Patent Publication Number: US-11647574-B2

Title: Voltage regulator circuit for LED luminaire

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a divisional of U.S. patent application Ser. No. 17/243,914, filed Apr. 29, 2021, which claims priority to U.S. Provisional Patent Application Ser. No. 63/130,521, filed Dec. 24, 2020. Both of those applications are incorporated by reference herein in their entireties. 
    
    
     TECHNICAL FIELD 
     The invention relates to linear lighting and, more specifically, to linear luminaires. 
     BACKGROUND 
     Linear lighting is a class of lighting in which an elongate, narrow printed circuit board (PCB) is populated with light-emitting diode (LED) light engines, typically spaced from one another at a regular spacing or pitch. The PCB may be either flexible or rigid. Although a strip of linear lighting is a microelectronic circuit on a PCB, for various reasons, lighting circuits are usually kept simple, often no more than the LED light engines and an element or elements to set the current in the circuit, typically a resistor or a current-source integrated circuit. Combined with an appropriate power supply, linear lighting is considered a luminaire in its own right, although it is frequently used as a raw material in the construction of more complex luminaires. 
     Linear luminaires, i.e., finished light fixtures based on linear lighting, are often made by placing a strip of linear lighting in a channel and covering it with a cover. The channels are typically extrusions, with a constant cross-sectional shape, and in most cases, the strip of linear lighting is mounted directly on the bottom or one of the sidewalls of the channel. Most channels are made of a metal, such as anodized aluminum, although some channels may be made of plastic. The ends of a channel are typically capped with endcaps. 
     The channel in a linear luminaire serves several functions. First and foremost, it provides some protection from dirt, dust, and the elements. Second, depending on the particular application, the channel cover may diffuse and direct the light emitted by the linear lighting. Finally, linear lighting generates heat, and the channel may act as a heat sink. 
     As linear luminaires have become more prevalent in the market, they are often called upon to perform in more and more extreme environments, for example, weathering long outdoor exposures. Moreover, while many designers and consumers were once content to save energy merely by switching from incandescent, neon, or fluorescent lighting to LED lighting, modern designers and consumers expect better energy efficiency from modern linear luminaires, as well as greater functionality and more control over that functionality. 
     BRIEF SUMMARY 
     One aspect of the invention relates to a linear luminaire. The linear luminaire has a channel, which has a bottom and a pair of sidewalls that arise from the bottom, giving the channel a U-shape in cross-section. An elongate printed circuit board (PCB) is mounted on stand-offs above the bottom, leaving a lower compartment or portion of the channel open. The PCB has a plurality of LED light engines mounted on it, and those LED light engines may be spaced at a close pitch along the length of the PCB. The PCB may be rigid, made, for example, of aluminum, FR4, or another such material. The mounting of the PCB causes it to extend within an upper compartment or portion of the channel. At its ends, the upper compartment of the channel overhangs the lower compartment. That is, the upper compartment of the channel extends beyond the lower compartment. The PCB has an extent such that it ends almost exactly at the ends of the upper compartment of the channel. With this arrangement, several linear luminaires can be placed end-to-end with virtually no dark spots or light holes between them. The open lower compartment of the linear luminaire provides a raceway for wiring, and to the extent that wiring passes between adjacent linear luminaires, it is shielded from view by the overhung upper compartments of the adjacent linear luminaires. 
     Another aspect of the invention relates to drive circuits for linear luminaires. In a drive circuit according to this aspect of the invention, several series of LED light engines are connected in parallel to voltage and, through a driver integrated circuit (IC), to ground. The series of LED light engines may be of the same type or of different types, and thus, the series of LED light engines may take the same voltage or different voltages. Typically, series of LED light engines that take the same voltage are grouped together. The driver IC sets the current in each series of LED light engines. Power supply circuits under the control of one or more power control ICs take an input voltage and supply the voltages needed to activate the series of LED light engines and other electronic components. In each series, the voltage remaining after the last LED light engine in the series is detected and sent into a power feedback circuit coupled to the one or more power control ICs. The power feedback circuit provides a feedback signal to the power control ICs that causes the voltage applied to the series of LED light engines to be increased or decreased. In some cases, the power feedback signal may be generated by an integrator. This may have the effect of compensating for variations in the forward voltages of the various LED light engines. 
     In some embodiments according to this aspect of the invention, the driver IC may modulate the power applied to the series of LED light engines with a pulse-width modulation (PWM) signal, such as a PWM current signal. In this case, each series of LED light engines may have a parallel leg that connects after cathode of the last LED light engine in the series. The parallel leg may have a filter, such as an RC low-pass filter, that filters out the PWM modulation so that a generally steady-state remaining voltage can be detected and sent to the power feedback circuit. Based on the remaining voltage, the applied voltage may be increased or decreased to ensure that the driver IC receives at least a threshold minimum voltage. 
     Yet another aspect of the invention also relates to drive circuits for linear luminaires. In a drive circuit according to this aspect of the invention, at least one series of LED light engines is arranged between voltage and ground. A driver IC sets the current in the series of LED light engines. A switching element, such as a bipolar junction transistor (BJT) is arranged between the series of LED light engines and the driver IC such that its collector is connected to the series of LED light engines and its emitter is connected to the driver IC. When the driver IC sets the current in the series of LED light engines to a nonzero value, a steady voltage supplied to the base of the BJT allows power to flow between collector and emitter. When the driver IC sets the current in the series of LED light engines to zero, the voltage at the base of the BJT trends toward zero, such that the BJT does not allow power to flow and protects the driver IC from high voltages. The driver IC may modulate the power applied to the series of LED light engines with a pulse-width modulation (PWM) signal. 
     A further aspect of the invention relates to control methods for luminaires. In one method using the kind of drive circuits described above, a particular drive circuit has a fixed power budget. When instructions to activate one or more series of LED light engines are received, a central unit of the drive circuit examines the instructions, determines if any available series of LED light engines will be unused when the instructions are executed, and if so, reallocates the unused power among the series of LED light engines that are or will be active when the instructions are executed. 
     Yet another further aspect of the invention relates to color transitions in luminaires having LED light engines capable of emitting different color temperatures of white light. In these types of luminaires, if a transition between a first color temperature of white light and a second color temperature of white light is detected, a central unit may alter the transition instructions such that the transition occurs along the Planckian locus. 
     Other aspects, features, and advantages of the invention will be set forth in the description that follows. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWING FIGURES 
       The invention will be described with respect to the following drawing figures, in which like numerals represent like features throughout the description, and in which: 
         FIG.  1    is a perspective view of a linear luminaire according to one embodiment of the invention; 
         FIG.  2    is a cross-sectional view taken through Line  2 - 2  of  FIG.  1   ; 
         FIG.  3    is a side elevational view of two linear luminaires abutted end-to-end; 
         FIG.  4    is a perspective view of the underside of two adjacent printed circuit boards, illustrating harnesses or electrical connectors that connect between them; 
         FIG.  5    is a view similar to the view of  FIG.  2   , illustrating the linear luminaire encapsulated with resin; 
         FIG.  6    is a schematic diagram of a first portion of a lighting circuit for a linear luminaire, illustrating series of different types of LED light engines; 
         FIG.  7    is a schematic diagram of a first voltage feedback circuit for voltage adjustment in a linear luminaire; 
         FIG.  8    is a schematic diagram of a second voltage feedback circuit for voltage adjustment in a linear luminaire; 
         FIGS.  9 - 1  and  9 - 2    are, collectively, a schematic diagram of power circuitry for a linear luminaire, illustrating boost and buck converter circuit topologies with controllers that are responsive to voltage feedback from circuits like those shown in  FIGS.  7  and  8   ; 
         FIG.  10    is a schematic overall diagram of a lighting circuit for a linear luminaire; 
         FIG.  11    is a schematic diagram of a method for allocating power among series of LED light engines in a linear luminaire; 
         FIG.  12    is a schematic diagram of a method for color-correcting transitions between one color temperature of white light and another; 
         FIG.  13    is a schematic diagram of a first alternative voltage feedback circuit for voltage adjustment in a linear luminaire; 
         FIG.  14    is a schematic diagram of a second alternative voltage feedback circuit for voltage adjustment in a linear luminaire; 
         FIG.  15    is a schematic diagram of a method for imposing a power consumption limit on the LED light engines of a luminaire; and 
         FIG.  16    is a schematic diagram of a method for controlling the temperature of a linear luminaire by controlling the power consumption of LED light engines installed in the linear luminaire. 
     
    
    
     DETAILED DESCRIPTION 
       FIG.  1    is a perspective view of a linear luminaire, generally indicated at  10 , according to one embodiment of the invention. The luminaire  10  comprises a channel  12  and a strip of linear lighting  14 . The strip of linear lighting  14  includes a plurality of LED light engines  16  disposed linearly along a printed circuit board (PCB)  18 . 
     As the term is used here, “light engine” refers to an element in which one or more light-emitting diodes (LEDs) are packaged, along with wires and other structures, such as electrical contacts, that are needed to connect the light engine to a PCB. LED light engines may emit a single color of light, or they may include red-green-blue (RGBs) that, together, are capable of emitting a variety of different colors depending on the input voltages. If the light engine is intended to emit “white” light, it may be a so-called “blue pump” light engine in which a light engine containing one or more blue-emitting LEDs (e.g., InGaN LEDs) is covered with a phosphor, a chemical compound that absorbs the emitted blue light and re-emits either a broader or a different spectrum of wavelengths. The particular type of LED light engine is not critical to the invention. In the illustrated embodiment, the light engines are surface-mount devices (SMDs) soldered to the PCB  18 , although other types of light engines may be used. For reasons that will be explained below in more detail, the LED light engines  16  may include individual red, green, and blue LEDs as well as two color temperatures of “white” LEDs, typically a “cool” white and a “warm” white. 
     In the illustrated embodiment, the LED light engines  16  are in the form of small, rectangular  2110  surface-mount packages. Such small packages may make it easier to mix and diffuse the resulting light. Of course, other sizes and packages are possible. 
     The channel  12  has a bottom  20  and a pair of sidewalls  22 ,  24  that arise from the bottom  20 . As shown in  FIG.  2   , a cross-sectional view taken through Line  2 - 2  of  FIG.  1   , the sidewalls  22 ,  24  are straight-sided in the illustrated embodiment, making rounded corners where they meet the bottom  20  and giving the channel  12  a U-shape as viewed in cross-section or from one of the ends. The channel  12  also has an upper portion  26  and a lower portion  28 . For reasons that will be described below in more detail, the upper portion  26  overhangs and extends out beyond the lower portion  28  at respective ends of the channel  12 . 
     In the illustrated embodiment, the strip of linear lighting  14  has a rigid PCB  18 . The PCB  18  may be made of, e.g., FR4 composite material, ceramic, or aluminum, to name a few possible materials. In most linear luminaires, the strip of linear lighting would be mounted to the bottom of the channel, or to one of the sidewalls. That is not the case in the linear luminare  10 . Instead, as can best be seen in the perspective view of  FIG.  1   , the PCB  18  is mounted on a series of stand-offs  30  that are connected directly between the PCB  18  and the bottom  20 . (In  FIG.  1   , a portion of the sidewall  24  is cut away to show the internal configuration of the channel  12 .) The stand-offs  30  have sufficient height such that the PCB  18  defines the boundary between the lower portion  28  and the upper portion  26 . The stand-offs  30  mount in through holes  32  through the PCB  18  and in through holes  34  through the bottom  20  of the channel  12 . The stand-offs  30  of the illustrated embodiment are hollow and threaded along their interior to receive screws or bolts, although rivets and other such securing structure may be used in other embodiments. 
     In addition to securement and positioning, the stand-offs  30  may serve as heat sinks, connecting the PCB  18  thermally with the channel  12  and serving to draw heat away from the PCB  18 . 
     The PCB  18  is coextensive with the full length of the overhung upper portion  26 , terminating essentially where the upper portion  26  terminates. The PCB  18  includes a line of LED light engines  36  that extends to the very ends of the PCB  18 . The LED light engines  36  are spaced together at a very close pitch, essentially as close to one another as practical. The line of LED light engines  36  is offset from the centerline of the PCB  18  so as to accommodate the through holes  32  for the stand-offs  30 . In addition to the through holes  32  for the stand-offs  30 , the PCB  18  has sets of through holes  38  spaced at intervals from one another along its length. In the illustrated embodiment, there are five through holes  38  in each set, aligned linearly with one another, and also in general alignment with the through holes  32  for the stand-offs  30  on the same side of the PCB  18 . The sets of through holes  38  provide channels through which wires for power and data can pass. With this arrangement, wires for power and data would pass through the sets of through-holes  38  and be soldered or otherwise connected to solder pads (not shown in the view of  FIGS.  1  and  2   ). 
     The luminaire  10  is arranged to provide a continuous line of light with as few interruptions (i.e., dark spots) as possible. Several features contribute to this. First, as noted above, the LED light engines  16  are spaced closely together, in this case typically 0.030 inches (0.762 mm) apart. Additionally, the overhung upper compartment  26  may contribute to this in some embodiments. 
     All channels  12  used for the luminaire  10  and for other linear luminaires have a finite maximum length. For example, for shipping and handling reasons, channels  12  may be limited in length to approximately 8 feet (2.4 meters). If a longer luminaire is needed, individual luminaires are placed end-to-end. When two typical luminaires are abutted end-to-end, there can be a gap, and thus, a dark spot, between the end of one luminaire and the beginning of the next. Several factors contribute to this gap, including endcaps in the ends of the luminaires and space needed between adjacent luminaires to allow for the passage of cables and wires. 
     The luminaire  10  is designed to reduce the gap between adjacent luminaires  10  as much as possible when two luminaires  10  are abutted end-to-end.  FIG.  3    is a side elevational view of two luminaires  10  abutted end-to-end. As will be described below in more detail, the design of the luminaires  10  may allow the luminaires to be without endcaps. However, as can also be appreciated from  FIG.  3   , the overhung upper compartments  26  assist in producing a gapless spacing between adjacent luminaires  10 . As shown, the upper compartments  26  abut in  FIG.  3   , while the lower compartments  28  stop well short of the extent of the upper compartments  26 . The linear lighting  14  comes to the edge or almost to the edge of each upper compartment  26 . 
     The overhung upper compartments  26  and shorter lower compartments  28  leave a space  40  between the two luminaires  10 , i.e., a space between adjacent lower compartments  28 , for the insertion of cables and wires. That space  40  may serve as a cableway, permitting wires or cables from one luminaire  10  to be connected to wires or cables from the abutted or adjacent luminaire  10 . Any cables or wires that may be in the cableway space  40 , are shielded from view by the abutted upper compartments  26 . The two lower compartments  28  each have openings, or knock-outs for openings  42 , at their ends, allowing cables and wires to enter the cableway space  40 , as can be seen in  FIG.  2   . 
     In embodiments of the luminaire  10 , the linear lighting  14  may be made to particular lengths that are shorter than the channels  12  in which they are to be placed. For that reason, individual lengths of linear lighting  14  may be joined together using harnesses or electrical connectors  44  to bring the power and control signals from one length of linear lighting  14  to the next.  FIG.  4    is a perspective view of the underside of two adjacent PCBs  18 , illustrating their joinder with connectors  44 . The connectors  44  would typically be press-fit connectors, although any type of connectors  44  may be used. The placement of the connectors  44  on the underside of the PCBs  18  prevents the connection from obscuring or obstructing the light output. Additionally, as shown, one connector  44  extends past the end of its PCB  18  while the complementary connector  44  is set back from the end of its PCB  18 . This allows for a connection with no gap between adjacent strips of linear lighting  14 . Connectors  44  like those shown in  FIGS.  2  and  4    may be used between strips of linear lighting  14  in the same channel  12 , and they may also be used to electrically connect two adjacent luminaires  10  in some cases. 
     Most linear luminaires include a cover on the channel that serves to cover and protect the linear lighting. As was described briefly above, the ends of channels may be capped with endcaps in order to close off the channel entirely. Linear luminaires  10  according to embodiments of the invention may use these elements. 
     However, the illustrated embodiment of the linear luminaire  10  is designed to be entirely encapsulated with a resin. Resin encapsulation is more likely than covers and endcaps to provide complete protection for the linear lighting  14  while at the same time providing other benefits, like heat transmissibility. Fully encapsulated by resin, a linear luminaire  10  may have a high ingress protection (IP) rating, up to and including IP68, a rating which permits full submersion of the luminaire  10  for some period of time. 
     U.S. Pat. No. 10,801,716 to Lopez-Martinez et al., the work of the present assignee, describes procedures for resin encapsulation of linear lighting, and is incorporated by reference in its entirety. For purposes of this description, the terms “resin encapsulation” and “potting” are used interchangeably. The linear luminaire  10  may be potted using a polyurethane resin, a silicone resin, or any other suitable resin. In a typical potting operation, the channel  12  would act as a mold for the resin, and the ends of the channel  12  may be capped or blocked temporarily to allow for the inpour of resin. Ports  46  in the channel  12 , shown particularly in  FIG.  2   , may be provided at regular intervals to allow for inflow of resin for potting, although in some embodiments, resin may simply be introduced by pouring it into the channel  12  from the top. 
     As the Lopez-Martinez et al. patent explains, during potting, resin can be deposited in several layers, and cured or partially cured between layers. In encapsulating a linear luminaire  10 , resins may be chosen specifically so that the encapsulation of the lower compartment  28  is optimized for heat transfer while the encapsulation of the upper compartment  26  is optimized for light emission. For example, the resin of the lower compartment  28  may be doped with ceramic or metal particles to aid in heat transmission, while the upper compartment  26  may use a clear, transparent resin. The resin of the upper compartment  26  may also be formed into a lens, e.g., a convex lens, a concave lens, etc. by using the meniscus of the liquid material or by filling the upper compartment  26  while capped with a mold. Diffusing additives may be used in the resin if greater light diffusion is desired. 
     When polymeric resins come into direct contact with light engines  16 , the quality of the light emitted by some types of light engines may change. Specifically, in blue-pump LED light engines that are topped with a phosphor, that phosphor is usually held within a silicone polymer matrix. Direct contact between an encapsulating resin and the silicone matrix that holds a phosphor allows more blue light to escape from the LED light engine for refractive reasons, causing a change in the color of the emitted light. 
     There are a number of different internationally-recognized systems for describing and reporting the color of light emitted from LED light engines. A full description of these systems is not necessary to understand the present invention. For these purposes, it is sufficient to say that the color of so-called “white light” LED light engines is usually described in terms of color temperature, measured in degrees Kelvin. The color temperature scale is a descriptive shorthand that compares the color emitted by a “white” LED light engine to the color of a blackbody radiator—an incandescent object whose color is determined only by its temperature. Stars, like our sun, provide natural light, are considered to be blackbody radiators. We compare artificial light sources, like LED light engines, to the light emitted by stars. For example, LED light engines that provide a “warm” white light with a large proportion of yellow and red in their spectra typically have a color temperature in the range of about 2400K to about 3500K. “Cooler” white LED light engines, with more blue in their spectra, typically have color temperatures in the range of 5000K to 6500K. For reference, the color temperature of sunlight varies throughout the day, but at noon on a clear summer day, the color temperature of sunlight is about 5500K. 
     The present assignee&#39;s own photometric measurements have shown that encapsulation with polyurethane resins can drive an increase in color temperature of several hundred degrees Kelvin, depending on the original color temperature of the LED light engines and the nature of the resin. In other words, significantly more blue light may be emitted by an encapsulated blue-pump “white” LED light engine. However, there may be other shifts as well. For example, the resin material itself may selectively absorb or attenuate certain wavelengths of light, for reasons having to do with its fundamental chemistry. For example, the present assignee has found that encapsulation with certain polyurethane resins can cause both an overall color temperature shift and a shift toward green. If a linear luminaire  10  according to an embodiment of the invention is encapsulated, and if it carries RGB LED light engines that are capable of producing many different colors, the light output of the RGB LED light engines may be used to compensate for color and color temperature shifts caused by the encapsulation process. This will be described below in more detail. 
     In any case,  FIG.  5    is an end elevational view of the luminaire  10 , similar to the view of  FIG.  2   , showing the luminaire  10  with a first potting material  60  in the upper compartment  26  and a second potting material  70  in the lower compartment  26 . As explained in the Lopez-Martinez et al. patent and above, the two potting materials  60 ,  70  may be the same, or they may have the same base with different additives, thus adapting the second potting material  70  for heat transmission and the first potting material  60  for light transmission. 
     Lighting Circuits 
     As those of skill in the art will appreciate, LED light engines  16  are solid-state semiconductor devices that are powered and controlled by a microelectronic circuit. (In this description, the term “drive” will be used as a synonym for “power and control.”) The exact type of circuit that is used to drive the LED light engines  16  will vary from embodiment to embodiment, depending on the nature of the LED light engines  16  (e.g., single-color or RGB) and the functions that the LED light engines  16  are to perform. 
     At its most basic, a drive circuit for LED light engines  16  of a single color may comprise a plurality of LED light engines  16  and a component or components to set the current in the circuit. The current-setting components may be either on the PCB  18  or in the power supply. The simplest current-setting component is a resistor, although current-source integrated circuits (ICs) may also be used. U.S. Pat. Nos. 10,928,017 and 10,897,802 provide more detail on basic LED lighting circuits and simple variations to those circuits that allow them to work with different input voltages and to provide different light outputs. Both of those patents are incorporated by reference herein in their entireties. 
     Many existing linear lighting circuits operate on direct current (DC) power at low voltage. For purposes of this description, the term “low voltage” refers to voltages under about 50V. However, there is no requirement that the voltage be low voltage. U.S. Pat. No. 10,028,345 gives examples of simple drive circuits for high-voltage linear lighting, and is incorporated by reference in its entirety. 
     If the LED light engines  16  are RGB LED light engines, drive circuits and systems can be more complex. First, RGB LED light engines typically have a separate circuit for each of the red LEDs, the green LEDs, and the blue LEDs. Second, red, green, and blue LEDs each have different forward voltages, which means that the configuration of, e.g., the red circuit may be different from the configuration of the blue circuit. 
     The elements described above are the elements that constitute a basic, functional lighting circuit. A basic lighting circuit will cause a luminaire to light when power is applied, but otherwise offers very little in the way of control or interface possibilities. With a basic lighting circuit, control elements external to a linear luminaire can be connected to it to allow more functionality. For example, external dimmers may allow a linear luminaire to dim. Additionally, if the linear luminaire has RGB LED light engines, it may be desirable to control the luminaire with an external controller that can translate a digital lighting control protocol, such as DMX512, into analog voltage signals for the LED light engines. The need for an external controller may also arise if a digital lighting control protocol like the digital addressable lighting interface (DALI). 
     Although complex lighting circuits are not necessarily the norm in the industry, since a strip of linear lighting  14  is a microelectronic circuit on a PCB  18 , it is perfectly possible to place control elements on the PCB  18  with the LED light engines  16 . Including control elements on the PCB  18  increases the functionality of the luminaire  10 , reduces the number and type of external control modules that are required, and may improve the ability of the luminaire  10  to manage its own particular output issues, like color shifts caused by encapsulation. 
     Thus, in some embodiments, a linear luminare  10  may include the electronics necessary to decode digital control signals and drive the LED light engines  16  accordingly, or to perform any subset of those functions. Any lighting control methods or protocols may be implemented in hardware on the PCB, including DMX512, DALI, 0-10V dimming, etc. The following description provides an example of digital control circuitry for a linear luminaire  10  that, among other things, implements DMX512 to control a number of different types of LED light engines  16 . Although the following description makes specific reference to the linear luminaire  10 , the described drive circuitry may be implemented in other types of solid-state luminaires. 
       FIG.  6    is a schematic diagram of a first portion of an LED drive circuit, generally indicated at  100 , according to an embodiment of the invention. The LED drive circuit  100  illustrated in  FIG.  6    assumes that the light engines  16  are actually of five different types: red LED light engines, green LED light engines, blue LED light engines, warm white LED light engines, and cool white LED light engines. As was described briefly above, the warm white LED light engines are blue-pump LED light engines topped by a phosphor that absorbs the blue light and emits a broader spectrum. The warm white LED light engines may have a color temperature of, e.g., 2700K, while the cool white LED light engines may have a color temperature of, e.g., 5000K. 
     The LED drive circuit  100  assumes that the linear luminaire  10  has 180 LED light engines per foot, 36 of each type. The LED light engines  16  are physically aligned with one another and spaced at a regular pitch along the PCB  18 . Yet as shown in  FIG.  6   , electrically, the  36  LED light engines of each type are arranged as three parallel series of twelve LED light engines each: red R 1 , R 2 , R 3 ; green G 1 , G 2 , G 3 ; warm white WW 1 , WW 2 , WW 3 ; cool white CW 1 , CW 2 , CW 3 ; and blue B 1 , B 2 , B 3 . 
     At one end, each series of LED light engines R 1  . . . B 3  is connected to a voltage source  106 ,  108  that is adapted to forward bias the LED light engines in each series R 1  . . . B 3  to light. At the other end, each series of LED light engines R 1  . . . B 3  is connected to a driver integrated circuit (IC)  102 . In this embodiment, the driver IC  102  is a TLC59116 16-channel constant-current LED driver (Texas Instruments, Dallas, Tex., United States). Thus, the driver IC  102  acts as the current-setting element in the circuit; the individual series R 1  . . . B 3  do not have any resistors or other current-setting elements. Additionally, the driver IC  102  is capable of controlling the output of each series of LED light engines R 1  . . . B 3  by applying a pulse-width modulation (PWM) current signal. The driver IC  102  of this embodiment has a maximum frequency in the low-megahertz range, and is capable of modulating the LED light engines in each series R 1  . . . B 3  at frequencies in the kilohertz range. 
     The TLC59116 has an 8-bit resolution for light output control, meaning that 256 individual light output levels are possible. Notably, this particular driver IC  102  requires a minimum applied voltage of about 0.3V in order to function. As will be described below in more detail, the driver IC  102  is under the control of a central unit  104  (not shown in  FIG.  6   ), such as a microprocessor or microcontroller, that serves as an interface and decodes control signals in order to instruct the driver IC  102 . 
     Because the circuit  100  contains several types of LED light engines, it has several voltage sources of different voltages. In particular, because the forward voltage of red LEDs is typically around 2V, the voltage source  106  that supplies the series of red LEDs R 1 , R 2 , R 3  is a 28V source. The voltage source  108  that supplies the other series of LEDs G 1  . . . B 3  is a 40V source, because green and blue LEDs typically have higher forward voltages (the “white” light series WW 1  . . . CW 3  are blue-pump LED light engines with the same forward voltages as blue LEDs). The driver IC  102  sets the current in each series of LEDs to about 11 mA when the series of LED light engines R 1  . . . B 3  are on. 
     The difficulty with a circuit like this lies in the variation in forward voltages from one LED light engine  16  to the next. For example, the forward voltages of blue-light LEDs typically vary in the range of 3V-3.3V, with the precise forward voltage of any one LED light engine usually unknown to the designer. If one assumes the worst-case scenario—in this example, that the forward voltages are all 3.3V—that has the potential to waste power if, in fact, some of the LED light engines have lesser forward voltages. On the other hand, if one underestimates the required voltage, it may be difficult to bring all of the LED light engines  16  to full brightness. 
     The typical solution to this problem is to use a higher voltage and waste some power for the sake of bringing all of the LED light engines  16  to full brightness. However, there is another potential adverse impact of setting the voltage high enough to accommodate the worst-case forward voltage for every LED light engine  16 : excess heat. In this circuit, any excess voltage is applied to the driver IC  102 , and the transistors in the driver IC  102  generate heat in proportion to that applied voltage. The resultant heat can shorten the lifetimes of the LED light engines  16  as well as the components that drive them. 
     Thus, the LED drive circuit  100  is designed to adjust the applied voltage to the minimum value needed for a series of LEDs. There is also an additional mechanism to ensure that the driver IC is not exposed to transitory increases in voltage that may cause damage. 
     With respect to high voltage protection, a switching element is installed in each series of LED light engines R 1  . . . B 3 . In this embodiment, at the bottom of each series of LED light engines R 1  . . . B 3 , an NPN bipolar junction transistor (BJT) Q 101 , Q 102 , Q 103  . . . Q 503  is installed with its collector connected to one of the series of LED light engines R 1  . . . B 3  and its emitter connected to the driver IC  102 . Each series of LED light engines R 1  . . . B 3  has its own BJT Q 101  . . . Q 503 ; thus, the BJTs Q 101  . . . Q 503  are interposed between the series of LED light engines R 1  . . . B 3  and the driver IC  102 . The bases of the BJTs Q 101  . . . Q 503  are connected to a 1.2V DC source  105 , which is enough voltage to exceed the base-emitter “on” voltage. Thus, when the series of LED light engines R 1  . . . B 3  are turned on by the driver IC  102 , the BJTs Q 101  . . . Q 503  allow current to flow. 
     As those of skill in the art might observe, for the purpose of switching individual series of LED light engines R 1  . . . B 3  on and off, the BJTs Q 101  . . . Q 503  are redundant and unnecessary: the driver IC  102  handles that switching function itself. 
     However, the BJTs Q 101  . . . Q 503  may serve a useful function in protecting the driver IC  102  from high voltages that may cause damage, particularly in transitional and non-steady state situations. For example, in the instant after the driver IC  102  shuts down a series of LED light engines R 1  . . . B 3 , the voltage approaches 28V in the series of red LED light engines R 1 , R 2 , R 3 , and the voltage approaches 40V in the other series of LED light engines G 1  . . . B 3 . In other words, for an instant after a series of LED light engines R 1  . . . B 3  is shut down, the voltage approaches the full voltage of the voltage source  106 ,  108 . If one considers that the driver IC  102  is driving the series of LED light engines R 1  . . . B 3  with a PWM current at a frequency that will often be in the kilohertz range, such non-steady state occurrences are frequent and become a greater concern. 
     A device like the driver IC  102  may only be able to take about 20V on a pin before the applied voltage could cause possible damage. The BJTs Q 101  . . . Q 503 , which may be, e.g., MMBT3904 BJTs, may be able to take up to 60V without damage. The BJTs Q 101  . . . Q 503  are also able to switch off very quickly, in the range of a few tens of nanoseconds, once the voltage on the base is removed. Thus, the BJTs Q 101  . . . Q 503  serve to protect the driver IC  102  from transitory increases in voltage. 
     The BJTs Q 101  . . . Q 503  are one example of a switching device that could be used to perform this protective function. In other embodiments, other kinds of switching devices could be used. For example, a field-effect transistor (FET) may be used in some embodiments. In that case, the 1.2V source would be adjusted as appropriate. 
     As was described above, the drive circuit  100  also preferably includes a mechanism to adjust the applied voltages in order to compensate for variations in LED forward voltage without wasting power and generating excess heat. The first part of that mechanism involves sensing how much voltage remains at the bottom of a series of LED light engines R 1  . . . B 3 , i.e., the total voltage drop in that series. 
     To that end, each series of LED light engines R 1  . . . B 3  has a parallel leg  110 ,  112  connected to the series R 1  . . . B 3  just below the cathode of the last LED light engine D 112  . . . D 336  in the series R 1  . . . B 3 . The parallel legs  110 ,  112  join the series R 1  . . . B 3  just above the collectors of the BJTs Q 101  . . . Q 503 . Although each series has such a parallel leg  110 ,  112  to simplify the diagram of the drive circuit  100  of  FIG.  6   , the full parallel leg  110 ,  112  is shown only on series R 1  and series B 3 . 
     Each parallel leg  110 ,  112  contains an RC low-pass filter. More specifically, each parallel leg  110 ,  112  includes a large, 0.1 μF capacitor C 103 , C 104  connected to a 1MSΩ resistor R 105 , R 106 . A small voltage source  114 , in this embodiment, 3.3V, charges the capacitor C 103 , C 104 . The resistor R 105 , R 106  and the capacitor C 103 , C 104  form an RC circuit. In this case, the time constant of that circuit is approximately 0.1 s, sufficient to filter out a kilohertz-range PWM modulation. A diode D 112 A, D 336 A with a small forward voltage (e.g., in the range of 0.6-0.7V) is arranged in parallel with the last LED D 112 , D 136  in the series, with its cathode connected below the cathode of the last LED D 112 , D 136  in the series. As was described above, under non-steady state conditions, voltage can build up at the collector of the BJT Q 101  . . . Q 503 . The diode D 112 , D 136  in the parallel leg  110 ,  112  prevents any large, transient voltages from charging the capacitor C 103 , C 104 , allowing the parallel leg  110 ,  112  and its low-pass RC filter to function as expected. The low-pass filtered voltages in the parallel legs  110 ,  112 , which correspond to the steady-state voltages that remain after the last LED D 112 , D 136  in the series R 1  . . . B 3 , are indicated as 28ADJ and 40ADJ, respectively, in  FIG.  6   . 
     The 28ADJ and 40ADJ voltages drawn from the parallel legs  110 ,  112  at the bottoms of the series of LED light engines R 1  . . . B 3  are sent into feedback circuits, described in more detail below, that either raise the voltage applied to the series of LED light engines R 1  . . . B 3  or decrease that voltage. For the sake of simplicity in design, the applied voltage is not adjusted for each individual series of LED light engines R 1  . . . B 3 . Instead, whichever 28V series R 1  . . . R 3  has the lowest voltage controls whether the 28V source is increased or decreased in voltage, and whichever 40V series G 1  . . . B 3  has the lowest voltage controls whether the 40V source is increased or decreased in voltage. In some embodiments, the voltages to the series of LED light engines R 1  . . . B 3  may be individually controlled. 
       FIG.  7    is a schematic diagram of the first portion of a feedback control circuit, generally indicated at  150 . The left side of the circuit, generally indicated at  152 , is a buffered voltage source that takes a voltage source  154 , in this embodiment, a 3.3V source, and uses an op amp U 104 A in a voltage-follower configuration to produce a buffered voltage output. 
     More specifically, the voltage from the voltage source  154  goes to a voltage divider comprised of two resistors R 112 , R 113 . The output from the voltage divider is sent to the noninverting input of the op amp U 104 A; the inverting input of the op amp U 104 A is connected to the output, such that the op amp U 104 A is in a voltage follower configuration. Thus, the voltage output of the left side of the circuit  152  is entirely dependent on the voltage supplied by the voltage source  154  and on the values of the resistors R 112 , R 113  that comprise a voltage divider. In this embodiment, that output voltage is designed to be 1.549V. Capacitors C 106 , C 108  are placed on each leg of the circuit that connects with the noninverting input of the op amp U 104 A to filter noise. The op amp U 104 A itself is connected to the 3.3V source  154  and to ground. 
     The advantage of the buffered voltage source  154  is that it is simple and requires relatively few components; however, any topology that produces a stable voltage may be used. 
     The second side  156  of the feedback control circuit  150  is connected to the first side  152  of the circuit  150  and includes a second op amp U 104 B. The second op amp U 104 B has a gain determined by the ratio of the values of two resistors, resistor R 109  and resistor R 114 . The inverting input of the second op amp U 104 B connects between the two resistors R 109 , R 114 , with resistor R 114  connected between the inverting input and the output of the second op amp U 104 B and resistor R 109  connected in series with the output of the first side  152  of the circuit  150 . 
     The noninverting input of the second op amp U 104 B receives the voltage 28ADJ drawn from the parallel leg  110 . In the illustrated embodiment, resistor R 114  is a 154 kΩ resistor and resistor R 109  is a 100kΩ resistor. If the voltage 28ADJ that is received by the noninverting input of the second op amp U 104 B is zero, the second op amp U 104 B acts as a traditional inverting amplifier with a gain equal to Expression 1: 
     
       
         
           
             
               
                 
                   - 
                   
                     
                       V 
                       fs 
                     
                     ⁡ 
                     
                       ( 
                       
                         
                           R 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           114 
                         
                         
                           R 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           109 
                         
                       
                       ) 
                     
                   
                 
               
               
                 
                   ( 
                   1 
                   ) 
                 
               
             
           
         
       
     
     where V fs  is the voltage from the first side  152 , and R 109  and R 114  are the values in Ohms of those resistors. 
     When the voltage on the noninverting input of the second op amp U 104 B is nonzero, with the arrangement shown, that voltage has a gain equal to Expression 2: 
     
       
         
           
             
               
                 
                   28 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   
                     ADJ 
                     ⁡ 
                     
                       ( 
                       
                         1 
                         + 
                         
                           
                             R 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             114 
                           
                           
                             R 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             109 
                           
                         
                       
                       ) 
                     
                   
                 
               
               
                 
                   ( 
                   2 
                   ) 
                 
               
             
           
         
       
     
     where 28ADJ is the voltage drawn from the parallel leg  110  and received by the noninverting input of the second op amp U 104 B, as described above. Thus, the output voltage ADJ28V of the second side  156  of the circuit  150 , i.e., the output of the circuit  150  is given by Equation 1: 
     
       
         
           
             
               
                 
                   
                     ADJ 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     28 
                     ⁢ 
                     V 
                   
                   = 
                   
                     
                       28 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       
                         ADJ 
                         ⁡ 
                         
                           ( 
                           
                             1 
                             + 
                             
                               
                                 R 
                                 ⁢ 
                                 
                                     
                                 
                                 ⁢ 
                                 114 
                               
                               
                                 R 
                                 ⁢ 
                                 
                                     
                                 
                                 ⁢ 
                                 109 
                               
                             
                           
                           ) 
                         
                       
                     
                     - 
                     
                       
                         V 
                         fs 
                       
                       ⁡ 
                       
                         ( 
                         
                           
                             R 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             114 
                           
                           
                             R 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             109 
                           
                         
                         ) 
                       
                     
                   
                 
               
               
                 
                   Eq 
                   . 
                   
                       
                   
                   ⁢ 
                   
                     ( 
                     1 
                     ) 
                   
                 
               
             
           
         
       
     
     In Equation 1 above, ADJ28V is the feedback voltage that is supplied to the circuit controlling the 28V source  106 . That circuit will be described below in more detail. An additional resistor R  107  is at the output of the second op amp U 103 B. 
       FIG.  8    is a schematic diagram of the corresponding feedback circuit  160  for the 40V voltage sources. The feedback circuit  160  of  FIG.  8    is identical in overall topology to the feedback circuit  150  of  FIG.  7   . The differences lie in the values of the resistors and other components. 
     Specifically, the feedback circuit  160  has a first side  160  that produces a buffered voltage output. A voltage divider comprised of resistors R 103  and R 104  takes a 3.3V voltage source  154  and directs its output to the non-inverting input of a first op amp U 103 A. The voltage source  154  also powers the first op amp U 103 A itself. The first op amp U 103 A is configured as a voltage follower, with the inverting input of the first op amp U 103 A connected to the output. The values of the voltage-divider resistors R 103 , R 104  are selected such that, in this case, the output of the first side  162  of the circuit is a buffered 1.7V. Capacitors C 105 , C 107  are placed on the legs of the first side  160  circuit that feed into the non-inverting input of the first op amp U 103 A to filter noise. 
     The second side  164  of the feedback circuit  160  of  FIG.  8    is configured essentially identically to the second side  156  of the circuit  150  of  FIG.  7   , with a second op amp U 103 B. In this case, the output of the second side, ADJ40V, is given in Equation (2): 
     
       
         
           
             
               
                 
                   
                     ADJ 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     40 
                     ⁢ 
                     V 
                   
                   = 
                   
                     
                       40 
                       ⁢ 
                       
                         ADJ 
                         ⁡ 
                         
                           ( 
                           
                             1 
                             + 
                             
                               
                                 R 
                                 ⁢ 
                                 
                                     
                                 
                                 ⁢ 
                                 111 
                               
                               
                                 R 
                                 ⁢ 
                                 
                                     
                                 
                                 ⁢ 
                                 108 
                               
                             
                           
                           ) 
                         
                       
                     
                     - 
                     
                       
                         V 
                         fs 
                       
                       ⁡ 
                       
                         ( 
                         
                           
                             R 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             111 
                           
                           
                             R 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             108 
                           
                         
                         ) 
                       
                     
                   
                 
               
               
                 
                   Eq 
                   . 
                   
                       
                   
                   ⁢ 
                   
                     ( 
                     2 
                     ) 
                   
                 
               
             
           
         
       
     
     Where 40ADJ is the voltage drawn from the parallel legs  112  of the 40V series of LED light engines G 1  . . . B 3 , ADJ40 is the output of the feedback circuit  160 , and R 108  and R 111  are the resistance values of those resistors. 
     In the illustrated embodiment, resistor R 108  is a 100kΩ resistor and R 111  is a 343kΩ resistor. All of the op amps U 103 A, U 103 B, U 104 A, U 104 B in both feedback circuits  150 ,  160  are TSZ122IQ2T op amps (STMicroelectronics, Geneva, Switzerland). 
     Other voltage sources in the lighting circuit  100  may have the same buffered voltage-follower topology as the first sides  152 ,  162  of the feedback circuits  150 ,  160 . For example, the 1.2V source  105  that is applied to the base of the BJTs Q 101  . . . Q 503  may have this topology. 
     In order to understand how the feedback voltages ADJ28V, ADJ40V output from the circuits of  FIGS.  7 - 8    are used, it is helpful to look at the power circuitry for the lighting circuit  100 .  FIGS.  9 - 1  and  9 - 2    are, collectively, a schematic circuit diagram of the power circuitry  200  of the lighting circuit  100 . The power circuitry  200  is designed to receive 24V DC from an input harness  202 . The precise characteristics of the power circuitry  200  are not critical to an understanding of the invention. For these purposes, it is sufficient to say that from the input harness  202 , power flows into a boost converter  204 , i.e., a step-up converter, that produces the 40V voltage source  108 . From the boost converter  204 , 40VDC is sent to a buck converter  206 , i.e., a step-down converter, that produces the 28V voltage source  106 . An output harness  208  receives 24VDC so that the linear luminaires  10  can be “daisy chained” with one luminaire  10  supplying power for the next. 
     The boost converter  204  is controlled by a high voltage switch-mode regulator integrated circuit  210 , such as the LM5000SD-3/NOPB (Texas Instruments, Dallas, Tex., United States). The buck converter  206  is controlled by a buck regulator integrated circuit  212 , such as the LMR16006 (Texas Instruments, Dallas, Tex., United States). Both of these integrated circuits  210 ,  212  have feedback pins FB to regulate the output voltage. 
     The feedback voltage for each of the regulator ICs  210 ,  212  is set by a voltage divider network. In the illustrated embodiment, the boost regulator IC  210  has a voltage divider  214  comprised of resistors R 2  and R 4 , which in this case are a 324kΩ resistor and a 10kΩ resistor, respectively. The buck regulator IC  212  has a voltage divider  216  comprised of resistors R 5  and R 6 , which in this case are a 287kΩ resistor and a 10kΩ resistor, respectively. 
     The voltage output ADJ40 from the feedback circuit  160  is received at a terminal between the voltage divider  214  and the feedback pin FB of the boost IC  210 . Similarly, the voltage output ADJ28V from the feedback circuit  150  is received at a terminal between the voltage divider  216  and the feedback pin FB of the buck IC  212 . With this layout, if the feedback voltages ADJ28V, ADJ40V are positive, they add to the voltage seen by the feedback pins FB of the boost and buck ICs  210 ,  212 . If the feedback voltages ADJ28V, ADJ40V are negative, they subtract from the voltage seen by the feedback pins of the boost and buck ICs  210 ,  212 . The total voltages seen by the respective feedback pins FB of the boost and buck ICs  210 ,  212  determine whether the voltages of the voltage sources  106 ,  108  are upregulated or downregulated. 
     The voltage setpoints that cause the voltage of the voltage sources  106 ,  108  to be upregulated or downregulated depend on the particular characteristics of the lighting circuit  100 . In this embodiment, because of the minimum-voltage requirements of the driver IC  102 , the basic assumption is that the voltage at the bottoms of the series of LED light engines R 1  . . . B 3  should not fall below 0.5V. The feedback circuits  150 ,  160  are configured to produce output voltages ADJ28V, ADJ40V in accordance with that goal. 
       FIG.  10    is a schematic diagram of the LED drive circuit  100  in its entirety. The driver IC  102  that drives the series of LED light engines R 1  . . . B 3  is connected to the central unit  104 . The central unit  104  in this case is a microprocessor, namely an MSP430FR2153TRSMR (Texas Instruments, Dallas, Tex., United States). Typically, the central unit  104  provides clock, data, and reset signals to the driver IC  102 . The central unit  104  itself is monitored by a supervisor/monitor IC  118 , such as a TPS3851E (Texas Instruments, Dallas, Tex., United States), which has the ability to reset the central unit  104  when needed. The central unit  102  receives input through an input controller  120  and an output controller  122 , which are bus line transceiver ICs. 
     As was described previously, the power circuitry  200  provides separate voltage sources  105 ,  106 ,  108 ,  114  of various voltages using boost and buck converters  204 ,  206 . While not described in detail above, the 3.3V source may be provided by a single, integrated power step down module that receives 24VDC and outputs the 3.3V, such as a LMZM23600V3SILR (Texas Instruments, Dallas, Tex., United States), or by the kind of custom voltage division and regulation/buffering circuit described above. The power circuitry receives feedback from the two feedback circuits  150 ,  160  as was also described above. 
     The lighting circuit  100  described above has a number of advantages. First among them is more efficient use of power. There are other advantages as well. For example, one conventional way to resolve the problem of LED light engines with varying forward voltages is to buy LED light engines that have been tested and confirmed to have the same forward voltage to within a particular tolerance. However, LED light engines that have been specified or confirmed to have the same forward voltage are more expensive. The lighting circuit  100  described above may allow an LED luminaire  10  to use less expensive LED light engines  16 , because LED light engines  16  of the same type need not have the same forward voltages; instead, the lighting circuit  100  can compensate for variations. 
     It may be possible to derive additional power savings and additional benefits in some embodiments. More specifically, the feedback control circuits  150 ,  160  described above produce a single voltage output, ADJ28V or ADJ40V, to upregulate or downregulate the voltage applied to the series of LED light engines R 1  . . . B 3  for each voltage input. With these feedback control circuits  150 ,  160 , it is possible that there may be some error in the ADJ28V or ADJ40V output. For example, the ADJ28V or ADJ40V output voltage may overshoot or undershoot the voltage required for the circuitry to provide the exact voltage necessary to power the series of LED light engines R 1  . . . B 3  in any given instant. This may be especially true if the necessary voltage changes rapidly, which it may, depending on how the series of LED light engines R 1  . . . B 3  is driven. 
       FIG.  13    illustrates an alternative feedback control circuit  400  that takes as input the remainder voltage 40ADJ described above and outputs a feedback control voltage ADJ40V that is applied to the feedback pin FB of the regulator  210  in the boost converter  204 . 
     The feedback control circuit  400  has a very similar topology to the feedback control circuit  160  of  FIG.  8   , including a first side  402  that produces a buffered voltage output using an op amp U 403 A in a voltage follower configuration, and a second side  404 , connected to the first side, that receives the remainder voltage 40ADJ at the non-inverting input of a second op amp U 403 B. The main difference between the feedback control circuit  400  and the feedback control circuit  160  described above is that in the feedback control circuit  400 , the second op amp U 403 B is configured as an op amp integrator. Specifically, an RC network is connected across the op amp&#39;s feedback path, with a 1MΩ, resistor R 408  in the path to the op amp U 403 B inverting input, and a 0.1 μF capacitor C 413  connected between the non-inverting input and the output. 
     As was described above, the goal is to provide enough voltage to power the LEDs in each series R 1  . . . B 3  while leaving sufficient remaining voltage on the driver IC  102  to allow it to function. The minimum voltage allowable on the driver IC  102  may be a small voltage like 0.3V, as described above, but for design purposes, it is better to keep the voltage above the design minimum of the driver IC  102 . For that reason, the voltage at the emitter of the BJTs Q 101  . . . Q 503 , which is the voltage applied to the driver IC  102 , is preferably at least about 0.5V in this embodiment. If the emitter voltage of any one of the BJTs Q 101  . . . Q 503  is 0.5V, its collector voltage is most likely at 1V, and the 40ADJ voltage drawn from the parallel leg  110 ,  112  is likely 1.5V. 
     For this reason, the first side  402  of the feedback control circuit  400  provides a buffered voltage output of about 1.5V using the op amp U 403  configured as a voltage follower. That buffered 1.5V is input to the inverting input of the op amp U 403 B through the 1MΩ resistor R 408 . The 40ADJ voltage is input to the non-inverting input of the op amp U 403 B. Any difference between the buffered 1.5V input to the inverting input of the op amp U 403 B and the 40ADJ voltage applied to the non-inverting input of the op amp U 403 B results in a ramped positive or negative voltage output for ADJ40V that continues to increase or decrease until the voltage applied to the feedback pin FB of the regulator IC  210  causes 40ADJ to return to 1.5V. The regulator IC  210  itself and the voltage divider network around it is designed such that the feedback pin FB of the regulator IC  210  sees a reference voltage of 1.259V when no changes are necessary to the voltage output; as described above, the ADJ40V output changes the voltage seen by the feedback pin FB of the regulator IC  210 . 
     The continuously increasing or decreasing ramp created by the op amp integrator U 403 B, C 413 , R 408  in the second side  404  of the feedback circuit  400  tends to zero any error that occurs, causing the circuit to follow more closely any changes to the voltage 40ADJ found in the parallel legs  112 . 
       FIG.  14    is a circuit diagram of the corresponding 28V feedback control circuit  450  for the red series of LED light engines R 1 , R 2 , R 3 . The feedback control circuit  450  is essentially identical to the feedback control circuit  400  described above, with a first side  452  that produces a buffered voltage output of 1.5V using an op amp U 404 A in a voltage follower configuration, and a second side  454  that takes the 28ADJ voltage and produces a ramped output voltage ADJ28V using an op amp U 404 B in an integrator configuration with a 0.1 μF capacitor C 414  between the inverting input and the output of the op amp U 404 B and a 1MΩ resistor R 409  in the path to the inverting input. 
     The rate of rise or fall of the voltage output, which is determined in part by the RC time constants of the resistor-capacitor networks (R 408  and C 413 ; R 409  and C 414 ), is not critical, so long as it is slow enough so as not to cause any instability. 
     Software Control 
     Because the central unit  104  is a programmable component, a number of useful control methods for a linear luminaire  10  can be implemented either entirely in software, or in a combination of hardware and software. “Software,” for purposes of these instructions, refers to a set of machine-readable instructions that, when executed by a machine like the central unit  104 , cause the machine to perform certain tasks. Software is typically embodied or stored in some form of non-transitory machine-readable medium. As was noted above with respect to circuitry, although portions of the following description make reference to the linear luminaire  10  and its central unit  104 , these methods, and the software that embodies these methods, may be implemented on other types of luminaires using other types of hardware. 
     With the linear luminaire  10 , the machine-readable medium will typically be firmware or onboard memory programmed at the time of manufacture. However, if needed, a linear luminaire  10  could have other types of machine-readable media, like flash memory, a solid-state drive, or the like. Software and related commands may be communicated via the input controller  120  and the output controller  122  and sent through the input and output harnesses  202 ,  208 . In some cases, the lighting circuit  100  may have an interface such as a universal serial bus (USB) interface, with an appropriate port, to allow for upload of firmware updates and other forms of software installation. If the lighting circuit  100  has a USB interface, a USB drive may serve as a non-transitory machine-readable medium to transfer software from, e.g., a development computer to the linear luminaire  10 . In yet other embodiments, the lighting circuit  100  may include a wireless interface to allow for communication and programming functions. 
     As was described in detail above, one concern for linear luminaires  10  is power usage. In the design of an installation that uses linear luminaires  10 , it is assumed that there is some power budget that should not be exceeded, either because of limitations on the power supplies that supply the input power to the linear luminaires  10 , because of safety regulations, or because of a general desire to conserve power. For example, a linear luminaire  10  may have a power budget of 6 W per foot. In the illustrated embodiment, that power must be divided among the various series of LED light engines R 1  . . . B 3 . In keeping with this power budget, the driver IC typically sets the current in each series of LED light engines R 1  . . . B 3  to 11 mA. 
     As important as power budgeting and power conservation may be, brightness is also relevant. “Brightness,” as the term is used in this description, refers to the human perception of radiant or reflected light. Brightness is related to the luminous flux (i.e., the light output) of a light source, but it is not entirely dependent on it. For example, the Helmholtz-Kohlrausch effect is a perceptual phenomenon in which intensely saturated colors are seen by the human eye as brighter than “white” light of equal luminous flux. Simply put, a linear luminaire  10  may not be adequate for its task if it is not bright enough to be seen in its environment. 
     To that end,  FIG.  11    illustrates a method, generally indicated at  300 , for budgeting and shifting power among the series of LED light engines R 1  . . . B 3  installed in a linear luminaire  10 . The following description of method  300  assumes that the linear luminaire in question is the linear luminaire  10  with the series of LED light engines R 1  . . . B 3  described above, although method  300  is applicable to any linear luminaire that uses multiple sets of LED light engines  16 . Method  300  begins at task  302  and continues with task  304 . 
     In task  304 , the central unit  104  receives instructions to activate one or more series of LED light engines R 1  . . . B 3 . These instructions may be in any format and using any protocol. For example, the instructions in question could be instructions in the DMX512 protocol, or they could be simple 0-10V signals indicating brightness. Method  300  continues with task  306 . 
     Task  306  is a decision task. In task  306 , the central unit  104  parses the instructions received in task  304  to determine which of the series of LED light engines R 1  . . . B 3  will be active when executing the instructions. If all of the series of LED light engines R 1  . . . B 3  will be active when executing the instructions (task  306 :YES), method  300  continues with task  312 , and the instructions are executed. (The central unit  104  may alter or offset the instructions before executing them, as will be explained below in more detail.) 
     If all of the series of LED light engines R 1  . . . B 3  will not be active when executing the instructions (task  306 :NO), method  300  continues with task  308 . In this case, with some of the series of LED light engines R 1  . . . B 3  off, there is some amount of unbudgeted power. In task  308 , the central unit  104  calculates how much of the power budget will be unspent if the instructions are executed. For example, if the red series of LED light engines R 1 , R 2 , R 3  are unused, there may be nearly a watt of unused power. In calculating the power that will be unspent, the central unit  104  may use the ideal voltage that is intended to be used (e.g., 28V, 40V), or the central unit  104  may use the actual applied voltage generated by the feedback circuits  150 ,  160  to compensate for forward voltage variations. 
     Once the central unit  104  has calculated the unused power in task  308 , method  300  continues with task  310 . In task  310 , the central unit  104  distributes the unused power among the series of LED light engines R 1  . . . B 3  that will be used when the instructions are executed. This would typically be done by instructing the driver IC  102  to increase the current level in each of the series R 1  . . . B 3  that will be active when the instructions are executed. This, in turn, would typically be done by increasing the duty cycle of the series R 1  . . . B 3 . This is possible because the individual LED light engines  16  will typically be rated for more current than is applied when all of the series of LED light engines R 1  . . . B 3  are active. For example, individual LED light engines  16  may be rated for a current of 30 mA or more. 
     In some implementations of task  310 , the unused power may be evenly divided among the active series of LED light engines R 1  . . . B 3 . However, that need not always be the case. Instead, in some implementations, task  310  may put more of the unused power into the “white” light series of LED light engines WW 1 , WW 2 , WW 3 , CW 1 , CW 2 , CW 3  in view of the Helmholtz-Kohlrausch effect. Other perceptual phenomena involving brightness may also be taken into account in allocating unused power. To the extent possible, however, any power increases should across-the-board, applied to all active series. Increasing the power to or duty cycle of only one series R 1  . . . B 3  relative to the others may cause color shifts relative to the color that was intended or commanded. 
     In task  312 , the instructions are executed and the series of LED light engines R 1  . . . B 3  are activated as instructed. If control of method  300  passed directly from task  306  to task  312 , this would be done without power adjustments. If control of method  300  passed from task  310  to  312 , the instructions are executed with unused power distributed among the active series of LED light engines R 1  . . . B 3 . 
     Method  300  terminates and returns at task  314 . Generally speaking, if method  300  is implemented, it would be executed every time a new instruction or set of instructions is received. As those of skill in the art may realize, although power utilization and allocation determinations (tasks  308  and  310 ) are followed immediately by execution of instructions (task  312 ) in the description above, in some embodiments, the central unit  104  may pre-process instructions and determine power allocations for later execution. 
     As those of skill in the art will note, method  300  is a method for power control that refers to a set power budget. In some cases, simpler methods may be used. For example, in some embodiments, it may be sufficient to set every series of LED light engines R 1  . . . B 3  to a particular current setpoint, except when all of the series of LED light engines R 1  . . . B 3  are active, in which case a lower current setpoint is enforced. For example, each series of LED light engines R 1  . . . B 3  could be set to 120% of nominal current, unless all of the series R 1  . . . B 3  are active, in which case the lower, 100% nominal current level is set and enforced for each series. Such setpoint-based methods may be simpler to use. 
     In the context of the luminaire  10 , enforcing a current limit may mean limiting each series of LED light engines R 1  . . . B 3  to a particular maximum duty cycle that is less than 100%. For example, if the driver IC  102  permits an 8-bit resolution for duty cycle, allowing 256 possible duty cycles for each series of LED light engines R 1  . . . B 3  where 0 represents 0% duty cycle and  255  represents 100% duty cycle, the series of LED light engines R 1  . . . B 3  in a group may be limited to a duty cycle of, e.g.,  204 . If the commanded duty cycle for any of the series of LED light engines R 1  . . . B 3  exceeds that defined threshold, a scaling fraction (90% of commanded duty cycle, 80%, etc.) is applied to each of the series of LED light engines R 1  . . . B 3  until all series R 1  . . . B 3  are back below the threshold. By scaling back all series of LED light engines R 1  . . . B 3  together, the luminaire  10  can achieve a power budget target without creating a color shift that would otherwise occur if only one or two series R 1  . . . B 3  were scaled back. 
     This basic method is generally indicated at  500  in  FIG.  15    and begins at task  502 . Method  500  is the type of method that would be executed by the central unit  104  any time the luminaire  10  is accepting instructions for driving the series of LED light engines R 1  . . . B 3 . In task  504 , a new instruction is received. This new instruction presumably commands a particular duty cycle for each of the series of LED light engines R 1  . . . B 3 . In keeping with the description above, the description of method  500  will assume that that duty cycle is an 8-bit number from 0-255. Method  500  continues with task  506 . In task  506 , the duty cycle instructions are checked against a PWM/current limit threshold that is pre-set and programmed into the central unit  104 . If the instructions are all below the pre-set limit (task  506 :YES), the instructions are executed in task  512 . If any of the instructions designate a duty cycle that would bring a series R 1  . . . B 3  above the pre-set limit (task  506 :NO), method  500  continues with task  510 . In task  510 , an across-the-board scaling factor or fraction is applied to the duty cycles of all active series R 1  . . . B 3  to bring them all below the threshold. Method  500  completes and returns in task  514 . 
     Power control methods are only one possible type of supervisory or control methods that may be executed by the central unit  104  and other components based on software instructions. Software may also be used to make color adjustments. For example, as was described above, the central unit  104  may intercede to offset particular color instructions to compensate for color or color temperature shifts due to encapsulation. 
     One particular area in which additional control may be useful is in transitions from one color or one type of LED light engine to another. For example, the linear luminaire  10  has both “cool white” and “warm white” LED light engines  16 . As was explained above, these are blue-pump LED light engines with different phosphors that allow them to emit light with different overall color temperatures. 
     If one wishes to transition between “cool white” and “warm white,” for example, it may seem logical simply to turn the cool white series CW 1 , CW 2 , CW 3  off and turn the warm white series WW 1 , WW 2 , WW 3  on. However, there can be problems with such transitions. The speed at which one makes such a transition is one issue, and a fast transition can cause problems of its own. However, transitions can create color problems as well. 
     Specifically, linear transitions from white light of one color temperature to white light of another color temperature run into a problem that becomes evident when one looks at a color chart, be it the CIE 1931, the CIE 1960, or the CIE 1976 color chart. On a color chart, the colors of natural “white” light all fall along a curve—the Planckian locus. That is, the Planckian locus is a curve on the CIE 1931, CIE 1960, and CIE 1976 color charts along which lie all of the colors that are emitted by blackbody radiators. While the light emissions of practical LED light engines  16  do not lie exactly along the Planckian locus, the color of light they emit is usually engineered to be as close to that of a blackbody radiator as possible. In the CIE color charts, the pink-hued colors lie below the Planckian locus, and the yellow and green-hued colors lie above it. 
     The straightest path between two points is a line. Yet, given the shape of the Planckian locus, if one implements a straight-line transition between one color temperature of white light and another, the light often acquires a pinkish hue during the transition. This hue appears unnatural to most observers and is thus undesirable. 
     For that reason, method  350  is a method for correcting transitions between one color temperature of light and another. Method  350  begins at task  352  and continues with task  354 . In task  354 , the central unit  104  receives instructions for activating one or more series of LED light engines R 1  . . . B 3 . Those instructions may be received from an external device, such as a control computer or another linear luminaire  10 , or they may be received (i.e., passed) from another control method that is also being executed by the central unit  104 . If the central unit  104  is running multiple control methods, methods like method  350  that change the colors that are used will generally be run before methods like method  300  that determine how power is allocated among series of LED light engines R 1  . . . B 3 . Method  350  continues with task  356 . 
     Task  356  is a decision task. If the central unit  104  detects that the instructions necessitate a transition between white light of one color temperature and white light of another (task  356 :YES), control of method  350  passes to task  358 , and the central unit  104  corrects the instructions such that the transition occurs along the Planckian locus. This typically involves activating red, green, and blue colored LED light engines  16  in appropriate instants to create a nonlinear transition. Once that is done, or if no modifications are necessary because the instructions do not contain or imply a transition (task  356 :NO), method  350  terminates and returns at task  360 . 
     More generally, the design of the linear luminaire  10 , with red, green, and blue LED light engines in addition to dedicated “white” LED light engines, has some specific advantages. For example, the linear luminaire  10  has dedicated cool white CW 1 , CW 2 , CW 3  and warm white WW 1 , WW 2 , WW 3  series of LED light engines. However, if desired, it is possible to use the RGB series of LED light engines R 1  . . . B 3  to interpolate between cool and warm to produce other color temperatures of white light. Potentially, any desired color temperature of white light could be produced by color mixing. 
     When producing white light of other color temperatures, it is likely that either the cool or the warm series of LED light engines CW 1  . . . WW 3  will be active along with red, green, or blue series R 1  . . . B 3 , depending on the desired color temperature. The central unit  104  can be calibrated or otherwise set, given the particular characteristics of the linear luminaire, to produce mixed white lights of arbitrary color temperatures that are as close to the Planckian locus as possible. (That is, in formal terms, the Duv of the light relative to the Planckian locus should be minimized.) 
     As those of skill in the art may appreciate, the ability to mix RGB light precisely would also allow the central unit  104  to compensate for blue-pump white light LED light engines with suboptimal characteristics, for example, warm white LED light engines with a large Duv or a low color rendering index (CRI). This, in turn, may allow for the use of less desirable, and thus less expensive, white LED light engines. 
     If red, green, and blue lights are mixed to create or augment white light, that mixing may be controlled by a method like method  350  in order to keep the emitted light along the Planckian locus to avoid any unnatural colors during startup or transition. 
     Other supervisory and control methods may be implemented for purposes of safety, or in order to ensure the longevity of the luminaire  10 . For example, the boost converter  204  will work with very low input voltages, e.g., under 5 volts. In boosting these low voltages, the boost converter  204  may draw so much current that the input harness  202  exceeds its rated ampacity. To avoid these issues, the central unit  104  or the regulator IC  210  may be programmed not to allow the luminaire  10  to function unless the voltage in the input harness  202  exceeds a threshold, e.g., 19V. 
     Another possible safety or longevity issue is heat. If the luminaire  10  gets too hot, it may damage the PCB  18  and the electronics. For that reason, luminaires according to embodiments of the invention may include at least one temperature sensor. In the illustrated embodiment, the luminaire  10  includes two temperature sensors  103 ,  107  connected to the central unit  104 . These two temperature sensors  103 ,  107  may be in different locations within the luminaire  10 . For example, one temperature sensor  103  may be positioned to read the temperature on the PCB  18 , while the other temperature sensor  107  may be positioned to read the temperature at or near the stand-offs  30  that serve as heat sinks. The temperature sensors  103 ,  107  may be, for example, thermistors. 
     The central unit  104  may be programmed to read and use the data from the temperature sensors  103 ,  105  in specific ways.  FIG.  16    is a flow diagram of one such method, generally indicated at  550 . Method  550  begins at task  552  and continues with task  554 , in which the temperatures are read from the temperature sensors  103 ,  107 . Method  550  then continues with task  556 , a decision task. In task  556 , if the temperatures are too high as compared with pre-set thresholds (task  556 :YES), method  550  continues with task  558 . If not (task  556 :NO), method  550  returns at  560 . 
     In task  558 , the central unit  104  implements the kind of across-the-board decrease in the PWM duty cycle of each active series of LED light engines R 1  . . . B 3  that was described above. This helps to ensure that color change or shift caused by the decrease will be minimal to none. While the central unit  104  may implement this decrease instantaneously, by instructing the PWM duty cycle of each series R 1  . . . B 3  to fall immediately to some fraction of its original instructed duty cycle (e.g., 90%, 80%, etc.), this has particular disadvantages. For example, immediate decrease in intensity of the series of LED light engines R 1  . . . B 3  could be perceived by the human eye as flicker. For that reason, in task  558 , the central unit  104  preferably implements a gradual, ramped decrease in duty cycle to the target. The rate of decrease of that ramp may vary, but it should be slow enough that the human eye will not perceive the change as flicker. 
     In this description, the term “about,” when applied to a number or value, should be construed to mean that that number or value can vary somewhat, as long as the variation does not affect the described circumstances or result. As one example, when describing color temperatures of white light, variations of up to 300K are accepted in some contexts in industry. If it cannot be determined what value or threshold would change the described circumstances, the term “about” should be construed to mean the stated value plus or minus 5%. As those of skill in the art will realize, the stated values of resistors, capacitors, and other circuit elements have their own tolerances. Unless otherwise stated, the tolerances for circuit elements should be construed to be ±1%. 
     While the invention has been described with respect to certain embodiments, the description is intended to be exemplary, rather than limiting. Modifications and changes may be made within the scope of the invention, which is defined by the appended claims.