Abstract:
A dimmable, light-emitting diode (LED) power supply adapted to provide a direct current (DC), constant current (“constant current source”) from a conventional, phase-controlled 120 VAC, 60 Hz power source is disclosed. The constant current source of the present invention utilizes two processes to control dimming. In a first process, the phase angle of the input voltage is used to control the duty cycle of a line frequency pulse width modulation (PWM). In a second process, a proportional-current limit adjustment is used to control the average current to the LED during the ON time of the line frequency by PWM. As a result, at relatively low phase angles, peak currents can be lowered, reducing flicker and improving the audible noise levels generated by the circuit.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    Not Applicable 
       STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
       [0002]    Not Applicable 
       BACKGROUND OF THE INVENTION 
       [0003]    Light-dimming is effected by adjusting the magnitude and/or the time of application of current to a light source(s). For example, control and dimming of a light source(s) using pulse width modulation (PWM) is well-known to those of ordinary skill in the relevant art. PWM-dimming works by varying the amount or length of time a switching device, such as a TRIAC, is in an ON state during a cycle relative to the total cycle time. This is referred to as the “duty cycle”. Typically, PWM-dimming supplies power (current) to the light source when an input voltage is above a threshold voltage (ON), and withholds power when the input voltage is below the threshold voltage (OFF). The longer the duty cycle, the relatively-brighter the light source(s), and the shorter the duty cycle, the relatively dimmer the light source(s). 
         [0004]    Conventional, solid-state, PWM-dimming switches typically include a switching device, e.g., a TRIAC, and a firing capacitor. The charged time of the capacitor is delayed by the setting of the dimming device. Once the firing capacitor has reached a predetermined voltage, the capacitor discharges or “fires”, triggering the TRIAC. The TRIAC will continue to conduct electricity to the light source(s) until there is no current, which happen twice per cycle at each of the zero crossings of the AC current sine wave. Accordingly, by controlling the rate at which the capacitor is charged, one can control the firing time of the capacitor. 
         [0005]    As an alternative to PWM-dimming directly from the TRIAC, relatively-large input and/or output storage capacitative devices can be used to capture and store charge (energy). Dimming can then be achieved by PWM-dimming at a higher frequency, to prevent flicker. Relatively large input and/or output storage capacitative devices, however, are expensive and consume space, hence, their use is undesirable in many applications. 
         [0006]    However, as high-power, light-emitting diodes (LEDs) and LED clusters are used more and more in housing, commercial, and industrial applications to replace conventional light sources, e.g., incandescent light bulbs, the need to retrofit conventional, in situ light-dimming switches economically is desirable. However, conventional dimming switches cannot dim LEDs or LED clusters effectively, especially at low or relatively-low current levels, using just the phase angle of the input voltage. 
         [0007]    Another problem with prior art PWM-dimming techniques includes flicker resulting from TRIAC jitter and noise. Analog-driven TRIACs experience jitter at their turn-on phase angle. The effect of the jitter is a function of current magnitude and the duty cycle. 
         [0008]    U.S. Pat. No. 7,038,399 to Lys, et al. discloses an apparatus and method for powering solid-state, LED-based light sources using non-standard line voltage signals generated by a conventional AC dimming circuits. According to Lys, et al., a controller can be used to condition the AC signals generated by the AC dimming circuit, to drive the LED light sources. 
         [0009]    Accordingly, it would be desirable to provide a power control system that adapts standard, phase-angle, wall-dimming switches for use with high-power LEDs and LED clusters similar to what has been disclosed by Lys, et al. but by using a sensed or average input voltage. Moreover, it would be desirable to provide a power control system that reduces flicker resulting from jitter using proportional current PWM-dimming. 
       BRIEF SUMMARY OF THE INVENTION 
       [0010]    A dimmable, light-emitting diode power supply adapted to provide an adjustable, direct current (DC), constant current (“constant current source”) from a conventional, phase-controlled 120 VAC, 60 Hz power source is disclosed. The constant current source of the present invention utilizes two processes to control dimming. In a first process, the phase angle of the input voltage is used to control pulse width modulation (PWM). More specifically, whenever the input voltage exceeds a pre-determined threshold voltage, a PWM-dimming signal provides power to the output. However, whenever the input voltage drops below the predetermined threshold voltage, the PWM signal is turned off, to avoid flicker. 
         [0011]    In a second process, a proportional current limit adjustment is used to control the duty cycle. As a result, at relatively low phase angles, peak and average currents can be lowered, improving the audible noise levels generated by the circuit. 
         [0012]    Advantageously, 120 Hz audible noise generated by the TRIAC-dimming circuit is reduced; dimming performance at low-light power levels can be enhanced without flicker; and non-linear dimming techniques can be implemented that, to the human eye, appears to be more “linear”. 
     
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         [0013]    The invention will be more fully understood by reference to the following Detailed Description of the invention in conjunction with the Drawing, of which: 
           [0014]      FIG. 1  shows an illustrative schematic diagram of a light-emitting diode dimming system in accordance with the present invention; 
           [0015]      FIG. 2  shows comparative duty cycles for traditional PWM-dimming (top) and proportional-current PWM-dimming (bottom); 
           [0016]      FIG. 3  shows the relationship between input voltage and various phase delay stages; 
           [0017]      FIG. 4  shows light-source output current at various phase delay stages; and 
           [0018]      FIG. 5  shows an illustrative functional block diagram of a main switch driving circuit in accordance with the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0019]    An illustrative schematic diagram for a phase-control, PWM-based dimming circuit  10  is shown as  FIG. 1 . The dimming circuit  10  provides power to a light source(s)  80  and includes a phase-angle detection circuit  20 , a controller power supply  30 , a controller  40 , a power stage  50 , a dv/dt filter  60 , an optional output filter  70 , and an optional slope compensation circuit  90 . 
         [0020]    Although the invention will be described in connection with a TRIAC, those of ordinary skill in the art can appreciate that proportional-current PMW-dimming control can also be accomplished using an integrated gate bi-polar transistor (IGBT). 
       Phase-Angle Detection Circuit 
       [0021]    The phase-angle detection circuit  20  is structured and arranged to sense input voltage (V IN ) from a conventional, phase-controlled 120 VAC, 60 Hz power source  15 , such as a standard AC outlet, and to provide a filtered, DC pulse signal to the controller  40  that is based on the sensed input voltage and pseudo-representative of the input voltage&#39;s RMS value (V RMS ). 
         [0022]    The phase-angle detection circuit  20  includes a voltage divider  22 , a capacitive element  24 , which can be part of an RC filtering circuit  28 , and a comparator  25 . As shown in  FIG. 1 , the output of the comparator  25  of the phase-angle detection circuit  20  is electrically coupled to one of the input pins of the controller  40 , e.g., PIN  2 . 
         [0023]    The voltage divider  22  includes resistive elements (R 39  and R 40 ), which are electrically coupled in series between a voltage source having a predetermined, fixed voltage, e.g., 3.3V, and ground. The voltage divider  22  is used to set the predetermined threshold voltage at one of the inputs of the comparator  25 , where it is compared to the input voltage from the source  15 . 
         [0024]    When the input voltage from the source  15  exceeds the threshold voltage (V IN &gt;V TH ), the comparator  25  outputs a voltage HI signal pulse. Otherwise, when the input voltage is less than the threshold voltage (V IN &lt;V TH ), the comparator  25  outputs a voltage LO signal pulse or no signal pulse. 
         [0025]    The voltage HI signal pulse charges the capacitive element (C 34 )  24 . The size and capacity of the capacitive element  24  is structured and arranged so that as the capacitive element  24  charges, the signal to the controller  40  becomes pseudo-representative of the input voltage&#39;s RMS voltage (V RMS ). 
         [0026]    The phase-angle detection circuit  20  shown in  FIG. 1  includes an RC filter  28 , which filters noise that, inter alia, causes flicker at low current light levels. The RC filter  28 , however, is optional. When the RC filter  28  is not included with the dimming circuit  10 , the comparator  25  outputs an unfiltered, DC pulse signal directly into the controller  40 . This option, however, requires greater processing power. 
       Controller Power Supply 
       [0027]    The dimming circuit  10  includes a relatively-low voltage power supply  30  for powering the controller  40  and, more particularly, the main switch driving circuit  35 . The controller power supply  30  includes a start-up linear regulator  32 , an over-voltage clamp circuit  34 , and a “flying capacitor” charge pump  36 . The start-up linear regulator  32  and the over-voltage clamp circuit  34  are each electrically coupled to at least one of the pins, e.g., pins  13  and  14 , of a main switch driving circuit  35 . The flying capacitor charge pump  36  is electrically coupled to the output of the linear regulator  32  as well as to the power stage  50 . 
         [0028]    Because the main switch driving circuit  35  and the main switch  42  are frequently in an OFF state, the start-up linear regulator  32  is adapted to deliver a predetermined voltage to the main switch driving circuit  35  until switching of the main switch  42  has been initiated. Once switching has been initiated, the flying capacitor (C 15 )  38  can supply additional current to the main switch driving circuit  35  by returning some of the power that flows from the power stage  50  when the main switch  42  is ON. 
         [0029]    The start-up linear regulator  32  is adapted to maintain the line voltage (V DD ) at a desired voltage, e.g., approximately 10.3V. More particularly, when power is first applied to the power stage  50 , current from a DC voltage source is made to flow through a switching device (Q 2 )  31  until the voltage sensed at a capacitive element (C 2 )  33  is approximately 10.3V. However, once the sensed voltage at capacitive element (C 2 )  33  exceeds a predetermined maximum voltage, e.g., approximately 10.5V, the switching device (Q 2 )  33  of the linear regulator  32  will automatically turn OFF for increased efficiency. 
         [0030]    The flying capacitor charge pump  36  includes a flying capacitor (C 15 )  38  that is adapted to bleed off some of the power (current) from the power stage  50  when the main switch  42  is in an ON state and to provide this voltage to power the main switch driving circuit  35 . The over-voltage clamp circuit  34  is provided to dissipate energy whenever the flying capacitor charge pump  36  generates and delivers more current than the main switch driving circuit  35  demands. 
       Controller 
       [0031]    The controlling circuitry for the dimming circuit  10  includes a controller  40 , a main switch (Q 3 )  42 , and a main switch driving circuit  35 . For illustrative purposes only, the main switch  42  in  FIG. 1  is shown as a field effect transistor (FET). Those of ordinary skill in the art can appreciate that other solid-state switching devices can also be used. As shown in  FIG. 1 , the drain of the main switch is electrically coupled to the power stage  50 ; the source of the main switch is electrically coupled to one of the pins of the main switch driving circuit  35 , e.g., the current sense (CS) signal PIN; and the gate of the main switch  42  is electrically coupled to at least one of the output pins of the main switch driving circuit  35 , e.g., pins  11  and  12 . 
         [0032]    The controller  40  is a programmable microprocessor, such as a low-power MSP430 manufactured by Texas Instruments of Dallas, Tex., that is capable of receiving and processing signal and other data and of executing a plurality of software and/or hardware applications, driver programs, algorithms, and the like, to control the peak current and duty cycle to the light source(s)  80 . 
         [0033]    As previously mentioned, the controller  40  is electrically coupled to the phase-angle detection circuit  20 , e.g., PIN  2 , and to the main switch driving circuit  35 , e.g., PINS  4  and  6 . The controller  40  is adapted to convert pulse signals from the comparator  25  of the phase-angle detection circuit  20  into control signals, for the purposes of controlling, i.e., setting, the peak current (I_LIM)  43  to the power stage  50  and for proportionally-adjusting a pulse width modulation (PWM) duty cycle associated with a 1-2 kHz PWM signal, which is based on the limiting, peak current (I_LIM). The controller  40  is further adapted to provide a PWM clock signal to the main switch driving circuit  35 . 
         [0034]    In short, the controller  40 , via the software and/or hardware applications, driver programs, algorithms, and the like, is adapted, inter alia, to establish the switching frequency of the main switch driving circuit  35  using a first PWM signal having a frequency of approximately 125 kHz; to adjust the duty cycle a second PWM signal having a frequency of about 1-2 kHz based on signal data from the phase-angle detection circuit  20 ; and to proportionally-adjust the peak level of the current to the light source(s) vis-à-vis the duty cycle at low or relatively-low current levels. 
         [0035]    The main switch driving circuit  35  is electrically coupled to the controller  40  and is further adapted to receive the limiting, peak current (I_LIM) signals  43  and the PWM clock signals  44  generated by the controller  40 . The main switch driving circuit  35  is also electrically coupled to the flying capacitor charge pump  36  and is further adapted to receive the CS signal therefrom. 
         [0036]    Among the plurality of software and/or hardware applications, driver programs, algorithms, and the like is an algorithm that compares CS signals to the limiting, peak current (I_LIM) signals  43 . Based on that comparison, the main switch driving device  35  is structured and arranged to control the gate of the main switch  42 . By controlling the timing of opening and closing of the gate of the main switch  42 , the main switch driving circuit  35  proportionally-adjusts the duty cycle of the first PWM signal  44  for a particular limiting, peak current (I_LIM). 
       Power Stage 
       [0037]    The power stage  50  is electrically coupled to a DC voltage source, to the light source(s)  80 , and to the main switch  42 . For illustrative purposes only, the power stage  50  shown in  FIG. 1  is a buck regulator power stage. 
         [0038]    When the main switch  42  is in an ON state, the power stage  50  delivers current from the light source(s)  80  through an inductive element  52  and the main switch  42  to a current-sensing device  37 , which, for illustrative purposes only is shown in  FIG. 1  as two resistive elements (R 3  and R 20 ). Current passing through the resistive elements (R 3  and R 20 ) generates voltage, which, after filtering by an RC filter  39 , e.g., resistive element R 38  and capacitive element C 29 , is applied to the CS signal PIN of the main switch driving circuit  35  where the CS signal is compared to the voltage associated with the limiting, peak current (I_LIM) and the RC filter  47 . 
         [0039]    As will be described in greater detail below, once the voltage at the CS signal PIN exceeds the voltage associated with the limiting, peak current (I_LIM), the main switch driving circuit  35  outputs a gate drive signal to open (turn OFF) the main switch  42 , preventing further current flow therethrough until the next leading- or rising-edge clock cycle. Thus, current flowing in the light source(s)  80  is used to control the opening and closing of the main switch  42  and, resultingly, PWM-dimming by proportionally-limiting the peak current is achieved, e.g., at low or relatively-low current levels. More particularly, the peak current is proportionally-scaled to the pseudo-representative RMS voltage of the AC input voltage. 
       Dv/Dt Filter 
       [0040]    The dv/dt filter  60  is electrically coupled between the rectified AC current delivered by the utility grid  15  and the DC power source to the power stage  50 . The dv/dt filter  60  is used to protect the input capacitor from the large change in input voltage generated from a TRIAC-driven phase controlled circuit. This circuit limits the inrush current when the TRIAC turns on and prevents the line current from ringing back to zero or near zero. If this circuit were not present, the line current could ring back to zero and cause the TRIAC to turn off prematurely. The dv/dt filter  60  includes a dampening resistive element  62  and a relatively-low equivalent series resistance (ESR) inductive element  64 . As shown in  FIG. 1 , the resistive element  62  and the inductive element  64  are arranged in parallel. 
       Output Filter 
       [0041]    In order to provide an expanded dimming range, delivery of low or relatively-low current levels to the light source(s)  80  is often desirable. Hence, if the dimming circuit  10  is designed to operate at an expanded dimming range and/or to deliver low or relatively-low light current levels, an optional output filter  70  can be added to the dimming circuit  10  at the output of and parallel to the power stage  50 . Advantageously, with an output filter  70 , the LED light source(s)  80  can be dimmed lower while the TRIAC remains turned ON. 
         [0042]    The output filter  70  can be a RC circuit comprising at least one capacitive element  72  and at least one resistive element  74  and  76 . The output filter  70  enhances the behavior of the light source(s)  80  by dissipating energy through the resistive elements  74  and  76  and also by filtering out very low current output pulses via the capacitive element  72 . The capacitive element  72  also reduces ripple in the output current. 
       Light Sources 
       [0043]    As is well-known to those of ordinary skill in the art, high-power LED light sources  80 , e.g., 1 to 2 W LEDs, can produce significant levels of light even when current levels are on the order of approximately 10 mA. Conventional systems at this low current level, however, may cause the TRIAC to turn OFF (or not to turn ON) and/or can result in flicker that is caused by TRIAC jitter. 
       Slope Compensation Device 
       [0044]    To prevent or minimize sub-harmonic oscillation, an optional slope compensation device  90  can be included with the dimming circuit  10 . However, when the maximum duty cycle is pre-set at 50 percent, e.g., by an application and/or by being hardwired in the controller  40 , addition of a slope compensation device is redundant and not needed. 
         [0045]    The slope compensation device  90  is electrically coupled to the controller  40  and to the CS PIN of the main switch driving circuit  35  and is adapted to add a ramp signal on the CS pin to minimize or prevent sub-harmonic oscillation. The slope compensation device includes a switching device (Q 4 )  95  and an RC filter  92  (resistive element R 27  and capacitive element C 22 ). 
         [0046]    The base of the amplifier device (Q 4 )  95  is electrically coupled to capacitive element C 22 , so as to be an impedance-matching amplifier. The 125 KHz PWM signal output  44  of the controller  40  generates a ramp wave at  92  when filtered by resistive element R 27  and capacitive element C 22 . This ramp wave is buffered by the amplifier device (Q 4 )  95  then superimposed on CS by capacitive element C 36  and resistive element R 14 . 
       Method of PWM-Dimming 
       [0047]    Having described the components of the dimming circuit, a method of proportional-current PWM-dimming using the same will now be described. For illustrative purposes only, the controller will assumedly operate at 1 MHz and the main clock of the controller will be divided in eighths to provide a PWM main clock signal of approximately 125 kHz to the main switch driving circuit, which, correspondingly, sets the switching frequency of the main switch at 125 kHz. 
         [0048]    Comparative duty cycle pulses for conventional PWM-dimming (top) and for proportional-current PWM-dimming (bottom) are shown in  FIG. 2 . The areas under the pulses in both the top and bottom waveforms represents the average output current to the lights source(s). It should be noted that, in both the top and bottom waveforms, the areas under each pulse are the same or substantially the same. Hence, the intensity of the light sources associated with each waveform should appear to the human eye as the same or substantially the same. 
         [0049]    Although the areas under the pulses are the same or substantially the same, those of ordinary skill in the art, however, can appreciate that TRIAC jitter and the resulting flicker associated with the leading or rising and trailing or falling edges of the (top) traditional PWM-dimming duty cycle is significantly greater than the TRIAC jitter associated with the leading or rising and the trailing or falling edges of the (bottom) proportional current PWM-dimming duty cycle. The intent of the present invention, then, is to control the amplitude of the current proportional, i.e., by establishing a limiting, peak current (I_LIM), and the duration of the duty cycle, to reduce jitter and the resulting flicker. 
         [0050]      FIG. 3  and  FIG. 4  further illustrate the benefits of proportional-current PWM-dimming.  FIG. 3  shows the TRIAC dimmer voltage output for various phase-delay states and  FIG. 4  shows the related LED current waveform. Advantageously, as shown in  FIG. 4 , there is no or substantially no current flowing to the light source(s) when the phase-delay is greater than approximately 5.9 msec. Hence, there is no flicker. 
         [0051]    When the rheostat or variable resistor of the dimming device is adjusted to dim the light source(s), the flow of current to the firing capacitor is reduced and, correspondingly, so is the duty cycle of the TRIAC. Although the following description will involve a dimming action, those of ordinary skill in the art can appreciate the application of the same when the desired action is light brightening rather than light dimming. 
         [0052]    In a first step, phase-controlled, 120 VAC, 60 Hz power is provided to the phase-angle detection circuit and to the dv/dt filter. The comparator of the phase-angle detection circuit compares the input voltage from the phase-controlled, 120 VAC, 60 Hz power source and outputs a digital DC pulse signal to the controller whenever the input voltage exceeds a threshold voltage, e.g., 100 VDC. 
         [0053]    In a next step, using the voltage data from the comparator, the controller calculates and outputs a limiting, peak current value (I_LIM) to the main switch driving circuit in the form of a second PWM signal. The frequency of the second PWM signal is between 1 and 2 kHz as compared to a frequency of 125 kHz for the main clock PWM signal. Similar to what is shown in  FIG. 2 , the application, driver program, algorithm, and the like used by the controller calculates a limiting, peak current (I_LIM) having a lower amplitude and a longer duration than is traditionally output when conventional light sources are being dimmed. The scaled current value will be proportional to the pseudo-representative RMS value of the AC input voltage. 
         [0054]    In a next step, the main switch driving circuit compares the limiting, peak current (I_LIM) to a current sense (CS) signal. More specifically, as shown in  FIG. 5 , the voltage associated with the CS signal and the voltage associated the limiting, peak current (I_LIM) are compared. 
         [0055]    Depending on the relationship between the voltages associated with limiting, peak current (I_LIM) and with the CS signal, the main switch driving circuit will cause the gate of the main switch to be closed (turned ON) or open (turned OFF). As long as the voltage associated with the limiting, peak current (I_LIM) is greater than the voltage associated with the CS signal, the main switch remains ON. However, when the voltage associated with the CS signal exceeds the voltage associated with the limiting, peak current (I_LIM), the main switch is turned OFF until the next leading or rising edge of the main clock cycle from the first PWM signal. This keeps the current to the light sources at or below the limiting, peak current (I_LIM) at all times. 
         [0056]    Referring to  FIG. 5 , the illustrative logic circuitry enabling a main switch driving circuit to accomplish proportional-current dimming is shown. Voltage associated with the CS signal on the CS pin (pin  8 ) is compared to the voltage output by the RC filter associated with the limiting, peak current (I_LIM) (pin  6 ). The comparator  82  output SETS (S) an R/S flip flop  84  that has been RESET (R) from the rising edge of the main clock (IN at PIN  2 ). For example, as long as the voltage associated with the CS signal is less than the voltage associated with the limiting, peak current (I_LIM), the comparator will keep the flip-flop  84  in RESET and the gate of the main switch will remain closed and turned ON. However, when the voltage associated with the CS signal equals or exceeds the voltage associated with the limiting, peak current (I_LIM), the comparator  82  will output a signal to SET flip-flop  84 , which sends a voltage LO signal to the logic AND-gate  86 , causing the operational amplifier  85  to open the gate (turned OFF) of the main switch. The main switch will remain turned OFF until the next leading or rising edge of the PWM main clock switching cycle. 
         [0057]    The impact of the state of the main switch affects the power stage and current to the light source(s) as follows. As power (current) is delivered to the light source(s), the input current is also fed to the drain of the main switch via an inductive element (L 1 )  52  ( FIG. 1 ). When the gate of the main switch is open (turned OFF), the current is prevented from flowing through the main switch and, instead, flows through a diode (D 8 ). As a result, the power (current) to the light source(s) ramps down, dimming the light source(s). When, on the other hand, the gate of the main switch is closed (turned ON), current flows through the main switch and, accordingly, power (current) to the light source(s) ramps up until the voltage associated with the CS signal again equals or exceeds the voltage associated with the limiting, peak current (I_LIM). 
         [0058]    The current that flows through the main switch is, in turn, measured and filtered, to provide the CS signal. More particularly, a current-sensing means  37 , e.g., a pair of resistive elements (R 3  and R 20 ), samples the current, producing an associated voltage, which, as previously mentioned, is applied to the CS pin and compared to the voltage associated with the limiting, peak current (I_LIM). As a result, the sensed current—along with any ripple current in the inductive element (L 1 )  52 —is used to set the RMS current to the light source(s)  80 . In this process, DC constant current from a phase-controlled, AC power source can be controlled to high-power LEDs or LED clusters can be controlled to minimize jitter and flicker. 
         [0059]    Optionally, one of the software and/or hardware applications, driver programs, algorithms, and the like can be structured and arranged to receive and process the pulse signal(s) from the comparator; to discard anomalies or “outriders” that exceed predetermined voltage limits; and to average the pulse signal. Alternatively or additionally, when the phase-angle detection circuit detects a large, instantaneous change in input voltage, but which still falls within tolerable voltage limits, another of the software and/or hardware applications, driver programs, algorithms, and the like can be structured and arranged to provide a non-immediate, gradual dimming. Optionally, the controller can also be adapted to filter the pulse signals from the comparator to gradually dim the light source(s), e.g., during power UP or power DOWN sequencing. 
         [0060]    In another software- and/or hardware-embodied application, driver program, algorithm, and the like, the controller can provide over-temperature protection. For example, the controller can monitor a plurality of temperature sensing devices  48  and  49 , e.g., negative temperature coefficient thermistors. A first temperature sensing device  48  can be disposed proximate the main switch and a second temperature sensing device  49  can be disposed proximate the heat removal device(s) for the light source(s). 
         [0061]    During operation, when the sensed first and/or second temperatures exceed predetermined threshold temperatures for the main switch and/or heat removal device, the controller is adapted to throttle back or lower the limiting, peak current (I_LIM). For example, current reduction can be performed using a linear relationship that decreases from full current to zero current over a 20° C. range. Furthermore, if the light source(s) is/are disposed in a cooling-restricted area, the controller can reduce the limiting, peak current until the temperature stabilizes somewhere along the linear reduction curve. 
         [0062]    It will be apparent to those skilled in the art that modifications to and variations of the disclosed method and system are possible without departing from the inventive concepts disclosed herein, and therefore the invention should not be viewed as limited except to the full scope and spirit of the appended claims.