Patent Publication Number: US-9894725-B2

Title: Current feedback for improving performance and consistency of LED fixtures

Description:
CROSS-REFERENCE TO PRIOR APPLICATIONS 
     This application is the U.S. National Phase application under 35 U.S.C. §371 of International Application No. PCT/IB14/059450, filed on Mar.05, 2014, which claims the benefit of U.S. Provisional Patent Application Ser. No. 61/783,714, filed on Mar. 14, 2013. These applications are hereby incorporated by reference herein. 
    
    
     TECHNICAL FIELD 
     The present invention is directed generally to control of solid state lighting devices. More particularly, various inventive apparatuses and methods disclosed herein relate to implementing feedback control to improve performance and consistency of solid state lighting devices. 
     BACKGROUND 
     Existing solid state fixtures including light emitting diodes (“LEDs”) commonly include power supplies that utilize offline power converter topologies and operate in an open loop manner. The power supply may include a microcontroller (μC) that stores a power curve and outputs a pulse-width modulated (PWM) signal as a control signal to a power factor control (PFC) chip, which adjusts wattage of the buck power converter over a universal input voltage range from 90 volts AC to 480 volts AC. PFC chips may typically have tolerances of up to about 12% with respect to gain. Moreover, the forward voltage drops of LEDs also vary by bin and drive current. As a result, it is usually necessary to rework and/or change resistors within the power supplies of existing solid state fixtures during manufacture to adjust the power rating of the supply/fixture to meet desired specifications prior to finalizing the product for shipment or consumer use so that the supply/fixtures are calibrated to emit light having brightness that meets desired specifications. Such rework may be a time consuming and inefficient process, and may result in problems when the AC input voltage is above or below its nominal value or on the low end of an electronic low voltage (ELV) dimmer, where inconsistencies in drive current may visibly appear from fixture to fixture. Typically solutions to these problems include limiting low end dimming to obscure low end inconsistencies in driving current. This would however result in dead travel near the low end of the dimmer. 
     Thus, it would be desirable to provide a solid state lighting system that maintains consistent lighting current and brightness over time, reduces or eliminates the need to rework supply/fixtures during manufacture, enables consistent low end dimming of cascaded fixtures, improves dimmer compatibility and/or and sets a hard upper limit for lighting current. 
     SUMMARY 
     Generally, in one aspect, a lighting system includes a power converter connected to mains voltage and configured to provide a driving current responsive to a control signal; a voltage measurement circuit configured to provide a voltage sense signal indicative of an amplitude of the mains voltage; a light emitting diode (LED) module including at least one string of LEDs that emit light responsive to the driving current, and configured to detect an LED current through the at least one string and output a current feedback signal indicative of the detected LED current; and a driver controller configured to output the control signal responsive to the voltage sense signal and the current feedback signal. 
     In another aspect, a lighting driver includes a power converter connected to mains voltage and configured to provide a driving current to a solid state lighting load responsive to a control signal; a voltage measurement circuit configured to provide a voltage sense signal indicative of an amplitude of the mains voltage; and a driver controller configured to output the control signal responsive to the voltage sense signal and a current feedback signal indicative of a lighting current through the solid state lighting load, wherein the power converter provides the driving current to maintain the lighting current at a selected constant level regardless of the amplitude of the mains voltage. 
     In another aspect, a method of controlling a solid state lighting load includes converting mains voltage to provide a driving current to the solid state lighting load; generating a current feedback signal indicative of a lighting current through the solid state lighting load; and detecting an amplitude of the mains voltage, wherein said converting comprises providing the driving current to maintain light emitted from the solid state lighting load at a selected constant brightness responsive to the detected amplitude of the mains voltage and the current feedback signal. 
     As used herein for purposes of the present disclosure, the term “LED” should be understood to include any electroluminescent diode or other type of carrier injection/junction-based system that is capable of generating radiation in response to an electric signal. Thus, the term LED includes, but is not limited to, various semiconductor-based structures that emit light in response to current, light emitting polymers, organic light emitting diodes (OLEDs), electroluminescent strips, and the like. In particular, the term LED refers to light emitting diodes of all types (including semi-conductor and organic light emitting diodes) that may be configured to generate radiation in one or more of the infrared spectrum, ultraviolet spectrum, and various portions of the visible spectrum (generally including radiation wavelengths from approximately 400 nanometers to approximately 700 nanometers). Some examples of LEDs include, but are not limited to, various types of infrared LEDs, ultraviolet LEDs, red LEDs, blue LEDs, green LEDs, yellow LEDs, amber LEDs, orange LEDs, and white LEDs (discussed further below). It also should be appreciated that LEDs may be configured and/or controlled to generate radiation having various bandwidths (e.g., full widths at half maximum, or FWHM) for a given spectrum (e.g., narrow bandwidth, broad bandwidth), and a variety of dominant wavelengths within a given general color categorization. 
     For example, one implementation of an LED configured to generate essentially white light (e.g., a white LED) may include a number of dies which respectively emit different spectra of electroluminescence that, in combination, mix to form essentially white light. In another implementation, a white light LED may be associated with a phosphor material that converts electroluminescence having a first spectrum to a different second spectrum. In one example of this implementation, electroluminescence having a relatively short wavelength and narrow bandwidth spectrum “pumps” the phosphor material, which in turn radiates longer wavelength radiation having a somewhat broader spectrum. 
     The term “light source” should be understood to refer to any one or more of a variety of radiation sources, including, but not limited to, LED-based sources (including one or more LEDs as defined above), incandescent sources (e.g., filament lamps, halogen lamps), fluorescent sources, phosphorescent sources, high-intensity discharge sources (e.g., sodium vapor, mercury vapor, and metal halide lamps), lasers, other types of electroluminescent sources, and others. 
     A given light source may be configured to generate electromagnetic radiation within the visible spectrum, outside the visible spectrum, or a combination of both. Hence, the terms “light” and “radiation” are used interchangeably herein. Additionally, a light source may include as an integral component one or more filters (e.g., color filters), lenses, or other optical components. Also, it should be understood that light sources may be configured for a variety of applications, including, but not limited to, indication, display, and/or illumination. An “illumination source” is a light source that is particularly configured to generate radiation having a sufficient intensity to effectively illuminate an interior or exterior space. In this context, “sufficient intensity” refers to sufficient radiant power in the visible spectrum generated in the space or environment (the unit “lumens” often is employed to represent the total light output from a light source in all directions, in terms of radiant power or “luminous flux”) to provide ambient illumination (i.e., light that may be perceived indirectly and that may be, for example, reflected off of one or more of a variety of intervening surfaces before being perceived in whole or in part). 
     The term “spectrum” should be understood to refer to any one or more frequencies (or wavelengths) of radiation produced by one or more light sources. Accordingly, the term “spectrum” refers to frequencies (or wavelengths) not only in the visible range, but also frequencies (or wavelengths) in the infrared, ultraviolet, and other areas of the overall electromagnetic spectrum. Also, a given spectrum may have a relatively narrow bandwidth (e.g., a FWHM having essentially few frequency or wavelength components) or a relatively wide bandwidth (several frequency or wavelength components having various relative strengths). It should also be appreciated that a given spectrum may be the result of a mixing of two or more other spectra (e.g., mixing radiation respectively emitted from multiple light sources). 
     The term “lighting fixture” is used herein to refer to an implementation or arrangement of one or more lighting units in a particular form factor, assembly, or package. The term “lighting unit” is used herein to refer to an apparatus including one or more light sources of same or different types. A given lighting unit may have any one of a variety of mounting arrangements for the light source(s), enclosure/housing arrangements and shapes, and/or electrical and mechanical connection configurations. Additionally, a given lighting unit optionally may be associated with (e.g., include, be coupled to and/or packaged together with) various other components (e.g., control circuitry) relating to the operation of the light source(s). An “LED-based lighting unit” refers to a lighting unit that includes one or more LED-based light sources such as one or more strings of LEDs as discussed above, alone or in combination with other non LED-based light sources. A “multi-channel” lighting unit refers to an LED-based or non LED-based lighting unit that includes at least two light sources configured to respectively generate different spectrums of radiation, wherein each different source spectrum may be referred to as a “channel” of the multi-channel lighting unit. 
     The term “controller” is used herein generally to describe various apparatus relating to the operation of one or more light sources. A controller can be implemented in numerous ways (e.g., such as with dedicated hardware) to perform various functions discussed herein. A “processor” is one example of a controller which employs one or more microprocessors that may be programmed using software (e.g., microcode) to perform various functions discussed herein. A controller may be implemented with or without employing a processor, and also may be implemented as a combination of dedicated hardware to perform some functions and a processor (e.g., one or more programmed microprocessors and associated circuitry) to perform other functions. Examples of controller components that may be employed in various embodiments of the present disclosure include, but are not limited to, conventional microprocessors, application specific integrated circuits (ASICs), and field-programmable gate arrays (FPGAs). 
     In various implementations, a processor or controller may be associated with one or more storage media (generically referred to herein as “memory,” e.g., volatile and non-volatile computer memory such as RAM, PROM, EPROM, and EEPROM, floppy disks, compact disks, optical disks, magnetic tape, etc.). In some implementations, the storage media may be encoded with one or more programs that, when executed on one or more processors and/or controllers, perform at least some of the functions discussed herein. Various storage media may be fixed within a processor or controller or may be transportable, such that the one or more programs stored thereon can be loaded into a processor or controller so as to implement various aspects of the present invention discussed herein. The terms “program” or “computer program” are used herein in a generic sense to refer to any type of computer code (e.g., software or microcode) that can be employed to program one or more processors or controllers. 
     The term “addressable” is used herein to refer to a device (e.g., a light source in general, a lighting unit or fixture, a controller or processor associated with one or more light sources or lighting units, other non-lighting related devices, etc.) that is configured to receive information (e.g., data) intended for multiple devices, including itself, and to selectively respond to particular information intended for it. The term “addressable” often is used in connection with a networked environment (or a “network,” discussed further below), in which multiple devices are coupled together via some communications medium or media. 
     In one network implementation, one or more devices coupled to a network may serve as a controller for one or more other devices coupled to the network (e.g., in a master/slave relationship). In another implementation, a networked environment may include one or more dedicated controllers that are configured to control one or more of the devices coupled to the network. Generally, multiple devices may be coupled to some network and each may have access to data that is present on the communications medium or media; however, a given device may be “addressable” in that it is configured to selectively exchange data with (i.e., receive data from and/or transmit data to) the network, based, for example, on one or more particular identifiers (e.g., “addresses”) assigned to it. 
     The term “network” as used herein refers to any interconnection of two or more devices (including controllers or processors) that facilitates the transport of information (e.g. for device control, data storage, data exchange, etc.) between any two or more devices and/or among multiple devices coupled to the network. As should be readily appreciated, various implementations of networks suitable for interconnecting multiple devices may include any of a variety of network topologies and employ any of a variety of communication protocols. Additionally, in various networks according to the present disclosure, any one connection between two devices may represent a dedicated connection between the two systems, or alternatively a non-dedicated connection. In addition to carrying information intended for the two devices, such a non-dedicated connection may carry information not necessarily intended for either of the two devices (e.g., an open network connection). Furthermore, it should be readily appreciated that various networks of devices as discussed herein may employ one or more wireless, wire/cable, and/or fiber optic links to facilitate information transport throughout the network. 
     It should be appreciated that all combinations of the foregoing concepts and additional concepts discussed in greater detail below (provided such concepts are not mutually inconsistent) are contemplated as being part of the inventive subject matter disclosed herein. In particular, all combinations of claimed subject matter appearing at the end of this disclosure are contemplated as being part of the inventive subject matter disclosed herein. It should also be appreciated that terminology explicitly employed herein that also may appear in any disclosure incorporated by reference should be accorded a meaning most consistent with the particular concepts disclosed herein. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the drawings, like reference characters generally refer to the same parts throughout the different views. Also, the drawings are not necessarily to scale, emphasis instead generally being placed upon illustrating the principles of the invention. 
         FIG. 1  illustrates a lighting system including a lighting driver and a light emitting diode (LED) module, according to a representative embodiment. 
         FIG. 2  illustrates a flow diagram showing a process of generating the control signal, according to a representative embodiment. 
         FIG. 3A  illustrates a lighting driver, according to a representative embodiment. 
         FIG. 3B  illustrates an LED module usable with the lighting driver of  FIG. 3A , according to a representative embodiment. 
         FIG. 4  illustrates an LED module usable with the lighting driver of  FIG. 1 , according to a representative embodiment. 
         FIG. 5  illustrates an LED module usable with the lighting driver of  FIG. 1 , according to a representative embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     In the following detailed description, for purposes of explanation and not limitation, representative embodiments disclosing specific details are set forth in order to provide a thorough understanding of the present teachings. However, it will be apparent to one having ordinary skill in the art having had the benefit of the present disclosure that other embodiments according to the present teachings that depart from the specific details disclosed herein remain within the scope of the appended claims. Moreover, descriptions of well-known apparatuses and methods may be omitted so as to not obscure the description of the representative embodiments. Such methods and apparatuses are clearly within the scope of the present teachings. 
     Generally, it is desirable that light from a solid state lighting load, such as a light emitting diode (LED) module for example, may be emitted at a selected constant brightness or lumens. It is desirable that the LED current through the LED module is maintained at a selected constant level over the lifetime of the LED module so that light of the selected brightness may be emitted by the LED module, regardless of the amplitude of the mains voltage powering the lighting system, and despite aging and/or temperature variations of the LED module and tolerances of the power supply and/or lighting drivers. It is also generally desirable that when LED modules each designed to emit light of a selected brightness are disposed near each other, they consistently emit light of relatively the same brightness. It is still further desirable that such respective LED modules of similar design and disposed near each other may be controllable by a same dimming device to emit light of relatively the same brightness. In the various embodiments, these objectives and others may be achieved by controlling the driving current provided to an LED module responsive to an amplitude of the mains voltage and a current feedback signal indicative of the detected LED current through the LED module. 
       FIG. 1  illustrates a lighting system  10  including a lighting driver  100  and a light emitting diode (LED) module  200 , according to a representative embodiment. Lighting driver  100  may include mains voltage source  110 , dimmer  120 , power converter  130 , voltage measurement circuit  140 , dimmer measurement circuit  150 , driver controller  160  and power controller  170 . 
     In some embodiments, mains voltage source  110  may provide AC mains voltage of 120 volts AC, 220 volts AC, 277 volts AC, 480 volts AC, or any other AC voltage, depending on the power supply connected to lighting system  10 . Mains voltage source  110  may be characterized as a universal AC mains voltage source providing any mains voltage within a range of about 90 volts AC to 480 volts AC, for example. Lighting system  10  is thus designed as operable responsive to various different AC main voltages. In some embodiments, dimmer  120  may be an electronic low voltage (ELV) dimmer, a triac dimmer, or other type dimmers that cut or modify a phase of the mains voltage provided to power converter  130  to adjustably dim the light emitted by LED module  200  to a desired dimming level. Dimmer  120  may be responsive to a wall mounted switch or potentiometer manipulated by a system user, 
     Voltage measurement circuit  140  as shown in  FIG. 1  is connected to mains voltage source  110 , and is configured to measure the amplitude of the mains voltage, and output a voltage sense signal indicative of the amplitude of the mains voltage to driver controller  160 . Since rectification of the mains voltage may typically be a function of power converter  130 , the mains voltage provided to voltage measurement circuit  140  may or may not be rectified. Voltage measurement circuit  140  thus may or may not rectify the mains voltage prior to measurement. The voltage sense signal indicates whether the AC mains voltage provided by mains voltage source  110  is 120 volts AC, 277 volts AC, or 480 volts AC for example. In some embodiments, voltage measurement circuit  140  may include diodes for rectifying the AC mains voltage. The voltage sense signal may be an analog signal. 
     Dimmer measurement circuit  150  as shown in  FIG. 1  is connected to the mains voltage output from dimmer  120 , and is configured to detect if the phase of the mains voltage output from dimmer  120  is cut or modified and output a dimmer sense signal to driver controller  160  responsive to the detected cut or modified phase of the mains voltage. In some embodiments, dimmer measurement circuit  150  may include filters and analog to digital converters for example, and may convert the mains voltage output from the dimmer  120  into a square wave and output the square wave as the dimmer sense signal. The square wave may have a duty cycle corresponding to the amount of phase cut from the mains voltage by dimmer  120 . For example, in some embodiments dimmer measurement circuit  150  may convert mains voltage that does not have any phase cut into a square wave having 50% duty cycle indicative of a maximum desired lighting level (no dimming), and may convert mains voltage having a maximum amount of phase cut into a square wave having a minimal duty cycle indicative of a minimal desired lighting level (maximum dimming). 
     Power converter  130  is connected to the mains voltage provided from dimmer  120 , and is controlled by power controller  170  responsive to a control signal provided from driver controller  160  to provide a driving current to LED module  200 . As will be subsequently described in further detail, power converter  130  may be characterized as a constant power source configured to provide a driving current to LED module  200 , to maintain the LED current through LEDs  211 ,  212 ,  213 ,  214 ,  215 , . . . ,  21   n  at a selected constant level, to consequently maintain light emitted from LED module  200  at a selected constant brightness. In the representative embodiment shown in  FIG. 1 , power converter  130  includes a buck power converter. In some representative embodiments, power converter  130  may instead include a flyback power converter. Power controller  170  may include a power factor correction (PFC) chip configured to control power converter  130  responsive to a control signal output from driver controller  160  through resistor  180 . In some representative embodiments, the control signal may be a pulse-width modulation (PWM) signal, and/or power controller  170  may be integrated within power converter  130 . Resistor  180  as shown includes a first terminal end connected to driver controller  160 , and a second terminal end connected to power controller  170 . As further shown, capacitor  190  includes a first terminal end connected to the second terminal end of resistor  180 , and a second terminal end connected to ground. The operation and structure of power converter  130 , which as noted above may be a buck power converter, a flyback power converter, or other types of power converters in certain representative embodiments, are well known and further description thereof is omitted so as to not obscure the description. Likewise, the operation and structure of power controller  170 , which as noted above may be a PFC chip or the like in certain representative embodiments, are well known and further description thereof is also omitted. 
     LED module  200  as shown in  FIG. 1  includes a string of LEDs  211 ,  212 ,  213 ,  214 ,  215 ,  . . . ,  21   n  connected in series. Although the string is shown as including a plurality of LEDs, in some representative embodiments the string may include a single LED. Cable  300  interconnects lighting driver  100  and LED module  200 . Cable  300  includes a first wire connected between power converter  130  and a first end of the string at an anode of LED  211 , and a second wire connected between power converter  130  and a second end of the string at a cathode of LED  21   n  via resistor  270 . LEDs  211 ,  212 ,  213 ,  214 ,  215 , . . . ,  21   n  are driven to emit light responsive to the driving current provided from power converter  130  to the string via the first wire of cable  300 . 
     LED module  200  as shown in  FIG. 1  further includes amplifier  240  having an input connected to a node between LED  21   n  of the string and resistor  270 . Amplifier  240  may be an operational amplifier (op-amp), and is configured to amplify the LED current (lighting current) that has passed or flowed through the string at the node between LED  21   n  and resistor  270 , and provide the amplified LED current as a detected LED current to analog to digital (A/D) converter  250 . A/D converter  250  is configured to convert the detected LED current into a digital signal. The digital signal output from A/D converter  250  may be characterized as a current feedback signal indicative of the detected LED current through the string. An optical isolator (opto-coupler)  260  is connected to the output of A/D converter  250 , and is configured to transmit the current feedback signal from LED module  200  via cable  300  to driver controller  160  within lighting driver  100 . In a representative embodiment, A/D converter  250  may include an N-bit analog to digital converter where N is a real number greater than or equal to 2. For example, A/D converter  250  may include a 12 bit analog to digital converter. Optical isolator  260  may include a digital I2C opto-coupler, or any other sufficiently fast digital opto-coupler, and is configured to provide the current feedback signal to lighting driver  100  via two additional wires of cable  300 . Optical isolator  260  may be disposed exteriorly of LED module  200 . 
     As noted above, power converter  130  in the representative embodiment of  FIG. 1  includes a buck power converter, and is thus connected to a different ground than driver controller  160 . That is, power converter  130  and driver controller  160  have isolated ground references. Since the ground of LED module  200  is floating with respect to the ground of driver controller  160 , LED module  200  further includes local voltage source  230  connected to power converter  130 . Local voltage source  230  is configured to provide a local voltage to power amplifier  240 , A/D converter  250  and optical isolator  260 . In a representative embodiment, local voltage source  230  may include one or more zener diodes or DC-DC switches, and may provide a local voltage of 5 volts DC for example. In representative embodiments where power converter  130  includes a flyback power converter instead of a buck power converter, if the ground connected to the flyback power converter may be the same as the ground connected to driver controller  160 , local voltage source  230  and optical isolator  260  may be excluded from LED module  200 , A/D converter  250  and amplifier  240  may be powered off the same source as driver controller  160 , i.e., via an auxiliary rail (not shown) from power converter  130 , and the current feedback signal may be provided directly to driver controller  160  as a digital signal from A/D converter  250  or as an analog signal in the case that A/D converter  250  is further excluded from LED module  200 . In general, in the case that power converter  130  and driver controller  160  share a common ground reference and thus have non-isolated ground references, local voltage source  230  and optical isolator  260  may be excluded from LED module  200 . In the case that A/D converter  250  is further excluded, driver controller  160  may be configured as an including an A/D converter for converting the current feedback signal received in analog form. 
     In a representative embodiment, driver controller  160  within lighting driver  100  is connected to voltage measurement circuit  140 , dimmer measurement circuit  150  and cable  300 , and is configured to output the control signal responsive to the voltage sense signal, the dimmer sense signal and the current feedback signal. In some representative embodiments, lighting driver  100  may be implemented without a dimming feature, and thus dimmer  120  and dimmer measurement circuit  150  may be excluded and the mains voltage from mains voltage source may be provided directly to power converter  130 . In such a case, driver controller  160  may be configured to output the control signal responsive to the voltage sense signal the current feedback signal. 
     As described previously, in a representative embodiment the control signal may be a PWM signal, or an analog signal in the case that driver controller is configured to include a digital to analog converter, and power controller  170  may be configured as responsive to the PWM signal to control power converter  130  to adjust the driving current so that the LED current (lighting current) passed through the string is maintained at a selected constant level. In a representative embodiment, driver controller  160  may be a microprocessor or microcontroller, and may include memory and/or be connected to memory. The functionality of driver controller  160  may be implemented by one or more processors or controllers. In either case, driver controller  160  may be programmed using software or firmware (e.g., stored in memory) to perform the corresponding functions described, or may be implemented as a combination of dedicated hardware to perform some functions and a processor (e.g., one or more programmed microprocessors and associated circuitry) to perform other functions. Examples of controller components that may be employed in various representative embodiments include, but are not limited to, conventional microprocessors, microcontrollers, application specific integrated circuits (ASICs) and field programmable gate arrays (FPGAs). 
       FIG. 2  illustrates a flow diagram showing a process of generating the control signal described with respect to  FIG. 1 , according to a representative embodiment. In this representative embodiment, the control signal is understood to be a PWM signal, although in other representative embodiments control signal may have a different format. Upon starting the process responsive to turning on mains voltage source  110  of lighting driver  100  to provide mains voltage for powering LED module  200  of lighting system  10 , driver controller  160  outputs a PWM signal in step S 1  that has a duty cycle based on a last saved PWM value to power controller  170 . Thereafter driver controller  160  determines in step S 2  whether or not lighting system  10  is configured as including a dimmer such as dimmer  120 , according to configuration information that may be stored in memory for example or responsive to a change in the phase of the mains voltage indicative that a dimmer such as dimmer  120  has been enabled or placed in the circuitry of lighting driver  100 . In the event that driver controller  160  determines in step S 2  that lighting system  10  is configured as including a dimmer, driver controller  160  subsequently sets a lowest dimming level limit in step S 3 . The purpose of setting the lowest dimming level in step S 3  is so that driver controller  160  does not brown out or lose control of lighting system  10  in the event that dimmer  120  is able to go to levels close to zero. Hence, the minimum dimming level is used to always keep power converter  130  on to the extent that an auxiliary rail (not shown) of power converter  130  can provide enough power to driver controller  160 . In the event that driver controller  160  determines in step S 2  that lighting system  10  is not configured as including a dimmer, the process proceeds to step S 4  where driver controller  160  determines the LED current according to the current feedback signal. Thereafter driver controller  160  determines in step S 5  if the LED current is at a required level according to either the voltage sense signal and the dimmer sense signal in the case that lighting system  10  includes a dimmer, or according to the voltage sense signal in the case that lighting system  10  does not include a dimmer. In the event that it is determined in step S 5  that the LED current is at the required level, driver controller  160  maintains the duty cycle of the PWM signal in step s 6 . In the event that it is determined in step S 5  that the detected LED current is not at the required level, driver controller  160  adjusts the duty cycle of the PWM signal in step s 7  so that the driving current provided by power converter  130  may consequently adjust the driving current so that the LED current through the string in LED module  200  may be returned to the selected constant level. The process subsequently loops through steps S 4 -S 7  to maintain the LED current through the string in LED module  200  at the selected constant level. 
     In accordance with the representative embodiment described with respect to  FIGS. 1 and 2 , the current feedback signal indicative of the LED current through the string is used to adjust the control signal (PWM signal) output from driver controller  160 , to compensate for any inherent design/manufacturing tolerances in power controller  170  and/or power converter  130 , and to consequently ensure that the appropriate driving current is provided to LED module  200 . Accordingly, the LED current (lighting current) passed through the string may be maintained at a selected constant level, and consequently the light emitted by LED module may be maintained at a selected constant brightness, despite such tolerances. Also, the LED current through LED module  200  may be maintained at a selected constant level over the lifetime of LED module  200 , regardless of the amplitude and/or variations of the mains voltage powering lighting system  10 , and despite aging and/or temperature variations of LEDs  211 ,  212 ,  213 ,  214 ,  215 , . . . ,  21   n  within LED module  200 . Moreover, power converter  130  may be controlled responsive to the current feedback signal to reduce and/or eliminate flicker at lower dimming levels, so that lighting system  10  may be compatible with a wide range of different dimmers. Also, in the event of a shorted LED within the string, the current could be maintained constant responsive to the current feedback signal. Additionally, a maximum string current may be set in the case of a failure in the system. 
       FIG. 3A  illustrates a lighting driver  400  and  FIG. 3B  illustrates an LED module  500  usable with the lighting driver  400  of  FIG. 3A , according to a representative embodiment. Lighting driver  400  and lighting module  500  include similar components as lighting driver  100  and LED module  200  shown in  FIG. 1  which may be denoted with similar reference numerals. Detailed description of the similar components may hereinafter be omitted so as to not obscure the description of this representative embodiment. 
     As shown in  FIG. 3B , LED module  500  is configured as including a plurality of strings connected to different driving currents respectively provided by power converters  131 ,  132 , . . . ,  13   m  within lighting driver  400 . The LED currents (lighting currents) through each of the strings within lighting module  500  may thus be independently controlled so as to be maintained at a same selected constant level, so that the light emitted from the strings may consequently be maintained at selected constant brightness. 
     Lighting driver  400  as shown in  FIG. 3A  includes mains voltage source  110 , dimmer  120 , voltage measurement circuit  140  and dimmer measurement circuit  150  of similar function and interconnection as described with respect to  FIG. 1 . Dimmer  120  is configured as previously described to output mains voltage which may or may not have cut or modified phase to each of power converters  131 ,  132 , . . . ,  13   m.    
     Driver controller  360  shown in  FIG. 3A  is configured to provide a first control signal to power controller  171  through resistor  181 . Resistor  181  includes a first end terminal connected to driver controller  360 , and a second end terminal connected to power controller  171 . Capacitor  191  includes a first end terminal connected to the second end terminal of resistor  181 , and a second end terminal connected to ground. Power controller  171  controls power converter  131  to provide a first driving current to lighting module  500  via wiring pair w 1 . Driver controller  360  is further configured to provide a second control signal to power controller  172  through resistor  182 . Resistor  182  includes a first end terminal connected to driver controller  360 , and a second end terminal connected to power controller  172 . Capacitor  192  includes a first end terminal connected to the second end terminal of resistor  182 , and a second end terminal connected to ground. Power controller  172  controls power converter  132  to provide a second driving current to lighting module  500  via wiring pair w 2 . Driver controller  360  is still further configured to provide an mth control signal to power controller  17   m  through resistor  18   m . Resistor  18   m  includes a first end terminal connected to driver controller  360 , and a second end terminal connected to power controller  17   m . Capacitor  19   m  includes a first end terminal connected to the second end terminal of resistor  18   m , and a second end terminal connected to ground. Power controller  17   m  controls power converter  13   m  to provide an mth driving current to lighting module  500  via wiring pair wm. 
     Lighting module  500  as shown in  FIG. 3B  includes local voltage source  230 , A/D converter  250  and optical isolator  260  of similar function and interconnection as described with respect to  FIG. 1 . In this representative embodiment, local voltage source  230  is connected to a first wire of wiring pair w 1 , but may in the alternative be connected to a first wire of wiring pair w 2  or a first wire of wiring pair wm. 
     Lighting module  500  shown in  FIG. 3B  includes a first string of LEDs  211 ,  212 ,  213 ,  214 ,  215 , . . . ,  21   n  connected in series. An anode of LED  211  is connected to a first wire of wiring pair w 1  and a cathode of LED  21   n  is connected to a second wire of wiring pair w 1  through resistor  271 . The first string of LEDs  211 ,  212 ,  213 ,  214 ,  215 , . . . ,  21   n  is driven to emit light responsive to the first driving current. Amplifier  241  has an input connected to a first node between LED  21   n  of the first string and resistor  271 , and is configured to amplify the LED current that has passed through the first string at the first node and provide a first amplified LED current as a first detected LED current to multiplexer  280 . Lighting module  500  further includes a second string of LEDs  221 ,  222 ,  223 ,  224 ,  225 , . . . ,  22   n  connected in series. An anode of LED  221  is connected to a first wire of wiring pair w 2  and a cathode of LED  22   n  is connected to a second wire of wiring pair w 2  through resistor  272 . The first string of LEDs  221 ,  222 ,  223 ,  224 ,  225 , . . . ,  22   n  is driven to emit light responsive to the second driving current. Amplifier  242  has an input connected to a second node between LED  22   n  of the second string and resistor  272 , and is configured to amplify the LED current that has passed through the second string at the second node and provide a second amplified LED current as a second detected LED current to multiplexer  280 . Lighting module  500  still further includes an mth string of LEDs  2   m   1 ,  2   m   2 ,  2   m   3 ,  2   m   4 ,  2   m   5 , . . . ,  2   mn  connected in series. An anode of LED  2   m   1  is connected to a first wire of wiring pair wm and a cathode of LED  2   mn  is connected to a second wire of wiring pair wm through resistor  27   m . The mth string of LEDs  2   m   1 ,  2   m   2 ,  2   m   3 ,  2   m   4 ,  2   m   5 , . . . ,  2   mn  is driven to emit light responsive to the mth driving current. Amplifier  24   m  has an input connected to an mth node between LED  2   mn  of the mth string and resistor  27   m , and is configured to amplify the LED current that has passed through the mth string at the mth node and provide an mth amplified LED current as an mth detected LED current to multiplexer  280 . 
     Multiplexer  280  is configured to selectively output the first, second and mth detected LED currents to A/D converter  250  in sequence responsive to multiplex control signal mux_ctrl. In a representative embodiment, multiplexer  280  may be a switch that toggles between three input terminals respectively connected to the first, second and mth detected LED currents to selectively provide the detected LED currents to A/D converter  250  via an output terminal. A/D converter  250  converts the first, second and mth detected LED currents selectively provided from multiplexer  280  in sequence into respective digital signals that may be characterized as corresponding first, second and mth current feedback signals which are sequentially transmitted via wiring pair wfb to driver controller  360  within lighting driver  400 . Driver controller  360  is configured to output the first, second and mth control signals responsive to the respective first, second and mth current feedback signals, and further responsive to the voltage sense signal and the dimmer sense signal, to independently control the LED currents (lighting currents) through each of the strings within lighting module  500  to be maintained at a same selected constant level, so that the light emitted from the strings may consequently be maintained at selected constant brightness. Multiplex control signal mux_ctrl may be a clocked signal or the like generated within LED module  500 , and driver controller  360  may be configured as operable in synchronization with a similarly provided or generated clock to output the first, second and mth control signals responsive to the respective first, second and mth current feedback signals. In a representative embodiment, driver controller  360  may be configured to generate and send the mux_ctrl signal to lighting module  500  through an opto-coupler, or directly in the case where lighting driver  400  and lighting module  500  share a common ground reference. In accordance with this representative embodiment, strings having different numbers of LEDs and/or different color LEDs may also be independently controlled. 
       FIG. 4  illustrates an LED module  600  usable with the lighting driver  100  of  FIG. 1 , according to a representative embodiment. Lighting module  600  includes similar components as LED module  200  shown in  FIG. 1  which may be denoted with similar reference numerals. Detailed description of the similar components may hereinafter be omitted so as to not obscure the description of this representative embodiment. 
     LED module  600  as shown in  FIG. 4  includes a string of LEDs  211 ,  212 ,  213 ,  214 ,  215 ,  . . . ,  21   n  connected in series. Cable  300  interconnects lighting driver  100  and LED module  600 . Cable  300  includes a first wire connected between power converter  130  and a first end of the string at an anode of LED  211 , and a second wire connected between power converter  130  and a second end of the string at a cathode of LED  21   n  via resistor  270 . LEDs  211 ,  212 ,  213 ,  214 ,  215 , . . . ,  21   n  are driven to emit light responsive to the driving current provided from power converter  130  to the string via the first wire of cable  300 . LED module  700  further includes local voltage source  230  and optical isolator (opto-coupler)  260  as shown and described with respect to  FIG. 1 . 
     As further shown in  FIG. 4 , an LED current (lighting current) that has passed or flowed through the string at the node between LED  21   n  and resistor  270  is provided to microcontroller  410 . As further shown, resistor  422  includes a first end terminal connected to the first wire of cable  300 . Resistor  424  includes a first end terminal connected to a second end terminal of resistor  422 , and a second end terminal connected to the second wire of cable  300  that is connected to resistor  270 , which is the microcontroller  410  side ground. A sensed voltage level indicative of a voltage across the LED string is provided from the node between resistors  422  and  424  to microcontroller  410 . A temperature sensor  420  is configured to sense a temperature of the LEDs  211 ,  212 ,  213 ,  214 ,  215 , . . . ,  21   n  and provide a temperature sense signal indicative of the detected temperature to microcontroller  410 . Microcontroller  410  is configured to output a digital signal including a current feedback signal responsive to the LED current at the node between LED  21   n  and resistor  270 , an LED voltage feedback signal responsive to the voltage level at the node between resistors  422  and  424 , and an LED temperature feedback signal responsive to the temperature sense signal provided by temperature sensor  420 . Optical isolator (opto-coupler)  260  is connected to the output of microcontroller  410  and is configured to transmit the digital signal from microcontroller  410  via cable  300  to driver controller  160  within lighting driver  100  shown in  FIG. 1 . In this representative embodiment, driver controller  160  is configured to output the control signal to power controller  170  responsive to the current feedback signal, the LED voltage feedback signal and the LED temperature feedback signal, in addition to the voltage sense signal output from voltage measurement circuit  140  and the dimmer sense signal output from dimmer measurement circuit  150 , to control the driving current output from power converter  130  to LED module  600 . 
       FIG. 5  illustrates an LED module  700  usable with the lighting driver  100  of  FIG. 1 , according to a representative embodiment. Lighting module  700  includes similar components as lighting module  600  shown in  FIG. 4  which may be denoted with similar reference numerals. Detailed description of the similar components may hereinafter be omitted so as to not obscure the description of this representative embodiment. In this representative embodiment, power converter  130  may include a flyback power converter for example, and the ground of the flyback power converter may be the same as the ground connected to driver controller  160 . Accordingly, the current feedback signal responsive to the LED current at the node between LED  21   n  and resistor  270 , the LED voltage feedback signal responsive to the voltage level at the node between resistors  422  and  424 , and an LED temperature feedback signal provided by temperature sensor  420  may be directly transmitted to driver controller  160  of lighting driver  100  via cable  300 . 
     While several inventive embodiments have been described and illustrated herein, those of ordinary skill in the art will readily envision a variety of other means and/or structures for performing the function and/or obtaining the results and/or one or more of the advantages described herein, and each of such variations and/or modifications is deemed to be within the scope of the inventive embodiments described herein. More generally, those skilled in the art will readily appreciate that all parameters, dimensions, materials, and configurations described herein are meant to be exemplary and that the actual parameters, dimensions, materials, and/or configurations will depend upon the specific application or applications for which the inventive teachings is/are used. Those skilled in the art will recognize, or be able to ascertain using no more than routine experimentation, many equivalents to the specific inventive embodiments described herein. It is, therefore, to be understood that the foregoing embodiments are presented by way of example only and that, within the scope of the appended claims and equivalents thereto, inventive embodiments may be practiced otherwise than as specifically described and claimed. Inventive embodiments of the present disclosure are directed to each individual feature, system, article, material, kit, and/or method described herein. In addition, any combination of two or more such features, systems, articles, materials, kits, and/or methods, if such features, systems, articles, materials, kits, and/or methods are not mutually inconsistent, is included within the inventive scope of the present disclosure. 
     All definitions, as defined and used herein, should be understood to control over dictionary definitions, definitions in documents incorporated by reference, and/or ordinary meanings of the defined terms. 
     The indefinite articles “a” and “an,” as used herein in the specification and in the claims, unless clearly indicated to the contrary, should be understood to mean “at least one.” 
     The phrase “and/or,” as used herein in the specification and in the claims, should be understood to mean “either or both” of the elements so conjoined, i.e., elements that are conjunctively present in some cases and disjunctively present in other cases. Multiple elements listed with “and/or” should be construed in the same fashion, i.e., “one or more” of the elements so conjoined. Other elements may optionally be present other than the elements specifically identified by the “and/or” clause, whether related or unrelated to those elements specifically identified. Thus, as a non-limiting example, a reference to “A and/or B”, when used in conjunction with open-ended language such as “comprising” can refer, in one embodiment, to A only (optionally including elements other than B); in another embodiment, to B only (optionally including elements other than A); in yet another embodiment, to both A and B (optionally including other elements); etc. 
     As used herein in the specification and in the claims, “or” should be understood to have the same meaning as “and/or” as defined above. For example, when separating items in a list, “or” or “and/or” shall be interpreted as being inclusive, i.e., the inclusion of at least one, but also including more than one, of a number or list of elements, and, optionally, additional unlisted items. Only terms clearly indicated to the contrary, such as “only one of” or “exactly one of,” or, when used in the claims, “consisting of,” will refer to the inclusion of exactly one element of a number or list of elements. In general, the term “or” as used herein shall only be interpreted as indicating exclusive alternatives (i.e. “one or the other but not both”) when preceded by terms of exclusivity, such as “either,” “one of,” “only one of,” or “exactly one of.” 
     As used herein in the specification and in the claims, the phrase “at least one,” in reference to a list of one or more elements, should be understood to mean at least one element selected from any one or more of the elements in the list of elements, but not necessarily including at least one of each and every element specifically listed within the list of elements and not excluding any combinations of elements in the list of elements. This definition also allows that elements may optionally be present other than the elements specifically identified within the list of elements to which the phrase “at least one” refers, whether related or unrelated to those elements specifically identified. 
     It should also be understood that, unless clearly indicated to the contrary, in any methods claimed herein that include more than one step or act, the order of the steps or acts of the method is not necessarily limited to the order in which the steps or acts of the method are recited. Also, reference numerals appearing the claims, if any, are provided merely for convenience and should not be construed as limiting the claims in any way. 
     In the claims, as well as in the specification above, all transitional phrases such as “comprising,” “including,” “carrying,” “having,” “containing,” “involving,” “holding,” “composed of,” and the like are to be understood to be open-ended, i.e., to mean including but not limited to. Only the transitional phrases “consisting of” and “consisting essentially of” shall be closed or semi-closed transitional phrases, respectively, as set forth in the United States Patent Office Manual of Patent Examining Procedures, Section 2111.03.