The gate terminal voltage on all of the MOSFETS Q 1 , Q 2 , Q 3  and Q 4  ( 107 ,  110 ,  113  and  116 ) is greater than 8.5 VDC, and they are all fully turned ON. 
         [0063]    In  FIG. 2  region 1, MOSFET Q 1  ( 107 ) is turned ON in its “linear region” and Q 2 , Q 3  and Q 4  ( 110 ,  113  and  116 ) are turned ON but the voltage from the bridge rectifier is too low for LED string #2 #3 and #4 ( 102 ,  103  and  104 ) to conduct current. The current through LED string #1 ( 101 ) (ILED 1 ) goes through MOSFET Q 1  ( 107 ), then goes through RS 1 , RS 2 , RS 3  and RS 4  ( 130 ,  131 ,  132  and  133 ) in series. The voltage at the top side of RS 1  which=ILED 1 ×(RS 1 +RS 2 +RS 3 +RS 4 ), is fed back to opamp U 1  ( 126 ) “− Input” pin via feedback resistor R 1 _ 1  ( 134 ). In addition, there is another feedback resistor R 2 _ 1  ( 135 ) connecting from the U 1  ( 126 ) output pin to U 1  ( 126 ) “− Input” pin. This feedback mechanism regulates ILED 1  at the desired level until LED string #2 starts up. 
         [0064]    In  FIG. 2  region 2, MOSFET Q 2  ( 110 ) is turned ON in its “linear region”, Q 1  ( 107 ) is turned OFF because of the excessive voltage now present across its sense resistor RS 1  ( 130 ) and Q 3  and Q 4  ( 113  and  116 ) are turned ON although the output voltage from the bridge rectifier is too low for LED strings #3 and #4 ( 103  and  104 ) to conduct current. The bridge rectifier output voltage is high enough for LED strings #1 and #2 ( 101  and  102 ) in series to conduct current. The LED string #1 ( 101 ) current ILED 1  is identical to the current through LED string #2 ( 102 ) ILED 2 . LED string #2 ( 102 ) current ILED 2  is going through MOSFET Q 2  ( 110 ), then goes through RS 1 , RS 2 , RS 3  and RS 4  ( 130 ,  131 ,  132  and  133 ) in series. The voltage on the top side of RS 2  which=ILED 2 ×(RS 2 +RS 3 +RS 4 ), is fed back to the opamp U 2  ( 127 ) “− Input” pin via a feedback resistor R 1 _ 2  ( 136 ). In addition, there is another feedback resistor R 2 _ 2  ( 137 ) connecting from U 2  ( 127 ) output pin to U 2  ( 127 ) “−Input” pin. This feedback mechanism regulates ILED 2  at the desired level until LED string #3 starts up. 
         [0065]    In  FIG. 2  region 3, MOSFET Q 3  ( 113 ) is now turned ON in its “linear region”, Q 1  and Q 2  ( 107  and  110 ) are turned OFF by the excess voltage present at the tops of RS 1 ( 130 ) and RS 2 ( 131 ) respectively and Q 4  ( 116 ) is turned ON but the bridge rectifier output voltage is too low for LED string #4 ( 104 ) to conduct current. The bridge rectifier output voltage is high enough for LED strings #1 #2 and #3 ( 101 ,  102  and  103 ) in series to conduct current. The current through LED strings #1 and #2 ( 101  and  102 ) is identical to the current ILED 3  through LED string #3 ( 103 ). The LED string #3 ( 103 ) current ILED 3  goes through MOSFET Q 3  ( 113 ), then through RS 1 , RS 2 , RS 3  and RS 4  ( 130 ,  131 ,  132  and  133 ) in series. The voltage at the top side of RS 3  which=ILED 3 ×(RS 3 +RS 4 ), is fed back to opamp U 3  ( 128 ) “− Input” pin via the feedback resistor R 1 _ 3  ( 138 ). In addition, there is another feedback resistor R 2 _ 3  ( 139 ) connecting from U 3  ( 128 ) output pin to U 3  ( 128 ) “− Input” pin. This feedback mechanism regulates ILED 3  at the desired level until LED string #4 starts up. 
         [0066]    In  FIG. 2  region 4, MOSFET Q 4  ( 116 ) is turned ON in its “linear region”, Q 1  Q 2  and Q 3  ( 107 ,  110  and  113 ) are turned OFF by the excess voltage at the tops of RS 1 ( 130 ), RS 2 ( 131 ) and RS 3 ( 132 ) respectively. The bridge rectifier output voltage is high enough for LED strings #1 #2 #3 and #4 ( 101 ,  102 ,  103  and  104 ) in series to conduct current. The current through LED strings #1 #2 and #3 ( 101 ,  102  and  103 ) is identical to the current ILED 4  in LED string #4 ( 104 ). The current ILED 4  through LED string #4 ( 104 ) goes through MOSFET Q 4  ( 116 ), then goes through RS 1 , RS 2 , RS 3  and RS 4  ( 130 ,  131 ,  132  and  133 ). The voltage at top side of RS 4  which=ILED 4 ×RS 4 , is fed back to opamp U 4  ( 129 ) “− Input” pin via a feedback resistor R 1 _ 4  ( 140 ). In addition, there is another feedback resistor R 2 _ 4  ( 141 ) connecting from U 4  ( 129 ) output pin to U 4  ( 129 ) “− Input” pin. This feedback mechanism holds ILED 4  at the desired constant level regardless of the input voltage. 
         [0067]    In the foregoing description of the bridge rectifier output voltage rising from 0 to peak, all four MOSFETs were turned ON in region 0 but there was no current going through them because the voltage was lower than LED forward voltages of the four LED strings. In region 1, only LED string #1 ( 101 ) is turned ON in constant current mode with LED string #1 ( 101 ) current=ILED 1 , controlled by 4 sense resistors in series (RS 1 +RS 2 +RS 3 +RS 4 ). In region 2, LED string #1 and #2 ( 101  and  102 ) are turned ON in constant current mode, with ILED 1 =ILED 2 , controlled by only 3 sense resistors (RS 2 +RS 3 +RS 4 ). Therefore, LED string #1 ( 101 ) is conducting current in both region 1 and region 2. Due to the 3 sense resistors having total series resistance lower than all 4 sense resistors, the ILED 2  current in region 2 is greater than the ILED 1  current in region 1. Therefore, the LED string #1 has ILED 1  current in region 1, and ILED 2  current in region 2. The LED string #1 ( 101 ) current is stepping upward in the same manner in region 3 and region 4. The same calculation gives the relation ILED 4 &gt;ILED 3 &gt;ILED 2 &gt;ILED 1 . This is the basic principle for the staircase like LED current waveform in this AC LED light engine. The LED string #1 ( 101 ) has 4-steps going up from region 0 to region 4. LED string #2 ( 102 ) has 3-steps going up from region 1 to region 4. LED string #3 ( 103 ) has 2-steps going up from region 2 to region 4. LED string #4 ( 104 ) transitions from zero current to current ILED 4  in region 4. 
         [0068]    When the AC voltage reaches the peak at the center of region 4, or at AC Phase 90 degrees, it starts falling. In region 5, 6 and 7, the LED strings follow the same principle as in regions  3 ,  2  and  1  with the same values of ILED 3 , ILED 2  and ILED 1 . Finally in region 8, the AC voltage is too low and none of the LEDs conducts any current. In  FIG. 4  the currents of LED strings #1 #2 #3 and #4 ( 101 ,  102 ,  103  and  104 ) are shown at 22 mA, 24 mA, 28 mA and 32 mA respectively with 4, 3, 2 and 1 steps. The phase of the voltage half wave between 0 and 180 Degrees is shown for reference. It can be seen that proportionately the current drawn from the power line rises faster than the power line voltage, leading to excessive current harmonic distortion. 
       b) Preferred Embodiment with Improved THD Performance 
       [0069]    As just remarked above, the simplistic embodiment in which all four R 1  resistors have the same value of 14K leads to the current waveform not following well the voltage waveform. In the preferred embodiment the four R 1  resistors are adjusted so that the 4 steps of LED current will closely follow the AC voltage sine wave contour making possible low current total harmonic distortion. 
         [0070]    The three resistors R 1 _ 1  ( 134 ), R 1 _ 2  ( 136 ) and R 1 _ 3  ( 138 ) which were originally all 14K Ohm are changed to 39K, 22K and 16K Ohm respectively.  FIG. 5  shows the AC power line current with these 3 new resistors. The 3 flat LED current readings are now at 8 mA, 14 mA and 20 mA which coincide with the AC Sine wave contour. These modified resistor values produce significantly improved current total harmonic distortion. 
         [0071]    Although specific components have been described in the above embodiments for illustrative purposes, those skilled in the art will be able to immediately see numerous variations of the invention using different components and ratios. Even though four LED strings were described, the principles of the invention can be applied to any number of LED strings. The strings described had 16 LEDs each, however with different power line voltages and new LED technology essentially any number of LEDs could be present in a string. The strings depicted had equal numbers of LEDs, however it could be advantageous under certain circumstances for the strings to have unequal numbers of LEDs. Single LEDs were depicted in the strings, however under some circumstances packages with two or more LEDS could be used advantageously. Although light emitting diodes were described, any device which emits light with the application of an electric current could be used. The switches were described as being n-channel silicon MOSFETS, however IGBTs, bipolar transistors and any of the numerous solid state switches known to those of skill in the art could be used. Discrete silicon diodes were described for the bridge rectifier function, but any kind of diode, including light emitting diodes, could be used and the bridge rectifier might be formed from a single integrated package. Any of the resistors and current limiters described could be replaced by constant current resistors or other active devices while following the same principles. Capacitors could be added to store energy to prevent a dark period around the line voltage zero crossing time. The exemplary circuit described operated at 230V, however the principles of the invention could be applied to circuits operating at any of the standard world utility voltages, such as 90V, 120V, 220V, 240V, 277V and 347V. 
         [0072]    Accordingly it will be apparent to those skilled in the art that many modifications and variations can be made in the present invention without departing from the spirit or scope of the invention. Thus it is intended that the present invention cover the modifications and variations of this invention provided they come within the scope of the appended claims and their equivalents.