LED-based lighting fixtures and related methods for thermal management

Disclosed is a light emitting diode (LED)-based lighting fixture including an LED and a voltage supply configured to provide electrical power to the LED. The LED-based lighting fixture also includes a temperature sensor configured to determine a temperature at a selected location of the lighting fixture; and a controller connected between the temperature sensor and the voltage supply and configured to determine an ambient temperature and a drive current based on the ambient temperature and to provide an input voltage to the LED based on the drive current. A method of controlling the operational lifetime of an LED, a computer readable medium and an apparatus are also described.

TECHNICAL FIELD

The present disclosure is directed generally to LED-based lighting fixtures. More particularly, various inventive methods and apparatus disclosed herein relate to thermal management of LED-based lighting fixtures.

BACKGROUND

Digital lighting technologies, i.e. illumination based on semiconductor light sources, such as light-emitting diodes (LEDs), offer a viable alternative to traditional fluorescent, HID, and incandescent lamps. Functional advantages and benefits of LEDs include high energy conversion and optical efficiency, durability, lower operating costs, and many others. Recent advances in LED technology have provided efficient and robust full-spectrum lighting sources that enable a variety of lighting effects in many applications. Some of the fixtures embodying these sources feature a lighting module, including one or more LEDs capable of producing different colors, e.g. red, green, and blue, as well as a processor for independently controlling the output of the LEDs in order to generate a variety of colors and color-changing lighting effects, for example, as discussed in detail in U.S. Pat. Nos. 6,016,038 and 6,211,626, the disclosures of which are specifically incorporated herein by reference.

As is known, the lifetime of an LED is related to the junction temperature; the greater the junction temperature, the shorter the lifetime of the LED. LED lifetime requirements based on the junction temperature of the LEDs are often specified at the maximum ambient temperature rating of the product. Illustratively, the lifetime requirement is fifty thousand hours of operation at 50° C., with the understanding that the higher the ambient temperature, the higher junction temperature of the LED, leading to shorter lifetime. Often, LEDs designed to this standard are driven at a particular drive current to attain an output power. In order to meet the lifetime requirements, the power output to the LEDs in known LED-based lighting fixtures is set at the same level regardless of the ambient temperature. For example, the power output level is selected for the maximum ambient temperature and junction temperature to meet the lifetime specification. Naturally, at a lower ambient temperature and junction temperature, the drive current to the LEDs is lower for the output power selected for maximum ambient and lifetime criteria. Illustratively, at ambient temperatures in the range of 25° C. to 30° C., at the selected output level, the junction temperature of the LEDs, the lifetime is increased over that of the requirements, but is realized at the cost of reduced output power. Accordingly, because of the design criteria for LED lifetime are based on comparatively high ambient temperatures (e.g., 50° C.), known LED-based lighting fixtures operating at typical ambient temperatures (e.g., 25° C. to 30° C.), are not driven with the maximum current possible for the lifetime requirements.

Thus, there is a need in the art to provide LED-based lighting fixtures that have a greater power output over typical ambient temperature ranges while complying with lifetime specifications for higher ambient temperatures.

SUMMARY

Applicants have recognized and appreciated that it would be beneficial to provide better control over the drive current based on temperature at the junction of LED light sources, such that their lifetime requirements are met, while improving their light output performance over a wide range of junction temperatures. In addition, Applicants have recognized and appreciated that the LED junction temperature advantageously can be determined in the controller for an LED-based lighting fixture, rather than measured directly via a dedicated temperature sensor for the LED. Furthermore, Applicants have recognized that temperature sensing at one or more locations of the LED-based lighting fixture itself can be used to correlate to an ambient temperature, which in-turn can be used to correlate to a junction temperature.

Generally, in one aspect, the present disclosure focuses on an LED-based lighting fixture, employing an LED and a power source configured to provide electrical power to the LED. The lighting fixture includes a temperature sensor configured to measure a temperature at a selected location of the lighting fixture; and a controller connected between the temperature sensor and the power source and configured to determine an ambient temperature and a drive current based on the ambient temperature, and to provide an input signal to the power source based on the drive current.

In accordance with another aspect, a method of controlling the operational lifetime of an LED includes measuring a temperature at a location of an LED-based lighting fixture; calculating a temperature of a junction of the LED based on the measured temperature; and based on the calculating, either adjusting a drive current so that the temperature at the junction remains below a threshold level, or adjusting the drive current to attain a particular luminous output level by the LED, or both.

The present disclosure also focuses on a computer-readable medium storing a program, executable by a controller, for controlling the operational lifetime of an LED. The computer readable medium comprises a measuring code segment for measuring a temperature at a location of an LED-based lighting fixture; a calculating code segment for calculating a temperature of a junction of the LED based on the measured temperature; and an adjusting code segment for adjusting a drive current so that the temperature at the junction remains below a threshold level, or adjusting the drive current to attain a particular luminous output level by the LED, or both.

In accordance with yet another aspect, an apparatus for controlling the operational lifetime of an LED includes a power source configured to provide electrical power to the LED; a temperature sensor configured to determine a temperature at a selected location of the lighting fixture; a controller connected between the temperature sensor and the power source and configured to correlate a measured temperature to a drive current, and to provide an input signal based on the drive current.

As used herein for purposes of the present disclosure, the term “LED” should be understood to include any electroluminescent diode or other type of carrier injection/junction-based system that is capable of generating radiation in response to an electric signal. Thus, the term LED includes, but is not limited to, various semiconductor-based structures that emit light in response to current, light emitting polymers, organic light emitting diodes (OLEDs), electroluminescent strips, and the like. In particular, the term LED refers to light emitting diodes of all types (including semi-conductor and organic light emitting diodes) that may be configured to generate radiation in one or more of the infrared spectrum, ultraviolet spectrum, and various portions of the visible spectrum (generally including radiation wavelengths from approximately 400 nanometers to approximately 700 nanometers). Some examples of LEDs include, but are not limited to, various types of infrared LEDs, ultraviolet LEDs, red LEDs, blue LEDs, green LEDs, yellow LEDs, amber LEDs, orange LEDs, and white LEDs (discussed further below). It also should be appreciated that LEDs may be configured and/or controlled to generate radiation having various bandwidths (e.g., full widths at half maximum, or FWHM) for a given spectrum (e.g., narrow bandwidth, broad bandwidth), and a variety of dominant wavelengths within a given general color categorization.

For example, one implementation of an LED configured to generate essentially white light (e.g., a white LED) may include a number of dies which respectively emit different spectra of electroluminescence that, in combination, mix to form essentially white light. In another implementation, a white light LED may be associated with a phosphor material that converts electroluminescence having a first spectrum to a different second spectrum. In one example of this implementation, electroluminescence having a relatively short wavelength and narrow bandwidth spectrum “pumps” the phosphor material, which in turn radiates longer wavelength radiation having a somewhat broader spectrum.

It should also be understood that the term LED does not limit the physical and/or electrical package type of an LED. For example, as discussed above, an LED may refer to a single light emitting device having multiple dies that are configured to respectively emit different spectra of radiation (e.g., that may or may not be individually controllable). Also, an LED may be associated with a phosphor that is considered as an integral part of the LED (e.g., some types of white LEDs). In general, the term LED may refer to packaged LEDs, non-packaged LEDs, surface mount LEDs, chip-on-board LEDs, T-package mount LEDs, radial package LEDs, power package LEDs, LEDs including some type of encasement and/or optical element (e.g., a diffusing lens), etc.

A given light source may be configured to generate electromagnetic radiation within the visible spectrum, outside the visible spectrum, or a combination of both. Hence, the terms “light” and “radiation” are used interchangeably herein. Additionally, a light source may include as an integral component one or more filters (e.g., color filters), lenses, or other optical components. Also, it should be understood that light sources may be configured for a variety of applications, including, but not limited to, indication, display, and/or illumination. An “illumination source” is a light source that is particularly configured to generate radiation having a sufficient intensity to effectively illuminate an interior or exterior space. In this context, “sufficient intensity” refers to sufficient radiant power in the visible spectrum generated in the space or environment (the unit “lumens” often is employed to represent the total light output from a light source in all directions, in terms of radiant power or “luminous flux”) to provide ambient illumination (i.e., light that may be perceived indirectly and that may be, for example, reflected off of one or more of a variety of intervening surfaces before being perceived in whole or in part).

The term “spectrum” should be understood to refer to any one or more frequencies (or wavelengths) of radiation produced by one or more light sources. Accordingly, the term “spectrum” refers to frequencies (or wavelengths) not only in the visible range, but also frequencies (or wavelengths) in the infrared, ultraviolet, and other areas of the overall electromagnetic spectrum. Also, a given spectrum may have a relatively narrow bandwidth (e.g., a FWHM having essentially few frequency or wavelength components) or a relatively wide bandwidth (several frequency or wavelength components having various relative strengths). It should also be appreciated that a given spectrum may be the result of a mixing of two or more other spectra (e.g., mixing radiation respectively emitted from multiple light sources).

For purposes of this disclosure, the term “color” is used interchangeably with the term “spectrum.” However, the term “color” generally is used to refer primarily to a property of radiation that is perceivable by an observer (although this usage is not intended to limit the scope of this term). Accordingly, the terms “different colors” implicitly refer to multiple spectra having different wavelength components and/or bandwidths. It also should be appreciated that the term “color” may be used in connection with both white and non-white light.

The term “color temperature” generally is used herein in connection with white light, although this usage is not intended to limit the scope of this term. Color temperature essentially refers to a particular color content or shade (e.g., reddish, bluish) of white light. The color temperature of a given radiation sample conventionally is characterized according to the temperature in degrees Kelvin (K) of a black body radiator that radiates essentially the same spectrum as the radiation sample in question. Black body radiator color temperatures generally fall within a range of from approximately 700 degrees K (typically considered the first visible to the human eye) to over 10,000 degrees K; white light generally is perceived at color temperatures above 1500-2000 degrees K.

Lower color temperatures generally indicate white light having a more significant red component or a “warmer feel,” while higher color temperatures generally indicate white light having a more significant blue component or a “cooler feel.” By way of example, fire has a color temperature of approximately 1,800 degrees K, a conventional incandescent bulb has a color temperature of approximately 2848 degrees K, early morning daylight has a color temperature of approximately 3,000 degrees K, and overcast midday skies have a color temperature of approximately 10,000 degrees K. A color image viewed under white light having a color temperature of approximately 3,000 degree K has a relatively reddish tone, whereas the same color image viewed under white light having a color temperature of approximately 10,000 degrees K has a relatively bluish tone.

The term “lighting fixture” is used herein to refer to an implementation or arrangement of one or more lighting units in a particular form factor, assembly, or package. The term “lighting unit” is used herein to refer to an apparatus including one or more light sources of same or different types. A given lighting unit may have any one of a variety of mounting arrangements for the light source(s), enclosure/housing arrangements and shapes, and/or electrical and mechanical connection configurations. Additionally, a given lighting unit optionally may be associated with (e.g., include, be coupled to and/or packaged together with) various other components (e.g., control circuitry) relating to the operation of the light source(s). An “LED-based lighting unit” refers to a lighting unit that includes one or more LED-based light sources as discussed above, alone or in combination with other non LED-based light sources. A “multi-channel” lighting unit refers to an LED-based or non LED-based lighting unit that includes at least two light sources configured to respectively generate different spectrums of radiation, wherein each different source spectrum may be referred to as a “channel” of the multi-channel lighting unit.

The term “addressable” is used herein to refer to a device (e.g., a light source in general, a lighting unit or fixture, a controller or processor associated with one or more light sources or lighting units, other non-lighting related devices, etc.) that is configured to receive information (e.g., data) intended for multiple devices, including itself, and to selectively respond to particular information intended for it. The term “addressable” often is used in connection with a networked environment (or a “network,” discussed further below), in which multiple devices are coupled together via some communications medium or media.

In one network implementation, one or more devices coupled to a network may serve as a controller for one or more other devices coupled to the network (e.g., in a master/slave relationship). In another implementation, a networked environment may include one or more dedicated controllers that are configured to control one or more of the devices coupled to the network. Generally, multiple devices coupled to the network each may have access to data that is present on the communications medium or media; however, a given device may be “addressable” in that it is configured to selectively exchange data with (i.e., receive data from and/or transmit data to) the network, based, for example, on one or more particular identifiers (e.g., “addresses”) assigned to it.

The term “network” as used herein refers to any interconnection of two or more devices (including controllers or processors) that facilitates the transport of information (e.g. for device control, data storage, data exchange, etc.) between any two or more devices and/or among multiple devices coupled to the network. As should be readily appreciated, various implementations of networks suitable for interconnecting multiple devices may include any of a variety of network topologies and employ any of a variety of communication protocols. Additionally, in various networks according to the present disclosure, any one connection between two devices may represent a dedicated connection between the two systems, or alternatively a non-dedicated connection. In addition to carrying information intended for the two devices, such a non-dedicated connection may carry information not necessarily intended for either of the two devices (e.g., an open network connection). Furthermore, it should be readily appreciated that various networks of devices as discussed herein may employ one or more wireless, wire/cable, and/or fiber optic links to facilitate information transport throughout the network.

The term “user interface” as used herein refers to an interface between a human user or operator and one or more devices that enables communication between the user and the device(s). Examples of user interfaces that may be employed in various implementations of the present disclosure include, but are not limited to, switches, potentiometers, buttons, dials, sliders, a mouse, keyboard, keypad, various types of game controllers (e.g., joysticks), track balls, display screens, various types of graphical user interfaces (GUIs), touch screens, microphones and other types of sensors that may receive some form of human-generated stimulus and generate a signal in response thereto.

DETAILED DESCRIPTION

Referring toFIG. 1A, an LED-based light fixture (“fixture”)100is illustrated in perspective view. The fixture100includes a housing101and LEDs102as a unit. As described more fully below, electronic components and devices useful in driving the LEDs102are provided in the housing100. In a representative embodiment, the electronic components may be provided in one or more separate packages (not shown inFIG. 1A) and disposed in the housing101. Moreover, the LEDs102may be provided in a separate package (not shown inFIG. 1A) and disposed in the housing101. The packages that are disposed in the housing101may include one or more substrates each including one or more electrical and electronic devices. As will become clearer as the present description continues, embodiments are described in the context of certain architectures having electronic components and devices that can be integrated and packaged to different degrees. It is emphasized that the architectures described in connection with the representative embodiments are intended to be illustrative and that other architectures are contemplated.

Referring toFIG. 1B, a simplified schematic block diagram of the LED-based lighting fixture100in accordance with a representative embodiment is shown. The lighting fixture100includes a temperature sensor103, which provides an input to a controller104, which includes a memory105. The controller104provides an output to a power source106. The power source106in turn provides electrical power to LEDs102. The temperature sensor103is illustratively a thermistor, or similar device that takes measurements at one or more locations of the lighting fixture100and gathers temperature data during operation of the LEDs102. Illustratively, the temperature sensor103is a thermistor integrated circuit (IC), commercially available from Microchip Technology, Inc., Chandler, Ariz. USA.

In a representative embodiment, the temperature sensor103, the controller104(with memory105), the power source106and the LEDs102are provided over a common substrate (not shown) such as a printed circuit board (e.g., FR4). The common substrate is then provided in the housing101. Alternatively, one or more of these components may be located on different substrates. In a representative embodiment, the power source106may be provided over a separate substrate (e.g., circuit board) and in a first package107due to its heat generating characteristics; and the LEDs102may be provided over a second substrate and in a second package108. The packages107,108may then be provided in the housing101of the fixture100. Still alternatively, the first package107and the second package108may not be provided in a common housing (e.g., housing101), but rather in separate housings (not shown) with required electrical connections therebetween.

Some or all of the temperature sensor103, the controller104, the power source106and the LEDs102of the fixture100may be integrated. In this case, one or more of these components may be provided over the common substrate from which the selected components are integrated. For example, some or all of the temperature sensor103, the controller104, the power source106and the LEDs102may be integrated circuit (IC) in semiconductor (e.g., Si or Group III-V semiconductor). This IC may then be provided over the substrate for the temperature sensor103, the controller104, the power source106and the LEDs102of the fixture100, or may include a selected number of these components. In the latter example, another substrate comprising the remaining components may be provided in addition to the IC. Finally, connections to and between the components of the substrate are effected using one of a variety of known techniques and materials.

In operation, the temperature sensor103takes temperature measurements of the fixture100generally, and particularly at one or more selected points or components of the first package107continuously or at predetermined time intervals. Notably, when the sensor103, the processor104, the power source106and the LEDs102are provided over a common substrate, the sensor103is configured to take temperature measurements at one or more locations on the common substrate, or within the housing101, or both. Alternatively, when the components of the lighting fixture100are provided in first package107and second package108, such as described above, the sensor103is configured to take temperature measurements at one or more locations in the first package107, such as at one or more locations on the substrate(s) provided in the first package107.

As described through illustrative embodiments herein, the temperature measurements taken by the sensor103of the fixture100are correlated to a junction temperature of the particular LEDs in use. Based on these correlations, the drive current to the LEDs102may be altered to optimize the light output at each LED, or to optimize the lifetime of each LED, or both. As will become clearer as the present description continues, when the correlated junction temperature is below a certain temperature, the drive current may be increased to increase luminous output of the LEDs102, without significantly impacting the lifetime of the LED. By contrast, when the correlated junction temperature exceeds a certain temperature, in order to meet standards for LED lifetime, the drive current must be lowered.

The controller104comprises software, hardware or firmware, or a combination thereof, to determine the drive current for the correlated junction temperature based on the ambient temperature. To this end, the controller104may be an FPGA with software cores instantiated therein, a programmable microprocessor (e.g., Harvard architecture microprocessor) with suitable memory105, or an application specific integrated circuit (ASIC) with suitable memory105. The correlation of temperature comprises a first correlation of the temperature measured by the sensor103at one or more locations of the fixture100to the ambient temperature; and a second correlation between the temperature taken by the sensor103and the junction temperature. Based on the determined junction temperature, a drive current is chosen for operation of the LEDs102of the lighting fixture100. The output of the controller104is provided to the power source106, which converts an input signal from the controller into an output drive current for the LEDs104. The drive current is then provided by the power source106.

In accordance with a representative embodiment, the correlation of the temperature measured by the sensor103to the ambient temperature, and the correlation of the temperature measured by the sensor103to the junction temperature of the LEDs may be calculated algorithmically via computer readable code stored on a computer readable medium on the controller104. In accordance with another representative embodiment, the correlations between measured sensor temperature, ambient temperature, junction temperature and drive current may be stored in memory105, which may include a look-up table, instantiated in the controller104.

FIG. 1Cillustrates a simplified schematic block diagram of lighting fixture100in accordance with a representative embodiment. Many of the details of the embodiments described in connection withFIGS. 1A and 1Bare common to the embodiment described presently. Many of these details are not repeated in order to avoid obscuring the presently described embodiment.

The lighting fixture100comprises a microprocessor109and a transition mode power factor controller (PFC)111. In the representative embodiment, the microprocessor109and the PFC111are provided in a third package110. The temperature sensor103is provided in the first package107, and the LEDs102are provided in the second package108. Alternatively, the sensor103, the microprocessor109and the PFC111may be provided in first package107and the LEDs102in the second package108; or the microprocessor109, the PFC111and the LEDs102may be provided in the same package. In any case, the sensor103, the microprocessor109, the PFC111and the LEDs102are disposed in the housing101.

The sensor103measures the temperature at one or more locations of the lighting fixture100as described above. The microprocessor109converts the analog input from the sensor103to a digital value via an analog to digital (A/D) converter, which is used to determine a pulse width modulation (PWM) signal to be provided to the PFC111. To this end, the digital value indicative of the measured temperature is correlated to an ambient temperature, and then correlated to a junction temperature of the particular LEDs in use. Based on these correlations, the PWM signal from the microprocessor109to the PFC111may be altered and the drive current output of the PFC111to the LEDs102thereby altered to optimize the light output at each LED, or to optimize the lifetime of each LED, or both. In a manner similar to the embodiments described above in connection withFIG. 1B, when the correlated junction temperature is below a certain temperature, the PWM signal result in an increased drive current to the LEDs102with insignificant impact on the lifetime of the LED. By contrast, when the correlated junction temperature exceeds a certain temperature, in order to meet standards for LED lifetime, the drive current must be lowered.

The correlation of the temperature measured by the sensor103to the ambient temperature, and the correlation of the temperature measured by the sensor103to the junction temperature of the LEDs102may be calculated algorithmically via computer readable code stored on a computer readable medium on the microprocessor109in accordance with a representative embodiment. In accordance with another representative embodiment, the correlations between measured sensor temperature, ambient temperature, junction temperature and drive current may be stored in memory, which may include a look-up table, instantiated in the microprocessor109.

FIG. 2illustrates a table including data useful in determining the drive current to the LEDs102with consideration of light output and LED lifetime. The table includes the ambient temperature, the temperature measured by the sensor103, the average junction temperature and the estimated light output level in accordance with a representative embodiment. The table also includes the output voltage (Vout) of the temperature sensor, which is proportional to the temperature of the temperature sensor103during operation. As described above, an analog to digital (A/D) conversion translates the analog voltage (Vout) to a digital value as shown in the table. The table further includes an average LED case temperature, an average junction temperature, a steady state power level of the LEDs, and a light output level at the respective steady state power level. As alluded to previously, the temperature at the selected locations on the LED-based lighting fixture100is measured by the sensor103, and from these data the junction temperature is determined based on the thermal resistance of the LED package. Once the junction temperature is determined, the drive current is determined at the controller104or the microprocessor109as described above.

The data in the table ofFIG. 2correlate the LED junction temperature and steady state power of the LEDs102at a particular measured temperature, and also correlate the ambient temperature to the junction temperature. From these correlations, the power (i.e., drive current) provided by the LEDs102is determined to increase the luminous output of the LEDs102, or the lifetime of the LEDs102, or both. As can be readily appreciated, the less power that is provided to the LEDs, the less heat that is dissipated by the LEDs, independent of the ambient temperature. Notably, the correlation is somewhat independent of the measurements of the temperature sensor103. For example, in the embodiment described in connection withFIG. 1B, the power source106, the temperature sensor103and the controller104may be provided on a substrate and in the first package107, and the LEDs102may be provided on another (separate) substrate and in the second package108. As such, the first package107comprising the power source106has a first thermal mass, and the second package108comprising the LEDs102has a second thermal mass separate from that of the first package107. During operation, the temperature of the first package107comprising the temperature sensor103, the controller104and the power source106generally will remain at a consistent ambient temperature, even when the power provided to the LEDs is increased or decreased. Turning to the table ofFIG. 2, if for example, the power to the LEDs is maintained at 27.7 W, throughout the ambient temperature range (in this case 25° C. to 50° C.), the temperature measured by the sensor101will increase as shown in the table. The increase in temperature in the second package108comprising the LEDs102would result in an increase in the junction temperature of the LEDs102and therefore decrease the lifetime of the LEDs102due to the increase in ambient temperature. However, in accordance with representative embodiments, correlations of measured temperature to ambient temperature and to junction temperate are used to reduce the steady state power to the LEDs102as the temperature measured in the first package by the sensor103increases.

Beneficially, the method of altering the steady state power iteratively to maintain the LED junction temperature below a predetermined maximum level is effected independently of the ambient temperature. Thus, the LED lifetime is increased, but the light output is maintained at a relatively high level at normal ambient operating temperature (e.g., 25° C. to 35° C.).

FIG. 3illustrates a flowchart of a method300of controlling light output and lifetime of LEDs in accordance with a representative embodiment. The method is implemented in a lighting fixture such as lighting fixtures100described above in connection withFIGS. 1B and 1C. Notably, the method300comprises calculations that may be carried out via the controller104, or the microprocessor109, and may be instantiated in a computer-readable medium implemented in therein. To this end, the computer readable medium comprises a measuring code segment for measuring a temperature at a location of an LED-based lighting fixture. The computer readable medium comprises a calculating code segment for calculating a temperature of an ambient of the LED based on the measured temperature. The computer readable medium comprises a calculating code segment for calculating a temperature of a junction of the LED based on the measured temperature. The computer readable medium comprises an adjusting code segment for adjusting a drive current so that the temperature at the junction remains below a threshold level, or adjusting the drive current to attain a particular luminous output level by the LED, or both.

As note previously, the controller104and the microprocessor109comprise one or more of software, hardware and firmware configured to determine various settings for the LEDs102depending on current conditions (e.g., ambient temperature), desired output from the LEDs, and lifetime requirements. Many of the details of the calculations and settings are similar or identical to those described above in connection withFIGS. 1A-1Cand2, and are not generally repeated in order to avoid obscuring the description of the presently described embodiments.

At301, the method comprises measuring a temperature at a location of an LED-based lighting fixture. For example, according to an embodiment, the temperature sensor103measures the temperature of the ambient of the fixture100. Notably, the temperature sensor103may be in the first package107in an embodiment where the LEDs102are in the second package108. Alternatively, as described above, the temperature sensor103and all other components may be provided in the same package.

At302, the method comprises calculating a temperature of a junction of the LED based on the measured temperature. The calculation of the temperature of the junction may comprise an algorithmic calculation in the controller104or the microprocessor109. Alternatively, a look-up table or similar memory device in the controller104or the microprocessor109may comprise data compiled through multiple measurements that are statistically averaged. Still alternatively, the look-up table may be compiled by modeling the junction temperature incorporating various factors, such as the heat generation characteristics of the particular LEDs, heat dissipation capabilities of the first package107and the second package108, and the components thereof.

At303the method comprises adjusting a drive current so that the temperature at the junction remains below a threshold level, or adjusting the drive current to attain a particular luminous output level by the LED, or both. The adjustment of the drive current to the LEDs102is effected by providing a digital value corresponding to the voltage (Vout) of the temperature sensor103. The digital value is used at the controller104or the microprocessor109to correlate the temperature at the temperature sensor103to a junction temperature of the LEDs104via a computation or a look-up table, for example, and as described above. The correlated junction temperature of the LEDs is used to determine the drive current for the desired steady-state power level. For example, with reference toFIG. 2, the output from the controller104comprises a digital value that corresponds to a particular junction temperature and the required drive current for the desired steady state power level. By way of illustration, at am ambient temperature of 25° C. and a sensor temperature of 46.4° C., digital output of 263 is provided by an A/D converter to the controller104. The controller104correlates this digital value to a junction temperature and drive current for this junction temperature. In this example, the junction temperature determined at the controller104is approximately 73.5° C. A command is provided to the power source106to provide this drive current to the LEDs104. In this example, the drive current results in a power output of 27.7 W and 1050 L. In the present example, a maximum junction temperature of 90° C. is set for the LEDs104to ensure a lifetime within specifications or standards. Continuing with this example, if the correlated ambient temperature increases to 40° C., the digital value based on the voltage output from the temperature sensor101is changed to 327. This correlates to a junction temperature of 88.1° C., and the drive current is reduced to provide a steady-state power level of 26.5 W and 1002 L. As can be appreciated, the increased ambient temperature exacts a reduced steady state power level, and allows the LEDs104to function within lifetime specifications. Generally, therefore, the method300allows for a comparatively higher steady-state output for lower ambient temperatures and a comparatively lower steady-state output for higher ambient temperatures. Adjustment of the drive current can be made to provide a desired lifetime and desired light output.

FIG. 4illustrates a graph of temperature versus drive current in accordance with a representative embodiment. Notably, Tarefers to the ambient temperature, such as determined by the temperature sensor101; and Tjrefers to the junction temperature determined by the controller102as described above. At401, the ambient temperature is comparatively low, and the corresponding junction temperature at402is also comparatively low. At403, the ambient temperature is appreciably higher. The corresponding junction temperature is shown at403. These data are used by the controller102to determine the drive current for the desired light output, or desired LED lifetime, or both, and as described above.

Any reference numerals or other characters, appearing between parentheses in the claims, are provided merely for convenience and are not intended to limit the claims in any way.