Abstract:
An optimum regulation method is disclosed for reconfigurable LED arrays used for general illumination applications. This document describes a reconfigurable LED array formed by connecting in series LED lamps and LED pairs capable of being reconfigured in either series or parallel. The performance deficiencies of previous solutions are solved by changing the voltage rating of the array through the reconfiguration of LED pairs. The simplicity of the concept can make practical the implementation of driverless LED lighting fixtures.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority from, and incorporates by reference the entirety of, U.S. Provisional Patent Applications Ser. No. 61/561,914, filed on Nov. 20, 2011, and Ser. No. 61/587,106, filed on Jan. 16, 2012. 
     
    
     FIELD 
       [0002]    This invention relates to lighting devices used for general illumination purpose and constructed based on solid state devices such as Light Emitting Diodes better known as LED, which comprise LED arrays, electronic driving circuits, and reflectors enclosed in housing. 
       BACKGROUND OF THE INVENTION 
       [0003]    The use of LED lamps is a trend that continues and as the technology matures, it is expected that LED lamps will be the predominant source of artificial light for general illumination purposes. LED lamps are robust solid state devices capable of lasting 50,000 hours or more. The main electrical components of existing LED fixtures are the LED module comprising LED lamps organized in arrays and an electronic driver. The driver is a complex device used to control the voltage and current applied to the LED arrays based on high frequency switching of power electronics devices. The buck and boost converters are typical topologies of existing LED drivers. Because of the complexity of these drivers, they are usually the weakest link in the LED lighting fixture system, limiting the expected life and output of the existing fixtures. Additional disadvantages of the existing LED drivers are, the over sizing of the LED lighting fixtures in order to house the relatively large driver units, lower energy efficiency, and higher cost of the LED lighting fixtures among others. 
         [0004]    U.S. Pat. No. 7,936,135 B2 awarded on May 3, 2011 makes an attempt to solve the problems associated with the high frequency switching of existing drivers. This patent proposes to control the current of LED arrays by changing the configurations from series to parallel and vice versa. However, the solutions disclosed in this patent are still not practical and of low commercial value. First, when the proposed regulation scheme maintains a constant current, some LED lamps are turned off as illustrated in  FIGS. 1 ,  2 ,  3 ,  5 ,  6 ,  7 ,  8 , and  9  of the patent, making it not suitable for DC applications. On the other hand, when the solution scheme is to maintain a constant illumination level, the current of the array varies in a wide range as illustrated in  FIGS. 4A ,  4 B,  4 C, and  4 D of the patent, generating higher harmonics and increasing the design constrains of the driver. 
         [0005]    There still is a market need for an LED lighting fixture with a minimum amount of electronic components to drive the LED arrays at lower switching frequencies and with improved current-illumination regulation and efficiency performances. Furthermore, in addition to increasing the efficiency and life expectancy at a lower cost, the electronic components can be integrated with the LED modules substantially decreasing the footprint of the LED fixtures. 
       SUMMARY OF THE INVENTION 
       [0006]    An optimum current-illumination regulation scheme is proposed based on arrays formed by connecting in series LED lamps and LED pairs that can reconfigure their connections. The proposed inventive concept comprises regulating the current through an LED array by changing the array rated voltage as a consequence of reconfiguring the connections of the LED pairs, while substantially maintaining a constant illumination level. When the proposed inventive concept is applied to LED lighting fixtures, a simpler construction and a more reliable fixture is obtained thanks to the elimination of the high frequency drivers commonly used in existing LED lighting fixtures. In addition to the latter advantages, the proposed LED fixture has a smaller housing, higher energy efficiency, and lower cost. Furthermore, the simplicity of the concept makes it practical for integrating the control functions with the LED lamps allowing for the driverless solid state lighting fixtures. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS  
         [0007]      FIG. 1  depicts a graphical representation of an LED lamp. 
           [0008]      FIG. 2  illustrates the electrical model of the LED lamp shown in  FIG. 1 . 
           [0009]      FIG. 3  illustrates an array formed with four LED lamps. 
           [0010]      FIG. 4  shows the equivalent circuit of the LED array shown in  FIG. 3 . 
           [0011]      FIG. 5  represents a composite current-voltage plot of the curves corresponding to the LED lamp shown in  FIG. 1  and the array shown in  FIG. 3 . 
           [0012]      FIG. 6  illustrates an array with an LED-pair in series state. 
           [0013]      FIG. 7  illustrates an array with an LED-pair in parallel state. 
           [0014]      FIG. 8  illustrates an LED-pair with a control line. 
           [0015]      FIG. 9  illustrates the LED-pair shown in  FIG. 8  integrated in a single module. 
           [0016]      FIG. 10  illustrates the LED-pair module shown in  FIG. 9  integrated with some control functions in a single module. 
           [0017]      FIG. 11  shows an LED-pair with two control lines. 
           [0018]      FIG. 12  shows another LED-pair of  FIG. 11  having two lamps per branch. 
           [0019]      FIG. 13  illustrates a solid state lighting fixture powered from an AC voltage source with array having some of the LED modules shown in  FIG. 9 . 
           [0020]      FIG. 14  illustrates a control circuit based on analog devices. 
           [0021]      FIG. 15  depicts a control circuit based on a microprocessor unit. 
           [0022]      FIG. 16  illustrates the lighting fixture shown in  FIG. 13  with the array having some of the LED modules shown in  FIG. 10 . 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0023]    The disadvantages of reconfigurable LED arrays proposed by the prior arts are mitigated by only reconfiguring LED-pairs within the array. As later explained on this document, instead of being turned off, the LED-pairs are always on, while maintaining a substantially constant current flow through the array. 
         [0024]    The variations of the LED parameters with temperature will not be considered. This assumption can be acceptable for arrays having a higher number of low power LEDs as opposed to a single high power LED concentrated in a small area. 
         [0025]      FIG. 1  shows the standard symbol used for LEDs lamps. The negative terminal of the applied voltage Vd is connected to the ground terminal  1 . The equivalent electrical circuit  4  of an LED  2  is illustrated in  FIG. 2 . Curve  8  shown in  FIG. 5  is a typical plot of the forward current Id versus the voltage Vd of the LED  2 . The battery models the LED  2  knee voltage Vk and is considered not to be influenced by the LED forward current Id. Curve  8  indicates that when the applied voltage Vd is lower than the knee voltage Vk, no substantial current flows through the LED  2 . The forward voltage Vd in LED  2  is the sum of the knee voltage Vk plus the increment voltage ΔVRd due to the voltage drop across Rd due to the flow of the forward current Id. At rated forward current Idr, the rated voltage across the LED  1  is Vdr=Vk+ΔVRdr=Vk+Idr*Rd. The high sensitivity of LEDs due to variations in the applied voltage is indicated by the slope of the curve  8  and it can be estimated as 1/Rd, approximately. 
         [0026]      FIG. 3  shows an array  10  of four LEDs  2  and  FIG. 4  depicts its equivalent electrical circuit  6 . The array  10  knee voltage Vka and the forward resistance Rda are now four times larger than the one shown for a single LED  2  in  FIG. 2 . Curve  9  shown in  FIG. 5  is a plot of the array current Ida versus the array total voltage Vda. No substantial current Ida can flow through the LED array  10  if the applied voltage Vda is less than four times the knee voltage Vk of the individuals LEDs  2 , that is, if Vda&lt;4Vk. The array forward resistance Rda is approximately four times the forward resistance Rd of a single LED, and the slope shown in curve  9  is now smaller by a factor of four, that is, 1/Rda=1/(4Rd). In other words, the higher the number of LEDs  2  within the array, the less sensitive the array becomes to changes in the applied voltage Vda. As the number of LEDs  2  increases within the array, it becomes easier to control the current Ida. This is a fact that had not been exploited to its full potential. Because of the limitations of the high frequency drivers at higher voltages, the tendency of the present technology is to decrease the number of LEDs  2  within the array by increasing the power and voltage of a single LED  2 . Two major disadvantages of this tendency are increase heat management issues due to higher power densities in a single area of the LED device and a worse light distribution due to a single point light source. 
         [0027]      FIG. 6  and  FIG. 7  illustrate the inventive concept through a simple application of an LED array  20  consisting of a standard LED  2  and an LED-pair  12  connected in series.  FIG. 6  shows the LED-pair  12  in series state. The LED-pair  12  is in series state when switch ‘c’ is closed and switches ‘a’ and ‘b’ are open, making its voltage equal to 2 Vd. When the LED-pair  12  is in series state, the voltage of the array  20  is equal to 3 Vd. The current flowing through each LED of the pair  12  in series state is equal to the array current Ida. Notice that the current of the array Ida is equal to the current Id of the LED  2  located at the bottom.  FIG. 7  shows the LED-pair  12  in parallel state. The LED-pair  12  is in parallel state when switch ‘c’ is open and switches ‘a’ and ‘b’ are closed, making its voltage equal to Vd. When the LED-pair  12  is in parallel state, the voltage of the array  20  is equal to 2 Vd. The current flowing through each LED of the pair  12  is now equal to approximately 50% of the array current Ida. The regulation of the current, voltage, and illumination levels of the array  20  shown in  FIG. 6  and  FIG. 7  is poor, which can fluctuate up to 40% of its expected value. 
         [0028]    As the number of LEDs  2  increases within the array  20 , the regulation performance improves dramatically. The LED-pair  12  represents the optimum regulation scheme for reconfigurable LED arrays. When changing the state of an LED-pair  12  the voltage rating Vda of the array changes by the minimum amount of ±Vd, ant the array current Ida is kept substantially constant. While the illumination level of an LED-pair  12  changes by 50% approximately, the illumination level of the array is barely noticeable. If the DC voltage applied to the LED array contains 60 Hz ripples, the reconfiguration of the LED-pairs  12  occurs at a rate of 120 times per second, which can not be perceived by the human eye. There are additional advantages for using low frequency drivers in terms of lower design complexity and noise generation, higher efficiencies, and lower production cost. 
         [0029]    The proposed inventive concept can be extended to have three LEDs  2  configured in an LED-triple module (not shown). The LED-triple can be capable of reconfiguring its three LEDs  2  in series, parallel, or a combination of a series-parallel connections; changing the voltage rating of the LED-triple to Vd, 2 Vd, and 3 Vd. However, as the number of LEDs  2  increases, the complexity of the control circuit driving the LEDs within the module increases considerably. Furthermore, the illumination performance of the array is also negatively affected because some LEDs can be driven at currents lower than 33% of the array rated current. The advantages of having arrays with LED-pairs  12  are not anticipated by the prior arts in either the written specifications or the drawings. 
         [0030]    Since the configuration of the LED shown in  FIG. 1  does not change, the LED  2  can be considered static LED. On the other hand, The LEDs forming the LED-pair  12  can be considered dynamic LEDs because they can be reconfigured in series or parallel. For simplicity sake, the embodiments are shown with the switching devices being performed with mechanical switches, however, it is understood that the actual construction will be implemented by using electronic switching devices such as MOSFETs, BJTs, IGBTs, and FETs among other electronic devices capable of implementing the switching function. 
         [0031]    The states of the switching devices ‘a’, ‘b’ and ‘c’ of the LED-pair  12  can be changed with a single control line ‘C’ as illustrated in  FIG. 8 . This function can be implemented by replacing the mechanical switches (a) and (b) with enhancement mode MOSFETs and switch (c) with a depletion mode MOSFET. In this way the gates of the MOSFETs within an LED-pair  12  can be logically tight together to a single control line. When using a single control line ‘C’, the default state of the LED-pair  12  is series because the state of the depletion mode MOSFET represented by the switch ‘c’ is low impedance when no power is applied, while the state of the enhancement mode MOSFETs represented by the switches ‘a’ and ‘b’ is high impedance. Then, the LED-pair  12  can be configured in series when the status of the control line ‘C’ is logic low and configured in parallel when the status of the control line ‘C’ is logic high. The default state of this LED-pair  12  can also be considered fail safe since the array containing these LED-pairs  12  presents its highest impedance when initially connected to a voltage source, exposing the LEDs within the array to the minimum current when the control lines are not yet stable due to initialization delays within the control circuit. 
         [0032]    The LED-pair  12  shown in  FIG. 8  can be integrated in a single LED-pair module  30  shown in  FIG. 9 . Module  30  can ease the implementation of the solid state lighting fixtures  70  based on LED-pairs  12  as illustrated in  FIG. 13 . The AC voltage source  16  is converted to a full wave DC voltage by the bridge rectifier  22 . The fuse  18  can protect the fixture  70  against current overloads while the metal oxide varistor  24  can protect against momentary line over voltages. A capacitor  26  can be added to minimize the AC ripples of the voltage+VDC. The array shown in fixture  70  comprises dynamic LEDs represented by the LED-pair modules  30  and static LEDs  2  connected in series. A shunt resistor  28  can be added to monitor and control the current Ida flowing through the array. 
         [0033]      FIG. 14  and  FIG. 15  illustrate two possible implementations of the control circuits used to change the states of the modules  30  forming the LED array of the lighting fixture  70  shown in  FIG. 13 . The analog control circuit  80  shown in  FIG. 14  can be implemented with operational amplifiers  32  or other types of analog electronic devices. The changes of the array current Ida can be amplified and used to activate the control lines Vc 1  through Vcn. A microprocessor version of the control circuit  90  is shown in  FIG. 15 . The microprocessor unit  34  can read the changes of the array current Ida and activate the control lines Vc 1  though Vcn in accordance with the software algorithm stored in the unit  34 . The control circuit can also be implemented with other electronic devices, for example, it can be constructed with logic gates only. The control circuits  80  and  90  can also be designed to monitor the array voltage instead of the current or to accept inputs for other important parameters affecting the performance of the LEDs. For instance, the temperature of the LEDs can be factored into the control function to improve the overall performance of the LED lighting fixture  70 . 
         [0034]    The implementation details of the integrated control circuit  14  and the control circuit driving the LED-pair  12  are not shown for clarity. It is understood that a person with ordinary skills in the art can design these control circuits when the control specifications are provided. 
         [0035]    As an example of the application of the disclosed inventive concept, assume the lighting fixture  70  shown in  FIG. 13  is a retrofit that can be screwed into a standard 120 Vac light bulb socket. The 120 Vac represents the Mean Square Root (RMS) value of the voltage source  16 . After rectification and filtering, the +VDC value is approximately equal to the peak voltage Vp=√ 2 *120V=169.7 Vdc. The LED used for this example is a white color LED series 61-238 as manufactured by Everlight Electronics Co., LTD., with the following electrical characteristics: when the forward rated current Idr=20 ma, the LED forward voltage Vdr=3.1V, and the illumination is 3,300 mcd, approximately. When the forward current drops to 50%, Idh=10 ma, the LED forward voltage is Vdh=2.9V, and the illumination level is 2,640 mcd, approximately. If the rated voltage of each LED is Vdr=3.1V, then, the approximate number of LEDs required for the array of the fixture  70  can be estimated as 169.7V/3.1≈170V/3.1≈55. That is, about 55 LEDs connected in series add up to approximately 170.5Vdc closely matching the magnitude of the voltage source  16 . If a total of 58 LEDs were used to construct the array, the number of static and dynamic LEDs can be equal to 42 and 16, respectively. The sixteen dynamic LEDs are represented by eight LED-pair modules  30 . Because the eight LED-pair modules  30  are initially connected in series, the initial impedance of the array occurs when all 58 LEDs are configured in series, representing the highest possible impedance of the LED array. Therefore, and momentarily, the magnitude of the voltage source  16  rated at 169.7V is smaller than the array rated voltage Vdar, which is approximately equal to Vdar=58×3.1V=179.8V. As a consequence, during the initialization period the LEDs are guaranteed to be driven at a lower current value than their rated value. As the control circuit samples and processes the array current Ida through the shunt resistor  28 , it starts activating the control lines and configuring the LED-pair modules  30  in parallel until the array forward current Ida is approximately equal to the rated current Idar=20 ma. In this case, when the array rated current Idar flows, the array rated voltage should match the 170V of the source, approximately. Then, the control circuit starts activating the control lines Vc 1  through Vc 8  to configure four LED-pair modules  30  in parallel for a new array rated voltage Vdar=42*Vdr+4*Vdh+8*Vdr=42*3.1V+4*2.9V+8*3.1V=1.66.6V. The voltage difference Vdiff≈169.7−166.6=3.1V can represent voltage distribution losses among the shunt resistor  28 , the switching transistors, and other Joules&#39; losses in the wiring, terminations, etc. 
         [0036]    As described above, a change in configuration of an LED-pair module  30  produces a change in voltage drop equal to Vdr≈3.1V. Then, the control circuit can be set to respond to variations in the input voltage equal to ±Vdr or its equivalent variations in the array current Ida. For instance, if the voltage source  16  is increased by a magnitude Vdr, the control circuit can activate three control lines to configure three LED-pair modules  30  in parallel. The new array rated forward voltage Vdar can be estimated as Vdar=42Vdr+3Vdh+10Vdr≈42*3.1V+3*2.9V+10*3.1V=169.9V. On the contrary, if the voltage source  16  is decreased by a magnitude −Vd, the control circuit can reconfigure the array to have five LED-pairs modules  30  in parallel. The new array rated voltage Vdar can now be estimated as Vdar=42Vdr+5Vdh+6Vdr≈42*3.1V+5*2.9V+6*3.1V=163.3V. 
         [0037]    The regulation of the above lighting fixture  70  can be estimated as follows, at rated voltage source  16  there are four modules  30  configured in series and four configured in parallel for an approximate array rated voltage of Vdar=166.6V, as described above. The array luminosity can be estimated as 50*3,300+8*2,640=1.86,120 mcd. That is, 50 LEDs are driven at about 20 ma, while 8 LEDs are driven at about 10 ma. The maximum array rated voltage Vdar=179.8V occurs when all eight modules  30  are configured in series. The array maximum luminosity can now be estimated as 58*3300=191,400 mcd. That is, all 58 LEDs are driven at the rated current of 20 ma. The array minimum rated voltage is Vdar=42*Vdr+8*Vdh≈42*3.1V+8*2.9V=153.4V, which occurs when all eight modules  30  are configured in parallel. That is, 42 LEDs are driven at 20 ma while 16 LEDs are driven at 10 ma The array minimum luminosity can be estimated as 42*3,300+16*2,640=180,840 mcd. The luminosity tolerance is equal to (191,400˜180,840)/2=±5,280 mcd. And, the percentage regulation can be estimated approximately as (±5,280/186,120)*100=2.84%. The range of the voltage regulation can be estimated approximately as 179.8V−153.4V−26.4V, and the percentage regulation as (26.4V/166.6V)* 100=15.8% or ±7.9%. In summary, an 8% change of the input voltage  16  generates an array luminosity change of less than 3%. The regulation range can be increased by adjusting the numbers of static LEDs  2  and dynamic LED-pair modules  30  within the array. 
         [0038]    Even though  FIG. 13  shows the lighting fixture  70  constructed of a single LED array with an approximate average power rating of 166.6V*20 ma=3.3W. The power rating of the lighting fixture  70  can easily be increased to 6.6W, 9.9W, etc., by simply adding LED arrays in parallel. In addition the complexity of the control circuit is not substantially increased because any additional array can share the same control lines Vc 1  through Vc 8 . The LED lighting fixtures  70  can also be designed with dual rated voltages by changing the series-parallel configurations (not shown) among multiple LED arrays. For example, a dual rated LED lighting fixture  70  can be plugged into a 120 Vac standard socket when the LED arrays within the fixture are configured in parallel or to a 277 Vac electrical system when the LED arrays are configured in series. The users, through an external switch, can perform the voltage ratings transition manually. Alternatively, the LED fixture can be furnished with an auto detection circuit (not shown) to automatically adjust the voltage rating of the electrical system by changing the configuration of the LED arrays within the fixture  70 . 
         [0039]    The control circuits shown in  FIG. 14  and  FIG. 15  can be further simplified if the LED-pair modules  30  are constructed with additional control functions.  FIG. 10  illustrates an LED-pair module  40  which includes the integration of the LED-pair  12  and an integrated control circuit  14 . The integrated control circuit  14  can read the states of other modules  40  located above and below, and it can also broadcast its slate to other modules  40 . The control lines ‘ED’ can be the output of a circuit that determines if the array current Ida is outside an allowable range. The control line ‘E’ is the enable function. When the enable line ‘E’ is logic ‘0’, for example, the array rated current Ida is within permissible values and the actual configurations of the LED-pairs  12  within the modules  40  do not change. When the array current Ida is outside the allowable range, the enable line is set to logic ‘1’. Then, the states of the module  40  can change based on the states of the modules  40  located immediately above and below, and the state of the control line ‘D’. The control line ‘D’ can be set to logic ‘0’, for example, when the array current Ida is below the lower limit. And, the control line ‘D’ can be set to logic ‘1’, when the array current Ida is above the upper limit. The actual state of a module  40  does not change when the states upper and lower modules  40  are the same. On the other hand, if the states of the neighbors modules  40  are not equal, the present state of a module  40  can either change to series if the control line ‘D’, is set to logic ‘1’, or to parallel if the control line ‘D’ is set to logic ‘0’. 
         [0040]      FIG. 16  illustrates one embodiment of a solid state lighting fixture  100  with the array having some LED-pair modules  40 . As previously indicated, by default, the initial configuration of each LED-pair  12  is in series presenting the array highest possible impedance. After the initialization time delay, the directional circuit  36  detects that the array current Ida is below the permissible lower limit, which sets control lines to the logic states ED=10. The states of the lower modules  40  do not change because their neighbors have equal state. However, the top module  40  changes to parallel because the module  40  below is in series. If the control lines ‘ED’ remain logic ‘10’, the second module  40  from the top can change to parallel. This process can continue until the enable line ‘E’ changes to logic ‘0’ when the array current Ida rises to a value in between the upper and lower permissible limits. If the array current Ida increases beyond the upper limit due to an increase in the applied voltage  16 , the control lines change to the logic state ED=‘11’, and the states of the modules  40  are sequentially reconfigured in series until the array current Ida falls again within the permissible limits. It should also be noted that the control lines ‘E’ and ‘D’ logic functions can also be integrated with the LED-pair modules  40  eliminating the need for a separate directional circuit  36 . This new LED module  41  (not shown) can be similar to the LED-pair module  40  shown in  FIG. 10  except without the ‘E’ and ‘D’ control lines allowing for a driverless solid state lighting fixture (not shown). 
         [0041]    Additional embodiments of the LED-pairs can have more than one control line.  FIG. 11  and  FIG. 12  illustrate additional embodiments  50  and  60  of the LED-pair  12 . These embodiments require two control lines ‘C 1 ’ and ‘C 2 ’ to select one of four possible states such as high impedance state (or open circuit), zero impedance state (or short circuit), parallel state, and series state. The parallel and series states are similar to those already described for the LED-pair  12 . In high impedance state, all switching devices ‘a’, ‘b’ and ‘c’ are open. In zero impedance state, all switching devices ‘a’, ‘b’, and ‘c’ are closed. In addition, the LED-pair  12  can have more than one LED per branch.  FIG. 12  illustrates and embodiment of an LED-pair  60  comprising two LEDs per branch. The current rating of the LED-pair  60  is the same as the LED-pair  12 . However, the voltage rating is different. In parallel, the voltage rating of the LED-pair  60  is 2Vd, while in series, the voltage rating is 4Vd. 
         [0042]    This written description uses examples to disclose the invention, including the best mode, and also to enable any person skilled in the art to make and use the invention. The patentable scope of the invention is defined by the claims, and may include other embodiments that occur to those skilled in the art. Such other embodiments are intended to be within the scope of the claims if they have structural elements that do not differ from the literal language of the claims, or if they include equivalent structural/functional elements with insubstantial differences from the inventive concept herein claimed.