Patent Publication Number: US-RE49137-E

Title: Illumination device and method for avoiding an over-power or over-current condition in a power converter

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
     The present application is a reissue of U.S. Pat. No. 9,485,813, issued on Nov. 1, 2016 from U.S. application Ser. No. 14/604,870, filed Jan. 26, 2015, which is hereby incorporated by reference herein in its entirety.  
     RELATED APPLICATIONS 
     This application is related to commonly assigned U.S. patent application Ser. Nos. 14/314,451, which published on Dec. 31, 2015 as U.S. Patent Application Publication No. 2015/0377699; 14/314,530, which published on Dec. 31, 2015 as U.S. Patent Application Publication No. 2015/0382422; 14/314,580, which issued on Jul. 12, 2016 as U.S. Pat. No. 9,392,663; 14/471,057, which issued on Jul. 12, 2016 as U.S. Pat. No. 9,392,660; and 14/471,081, which published on Mar. 3, 2016 U.S. Patent Application Publication No. 2016/0066384. The entirety of these applications is incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to illumination devices comprising light emitting diodes (LEDs) and, more particularly, to illumination devices and methods for avoiding an over-power or over-current condition. Specifically, illumination devices and methods are provided herein for adjusting the drive currents supplied to the LED chains, so as not to exceed a maximum safe power level or a maximum safe current level attributed to one or more power converters included within the illumination device. 
     2. Description of the Relevant Art 
     The following descriptions and examples are provided as background only and are intended to reveal information that is believed to be of possible relevance to the present invention. No admission is necessarily intended, or should be construed, that any of the following information constitutes prior art impacting the patentable character of the subject matter claimed herein. 
     Lamps and displays using LEDs (light emitting diodes) for illumination are becoming increasingly popular in many different markets. LEDs provide a number of advantages over traditional light sources, such as incandescent and fluorescent light bulbs, including low power consumption, long lifetime, no hazardous materials, and additional specific advantages for different applications. When used for general illumination, LEDs provide the opportunity to adjust the color (e.g., from white, to blue, to green, etc.) or the color temperature (e.g., from “warm white” to “cool white”) to produce different lighting effects. 
     In some cases, a number of differently colored emission LED chains may be combined into a single package, or emitter module, to provide a multi-colored LED illumination device. A multi-colored LED illumination device may be described as comprising two or more different colors of LED chains combined within an emitter module, typically to produce white or near-white light. Some multi-colored illumination devices may comprise only one emitter module, whereas others may include a plurality of emitter modules arranged, e.g., in a line or an array. There are many different types of white light illumination devices on the market, some of which combine red, green and blue (RGB) LED chains, red, green, blue and yellow (RGBY) LED chains, phosphor-converted white and red (WR) LED chains, RGBW LED chains, etc. within a single emitter module. By combining different colors of LED chains within the same emitter module, and driving the differently colored LED chains with different drive currents, these illumination devices may be configured to generate white or near-white light within a wide gamut of color set points or correlated color temperatures (CCTs) ranging from “warm white” (e.g., roughly 2600K-3700K), to “neutral white” (e.g., 3700K-5000K) to “cool white” (e.g., 5000K-8300K). 
     Some multi-colored LED illumination devices enable the brightness and/or color of the illumination to be changed to a particular set point. For example, some prior art illumination devices allow the target chromaticity or color set point to be changed by altering the ratio of drive currents supplied to the individual LED chains. As known in the art, the target chromaticity may be changed by adjusting the drive current levels (in current dimming) or duty cycle (in PWM dimming) supplied to one or more of the emission LED chains. For example, an illumination device comprising RGB LED chains may be configured to produce a “warmer” white light by increasing the drive current supplied to the red LEDs and decreasing the drive currents supplied to the blue and/or green LEDs. 
     Some prior art illumination devices also provide dimming capabilities, i.e., the ability to change the brightness level or target lumens output from the emission LEDs, in addition to (or instead of) color tuning. In most cases, the brightness level may be changed by adjusting the drive current levels (in current dimming) or the duty cycle of the drive currents (in PWM dimming) supplied to all emission LED chains to produce a new target lumens output. For example, the drive currents supplied to all emission LED chains may be increased to increase the target lumens output of the illumination device. 
     When the drive current supplied to a given LED is adjusted to change the brightness level or color set point of the illumination device, the junction temperature of that LED is inherently affected. As expected, higher drive currents result in higher junction temperatures, while lower drive currents result in lower junction temperatures. Below a certain junction temperature (e.g., about 25° C.), the lumen output of a given LED is generally unaffected by temperature. Beyond this temperature, however, the lumen output of an LED decreases significantly with increasing junction temperatures, thereby requiring higher drive currents to maintain the target lumens and target chromaticity settings of the illumination device. In some cases, the drive currents needed to maintain a certain target lumens and/or target chromaticity setting at a particular operating temperature may exceed a maximum current or power level, which can be safely provided by the power converters, which are included within the illumination device for supplying power to the LED chains. 
     As the brightness level and target chromaticity settings change, the power delivered to each LED chain by the power converters changes. At certain brightness levels and target chromaticity settings, the power drawn by the combined load (i.e., all LED chains combined) may exceed a maximum safe current or power level attributed to the power converters. This may cause the transformer core of one or more of the power converters to saturate, over-heat and possibly fail, unless counteractive measures are taken. 
     Some prior art illumination devices include power control circuitry for regulating LED power consumption or for protecting the LEDs from an over-voltage condition. For example, some devices may use current/voltage sensing and feedback to adjust the amount of power supplied to the LED chains by the power converter, and may use voltage clamps to protect the LEDs from electrical damage when the output voltage of the power converter exceeds a maximum value. However, the power control circuitry used in these devices does not protect the power converter from excessive current or power draws when the LEDs are operated at or near maximum operating levels. 
     A need remains for improved illumination devices and methods for limiting the amount of power drawn from the power converters of the illumination device, so as not to exceed a maximum safe current level or a maximum safe power level when brightness and/or target chromaticity settings are changed. 
     SUMMARY OF THE INVENTION 
     The following description of various embodiments of an illumination device and a method for controlling a lumen output of an illumination device is not to be construed in any way as limiting the subject matter of the appended claims. 
     According to one embodiment, an illumination device is provided herein comprising at least a plurality of light emitting diode (LED) chains, a driver circuit, at least one power converter, a temperature sensor and a control circuit. The LED chains are generally configured to produce illumination for the illumination device, and in particular, may produce illumination corresponding to desired lamp settings (e.g., a chromaticity setting, a brightness setting or a white mix setting) set within the illumination device. The driver circuit is coupled for generating and supplying a respective drive current to each of the plurality of LED chains, so as to achieve the desired lamp settings. The lamp settings may generally be changed, for example, by a user or building controller. In some embodiments, the illumination device may include an interface for receiving the desired lamp settings. 
     At least one power converter is coupled for supplying power to the driver circuit. Ideally, the at least one power converter may supply the amount of power required by the driver circuit to produce the respective drive currents needed to produce the illumination at the desired lamp settings. In some embodiments, the at least one power converter may comprise a first power converter (e.g., an AC/DC converter), which is coupled for supplying a DC voltage to a plurality of second power converters (e.g., a plurality of DC/DC converters), each of which are coupled for producing a forward voltage on a respective one of the LED chains. As described in more detail herein, the first power converter may have a maximum safe power level and the second power converters may each have a maximum safe current level, above which the inductive core of the power converters saturates, potentially causing the power converter to overheat and fail. The maximum safe power/current levels attributed to the power converters are not always consistent and tend to vary with operating temperature, once the operating temperature exceeds a predetermined safe temperature. 
     As lamp settings change, the drive currents supplied to the LED chains by the plurality of DC/DC converters change, which in turn, affects the operating temperature of the illumination device. At certain brightness and chromaticity settings, the drive current that should be supplied to a given LED chain to achieve the desired lamp settings may exceed a maximum safe current level attributed to a corresponding DC/DC converter at the present operating temperature, resulting in an “over-current condition.” At other brightness and chromaticity settings, the total power drawn by all LED chains combined may exceed a maximum safe power level attributed to the AC/DC converter at the present operating temperature, resulting in an “over-power condition.” In either case, an over-current or over-power condition may saturate the inductive core of the power converter, possibly causing the power converter to overheat and fail. 
     Improved illumination devices and methods are provided herein for limiting the load requirements placed on one or more power converters of the illumination device, so as not to exceed a maximum safe current/power level attributed to the power converters when lamp settings are changed. This need is particularly relevant to multi-colored LED illumination devices that provide dimming and/or color tuning capabilities, since changes in drive current inherently affect the lumen output, color and temperature of the illumination device, as well as the load requirements placed on the power converters. This need is also relevant to illumination devices with power converters rated with appropriate or reduced load ratings (i.e., not over-engineered to handle excessive loads), as such power converters are particularly susceptible to over-current and over-power conditions. 
     The improved illumination device and methods described herein avoid over-current and over-power conditions by including a control circuit, which is generally configured for determining the maximum safe current level and/or the maximum safe power level of the power converter(s) at a present operating temperature, and adjusting the respective drive currents supplied to the plurality of LED chains by the driver circuit, so as not to exceed the maximum safe current level or the maximum safe power level at the present operating temperature. In addition to a control circuit, a temperature sensor may be included for measuring the operating temperature presently associated with the power converter(s). 
     The method steps described herein are performed by several different components of the illumination device, such as the interface, the temperature sensor, the driver circuit and the control circuit. The method steps implemented by the control circuit may, in some embodiments, be performed by program instructions that are stored within a storage medium and executed by a processing device of the illumination device. Alternatively, the control circuit could comprise hardware logic for implementing the method steps. 
     In some embodiments, the control circuit and method may determine a maximum lumens value that can be safely produced by all LED chains at a predetermined safe temperature (e.g., 25° C.) to achieve a particular chromaticity setting. The chromaticity setting may be received by the interface or may be stored within the illumination device, and may be detected by the control circuit. In some embodiments, the control circuit and method may be configured for determining the maximum lumens value upon receiving the chromaticity setting, or only upon detecting a change in the chromaticity setting. 
     In some embodiments, the control circuit and method may determine the maximum lumens value by determining a lumen proportion, which is needed from each LED chain to achieve the chromaticity setting at the predetermined safe temperature. In some embodiments, the control circuit and method may determine the lumen proportions by determining, for each LED chain, chromaticity values that are expected for the LED chain using a forward voltage calibrated for the LED chain at the predetermined safe temperature, the respective drive current supplied to the LED chain by the driver circuit, a table of stored calibration values correlating forward voltage and drive current to chromaticity at a plurality of different temperatures, and one or more interpolation techniques. The control circuit and method may then use the expected chromaticity values to calculate the lumen proportions needed from each LED chain to achieve the chromaticity setting at the predetermined safe temperature. 
     Once the lumen proportions are determined, the control circuit and method may determine a relative lumens needed from each LED chain to achieve the lumen proportion determined for that LED chain, assuming only one of the plurality of LED chains is driven with a maximum drive current. For each LED chain, the control circuit and method may then calculate a ratio of the relative lumens determined for the LED chain over a maximum lumen output for that LED chain, and may determine an actual lumens needed from each LED chain to achieve the chromaticity setting at the predetermined safe temperature by dividing the relative lumens calculated for each LED chain by a largest of the calculated ratios. Finally, the control circuit and method may sum the actual lumens needed from each LED chain to determine the maximum lumens value that can be produced by all LED chains combined at the predetermined safe temperature to achieve the chromaticity setting. 
     In some embodiments, the control circuit and method may perform additional steps to determine the maximum lumens value. For example, the control circuit and method may determine a drive current, which is needed to produce the actual lumens needed from each LED chain to achieve the chromaticity setting at the predetermined safe temperature, and may further estimate a total power drawn by all LED chains combined at the predetermined safe temperature. The control circuit and method may also determine a maximum safe power level and a maximum safe current level for the power converters at the predetermined safe temperature. For the AC/DC converter, the control circuit and method may calculate a ratio of the maximum safe power level at the predetermined safe temperature over the total power estimated at the predetermined safe temperature. For each DC/DC converter, the control circuit and method may calculate a ratio of the maximum safe current level of the DC/DC converter at the predetermined safe temperature over the drive current determined for each corresponding LED chain at the predetermined safe temperature. A smallest of the calculated ratios may then be used to generate a scale factor, which can be applied to the maximum lumens value. 
     In some embodiments, the control circuit and method may determine a target lumens value that can be safely produced by all LED chains at the present operating temperature to achieve a particular chromaticity setting and a particular brightness setting without exceeding the maximum safe current level or the maximum safe power level associated with the at least one power converter. The chromaticity and brightness settings may be received by the interface or may be stored within the illumination device, and may be detected by the control circuit. In some embodiments, the control circuit and method may be configured for determining the target lumens value upon detecting the brightness setting, or only upon detecting a change in the brightness setting. 
     In some embodiments, the method steps described above for determining a maximum lumens value upon detecting a chromaticity setting (or a change in a chromaticity setting) may be performed before the method steps described below for determining a target lumens value. In other embodiments, the method steps described above for determining a maximum lumens value may be skipped if the chromaticity setting is not changed and a default or previously calculated maximum lumens value is assumed. 
     In some embodiments, the control circuit and method may determine the target lumens value by applying the brightness setting to the maximum lumens value to generate a temporary target lumens value, and thereafter, determining an actual lumens needed from each LED chain to achieve the temporary target lumens value at the present operating temperature. 
     In some embodiments, the control circuit and method may determine the actual lumens needed from each LED chain to achieve the temporary target lumens value at the present temperature by periodically turning the plurality of LED chains off for short durations of time, and measuring a forward voltage presently developed across each LED chain by applying a non-operative drive current to each LED chain, one chain at a time, during the short durations of time the plurality of LED chains are periodically turned off. In addition, the control circuit and method may be configured for determining chromaticity values that are expected for each LED chain using the forward voltage measured across each LED chain, the respective drive current supplied to each LED chain by the driver circuit, a table of stored calibration values correlating forward voltage and drive current to chromaticity at a plurality of different temperatures, and one or more interpolation techniques. Next, the control circuit and method may use the expected chromaticity values and the chromaticity setting to calculate the actual lumens needed from each LED chain to achieve the temporary target lumens value. 
     In some embodiments, the control circuit and method may perform additional steps to determine the target lumens value. For example, the control circuit and method may determine a drive current, which is needed for each LED chain to produce the actual lumens at the present operating temperature, and may further estimate a total power drawn by all LED chains combined at the present operating temperature. The control circuit and method may also determine the maximum safe power level and the maximum safe current level of the power converters at the present operating temperature, if this step was not previously performed. For the AC/DC converter, the control circuit and method may calculate a ratio of the maximum safe power level at the present operating temperature over the total power estimated at the present temperature. For each DC/DC converter, the control circuit and method may calculate a ratio of the maximum safe current level of the DC/DC converter at the present operating temperature over the drive current determined for each corresponding LED chain at the present temperature. A smallest of the calculated ratios may then be used to generate a scale factor. The target lumens value can then be calculated by applying the scale factor and the brightness setting to the maximum lumens value. 
     When operating temperatures are less than or equal to the predetermined safe temperature (e.g., 25° C.), the method described thus far provides a precise lumen output for the particular lamp settings (e.g., chromaticity, brightness and/or white mix settings) selected for the illumination device. Above the predetermined safe temperature, the scale factor applied to the target lumens value scales the lumen output with temperature, so as not to exceed the maximum safe power level or the maximum safe current level associated with the power converters at the present operating temperature. This avoids an “over-power” or “over-current” condition in the power converters, which improves lamp efficiency and prevents saturation of the inductive core. 
     In some embodiments, the control circuit and method may adjust the respective drive currents supplied to the plurality of LED chains by the driver circuit, so as to achieve the target lumens value. The illumination device may produce illumination at the new drive current levels, and the control circuit and method may continue to monitor for changes in lamp settings. 
     In other embodiments, one or more of the method steps described above may be repeated a predetermined number of times to minimize errors before the drive currents are adjusted. For example, as the scale factor reduces with increasing temperatures above 25° C., the target lumens value decreases, which improves LED efficiency and changes the relative drive currents between the LED chains. When this occurs, it may be beneficial to recalculate the chromaticity values that are expected for each LED chain at the new drive currents to provide a more accurate representation of the maximum lumens value. 
     The method steps described above are generally performed when the illumination device is first turned “on,” and any time a change in lamp settings (e.g., chromaticity, brightness and/or white mix settings) is detected during normal operation of the illumination device. In some embodiments, additional steps may be used to fine tune the drive currents during operation of the illumination device when no changes in lamp settings are detected. 
     During normal operation, in which no changes are made to the lamp settings, it may be beneficial to continually or periodically update the scale factor applied to the target lumens value as operating temperatures change over time. For example, when drive currents increase, the operating temperature increases, which in turn, decreases the maximum safe power level and the maximum safe current level associated with the power converters. In order to avoid saturating the inductive core of the power converters, the control circuit and method may continually or periodically update the scale factor applied to the target lumens value to compensate for temperature related changes that occur, even when lamp settings remain the same. 
     In some embodiments, the control circuit and method may continually or periodically readjust the drive currents supplied to the LED chains to account for changes in the present operating temperature. This may be a achieved, in some embodiments, by measuring a new present temperature, determining an actual lumens needed from each LED chain to achieve the chromaticity setting and the target lumens value at the new present temperature, and determining a drive current that should be supplied to each LED chain to produce the actual lumens at the new present temperature. After the determined drive currents are applied to the LED chains, the control circuit and method may determine (not estimate) a total power drawn by all LED chains combined at the new present temperature, and may update the scale factor to account for changes in the maximum safe power level and/or the maximum safe current level of the power converters at the new present temperature. After the target lumens value is recalculated using the updated scale factor, the control circuit and method may adjust the respective drive currents supplied to the plurality of LED chains by the driver circuit, so as to achieve the recalculated target lumens value. 
     In some embodiments, the control circuit and method may update the scale factor by determining the maximum safe power level and the maximum safe current level of the power converters at the new present temperature. For the AC/DC converter, the control circuit and method may calculate a ratio of the maximum safe power level at the new present temperature over the total power determined at the new present temperature. For each of the DC/DC converters, the control circuit and method may calculate a ratio of the maximum safe current level at the new present temperature over the drive current determined for each LED chain. In some embodiments, the control circuit and method may subtract “1” from a smallest of the calculated ratios to generate a subtraction result, and may add the subtraction result to the scale factor to update the scale factor. In other embodiments, the control circuit and method may subtract “1” from a smallest of the calculated ratios to generate a subtraction result, multiply the subtraction result with a coefficient value to generate a multiplication result, and add the multiplication result to the scale factor to update the scale factor. 
     Depending on the brightness settings and operating temperature, the step of adding the subtraction (or multiplication result) to the scale factor to update the scale factor may cause the scale factor to gradually increase or decrease. As expected, increasing scale factor values increase the target lumens value, and thus, increase the drive currents supplied to the LED chains, the total power drawn by all LED chains, and eventually the operating temperature. Decreasing scale factor values have the opposite effect. By updating the scale factor value to account for changes in operating temperature over time, the control circuit and method ensures that the respective drive currents supplied to the LED chains will not cause an over-power or over-current condition in the power converters. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Other objects and advantages of the invention will become apparent upon reading the following detailed description and upon reference to the accompanying drawings. 
         FIG. 1  is one example of a block diagram of an illumination device comprising at least one or more power converters, LED driver circuitry, control circuitry and a plurality of LED chains, according to one embodiment of the invention; 
         FIG. 2  is an exemplary block diagram of LED driver circuitry that may be included within the illumination device of  FIG. 1 ; 
         FIG. 3  is an exemplary graph illustrating how the saturation current (I sat ) of a power converter decreases with increasing temperatures above a predetermined safe temperature; 
         FIG. 4  is an exemplary graph illustrating how the drive current supplied to an LED chain decreases roughly linearly with increasing temperatures above a predetermined safe temperature, due to decreasing I sat  in the power converter; 
         FIG. 5  is an exemplary graph illustrating how the lumen output of an LED chain decreases roughly with the square of temperature above a predetermined safe temperature, since both drive current and LED efficacy decrease with increasing temperature; 
         FIGS. 6, 7A, and 7B  are flow chart diagrams illustrating exemplary methods used to determine a Max Lumens that can be produced by all LED chains at a predetermined safe temperature to achieve a particular target chromaticity setting; 
         FIG. 8  is a graphical representation depicting how one or more interpolation technique(s) may be used in the methods of  FIGS. 7B and 12B  to determine the expected x chromaticity value for a given LED chain using a calibrated or measured forward voltage (Vfe_safe or Vfe_present), the present drive current and a table of stored calibration values; 
         FIG. 9  is a graphical representation depicting how one or more interpolation technique(s) may be used in the methods of  FIGS. 7B and 12B  to determine the expected y chromaticity value for a given LED chain using a calibrated or measured forward voltage (Vfe_safe or Vfe_present), the present drive current and a table of stored calibration values; 
         FIG. 10  is a graphical representation depicting how one or more interpolation technique(s) may be used to determine the Drive Currents (Ix) needed from each LED chain to produce the Actual Lumens at the predetermined safe temperature in step  56  of  FIG. 6 , and the Drive Currents (Ix) needed from each LED chain to produce the Actual Lumens at the present operating temperature in step  118  of  FIG. 12A ; 
         FIG. 11  is a flow chart diagram illustrating an exemplary method for determining a scale factor for adjusting the Target Lumens value determined in  FIGS. 6 and 12A  to ensure that the individual drive currents needed to achieve the target chromaticity setting and the total power drawn by all LED chains at the predetermined safe temperature do not exceed a maximum safe current level or a maximum safe power level of the power converters at a present operating temperature; 
         FIGS. 12A and 12B  are flow chart diagrams illustrating an exemplary method for adjusting the Target Lumens value to account for changes in the brightness setting and/or to adjust the scale factor to account for changes in brightness due to temperature changes; and 
         FIGS. 13A and 13B  are flow chart diagrams illustrating an exemplary method for continually or periodically updating the scale factor to account for temperature related changes in the maximum safe power level attributed to the power converters during normal operation of the illumination device when no changes are made to the target chromaticity or brightness settings. 
     
    
    
     While the invention is susceptible to various modifications and alternative forms, specific embodiments thereof are shown by way of example in the drawings and will herein be described in detail. It should be understood, however, that the drawings and detailed description thereto are not intended to limit the invention to the particular form disclosed, but on the contrary, the intention is to cover all modifications, equivalents and alternatives falling within the spirit and scope of the present invention as defined by the appended claims. 
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     An LED generally comprises a chip of semiconducting material doped with impurities to create a p-n junction. As in other diodes, current flows easily from the p-side, or anode, to the n-side, or cathode, but not in the reverse direction. Charge-carriers—electrons and holes—flow into the junction from electrodes with different voltages. When an electron meets a hole, it falls into a lower energy level, and releases energy in the form of a photon (i.e., light). The wavelength of the light emitted by the LED, and thus its color, depends on the band gap energy of the materials forming the p-n junction of the LED. 
     Red and yellow LEDs are commonly composed of materials (e.g., AlInGaP) having a relatively low band gap energy, and thus produce longer wavelengths of light. For example, most red and yellow LEDs have a peak wavelength in the range of approximately 610-650 nm and approximately 580-600 nm, respectively. On the other hand, green and blue LEDs are commonly composed of materials (e.g., GaN or InGaN) having a larger band gap energy, and thus, produce shorter wavelengths of light. For example, most green and blue LEDs have a peak wavelength in the range of approximately 515-550 nm and approximately 450-490 nm, respectively. 
     In some cases, a “white” LED may be formed by covering or coating, e.g., a blue LED having a peak emission wavelength of about 450-490 nm with a phosphor (e.g., YAG), which down-converts the photons emitted by the blue LED to a lower energy level, or a longer peak emission wavelength, such as about 525 nm to about 600 nm. In some cases, such an LED may be configured to produce substantially white light having a correlated color temperature (CCT) of about 3000K. However, a skilled artisan would understand how different colors of LEDs and/or different phosphors may be used to produce a “white” LED with a potentially different CCT. 
     When two or more differently colored LEDs are combined within a single package, the spectral content of the individual LEDs are combined to produce blended light. In some cases, differently colored LEDs may be combined to produce white or near-white light within a wide gamut of color points or CCTs ranging from “warm white” (e.g., roughly 2600K-3000K), to “neutral white” (e.g., 3000K-4000K) to “cool white” (e.g., 4000K-8300K). Examples of white light illumination devices include, but are not limited to, those that combine red, green and blue (RGB) LEDs, red, green, blue and yellow (RGBY) LEDs, white and red (WR) LEDs, and RGBW LEDs. 
     The present invention is generally directed to illumination devices having a plurality of light emitting diodes (LEDs) that are configured to provide illumination for the illumination device. For the sake of simplicity, the term “LED” or “emission LED” will be used throughout this disclosure to refer to a single LED, or a chain of serially connected LEDs supplied with the same drive current. Although not limited to such, the present invention is particularly well suited to illumination devices (i.e., multi-colored illumination devices) in which two or more different colors of emission LEDs are combined within a single package or emitter module to produce blended white or near-white light. The “color” of an LED is generally understood as referring to the peak emission wavelength of the light produced by the LED when forward biased. While examples of peak emission wavelengths for different colors of LEDs are provided above, the illumination device described herein is not limited to only the exemplary colors of LEDs mentioned herein and may comprise substantially any combination of LEDs. 
     The present invention is also particularly well suited to illumination devices (i.e., tunable illumination devices) that enable the target brightness level and/or the target chromaticity setting to be changed by adjusting the drive currents supplied to one or more of the emission LEDs. In addition to changing the lumen output and/or the color point setting of the illumination device, adjusting the drive currents supplied to one or more of the emission LEDs inherently affects the temperature of the illumination device and changes the load requirements placed on one or more power converters included within the illumination device. According to one embodiment, the present invention provides an improved illumination device and methods for limiting the amount of power drawn from the power converters of the illumination device, so as not to exceed a maximum safe current/power level when target brightness and/or target chromaticity settings are changed. 
       FIG. 1  illustrates an exemplary block diagram of an improved illumination device  10 , according to one embodiment of the invention. The illumination device illustrated in  FIG. 1  provides one example of the hardware and/or software that may be used to implement the methods shown in  FIGS. 6-13  and described below. 
     In the illustrated embodiment, illumination device  10  comprises a plurality of emission LEDs  26 , and in this example, comprises four chains of any number of serially connected LEDs. In typical embodiments, each chain may have 2 to 4 LEDs of the same color, which are coupled in series and configured to receive the same drive current. In one example, the emission LEDs  26  may include a chain of red LEDs, a chain of green LEDs, a chain of blue LEDs, and a chain of white or yellow LEDs. However, the present invention is not limited to any particular number of LED chains, any particular number of LEDs within each chain, or any particular color or combination of LED colors. In some embodiments, the emission LEDs  26  may be mounted on a substrate and encapsulated within a primary optic structure of an emitter module, possibly along with one or more photodetectors (not shown in  FIG. 1 ). In some embodiments, an illumination device may include more than one emitter module. 
     In addition to emission LEDs  26 , illumination device  10  includes various hardware and software components for powering the illumination device and controlling the light output from the one or more emitter modules. In the embodiment shown in  FIG. 1 , illumination device  10  is connected to AC mains  12  and includes an AC/DC converter  14  for converting the AC mains voltage (e.g., 120V or 240V) to a DC voltage (V DC ). The DC voltage (e.g., 15V) is supplied to LED driver circuits  24  to produce the drive currents, which are supplied to the emission LEDs  26  for producing illumination. In the embodiment of  FIG. 1 , a DC/DC converter  16  is included for converting the DC voltage V DC  (e.g., 15V) to a lower voltage V L  (e.g., 3.3V), which is used to power the low voltage circuitry of the illumination device, such as PLL  18 , interface  20 , and control circuit  22 . In other embodiments, illumination device  10  may be powered by a DC voltage source (e.g., a battery), instead of AC mains  12 . In such embodiments, the illumination device may be coupled to the DC voltage source and may or may not include a DC/DC converter in place of the AC/DC converter  14 . Additional timing circuitry may be needed to providing timing and synchronization signals to the control and driver circuits. 
     In the illustrated embodiment, a phase locked loop (PLL)  18  is included within illumination device  10  for providing timing and synchronization signals. Generally speaking, PLL  18  locks onto the AC mains frequency (e.g., 50 or 60 HZ) and produces a high speed clock (CLK) signal and a synchronization signal (SYNC). The CLK signal provides timing signals for control circuit  22  and LED driver circuits  24 . In one example, the CLK signal frequency is in the tens of mHz range (e.g., 23 MHz), and is precisely synchronized to the AC Mains frequency and phase. The SNYC signal is used by the control circuit  22  to create the timing signals used to control the LED driver circuit  24 . In one example, the SNYC signal frequency is equal to the AC Mains frequency (e.g., 50 or 60 HZ) and also has a precise phase alignment with the AC Mains. 
     In some embodiments, interface  120  may be included within illumination device  10  for receiving calibration data from an external calibration tool during manufacturing of the device. The calibration values received via interface  20  may be stored in a table of calibration values within storage medium  23  of control circuit  22 , for example. Examples of calibration values that may be received via interface  20  include, but are not limited to, the luminous flux, intensity, wavelength, and chromaticity of the light emitted by each LED chain, as described in co-pending application Ser. Nos. 14/314,451 and 14/471,057. In some embodiments, efficiency values corresponding to one or more of power converters of the illumination device may also be received via interface  20  and stored within storage medium  23 . If included, these efficiency values may be used to determine the maximum loads that may be placed on the power converters without saturating the transformer core. 
     Interface  20  is not limited to receiving calibration data and may be used, in some embodiments, for communicating information and commands to and from illumination device  10 . During normal operation of illumination device  10 , for example, interface  20  could be used to communicate commands used to control the illumination device, or to obtain information about the illumination device. For instance, commands may be communicated to illumination device  10  via interface  20  to turn the illumination device on/off, to control the brightness level and/or color set point of the illumination device, to initiate a calibration procedure, or to store calibration results in memory. In other examples, interface  20  may be used to obtain status information or fault condition codes associated with illumination device  10 . 
     In some embodiments, interface  20  may comprise a wireless interface that is configured to operate according to ZigBee, WiFi, Bluetooth, or any other proprietary or standard wireless data communication protocol. In other embodiments, interface  20  could communicate optically using infrared (IR) light or visible light. Alternatively, interface  20  may comprise a wired interface, which is used to communicate information, data and/or commands over the AC mains  12  or a dedicated conductor, or a set of conductors. In another alternative embodiment, interface  20  may additionally or alternatively comprise a user interface, such as a display screen and/or one or more buttons, sliders, knobs or switches for controlling and/or diagnosing illumination device  10 . A skilled artisan would recognize that a number of different interfaces may be included within the illumination device for communicating information, commands and control signals. 
     According to one preferred embodiment, interface  20  is coupled for receiving control signals from a building controller and/or from a user for altering an illumination state of illumination device  10 . For example, interface  20  may receive control signals for turning the illumination device on/off, for changing a brightness level, or for changing a color point setting of the illumination device. In some embodiments, the brightness level may be adjusted substantially continuously between a minimum level (e.g., 0% brightness) and a maximum level (e.g., 100% brightness), according to a linear or logarithmic scale, by defining the brightness level as a 16-bit variable. In other embodiments, the brightness level may be adjusted between a limited number of predefined steps, wherein each step corresponds to a percent change in brightness (e.g., 0%, 25%, 50%, 75% or 100% maximum brightness) or a decibel change (e.g., +/−1 dB) in lumen output. 
     In some embodiments, the color point setting may be defined by a set of target chromaticity coordinates, such as x and y chromaticity values from the CIE 1931 Chromaticity Diagram, but is not limited to such. In some embodiments, the color point setting may be adjusted by selecting substantially any pair of x and y chromaticity values that fall with the color gamut producible by the combination of emission LEDs  26  included within the illumination device  10 . In some embodiments, the x and y chromaticity values may each comprise 16-bit variables. If a white LED chain is included within illumination device  10 , a 16-bit white mix variable may be combined with the 16-bit x and y chromaticity values to further define the color point setting. 
     As known in the art, the color gamut producible by a particular combination of emission LEDs  26  is defined by and constrained within the lines connecting the chromaticity coordinates of the emission LEDs. For example, a red (R) LED with a peak wavelength of 625 nm may have a chromaticity coordinate of (0.69, 0.31), a green (G) LED with a peak wavelength of 528 nm may have a chromaticity coordinate of (0.18, 0.73), and a blue (B) LED with a peak wavelength of 460 nm may have a chromaticity coordinate of (0.14, 0.04). When the chromaticity coordinates of the RGB LEDs are connected together, they form a triangle representing the color gamut producible by that particular combination of LEDs. With four different chains of LEDs (e.g., RGBW), there is an infinite number of different spectrums that can be combined to produce the same target chromaticity (x,y) within the color gamut triangle, since two different sets of three color LED chains can be used to produce the same target chromaticity. For example, magenta can be produced by the combination of RGB or RWB. The white mix variable defines the proportion of the total lumens produced by each color gamut triangle. For example, 100% white mix includes no green component, while 0% white mix contains no white. 
     Using the timing signals received from PLL  18  and the control signals from interface  20  (e.g., a desired brightness level and target chromaticity), control circuit  22  calculates and produces values indicating a desired drive current to be supplied to each of the LED chains  26 . This information may be communicated from control circuit  22  to LED driver circuits  24  over a serial bus conforming to a standard, such as SPI or I 2 C, for example. In addition, control circuit  22  may provide a latching signal that instructs the LED driver circuits  24  to simultaneously change the drive currents supplied to each of the LED chains  26  to prevent brightness and color artifacts. 
     In some embodiments, control circuit  22  may be configured for determining the respective drive currents needed to achieve a desired luminous flux and/or a desired chromaticity for the illumination device in accordance with one or more of the compensation methods described in co-pending application Ser. Nos. 14/314,530; 14/314,580; and 14/471,081, which are commonly assigned and incorporated herein in their entirety. In a preferred embodiment, control circuit  22  may be further configured for adjusting the drive currents supplied to the emission LEDs  26 , so as not to exceed a maximum safe current level or a maximum safe power level attributed to one or more power converters of the illumination device  10  at a present operating temperature. 
     As shown in  FIG. 1 , a temperature sensor  28  may be included within the illumination device  10  for measuring a present operating temperature of the illumination device. In some embodiments, temperature sensor  28  may be a thermistor, which is thermally coupled to a circuit board or chip comprising one or more of the components shown in  FIG. 1 . For example, temperature sensor  28  may be coupled to a circuit board comprising AC/DC converter  14 , DC/DC converter  16 , PLL  18  and interface  20 . In another example, temperature sensor  28  may be thermally coupled to the chip comprising LED driver circuits  24  and emission LED chains  26 . In other embodiments, temperature sensor  28  may be an LED, which is used as both a temperature sensor and an optical sensor to measure ambient light conditions or output characteristics of LED chains  26 . The temperature measured by the sensor  28  is supplied to the control circuit  22  for adjusting the drive currents. 
     In some embodiments, control circuit  22  may determine the respective drive currents by executing program instructions stored within storage medium  23 . In one embodiment, the storage medium may be a non-volatile memory, and may be configured for storing the program instructions along with a table of calibration values, as described for example in co-pending application Ser. Nos. 14/314,451 and 14/471,057. Alternatively, control circuit  22  may include combinatorial logic for determining the desired drive currents, and storage medium  23  may only be used for storing the table of calibration values. 
     In general, LED driver circuits  24  may include a number (N) of driver blocks  30  equal to the number of emission LED chains  26  included within the illumination device  10 . In one exemplary embodiment, LED driver circuits  24  comprise four driver blocks  30 , each configured to produce illumination from a different one of the emission LED chains  26 . In some embodiments, LED driver circuits  24  may comprise circuitry for measuring ambient temperatures, measuring photodetector and/or emitter forward voltages and photocurrents, and adjusting the LED drive currents. Each driver block  30  receives data indicating a desired drive current from control circuit  22 , along with a latching signal indicating when the driver block  30  should change the drive current. 
       FIG. 2  is an exemplary block diagram of LED driver circuits  24 , according to one embodiment of the invention. In the exemplary embodiment of  FIG. 2 , LED driver circuits  24  include four driver blocks  30 , each block including a DC/DC converter  32 , a current source  34 , and an LC filter  36  for generating the operative drive currents (Idrv) supplied to a connected chain of emission LEDs  26 a to produce illumination, and the relatively small drive currents (Idrv) used to obtain emitter forward voltage (Vfe) measurements. In some embodiments, DC/DC converter  32  may convert the DC voltage (V DC ) into a pulse width modulated (PWM) voltage output (Vdr) when controller  40  drives the “Out_En” signal high. This PWM voltage signal (Vdr) is filtered by LC filter  36  to produce a forward voltage on the anode of the connected LED chain  26 a. The cathode of the LED chain is connected to current source  34 , which forces a fixed drive current (Idrv) equal to the value provided by the “Emitter Current” signal through LED chain  26 a when the “Led_On” signal is high. The “Vc” signal from current source  34  provides feedback to the DC/DC converter  32  to output the proper duty cycle and minimize the voltage drop across current source  34 . 
     As shown in  FIG. 2 , each driver block  30  may also include a difference amplifier  38  for measuring the forward voltage (Vfe) drop across the connected chain of emission LEDs  26 a. When measuring Vfe, DC/DC converter  32  is turned off and current source  34  is configured for drawing a relatively small drive current (e.g., about 1 mA) through the connected chain of emission LEDs  26 a. The forward voltage drop (Vfe) produced across LED chain  26 a by that current is measured by the difference amplifier  38 , which produces a signal equal to Vfe. The forward voltage (Vfe) is converted to a digital signal by analog to digital converter (ADC)  42  and supplied to controller  40 . Controller  40  determines when to take forward voltage measurements and produces the Out_En, Emitter Current and Led_On signals, which are supplied to each of the driver blocks  30 . 
     LED driver circuit  24  is not limited to the embodiment shown in  FIG. 2 . In some embodiments, each LED driver block  30  may include additional circuitry for measuring photocurrents, which are induced across one or more of the emission LED chains  26 , when these chains are configured for detecting incident light (e.g., ambient light or light emitted from other emission LEDs). In some embodiments, LED driver circuit  24  may additionally include one or more receiver blocks (not shown) for measuring forward voltages and/or photocurrents induced across one or more photodetectors, which may also be included within the emitter module. In some embodiments, LED driver circuit  24  may include a temperature sensor for measuring a temperature of the driver circuitry and a multiplexer for multiplexing the emitter forward voltages (Vfe) and measured temperatures to the ADC  42 . Exemplary embodiments of such a driver circuit are described in the previously mentioned co-pending applications. 
     DC/DC converter  16  and DC/DC converters  32  may include substantially any type of DC/DC power converter including, but not limited to, buck converters, boost converters, buck-boost converters, Ćuk converters, single-ended primary-inductor converters (SEPIC), or flyback converters. AC/DC converter  14  may likewise include substantially any type of AC/DC power converter including, but not limited to, buck converters, boost converters, buck-boost converters, Ćuk converters, single-ended primary-inductor converters (SEPIC), or flyback converters. Each of these power converters generally comprise a number of inductors (or transformers) for storing energy received from an input voltage source, a number of capacitors for supplying energy to a load, and a switch for controlling the energy transfer between the input voltage source and the load. The output voltage supplied to the load by the power converter may be greater than or less than the input voltage source, depending on the type of power converter used. 
     According to one preferred embodiment, AC/DC converter  14  comprises a flyback converter, while DC/DC converter  16  and DC/DC converters  32  comprise buck converters. AC/DC converter  14  converts the AC mains power (e.g., 120V or 240V) to a substantially lower DC voltage V DC  (e.g., 15V), which is supplied to the buck converters  16 / 32 . The buck converters  16 / 32  step down the DC voltage output from the AC/DC converter  14  to lower voltages, which are used to power the low voltage circuitry and provide drive currents to the LED chains  26 . 
     As known in the art, each of the power converters  14 / 16 / 32  has a saturation current (I sat ) associated therewith, above which the inductive core saturates, potentially causing the power converter to overheat and fail. These saturation currents limit the maximum current that DC/DC converters  32  can safely deliver to the emission LED chains  26 , and the maximum total power AC/DC converter  14  can safely draw from the AC mains power line  12  (or other input voltage source). These saturation currents are generally dependent on the magnetic flux density of the inductors or transformers used within the power converters, and in some embodiments, may range between about 30 mA and about 3 A for the AC/DC converter  14  and the DC/DC converters  32 . In one embodiment, a typical saturation current may be about 1 A for both the AC/DC and DC/DC converters. The maximum safe power level provided by the AC/DC converter is generally defined as the saturation current (I sat ) times the AC mains voltage  12 , and in one embodiment, may be approximately 18.5 W when drawn from a 120 Vrms AC power line. Assuming 80% efficiency, the AC/DC converter  14  may, in some cases, safely provide about 16 W to the load. 
     It is worth noting that the saturation currents may not always be the same for all power converters, and may be substantially different for one or more of the DC/DC converters. In one particular embodiment, the saturation current for the DC/DC converters  32  coupled to the red, green and white LED chains may be about 900 mA. However, since the smaller blue LEDs require less drive current, the DC/DC converter  32  coupled to the blue LED chain may exhibit a saturation current of about 400 mA. The maximum safe current level provided by the DC/DC converters is generally defined as the saturation current (I sat ) of that converter, and thus, may be about 900 mA for the DC/DC converters coupled to the red, green and white LED chains and about 400 mA for the DC/DC converter coupled to the blue LED chain, in one embodiment. 
     The saturation current of a power converter is affected by temperature and begins to decline above a certain temperature (e.g., 25° C.). As shown in  FIG. 3 , for example, the saturation current decreases roughly linearly with increasing temperatures above 25° C., and may sometimes decrease as much as 30-40% over a 25° C.-100° C. temperature range. This decreasing saturation current reduces the maximum safe current level associated with the DC/DC converters  32  and the maximum safe power level associated with the AC/DC converter  14 . At 75° C., for example, the maximum safe power level of AC/DC converter  14  may only be about 15 W, and the maximum safe current level of DC/DC converters  32  may only be about 750 mA for the red, green and white LED chains and about 330 mA for the blue LED chain. 
     As known in the art, temperature also affects the drive currents supplied to the LED chains and the lumen output produced thereby. As shown in  FIGS. 4-5 , for example, increasing drive currents are typically needed to maintain a consistent lumen output from the LED chains up to a certain temperature (e.g., 25° C.). Above this temperature, decreasing saturation currents in the power converters limit the amount of drive current that can be supplied to the LED chains, which in turn, limits the lumen output. While drive current decreases roughly linearly with increasing temperatures above 25° C. ( FIG. 4 ) due to decreasing I sat  ( FIG. 3 ), lumen output decreases roughly with the square of temperature ( FIG. 5 ), since both drive current and LED efficacy decrease with increasing temperature. While temperature related changes in lumen output may not be noticeable at certain levels of brightness (e.g., 50% or less), a user would observe lumen output decreasing with increasing temperatures above 25° C. at higher levels of brightness (e.g., greater than 50%). If the brightness is set to full scale (100%), for example, the lumen output of the illumination device may decrease as much as 30-40% over the 25° C.-100° C. temperature range, and thus, would be very noticeable. 
     As the brightness level and color point setting of the illumination device  10  change, the drive currents individually supplied to the LED chains  26  by the DC/DC converters  32  change, which in turn, affects the temperature of the illumination device. At certain brightness levels and color point settings, the drive current that should be supplied to a given LED chain to achieve the desired settings may exceed a maximum safe current level attributed to a corresponding DC/DC converter  32  at the present operating temperature. For example, if the illumination device is configured to produce saturated green light at 100% brightness, the LED driver circuit  24  may be configured to supply approximately 900 mA of drive current to the green LED chain at 25° C. At 75° C., the maximum safe current attributed to the DC/DC converter  32  may only be 750 mA, which is less than the drive current that should be supplied to the green LED chain. Unless the drive current is reduced from 900 mA to 750 mA or below, an “over-current condition” results, causing the inductive core of the DC/DC converter  32  to saturate. At best, an “over-current condition” would significantly reduce the efficiency of the illumination device. At worst, such condition may cause the power converter to overheat and fail. 
     At other brightness levels and color point settings, the total power drawn by the combined load (i.e., all LED chains  26 ) could exceed a maximum safe power level attributed to the AC/DC converter  14 . For example, if the illumination device were to include chains of RGBW LEDs, and all LED chains were driven with maximum drive currents (e.g., about 900 mA for the white, red and green chains and about 400 mA for the blue chain) to achieve 100% brightness and about 10K light, the white, red, green and blue emission LED chains could consume up to about 10 W, 8 W, 10 W and 5 W, respectively, which is more than twice the maximum power level (e.g., about 16 W) that can be safely drawn from AC/DC converter  14 . This “over-power condition” would saturate the inductive core of the AC/DC converter  14 , and most likely cause the power converter to overheat and fail. 
     Prior art illumination devices typically address this issue by over engineering the power converters, so that the user cannot specify brightness and color point settings that would result in an over-power or over-current condition. For instance, prior art illumination devices may use an AC/DC converter that provides up to about 40 W instead 16 W of maximum power, and may use inductors with saturation currents of 1 A at 100° C., instead of 1 A at 25° C. However, these power converters increase the cost of the illumination device, consume more space and generate more heat than the preferred embodiments of power converters disclosed herein. 
     A need remains for improved illumination devices and methods for limiting the load requirements placed on one or more power converters of the illumination device, so as not to exceed a maximum safe current/power level attributed to the power converters when brightness levels and/or color point settings are changed. This need is particularly relevant to multi-colored LED illumination devices that provide dimming and/or color tuning capabilities, since changes in drive current inherently affect the lumen output, color and temperature of the illumination device, as well as the load requirements placed on the power converters. This need is also relevant to illumination devices with power converters rated with reduced load ratings, as such power converters are particularly susceptible to over-current and over-power conditions. 
     In order to meet these needs, improved illumination devices and methods are provided herein for adjusting the drive currents supplied to the emission LEDs  26 , so as not to exceed a maximum safe current level or a maximum safe power level attributed to one or more power converters of the illumination device at the present operating temperature. Specifically, improved illumination devices and methods are provided herein for determining a target lumens that can be safely provided by the illumination device at the present operating temperature, and for recalculating the target lumens in response to a change in brightness level, chromaticity setting and/or temperature. 
       FIGS. 6-13  illustrate various embodiments of methods for adjusting the drive currents supplied to one or more of the emission LED chains, so as not to exceed a maximum safe current level or a maximum safe power level attributed to one or more of the power converters at a present operating temperature. As described in more detail below, the method steps shown in  FIGS. 6, 7A and 7B  may be used to determine a maximum lumens value (“Max Lumens”) that can be produced by all LED chains at a predetermined safe temperature (e.g., 25° C.) to achieve a particular target chromaticity (“Target Chromaticity”) setting. These steps may be performed when the illumination device is first turned “on,” and may be repeated whenever a change in the Target Chromaticity setting is detected. 
     In one embodiment, a “predetermined safe temperature” may be a typical ambient temperature. Although an exemplary safe temperature of 25° C. is used herein, a skilled artisan would understand how any temperature, which is within a normal operating range of the illumination device may alternatively be used. 
     Method steps shown in  FIGS. 6 and 11  may be used to determine a scale factor (“Scale Factor”), which can be used to adjust a target lumens (“Target Lumens”) value to ensure that the individual drive currents (“Drive Currents”) needed to achieve the Target Chromaticity setting and the total power (“Total Power”) drawn by all LED chains at the predetermined safe temperature do not exceed a maximum safe current level (“Max Current”) or a maximum safe power level (“Max Power”) of the power converters at a present operating temperature. The method steps shown in  FIGS. 12A and 12B  may be used to adjust the Target Lumens value to account for changes in the brightness (“Brightness”) setting and/or to adjust the Scale Factor to account for changes in brightness due to temperature changes. In most cases, the method steps shown in  FIGS. 12A-12B  may be performed after those shown in  FIGS. 6, 7A and 7B . If no changes are made to either the Target Chromaticity or Brightness settings, the method steps shown in  FIGS. 13A-13B  may be used to continually or periodically update the Scale Factor to account for temperature related changes in the Drive Currents, Total Power, Max Current and Max Power. 
     The methods illustrated in  FIGS. 6-13  may be utilized by an improved illumination device in accordance with the present invention, and may be performed by several different components included within such a device. In some embodiments, the method steps performed by the control circuit  22  may be implemented as program instructions, which are stored on a storage medium (e.g., storage medium  23 ) and executed on a processing device (included, e.g., within control circuit  22 ). In other embodiments, the control circuit may comprise hardware logic, or a combination of hardware logic and program instructions, for implementing the method steps described herein. In either embodiment, at least some of the method steps shown in  FIGS. 6-13  are performed outside of a processing device by other components of the illumination device. For example, an operating temperature of the illumination device can be measured by a temperature sensor  28  included within the illumination device  10 , and lamp settings can be set or changed through an interface  20  of the illumination device  10 . Once a desired Target Lumens value is determined for a particular set of lamp settings at a present operating temperature, drive circuitry  24  within the illumination device  10  may be used to adjust the drive currents supplied to the respective LED chains to achieve the desired Target Lumens value. 
     The methods illustrated in  FIGS. 6-13  assume that an illumination device employing such methods includes at least one emitter module comprising a plurality of emission LED chains, wherein at least one LED chain is configured for emitting a different peak wavelength (i.e., a different color of light) than the other LED chains. According to one embodiment, each emitter module may include a chain of red LEDs, a chain of green LEDs, a chain of blue LEDs and a chain of white LEDs, as discussed above. However, the methods described herein are not limited to any particular number or color of LED chains, and may be generally applied to an emitter module comprising at least two chains of differently colored LEDs (e.g., white and red LEDs, or white and blue LEDs). 
     The methods illustrated in  FIGS. 6-13  also assume that the brightness level and color point setting of the illumination device can be changed. According to one embodiment, a 16-bit brightness variable may be supplied to the illumination device to set the brightness level, while a 16-bit x chromaticity variable, a 16-bit y chromaticity variable and a 16-bit white mix variable may be supplied to set the color point or target chromaticity of the illumination device. However, the methods described herein are not limited to 16-bit variables, nor are they limited to defining chromaticity values in terms of x and y coordinates. A skilled artisan would understand how the brightness level and color point setting of the illumination device may be alternatively defined without departing from the scope of the invention. 
     The methods illustrated in  FIGS. 6-13  further assume that various electrical and optical characteristics of the emission LEDs were previously calibrated over a plurality of different drive currents and ambient temperatures during manufacturing of the illumination device, and that the calibration results were stored within a table of calibration values within a storage medium of the illumination device. According to one embodiment, an exemplary calibration table may include a plurality of luminous flux values, a plurality of x chromaticity values, a plurality of y chromaticity values and a plurality of emitter forward voltage values, which were previously obtained from each emission LED chain at a plurality of different drive currents (e.g., 10%, 30% and 100% of a max drive current) and at least two different ambient temperatures (e.g., T 0  and T 1 ). Exemplary calibration methods that may be used to obtain such calibration values are described, e.g., in co-pending application Ser. Nos. 14/314,451 and 14/471,057. In one example, the calibration values may be obtained at the predetermined safe temperature (e.g., T 0 =25° C.) associated with the power converters and also at a substantially higher temperature (e.g., T 1 =85° C.). However, the calibration values may be obtained at other ambient temperatures, as described further in the previously mentioned co-pending applications. 
     As shown in  FIG. 6 , embodiments of an improved method may generally begin when a change in one or more lamp settings is detected (in step  50 ). Examples of lamp settings that can be changed include, but are not limited to, turning the illumination device “on,” or changing a brightness level or a color point setting of the illumination device. In  FIG. 6 , a change in lamp settings is detected (in step  50 ) whenever a change in the target chromaticity setting (step  52 ) and/or a change in the brightness level (step  64 ) is detected during operation of the illumination device (i.e., when the illumination device is “on”). As noted above, these settings may be changed by a building controller and/or by a user (via interface  20 , for example) for altering an illumination state of the illumination device, and changes in such settings may be detected by control circuitry (e.g., control circuit  22 ,  FIG. 1 ) included within the illumination device. 
     In some embodiments, a change in lamp settings may be detected (in step  50 ) when the illumination device is first turned “on,” so that a Max Lumens value and a Scale Factor value may be initially calculated. In other embodiments, the method may reset the Scale Factor value to “1” and retrieve a previously calculated Max Lumens value from memory (e.g., storage medium  23 ,  FIG. 1 ) when the illumination device is first turned “on.” The previously calculated Max Lumens value retrieved from memory may be, for example, a Max Lumens value calculated for a default chromaticity setting or the last target chromaticity setting stored within the illumination device before the illumination device was turned “off.” 
     If a change in target chromaticity is detected (in step  52 ), the method may determine or recalculate the maximum lumen value (“Max Lumens”) produced by all LED chains to achieve the target chromaticity setting at the predetermined safe temperature (e.g., 25° C.) (in step  54 ). When driven with a maximum drive current, each LED chain produces a certain number of maximum lumens at 25° C. (otherwise referred to herein as a “maximum lumens output”). In one embodiment, the maximum lumens output produced by chains of four white, red, green and blue LEDs may be 1000 lumens, 250 lumens, 400 lumens, and 50 lumens, respectively, at 25° C. when each chain is driven with its maximum drive current. The maximum lumens output produced by each LED chain at the predetermined safe temperature may be stored within a storage medium of the illumination device and used to determine to determine the Max Lumens value that can be safely produced by all LED chains combined. 
     In order to determine the Max Lumens that can be safely produced by all LED chains combined, one LED chain is chosen to provide its maximum lumens output at 25° C. and the lumens needed from the other chains to produce the Target Lumens value are determined. If the needed lumens are greater than the Max Lumens, the lumens of all chains are scaled down proportionally by a Scale Factor value. In some embodiments, the LED chain providing maximum lumens output may be chosen based on the target chromaticity and white mix settings chosen for the illumination device. 
       FIGS. 7A and 7B  are flowchart diagrams illustrating one embodiment of a method, which can be used to determine the Max Lumens value (in step  54 ). As shown in  FIG. 7A , the method may generally begin (in step  70 ) by determining the lumen proportions that are needed from each LED chain to achieve the Target Chromaticity setting at the predetermined safe temperature. Exemplary method steps for determining the lumen proportions for each LED chain are shown in  FIG. 7B . In step  72 , for example, the method may determine the drive currents (Idrv), which are presently supplied to each of the LED chains by the LED driver circuitry. In step  74 , the method may determine the chromaticity values (x i , y i ) that are expected for each LED chain using a forward voltage (Vfe_safe) value, which was previously calibrated for each LED chain at the predetermined safe temperature, the drive current (Idrv) presently supplied to each LED chain, a table of calibration values stored within the illumination device, and one or more interpolation techniques. 
     The graphs shown in  FIGS. 8-9  depict how one or more interpolation technique(s) may be used to determine the expected x and y chromaticity values (x i , y i ) for a given LED chain at the predetermined safe temperature (Vfe_safe) and the present drive current (Idrv) from a table of stored calibration values. In  FIGS. 8-9 , the solid dots (●) represent examples of x and y chromaticity calibration values, which were previously obtained during calibration of the illumination device at three different drive currents (e.g., 10%, 30% and 100% of the maximum drive current) and two different temperatures (e.g., T 0  and T 1 ) and stored within the table of calibration values. Exemplary methods for obtaining such calibration values are described in co-pending application Ser. Nos. 14/314,451 and 14/471,057, which are incorporated herein in their entirety. The stored calibration values are not limited to only those shown in  FIGS. 8-9 . 
     In some embodiments, two interpolation techniques may be needed to determine the expected x and y chromaticity values (x i , y i ) for a given LED chain at the predetermined safe temperature (Vfe_safe) and the present drive current (Idrv). As shown in  FIGS. 8-9 , e.g., a first linear interpolation may be applied to the stored calibration values (●) to calculate the x and y chromaticity values (Δ), which should be produced at the predetermined safe temperature (Vfe_safe) when using the same three drive currents (e.g., 10%, 30%, and 100% of the maximum drive current) used during the calibration phase. If the drive current (Idrv) presently supplied to the LED chain differs from one of the calibrated drive current levels, a second interpolation may be applied to the calculated x and y chromaticity values (Δ) to generate a relationship there between (denoted by the solid line in  FIGS. 8-9 ). The second interpolation may be linear or non-linear depending on the color of the LED chain. From this relationship, the expected x and y chromaticity values (x i , y i ) for a given LED chain may be determined for the present drive current (Idrv). 
     In other embodiments, only one interpolation technique may be needed to determine the x and y chromaticity values (x i , y i ) that are expected for a given LED chain at the predetermined safe temperature (Vfe_safe) and the present drive current (Idrv). For example, if at least some of the x and y chromaticity calibration values (●) were previously measured at the predetermined safe temperature (i.e., if T 0 =25° C.), a linear interpolation technique may be applied directly to the stored calibration values (●) to determine a relationship there between (denoted by the dashed line at Vfe @ T 0  in  FIGS. 8-9 ). From this relationship, the expected x and y chromaticity values (x i , y i ) for a given LED chain may be determined for the present drive current (Idrv). 
     The x and y chromaticity values expected for each emission LED chain may be expressed as a color point in the form of (x i , y i ). In an illumination device comprising four LED chains, for example, step  74  of  FIG. 7B  may result in the generation of four expected color points: (x 1 , y 1 ), (x 2 , y 2 ), (x 3 , y 3 ), and (x 4 , y 4 ). Once the expected color points are determined, the expected color points can be used to determine the lumen proportions that are needed from each of the LED chains to achieve the Target Chromaticity (xm, ym) setting for the illumination device (in step  76 ). As indicated above, the Target Chromaticity (xm, ym) setting may be stored with a storage medium of the illumination device, and in some embodiments, may include a 16-bit x chromaticity variable and a 16-bit y chromaticity variable. If a white LED is included within the illumination device, a 16-bit white mix variable may be combined with the 16-bit x and y chromaticity values to further define the Target Chromaticity (xm, ym) setting. 
     Since lumen proportions are desired, a Target Lumens (Ym) value of “1” is assumed in the calculation of the lumen proportions in step  76  of  FIG. 7B . For example, if four emission LED chains are included within the illumination device, the Target Lumens (Ym) for the combined light from all LED chains may be expressed as:
 
Ym=Y 1 +Y 2 +Y 3 +Y 4 =1
 
where Y 1 , Y 2 , Y 3 , and Y 4  represent the lumen proportions of the four emission LED chains. These lumen proportions (Y 1 , Y 2 , Y 3  and Y 4 ) may be calculated using well-known color mixing equations, the Target Chromaticity (xm, ym) values set within the illumination device, and the expected color points (x 1 , y 1 ), (x 2 , y 2 ), (x 3 , y 3 ), (x 4 , y 4 ) determined in step  74  of  FIG. 7B . As these equations are well-known and readily understood by a skilled artisan, further description of such equations will be omitted herein. In one example, the lumen proportions determined in step  76  may be 0.2, 0.2, 0.2 and 0.4 for chains of red, green, blue and white LEDs, respectively. Of course, substantially different lumen proportions may be determined for different Target Chromaticity values and different combinations of LED chains.
 
     Once the lumen proportions (e.g., Y 1 , Y 2 , Y 3 , and Y 4 ) are calculated for each emission LED chain in step  76  of  FIG. 7B , the method calculates the Relative Lumens needed from each LED chain at 25° C. to achieve the lumen proportions in step  78  of  FIG. 7A . In calculating the Relative Lumens, one of the LED chains is assumed to be driven with a maximum drive current to produce a maximum lumen output, as described above. For example, the method may assume that a chain of white LEDs is driven with a maximum drive current (e.g., 900 mA) to produce a maximum lumen output of, e.g., 1000 lumens. If the lumen proportions determined in step  76  are 0.2, 0.2, 0.2 and 0.4 for chains of red, green, blue and white LEDs, the Relative Lumens needed from each LED chain to achieve the lumen proportions would be 500 lumens from the red LED chain, 500 lumens from the green LED chain, 500 lumens from the blue LED chain, and 1000 lumens from the white LED chain. 
     In step  80 , the Relative Lumens from step  78  are divided by the maximum lumens that can be produced by each LED chain at 25° C. (which is known and stored in memory as discussed above) to determine a ratio of Relative Lumens over maximum lumens for each LED chain. In the above example, a ratio of Relative Lumens over maximum lumens may be: 
     500/250=2 for the red LED chain; 
     500/400=1.25 for the green LED chain; 
     500/50=10 for the blue LED chain; and 
     1000/1000=1 for the white LED chain. 
     In step  82 , the Actual Lumens needed from each LED chain to achieve the Target Chromaticity at 25° C. is determined by dividing the Relative Lumens from step  78  by the largest ratio calculated in step  80 . In the above example, the LED chain with the largest ratio (e.g., 10) is the blue LED chain. Thus, the Actual Lumens may be determined in the current example by dividing the Relative Lumens (e.g., 500, 500, 500 and 1000 lumens) determined in step  78  for the red, green, blue and white LED chains by 10 to achieve an Actual Lumens of 50 lumens from the red LED chain, 50 lumens from the green LED chain, 50 lumens from the blue LED chain, and 100 lumens from the white LED chain. 
     In step  84 , the Actual Lumens from all LED chains are summed to determine the Max Lumens that can be produced by all LED chains at 25° C. In the current example, a Max Lumens of 50+50+50+100=250 lumens is determined (in step  84 ) and temporarily stored in memory (in step  86 ). Once the Max Lumens value is determined, process flow returns to step  56  of  FIG. 6 . 
     Step  56  of  FIG. 6  determines the Drive Currents (Ix) that are needed from each LED chain to produce the Actual Lumens for each chain at the predetermined safe temperature. According to one embodiment, the Drive Currents (Ix) may be determined using a forward voltage (Vfe_safe), which was previously calibrated for each LED chain at the predetermined safe temperature, the Actual Lumens values determined in step  82  for each LED chain, the table of calibration values stored within the illumination device, and one or more interpolation techniques. 
     The graph shown in  FIG. 10  depicts how one or more interpolation technique(s) may be used to determine the Drive Currents (Ix) needed from each LED chain (in step  56 ) to produce the Actual Lumens (Lx) determined in step  82  of  FIG. 7A . In  FIG. 10 , the solid dots (●) represent exemplary luminous flux calibration values, which were previously obtained during calibration of the illumination device at three different drive currents (e.g., 10%, 30% and 100% of the maximum drive current) and two different temperatures (e.g., T 0  and T 1 ) and stored within the table of calibration values. Exemplary methods for obtaining such calibration values are described in co-pending application Ser. Nos. 14/314,451 and 14/471,057, which are incorporated herein in their entirety. The stored calibration values are not limited to only those shown in  FIG. 10 . 
     In some embodiments, two interpolation techniques may be needed to determine the Drive Currents (Ix) that are respectively needed for each LED chain to produce the Actual Lumens (Lx) determined in step  82 . For example, a first linear interpolation may be applied to the stored luminous flux calibration values (●) to calculate the luminous flux values (Δ), which should be produced at the predetermined safe temperature (Vfe_safe) when using the same three drive currents (e.g., 10%, 30%, and 100% of the maximum drive current) used during the calibration phase. If the Actual Lumens (Lx) produced by a given LED chain differs from one of the calculated luminous flux values (Δ), a second interpolation may be applied to the calculated luminous flux values to generate a relationship there between (denoted by the solid line in  FIG. 10 ). The second interpolation may be linear or non-linear depending on the color of the LED chain. From this relationship, the Drive Currents (Ix) needed for a given LED chain to produce the Actual Lumens (Lx) may be determined. 
     In other embodiments, only one interpolation technique may be used to determine the Drive Currents (Ix) that are needed for each LED chain to produce the Actual Lumens (Lx) determined in step  82 . For example, if the luminous flux calibration values (●) were previously measured at the predetermined safe temperature (i.e., if T 0 =25° C.), a linear or non-linear interpolation technique may be applied directly to the stored luminous flux calibration values (●) to determine a relationship there between (denoted by the dashed line at Vfe @ T 0  in  FIG. 10 ). From this relationship, the Drive Currents (Ix) needed for a given LED chain to produce the Actual Lumens (Lx) may be determined. 
     Once the Drive Currents are known, the total power (“Total Power”) drawn by all LED chains at the predetermined safe temperature may be estimated (in step  58 ). The Total Power drawn by all LED chains is the sum of the power drawn by each individual chain (e.g., P 1 +P 2 +P 3 +P 4  when four LED chains are included). In one embodiment, the power drawn by each individual LED chain can be estimated by multiplying a respective Drive Current (Ix) with a forward voltage value (Vfe_est) estimated for that Drive Current at 25° C. In one example, the forward voltage values (Vfe_safe) that were previously calibrated for each LED chain at 25° C. may be scaled (e.g., by some fixed amount or by using characterization data and a curve fitting approach) to estimate the forward voltage values (Vfe_est) corresponding to the Drive Currents. 
     In step  60 , a Scale Factor is generated for adjusting a Target Lumens value set for the illumination device to ensure that the Drive Currents determined for each LED chain (in step  56 ) and the estimated Total Power drawn by all LED chains (in step  58 ) at the predetermined safe temperature will not exceed a maximum safe current level (“Max Current”) or a maximum safe power level (“Max Power”) attributed to the power converters (e.g., power converters  14  and  32  of  FIGS. 1-2 ) of the illumination device at the predetermined safe temperature. 
       FIG. 11  is a flowchart diagram illustrating one embodiment of a method, which can be used to generate a Scale Factor for a predetermined safe temperature. As shown in  FIG. 11 , the Scale Factor value may be temporarily set to “1” (in step  90 ) for a first iteration of possibly multiple iterations used to generate the Scale Factor value. In this embodiment, the maximum safe current level (“Max Current”) and the maximum safe power level (“Max Power”) attributed to the power converter(s) may then be determined for the predetermined safe temperature (in step  94 ). 
     As shown in  FIG. 3 , the saturation current (I sat ) associated with a power converter decreases linearly with increasing temperatures above the predetermined safe temperature (e.g., 25° C.), and in some cases, may decrease as much as 30-40% over a 25° C.-100° C. temperature range. This decreasing saturation current decreases the Max Current associated with the DC/DC converters  32  and the Max Power associated with the AC/DC converter  14 . In some embodiments, a relationship of I sat  vs. temperature may be stored within a storage medium of the illumination device (e.g., storage medium  23 ) for each power converter. In one example, the slope and intercept of the I sat  vs. temperature relationship shown in  FIG. 3  may be stored for each power converter. While the relationship of I sat  vs. temperature may be similar for each power converter, slightly different slope and intercept values may be stored so that each power converter can be individually characterized. 
     From the stored I sat  vs. temperature relationships, the Max Current associated with each of the DC/DC converters  32  and the Max Power associated with the AC/DC converter  14  may be determined at the predetermined safe temperature by linearly interpolating between the stored values (in step  94 ). In one embodiment, the Max Current at 25° C. may be approximately 900 mA for the white, red and green LED chains and approximately 400 mA for the blue LED chain, and the Max Power at 25° C. may be approximately 16 W. 
     In step  96 , a ratio of Max Power (from step  94 ) over Estimated Total Power (from step  58 ) is calculated for the AC/DC converter  14 . In step  98 , a ratio of Max Current (from step  94 ) over Drive Current for each LED chain (from step  56 ) is calculated for each of the DC/DC converters  32 . The smallest of the ratios calculated in steps  96  and  98  is multiplied with the Scale Factor value (e.g., “1” from step  90  if on first iteration) and the result is stored as a new Scale Factor value (in step  100 ). If the result is greater than 1, the new Scale Factor value is clipped at  1 . 
     As noted above, the Drive Currents (Ix) determined in step  56  of  FIG. 6  were calculated under the assumption that one LED chain was driven with a maximum drive current to provide a maximum lumen output (step  78  of  FIG. 7A ). This means that one of the Max Current/Drive Current ratios determined in step  98  will be “1” (for the LED chain driven with maximum drive current), and the Max Current/Drive Current ratios for the other LED chains should be values less than or equal to “1.” The Max Power/Total Power ratio may be more or less than one, depending on the combined Drive Currents (step  56 ) needed to achieve the Target Chromaticity. The smallest of the ratios calculated in steps  96  and  98  is used in step  100  to generate the Scale Factor. In one example, a Scale Factor of 0.5 may be generated in step  100  if the estimated Total Power is twice as much as the Max Power. 
     Once the Scale Factor is determined (in step  100 ), a Target Lumens value is calculated (in step  62  of  FIG. 6 ) according to the equation:
 
Target Lumens=Brightness*Max Lumens*Scale Factor
 
where “Brightness” typically refers to the brightness setting stored within the illumination device, “Max Lumens” refers to the Max Lumens value calculated in step  54 , and “Scale Factor” refers to the scale factor generated in step  100 . In this step, however, the Target Lumens value is calculated with the Brightness value temporarily set to “1,” and the results of the calculation are used to update the stored Max Lumens value. In some embodiments, the method may proceed immediately to  FIG. 12A  to adjust the Target Lumens value to account for changes in the Brightness setting and/or to adjust the Scale Factor value to account for changes in brightness due to temperature changes.
 
     In some embodiments, steps  54 - 62  of  FIG. 6  may be repeated a number of times to minimize errors. For example, as the Scale Factor reduces, the Target Lumens value determined in step  62  reduces, which decreases the drive currents supplied to the LED chains, improves LED efficiency and changes the relative drive currents between the LED chains. When this occurs, it may be beneficial to repeat steps  54 - 62  to determine the chromaticity values that are expected for each LED chain at the new drive currents supplied thereto to provide a more accurate representation of the Max Lumens value. For all subsequent iterations of steps  54 - 62 , step  90  of  FIG. 11  is not performed and the previous Scale Factor value is used instead, so that the Scale Factor changes less and less with each iteration. The Max Lumens value is not updated with the Target Lumens value calculated in step  62  until all iterations are complete. 
     If a change in Brightness setting is detected (in step  64  of  FIG. 6 ), the method may also proceed to  FIG. 12A  (in step  66 ). Although similar method steps are shown in  FIGS. 6 and 12A , the method shown in  FIG. 12A  determines the Drive Currents that should be supplied to the LED chains, estimates the Total Power that should be drawn by the LED chains, and generates the Scale Factor at the present operating temperature, instead of the predetermined safe temperature used in steps  56 ,  58  and  60  of  FIG. 6 . This provides more accurate Drive Currents, Estimated Total Power and Scale Factor values for the present operating temperature. The method shown in  FIG. 12A  also calculates the Target Lumens using the brightness setting stored within the illumination device, and thus, provides a more accurate Target Lumens value. 
     In some embodiments, the method shown in  FIG. 12A  may begin by temporarily resetting the Scale Factor to “1” and loading the brightness setting (in step  102 ), for example, from the interface  20  or storage medium  23  of the illumination device. In step  104 , the Target Lumens value is again calculated according to the equation:
 
Target Lumens=Brightness*Max Lumens*Scale Factor
 
this time using the brightness setting stored within the illumination device and retrieved in step  102 , the Max Lumens value stored in step  62  of  FIG. 6 , and the Scale Factor set to “1.” Since the Scale Factor is temporarily set to “1” in step  102 , the Target Lumens value calculated in step  104  may be considered a temporary Target Lumens value.
 
     In step  106 , the method determines the Actual Lumens needed from each LED chain to achieve the Target Lumens value (from step  104 ) at the present operating temperature. Exemplary method steps for determining the Actual Lumens needed from each LED chain are shown in  FIG. 12B . While the method steps shown in  FIG. 12B  are similar to those shown in  FIG. 7B  and discussed above, there are two exceptions. 
     First, the x and y chromaticity values expected for each LED chain are determined (in step  114 ) at the present operating temperature, instead of the predetermined safe temperature, by measuring a forward voltage (Vfe_present) presently developed across each LED chain. This is achieved during operation of the illumination device by periodically turning all LED chains “off” for short periods of time (in step  108 ), applying a relatively small, non-operative drive current to each LED chain, one chain at a time, during the short durations of time, and measuring the forward voltage (Vfe_present) developed there across (in step  110 ). Methods for measuring a forward voltage are described further in co-pending application Ser. Nos. 14/314,530; 14/314,580; and 14/471,081. After the forward voltages are measured across each LED chain, the drive currents (Idrv) supplied to the LED chains to produce illumination are determined (in step  112 ) from the LED driver circuitry. In step  114 , the x and y chromaticity values expected for each LED chain (x i , y i ) are determined using the forward voltage (Vfe_present) measured in step  110 , the drive current determined in step  112 , a table of stored calibration values and one or more interpolation techniques. The x and y chromaticity values expected for each LED chain (x i , y i ) may be determined in the same manner described above in step  74  of  FIG. 7B  and shown in  FIGS. 8-9 , except that the chromaticity values are determined for Vfe_present, instead of Vfe_safe. 
     As a second distinction, the method shown in  FIG. 12B  calculates the Actual Lumens needed from each LED chain to achieve the Target Chromaticity (xm, ym) setting and the Target Lumens (Ym) in step  116 . Although Actual Lumens are calculated (in step  116  of  FIG. 12B ) instead of lumen proportions (in step  76  of  FIG. 7B ), the process is essentially the same. For example, the Target Lumens (Ym) for the combined light from four LED chains may be expressed as:
 
Ym=Y 1 +Y 2 +Y 3 +Y 4  
 
In this case, however, Ym is not set to “1,” so that Y 1 , Y 2 , Y 3 , and Y 4  represent the Actual Lumens needed from the four LED chains to produce the Target Lumens (Ym) value determined in step  104 . The Actual lumens (Y 1 , Y 2 , Y 3  and Y 4 ) may be calculated using well-known color mixing equations, the Target Chromaticity (xm, ym) values set within the illumination device, and the expected color points (x 1 , y 1 ), (x 2 , y 2 ), (x 3 , y 3 ), (x 4 , y 4 ) determined in step  114  of  FIG. 12B . As these equations are well-known and readily understood by a skilled artisan, further description of such equations will be omitted herein.
 
     In step  118 , the Drive Currents (Ix) needed for each LED chain to produce the Actual Lumens at the present operating temperature are determined. According to one embodiment, the Drive Currents may be determined using the forward voltage (Vfe_present) measured for each LED chain in step  110 , the Actual Lumens determined for each LED chain in step  106 / 116 , the table of calibration values stored within the illumination device, and one or more interpolation techniques. The Drive Currents needed for each LED chain may be determined in the same manner described above in step  56  of  FIG. 6  and shown in  FIG. 10 , except that the Drive Currents are determined at Vfe_present, instead of Vfe_safe. 
     In step  120 , the total power (“Total Power”) drawn by all LED chains at the present operating temperature is estimated. As noted above, the power drawn by each LED chain can be estimated by multiplying a respective Drive Current determined in step  118  with a forward voltage value (Vfe_est), which is estimated for that Drive Current level at the present operating temperature. The Total Power drawn by all LED chains can then be calculated by summing the power drawn by each chain (e.g., P 1 +P 2 +P 3 +P 4  when four LED chains are included). In one example, the forward voltage (Vfe_safe) values that were previously calibrated for each LED chain at 25° C. may be scaled (e.g., by some fixed amount or by using characterization data and a curve fitting approach) to estimate the forward voltage (Vfe_est) values corresponding to the respective Drive Currents at the present operating temperature. Alternatively, the forward voltages (Vfe_present) measured for each LED chain in step  110  may be scaled to estimate the forward voltage (Vfe_est) values corresponding to the respective Drive Currents at the present operating temperature. 
     In step  122 , a Scale Factor is generated for adjusting the Target Lumens value to ensure that the Drive Currents determined for each LED chain (in step  118 ) and the estimated Total Power drawn by all LED chains (in step  120 ) at the present operating temperature will not exceed a maximum safe current level (“Max Current”) or a maximum safe power level (“Max Power”) attributed to the power converters (e.g., power converters  14  and  32  of  FIGS. 1-2 ) at the present operating temperature. 
     An exemplary method for generating a Scale Factor for a predetermined safe temperature was described above with respect to  FIG. 11 . In step  122  of  FIG. 12A , a Scale Factor is generated at the present operating temperature, instead of the predetermined safe temperature. While a similar method is used, additional method steps may be needed to generate the Scale Factor at the present operating temperature. 
     Returning to  FIG. 11 , the Scale Factor value is again temporarily set to “1” (in step  90 ). However, in this case, the present operating temperature is measured (in step  92 ) before the Max Power and Max Currents are determined for the power converters (in step  94 ). According to one embodiment, the present operating temperature can be measured by a temperature sensor (e.g., temperature sensor  28 ,  FIG. 1 ), which is coupled to a circuit board or chip comprising one or more of the power converters, control circuit, driver circuitry and emission LEDs. Once the present operating temperature is measured (in step  92 ), the Max Current associated with each of the DC/DC converters  32  and the Max Power associated with the AC/DC converter  14  may be determined at the present operating temperature (in step  94 ), instead of the predetermined safe temperature. 
     As noted above, the Max Current may be approximately 900 mA for the white, red and green LED chains and approximately 400 mA for the blue LED chain at 25° C., and the Max Power may be approximately 16 W at 25° C. However, these values decrease significantly above the safe operating temperature. At a present operating temperature of about 75° C., for example, the Max Current of the DC/DC converters  32  and the Max Power of the AC/DC converter  14  may only be about 80% of their safe temperature (25° C.) values. Step  94  of  FIG. 11  determines the Max Current and Max Power values for the power converters at the present operating temperature. According to one embodiment, the Max Current and Max Power values may be determined by linearly interpolating between the stored slope and intercept values corresponding to the I sat  vs. temperature relationships ( FIG. 3 ) attributed to each of the power converters. 
     In step  96 , a ratio of Max Power (from step  94 ) over Estimated Total Power (from step  58 ) is calculated for the AC/DC converter  14 . In step  98 , a ratio of Max Current (from step  94 ) over Drive Current for each LED chain (from step  56 ) is calculated for each of the DC/DC converters  32 . The smallest of the ratios calculated in steps  96  and  98  is multiplied with the Scale Factor value (e.g., “1” from step  90  if on first iteration) and the result is stored as a new Scale Factor value (in step  100 ). If the result is greater than 1, the new Scale Factor value is clipped at  1 . 
     Once the Scale Factor is generated (in step  122 ), the Target Lumens value is again calculated (in step  124 ) according to the equation:
 
Target Lumens=Brightness*Max Lumens*Scale Factor
 
using the brightness setting stored within the illumination device, the Max Lumens value calculated in step  62  of  FIG. 6 , and the scale factor generated in step  122  of  FIG. 12A . When operating temperatures are less than or equal to the predetermined safe temperature, the method described thus far provides a precise lumen output for the particular chromaticity, white mix and brightness level settings selected for the illumination device. Above the predetermined safe temperature, the Scale Factor generated in step  122  scales the lumen output with temperature, so as not to exceed the Max Power or Max Current associated with the power converters at the present operating temperature. This avoids an “over-power” or “over-current” condition in the power converters, which improves lamp efficiency and prevents saturation of the inductive core.
 
     In some embodiments, the drive currents supplied to the LED chains may be adjusted in step  126  (via driver circuitry  24 , for example) to achieve the new Target Lumens value calculated in step  124 . The illumination device may produce illumination at the new drive current levels, and the method may continue to monitor for changes in lamp settings in step  50  of  FIG. 6 . 
     In other embodiments, steps  106 - 124  of  FIGS. 12A and 12B  may be repeated a predetermined number of times to minimize errors before the drive currents are adjusted in step  126 . For example, as the Scale Factor reduces with increasing temperatures above 25° C., the Target Lumens value determined in step  124  decreases, which improves LED efficiency and changes the relative drive currents between the LED chains. When this occurs, it may be beneficial to repeat steps  106 - 124  to determine the chromaticity values that are expected (in step  114 ) for each LED chain at the new drive currents to provide a more accurate representation of the Max Lumens value. For all subsequent iterations of steps  106 - 124 , however, step  90  of  FIG. 11  is not performed and the previous Scale Factor value is used instead, so that the Scale Factor changes less and less with each iteration. 
     In yet other embodiments, one or more of the compensation methods described in co-pending application Ser. Nos. 14/314,530; 14/314,580; and 14/471,081 may be performed to fine tune the drive currents before the adjusted drive currents are supplied to the LED chains (in step  126 ). The method shown in  FIGS. 12A and 12B  is assumed to include all such embodiments. 
     By performing the method steps illustrated in  FIGS. 6-12B  and described above, the control circuitry (e.g., control circuit  22 ,  FIG. 1 ) of an illumination device is able to control the respective drive currents supplied to the emission LED chains (e.g., LED chains  26 ) by the driver circuits (e.g., driver circuitry  24 ), so as not to exceed a maximum safe power level (“Max Power”) and/or a maximum safe current level (“Max Current”) attributed to the power converters (e.g., AC/DC converter  14 , DC/DC converters  32 ) at the present operating temperature. As noted above, the methods shown in  FIGS. 6-12B  are generally performed when the illumination device is first turned “on,” and any time a change in lamp settings (e.g., target chromaticity, white mix and/or brightness level) is detected during normal operation of the illumination device. However, since the Max Power and Max Current that can be safely produced by the AC/DC and DC/DC power converters are affected by changes in temperature (above the predetermined safe temperature), additional steps may be needed to fine tune the drive currents during operation of the illumination device when no changes in lamp settings are detected. 
     If no changes in lamp settings are detected in step  50  of  FIG. 6 , the method may proceed (in step  68 ) to the normal operation mode shown in  FIG. 13A . During normal operation, in which no changes are made to the brightness level or the target chromaticity or white mix variables stored within the illumination device, the drive currents supplied to the LED chains are continually or periodically updated as the operating temperature changes over time. As drive currents increase, the operating temperature increases, which decreases the Max Current and the Max Power associated with the power converters. The method shown in  FIG. 13A  is used during normal operation of the illumination device to continually or periodically adjust the Scale Factor value, so as to account for temperature related changes in the Drive Current, Total Power, Max Current and/or Max Power. 
     In some embodiments, the method shown in  FIG. 13A  may begin by re-measuring the present operating temperature (in step  128 ). As noted above, the present operating temperature may be measured by a temperature sensor (e.g., temperature sensor  28 ,  FIG. 1 ), which is coupled to a circuit board or chip comprising, e.g., one or more of the power converters, control circuit, driver circuitry and/or emission LEDs. Other means for measuring the present operating temperature may also be used. 
     In some embodiments, the operating temperature measured in step  128  of  FIG. 13A  may be compared to a previously measured operating temperature to determine if the operating temperature has changed by a certain amount. In one embodiment, a change in temperature may be detected (in optional step  130 ) if the operating temperature changes by about 1° C. However, the detecting step is not limited to any particular increment of temperature, may be configured to detect substantially any predetermined difference in operating temperature, and may not be performed in all embodiments. 
     If no change in temperature is detected (in optional step  130 ), the method may proceed to step  50  of  FIG. 6  to continue monitoring for changes in lamp settings. If no changes in lamp settings are detected in step  50  of  FIG. 6 , the method may continually or periodically monitor the present operating temperature in step  128  of  FIG. 13A  until a change in operating temperature is detected (in step  130 ). If optional step  130  is not included, method steps  128 - 156  may be performed continually or periodically, whilst no changes in lamp settings are detected, to update the Scale Factor to account for temperature related changes. 
     If a change in operating temperature is detected (in optional step  130 ), the Actual Lumens needed from each LED chain to achieve the Target Chromaticity (xm, ym) setting stored within the illumination device and the most recently calculated Target Lumens (Ym) may be determined in step  132  for the new present operating temperature, as described above in step  106  of  FIG. 12A . In step  134 , the Drive Currents (Ix) needed to produce the Actual Lumens at the present operating temperature may be determined for each LED chain, as described above in step  118  of  FIG. 12A . In step  136 , the Drive Currents (Ix) determined in step  134  may be supplied to the LED chains via the LED driver circuitry. 
     In step  138 , the Total Power actually drawn by all LED chains at the present operating temperature is calculated by summing the power drawn by each individual LED chain (e.g., P 1 +P 2 +P 3 +P 4 ). As noted above, the power drawn by each LED chain may be calculated by multiplying the drive current presently supplied to the LED chain with a forward voltage corresponding to that drive current. In this case, however, the forward voltage values are not estimated. Instead, each forward voltage value is calculated by multiplying an input voltage supplied to a respective DC/DC converter (e.g., DC/DC converters  32  of  FIG. 2 ) by the duty cycle of that converter. This provides a more accurate representation of the Total Power actually being drawn by all LED chains, compared to the estimates determined in steps  58  and  120 . 
     In step  140 , the Scale Factor value is updated to account for any changes in the maximum safe current level (“Max Current”) and/or the maximum safe power level (“Max Power”) of the power converter(s) at the new present operating temperature. An exemplary method for updating the Scale Factor value is shown in  FIG. 13B . 
     Several of the method steps used in  FIG. 13B  to update the Scale Factor value are similar to the ones used in  FIG. 11  to generate the Scale Factor value. For example,  FIG. 13B  may begin (in step  142 ) by determining the Max Power and the Max Current attributed to the power converters at the new operating temperature. The Max Power and Max Current may be determined in step  142  in the same manner as described above in step  94  of  FIG. 11 . In step  144 , a ratio of the Max Power (from step  142 ) over Total Power (from step  138 ) is calculated for the AC/DC converter  14 , similar to step  96  of  FIG. 11 . In step  146 , a ratio of the Max Current (from step  142 ) over the Drive Current determined for each LED chain (in step  134 ) is calculated for each of the DC/DC converters  32 , similar to step  98  of  FIG. 11 . However, the similarities between  FIGS. 11 and 13B  end here. 
     In step  148  of  FIG. 13B , “1” is subtracted from the smallest of the ratios calculated in steps  144  and  146  and the result of such subtraction is used to generate a new or updated Scale Factor value. In some embodiments, the subtraction result from step  148  is added to a previously generated Scale Factor value to produce a new Scale Factor value, which is stored (in step  152 ). Depending on the brightness setting and the present operating temperature, the subtraction result from step  148  may be a positive value (which increases the Scale Factor value) or a negative value (which decreases the Scale Factor value). 
     As long as the brightness setting is small enough (e.g., roughly 50% or less), all Drive Currents determined in step  134  and the Total Power calculated in step  138  will be less than their maximum safe levels at the present operating temperature. When this occurs, the smallest of the ratios calculated in steps  144  and  146  will be some value greater than “1.” After “1” is subtracted from this value in step  148 , a positive result is added to the previously generated Scale Factor to generate a new Scale Factor value, which gradually increases towards “1,” until it is clipped at  1 . On the other hand, if the brightness setting and operating temperature are both high, at least one of the Drive Currents or the Total Power will exceed its maximum safe level, resulting in at least one ratio (from steps  144  or  146 ) that is less than “1.” When “1” is subtracted from this ratio (in step  148 ), a negative result is added to the previously generated Scale Factor to generate a new Scale Factor value, which gradually decreases away from “1.” 
     In some embodiments, the new Scale Factor value is used to calculate a new Target Lumens value (in step  154  of  FIG. 13A ) according to the equation provided above. As expected, increasing Scale Factor values increase the Target Lumens value, and thus, increase the drive currents supplied to the LED chains, the Total Power drawn by all LED chains, and eventually the operating temperature. Decreasing Scale Factor values have the opposite effect. 
     In some embodiments, the drive currents supplied to the LED chains (in step  136 ) may be adjusted to achieve the new Target Lumens value (in step  156 ). The illumination device may produce illumination at the new drive current levels, and the method may return to step  50  of  FIG. 6  to monitor and detect changes in lamp settings. In other embodiments, one or more of the compensation methods described in co-pending application Ser. Nos. 14/314,530; 14/314,580; and 14/471,081 may be performed to fine tune the drive currents before the adjusted drive currents are supplied to the LED chains (in step  156 ). The method shown in  FIG. 13A  is assumed to include all such embodiments. 
     In some embodiments, the positive or negative subtraction result from step  148  of  FIG. 14  may be scaled by a coefficient value (Ki) (in optional step  150 ) before the result is added to the previously generated Scale Factor to generate a new Scale Factor value (in step  152 ). The coefficient value (Ki) is typically much less than “1” and may be used, in some embodiments, to ensure that the control loop shown in  FIG. 13  responds much faster than temperature changes. In optional step  150 , the positive or negative subtraction result from step  148  is multiplied by the coefficient value (Ki) and the multiplication result is added to the previously generated Scale Factor to generate the new Scale Factor value. The new Scale Factor value may be stored (in step  152 ) and applied to the Target Lumens value (in step  154 ), as described above. 
     It will be appreciated to those skilled in the art having the benefit of this disclosure that this invention is believed to provide an improved illumination device and improved methods for avoiding an over-power or over-current condition in a power converter. Specifically, illumination devices and methods are provided herein for adjusting the drive currents supplied to the LED chains, so as not to exceed a maximum safe power level or a maximum safe current level attributed to one or more power converters included within the illumination device. Further modifications and alternative embodiments of various aspects of the invention will be apparent to those skilled in the art in view of this description. It is intended, therefore, that the following claims be interpreted to embrace all such modifications and changes and, accordingly, the specification and drawings are to be regarded in an illustrative rather than a restrictive sense.