Abstract:
A variable intensity LED illumination system is configured to provide a change in luminance versus input voltage that corresponds to a desired transfer function, such as the dimming characteristics of an incandescent lamp, which more closely resembles the response of the human eye. The system also advantageously provides overvoltage protection, increased brightness, energy efficiency, and significantly better longevity and ruggedness, compared to incandescent lamps.

Description:
GOVERNMENT INTEREST 
       [0001]    The invention described herein may be manufactured, licensed, and used by or for the U.S. Government. 
     
    
     TECHNICAL FIELD 
       [0002]    The present invention relates generally to variable intensity light sources and more particularly to a light emitting diode illumination system that can be configured to predetermined luminescence vs. supply voltage characteristics. 
       BACKGROUND 
       [0003]    LED light sources are rapidly replacing incandescent and fluorescent light sources in many applications. The term light emitting diode (LED) is used to refer to a semiconducting material impregnated, or doped, with impurities to create a p-n junction capable of emitting light, and includes both a laser diode and a spontaneously emitting LED. LED light sources are much more efficient, are more rugged and have longer life cycles, produce little heat, are generally smaller, and are lighter in weight than their incandescent counterparts. 
         [0004]    Despite their many advantages, conventional LED light sources have some limitations. In particular, LEDs are not well suited to applications that require variable intensity. Conventional incandescent bulbs work well in such applications because their luminous intensity varies exponentially in response to a linear change in voltage, closely matching the response of the human visual system. Changes in luminance of four orders of magnitude can be achieved with incandescent light sources. LEDs, however, essentially provide a linear change in intensity in proportion to a change in current and variability over only two orders of magnitude is typically achieved. The luminous intensity of a conventional LED thus appears to the eye to change slowly at higher light levels and abruptly at lower levels of luminance when an LED light source is placed in a conventional dimming circuit. Embodiments according to the present invention are directed to solving these problems by providing an LED light source that may be configured to vary in intensity according to a predetermined voltage-to-luminance transfer function, to provide dimming characteristics that are closer to the response of the human eye, and over the full four orders of luminance change. 
       SUMMARY 
       [0005]    In general, in one aspect, an embodiment of a variable intensity LED light system according to the present invention includes a number of circuit legs coupled in parallel. Each of the circuit legs includes a current limiting device in series with a first circuit element that provides a distinct voltage set point for that circuit leg, and a second circuit element that provides a regulated voltage. An LED is coupled in parallel with the second circuit element of each circuit leg. In general, the regulated voltage of the second circuit element is greater than the forward voltage drop of the LED. The distinct voltage set points of the first circuit elements and the resistances of the current limiting devices are calibrated to correspond to points on a target luminance-versus-voltage curve. The target luminance-versus-voltage curve may approximate luminance-versus-voltage curve for an incandescent lamp. In another aspect, an embodiment of a variable intensity LED light system according to the present invention includes an additional circuit leg coupled in parallel with the plurality of circuit legs, in which the additional circuit leg has a resistor in series with a Zener diode that provides a regulated voltage, and an LED coupled in parallel with the Zener diode This additional circuit leg is provided for low luminance operation. 
         [0006]    In general, in another aspect, an embodiment of a method for varying the intensity of an LED lighting system in response to a change in supply voltage includes providing a plurality of circuit legs each having a first node for coupling to a positive terminal of a variable voltage DC source, a second node for coupling to the anode of an LED and a third node for coupling to the cathode of the LED and to a negative terminal of the variable voltage DC source. The method further includes providing a current limiting device in each circuit leg having a resistance to limit the current delivered to the LED to a predetermined value; providing a first circuit element in each circuit leg which, at a given supply voltage, changes from a high impedance to a low impedance when a voltage set point has been exceeded at the input node; and a second circuit element in each circuit leg to regulate the voltage across the LED. In another aspect, the method for varying the intensity of an LED lighting system in response to a change in supply voltage includes providing an additional circuit leg coupled in parallel with the plurality of circuit legs, in which the additional circuit leg has a resistor in series with a Zener diode that provides a regulated voltage, and an LED coupled in parallel with the Zener diode. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0007]    Embodiments according to the invention are illustrated in the accompanying drawings in which like reference numerals represent like parts throughout and in which: 
           [0008]      FIG. 1A  shows a circuit diagram of an embodiment of a variable intensity LED illumination system according to the present invention; 
           [0009]      FIG. 1B  shows a circuit diagram of an alternative embodiment of a variable intensity LED illumination system according to the present invention; 
           [0010]      FIG. 2A  shows a bottom plan view of a printed wiring board for mounting component parts in an embodiment of a variable intensity LED illumination system according to the present invention; 
           [0011]      FIG. 2B  shows a top plan view of the printed wiring board shown in  FIG. 2A ; 
           [0012]      FIG. 3A  shows a top plan view of a second printed wiring board for mounting component parts in an embodiment of a variable intensity LED illumination system according to the present invention; 
           [0013]      FIG. 3B  shows a bottom plan view of the second printed wiring board shown in  FIG. 3A ; 
           [0014]      FIG. 4  shows a perspective view of an embodiment of a variable intensity LED illumination system according to the present invention that is dimensioned to correspond to a standard No. 327 instrumentation bulb; and 
           [0015]      FIG. 5  shows a graph of luminance versus voltage for an embodiment of a variable intensity LED illumination system according to the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0016]    In the following detailed description, reference is made to the accompanying drawings which are a part of this patent disclosure, and in which are shown by way of illustration specific embodiments in which the invention, as claimed, may be practiced. This invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth; rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. 
         [0017]      FIG. 1A  shows a schematic diagram of an embodiment of a variable intensity LED illumination system  100  according to the present invention. LED illumination system  100  is designed for small, low voltage DC, variable intensity lighting applications, such as the illumination of a vehicle instrument panel or dashboard, or for providing internal lighting for avionics, flight instruments, indicators, and the like. LED illumination system  100  includes a positive input node  120  and a ground node  122  for connection to a variable voltage direct current source  116 . In this embodiment LED illumination system  100  is designed to be powered by a 5-28 Volt DC variable voltage source such as the low voltage power supply for avionics and instrumentation commonly found on commercial and military aircraft. In alternative embodiments, the invention may be adapted for use with higher or lower variable voltage sources. 
         [0018]    LED illumination system  100  includes four LED circuit legs  101   a - d  coupled to second Zener diodes  114   a - d  which function as fixed voltage sources for LEDs coupled to each circuit leg. Each LED circuit leg  101   a - d  includes a resistor  102   a - d  coupled to positive input node  120  which act as current limiting devices. 
         [0019]    Resistors  102   a - d  are each in series with first Zener diodes  110   a - d  which, in turn, are in series with second Zener diodes  114   a - d  coupled to ground node  122 . First Zener diodes  110   a - d  and second Zener diodes  114   a - d  are oriented to operate in conventional reverse breakdown mode. 
         [0020]    LEDs  112   a - d  are coupled in parallel with corresponding second Zener diodes  114   a - d  of each circuit leg at coupling nodes  113   a ,  113   b    113   c  and  113   d , respectively. Second Zener diodes  114   a - d  are selected to have Zener voltages sufficient to ensure voltages across LEDs  112   a - d  are in excess of the LEDs&#39; forward voltage drops. For example, in LED illumination system  100 , a Zener voltage of about 5.1 volts has been selected for Zener diodes  114   a - d  since LEDs  112   a - d  have turn-on voltages of between 3.5V to 4.1V. Second Zener diodes  114   a - d  also provide over-voltage/spike protection for LEDs  112   a - d  by safely shunting away any transients that might otherwise damage the LEDs. 
         [0021]      FIG. 1B  shows an embodiment of an LED illumination system  100  where first Zener diode  110   a  has been eliminated to provide for very low luminance operation. 
         [0022]    Since, as noted above, the luminance of an LED is proportional to current, a luminance/voltage characteristic of an LED illumination system according to the present invention may be approximated by taking the current through each of the LEDs at a given supply voltage and using the luminance versus current curve from the LED manufacturer to establish individual LED luminance outputs. The luminance of an LED illumination system  100  may then be determined by summing the luminance of each of the LED circuit legs. Current supplied to LEDs  112   a - d  in circuit legs  101   a - d  is controlled by the value of current limiting resistors  102   a - d . Current limiting resistors  102   a - d  are selected so that the sum of the currents through each LED  112   a - d  will deliver a combined luminance from the LEDs  112   a - d  corresponding to target luminances at voltage set points. 
         [0023]    LED illumination system  100  is designed to approximate a luminance-versus-supply voltage profile for a small low voltage incandescent lamp. In this example, as shown on graph  500  of  FIG. 5 , four set points  506   a - d  were selected to model the luminance versus-supply voltage profile for a typical incandescent Miniature Lamp ML-327/#327. The No. 327 lamp, or “peanut bulb,” is used in many aircraft instrumentation lighting applications. A Curve  502  on graph  500  shows a luminance versus-supply voltage profile averaged from a sample of six No. 327 lamps. A curve  504  represents the actual luminance versus supply voltage characteristics measured for a prototype variable intensity LED illumination system  100  according to the present invention. As can be seen from curve  504 , the prototype provides higher luminescence in the higher voltage range compared to the incandescent lamp. This represents a design improvement for any application where sunlight backlighting provides difficulty in seeing instrument indicators. 
         [0024]    Some adjustment of resistor values may be required after the luminance from LED illumination system  100  is measured to compensate for performance characteristics of components. Accordingly, the actual luminance curve for a particular implementation should be verified experimentally to allow for any changes that may result from variations in component specifications. 
         [0025]    The voltage set points for circuit legs  101   a - d  are determined by summing the Zener voltage of first Zener diodes  110   a - d  with the Zener voltage of second Zener diodes  114   a - d  in their respective circuit legs  101   a - d . Voltage set points for the prototype of LED illumination system  100  are as follows: LED circuit leg  101   a , 10.7 volts, LED circuit leg  101   b , 15.1V, LED circuit leg  101   c , 18.1V, and LED circuit leg  101   d , 23.1V. 
         [0026]    The use of four set points and four LEDs provides a good approximation of the voltage versus luminous intensity characteristics of the No. 327 lamp. In alternative embodiments more or fewer set points, circuit legs and LEDs may be employed, depending on the requirements of a particular application. Other luminance profiles may similarly be modeled in alternative embodiments. 
         [0027]    In many lighting applications a white or yellow-white light is desired. Typically, white LEDs use a blue emitting chip in conjunction with a light converter such as yellow phosphor, which absorbs the blue light and emits white or yellow white light. In this embodiment, LEDs  112   a - d  are InGaN blue with a yellow phosphor blob coating. InGaN LEDs have high intensity and excellent reliability. The yellow phosphor coating converts the blue light emitted by the LED to a white light spectrum but also advantageously has reduced output in the infrared range. 
         [0028]    LEDs emitting other light colors or combinations of colors may likewise be employed in alternative embodiments. 
         [0029]    A wide variety of stock LEDs may be used in embodiments of variable intensity LED illumination systems according to the present invention depending on the requirements of a particular application. For the prototype LED illumination system  100 , a Fairchild FOLF598CIWTR, commonly used in camera flash applications, was selected. The Fairchild package was chosen based on its availability, small size, packaging, performance characteristics, luminance, and price. The Fairchild FOLF598CIWTR provides an array of four InGaN LEDs mounted on a circuit board. Another LED that may be used is a discrete surface mounted LED manufactured by Dominant. The Dominant LEDs are significantly less expensive at the present time and provide comparable performance. 
         [0030]    The details of construction of a lamp  400  incorporating an LED illumination system  100  according to the present invention will now be described. Lamp  400 , shown in perspective in  FIG. 4 , is designed as a replacement for the No. 327 instrumentation lamp. 
         [0031]    The exterior dimensions of lamp  400  correspond to the exterior dimensions of a conventional No. 327 instrumentation light bulb in accordance with MS25237 and multiple industry supplier data. Lamp  400  includes an envelope or lens  402  of a clear polycarbonate injection molded material having a domed top portion  401  and a cylindrical sidewall  403  extending down and terminating at a lower edge  411 . At approximately the midpoint of sidewall  403 , a ridge  407  projects radially outwardly to a widened bottom portion of sidewall  403  and continues to lower edge  411 . Ridge  407  provides a stop to engage a corresponding inwardly projecting rim  409  formed at the top of a cylindrical metal sleeve  408  that is dimensioned to slide mount over the dome  401  of lens  402  and cover the bottom portion of sidewall  403 . Sleeve  408  extends a short distance down beyond edge  411  where it flares out to provide a mount for a disc shaped base  406  of a phenolic insulating material. 
         [0032]    In a variety of aircraft applications it is desirable to reduce the infrared output of instrumentation lamps. Although the infrared output of LEDs  112   a - d  is already significantly lower then that of a comparable No. 327 incandescent lamp, infrared output may be further reduced by the addition of an infrared inhibitor to the clear polycarbonate material of lens  402 . 
         [0033]    Internally, lamp  400  includes two wiring boards, a disc-shaped printed wiring board (PWB)  200  for mounting LEDs (LED board  200 ) and a PWB  300  for mounting LED driver components of LED illumination system  100  (Driver Board  300 ). PWBs  200  and  300  are conventional circuit boards formed of a dielectric resinous substrate material over which a conductive layer such as copper, gold, or aluminum is applied and etched to form traces for electrically coupling components mounted thereto. Driver board  300  is mounted edgewise perpendicularly beneath PWB  200  inside of lamp  400 . 
         [0034]      FIG. 2B  shows a plan view of the top surface  201  of PWB  200  on which LEDs  112   a - d  are positioned. Top surface  201  faces up toward domed top portion  401  of lens  402 .  FIG. 2A  shows the bottom surface  202  of PWB  200 . Bottom surface  202  includes two center contacts  206  and  207  which are coupled via through-holes to traces on top surface  201 . When PWB board  300  is mounted perpendicularly beneath PWB board  200 , contacts  206  and  207  are coupled via traces on PWB  300  to ground terminal pad  306  which is coupled via pigtail or the like to metal sleeve  408 . Bottom surface  202  also has four circumferential contacts,  212   a ,  212   b ,  212   c  and  212   d  that are through coupled to trace areas  115   a - d , respectively, directly above on top surface  201 . Circumferential contacts,  212   a - d  are coupled to LED coupling nodes  113   a - d , respectively, on PWB  300 . 
         [0035]    PWB  300  is positioned perpendicularly to PWB  200  and is mounted edgewise to the bottom of PWB  200  by soldering contacts  206  and  207  to pad  306  on PWB  300 . A good electrical coupling of the two boards is all that is required to attach the boards at this stage, since a potting compound will be added during final assembly to secure and protect all of the internal components. Two radially opposing alignment slots  405  are formed in sidewall  403  of lens  402  and extend up from lower edge  411  to just above the ridge  407  (only one is shown in the perspective view of  FIG. 4 ). Slots  405  are dimensioned to engage the outer edges of PWB  300  and are cut deep enough to ensure that the top surface  301  of PWB  300  lies at or above ridge  407  when PWB  300  is inserted. 
         [0036]    After PWBs  200  and  300  have been joined together and inserted into slots  405 , a suitable potting compound  414  such as CONAP® epoxy, or similar, is injected into the bottom area of  408  to secure the boards in position, encapsulate all components. Potting compound  414  seals out moisture, prevents corrosion, and provides vibration damping and electrical insulation. Potting compound  414  should have a low coefficient of thermal expansion. 
         [0037]    After the potting compound has been added, base  406  is affixed to the bottom of sleeve  408 . Base  406  includes an axial hole  413  that is centered over a positive power terminal  310  on PWB  300 . A positive center terminal  410  is formed by soldering a contact in an eyelet  415 . Center terminal  410  is electrically coupled to positive power terminal  310 . 
         [0038]    PWBs  200  and  300  are designed for surface mount components; however, in alternative embodiments leaded components may be employed as well as a number of alternative component layouts. In still other embodiments, components such as resistors may be formed directly on the substrate such as by an inking process, or the like. In yet other embodiments, some or all of the components of an LED illumination system according to the present invention may be incorporated in an integrated circuit. 
         [0039]    Operation of an embodiment of a variable intensity LED illumination system  100  according to the present invention is substantially as follows. When supply voltage  116  is at its maximum value Zener diodes  114   a - d  and  110   a - d  are in a low impedance (reverse breakdown) state and LEDs  112   a - d  are in a forward biased light emitting state. Current through Zener diodes  114   a - d  is enough to maintain the Zener voltage of 5.1V with current divided according to the impedance of the zener diode  114   a - d  and the associated LEDs  112   a - d  that are in parallel with the zener diodes. As supply voltage  116  is reduced, the circuit legs “turn off” at the voltage set points one by one and overall luminance is correspondingly reduced. 
         [0040]    Embodiments according to the present invention may be configured to approximate the luminance/voltage characteristics of a conventional incandescent bulb or to conform to a variety of other luminance to voltage transfer functions. 
         [0041]    Embodiments according to the present invention also require less current, have substantially longer life cycles and better ruggedness compared to incandescent bulbs. A further advantage of embodiments of variable intensity LED illumination systems according to the present invention is the ability to provide higher intensity luminance compared to conventional incandescent lamps, resulting in better visibility of instrument dials and the like when high ambient backlight conditions exist. 
       CONCLUSION 
       [0042]    Embodiments of voltage variable programmable intensity LED illumination systems according to the present invention provide significant improvements over existing variable intensity light systems and are capable of being programmed to closely match a number of different intensity response curves including the intensity response curve of the human visual system. It will be clear to one skilled in the art that the above embodiments may be altered in many ways without departing from the scope of the invention. Although embodiments described herein have been described in terms of off-the-shelf surface mount components mounted to a printed wiring board, other assembly and packaging techniques and technologies may be employed in alternative embodiments based on production quantities, cost considerations, and the like. For example, where production quantities are sufficiently large, application specific monolithic or hybrid integrated circuit designs may be employed to realize variable intensity LED illumination systems according to the present invention. In other alternative embodiments, a stand-alone driver circuit for a separable array of LEDs may be provided. Likewise, a number of different lamps of different voltage ranges, or having leads or different socketing arrangements may be produced in alternative embodiments of voltage variable programmable intensity LED light systems according to the present invention. Accordingly, the scope of the invention should be determined by the following claims and their legal equivalents.