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Posted on 29 June 2019

Spectral Tuning for White LED based Luminaries

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A smart LED driver can be adjusted for brightness degradation over lifetime

If you ask a consumer to characterize the perfect light, you might hear in their description a light requiring the lowest amount of energy, the light output and color is adjustable, it will last a very long time, etc. to list but a few of the many desirable characteristics.

By Brian Johnson, Lighting Specialist, Fairchild

 

Lowest input energy is the efficient conversion of input power to lumen output known as efficacy, adjusting light output relates to dimming to soften the light’s brightness, or better yet add in adjustment of the light fixture color in order to simulate day versus night conditions and can maintain the light output for a long useful lifetime by tuning the bias current through the LEDs. Incandescent lamps suffer from low efficacy and low usage life times, Sodium lamps offer little color options and low usage life times, and fluorescent lamps have fewer dimming options in retro-fit applications and low usage life times. High brightness LEDs however, promise good efficacy, excellent life time, color choices with easy dimming control and no UV rays. These promises are realized and dependent on the smart design of the control gear or LED driver electronics. A smart LED driver can adjust for brightness degradation over lifetime, can provide the drive characteristics to adjust for color; and replace the need for LED binning to a desired color and brightness by using spectral tuning on a set of different LED’s in a system to dial in to a desired color  and brightness characteristic.

Spectral Tuning versus Binning

Spectral tuning is combining the spectral power distributions of several LEDs, for example mixing red, green, and blue LEDs properly resulting in white light. This RGB combination is also used to create almost any color of light. If the LED driver is not designed to accommodate a set of different LEDs, then the designer has to choose from binned LEDs in order to create a specific color. Binning is the process manufacturers use to group LEDs based on luminous flux and color. As an example, Figure 1 shows the “bins” for an industry standard set of LEDs.

LED Bins on the Chromaticity Diagram

The bins are showed by the rectangular regions plotted on the Chromaticity Diagram. A set of light fixtures that contain LEDs from a specific bin will be close in color and brightness characteristics. However, in a large office or factory environment containing many light fixtures, binning may still result in non-uniform light color that will be noticeable across a large set of light fixtures. A binned LED design will not provide a way to vary the color of the light fixture. A set of different LED colors using feedback to tune the spectral characteristics of the different LED’s in a system can create a compensated lighting system in the office environment that will create a uniform effect across the room. Spectral tuning can also compensate for other effects, i.e. natural light from the side of the room with windows to the outside or hallway lighting reflecting into a room.

Another effect of LEDs is the shift in color from a change in the LED forward current. Figure 2 shows color change versus forward current for an industry set of LEDs.

LED Lamp Units by Application

The LED driver can be designed with a tight constant current (CC) output tolerance, however tightening the CC tolerance will increase in cost of the LED driver. A lower cost solution is when the designer uses a feedback system to adjust for LED color shift where the feedback compensates the color variance due to forward current across a set of LEDs.

Binnig LEDs usually has manufacturing implications that result in increased cost of the LEDs purchased. Just because a binned set of LEDs is specified, a number of LED drivers may still not match the binned application bias forward current set point across a large number of fixtures. There are also temperature effects and lifetime degradation effects that can cause variation in fixture color.

Feedback using Spectral Tuning

Feedback or the use of a control scheme that can counter the variation effects of the system will be described to create a fixture that can automatically adjust color and brightness. Color sensors and a microcontroller are used to process the sensor inputs. An example color sensor uses photo diodes with a non-organic three way color filter, offer exceptional stability and very low drift over temperature and ageing, and the filters are designed to implement the spectral sensitivity curve of the human eye (CIE1931).

The schematic of the closed loop spectral tuning luminaire is shown in Figure 3.

Spectral Tuning Luminaire

The control loop is shown implemented with a micro controller. The control loop measures both brightness and color through the sensor and uses the PWM signal to adjust the currents in the LED strings. The FAN7346 has the ability to control the current in the individual LED strings with a PWM input signal. The power supply can be a power factor correction front stage followed by a LLC dc-dc second stage to provide power to multiple LED strings, Figure 4. The power supply could also be an off-the-shelf design with the FAN7346 controlling the feedback to the power supply. Alternative designs can consist of three power converters (30W/10W/10W) independently controlling three sets of LED strings using white, green, and amber colors creating a white based tuning system or use three strings with identical power supplies to “mix” three strings with red, green, and blue LEDs for a wider color tuning range. The need to bin the LED colors is not required; pick low cost LEDs that have the performance needed for the light application.

Spectral Tuning Luminaire Power Supply

Example System

An example system was created to tune for white lighting in an office environment. Three flyback PFC power supplies were operated in parallel with the main power supply operating with output power up to 30W driving the main string of white LEDs along with two additional power supplies providing up to 10W each for LED strings containing amber and green LEDs. This gives a total electrical power of 50W. Figure 5 shows this luminaries design at full power. The color sensor is located in the middle of the luminaire array, facing down, to achieve a proper measurement of the light color and intensity.

No color or brightness differences could be seen by test personnel.

Spectral Tuning Luminaries at full power

Figure 6 shows luminaries dimmed down. Notice some of the luminaries actually turned off due to high ambient light conditions, i.e. sun light from the windows.

Spectral Tuning Luminaries dimming

Conclusion

A LED system that uses spectral tuning is shown to provide uniform color characteristics in office or factory environments. Spectral tuning allows color compensation from sunlight or other light sources that may be affecting the space where brightness and color control are desirable. The feedback system can also offset aging or drift effects associated with LED lifetime and color shift. Cost effects from tightly binned LEDs are also eliminated since spectral tuning feedback is used to control the color. Other benefits from the example presented can include calibration, implementing advance protection features, setting safety light conditions to balance light output versus power used from a battery backup versus lighting a desired escape route, and dimming can be remotely controlled through a wireless interface targeting specific fixtures versus controlling the entire system.

 

About The Author

Brian Johnson is the America and Europe Lighting Specialist for Fairchild’s LED Lighting Product Segment. He has been with Fairchild for over a year after spending 20+ years rotating in Development and Marketing positions in the Power Electronics Industry. He graduated from Purdue University with a B.S.E.E. and M.S.E.E.

 

 

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