In solid state lighting devices, including a plurality of LEDs of different colors, control of both intensity and color is commonly achieved using pulse width modulation (PWM). Such PWM control is well-known, and indeed, commercial PWM controllers have long been available specifically for driving LEDs. See, e.g., Motorola Semiconductor Technical Data Sheet for MC68HC05D9 8-bit microcomputer with PWM outputs and LED drive (Motorola Ltd., 1990). In PWM, a train of pulses is applied at a fixed frequency, and the pulse width (that is, the time duration of the pulse) is modulated to control the time-integrated power applied to the light emitting diode. Accordingly, the time-integrated applied power is directly proportional to the pulse width, which can range between 0% duty cycle (no power applied) to 100% duty cycle (power applied during the entire period).
Known PWM illumination control has certain disadvantages. In particular, known systems and methods introduce a highly non-uniform load on the power supply. For example, if the illumination source includes red, green, and blue illumination channels and driving all three channels simultaneously consumes 100% power, then at any given time the power output may be 0%, 33%, 66%, or 100%, and the power output may cycle between two, three, or all four of these levels during each pulse width modulation period. Such power cycling is stressful for the power supply, and dictates using a power supply with switching speeds fast enough to accommodate the rapid power cycling. Additionally, the power supply must be large enough to supply the full 100% power, even though that amount of power is consumed only part of the time.
Power variations during PWM may be avoided by diverting current of each “off” channel through a “dummy load” resistor. However, the diverted current does not contribute to light output and hence introduces substantial power inefficiency.
Known PWM control systems are also problematic as relating to feedback control. To provide feedback control of a color-adjustable illumination source employing known PWM techniques, the power level of each of the red, green, and blue channels must be independently measured. This typically dictates the use of three different light sensors each having a narrow spectral receive window centered at the respective red, green, and blue wavelengths. If further division of the spectrum is desired, the problem becomes very expensive to solve. If, for instance, a five channel system has two colors that are very close to one another, only a very narrow band detector is able to detect variations between the two sources.
In order to overcome these problems, one known illumination system utilizes a multi-channel light source having different channels that generate illumination of different colors corresponding to the different channels. The system includes a power supply that selectively energizes the channels by utilizing time division multiplexing (TDM) to generate illumination of a selected time-averaged color. However, this system was designed to cover a large color space. In order to achieve this large color space, the system uses TDM to selectively vary the “on” time of one individual LED color at a time for a specified duration. Therefore, because only one color of LED is used at a time, a large number of LEDs are required to produce some colors, particularly white light. Further, while this approach can provide any color within the full range of available LED chips, it has a low utilization of LEDs. This large quantity of LEDs provides a large Gamut, but does not make efficient use of LEDs.
Therefore, there remains a need for an illumination system that economically and effectively produces white light by concurrently utilizing a majority of the LED chips in the system. There also remains a need for an illumination system that quickly and efficiently stabilizes the color-shifting or degradation that gradually occurs in LEDs.